MODELLING OF METHANE HYDRATE FORMATION

MODELLING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS BY CHEMICAL AFFINITY Final Dissertation Submitted

In partial fulfilment of the requirement for The award of the degree of

MASTER OF TECHNOLOGY In PETROLEUM ENGINEERING GUDALA MANOJKUMAR Admission No. 2011MT0141

Under the guidance of

Dr. SUKUMAR LAIK Professor Department of Petroleum Engineering, Indian School of Mines, Dhanbad.

Department of Petroleum Engineering Indian School of Mines, Dhanbad May-2013.

CERTIFICATE This is to certify that dissertation entitled “Modelling of methane hydrate formation an dissociation in presence of surfactants by chemical affinity” has been carried out by Gudala Manojkumar (Admission No. 2011MT0141), student of M.Tech (Petroleum Engineering), is a record of bonafide research work carried out by the candidate under my supervision in the Department of Petroleum Engineering, Indian School of Mines, Dhanbad. The dissertation submitted is in the partial fulfilment of the requirement of the award of the degree of Master of Technology in Petroleum Engineering from Indian School of Mines, Dhanbad, during the academic session of 2011-2013. To the best of my knowledge, the work carried out has not been submitted elsewhere for award of any degree.

Dr. Sukumar Laik Professor Department of Petroleum Engineering Indian School of Mines, Dhanbad May, 2013

ACKNOWLEDGEMENT I take this opportunity to express

my deep sense of

gratitude to my project guide

Dr. Sukumar Laik , Professor, Department of petroleum engineering , Indian School of Mines, Dhanbad for his excellence guidance and meticulous examination of the thesis manuscript. Without his help it would not have been possible for me to carry out the work smoothly. I thank him for his valuable suggestion supervision and constant inspiration throughout the course of this work without it would not have been possible to complete this work and I am deeply indebted to him. I take this opportunity to express my deep sense of gratitude to my project co-guide Dr. Ajay Mandal, Associate Professor, Department of petroleum engineering , Indian School Of Mines Dhanbad for his excellence guidance and meticulous examination of the thesis manuscript I would also like to express deep gratitude to Dr. A.K. Pathak, Professor& Head of Department of Petroleum Engineering and Dr.TK Naiya, Course Co-ordinator for their periodic suggestion and corporation during the course of work. Lastly, I sincerely pay homage to Almighty God, Parents, members of my family and all others from the Indian School of Mine, Dhanbad for their blessings as well as direct and indirect assistance that has been a source of energy and inspiration for me throughout the period of this work.

Date: 13/05/2013

Gudala Manojkumar

Place: Dhanbad.

M.Tech (Petroleum Engineering) Admn. No. (2011MT0141)

CONTENTS CONTENTS

i-iii

LIST OF TABLES

iv

LIST OF FIGURES

v-viii

ABSTRACT

ix

CHAPTER 1: INTRODUCTION

01-04

1.1.Gas hydrates of interest in hydrocarbon industry

01

1.2.Objective of the present research work

03

CHAPTER 2: FUNDAMENTAL CONCEPCTS OF GAS HYDRATE

05-17

2.1. Introduction

05

2.2. Hydrate Nucleation

05

2.2.1. Homogeneous nucleation

06

2.2.2. Embryo and Critical Nucleus

07

2.2.3. Heterogeneous nucleation

09

2.2.4. Nucleation Sites

11

2.3. Growth of Gas Hydrate

11

2.4. Structure

14

2.5. Hydrate stability zone

16

CHAPTER 3: LITERATURE REVIEW

18-24

3.1. Introduction

18

3.2. Gas hydrate study in presence of surfactants

20

CHAPTER 4: EXPERIMENTAL SET UP AND PROCEDURE

25-35

4.1. Experimental set up for gas hydrate

25

4.2. Instrumental section

27

4.2.1. Cell

27

4.2.2. Thermostatic bath

27

4.2.3. Software

28

i

CONTENTS 4.3. Experimental procedure

29

4.3.1. Experimental Procedure of HSZ Experiment

29

4.3.2. Stability Criteria

30

4.3.3. Experimental Procedure of Hydrate Kinetic Study

32

CHAPTER 5: MODELING OF METHANE HYDRATE FORMATION

36-63

AND DISSOCIATION IN PRESENCE OF SURFACTANTS 5.1.Introduction

36

5.2.Modeling of methane hydrate formation and dissociation

36

5.2.1. Gibbs–Duhem relation in presence of surfactants

36

5.2.2. Chemical affinity

38

5.2.3. Rate of Affinity decay

40

5.2.4. Modeling of methane hydrate formation

41

5.2.5. Modeling of methane hydrate dissociation

42

5.3.Experimental section

43

5.3.1. Materials Used

43

5.3.2. Procedure

44

5.4.Experimental results and discussions

44

5.4.1. Methane hydrate formation and dissociation in presence of pure water

44

5.4.2. Chemical affinity and affinity decay rate changes in presence of pure

46

water during formation and dissociation 5.4.3. Methane hydrate formation in presence of SDBS

47

5.4.4. Chemical affinity and affinity decay rate changes in presence

51

of SDBS during formation and dissociation 5.4.5. Methane hydrate formation in presence of CTAB

53


55

of CTAB during formation and dissociation. 5.4.7. Methane hydrate formation in presence of Tergitol

57


60

of Tergitol during formation and dissociation

ii

CONTENTS CHAPTER 6: STORAGE CAPACITY AND KINETICS OF METHANE

64-73

HYDRATE FORMATION IN PRESENCE OF SURFACTANTS 6.1. Introduction

64

6.2. Storage capacity of methane hydrate in presence of SDBS, CTAB and Tergitol

64

6.3. Kinetics of methane hydrate formation in presence of SDBS, CTAB, and Tergitol 66 6.4. Induction time of Hydrate Formation in presence of SDBS, CTAB, and Tergitol 71 CHAPTER 7:

SUMMARY AND CONCLUSIONS

WORKSHOPS/CONFERENCES

74-75 76

REFERENCES

77-79

iii

LIST OF TABLES Table

Description

Page

2. 1

Physical properties of the three types of gas hydrate structures (Sloan, 1998)

15

4. 1

Technical Description and Operating Conditions of Video Hydrate Cell

29

4. 2

Measurement of Temperature-Pressure data during HSZ experiment

31

4. 3

Temperature-Pressure data during measurement of induction time

34

5. 1

Formation and Dissociation parameters of methane hydrate in

50

presence of SDBS in water 5. 2

Chemical affinity decay rates during formation and dissociation in

51

presence of SDBS 5. 3


55

presence of CTAB in water 5. 4


56

presence of CTAB 5. 5


59

presence of Tergitol in water 5. 6


61

presence of Tergitol 6. 1

Storage Capacity of Methane hydrate in presence of Surfactant

66

6. 2

Hydrate formation rate and Rate constant

70

6. 3

Measured Induction time of hydrate formation in presence of surfactant

72

iv

LIST OF FIGURES Figure No

Description

Page

2. 1

Formation of a crystal according to classical nucleation theory

7

2. 2

The process and the energy change of spontaneous nucleation

9

(Homogeneous nucleation) 2. 3

Gibbs free energies of heterogeneous nucleation compared

10

to homogenous nucleation 2. 4

The process from nucleation to crystal growth

12

2. 5

Phase diagram for gas hydrates indicating the location relative to the

13

phase boundary of various chemical processes 2. 6

Two staged growth using a mixture of methane and ethane

14

2. 7

Hydrate structures (From Center for Gas Hydrate Research – Heriot Watt)

16

2. 8

Hydrate stability zone in offshore environments

17

3. 1

Clathrate lattice with methane molecules entrapped in

19

water molecule (Structure I). 4. 1

Hydrate Cell

25

4. 2

Autoclave Gas Hydrate Set Up

26

4. 3

Schematic diagram of Gas Hydrate Set up

26

4. 4

Video Hydrate Cell

27

4. 5

Thermostatic bath for both cooling and heating

28

4. 6

Graphic display of pressure, temperature and bath set point temperature

28

during experiment 4. 7

Online Graph of experiment

31

4. 8

Online video pictures at the intervals of 10min after the nucleation of hydrate

32

4. 9

Excel spreadsheet

34

4. 10

Pressure-Temperature change during measurement of induction time

35

5. 1

Schematic graph of Pressure- Temperature cure of hydrate formation

41

5. 2

Schematic graph of Pressure- Temperature cure of hydrate dissociation

43

5. 3

Different types of surfactants used for methane hydrate formation

43

and dissociation

v


5. 4

Description

Page

Pressure-Temperature trace of methane hydrate formation and

45

dissociation in pure water 5. 5

Pure methane hydrate prepared in laboratory (2011)

45

5. 6

variation Chemical affinity and thermodynamic extent of reaction

47

w.r.to time in presence of pure water during methane hydrate formation and dissociation. 5. 7

Temperature pressure profile of methane hydrate formation and

48

dissociation in presence of 1000ppm SDBS in distilled water. 5. 8


49

dissociation in presence of 5000ppm SDBS in distilled water. 5. 9


50

dissociation in presence of 10,000ppm SDBS in distilled water. 5. 10

Variation of chemical affinity and thermodynamic extent of reaction

51

w.r.to time in presence of 1000ppm of SDBS during methane hydrate formation and dissociation. 5. 11


52



52



53

dissociation in presence of 1000ppm CTAB in distilled water. 5. 14


54

dissociation in presence of 5000ppm CTAB in distilled water. 5. 15


54

dissociation in presence of 10, 000ppm CTAB in distilled water. 5. 16

Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 1000ppm CTAB during methane hydrate formation and dissociation. vi

55


5. 17

Description


Page

56

w.r.to time in presence of 5000ppm CTAB during methane hydrate formation and dissociation. 5. 18


57

w.r.to time in presence of 10000ppm CTAB during methane hydrate formation and dissociation 5. 19

Temperature-Pressure profile of methane hydrate formation and

58

dissociation in presence of 1000ppmTergitol in distilled water. 5. 20


58

dissociation in presence of 5000ppmTergitol in distilled water. 5. 21


59

dissociation in presence of 10,000ppmTergitol in distilled water. 5. 22


60

w.r.to time in presence of 1000ppm Tergitol during methane hydrate formation and dissociation. 5. 23


61



62


Change in chemical affinity decay rates in presence of different

62

concentrations of SDBS, CTAB, and Tergitol during methane hydrates formation 5. 26

Change in chemical affinity decay rate in presence of different concentrations of SDBS, CTAB, and Tergitol during methane hydrate dissociation

vii

63

LIST OF FIGURES

Figure No

Description

Page

6. 1

Storage capacity of methane hydrate in presence of surfactant

65

6. 2

Change of moles of methane during hydrate formation in

67

presence 1000ppm SDBS 6. 3

Semi-logarithmic plot of change of moles of gas during hydrate

67

formation in presence of 10000ppm SDBS 6. 4

Hydrate formation rate in presence of different surfactant

68

6. 5

Temperature-pressure profile during measurement of

72

induction time in 1000ppm SDBS

viii

ABSTRACT Natural gas hydrates have been an area of active research in the oil and gas industry since their role in plugging or blocking fluid flow in oil and gas pipelines was demonstrated and also the largest source of natural gas. Gas hydrates are also used as storing and transportation purposes. Chemical affinity used as a one of the driving force for formation and dissociation of methane hydrate formation and dissociation after nucleation. Chemical affinity used in the modelling of methane hydrate formation and dissociation in presence of surfactants. Chemical potential of solution and micelles are equal at the equilibrium point. Chemical affinity decay rate vary with the concentration of surfactants. Methane hydrates were produced in a high pressure autoclave cell and kinetic rates were investigated in the presence of SDBS, CTAB, and TERGITOL at different concentrations (0ppm, 1000ppm, 5000ppm, 10000ppm). The degree of sub-cooling and chemical affinity have been used as the driving force for hydrate formation. The experimental results show that the rate of hydrate formation is strongly influenced by pressure, temperature and driving force. Experimental results were used in the mathematical modeling of methane hydrate formation and dissociation. By adding surfactants the chemical affinity and extent of reaction are varying with concentration of surfactants. The results shows that decay rate is varying with concentration of surfactants. Formation pressure and formation temperatures are varying after surfactants addition, and also the dissociation temperature and pressure. The results show that all additives increase the dissociation rate of methane hydrate below the ice point. The induction time decrease and growth rate increases by adding surfactant. The effect of surfactants on methane hydrate formation and dissociation in thoroughly emphasized in this work in future thermodynamic modeling is going to be working on this changes.

ix

CHAPTER 1 INTRODUCTION Natural gas hydrates have been an area of active research in the oil and gas industry since their role in plugging or blocking fluid flow in oil and gas pipelines was demonstrated by Hammerschmidt (1934). Makogon (1965) first proposed that natural gas hydrates could exist in the earth’s subsurface. Since then, research has been performed to estimate and quantify the volume of naturally occurring gas hydrates both onshore (beneath the permafrost) and offshore (in deep water marine sediments). Natural gas hydrates are an outstanding energy source of future. They are occurring worldwide, mainly in off shore (90% of the oceans aerial extent) in outer continental margins sediments and to a lesser extent in Polar Regions where they are associated with permafrost (23% of land mass). Both biogenic and thermogenic gas hydrates are found in oceanic sediments. The gas hydrates appear to contain largest mass of organic carbon in earth in the form of trapped methane. The amount of this methane is likely to be twice times more of the combined oil and gas reserves known to exist in the world. Because hydrates concentrates methane (at STP) by as much as a factor of 164 and since less than 15% of the recovered energy is required for dissociation, the Natural gas hydrates are solids that form from a combination of water and one or more hydrocarbon or non-hydrocarbon gases. In physical appearance, gas hydrates resemble packed snow or ice. In a gas hydrate, the gas molecules are "caged" within a crystal structure composed of water molecules. Sometimes gas hydrates are called "gas clathrates". Clathrates are substances in which molecules of one compound are completely "caged" within the crystal structure of another. Therefore, gas hydrates are one type of clathrate. 1.1. Gas hydrates of interest in hydrocarbon industry Gas hydrates are a potential energy resource: Considering the planet as a whole, the quantity of natural gas in sedimentary gas hydrates greatly exceeds the conventional natural gas resources (e.g., Kvenvolden, 1993). As a result, numerous studies have discussed the energy resource potential of gas hydrates (see, for example, Collett, 1993, 1997, 2002; Iseux, 1992; Kvenvolden, 1993; Milkov and Sassen, 2003).However, utilization of gas

CHAPTER 1

INTRODUCTION

hydrates as an energy resource has been largely inhibited by the lack of economical methods for production for most hydrate accumulations, especially marine shelf hydrates. A variety of different mechanisms have been proposed for economically developing gas hydrates as an unconventional gas source (e.g., see discussions in Goel et al., 2001, Sawyer et al., 2000). Thus far, the only method that has been successful used to economically produce gas from gas hydrates is the "depressurization method". This method is applicable only to hydrates that exist in Polar Regions beneath permafrost. This method is applicable when a free gas phase exists beneath the hydrate accumulation. Under such circumstances, production of the free gas leg using conventional gas development techniques produces a pressure drop. This pressure drop causes the overlying hydrate to become unstable and to progressively disassociate into free gas + water, a process that adds gas to the underlying free gas accumulation. Gas hydrates estimation of reserves: USA – 318,000 TCF (Collett 1995) Alaska North Slope – 590 TCF (Collett 1997) Japan – 1,765 TCF (MITI/JOGMEC 1998) India – 4,307 TCF (ONGC 1997) Canada - 1,550 – 28,600 TCF (Majorowicz and Osadetz 2001) Beaufort/Mackenzie Delta – 311 TCF (Osadetz and Chen 2005) The role of gas hydrates in past and future climate changes Gas hydrates are also of interest because of their potential role in climate change. Gas hydrates in continental shelf sediments can become unstable either as a result of warming bottom water, or as a result of a pressure drop due to a reduction in sea level (such as during an ice age). If these marine gas hydrates begin to rapidly disassociate into gas + water, then the methane trapped in the gas hydrates can be released to the atmosphere. Methane is a greenhouse gas. In fact, methane is many times more effective as a greenhouse gas than is CO2. Therefore, if the flux of methane to the atmosphere from dissociating hydrates is sufficient in quantity, this methane can cause global warming. This process is believed to have influenced past climate changes (see, for example, Henriet, 1998; Haq, 1998; Hesselbo et al., 2000; Kvenvolden, 1991), and may enhance the current global warming episode by way of a "positive feedback" loop. Specifically, as the earth warms, 2

CHAPTER 1

INTRODUCTION

increasing bottom water temperatures could cause gas hydrate disassociation in many marine shelf locations. This gas hydrate disassociation would cause further warming due to the greenhouse effects of the gas which is released. Production (flow assurance) problems Anthropogenically formed gas hydrates create another reason that these substances are of interest. Gas hydrates can spontaneously form in petroleum production equipment and pipelines associated with deep-water petroleum production and arctic on-shore petroleum production. These unwanted hydrates can clog equipment, preventing the optimum production of hydrocarbons. Various methods are used to prevent hydrate formation in petroleum production and transportation equipment (see, for example, Paez et al., 2001; Reyma and Stewart, 2001; Yousif and Dunayevsky, 1997; Behar et al.,1994). 1.2. Objective of the present research work The gas hydrates are encountered either in natural environment or in production operations of oil and gas always associated with various additives like clays, sand, silts, salts, etc. Naturally its property gets changed during formation as well as in dissociation. To exploit natural gas hydrates economically or inhibit hydrate in order to avoid its formation in production operations or develop hydrate as a new means of storage and transportation, the understanding of gas hydrate formation and dissociation is crucial in presence of various additives. The understanding of kinetics of hydrate formation and dissociation is very important for production operations of gas form hydrate accumulated basins. Recently gas hydrate technology was used for storing and transportation purposes. Therefore the main objective of this work is to develop fundamental understanding of gas hydrate formation and dissociation in presence of different types of surfactants. The present research work is undertaken as follows:  Methane is the major constituent of natural gas hydrate is used as hydrate forming gas molecules, water as host to encapsulate methane in cage of water molecules and effect of different surfactants after detailed literature survey to observe the effects of these materials on methane hydrate formation and dissociation.

3

CHAPTER 1

INTRODUCTION

 Mathematical modeling in which chemical affinity is used as new driving force for

hydrate formation and dissociation after nucleation/formation starts and sub-cooling is before nucleation starts.  Chemical affinity decay rate is calculating to determine the how fast the system reaches

to the equilibrium point. During formation, equilibrium point was defined as the point where there is no further consumption of methane gas. During Dissociation, equilibrium point was defined as the point where there is no further release of methane gas from formed methane hydrate.  Hydrate formation is greatly dependent on the pressure and temperature. Therefore experiments were carried out with fixed temperature and pressure for a given sets of experiments to observe the effects of different surfactants with different concentration in water on hydrate formation and dissociation. The temperature and pressure conditions of hydrate formation and dissociation are measured experimentally in presence of surfactants.  Hydrate formation rate, rate constant and induction time of hydrate formation these are major kinetics parameter of hydrate formation is measured. Once we could know the temperature-pressure conditions of hydrate formation, how long it would be inhibited without hydrate formation which is determined by the measurement of induction time.

4

CHAPTER 2 FUNDAMENTAL CONCEPCTS OF GAS HYDRATE 2.1. Introduction Gas hydrates are ice-like crystalline compounds that are composed of water molecules (host) with hydrate forming gas molecules (guests). The gas molecules interact with water molecules through van der Waals (non-polar) forces. Since no bonding exists between the guest and host molecules, the guest molecules are free to rotate inside the cages, and this rotation can be measured by spectroscopic techniques (Gutt et al. 1999).Natural gas hydrates (NGH) are crystalline compounds formed by the association of molecules of water with natural gas. Makogon (1997) illustrates the methane hydrate formation reactions as: CH4

+

(Methane) CH4 (Methane)

nh H 2 O (Water)

+

nh H 2 O (Ice)

↔ CH4 nh H2 O + ∆H1

(2.1)

(Hydrate) ↔ CH4 nh H2 O + ∆H1

(2.2)

(Hydrate)

Here nh is the hydration number approximately equal to 6 for methane hydrates (Sloan and Koh, 2008). The hydrate formation reaction is an exothermic process (generates heat) and the hydrate dissociation reaction is an endothermic process (absorbs heat). And ∆H1 is the heat of formation of methane hydrate. The formation of natural gas hydrates depends on pressure, temperature, gas composition, driving force and presence of various additives. The purpose of this chapter to identify some of the important factors influencing the nucleation process, proposed nucleation models of gas hydrates that may be utilized to interpret the data obtained during the experiment of hydrate formation and dissociation and properties of crystal structures. 2.2. Hydrate Nucleation To exploit natural gas hydrates economically understanding of gas hydrate formation is crucial. The efficiency of any exploitation strategy depends on the knowledge of kinetics of hydrate formation and dissociation. An essential ingredient of such phenomena is the development of appropriate theoretical tools that can describes all stages of the processes involved. The little understood steps of hydrate formation are nucleation and its growth. 5

CHAPTER 2

FUNDAMENTAL CONCEPTS OF GAS HYDRATE

Nucleation is perhaps the most challenging step in understanding the process of crystallization of gas hydrates. At the same time, its understanding could be the key to the kinetic inhibition or promotion of this process. Nucleation of hydrates is a microscopic stochastic phenomenon where gas-water clusters (nuclei) grows and disperses until the nuclei have grown to a critical size (Natarajan et al., 1994). Primarily hydrate nucleation takes place at the vapor-liquid (V-Lw) interface (Sloan, 1998), thus the theories dealing with describing this phenomenon have focused on this surface. Two theories dealing with describing the nucleation mechanism have gained acceptance in literature although they are hypothetical. 1. One of these is the cluster nucleation theory which proposes that water molecules form labile clusters around dissolved gas molecules. These clusters combine due to hydrophobic bonding between the polar molecules inside the clusters, to form hydrate unit cells. 2. The other theory assumes that nucleation is taking place on the vapor side at the (V-Lw) interface. First gas molecules are transported to the interface and absorbed by the aqueous surface. At suitable adsorption sites water molecules will form first partial and then complete cages around the adsorbed gas molecules. Clusters will join and grow on the vapor side until the critical size is reached. 2.2.1. Homogeneous nucleation Homogeneous nucleation is a solidification process occurring in the absence of impurities. It involves many more molecules than could collide simultaneously, so a sequence of bimolecular collisions of an autocatalytic nature is more probable. That is, there is a sequential formation of clusters (embryos) within the supercooled liquid of increasing cluster size, until the critical cluster size, An is reached. The critical cluster size (also called critical nucleus) is the cluster size that must be reached before nuclei/clusters can grow spontaneously. Before achieving the critical size, clusters (or embryos) of molecules form in the bulk metastable liquid, and these clusters may either grow or shrink as a result of density or composition fluctuations. When the cluster attains a critical size, monotonic growth occurs. Such a phenomenon of critical cluster size formation and spontaneous growth may be interpreted by the excess Gibbs free energy (ΔG) between a small solid particle of solute and 6

CHAPTER 2


the solute in solution. ΔG is equal to the sum of the surface excess free energy ΔGs (for solute molecules becoming part of the surface of the crystal nuclei), and the volume excess free energy ΔGV (for solute molecules ending up in the bulk/interior of the crystal nuclei).

Figure 2.1 Formation of a crystal according to classical nucleation theory

2.2.2. Embryo and Critical Nucleus: If a globular particle of a new phase (a liquid phase or a solid phase) is produced in a supercooled phase (a gas phase or a liquid phase), the change in free energy per particle can be expressed by ∆G = ∆Gs + ∆GV

(2.3)

4

∆G = 4πr 2 σ + 3 πr 3 ∆g v

(2.4)

Where ∆g v , is free energy change per unit volume and σ is the surface tension of the crystalliquid interface. The addition of the surface and volume effects causes a maximum (∆Gcri ) in the value of ∆G at the radius corresponding to the critical nucleus, rc . That is, the free energy barrier ∆Gcri must be surmounted to form a cluster of a critical size, beyond which the nuclei/clusters grow spontaneously. The maximum value of ∆G and the critical radius are obtained by differentiating Equation 2.4 and setting the result to zero to obtain: ∂∆G

[ ∂r ] = 0

(2.5)

rc = −2σ/∆g v

(2.6)

∆Gcri = 4πσrc2 /3

(2.7)

rc

∆Gcri =

16πσ3

(2.8)

3∆g2v

7

CHAPTER 2


Because the change in free energy is upward while the radius is less than rc , the new phase particle is metastable and it will be annihilated immediately after it is generated. The new phase particle in such a state is called an embryo. However, because once the radius goes beyond rc the change in free energy becomes downward; the new phase particles will go on growing. This rc is called the critical radius, and a new phase particle of radius rc is called the critical nucleus. In general, the free energy of transformation ∆g v is proportional to the degree of supercooling ∆T, ∆g v =

∆H

∆T

Te

(2.9)

Equations 2.6 and 2.7 are rewritten as rc =

−2σTe

(2.10)

∆H.∆T 16πσ3 T2

∆Gcri = (3.∆H2 .∆Te2)

(2.11)

The rate at which critical sized clusters are formed is very sensitive to the height of the free energy barrier (∆G), or equivalent to the extent of penetration into the metastable region. As the critical cluster size becomes smaller, so does the free energy barrier that must be overcome to form the critical cluster. With increasing super saturation, the free energy barrier eventually becomes small enough for nucleation to become spontaneous. Englezos et al. (1987a) and Englezos and Bishnoi (1988) determined an expression for the radius of the hydrate critical nucleus using the Gibbs free energy per unit volume of hydrate formed (∆g v ) in a modification of Equations 2.6 and 2.7 as rc =

−2σ

(2.12)

∆gv

(−∆g v ) =

RT vh

f

[∑21 θj ln (f b,j ) + ∞,j

nw vw (P−P∞ ) RT

]

(2.13)

Where the surface tension σ is the value of ice in water vh and vw are the molar volumes of hydrate and water, respectively, θj is the fractional filling of the hydrate cages on a water free basis,fb,j and f∞,j are the bulk phase experimental and equilibrium fugacities, respectively, of component j at temperature T, (P − P∞ ) represents the overpressure, and nw is the number of water molecules per gas molecule. The equation contains the assumption that the hydrate phase is at equilibrium, rather than at operating conditions.

8

CHAPTER 2


Crystal

r

Radius

Gibbs free energy

Interface energy

Cluster size

Volume energy

Critical nucleus

Growing Embryo Atom Induction period

Growing

Nucleation Radius of nucleation,r

Time, t

Figure 2.2 The process and the energy change of spontaneous nucleation (Homogeneous nucleation)

Using Equations 2.12 and 2.13, Englezos et al. (1987a) calculated the critical radius of methane hydrate to be 30–170 Å. In comparison, critical cluster sizes using classical nucleation theory are estimated at around 32Å (Larson and Garside, 1986), while computer simulations predict critical sizes to be around 14.5 Å (Baez and Clancy, 1994; Westacott and Rodger, 1998; Radhakrishnan and Trout, 2002). Chen (1980, p. 7) suggested that the use of bulk phase properties, Microscopic critical clusters contain several tens to thousands of molecules, and as such have a spectrum of sizes and properties, which may be difficult to quantify with a single number on a macroscopic scale. 2.2.3. Heterogeneous nucleation: Heterogeneous nucleation occurs much more often than homogeneous nucleation. Heterogeneous nucleation applies to the phase transformation between any two phases of gas, liquid, or solid, typically for example, condensation of gas/vapor, solidification from liquid, bubble formation from liquid, etc. Heterogeneous nucleation forms at preferential sites such as phase boundaries, surfaces (of container, bottles, etc.) or impurities like dust. At such preferential sites, the effective surface energy is lower, thus diminishes the free 9

CHAPTER 2


energy barrier and facilitating nucleation. Surfaces promote nucleation because of wetting – contact angles greater than zero between phases facilitate particles to nucleate. The free energy needed for heterogeneous nucleation is equal to the product of homogeneous nucleation and a function of the contact angle: ∆G′ cri(hetrogeneous) = ∅ × ∆Gcri(homogeneous)

(2.14)

Where ∅=

2−3cosθ+cos3 θ

(2.15)

4

The barrier energy needed for heterogeneous nucleation is reduced and less super-cooling is needed. The wetting angle determines the ease of nucleation by reducing the energy needed.

Figure 2.3 Gibbs free energies of heterogeneous nucleation compared to homogenous nucleation

When the angle of contact θ =180 (complete non-wetting of the substrate) then ∆G′ cri(hetrogeneous) = ∆Gcri(homogeneous) When the angle of contact θ =0 (complete wetting of the substrate) then ∆G′ cri(hetrogeneous) = 0 The foreign surface effectively lowers the ∆G′ cri(hetrogeneous) and critical radius (rc ) required for catastrophic growth, as shown in Equations 2.14 and 2.15. Homogeneous nucleation of hydrates is an anomaly. Hence, heterogeneous nucleation occurs much more frequently.

2.2.4. Nucleation Sites A nucleation site is a point where a phase transition is favored, and in this case the formation of a solid from a fluid phase. An example of nucleation is the deep fryer used to 10

CHAPTER 2


make French fries in fast-food restaurants throughout the world. In the fryer, the oil is very hot but does not undergo the full rolling boil because there are no suitable nucleation sites. However, when the potatoes are placed in the oil, it vigorously boils. The French fries provide an excellent nucleation site. Importance and purpose of nucleation sites: Heterogeneous nucleation is far more common place when compared to homogeneous nucleation, as started before. Heterogeneous nucleation takes place in the presence of foreign bodies such as dust particles, a fluid interface or a container wall. From a free energy point of view, it has been found that it is easier to form hydrate nuclei in a two dimensional surface like dust or container wall, rather than a three dimensional surface free volume of water (Sloan and Koh 2007). In the case of heterogeneous nucleation, the presence of these foreign particles reduces the excess free energy and the critical radius required to form crystal clusters of hydrates. 2.3. Growth of Gas Hydrate The hydrate growth stage is an immediate process that follows the nucleation stage. A very simple but powerful method of studying and analyzing growth experiments is by using the real gas equation. Most of the nucleation parameters (displacement from equilibrium conditions, surface area, agitation, water history, and gas composition) continue to be important in hydrate growth.

11

CHAPTER 2


Figure 2.4 The process from nucleation to crystal growth

The process of making a crystal is broken into two parts. The first is the process of nucleation. The solution organizes into small units of the crystal. These small units are not stable: they can re-dissolve easily. Dissolution is no longer likely once the nucleus reaches a critical size. Growth of the crystal can then occur by addition of more structural units. Like nucleation rates, growth rates increase with driving force. Unlike nucleation, the end result of a growth process may involve different types, shapes, and sizes of crystals depending on the driving force (Myerson, 2002, Chapter 6). Where there is the greatest potential for growth outside of the region of homogeneous nucleation, within the dendritic and crystallite product zones (Fig. 3.5), growth may be very rapid. Imperfect crystal structures often result, in addition to overgrowth of other material, including growth media and its dissolved constituents. Growth that is most likely to take place under natural conditions will lie in the pressure-Temperature region of the metastable zone (MSZ) (Fig 2.5). This is a region where primary nucleation does not normally occur but growth takes place. Unlike the normal phase boundary, which describes the location of a thermodynamic phase transition, the MSZ is a kinetic value. As time progresses the MSZ boundary approaches the thermodynamic phase boundary. At some point in time the MSZ and thermodynamic phase boundary may be coincident. 12

CHAPTER 2


Figure 2.5 Phase diagram for gas hydrates indicating the location relative to the phase boundary of various chemical processes

Two staged growth using a mixture of methane and ethane as an example. When multiple hydrate forming gases are present the first hydrate formed may be a compound hydrate, a hydrate that contains both gases. If one of the gases is completely consumed (in this case ethane), a second round of hydrate growth may occur to produce pure methane hydrate. The second induction time can be of variable length. Pure methane hydrate formation may possibly begin while compound hydrate formation is ongoing (figure 2.6). The ratio of methane to heavier hydrocarbon consumption during compound hydrate formation depends on the initial composition of the HFG presents the decrease in partial pressure of methane and propane during the initial precipitation of compound hydrate with a ratio of 11 mole methane: 1 mole propane. Dendrites are solids that grow in such a way that any given point of the growing material may serve as a nucleation site for new crystal growth. The pattern is essentially that of a river, which gives way upstream to smaller and smaller tributaries and so on. Each of these dendritic crystals provides the roots for other crystals until the crystals may fill the entire space and overgrows and traps non-crystalline material. This growth mode will result in structures that have a very high surface area with respect to both the mass of hydrate and the volume in which the growth has taken place. 13

CHAPTER 2


Figure 2.6 Two staged growth using a mixture of methane and ethane

2.4. Structure Gas hydrates are solid crystalline compounds in which gas molecules are encaged inside the lattices of ice crystals. These light non-polar gases are referred to as guests, whereas the ice crystals are called hosts. The following four structure rules of thumb: i. Fit of the guest molecule within the host water cage determines the crystal structure. ii. Guest molecules are concentrated in the hydrate, by a factor as high as 180. iii. Guest: cage size ratio controls formation pressure and temperature. iv. Because hydrates are 85 mole % water and 15 mole % gas, gas–water interfacial formation dominates. The water molecules through hydrogen bonding form a lattice structure with interstitial cavities. These cavities are occupied by gas molecules with molecular size smaller than the diameter of the cavities, thereby stabilizing the crystal lattice framework. It has been established that a majority of hydrates crystallize into two types of structures, commonly known as sI and sII, which have been investigated with X-ray diffraction methods by von Stackelberg and Müller (1954). They found that the unit cell of sI is a 12Å cube, consisting of 46 water molecules, which has two types of cavities. The two small cavities are pentagonal dodecahedra (512), whereas the six large cavities are tetradecahedra (51262) and have two opposite hexagonal faces and twelve pentagonal faces, giving an average 14

CHAPTER 2


coordination number for the hydration shell in the crystalline state of 22 water molecules at a radius of about 3.91 Å (Table 2.1). The smaller cavities are almost spherical, whereas the larger cavities of sI are slightly oblate. The unit cell of sII, which is a 17.3 Å cube with 136 water molecules, also contains two types of cavities. The 16 smaller cavities are distorted pentagonal dodecahedra and the 8 larger cavities are hexadecahedra (51264), having 4 hexagonal faces and twelve pentagonal faces. The latter cavities are almost spherical in shape. Table 2. 1 Physical properties of the three types of gas hydrate structures (Sloan, 1998).

Structure I Small Large

Structure II Small Large

Structure H Small Medium Large

Cavity Types Radius (Ǻ) Cage/Unit cell

512 3.91 2

51262 4.33 6

512 3.902 16

51264 4.683 8

512 3.91 3

Co-ordination number Crystal type nH2O/unit cell

20

24

20

28

20

Cubic 46

Cubic 136

435663 4.06 2 20 Hexagonal 34

51268 5.71 1 36

Ripmeester et al. (1987) reported a new hexagonal hydrate structure, known as sH which requires both large and small molecules to stabilize the structure. According to the authors, the unit cell oh sH hydrate has 34 water molecules forming a hexagonal lattice. The sH has three different types of cavities, three 512 cavities which are common to all known hydrate structures, two new 12 face 435663 cavities and one new large 51268 cavity. The 435663 cavity has three square faces, six pentagonal faces, and three hexagonal faces; whereas the 51268 cavity has 12 pentagonal faces and eight hexagonal faces. The first two cavities accommodate the small gas molecules. The large cavity in this structure can accommodate even larger molecules, so molecules in the size range of 7.5 Å to 8.6 Å can potentially form gas hydrates. The structure formed is a function of the molecular size of the gas molecules, with smaller molecules such as methane, ethane, nitrogen, and carbon dioxide forming sI and larger gas molecules such as propane and isobutene forming sII. The sH is formed from components of the light naphtha fraction or components of gasoline, thus indicating a hydrate structure that can participate in petroleum as well as natural gas processes. The 15

CHAPTER 2


arrangement of molecules and properties of sI, sII, and sH are shown in Fig. 2.7 and Table 2.1.

Figure 2.7 Hydrate structures (From Center for Gas Hydrate Research – Heriot Watt) 2.5. Hydrate stability zone

Naturally occurring hydrates are known to exist in two different types of environments, arctic permafrost and deep water oceanic sediments. A majority of the hydrates occur in oceanic sediments because of active production of methane by methanogenesis in marine sediments (Claypool and Kaplan, 1974). The methane formed then reacts with pore water and forms methane hydrate when the correct pressure and temperature conditions occur. This chapter deals with the detailed characteristics of offshore hydrate deposits, as the main purpose of this dissertation is to study the geo-mechanical stability of offshore hydratebearing sediments. Because so little data are available on gas hydrate deposits in the ocean, considerable uncertainty remains concerning how the gas hydrate is distributed in the sediment and how much gas is really trapped in the form of hydrates.

16

CHAPTER 2


Figure 2.8 Hydrate stability zone in offshore environments

The amount of methane available as hydrates has been estimated by a number of researchers. Makogon (1966) first published the idea of occurrence of hydrates in nature and proved it through experimental work. He also first generated a methodology to estimate the in-place hydrates in the subsurface. A lot of studies to estimate the hydrate resource have been done since and has been described in detail by Milkov (2004). Although knowledge on the total hydrate inventory and its global distribution is fraught with significant uncertainties, it is rather well established that the oceanic hydrate deposits constitute the bulk of natural hydrates (Sloan and Koh, 2008). In offshore environments, hydrates are stable in water depths greater than 200 to 600 meters depending on the gas composition and seafloor temperatures (Milkov and Sassen, 2002). Fig. 2.8 (data from Milkov and Sassen, 2003) shows the pressure and temperature conditions that can lead to a typical offshore hydrate deposit in Gulf of Mexico.

17

CHAPTER 3 LITERATURE REVIEW 3.1. Introduction To achieve demand for fossil fuel resources requirement and depletion of global energy reserves has necessitated exploring possible alternative sources of hydrocarbons. An alternative energy source that has caught considerable attention during the recent times in hydrocarbon industry is the gas hydrates. Gas hydrate deposits exist beneath of the ocean and under many regions of permafrost. Methane, a major constituent of natural gas hydrates, is formed in these places because it is stable only at high pressure and low temperature condition. In the ocean bottom, gas hydrates are formed in the form of nodules, mound etc., it mixes up with clay and other substances like natural surfactants, sands, salts etc. at subsea soil. Naturally its property gets changed during formation as well as dissociation. Therefore the nature of pure gas hydrate is different from that of hydrates in presence of additives. A review of gas hydrate formation and its dissociation in presence of surfactants is presented in this chapter. The objective of this chapter is to provide the insights to work on the proposed research title “Modeling on Methane Hydrate Formation and Dissociation in Presence of Surfactants by Chemical Affinity”. The industrial interest in gas hydrates began with the discovery that hydrate formation could plug natural gas pipelines. Several theoretical and experimental works were focused on studying gas hydrates. The modern studies of hydrates are aimed at some goals. These are: (a) Prevention of formation and removal of large hydrate accumulation in natural gas production, transportation and processing systems. (b) Exploitation of natural gas reserves accumulated in the earth in hydrate state and studying the ecological aspects of gas hydrates. (c) Creation of new technologies by utilizing properties of hydrates for storage and transportation of gas in hydrate state. Study of gas hydrates is a very challenging job which got initiated from 1811, when Humphery Davy first produced chlorine clathrate hydrate by cooling an aqueous solution 18

CHAPTER 3

LITERATURE REVIEW

saturated with chlorine gas below 9oC. It was just a chemical curiosity. Within 200 years lot of work has been carried out on gas hydrates and it became such a vast subject that it has necessitated subdivisions and identification of various components in this, so that in-depth study in any area is possible for better understanding.

Figure 3.1 Clathrate lattice with methane molecules entrapped in water molecule (Structure I).

The process of formation of gas hydrates begins with appearance of crystallized nuclei on the gas water contact surface. The hydrate formation takes place when mutual orientation forces between the water molecules and the molecules of hydrate forming gases overcomes the disrupting forces of thermal motion of molecules. Actually, as the temperature drops or pressure increases, first a moment comes when equilibrium is reached between the orienting and disrupting forces. Then at still lower temperature or higher pressure, a transition takes place from the point of equilibrium to a region in which the orienting forces of mutual molecular attraction overcomes the disrupting forces that hinder the mutual orientation. After the formation of nuclei of a critical size, the hydrate crystal grows depending on temperature, pressure and surface area of contact between water and gases. Gas hydrate formation is strongly dependent on pressure, temperature and composition of gas. However it can be affected if the interfacial surface area of liquid-gas, gas-solid, liquid-solid surfaces change. The interface between water and gas phases is an ideal location for the formation of gas hydrates. Besides pressure, temperature and 19

CHAPTER 3

LITERATURE REVIEW

composition of gas mixture, the formation and dissociation of gas hydrates are affected by chemicals such as salts, alcohols, glycols, polymers, surfactants etc.; some of these act as gas hydrate inhibitors, and some act as promoters. 3.2. Gas hydrate study in presence of surfactants: Gas hydrates have drawn much attention not only as a new natural energy resource but also as a new means for natural gas storage and transportation. The storage of natural gas hydrate is appealing for industrial utilization because of not only its high storage capacity, but also its high safety. Hydrates can store large quantities of natural gas e.g. 180 SM 3 per M3 of hydrates (Makogon, 1997; Khokhar et al., 1998). Gudmundsson et al. (1994) reported that hydrate could be stored at -15o C under atmospheric pressure for 15 days, retaining almost all gas. Gudmundsson and BØrrehaug (1996) showed a substantial cost saving (24%) for the transport of natural gas in hydrate form compared to liquefied natural gas from the North Sea to Central Europe. Slow formation rate of natural gas hydrate has been considered to be a critical problem hindering the industrial application of gas hydrates for storage and transportation of natural gas, unreacted interstitial water as a large percentage of the hydrate mass, reliability of hydrate storage capacity, and economy of process scale-up. Since the solubility of natural gas in water is very low, only a thin hydrate film is formed at the interface between the water and gas without stirring or other enhancing measures. To solve these problems, two approaches (mechanical and chemical means) are generally adopted. The mechanical method includes stirring (Iwasaki et al, 2005; Takaoki et al., 2005) spraying of liquid in continuous gas phase (Fukumoto et al., 2001; Ohmura et al., 2002) bubbling of gas in continuous liquid phase, microbubbling and icing. The chemical method consists of changing the properties of reactant system by adding low dose of surfactants to decrease gas/liquid interfacial tension and to increase the solubility of gas in liquid water. For example, surfactant such as SDS is used to reduce the formation time of hydrates and increase the gas storage efficiency (Karaaslan and Parlaktuna, 2000; Sun et al., 2004). In recent years, researchers have reported the promotion effect of some surfactants on gas hydrate formation and gas content. Not only high formation rate and gas content of natural gas hydrate are very important in commercializing the technology of storage and transportation of gas hydrates, but also the stability of the hydrate formed. 20

CHAPTER 3

LITERATURE REVIEW

The importance of studying hydrate formation and dissociation in the presence of surfactants is due to the fact that some surfactants are naturally formed from the crude oil itself under suitable conditions in the reservoir (Yarranton et al., 2000; Gafonova and Yarranton, 2001; Moran and Czarnecki, 2007). During the last two decades, several studies have been reported showing a significantly increased hydrate formation rate with the addition of surfactant molecules (Kalogerakis et al., 1993; Karaaslan and Parlaktuna, 2000; Karaaslan et al., 2002; Sun et al., 2004). The success of the potential applications based on hydrate is mainly hindered by some technological problems associated with hydrate formation, including slow formation rates, low conversions, and the economics of process scale-up (Ribeiro et al., 2008). Some kinds of additives have been used to overcome such difficulties. The addition of THF reduces the induction time and the hydrate formation pressure. However, the rate of hydrate growth is reduced (Linga et al., 2008). Surfactants are amphiphilic molecules which exhibit a dual affinity for polar and nonpolar substances. The properties of surfactant molecule determined by the balance effects produced by the type, size, and strength of the hydrophilic and liophilic groups. The surfactant molecules tend to be water soluble if the hydrophilic group are more and if surfactant is ionized and the liophilic hydrocarbon chain is short (< 12 carbon atoms). A long hydrocarbon chain (> 16 carbon atoms) will make surfactant molecule oil soluble. The chemical structure of surfactant and other parameters depending on the different fluids, temperature and pressure can alter the affinity of surfactants with water and oil phases. Surfactant molecules present two fundamental properties: interfacial adsorption and selfassociation, which are the essential processes leading to surfactants forming structures enhancing solubilisation. The adsorption of surfactant molecules to interfaces is driven by its double affinity to polar and nonpolar substances, where free energy of the system is minimum. The surfactant can diffuse from bulk phase to an interface such as gas/liquid, liquid/liquid and liquid/solid, decreasing the interfacial tension, modifying the contact angle between the phases and wettability of solid surfaces and changing surface charge and surface viscosity (Shah, 1997). When water soluble surfactants are added to an aqueous phase, surfactant molecules adsorb to any available interface until the interface is saturated. With increasing surfactant 21

CHAPTER 3

LITERATURE REVIEW

concentration, surfactant molecules start to associate, forming spherical aggregates called micelles. The surfactant concentration at which the first micelle is formed is known as the critical micelle concentration (CMC) and may be detected from a discontinuity in the change in several variables such as surface tension, viscosity, osmotic pressure, electrical conductivity and density (Preston, 1948). The association of surfactant molecules in the form of micelles at concentration above the CMC accelerates gas hydrate formation and reduces the induction time in quiescent systems, which is of special interest for the application of gas hydrates in the storage and transportation of natural gas (Kalogerakis et al., 1993). Zhong and Rogers (2000) studied the effect of sodium dodecyl sulfate (SDS) on the hydrate formation mechanism using ethane and natural gas as guest molecules. They suggested that micelles act as nucleation point by increasing the solubility of hydrocarbon gas in the aqueous phase and by inducing the formation of hydrate crystals around the micelle in the bulk water phase below the gas/water interface. Consequently, the hydrate formation rate was observed to increase by more than 700 times and the induction time for nucleation decreased significantly compared to systems without surfactants. Other studies had obtained the similar effects with different surfactants, but invoked different mechanistic explanations, questioning the CMC requirement (Zhang et al.; 2004, Lee et al., 2010). The morphology of the hydrate film that forms at hydrocarbon/water interfaces is affected by the adsorption of surfactant molecules at the interface. Luo et al. (2007) studied gas hydrate formation in methane bubble column without surfactants, observing the hydrate shell around gas bubbles, which hindered further formation of gas hydrates. The effect of SDS below the CMC on hydrate formation of gas bubbles using static mixture was studied by Tajima et al. (2010), who reported an increase in hydrate formation rate and a change in the morphology of the hydrate film with addition of SDS. The adsorption of surfactant at the bubble interface promoted the formation of a rougher hydrate film with a weaker structure that easily collapsed; the increase in hydrate formation rate was attributed to higher mass transfer through hydrate film and surface renewal by the film collapse. Kalogerakis et al. (1993) compared the effect of anionic and non-ionic surfactants in a stirred cell, reporting a greater increase in methane hydrate formation rate with an anionic surfactant (SDS). From 22

CHAPTER 3

LITERATURE REVIEW

the experiments with anionic surfactant, aggregates of hydrate particles suspended in the liquid phase were observed, increasing the slurry viscosity. Hydrate growth on the reactor wall was attributed to a more water-wet wall due to the anionic surfactant solution. The effect of anionic surfactants seems to be detrimental for the transportability of hydrate slurries and should be avoided in pipelines, while non-ionic surfactants seem to prevent agglomeration of hydrate particles. Karaaslan et al. (2002) studied the effect of linear alkyl benzene sulfonic acid on the formation rate of hydrates with structures I and II. The work revealed that this compound increases the rate of production of both types of hydrate, but its effect on structure I is more significant. Sun et al. (2003) studied the effect of an anionic surfactant (SDS), a non-ionic surfactant (dodecyl polysaccharide glycoside) and cyclopentane on the gas content of the hydrate formed from natural gas containing 92 mol% methane. The effect of the anionic surfactant was more pronounced compared to the non-ionic surfactant. Cyclopentane reduced the induction time but could not improve the storage capacity. Gnanendran and Amin (2003) used various concentrations of para-toluene sulfonic acid as hydrate promoter and found that its optimum concentration for hydrate formation as 3.5 g/l. Link et al. (2003) observed that by addition of sodium dodecyle sulfate, the gas content of methane hydrate could reach 97% of the theoretical value. Zhang et al. (2004) reported that alkylpolyglycoside , sodium dodecyl benzene sulfonate and potassium oxalate monohydrate increase the natural gas hydrate formation rate and its storage capacity. Ganji et al. (2007) studied the effects of anionic surfactants sodium dodecyl sulphate (SDS) and linear alkyl benzene sulfonate (LABS), cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and non-ionic surfactant ethoxylated nonyphenol (ENP) on the formation, dissociation and storage capacity of methane hydrate. Each surfactant was tested with three different concentrations 300, 500 and 1000 ppm. SDS was found to accelerate the hydrate formation rate effectively with these concentrations. LABS increased the hydrate formation rate at 500 and 1000 ppm but decreased it at 300 ppm. CTAB and ENP showed 23

CHAPTER 3

LITERATURE REVIEW

promotion effect at 1000ppm while decreased the rate at 300 and 500 ppm. Hydrate stability tests have also been conducted at three temperatures, 268.2, 270.2 and 272.2 K, with and without surfactant. All three additives increase the dissociation rate of methane hydrate below the ice point. CTAB showed the minimum and LABS the maximum effect on the methane hydrate dissociation rate. Ganji et al. (2007) reported that by addition of minor amount xanthan or starch in SDS solution decreased the dissociation rate of methane hydrate effectively. The effect of surfactant carbon chain lengths (A series of surfactants with sodium sulfonic acid group in common C4, C12, and C18) on kinetics of hydrate formation was also studied. The surfactant with shortest chain length (C4, butanesulfonic acid sodium salt) showed the highest acceleration of about 2.5 times larger than pure water for methane hydrate formation (Daimaru et al., 2007). Mandal and Laik (2008) investigated the effect of SDS on ethane hydrate formation and dissociation. They observed that hydrate formation rate increases with the surfactant concentration above the critical micelle concentration (CMC) and also found increased dissociation rate with surfactant concentration under similar operating condition of pressure and temperature. Zhang et al. (2010) attempted to reduce the dosage of SDS for methane enclathration by adding a small amount of salt and cyclopentane(CP). The results showed that a small amount of CP reduces the SDS dosage. At a concentration of 20ppm or less, SDS cannot promote methane enclathration even with CP. Under the above condition, methane enclathration is accelerated by adding salts. Among the two salts investigated, NaCl was more effective than NaClO4 in promoting the enclathration.

24

CHAPTER 4 EXPERIMENTAL SET UP AND PROCEDURE 4.1. Experimental set up for gas hydrate The video hydrate cell (Fig. 4.1) of gas hydrate set up is a mercury free cell designed by Vinci Technology, France, to study gas hydrate formation and dissociation; measurement of induction time for formation of hydrates; monitoring of pressure drop as a function of time during hydrate formation. It has also provision for capturing pictures during the experiments. The system consists of a constant volume hydrate cell with a capacity of 250cc and pressure rating up to 3000psi. The cell temperature is controlled by a thermostatic bath. A thermocouple measures the temperature inside the cell with an accuracy of 0.1oC. The cell pressure is monitored by a pressure transducer. A magnetic stirrer with adjustable rotation speed is used to agitate the test fluid.

Magnetic stirrer

Figure 4.1 Hydrate Cell

A computer is used for data acquisition of temperature and pressure versus time. Figure 4.2 is the complete gas hydrate set up and its schematic flow diagram is shown in Figure 4.3. Autoclave set up is also attached with gas booster to build up high pressure inside hydrate cell and vacuum pump is used to evacuate air in the cell before inserting gas.

25

CHAPTER 4

EXPERIMENTAL SETUP AND PROCEDURE

Autoclave Set Up

Computer Interface

Gas Booster Vacuum Pump

Methane Gas Cylinder

Figure 4.2 Autoclave Gas Hydrate Set Up

7 3

2

4 6 1 5

1. 2. 3. 4. 5. 6. 7.

Gas Cylinder Gas Booster Pressure Gauge Thermostatic Bath Video Hydra Cell Computer Interface/ Data Acquisition Temperature Probe Figure 4.3 Schematic diagram of Gas Hydrate Set Up

26

CHAPTER 4


4.2. Instrumental section The instrument basically consists of three main parts: Cell, Thermostatic bath and Software to monitor the experiment through program and record temperature, pressure and time data during the experiment. 4.2.1. Cell The cell (Fig. 4.4) is made of stainless steel, equipped with a PT100 (temperature sensor), to measure temperature through the port (a) as shown in the picture (Fig. 4.4). Inside the cell, there is a magnetic stirrer which is controlled independently by an external current. At the top of the cell, the port (b) is used for housing the camera. The port (c) is used in case of condensation; a vacuum is applied to improve the visualization of the camera by application of vacuum. The technical description and operating conditions of video hydrate cell is given in table 4.1 c a b

Figure 4.4 Video Hydrate Cell

4.2.2. Thermostatic bath: The dimension of the bath (Fig. 4.5) is 225×370× 429mm, the fluid capacity is 2.7L. The bath works with a thermostatic fluid (water and glycol at 25%) to reach working temperature up to 600C. The bath has maximum fluid pressure of 700mbar and maximum fluid flow rate of 27L/min. Its operation is very simple to obtain an excellent thermal stabilization and 27

CHAPTER 4


control of temperature during the processes by chilling or heating at a fast rate can be achieved.

Camera

Compatible Control-Thermostats Pressure Guage

Bath

Figure 4.5 Thermostatic bath for both cooling and heating

4.2.3. Software: A color graphics screen (Fig. 4.6) displays information clearly, such as: set point temperature of bath, actual measured temperature of process fluid inside the cell. RS232 interface allows monitoring and record data (pressure, temperature and time) from Applib Lab interface.

Figure 4.6 Graphic display of pressure, temperature and bath set point temperature during experiment 28

CHAPTER 4


Table: 4.1 Technical Description and Operating Conditions of Video Hydrate Cell

Description

Operating Conditions

Operating Pressure

3,000 psi

Pressure accuracy

0.1% Full Scale

Process Fluid

Hydrocarbons, water, solvent

Cell’s volume

250cc

Operating Temperature

-10 0C to 60 0C

Temperature accuracy

0.10C

Stirring Mechanism

Magnetic

Stirring speed

Up to 1000RPM

4.3. Experimental procedure Two types of experiments are carried out in this instrument automatically through a program configured by VINCI Technology a. Hydrates stability zone (HSZ) b. Hydrates kinetic study Hydrate stability zone experiment reveals the formation and dissociation conditions of hydrate formation while hydrate kinetic study is used to measure the induction time of hydrate formation. 4.3.1. Experimental Procedure of HSZ Experiment: The hydrate cell was filled with known volume of pure water (without additive) or water having additives depending on our interest of investigation during the course of experiment and immersed into a temperature controlled bath to maintain a constant temperature of the cell. The liquid in the bath is a mixture of water and ethylene glycol (25%). Before charging methane gas, the cell was evacuated by vacuum pump. Cell was then pressurized with methane gas up to the desired pressure and test liquid in the cell was agitated with magnetic stirrer to saturate the liquid. Decrease in pressure was observed due to dissolution of gas in liquid. Methane gas was again charged to preset pressure to compensate pressure loss. The stirring rate is an important parameter that has an effect on the kinetics of hydrate formation. Therefore the test sample is agitated with a constant RPM with the stirrer. The cell was cooled step-wise in the programmable bath (Set point temp., Table 4.2) 29

CHAPTER 4


and sufficient time (1-3 hour) was given as per requirement to attain the equilibrium condition at each temperature. The hydrate formation was detected by a sudden pressure drop (Fig 4.8) under constant cell temperature and observation by online video picture (Fig 4.9). After hydrate formation, the bath temperature is brought to 271.15K and hydrate is kept at that temperature for 3-6 hours to stabilize the hydrate and to allow it to attain equilibrium state. To observe the dissociation of hydrate the entire cell was heated slowly, step wise so that equilibrium of the system is not disturbed. On increasing temperature, dissociation of hydrate was observed with substantial increase in pressure. Hydrate dissociation is assumed to be complete when the heating curve joined the cooling curve i.e. loop is closed (Fig 4.8). The time, temperature and pressure data obtained (Table 4.2) during the experiments were stored in the computer and also displayed as online graphs (Fig 4.8). At least 30 hours is required to complete one HSZ experiment. Before running the macro (software) of HSZ experiment, the stability criteria must be filled in the excel spreadsheet in order to get data. 4.3.2. Stability Criteria: At each temperature step, the system will analyze if the mixture is thermodynamically stable. In order to do so, the software will record P, T parameters and make the measurement when P, T is not fluctuating any more. For example, if the stability criteria are: P=5psi, T=0.5oC, duration=20min and skip stability=60min, then when (P, T) is fluctuating less than (5psi, 0.5oC) during 20min, then the software will make the measurements and go to the next pressure step. If stability is not reached during 60min, then the software will make the measurements (even if it is not stable) and go to the next step.

HSZ OIL FIELD Stability Criteria Pressure (psi)

Temp. (°C)

10 0.5 skip stability

Duration (min) 20.00 60

More or less same procedure is used in the entire work to carry out the hydrate stability zone (HSZ) experiment to investigate the effects of extraneous materials on methane hydrate formation and dissociation conditions. 30

CHAPTER 4


Table 4.2: Measurement of Temperature-Pressure data during HSZ experiment Time (min)

Temp. Setpoint (°C)

Temperature Measurement (°C)

Pressure Measurement (psig)

0 67 135 202 270

20 18 16 14 12

20.57 18.6 16.52 14.55 12.6

1668 1647 1627 1606 1589

Date & Time

P-T curve 1880

Start Cooling 1680

FormationDissociation loop closed

Pressure (psi)

1480 1280 1080 880 680

480 280

Heating

80 -2.00

3.00

8.00

13.00

Temperature (°C) Figure 4.7 Online Graph of experiment

31

18.00

23.00

CHAPTER 4


0min

10min

20min

30min

Figure 4.8: Online video pictures at the intervals of 10min after the nucleation of hydrate

4.3.3. Experimental Procedure of Hydrate Kinetic Study: Basically, the objective of the kinetic experiment is to know the inhibition time or induction time of the hydrate formation. In order to do so, experiment is started with P, T conditions (determined by HSZ experiment) where hydrate does not exist. Then the temperature is decreased to that temperature (hydrate formation temperature) where hydrate forms. Then it would be easy to analyze the inhibition time of hydrate formation. Therefore before starting the kinetic experiment, the mixture (methane + water) is adjusted to a temperature where hydrates are not stable and wait for the stability of temperature and pressure for a few moments. Before running the macro (software) of kinetic experiment, the following parameters must be filled in the cells of the excel spreadsheet (Fig. 4.9) in order to determine inhibition time or induction time of hydrate formation: 32

CHAPTER 4


Time Increment (min): The system will record automatically each parameter (P, T) of the cell after this duration. For example, = 5min, then the P, T parameters will be recorded at an interval of 5 minutes. Experimental duration (min): This parameter is the total length of the experiment. After this duration, the system will turn automatically the temperature to temperature. Final Conditions (oC): After the end of the temperature of the cell will change to temperature. Subcooling is determined as: Subcooling = T (hydrate formation/onset) – T (equilibrium). To do a kinetic experiment, previously hydrates stability zone (HSZ) experiment has to perform in order to determine the subcooling parameter. The P, T values of hydrates stability zone is the input of the cell in excel spreadsheet. Temp. Set Point (oC): The temperature of the system adjusted to this value in order to get the induction time of hydrate formation. Hydrate Stability Zone: The region of temperature and pressure where the hydrate exists is the hydrate stability zone. This Temperature and pressure conditions are reported in the cell. The variation in pressure during measurement of induction time of hydrate formation is shown in Figure 4.11 shows the variation of temperature and pressure simultaneously during induction time measurement. It can be seen in Figure 4.11 that the pressure of the system decreases with temperature initially due to cooling of gas at constant volume. Slow decrease in pressure is still observed at constant temperature followed by drastic drop in pressure with rise in cell temperature indicating hydrate formation.

The time period

between start of experiment (ts) and hydrate onset time (to) is the induction time of hydrate formation (Fig. 4.11).

33

CHAPTER 4


The data of time, temperature and pressure generated during the measurement of induction presented in Table 4.3 and are plotted in Figure 4.11.

Figure 4.9 Excel spreadsheet

Table 4.3: Temperature-Pressure data during measurement of induction time Time (min) 0 20 41 62 83 104

Temp. Setpoint (°C)

Temperature Measurement (°C)

Pressure Measurement (psig)

8

15.0 10.1 8.4 8.3 8.2 8.2

1508 1466 1446 1443 1441 1438

34

Date & Time

EXPERIMENTAL SETUP AND PROCEDURE 1600

16.0

1400

14.0

1200

12.0

Induction Time (min) = to−ts

Pressure (psi)

1000

10.0

800

8.0 Pressure… Temperature…

600

6.0

400

4.0

200

2.0

0

0.0 0

10

20

30

40

50

60

Time (min)

Figure 4.10 Pressure-Temperature change during measurement of induction time

35

Temperature(oC )

CHAPTER 4

CHAPTER 5 MODELING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS 5.1.Introduction: Modeling of methane hydrate formation and dissociation by chemical affinity and modeling required a lot of experiments to be performed, and also the kinetics of hydrate formation investigated. The Sub-cooling and chemical affinities are used as a driving force before and after the formation critical nucleus (nucleation/ formation). The following results and discussions are based on many experimental data with extensive theoretical discussions. To be able to evaluate the kinetics of formation, and using the experimental data for the modeling analysis and the real gas equations have been employed primarily. To observe the effect of surfactant experiments are carried out with different concentration. Experiments are carried out at 0ppm, 1000ppm, 5000ppm, and 10000ppm for each surfactant. The main results of the thesis on affinity decay rate, the kinetics of methane hydrate formation are presented in this chapter, and effects of surfactants are studies with different concentrations. The main objective of the present study is to investigate the formation and dissociation conditions of methane hydrate in presence of different kinds of surfactants such as anionic (SDBS), cationic (CTAB), and non-ionic (Tergitol) and, using of chemical affinity modeling to evaluate the experimental results during the formation and dissociation of methane hydrate. 5.2. Modeling of methane hydrate formation and dissociation: 5.2.1. Gibbs–Duhem relation in presence of surfactants: Let us consider a two component system containing of water (w) and surfactants. Superscripts s and m refers, to the solution part and those constituting mixed micelles respectively. The m number of molecules and the chemical potential of solution parts are denoted with nm i and μi

, respectively for ith species. The number of micelles and the chemical potential of the mixed m th micelle are denoted as nm i and μi respectively for i species. For the Gibbs free energy of the

whole system containing nw water molecules, nsi molecules/ions in the solution part, nm i molecules/ions in the mixed micelles, we have Eq. (5.1) at an absolute temperature T and a pressure P with the subdivision potential ε for the micelle (D.G Hall et.al.1987). 36

CHAPTER 5

MODELING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS

m dG = VdP − SdT + ∑ μsi dnsi + ∑ μm i dni + εdn

(5.1)

Here ‘i’ suffix refers to the water and surfactants for the solution part but for micelles it refers to only surfactants. Chemical potentials of ionic species can be regarded as electrochemical potentials if surface potential of micelles is explicitly taken into account. The aggregation number of micelles ‘mi’ can be referred as the mi =

nm i

(5.2)

n

n × mi = nm i

(5.3)

ndmi + mi dn = dnm i

(5.4)

Substitute equation 5.4 in equation 5.1 dG = VdP − SdT + ∑ μsi dnsi + ∑ μm i (ndmi + mi dn) + εdn

(5.5)

m dG = VdP − SdT + ∑ μsi dnsi + n ∑ μm i dmi + ∑ μi mi dn + εdn

(5.6)

Now the equation 5.1 can be rewritten as equation 5.6 dG = VdP − SdT + ∑ μsi dnsi + n ∑ μm i dmi + μm dn

(5.7)

Here μm = ∑ mi μm i +ε

(5.8)

By integration equation (5.7) at constant intensive properties including mi G = ∑ μsi nsi + nμm

(5.9)

dG = ∑(nsi dμsi + μsi dnsi ) + (μm dn + ndμm )

(5.10)

Combining equations (5.7) and (5.10) we have the Gibbs–Duhem relation for the whole solution VdP − SdT − ∑ nsi dμsi + n ∑ μm i dmi − ndμm = 0

(5.11)

Consider the exchange reactions of surfactants between the micelles and the solution part under constant total number of moles nti (=nsi + nm i ) for all surfactants systems, while the change of the number of micelles ‘n’ is allowed. The associated change in the free energy dG under constant pressure (P) and temperature (T) is s nti = nsi + nm i = ni + nmi

(5.12)

Differentiation of equation 5.12 gives dnti = dnsi + ndmi + mi dn

(5.13) 37

CHAPTER 5


There is no change number of molecules in all surfactants systems, i.e. dnti = 0. ndmi = −dnsi − mi dn

(5.14)

Combining equation 5.7 and equation 5.14 we get s dG = VdP − SdT + ∑ μsi dnsi + ∑ μm i (−dni − mi dn) + μm dn

(5.15)

s m dG = ∑i(μsi − μm i ) dni + (μm − ∑i μi mi )dn

(5.16)

m At equilibrium dG=0, μsi = μm i and μm = ∑i μi mi s m ∑i(μsi − μm i ) dni + (μm − ∑i μi mi )dn = 0

(5.17)

Equation (5.17) can be applied for both critical micelle concentrations near and far from the cmc. 5.2.2. Chemical affinity: The chemical affinity was officially introduced as a thermodynamic function by DeDonder (1922) and defined as the chemical potential of a reaction, with the reaction proceeding in the direction to minimize the potential. Chemical affinity is defined as the “tendency of atom/compound to combine by chemical reaction with atom/compound of unlike compositions”. The IUPAC definition of affinity the negative partial differential of free energy (G) with respect to extent of reaction (ζ) is ∂G

A = − ( ∂ζ )

(5.18) p,T

More than just the presence of the surfactant, the concentration of surfactants specifically the presence of micelles is of key importance. Hydrate formation, in general follows mechanism of nucleation on certain regions called the nucleation sites. Micelles serves as nucleation sites for the gas to accumulate and water to envelope it. Heterogeneous nucleation is more appropriate than the homogeneous nucleation in now a days. Heterogeneous nucleation takes place in the presence of foreign bodies like dust, slats, clays, etc. Divya Ramaswamy and Mukul M. Sharma are told that micelles are serves as a ‘foreign particles’ that act as a nucleation site. P. Di Profio, et al. (2005) are reported that the surfactant micelles are not present under hydrate forming conditions where a substantial promotion of hydrate formation. 38

CHAPTER 5


Naoki Ando et al. (2012) are reported that the micelle formation in the aqueous phase neither promotes nor retards the hydrate formation. At high pressure there is no formation of micelles. Our experimental conditions are not suitable to form micelles. Then the equation (5.6) reduces to equation 5.19 at constant pressure and temperature dG = ∑i μsi dnsi

(5.19)

At constant pressure and temperature μsi = (

∂G

∂nsi

)

(5.20)

P,T

At constant pressure and temperature, the change in free energy with respect to extent of reaction is defined as the ∂G

dG = ( ∂ζ )

dζ

(5.21)

P,T

From equation (5.19) and equation (5.21) ∂G

∑i μsi dnsi = ( ) ∂ζ

dζ

(5.22)

P,T

∑i μsi

dnsi dζ

∂G

= ( ∂ζ )

(5.23) P,T

From equation (5.18) and equation (5.23) ∑i μsi vis = −Ai

Ai = − ∑i μsi vis

or

(5.24)

At equilibrium ∑i μsi vis = 0

(5.25)

Where μsi is chemical potential and vis (= dNis ⁄dζ) is stoichiometric coefficient of surfactant containing system. According to the definition, at equilibrium condition Ai = 0 and in any state prior to equilibrium is greater than zero (Ai > 0). Classically, the chemical potential μsi of system ith species can be related to its chemical potential μ0i in any arbitrary state by an equation of the form ∑i μsi vis = ∑i μ0i vi0 + RT ∑ ln (ai )vi

(5.26) 39

CHAPTER 5


Where R is the universal gas constant and ai is the activity of system of ith species. Equation (5.26) can be expressed in terms of affinity by substituting equation (5.24) into equation (5.26) Ai = A0 − RTlnQi

(5.27)

Here A0 is the affinity of standard/ arbitrary system, if the components are in their standard state and A0 is the only function of temperature, Qi (= ∏(ai )vi )) is defined as the affinity ratio. At equilibrium Ai = 0 and Qi = K(equilibrium constant) and therefore A0 = RTlnK

(5.28)

Then equation (5.16) can be rewritten as Ai = RTlnK − RTlnQi

(5.29)

Q

Ai = −RTln Ki = −RTlnζQi

(5.30)

Here ζQi (= Qi ⁄K) is thermodynamic extent of reaction. The affinity of the system is a function of extent of reaction. The extent of reaction is a function of both pressure and temperature, thus the affinity of system is function of both pressure and temperature. The thermodynamic extent of a reaction is defined as ζQi = 0 zero at the beginning of the reaction and at equilibrium is ζQi eq = 1. The extent of reaction is defined as the ζi =

∆Nj

(5.31)

vi

Here ∆Nj is change in moles and vi volume of the system. 5.2.3. Rate of Affinity decay: According to equation (5.30) reaction proceedings towards equilibrium the affinity decays towards from maximum to zero. The affinity decay rate indicates that the system how fast reaches to the equilibrium state. dAj dt

≤0

(5.32)

40

CHAPTER 5


The slope of the curves between chemical affinity and time would give the affinity decay rate. The affinity rate can also be defined as the dAi dt

= Ar =

Aj+1 −Aj

(5.33)

∆t

5.2.4. Modeling of methane hydrate formation As shown in the figure 2, the experimental results show that the equilibrium point B1 is too far from the experimental conditions in isochoric system. After the formation of hydrate crystals, pressure decreases gradually because of gas consumption, and the final pressure must be equal Peq1(at point B1). At this point, hydrate formation stops, there is no further consumption of gas and the system reaches equilibrium. Hydrate formation is a chemical reaction similar to the following equation CH4 (g) + nh H2 O(l) → CH4 nh H2 O(s)

(5.34)

Where, l, s, and g in parenthesis denote the different liquid state, solid state, gaseous state, and nh denote hydrate number respectively.

i

Figure 5.1 Schematic graph of Pressure- Temperature cure of hydrate formation

41

CHAPTER 5


For calculating affinity under different experimental conditions, we must measure the moles of methane gas consumed during hydrate formation at each stage. The number of moles (n) of methane at each state can be determined by the real gas equation (eq 5.35). V

P

n = R (ZT)

(5.35)

i

Where V is gas volume R is universal gas constant, P, T are pressure and temperatures of the cell and Z is gas compressibility factor. Compressibility factor (Z) is calculated by PengRobinson equation of state. Subscript ‘i’ indicates at a particular time. The amount of total gas consumed during the hydrate formation is (NA1 − NB1 ) and the thermo-dynamic extent of reaction is defined as the ζQi =

Qi K

(N

−N )

= (N A1−N i A1

(5.36)

B1 )

And we obtain chemical affinity and affinity decay rate by using equation (30) and equation (33) at each time step during formation. 5.2.5. Modeling of methane hydrate dissociation: As shown in the figure 3 the equilibrium condition B2 is too far from the experimental condition A2 in isochoric system. At point B2, hydrate dissociation completes, there is no further release of gas and the system reaches equilibrium. Hydrate dissociation is a chemical reaction similar to the following equation CH4 nh H2 O(s) → CH4 (g) + nh H2 O(l)

(5.37)

Where, l, s, and g in parenthesis denote the different liquid state, solid state, gaseous state, and nh denote hydrate number respectively. For calculating affinity under different experimental conditions, we must measure the moles of methane gas at each step during hydrate dissociation by using real gas equation (eq 5.35). The total amount of moles decomposed during the dissociation process is (NB2 − NA2 ) and thermodynamic extent of reaction is defined as the ζQj =

Qj K

(N −NA2 )

= (N j

(5.39)

B2 −NA2 )

And we obtain chemical affinity and affinity rate by using equation (5.30) and equation (5.33) at each time step during dissociation.

42

CHAPTER 5


Figure 5.2 Schematic graph of Pressure- Temperature cure of hydrate dissociation 5.3.Experimental section 5.3.1. Materials Used: Three different kinds of surfactant are used in the present study. These are: a) Anionic: Sodium dodecyl benzene sulfonate (SDBS), b) Cationic: Cetyl tri-methyl ammonium bromide (CTAB), and c) Non-ionic: Tergitol. These chemicals were procured from Merck Specialities Pvt. Ltd., Mumbai, India and Tergitol from Sigma Aldrich, Germany.

a) Anionic Surfactant: Sodium dodecyl benzene sulfonate (SDBS) CH3 N

+

Br

CH3

CH3

b) Cationic Surfactant: Cetyl tri-methyl ammonium bromide (CTAB)

𝐂𝟏𝟐−𝟏𝟒 𝐇𝟐𝟓−𝟐𝟗 𝐎 [𝐂𝐇𝟐 𝐂𝐇𝟐 𝐎]𝟏𝟐 𝐇 c) Non-ionic Surfactant: Tergitol, 15-S-12 Figure 5.3 Different types of surfactants used for methane hydrate formation and dissociation 43

CHAPTER 5


5.3.2. Procedure: The different concentrations of surfactants in distilled water were tested in the present study. These are 1000ppm, 5000ppm, and 10,000ppm.130 mL test sample was loaded in hydrate cell and air in cell was then removed using a vacuum pump. Thereafter, cell temperature was brought to 20 oC (293.15K) and allowed to stabilize for one hour. Once it was stabilized, methane was charged up to desired pressure and kept for 1hour at the same temperature to ensure the stabilization of pressure. Now, the temperature of the hydrate cell was lowered step-wise at the rate of 1K/h. The hydrate formation was detected when sudden drop in pressure at a particular temperature accompanied by sharp temperature rise of the cell was observed. After complete formation of hydrate, the temperature of the system was brought to −2 oC (271.15K) and kept at that temperature for 5-6 hours to stabilize the hydrate. The dissociation of hydrate is observed under heating condition. The dissociation of hydrate was observed with substantial rise in pressure. During the dissociation the temperature of the system was increased slowly (0.1K/h) so that the equilibrium of the hydrate is disturbed. 5.4.Experimental results and discussions 5.4.1. Methane hydrate formation and dissociation in presence of pure water The formation and dissociation behavior of pure methane hydrate is shown in Figure 5.4. Pure methane hydrate (Figure 5.5) is prepared with 99.99% pure methane and distilled water. Methane and water are brought in contact at low temperature and high pressure to form hydrate. The formation of hydrate is divided into three different periods of time: dissolution of methane gas, induction, and growth periods. During dissolution and induction periods, gas and liquid phases are present in the reactor cell. The governing process during dissolution is the mass transfer from gas to the liquid phase. The pressure of the reactor was slowly decreasing due to the dissolution of gas. This period ends when the gas saturates the water. Thus, the concentration of the gas in the water increases and super saturated regions appear leading to a clustering of water molecules around gas molecules. Induction period or nucleation period ends when clusters attain a critical size. The process from solution to nucleation and nucleation to crystal growth is shown in schematically in figure 2.4. The growth period assumed to start after the turbidity i.e. transition from a clear to translucent solution followed by sudden drop in the pressure of gas in the reactor cell due to the sudden 44

CHAPTER 5


loss of super-saturation in the liquid and continued until the experiment concluded. A sharp discontinuity is observed in the curve due to encapsulation of gas in the cage of water molecules, with a slight rise in the solution temperature due to the hydrate formation is exothermic reaction. It is observed that hydrate formation does not take place in a single stage however this process occurred in multiple stages (Figure 5.4). The rate of the phase transformation is directly related to three-phase equilibrium pressure. The driving force for the crystallization of water and gas assumed to be the difference between the dissolved gas fugacity and the three-phase equilibrium fugacity at the experimental conditions [Englezos et al., 1987]. 11

Formation Dissociation

Cooling Start

Tonset

10

(Teq, Peq) Pressure (MPa)

Growth period 9

8

Induction Period 7

Subcooling (Sub= Teq-Tonset 6

Sub

Start Heating 281

282

283

284

285

286

287

288

289

Temperature(K)

Figure 5.4 Pressure-Temperature trace of methane hydrate formation and dissociation in pure water

Figure 5.5 Pure methane hydrate prepared in laboratory 45

CHAPTER 5


5.4.2. Chemical affinity and affinity decay rate changes in presence of pure water during formation and dissociation The figure 5.4 and 5.6 shows the formation (P-T) curve and the changes in the affinity and thermodynamic extent of reaction during hydrate formation and dissociation in presence of pure water. After starting of the experiment the pressure of the system continues to decrease for a number of hours without hydrate formation D to A1 (Figure 5.1). This is due to dissolution of gas. This period ends when the gas saturates the liquid. Hydrate formation start at point “A1” with metastability, i.e. the hydrate nucleus reaches to its critical nucleus radius (Figure 5.1). Thus the concentration of gas in liquid increases and becomes super saturated leading to clustering of water molecules around gas molecules with sudden drop in pressure at constant temperature. The sharp drop in pressure at a particular temperature is due to hydrate nucleation (at point “A1” of Figure 5.1) with appearance of turbidity in liquid phase and subsequent growth of hydrates. A sharp discontinuity is observed in the formation curve due to encapsulation of gas in the cage of water molecules, with a slight rise in the solution temperature due to the heat release during the stable hydrate formation. Heat is released during the hydrate formation due to the hydrate crystal formation is an exothermic reaction. Hydrate formation is a multi-stage task. The variation in the pressure and temperature with time are used to estimate chemical affinity and thermodynamic extent of reaction by using equation 5.19 and equation 5.25. Figure 5.6 is shows the chemical affinity, extent of reaction and decay rate changes with time. The chemical affinity goes on decreasing from maximum value at starting of the nucleation and zero at equilibrium condition where there is no further consumption methane gas. In the same way the thermodynamic extent of reaction increases from zero at starting of nucleation to 1 equilibrium condition (𝑖. 𝑒. 0 ≤ 𝜁𝑄 ≥ 1). At starting, affinity values are more during dissociation than formation, affinity decay rates are calculated by using the chemical affinity curves, the slope of the chemical affinity curves w.r.to time gives the chemical affinity decay rate. During the formation of hydrate, affinity decay rate defined as a how fast the system reaches to equilibrium state where there is no further consumption of methane gas.. During the dissociation of hydrate, affinity decay rate defined as a how fast the system reaches to equilibrium state where there is no further release of methane gas from the methane hydrate. 46

CHAPTER 5


In presence of pure water, the system reaches to equilibrium in very slow rate with respect to

1.0

Pure water Formation

4000

0.8

3000

0.6

2000

0.4

1000

0.2

0

0.0 0

200

400

600

800

5000

Chemical affinity (KJ/mol)

5000

Thermodynamic extent o reaction


solutions/system having surfactants.

Pure water Formation

4000 3000

Affinity decay rate

2000 1000 0 0

1000 1200 1400 1600

200

400

Time (min)

0.8

4000 0.6 3000 0.4 2000 0.2

1000

0.0

0 0

200

400

600

800

1000

6000



1.0

Pure water Dissociation

5000

800

1000

1200

1400

Time (min) Thermodynamic extent o reaction

6000

600

Pure water Dissociation

5000 4000

Affinity decay rate

3000 2000 1000 0 0

1200

200

Time (min)

400

600

800

1000

1200

Time (min)

Figure 5.6 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of pure water during methane hydrate formation and dissociation. 5.4.3. Methane hydrate formation in presence of SDBS The behavior of methane hydrate formation and dissociation in presence of 1000ppm SDBS is presented in Figure 5.7. When the temperature of the reactor reached 284.76K sharp drop in pressure was observed due to hydrate nucleation followed by sudden increase in temperature (285.25K) of the hydrate cell suggesting hydrate nucleation is exothermic in nature. The pressure of the cell continued to decrease with slow decrease in temperature indicating that hydrate formation takes place in a number of stages.

47

CHAPTER 5


12

Pressure (MPa)

10

8

Formation Dissociation Hydrate Onset

Dissociation point (Te, Pe)

P (Pressure drop) = 8.91MPa

6



4

Subcooling (enset

2 270

275

280

285

290

295

Temperature (K)

Figure 5.7 Temperature pressure profile of methane hydrate formation and dissociation in presence of 1000ppm SDBS in distilled water. When the temperature of the hydrate cell was brought to 272.18K no significant pressure change was observed indicating complete formation of hydrate. The dissociation of hydrate was studied under heating condition. Up to 274.36K no significant pressure rise was observed. The substantial increase in pressure was observed when temperature of the system reached 276.91K indicating start of the hydrate dissociation. The dissociation of hydrate was completed at dissociation point (289.81K) where the slope of the curve changed sharply i.e. the heating curve joins the cooling curve (Fig. 5.7). The dissociation point is the highest temperature where all the three phases are in equilibrium. No more hydrate is observed after dissociation point (Fig. 5.7). Methane hydrate formation and dissociation is also studied with increasing the concentration of SDBS in water under similar condition of temperature and pressure. Figure 5.8 shows the formation and dissociation nature of hydrate in presence of 5000ppm SDBS in water.

48

CHAPTER 5

MODELING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS Start Cooling


12

Pressure (MPa)

10

Dissociation 8

6

Temperature is suddenly increases to 285.63 K when hydrate formation begins

Point (Te,Pe)

4

Start Heating 2 270

275

280

285

290

295

Temperature (K)

Figure 5.8 Temperature pressure profile of methane hydrate formation and dissociation in presence of 5000ppm SDBS in distilled water.

It is observed that hydrate nucleation begins at 282.02K with drastic change in pressure and followed by sudden rise in temperature to 285.63K. The formation curve tried to achieve the equilibrium condition of the phase boundary of hydrate when substantial amount of hydrate is formed as it is obvious from the Figure 5.8 in which formation curve touched the dissociation curve. This result suggests that when the sufficient amount of hydrate is formed, system tried to achieve the phase equilibrium condition. Once hydrate nucleus is formed with high pressure drop, the pressure continues to decrease for hydrate growth. The experiment was then carried out with increase in concentration of SDBS increased to 10,000ppm in water. The formation and dissociation of methane hydrate under this experimental condition is shown in Figure 5.9. The results obtained in this condition are appreciably different from the results of hydrate formation in lower concentration of SDBS. It can be noticed that hydrate nucleation started at 282.85K and 10.73MPa pressure. During nucleation of hydrate the temperature of the hydrate cell slightly increases which is far less than that was observed in lower concentration of SDBS. Two major drop in pressure, at different temperatures, 282.85K and 276.6K were noticed unlike in earlier cases. Also it was observed that no significant change in pressure occurred during hydrate growth in the temperature range from 280.8K to 278.2K suggesting slow growth of hydrate. During hydrate dissociation it can be seen that hydrate

49

CHAPTER 5


begins to decompose at 279.65K which is higher than that of dissociation temperature observed in lower concentration of SDBS i.e. 1000ppm and 5000ppm. 12

Start Cooling


Pressure (MPa)

10

Dissociation Point 8

6

4

Hydrate Dissociation start Start Heating

270

275

280

285

290

295

Temperature (K)

Figure 5.9 Temperature pressure profile of methane hydrate formation and dissociation in presence of 10,000ppm SDBS in distilled water. Table 5.1 Formation and Dissociation parameters of methane hydrate in presence of SDBS in water Test Sample 0.0ppm 1000ppm 5000ppm 10000ppm

Nucleation Temp. (K) 284.29 284.76 282.02 282.85

Nucleation Pressure (MPa) 10.21 10.87 10.67 10.73

Dissociation Temp. (K) 284.82 276.91 277.75 279.65

Subcooling (K) 3.36 5.05 9.67 7.75

Pressure Drop (∆P/MPa) 2.11 8.91 7.82 6.40

Dissociation Point (K) 285.75 289.81 291.69 290.6

The formation and dissociation characteristics of methane hydrate in different concentrations of SDBS is summarized in Table 5.1. From the summary of the table it is clear that 1000ppm SDBS resulted in highest drop in pressure due to encapsulation of gas while 10,000ppm SDBS lowest. As the concentration of SDBS increases the amount of gas consumed during hydrate formation decreases. The hydrate dissociation temperature is increased with increase in concentration of SDBS. Hydrate nucleation temperature and pressure is also affected with variation in concentration of SDBS. With increasing concentration of SDBS depression in hydrate nucleation temperature was noticed. A certain degree of subcooling which is a driving

50

CHAPTER 5


force of hydrate formation is required to initiate hydrate formation. The subcooling of hydrate formation varies with concentration SDBS used. (Table 5.1). 5.4.4. Chemical affinity and affinity decay rate changes in presence of SDBS during formation and dissociation Figure 5.10, 5.11 and 5.12 shows the changes in chemical affinity changes during formation and dissociation in presence of SDBS with different concentrations. The chemical affinity values are very high when compared to the pure water. Time taken by system reach to equilibrium is less when compared to pure water systems that means the rate of decay of the SDBS containing systems are more than the pure water system. The chemical affinity values are varying when concentration of SDBS varies. Affinity decay rates are decreasing while increasing concentration of SDBS during formation and dissociation (table 5.2).

3000

0.8

2500 2000

0.6

1500

0.4

1000 500

0.2

0 0

100

200

300

400

4000

1.0

Formation

500

600

700

Formation

3500



3500

Thermodynamic extent of reaction

4000

3000

Affinity decay rate

2500 2000 1500 1000 500 0 0

800

100

0.8

4000 0.6

3000

0.4

2000 1000

0.2

0 0

100

200

300

400

300

400

500

600

500

600

0.0 700

Time (Min)

6000



1.0

Dissociation


6000 5000

200

Time (Min)

Time (Min)

Dissociation

5000 4000

Affinity decay rate 3000 2000 1000 50

100 150 200 250 300 350 400 450 500

Time (Min)

Figure 5.10 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 1000ppm of SDBS during methane hydrate formation and dissociation. Table 5.2 Chemical affinity decay rates during formation and dissociation in presence of SDBS Concentration In PPM 0 1000 5000 10000

SDBS (in KJ/mole/min) Formation Dissociation 1.73074 5.07398 6.71722 11.99305 3.89386 8.36303 2.93871 8.02451 51

CHAPTER 5

MODELING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS 1.1

2500

0.8

2000

0.7

1500

0.6

1000

0.5 0.4

500

0.3

0

0.2 0

100

200

300

400

500

600

700

2500 2000 1500 1000 500 0 -500

Time (Min)

Dissociation

6000

0.8

5000 0.6

4000 3000

0.4

2000 0.2

1000 0

0.0

-1000

0

200

400

600

800

1000


7000

0

100

200

300

400

500

600

700

Time (Min)

8000 1.0


8000

Formation

3000


0.9

3500

7000

Dissociation

6000 5000 4000 3000 2000 1000 0 -1000

0

200

400

600

800

1000

Time (Min)

Time (Min)

Figure 5.11 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in


Formation

0.9

1500

0.8

1000

0.7 0.6

500

0.5 0

0.4 0

100 200 300 400 500 600 700 800 900


1.0

2000


presence of 5000ppm of SDBS during methane hydrate formation and dissociation.

2000

Formation

1500 1000 500 0 0

100

200


1.0

Dissociation

0.8

4000 3000

0.6

2000

0.4

1000

0.2

0 0

100

200

300

400

Time (Min)

500

600

700

0.0


6000 5000

300

400

500

600

700

800

Time (Min)

Time (Min) 6000



3000

-500


1.0

Formation


3500

Dissociation

5000 4000 3000 2000 1000 0 0

100

200

300

400

500

600

700

Time (Min)

Figure 5.12 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 10000ppm of SDBS during methane hydrate formation and dissociation. 52

CHAPTER 5


5.4.5. Methane hydrate formation in presence of CTAB

12

Pressure (MPa)

10

Start Cooling

Formation Dissociation Hydrate Onset

8

6

Hydrate Growth

4

Strat Heating

2 270

275

280

285

290

295

Temperature (K)

Figure 5.13 Temperature pressure profile of methane hydrate formation and dissociation in presence of 1000ppm CTAB in distilled water.

The formation and dissociation of methane hydrate was also studied in presence of cationic surfactant CTAB keeping same variations in concentration. Figure 5.13 illustrates the formation and dissociation characteristics of methane hydrate in presence of 1000ppm of CTAB. The pressure of the system started to fall rapidly when hydrate nucleation start at 283.75K followed by sudden rise in temperature of the hydrate cell to 286.14K which was higher than the phase equilibrium temperature of the hydrate (Fig 5.13). The results obtained during formation and dissociation of methane hydrate in higher concentration of CTAB is presented in Figures 5.14 and 5.15. It is important to note that during hydrate nucleation, the temperature of the system going beyond the hydrate phase equilibrium temperature. This result suggests that hydrate nucleation is a highly exothermic reaction in presence of CTAB. The nucleation temperature and pressure of hydrate is not affected much when the concentration of the CTAB is increased from 1000ppm to 5000ppm while 10,000ppm CTAB causes depression in nucleation temperature and pressure compared to hydrate nucleation temperature and pressure in lower concentration of CTAB. It is observed that the dissociation temperature increasing with increase in concentration of CTAB. The subcooling of hydrate formation is influenced by the 53

CHAPTER 5


variation in concentration of the CTAB. It is seen that subcooling increases with increase in

Pressure (MPa)

concentration of CTAB.

12


10

Hydrate Nucleation

8

Start Cooling

Dissociation Point Hydrate Growth

6

4

2 270

Start Heating 275

280

285

290

295

Temperature (K)

Figure 5.14 Temperature pressure profile of methane hydrate formation and dissociation in presence of 5000ppm CTAB in distilled water.

The amount of gas consumed during hydrate formation is found to increase slightly when the concentration of CTAB is increased from 1000ppm to 5000ppm while it decreased with further increase in concentration to 10,000ppm (Table 5.3).

12


Cooling

10

Dissociation point

Pressure(MPa)

Nucleation 8

6

Growth

4

2 270

Start Heating 275

280

285

290

295

Temperature (K)

Figure 5.15 Temperature pressure profile of methane hydrate formation and dissociation in presence of 10, 000ppm CTAB in distilled water. 54

CHAPTER 5


Table 5.3 Formation and Dissociation parameters of methane hydrate in presence of CTAB in water Test Sample

Nucleation Nucleation Dissociation Pressure Subcooling Dissociation Temp. Pressure Temp. Drop (K) Point (K) (K) (MPa) (K) (∆P/MPa) 1000ppm 283.75 10.81 275.65 3.83 8.31 287.58 5000ppm 283.80 10.87 277.15 4.98 8.43 289.78 10000ppm 281.75 10.39 277.34 6.86 7.98 288.61

5.4.6. Chemical affinity and affinity decay rate changes in presence of CTAB during

0.9

1500

0.8

1000

0.7 0.6

500

0.5 0

0.4 0

100 200 300 400 500 600 700 800 900

6000 1.0

Dissociation

5000

0.8

4000 3000

0.6

2000

0.4

1000

0.2

0 0

100

200

300

400

500

600

700

0.0 800



Time (min)

Time (min)


1.0

Formation

2000

Formation

1500 1000 500 0 0

100 200 300 400 500 600 700 800 900

Time (min) 6000


2000



formation and dissociation.

Dissociation

5000 4000 3000 2000 1000 0 0

100

200

300

400

500

600

700

800

Time (min)

Figure 5. 16 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 1000ppm CTAB during methane hydrate formation and dissociation.

Figure 5.16, 5.17 and 5.18 shows the changes in chemical affinity changes during formation and dissociation in presence of CTAB with different concentrations. The chemical affinity values are very high when compared to the pure water. The chemical affinity values are varying when concentration of CTAB varies. 5000ppm of CTAB is having higher values than the other concentrations of CTAB (1000ppm and 10000ppm) during formation and 55

CHAPTER 5


dissociation. Affinity decay rates are varying while changing concentration of CTAB during formation and dissociation. Chemical affinity decay rates lower than the all concentrations of SDBS and Tergitol but higher than the pure water containing systems (table 5.4). Table 5.4 Chemical affinity decay rates during formation and dissociation in presence of CTAB

1.0

Formation

0.9 1500

0.8

1000

0.7 0.6

500

0.5 0

0.4 0

200

400

600

800

1000


2000

CTAB (in KJ/mole/min) Formation Dissociation 1.73074 5.07398 2.74623 7.51366 3.08057 7.79159 2.065 7.8603



Concentration In PPM 0 1000 5000 10000

0.8

4000

0.6

3000

0.4

2000 1000

0.2

0

0.0 0

200

400

600

1000 500 0 100

200

300

400

500

600

700

800

1000

Time (min)

8000



Dissociation


1.0

5000

-1000

1500

Time (min)

8000 6000

Formation

0

Time (min) 7000

2000

7000

Dissociation

6000 5000 4000 3000 2000 1000 0 -1000

0

200

400

600

800

1000

Time (min)

Figure 5.17 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 5000ppm CTAB during methane hydrate formation and dissociation.

56

1.0 0.9

1400 1200

0.8

1000 800

0.7

600 0.6

400 200

0.5

0 -200

0

0.4 100 200 300 400 500 600 700 800 900

7000

Dissociation

6000

4000

0.6

3000

0.4

2000

0.2

1000 0 -1000

1.0 0.8

5000

0.0 0

Formation

1600 1400 1200 1000 800 600 400 200 0 -200

0

100 200 300 400 500 600 700 800 900

Time (min)

100 200 300 400 500 600 700 800 900



Time (min)

1800


Formation

1600


1800

MODELING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS Thermodynamic extent of reaction


CHAPTER 5

7000

Dissociation

6000 5000 4000 3000 2000 1000 0 -1000

0

100 200 300 400 500 600 700 800 900

Time (min)

Time (min)

Figure 5.18 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 10000ppm CTAB during methane hydrate formation and dissociation

5.4.7. Methane hydrate formation in presence of Tergitol Methane hydrate formation and dissociation was further investigated in presence of non-ionic surfactant Tergitol. Same concentration of Tergitol was used as in case of SDBS and CTAB. The obtained temperature and pressure data during hydrate formation and dissociation are presented in Figures 5.19, 5.20 and 5.21, and the thermodynamic parameters of hydrate formation and dissociation are summarized in Table 5.5. The thermodynamic parameters of hydrate formation and dissociation are noticeably affected upon the variation in concentration of Tergitol. When the concentration of Tergitol is increased from 1000ppm to 5000ppm, the nucleation temperature and pressure of hydrate formation shifted to higher values and also less subcooling is needed to initiate hydrate formation. Hydrate formation and dissociation is further investigated with increase in concentration of Tergitol to 10,000ppm. It is observed that the nucleation temperature and pressure of hydrate formation decreased to lower values compared to that of system having 5000ppm but it is high for the system having 1000ppm Tergitol. It can also be seen that the 57

CHAPTER 5


temperature required to initiate hydrate dissociation decreases with increase in concentration of Tergitol suggest that higher concentration of Tergitol promote hydrate dissociation (Table 5.5). The pressure drop of hydrate formation is slightly influenced on the variation of concentration of Tergitol. 1000ppm Tergitol resulted more pressure drop than the system having higher concentration of Tergitol.

12

Pressure (MPa)

10

8

Strat Cooling


Hydrate Nucleation

Dissociation Point

Hydrate Growth

6

4

Dissociation 2 270

Start Heating 275

280

285

290

295

Temperature (K)

Figure 5.19 Temperature-Pressure profile of methane hydrate formation and dissociation in presence of 1000ppmTergitol in distilled water.

12

Formation Dissociation Nucleation

10

Pressure (MPa)

Cooling


6

4

Heating Start 2 270

275

280

285

290

295

Temperature (K)

Figure 5.20 Temperature pressure profile of methane hydrate formation and dissociation in presence of 5000ppmTergitol in distilled water. 58

CHAPTER 5


12


10

Cooling

Nucleation

Pressure (MPa)


6

4

2 270

Heating up 275

280

285

290

295

Temperature (K)

Figure 5.21 Temperature pressure profile of methane hydrate formation and dissociation in presence of 10,000ppmTergitol in distilled water.

The amount of methane encapsulation by hydrate increases, when surfactant is added to the aqueous phase. This is over five times more compared to the hydrate formed in absence of surfactant. This increase of methane uptake may be explained by examining the processes that occur during hydrate formation. Since methane hydrate is less dense than water, hydrates form on the surface of water and cover the gas-water interface with a thin layer of methane hydrate. This hydrate layer restricts the absorption of additional methane even after vigorous stirring, as it must diffuse through the edge of the hydrate cell in order to encounter free water. When surfactant is present, the hydrate formed is forced to the edges of the surface of the reactor cell, allowing significantly more contact between methane gas and free water surface. Thus the concentration of methane in water continues to increases and hence hydrate formation is enhanced [Taylor et al., 2003]. Table 5.5 Formation and Dissociation parameters of methane hydrate in presence of Tergitol in water Test Sample (Tergitol)

Nucleation Nucleation Dissociation Subcooling Temp. Pressure Temp. (K) (K) (MPa) (K)

Pressure Drop (∆P/MPa)

Dissociation Point (K)

1000ppm

283.66

10.88

277.55

5.59

7.60

289.25

5000ppm

285.64

11.04

276.89

3.59

7.41

289.23

10000ppm

283.75

10.85

276.15

4.92

7.54

289.46

59

CHAPTER 5


5.4.8. Chemical affinity and affinity decay rate changes in presence of Tergitol during

1.0

Formation

0.9

2000

0.8

1500

0.7 0.6

1000

0.5

500

0.4

0

0.3 0

100 200 300 400 500 600 700 800 900


1.0

Dissociation

0.8

6000 5000

0.6

4000 3000

0.4

2000

0.2

1000 0

0.0 0

200

400

600

800

1000


8000

-1000

Formation

2500 2000 1500 1000 500 0

100 200 300 400 500 600 700 800 900

Time (min)

Time (min) 7000

3000

0

8000


2500


1.1 3000



formation and dissociation.

Dissociation

7000 6000 5000 4000 3000 2000 1000 0 -1000

0

200

400

600

800

1000

Time (min)

Time (min)

Figure 5.22 Variation of chemical affinity and thermodynamic extent of reaction w.r.to time in presence of 1000ppm Tergitol during methane hydrate formation and dissociation.

Figure 5.22, 5.23 and 5.24 shows the changes in chemical affinity changes during formation and dissociation in presence of Tergitol with different concentrations. The chemical affinity values are very high when compared to the pure water. The chemical affinity values are varying when concentration of Tergitol varies. 10000ppm of Tergitol is having higher values than the other concentrations of Tergitol (1000ppm and 5000ppm) during formation and dissociation. Affinity decay rates are varying while changing concentration of Tergitol during formation and dissociation. During dissociation, chemical affinity decay rates increasing while increasing concentrations from 1000ppm to 10000ppm. But chemical affinity decay rates are lower than the all concentrations of SDBS and higher than CTAB and the pure water containing systems (table 5.6). Figure 5.25 and 5.26 shows the chemical affinity decay rates during the formation and dissociation in presence of pure water, SDBS, CTAB and Tergitol.

60

CHAPTER 5


Table 5.6 Chemical affinity decay rates during formation and dissociation in presence of Tergitol Tergitol (in KJ/mole/min) Formation Dissociation 3.1756 7.98784 3.03271 8.0714 4.65223 8.66029


1.0

Formation

3000

0.9

2500

0.8

2000

0.7

1500

0.6 0.5

1000

0.4

500

0.3

0 -500

0.2 0

200

400

600

800

1000


1.1 3500


Concentration In PPM 1000 5000 10000

3500

Formation

3000 2500 2000 1500 1000 500 0 -500

0

200

Time (min) 1.0

Dissociation

6000

0.8

5000 4000

0.6

3000

0.4

2000 1000

0.2

0

0.0

-1000

0

200

400

600

800

1000

600

800

1000

8000



7000


8000

400

Time (min)

7000

Dissociation

6000 5000 4000 3000 2000 1000 0 -1000

0

200

400

600

800

1000

Time (min)

Time (min)


61

1.0 0.8

4000 0.6

3000

0.4

2000 1000

0.2

0 0

0.0 100 200 300 400 500 600 700 800 900

8000

1.0

Dissociation

7000

0.8

6000 5000

0.6

4000 3000

0.4

2000

0.2

1000 0 -1000

0.0 0

200

400

600

6000

Formation

5000 4000 3000 2000 1000 0 0

100 200 300 400 500 600 700 800 900

Time (min)

800

1000



Time (min)


Formation

5000


6000

MODELING OF METHANE HYDRATE FORMATION AND DISSOCIATION IN PRESENCE OF SURFACTANTS Thermodynamic extent of reaction


CHAPTER 5

8000

Dissociation

7000 6000 5000 4000 3000 2000 1000 0 0

100 200 300 400 500 600 700 800 900

Time (min)

Time (min)


Affinity decay rate (KJ/mole/min)

7

SDBS CTAB TERGITOL PURE WATER

6 5 4 3 2 1 0

0

1000

5000

10000

Concentration of surfactants (ppm)

Figure 5.25 Change in chemical affinity decay rates in presence of different concentrations of SDBS, CTAB, and Tergitol during methane hydrates formation 62

CHAPTER 5


SDBS CTAB TERGITOL PURE WATER

Affinity decay rate (KJ/mole/min)

12

10

8

6

4

2

0

0

5000 1000 10000 Concentration of surfactants (ppm)

Figure 5.26 Change in chemical affinity decay rates in presence of different concentrations of SDBS, CTAB, and Tergitol during methane hydrates dissociation

63

CHAPTER 6 STORAGE CAPACITY AND KINETICS OF METHANE HYDRATE FORMATION IN PRESENCE OF SURFACTANTS 6.1. Introduction: The kinetics of hydrate formation required a lot of experiments to be performed. The Subcooling and chemical affinities are used as a driving force before and after the nucleation/ formation stars. The following results and discussions are based on many experimental data with extensive theoretical discussions. To be able to evaluate the kinetics of formation, induction time and storage capacity lot of experimental data is required. To observe the effect of surfactant experiments are carried out with different concentration. Experiments are carried out at 0ppm, 1000ppm, 5000ppm, and 10000ppm for each surfactant. The objective of this chapter is to evaluate the kinetics of hydrate formation, effect of surfactants on induction time and storage capacity of gas in hydrate form in presence of surfactants 6.2. Storage capacity of methane hydrate in presence of SDBS, CTAB and Tergitol The storage capacity is the standard volume of gas stored in per volume of hydrate. Large drop in pressure was observed during hydrate formation in presence of surfactant in water compared to the system without surfactant. The presence surfactant in water enhances amount of gas consumption. The storage capacity of hydrate may be calculated using the following equation (Guo et al., 2002): Storage Capacity =

VSTP VH

=

(1000 × 22.4)

6.1

M ( w +∆V)×nH ρw

Where VSTP is the volume of gas entrapped in hydrate at standard conditions of temperature and pressure and VH is the volume of gas hydrate.MW and ρw are the molar mass and density of water respectively. nH is the hydrate number, calculated by dividing the moles of water to consumed moles of gas at each time. ∆V is the difference of molar volumes of water in the hydrate and liquid phases and is 4.6 cm3/mol for hydrate structure I (Makogon, 1997). It may be assumed that the volume of hydrate is equal to the empty hydrate lattice. This can be calculated using the equation (Klauda and Sandler, 2000): 64

CHAPTER 6

STORAGE CAPACITY AND KINETICS OF METHANE HYDRATE FORMATION IN PRESENCE OF SURFACTANTS

VH = (11.835 + 2.217 × 10−5 + 2.242 × 10−6 )3

10−30 NA − 8.006 × 10−9 P + 5.448 × 10−12 × P 2 46

(6.2) The storage capacity of hydrate is calculated using Equation 6.1 which is summarized in Table 6.1 and presented in Figure 6.1. 200 0ppm 1000ppm 5000ppm 10000ppm

180

Storage capacity (V/V)

160 140 120 100 80 60 40 20 0 SDBS

CTAB

TERGITOL

Figure 6.1 Storage capacity of methane hydrate in presence of surfactant

The presence of surfactant in water enhances the storage capacity of hydrate 1.5 times more than that of system having no surfactant. It is also found that the storage capacity of hydrate vary with surfactant concentration and on types of surfactant. As the concentration of SDBS increased from 1000ppm to 10000ppm the storage capacity is found to decrease. 1000ppm SDBS results in more storage of gas compared to others at higher concentration of SDBS. The storage capacity of methane hydrate in presence of CTAB decreases with increase in concentration. Tergitol causes increase in storage capacity with concentration. 1000ppm CTAB results in maximum storage of gas. Hydrates generally formed at the interface of gas and water and thus gas-water interface is blocked for further conversion of water to hydrate even on stirring. The solubility of gas in water increases on adding surfactants and finer hydrate particles are formed with water. This causes a high surface area and hence high mass transfer rate during hydrate formation.

65

CHAPTER 6


Table 6.1 Storage Capacity of Methane hydrate in presence of Surfactant

Test Sample 0.0ppm 1000ppm

SDBS 94 160

5000ppm 10,000ppm

159 141

Storage Capacity (V/V) CTAB Tergitol 94 94 169 142 162 160

145 157

6.3. Kinetics of methane hydrate formation in presence of SDBS, CTAB, and Tergitol Our interest in the present study is to find the rate of hydrate formation in presence of different concentration of surfactant in water. It is observed that the rate of conversion of gas into hydrate is not constant. It varies with progress of hydrate growth. As observed, the slope of the curve after nucleation exhibits an exponential behavior (Fig. 8.12) from which a firstorder reaction may be assumed. The expression of first order kinetic is 𝑁 = 𝑁𝑜 𝑒 −𝑘𝑡

(6.3)

Where 𝑁 is the number of moles at any time t, 𝑁𝑜 is the initial number of moles of methane, k is the rate constant (s-1), and t is time in second. The derivative of N with respect to time t, gives the rate of hydrate formation. 𝑑𝑁 𝑑𝑡

= −𝑁𝑜 𝑘𝑒 −𝑘𝑡

(6.4)

The slope of natural logarithm of Equation (6.3) gives rate constant k. N

ln (N ) = −𝑘𝑡

(6.5)

o

The hydrate formation in region (Fig. 8.12) is divided into three zones of time as shown in Figure 8 (0-50, 50-100 and 100-150min) after nucleation. This may be due to different rate of hydrate formation give different reaction rates and hence different rate constants. The data of three zones are used to estimate the rate constant of hydrate formation (Fig. 6.2). Hydrate formation is different in all the three zones of time. Initially (0-50 min after nucleation) the rate of formation is fast which decreases exponentially with time. 66

CHAPTER 6

STORAGE CAPACITY AND KINETICS OF METHANE HYDRATE FORMATION IN PRESENCE OF SURFACTANTS 0.1 0.0

Zone 1

Zone 2

Zone 3

-0.1 -0.2

N/N0

-0.3 -0.4 -0.5 -0.6 -0.7 -0.8 0

50

100

150

200

Time (Min)

Figure 6.2 Change of moles of methane during hydrate formation in presence 1000ppm SDBS 0.0 -0.50

-0.55

-0.2

ln(N/N0)

ln(N/N0)

-0.1

-0.3

-0.60

-0.4 -0.65

-0.5 0

10

20

30

40

50

50

60

Time (Min)

70

80

90

100

110

Time (Min)

-0.66

ln(N/N0)

-0.67 -0.68 -0.69 -0.70 -0.71 100

110

120

130

140

150

160

Time (Min)

Figure 6.3 Semi-logarithmic plot of change of moles of gas during hydrate formation in presence of 10000ppm SDBS

The slope of the curve of ln(N/N0) vs time gives the rate constant (k) of hydrate formation (Fig. 6.3). It is found that hydrate formation rate is increased many folds in presence of surfactants compared to hydrate formation in pure water which decreases with progress time. 67

CHAPTER 6

STORAGE CAPACITY AND KINETICS OF METHANE HYDRATE FORMATION IN PRESENCE OF SURFACTANTS 0.011

Hydrate Formation rate (mol/min)


0.000025

PURE WATER

0.000020

0.000015

0.000010

0.000005

0.000000

0-50

50-100

0.010

1000 ppm 5000 ppm 10000 ppm

0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000

100-150

SDBS

0.009

0-50

Time zone (min)

50-100

100-150

Time zone (min)



0.0065 0.008

CTAB

0.007

1000 ppm 5000 ppm 10000 ppm

0.006 0.005 0.004 0.003 0.002 0.001 0.000

0.0060

TERGITOL

0.0055

1000 ppm 5000 ppm 10000 ppm

0.0050 0.0045 0.0040 0.0035 0.0030 0.0025 0.0020 0.0015 0.0010 0.0005

0-50

50-100

0.0000

100-150

Time zone (min)

0-50

50-100

100-150

Time zone (min)

Figure 6.4 Hydrate formation rate in presence of different surfactant

The rate of hydrate formation also varies with the concentration of surfactant. As the concentration of SDBS increases from 0ppm to 10,000ppm, the formation rate is found to increase effectively while CTAB has different impact on hydrate formation rate with increase in concentration. The rate is increased with increase in the concentration of CTAB from 0ppm to 5000ppm while it slows down with further increase in concentration to 10,000ppm. It reveals that CTAB has inhibition effect at concentration of 10,000ppm. Ganji et al. (2007) studied methane hydrate formation rate in presence of CTAB at 300ppm, 500ppm and 1000ppm. The results indicated that at 300ppm and 500ppm CTAB lowered the formation rate than that in pure water while at 1000ppm CTAB increased the hydrate formation rate. Tergitol is also found to enhance the rate of hydrate formation with increase in concentration but less effectively than SDBS and CTAB. It is seen that hydrate formation rate is influenced effectively in the presence of all the three surfactants at a given concentration. Therefore it is crucial to compare the effectiveness of different surfactants at a particular concentration on hydrate formation rate. The hydrate formation rate is 174 times more at 1000ppm of SDBS 68

CHAPTER 6


than that in water, 1000ppm CTAB increased the rate 271 times more while 1000ppm Tergitol causes 145 times more compared to formation rate in water. The effect of further increase in concentration of surfactant on formation rate is also investigated. It is observed that the hydrate formation rate is increased by 383 folds at 5000ppm of SDBS than that in system without SDBS, 5000ppm CTAB enhances the rate 332 folds more compared to hydrate formation without CTAB and Tergitol also found to increase 155 times more but it is less than SDBS and CTAB. The formation rate is further increased by 443times than that in pure water with increase in concentration of SDBS to 10,000ppm while 10,000ppm CTAB reduced the rate to that in 1000ppm CTAB and 5000ppm CTAB. The surfactant molecule contains polar hydrophilic group and non-polar hydrophobic group. Surfactant molecules in water tend to aggregate to form various kinds of structures such as spherical and rod -like micelles and multilayer structures. A peculiar feature of ionic surfactants in water is that their specific conductivity increases as a function of surfactant concentration. At the critical micelle concentration (CMC) and above, molecules of surfactants in excess of the CMC aggregate to form the micellar macro ion which has a low mobility and thus contributing less to the overall conductivity of the solution. As a result, the conductivity vs. surfactant concentration curve shows a point where its slope changes detectably is noted as CMC (Mukerjee and Mysels, 1971).

69

CHAPTER 6

STORAGE CAPACITY AND KINETICS OF METHANE HYDRATE FORMATION IN PRESENCE OF SURFACTANTS Table 6.2 Hydrate formation rate and Rate constant Test Sample

Time zone

k(min-1)×10−3

Formation rate (mole/min)×10−3

Pure water

0-50 50-100 100-150

0.391 0.21 0.879

0.024097 0.008801 0.001543

1000ppm

0-50 50-100 100-150

4.75 1.28 0.622

4.21 0.955 0.442

5000ppm

0-50 50-100 100-150

8.58 3.71 0.983

9.231 3.129 0.723

0-50 50-100 100-150

9.48 3.4 0.887

10.691 2.83 0.651

1000ppm

0-50 50-100 100-150

6.7 2.45 0.143

6.541 0.192 0.106

5000ppm

0-50 50-100 100-150

7.71 4.8 0.124

8.011 4.31 0.9323

0-50 50-100 100-150

6.01 3.28 0.132

5.53 2.30 0.959

0-50 50-100 100-150

4.04 2.42

3.51 1.93

1.47

1.12

5000ppm

0-50 50-100 100-150

4.42 2.59 2.00

3.74 2.02 1.51

10000ppm

0-50 50-100 100-150

6.40 3.43 1.87

6.03 2.78 1.40

SDBS

10000ppm

CTAB

10000ppm

1000ppm

Tergitol

70

CHAPTER 6


6.4. Induction time of Hydrate Formation in presence of SDBS, CTAB, and Tergitol: A typical temperature-pressure profile of methane hydrate formation during measurement of induction time in presence of surfactant is shown in Figure 6.5. The difference of hydrate onset time (to) and the start time (ts) of experiment is the induction time of hydrate formation. Induction time measured during hydrate formation with variation in concentration of surfactant is presented in Table 6.2. The point where slope changes drastically followed by sudden rise in temperature is the hydrate onset time. Pressure of the system continues to decrease with time at constant temperature due to growth of hydrate. It is found that the nucleation time of hydrate formation is greatly affected on addition of surfactant in the pure water system. Induction time of hydrate formation is shortened compared to hydrate formation in pure water. Also, it is observed that induction time varies with change in concentration of surfactant in water. As the concentration of SDBS increases from 1000ppm to 5000ppm, induction time becomes just half while no impact on induction time has been observed with further increase in concentration of SDBS to 10000ppm. Similar influence of concentration on induction time has been observed in case of CTAB. It is seen that the induction time is decreased by one third as the concentration of CTAB is increased from 1000ppm to 5000ppm while further increase in concentration of CTAB has no effect on induction time. Tergitol has significantly different effect on induction time. It reduced the induction time noticeably more but as the concentration is increased from 1000ppm to 10,000ppm, hydrate formation is delayed several folds than that in 1000ppm Tergitol.

71

CHAPTER 6

STORAGE CAPACITY AND KINETICS OF METHANE HYDRATE FORMATION IN PRESENCE OF SURFACTANTS 11.5

Pressure Temperature

11.0

290 289 288

10.0

Induction time (ti)= to-ts

9.5

287 286

9.0

285

8.5 8.0

ts

to

284

7.5 0

50

100

Temperature (K)

Pressure (MPa)

10.5

150

200

250

300

350

283 400

Time(min)

Figure 6.5 Temperature-pressure profile during measurement of induction time in 1000ppm SDBS Table 6.3 Measured Induction time of hydrate formation in presence of surfactant

0.0ppm (water) 1000ppm SDBS 5000ppm SDBS 10000ppm SDBS

Onset Temp. (To/K) 284.29 284.76 282.0 282.85

Dissociation Temp. (Tdiss/eq/K) 288.75 289.81 287.78 288.40

1000ppm CTAB 5000ppm CTAB 10000ppm CTAB

283.75 283.80 281.75

1000ppm Tergitol 5000ppm Tergitol 10000ppm Tergitol

283.66 285.64 283.75

Test Sample

Subcooling (K) ∆T = Tdiss/eq− Tonset

Induction Time (min)

4.46 5.05 5.78 5.65

216 82 41 40

288.91 289.78 286.96

5.15 4.98 5.21

30 11 10

289.25 290.72 289.21

5.59 5.18 5.46

20 71 101

It is observed that the induction time of hydrate formation decreases gradually when the same experiment is repeated (Table 8.7). It means relatively less time is required to initiate hydrate formation compared to the previous run of the same experiment. The third formation 72

CHAPTER 6


of the hydrate occurs at even more less time than first and second formation. This decrease in formation time may be due to microscopic hydrate crystals that are still remaining in solution after hydrate dissociation. These crystals would act as seeds for future formations and provide carrier for mass transfer and hence reaction among gas and water molecules. Consequently, the hydrate formation becomes easy [Parent and Bishnoi, 1996; Makogon, 1997; Koh, 2002; Zatsepina et al., 2004; Wu and Zhang, 2010]. The CMC is affected by the structure of the surfactant and its state in solution. The surfactant molecule adsorbs to the interface of gas-liquid phase in regular arrangement when the concentration of solution is lower than CMC. Hydrophobic groups move toward the gaseous phase, while hydrophilic groups move towards the aqueous phase in symmetric ways of arrangement. The adsorption of surfactant molecule to the interface causes decrease in surface tension. When the concentration of solution exceeds the CMC, the surface tension will not decrease further with increase in concentration of solution and no more surfactant molecules adsorb at the surface and these hydrophobic groups start to agglomerate to form clusters in the aqueous phase. These surfactant clusters in aqueous phase accelerate the uniform distribution of guest molecule (methane). Methane molecules are restricted by these surfactant clusters and thus the ability to move is lost compared to that in pure water. They move only with clusters surrounded by water molecules which help to build up cages of hydrate crystal. Gas molecules adsorbed to the inside of cages and then hydrate crystal clusters formed. As the number of hydrate crystal clusters increases the opportunity to contact among each other increases and consequently the formation of hydrate crystal nuclei becomes easier which accelerate hydrate formation (Zhang Bao-yong et al., 2008).

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CHAPTER 7 SUMMARY AND CONCLUSIONS A series of experiments were conducted to investigate the influence of different types of surfactants on methane hydrate formation and dissociation. The rate of hydrate formation is also observed with variation in concentrations of surfactants. It is also studied in presence of different types of surfactant. Based on experimental results and observations conclusions may be made as follows:  Modeling of methane hydrate formation and dissociation in presence of surfactants by chemical affinity was modeled by using fundamentals of chemical affinity, chemical potential of micelles and solution, thermodynamic extent of reaction.  Chemical potential of the micelles and chemical potential of the solution are equal at equilibrium during formation and dissociation of methane hydrate.  Chemical affinity and subcooling are used as driving force for formation of methane hydrate formation after nucleation starts. Before nucleation subcooling was the driving force for advancement of reaction. After nucleation chemical affinity used as a driving force.  The chemical affinity decay rate is more in presence of 1000ppm of SDBS than that of other concentrations of SDBS, CTAB and Tergitol. i.e 1000ppm of SDBS system reaches the equilibrium state faster than the other surfactant concentrations.  In presence of CTAB, decay rates are lower than the other surfactant concentrations.  The formation of methane hydrate is more exothermic in presence of surfactant than that of hydrate formation without surfactant. This exothermic nature of hydrate formation is more pronounced in the presence of CTAB.  The storage capacity of hydrate in presence of surfactant is enhanced 1.5 times than that of hydrate formation without surfactants.  All the surfactants enhanced the rate of hydrate formation many folds than that of hydrate formation in absence of surfactant. Tergitol is less effective than SDBS and CTAB in enhancing the rate.

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CHAPTER 7

SUMMARY AND CONCLUSIONS

Thus it may be concluded that methane hydrate formation is promoted thermodynamically as well as kinetically by the presence of surfactants. Future work proposition: This kind of work I would like to propose for the future work on gas hydrate formation and dissociation  Work on methane hydrate formation and dissociation in presence of natural gas with different additives like salts, clays, surfactants, or mixtures of extraneous materials.  Work on container design for storing of hydrate for storing and transportation purpose.  Work different dissociation techniques like depressurization, thermal recovery, inhibition technique and combination of dissociation techniques.  Work on hydrate formation and dissociation in presence of naturally occurring substances, core sample etc.

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WORKSHOPS/ CONFERENCE 1. International workshop on natural gas hydrates: Exploration and Production, IIT Madras, India, August 7-9, 2012 2. Manojkumar Gudala, Vikash Kumar Saw, G. Udayabhanu, Ajay Mandal and Sukumar Laik. “Kinetics of Methane Hydrate Formation and its Dissociation in presence of Nonionic Surfactant Tergtol” International Conference on Developing Oil and Gas Resources “DUOG 13”, IIT Madras, India, March 1-3, 2013.

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