Figure 4-13: Hydroisomerization Yield as a function of conversion - USY-E ........... 60 · Figure 4-12: .... making this dream comes true. Their encouragement and ...
The Effect of Key Parameters Such as Temperature and Contact Time on The Hydroisomerization of n-heptane Over Platinum Loaded Zeolite USY Catalysts
A dissertation submitted to The University of Manchester for the degree of Master of Science in the Faculty of Engineering and Physical Science
2012
Essa Ibrahim Alnaimi
School of Chemical and Analytical Science
Table of Contents Abstract: ........................................................................................................................ 6 DECLARATION .......................................................................................................... 7 INTELLECTUAL PROPERTY STATEMENT ........................................................... 7 ACKNOWLEDGEMENTS .......................................................................................... 8 CHAPTER ONE ........................................................................................................... 9 INTRODUCTION ......................................................................................................... 9 1.1 Introduction ......................................................................................................... 9 1.2 Aims and Objectives ......................................................................................... 10 CHAPTER TWO......................................................................................................... 11 LITERATURE REVIEW: ZEOLITE ......................................................................... 11 2.1
Introduction .................................................................................................. 11
2.2
Zeolite structure and composition ................................................................ 12
2.3
Synthesis: ...................................................................................................... 13
2.4
Types of Zeolites .......................................................................................... 14
2.4.1 Zeolite Y ..................................................................................................... 14 2.4.2 Zeolite Beta ................................................................................................ 15 2.4.3 Zeolite Mordenite ....................................................................................... 16 2.5
Post Treatment .............................................................................................. 17
2.5.1 Ion Exchange .............................................................................................. 17 2.5.2 Dealumination: ........................................................................................... 17 2.5.2.1 Steaming .................................................................................................. 17 2.5.2.2 Acid Leaching ......................................................................................... 18 2.5.2.3 Metal loading: ......................................................................................... 18 2.6
Zeolite Properties .......................................................................................... 19
2.6.1 Zeolite Acidity............................................................................................ 19 2.6.2 Shape selectivity ......................................................................................... 20 2.7
Hydroisomerization ...................................................................................... 21
2.8
Reaction Mechanism .................................................................................... 22
2.9
Effect of Key Parameters ............................................................................. 25
2.9.1 Reaction Temperature ................................................................................ 25 2.9.2 Space Time ................................................................................................. 26 2.9.3 Chain Length .............................................................................................. 27
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2.9.4 Catalyst Acidity .......................................................................................... 27 2.9.5 Catalyst Geometry ...................................................................................... 28 2.9.6 Platinum Content ........................................................................................ 29 CHAPTER THREE ..................................................................................................... 31 EXPERIMENTAL ...................................................................................................... 31 3.1 Introduction: ...................................................................................................... 31 3.2 Catalyst Preparation .......................................................................................... 31 3.3 Catalyst Characterization .................................................................................. 34 3.3.1 Scanning Electron Microscopy .................................................................. 34 3.3.2 Transmission Electron Microscopy – TEM. .............................................. 35 3.3.3. X-ray Diffraction ....................................................................................... 37 3.4 Catalyst Testing ................................................................................................. 38 3.4.1 The hydroisomerization unit: ..................................................................... 38 3.4.2 Equipment Calibration ............................................................................... 42 3.4.3 GC Analysis – gas and liquid ..................................................................... 47 3.4.4 Hydroisomerization Testing of n-heptane: ................................................. 49 3.4.5 Mass Balance.............................................................................................. 52 CHAPTER FOUR ....................................................................................................... 54 RESULTS AND DISCUSSION ................................................................................. 54 4.1 Introduction ....................................................................................................... 54 4.2 Temperature Stability in the Reactor ................................................................ 54 4.3 Effect of Key Parameters .................................................................................. 55 4.3.1 Effect of Reaction Temperature and Contact Time.................................... 56 4.3.2 Yield of C7 Isomers as a Function of Conversion ...................................... 60 4.3.3 Effect of Metal/Acid Balance ..................................................................... 61 CHAPTER FIVE ......................................................................................................... 63 CONCLUSION AND FUTURE WORK .................................................................... 63 References ................................................................................................................... 66
Word Count (15,416)
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Table of Figures Figure 2-1: Some forms of Natural Zeolite [2] ........................................................... 11 Figure 2-2: Zeolite common cages and their secondary building units (SBU) [3] ..... 13 Figure 2-3: Pore diameter in Angstroms and 3-D view of the structure of faujasite zeolite [3]..................................................................................................................... 15 Figure 2-4: : Pore diameters in Angstroms and 3-D view of the structure of Beta zeolite [10]................................................................................................................... 15 Figure 2-5:Pore diameters in Angstroms and 3-D view of the structure of Mordenite zeolite [10]................................................................................................................... 16 Figure 2-6: Brønsted acid site created by the formation of hydroxyl bride [7]........... 19 Figure 2-7: Schematic representation of the three types of shape-selectivity [30] ..... 21 Figure 2-8: Monomolecular Isomerization of Heptane into 2-methylhexane on bifunctional catalyst - Modified from [32] .................................................................. 23 Figure 2-9: Bimolecular Isomerization of Heptane into 2-methylhexane on bifunctional catalyst - Modified from [32] .................................................................. 23 Figure 2-10: Monomolecular mechanism of type A isomerization ............................ 24 Figure 2-11: Monomolecular mechanism of type B isomerization ............................. 25 Figure 2-12: Effect of contact time (W/F) on n-heptane hydroisomerization[36]] ..... 26 Figure 2-13: Effect of chain length on isomerization of n-alkane - Adapted from [37] ..................................................................................................................................... 27 Figure 2-14: 7 Steps of a heterogeneous catalytic reaction [38] ................................. 28 Figure 2-15: Effect of Catalyst Geometry on The Isomerization of n-heptane, Adapted from [42] ....................................................................................................... 29 Figure 2-16: Effect of Platinum Content on The Isomerization of n-heptane, Adapted from [36] ..................................................................................................................... 29 Figure 3-1: SEM micrograph of CBV712 [1] 34 Figure 3-2: Illustration of the diffracted electron beam when it hits the sample [2,3] 35 Figure 3-3: Illustration of Transmission Electron Microscopy - adopted from [5]..... 36 Figure 3-4: The sample was injected onto the carbon-coated copper grid – adapted from [59] ..................................................................................................................... 36 Figure 3-5: Illustration of the XRD principle – Adopted form [7] ............................. 37 Figure 3-6: XRD pattern of CBV712 [1] .................................................................... 38 Figure 3-7: Process & Instrumentation Diagram of the Hydroisomerization unit [11] ..................................................................................................................................... 39 Figure 3-8: Calibration of H2 mass flow controller..................................................... 43 Figure 3-9: Calibration of HPLC Pump ...................................................................... 44 Figure 3-10: Stable Temperature Zone in the Reactor with Pure H2 Flow. ................ 45 Figure 3-11: Estimating the gas GC RF for C6 and C7 .............................................. 48 Figure 3-12: Temperature program of the liquid GC .................................................. 48 Figure 3-13: Temperature program during catalyst calcination and activation .......... 50 Figure 4-1: Reactor temperature with only H2 flow 55 Figure 4-2: Temperature profile across the reactor during the experiment at 250°C . 55
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Figure 4-3: The conversion of n-heptane as a function of temperature at different contact times - USY-E................................................................................................. 56 Figure 4-4: The conversion of n-heptane as a function of temperature at different contact times – CBV712 ............................................................................................. 56 Figure 4-5: Hydroisomerization yield ( to isomers) as a function of temperature USY-E ......................................................................................................................... 57 Figure 4-6: Hydroisomerization yield (to isomers) as a function of temperature CBV712 ....................................................................................................................... 57 Figure 4-7: Hydroisomerization yield (to isomers) as a function of temperature USY-E ......................................................................................................................... 58 Figure 4-8: Hydroisomerization selectivity to isomers as a function of temperature CBV712 ....................................................................................................................... 58 Figure 4-9: Hydroisomerization selectivity to isomers as a function of temperature USY-E ......................................................................................................................... 58 Figure 4-10: The overall effect of Temperature on at 140.6 kg.s/mole - USY-E ....... 59 Figure 4-11: The overall effect of temperature at 140.6 kg.s/mole - CBV712 ........... 59 Figure 4-13: Hydroisomerization Yield as a function of conversion - USY-E ........... 60 Figure 4-12: Hydroisomerization Yield as a function of conversion - CBV712 ........ 60 Figure 4-15: The ratio of Mono-branched isomers to Multi-branched isomers of USYE as a function of contact time .................................................................................... 62 Figure 4-14: The ratio of Mono-branched isomers to Multi-branched isomers of CBV712 as a function of contact time ........................................................................ 62
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ABSTRACT In order to upgrade the heavy naphtha products (C7 - C10) from low octane stream to high octane gasoline stream, n-heptane was used as a model compound reaction. Therefore, the effect of key parameters such as temperature and contact time on the hydroisomerization of n-heptane over platinum loaded zeolite USY was investigated to show the optimum conditions that would results in high C7 isomers yield and selectivity while minimizing cracked products. The USY catalysts, CBV712 and USY-E, were dealuminated by steaming to enhance their physical properties and their performance, then, CBV712 was further treated by acid leaching to remove extra framework aluminium (EFAL) species. Interpreting the acquired results require deep understanding of the used catalysts. The obtained catalysts were characterized by several methods and techniques such as BET, elemental analysis, H2 chemisorption, NMR, and XRD. The hydroisomerization of n-heptane was tested over USY catalysts at a range of temperatures (190 - 250 °C) and contact times (46.8, 70.6, and 140.6 kg.s/mole). Both catalysts showed that as temperature increased, the n-heptane conversion increased. At any given temperature, the conversion of n-heptane was higher for longest contact time. However, the selectivity to C7 isomers decreased with temperature whereas the yield of C7 isomers increased with temperature until it hit a maximum, then, dropped down. This indicates that the hydroisomerization reaction is the dominant reaction up to the maximum the yield point, then, the cracking reaction dominates causing the C 7 isomers yield to decrease. Although both catalysts have similar Si/Al ratio (6 - 8), they have been post treated differently as CBV712 catalyst were steamed and acid leached while USY-E were just steamed. The dealumination of USY-E via steaming created EFAL species (Lewis acid sites) in the catalyst structure. The increased cracking activity USY-E could be attributed to these EFAL species. There is also a balance between pore blocking due to EFAL species and overall additional acid sites and the resulting additional activity. Based on the results and analysis obtained during the conducted experiments, there are few areas that could be researched and investigated to close the gap between the application of these catalysts with normal alkane and formulating catalysts that could convert heavy naphtha stream to high octane gasoline.
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DECLARATION No portion of the work referred to in the dissertation has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning.
INTELLECTUAL PROPERTY STATEMENT I.
The author of this dissertation (including any appendices and/or schedules to this dissertation) owns certain copyright or related rights in it (the “Copyright”) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes.
II.
Copies of this dissertation, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has entered into. This page must form part of any such copies made.
III.
The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the dissertation, for example graphs and tables (“Reproductions”), which may be described in this dissertation, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. Further information on the conditions under which disclosure, publication and commercialisation of this dissertation, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/display.aspx?DocID=487), in any relevant Dissertation restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s Guidance for the Presentation of Dissertations.
IV.
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ACKNOWLEDGEMENTS First of all, I would like to thank Allah for his guidance, support, and mercy
“ .. فبأي ألآء ربكام تكذابن، وخلق الإنسان، وعَ مَّل البيان، أعطى اللسان،امحلد هلل رب العاملني .حق ما قال العبد ولكنا كل عبد ُّ أ، أهل الثناء واجملد،كل امحلد اي من هو للحمد أهل ..يطلب نرصتَك ُ من ضعيف..كل امحلد ..يطلب غناك ُ من فقري..كل امحلد ..يطلب عزك ٍ من..كل امحلد ُ ذليل .. وما خفناك اإل تصديق ًا بوعدك ووعيدك، وما رجوانك اإل ثق ًة فيك..ظن بك ٍ حسن َ ما دعوانك اإل..كل امحلد فكل امحلد "]52 ـ52 :" قال رب ارشح يل صدري ويرس يل أمري واحلل عقدة من لساين يفقهوا قويل" [طه
I would like to thank Dr. Garforth, A., my academic supervisor, for his continuous support and guidance throughout the MSc project. I would also like to acknowledge the effort and support of all academic stuff form post graduate receptionists to lab staff. Especial thanks goes for Dr. Holmes, R., Dr. Akah, A., my colleagues Mr Farzi, F., Ms Paranitha, Mr Groom, J. for their help and support. In addition, I would to show gratitude to Saudi Aramco for sponsoring me and making this dream comes true. Their encouragement and support in both academically and socially were invaluable which eased my study. Especial thanks go for the Research and Development Centre and ACS, London office for their continuous support. Finally and most importantly, I would like to thank each and every single person in my family especially my beloved parents, siblings, and my wife for their beyond belief support and encouragements which made my study possible. I would like to dedicate this work to my parent, wife, and especially my beloved newly born daughter Ms Dana Alnaimi and wish her a great and prosper life. 8
CHAPTER ONE INTRODUCTION 1.1 Introduction As a result of changing life styles, the number of vehicles has increased tremendously in the past few decades as well as gasoline consumptions [1, 2]. Environmentally, the increase use of low grade fuel imposes great accumulative hazards due to the high quantities of harmful impurities and carcinogenic materials contained in the gasoline such as sulphur and benzene rings. This means that attention needs to be focussed on developing high performance engines and cleaner fuels if the impact on the environment and health must be reduced. In order to produce high performance fuel, the octane number, which is an indicative parameter on how much compression the fuel can withstand before detonating, of the fuel has to be increased [3]. The higher the octane number, the better the engine performance and therefore, gasoline treatment and enhancement such as the addition of benzene, olefins, and octane boosters has been implemented in the oil industry. Component n-heptane RON
0
n-hexane
2-methylhexane
Benzene
Toluene
25
44
101
111
Table 1- 1: Research octane number of different hydrocarbons
Although the addition of benzene rings (aromatics) and olefins increases the octane number of gasoline, research has shown that benzene and aromatics cause cancers whilst a high amount of olefins cause gum formation in the gasoline pool – see Table 1-1. Recently, strict legislation has been issued to steeply decrease the content of benzene and aromatics in gasoline to 1.3 - 3 volume % [4]. As a result, refiners struggled with producing the same gasoline quality without spending more money on additives that boost the octane number such as tetraethyl lead (TEL) which increases the octane number to around 150 and MTBE which is an oxygenate that helps in burning fuel completely [3]. However, TEL was phased out due to its toxicity to air and soil around the road. In addition, MTBE faced the same problem where its high affinity to underground water caused it to be banned in many countries.
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Therefore, upgrading low-value naphtha stream via hydroisomerization reaction would not only increase the branched alkanes in the gasoline pool, it would lower the cost of gasoline production, hence, maximizing the refinery profitability as compared to additives [6, 7]. This principle has been commercially adopted for upgrading light naphtha (C4 – C6) over bifunctional zeolite catalyst, however, the hydroisomerization of heavy naphtha (C7 – C10) has not been commercially explored [8-10].
1.2 Aims and Objectives The aim of this research is to study the effect of key parameters such as temperature, space time, and acidity on the hydroisomerization of n-heptane as a model compound over platinum loaded zeolite USY, as way of studying effect of heavy naphtha in gasoline enhancements.. The objective of this study is as follows:
Present a general background about the most used zeolites and their advantageous characteristics.
Illustrate the optimum conditions over a range of zeolite catalysts via reviewing the literature of hydroisomerization of n-heptane and the effect of key parameters such as temperature, space time, and acidity on the hydroisomerization over bifunctional catalysts. This would help in determining the starting point of this research as the optimum temperature range and catalyst types would be chosen.
Test the effect of key parameters on the hydroisomerization of n-heptane over platinum loaded zeolite catalysts experimentally. Then, compare the selectivity, activity, and stability of the in-house catalyst to the commercial one.
Provide findings and insights that would have a direct impact on the catalysts market and refining process economics as well as catalyst research
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CHAPTER TWO LITERATURE REVIEW: ZEOLITE 2.1 Introduction In 1756, the Swedish mineralogist, Cronstedt, observed stones that danced about as the heat of the volcano was rising in the blowpipe flame and causing the entrapped water inside the stone to evaporate. Cronstedt named the stone zeolite from the Greek words “zeo” and “lithos” meaning “to boil” and “stones” – see Figure 2-1. At first, the conception about natural occurring zeolite was that it only forms in hot volcanic rocks over thousands of years, however, the discovery of sedimentary depositions in Western United State, proved that natural zeolite could be mined commercially [10]. A zeolite is a crystalline hydrate aluminosilicate with - cage like - enclosed cavities occupied by cations and water molecules that have freedom of movement and those water molecules could be removed without affecting the zeolite cage structure as observed by Damour [11]. The dehydrated zeolite with spongy framework as reported by Weighl and Stenihoff in 1925, could work as molecular sieve. In 1959, the door of opportunities opened up for zeolite in the oil and gas industry as well as other industries when Union Carbide presented the first major bulk separation process using zeolite Y, the “ISOSIV” process [11].
Figure 2- 1: Some forms of Natural Zeolite [12]
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The importance of zeolites arises from the fact that they have uniformly-sized pores in their three-dimensional framework, which allows them to act as sieves on a molecular level, that is, allow the passage of a certain size of molecules, while preventing larger ones. For example, the zeolite catalysts could specifically be designed to target or deliver one task such as gas separation and purification where the zeolite catalyst would remove air from the hydrocarbon gas stream and hydrocarbon cracking where big molecules get trapped in the zeolite cage, then, cracked to exit [11]. Zeolite catalysts have different forms and structures based on their synthesis methodology and as a result every zeolite has its own properties and characterization.
2.2 Zeolite structure and composition Zeolite catalysts have different forms and structures based on their synthesis methodology where every zeolite has its own properties and characterization. The tetrahedral atoms, TO4, are the primary building units (PBU) of the zeolite structure, where every T may be Si, Al, P, and Ga. Si atom has four positive charges in their orbits so upon connection with four oxygen atoms the net charge is balanced, while Al has just three positive charges which leaves the atom net charge negative after linkage with four oxygen atoms. The negative charges caused by the AlO4-5 are balanced by attracting cations such as Na+, K+, Mg2+, or Ca2+ to stabilize the negative charge [13]. The zeolite structure is built from the PBUs, SiO4-4 and AlO4-5, as they link together via shared oxygen atoms to yield different structural forms such as sheets, channels or cages like β-cage. The zeolite structure contains water molecules that move around the framework freely. This property enhances the ion exchange procedure with cations and metals. The concept of secondary building units (SBU) and structural subunits (SSU) where introduced to describe the structure of the zeolite and distinguish between different topology. SBU is the minimum number of units based on group of tetrahedral linked together, from which a known zeolite topology could be built [14].
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As a result of the structural diversity of zeolites, they are described by both the size of the secondary building units (SBU), which determines the pore size of the structure, and by structural subunits (SSU) to indicate the framework topology [11] – see Figure 2-2.
Figure 2- 2: Zeolite common cages and their secondary building units (SBU) [11]
Figure 2-2 shows different types of zeolite’s primary and secondary building units. Secondary building units are donated by naming and numbering system such as D4R and D6R where D stands for double, number indicates the number of planes, and R stands for ring. The donated numbers such as 46 mean that there are 6 4-membered planes in SBU. For example, α-cavity is named as [4126886], which indicates that there are 12 4-membered planes, 8 6-membered planes, and 6 8-membered planes connected together, and that means there are 9 hexagonal prism connected to 8 sodalite units, and 1 supercage to form the α-cavity [15].
2.3 Synthesis: In 1954, R. M. Milton and co-worker D. W. Breck succeeded in commercially manufacturing synthetic zeolites that have wider range of properties and larger cavities than their natural counterparts and the earliest applications of zeolites were the drying of refrigerant gas and natural gas. In 1970s, following the advancements in zeolite ZSM-5 and its applications in industries, the interest of developing zeolites rose sharply [11]. 13
Currently, the manufacturing of synthetic zeolites exceeds 12,000 tons a year [13]. The starting compositions of the zeolite along with the conditions of the reaction such as acidity, temperature, and water pressure affect the shape and properties of the produced zeolite. There are a number of procedures to produce zeolites one of which involves mixing sodium, aluminium, and silica chemicals with steam to create an amorphous and water-rich solid gel. Then, the gel is aged and heated to about 90°C [11]. The empirical formula of zeolites is as follows: (Eq. 2-1)
⁄
Where n represents the cation valence, y ranges between 2 and 10, and w is the number of water molecules present in the framework.
2.4 Types of Zeolites There are over 100 types of zeolite most of which serve different purposes [16]. The most common types are zeolite Y, Beta, and Mordenite. 2.4.1 Zeolite Y In 1842, zeolite Y was discovered by Damour and named after the French geologist and volcanologist, Barthelemy-Faujas de Saint-Fond. It has a faujasite type phase and its structure consists of 12-membered rings, which creates large cavity with 12 Å diameter, and cubic cell with 24.7 Å long where the pores of the structure run at right angles to each other. The structure of the zeolite Y has Si/Al ratio of 2.43 and could be described by its SBUs, which are 4, 6, and 6-6, as illustrated in Figure 2-3 [11,17]. Zeolite USY is a modified version of zeolite Y as USY catalyst has higher Si/Al ratio than that of zeolite Y in the range of 4 and more. Subsequently, Zeolite USY has more silicon atoms and less aluminium atoms which reduce the acid density as the density of aluminium is reduced. The dealumination of USY helps improve its thermal stability compared to that of the zeolite Y. The acid sites in the catalyst are isolated and this reduces the possibility of catalysing more than one molecule and enhances the acidity [11]. 14
Figure 2- 3: Pore diameter in Angstroms and 3-D view of the structure of faujasite zeolite [11]
Having many advantages such as high acidity, large pores, supercage, and cation incorporation, zeolite Y and its derivatives are used extensively in the refining and petrochemical industries where USY catalyst is deployed to increase the octane and olefin yield, by reducing the effect of hydrogen transfer. Zeolite Y is also used in cracking crude oil into lighter products due to its high acidity [18]. 2.4.2 Zeolite Beta Although zeolite beta was first synthesized by Mobil Oil in 1967, no practical applications were found for the catalyst until recently due to the disorder inside the zeolite structure which consists of three interconnecting pore system, two of which are 12-ring linear channels (5.7 x 7.5 Å) in different crystallographic direction intersecting partially and the third pore is a sinusoidal channel (5.6 x 6.5 Å) formed at the intersection. There are three crystalline phases of zeolite beta; one tetragonal and two monoclinic – see Figure 2-4 [17,19] .
Figure 2- 4: : Pore diameters in Angstroms and 3-D view of the structure of Beta zeolite [19]
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One of the prevail advantages of zeolite beta, in addition to the large pores which eliminate the diffusion limitations, especially in the petrochemical industry is the small crystal size of the catalyst between 20 – 50 nm which makes it ideal for synthesis of bulky molecules such as polymerization reaction. Moreover, there is a big market for zeolite beta in the oil and gas industry as it’s used in hydrotreating and fine organic cracking such as benzene alkylation with propene to produce cumene and alkylation of isobutene and isobutane [17]. 2.4.3 Zeolite Mordenite The mordenite catalyst consists of two types of pores that form the catalyst; First, 12ring elliptical channels (6.5 x 7.2 Å) in the c-axis; Second, 8-ring channels (3.4 x 4.8 Å) in the b-axis which are not accessible to molecules bigger than that of C3 as shown in Figure 2-5. The structure of the catalyst consists of five rings which are connected together to form a chain [20,21].
Figure 2- 5:Pore diameters in Angstroms and 3-D view of the structure of Mordenite zeolite [19]
Mordenite is a very active catalyst and it is used in light paraffins (C5/C6) isomerization. However, the disadvantage of using H-mordenite isomerization catalyst is the rapid deactivation rate due to channel blockage by coke formation although it has good selectivity and high amount of silicon atoms. As a result, Hmordenite catalyst is often post treated with either steaming or acid leaching to overcome the high acidity of the catalyst and the fact that it has one dimensional channels. Dealumination of H-mordenite changes the structure of mordenite creating mesopores which reduces the deactivation rate greatly, however, the acidity reduces as Brønsted acid site reduces with the removal of Al atoms [22,23]. 16
2.5 Post Treatment The synthesis of zeolite catalyst could only go so far in developing the properties without post treatment. In order to enhance catalyst performance and design new catalysts for specific purpose, further treatment of the zeolite is required. The major zeolite treatments are ion exchange, dealumination, and metal loading. 2.5.1 Ion Exchange The synthesized zeolite shows ion exchange tendency with cations as a result of its unbalanced overall charge that was caused by the existence of one of the primary building units – AlO4-5 (PBU). In order to stabilize the zeolite structure, AlO4-5 is ionexchanged with cations such as NH4+ or alkali group in an aqueous solution. The zeolite is mixed constantly with a solution that contains the desired cation at the right proportion at room temperature or higher for enhanced ion-exchange rate [24,25] As the number of tetrahedral aluminium atoms increases, the Si/Al ratio decreases which increase the ion exchange capacity. The acidity of the zeolite is characterized by the number of Brønsted acid site in the framework which is reduced with the removal of aluminium atoms. The rate of ion-exchange is dependent upon the size of the cation, the size of pores in the zeolite structure and the temperature of the solution [25]. 2.5.2 Dealumination: In some processes like hydrocracking, dealumination of zeolite catalysts is a necessity to enhance catalysts’ performance and improve their hydrothermal stability. Dealumination could be achieved by either hydrothermal treatment – steaming – or acid leaching. 2.5.2.1 Steaming The zeolite is calcined in their hydrogen or ammonium form while steaming with water vapour at higher temperature around 500 °C to reduce the lattice aluminium concentration. Steaming the zeolite catalyst removes the tetrahedral aluminium atoms from the framework without modifying the structure and leaves the aluminium atoms – extra framework aluminium atoms (EFAL) – move freely and form new Lewis acid 17
sites at the surface of the catalyst. The removed aluminium atoms from the zeolite framework are replaced by silicon atoms which increases the Si/Al ratio [26,27]. For example, by partial dealumination of zeolite Y, aluminium atoms would be removed without modifying the zeolite structure which results in ultrastable zeolite Y – USY. The modifications in the framework, Si/Al ratio, structure, and acidity show improvements in zeolite USY performance, selectivity, stability, and coking behaviour. Therefore, USY is used extensively in the oil and gas industry. However, the removed aluminium – EFAL – species affect the cracking activity and the selectivity of the catalyst and eventually cause coking as they hinder the catalytic and transport properties [28,29]. 2.5.2.2 Acid Leaching Acid leaching induces the same effect on the zeolite catalyst as that of steaming except that acid leaching removes the EFAL species completely from the structure which increases the Si/Al ratio, enhances the thermal stability, and causes mesopores (20 – 500 Å diameter [30]) in the structure. Acid leaching is usually performed with a solution of hydrochloric acid (HCl), ammonium hexafluorosilicate [20], and silicon tetrachloride [31,32] at a pH close to 4 to enhance the formation of hydroxyl nest in place of aluminium atoms through hydrolysis [7,16] 2.5.2.3 Metal loading: In order to synthesize the zeolite catalyst for specific processes, certain properties have to be either enhanced or introduced to the catalyst. Metals from group VIII usually are incorporated into the zeolite structure such as platinum and palladium. The resultant zeolite is described as bifunctional catalyst where the zeolite acid sites provide the acid function for cracking activities and metal sites to provide the hydro/dehydrogenation functions [32].
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2.6 Zeolite Properties The use of zeolite catalysts in the oil and gas and petrochemical industries has exceeded any other type of catalysts due to zeolite unique properties such as shape selectivity and high acidity [26,33]. 2.6.1 Zeolite Acidity In the zeolite framework, silicon atoms and aluminium atoms are connected by hydroxyl bridge which result in the formation of Brønsted acid sites – see Figure 2-6 [9,16]. The acidity of the zeolite increases as Si/Al ratio decreases since the Si/Al ratio indicates the number of potential acid sites; at a specific Si/Al ratio, the zeolite reaches its highest acidity value per active site [27,29]. It has been indicated that high crystalline zeolite hosts stronger Brønsted acid sites, while weaker acid sites could be found in less crystalline structure. Usually, a zeolite is modified after synthesis by dealumination or ion exchange treatments to improve its properties. However, dealumination process removes aluminium atoms from zeolite structure, hence, Si/Al ratio increases leading to a decrease the number of Brønsted acid sites. EFAL species resulting from dealumination by hydrothermal treatment migrate to the zeolite surface to form new Lewis acid site, in addition to partly linked aluminium atoms. According to the study [34], superacid sites are created when Lewis acid gets in close proximity with Brønsted acid.
Figure 2- 6: Brønsted acid site created by the formation of hydroxyl bride [16]
19
2.6.2 Shape selectivity The zeolite is built from primary and secondary building units that are replicated several times to form a specific zeolite structure. As a result, a synthetic zeolite has a uniform structure which means that the dimensions of the pores of the framework are exactly the same throughout the whole zeolite structure and cages have similar properties. The shape selectivity of zeolite could be classified into three categories as illustrated in Figure 2-7 (A - C); reactant selectivity, product selectivity, and restricted transition state selectivity [24, 35-37]. Reactant selectivity The zeolite structure and pore dimensions limit the reaction to those molecules that could pass through the pores due to their small size and high Gibbs free energy of adsorption. In order for a molecule to diffuse through the pore into the active sites of the zeolite, it has to overcome the Gibbs free energy barrier and possesses the right shape [39]. Product selectivity The outcomes of the reaction inside the zeolite cages have to obtain a certain shape to fit through the pore. Once the new molecule enters the crystal, it pushes the products out of the cage – giving the products some Gibbs free energy to desorb of the catalyst structure and overcome the low diffusion barrier. Restricted transition state selectivity This type of selectivity differs from the other two selectivity methods based on the basis of screening as the later types limit the reaction based on the diffusion barrier and the size of either products or reactants, while the restricted transition state selectivity limits the reaction by its mechanism. Moreover, this type of selectivity can prevent the formation of intermediates which would lead to reaction with small transition state [40]
20
A
B
C
Figure 2- 7: Schematic representation of the three types of shape-selectivity where A is reactant selective; B is product selective; and C is transition state selective– Adapted from [37,39-41]
2.7 Hydroisomerization In order to comply with the clean fuel act and the strict environmental legislation regarding gasoline impurities, the quantity of aromatics, olefins, and benzene rings have to be sharply reduced. The uses of octane boosters such as TEL and MTEB were banned in many countries due to their toxicity and high affinity to underground water, respectively [38]. As a result, refiners have invested more in processes rather than chemicals to upgrade the gasoline quality and increase its octane number such as catalytic reformer and hydroisomerization units, however reformate contains high
21
quantities of aromatics. So, hydroisomerization of light straight – chain paraffins into branched alkanes was the most viable and cheapest option. Hydroisomerization of light naphtha (C5-C6) has commercial applications in the industry. The hydroisomerization reaction occurs in bifunctional catalysts where metal sites are supported on acidic structure. Although platinum loaded chloride alumina – the commercial catalyst – is a very active hydroisomerization catalyst and it operates at low temperatures, it causes corrosion and pollution problems during the reactivation process and it is very sensitive to water and sulphur. Moreover, there is extensive research regarding the hydroisomerization of heavy naphtha (C7-C10) to upgrade it from low quality stream to products with high octane number. There is not any commercial process that could hydroisomerize C7-C10 yet due to their high tendency to crack. Research is focusing on developing a wellbalanced catalyst in terms of hydrogenation – dehydrogenation functions and acidity to convert heavy naphtha to high octane isomers. In order to do so, reaction mechanism and the effect of key parameters such as temperature, space time, and acidity have to be explored and understood.
2.8 Reaction Mechanism There are several routes for the hydroisomerization of n-alkanes, which takes place over bifunctional zeolite catalysts, such as monomolecular mechanism and bimolecular mechanism – see Figure 8-8 & 8-9. The path of n-alkanes hydroisomerization reaction is greatly affected by the balance of hydrogenationdehydrogenation function and acidity. As in a well-balanced catalyst, the hydroisomerization reaction follows the monomolecular path– the common path, whereas in unbalanced catalyst dimerization cracking path is followed. During such a mechanism alkylcarbenium ions are dimerized, then, cracked at acidic sites which results in cracked products ranging from C3 to C6 whereas on well-balanced catalyst the cracked products are limited to C3 and C4 [42].
22
Figure 2- 8: Monomolecular Isomerization of Heptane into 2-methylhexane on bifunctional catalyst Modified from [42]
Figure 2- 9:Bimolecular Isomerization of Heptane into 2-methylhexane on bifunctional catalyst - Modified from [42]
Monomolecular mechanism, in well balanced catalyst, states that alkenes, which are formed by the dehydrogenation of alkanes over metal phase, are protonated at the Bronsted acid sites resulting in alkylcarbenium ions. Then, rearrangement of C-C bond and scission occur to the alkylcarbenium ions prior to desorption as rearranged alkenes. After desorption, product alkenes are hydrogenated by the metal phase to yield the final isomers. As a result of the fast rate of hydride shift compared to C-C bond rearrangement, there are two types of isomerisation in monomolecular mechanism that n-alkanes could follow to be rearranged to isomers; Type A isomerization; Type B isomerization [43].
23
Type A isomerization During this type of isomerisation, the side chain of the straight-chain alkane is moved to a new position with no alteration of the number of primary, secondary, tertiary, and quaternary C atoms in the molecule as isomerization proceeds through cyclization of the alkylcarbenium ion into an intermediate corner protonated cyclopropane ring (CPCP) – see Figure 2-10.
Figure 2- 10:Monomolecular mechanism of type A isomerization [42]
Type B isomerization According to this type of isomerisation, a proton has to overcome an important energy barrier to jump from corner to corner leaving a positive charge toward a corner carbon atom free of alkyl substituent which results in creating a branch in a linear carbon chain. After the proton jump, the CPCP intermediate ion opens up to become the final isomer product – see Figure 2-11. Due to the fact that type B isomerisation has to migrate overcoming an energy barrier, type A isomerisation is much faster than type B isomerisation. The number of the number of primary, secondary, tertiary, and quaternary C atoms is changed as the degree of branching is changed. Sometimes type B isomerisation occurs via substituted protonated cyclobutanes (CPCB) to account for the formation of ethyl side chain [42].
24
Figure 2- 11: Monomolecular mechanism of type B isomerization [42]
2.9 Effect of Key Parameters The hydroisomerization reaction is influenced by the activity, selectivity, stability of the catalyst. The zeolite catalysts have many advantages over other catalyst such as shape selectivity and high acidity; however, these properties could either be enhanced or hindered by reaction conditions. There are major factors that could affect the zeolite performance during hydroisomerization of n-heptane such as reaction temperature, space time, chain length, catalyst geometry, acidity, and metal-acid balance. 2.9.1 Reaction Temperature In general, zeolite catalysts and chemical reactions are affected by the reaction temperature. The activity of the catalyst increases with temperature, which is most noticeable during hydrocracking and hydroisomerization reaction, as temperatures are often slightly increased to compensate for the loss of activity due to coke formation. The selectivity of alkane hydroisomerization decreases as temperature increases especially with long-chain hydrocarbon [34]. Although total conversion of hydroisomerization tends to increase with temperature, the n-alkane conversion to isomers increases with temperature until cracking reaction becomes predominant – see Figure 2-12 [44,45].
25
Figure 2- 12: Effect of reaction temperature on n-heptane hydroisomerization. Adapted from [46]
2.9.2 Space Time Space time is a representation of the feed flow in terms of space velocity and catalyst weight. For a fixed amount of catalyst weight, the lower the space time the higher the feed flow. The effect of space time on the isomerization of n-alkanes is similar to that of temperature [46]. At constant temperature, contact time between the hydrocarbon feed and the catalyst increases as space time increases which results in high overall conversion. The selectivity of isomerization decreases with increasing space time – see Figure 2-13.
Figure 2- 13: Effect of contact time (W/F) on n-heptane hydroisomerization [46]
26
2.9.3 Chain Length The total conversion of the reactant is affected by the hydrocarbon chain length where the conversion increases as the chain length increase. Due to the high tendency of long-chain hydrocarbon (C7-C10) to crack, the hydroisomerization yield decreases compared with the high conversion which is illustrated in Figure 2-14. However, product distribution is also affected by the length of the hydrocarbon chain as yield of multi-branched isomers increase with increasing carbon chain [47].
Figure 2- 14: Effect of chain length on isomerization of n-alkane - Adapted from [47]
2.9.4 Catalyst Acidity The Si/Al ratio indicates the number of potential acid sites; at a specific Si/Al ratio, the zeolite reaches its highest acidity value per active site and therefore, as the Si/Al ratio increases, the zeolite acidity increases. During the hydroisomerization reaction, alkanes are dehydrogenated and/or hydrogenated on the metal phase, whereas cracking occurs on the acidic sites, consequently the cracking activity increases as the acidity of the catalyst increases. The selectivity of the catalyst to isomerization compared to cracking decreases with acidity while selectivity to multi-branched isomers increases with acidity [46].
27
2.9.5 Catalyst Geometry Zeolite catalysts have various geometry types and acidity strength, thus having different properties. Most heterogeneous catalytic reactions, except those reacting on the external surface follow the 7-step mechanism in order to convert the feeds to products – see Figure 2-15 [48]. The 7-step mechanism states that a feed molecule has to i.
Overcome the bulk diffusion barrier;
ii.
Diffuse through the structural pores;
iii.
Be adsorbed by the active site;
iv.
React over the active site;
v.
Desorb from the active site;
vi.
Diffuse back to the surface;
vii.
Diffuse through the bulk
Bulk Phase
Boundary Layer
Pellet
Figure 2- 15: 7 Steps of a heterogeneous catalytic reaction [38]
Therefore, the structure of the zeolite such as pore diameter and cage dimensions affects the reaction mechanism and imposes some challenges on the reacting molecules such as diffusion barriers. 28
For example, some studies [8,46, 49-50] have shown that zeolite USY produces less multi-branched isomers and cracked products than zeolite Mordenite due to the structural dimension of Mordenite pores and the higher acid site density. Mordenite is used as standard catalyst for C5 and C6 hydroisomerization instead of USY. This is interestingly unexpected because as a result of the higher cracking rate of Mordenite, the overall isomers yield is less than that of zeolite USY – see Figure 2-16.
Figure 2- 16: Effect of Catalyst Geometry on The Isomerization of n-heptane, Adapted from [47]
2.9.6 Platinum Content The platinum ion phase is responsible for the hydrogenation-dehydrogenation function in the catalyst where reactants would be dehydrogenated to create olefins and once alkylcarbenium ion is detached from the acid site, it would be hydrogenated by platinum as explained in Section 2.8. According to an earlier study [46], where the effect of platinum content over zeolite Beta was investigated at a constant temperature of 230 °C, the selectivity to isomers was found to increase as the platinum content was increased. This could be attributed to the role of platinum in hydrogenating the intermediate olefins. However, the nheptane conversion and the yield of multi-branched isomers were not affected by the platinum content as shown in Figure 2-17.
29
Figure 2- 18: Yield and selectivity as a function of Pt content (wt%) [46]
This dissertation will focus on platinum loading of two USY catalysts which have been post synthetically modified via steaming and steaming with acid leaching. These treatments would be expected to change the Si/Al ratio, nature of acid sites and the structural geometry.
30
CHAPTER THREE EXPERIMENTAL 3.1 Introduction: The aim of the n-heptane hydroisomerization experiment is to investigate the effect of temperature and residence time on n-heptane hydroisomerization reaction over platinum loaded zeolite USY. In this chapter, catalysts’ preparation, characterization, and testing are going to be discussed in details to illustrate the methodology that has been followed throughout the dissertation. In order to promote results reproducibility and repeatability, the unit specifications, experimental procedures and mass flow controller, furnace, and GC calibrations are explained thoroughly. The experiments were conducted in a stainless steel reactor heated by a furnace that provides the required temperature for the reaction to take place. The liquid and gas feeds were introduced to the system by HPLC pump and high pressure cylinder connected to mass flow controllers. The products of the hydroisomerization reaction were analysed via the Varian 3400 gas chromatography (GC).
3.2 Catalyst Preparation As shown previously in the literature review, zeolite Mordenite has high acidity and selectivity to C5/C6 hydroisomerization whereas zeolite USY and Beta have lower acidity and selectivity to isomers. However, zeolite Mordenite was not chosen for the hydroisomerization of n-heptane due to its high cracking activity which resulted from both the high acidity and 1-dimensional structure. Zeolite USY and Beta catalysts were initially considered in this dissertation for nheptane isomerisation experiment due to their large pore openings and wide channels which could accommodate n-heptane structure and its isomers. In addition both zeolites have high acidity compared to other types of zeolites. Zeolite USY was favoured over Beta due to its unique pore size and structure and also as a result of
31
time constraints required for testing. Two samples of the zeolite catalysts, USY, were post synthetically treated differently and metal loaded with 1 wt% platinum. As discussed in the literature review section, there are several noble metals that could promote the hydrogenation-dehydrogenation function such as palladium and platinum. Owing the superiority of platinum over palladium in hydrogenationdehydrogenation function, the two samples were loaded with 1 wt% of platinum which is the recommended amount to maintain the metal – acid balance [51] In these experiments, two catalysts were prepared and tested; first catalyst, CBV712, was a commercial catalyst which was dealuminated by thermal treatment, then, acid leached to remove EFAL species created by the thermal treatment. Second catalyst, USY-E, was an in-house catalyst prepared from the parent zeolite Y with 0.57 wt% soda – see Table 3-1. Material Name
Material Type
Post synthesis modification
Supplier
CBV712
USY
Steamed and Acid leached
Zeolyst International
USY-E
USY
Moderately Steamed
Crosfield
Material Name
Formula
Purity (wt%)
Tetraamine platinum(II) chloride
[Pt(NH3)4Cl2.H2O]
99.5
Table 3- 1: Type of catalysts used, treatment methods, and supplier
The in-house catalyst was ion exchanged with NH4NO3 to reduce the sodium content in the structure to 0.1 wt% soda. Then, the catalyst was dealuminated by mild steaming to improve the catalyst thermal stability and other physical properties The resultant catalyst was ion exchanged with [Pt(NH3)4Cl2.H2O] salt to load the required metal, platinum. The catalysts were subjected to characterization testing and the results are presented in Table 3-2.
32
Elemental Analysis (bulk)
Prior to Ion Exchange
Ion exchanged with Pt salt
Si/Al mole ratio
Pt (wt%)
CBV712
USY-E
CBV712
USY-E
5.93
2.67
0.98
0.93
Crystallinity*
XRD
CBV712
USY-E
87.50
94.83
Crystallite Size(nm) *Related to in-house as-synthesised Y
CBV712
USY-E
2.95
2.06
Si/Al framework
NMR
CBV712
USY-E
8.34
7.14
Three chemical environments for Si-NMR: Si(0Al), Si(1Al), and Si(2Al) Lewis acid sites are existed: Al(0Si)4, (Al(H2O)5)3+, and (Al(H2O)6)3+
Metallic surface area (m2g-1 metal)
Metal dispersion (%) Hydrogen chemisorption
CBV712
USY-E
CBV712
USY-E
38.44
54.95
94.95
135.71
Surface area (m2g-1)
BET
Pore Size Å
CBV712
USY-E
CBV712
USY-E
816.26
690.62
42.75
36.14
Both catalyst show micro and mesoporosity
Table 3- 2: Previous characterization of CBV712 and USY-E [52]
33
3.3 Catalyst Characterization 3.3.1 Scanning Electron Microscopy The scanning electron microscopy – SEM – works in the basis that an electron intensive beam, which is fired from a light source, hits the specimen to identify some of its properties. However, SEM focuses on the reflected beams after the electron interaction with the specimen surface to study the surface topology, morphology, chemical composition, and crystalline structure rather than what’s beyond the surface – see Figure 3-1.
Figure 3- 1: SEM micrograph of CBV712 [52]
The fundamental principle behind the SEM is that a huge amount of kinetic energy, which is generated from an accelerated electron beam, is directed to the tested object. The incident beam interacts with the object’s surface and subsequently decelerates as it gets deflected – see Figure 3-2. The topology and morphology images of the object’s surface such as the zeolite catalysts are extracted from the secondary electrons while the contrasts in composition in multiphase samples are illustrated by the backscattered electrons (i.e. for rapid phase discrimination). In addition, photons are used for elemental analysis and X-ray, since X-ray is generated from the inelastic collisions of the incident beam with electrons in the shells of atoms in the sample [53]. 34
Figure 3- 2: Illustration of the diffracted electron beam when it hits the sample [54,55]
3.3.2 Transmission Electron Microscopy – TEM. The discovery of high magnification made by transmission electron microscopy – TEM – has made the study of medical, biological, and material possible. As the light microscopy, the TEM operates to magnify small objects. However, TEM uses electrons instead of light to be able to magnify objects at near atomic level with high resolution due to their short wavelength [56]. The resolution of the image could be enhanced according to Equation 3-1 [57]:
[
√
]
Where d is the maximum resolution, h is Planck’s constant,
(Eq. 3-1) is the rest mass of an
electron, E is the energy of the accelerated electron, c is the speed of light, and NA is the numerical aperture of the system. Through the vacuum column of the TEM, the fired electrons from a light source get concentrated into a very thin beam by the electromagnetic lenses. Then, the electron beam hits the studied specimen and subsequently some electrons get scattered and disappear. A fluorescent screen receives the scattered electrons and displays a shadow image of the specimen with its different parts displayed in varied darkness according to their density. This image could be studied directly or saved on the microfilm screen for further investigation – see Figure 3-3 [56]. 35
Electron source Electron beam Specimen Electromagnetic lense
Image Screen
Figure 3- 3: Illustration of Transmission Electron Microscopy - adopted from [56]
TEM is used in the characterization of catalysts with the aid of electron diffraction and X-ray microanalysis to show the quality of metal dispersion. The TEM images display the surface of the catalysts with different colour contrast depending on the site density which shows the metallic sites in catalytic phase and the sintering of metals before and after the catalytic reaction. The catalyst should be activated prior to testing by TEM in order to show the metallic platinum crystallite clusters. For a typical experiment, the samples were grounded and diluted with acetone, then; the mixture was ultrasonicated to help in dispersing the particles. A thin sample layer was produced by injecting a droplet of the solution onto the carbon-coated copper grids for analysis see Figure 3-4.
Figure 3- 4: The sample was injected onto the carbon-coated copper grid – adapted from [58]
36
3.3.3. X-ray Diffraction X-ray diffraction – XRD – is a fast way to analyse the crystalline phase of a solid subject as 95% of solids are crystalline. In 1919, A. W. Hull discovered that every crystalline substance has its own finger print in terms of X-ray pattern where the same pattern will always be the results of the same substance. The XRD is usually used to identify the types of crystalline components present in a polycrystalline phase and their quantity which are represented by the area under the components’ peak [59]. The monochromatic X-rays are fired toward the sample surface at a range of 2 angles to cover all possible directions of the structure – see Figure 3-5. According to the topology of the surface, the X-rays would be deflected, then, detected and processed to produce the appropriate peaks. In order to identify the different components which make up the sample structure, the obtained peaks would need to be converted to d-spacing.
Figure 3- 5: Illustration of the XRD principle – Adopted form [59]
XRD is based on the constructive interference of the monochromatic X-rays and a crystalline sample which occurs when the monochromatic X-rays are deflected by two consecutive planes and Bragg’s law is satisfied. From Equation 3-2, Bragg’s law relates the wavelength of the electromagnetic radiations to angle of diffraction and spacing of planes in the sample. Bragg's law
(Eq. 3-2) Where n is the order of diffraction, is the Bragg’s angle, and
is the wavelength of the incident X-ray beam,
is the interplanar distance for a set of planes. 37
Every crystalline sample is constructed by different atoms, which are represented by parallel planes in three dimensional distribution and these planes are separated from one another by distant d. The distant between planes are affected by the atom size where different atoms would yield various d measurements and every crystal has its own fixed d-spacing measurement. In fact, samples are identified by comparing the obtained XRD peaks with the standard d-spacing data base [60].
Figure 3- 6: XRD pattern of CBV712 [52]
3.4 Catalyst Testing 3.4.1 The hydroisomerization unit: The materials that have been used in these experiments are shown in Table 3- 3 which includes liquid and gas feed and any equipment that are used in catalyst preparation and loading. The hydroisomerization unit was built to provide flexibility in regards with system pressure as it could withstand operation pressure up to 2 MPa. The design specifications of the hydroisomerization unit are shown in Table 3-3. Design Parameter
Specification
Design Pressure
5 MPa
Design Temperature
650 °C
Reactor Total Volume
43.7 mL
Operating Pressure
0.1 MPa
Operating Temperature
190 – 250 °C
nC7 Flow Rate
0.125-0.5 mL/min
H2 Flow rate
100-1000 mL/min
Air Flow rate
50-500 mL/min
Table 3- 3: Design Specifications of The Hydroisomerization Unit
38
Figure 3- 7: Process & Instrumentation Diagram of the Hydroisomerization unit [62]
39
The hydroisomerization unit consists of four main modulus; First, the feed supply section; Second, the reactor section; Third, products separation section and finally, pressure controlling section. These different parts of the unit are shown in details in the process and instrumentation diagram (P&ID) – see Figure 3-7. The Feed Supply Module First of all, the feed supply section encompasses high pressure cylinders, which contains the gas feed and liquid feed pump. The liquid feed pump delivers liquid reactant to the reactor section at the appropriate flow rate. The gas supply segment is responsible for providing high pressure gases to the unit such as hydrogen, nitrogen, and air. These gases are usually stored in high pressure cylinders – up to 17.5 MPa – and connected to the system through pressure regulators which adjust the high pressure in the cylinder to the required pressure where the flow of the incoming gases is controlled by the mass flow controller. Check valves in the gas streams are used to prevent contamination to the main cylinders in case of back pressure due to plug or blockages. The liquid feed segment consists of a bottle that holds up to 1.0 litre, a balance to check the consumption and verify the mass flow rate, and a pump. The n-heptane with 99% purity is pump by an HPLC pump (Gilson 305 piston pump) – see Table 33. The Reactor Module Secondly, the reactor module, which is the main part of the system, comprises of a mixer that allows the gas and liquid feed to mix, a stainless steel reactor (SS-316) – see Table 3-4 for reactor specification, and a furnace. In the reactor section, the hydrogen with 99.99% purity is mixed prior to the reactor inlet with the n-heptane at constant flow ratio. The mixture enters the reactor in a down flow mode to mimic the actual process at the refinery although up flow mode shows better liquid distribution. The reactor has a three zone electrical furnace that allows independent control of each heating zone and every zone has its own K-type thermocouple to measure the reactor skin temperature. The furnace provides an extra preventive measure by employing a 40
fourth thermocouple that is connected to an emergency off switch which is activated once the maximum set temperature is reached to prevent any temperature run away. The temperature of the catalyst bed is monitored via a movable thermocouple that is inserted through the ceramic thermowell –see Table 3-4. The products of the reaction exit the reactor at the bottom to the next the section – separation section. Description
Specification
Body Material
SS-316
Reactor Total Length
64.5 cm
Reactor Bed Length
53.5 cm
External Diameter
1.27 cm
Internal Diameter
1.02 cm
Wall Thickness
0.1245 cm
Thermowell Diameter
0.3175 cm
Reactor Total Volume
43.70 mL
Reactor Bed Volume (Total Vol. – Themowell Vol.)
39.92 mL
Maximum Operating Temperature
550 °C
Allowable Working Pressure @ 550°C
3 MPa
Table 3- 4: Reactor specifications [62]
The Separation Module Thirdly, the separation module, where the products are separated into gas and liquid is made of a separator, a heat exchanger, and sampling points for gas and liquid. The separator vessel enhances the separation of gases and liquid especially hydrogen gas due to its poor solubility in liquid. All gases and vapours passes through a concentric pipe that is used as counter-current heat exchanger to condense vapours and improve the overall mass balance. The duty of the condenser is controlled by the flow rate and temperature of the coolant bath. In addition, there are two sampling points in the system for the liquid and gas products. 41
The Backpressure Control Module Once the gas stream leaves the heat exchanger, it enters the pressure controlling section which starts with a knock-out vessel and an inline filter (with 7 micron mesh) to capture any vapour and liquid carried in the gas stream, especially in high hydrogen flow rates. Afterward, the gas passes through a pressure control loop which consists of a back pressure control valve that controls the system pressure and a bypass double valve line that is connected to the vent system, in case of an emergency. As safety counter measure, the unit has two relief valves, one in the gas line and one in the liquid line, both are set at 3 MPa in case of any blockages in the reactor or the system. 3.4.2 Equipment Calibration The hydroisomerization unit was not functional for some time and prior to conducting these experiments, so equipment such as gas mass flow controllers, HPLC pump, GC, and furnace temperatures had to be calibrated and verified. 3.4.3.1 Mass Flow Controller
The calibration of the hydrogen mass flow controller was conducted as follows: setting the opening of the controller valve to a set point; closing every exit in the system other than that of gas sampling; measuring the flow rate via a bubble graduated cylinder with a capacity of 25 mL. This procedure was performed 3-4 times at different flow rates, then, the average was calculated for every flow rate – see Figure 3-8 & 3-9. The calibration of air mass flow controller was done following the same procedure as explained earlier – see Table 3-5. The figures of mass flow controllers show that the opening percentage of the mass controlling valve is linear with the flow rate of the gas as R2 is very close to 1.
42
Valve opening % 0 2.5 4.0 6.5 8.0 14 29 44
Time (sec) 0 12 9 6 5 3 3 1
Volume (ml) 10 10 10 10 10 10 20 10
Flow Rate (ml/min) 0.0 50.0 66.7 100.0 120.0 200.0 400.0 600.0
Table 3- 5: Calibration of H2 mass flow controller
H2 M.F.C 700.0
H2 Flow (ml/min)
600.0 500.0 400.0 300.0
y = 13.303x + 14.273 R² = 0.999
200.0 100.0 0.0 0
10
20
30
40
50
Valve Opening (%) Figure 3- 9: Calibration of H2 mass flow controller
Air M.F.C
90 80 Flow (mL/min)
70 60 50 40
y = 11.169x - 28.532 R² = 0.9939
30 20 10 0 5
6
7
8
9
Valve Opening (%) Figure 3- 8: Calibration of air mass flow controller
43
10
11
3.4.3.2 HPLC Pump
First of all, the K-1001 HPLC pump was primed with n-heptane as it is necessary to purge it whenever the pump is restarted to avoid damaging the piston. Then, the HPLC pump was set at different flow rates and at each flow rate, a sample was collected and weighed to verify both the volume flow rate and the mass flow rate – see Table 3-6 and Figure 3-10. Pump setting (mL/min)
Actual flow (mL/min)
Error %
0
0
0
1
1.1
9.1
2
2.1
4.8
3
3.1
3.2
4
4.1
2.4
5
5.1
1.9
6
6.1
1.6
7
7.1
1.4
8
8.1
1.2
9
8.95
-0.56
Table 3- 6: Calibration of HPLC Pump
Actual Flow (ml/min)
Pump Calibration 10 9 8 7 6 5 4 3 2 1 0
y = 0.99x + 0.1333 R² = 0.9998
0
2
4
6
Flow Settings (ml/min) Figure 3- 10: Calibration of HPLC Pump
44
8
10
3.4.3.3 Furnace Temperature
The furnace of the reactor is divided into three sections (Top, Middle, Bottom) and each section is controlled independently. In order to achieve a stable reaction temperature across the reactor especially at the catalyst section, calibration of the furnace temperature across the reactor length (30 cm) was carried out – see Table 3-7 & 3-8. Firstly, the temperature of the furnace was set to the reaction temperature while the desired flow of hydrogen was maintained. Secondly, a marked thermocouple was inserted in the ceramic thermo-well to measure the temperature inside the reactor, then, compare it with the set points of the furnace as the furnace temperature indicates the skin temperature of the reactor. Finally, the set points temperatures of the furnace were adjusted till the reaction temperature was met. This procedure was repeated for all reaction temperatures such as 150°C, 190°C, 210°C, 230°C, and 250°C – see Table 3-5. From Figure 3-11, the stable temperature zone where the catalyst bed would be loaded in was between 13 – 24 cm.
Reactor Temp. Profile
°C
°C
°C
Figure 3- 11: Stable Temperature Zone in the Reactor with Pure H2 Flow.
45
°C
190 °C
210 °C
230 °C
250 °C
Top
Mid.
Btm
Top
Mid.
Btm
Top
Mid.
Btm
Top
Mid.
Btm
200
185
201
221
205
225
240
225
241
260
245
261
Table 3- 7: Furnace setting points for the different heating zones
Length 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Reactor Length Temperature °C 73.6 1 83.6 2 99.5 3 121.8 4 136.8 5 149.2 6 159.7 7 168.2 8 175.6 9 180.1 10 183.6 11 185.9 12 188 13 189.2 14 190 15 190 16 190 17 190 18 190 19 190 20 190 21 190 22 190 23 190 24 189.8 25 189.7 26 189.3 27 188.1 28 187 29 185.2 30
Reactor Length Temperature °C 78.2 1 86 2 100 3 122 4 143 5 159.5 6 173.2 7 183 8 190 9 196.3 10 201 11 206 12 207 13 208 14 209 15 210 16 210 17 210 18 210 19 210 20 210 21 210 22 210 23 210 24 211 25 211 26 210 27 210 28 209 29 208 30
Reactor Length Temperature °C 83.7 1 94.1 2 110.3 3 133.9 4 156.7 5 176 6 192 7 203 8 209 9 215 10 221 11 225 12 227 13 228 14 230 15 230 16 230 17 230 18 230 19 230 20 230 21 230 22 230 23 229.5 24 229 25 229 26 229 27 229 28 227 29 226 30
Reactor Temperature°C 91 100.8 119.3 145.4 171.5 190.9 209 221 233 240 244 247 249 250 250 250 250 250 250 250 250 249 249 249 249 248 247 246 245 245
Table 3- 8: Temperature readings of the reactor measured by the thermocouple with pure H2 flow
46
3.4.3 GC Analysis – gas and liquid Gas GC Analysis The performance of the gas GC had to be determined by measuring its response factor – RF – for a standard gas containing 1% by volume of C1, C2, C3, i-C4, n-C4, iC5 and n- C5 and use it as a baseline. The gas GC used in these experiments is a Varian 3400 GC, which is equipped with a 50m x 0.32 mm i.d. PLOT Al2O3/KCL capillary column fitted to an FID detector. In order to calculate the RF of the gas GC, the standard gas was injected into the gas GC several time prior to starting the analysis procedure and whenever space time was changed to account for the systematic errors using gas bags of 1.0 L, which are used to fill a gas service valve with a 250 mL loop. The elution times of the tested components and their average peak area counts are acquired, then, the response factor – RF – of each component are calculated by dividing its mole % by its average peak area count – see Table 3-9. The mass balance and the gas yield were calculated by extrapolating the RFs of heavier hydrocarbons, which have higher carbon number, utilizing the trend line equation that’s generated by plotting the carbon number of the gas mixture against the RFs in a log-log scale as illustrated in Figure 3-12. Number
Molecule Name
Area (μV.min)
RF=x/Area
log Carbon Number
log RF
1
Methane
173.7
0.006
0.00
-2.24
2
Ethane
342.3
0.003
0.30
-2.53
3
Propane
491.9
0.002
0.48
-2.69
4
iso-Butane
643.8
0.002
0.60
-2.81
5
n-Butane
619.7
0.002
0.60
-2.79
6
iso-Pentane
726.7
0.001
0.70
-2.86
7
n-Pentane
669.4
0.001
0.70
-2.83
8
hexane
0.001
0.78
-2.94
9
n-heptane
0.001
0.85
-2.99
Table 3- 9: Calculation of gas GC response factor
47
RF Calibration -1 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Log RF
-1.5 y = -0.9334x - 2.3528 R² = 0.998
-2
-2.5
-3
-3.5
Log Carbon number Figure 3- 12: Estimating the gas GC RF for C6 and C7
In order to capture the wide range of products by the gas GC, its temperature program had to be set as the followings; the initial column temperature was held at 110 C for 5 minutes, then, it was ramped to 225 C at a rate of 7.5 C/min and held for 10 minutes. Liquid GC Analysis FID response for the hydrocarbons is generally estimated to be linear over several dynamic ranges. This could normally be checked by the analysis of standard (typically 10:90, 50:50, 90:10) mixture of i-C7 and C7 or if cracking was the objective, then, a mixture of C5-C7 would be used. However, this was not carried out here as the response for all products were assumed to be linear.
Liq. GC - Temp. Program 250
Temperature (°C)
200 150 100 50 0 0
10
20
30 Time ( min)
40
Figure 3- 13: Temperature program of the liquid GC
48
50
60
The liquid samples were analysed by injecting 0.2 microliter into a Varian 3400 gas chromatograph (GC) with a capillary column type a 50m x 0.25 mm i.d. CP-Sil PONA CB optimized gasoline column fitted to a flame ionization detector (FID). The GC was programmed in such a way to improve peak identification and separation – see Figure 3-13.
3.4.4 Hydroisomerization Testing of n-heptane: 3.4.4.1 Reactor loading & Catalyst activation
The 1 wt% platinum - USY catalyst, which was prepared previously [52], were pelletized and sieved in a 250 – 425 micrometre mesh. Then, 2 grams of catalyst were loaded in the most stable zone in the reactor between fine glass wool to withhold the catalyst pellets from moving and to enhance liquid distribution. Table 3-10 shows the materials that were used in this process and their suppliers. Material Name
Dimensions
Supplier
Glass Beads
d= 1.0-1.3 mm
Farnell
Glass Wool
Fibre-glass
Farnell
Ceramic Thermowell
d=1.1mm, L= 0.65 m
Farnell
Thermocouple
d=1 mm, L=1 m
Farnell
Glass Liquid Vials
2.15 ml
Farnell
Gas Bags
0.5 L
Farnell
Specac Atlas Hydraulic Press
5 tons/cm2 (force applied)
Available in the lab
Mortar and Pestle
Standard
Available in the lab
Feed
Purity (wt%)
Dimensions
Supplier
Hydrogen
99.99
65.5 kg
BOC
Air
99.99
65.5 kg
BOC
n-heptane
99.33
2L
Sigma Aldrich
Table 3- 10: Material used in catalyst loading and testing
49
The activation of the catalyst was performed in-situ once the reactor was mounted and leak tested. As illustrated in Figure 3-14, the catalyst was heated by air to 500 °C at 2 °C/min to remove ammonium from the catalyst. Then, the temperature was decreased to 450 °C and hydrogen was introduced to reduce the catalyst which allows the conversion of platinum ions into their elemental form. The ramp temperature is very critical as faster heating rate might cause platinum sintering and agglomeration which leads to poor catalyst performance
Temperature Program
600
Temperautre (°C)
500 400 300 200 100 0 0
100
200
300
400
500
600
700
Time (minutes) Calcination by Air
Reduction by Hydrogen
Figure 3- 14: Temperature program during catalyst calcination and activation
Once the catalyst was activated, the temperature of the reactor was set to the required reaction temperature with the appropriate hydrogen flow. After the temperature of the reactor stabilized, the n-heptane feed with purity of 99.33 wt% was introduced at molar ratio of H2/HC = 9. Table 3-11 shows some physical properties of the nheptane feed which was obtained from Sigma Aldrich; impurities are mostly heptane isomers. Boiling point, oC
98.4
Specific gravity
0.684
Vapour pressure @ 20oC, mmHg
4
Viscosity @ 25oC, cP
0.386
Table 3- 11: n-heptane physical properties
50
3.4.4.2 Reactor and Feed Conditions
The optimum conditions for hydroisomerization experiments were chosen based on extensive search in the literature where similar experiments were conducted over different catalysts. From literature review (Section 2.8), the hydroisomerization reaction yields best results over a range of temperatures between 170 – 270 °C. For this investigation, four temperatures were selected, which are 190 °C, 210 °C, 230 °C, and 250 °C, to test the selectivity, activity, and stability of two different catalysts. At each temperature, three flow rates were tested to alter the space time (at a constant H2: HC ratio = 9:1) – see in Table 3-12. Firstly, the hydrogen flow rate was set to the required flow by changing the valve opening. After the unit is stable, a temperature profile along the reactor tube was recorded as a baseline for further comparison once the liquid was introduced. Secondly, the n-heptane feed was introduced at the appropriate molar flow ratio via the HPLC pump. Thirdly, every 30 minutes, a temperature profile was recorded to ensure that the reaction is stable thermally and a gas sample was collected, then, a liquid sample was collected and weighted. For every collected sample the gas flow rate was measured using a liquid bubbler and the liquid sample was weighted and compared against the weight of the feed tank.
Space Time (W/F) (kg.s/mol)
Temperatures (°C)
1
2
3
190
46.84
70.32
140.63
210
46.84
70.32
140.63
230
46.84
70.32
140.63
250
46.84
70.32
140.63
Table 3- 12: Experimental conditions for both CBV712 and USY-E at (H2: HC = 9:1)
During the activity, selectivity, and conversion experiments, 3 to 4 samples were collected at different space time (W/F). At higher temperatures, extra samples are collected at the initial flow rate, once samples of all flow rates were taken to check for short term deactivation and repeatability. This procedure was followed throughout all experiments to provide consistency and minimize human errors. 51
3.4.5 Mass Balance The products of the hydroisomerization are separated in the separation module (Section 3.4.2) into gas and liquid streams. In order to calculate the mass balance for this unit, the products have to be analysed and converted to mass basis. The liquid samples are analysed by Varian 3400 GC, as explained in Section 3.4.4, and the peaks are drawn in the software (Galaxie) for further analysis and storage. For example, with feed flow rate of 22.5 mL/hr and 190 °C reaction temperature, the results of liquid sample are shown in Table 3-13. The area under each peak, which is calculated by multiplying the mole% by its molecular weight, represents the weight % of the hydrocarbon which, then, is multiplied by the total mass of the sample to get
C7 Isomers
the mass liquid out.
n-C7
Mole%
MW
wt%
Weight (g)
1.17
100.2
1.18
0.027
1.54
100.2
1.54
0.035
0.07
100.2
0.07
0.002
0.81
100.2
0.81
0.018
16.37
100.2
16.40
0.373
2.47
100.2
2.47
0.056
17.58
100.2
17.61
0.400
1.75
100.2
1.75
0.040
58.25
101.2
58.94
1.339
Total
2.77
Table 3- 13: Mass balance calculation for gas sample at T= 190 °C and feed flow rate = 22.5 mL/hr
For gas samples, the gas is transferred to the gas GC (Varian 3400 – Section 3.4.4) via 1.0 litre gas bags. The gas stream comprises of hydrocarbons and mostly hydrogen which cannot be detected by the gas GC because it’s used with addition of air to flame the gases while helium gas is used as carrier. Therefore, the area % of each gas peak does not reflect the weight % of hydrocarbon gas sample. In order to calculate the weight of hydrocarbons in the gas stream, the total number of moles has to be calculated by applying the ideal gas law since the unit pressure is 101.325 kPa: 52
(Eq. 3-3) Where P is the unit pressure, V is the volume of the exiting gas, R is constant = 8.314, T is the return temperature of the coolant = 281 K. Once the total number of moles is calculated, the mole % of the gas sample is multiplied by the total number of moles, then, multiplied by the molecular weight that correspond to the carbon number – see Table 3-14. The overall mass balances of all experiments were maintained around ± 90%. Area (μV.min)
RF=x/Area
Mole%
Total number of moles
mole
MW
Weight (g)
0.3
2.92E-03
8.76E-06
0.52
2.8E-06
30
0.00028
1.6
2.03E-03
3.25E-05
0.52
5.0E-06
44
0.0005
0.2
1.55E-03
3.11E-06
0.52
8.0E-06
58
0.0008
1.4
1.61E-03
2.26E-05
0.52
4.0E-06
58
0.0004
1.2
1.38E-03
1.65E-05
0.52
6.0E-06
72
0.0006
2.2
1.01E-03
2.23E-05
0.52
9.0E-06
100
0.0009
4.2
1.01E-03
4.26E-05
0.52
2.4E-05
100
0.0024
5.4
1.01E-03
5.47E-05
0.52
4.6E-05
100
0.0046
107
1.01E-03
0.001085
0.52
7.4E-04
100
0.0748
1265.9
1.01E-03
0.012833
0.52
1.1E-02
100
1.135
Total
1.30
Table 3- 14: An example of gas weight calculations at T = 190 °C & feed flow rate = 22.5 mL/hr
The rig was run according to precise procedures to maintain consistent and stable operation and avoid upsets that could affect the hydroisomerization reaction. Products (liquid and gas) were separated, then, collected on constant time intervals until consistent results were acquired. Product samples were analysed via GCs to calculate the mass balance and to interpret the results.
53
CHAPTER FOUR RESULTS AND DISCUSSION 4.1 Introduction The objective of this chapter is to interpret and evaluate the obtained results via comprehensive analysis and illustrate the optimum conditions of n-heptane hydroisomerization reaction that promote high selectivity, stability, and activity. During this investigation, the n-heptane hydroisomerization experiments were conducted over two zeolite USY catalysts with different post treatment conditions, which affected the Si/Al ratio and Brønsted/Lewis acid sites ratio, at similar reaction conditions – see Table 3-2. The commercial catalyst, CBV712, and in-house catalyst, USY-E, were loaded with 1 wt% platinum and tested over a range of temperatures (190 – 250 °C), at various space times (140.6, 70.6, 46.8 kg.s/mol), and constant molar ratio of H2:HC (n-C7) at 9:1.
4.2 Temperature Stability in the Reactor Catalysts were both positioned in the stable temperature zone in the reactor to maintain the temperature at the required reaction temperature over the whole catalyst bed. The temperature profile of the reactor was recorded prior to any experiment to establish a base case and ensure that the furnace settings were correct at the specified flow rate, taking into account the conditions of the room–see Figure 4-1. However, once n-heptane was introduced and the reaction was taking place over the catalyst bed, the temperature profile across the reactor changed due to the slightly exothermic nature of the reaction as noticed during the experiments – see Figure 4-2 and Table 4-1. The degree of temperature increase changes with catalyst type and with time on stream due to catalyst loss of activity. Temperature Settings 190 °C 210 °C 230 °C 250 °C
Actual Temperature of the Reactor ±1.5 °C ±2.0 °C ± 2.5°C ±4.0 °C
Table 4- 1: Effect of hydroisomerization exothermic reaction
54
Reactor Temperature 260
Catalyst Bed
250 Temperature °C
240 230
190
220
210
210
230
200
250
190 180 10
11
12
13
14
15
16
17
18
19
20
21
22
Reactor Depth (cm)
Figure 4- 1: Reactor temperature with only H2 flow
Reactor Temperature 270
Temperature (°C)
265
Catalysts Bed
260 255 250 245 240 235 230 10
11 BASELINE
12
13
14
15 16 17 18 Reactor Length (cm)
Sample1
Sample2
19
20
21
22
Sample3
Figure 4- 2: Temperature profile across the reactor during the experiment at 250°C
4.3 Effect of Key Parameters The variation of reaction temperature and contact time has affected the hydroisomerization of n-heptane and yielded different results according to the catalyst properties. 55
4.3.1 Effect of Reaction Temperature and Contact Time As expected, the USY catalysts, which have similar Si/Al ratio of 6 - 8 (see Table 32), showed similar conversions of n-heptane (10 – 98 wt%) over temperature range tested (190 - 250 °C) – see Figure 4-3 & 4-4. Both catalysts, CBV712 and USY-E, showed a large jump in the conversion of n-heptane at 230 °C. In addition, as the contact time between the feed and the catalyst bed increased, the total conversion of n-heptane increased at any given temperature. The general effect of temperature on the n-heptane conversion had followed an S-shape which agrees with previous literature [46,47].
Conversion vs Temperature
Conversion (wt%)
100% 90% 80% 70%
140.6
60% 50%
46.8
70.6
40% 30% 20% 10% 0% 170
180
190
200
210
220
230
240
250
260
270
Temperature (°C) Figure 4- 3: The conversion of n-heptane as a function of temperature at different contact times – CBV712
Conversion Temperature 100% 90%
140.6
Conversion (wt%)
80%
46.8
70% 60% 50% 40% 30% 20% 10% 0% 170
180
190
200
210
220
230
240
250
260
270
Temperature (°C) Figure 4- 4: The conversion of n-heptane as a function of temperature at different contact times – USY-E
56
Figure 4-5 and 4-6 show that as temperature increased, the yield of C7 isomers increased until it hit a maximum. For example, the yield of C7 isomers over CBV712 catalyst increased with temperature until it hit a maximum of 50 wt% at reaction temperature of 230 °C, then, at 250 °C the yield dropped down for all contact time except for the shortest (46.8 kg.s/mol). It was noted that the C7 isomers yield over CBV712 and 46.8 kg.s/mol contact time (W/F) had reached 60 wt% and did drop down indicating that at 250 °C the dominant reaction was hydroisomerization. The obtained yield of C7 isomers over USY-E catalyst followed similar trend to that of CBV712 and as contact time increased, the yield of C7 isomers of both decreased with temperature while the cracked products increased.
Yield vs Temperature 70% 60%
46.8 70.6 140.6
YIELD (wt%)
50% 40% 30% 20% 10% 0% 190
200
210
220 Temperature (°C)
230
240
250
Figure 4- 5: Hydroisomerization yield ( to isomers) as a function of temperature – CBV712
Yield vs Temperature 70% 46.8 70.6 140.6
YIELD (wt%)
60% 50% 40% 30% 20% 10% 0% 190
200
210
220
230
240
Temperature (°C) Figure 4- 6: Hydroisomerization yield (to isomers) as a function of temperature – USY-E
57
250
USY-E catalyst showed a steep decrease in the selectivity with temperature increase especially from 230 °C to 250 °C (from 85 to 50 wt%) where CBV712 catalyst demonstrated superior selectivity to C7 isomers (100-90 wt%) over the temperature range tested except at 140.6 kg.s/mole contact time – see Figure 4-8 & 4-9. However, the selectivity to C7 isomers over CBV712 dropped down to 40 wt% at 140.6 kg.s/mole. As shown in the Figures, as contact time increased the selectivity to C 7 isomers decreased at any given temperature.
Select. vs Temperature 100%
SELECTIVITY (wt%)
90%
Figure 4- 7: Hydroisomerization yield (to isomers) as a function of temperature - USY-E
80% 70% 60%
46.8
50%
70.6
40%
140.6
30% 20% 190
200
210
220
230
240
250
Temperature (°C) Figure 4- 8: Hydroisomerization selectivity to isomers as a function of temperature - CBV712
Select. vs Temperature 100% 90%
SELECTIVITY
80% 70%
46.8
60%
70.6
50%
140.6
40% 30% 20% 190
200
210
220
230
240
Temperature (°C) Figure 4- 9: Hydroisomerization selectivity to isomers as a function of temperature - USY-E
58
250
The overall performance of both catalysts is illustrated in Figure 4-10 & 4-11 where the conversion and the yield and selectivity to C7 isomers are shown as a function of temperature at the longest contact time (140.6 kg.s/mole). Although both catalysts have similar Si/Al ratio, they have been post treated differently as CBV712 catalyst were steamed and acid leached while USY-E were just steamed. The dealumination of USY-E via steaming created EFAL species (Lewis acid sites) in the catalyst structure. The increased cracking activity USY-E could be attributed to these EFAL species. There is also a balance between pore blocking due to EFAL species and overall additional acid sites and the resulting additional activity.
wt%
Effect of Temp. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%
Conversion Yield Selectivity
180
190
200
210
220
230
240
250
260
Temperature (°C) Figure 4- 11: The overall effect of Temperature on at 140.6 kg.s/mole – CBV712
Effect of Temp. 100% 90%
Conversion
wt%
80% 70%
Yield
60%
Selectivity
50% 40% 30% 20% 10% 0% 180
190
200
210
220 230 Temperature (°C)
240
Figure 4- 10: The overall effect of temperature at 140.6 kg.s/mole – USY-E
59
250
260
4.3.2 Yield of C7 Isomers as a Function of Conversion The yield of C7 isomers is plotted as a function of the overall conversion for different temperature – see Figure 4-12 & 4-13. It is clear that the yield of C7 isomers increased as the conversion increased. CBV712 catalyst showed very consistent yield results at similar n-heptane conversion which indicates that the cracking activity where maintained very low between (190-250 °C) due to the mild acid leaching treatment that followed CBV712 steaming to remove the EFAL species. On the other hand, the yield of C7 isomers over USY-E varied with temperature indicating that cracking activity is sensitive to temperature i.e. at the same n-heptane conversion, the yield of C7 isomers varied between (17 and 35 wt%) due to temperature changes between 190 to 250 °C. CBV712 is preferred over USY-E due to the consistency and higher yield obtained at any conversion over the temperature range tested.
Yield vs Conv.
C7 Isomers yield (wt%)
70% 60% 50% 40% 30% 20% 10% 0% 0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
Conversion (wt%) Figure 4- 12: Hydroisomerization Yield as a function of conversion – CBV712
Yield vs Conv.
C7 Isomers yield (wt%)
70% 60% 50% 40% 30% 20% 10% 0% 0%
10%
20%
30%
40% 50% 60% Conversion (wt%)
70%
80%
90%
Figure 4- 13: Hydroisomerization Yield as a function of conversion - CBV712
60
100%
4.3.3 Effect of Metal/Acid Balance The route of n-heptane hydroisomerization reaction mechanism is dictated by the balance between the hydrogenation – dehydrogenation function, which is represented by platinum sites, and the acid function provided by both Brønsted and Lewis acid sites. This Metal/Acid ratio is an indication of the rate at which the metal sites are performing their functions compared to that of acid sites. The optimum Metal/Acid ratio at which the hydroisomerization reaction mechanism delays cracking products through sequential scheme and follow Equation 4-1 is six acid sites per available platinum atom (nPt/nH+ = 0.17) [50]. (Eq. 4-1) Where MB is mono-branched; MuB is multi-branched; and CP is cracked products However, as the number of acid sites exceeds the optimum ratio, the yield of cracked products increase at the expense of C7 isomers yield. Due to the unbalanced Metal/Acid ratio, the reactants and intermediates alkenes are cracked over acid sites before reaching metal sites to be hydrogenated – dehydrogenated following different reaction mechanism – see Equation 4-2. This reaction mechanism shows that monobranch (MB) and multi-branch (MuB) isomers are primary products which, then, get converted into cracked products [34]. (Eq. 4-2) Figure 4-14 & 4-15 show the yield distribution ratio of mono-branched isomers and multi-branched isomers (MB/MuB) as a function of contact time. CBV712 catalyst showed that as contact time increased, the ratio of MB/MuB isomers decreased until it hit a minimum, then, increased again. It was noted that the ratio of MB/MuB reached a maximum of 21 – 22 % at 210 °C and 70.6 kg.s/mole, despite the general trend. Moreover, USY-E catalyst showed similar trends to that of CBV712 and lower MB/MuB ratio. This result indicates that the isomerization reaction mechanism followed Eq. 4-1 where the mono-branched isomers were the first products, then, multi-branched ones and finally the cracked products. This suggests that at a certain C7 isomers yield, the 61
research octane number could be maximized by producing multi-branched isomers through choosing the optimum temperature and contact time.
Ratio (%)
MB/MuB Ratio 24 22 20 18 16 14 12 10 8 6 4 2 0
190 °C 210 °C 230 °C 250 °C
40
50
60
70
80 90 100 110 Contact Time (kg.s/mole)
120
130
140
Figure 4- 14: The ratio of Mono-branched isomers to Multi-branched isomers of CBV712 as a function of contact time
MB/MuB Ratio
190 °C
10 9
210 °C
8
230 °C
Ratio (%)
7
250 °C
6 5 4 3 2 1 0 40
50
60
70
80
90
100
110
120
130
140
Contact Time (kg.s/mole) Figure 4- 15: The ratio of Mono-branched isomers to Multi-branched isomers of USY-E as a function of contact time
62
CHAPTER FIVE CONCLUSION AND FUTURE WORK A fixed bed reactor was rebuilt and calibrated within 4 - 6 weeks for the n-heptane hydroisomerization reaction over platinum loaded zeolite USY. Although the rig was optimized to be operated for n-heptane hydroisomerization, with more preparation time it could be further improved by modifying some sections such as changing the condenser configurations in the separation module to capture most of the products in vapour phase. The selectivity, activity, and stability of USY zeolites were promising during the hydroisomerization reaction as they demonstrated high yield and selectivity to C7 isomers (60 wt% & 95 wt%, respectively) promoting the objective of these experiments which is increasing the research octane number - RON - of gasoline through replacing aromatics with highly branched alkanes. Two samples of USY catalysts were post synthetically treated to help improve the catalyst properties in the hydroisomerization reaction. CBV712, commercial catalyst, was dealuminated via mild steaming, then, acid leached to remove detached Al atoms from the framework USY-E, in-house catalyst was also dealuminated by steaming only, which created EFAL species in the framework. Moreover, both catalysts were loaded with 1 wt% platinum through ion-exchange treatment with platinum salt [Pt(NH3)4Cl2.H2O] to produce bifunctional catalysts. Two grams of each catalyst was loaded at the stable temperature zone in the fixed bed reactor – see Figure 3-10. During the experiments, the reaction temperature and contact time between the feed and the catalyst bed were varied to study their effect on the hydroisomerization of nheptane – see Table 3-12. As shown earlier, the effect of catalyst post treatment such as dealumination and metal loading could enhance the physical properties such as thermal stability and the performance of zeolite USY in terms of yield and selectivity to C7 isomers and stability. Both catalysts tested gave results similar to literature [34,42,46,50] in terms of conversion and selectivity to C7 isomers. USY-E had a higher activity than that of CBV712 due to the fact that steaming creates EFAL species which could form new 63
acid sites (Lewis acid) and hinder the movement of bulky molecules enhancing cracking activity. However, CBV712 was more selective because of the mesoporous structure created by steaming which was then, acid leached to remove EFAL species from the framework. Therefore, structure modifications of CBV712 catalyst by post treatment have improved the hydroisomerization reaction by enlarging the pore sizes which minimized the diffusion energy barrier. From Figure 4-12 & 4-13, it is clear that the CBV712 is preferred over USY-E due to the higher yield obtained at any conversion and the yield consistency over the temperature range tested. The yield of C7 isomers over both catalysts had increased as the conversion increased. CBV712 catalyst showed very consistent yield results at similar n-heptane conversion due to the modified structure after the acid leaching treatment which removed the EFAL species from the framework. On the other hand, the yield of C7 isomers over USY-E varied with temperature indicating that cracking activity is sensitive to temperature i.e. at the same n-heptane conversion, the yield of C7 isomers varied between (17 and 35 wt%) due to temperature changes between 190 to 250 °C. From Equation 4-1, a sequential reaction mechanism is followed to reduce cracking products (C1 - C6) whenever the optimum ratio of Metal/Acid is met. The optimum Metal/Acid ratio, which is reported in literature [50], is six acid sites per available platinum atom (nPt/nH+ = 0.17). (Eq. 4-1) Although the obtained yield and selectivity to C7 isomers over CBV712 and USY-E were promising, experimental improvements could be implemented such as better coolant medium at the separator condenser to simulate refinery conditions and GC columns to help separating gaseous product for better analysis. Based on the results and analysis obtained during these experiments, there are few areas that could be researched and investigated to close the gap between the application of these catalysts with normal alkane and formulating catalysts that could
64
convert heavy naphtha stream to high octane gasoline. The proposed areas that could be explored are as follows;
Testing a mixture of C7-C10 as a model compound instead of n-heptane to simulate the effect of using paraffinic heavy naphtha without any aromatics. This would show more realistic behaviour of the catalysts toward hydroisomerization of heavy naphtha.
Comparing USY with post synthetically treated Beta, and Mordenite would show the effect and enhancement of post treatment as moderate level of acid leaching would greatly enhance the yield and selectivity to alkane isomers of steamed catalysts. As proved earlier in the literature, comparing catalysts with different structural geometry would show their effect on the yield and selectivity to alkane isomers and the stability of the catalyst.
Poisoning the external surface of the catalysts to prevent any hydrogenationdehydrogenation or cracking functions at the surface. This would allow a more comprehensive study about the performance of catalyst transport phenomena and Metal/Acid ratio inside the catalyst structure.
Loading catalysts with metals other than platinum or combination with different loading methods. As illustrated earlier in section 2.5.2.3, the performance of hydrogenation-dehydrogenation function differs from one metal to another. In addition, the level of metal loading could be changed to reflect the effect of Metal/Acid ratio on the hydroisomerization reaction.
Varying the partial pressure of hydrogen by changing the molar flow ratio of hydrogen to feed (n-C7) which was fixed at 9:1. This would allow the effect of hydrogen partial pressure on the hydroisomerization of n-heptane to be explored and the reaction order with respect to hydrogen to be determined empirically.
Testing at different operation pressure to study the effect of pressure on the hydroisomerization of n-heptane.
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