HYDRODYNAMIC CHARACTERISTICS OF THREE-PHASE NON ...

Emirates Journal for Engineering Research, 15 (1), 41-49 (2010) (Regular Paper)

HYDRODYNAMIC CHARACTERISTICS OF THREE-PHASE NONNEWTONIAN LIQUID-GAS - SOLID FLUIDIZED BEDS

Abbas H. Sulaymon1,Thamer J. Mohammed.2 Ali H. Jawad2 1 2

Department of Chemical Engineering, The University of Baghdad, Baghdad, Iraq Department of Chemical Engineering, The University of Technology, Baghdad, Iraq E-mail: [email protected] (Received March 2009 and accepted February 2010)

‫ اﻟﻐﺎز اﻟﻤﺘﺒﻘﻲ‬،‫إن هﺪف اﻟﺒﺤﺚ دراﺳﺔ اﻟﺨﻮاص اﻟﻬﺪروﻟﻴﻜﻴﺔ ﻟﻠﻄﺒﻘﺎت اﻟﻤﻤﻴﻌﺔ اﻟﺜﻼﺛﻴﺔ اﻷﻃﻮار ﻣﺜﻞ اﻟﻐﺎز اﻟﻤﺘﻴﻘﻲ‬ .‫ اﻟﺨﻮاص اﻟﻔﻴﺰﻳﺎوﻳﺔ ﻟﻠﺴﻮاﺋﻞ واﻟﻐﺎزات‬،‫ ﺳﺮﻋﺔ اﻟﻐﺎز‬،‫ اﻟﺴﺮﻋﺔ اﻟﺪﻧﻴﺎ ﻟﻠﺘﻤﻴﻊ‬،‫ ﺳﺮﻋﺔ ﺻﻤﻮد اﻟﻔﻘﺎﻋﺎت‬،‫اﻟﻤﻮﻗﻌﻲ‬ ‫ م وارﺗﻔﺎﻋﻪ‬0.106 ‫أﺟﺮﻳﺖ اﻟﺘﺠﺎرب ﻟﻠﻄﺒﻘﺎت اﻟﻤﻤﻴﻌﺔ )ﻏﺎز – ﺳﺎﺋﻞ – ﺻﻠﺐ ( ﻓﻲ ﻋﻤﻮد زﺟﺎﺟﻲ ﻗﻄﺮﻩ اﻟﺪاﺧﻠﻲ‬ ‫ ارﺑﻌﺔ ﺗﺮاآﻴﺰ‬. %23.11 ‫ ﻣﻠﻢ وﻣﺴﺎﺣﺔ ﺻﺎﻓﻴﺔ‬7 ‫ ﺛﻘﺒﺎ وﺑﻘﻄﺮ‬53 ‫ ﻣﻨﻈﻢ اﻟﻐﺎز ﻗﺮص ﺑﻼﺳﺘﻚ ﻣﺜﻘﺐ ﻳﺤﺘﻮي ﻋﻠﻰ‬. ‫ م‬2 ‫ ﺛﺎﻧﻲ‬،‫ اوآﺴﺠﻴﻦ‬،‫ وﻏﺎزات )هﻮاء‬.‫( ﺳﻮاﺋﻞ ﻏﻴﺮ ﻧﻴﻮﺗﻴﻨﻴﺔ‬% 2,1,0.5,0.1 ‫ﻟﻤﺎدة آﺎرﺑﻮآﺴﻲ ﻣﻴﺜﺎﻳﻞ ﺳﻴﻠﻴﻠﻮز )وزﻧﺎ‬ /‫ آﻠﻐﻢ‬770 = ‫ ﻣﻠﻢ )اﻟﻜﺜﺎﻓﺔ‬0.25 – 0.75 ‫ م | ﺛﺎ واﻟﻜﺮﺑﻮن اﻟﻤﻨﺸﻂ ﺑﻘﻄﺮ‬0.03-1.4 ‫أوآﺴﻴﺪ اﻟﻜﺮﺑﻮن ( ﺑﺴﺮﻋﺔ‬ ‫ ﺗﻢ إﻳﺠﺎد ﻋﻼﻗﺔ‬.‫( اﺳﺘﺨﺪﻣﺖ ﻓﻲ اﻟﺘﺠﺎرب‬3‫ م‬/ ‫ آﻠﻐﻢ‬2500 = ‫ ﻣﻠﻢ )اﻟﻜﺜﺎﻓﺔ‬1.8 ‫ ﺑﻘﻄﺮ‬Ni-Mo|Al2O3 ‫ ﻣﺎدة‬، (3‫م‬ .‫اﻟﻐﺎز اﻟﻤﺘﺒﻘﻲ ﺑﺪﻻﻟﺔ اﻟﻤﺠﺎﻣﻴﻊ اﻟﻌﺪﻳﻤﺔ اﻷﺑﻌﺎد‬ The aim of this work is to study the hydrodynamic characteristics of three-phase fluidized beds. Such as gas- holdup, localgas- holdup, bubble rise velocity, minimum fluidization velocity, superficial gas velocity, the physical and liquid rheological properties. The experimental work of gas-liquid-solid fluidized beds system was carried out in QVF glass column 0.106m I.D. and 2 m height. A perforated teflone plate was used as gas distributor, with 53 holes , 7 mm diameter and free surface area of 23.11%.Four of Carboxy Methyl Cellulose concentrations 0.1 %, 0.5 %, 1 %, and 2 wt%, were used as non-Newtonian (pseudoplastic) liquids. Air, O2 and CO2 were used with superficial velocity 0.03 -1.4 m/s. Activated carbon with diameter 0.25 -0.75mm and density 770 kg/m3 , Ni-Mo/Al2O3 with diameter 1.8 mm and density 2500 kg/m3 were used as solid phase. The gas holdup was correlated with dimensionless groups and independent parameters with correlation coefficient is 0.9929, the following correlation is obtained. 2

c

c5

c6

c

c

c

c

3 ⎡ ⎤ ⎡ ρ l ⎤ ⎡ d p ⎤ 7 ⎡ d p ⎤ 8 ⎡ ρ l ⎤ 9 ⎡ ρ l ⎤ 10 ⎡U sg ⎤ c11 c4 ρ l [ Re ] ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ [We] ⎥ ρ ρ ρ U D H C Δ ⎣ l ⎦ ⎣⎢ g ⎥⎦ ⎢⎣ p ⎥⎦ ⎣ h ⎦ ⎣ o ⎦ ⎣ ⎦ ⎣ w ⎦

ε g = C1 [Fr]c ⎢

Keywords: Gas holdup, localgas- holdup, bubble rise velocity, minimum fluidization velocity, Non-Newtonian

1. INTRODUCTION A gas-liquid-solid three-phase fluidized beds is one of the most important multiphase reactors for physical, chemical, petrochemical, electrochemical and biochemical processing[1, 2]. In spite of the fact that in biochemical industries a number of fluids represent non-Newtonian behaviors, only a few studies on nonNewtonian fluids in three-phase fluidized beds have been published[3,4]. There are numerous applications of bubble motion in non-Newtonian fluids e.g. antibiotic fermentations, biotechnology, food treatment, polymer processes and food processing. Biotechnology, food processing and pharmaceutical processes constitute a wide spectrum of chemical industries where bubble columns are used. The rheological behavior of such

viscous pseudoplastic non-Newtonian solution can be fairly well simulated by the solutions of carboxymethyl cellulose (CMC)[5]. Barghi et al.[6] studied fluidization regimes in liquid-solid and gasliquid-solid fluidized beds. The liquid velocities at which regime transition occurs in liquid-solid and gasliquid-solid systems were obtained for monosize and multicomponent systems. Minimum fluidization, complete fluidization and complete mixing velocities of particles were obtained from pressure drop measurements, a collision technique or a conductivity method. In order to determine the gas holdup in bubble columns several measurement techniques have been developed and applied during the last years. The measurement techniques, which also provide highly

41

Abbas H. Sulaymon, Thamer J. Mohammed. Ali H. Jawad

precise information about bubble sizes and velocities of bubbles, were reviewed extensively by Veera[7]. The most applied techniques are: • Bed expansion technique by direct visual observation. • Electro-resistivity probe: This technique utilizes the difference in electrical conductivity between the liquid and gas phases[8, 9. 10]. • Photographing the bubbles: Many researchers used high-speed camera and video camera to measure the hydrodynamics parameters of bubbles. • Gamma ray, Beta ray, X-ray, and neutrons have been used by various investigators for determining the gas holdup in two phase flow[10.7] Lee et al.[11] predicted gas phase hold-up in three phase fluidized beds of different particles size (dp=1-8 mm), different liquids (water, ethanol, acetone and CMC solutions). The rise velocity of bubbles can be affected to a significant extent by the dimensions of the bubble column[12]. There is a large variety of experimental data on rise velocity available in the literature for different bubble sizes and column diameters; it is difficult to compare the results of different authors with those of others because of: • Difference in the physical properties of the liquids. • Presence of impurities in the liquid phase. • Each study often restricted to a narrow bubble size range in a given column diameter.

Therefore the aim of the present work was studied multi variables which more effects in a system of three-phase non- Newtonian liquid-gas -solid fluidized beds.

2. EXPERIMENTAL DETAILS A schematic diagram of the experimental setup is shown in Fig.1. A Q.V.F.glass column of (0.106 m) I.D. and (2 m) height was used. Two types of cameras were connected on-line to the computer. The photographs and the films of the cameras were stored in the hard disk of the computer and in 8 mm film disk. The recorded films of the three-phase in fluidization system (circulation phenomena, bubble rise velocity and bubble diameter) were analyzed by using frame-by-frame analysis technique with the help of a computer program. On the other hand, in order to get more information about the gas void fraction, the rise velocity and bubble size under realistic conditions, an additional investigation was carried out by using conductivity probe interface system. Compressed air and two bottled gases (CO2 and O2) respectively were used in the present study. The physical properties of these gases at 38 0C are listed in Table 1.

Figure 1 Schematic diagram of experimental setup .1, Gas distributor; 2, Control valve; 3, Filter dryer; 4, Needle valve; 5, Rotameter; 6, Column; 7, Resistivity probe; 8, Pressure tap; 9, Gas distributor; 10, Gas chamber; 11, High speed camera; 12, Interface; 13, Personal computer; 14, Perostatic pump.

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Emirates Journal for Engineering Research, Vol. 15, No.1, 2010


Table 1. Properties of gases used Gas Air CO2 O2

Density (ρg), kg/m3 1.20 1.840 1.4289

-3

Viscosity (μg×10 ), kg/m.s 0.0178 0.0143 0.0212

Liquids with four different concentrations (0.1, 0.5, 1 and 2 wt.%) carboxy-methyl cellulose (CMC) were used. The densities of the liquids were measured by means of the Pycnometer method. Liquids concentrations, and viscosities were measured at 25 oC and checked with literature as shown in Table 2,[13, 14, 5, 15 and 16] . The accurate knowledge of the hydrodynamic parameters is important for design and performance of bubble columns. Therefore, the hydrodynamic parameters of the bubble column (local gas holdup, rise velocities ,sizes of bubbles and frequency) have been studied by using the conductivity probe detector[8,17,9,10] The basic object behind the measurement of gas holdup profile (local gas holdup) in bubble column using this technique is to get more information on gas holdup distribution. The basic principle of this probe is the difference in electrical conductivity between the liquid and gas phases, generating a pulse of voltage when the probe sensor encounters a bubble in the liquid. Therefore, as the bubble intercepts the tips, the interface system will convert these signals to a series of pulses denoting the arrival of front and rear of a bubble. Details of this system are shown in Figure 2.

Figure 2 Schematic diagram of the conductivity bubble detector system with probe details.

Three methods were used to measure the gas, holdup. The first method was using CCPS (Computerized Conductivity Probe System) with high accuracy (less than 1 % error). The second method was the manometric method and the third method was the quick –closing valves. Figures (3-8), show the relation between the superficial gas velocity and gas holdup for a given gas phase (O2,CO2 and air), water with different concentration of (CMC) and activated carbon, Ni.MO/Al2O3 as solid phase

42

Table 2. Physical Properties of CMC Solutions at o 25 C. Test Conc. Density Consistency Flow Surface Liquid wt% (kg/m3) Index (Pa.sn) Behaviour Tension Index (Pa.m) CMC-1 2 % CMC-2 1 % CMC-3 0.5 % CMC-4 0.1 %

1008 1005 1002 1000

6.74 4.52 2.570 0.185

0.69 0.82 0.92 0.97

0.0680 0.0702 0.0718 0.0730

3. RESULTS AND DISCUSSION 3.1 Gas Holdup The first topic of study for the hydrodynamics of threephase fluidization is gas holdup. Gas holdup is a key parameter to characterize the macroscopic hydrodynamics of three-phase fluidization systems. The gas holdup depends on gas and liquid velocities, gas distributor design, column geometry (diameter and height), physical and rheological properties of the gas and liquid, particle concentration, and physical properties of the particles. It is generally agreed that the gas velocity, .holdup relationship is the most important The gas holdup generally increases with gas velocity, with a higher rate of increase in the dispersed bubble regime than in the churn-turbulent regime. Such distributors as perforated plates, nozzle injectors, and spargers affect the gas holdup only at low gas velocities. The effect of column size on gas holdup is negligible when the column diameter is larger than 0.1 to 0.15 m. the influence of the column height is insignificant if the height is above 1 to 3 m and the ratio of the column height to the diameter is larger than 5. Gas holdup decreases as liquid viscosity and/or gas/liquid surface tension increase; however, the effect of liquid density is not clear. The addition of particles into a bubble column leads to a larger bubble size and thus a decreased gas holdup, especially when the particle concentration is low. The particle size effect on the gas holdup can be ignored in the particle size range of 44 to 254 µm. From Figures (3 and 4) it can be observed that, the gas holdup increased slowly with the superficial gas velocity up to 0.8 m/s, for all CMC concentrations. On the other hand, for high superficial gas velocities greater than 0.8 m/s, the gas holdup was increased rapidly with superficial gas velocity for various CMC concentrations. Examining figures (5 and 8) it can be seen that, the gas holdup increased rapidly with the superficial gas velocity up to 0.12 m/s, for all CMC concentrations. On the other hand, for high superficial gas velocities greater than 0.12 m/s, the gas holdup was increased slowly with superficial gas velocity. According to the visual flow observations and the shapes of graphs in Figures (3-8), there are three regimes in the bubble column of highly viscous liquids, bubbly flow, transition and churn- turbulent. In


Hydrodynamic Characteristics of Three-phase non-NewtonianLiquid-Gas - Solid fluidized beds

the bubbly flow regime, the bubbles generated at the gas distributor are uniform in size and disperse homogeneously throughout the liquid phase. In the regime that appears at high superficial gas velocities, bubbles agglomerate within a few centimeters from the gas distributor and form large bubbles. This large bubble is responsible for the lower gas holdup values.The transition from bubbly to churn-turbulent flow regime occurred at superficial gas velocities between 0.09-0.21 m/s. These results are in agreement with the work of Viswanathan[18], and Forret et al[19] It can be concluded also that, the gas holdups in bubble columns of highly viscous liquids are clearly different from that of the low viscosity liquids in the literature reported[8,20,21]. This conclusion, indicating an important influence of the liquid physical properties on the fluid dynamics and hydrodynamic parameters.

Figure 5 Effect of superficial gas velocity on the experimental gas hold up for Air .CMC. activated carbon

Figure 3 Effect of superficial gas velocity on the experimental gas hold up for O2.CMC.activated carbon Figure 6 Effect of superficial gas velocity on the experimental gas hold up for O2.CMC.Ni.Mo/Al2O3

Figure 4 Effect of superficial gas velocity on the experimental gas hold up for CO2 .CMC. activated carbon


Figure 7 Effect of superficial gas velocity on the experimental gas hold up for CO2.CMC.Ni.Mo/Al2O3

43


Figure 8 Effect of superficial gas velocity on the experimental gas hold up for Air. CMC.Ni.Mo/Al2O3.

The effect of gas density on holdup are by employing of different gases investigated as illustrated in Figure (9). At low superficial gas velocity gas holdup is nearly directly proportional to gas velocity. This behaviour is characteristic of bubble columns operating at bubbly flow regime, and continues until a point is reached where a rather abrupt change occurs. At gas velocity above this point, gas holdup continues to increase with increasing gas velocity, out of this rate the change is significantly lower. This regime of operation is characteristic of the churn-turbulent regime. The velocity at which transition from bubbly to churn- turbulent flow occurs to be strongly dependent upon gas phase density, These results are in accord with the work of Reilly et al.[22].

Figure 9 Effect of gas density on the experimental gas hold up for different gases at 0.1 wt% CMC (n=0.97).

Conductivity Probe System (CCPS) at different heights above the gas distributor. It is important to mention here that; most of experimental methods described in literature[23, 15, 24] are incapable of detecting radial variations in gas holdup. For this reason, the computerized conductivity probe interface system is regarded as suitable method for measuring local gas holdup in both axial and radial directions in the bubble column. Therefore, the conductivity probe interface system is developed for the detection of bubbles in three-phase fluidized bed of high viscosity liquids. The probe utilizes the difference in the conductivity of gas and liquid to distinguish the gas phase from the liquid phase. The local distribution of gas holdup, the bubble rise velocity, bubble diameter, and bubble frequency were measured with this technique at different positions above the gas distributor. Radial gas holdup profile can be obtained by using CCPS as function of dimensionless radial distances for different values of superficial gas velocities. Figures (10-13) show the radial gas holdup profiles at different superficial gas velocities for O2- CMC-activated carbon systems, The general trend of these figures is that, the gas holdup increases with the increasing gas superficial velocity. Also these figures point out, that the gas holdup is high in the center and low near the wall of the column, which leads to a liquid circulation through the column with liquid flowing up in the center and down near the wall. This kind of distribution in bubble columns is in agreement with the finding of Chen et al.[25] and Yuanxin et al.[21]

Figure 10 Radial distribution for experimental gas hold up for O2-CMC-Activated carbon at 0.1m above gas distributor and Usg =0.07 m/s.

3.2 Local gas holdup distribution In the present experimental work, the hydrodynamic parameters were measured in the bubble column such as local gas holdup, bubble diameter, and bubble rise velocity. The measurements were obtained with the help of the Computerized

44



3.3 Bubble rise velocity

Figure 11 Radial distribution for experimental gas hold up for O2-CMC-activated carbon at 0.5m above gas distributor and Usg =0.07 m/s.

The bubble rise velocity was calculated using (CCPS), and in order to ensure the consistency of the present technique data, the bubble rise velocity was calculated by analyzing of set of video films at each respective condition. The results show very high accuracy of data that measured with CCPS technique for bubble rise velocity measurement. Table 3 shows the comparison between the results of the bubble rise velocities by using frame-by frame analysis technique (video film analysis) and CCPS unit, at different superficial gas velocities. All measurements were conducted in the column center at 0.85 m above the gas distributor. Figures (14-19) shows the effect of superficial gas velocity on the bubble rise velocity. These figures indicated that the bubble rise velocity increased with increasing gas velocity because when increasing gas velocity, substantial liquid circulation occurs and the bubbles tend to be present mainly in the upward flow regions, thereby increasing the bubble velocity, which is the sum of liquid velocity and bubble velocity. These results are in agreement with the observation of Towell[26] and Koide et al.[27].

Figure 12 Radial distribution for experimental gas hold up for O2-CMC-Activated carbon at 0.75m above gas distributor and Usg =0.07 m/s. Figure 14 Effect of superficial gas velocity on the experimental bubble rise velocity for O2-CMC-activated

Figure 13 Radial distribution for experimental gas hold up for O2-CMC-Activated carbon at 0.85m above gas distributor and Usg =0.07 m/s.


Figure 15 Effect of superficial gas velocity on the experimental bubble rise velocity for CO2-CMC-activated carbon.

45


Figure 19 Effect of superficial gas velocity on the experimental bubble rise velocity for Air-CMC-NiMo/Al2O3.

Figure 16 Effect of superficial gas velocity on the experimental bubble rise velocity for Air-CMC-activated carbon

Figure 17 Effect of superficial gas velocity on the experimental bubble rise velocity for O2-CMC-NiMo/Al2O3.

Table 3. Comparison of bubble rise velocity with superficial gas velocities for two different methods of measurement. Usg 0.1 % 2% m/s CMC (cm/s) CMC (cm/s) A B A B 43.21 26 0.04 43.92 26.25 47.1 8.2 0.06 46.54 28.06 48 29.1 0.07 49.3 29.88 50.2 31.4 0.08 50.2 30.53 51 31.5 0.09 50.92 31.12 52.5 32 0.12 52.8 32 53.4 32.5 0.15 55.19 32.80 55.5 33 0.18 56.78 33.81 57 34.56 0.21 58.12 34.71 60.3 36 0.24 59.22 35.47 62.5 36.5 0.27 61 36.17 A= B=

Bubble rise velocity measured by CCPS. Bubble rise velocity measured by using frame-by-frame analysis.

3.4 Minimum fluidization velocity

Figure 18 Effect of superficial gas velocity on the experimental bubble rise velocity for CO2-CMC-NiMo/Al2O3.

The minimum liquid flow rate required to fluidize a bed of particles (Ulmf) may generally be determined from the change in the bed dynamic pressure drop behavior that occurs as the bed changes from a fixed bed to a fluidized bed. There are considerable variations on minimum fluidization phenomena among small/light, large/heavy, and mixed particle systems. However, Ulmf in general can be evaluated mechanistically by considering an intrinsic condition for minimum fluidization where the total pressure drop over a bed of particles at the fixed state is equal to the total bed weight per unit bed cross-sectional area as formulated by Song et al.[28].In this evaluation, the pressure drop in the fixed bed can be described by a flow model developed by Chern et al.[29,30], the following equation have typical been used to determine the holdup of each phase in three phase fluidized bed. εg +εl +εs =1

46

(1)



Δp= gH (ρgεg +ρlεl + ρsεs) εs =

(2)

Ms ρ s AH o

(3)

where the bed height in equations (2 and 3) is obtained either visually or from the measured pressure drop gradient. A more direct method of measuring εg is to simply isolate a representative portion of the test section by simultaneously shutting two quick closing valves and measuring the fraction of the isolated volume occupied by the gas, there method were adapted in the present work. Figures (20 and 21) show the variation of pressure drop with superficial liquid velocity for gas-liquidsolid system for different superficial gas velocities, the minimum fluidization velocity (Ulmf) decreases with increases in gas velocity. The minimum fluidization velocity decreases with increase in gas velocity in due to the increase in gas velocity tend to increase the holdup and density reduces for whole mixture. While the minimum fluidization velocity increases with increases in particle of higher size. From this it is observed that bed mass has no effect on the minimum fluidization velocity. There results are coincided with the observation of Jena et al.[31].

Figure 21 Typical pressure drop measurements for 1.8 mm Ni-Mo /Al2O3 with 0.1% CMC (n=0.97)

3.5 Empirical Correlation of Gas holdup Dimensional analysis is used to correlate diameter of holes in the distributor , superficial gas velocity , superficial liquid velocity , height of bed , diameter of particle , density of particle and physical properties gas phase and liquid phase which affect the gas hold-up in three-phase fluidized beds. εg is assumed to be a function of the following parameters. εg =f(Ug,Ul,ρg,ρl,g,Δρ (ρs-ρl), μeff, dp, Dh, Ho, ρp, σ, Cw)

(4)

Applying Buckingham’s π theorem for dimensional analysis, the following correlation is obtained. ε g = C 1 [Fr ]

c2

c

Figure 20 Typical pressure drop measurements for 0.5 mm activated carbon with 0.1% CMC (n=0.97)

⎡ U sg ⎤ ⎢ ⎥ ⎣ Ul ⎦ c

c3

[Re ]

c4

c

c5

⎡ ρl ⎤ ⎡ ρl ⎤ ⎢ ⎥ ⎢ ⎥ ⎣⎢ ρ g ⎦⎥ ⎣⎢ ρ p ⎥⎦

7 8 ⎡ d p ⎤ ⎡ d p ⎤ ⎡ ρl ⎤ 9 ⎡ ρl ⎤ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎥ ⎢ ⎣ D h ⎦ ⎣ H o ⎦ ⎣ Δρ ⎦ ⎣ C w ⎦

c10

c6

[We ]c

11

(5)

Table 4. values of parameters in Equation (4) C1

C2

C3

C4

0.05874

-0.1648

-0.8194

1.0771

C8

C9

C10

C11

-0.2649

0.2436

0.0213

-0.0544

The constant C1and powers of the dimensionless group in Equation (5) were calculated using statistica computer program version 6.5, Table (4) lists the values of constants , correlation coefficient , variance and standard error. A comparison between the


C5

C6

C7

0.4022

-0.5134

-0.3688

Correlation coefficient 0.9929

Standard error 0.00031

Variance 98.58

observed and predicted results of gas hold-up is shown in Figure. (22).

47


Observed versus Predicted Values 0.15

Observed Values

0.13

0.11

0.09

0.07

0.05 0.04

0.06

0.08

0.10

0.12

0.14

0.16

Predicted Values

Figure 22. Comparison between the observed and predicted values of gas hold-up .

4-CONCLUSION

Greek symbols

Depending on operating conditions of bubble column and physical properties of liquids, the liquid circulation phenomenon can be observed clearly for highly viscous Newtonian and non-Newtonian liquids. Gas-holdup in three-phase fluidized beds increases with increasing superficial gas velocity for all particle sizes studied. It was observed that with an increases in superficial gas velocity above 0.12 m/s, more complex hydrodynamic behaviour occurs, in which the transition from bubbly to churn-turbulent flow regime appeared. Moreover, large bubbles were formed with high bubble rise velocity due to coalescence process that appeared. The minimum liquid fluidization velocity increases with increase in particle size at constant gas velocity but decrease with increase .

Δρ

Nomenclature A CCPS CW Dh db dp Fr g H Re Usg Ul Ulmf Ubr We

48

Cross sectional area of the bubble column (m2) Computerized Conductivity Probe System Solid concentration, weight of solid/vol. of slurry (kg/m3) Holes diameter in distributor (m) Mean bubble diameter (cm) Particle diameter (size) (mm) Froude number, Usg2/gdp Gravitational acceleration (m/s2) Average height of expanded bed (m). Reynolds number, UsgdpρL/μL Superficial gas velocity (m/s) Superficial liquid velocity(m/s) Minimum fluidization velocity (m/s) Bubble rise velocity(cm/s) Weber number, ρLdpUsg2/σL

εg εl εs μeff

ρg

ρl ρp, ρs σ

Density difference between solid and liquid(kg/m3) Gas hold-up Liquid hold-up Solid hold-up Effective viscosity of continuous (mPa.s) Gas density(kg/m3) Liquid density(kg/m3) Particle density(kg/m3) Liquid surface tension(N/m)

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