0263–8762/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part A, March 2004 Chemical Engineering Research and Design, 82(A3): 381–389
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INFLUENCE OF GAS SEPARATOR AND SCALE-UP ON THE HYDRODYNAMICS OF EXTERNAL LOOP CIRCULATING BUBBLE COLUMNS W. A. AL-MASRY* Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia
G
as separators play a significant role on the operation of circulating bubble columns (CBCs), and affect the system hydrodynamics of gas hold-up and the liquid circulation. To study these effects experimentally, a 135 l external loop CBC made from Perspex was used in this work. The CBC had riser and downcomer diameters of 0.153 and 0.082 m, respectively. The liquids used were water and glycerol, while the sparging gas was air. The ratio of the liquid volume in the gas separator to the liquid volume in the reactor, volume ratio TVR, was varied up to 57%. Discernible effects of the volume-ratio on riser, downcomer gas hold-ups and liquid circulation velocity were observed at TVR 18%. Smooth operation of the CBC was observed at an optimum volume ratio TVRO ¼ 30%. Transition of operation between that of bubble column BC and CBC occurred at TVR ¼ 0% and UGR ¼ 0.07 m s1 for air–water system, and TVR ¼ 0% and UGR ¼ 0.03 m s1 for air–glycerol system. The drift flux model was found useful, with an average distribution parameter C0 equal to 0.965, 1.91, 0.78 for homogenous, transitional and heterogenous regimes, respectively. Simple correlations for predicting gas hold-up in the riser, gas hold-up in the downcomer, and liquid circulation velocity were developed taking into account the effect of the volume ratio and liquid viscosity. To account for the scale-up factor AD=AR, the 170 l external loop CBC data of Al-Masry (1999b), with dR ¼ 0.19 m and dD ¼ 0.14 m, were utilized along the data from this work. The two combined effects, i.e. TVR and AD=AR were recorded in new correlations which predict gas holdup and liquid circulation with good accuracy. Keywords: circulating bubble column; hydrodynamics; gas separator; scale-up; liquid circulation; gas hold-up.
INTRODUCTION
The gas–liquid dispersion then flows into the downcomer and travels to the bottom, through the base where it re-enters the riser. Thus, the liquid phase circulates continuously around the loop. While retaining some of the characteristics of conventional BCs, the macro-scale liquid circulation exhibited by CBCs is a unique feature. This circulation is an effect caused by the difference in fractional gas hold-up that exists between the riser and the downcomer. In turn, this creates a hydrostatic pressure difference between the bottom of the riser and the bottom of the downcomer, which acts as the driving force for the fluid circulation (Onken and Weiland, 1983; Nicol and Davidson, 1988a; Glennon et al., 1992; Joshi et al., 1990; Verlaan et al., 1989). Gas hold-up and liquid circulation velocity are the most important parameters characterizing the hydrodynamics, and they critically affect oxygen transfer in CBCs. Geometrical variations in the gas–liquid disengagement section play an important role in the performance of CBCs, especially when foaming systems are considered (Al-Masry and Dukkan, 1997; Al-Masry, 1999a). Consequently, it is important in the design and scale-up of CBCs that the gas separator configuration should be critically considered.
Conventional bubble columns in which gas is sparged through a pool of liquid are now widely used in the chemical and biochemical industries. Several modifications of the bubble column (BC) concept have emerged to cope with new processes. Circulating bubble columns (CBCs), also known as airlift reactors, form one of these important classes of modified bubble columns. CBCs are pneumatic reactors comprising riser, downcomer and gas–liquid disengagement section (gas separator). In the riser, the gas is sparged and, the gas–liquid dispersion travels upward in concurrent two phase flow. The riser has the higher fractional gas hold-up and this is where most of the gas–liquid mass transfer takes place. The liquid leaving the top of the riser enters the gas disengagement section, where, depending on its specific design, some or most of the dispersed gas is removed. *Correspondence to: Professor W.A. Al-Masry, Department of Chemical Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia. E-mail:
[email protected]
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AL-MASRY GAS SEPARATOR
The different designs of external loop CBCs and of gas– liquid separator geometries have made it difficult to model much of the data presented in the literature. In general, two distinct types of phase separators are identified in the literature for external loop CBCs. The first configuration is that of closed channel type, which is a horizontal tube connecting the riser and downcomer. The other type is the open channel configuration, which is a rectangular tank placed at the top of the riser and downcomer (see Figure 1). The data published in the literature to date for hydrodynamics and mass transfer in CBCs do not fully consider the influence of the gas separator design. Indeed many of the researchers do not clearly describe the gas separator geometry or liquid height in it. Much of the published works concentrate on internal loop CBCs (Siegel and Merchuk, 1988; 1991; Merchuk et al. 1994; Russell et al. 1994; Choi et al. 1995; Klein et al. 2001). The limited works on external loop CBCs available in the literature are summarized below. Ghiradini et al. (1992) presented hydrodynamic data for three external loop CBCs of equal heights but different sizes. In each reactor, the riser and downcomer diameters were equal, giving AD=AR ¼ 1.0 for all the three reactors. The range of gas superficial velocity used was extremely high (between 0.2 and 1.2 m s1) compared with the normal values reported in the literature 1.2 m s1. The filling factor, i.e. the volume ratio of non-aerated liquid in the phase separator to the total non-aerated liquid, was taken into consideration. At constant filling factor, the gas hold-up and liquid circulation velocity increased with increasing reactor size. For a given reactor, increasing the filling factor from 0.1 to 0.32 increased both the gas hold-up and the liquid circulation velocity. Bentifraouine et al. (1997) presented hydrodynamic results for two external loop CBCs. The phase separator in the first reactor was a horizontal tube junction, equipped with twin nozzles, which could be opened or closed, while the second reactor had a tank as the phase separator. When the twin nozzles were opened, the liquid
circulation velocity was increased and gas hold-up was decreased. The container phase separator improved liquid circulation over the horizontal tube junction (when the two nozzles were opened) by less than 10% at the highest gas flow rate used. Improvement of gas hold-up was minimal. Benyahia and Jones (1997) presented data for two geometrically similar external loop CBCs with a linear scale ratio equal to 2. The volume of the liquid in the phase separator had to be significantly increased (to 74.18% of the reactor volume of both reactors) to obtain almost air-free liquid in the downcomer. Gas hold-up was found to be higher in the large reactor compared with the small one. A constant gas hold-up could be maintained on scaling from the small reactor to the large one if the power input per unit volume was increased by 25%. Likewise a constant mass transfer coefficient could be obtained on increasing the power input per unit volume by 27% between small and large reactors. Al-Masry and Dukkan (1998) studied the role of the gas disengagement section as well as surface active agents on the hydrodynamics and mass transfer characteristics of a 700 l external loop CBC. Silicone polymer antifoam, polypropylene glycol and octanol were added to the air–water system as surface-active agents. Polypropylene glycol increased the number of small bubbles leading to excessive foaming in the system. Two types of gas disengagement section were investigated: closed channel and open channel. The closed channel configuration failed to remove the increased number of small bubbles and the reactor continuity was disrupted at high gas throughputs. In contrast, the open channel configuration improved the reactor performance with a substantial reduction in the downcomer gas hold-up and an increase in volumetric mass transfer coefficient. The volume of degassed liquid in both cases was kept constant. Al-Masry (1999) studied the effect of liquid volume in the gas separator on the hydrodynamics of 170 l external loop CBC. The reactor height was 2.5 m with riser and downcomer diameters of 0.19 and 0.14 m respectively. The scale-up factor AD=AR ¼ 0.54. The systems investigated were air–water and air–glycerol. A discernible effect of the liquid volume in the gas separator was observed on gas hold-up and liquid circulation. The critical volume ratio was found to be 7% for the air–water system and 19% for the air–glycerol system.
EXPERIMENTAL METHODS Gas hold-up and liquid circulation velocity were investigated in a circulating bubble column reactor made from Perspex. The CBC had a working volume of 135 l with riser and downcomer diameters of 0.153 and 0.082 m, respectively. The scale-up factor AD=AR ¼ 0.29. Gas hold-up in the Table 1. Liquid volume ratio. ZL (m)
Figure 1. Schematic of gas-separators in external loop CBCs.
0.0 0.05 0.10 0.15 0.20 0.25 0.30
VLGS (l)
TVR (%)
0 14 28 42 56 70 84
0 17.95 30.43 39.62 46.67 52.24 56.76
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with the corresponding TVR given in Table 1. Details of the experimental set-up are shown in Figure 2, with dimensions listed in Table 2. RESULTS AND DISCUSSION The resulting influence of geometrical design of the gas separator on the CBCs performance is the combination of its ability to separate gas from liquid and its hydraulic resistance. The gas separator markedly influences the extent of bubble penetration into the downcomer, whereby it determines the magnitude of gas hold-up and consequently the total driving force of liquid flow in CBCs. The ratio of the liquid volume in the gas separator to the total volume of the CBC at zero gas throughputs is used as the main parameter in the analysis of the results of this work (Al-Masry, 1999b). The volume ratio TVR is expressed as: TVR ¼
Figure 2. Schematic diagram of experimental setup: A, air; F, filter and pressure regulator; M, magnetic flow meter; P, manometer tappings; S, sparger; T, flow meter.
riser and downcomer were inferred from inverted U-tube manometers filled with water. The liquid circulation velocity in the downcomer was monitored using an electromagnetic flowmeter (Flowmetrix, South Africa). The liquid velocity in the riser was calculated from the continuity equation. Water (r ¼ 999 kg m3, m ¼ 0.001 Pa s) and viscous foaming glycerol (r ¼ 1264 kg m3, m ¼ 1.0 Pa s) were investigated for superficial gas velocities up to 0.12 m s1. The gas used was filtered air. The gas distributor was a perforated plate with 1 mm holes on a triangular pitch, and it was designed according to the criteria defined by Ruff et al. (1978) to prevent weeping. The gas–liquid disengagement section configuration was of the open channel type. The liquid height in the gas separator was varied from 0.0 to 0.3 m
VLGS 100 VL
where VLGS is the liquid volume in the gas separator and VL is the total liquid volume in the CBC. In general there are three behaviours of the bubbles in the gas separator of external loop CBCs. The first flow behaviour occurs when there is no liquid in the gas separator, TVR ¼ 0%. As the gas sparging begins, the two-phase volume in the riser increases, and as a result a small layer of liquid and gas mixture form in the gas separator. At this case, most of the bubbles are dragged into the downcomer as seen in Figure 3, and recirculate into the riser. The second flow behaviour occurs when the volume ratio is increased to a certain transitional value TVRS, but not enough to prevent the entrainment of small bubbles into the downcomer. The entrainment is limited to the top section of the downcomer as shown in Figure 4, with limited recirculation into the riser. In the third flow behaviour, the entrainment of bubbles into the downcomer becomes negligible as the volume ratio is increased beyond TVRS to reach the optimum value TVRO as shown in Figure 5. It is the aim of this work to find the optimum volume ratio TVRO for the smooth operation of the CBC. The smooth operation is attained when the CBC works with the maximum gas removal in the gas separator, and the least gas entrainment in the downcomer. Experiments were conducted for TVR ¼ 0–57%. The value of TVR is zero when the liquid volume in the gas liquid separator is zero. At TVR ¼ 0% the liquid continuity is broken and liquid circulation velocity in
Table 2. Circulating bubble column dimensions. Item Volume (l) Gas separator characteristic length (m) Width of gas separator (m) Length of gas separator (m) Height of gas separator (m) Degassed liquid height in gas separator (m) Riser diameter (m) Downcomer diameter (m) Nominal riser length (m) Nominal downcomer length (m) Dispersion height (m)
Notation
Value
V LRD
135 0.28
W L Z ZL
0.36 0.78 0.628 0.0–0.3
dR dD HR HD DH
0.153 0.082 2.40 3.26 2.187
(1)
Figure 3. Top section flow behaviour at TVR < TVRS.
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Figure 4. Top section flow behaviour at TVR ¼ TVRS.
Figure 6. Effect of TVR on riser gas hold-up for an air–water system.
Figure 5. Top section flow behaviour at TVR > TVRS.
the downcomer is zero, i.e. the mode of operation is that of BC. When there is enough gas present in the riser, the two-phase volume in the riser is increased, and hence the continuity re-established again, allowing the mode of operation to return to CBC, with appreciable waterfall effects. Therefore, the operation at TVR ¼ 0% is bimodal between that of BC and that of CBC depending on superficial gas velocity. For the air–water system the transition between BC and CBC occurs at TVR ¼ 0% and UGR ¼ 0.07 m s1, while for the air–glycerol system the transition occurs at TVR ¼ 0% and UGR ¼ 0.03 m s1. Figures 6 and 7 show the riser gas hold-up eGR for air– water and air–glycerol systems, respectively. The gas holdup is characterized by three regimes: homogenous (UGR < 0.05 m s1), transitional (0.05 < UGR < 0.1 m s1) and heterogeneous (UGR 0.10 m s1). The gas hold-up for air–glycerol in the homogenous region is lower than that for air–water due to increase of coalescence. This is similar to the results reported by Al-Masry (1998) and Molina et al. (1999). However, when the gas throughput is increased to the transitional and then to the heterogenous regime the amount of bubbles break-up is increased, leading to slight increase in gas hold-up. Increasing the volume ratio beyond 30% has no effect on the gas hold-up for both air– water and air–glycerol systems. It is interesting to note that, in Figure 7, the profile of gas hold-up increased for TVR ¼ 52.24 and 56.76%, while the others decrease. This could be explained by the accumulation of small bubbles with experiment time, adding up to the large bubbles and leading to increase and change in direction of gas hold-up
Figure 7. Effect of TVR on riser gas hold-up for an air–glycerol system.
profile in the riser. The effect of small bubbles in viscous systems in CBCs is discussed elsewhere (Muller and Davidson, 1992). From Figures 6 and 7 it is clear that TVRS ¼ 17.95% and TVRO ¼ 30.43%. Also, it is evident that the higher gas hold-up for BC mode of operation (TVR ¼ 0%) than CBC mode of operation agrees well with the literature (Weiland and Onken, 1981). Figure 8 shows the downcomer gas hold-up eGD for the air–water system, which indicates that the maximum value is obtained at zero volume ratio, corresponding to BC operation. At volume ratio higher than 30% the effect on the downcomer gas hold-up is negligible. Figure 9 shows the downcomer gas hold-up for the air–glycerol system. The maximum gas hold-up is shown at volume ratio equal to zero, similar to the air–water system. Again, the average value of volume ratio corresponding to 30% could be estimated for the stable operation of the column. The same trend of gas hold-up for TVR ¼ 52.24 and 56.7% is repeated here. Liquid circulation curves are shown in Figures 10 and 11 for air–water and air–glycerol systems, respectively. Both systems show optimum operation at an average volume ratio of about 30%. At TVR ¼ 0%, when the gas throughput is zero, the liquid circulation is zero. However, once the gas throughput is increased, the liquid level in the gas liquid separator starts to increase until the continuity of the circulation is re-established. This corresponds to
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Figure 8. Effect of TVR on downcomer gas hold-up for an air–water system.
Figure 10. Effect of TVR on liquid circulating velocity for an air–water system.
Figure 9. Effect of TVR on downcomer gas hold-up for an air–glycerol system.
Figure 11. Effect of TVR on liquid circulating velocity for an air–glycerol system.
UGR ¼ 0.07 and 0.03 m s1 for air–water and air–glycerol, respectively. At TVR ¼ 17.9%, the level of the liquid in the gas separator is still not high enough to generate smooth liquid circulation, and the waterfall effects in the downcomer predominate. The waterfall effect diminishes at TVR exceeding TVRS ¼ 18%, with the smooth operation recovering fully at TVR 30%. It is interesting to note here that Figure 10 shows ULR ¼ 0 for TVR ¼ 0% when 0.012 < UGR < 0.07 m s1, and the expected eGD should be zero. However, the corresponding eGD values shown in Figure 8 are greater than zero. This special case happened at the beginning of the waterfall phenomenon, and is related to the installed magnetic flowmeter and the manometer. When the waterfall started, it entrained gas bubbles in the top section of the downcomer. The entrained bubbles did not travel to the bottom of the downcomer (due to the absence of liquid continuity) and remained at the top section of the downcomer only. Hence the pressure difference between the top and bottom tappings of the manometer in the downcomer was not zero, but increasing with UGR as we can see from Figure 8. At the same time the presence of the entrained bubbles at the top section of the downcomer without liquid continuity did not create liquid flow in the downcomer, and hence the magnetic flow meter was reading zero. The same explanation applies to Figures 9 and 11.
The overall behaviour of the gas separator in this work is similar to the observations noted by Al-Masry (1999b), that there is an optimum level of liquid in the gas separator which provides the smooth operation of CBC with minimum gas entrainment in the downcomer. In this work, AD=AR ¼ 0.29, and the volume ratios are TVRS ¼ 18% and TVRO ¼ 30% for both air–water and air–glycerol systems. Al-Masry (1999b) reported for a larger column, AD=AR ¼ 0.54, volume ratios of TVRS ¼ 7% and TVRO ¼ 18% for air– water and air–glycerol systems, respectively. The larger column had attained the stability of operation much earlier than the small column, with clear scale effects on the operation. The dimensions of the gas separators (the width and length) are the same in both columns. Drift-flux Model The generalized model for slip by Zuber and Findlay (1965) provides a useful basis for the evaluation of the gas hold-up in terms of mean velocities, and has been adopted by many authors (Nicol and Davidson, 1988b; Snape et al., 1995; Miyahara et al., 1999; Vial et al., 2003). The model allows for the actual slip between phases as well as the effective slip between phases due to non-uniform voidage distribution. The basis of this is that the actual gas velocity
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is a function of the superficial velocity as well as the gas drift velocity. The model in its simplest form is given by: UGR ¼ C0 (UGR þ ULR ) þ C1 eGR
(2)
where C0 is a distribution parameter which allows for the effect of non-uniform voidage and has a value of 1.0 for radially uniform flow, and C1 is often equated to the terminal rise velocity of a gas bubble in the liquid medium. The drift-flux model is applied the experimental data, and the best fit lines for air–water system at TVRO are: UGR ¼ 0:97(UGR þ ULR ) þ 0:36 eGR UGR < 0:05 m s1
(3)
UGR ¼ 1:95(UGR þ ULR ) þ 0:11 eGR 0:05 UGR 0:10 m s1
(4)
UGR ¼ 0:79(UGR þ ULR ) þ 0:53 eGR UGR > 0:1 m s1
(5)
The best fit lines for air–glycerol system at TVRO are: UGR ¼ 0:96(UGR þ ULR ) þ 0:35 eGR UGR < 0:05 m s1
(6)
UGR ¼ 1:87(UGR þ ULR ) þ 0:22 eGR 0:05 UGR 0:10 m s1
(7)
UGR ¼ 0:77(UGR þ ULR ) þ 0:21 eGR UGR > 0:10 m s1
(8)
It can be seen from both equations (3) and (6) that the distribution parameter C0 1.0, indicating uniform radial bubble distribution. The value of C0 as seen in equations (4) and (7) is between 1.95 and 1.87, indicating significant non-uniformity of radial flow profile in the riser which is, indeed, a predictable result for the transitional bubbling mode and the riser diameter used. The transition regime is related to the existence of a macro-circulation pattern, which disappear when the fully established heterogeneous regime begins, decreasing C0 values to 0.8. Many works in the literature when using the drift-flux model have ignored the three regimes and thus either report one single distribution parameter for the entire flow, or two values for the bubbly and heterogenous flow regime. For comparison purposes with other workers, Nicol and Davidson (1988b) presented their results in external loop CBC of equal riser and downcomer diameters of 0.24 and 9 m height for air– water with a single equation: UGR ¼ 1:13(UGR þ ULR ) þ 0:28 eGR 0:02 UGR 0:71 m s
1
diameter of 0.158 m and downcomer diameter of 0.05 m, height 2.62 m, with various electrolytes and saccharose solutions. When using sparger with 0.5 mm hole diameter, the authors reported three regimes with C0 ¼ 1.08 in the bubbly flow regime up to UGR ¼ 0.067 m s1. The heterogenous regime began at UGR ¼ 0.14 m s1, but no distribution parameter were given for both transitional and heterogenous regimes. When the authors used a sparger with 1.6 mm hole diameter, a single distribution parameter for the entire flow (0.028 UGR 0.198 m s1) was obtained C0 ¼ 1.5 and C1 ¼ 0.263 m s1, with clear indication of sparger design effects. Miyahara et al. (1999) presented their data in external loop CBC of equal riser and downcomer diameters of 0.14 and 3.5 m in height for an air–distilled water system. The authors found a single distribution parameter for the entire flow (UGR < 0.1 m s1) with C0 ¼ 0.99 and C1 ¼ 0.41. Vial et al. (2003) reported distribution parameters for an air–water system in a CBC with 0.15 m riser diameter, 0.08 m downcomer diameter, and 6 m height. Three regimes were found with their corresponding distribution parameters: C0 ¼ 0.99 (UGR < 0.05 m s1) and C0 ¼ 2.15 (0.05 < UGR < 0.1 m s1) and C0 ¼ 1.62 (UGR > 0.1 m s1). The distribution parameters presented by Vial et al. (2003) are higher than those obtained in this work. Although the diameters of both columns are the same, however, the spargers and gas separators are not. Vial et al. (2003) used multiple-orifice sparger and expanded cylindrical gas separator overflows in a rectangular tank. The results reported indicate that the effects of gas separator geometry and sparger design need to be studied in more detail.
(9)
The authors explained the value of 1.13 for the distribution parameter to presence of central maximum of voidage. Snape et al. (1995) used 65 l external loop CBC with riser
Design of Gas–Liquid Separator A gas bubble exiting the riser will disengage from the liquid phase when the mean residence time in the horizontal flow section of the gas separator tRD is greater than the time needed by the bubble moving at the free rise velocity tZGL to reach the surface of the gas separator. The condition for complete deaeration at the top of a CBC is given by tRD > tZGL
(10)
For a rectangular gas separator, the time needed by a bubble to cross the horizontal distance LRD is given by tRD ¼
LRD ULGS
(11)
where ULGS is the superficial liquid velocity in the gas separator. The time necessary for a bubble to travel to the surface is given by: tZGL ¼
ZGL UB
(12)
where UB is the bubble velocity. Rewriting equation (10) using equations (11) and (12), the condition becomes LRD Z > GL ULGS UB Expressing equation (13) in terms of ULR: LRD WZGL ZGL > ULR AR UB
(13)
(14)
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Figure 12. Three-dimensional plot for air–water downcomer gas hold-up (this work). Figure 14. Parity plot for downcomer gas hold-up for AD=AR ¼ 0.54 and m ¼ 1.
It can be seen from both sides of the inequality [equations (15) and (16)] that it satisfies the theoretical condition for complete gas removal at top of the CBC, which is not the case from the experimental observations. To satisfy the experimental conditions, the bubble rise velocity is calculated from equation (14) to be UB ¼ 0.06 m s1 corresponding to 0.5 mm in diameter (McCabe et al., 2001). Practically, the variables in the above inequality cannot be adjusted once the reactor has been designed and constructed, except ZL or ZGL. Therefore, to account for ZGL influences on the hydrodynamics of CBCs, the data in this work were fitted empirically with TVR as a parameter using non-linear regression analysis. The results are given in the following equations
Figure 13. Three-dimensional plot for air–water downcomer gas hold-up of Al-Masry (1999).
The rise velocity of the smallest bubble to be excluded from the downcomer should be such that the inequality [equation (14)] is satisfied at the maximum anticipated liquid circulation (Al-Masry, 1999b; Verlaan et al., 1986). This is generally true when a certain critical degassed liquid height or volume in the gas-separator is exceeded, as verified experimentally in this work. For instance, the downcomer gas hold-up in the case of air–water system are shown in three-dimensional plots in Figures 12 and 13. It is clear that complete separation was not possible for the small reactor, while for the large reactor it was possible when TVR exceeded 11%. Take, for example, UGR ¼ 0.0163 m s1, TVR ¼ 52.24%, the left-hand side of the inequality [equation (14)] is calculated to equal: LRD WZGL 0:28 0:36 ZGL ¼ 16:82 ZGL ¼ ULR AR 0:278 0:0177
(17)
0:88 0:32 0:08 TVR mL eGD ¼ 0:59 UGR 0:433 0:3 0:03 ULR ¼ 0:003 UGR TVR mL
(18) (19)
The above equations are regressed with R2 > 0.9 and valid for the small reactors. To account for the scale-up effects, the data of this work and those of Al-Masry (1999b) were combined in a single correlation of the form: y ¼ a[(bUGR þ 1)n 1]1=n exp (TVR c þ ADR d þ mL f ) (20) where y could be eGR, eGD or ULD and ADR ¼ (AD þ AR)=AR. The coefficients a, b, c, d and f are the regression parameters given in Table 3. The new correlation predicts the hydrodynamics parameters with good accuracy taking into account the effect of the volume ratio TVR and the scaleup factor ADR. As an example of the accuracy of equation (20), a parity plot for experimental and predicted eGD is shown in Figure 14 with 20% error margins. Most of the data are predicted satisfactorily with some deviation at low
(15)
The right-hand side can be calculated assuming UB ¼ 0.25 m s1 with 3 mm in diameter (McCabe et al., 2001): ZGL Z ¼ GL ¼ 4 ZGL UB 0:25
0:86 0:065 0:024 eGR ¼ 0:023 UGR TVR mL
(16)
Table 3. Regression parameters for equation (20). Parameter Variable eR eD nLR
a
b
c
d
f
n
Average error (%)
3.8351 2.6928 0.4157
0.7332 0.4716 0.0538
0.0059 0.0112 0.01266
1.0954 0.6946 0.59639
0.08396 0.07577 0.1367
1.3315 1.2986 2.6649
3.588 5.159 1.109
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superficial gas velocities. Although equation (20) is empirical in its nature, it is a good design correlation for external loop CBCs. CONCLUDING REMARKS Circulating bubble column reactors are characterized by their liquid circulation, which provides better mixing than simple bubble column reactors. In this study the hydrodynamics of air–water and air–glycerol systems were investigated in a circulating bubble column reactor with an open channel rectangular gas separator. The volume ratio of the liquid in the gas separator to the reactor volume was found to affect the downcomer gas hold-up for both systems investigated. The volume ratio is a design parameter, which needs to be taken into consideration, particularly when gas recirculation needs to be minimized. A critical optimum volume ratio was found to be 30% for operation of the CBC with the least gas entrainment in the downcomer. New empirical correlations are proposed for external loop CBCs with Newtonian systems. By adjusting the volume ratio (the gas separator design) in the CBC one can establish the suitable conditions for the implementation of a required chemical or biochemical process, with respect to gas holdup in the downcomer. The quantification of the gas recirculation, which is a direct consequence of the gas separator design, is important for the overall performance of the reactor. Therefore, the effect of the gas separator should be taken into account in the design and scale-up procedure of CBCs. The volume ratio can be adjusted by varying the size, the shape of gas separator or the height of the liquid in the gas separator. The drift-flux model is a valuable analysis tool, and has shown that three flow regimes exist, particularly the transitional regime which has always been neglected in the literature. To better understand the hydrodynamics of the gas separator a computational fluid dynamics CFD analysis is needed. Such analysis will provide better insight into the various effects on the two-phase behaviour in gas separator. This exercise is currently performed in our multiphase laboratory. NOMENCLATURE AD ADR AR dD DH dR HD HR L LRD tRD TVR TVRS TVRO tZLG UB UGR VLGS VL ULD ULGS ULR W
downcomer cross-sectional area, m2 scale-up parameter in equation (20), ADR ¼ (AD þ AR)=AR riser cross-sectional area, m2 diameter of downcomer, m dispersion height, m diameter of riser, m nominal downcomer length, m nominal riser length, m length of gas separator, m characteristic length in gas separator, m time for bubble to cross distance LRD, s volume ratio transitional volume ratio optimum volume ratio time for bubble to travel distance ZLG, s bubble free rise velocity, m s1 superficial riser gas velocity, m s1 degassed liquid volume in the gas separator, l degassed liquid volume in the reactor, l superficial liquid velocity in the downcomer, m s1 superficial liquid velocity in the gas separator, m s1 superficial liquid velocity in the riser, m s1 width of the gas separator, m
Z ZGL ZL
height of gas separator, m height of gas–liquid dispersion in gas separator, m height of degassed liquid in gas separator, m
Greek symbols m liquid viscosity, Pa s downcomer gas hold-up eGD riser gas hold-up eGR
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ACKNOWLEDGEMENT The author wishes thank to Dr Nayef Ghasem for his help in the regression analysis of the data. The manuscript was received 9 June 2003 and accepted for publication after revision 24 October 2003.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A3): 381–389