Ind. Eng. Chem. Res. 2009, 48, 1621–1628
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Flow Pattern, Pressure Drop, and Mass Transfer in a Gas-Liquid Concurrent Two-Phase Flow Microchannel Reactor Haining Niu,†,‡ Liwei Pan,† Hongjiu Su,† and Shudong Wang*,† Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, China
Flow pattern, pressure drop, and mass transfer characteristics have been studied for the gas-liquid twophase flow in a 1.0 mm inner diameter circular microchannel reactor. A mixture of CO2, N2, and polyethylene glycol dimethyl ether was used to represent the gas and liquid phases, respectively. Bubbly, slug, churn, and slug-annular flow patterns were observed in the present work. A flow pattern map using superficial gas and liquid velocities as coordinates has been developed and compared to the existing flow pattern maps for ∼1 mm diameter channels. The data obtained for the pressure drop of the two-phase flow were analyzed and compared with the homogeneous model and the separate flow model to assess their predictive capabilities. The liquid side volumetric mass transfer coefficient increased with an increase of the superficial gas and liquid velocities, and the influences of the superficial gas and liquid velocities on it were demonstrated. The liquid side mass transfer coefficient, which was as high as 3.34 s-1, was 1 or 2 orders of magnitude higher than the traditional industrial gas-liquid contactors. 1. Introduction The objective of this work is to study experimentally the characteristics of the gas-liquid two-phase flow pattern, pressure drop, and mass transfer in horizontal circular microchannel reactors. As a new means of process intensification, the microreaction technology has attracted great interest during the past 20 years for some of its distinguished advantages differing from traditional processes.1-4 The microreaction technology is featured by enhanced heat and mass transfer, inherent safety characteristics, high surface to volume ratio, and so on. Owing to these unique advantages, some novel microreactors for gas-liquid reactions that are difficult to operate in conventional reactors such as direct fluorination of aliphatics5,6 and benzenoid bromatics,7 and hydrogenation of cyclohexene over Pt/Al2O3 catalysts,8 have already been commercially available. For such microchannel reactors involving gas-liquid two phase flows, the hydrodynamics, including the flow pattern, the pressure drop, and the characteristics of mass transfer are of great importance. 1.1. Flow Pattern. Using oil and gas as the working fluids in macrochannels, maybe the first flow pattern map available in literature was created by Baker,9 who had defined flow pattern transitions based on superficial gas and liquid velocities. Flow patterns such as bubble, plug, slug, stratified, wavy, annular, and spray could be discerned. As far as larger channels were concerned, the effects of fluid properties, including flow rates, densities, liquid viscosity, surface tension, and pipe diameter, on two-phase flow patterns were investigated extensively in the past,10-12 and the fluid properties and pipe diameters usually had only moderate influences on the transition lines. A decrease of the inner diameter of the channels would lead to a change in relative effects of the gravitational, shear, and surface tension forces, and the flow pattern transition line correlations concluded from larger channels might not be applicable to those of the smaller ones. Triplett et al.13 studied the air-water two-phase characteristics in long horizontal circular microchannels having inner diameters * To whom correspondence should be addressed. E-mail: wangsd@ dicp.ac.cn. † Dalian Institute of Chemical Physics. ‡ Graduate School of Chinese Academy of Sciences.
of 1.1 and 1.45 mm, respectively. The discernible flow patterns were bubbly, churn, slug, slug-annular, and annular flow. Using superficial air and liquid velocities as coordinates, they developed flow pattern maps and compared them with other available relevant flow regime transition models. The effect of channel diameter and surface tension on the flow pattern of air-water mixtures in horizontal round and rectangular channels were also studied by Colman et al.14 They showed that the aspect ratio, hydraulic diameter, and surface tension played an important role in determining the flow patterns and transitions. With a channel inner diameter ranging from 1 to 4.9 mm, and with other data of larger channels from the literature, Ide et al.15 have proposed that the critical channel size at which the surface tension forces surpass the gravitational force was around 5 mm, which has also been described previously by Bretherton,16 and the effect of the channel diameter and flow direction on the flow pattern was smaller as long as the gravitational effect was diminished. The flow pattern maps discussed above usually adopted air or nitrogen and water as the working fluids, and the mass transfer from gas to liquid can be ignored. Since physical absorption was present, and mass transfer from gas to liquid was obvious in these experiments, so, whether the flow regime transition or the correlations obtained by air or nitrogen and water could be applied to these cases are subjected to further studies. 1.2. Pressure Drop. Accurate prediction of a two-phase flow pressure drop is of paramount importance to design engineers. Homogeneous and separated flow models have been developed for the prediction of two-phase frictional pressure drops. With a homogeneous pressure drop model, Triplett et al.17 have predicted well experimental data in bubbly and slug flow patterns at high ReL, but overpredicted the pressure drop in an annular flow pattern, and this suggested that for annular flow the gas-liquid interfacial momentum transfer and the wall friction in microchannels may be significantly different from those similar processes in large channels. With the separated flow model, Lowry and Kawaji18 have measured the pressure drop for a concurrent upward air-water flow in narrow passages and concluded that the Lockhart-Martinelli correlation was an adequate predictor of the two-phase frictional multiplier for the
10.1021/ie801095a CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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pressure drop. However, Mishima et al.19,20 have reported that the parameter C in the Lockhart-Martinelli correlation should include the effect of tube sizes to represent the frictional pressure drop for small tubes less than 5 mm in hydraulic diameters. Fujita et al.21 have suggested that the Lockhart-Martinelli correlation could not predict two-phase pressure drops satisfactorily when the liquid superficial velocity was low for the intermittent flow. The above discussions implied that though different models have been applied to assess the pressure drop of the gas-liquid two phase flow, certain inconsistent results still existed. The experimental pressure drop of the present study will be used to test their predictive capabilities. 1.3. Mass Transfer. Having advantages of high mass transfer rates and high flow rates of both the gas and the liquid which are unlimited by flooding, the mass transfer of concurrent gas-liquid two phase flow has been studied. Jepson22 has determined the effect of gas-liquid flow rates, channel diameter, and channel orientation on the rate of the liquid-phase controlled mass transfer in concurrent two-phase flow, and the mass transfer data was correlated with two-phase frictional energy dissipation. The interfacial area, as an important parameter for the mass transfer rates, was determined in a concurrent gas-liquid two-phase flow by Kasturi et al.23 They showed that the interfacial area increased with an increase of the liquid velocity and reached a maximum value at about the same gas velocity for all liquid velocities. The average values of the interfacial area obtained in their investigation were much higher than those reported for conventional packed or bubble columns, and an annular or annular-mist flow would be preferred if high interfacial areas were required. Under similar conditions gas and liquid side mass transfer coefficients were also measured, and their results were similar as those presented by Shilimkan et al.24 Tortopidis et al. have concluded that variations in mass transfer were related to flow regime transitions, and a stratified flow gave the lowest mass transfer coefficient, while annular flow gave the highest, and slug flow led to mass transfer rates fairly independent of the gas velocity.25 Recently, with a hydraulic diameter of 0.667 mm, the mass transfer characteristics in a rectangular microchannel have been investigated by Yue et al.26 Their results showed that both the liquid side volumetric mass transfer coefficient and the interfacial area increased with an increase of the superficial gas velocities for a fixed superficial liquid velocity. At a fixed superficial gas velocity, the increase in superficial liquid velocity could affect the liquid side mass transfer coefficient greatly, but not the interfacial area. These showed that the mass transfer characteristics in a microchannel were different from those in a conventionally larger channel and more data should be accumulated. 2. Experimental Setup A sketch of the experimental setup of the present study is depicted in Figure 1. A mixture of CO2, N2, and polyethylene glycol dimethyl ether was used to represent the gas and liquid phases, respectively. The gas phase was first conveyed by a pressure-regulating valve from a gas cylinder. Passing through a valve, the gas phase was controlled by a mass flow meter so as to obtain a given flow rate before it was introduced to the gas inlet of a T-type mixer (see Figure 2). The liquid flow rate was regulated by a 0∼60 mL/min positive displacement pump and met the gas stream through a T-type mixer. After flowing through the microchannel reactor horizontally, the gas-liquid two phase mixture was separated in a phase separator. Most of the gas from the separator was vented, and a small amount of it was introduced to a chromatograph for analysis. The test
Figure 1. Schematic diagram of the experimental setup.
Figure 2. Detailed structure of the T-type mixture.
section was placed horizontally and the T-shaped mixer was shown in Figure 2. Two circular microchannels, which were made of stainless steel and quartz-glass, respectively, were used to investigate the hydrodynamics and the mass transfer throughout the experiment. The length of all the channels was 180 mm and the inner diameter was determined by imaging the channel cross-section with a microscope. The pressure drop was measured by two pressure transducers P1 and P2 (200 ( 0.5 kPa). The pressure transducer P1 was connected to the pressure detection port of the T-type mixture and P2 was placed near the microchannel outlet. A visual basic program was developed to manage the data signal acquired by the two pressure transducers. Figure 2 showed that the distance from the T-type mixer’s pressure detection port to the capillary inlets was about 2 mm, so it was negligible, in regard to the local effect of the T-mixer on the total pressure drop, with respect to the total lengths of the mirochannels. On the outlet, the pressure loss due to the sudden flow area expansion was considered. The quartz-glass microchannel could be used to investigate the flow patterns with a digital charge-coupled device (CCD) camera. All of the experiments were performed at room temperature (ca. 295 K) and at near-atmospheric pressure. 3. Results and Discussion 3.1. Two-Phase Flow Pattern. 3.1.1. Flow Pattern. It is impossible to understand the two-phase flow phenomenon without a clear understanding of the flow patterns encountered. The study of two-phase flow patterns in circular tubes has covered a period of more than 40 years. One of the problems in studying and reporting two-phase flow patterns was the lack of uniformity in terminology used by various investigators for different flow regimes. For instance, it is common that a flow regime with the same characteristics to have two or even three different names. It is important that specific definitions for these regimes be established and described before the flow regime maps are presented. In this work, the five major flow regimes,
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Figure 3. Representative photographs of flow patterns at low superficial liquid velocities: (a) slug UL ) 0.05, UG ) 0.09 m/s; (b) slug UL ) 0.05, UG ) 0.91 m/s; (c) slug-annular UL ) 0.05, UG ) 3.63 m/s; (d) slug-annular UL ) 0.05, UG ) 9.98 m/s.
including stratified, bubbly, slug-annular, and churn, are described as follows: 3.1.1.1. Stratified Flow. At low superficial gas and liquid velocities, a complete separation of the gas-liquid two-phase is observed. The liquid flows at the bottom of the channel and the gas flows over the liquid. According to whether there is the appearance of fluctuation at the flow interface or not, the stratified flow can be further divided into stratified wavy and stratified smooth flows. 3.1.1.2. Bubbly Flow. A bubbly flow usually occurs when the superficial gas velocity is low while the superficial liquid velocity is high. When the superficial gas velocity is low, small bubbles are driven by buoyancy forces and they flow primarily in the top half of the channel. This can be characterized by the presence of distinct and distorted bubbles in the continued liquid phase, the diameters of which are less than or equal to the inner diameter of the channel. 3.1.1.3. Slug Flow. Slug flow is characterized by discontinuities in the liquid and gas flow. Surrounded by a thin film of liquid, a gas bubble is separated by the liquid slug and becomes longer when the superficial gas velocity increases. Usually the lengths of the bubbles are longer than the inner diameter of the channel. 3.1.1.4. Churn Flow. Churn flow appears at moderate superficial gas velocities and high superficial liquid velocities. The elongated bubbles become unstable near their trailing ends, leading to their disruption. 3.1.1.5. Slug-Annular Flow. Slug-annular flow usually appears with high superficial gas velocities and low superficial liquid velocities. The liquid is pushed around the channel wall to form an annular ring of the liquid phase and the gas flows through the core of the channel. Figures 3 and 4 show typical two-phase flow patterns observed at different superficial gas and liquid velocities. At
Figure 4. Representative photographs of flow patterns at high superficial liquid velocities: (a) bubble UL ) 0.9, UG ) 0.09 m/s; (b) transition UL ) 0.9, UG ) 0.45 m/s; (c) slug UL ) 0.9, UG ) 1.81 m/s; (d) churn UL ) 0.9, UG ) 0.91 m/s.
low superficial liquid velocities and low superficial gas velocities, the slug flow could be seen clearly (Figure 3a). With an increase of the superficial gas velocity, the gas bubbles become longer and the liquid slugs become shorter (Figure 3b). Increasing the superficial gas velocity further eventually leads to the merging of the elongated bubbles and the development of the slug-annular flow pattern (Figure 3c,d). The annular flow pattern depicted by Triplett13 has not been disclosed yet, maybe the reason was that the superficial gas velocities were not high enough in their experiments. When the superficial liquid velocity was high, bubbly flow was observed with low superficial gas velocity. The small bubbles flow primarily in the top half of the channel (Figure 4a), which is similar to the phenomenon observed by Coleman et al.14 With the increase of the superficial gas velocity, a transition flow pattern could be observed (Figure 4b). The transition flow, consisting of a series of bubbles, is similar to trains and with a diameter similar to the channel diameter, and passes regularly through the channel. Except for the slug flow (Figure 4c), a churn flow will be seen with high superficial gas velocities. The stratified flow was not detected in our work, and this is consistent with the previous observation of Fukano and Kariyasaki.27 3.1.2. Flow Pattern Map. Using the superficial gas and liquid velocities as coordinates, the observed flow regime map and the flow pattern transition lines are depicted in Figure 5. The superficial velocities covered a range of UL ) 0.03∼0.9 m/s and UG ) 0.03∼9.98 m/s for the present gas-liquid system. The solid lines, representing the boundaries where the flow pattern transitions occurred, divided the flow pattern maps into four regions. The churn and the slug-annular flows, occurring with higher superficial gas velocities, are situated in the top right corner and bottom right corner of the flow map, respectively. The bubbly flow is situated in the top left corner, and the rest is the slug flow pattern.
1624 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Table 1. Expressions for the Two-Phase Mixture Viscosity Two-phase viscosity model 29
McAdams Dukler et al.30 Beatie and Whalley31 Lin et al.32
Figure 5. Flow pattern and flow pattern transition lines.
function ηtp ηtp ηtp ηtp
(mηG-1
number
(1-m)ηL-1)-1
) + ) βηG + (1-β)ηL ) βηG + (1-β)(1 + 2.5β)ηL ) ηGηL(ηG + m1.4(ηL - µG))-1
a b c d
where (dp/dz)tpf is the pressure drop gradient due to wall friction, (dp/dz)tpa is the pressure drop gradient due to acceleration, (dp/ dz)tpg is the gravitational pressure gradient and (dp/dz)expan is the pressure drop gradient due to the sudden flow area expansion. According to Fridel28 the accelerative component is of significance only in evaporating or condensing flows and can be neglected when no phase changes occurred, and the pressure drop due to gravity (or the static head pressure gradient) was zero because the channel was horizontally oriented. Thus the equation can be simplified as: (dp/dz)tpf ) (dp/dz)tp - (dp/dz)expan
(2)
3.2.1. Comparison with the Homogeneous Model. The homogeneous model is obtained by assuming that there is no slip between the gas and the liquid phases. The two-phase pressure gradient (dp/dz)tpf is calculated from Figure 6. Comparison of two-phase flow regimes with those of Triplett et al.13
(dp/dz)tpf ) ftp
1 G2 D 2Ftp
(3)
where Ftp is the homogeneous mixture density defined as 3.1.3. Comparison with the Results of Triplett et al.13 In 1998, Triplett et al. had conducted experiments in circular and semitriangular cross-section channels, and the fluids were air and water, respectively. The result in a circular channel with 1.1 mm inner diameter was compared with the present study in Figure 6. The channels were similar but the fluids were different. We all know that the absorption of a gas into a liquid is negligible when air and water flow concurrently through a channel, while for the N2/CO2/polyethylene glycol dimethyl ether system, the absorption is obvious and should not be ignored. Figure 6 shows that the transition from the slug flow to the slug-annular flow was in fairly good agreement with the results obtained by Triplett et al. Although absorption existed during the process, only a small quantity of the CO2 was absorbed into the liquid since the liquid flow was very slow. However, when the superficial gas and liquid velocities were all very high, the absorption of CO2 into the liquid could not be ignored. The experimental transition line from the slug flow to the churn flow has shifted slightly to the left, while the transition line from the churn flow to the slug-annular occurred at lower superficial liquid velocities. In the top left corner of the flow map, the transition line from the slug to the bubbly flow shifted to a lower position. Since the superficial liquid velocities were high and the superficial gas velocities were low, the mass transfer from gas to liquid phase was significant. The shift of the transition line to a lower superficial liquid velocity caused by the absorption was obvious. 3.2. Two-Phase Pressure Drop Gradient. The total pressure drop of a fluid is due to the variation of kinetic and potential energies as well as due to friction, so that the pressure is the sum of the static pressure drop, the momentum pressure drop, and the frictional pressure drop. The general equation for the pressure drop gradient in two-phase flow can be expressed as (dp/dz)tp ) (dp/dz)tpf + (dp/dz)tpa + (dp/dz)tpg + (dp/dz)expan (1)
Ftp )
m 1-m + FG FL
(4)
As a function of the homogeneous Reynolds number Retp (defined as GD/ηtp), the two-phase Darcy friction factor ftp is calculated from ftp )
64 Retp
(5)
Several models have been proposed by different investigators to evaluate the two-phase mixture viscosity, and the viscosity models selected are listed in Table 1. From calculations using the following two-phase viscosity models, the present experimental data are compared with the predictions of the homogeneous flow model in Figure 7. Figure 7 shows that the agreement between the experimental data and the homogeneous model calculated with different mixture viscosity models was generally poor, except for the model of McAdams and the viscosity model of Beatie and Whalley, which had an absolute mean deviation of 22.78% and 31.48%, respectively. The Dukler’s model has underpredicted significantly the frictional pressure drop gradient and is contrary to the model of Lin et al. Thus, the two-phase viscosity model played an important role for the proper prediction of the pressure drop gradient. 3.2.2. Comparison with the Separated-Flow Model. On the basis of a separated flow assumption, Lockhart and Martinelli33 have proposed the first predictive method for estimating the frictional two-phase pressure gradient. They put forward a Martinelli parameter, X, which was based on the ratio of the single-phase frictional pressure drops calculated by assuming that the liquid-phase and the gas-phase were both flowing alone, that is, X2 )
(dp/dz)L (dp/dz)G
(6)
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1625
Figure 7. Comparisons of the experimental frictional pressure drop gradient based on the homogeneous model.
Figure 8. Measured two-phase friction multiplier as a function of the Martinelli parameter.
Figure 10. Comparisons of the experimental frictional pressure drop gradient based on the separated flow model for the stainless steel microchannel reactor.
and X was presented originally in a graphical form. Then Chisholm34 approximated the relationship by the following simple expression: φL2 ) 1 +
Figure 9. Comparisons of the experimental frictional pressure drop gradient based on the separated flow model for the quartz-glass microchannel.
(dp/dz)L and (dp/dz)G are the liquid and gas frictional pressure gradients if each phase flows alone in the same channel with its own mass flow rate. They have also defined the two-phase multipliers: (dp/dz)tpf (7) φL2 ) (dp/dz)L Their data indicated that the multipliers were a function of the Martinelli parameter alone. The relationship between φL2
1 C + X X2
(8)
where C denotes the Chisholm’s factor and its value depends only on whether the liquid and gas flows are laminar or turbulent. For example, when the two phases are both laminar or turbulent, C ) 5 and 20, respectively. However, Mishima et al.20 have found that the value of the Chisholm parameter decreased as the channel diameter decreased, and the C value when taking the channel diameter into account can be defined as C ) 21[1 - exp(-0.319Dh)] 26,35-37
(9)
Recently, more literature results have revealed the effect of mass flux on the Chisholm parameter, which also can be seen from the present experiment in Figure 8. The Chisholm parameter increased with the increase of the mass flux and was not a constant value any more. Considering the effect of the
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Figure 11. Effect of superficial gas and liquid velocities on liquid side mass transfer coefficient: (a) quartz-glass microchannel reactor, (b) stainless steel microchannel reactor.
mass flux, the tube diameter, and other physical properties, the C value is correlated as C ) 0.0049 · ReL0.98 · ReL1.08 · Wetp-0.86
(10)
Figure 9 demonstrates the relationship between the experiments and the theoretical results obtained for different c values. The agreement between the experimental data and the separated flow model with the c values of the Chisholm equation and the Mishima equation was generally poor, and each of them has an absolute mean deviation of 30.71% and 22.69%, respectively. The prediction for the c value obtained by eq 10 agreed well with the experimental data, and the absolute mean deviation was only 3.45%. The validity of eq 10 for the prediction of the frictional pressure drop gradient was also tested for a stainless steel microchannel reactor. The comparison between the experimental and the predicted data is shown in Figure 10. The agreement between the experimental data and the separated flow model with the c value of eq 10 was generally good, with an absolute mean deviation of 6.8%. 3.3. Mass transfer characteristics. Mass transfer is characterized by a mean liquid side volumetric mass transfer coefficient, which can be defined as
(
UL Ce - Cin ln kLa ) L Ce - Cout
)
(11)
where Cin and Cout are the liquid phase CO2 concentrations at the inlet and outlet of the microchannel and Ce is the equilibrium
concentration which can be calculated by Henry’s law. Cin can be assumed as zero since the inlet liquid is fresh. A chromatogram is selected to measure the moral fraction of the gas phase in the microchannel inlet and outlet. The gas phase flow rate is exactly controlled by a mass flow controller. With the gas phase flow rate of the microchannel inlet, the liquid phase flow rate, and gas phase CO2 concentrations of the microchannel inlet and outlet, the concentration of CO2 in the outlet, Cout, can be calculated by the material balance. A blank experiment showed that the absorption in the phase separator was insignificant and the results were satisfactorily accurate. As shown in Figure 11, the liquid side mass transfer coefficients increased with the increase of the superficial gas and liquid velocities in both channels made of different materials. Comparison between the liquid side mass transfer coefficients in a stainless steel microchannel and a quartz-glass one also showed that the difference caused by channel material is insignificant (0.67-3.34 s-1 against 0.61-3.06 s-1). It was found by Yue et al.26 that the liquid side mass transfer coefficients can be well predicted by using (dp/dz)tp as the fitting parameter. The correlation in the present experiment can be shown as kLa ) 0.017(dp/dz)0.9 tp
(12)
The comparison of the experimental value and the predicted one from eq 12 can be seen in Figure 12. The prediction results agree well with the experimental value, with an absolute mean deviation of 10.04%.
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1627
the separated flow model. For the homogeneous mixture model the selection of a suitable viscosity model is crucial to the proper prediction of the frictional pressured drop. The LockhartMartinelli method is still suitable for the prediction of frictional pressure drops if a proper c value has been chosen. 4. The liquid side volumetric mass transfer coefficients increased with the increase of the superficial gas and liquid velocities. The effect of the superficial liquid velocity on the liquid side mass transfer coefficients was more remarkable than that of the superficial gas velocity. The liquid side volumetric mass transfer coefficient can be well correlated via the frictional pressure drop gradient and the dimensionless empirical equation. Figure 12. Comparisons between liquid side mass transfer coefficients by physical absorption and predicted by eq 12.
Appendix Nomenclature
Figure 13. Comparison between liquid side mass transfer coefficients from physical absorption and from prediction obtained by eq 13.
Dimensionless empirical correlations are also used frequently to correlate the liquid side mass transfer. With a least-squares regression method the liquid side mass transfer coefficients of the present experiment can be expressed as ShL · a · dh ) 0.116ReG0.39 · ReL0.7 · ScL0.5
(13)
The comparison between the experimental liquid side volumetric mass transfer coefficients with the predictions based on eq 13 are shown in Figure 13, which indicated that the agreement between the experimental data and the predictions was generally good, and the standard deviation was only 8.42%. 4. Conclusions From the absorption of CO2 into the polyethylene glycol dimethyl ether, the characteristics of the gas-liquid two-phase flow and the mass transfer were investigated in a circular microchannel reactor. A flow pattern map was developed and compared with the results of Triplett et al. The frictional pressure drop gradient was calculated and analyzed by means of the existing models. Major conclusions of this study are summarized as follows: 1. Two-phase flow patterns were observed for bubbly, slug, slug-annular, and churn flows. The stratified flow could not be observed in the present work, even at the lowest gas and liquid superficial velocities. 2. A flow pattern map was developed on the basisof the appearance of each flow type. The agreement of the flow pattern map in the present experiment with that of Triplett et al. was generally good. The effect of mass transfer on the shift of the transition lines was significant, particularly for high superficial liquid velocities. 3. The experimental data of pressure drop were compared with the predictions from the homogeneous mixture model and
C ) Chisholm parameter or molar concentration of CO2 in the liquid (m3/mol) Dh ) inner diameter (m) dp/dz ) pressure drop gradient (kPa/m) f ) two-phase friction factor G ) mass flux (kg/m2 s) kLa ) liquid side volumetric mass transfer coefficient (s-1) m ) gas phase mass fraction p ) pressure (kPa) Re ) Reynolds number Sc ) Schmidt number Sh ) Sherwood number U ) superficial velocity (m/s) We ) Webber number X ) Martinelli parameter z ) length of the microchannel (m) Greek letters β ) void fraction η ) viscosity (Pa s) F ) density (kg/m3) ΦL2 ) two-phase frictional multiplier Subscripts a ) acceleration cal ) calculation or prediction e ) equilibrium exp ) experimental measurement expan ) flow area expansion f ) friction G ) gas phase in ) microchannel inlet L ) liquid phase out ) microchannel outlet tp ) two-phase mixture
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ReceiVed for reView July 17, 2008 ReVised manuscript receiVed November 11, 2008 Accepted November 12, 2008 IE801095A
