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Ind. Eng. Chem. Res. 2008, 47, 9754–9758
Heat-Transfer Performance of a Liquid-Liquid Microdispersed System Kai Wang, Yangcheng Lu, Huawei Shao, and Guangsheng Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, People’s Republic of China
In this work, the heat-transfer performance of liquid-liquid microdispersed systems was tested for the first time with a microdispersion contactor named a membrane dispersion minicontactor and octane/water as the working system. The volumetric heat-transfer coefficients ranging from 5 to 25 MW/(m3 · °C) were obtained, which were 15-20 times higher than that of the conventional dispersion heat-transfer methods. The heattransfer Murphree efficiencies, defined to evaluate the heat-transfer performance, reached 85-99% for the microdispersed system with residence times less than 0.1 s. By introducing the two parameters of Ca number and oil volumetric fraction, a semiempirical model for predicting the volumetric heat-transfer coefficient has been developed, which fits the experimental results very well. Furthermore, the similarity between the heat and mass transfer in the microdispersed system was demonstrated. 1. Introduction Liquid-liquid microdispersed systems have received great attention for the scientific interests in recent years. Some attractive characteristics of microdispersed systems in the fields of chemical reaction,1 separation,2 colloid,3 measurement,4 and analysis5 have been exhibited. For example, the effects of interfacial tension and viscous force become important for droplet formation, but the effects of gravity become less.6,7 The mixing is enhanced with the decreasing of the mixing scale and the mass-transfer coefficients in microdevices are much higher than that in conventional devices for the large specific interfacial area.8 In the past decade many works relating to the momentum and mass-transfer properties of microdispersed systems have been carried out,6,9 but few talk about the heat-transfer properties. Although there are some results on heat-transfer properties for microstructured heat exchangers, most of them focus on the heat transfer between the fluid and the walls of microchannels.10,11 The heat-transfer properties for two-phase microdispersed systems have not been profoundly discussed. However, the heat transfer of a liquid-liquid microdispersed system is very important for many chemical processes, especially for multiphase chemical reaction processes. Besides, for the large interfacial specific area provided by microdroplets, liquid-liquid microdispersed systems can be used to develop an internal heat removal method in microreactors by removing reaction heat directly with inert solvent. This may become one of the promising alternatives for realizing temperature control in microreactors. In previous studies the heat-transfer performances of two immiscible liquids in pipe-flowing contactors and spray columns have been studied.12-14 The dispersed scales in those contactors ranged from 0.8 to 10 mm.15,16 In our previous works, a microdispersion contactor named a membrane dispersion minicontactor, with a microfiltration membrane as the dispersion medium, was developed for extraction and fast chemical reactions.17,18 Microscaled droplets ranging from 20 to 300 µm on average were produced with the minicontactor, and high mass-transfer rates have been achieved.19 In this work, this minicontactor was applied for the first time to test the heat* To whom correspondence should be addressed. E-mail: gsluo@ tsinghua.edu.cn.
transfer performance of liquid-liquid microdispersed processes with octane/water as the working system. The heat-transfer efficiency in this minicontactor was investigated, and the effects of operating parameters on the volumetric heat-transfer coefficients were discussed with the enhancement of a heat-transfer process by comparison with conventional pipe-flowing contactors. Additionally, considering the characteristics of the microdispersion process, a semiempirical model was established to predict the volumetric heat-transfer coefficients and the similarity between heat and mass transfer in this minicontactor was identified. 2. Experimental Setup and Method The experimental setup is shown in Figure 1, and the principle of membrane dispersion is illustrated. The dispersed phase passes through the membrane in this minicontactor and forms microscaled droplets under the action of shearing force of the cross-flowing continuous phase. A stainless steel microfiltration membrane with average pore size of 5 µm was placed on the top of the mixing chamber. The effective membrane was 0.2 cm × 1 cm, and the volume of the mixing chamber was 0.088 mL. The contactor was made of Teflon and coated with a 3 cm thick insulation layer (NBR-PVC rubber foam). Two volumecontrolled pumps (LB-80, Beijing Xingda Co. Ltd.) were used
Figure 1. Experimental setup and the principle of membrane dispersion.
10.1021/ie8005484 CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9755
Figure 3. Heat-transfer efficiency in the minicontactor. Figure 2. Transparent contactor to observe the phase separation. (FC ) 40 mL/min; FD ) 30 mL/min.) Table 1. Properties of the Working Systems (20-60 °C, 1 atm) density of the continuous phase, FC (g/mL) density of the dispersed phase, FD (g/mL) heat capacity of the continuous phase, (Cp)C (kJ/(kg · °C)) geat capacity of the dispersed phase, (Cp)D (kJ/(kg · °C)) viscosity of the continuous phase, µC (mPa · s) interfacial tension, σ (mN/m)
0.998 0.703 4.18-4.17 2.22-2.32 1.06-0.85 50.8
to deliver the two phases. The feed of the dispersed phase was heated by a thermostat water bath (HH-S2, Beijing Baoyan Co. Ltd.). Fast phase separation is required to precisely measure the temperature of each phase, so a double-layer fiber bundle was designed and placed at the outlet of the mixing chamber. The top and bottom layers were made of polypropylene and iron fibers, oxidized by oleum within an hour, respectively. This bundle with high specific surface and special wetting ability can improve phase separation effectively. Figure 2 shows the photographss of the outlet fluids without and with the fiber bundle in a transparent contactor. With the help of the bundle (Figure 2b), the two phases are separated efficiently, but without the bundle (Figure 2a) the outlet fluids are still mixed. Two pieces of this bundle were used in the heat-transfer experimental setup to keep the flow pattern stable. The porosity of the fiber bundle was about 50%. The length of each fiber bundle was 4 mm, and the diameter of the bundle was 5 mm. Four commercial temperature sensors (PT 100 A, Japan, 2 mm × 1.5 mm × 0.5 mm), which have been calibrated by a precise thermometer, were placed at the inlets and the outlet to measure the temperatures of the two phases. In the experiment hot octane (55-65 °C) was used as the dispersed phase. Water at room temperature (17-19 °C) was used as the continuous phase. In each run the continuous phase was pumped into the minicontactor first, and then followed with the dispersed phase. Within the 2 min after the beginning of each run, the temperatures were stable enough to be recorded. The heat transfer between two phases and the heat dissipation to the environment occurred simultaneously in experiments. Their ratio was calculated in each run with eq 1, and it was less than 5% in the experiments. e)
Iin - Iout × 100% (QC + QD) ⁄ 2
(1)
where Iin is the total enthalpy rate of the two phases at the inlet (kW), Iout is the total enthalpy rate of the two phases at the outlet (kW), QC is the heat-transfer rate calculated from the temperature change of the continuous phase (kW), and QD is the heat-transfer rate calculated from the temperature change of the dispersed phase (kW). Here the enthalpy at 0 °C is set to zero.
According to previous literature,12,13 the volumetric heattransfer coefficient (kW/(m3 · °C)), defined in eq 2, was introduced in this work to characterize the heat-ransfer performances. UV )
Q⁄V UA ) V (∆Tin - ∆Tout) ⁄ ln(∆Tin ⁄ ∆Tout)
(2)
where U is the heat-transfer coefficient (kW/(m2 · °C)), A is the total interfacial area (m2), V is the volume of the mixing chamber (m3), ∆Tin is the temperature difference between the inlet fluids (°C), and ∆Tout is the temperature difference between the outlet fluids (°C). Another characteristic parameter was the heat-transfer Murphree efficiency in the minicontactor, and it was defined as E)
TDin - TDout TDin - TDout*
× 100%
(3)
where TDin is the inlet temperature of the dispersed phase (°C), TDout is the outlet temperature of the dispersed phase (°C), and Tout* D is the ideal equilibrium temperature of the two phases (°C). Here TDout* is calculated from the inlet total enthalpy rate by the heat conservation as shown in Iin ) TDout*((Cp)CFCFC + (Cp)DFDFD)
(4)
where FC is the continuous-phase flow rate (mL/s); FD is the dispersed-phase flow rate (mL/s), (Cp)C is the isobaric heat capacity of the continuous phase (kJ/(kg · °C)), (Cp)D is the isobaric heat capacity of the dispersed phase (kJ/(kg · °C)), FC is the density of the continuous phase (g/mL), and FD is the density of the dispersed phase (g/mL). These physical properties are listed in Table 1. 3. Results and Discussion Heat-Transfer Murphree Efficiency. In our previous studies on a similar dispersion system (hexane/water), the membrane dispersion minicontactor produced microdispersed droplets with diameters ranging from 60 to 300 µm.19 Such microscaled droplets can provide large interfacial area and short heat-transfer distance, so fast mixing and heat-transfer processes may be achieved in the minicontactor. The heat-transfer Murphree efficiencies in this minicontactor are given in Figure 3. It shows that higher than 85% heat-transfer Murphree efficiencies were obtained in this minicontactor, indicating that the heat-transfer process can be carried out efficiently just in the residence time (τ) less than 0.1 s. Φo represents the oil volumetric fraction in the figure. Considering that the flow in the minicontactor was close to a plug flow, Φo can be calculated by Φo )
VD ) FD ⁄ (FD + FC) V
(5)
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Figure 4. Effect of cross-flow velocity on the volumetric heat-transfer coefficient.
Figure 6. Effect of oil volumetric fraction on the volumetric heat-transfer coefficient.
Figure 5. Effect of cross-flow velocity on the volumetric heat-transfer coefficient at different oil volumetric fractions.
where VD is the volume of the dispersed phase (mL/s) and FD is the flow rate of the dispersed phase (mL/s). Volumetric Heat-Transfer Coefficients of the Microdispersed System. The volumetric heat-transfer coefficient is a commonly used character for representing the heat-transfer device’s efficiency. As seen from eq 2, it can be enhanced with the increasing of the interfacial area. The advantage of the minicontactor is that it can produce a large specific interfacial area. Besides, the holdup of the dispersion phase has a decisive effect on the volumetric heat-transfer coefficient as well. Our previous works showed that the velocity in the mixing chamber was an important parameter to affect the droplet size.19,20 In this work the effect of cross-flowing velocity and the volumetric fraction of the dispersed phase were investigated. The definition of the cross-flowing velocity was given in uC ) FC ⁄ s
(6)
where FC is the flow rate of the continuous phase (mL/s) and s is the cross-section area of the mixing chamber (m2). Figure 4 shows that the volumetric heat-transfer coefficients at different cross-flowing velocities. According to the studies of Ford et al.21 and Bora et al.22 it is reported that the heattransfer resistance of the water phase is much lower than that of the oil phase, so the reduction of the oil droplet is beneficial for decreasing the heat-transfer resistance and increasing the heat-transfer area. The results in Figure 4 demonstrate this phenomenon. UV increases from 10000 to 23000 kW/(m3 · °C) with the increasing of uC. Figure 4 also represents that the coefficients are different at different dispersed-phase flow rates. This is related to the differences of the oil volumetric fraction in the mixing chamber. For excluding the oil volumetric fraction’s effect, another investigation on the cross-flowing velocity was preceded at fixed oil volumetric fractions, and the results are plotted in Figure 5. The volumetric heat-transfer coefficients show a linear relation with the increasing of the cross-flowing velocity and become large with the increasing of the volumetric fraction.
Figure 7. Comparison of volumetric heat-transfer coefficients in different contactors.12,13 (uT is the total velocity of the two phases in the mixing chamber.)
The volumetric fraction of the oil phase is another important parameter affecting the total interfacial area in the minicontactor. More oil phase can form more droplets in the contactor to increase the total interfacial area. Thus, the volumetric heattransfer coefficient is increased with the increasing of the oil volumetric fraction. Figure 6 shows that the coefficients are increased from 10000 to 17000 kW/(m3 · °C) with the increasing of Φo from 0.25 to 0.49 when uC is fixed at 0.37 m/s, and increased from 15000 to 22000 kW/(m3 · °C) with the increase of Φo from 0.25 to 0.45 when uC is fixed at 0.48 m/s. Enhancement of Heat Transfer by Dispersed Scale Miniaturization. The enhancement of mixing with microdispersed liquid-liquid systems has been studied by many works, and the fast mass-transfer rates in microdispersion devices have been fully represented.8,23 Considering the similarity of heat transfer with mass transfer, fast heat-transfer rates may be also available. To compare the heat-transfer performances in the minicontactor with the conventional contactors, two previous studies about the pipe-flowing contactors were cited in this paper,12,13 since both the membrane dispersion process and the pipe-flowing process are single-stage heat-transfer processes. Figure 7 shows that the volumetric heat-transfer coefficients in the microdispersed contactor are about 15-20 times higher than that in the pipe-flowing contactors. The heat-transfer performance is significantly enhanced by the microdispersed droplets. This is important for developing new heat-exchange technology in microreactors by removing reaction heat directly with inert solvents. Heat-Transfer Model of the Microdispersion System. From the above experimental results it is found that the volumetric heat-transfer coefficient is mainly determined by the crossflowing velocity and the oil volumetric fraction. The crossflowing velocity provides shearing force to form the micro-
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9757
Figure 8. Comparison of experimental data with predicted results.
droplet, and the volumetric fraction affects the total interfacial area in the contactor. According to previous works,6,24 the shearing force in microdevices is commonly expressed by the Ca number (Ca ) uCµC/σ, where µC is the viscosity of the continuous phase and σ is the interfacial tension, as shown in Table 1), since the droplet size in the cross-flowing microdevice is determined by the balance of the shearing force and the interfacial tension. Thus, the volumetric heat-transfer coefficient is proposed to be calculated with the following semiempirical model: UV ) pCaRΦoβ
(7)
where p is a constant and R and β are the index numbers. With the experimental data, the parameters in eq 7 are regressed and the result is given in UV ) 3.7 × 106Ca1.0Φo0.69 (0.1 < Φo < 0.6)
(8)
The comparison of experimental data with the predicted results is shown in Figure 8, which shows the predicted results fit very well with the experimental data. In our previous work, the mass-transfer performance was investigated in a similar membrane dispersion minicontactor with an n-butanol/succinic acid/water system.19 The microfiltration membrane used in that work is the same as this work, and the structure of the device is similar. To examine the similarity between heat and mass transfer of the liquid-liquid microdispersed system in this minicontactor, the volumetric mass-transfer coefficient, defined by eq 9, was calculated from our previous results. KV )
M⁄V KA ) in out V (∆C - ∆C ) ⁄ ln(∆Cin ⁄ ∆Cout)
the heat-transfer performance of a liquid-liquid microdispersed system. The heat transfer between octane and water was significantly enhanced by the effects of the microdispersed scale, and 85-99% heat-transfer Murphree efficiencies were obtained in this minicontactor with residence times less than 0.1 s. In comparison with the conventional pipe-flowing contactors, the volumetric heat-transfer coefficient of the microdispersed system was 15-20 times higher than that of conventional dispersed system. So it is highly promising for developing new heat exchange technology in microreactors. Furthermore, the effects of operating parameters on the volumetric heat-transfer coefficients in the minicontactor have been investigated, and it was found that the cross-flowing velocity and the oil volumetric fraction were the main factors to affect the heat-transfer coefficient. Accordingly, a semiempirical equation was established to correlate the volumetric heat-transfer coefficient with the Ca number and the oil volumetric fraction. Furthermore, the similarity of the heat and mass transfers was demonstrated by comparing the volumetric heat-transfer coefficient with the volumetric mass-transfer coefficient in the microdispersed process. Acknowledgment We gratefully acknowledge the support of the National Natural Science Foundation of China (Grants 20490200 and 20525622) and the National Basic Research Program of China (Grant 2007CB714302) for this work.
(9) Literature Cited
where K is the total mass-transfer coefficient (m/s), M is the mass-transfer rate (mol/s), ∆Cin is the concentration difference between the inlet fluids (mol/m3), and ∆Cout is the concentration difference between the outlet fluids (mol/m3). The volumetric mass-transfer coefficient was correlated by a semiempirical model such as eq 8 with the same index number for the Ca number and the oil volumetric fraction. A comparison of experimental data with the predicted results is given in Figure 9, which shows that the predicted results fit well with the experimental data. The semiempirical equation of the volumetric mass-transfer rate is given in eq 10. Comparing eq 10 with eq 8, one can conclude that heat and mass transfer in liquid-liquid microdispersed systems are similar. KV ) 3.0 × 105Ca1.0Φo0.69 (0.2 < Φo < 0.75)
Figure 9. Comparison of experimental data and predicted results for the volumetric mass-transfer coefficient.
(10)
4. Conclusion In this work a microdispersion contactor named a membrane dispersion minicontactor was applied for the first time to test
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ReceiVed for reView April 5, 2008 ReVised manuscript receiVed July 26, 2008 Accepted September 8, 2008 IE8005484
