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Physicochemical Characteristics of Hollow Fiber Structured Packings in Isopropanol/Water Distillation Zhihong Yang,† Guoliang Zhang,*,† Feini Liu,† and Qin Meng‡ College of Biological and EnVironmental Engineering, Zhejiang UniVersity of Technology, Hangzhou 310014, People’s Republic of China, and College of Materials Science and Chemical Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China
Three kinds of hollow fiber membrane contactors, polyether sulfone (PES), polysulfone (PS), and polypropylene (PP), were applied in IPA/water distillation to check how the material and structure of membranes would affect the performance. The Hansen solubility parameter (HSP) was calculated to analyze the compatibility between polymers and IPA in operation. It was found that a hydrophobic asymmetric microporous membrane coating thin dense layer on the liquid side with good compatibility worked better in the process, while the membrane with interconnected porous morphology was more favorable for decreasing Rm and more sensitive to vapor load, which might worsen operation stability. Even a slight wetting ( 1.6 cm/s. It is clear that HTU is mainly affected by a, υL, and KL (see eq 3). The comparison of the area per volume (a) among PES25, PS25, and PP100 is straightforward from Table 2. Because AL is nearly the same for the same mixture separation under equal heat rate, we speculate on that the difference of υL is slight. The difference of a and υL among these modules can be figured out clearly, but the comparison of KL is complicated. A further elaborate discussion should be essential to KL. It is worth quoting that an attractive low HTU as low as 0.1 m can be obtained by every module. From eq 2, we expect that the reciprocal of the number of transfer units (1/NTU) should be proportional to the liquid
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Figure 4. The number of transfer units (NTU) versus liquid flow. The results shown at total reflux support the prediction of eq 2. The overall mass transfer coefficients Kx for three modules are in the order of PES > PP > PS.
velocity L. This expectation is supported by the data in Figure 4, which means that we can obtain higher distillate purity in hollow fiber column by reducing the column’s flow rate. The overall mass transfer coefficients Kx for PES25, PS25, and PP100 can be calculated as 3.02 × 10-5, 2.34 × 10-5, and 2.43 × 10-5 mol/(cm2 · s), respectively. The KL values of 0.0026, 0.0019, and 0.0020 cm/s were then given by Kx/cL. However, some phenomena emerge contradictorily to Figure 3 when we have a back substitution with KL to eq 3. The HTU of PES25 is a little higher than PP100. That is maybe due to our rough estimate of υL. Because we cannot ensure precisely the same and complete thermal isolation when constructing these modules and during operations, there is a practical difference of υL, which might lead to the diversity of apparatuses causing such conflicts in the process. In Table 2, we find some difference with what Hansen17 once pointed out that when the HSPs of the barrier film and a solvent were very different, then the driving force for mass transfer was reduced. Among the three polymers, ∆S-M of PP (16.86 MP1/2) is the biggest, while its Kx is greater than that of PS. Another abnormality is that PS showed the most sensitivity to experimental conditions and presented a violent fluctuation, while its ∆S-M (13.86 MP1/2) is not the biggest. All these indicate that what Hansen mentioned is also coupled with constraints, although our previous work certainly fit this theory when PES25 modules were used for the separation of different alcohol-water solutions. The ∆S-M of PES and methanol, ethanol, and isopropanol decreased gradually with the value of 15.24, 12.27, and 10.26 MP1/2, respectively (see Table 1). It can well interpret the ascending order of Kx and the relative sensitivity of HTU changing with heating rate in Figure 3. The cause of difference between PP and PS possibly arises from the different microstructure of hollow fibers, which might lead to the wetting behavior. Figure 5 exhibits the morphology of different hollow fibers used in the experiments. Polymeric membranes generally undergo a trade-off limitation between mass transfer and thickness: as thickness increases, mass transfer decreases, and vice versa. Generally, the porous structure of membranes is helpful for mass transfer, maybe that is why the difference (in 40%) of Kx contrasted sharply with membrane thickness discrepancy (in about 500%) among different hollow fibers. On the other hand, the porous structure will be more liable to be wetted, which might lead to the swelling, deformation, and the irregular channel of shell side. All these will bring disadvanta-
geous influences on operation stability, especially obvious under high Cs. Therefore, PS showed poor operation stability and the smallest Kx caused by the thick wall and interconnected porous structure. Although the entire PES membrane is nearly filled by finger-like pores, which are very helpful for reducing membrane resistance, the dense thin coating (7 µm PDMS) on its inner surface can withstand wetting effectively. Therefore, it presents the best mass transfer and works better than PP. As compared to conventional intimate contactors, hollow fibers separate two fluids and can avoid many disadvantages usually faced in two-phase fluids separation at present. While due to the extra membrane resistance, there is a compromise of mass transfer, the fraction of individual resistance to overall resistance in Figure 6 figures out that the membrane resistance Rm is not the rate-controlling step for mass transfer if supposed in the no-wetted case. The proportion of PP is even less than 6%, a seemingly negligible effect. Maybe that is why the mass transfer model in many gas/liquid contacting processes can be simplified as liquid resistance dominant. However, keeping absolutely dry looks more like an idealization. In IPA/water separation, the temperature on liquid side is about 335 K, and the surface tension of IPA will be as low as 18.5 × 10-3 N/m. As a result, the hollow fibers are prone to being wetted even though all of them are strongly hydrophobic. Only fully wetted case with operation at high vapor velocity is addressed here because the wetting degree of membrane is always hard to determine exactly and the high vapor velocity that indicates high productivity is especially attractive in conventional distillation. According to Figure 6, the Rm/Rt of PES25 and PS25 is nearly at 70%, and PP100 can reach about 30%. As a consequence, Figure 7 presents that even a slight wetting ( PP > PS, which is in the same order of HTU. Maximum value at the lower flow rate, which is about 85% (mass fraction), was obtained by PES25, in the aftermath of better mass transfer and prolonged residence time. Because of the biggest area per volume (10.01 cm2/cm3) and the thinnest wall (0.0037 cm), PP100 with the worst compatibility performed more excellent than PS25, although these two hold comparable KL. From the results above illustrated, we tend to cognize that membrane material and morphology do have an impact on performance of distillation and are enthusiastic to see how the theoretical overall mass transfer coefficients fit to the experimental values for three kinds of hollow fibers (Figure 9). In comparison, the experimental overall mass transfer coefficients KG are higher than the theoretically predicted ones for PES and
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Figure 5. The SEM pictures of the across wall structure for different hollow fibers (PES/PS/PP, from left to right). There is relevancy between mass transfer and thickness for polymeric membranes. The morphology difference of hollow fibers might cause different wetting behavior, which affects operation stability and Kx.
Figure 6. Effect of individual resistance on overall mass transfer resistance. In no-wetted case, contribution of membrane resistance is not obvious. As hollow fibers are getting wetted in the operation, it grows up to be the controlling part.
PS modules. This has quantitatively the same consequences as the previous mentioned by Yang6 and Koonaphapdeelert;21 however, both of them did not provide a clear explanation. Maybe the correlations for mass transfer in the shell side of membrane contactors should be corrected at a particular condition. Also, such difference may be caused by different degrees of back mixing in different column geometries and membrane characteristics. The most-striking phenomenon is that the value consequence of KG in the PP module is inverse, and it also has been demonstrated by Wickramasinghe9 who attributed this to the overestimation of correlation at low velocity. Otherwise, the membrane used in former studies is at least 3 times thicker than PP hollow fibers; what inspires us is that the imprecise measurement of ε/τ (see eq 8) may lead to underestimation of KG. The affection of measurement discrepancy for KG should be highlighted as the thickness increases. Meanwhile, the tendency to wetting and deforming of different hollow fibers are also possible causes, and further investigations are necessary and essential.
Figure 7. The mass transfer coefficients in the membranes decrease with wetting fraction. During the wetting process, each curve on behalf of three membranes is defined by km ′ ) (1)/((H)/(km,w) × fw + (1)/(km,d) × (1 fw)).
Figure 8. Distillate concentration versus Cs. When vapor flow rate is getting lower, the distillate concentration in operation tends to be higher.
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Figure 9. The overall mass transfer coefficient KG for different hollow fiber modules. The curves show how theoretical overall mass transfer coefficients (filled) fit to the experimental values (unfilled) in various modules.
Figure 10. How much energy consumption can we decrease? The hybrid process consisting of distillation followed by PV leads to a sharp decrease of the energy costs of about 48%.22 Therefore, novel MC combined with PV should have great advantage in economics.
5. Concluding Remarks In the present study, the effects of hollow fibers morphology and the compatibility of membrane material with IPA on separation efficiency and operation stability have been focused. High porous substrate membrane coating thin dense layer with good compatibility will facilitate mass transfer and help the distillation process to operate more stable. Finger-like porous PES substrate with a 7 µm PDMS coating performs with the best separation efficiency and good operation stability. When membrane was wetted in the process, the resistance has an increase of at least 10 times. Judgment based on material and structure of membranes can afford a brief principle, which is convenient to the primary selection of packing for different mixtures in industrial apparatus design. As before, all three kinds of modules designed here can successfully run at the vapor velocity higher than flooding velocity 5-6 times. An attractive value of HTU lower than 0.1 m was also found in this work.
Although the IPA/water azeotrope (87.8%) still cannot be broken through by these short 30 cm hollow fiber columns in the experiments, a process integrated pervaporation (PV) with our modules is expected to straightly separate azeotropes and deserves further practice aiming at saving energy consumption. Even if the membrane contactor might have longitudinal temperature drops and inefficient use of downstream surface area, the hybrid process can make two segments illuminate each other, such as what was conducted by V. V. Hoof.22 A coupling process (distillation + PV, see Figure 10) can save about 48% energy consumption as compared to conventional azeotropic distillation when it is used for the separation of IPA/water mixture. If our novel membrane contactors (MC) are combined with PV, how much energy consumption will be cut in this novel dual membrane process? Even if such a combination is only a preliminary idea and the interrelated economic comparison is not being carried out, a further cost decrease in operation and investment will be optimistically foreseeable.
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Acknowledgment We benefited a lot from conversation with Dr. E. L. Cussler while visiting CEMS, UMN. This work was primarily funded by the Research Fund of the National Natural Science Foundation of China (Grants 20776133 and 20976163). Other financial support came from the Zhejiang Provincial Bureau of Science and Technology, China (Grant 2008C13014-2). Nomenclature kG ) mass transfer in vapor side (cm/s) km ) mass transfer in the membrane (cm/s) kL ) mass transfer in liquid side (cm/s) KG ) overall mass transfer in vapor side (cm/s) KL ) overall mass transfer in liquid side (cm/s) di ) inner diameter of fibers (cm) do ) outer diameter of fibers (cm) dh ) hydraulic diameter of shell side (cm) DG ) the diffusion coefficient of gas (cm2/s) DL ) the diffusion coefficient of liquid (cm2/s) υG ) vapor velocity (cm/s) υL ) liquid velocity (cm/s) l ) the module length (cm) ν ) the kinematic viscosity of vapor (cm2/s) AL ) cross-section area of liquid flux (cm2) AG ) cross-section area of vapor flux (cm2) Sh ) Sherwood number (dimensionless) Re ) Reynolds number (dimensionless) δD ) dispersion interaction (MPa1/2) δP ) polar interaction (MPa1/2) δH ) hydrogen-bonding interaction (MPa1/2)
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ReceiVed for reView February 27, 2010 ReVised manuscript receiVed August 27, 2010 Accepted October 1, 2010 IE100437E
