Novel Hydrophobic Hollow Fiber Membrane Structured Packing And Its Performance In Distillation Separation

The hollow fiber structured packing in distillation was studied from polymer chemistry, contactor structure and operation performance with different organic solution systems. The methods for polymer selection, membrane preparation and surface modification were discussed in detail. Four types of polyethersulfone(PES) and polysulfone(PS) membranes were prepared and used in the evaluation of swelling effect caused by solvent and high temperature in distillation. The suitable membrane, which was PES-2, was selected from two types of different micro-porous membranes and used in purification of methanol/water, ethanol/water and IPA /water systems respectively. The flow direction and packing fraction in membrane contactors were also investigated. Baffles in the shell side were found to have good effect on separation and therefore set in counter flow operation of parallel mode. The mass transfer in separation, including the analysis of resistance on liquid, gas and membrane side respectively, and overall mass transfer coefficient were finally analyzed.First, chemical properties of polymers, membrane prepared and modification layer inside fibers were investigated. Results showed that the miroporous PES-2 and PS-1 membrane had better permeability and better developed finger cavities than PES-1 and PS-2 membrane with dense surface layer, and therefore more suitable for hollow fiber distillation. All the four types of membranes were of good thermal resistance and made by phase inversion technology. After effective surface modification using polydimethylsiloxane(PDMS), which is often with high permeability but low selectivity, the PES-2 and PS-1 membrane were more hydrophobic and contained fewer cavities in the surface of anisotropic membrane. It was also found that swelling from solvent and high temperature had an obvious effect on changes of surface layer, sub-layer of membrane and finger hole inside membrane which might hinder mass transfer. In the swelling process, the sub-layer of miroporous PES-2 and PS-1 membrane changed evenly while dense PES-1 and PS-2 membrane had a more irregular structure in the same position. The wall outside and finger cavities inside PES-2 and PS-1 membranes became thicker after solvation. The porosities of both miroporous PES-2 and PS-1 membrane decreased while those ofdifferent methanol, ethanol and isopropanol/water system. Both of them obtained a higher distillate concentration than 0.55 in molar fraction. When the vapor flowed faster, both two membranes had a distillate of lower concentration, in which PES-2 membrane faced more changes. At the low and high vapor velocity, the PES-2 membrane got an even higher NTU and an even lower HTU than PS-1 membrane, which presented higher separation efficiency. Therefore, PES-2 membrane contactor was more suitable for separation than PS-1 membrane. In the distillation of methanol, ethanol and isopropanol-water solutions using PES-2 membrane contactor, the fibers were non-selective with coating and had little resistance to mass transfer. Because liquid flowed inside the lumens of the fibers and vapors flowed counter currently outside fibers, the distillation process could avoid flooding effectively even at very high flows. As a result, an attractive low value of the height of mass transfer unit (HTU) less than 0.1m could be obtained at total reflux. Although the distillate concentration dropped as the heating rate rose, as noted before, the decreasing rate of concentration and HTU in the same module seemed to change inversely with the molecular weight of alcohols. Compared with normal structured packing such as ceramic intalox, the ordinate data of common capacity factor of hollow fiber column calculated on the Eckert version of GPDC correlation were 8.5-12.9 times higher above flooding. All these offered the possibility of distillation with better, more productive separations.Third, the flow direction and structure of membrane contactors were paid more attention to enhance separation. The parallel flow mode was first chose in separation due to the stronger driving force of concentration difference. After comparison, a parallel flow mode with liquid flow inside the lumens of the fibers while vapor counter flow outside along baffles was selected for gas/liquid membrane contactors. Two baffles were installed on the shell side of membrane contactors, which had a round shape with a demi-lune hole. Results showed that both baffled and unbaffled membrane contactors gave better, more productive separations than traditional packing in distillation, such as excellent Sulzer Gauze BX structured packing. The baffled membrane contactors performed better than unbaffled ones, especially at high vapor velocities. The minimal HTU of membrane contactor with baffles could reach as low as 5.4 cm, and almost all the contactors could work well above the limit whereflooding normally occurs in conventional cases. Further, like most other cases such as gas/liquid membrane contactors, the overall mass transfer coefficients K of this novel membrane contactor in distillation decreased while fiber packing fractions became bigger, which presented a strong relationship between shell mass transfer and fiber packing fraction. Compared with membrane extraction, membrane contactor in distillation had a 5-10 times larger K value.The mass transfer in hollow fiber distillation was finally analyzed in detail. Compared with theoretical estimation from earlier studies, it was found that the liquid side was mostly responsible for the total resistance of mass transfer, which was similar to the conventional distillation process at low concentration, although mass transfer resistance in the membrane and the vapor couldn't be negligible. When F-factor became bigger in separation of three different alcohol/water system, the mass transfer resistance in the membrane tended to increase and the mass transfer resistance in the liquid side almost kept still, but the vapor side resistance decreased. There is a big gap between the experimental Kexp value and the calculated Kcalc value from theory, the former was almost two times bigger than the latter. Since the baffled module showed better performance in distillation, it seemed that the theoretical correlations investigated both in the shell and tube side should be revalued in our process. When Graetz number was becoming smaller, the Sherwood number from experiments tended to be closer to that from Leveque correlation, which was obtained from heat transfer research several decades ago and showed good consistence with other processes. Like what some researchers predicted in membrane contactor studies elsewhere, the coefficient value of 1.62 in Leveque correlation should be treated bigger in this study. From this point of view, more progresses have to be made before theoretical modification.