Gas absorption studies using microporous hollow fiber membranes

Gas absorption in contactors using microporous hydrophobic hollow fibers is being increasingly used due to their enormous surface area/equipment volume. Experimental gas absorption studies were performed with such hollow fibers in modules having primarily a shell-and-tube type arrangement for parallel flow. Absorption of pure CO{dollar}sb2{dollar}, pure SO{dollar}sb2{dollar} and their individual mixtures in N{dollar}sb2{dollar} or air were studied systematically. Two modes of operation were adopted: nonwetted (gas in the membrane pores) and wetted (absorbent liquid in the membrane pores). For each mode, absorbent flowed either in the shell side or tube side. Absorbents used were pure water, 19.5 wt% diethanolamine (DEA) in water and 0.5 M sodium sulfite in water. Models utilizing basic partial differential equations of species conservation with or without reaction were developed from first principles. For CO{dollar}sb2{dollar} absorption in aqueous DEA, reactions schemes used include those by Sada et al. (1985), Danckwerts and Sharma (1966) and Laddha and Danckwerts (1981).; Numerical simulations of the models explained the observed gas absorption rates well in most cases when the absorbent flows in the tube side for both wetted and nonwetted modes. When the liquid flows in the shell side, a fitted parameter was used as a liquid phase axial dispersion coefficient for pure gas absorption to explain the experimental behavior; the absorption rates were always considerably lower compared to the tube side liquid flow cases due to shell side channeling. A much better gas absorption performance with shell side liquid flow was achieved using a crossflow module. For the reactive absorption of pure CO{dollar}sb2{dollar} in aqueous DEA in the wetted mode shell side liquid flow, the model, however, describes the experimental behavior well in the shell-and-tube devices. Since the reactions are fast and the membrane pores are filled with the absorbent liquid allowing the reaction to go to completion in the pore itself, shell side flow behavior does not have much influence.; The K{dollar}sb{lcub}rm L{rcub}{dollar}a calculations for the absorption of CO{dollar}sb2{dollar} shows that the performance of the hollow fiber modules are much better than those of conventional packed towers. A much lower HTU is obtained using a hollow fiber module suggesting dramatic reductions in contacting lengths.