Facilitated-transport membranes, observation

An alternative method for the preparation of facilitated transport membranes is the subject of the first paper in this section. Way and Noble (113) report a study of H,S facilitated transport in reactive ion exchange membranes. The use of a perfluorosulfonic acid lEM as a support for organic amine counterions avoids problems of solvent and carrier loss often encountered with ILMs. High carrier loadings of greater than 8 M in the lEMs were attained which helped to account for the high facilitation factors of 26.4 which are observed at low partial pressures. An analytical model predicted facilitation factors in excellent agreement with the experimental data. Separation factors for HjS over CH., of 792 to 1200 are reported. Implications of the mathematical model for industrial applications are also discussed. 

In this study, monopositive ethylene diamine (EDA) Ions were exchanged Into perfluorosulfonlo acid (PFSA) lonomer films to prepare facilitated transport membranes. The flux of HjS was measured with and without carrier present at ambient conditions as a function of HjS mole fraction In the feed gas stream. The selectivity of these membranes was determined by simultaneous measurements of HgS and CH fluxes from binary mixtures as a function of composition. Reaction equilibrium models were derived to predict the observed experimental data. 

The permeability of a facilitated transport membrane Is a function of the HjS partial pressure difference across the membrane. Consequently, use of the log-mean mole fraction difference accounts for the changes observed In H S mole fraction between the Inlet and outlet of the feed and sweep gas streams. 

This model suggests that //> is linearly proportional to pi, kj, Cb, and and logarithmically proportional to K. Furthermore, it increases with a decreasing po, which is an intrinsic advantage of facilitated transport membranes, as observed experimentally. The linear increase in / F relative to Cb and 2 represents the importance of both the carrier concentration and the releasing-reaction rate between the carrier and the small molecule. Note that pd can be empirically obtained from the slope of a plot of /F-1 versus (l/poX2 k, CC Xln(l+/i p )/po) [48,49]. 

The use of facilitated transport membranes for gas separation was first introduced by Ward and Robb [54] by impregnating the pores of a microporous support with a carrier solution, and a separation factor of 1500 was reported for CO2/O2. These membranes, or supported liquid membranes (SLMs), are discussed by several, and initially very good separation properties are observed [55-58]. They are however known to have serious degradation problems like loss of carrier solution due to evaporation or entrainment with the gas stream, and the complexing agent (carrier) can be deactivated. These problems have restricted further development... 

Third, on a new design, the addition of the membrane separation unit can lead to a reduction of the required number of theoretical stages, lowering thus the size of the distillation column. This trend was already observed in a former paper which demonstrated that the addition of relatively small area of a silver-based facilitated transport membrane (several dozen square meters) to a small capacity propylene/propane distillation column was leading to a reduction of the required number of trays from 135 to less than 105 (feed flow = 2.78 mol s propylene feed content = 0.44 mol mol propylene distillate content = 0.99). 

Experimentally, it has been observed that many substances are transported across plasma membranes by more complicated mechanisms. Although no energy is expended by the cell and the net flux is still determined by the electrochemical potential, some substances are transported at a rate faster than predicted by their permeability coefficients. The transport of these substances is characterized by a saturable kinetic mechanism the rate of transport is not linearly proportional to the concentration gradient. A facilitated mechanism has been proposed for these systems. Substances interact and bind with cellular proteins, which facilitate transport across the membrane by forming a channel or carrier. The two basic models of facilitated diffusion, a charmel or a carrier, can be experimentally distinguished (1,2). 

An alternative approach to solving stability problems with ILMs is presented by Bhave and Sirkar (114). Aqueous solutions are immobilized in the pore structure of hydophoblc, polypropylene hollow fibers by a solvent exchange procedure. Gas permeation studies are reported at pressures up to 733 kPa with the high pressure feed both on the shell and lumen sides of the laboratory scale hollow fiber permeator. No deformation of the hollow fibers is observed. Immobilizing a 30 weight % KjCO, solution in the hollow fibers greatly improved the separation factor, a(C02/Na). from 35.78 with pure water to 150.9 by a facilitated transport mechanism. Performance comparisons with commercial COj separation membranes are made. 

Hydrogen sulfide and methane fluxes were measured at ambient conditions for 200 um perfluorosulfonic acid cation exchange membranes containing monoposltlve EDA counterions as carriers. Facilitation factors up to 26.4 and separation factors for H2S/CH up to 1200 were observed. The HjS transport Is diffusion limited. The data are well represented by a simplified reaction equilibrium model. Model predictions Indicate that H S facilitated transport would be diffusion limited even at a membrane thickness of 1 um. 

Kumar et al. [40] reported interesting data for membranes where MCM-48 supports (pore size 2.4 mn) were modified with polyethyleneimine (PEI). They reported an N2/CO2 selectivity of 1.31 ( 293 K) in the absence of water, 17.6 ( 293 K) in the presence of water, and 1.35 ( 363 K) in the presence of water for a feed mixture of 80/20 N2/CO2 and feed pressure of 20 psi (103.4 cm Hg). In the presence of water, the size of the diffusing unit (CO2) increased due to the clustering of water molecules, which in turn reduced the CO2 diffusivity at room temperature, and hence, the PEI-MCM 48 membranes were highly N2 selective in the presence of water. This is opposite to what we and others [10, 12, 13] observe (CO2 selective membrane), and it may be due to the fact that in our case, the amine groups are readily accessible to the CO2 molecules (since they form a brush-like structure) for reactive separation whereas the PEI approach, in contrast, may be dominated by a solution-diffusion mechanism rather than reactive or facilitated transport. 

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