Facilitated transport performance

It is clear that numerous facilitated transport processes may still be set up, especially for anions, salts or neutral molecules, and that the active research in receptor chemistry will make available a variety of novel carrier molecules. Of special interest are those transport effectors, derived from coreceptors, that allow coupled transport (cotransport) to be performed. 

Carrier facilitated transport processes often achieve spectacular separations between closely related species because of the selectivity of the carriers. However, no coupled transport process has advanced to the commercial stage despite a steady stream of papers in the academic literature. The instability of the membranes is a major technical hurdle, but another issue has been the marginal improvements in economics offered by coupled transport processes over conventional technology such as solvent extraction or ion exchange. Major breakthroughs in performance are required to make coupled transport technology commercially competitive. 

Until quite recently, most of me facilitated transport results reported in me literature were obtained with supported liquid membranes held by capillarity in microporous films. The instability of these membranes has inhibited commercial application of me process. Three factors contribute to mis instability and me consequent loss of membrane performance over time ... 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Figure 11.26 Performance of a 37 m2 hollow fiber silver-nitrate-impregnated facilitated transport membrane for the separation of propylene/propane mixtures. The feed pressure was 5-13 atm the permeate was a hexane liquid sweep stream. The vertical dotted lines show when the membrane was regenerated with fresh silver nitrate solution [27]. Reprinted with permission from R.D. Hughes, J.A. Mahoney and E.F. Steigelmann, Olefin Separation by Facilitated Transport Membranes, in Recent Developments in Separation Science, N.N. Li and J.M. Calo (eds) (1986). Copyright CRC Press, Boca Raton, FL...

The first demonstration of facilitated transport of oxygen was performed by Scholander [22] using thin films of cellulose acetate impregnated with aqueous hemoglobin solutions. Later Bassett and Schultz [65] demonstrated the process with cobalt dihistidine, a synthetic carrier. The enhancements obtained in these... 

The prospects for facilitated transport membranes for gas separation are better because these membranes offer clear potential economic and technical advantages for a number of important separation problems. Nevertheless, the technical problems that must be solved to develop these membranes to an industrial scale are daunting. Industrial processes require high-performance membranes able to operate reliably without replacement for at least one and preferably several years. No current facilitated transport membrane approaches this target, although some of the solid polymer electrolyte and bound-carrier membranes show promise. 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

In the present study, a new solid facilitated transport membrane has been prepared by incorporating both fixed and mobile carriers in cross-linked PVA. Based on the membrane transport properties, we have also developed a mathematical model to study the performance of the C02-selective WGS membrane reactor. 

More recently, Teramoto et al. [62] performed experiments for facilitated transport of SO2 through a poly(vinilidene difluoride) (PVDF), used as supported membrane, containing pure water as a carrier. The permeance of SO2 at a partial pressure of 0.003 Pa was high as 1.04 X 10 barrer/cm and the best selectivity of SO2 over N2 was estimated to be more than 10000. 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

Theory. The relationship of the chemical aspects of complexatlon reactions to the performance of facilitated transport membranes Is discussed by Koval and Reyes (108). They describe a procedure which can be used to predict and optimize the facilitated transport of gases, Including measurement of the appropriate equilibrium, transport, and kinetic parameters and structural modification of the carrier to Improve the performance of the membrane. Examples of this procedure and carrier modification are given for derivatives of Fe(II) tetralmlne complexes which reversibly bind CO In nitrile solvents (118). Experimental challenges In the measurement of the appropriate properties for other membrane configurations such as reactive Ion exchange membranes and reactive polymer membranes are also discussed. 

The separation Is analyzed for the case of fully developed, one-dlmenslonal, laminar flow of a Newtonian fluid In the mass exchamge device. Parametric studies of the effects of the kinetic and transport properties are presented. The desirability of using facilitated transport membranes Is found to depend on the mass transfer resistances In the membrane. When the membrane resistance Is small, as In the case of many practical applications, the use of facilitated transport membranes Is desirable to Improve the separation performance of the device. 

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. 

An approximate analytical solution has been developed to calculate the exit concentration from a continuously recirculating facilitated transport liquid membrane system. The system is modeled as a series of SLM-CSTR pairs. The solution allows for two-dimensional transport (axial convective and radial diffusive) and laminar flow. The solution allows one to estimate the effect of a change in system variables on the operating performance. Comparison with experimental data was very good. 

Reversible complexatlon reactions have long been used to improve the speed and selectivity of separation processes, especially those Involving the separation or purification of dilute solutes (j ). Such reactions are the basis of a multitude of separation unit operations Including gas absorption, solvent extraction, and extractive distillation. When a reversible complexatlon reaction (carrier) Is Incorporated into a membrane, the performance of the membrane can be improved through a process known as facilitated transport. In this process, shown schematically In Figure 1, there are two pathways available for the transport of the solute through the membrane. The solute can permeate through the membrane by a solution-diffusion mechanism and by the diffusion of the solute-carrier complex. Other solutes are not bound by the carrier due to the specificity of the complexatlon reaction this Increases the selectivity of the process. 

In order to clarify the complexatlon reaction mechanisms, HjS transport experiments were performed using tetramethyl EDA [chemical formula (CHjljNCCHjlaNlCHjlj] as a carrier In an lEM. The HaS complexatlon reaction with EDA as well as most primary amines Is postulated to be an acid base reaction (10). Since TMEDA Is also a strong base, It should accept protons from the HaS and act as a carrier In an lEM environment. A facilitation factor of 1.93 was measured for a TMEDA lEM at an HaS feed mole fraction of 0.05. The degree of facilitation with the TMEDA membrane was much smaller than the EDA membrane ( EDA 15.8), but the data for the TMEDA lEM does support the acid-base complexatlon mechanism for HjS facilitated transport. The smaller F value for the TMEDA membrane may be due to the very low mobility of the TMEDA or because the binding between HaS and TMEDA Is so strong that the rate of the decomplexatlon reaction Is very slow. 

The separation of olefin/paraffin gas mixtures is one of the most energy-intensive processes in the petrochemicals industry, because it is mainly performed by cryogenic distillations. Membrane processes using the concept of facilitated transport have been considered as an intriguing alternative to cryogenic distillation, as they can simultaneously improve both permeability and selectivity. Silver ions incorporated in liquid membranes act as olefin... 

Lamb, J. D., West, J. N., Shaha, D. P., and Johnson, J. C. 2010. An evaluation of polymer inclusion membrane performance in facilitated transport with sequential membrane reconstitution. Journal of Membrane Science 365 256-259. 

Liquid-phase olefin separation was performed in perstraction mode at 25°C (Sungpet et al. 2001). For this to occur, it is postulated that simultaneous complex-ation of olefin-silver (I) ion-poly(pyrrole) is a necessary condition. Althongh silver is believed to be able to facilitate the transportation of certain components in this research, the researchers did not explain in detail the conditions or limitations for this facilitated transportation to occur. The researchers did not clearly state whether this facilitated transportation can only be achieved by silver (I) ions and not by any other metal ion with the same charges. Thus, there is room for future investigation on the properties of the different types of incorporated metal ions with the same charges, to facilitate the separation processes. 

Another type of membrane with high CO2 separation performances is the facilitated transport membrane. Membranes of this type are those in which an amine-based liquid supported by a solid membrane promotes a selective CO2 permeation. They exhibit generally a remarkably high selectivity and also high permeability at low CO2 pressure in the feed gas. Unfortunately, they suffer from very poor stability. For the separation of CO2, fixed carrier membranes were found to have high permselectivity without the drawback of poor stability, but with much lower permeability [13-15]. However, all membranes with amine-based carriers suffer from a strong reduction in performance with the reduction in the relative humidity in the gas mixtures, because the presence of water in the... 

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