Carrier-facilitated membrane separation systems

Mass transfer rates attainable In menbrane separation devices, such as gas permeators or dlalyzers, can be limited by solute transport through the menbrane. The addition Into the menbrane of a mobile carrier species, which reacts rapidly and reversibly with the solute of Interest, can Increase the membrane s solute permeability and selectivity by carrier-facilitated transport. Mass separation is analyzed for the case of fully developed, one-dimensional, laminar flow of a Newtonian fluid in a parallel-plate separation device with reactive menbranes. The effect of the diffusion and reaction parameters on the separation is investigated. The advantage of using a carrier-facilitated membrane process is shown to depend on the wall Sherwood number, tfrien the wall Sherwood nunber Is below ten, the presence of a carrier-facilitated membrane system is desirable to Improve solute separation. 

To-be-developed industrial membrane separation technologies Carrier facilitated transport Membrane contactors Piezodialysis, etc. Major problems remain to be solved before industrial systems will be installed on a large scale... 

The rising need for new separation processes for the biotechnology industry and the increasing attention towards development of new industrial eruyme processes demonstrate a potential for the use of liquid membranes (LMs). This technique is particularly appropriate for multiple enzyme / cofactor systems since any number of enzymes as well as other molecules can be coencapsulated. This paper focuses on the application of LMs for enzyme encapsulation. The formulation and properties of LMs are first introduced for those unfamiliar with the technique. Special attention is paid to carrier-facilitated transport of amino acids in LMs, since this is a central feature involved in the operation of many LM encapsulated enzyme bioreactor systems. Current work in this laboratory with a tyrosinase/ ascorbate system for isolation of reactive intermediate oxidation products related to L-DOPA is discussed. A brief review of previous LM enzyme systems and reactor configurations is included for reference. 

In this paper, we analyze mass transfer in a parallel-plate mass exchanger with reactive membranes. We consider the case of fully developed, one-dimensional laminar flow between two membranes. Equilibrium carrier-facilitated transport of the solute takes place in the membrane phase. The effect of the diffusion and reaction parameters of the carrier-facilitated system on solute separation is Investigated. 

In the search for improved metal ion separation schemes, considerable attention has been focused upon the use of liquid membranes (7,2). In a liquid membrane system, a liquid or quasi-liquid phase separates two other liquid phases in which the membrane is immiscible. In the most common arrangement, a hydrophobic liquid phase, such as chloroform or toluene, separates two aqueous phases. If chemical species have some solubility in the membrane, they may pass from one aqueous phase through the membrane into the second aqueous phase by simple diffusion. More frequently, a carrier molecule which resides in the membrane provides carrier-facilitated transport of metal ions across the membrane. Compared with simple diffusion, the carrier-facilitated transport is usually more efficient and selective. 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

Example 9.14 Nonisothermal facilitated transport An approximate analysis of facilitated transport based on the nonequilibrium thermodynamics approach is reported (Selegny et al., 1997) for the nonisothermal facilitated transport of boric acid by borate ions as carriers in anion exchange membranes within a reasonable range of chemical potential and temperature differences. A simple arrangement consists of a two-compartment system separated by a membrane. The compartments are maintained at different temperatures T] and T2, and the solutions in these compartments contain equal substrate concentrations. The resulting temperature gradient may induce the flow of the substrate besides the heat flow across the membrane. The direction of mass flow is controlled by the temperature gradient. 

It is usually assumed that the association-dissociation reactions occur at the membrane interfaces. Selectivity of this facilitated transfer is based on the different affinity of the carrier for the components of the source phase. In artificial liquid membrane systems, crown ethers are widely used as carriers to separate metal ions. Most of the ethers are photoresponsive and their structure and resulting metal-binding ability can be altered under irradiation. 

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

In bench-scale tests, using hoUow-fiber membrane as support and a carrier concentration of 2 M the ethylene permeance was 4.6 X 10 barrer/cm with an ethylene partial pressure of 65 psia, while the selectivity C2H4/C2H6 was about 240. Same tests were carried out for separation of propylene from propane. The selectivity obtained was greater than 100 but this result was confirmed only at bench scale. In fact, in the large pilot system, the selectivity and flux dechned over some weeks due to loss of solvent and carrier and to the necessity of remove hydrogen from the feed gas to prevent reduction of Ag f carrier. Despite the result, this remains the first study on the use of facilitated transport membrane for gas separations on a pilot scale. 

Immobilized Liquid Membranes. Many of the early studies on liquid membranes dealt with Immobilized liquid membranes. Therefore, a large amount of modeling describes these systems. Also, many of the modeling efforts have focused on facilitated transport where a nonvolatile carrier Is present In the membrane. The reaction scheme most often used Is = AB where A is the solute to be separated, B Is the nonvolatile carrier, and AB is the carrier-solute complex. 

Since IBM s offer greater stability than ILM s and greater selectivity and permeability than PM s, it would be useful to be able to model transport processes in these materials and to predict the effectiveness of facilitated transport based on relevant physical properties (RPP). Although it may be necessary to modify the model developed for ILM s in order to completely describe transport processes in IBM s, it is likely that moat of the same RPP s of the system will be Important. The purpose of this section is to point out that measurement of RPP s in IBM s, especially permselective IBM s, may be difficult. Although problems with model development and property measurement exist, carrier Impregnated IBM s can produce rapid and selective separations of gas mixtures. Way and co-workers have incorporated the monoprotonated ethylenediamlne cation into Nafion membranes to achieve the separation of carbon dioxide from methane (25). 

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