Nonselective membranes

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective transport of one of the enantiomers of a racemic mixture. These membranes can either be a dense polymer or a liquid. In the latter case, the membrane liquid can be chiral, or may contain a chiral additive (carrier). Nonselective membranes can also... 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

The toxicity of chromium is dependent on the oxidation state of the chromium atom, with chromium(VI) being significantly more toxic than chromium(III). One of the factors believed to contribute to this increased toxicity is the greater ability of chromium(VI) to enter cells, compared to chromium(III). Chromium(VI) exists as the tetrahedral chromate anion at physiological pH, and resembles the forms of other natural anions, such as sulfate and phosphate, which are permeable across nonselective membrane channels. Chromium(III), however, forms octahedral complexes and cannot easily enter through these channels. Therefore, the lower toxicity to chromium(III) may be due in part to lack of penetration through cell membranes. It follows that extracellular reduction of chromium(VI) to chromium(III) may result in a decreased penetration of chromium into cells, and therefore, a decreased toxicity. 

Furthermore, since the membrane selected is highly selective to methanol, it becomes insensitive to the operating conditions. In other words, the permeate stream will always be pure methanol irrespective of the flowrate and composition entering the membrane unit. This is different from the nonselective membrane discussed earlier in this chapter. For this reason, one may regard the profiles in the M-RCM as actual operating profiles, since they would not vary too much uuder finite reflux conditions. 

A schematic of a reactor made from a nonselective membrane for preventing the slip of an excess reactant is shown in Figure 24.2g. The principle of this reactor was outlined before. In the particular design shown, one of the reactants (5) is continuously recirculated on one side of the membrane so that complete conversion of A can be achieved on the opposite side without any slip. We refer to such a catalytic nonselective membrane reactor without packing as a CNMR-E. When packed, it is referred to as a CNMR-P. Another nonselective... 

Due to the availability of the mentioned overviews it is not the goal of this chapter to consider the whole field of membrane reactors. Rather, the discussion below will be focused on presenting simplified and more detailed mathematical models capable of describing the performance of membrane reactors. Although there are several studies available for analyzing the combination of reaction and membrane separation (e.g. Salomon et al., 2000 Struis and Stucki, 2001 Wielandet al., 2002 Patil etal., 2005 Rohde etal., 2005) there is a need to analyze in more detail specific features of membrane reactors. The focus of this chapter will be the development and application of simplified and also more detailed mathematical models for packed-bed membrane reactors in which certain reactants are dosed over the reactor wall using nonselective membranes. This type of membrane reactor is sometimes also-called a distributor (Dalmon, 1997 Julbe et al, 2001). Despite this restricted focus of the work, most of the concepts considered should be applicable also in the analysis of other types of membrane reactors. 

Urtiaga et al. (2001) studied the parallelism and the differences in PV and vacuum membrane distillation (VMD) in the removal of volatile organic compounds (VOCs) from aqueous streams. In their study, two gas-liquid separation processes, PV and VMD, were compared in the application to the separation of chloroform-water mixtures. Both technologies include the transfer of separated compounds initially in liquid phase through a membrane to a low-pressure gas phase. The use of a solid membrane enhances the separation efficiencies. However, PV and VMD are based on different mechanisms and employ membranes of different characteristics. Selective membranes need to be used in PV processes, while the VMD process requires the use of microporous nonselective membranes. 

Membrane separations are an accepted means for separating noncondensable gases that is, gases that ordinarily condense only under low-temperature or cryogenic conditions. The technology might be in wider use if (1) better and more selective membrane materials were available and (2) the necessary mathematical representations and calculations were better spelled-out for the separations attainable. In fact, the one sometimes depends on the other. Of particular interest are ways in which separations could be enhanced using relatively nonselective membranes. 

Catalytic nonselective membrane reactor with no packing (CNMR)... 

---