Membrane Column Sections

FIGURE 9 1 (a) A continuous membrane column, (b) Membrane column section break down [1]. 

FIGURE 9.2 A generalized membrane column section (MCS). Material moves through the membrane from the Yd pressure (ttr) retentate (R) side to the low pressure (np) permeate (P) side [1]. 

Earlier papers on the continuous membrane column (28,29) have discussed the separation of CO2-N2, CO2-O2 and O2-N2 (air) mixtures in stripper, enricher and total column units composed of 35 silicone rubber capillaries. A characterization of the membrane column using a membrane unit concept (analogous to transfer unit concept — HTU, NTU) has also been presented. The purpose of this paper is to present some new data and discussions on the extended study of continuous membrane column. Specifically, the topics of multicomponent separations, Inherent simulation difficulties, composition minima in the enriching section, variation of experimental parameters, and local HMU variation along the column will be covered. 

Usually in an enricher or the enriching section of the membrane column, the more permeable component is steadily concentrated from the feed inlet to the compressor. However, some of the results show that the shell-side and even the tube-side composition profiles can pass through a minimum. Note the experimental data in Figures 7 and 8. In these cases the feed flow is relatively slow and reflux action, rather than bulk flow, is predominant. Figure 8 Illustrates that a composition minimum can also occur dinring operation of the total column when the residue flow rate from the enriching section is too slow. 

The set of Equations 32 -r 37 has to be solved numerically. For the special case of total reflux, however, an analytical solution for the concentration profile along the membrane column can be derived if the pressure losses are neglected. In the case of total reflux, for every section of the column (Figure 6.21) ... 

Case 1 would represent, for example, the terminating sections (e.g., rectifying) of an entire membrane column configuration. Case 2, alternatively, would be a general MCS placed anywhere within the membrane column or cascade. 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

In order for solvent and solution to be in equilibrium in an apparatus such as that shown in Figure 3.2, the solution side must be at a higher pressure than the solvent side. This excess pressure is what is known as the osmotic pressure of the solution. If no external pressure difference is imposed, solvent will diffuse across the membrane until an equilibrium hydrostatic pressure head has developed on the solution side. In practice, to prevent too much dilution of the solution as a result of the solvent flow into it, the column in which the pressure head develops is generally of a very narrow diameter. We return to the details of osmotic pressure experiments in the next section. First, however, the theoretical connection between this pressure and the concentration of the solution must be established. 

Air analysis for some of the individual pesticides of this class has been published by NIOSH. These pesticides include mevinphos, TEPP, ronnel, malathion, parathion, EPN, and demeton (NIOSH Methods 2503, 2504, 1450). In general, pesticides in air may be trapped over various filters, such as Chro-mosorb 102, cellulose ester, XAD-2, PTFE membrane (1 pm), or a glass fiber filter. The analyte(s) are extracted from the filter or the sorbent tube with toluene or any other suitable organic solvent. The extract is analyzed by GC (using a NPD or FPD) or by GC/MS. The column conditions and the characteristic ions for compound identifications are presented in the preceding section. Desorption efficiency of the solvent should be determined before the analysis by spiking a known amount of the analyte into the sorbent tube or filter and then measuring the spike recovery. 

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