Generalized Membrane Column Sections

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]. 

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. 

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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