Automatic gas chromatographic evaluation of the performance of membranes used for permeation processes

Abstract

The separation of liquid mixtures by permeation through membranes frm a liquid stream to a gas phase is influenced by the permeability and the selectivity of the polymeric layer. These propeties depend on various parameters: time, temperature, swelling of the membrane in water and in organic solvents, compositiion of the permeating mixture, etc. The evaluation of a given membrane is accomplished by measuring the distribution of the compounds to be separated over the liquid and the gas phase. This procedure is time-consuming because many analyses of both phases must be performed in order to investigate the effect of the various parameters. Full automation of the analysis and of the evaluation of the results has been achieved by connecting an external events module to a digital integrator in order to connect an automatic gas-sampling valve and a small-volume liquid-sample valve to the carrier gas source and to the permeating streams, respectively. Each valve is automatically filled with samples coming from the liquid or gas circuit and activated to deliver the sample to the gas chromatographic unit, through a low-dead-volume injector, suitable for on-column introduction of both liquid and gaseous samples. The gas chromatographic columns were packed with various amounts of porous polymer-bead stationary phases (Porapak or Chromosorb “Century Series”) in order to separate water efficiently fromthe organic components of the permeating mixture (alcohols, glycols). A thermal conductivity detector was used. Computer programs were developed for automatic evaluation of the quantitative results, recording of the data, calculation of the retention-index values, plotting of the time—composition graphs, and calculation of the membrane selectivity as a function of time, temperature and composition of the permeating mixture. Low-cost personal computers are suitable for this applicatioin. With minor modifications and by using other columns and detectors, the system can be used for automatic analysis of any combination of liquid or gas streams.