Purification of chlorine gas with membranes -: an integrated process solution for magnesium production

Process solutions for membrane purification of chlorine gas are presented, based on experimental results from a comprehensive study of permeability and selectivity of the gases Cl-2, O-2, N-2 and H-2 as well as their durability in an aggressive gas environment. The objective is to establish a simple membrane scheme for purification of the chlorine gas coming from electrolysis in the production of magnesium. Removal of oxygen from this gas is crucial to avoid formation of H2O when the pure, dry chlorine gas reacts with hydrogen further down the process line. Three materials were chosen: a polydimethylsiloxane (PDMS) membrane, a carbon molecular sieve (CMS) membrane and a surface modified glass membrane. Separation experiments with mixed gas (Cl-2-O-2) were carried out for PDMS and the glass membrane over a wide range of chlorine compositions (5-95% Cl-2). In the rubbery PDMS material permeabilities of the mixtures deviated only little from those previously measured for pure gas - maintaining the pure gas selectivities. For the glass membrane, where separation was based on a surface selective flow (SSF), a slight decrease in selectivity was found mainly due to reduced Cl-2 permeation. Mixed gas experiments were not carried out for the CMS membrane since the mechanism for the separation is molecular sieving. The Cl-2 will then essentially be withheld; O-2 and N-2 will permeate. Based on the pure gas permeation data obtained for these materials, simulations were performed for an optimized membrane scheme for purification of Cl-2. The process solutions, which are suggested, are optimized with respect to minimum membrane area, minimum Cl-2 in waste stream and minimum energy requirements for recompression and recycling of the permeate. The solutions vary with respect to process conditions (temperature), membrane material, required permeation area and energy requirement. Preferred process temperature and the price of membrane material will be crucial factors for decision making. The findings related to process conditions and suitable materials for membrane purification of chlorine gas are general and applicable to various kinds of gas streams containing Cl-2. (C) 2001 Elsevier Science B.V. All rights reserved.