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 an d selectivity of the gases Cl-2, O-2, N-2 and H-2 as well as their durabili ty in an aggressive gas environment. The objective is to establish a simple membrane scheme for purification of the chlorine gas coming from electroly sis in the production of magnesium. Removal of oxygen from this gas is cruc ial to avoid formation of H2O when the pure, dry chlorine gas reacts with h ydrogen further down the process line. Three materials were chosen: a polyd imethylsiloxane (PDMS) membrane, a carbon molecular sieve (CMS) membrane an d 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 ran ge of chlorine compositions (5-95% Cl-2). In the rubbery PDMS material perm eabilities of the mixtures deviated only little from those previously measu red for pure gas - maintaining the pure gas selectivities. For the glass me mbrane, where separation was based on a surface selective flow (SSF), a sli ght 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 esse ntially be withheld; O-2 and N-2 will permeate. Based on the pure gas perme ation 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, m inimum Cl-2 in waste stream and minimum energy requirements for recompressi on and recycling of the permeate. The solutions vary with respect to proces s conditions (temperature), membrane material, required permeation area and energy requirement. Preferred process temperature and the price of membran e material will be crucial factors for decision making. The findings relate d to process conditions and suitable materials for membrane purification of chlorine gas are general and applicable to various kinds of gas streams co ntaining Cl-2. (C) 2001 Elsevier Science B.V. All rights reserved.