Hydrogen cyanide recovery by membrane gas separation

Abstract

Hydrogen cyanide is an important feedstock in many chemical industries, synthesized from natural gas and ammonia. To recovery unreacted ammonia and purify the hydrogen cyanide from water and hydrogen, additional chemical separation processes are required. The conventional approach is to undertake these separations through solvent absorption; but gas separation membranes are a competitive alternative. This investigation examines the potential for membrane gas separation to replace solvent absorption in hydrogen cyanide processing. Importantly, the HCN permeability through a range of common polymeric membranes were measured and resulting HCN selectivity reported. Rubbery polymeric membranes’ permeability strongly correlated with condensability of the gases, and therefore in HCN processing the order of permeability was H2O > H2 ≈ HCN > N2. In contrast, glassy polymeric membranes’ permeability correlated with kinetic diameter of the gases, and permeability order was H2O > H2 > HCN ≈ N2. The presence of N2 in the gas stream however presented a challenge, as the similar permeability between N2 and HCN meant that separation of these two gases was difficult. A facilitated transport mechanism was developed based on metal chlorides within both glassy and rubbery polymeric membranes. The complexation between the hydrogen cyanide and the metal ion increased the concentration of HCN within the polymeric membrane and facilitated the permeation of HCN. Zinc chloride demonstrated a clear increase in HCN permeability and improved HCN/N2 selectivity for the two membranes studied. This improvement was further enhanced by the presence of water vapour, which augmented the complexation between HCN and the metal ion, achieving an order of magnitude increase in selectivity. Hence, this investigation demonstrated the potential for membrane gas separation to compete in hydrogen cyanide processing.