(282a) Hybrid Membrane-Cryogenic Distillation Air Separation Processes for Oxygen Production

Hybrid separation processes are of commercial interest for many applications. Air separation is a prime target because cryogenic distillation, adsorption, and membranes are used commercially, and increasing demand for oxygen for alternative energy applications will increase construction of new plants. Hybrid adsorption-cryogenic distillation processes have been developed that potentially have lower operating and capital costs than their conventional counterparts [1].

In this paper, a novel membrane-cryogenic distillation air separation process for oxygen production is developed. This process uses a membrane gas permeator to increase the oxygen concentration of the feed to 23.5 % or less before the main air compressor of the cryogenic distillation plant. The reason for a 23.5% limit on oxygen is that above this concentration more expensive materials of construction are required. Although 23.5 % is a low concentration, it represents a more than 11 % reduction in gas flow rate. This reduction in flow rate results in reduced power requirements for compression and reduced sizes and costs of the downstream equipment. The hybrid approach can be applied either to new designs, or for retrofitting and debottlenecking existing plants. Achieving the 23.5% concentration with very low energy and reasonable membrane areas is surprisingly difficult with currently available membranes. To reduce the permeate concentration to 23.5% oxygen either a very high cut or a bypass stream is required. The use of a bypass stream and power recovery from the retentate proved to be the best configuration. High flux, low selectivity membranes resulted in low membrane areas, but the power requirements were greater than the savings from reduced gas flow rates. Highly selective, low flux membranes had low power but huge areas. None of the currently available commercial membranes have a combination of high flux and high selectivity that appears to be economical for this application. Carbon sieve membranes, on the other hand, have a combination of high flux and high selectivity that appears to be viable if they can be made with thin active layers and sell for a reasonable cost.

[1] Wankat, P. C.; Kostroski, K.P. Hybrid Air Separation Processes for Production of Oxygen and Nitrogen. Separ. Sci. Technol. (in press).