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 and 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 (Figure 1). 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. For a basis of 1.0 m3 air/s, the power requirement to compress air from 1.0 atm. to a typical distillation column operating pressure of 6.0 atm. is 322.7 kW. For the same amount of oxygen product the power requirement to compress air that has been enriched to 23.5% oxygen from 1.0 to 6.0 atm. is 287.0 KW. The difference, 35.7 kW, is the maximum power that can be used in the membrane permeator to have the membrane-cryogenic distillation system not require more energy. 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. Very high cuts are not economical because the membrane area becomes too large. The use of a bypass stream and power recovery from the retentate proved to be the best configuration (Figure 2). Abbreviated results for this configuration are given in Table 1. Note that all of the membrane systems have an optimum cut which minimizes the power. Power is also reduced by operating at as low a feed pressure as possible. On the other hand, membrane area is reduced by operating at a low cut with a higher feed pressure. High flux, low selectivity membranes (e.g., silicone PDMS membrane in Table 1) resulted in low membrane areas, but the power requirements were greater than the savings from reduced gas flow rates. Highly selective, low flux membranes (e.g., polystyrene membranes) had low power but huge areas (not shown in Table 1). The TMHFPSF composite membrane (Table 1) had reasonable power (net power < 35.7 kW), but based on capital cost estimate did not appear to be economical. A combination of high flux and high selectivity appears to be necessary to be economical for this application. Carbon sieve membranes (Table 1) 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.
展开▼
