Hybrid membrane/PSA processes

Figure 1 Flowsheet of hybrid membrane-PSA process. Steps pressurization (PR = PRi + PR2). high-pressure adsorption (HPA), co- and counter-current blowdown (CD, BD), Purge (PG) A stream enriched in less adsorbed component, B-. stream enriched in strongly adsorbed component. Pressure histories of the integrated cycle at CSS.

Hybrid adsorbent membrane - PSA process for improving H2 recovery... 

This chapter examines newly developed hybrid membrane processes for gas separation, which integrate membrane permeation with pressure swing adsorption (PSA) technology. A brief review of the theory underlying membrane and PSA processes is provided. The discussion will focus on the evaluation of the present state of the art of both processes and on the practical application of both technologies into a hybrid membrane/PSA concept. 

The concept is therefore straightforward the two membrane/PSA processes complement each other by removing diflerent components from the mixture. While adsorption removes impurities from the H2 product, the membrane takes H2 from the impurities that are retained as residue. Either because of the operational complexity of this type of integrated processes, either due to the mathematical complexity involved in the design procedures, research on feasible hybrid systems is still scarce. 

Figure 9 shows a schematic flow diagram and an example of the hybrid H2 PSA-SSF membrane concept. The fresh feed to the PSA process is SMROG. The PSA process cycle is an abridged version of the Poly-bed process with only two co-current depressurization steps, having a H2 recovery of 77.6%. The countercurrent depressurization effluent gas is fractionated. The initial part of this gas, which is richer in H2, is directly fed to a SSF membrane at a pressure of 3 bar. The H2 purge effluent gas is compressed to 3 bar and fed to the same membrane. The H2 enriched high pressure effluent gas from the membrane is recompressed and recycled as feed gas to the PSA process. This increased the overall H2 recovery of the hybrid process to 84.0% [23]. 

Figure 9. Schematic flow sheet of a hybrid H2 PSA-SSF membrane process...

Hybrid solutions composed by combining two different processes among PSA, membrane, or cryogenic systems can be adopted to enhance the performance of the process. 

The most economically attractive process is a hybrid system that combines cryogenic, PSA, and membrane units to produce 99.99% helium from dilute natural gas. The process is shown in F iire 15-22. The feed is natural gas containing 2.1% helium. The feed is first cooled to -60°F to condense the heavier hydrocarbons. Then the gas is cooled to -240°F to condense most of the methane and some nitrogen. At this point the gas contains 30-35% helium. The crude helium is then fed to a two-stage membrane unit that produces a 95% helium stream. PSA is used to upgrade this stream to Grade A purity. 

Figure 9.6 shows a schematic diagram for hybrid process A, consisting of a membrane permeator and a dual-bed PSA unit. The residue is sent directly to the PSA unit but, unlike scheme B, the permeate is temporarily stored in an intermediate tank before being sent to the adsorption process. Also, in scheme A the residue is used to feed the adsorption step instead of the permeate in configuration B. 

For brevity of presentation, we do not reproduce in full detail the model equations and parametric analysis for each scheme developed." " " " " Instead, we highlight some of the more representative results that demonstrate the hybrid concept and its enhancements relative to the more conventional units. The dimensions of the laboratory-scale PSA unit parameters, adsorbent properties, membrane properties and main process simulation parameters are listed in Tables 9.1 and 9.2. 

The hybrid scheme B was successfully applied to the bulk separation of CO2/N2 and CO2/CH4 mixtures over activated carbon." The process performance is reported in this chapter for CO2/N2 in terms of product recovery and purity at CSS. The numerical simulations are validated with experimental work on a composite membrane and a laboratory scale PSA unit. 

---