Co-current depressurization

When using co-current depressurization and purge steps, the concentration of the intermediate product increases towards the product end over the whole cycle. Selectivity enhancement is obtained provided that effective regeneration can be accomplished without significant increase of residence time. 

The basic PSA cycle was described by Skarstrom in 1960 (Skarstrom, 1960 1972). A similar cycle was the Guerin-Domine Cycle, invented at about the same time (Guerin-Domine, 1964 see a detailed account of these inventions in Yang, 1987). The latter was the basis for the modem vacuum swing cycle. The major additions to these cycles are co-current depressurization (CD) and pressure equalization (PE) steps. The CD step was added to increase product recovery, whereas the pressure equalization step was added to conserve the mechanical energy. 

Co-Current Depressurization. The first major process improvement after the invention of the Skarstrom and Guerin-Domine cycles was the introduction of the co-current depressurization step. To incorporate this step into the Skarstrom cycle, the adsorption step is cut short well before the breakthrough point, that is, the concentration front is far from reaching the outlet of the bed. The adsorption step is immediately followed by co-current depressurization before the bed is desorbed by further blowdown and purge, as required in the Skarstrom cycle. 

Ph = high-pressure, Pl = low-pressure, Pcd = pressure of co-current depressurization, Uh interstitial velocity in high-P step, Ul = interstitial velocity during desorption. 

Hydrogen purification was the first large-scale application of PSA technology. The first commercial PSA hydrogen purification unit was installed in conjunction with a steam reformer, in Toronto around 1966. The standard five-step PSA cycle (with a co-current depressurization step) is used. Three or more pressure... 

From the N2/CH4 isotherms and diffusivity data, the most promising sorbents appeared to be Sr-ETS-4, Mg-clinoptilolite, and purified cUnoptilolite. Thus, these three sorbents were compared for N2/CH4 separation (Jayaraman et al., 2002) by using a proven numerical PSA model (Rege et al., 1998). The overall diffusion time constants (D/R ) at 295 K for N2 were (in 1/s) 1.1 x 10 (purified cUnoptilolite) 1.8 x 10 (Mg-clinoptilolite) 3.1 x 10 (Sr-ETS-4). The corresponding values for CH4 were 2.0 x 10 (purified cUnoptilolite) 6.0 x 10 (Mg-cUnoptilolite) 1.2 x 10 (Sr-ETS-4) (Jayaraman et al., 2002). Heats of adsorption and other input data are available elsewhere (Jayaraman et al., 2002). The standard five-step PSA cycle was employed, which consisted of pressurization, high-pressure feed, co-current depressurization (or blowdown), counter-current blowdown and evacuation, and low-pressure purge. The cycle conditions were optimized for each run. The results are summarized in Table 10.11. 

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]. 

In recent PSA processes, three or more beds are used to synchronize and hold two additional steps to those in the Skarstrom cycle co-current depressurization or blowdown, and pressure equalization. From the open literature available, there are significant distinctions in the ways in which even the simplest PSA steps can be accomplished. For instance, pressurization with light product is generally superior to pressurization with feed. This is partially done by pressure equalization. Also, simultaneous depressurization with product withdrawal is advantageous over isobaric production or simultaneous pressurization with production. The co-current blowdown step has, likewise, appeared in several forms, either co- or counter-current to the feed step. The rinse step, which follows the feed step and allows the partial admittance of pure heavy product to the bed, shows benefits in terms of recovery by displacing the residual feed that is then recycled before the blowdown. 

Figure 9.5 Configurations of the coupled membrane/PSA system investigated by Feng and Ghosh. A two-bed PSA system is chosen for simplicity of the principle. I. Operating sequence of a PSA system with membrane-assisted feed gas pressurization. II. Operating sequence of a PSA system with membrane-assisted co-current depressurization. Adapted from Ref. 39.

In configuration II membrane-assisted co-current depressurization the membrane feed inlet is connected to bed B and the permeate outlet is connected to adsorber A that is being pressurized (step 1). Hence, the pressure diflerence... 

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