Most commonly, hydrogen is produced by the catalytic reforming of natural gas combined with a water gas shift reaction step, from which a hydrogen stream containing a number of impurities such as H2O vapor, CO2, CH4, CO, and N2 is obtained These impurities must be removed in order to attain the 99.95%+ H2 purity that is normally demanded.1 In fuel cell applications, the purity demands are as high as 99.99+%. The separation is usually carried out a large scale in PSA units, operating at pressures reaching about 7 MPa, using the Skarstrom, or modified Skarstrom PSA cycle. There are multiple steps, or stages, involved in the operation of each of the beds in the Skarstrom cycle
In the simplest case, the four steps in the sequence are as follows.
(a) Pressurization (with feed or raffinate product)
(b) High pressure adsorption separation with feed, with withdrawal of purified raffinate
(c) Depressurization, or “blowdown”, countercurrent to the feed
(d) Desorption at the lower operating pressure. This is accomplished by evacuation or purging the bed with (a portion) of the purified raffinate product