Wilbur Clymer Kratz

GAS MIXTURE FRACTIONATION TO PRODUCE TWO HIGH PURITY PRODUCTS BY PRESSURE SWING ADSORPTION

Abstract Pressure swing adsorption processes have been traditionally used to produce one high purity gas stream from a gas mixture. One of the most common uses of this technology is in the production of ultrahigh purity hydrogen from various gas streams such as steam methane reformer (SMR) off-gas. However, many of these gas streams contain a second gas in sufficiently high concentrations, e.g., carbon dioxide in SMR off-gas, that the recovery of this secondary gas stream along with the primary product is extremely desirable. A new pressure swing adsorption (PSA) process, GEMINI-8, has been developed at Air Products and Chemicals, Inc., to achieve this goal. Process cycle steps for the GEMINI-8 PSA process are illustrated by SMR off-gas fractionation for the production of hydrogen and carbon dioxide. Capital and power savings of this process as well as other advantages compared with the previous technology are discussed.

DESORPTION OF BENZENE-TOLUENE MIXTURES FROM ACTIVATED CARBON

Low temperature separation processes generally require warm end pretreatment steps to remove trace impurities prior to the bulk separation at cryogenic temperatures. The impurities must be removed to prevent freeze-up of the low temperature heat exchangers. In some hydrogen purification processes, benzene and toluene are separated by adsorption on activated carbon as part of the warm end pretreatment for impurity removal. The process is straightforward, using two beds of activated carbon operating in a cyclic fashion. Benzene and toluene are removed from the feed gas in one of two parallel beds to levels of <1 ppm, while the adsorbed components are removed from the other bed by purging with hot hydrogen gas at the adsorption pressure followed by cooling to the adsorption temperature. When the bed on adsorption is saturated, the beds are switched so the process is continuous. The reactivation pressure is not reduced because the effluent hydrogen reactivation gas is mixed with the remainder of the purified hydrogen at pressure. The benzene and toluene are impurities only in the sense that they will freeze in the low temperature exchangers. They are acceptable in the effluent purge gas.