Several processes are known in the art for the separation and recovery of hydrogen from hydrogen-hydrocarbon feed gas streams. Among these are the following:
Cryogenic Partial Condensation Processes--These processes can recover hydrogen as a high purity product, but without co-product heavy hydrocarbons. The capital expense is not justified for feed gases containing only small amounts of hydrogen. Recovery of heavy hydrocarbon co-products is possible, but purity will be low due to the high quantities of light hydrocarbons and other light components which will be condensed with the desired heavy hydrocarbons. The cost and energy consumption of downstream separation and purification (fractionation) facilities for the heavy hydrocarbon products will also be high. Several such processes are described in a paper by W. K. Lam, et al., titled "Recover Valuable OffGases by the Braun ROE Process." presented at the AIChE National Meeting, 6-10 April 1986 in New Orleans, LA.
Membrane Separation Processes--These processes can recover hydrogen but cannot separate light hydrocarbons from desirable heavy hydrocarbons. Hydrogen recovery is very low when the concentration of H.sub.2 in the feed gas is low. One such process is described in U.S. Pat. No. 4,180,552.
U.S. Pat. Nos. 4,548,618; 4,654,047 and 4,654,063 described combination membrane and cryogenic processes to recover hydrogen, however, these patents do not address the recovery of heavy hydrocarbons. These processes are most suitable for feed gases containing relatively large amounts of hydrogen, i.e. more than 50 mole% hydrogen.
Pressure Swing Adsorption (PSA) Processes--These processes have the same disadvantage as the membrane process; i.e., low hydrogen recovery for hydrogen-lean feed gases and inability to separate light and heavy hydrocarbons. One such process is described in U.S. Pat. No. 3,430,418.
U.S. Pat. No. 3,838,553 describes a combination PSA and cryogenic process to recover high purity hydrogen at high recovery, but again does not address the recovery of heavy hydrocarbons and is most suitable for hydrogen-rich feed gases.
Cryogenic Dephlegmation/Partial Condensation Processes--These processes, using dephlegmation for heavy hydrocarbon recovery followed by partial condensation for hydrogen recovery, can recover heavy hydrocarbon and high purity hydrogen hydrogen products. However, the power required to recompress the hydrogen and light gas reject streams which must be reduced to very low pressures to provide the necessary refrigeration for high hydrogen purity and recovery is very high. The capital cost of cryogenic equipment to separate non-hydrocarbon light impurities, such as N.sub.2 and CO, from hydrogen is also very high.