Recovery from refinery offgases

About 4 % of the world hydrogen production is made by electrolysis. 

Water electrolysis is basically applied, if cheap electricity is available or high-purity hydrogen is required. The electrolytic decomposition of water with the cell reaction 

The effective electricity consumption is approx. 4.5 kWh/Nm at standard conditions. The electrolysis of distilled water takes place without any side reactions resulting in high-purity gases. 

The minimum feed water demand of an electrolyzer is 0.8 1/Nm of hydrogen, in practice, about 1 1/Nm is required. Sea water must be desalinated first which consumes an energy of 40 - 1(X) kWh/m of water or in the order of 1 % of the H2 heat of combustion. 

Conventional electrolytic cells use aqueous solutions of KOH or NaOH or NaCl or use solid polymer matrices as the electrolyte. In industrial plants, the alkaline medium is preferred, because corrosion is more easily controlled and cheaper materials can be used than in acidic electrolysis technology. The alkaline water electrolysis using a 25 % potash lye as the electrolyte consumes about 4 kWh/Nm including energy losses and related energy demands for ancillary equipment. It is a mature technology since decades. 

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Faraji, S., Sotudeh-Ggarebagh, R., and Mostoufi, N. Hydrogen recovery from refinery offgases. Journal of Applied Sciences, 2005, 5, 459. 

PACALOWSKA, B., WHYSALL, M., NARASIMHAN, M.V., Improve Hydrogen Recovery from Refinery Offgases, Hydrocarbon Processing 75 (1996) November 55-59. 

Other sources of hydrogen are provided by the conversion of oil or other feedstocks such as municipal solid waste and biomass, decomposition of hydrogen sulfide, or recovery from refinery offgases. 

Membrane modules are arranged in parallel banks to achieve required capacity. Because economy of scale is not a factor they are used primarily for small- to medium-capacity hydrogen recovery from refinery and petrochemical offgases. 

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