Purification and liquefaction of carbon dioxide (CO2) from process gases to remove small traces of light hydrocarbons, e.g., amine stripper offgases. Purification of natural gas containing high levels of CO2 and separation of ethane/CO2, via a hybrid membrane/cryogenic process.
Products: Carbon dioxide of purities up to 99.998%. Gaseous CO2 for use in applications such as enhanced oil recovery (EOR).
Impure CO2-rich feed (typically 90% + CO2) is compressed and cooled to remove water (1). The gas is dried by molecular sieve driers (2) before entering the low-temperature section, where the gas is used to reboil the distillation column (3) before entering the column. The condenser is usually a reflux exchanger. Vapor rising through the exchanger passages is cooled, and the resulting liquid flows back down the same passages and has intimate contact with the rising vapor. The cold reflux to the column is provided by an external refrigeration cycle (4), typically
ammonia evaporating at –28°C. The bottoms product from the column is pure liquid CO2.
For recovery from lean-CO2 streams, a hybrid membrane/cryogenic process can produce pure CO2. Typically,
the membrane concentrates the CO2 content from 30% to 90%. The concentrated stream passes to a low-temperature unit for further purification. This method enables ethane/CO2 mixtures to be separated to pure products without multiple distillation columns. For recovery from rich-CO2 streams, enhanced recovery can be achieved with a membrane to process the distillation column overheads and recycling of the CO2-rich permeate into the cryogenic process.
Operating conditions: Plants from 5-tpd to 1,200-tpd pure CO2. Virtually any feed-gas pressure can be handled. Compression is required for feed-gas pressures below 14 barg.
Licensor: Costain Oil, Gas & Process Ltd.