Many facilities, including petroleum refineries, fertilizer plants, and fermentation plants, produce gases containing Carbon Dioxide (CO.sub.2). Often the Carbon Dioxide is considered a waste gas, and is merely vented to the atmosphere. In other instances the Carbon Dioxide can be separated out from the remaining gases, and utilized in some manner.
There are numerous known methods for separating Carbon Dioxide from other gases, including absorption by physical and chemical solvents, membranes and molecular sieves, and so forth. Such processes are, however, not particularly cost effective, if the Carbon Dioxide is required to be recovered as a liquefied product. Typically, Carbon Dioxide liquefaction utilized as part of a Carbon Dioxide separation process is performed using an added, (i.e., non-Carbon Dioxide) refrigerant. In prior art FIG. 1, for example, feed gas provided by a feed gas stream 100 is compressed in compressor 102, and is cooled against cooling water in water stream 106A. The compressed gas is then passed to a gas cooling unit 104, where it is cooled against cooling water in stream 106B and refrigerant in stream 108A. The cooled gas is then dried in gas drier 112,which typically uses molecular sieve or alumina desiccants. Gas drier 112 uses heat in stream 110. The desiccant bed is typically heat regenerated after it is fully loaded with moisture. Carbon Dioxide in the dried gas is then liquefied in liquefaction unit 114 against refrigerant in stream 108B, while other gases remain gaseous. Finally, since undesirable concentrations of impurities may be dissolved in the liquefied Carbon Dioxide, the impurities are removed using a stripping column 116. The output of the process is a purge stream containing impurities 118, and a purified Carbon Dioxide stream 120.
Liquefaction using an added refrigerant as depicted in FIG. 1 is problematic. For example, refrigerants having high operational efficiency, including ammonia and chlorofluorocarbons (CFCs), are often considered environmentally hazardous, and their use in many geographic areas may be severely restricted or even prohibited. Added refrigerants may also be expensive to purchase and maintain through the life span of the separation process. Still further, refrigerants introduce complexity, which results in increased capital and operational expenses, especially where regulatory changes require the use of a substitute refrigerant.
Thus, there remains a need for methods and apparatus that provides Carbon Dioxide separation without necessarily relying on refrigerants.