Olefin plant refrigeration system

#The present invention pertains to a refrigeration system to provide the cooling requirements of an olefin plant. More particularly, the invention is directed to the use of a tertiary or trinary refrigerant comprising a mixture of methane, ethylene and propylene for cooling in an ethylene plant.

#Ethylene plants require refrigeration to separate out desired products from the cracking heater effluent. Typically, a propylene and an ethylene refrigerant are used. Often, particularly in systems using low pressure demethanizers where lower temperatures are required, a separate methane refrigeration system is also employed. Thus three separate refrigeration systems are required, cascading from lowest temperature to highest. Three compressor and driver systems complete with suction drums, separate exchangers, piping, etc. are required. An additional methane refrigeration compressor, either reciprocating or centrifugal, can partially offset the capital cost savings resulting from the use of low pressure demethanizers.

#Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple refrigerants are utilized in a single refrigeration system to provide refrigeration covering a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. These mixed refrigeration systems have found widespread use in base load liquid natural gas plants. The application of a binary mixed refrigeration system to ethylene plant design is disclosed in U.S. Pat. No. 5,979,177 in which the refrigerant is a mixture of methane and either ethylene or ethane. However, such a binary refrigeration system cascades against a separate propylene refrigeration system to provide the refrigeration in the temperature range of −40° C. and warmer. Therefore, two separate refrigeration systems are required.

#The drawing is a schematic flow diagram of a portion of an ethylene plant illustrating one embodiment of the refrigeration system of the present invention.

#The present invention relates to an olefin plant wherein a pyrolysis gas is first processed to remove methane and hydrogen and then processed in a known manner to produce and separate ethylene as well as propylene and some other by-products. The process will be described in connection with a plant which is primarily for the production of ethylene. The separation of the gases in an ethylene plant through condensation and fractionation at cryogenic temperatures requires refrigeration over a wide temperature range. The capital cost involved in the refrigeration system of an ethylene plant can be a significant part of the overall plant cost. Therefore, capital savings for the refrigeration system will significantly affect the overall plant cost.

#Ethylene plants with high pressure demethanizers operate at pressures higher than 2.76 MPa (400 psi) with an overhead temperature typically in the range of −85° C. to −100° C. Ethylene refrigeration at approximately −100 to 102° C. is typically used to chill and produce overhead reflux. An ethylene plant designed with a low pressure demethanizer which operates below about 2.41 MPa (350 psi) and generally in the range of 0.345 to 1.034 MPa (50 to 150 psi) and with overhead temperatures in the range of −110 to −140° C. requires methane temperature levels of refrigeration to generate reflux. The advantage of the low pressure demethanizer is the lower total plant power requirement and the lower total plant capital cost while the disadvantage is the lower refrigeration temperature required and, therefore, the need for a methane refrigeration system in addition to the ethylene and propylene refrigeration systems.

#The tertiary refrigerant of the present invention comprises a mixture of methane, ethylene and propylene. The percentage of these components can vary depending on the ethylene plant cracking feedstock, the cracking severity and the chilling train pressure among other considerations, but will generally be in the range of 7 to 20 percent methane, 7 to 30 percent ethylene and 50 to 85 percent propylene. A typical composition for an ethylene plant with a low pressure demethanizer would be 10% methane, 10% ethylene and 80% propylene. The use of the tertiary refrigerant provides all the refrigeration loads and temperatures required for an ethylene plant while obviating the need for two or three separate refrigerant systems.