Cascade refrigeration process for liquefaction of natural gas

A process for liquefying a natural gas stream which comprises the steps of:

• (a) cooling the natural gas stream by one or more heat exchangers by means of a first refrigeration cycle of cascade refrigeration system having two cycles;
• (b) passing the cooled natural gas to a phase separator to produce a first vapor stream and a liquid stream;
• (c) passing the liquid natural gas stream to a demethanizer to produce a second vapor stream and a second liquid stream;
• (d) passing the second liquid stream to a fractionation plant to produce condensate product, make-up refrigerant and a third vapor stream;
• (e) combining the first vapor stream, the second vapor stream, and the third vapor stream and passing the combined vapor stream to one or more heat exchangers cooled by a second refrigeration cycle of the cascade refrigeration system to at least partially liquefy the combined vapor stream;
• and
• (f) passing the combined vapor stream of step (e) to an expansion means to produce liquefied natural gas having a temperature above about -112.degree. C. (-170.degree. F.) and a pressure sufficient for the liquid product to be at or below its bubble point.

A process for liquefying a pressurized methane-rich gas stream comprising the steps of:

• (a) introducing the gas stream into heat exchange contact with a first refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the gas stream is reduced by heat exchange with a first portion of a first refrigerant to produce a cooled gas stream;
• (b) introducing the cooled gas stream into heat exchange contact with a second refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the cooled gas stream is further reduced by heat exchange with a second refrigerant, thereby condensing at least a portion of the gas stream to produce a liquefied methane-rich stream having a temperature above -112.degree. C., said second refrigerant having a boiling point lower than the boiling point of the first refrigerant and the second refrigerant being partially cooled and condensed by heat exchange with a second portion of the first refrigerant;
• and
• (c) introducing the liquefied stream to a transportation means at a temperature above about -112.degree. C. (-170.degree. F.) and a pressure sufficient for such liquefied stream to be at or below its bubble point.

A process for liquefying a pressurized methane-rich gas stream comprising the steps of:

• (a) introducing the gas stream into heat exchange contact with a first refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the gas stream is reduced by heat exchange with a first portion of a first refrigerant to produce a cooled gas stream;
• (b) introducing the cooled gas stream into heat exchange contact with a second refrigerant cycle comprising at least one refrigeration stage whereby the temperature of the cooled gas stream is further reduced by heat exchange with a second refrigerant, said second refrigerant having a boiling point lower than the boiling point of the first refrigerant and the second refrigerant being partially cooled and condensed by heat exchange with a second portion of the first refrigerant;
• (c) expanding the gas stream cooled by step (b) to produce a liquid stream at a temperature above about -112.degree. C. (-170.degree. F.) and a pressure sufficient for the liquefied stream to be at or below its bubble point;
• and
• (d) introducing the liquefied stream to a storage or transportation means at a temperature above about -112.degree. C. (-170.degree. F.).