Process for purifying liquid natural gas

• US 5,359,856 A
• Filed: 10/07/1993
• Issued: 11/01/1994
• Est. Priority Date: 10/07/1993
• Status: Expired due to Fees

• Current Assignee: Praxair Technology Incorporated (Linde Plc)
• Original Assignee: Liquid Carbonic Corp. (Linde Plc)
• Inventors: Engdahl, Gerald E., Weber, Ronald C., Rhoades, George D.
• Primary Examiner(s): Capossela, Ronald C.
• Application Number:
  US08/133,667
• Time in Patent Office: 390 Days
• Field of Search: 62/9, 62/40
• US Class Current: 62/611
• CPC Class Codes: F25J 2215/60 Methane
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2240/40 Expansion without extracting work, i.e. isenthalpic throttling, e.g. JT valve, regulating valve or venturi, or isentropic nozzle, e.g. Laval
• CPC Class Codes: F25J 3/0615 Liquefied natural gas
• CPC Class Codes: F25J 3/0635 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/064 separation of CnHm with 2 carbon atoms or more

Abstract

The present invention is directed to a process for increasing the methane content of liquid natural gas to provide an essentially pure liquid methane product. In the process, a liquid natural gas feed stream is introduced into a first heat exchanger in indirect heat exchange with a purified methane vapor stream so as to liquify the purified methane vapor stream and to provide a purified liquid methane product. The heat exchange results in increasing the temperature of the liquid natural gas feed stream to a temperature just below the dew point of the liquid natural gas feed stream so as to partially vaporize the liquid natural gas feed stream to provide a mixture of a major amount of substantially pure methane vapor and a minor amount of liquid containing methane and higher hydrocarbons. The mixture of vapor and liquid from the first heat exchanger is transferred to a low pressure separator to provide a liquid bottom fraction and a purified methane top vapor fraction. At least a portion of the liquid bottom fraction and all of the top vapor fraction are transferred to a second heat exchanger in indirect heat exchange with the vapor fraction after the vapor fraction has been processed to change the temperature and pressure of the vapor fraction.

Claims

1. A process for increasing the methane content of liquid natural gas to provide a substantially pure liquid methane product comprising:

(a) introducing a liquid natural gas stream into a first heat exchanger in indirect heat exchange with a purified methane vapor stream so as to liquefy said purified methane vapor stream to provide a substantially pure liquid methane product and to partially vaporize said liquid natureal gas stream to provide a mixture of a major amount of substantially pure methane vapor and a minor amount of liquid containing methane and higher molecular weight hydrocarbons;

(b) transferring said mixture of vapor and liquid from said first heat exchanger to a low pressure separator to provide a liquid bottom fraction and a substantially pure methane top vapor fraction;

(c) introducing at least a portion of said liquid bottom fraction and all of said vapor fraction into a second heat exchanger in indirect heat exchange with said vapor fraction after said vapor fraction has been processed in accordance with step (d);

(d) transferring said vapor fraction exiting from said second heat exchanger through a compressor and an aftercooler to provide the said processed top vapor fraction for use as a heat exchange medium in said second heat exchanger; and

(e) introducing said processed top vapor fraction exiting from said second heat exchanger into said first heat exchanger to provide a heat exchange medium for said partial vaporization of said liquid natural gas and to provide said substantially pure liquid methane stream.

2. A process in accordance with claim 1 wherein said liquid natural gas feed stream is at a temperature of from about -260°

F. to about -200°

F. and a pressure of from about 15 psia to about 100 psia.

3. A process in accordance with claim 1 wherein said substantially pure methane stream entering said first heat exchanger is at a temperature of from about -245°

F. to about -170°

F. and a pressure of from 23 psia to about 88 psia.

4. A process in accordance with claim 1 wherein said liquid bottom fraction is from about 10 mole percent to about 20 mole percent based on said liquid natural gas feed stream.

5. A process in accordance with claim 1 wherein from about 10% to about 30% by weight of said liquid bottom fraction is introduced into said second heat exchanger.

6. A process in accordance with claim 1 wherein said liquid bottom fraction introduced into said second heat exchanger is at a temperature of about -225°

F. and a pressure of from about 15 psia to about 25 psia.

7. A process in accordance with claim 1 wherein said vapor fraction introduced into said second heat exchanger is at a temperature of from about -260°

F. to about -220°

F. and a pressure of from about 14 psia to about 55 psia.

8. A process in accordance with claim 1 wherein said processed vapor fraction entering said second heat exchanger is at a temperature of from about 60°

F. to about 125°

F. and a pressure of from about 24 psia to about 89 psia.

9. A process in accordance with claim 1 wherein said substantially pure liquid methane product is at a temperature of from about -255°

F. to about -220°

F. and a pressure of from about 22 psia to about 87 psia.

10. A process in accordance with claim 1 wherein the total work provided in compressing said vapor fraction exiting from said second heat exchanger is from about 0.032 to about 0.053 horsepower per pound/per hour of said liquid natural gas feed stream.

11. A process in accordance with claim 1 wherein the first heat exchanger and the second heat exchanger are parts of the same heat exchanger assembly.