Process for separating hydrocarbon gas constituents

• US 4,687,499 A
• Filed: 04/01/1986
• Issued: 08/18/1987
• Est. Priority Date: 04/01/1986
• Status: Expired due to Term

• Current Assignee: Mcdermott International Incorporated
• Original Assignee: Mcdermott International Incorporated
• Inventors: Aghili, Hafez K.
• Primary Examiner(s): Capossela, Ronald C.
• Application Number:
  US06/847,071
• Time in Patent Office: 504 Days
• Field of Search: 62/23, 62/24, 62/31, 62/32, 62/42
• US Class Current: 62/621
• CPC Class Codes: C07C 7/005 Processes comprising at least two steps in series
• CPC Class Codes: C07C 7/04 by distillation
• CPC Class Codes: C07C 9/04 Methane production by treatment of sewage C02F11/04; natural gas, synthetic natural gas C10L3/06
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/76 Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2230/24 Multiple compressors or compressor stages in parallel
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2240/30 Dynamic liquid or hydraulic expansion with extraction of work, e.g. single phase or two-phase turbine
• CPC Class Codes: F25J 2270/90 External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more

Abstract

A process for separating natural gas constituents wherein a portion of the residue gas from the demethanizer column is cooled, expanded, partially condensed and recycled to the top of this column. This alters the chemical equilibrium in the column causing the lighter methane to vaporize and separate from the heavier constituents such as ethane, propane, etc. The lighter methane constituent rises and is removed from the top of the demethanizer column while the heavier constituents fall and are removed from the bottom of this column.

Claims

1. A process for separating the constituents of a natural gas stream comprising:

(a) lowering the temperature of said gas stream;

(b) supplying said lower temperature gas stream to a high pressure separator, said high pressure separator separating said gas stream into predominantly vapor and predominantly fluid streams;

(c) lowering the pressure of said predominantly vapor stream;

(d) supplying said lower pressure vapor stream to an upper region of a demethanizer column;

(e) lowering the pressure of said predominantly fluid stream;

(f) supplying said lower pressure fluid stream to said demethanizer column at an elevation below said vapor stream;

(g) removing cold vapor residue gas from an upper region of said demethanizer column, said vapor residue gas comprising predominantly of methane and other residual light vapors;

(h) passing said vapor residue gas through at least one heat exchanger to raise the temperature of said vapor residue gas;

(i) compressing said vapor residue to a higher pressure;

(j) drawing off a portion of said higher pressure vapor residue;

(k) lowering the temperature of said drawn off higher pressure vapor residue.

(l) lowering the pressure of said higher pressure residue to produce a predominantly liquid stream;

(m) supplying said lower temperature predominantly liquid stream as reflux to the top of said demethanizer column whereby the addition of said reflux alters the chemical equilibrium existing in the top of said demethanizer column thereby enhancing the separation of the inlet stream constituents in said demethanizer column; and

,

(n) removing a demethanized liquid product from a lower region of said demethanizer column.

2. A process as set forth in claim 1 wherein the temperature of said gas stream is lowered by separating said stream into two streams and cooling said streams in gas heat exchangers.

3. A process as set forth in claim 2 wherein said gas heat exchangers use said cold vapor residue gas as a refrigerant and said demethanized liquid product as a refrigerant.

4. A process as set forth in claim 3 wherein the pressure of said predominantly vapor stream is lowered across expansion means.

5. The process as set forth in claim 4 wherein said expansion means comprise a main expander.

6. A process as set forth in claim 4 wherein said expansion means comprise on in-line expansion valve.

7. A process as set forth in claim 4 wherein the pressure of said predominantly fluid stream is lowered across and in-line expansion valve.

8. A process as set forth in claim 7 wherein said cold vapor residue gas is compressed on the compressor side of an expander.

9. A process as set forth in claim 8 wherein the temperature of said drawn off vapor residue gas is lowered in a heat exchanger using cold vapor residue gas a refrigerant.

10. A process as set forth in claim 8 wherein the pressure of said drawn off vapor is lowered across expansion means.

11. A process as set forth in claim 10 where said expansion means comprise a reflux expander.

12. A process as set forth in claim 10 wherein said expansion means comprise an in-line flash valve.

13. A process as set forth in claim 10 wherein the temperature of said gas stream is lowered in a chiller.

14. A process as set forth in claim 13 wherein said demethanizer tower in a natural gas extraction plant is a distillation tower having a reboiler system and a flux system.