Natural gas processing

• US 4,061,481 A
• Filed: 09/15/1976
• Issued: 12/06/1977
• Est. Priority Date: 10/22/1974
• Status: Expired due to Term

• Current Assignee: Elcor Corporation
• Original Assignee: The Ortloff Corporation
• Inventors: Campbell, Roy E., Tonne, Ronald Ray, Lawrence, John B.
• Primary Examiner(s): Lutter, Frank W.
• Assistant Examiner(s): Sever, Frank
• Application Number:
  US05/723,513
• Time in Patent Office: 447 Days
• Field of Search: 62/9, 62/11, 62/17, 62/12, 62/26, 62/23, 62/24, 62/31, 62/32, 62/34, 62/38, 62/42, 62/43, 62/44, 62/29, 62/33
• US Class Current: 62/621
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/50 using multiple (re-)boiler-condensers at different heights of the column
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2230/20 Integrated compressor and process expander; Gear box arrangement; Multiple compressors on a common shaft
• 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 2270/04 Internal refrigeration with work-producing gas expansion loop
• 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
• CPC Class Codes: F25J 3/0247 separation of CnHm with 4 carbon atoms or more

Abstract

The processing of gas streams containing hydrocarbons and other gases of similar volatility to recover high yields of components such as ethane, propane, and heavier hydrocarbons therefrom by expanding said gas stream in at least two stages through turboexpanders, each stage cooling the gas stream and producing energy in the form of horsepower used to drive a recompressor unit or other mechanical apparatus. Condensed liquids are collected intermediate the expansion stages. Preferably, a supplemental gas cooling stage and condensed liquid recovery occurs following each expansion stage.

Claims

1. In a process for obtaining a liquified fraction from a main stream of gas containing hydrocarbons selected from the group consisting of natural gas, refinery gas streams, and synthetic gas streams obtained from coal, crude oil, naphtha, oil shale, tar sands, and lignite which comprises the steps of

a. removing carbon dioxide, sulfur-containing gases, and other undesirable contaminants;

b. dehydrating the gas stream;

c. expanding said gas stream to a pressure lower than the previously existing pressure while extracting energy from the gas to provide a stream comprising gas and liquid portions; and

d. separating the resulting cold stream into residue gas and liquid portions,

the improvement consisting in the steps of;

9. The process of claim 1, wherein a gas cooling step using heat exchange with lower temperature residue gas, condensed liquid, external refrigeration, or any combination thereof is employed between each gas expansion stage prior to recovery of said condensed liquid.

2. expanding said gas stream in step (c) in at least two separate stages, each expansion stage being conducted at pressure-temperature conditions which cause hydrocarbons to condense;

2. directing the gas stream flowing from the first of said expansion stages to a condensed liquid separator where condensed liquid is separated and carried to a demethanizer;

3. directing the gas portion of the stream after separation in step (2) above into two vapor streams for further cooling, one of said streams flowing in heat exchange relationship with residue gas of lower temperature and the other of said streams flowing in heat exchange relationship with demethanizer liquid of lower temperature;

4. recombining the thus-cooled streams;

5. separating the liquid condensed by said further cooling step;

6. carrying the condensed liquid to a demethanizer;

7. directing the gas portion of the stream obtained in (5) above to the next expansion stage;

8. separating the stream leaving the said next expansion stage into gas and liquid portions;

9. directing the gas portion, from the separation following said next expansion stage, together with discharge gas from the demethanizer, into heat exchange relationship with at least one of the two vapor streams in (3) above; and

10. directing the liquid portion obtained in (8) above to a demethanizer.

5. The process of claim 2, wherein the expansion ratio of the process over all expansion stages combined is not in excess of about 5:

1.

6. The process of claim 2, wherein

i. the liquid portion from step (b) thereof is demethanized and the methane returned to the residue gas stream,

ii. the cold residue gas stream is utilized for refrigeration by passing it in heat exchange relation with the main stream of gas,

iii. the cold liquids condensed from the main stream of gas are utilized for refrigeration by passing them in heat exchange relation with the main stream of gas, and

iv. the horsepower created by expansion of said gases through the at least two separate stages is utilized.

10. The process of claim 2 wherein the feed gas containing hydrocarbons is a stream of natural gas.

3. In a process for separating a feed gas into a liquid product portion and a residue gas, said feed gas containing hydrocarbons and having methane and ethane as its major components, said process including the steps of

a. cooling said feed gas under substantially constant pressure;

b. expanding at least a portion of said cooled gas stream to a lower pressure while extracting energy from said gas portion to provide a partially condensed cold stream; and

c. treating at least the resulting partially condensed cold stream in a distillation column at said lower pressure to obtain said liquid product portion,

the improvement wherein said gas stream portion is expanded in step (b) in at least two separate stages by a process wherein

4. said portion of said cooled gas stream under pressure (a) is expanded in a first expansion means to an intermediate pressure while extracting energy from the gas, the outlet conditions of temperature and pressure in said first expansion stage being effective to cause the expanded feed gas portion to partially condense;

2. further cooling at least the gas remaining from said partial condensation immediately following said expansion in step (1) at a substantially constant pressure at said intermediate pressure;

3. separating said further cooled gas into a first vapor fraction at intermediate pressure and a first liquid fraction;

4. further expanding said first vapor fraction at said intermediate pressure in a second expansion stage, the outlet conditions of temperature and pressure in said second expansion stage being effective to cause hydrocarbons in the expanded vapor fraction to partially condense into a second gas and liquid portion;

5. separating said second liquid portion following said second expansion stage; and

6. thereafter treating said first and second liquid fractions in said distillation column (c) at said lower pressure.

7. The process of claim 4, wherein the expansion ratio of the process over all expansion stages combined is not in excess of 5:

1.

8. The process of claim 4, wherein the horsepower created by expansion of said gases through the at least two separate stages is utilized to recompress the warmed residue gas stream.

11. The process of claim 8 wherein the expansion ratio over all expansion stages combined is not in excess of about 5:

1.

12. In an apparatus for separating a feed gas into a liquid product portion and residue gas, said feed gas containing hydrocarbons and having methane and ethane as its major components, said apparatus including

a. means for cooling said feed gas under pressure at a substantially constant pressure;

b. expansion means connected to said cooling means to receive at least a portion of said cooled feed gas under pressure and to expand said gas portion to a lower pressure while extracting energy therefrom, said expansion means being adapted to partially condense said gas portion and to provide a cold gas stream; and

c. distillation means connected to said expansion means for receiving at least said partly condensed cold gas stream at said lower pressure, said distillation means being adapted to produce said liquid product portion,

the improvement wherein said expansion means (b) comprises

1. a first expansion means for expanding said gas stream portion to a pressure intermediate between the pressure of said feed gas and said lower pressure while extracting energy from the gas, said first expansion means being adapted to provide outlet conditions of temperature and pressure effective to cause hydrocarbons to partially condense from the gas stream portion at said intermediate pressure;

2. further cooling means connected directly to receive at least the gas remaining in said partially condensed feed stream from said first expansion means for further cooling the stream flowing from said first expansion means at a substantially constant pressure;

13. separation means connected to said further cooling means to receive the partially expanded and further cooled gas stream at said intermediate pressure and to separate it into a first liquid portion and first vapor portion;

4. a second expansion means connected to said separation means to receive said first vapor portion and to further expand it while extracting energy therefrom, said second expansion means being adapted to provide outlet conditions of temperature and pressure effective to cause hydrocarbons to partially condense from the further expanded gas stream;

5. separation means for removing a second liquid portion from the further expanded gas following said second expansion stage; and

6. means to receive said first and second liquid portions and supply them to said distillation column (c) at said lower pressure.