Claims
- 1. A process to produce high pressure methane gas comprising:
- (A) cooling a gaseous feed comprising methane and nitrogen;
- (B) introducing cooled feed into a single column cryogenic rectification plant and producing methane liquid therein;
- (C) partially vaporizing methane liquid and dividing remaining methane liquid into first and second portions;
- (D) expanding the first portion and heating the expanded first portion by indirect heat exchange with said cooling gaseous feed to produce methane gas; and
- (E) pumping the second portion to a high pressure and heating the high pressure portion by indirect heat exchange with said cooling gaseous feed to produce high pressure methane gas.
- 2. The process of claim 1 wherein the feed comprises less than 25 percent of nitrogen.
- 3. The process of claim 1 wherein the first portion comprises from 10 to 50 percent of the remaining methane liquid and the second portion comprises essentially all of the rest.
- 4. The process of claim 1 wherein the second portion is pumped to a pressure of at least 500 psia.
- 5. The process of claim 1 wherein the methane liquid is partially vaporized by recirculating heat pump fluid and the resulting vapor is passed to the column.
- 6. The process of claim 5 wherein the recirculating heat pump fluid is recirculated in a self-contained circuit independent of the column.
- 7. The process of claim 5 wherein the recirculating heat pump fluid is compressed prior to the partial vaporization of the methane liquid and then is subsequently expanded.
- 8. The process of claim 5 wherein the recirculating heat pump fluid is methane.
TECHNICAL FIELD
This application is a division of prior U.S. application Ser. No. 911,142 filing date Sept. 24, 1986 now a U.S. Pat. No. 4,710,212.
This invention relates to the separation of nitrogen from methane employing cryogenic rectification and is an improvement whereby methane product gas compression requirements are significantly reduced.
Natural gas, which is essentially methane, generally contains significant amounts of nitrogen contaminant as it emerges from a reservoir. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced gas recovery or enhanced oil recovery operation. Other contaminants which may be present in the natural gas from a reservoir include water, carbon dioxide, helium, hydrogen sulfide and higher hydrocarbons. In order to produce natural gas of a purity suitable for commercial use, the reservoir gas stream must be separated into components. Often the separation is by cryogenic rectification using either a single column or a double column separation plant. Generally, the nitrogen fraction comprises from 10 to 70 percent of the feed to the separation plant.
Generally the purified methane gas product from the cryogenic separation is introduced into a pipeline for delivery to end users and, in order to do so, the methane product gas must be compressed to the pipeline pressure which is generally at least about 500 psia. This methane product gas compression is quite costly and it is therefore desirable to eliminate or at least reduce methane product gas compression requirements.
Accordingly it is an object of this invention to provide a method for the separation by cryogenic rectification of nitrogen and methane wherein at least some methane gas product is produced at higher pressure thereby reducing the amount of methane gas product compression which is necessary to allow introduction of the methane gas product to a pipeline.
The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the present invention one aspect of which is:
A process to produce high pressure methane gas comprising:
(A) cooling a gaseous feed comprising methane and nitrogen;
(B) introducing cooled feed into the higher pressure column of a double column cryogenic rectification plant and producing methane-rich liquid therein;
(C) withdrawing methane-rich liquid and passing said liquid into the lower pressure column of the double column rectification plant and producing methane liquid therein;
(D) partially vaporizing methane liquid and pumping remaining methane liquid to a higher pressure;
(E) warming pumped methane liquid and further pumping at least a portion of the warmed methane liquid to a still higher pressure; and
(F) heating resultinq higher pressure methane by indirect heat exchange with said cooling gaseous feed to produce high pressure methane gas.
Another aspect of the present invention is:
A process to produce high pressure methane gas comprisinq:
(A) cooling a gaseous feed comprising methane and nitrogen;
(B) introducing cooled feed into a single column cryogenic rectification plant and producing methane liquid therein;
(C) partially vaporizing methane liquid and dividing remaining methane liquid into first and second portions;
(D) expanding the first portion and heating the expanded first portion by indirect heat exchange with said cooling gaseous feed to produce methane gas; and
(E) pumping the second portion to a high pressure and heating the high pressure portion by indirect heat exchange with said cooling gaseous feed to produce high pressure methane gas.
The term "column" is used herein to mean a distillation, rectification or fractionation column, i.e., a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column or alternatively, on packing elements with which the column is filled. For an expanded discussion of fractionation columns see the Chemical Engineer's Handbook, Fifth Edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York Section 13, "Distillation" B. D. Smith et al, page 13-3, The Continuous Distillation Process.
The term "double column", is used herein to mean a high pressure column having its upper end in heat exchange relation with the lower end of a low pressure column. An expanded discussion of double columns appears in Ruheman, "The Separation of Gases" Oxford University Press, 1949, Chapter VII, Commercial Air Separation, and Barron, "Cryogenic Systems", McGraw Hill, Inc., 1966, p. 230, Air Separation Systems.
The term "indirect heat exchange" is used herein to mean the bringing of two fluid steams into heat exchange relation without any physical contact or intermixing of the fluids with each other.
The term "pumped" is used herein to mean any means of increasing the pressure on a fluid and is not limited to the passing of the fluid through a pump.
US Referenced Citations (13)
Non-Patent Literature Citations (2)
| Entry |
| Energy Analysis Aids Equipment Design for Cryogenic Process, Chiu, Oil and Gas Journal, 1/18/1982. |
| Design and Operating Characteristics of the Sunflower Helium Plant, Crawford and Harlan, Journal of Petroleum Technology, 9/1970, pp. 1098-1102. |
Divisions (1)
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Number |
Date |
Country |
| Parent |
911142 |
Sep 1986 |
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Patent Grant
4778498
