Method for purifying natural gas using an economizer

Abstract

A process for purifying a gaseous feed stream of natural gas including methane, CO.sub.2 and heavy hydrocarbons including step a): cooling the gaseous feed stream in a heat exchanger; step b): introducing the cooled stream into a phase-separating chamber to produce a liquid stream depleted in methane and enriched in heavy hydrocarbons and a gaseous stream; step c): separating the gaseous stream obtained from step b) in a first membrane producing at least one CO.sub.2-enriched permeate stream and a residual stream enriched in methane; step d): introducing the residual stream obtained from step c) into a phase-separator to produce a liquid stream and a gaseous stream; step e): heating the gaseous stream obtained from step d) by introducing it into the heat exchanger used in step a) counter-currentwise with the feed stream thereby producing a gaseous stream depleted in CO.sub.2 and enriched in methane.

Claims

1. A process for purifying a gaseous feed stream of natural gas comprising methane, CO.sub.2 and hydrocarbons containing at least two carbon atoms, comprising the following steps: Step a): cooling the gaseous feed stream in a heat exchanger; Step b): introducing stream obtained from step a) into a phase-separating chamber to produce a liquid stream depleted in methane and enriched in hydrocarbons containing more than three carbon atoms and a gaseous stream; Step c): separating the gaseous stream obtained from step b) in a first membrane permeation unit including at least one main membrane separation stage from which emerges at least one CO.sub.2-enriched gaseous first permeate stream and a partially condensed first residual stream depleted in CO.sub.2 and enriched in methane; Step d): introducing the first residual stream obtained from step c) into a phase-separating chamber to produce at least two phases, a liquid stream including at least 0.5 mol % of hydrocarbons containing at least three carbon atoms initially contained in the feed stream, and a gaseous stream; Step e): heating the gaseous stream obtained from step d) by introducing it into the heat exchanger used in step a) counter-currentwise relative to the feed stream so as to produce a gaseous stream depleted in CO.sub.2 and enriched in methane relative to the feed stream; and Step f): introducing the gaseous stream obtained from step e) into a second membrane separation unit, from which emerges at least one gaseous second permeate stream enriched in CO.sub.2 and a gaseous second residual stream depleted in CO.sub.2 and enriched in methane.

2. The process as claimed in in claim 1, wherein the second residual stream contains less than 8 mol % of CO.sub.2 and more than 80 mol % of methane.

3. The process as claimed in claim 1, wherein the gaseous stream obtained from step d) undergoes Joule-Thomson expansion prior to step e).

4. The process as claimed in claim 1, wherein the feed stream to be purified comprises at least 15 mol % of CO.sub.2.

5. The process as claimed in claim 1, wherein the gaseous stream obtained from step e) is heated by introducing it into a heating means so as to produce a gaseous stream prior to step f).

6. The process as claimed in claim 1, wherein the liquid stream obtained from step b) is introduced into the heat exchanger used in step a) counter-currentwise relative to the feed stream.

7. The process as claimed in claim 1, wherein the liquid stream obtained from step d) is mixed with the liquid stream obtained from step b) before being introduced into the heat exchanger used in step a) counter-currentwise relative to the feed stream.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

(2) The FIGURE illustrates one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) In the FIGURE, a natural gas feed stream 1 is introduced into a heat exchanger 14 at a temperature T1.

(4) Typically, the feed stream 1 comprises at least 50 mol % of methane and at least 20 mol % of CO.sub.2.

(5) A partially condensed stream 2 leaves the heat exchanger 14 at a temperature T2 below T1.

(6) Stream 2 is introduced into a phase-separating chamber 15, from which emerges a liquid stream 3 and a gaseous stream 4.

(7) The gaseous stream 4 is then introduced into a first membrane separation unit 16 having a higher selectivity for CO.sub.2 than for methane and functioning in the presence of liquid. In this membrane unit, the stream is separated into a gaseous permeate stream 5 which is highly enriched in CO.sub.2 and a residual stream 6 which is partially condensed at a temperature T3 below T2.

(8) Stream 6 is introduced into a phase-separating chamber 17. A liquid stream 7 emerges therefrom, including at least 0.5 mol %, preferably at least 1% mol of hydrocarbons containing at least three carbon atoms initially contained in the feed stream 1. A gaseous stream 8 having a hydrocarbon dew point at least 2° C. lower than the feed stream (at an equivalent pressure), preferentially at least 5° C. lower (at an equivalent pressure) and more preferentially at least 10° C. lower, also emerges from the phase-separating chamber 17.

(9) The gaseous stream 8 is then heated 9 in the heat exchanger 14 up to a temperature quite close to T1 (i.e. to a temperature strictly greater than T2 and at least between T2 and T1). Before being introduced into the heat exchanger 14, stream 8 is optionally expanded, for example by means of a Joule-Thomson valve 19. Stream 8 heats up in the heat exchanger in a counter-current relative to the feed stream 1, which, itself, is cooled to the temperature T2. Stream 9 exiting the exchanger 14 is then introduced at a temperature T4 into a second membrane separation unit 18 after having been heated in a heating means 11.

(10) Typically, stream 9 is heated (it then becomes stream 10) by about 30° C. to 50° C. (i.e.: the difference between T1 and T4 is between 30° C. and 50° C.). The passage of stream 10 through unit 18 results in a gaseous residual stream 13 depleted in CO.sub.2 and enriched in methane and also a permeate stream 12 enriched in CO.sub.2 and depleted in hydrocarbons.

(11) Typically, stream 13 includes less than 8 mol % of CO.sub.2 and more than 80 mol % of methane and stream 12 includes at least 40 mol % of CO.sub.2.

(12) The membrane unit 18 comprises at least one membrane that is selective for CO.sub.2 but not selective for heavy hydrocarbons (of “glassy membrane” type, i.e. a membrane that is more selective for CO.sub.2 than for methane and more selective for methane than for heavy hydrocarbons). This membrane unit 18 does not function in the presence of liquid.

(13) The liquid streams 3 and 7 may be introduced, independently or after having been mixed, into the heat exchanger 14 in order to be heated and to serve to cool the feed stream 1, and then, at the outlet, to be mixed again with the feed stream in order to be recycled.

(14) The implementation of an embodiment according to the invention as described with the FIGURE is illustrated by the following summary table (in the material balance below, the liquids 3 and 5 are not heated in the exchanger to simplify the heat exchanger 14, although it may be possible to do so in order to further improve the performance of the system):

(15) TABLE-US-00001 -1- -2- -3- -4- -5- -6- -7- Vapour fraction % mol 1.000 0.997 0.000 1.000 1.000 0.997 0.000 Temperature ° C. 26.6 18.0 18.0 18.0 15.9 13.6 13.6 Pressure bara 62.8 62.5 62.5 62.5 1.3 62.5 62.5 Flow rate Nm3 h 4469 4469 13 4456 508 3948 10 Composition Methane Mol % 62.64% 62.64% 17.85% 62.78% 24.53% 67.70% 22.66% Ethane Mol % 2.51% 2.51% 2.72% 2.51% 0.50% 2.77% 3.61% Propane Mol % 1.84% 1.84% 5.26% 1.83% 0.25% 2.03% 7.13% i-Butane Mol % 0.61% 0.61% 3.49% 0.61% 0.02% 0.68% 4.79% n-Butane Mol % 0.54% 0.54% 3.99% 0.52% 0.02% 0.59% 5.49% i-Pentane Mol % 0.22% 0.22% 3.15% 0.21% 0.00% 0.24% 4.28% n-Pentane Mol % 0.17% 0.17% 3.07% 0.16% 0.00% 0.18% 4.15% n-Hexane Mol % 0.15% 0.15% 6.00% 0.13% 0.00% 0.15% 7.59% CO2 Mol % 29.34% 29.34% 18.34% 29.37% 73.23% 23.73% 17.48% Nitrogen Mol % 1.67% 1.67% 0.19% 1.67% 1.18% 1.74% 0.22% n-Heptane Mol % 0.13% 0.13% 10.75% 0.10% 0.00% 0.11% 11.69% H2S Mol % 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% H2O Mol % 0.09% 0.09% 11.82% 0.06% 0.27% 0.03% 0.03% n-Octane Mol % 0.09% 0.09% 13.36% 0.05% 0.00% 0.06% 10.87% -8- -9- -10- -11- -12- -13- Vapour fraction 1.000 1.000 1.000 1.000 1.000 1.000 Temperature 13.6 24.0 64.0 64.0 58.3 52.7 Pressure 62.5 62.2 61.7 61.7 1.3 61.6 Flow rate 3938 3935 3935 3935 1192 2743 Composition Methane 67.82% 67.85% 67.85% 67.85% 29.29% 84.61% Ethane 2.76% 2.77% 2.77% 2.77% 0.25% 3.86% Propane 2.02% 2.02% 2.02% 2.02% 0.04% 2.88% i-Butane 0.67% 0.67% 0.67% 0.67% 0.01% 0.96% n-Butane 0.58% 0.58% 0.58% 0.58% 0.01% 0.83% i-Pentane 0.22% 0.22% 0.22% 0.22% 0.00% 0.32% n-Pentane 0.17% 0.17% 0.17% 0.17% 0.00% 0.25% n-Hexane 0.13% 0.13% 0.13% 0.13% 0.00% 0.18% CO2 23.74% 23.70% 23.70% 23.70% 69.17% 3.94% Nitrogen 1.74% 1.74% 1.74% 1.74% 1.13% 2.01% n-Heptane 0.08% 0.08% 0.08% 0.08% 0.00% 0.12% H2S 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% H2O 0.03% 0.03% 0.03% 0.03% 0.10% 0.00% n-Octane 0.03% 0.03% 0.03% 0.03% 0.00% 0.05%

(16) To illustrate the main advantage of the invention, the hydrocarbon dew point (in ° C. at the pressure of the fluid) of the gas in the main points is calculated below (it may be observed that the dew point of product 13 is significantly lower than the temperature, which is critical for ensuring that no liquid forms anywhere in the second membrane):

(17) Feed (stream 1): 26.6° C.

(18) Stream 8: 13.6° C.

(19) Stream 13: 25° C.

(20) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.