AU2018279228B2

AU2018279228B2 - Method for purifying natural gas using membranes

Info

Publication number: AU2018279228B2
Application number: AU2018279228A
Authority: AU
Inventors: Nicolas CHANTANT; Sylvain Gerard; Pascal Marty; Martin Raventos; Paul Terrien
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2017-06-09
Filing date: 2018-06-04
Publication date: 2024-05-09
Anticipated expiration: 2038-06-04
Also published as: WO2018224763A1; FR3067261A1; BR112019025828A2; AU2018279228A1; FR3067261B1; MY203817A

Abstract

Method for purifying a natural gas feed stream comprising the following steps: - step a) : introduction of the feed gas stream (1) into a purification unit at a pressure P1; - step b) separation of the gas stream (1) in a membrane permeation unit comprising at least one principal membrane separation stage (17), from which exit at least one CO

Description

Method for purifying natural gas using membranes

[0001] The present invention relates to a process for purifying a hydrocarbon stream such as natural gas.

[0002] Crude natural gas may contain a large number of troublesome impurities to be removed. Carbon dioxide is an example of such. Above a certain concentration of CO 2 in natural gas, said gas typically cannot be marketed on account of its low calorific power. Several technologies exist for removing CO 2 from natural gas. When the content is relatively low (for example less than 10%), washing with amines is usually used. One of the weak points of this solution is the energy required to regenerate the amines which have absorbed the CO 2 . When the content is higher, this technology becomes prohibitive in terms of operating costs, for example.

[0003] Another previously described and more suited solution is the use of membranes for removing CO2 . This technology in itself is inexpensive, but requires systems for pretreating the gas to be treated with the membranes that are often very complex and expensive, in particular for removing heavy hydrocarbons and aromatics.

[0004] The typical scheme for extracting CO 2 from natural gas with membranes involves the preliminary use of adsorption systems to remove the heavy hydrocarbons, for example. The document Recent Developments in CO 2 Removal Membrane Technology by David Dortmundt and Kishore Doshi (1999 UOP LLC) explains various possible optimizations of such systems, but the idea of not using an adsorption system is never mentioned. The problem with adsorption solutions is that it is necessary to install an intermittent system requiring a large amount of equipment (heaters, blowers, coolers, etc.).

[0005] The inventors of the present invention have developed a solution for removing the CO 2 contained in a natural gas stream to be liquefied, minimizing the losses of methane during this removal while simultaneously minimizing the costs involved in the deployment of purification processes of this type.

[0006] One subject of the present invention is a process for purifying a natural gas feed stream comprising methane, CO 2 and hydrocarbons containing at least two carbon atoms, comprising the following steps:

[0007] Step a): Introduction of the feed gas stream into a purification unit at a pressure P1;

[0008] Step b): Separation of the gas stream in a membrane permeation unit including at least one main stage of membrane separation from which exits at least one gas enriched in C02 at a pressure P3 and a gas stream depleted in C02 at a pressure P2 above P3;

[0009] characterized in that the difference between P2 and P3 is greater than or equal to 20 bara; and:

[0010] when P1 is greater than or equal to a threshold pressure P,, step b) is preceded by steps b1) and b2) below:

[0011] Step b1): Joule-Thomson depressurization of the feed stream from the pressure P1 to a pressure between P2 and P,;

[0012] Step b2): Introduction of the stream obtained from step b1) into a phase separating pot to produce at least two phases, namely aliquid phase including at least 10 mol% and preferably at least 50 mol% of hydrocarbons containing at least four carbon atoms initially contained in the feed stream, and a gaseous phase;

[0013] when P1 is less than or equal to P, the membrane separation unit used in step b) includes a preliminary membrane separation stage producing a stream enriched in hydrocarbons containing at least four carbon atoms at the pressure P4 and a stream whose hydrocarbon dew point is at most equal to the hydrocarbon dew point of the feed gas from this step b) at the pressure P2.

[0013a] Another subject of the present invention is a process for purifying a natural gas feed stream comprising methane, C02 and hydrocarbons containing at least two carbon atoms, comprising the following steps:

[0013b] Step a): Introduction of the feed gas stream into a purification unit at a pressure P1;

[0013c] Step b): Separation of the gas stream in a membrane permeation unit including at least one main stage of membrane separation from which exits at least one gas

2a

enriched in C02 at a pressure P3 and a gas stream depleted in C02 (20) at a pressure P2 above P3;

[0013d] wherein the difference between P2 and P3 is greater than or equal to 20 bara; and:

[0013e] when P1 is greater than or equal to a threshold pressure P, step b) is preceded by steps b1) and b2) below:

[0013f] Step b1): Joule-Thomson depressurization of the feed stream from the pressure P1 to a pressure between P2 and P,;

[0013g] Step b2): Introduction of the stream (6) obtained from step b1) into a phase separating pot to produce at least two phases, namely aliquid phase including at least 10 mol% and preferably at least 50 mol% of hydrocarbons containing at least four carbon atoms initially contained in the feed stream, and a gaseous phase;

[0013h] when P1 is less than or equal to P, the membrane separation unit used in step b) comprises a preliminary membrane separation stage producing a stream enriched in hydrocarbons containing at least four carbon atoms at the pressure P4 and a stream whose hydrocarbon dew point is at most equal to the hydrocarbon dew point of the feed gas from this step b) at the pressure P2, wherein P1 decreases over time and the number of membrane units used increases over time.

[0014] According to other embodiments, a subject of the invention is also:

[0015] A process as defined previously, characterized in that the membranes used in the membrane separation unit are reverse-selectivity membranes for the preliminary stage and membranes that are more selective for C02 than for methane and more selective for methane than for heavy hydrocarbons for the main separation stage.

[0016] A process as defined previously, characterized in that P1 varies over time.

[0017] A process as defined previously, characterized in that the C02-depleted gas stream at a pressure P2 obtained from step b) comprises less than 10 mol% of C02.

[0018] A process as defined previously, characterized in that the feed stream to be purified comprises at least 15 mol% of C02.

[0019] A process as defined previously, characterized in that Ps is equal to 65 bara.

[0020] A process as defined previously, characterized in that the difference between P2 and P4 is greater than 20 bara.

[0021] Advantages associated with the preliminary use of a Joule-Thomson system for the depressurization of step b1) are in particular:

[0022] minimization of the number of pretreatment membranes to be used (reduced investment); and

[0023] minimization of the losses of methane which will pass through the membranes with the heavy hydrocarbons.

[0024] The invention is particularly advantageous for a gas field whose pressure decreases over time. In this case, it will be possible first to install a system consisting solely of the Joule-Thomson pretreatment and, after a certain number of years of exploitation, to add reverse-selectivity membranes (of "rubbery membrane" type or type 1 for this invention). These membranes are more selective for heavy hydrocarbons (i.e. hydrocarbons containing at least three carbon atoms) than for methane or even for C02. This makes it possible to reduce the hydrocarbon dew point on the high pressure side (i.e. on the residue side) while allowing the heavy hydrocarbons to pass through on the low pressure side (i.e. on the permeate side).

[0025] It is possible to start the first years with only a few membrane modules and to increase the number thereof when the inlet pressure decreases.

[0026] The hydrocarbon stream is generally a stream of natural gas obtained from a domestic gas network in which the gas is distributed via pipelines.

[0027] The term "natural gas" as used in the present patent application relates to any composition containing hydrocarbons, including at least methane. This comprises a "crude" composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.

[0028] The heat exchanger may be any heat exchanger, any unit or other arrangement suitable for allowing the passage of a certain number of streams, and thus allowing direct or indirect exchange of heat between one or more refrigerant fluid lines and one or more feed streams.

[0029] Generally, the natural gas stream is essentially composed of methane. Preferably, the stream that will be liquefied comprises at least 80 mol% of methane. Depending on the source, the natural gas contains amounts of hydrocarbons heavier than methane, for instance ethane, propane, butane and pentane and also certain aromatic hydrocarbons. The natural gas stream also contains non-hydrocarbon products, such as nitrogen (content variable but of the order of 5 mol%, for example) or other impurities H20, C02, H2S and other sulfur-based compounds, mercury and others (0.5 mol% to 5 mol% approximately).

[0030] An implementation example is illustrated in the figure by the example that follows.

[0031] In the figure, a natural gas feed stream 1 is introduced into a purification unit 2 at a pressure P1.

[0032] Typically, feed stream 1 comprises at least 50 mol% of methane and at least 20 mol% of C02.

[0033] When the pressure P1 is greater than or equal to a threshold pressure Ps determined beforehand, stream 1 is cooled in a heat exchanger 3. For example, Ps is equal to 65 bara. For example, stream 1 is cooled by about 200 C. Stream 4 thus cooled undergoes depressurization, for example by means of a Joule-Thomson valve 5. The pressure P2 of stream 6 thus depressurized is at least 5 bara below P1.

[0034] Stream 6 is introduced into a phase-separating pot 7. A liquid stream 8 comprising more than 99 mol% of water, another liquid stream 9 comprising at least 50 mol% of hydrocarbons containing more than 4 carbon atoms and at least 25 mol% of C02, and also a gas stream 10 comprising at least 50 mol% of methane exit therefrom. Optionally, gas stream 10 may be treated in a unit 11 which further filters the hydrocarbon streams containing more than 4 carbon atoms, in order to refine the separation performed previously in pot 7, so as to afford a gas stream 10' depleted in heavy hydrocarbons and a liquid stream 9' enriched in heavy hydrocarbons. Stream 9' is then mixed with stream 9.

[0035] Gas stream 10 or 10' is then heated in heat exchanger 3 by about 200 C. Stream 12 is sent to a unit 13 for removing the remaining impurities, for instance mercury. For example, unit 13 comprises an adsorption unit 14 followed by a particle filter 15. Stream 16 exiting unit 13 is then introduced at the pressure P2 into a membrane separation unit 17 after having been heated 19 in a heat exchanger 18. Typically, stream 16 is heated by about 300 C to 400 C. The passage of stream 19 through unit 17 results in a stream 20 depleted in C02 and enriched in methane and also a stream 21 enriched in C02 and depleted in hydrocarbons.

[0036] Typically, stream 20 includes less than 8 mol% of C02 and more than 80 mol% of methane and stream 21 includes at least 75 mol% of C02. Membrane unit 17 comprises at least one membrane that is selective for C02 but not selective for heavy hydrocarbons (of "glassy membrane" type or type 2 for this invention, i.e. a membrane that is more selective for C02 than for methane and more selective for methane than for heavy hydrocarbons).

[0037] The pressure P3 of stream 21 is less than 3 bara, preferably less than 1 bara.

[0038] Stream 20 exiting unit 17 at a pressure P2 is then compressed to a pressure above 100 bara and is then sent to exchanger 18. Stream 22 exiting therefrom is intended to be treated and/or transported as natural gas in a transportation means such as a pipeline or natural gas pipeline, for example, before being liquefied, for example.

[0039] When P1 is below Ps, feed stream 1 is sent directly via a valve 23 into the pipe which sends stream 12 (numbering employed in the case described in the preceding paragraph A) to unit 13. Stream 24 exiting unit 13 is then introduced into a first membrane separation unit 25 upstream of the separation unit 17. It is a matter here of a type 1 membrane according to the invention for reducing the hydrocarbon dew point on the high pressure side.

[0040] The passage of stream 24 through this unit 25 results in a stream 26 at a pressure P4 enriched in C02 and in hydrocarbons containing more than three carbon atoms and also a stream 27 enriched in methane and depleted in hydrocarbons containing more than three carbon atoms. Stream 27 is then introduced into membrane separation unit 17 after passing through heat exchanger 18 and follows the same process as stream 19 mentioned in the preceding paragraph A. Stream 21 is then mixed with stream 26 to provide the stream highly enriched in C02.

[0041] It is important to note that Ps may be lower if stream 22 is intended to be transported at a lower pressure, for example of the order of 30 bara.

[0042] Unless the context requires otherwise, where the terms "comprise", "comprises", "comprised" or"comprising" are used in this specification (including the claims), they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0043] The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for purifying a natural gas feed stream comprising methane, C02 and hydrocarbons containing at least two carbon atoms, comprising the following steps: - Step a): Introduction of the feed gas stream into a purification unit at a pressure P1; - Step b): Separation of the gas stream in a membrane permeation unit including at least one main stage of membrane separation from which exits at least one gas enriched in C02 at a pressure P3 and a gas stream depleted in C02 at a pressure P2 above P3; wherein the difference between P2 and P3 is greater than or equal to 20 bara; and: - when P1 is greater than or equal to a threshold pressure Ps, step b) is preceded by steps b1) and b2) below: Step b1): Joule-Thomson depressurization of the feed stream from the pressure P1 to a pressure between P2 and P,; Step b2): Introduction of the stream obtained from step b1) into a phase separating pot to produce at least two phases, namely a liquid phase including at least 10 mol% and preferably at least 50 mol% of hydrocarbons containing at least four carbon atoms initially contained in the feed stream, and a gaseous phase; - when P1 is less than or equal to P, the membrane separation unit used in step b) comprises a preliminary membrane separation stage producing a stream enriched in hydrocarbons containing at least four carbon atoms at the pressure P4 and a stream whose hydrocarbon dew point is at most equal to the hydrocarbon dew point of the feed gas from this step b) at the pressure P2, wherein P1 decreases over time and the number of membrane units used increases over time.

2. The process as claimed in claim 1, wherein the membranes used in the membrane separation unit are reverse-selectivity membranes for the preliminary stage and membranes that are more selective for C02 than for methane and more selective for methane than for heavy hydrocarbons for the main separation stage.

3. The process as claimed in claim 1 or claim 2, wherein the C02-depleted gas stream at a pressure P2 obtained from step b) comprises less than 10 mol% of C02.

4. The process as claimed in any one of claims 1 to 3, wherein the feed stream to be purified comprises at least 15 mol% of C02.

5. The process as claimed in any one of claims 1 to 4, wherein P, is equal to 65 bara.

6. The process as claimed in any one of claims 1 to 5, wherein the difference between P2 and P4 is greater than 20 bara.