WO2016156691A1

WO2016156691A1 - Natural gas treatment method for minimizing ethane loss

Info

Publication number: WO2016156691A1
Application number: PCT/FR2016/050549
Authority: WO
Inventors: Oumar KHAN; Mathieu LECLERC; Paul Terrien
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2015-04-02
Filing date: 2016-03-10
Publication date: 2016-10-06
Also published as: MY183059A; FR3034509A1; FR3034509B1

Abstract

The invention relates to a method for treating a gas stream (1) containing at least one acid gas, ethane and one light gas selected from among methane, carbon monoxide, hydrogen, and nitrogen. Said method includes the following steps: step a): supplying said gas stream (1) to a cryogenic separation unit (A) including at least one distillation column (14) for treating said gas stream (1), the treatment including partial elimination of the acid gas from said gas stream and production of a gas stream depleted of acid gas, and said at least one distillation column (14) including a main means (33) for supplying a liquid flow; step b): sampling, from the cryogenic separation unit (A), at least one portion (23) of the treated gas stream depleted of acid gas; step c): supplying at least one portion of the treated gas stream (23) depleted of acid gas to a physical separation unit (29) that separates the acid gas from the ethane, in which said stream is separated so as to form a residue, enriched with light gas and ethane (31), and a permeate stream (30) enriched with acid gas; and step d) returning the stream (30) enriched with acid gas into the cryogenic separation unit (A) by adding it separately from the supply, defined in step a), from the cryogenic separation unit (A) and below the main supply means (33) of the column (14) so as to extract the ethane from one of the at least one distillation column(s) (14).

Description

Process for treating natural gas to minimize ethane loss

The present invention relates to a method for minimizing the loss of ethane during the treatment of natural gas rich in CO2.

Natural gas is desirable for use as a fuel for use in heating buildings, to provide heat for industrial processes for the generation of electricity, for use as a feedstock for various synthesis processes for produce olefins, polymers and the like.

Natural gas is a resource still abundant. On the other hand, the amount of conventional natural gas represents a smaller and smaller share of production. Several types of unconventional gas exist. A prime example is shale gas, which accounts for a growing share of production.

Another example: the fields of acid gas, hitherto unexploited because of their high content of CO2 as well as H ₂ S represent a significant part of the potentially exploitable gas.

The use of an efficient and economical process for purifying natural gas into acidic compounds (typically H ₂ S and CO2) is therefore of great importance.

The efficiency of such a process can be measured in particular in terms of losses of hydrocarbons in the waste stream.

This yield can be quantitatively written as an energy efficiency or HHV yield (defined as the ratio of the calorific value of the natural gas treated by said process by the calorific value of the natural gas to be purified).

A method for separating CO2 from natural gas by cryogenic means is known and described in patent application FR 2 959 512.

This method makes it possible to purify a stream of natural gas in CO2 by means of a first column producing a liquid foot stream containing predominantly CO2 and a gaseous head containing predominantly CH. This column is equipped with a reboiler and a condenser. Part of the liquid foot stream of this first column is then used to produce the cold necessary for the process, by means of three detents at three different pressures followed by the vaporization of these same three relaxed streams.

The successive sprays being partial, the residual liquids (rich in CO2 but still containing a significant amount of hydrocarbons having at least two carbon atoms such as ethane, propane and butane) are harvested, pumped at a certain pressure and sent to a second column, this time only provided with a reboiler.

This column makes it possible to recover at its head a gaseous stream rich in purified CO2 and at the bottom a hydrocarbon-rich liquid stream having at least three carbon atoms such as propane and butane.

The overhead gas stream is then compressed, condensed and mixed with the liquid stream from the first column, before the mixture of these two streams is pumped to the required pressure.

The main disadvantage of the process which is the subject of the invention described in FR 2 959 512 concerns the fact that only the non-vaporized liquid during the three successive sprays is treated in the second column: the hydrocarbons having at least two carbon atoms such as Ethane, propane and butane contained in the gaseous phases are therefore sent to CO2 and thus not recovered in natural gas. In particular, such a process necessarily has a very high loss of ethane (typically at least 50%). Indeed ethane and carbon dioxide are very difficult to separate as shown in the curve of Figure 1 (binary ethane C2-CO2 at 30 bar).

The curve in Figure 1 illustrates that the separation between ethane and carbon dioxide is difficult:

• The two curves are very close to each other, especially when the CO2 content is high.

• The curve has an azeotrope, which shows that it is impossible without a particular device to make a total distillation in a simple distillation column with condenser and reboiler.

Moreover, it is known to separate CO2 and ethane relatively efficiently by membrane separation. On the other hand, membrane separation alone does not make it possible to separate CO2 from methane in a very efficient manner (there are many losses of methane in CO ₂ ).

The inventors of the present invention have then bridged a solution to solve the problems raised above.

An object of the present invention is to effectively separate any mixture containing both at least CO2, ethane and a light compound such as methane, nitrogen, CO or hydrogen while avoiding the disadvantages mentioned above.

The invention can more broadly be applied to the separation of a mixture of at least three gases: an acid gas, a light gas and a low permeability gas. The acid gas may be carbon dioxide or hydrogen sulphide, the light gas of methane and the low permeability gas of ethane. The process according to the invention is particularly advantageous when the cryogenic separation of the low permeability gas and the acid gas are difficult.

The present invention relates to a process for treating a gaseous stream containing at least one acid gas, ethane and a light gas selected from methane, carbon monoxide, hydrogen and nitrogen comprising the steps following:

a) supplying with said gas stream a cryogenic separation unit comprising at least one distillation column for treating said gas stream, the treatment comprising the partial removal of the acid gas from said gas stream and the production of a gaseous stream depleted in acid gas and said at least one distillation column comprising a main supply means of a liquid flow;

b) removing the cryogenic separation unit from at least a portion of the gaseous treated stream depleted in acid gas;

c) feeding with at least a portion of the acid gas-treated treated gaseous stream of a physical separation unit separating the acid gas from ethane, wherein said stream is separated to form a residue enriched with light gas and ethane and a current, permeate, enriched in acid gas;

d) Return of the stream enriched with acid gas in the cryogenic separation unit in order to extract ethane from one of the at least one distillation column by introducing it separately from the feed defined in step a) of the unit of cryogenic separation and below the main feed means of the column.

Cryogenic separation means separation effected at a temperature below -30 ° C and above -100 ° C, when the pressure is between 5 bara (absolute bar) and 100 bara. Preferably, the temperature of the cryogenic separation is between -40 ° C and -60 ° C for a pressure between 10 bara and 70 bara.

Physical separation means separation by one of the following means:

• Membrane separation; or

· Adsorption separation (eg pressure modulation -

PSA or by temperature modulation - TSA) Preferably, the temperature of the physical separation is between 5 ° C and 100 ° C when the pressure is between 1.5 bara and 90 bara.

The proposed method may further include at least the following features:

Process as defined above, characterized in that it comprises step e): recovery, from the cryogenic separation unit, of a stream containing more than 90% of the acid gas initially present in the gas stream treat.

· Method as defined above, characterized in that it comprises the additional step:

• Recovery, from the physical separation unit, of a gas stream containing more than 95% of the light gas initially present in the gas stream to be treated and more than 45% of the ethane initially present in the gas stream to treat.

Process as defined above, characterized in that the acidic gas is chosen from CO2 and H ₂ S.

• Process as defined above, characterized in that the acid gas is carbon dioxide and the light gas is methane.

Process as defined above, characterized in that the gaseous stream to be treated comprises from 10 mol% to 75 mol% methane, from 25 mol% to 90 mol% carbon dioxide and from 0.1 mol% to 15% molar. molar of ethane. • Process as defined above, characterized in that said gaseous stream to be treated further comprises hydrogen sulphide and hydrocarbons having at least three carbon atoms.

Process as defined above, characterized in that, at a pressure of between 5 bara and 100 bara, the temperature in the cryogenic separation unit is below -30 ° C. and preferably between -40 ° C. and -40 ° C. 60 ° C.

Process as defined above, characterized in that the physical separation unit is a membrane separation unit comprising at least one membrane.

• Method as defined above, characterized in that said at least one membrane is a polymer membrane.

Process as defined above, characterized in that the physical separation unit is a pressure modulation adsorption unit (PSA).

The present invention particularly relates to the purification of a stream of natural gas in CO2 by means of a cryogenic unit followed by a membrane separation, making it possible to maximize the HHV yield by minimizing the loss of hydrocarbons in the CO2, and more particularly the loss of methane and ethane in CO2, taking advantage of both the advantages of cryogenic separation and membrane separation.

An object of the present invention is therefore a process for separating CO2 from natural gas to obtain a methane-rich stream and a CO2 stream without methane (typically less than 1% by volume methane, preferably less than 0.5% by volume). methane). This process uses cryogenic separation (typically partial condensation and / or distillation around -40 ° C to -60 ° C) and membrane separation in a second time to both deplete the methane rich stream in CO2 and produce a current. of CO2 depleted in ethane so that it is recycled to the cryogenic separation.

As a remark, it can for example a separator pot operating at -20 ° C or -30 ° C before distillation in a column may be present.

The method typically comprises the following steps:

1) Drying a feed gas comprising methane, carbon dioxide and ethane. 2) First cooling in a heat exchanger and partial condensation to obtain a first liquid and a first gas.

3) Cooling of the first gas in a heat exchanger and partial condensation to obtain a second liquid and a second gas enriched in methane.

4) Mixing the two liquids and expanding the mixture in a valve to introduce it into a first distillation column.

5) Separation of most of the methane remaining in said distillation column to obtain a liquid depleted of ethane at the bottom of the column.

6) Relaxing the liquid from the first distillation column to introduce it into a second distillation column at lower pressure.

7) Compression of the current from the head of the two columns to recycle it to the feed gas.

8) Vaporization of the liquid depleted in methane, preferably at several pressure levels (for example about 5.5 bar, about 11 bar and about 16 bar) to provide at least a portion of the frigories required for steps 2) and 3) for obtain at least one low pressure gas, and optionally a medium pressure gas and a high pressure gas.

9) Compression of the low pressure gas, and possibly medium pressure gas to mix all the gases from the methane depleted liquid to obtain a high pressure gas depleted in methane.

10) Introduction of the methane-enriched gas into a membrane permeation unit to obtain a product gas and a carbon dioxide-enriched and ethanol-depleted gas which is compressed to the pressure of the first distillation column and introduced at least partly in the tank of the first distillation column to reboil and extract the ethane from the first distillation column by stripping. This (these) introduction (s) of (the) stream (s) depleted (s) can be done before or after a passage in an exchanger while being mixed or not with another current.

The invention utilizes a stream enriched with carbon dioxide and depleted in ethane to stripping the ethane from the first distillation column and thereby minimizing the loss of ethane in the carbon dioxide produced. This is possible by the nature of the additional separation process (membrane separation) which is very selective between carbon dioxide and ethane.

Several embodiments are possible:

· The membrane permeate charged with carbon dioxide and depleted in ethane can be cooled and partially condensed in a dedicated separator pot. The formed liquid can be sent directly expanded and sent to the second distillation column since it is depleted in ethane. The gas is used as the gas injected into the first column to extract by stripping the ethane from the first column.

• Several membranes in series can be used. A first membrane is used with a permeate at the pressure of the first distillation column. The amount of permeate being adjusted according to the amount of ethane to be extracted by stripping. The residue of this membrane being sent into a membrane. The number of membranes in series can be increased to obtain different permeate injected at different levels in the distillation column. The advantage of this variant is to obtain very low ethane permeates since the permeation yields through these membranes are low.

The invention will be described in more detail with reference to FIG. 2 which illustrates a method according to the invention.

A feed stream of gas 1, for example natural gas, is introduced into a pre-treatment unit 2 to remove impurities from said feed stream.

The gaseous stream 1 comprises, for example, methane, ethane and carbon dioxide.

The pre-treatment unit 2 typically comprises at least one adsorption device 34 for removing the water.

The gaseous stream 3 thus pretreated is cooled in a heat exchanger 4 at a temperature below -20 ° C and at a pressure of about 50 bara (or for example approximately the pressure of the feed gas). Preferably, the temperature remains above -60 ° C. The gaseous stream 5 thus cooled is partially condensed, for example in a phase separator pot 6, in order to obtain a first liquid stream 7 and a gas stream 8. This gas stream 8 is then cooled to a temperature below - 45 ° C in a heat exchanger 9, then partially condensed in a separator pot 10 to produce a second liquid stream 1 1 and a gas stream 23 enriched in methane and depleted of carbon dioxide. The first liquid stream 7 and the second liquid stream 11 are mixed. This mixture 12 is then expanded to a pressure of between 10 bara and 30 bara, for example using a valve 13, to be introduced into a first distillation column 14 via the main supply 33 of said column. 14. An ethane-depleted liquid stream is collected at the bottom of column 14 (i.e., below the lowest stage of column 14). The liquid stream 15 is expanded, for example by means of a valve 16, at a pressure of between 5 bara and 15 bara, then introduced into a second distillation column 17. The gas streams 18 and 19 collected at the top of the columns 14 and 17, are introduced into a heat exchanger 4 and then compressed using one or more compressors 20 before being recycled to about the pressure of the feed gas stream to the gas stream initial pretreated 3.

A liquid depleted of methane 21 is collected at the bottom of the second distillation column 17, and is vaporized to produce a gas stream 22.

Preferably, said liquid 21 is vaporized at several pressure levels (for example about 5.5 bar, about 11 bar and about 16 bar) to provide at least a portion of the frigories required for the previously described steps to obtain at least one gas at low pressure, and possibly a medium pressure gas and a high pressure gas.

The gas stream 22 is compressed using a compressor 24 to a pressure between 10 bara and 50 bara to produce a gas called "high pressure" depleted of methane 25.

This gas 25 can then be treated in a treatment unit 26 containing at least one distillation column in order to separate the carbon dioxide thus produced 27 from heavy hydrocarbons 28 having more than three carbon atoms, such as propane and butane, initially contained in the gaseous stream 1 to be treated. The gaseous stream enriched in methane 23 is introduced into a physical separation unit 29. Such a unit 29 is for example a membrane separation unit comprising at least one membrane allowing good separation of carbon dioxide and ethane.

The treatment of the gaseous stream 23 by the unit 29 makes it possible to produce a gaseous stream 30 enriched in carbon dioxide and depleted in ethane and a gaseous stream 31 enriched with methane and ethane that can optionally be subsequently mixed with the gas stream of heavy hydrocarbons. 28.

A stream of natural gas 31 enriched in methane, ethane and heavy hydrocarbons but depleted in carbon dioxide and other acid gases is thus produced by the implementation of the method which is the subject of the invention.

In fact, the gaseous stream 30 enriched in carbon dioxide and depleted in ethane is compressed using one or more compressors 20. This pressure is that of the stream supplying the first distillation column 14. The gaseous stream 30 thus compressed is introduced at least partly at the bottom 32 of the first distillation column 14. The foot 32 of the column 14 is located below the main supply 33. This makes it possible to produce a reboiler function for this first column 14 and in particular to extract ethane from this first column 14 because the gaseous stream rising in the column is very low in ethane (typically below 0.1 % mol of ethane and at most five times less ethane than in the feed gas), to the gaseous stream 18 extracted at the top of the column 14. This introduction 32 of the ethane-depleted gas in column 14 can be performed before or after a passage in a heat exchanger, said gas being mixed or not with another stream.

Another part of the gaseous stream 30 enriched with carbon dioxide and compressed is directly mixed with the gas stream 3 to produce a mixture 3 'introduced into the exchanger 4.

In FIG. 2, the cryogenic separation unit (A) comprises at least the elements 4, 6, 9, 10, 13, 14, 17.

The invention is not limited to only this cryogenic purification configuration. Some examples of modifications for which the invention can still be applied are listed below: • Number of exchangers different: exchangers 9 and 4 could possibly be combined in a single exchanger or in at least three exchangers.

• Number of different distillation columns: the invention also applies to processes comprising only one distillation column, whatever the pressure.

• Number of separator pots: the invention can be applied when there are at least three separator pots but it can also be applied with a, see no separator pot.

· Different refrigeration system: Figure 2 shows a self-refrigerated scheme, ie a scheme where refrigeration is provided by expansion of liquid CO2. It would be possible not to relax the liquid CO2, for example, and instead to pump it, using an external refrigeration system (CO2 cycle, propane cycle or any other refrigeration cycle).

Table 1 below summarizes the material balance made throughout the process illustrated in Figure 2.

The numbers of the "currents" (first column of the table) are the numbers in figure 2.

Table 1: Associated material balance (in Nm ³ / h). The distillation columns used in the process according to the invention are, for example, columns chosen according to one of the following two types: column with trays or column with packing (structured or not).

• The role of the distillation column is to promote the exchange of material and energy between the gas phase and the liquid phase, which increases the separating power of the column.

• Distillation columns use the difference in volatility of the components of a mixture to separate them. To improve the separation, a large exchange surface between the gas phase and the liquid phase is necessary. To increase the latter, elements are added in the column, such as trays or packings, the latter may be structured or not. In addition to the column and its lining, two heat exchangers can provide / remove the energy required for separation: a boiler located at the bottom of the column where the mixture is heated to boiling and the condenser at the top column that allows to liquefy the vapors to recover the purified product in liquid form. Part of the condensate is often reinjected into the column to increase the purity of the desired product, it is reflux. In the case of CO2, it can often happen, as is the case in the example of Figure 2 that there is no condenser as such, because the temperature of the column head is too close the temperature at which CO2 could solidify. In the example of Figure 2, the exchanger 9 and the separator pot 10 are in some ways the role of condenser.

The method which is the subject of the present invention mainly makes it possible to minimize the loss of hydrocarbons. This advantage is illustrated below by considering a conventional scheme where the stream from the membrane separation unit is simply mixed with the feed gas and comparing it to the process object of the present invention and by the example described in FIG. figure 2.

Diagram according to the example

Classical diagram of the illustrated invention fiqure 2

Yield C2 45% -47% 70%

PCS yield 93% -94% 95.4% The C2 yield is defined as the ratio of the molar amount of ethane in the final product to the molar amount of ethane in the feed gas.

The PCS yield is defined as the ratio of Qfinal by Qalim where Qfinal is the product of the higher heating value (in J / Nm ³ ) of the final product multiplied by the molar flow of the final product and Qalim is the product of the higher heating value (in J / Nm ³ ) of the feed gas multiplied by the molar flow rate of the feed gas.

Claims

1. A method of treating a gaseous stream (1) containing at least one acid gas, ethane and a light gas selected from methane, carbon monoxide, hydrogen and nitrogen comprising the steps of:

Step a): feeding with said gas stream (1) a cryogenic separation unit (A) comprising at least one distillation column (14) for treating said gas stream (1), the treatment comprising the partial removal of the gas acid of said gas stream and producing a gaseous stream depleted in acid gas and said at least one distillation column (14) comprising main supply means (33) of a liquid stream;

Step b): removal of the cryogenic separation unit (A) from at least a portion (23) of the gaseous treated stream depleted in acid gas;

Step c): feeding with at least a portion of the gaseous acid-treated gas stream (23) of a physical separation unit (29) separating the acid gas from ethane, wherein said stream is separated to form a residue enriched with light gas and ethane (31) and a stream (30), permeate, enriched with acid gas;

Step d): return of the stream (30) enriched in acid gas in the cryogenic separation unit (A) by introducing it separately from the feed defined in step a) of the cryogenic separation unit (A) and below the main feed means (33) of the column (14), for extracting ethane from one of at least one distillation column (14).

2. Method according to the preceding claim characterized in that it comprises step e): recovering, from the cryogenic separation unit (A), a stream containing more than 90% of the acid gas initially present in the gas stream to be treated.

3. Method according to one of the preceding claims characterized in that it comprises the additional step:

f) recovering, from the physical separation unit (29), a gaseous stream containing more than 95% of the light gas initially present in the stream gaseous to be treated and more than 45% of the ethane initially present in the gaseous stream to be treated (1).

4. Process according to any one of the preceding claims, characterized in that the acidic gas is chosen from CO2 and H ₂ S.

5. Method according to any one of the preceding claims, characterized in that the acid gas is carbon dioxide and the light gas is methane.

6. Process according to any one of the preceding claims, characterized in that the gaseous stream to be treated (1) comprises from 10 mol% to 75 mol% of methane, from 25 mol% to 90 mol% of carbon dioxide and from 0 mol% to , 1 mol% to 15 mol% of ethane.

7. Method according to the preceding claim characterized in that said gaseous stream to be treated (1) further comprises hydrogen sulphide and hydrocarbons having at least three carbon atoms.

8. Method according to any one of the preceding claims characterized in that, at a pressure between 5 bara and 100 bara, the temperature in the cryogenic separation unit (A) is less than -30 ° C and preferably included between - 40 ° C and - 60 ° C.

9. Method according to any one of the preceding claims characterized in that the physical separation unit (29) is a membrane separation unit comprising at least one membrane.

10. Method according to the preceding claim characterized in that said at least one membrane is a polymer membrane.

1 1. Process according to one of Claims 1 to 9, characterized in that the physical separation unit (29) is a pressure modulation adsorption unit (PSA).