NO318532B1 - Process for deacidification of a gas with a very high acid content - Google Patents

Description

The present invention relates to a method for treating a gas containing at least one acid gas and at least one hydrocarbon in order to remove at least part of the acid gases from the said fluid, as well as a device for processing a gas comprising at least one acid gas and at least one hydrocarbon, comprising at least one supply line (1) for said gas to be processed and at least one supply line (2) for a physical solvent capable of collecting acid gases and further application of the method.

The purpose of the present invention is to process a gas containing at least one hydrocarbon and at least one acid gas of high concentration in order to free it at least partially from the acid gas contained therein.

The method according to the invention is particularly suitable for deacidifying a natural gas.

French patents FR-2,605,241 and FR-2,636,857 describe refining processes using a cooled physical solvent which makes it possible to carry out the various stages of processing a gas, including deacidification and removal of water.

These processes have the particular advantage, compared to previous techniques, that they make it possible to use the same polar solvent for dewatering and deacidification of the gas, and make it possible to regenerate the solvent through direct contact, in an absorption tower or a contactor, between the solvent and the gas to be processed (patent FR-2,605,241), which leads to significant gains in terms of power consumption, investment and facility size.

However, for gases containing very high concentrations of acid gas, the implementation of such methods presents disadvantages. The gas to be processed must be cooled before it is fed into the sour gas absorption tower. This cooling stage leads to the condensation of a very large amount of hydrocarbons and of acid gases. This liquid phase must be stabilized, which leads to the production of stabilization gas in large quantities, which must then be recompressed and recycled. The presence of this recompression step damages this type of method because it involves high investment and operating costs.

The present invention overcomes the disadvantages of the prior art by proposing another possibility that uses an absorption tower operated with a temperature gradient. The gas to be processed is led, for example, without a prior cooling step, into the tower, where a cold, polar physical solvent circulates countercurrently.

Absorption of acid gases in the tower advantageously prevents condensation of these acid gases in a liquid hydrocarbon phase. The hydrocarbon dew point of the gas at absorption pressure actually drops sharply when the acid gas content decreases.

Figure 1 shows, in a temperature (T) - pressure (P) diagram, two enveloping curves A and B corresponding to the gas to be processed and to the processed gas, and the corresponding dew points bearing the reference letters a and b.

Line C represents the thermal profile AT existing in the absorption column, point a' corresponding to the operating conditions for the first stage, and b' for the upper stage.

Figure 1 shows the extent of the thermal profile in the absorption column for a given calculated example. The development of the enveloping curve of the gas in this tower makes it possible to prevent the formation of a liquid hydrocarbon phase; whatever stage is considered, the temperature is such that the gas is very far from the dew point of the hydrocarbon therein.

In the following description, the term "contact zone" or the term "contactor" is used arbitrarily to refer to the part of the device where a fluid to be processed is contacted with a solvent which is selective for acid gases.

Similarly, the term "rich solution phase" corresponds to a solution phase which is enriched in acid gas, and "lean solution phase" corresponds to a solution phase which is depleted of acid gas during the process.

The present invention relates to a method for treating a fluid, such as a gas containing at least one acid gas and at least one hydrocarbon, with the aim of freeing at least partially said fluid from acid gases.

It is characterized by the fact that it includes at least the following steps:

- countercurrent contact of said gas to be treated with a physical solvent which is selective towards said acid gases in a contactor, and simultaneously - formation of a temperature gradient in said contactor to prevent condensation of a liquid hydrocarbon phase and to obtain a processed gas depleted of acidic gases and a solvent phase enriched in acidic gases, said gas to be processed being introduced into the contactor at an inlet temperature Tg which is between 10°C and 50°C and said physical solvent being introduced into said contactor at an inlet temperature Tm which is between -50°C and 0°C. Various features and embodiments of the invention appear from claims 2-9.

The solvent is, for example, a physical, polar solvent.

The mixture of solvent and water is, for example, fed countercurrently to the gas to be processed, the gas to be processed being supplied at an initial temperature Tg and the solvent mixture at an initial temperature Tm, and the temperature values Tg and Tm are chosen with the aim of generating the emergency reverse gradient in the contactor, and Tg is higher than Tm.

At least a fraction of said gas to be processed can be extracted at a temperature level Ti, and said gas fraction can be cooled using a heat exchanger with at least a fraction of the processed gas, and injected again at a temperature level below Ti.

At least a fraction of the processed gas released from most of the acidic gases is used, for example, to cool the gas to be processed by circulating said fraction countercurrently in relation to said gas to be processed.

The temperature difference between the top and the bottom of the absorption column can vary, in absolute value, between 5 and 150°C, preferably between 30 and 100°C.

The solution phase enriched in acid gas can be regenerated by means of a simple expansion.

According to another embodiment, the solution phase, enriched in acid gas, is regenerated by means of a distillation process at a pressure below the absorption pressure.

The solution phase enriched in acid gas can be regenerated at least partially by expanding it and by contacting it with at least a fraction of regenerated solution phase and by producing a gas with a controlled content of acid gas.

According to another embodiment, the solution phase, enriched in acid gas, is at least partially regenerated using a distillation process with simultaneous heat exchange between said solution phase, which is gradually heated by the solution phase, as a result of said regeneration, which is cooled by circulating countercurrently in relative to the solution phase that is regenerated.

The water that accumulates during the process in the dissolution phase can be drained off.

The invention also relates to a device for processing a fluid such as, for example, a gas comprising at least one acid gas and at least one hydrocarbon.

It is characterized by the fact that it includes in compilation:

said supply line (1) and (2) is connected to a contactor (C1) to introduce said gas to be processed at an initial temperature that lies between +10°C and 50°C and to introduce said physical solvent at an initial temperature which lies between -50°C and 0°C, said contactor being suitable to work with a temperature gradient over at least part of the length of said contactor (C1) in order to obtain a gas depleted of acid gases at the top of the contactor and with outlet line (3) and for removal of said depleted gas a line (4), for withdrawal of solvent phase enriched in acid gases.

The contactor can include means for withdrawing and re-injecting the fluid and/or gas that circulates in said contactor.

The device includes, for example, a circuit that makes it possible to drain off at least part of the water that accumulates in the fluid that is capable of collecting the acidic gases.

The device and the method described above are used for processing a natural gas comprising the acidic gases such as C02 and/or H2S.

Other properties and advantages of the invention will become clear by reading the description which follows and which is given in the form of examples, with reference to accompanying drawings, in which: - Figure 1 shows the development of the phase diagrams of a gas in an absorption tower with a temperature gradient,

- Figure 2 shows in diagram form the principle behind the method according to the invention,

- Figures 3 to 5 describe embodiments that include different steps for regeneration of solvent, - Figures 6 and 7 show in diagram form examples of integration of energy around the contactor, - Figure 8 shows an example of a construction that makes it possible to remove the water that accumulates in the solvent, - Figure 9 shows in diagram form an exemplary embodiment of the method.

The principle behind the method according to the invention is described in connection with the diagram in Figure 2, which shows an example of a design of a device which is used, for example, for processing a natural gas in order to deacidify it.

The natural gas to be processed, containing acid gases and at least one hydrocarbon, is fed through line 1 into the absorption column or contact zone C1, where it is contacted, for example countercurrently, with a cold mixture of solvent and water flowing in through line 2, the dissolution phase comprising for example at least one polar solvent. A temperature gradient determined on the basis of the phase diagram of the gas to be processed (Figure 1) is generated in contact zone C1, so that condensation of a hydrocarbon phase containing acidic gases is prevented in this zone. In the contact zone, the acidic gases are selectively absorbed in the dissolution phase.

A gas phase depleted of acid gases is discharged from the upper part of contactor C1, through line 3.

In the lower part of the contact zone or contactor C1, a solution phase containing the acidic gases is drawn out through line 4.

The temperature gradient is generated, for example, by supplying the gas to be processed at the bottom of contactor C1, at a temperature varying between 10 and 50°C, and the solvent-water mixture from the top of the contactor at a temperature varying between 0 and -50°C .

The flow rate and temperature in the solvent at the top of the absorption column are advantageously controlled so that a desired concentration of acid gas is provided in the processed gas and that a condensation of hydrocarbons in the contactor is prevented.

The solution phase coming from contactor C1 is expanded through expansion valve V1, fed into separation cylinder B1, which makes it possible to separate it into a partially regenerated solution phase, drawn out from the bottom through a line 5, and a gas phase, released through a line 6 in top of the cylinder. This gas phase contains co-absorbed acid gases and hydrocarbons. It can optionally be used as fuel gas, or it can be recompressed and recycled to contactor C1.

The water content in the solution phase can preferably be at least 10% by weight.

The partially regenerated solution phase coming from cylinder B1 through line 5 is expanded through a valve V2, then fed into a cylinder B2, which separates the solution phase which was withdrawn from the bottom through a line 7 and the gas phase which was withdrawn from the top through a line 8. The solution phase is expanded through a valve V3, then fed into a separation cylinder B3, which separates a solution phase drawn out from the bottom of the cylinder through a line 9 and a gas phase drawn out from the top through a line 10.

The gas phases discharged through lines 6, 8 and 10 are rich in acid gases.

The solution phase from cylinder B3 is sufficiently depleted of acid gases to be recycled to contactor C1. It may first be necessary to withdraw at least part of the solvent through a line 11 to prevent the accumulation of water, which is carried with the gas to be processed and which is fed through line 1. The rest of the solvent is then recycled with a recycling pump P1, mixed with make-up solvent , which flows in through a line 13 to compensate for solvent loss, in the deacidified gas, in the acid gases produced and possibly in drain line 11, if the latter is not recycled after removal of the water it contains. The resulting mixture circulates in a line 14, it is cooled in an exchanger E1 using an external auxiliary coolant, for example, and recycled to contact zone C1 through line 2.

The refining process according to the invention essentially includes at least the following steps: - in an absorption tower, the gas to be processed with a fluid that is selective for the acidic gases is brought into contact with, for example, a mixture of polar solvent and water, and - causing a temperature rise in this absorption steam for the purpose of providing absorption of the acid gases without condensation of the hydrocarbons or of the acid gases, or at least, if such condensation occurs, that the condensed liquid phase is present in negligible quantities, - a processed gas depleted on acid gases and a solution phase enriched in acid gases is provided.

The absorption and temperature gradient can be provided by countercurrent circulation of the gas to be processed, which is supplied at the bottom of the contactor at a temperature Tg, and the solvent-water mixture, which is supplied from the top of the tower at a temperature Tm, with Tm < Tg.

The temperature difference | Tm - Tg | can vary between 5 and 100°C.

The value for the most common pressure in the absorption steam can vary between 1 and 20 MPa.

Figure 3 illustrates a variant of the regeneration stage, where a distillation tower C2 is used, placed after cylinder B1 in Figure 2, which makes it possible to carry out a deeper regeneration of the dissolution phase than that provided by using the construction in Figure 2.

The partially regenerated solvent from cylinder B1 is expanded through valve V2, then heated in an exchanger E2, before being fed, for example through a line 20, into the central part of distillation tower C2.

The acid gases produced by distillation are discharged from the top of tower C2 through a line 21, cooled in an exchanger E3 and fed into a decanting or separation cylinder D1. At the outlet from this decanting cylinder, a fraction of the acid gases is released through a line 23, and the liquid phase comprising the condensed solution phase is sent back through a line 22 to distillation tower C2 to be used as a return stream.

The regenerated solution phase is extracted from the bottom of distillation tower C2 through a line 24, cooled in an exchanger E2, before being sent through a line 25 to drain line 11 and recirculation pump P1 according to the construction described in connection with Figure 2.

Figure 4 illustrates an embodiment variant of the device in Figure 3, where the separation cylinder B1 is equipped with a contact zone, the purpose of which is to control the acid gas specification in a gaseous stream drawn out through line 6.

To achieve this, a fraction of the dissolution phase, regenerated and recycled according to the construction in Figure 3, is drawn out and sent through a line 26, located at the top of contact zone 27 in cylinder B1.

In this contact zone 27, part of the acid gases are absorbed using a specific amount of solvent, so that a gaseous discharge is provided, through line 6, with a controlled content of acid gas, which is selected on the basis of a desired specification. Figure 5 illustrates another type of solvent regeneration. Exchanger E2 in Figure 3 is replaced by a stripping exchanger ES1, which performs a distillation with simultaneous heat exchange between the rich solution phase, supplied through line 7 at the level of the upper part of the stripping exchanger, which is gradually heated, and the solution phase resulting from the regeneration stage is sent to the the lower part through line 24 which comes from distillation tower C2, and which is cooled by circulating countercurrently to the solution phase being regenerated.

At the outlet of stripping exchanger ES1, a rich solution phase is provided at the bottom and sent through line 20 to distillation tower C2, a rich gas is provided at the top and discharged through line 28, and in the upper part of the tower the regenerated solution phase is withdrawn through a line 29, which is connected to recirculation pump P1.

The internal heat exchange described above between the solution phase rich in acid gases and the solution phase depleted in acid gases can be carried out using different devices.

Integrated exchanger ES1 can be a shell-and-tube type exchanger. In this case, the regenerated solution phase, withdrawn from the bottom of tower C2, circulates in the tubes, distillation is carried out outside the tubes and inside the mantle.

Stripping exchanger ES1 can be a plate exchanger, where the tubes are placed above each plate so that they are dipped in the rich solution phase to be collected and circulated on each plate.

Stripping exchanger ES1 can be a continuous contact type exchanger, where the rich solution phase then sprinkles the pipes.

The regenerated dissolution phase, withdrawn from the bottom of tower C2, can also circulate outside the tubes and inside the jacket. Distillation is then carried out inside the tubes, which are preferably vertical and equipped with an internal filling material.

A vertical plate exchanger can also be used by the absorption towers and/or the pre-distillation exchanger described in the previous figures. The spaces between the different plates are alternately filled with the regenerated dissolution phase, extracted from the bottom of tower C2, and by the dissolution phase, as well as by the generated vapor phase circulating countercurrently in the tower during the distillation process. There is, for example, a type of exchanger made of brazed aluminum or stainless steel, where the plates are either butt-welded or diffusion-welded over the entire contact surface between the plates. Figure 6 shows an exemplary embodiment of the absorption stage, where the cold processed gas is used to at least partially cool the gas to be processed and which circulates in the absorption stream. The gas is withdrawn, during the processing, at the level of a plate in tower C1 or between two zones for filling material in tower C1 through a line 30, cooled in an exchanger E4 by the processed gas discharged through line 3, then reintroduced into tower C2 through a line 31. The levels for extraction and injection, as well as the number of these, are determined along the tower with the aim of optimizing cold reuse from the processed gas collected in line 3. Figure 7 shows another variant of the embodiment of the absorption stage, where simultaneous heat exchange is carried out between the processed gas, which is gradually heated, and the gas to be processed in contact with the solvent, which is cooled by countercurrent circulation. This type of "thermal exchange" can be carried out using various devices which are described above in connection with figure 5.

The processed gas coming from absorption tower C1 through line 3 is sent back through this line to the top of the tower, where it circulates countercurrently to the gas supplied at the bottom of the tower through line 1. After exchanging its cold with the gas to be processed , it flows out heated through a line 33 at the bottom of the tower, for example.

As the gas to be processed may contain water in various forms, it may in some cases be necessary to remove this water in order to prevent its accumulation in the dissolution phase.

Figure 8 shows a dewatering construction which makes it possible to remove the water which has accumulated in the dissolution phase during processing, and which comprises a contactor C3 placed in the stream below the temperature gradient contactor C1.

The raw gas to be refined is separated into two fractions which flow respectively through lines 35 and 36.

A first fraction passes through line 36 which joins line 1 (Figure 1), which feeds the gas into contactor C1.

The second gas fraction to be processed is sent through line 35 to the lower part of contactor C3.

The water-filled solution phase from drain line 11 (Figure 1) is fed to the top of contactor C3 by means of a pump P2 and by a line 34.

At the outlet from contactor C3, a solvent-laden gas is recovered and discharged from the top through a line 38 which connects with supply line 1, and a substantially solvent-free aqueous phase is recovered and discharged from the bottom of the contactor through a line 37.

The present invention will be made clear by reading the description below of a non-limiting example of gas processing.

This example, described in connection with Figure 9, makes it possible to deacidify a natural gas with a very high content of CO2.

The natural gas to be processed has, for example, the following composition in % based on moles:

This gas is saturated with water at a temperature of 30°C and at a pressure of 7 MPa. Its molar flow rate is 22,400 kmol/h. This gas is separated into two fractions in lines 35 and 36. The gas in line 35 is fed to contactor C3 at a molar flow rate of 17,000 kmol/h. A fraction of solvent containing approximately 6 kmol/h water and 54 kmol/h methanol is injected countercurrently in relation to this through line 34.1 the bottom of contactor C3, a stream of 17 kmol/h water, containing approximately 200 molar ppm methanol, is drawn out through line 37.

The methanol-filled gas coming from the top of contactor C3 is mixed with the gas from line 36, then with the recycled gas coming from regeneration cylinder B1 through a line 40. It is then sent to contactor C1. At the top of contactor C1, a solvent containing approximately 5,500 kmol/h of water and 49,500 kmol/h of methanol is injected through a line 2 at a temperature of -30°C. Under such conditions, contactor 1 is operated with a temperature gradient of 23°C at the bottom to -23°C at the top. The processed gas is withdrawn from the top of contactor C1 through line 3. It has a flow rate of 6,603 kmol/h, and contains approximately 87 mol% methane and 8 mol% CO2. This gas is heated in exchanger E to a temperature of 50°C before it is released through a line 41.

The rich solvent is withdrawn from contactor C1 through line 4. It is expanded to a pressure of 3.5 MPa through valve V1. At the outlet of separation cylinder B1, a solution phase is withdrawn through line 5, and a gas phase is discharged through line 6. The gas phase, containing approximately 6,800 kmol/h C02 and 1,300 kmol/h methane, is compressed to 7 MPa using a compressor K, cooled then to 30°C using an exchanger E to be recycled to the bottom of contactor C1 through line 40.

The solvent from line 5 is expanded to a pressure of 1 MPa through valve V2, separated in cylinder B2 into a gas phase drawn out through line 8 and a solvent phase drawn out through line 7. The gas from line 8 has a flow rate of 10,153 kmol/h and a concentration of 95 mol% CO 2 .

The solvent is expanded again to a pressure of 0.2 MPa through valve V3, separated in cylinder B3 into a gas phase drawn out through line 10 and a solution phase drawn out through line 9. The gas in line 10 has a flow rate of 5,666 kmol/h and a concentration of 98.5 mol% CO 2 .

A fraction of the solvent is withdrawn through drain line 11, sent to pump P2 for the purpose of being injected into contactor C3 through line 34. The other solvent fraction is sent to pump 1, cooled in exchanger E1 to -30°C before being fed into contactor C1 through line 2.

The method makes it possible to remove acid gases contained in a natural gas, as well as those included in a refinery gas or in any gaseous discharge that simultaneously contains hydrocarbons and acid gases. It allows the removal of acid gases such as H2S and CO2l and also mercaptans with the chemical formula R-SH, COS and CS2.

The various steps in the method according to the invention can be carried out in towers equipped with contact zones that allow substance exchange between the gas phases and the liquid phases. These contact zones are therefore equipped with perforated plates, bell plates, valve plates, or they can be continuous contact zones containing a filling material. This filling material can consist of filling elements, such as raschirings, pallet rings (liquid distributors), "Berl saddles" or any other filling material known to a person skilled in the field. It can also consist of stacked filling material made from gas, knitted fabric or from sheets that can be perforated or corrugated and/or shaped channels (forming channels) which make it possible to increase performance.

These plates or this filling material can be made of different materials, such as ceramics, aluminium, stainless steel, plastic mass.

The acid gas selective fluid may comprise a polar solvent selected from methanol, an alcohol, an ether, a polyethylene glycol ether, propylene carbonate.

It is also possible, without deviating from the scope of the invention, to use a solvent consisting of two polar solvents, or a polar solvent and an amine.

Claims (13)

1. Method for treating a gas containing at least one acid gas and at least one hydrocarbon to remove at least part of the acid gases from the said fluid, characterized in that it includes at least the following steps: - countercurrent contact of said gas to be treated with a physical solvent which is selective to said acidic gases in a contactor, and simultaneously - formation of a temperature gradient in said contactor to prevent condensation of a liquid hydrocarbon phase and to obtain a processed gas depleted in acid gases and a solvent phase enriched in acid gases, said gas to be processed being introduced into the contactor at an inlet temperature Tg which lies between 10°C and 50°C and said physical solvent being introduced in said contactor at an input temperature Tm which is between -50°C and 0°C. 2. Method according to claim 1, characterized in that at least one fraction of said gas to be processed is withdrawn at a temperature level Ti, said withdrawn gas fraction is cooled using a heat exchanger with at least one fraction of the processed gas and it is reinjected at a temperature level below Ti. 3. Method according to claim 1, characterized in that at least one fraction of the processed gas which has been freed of most of the acid gases is used to cool the gas to be processed by circulating said withdrawn fraction in countercurrent to the gas to be processed. 4. Method according to each of claims 1 to 3, characterized in that a temperature difference is generated, which varies in absolute temperature between 30 and 100°C between the top and bottom of the contactor. 5. Method according to each of claims 1 to 4, characterized in that the solvent phase enriched in acidic gases is regenerated by simple expansion. 6. Method according to each of claims 1 to 4, characterized in that the solvent phase enriched in acid gases is regenerated by a distillation process at a pressure below the absorption pressure. 7. Method according to each of claims 1 to 4, characterized in that the solvent phase enriched in acid gases is at least partially regenerated by expansion and contact of it with at least one fraction of regenerated solvent phase and by production of a gas with a controlled acid gas content. 8. Method according to each of claims 1 to 4, characterized in that the solvent phase enriched in acid gases is at least partially regenerated by a distillation process by simultaneous heat exchange between said solvent phase which is progressively heated with the solvent phase originating from said regeneration, which is cooled by circulation countercurrent to the solvent phase being regenerated. 9. Method according to each of the preceding claims, characterized in that the water accumulated during the process in the solvent phase is drained out. 10. Device for processing a gas comprising at least one acidic gas and at least one hydrocarbon, comprising at least one supply line (1) for said gas to be processed and at least one supply line (2) for a physical solvent capable of collecting acidic gases, characterized in that said supply line (1) and (2) are connected to a contactor (C1) to introduce said gas to be processed at an initial temperature that lies between +10°C and 50°C and to introduce said physical solvent at an initial temperature that is between -50°C and 0°C, said contactor being suitable to work with a temperature gradient over at least part of the length of said contactor (C1) to obtain a gas depleted of acid gases at the top of the contactor and with an outlet line (3) for removal of said depleted gas and a line (4) for withdrawal of solvent phase enriched in acid gases. 11. Device according to claim 10, characterized in that said contactor includes means for withdrawal and reinjection of the fluid and/or gas circulating in said contactor. 12. Device according to claim 10, characterized in that it comprises a circuit (11, C3) which enables emptying of at least part of the water accumulated in the fluid which is capable of collecting acid gases. 13. Use of the method according to each of claims 1 to 9 and the device required in each of claims 10 to 12 for processing a natural gas comprising acidic gases such as CO2 and/or H2S.