RU2774658C1 - Method for producing methanol - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method for joint synthesis of methanol, ammonia, and carbon monoxide, and to a unit for implementation thereof. The proposed method includes the stages of: a) synthesising methanol by means of catalytic conversion of a first hydrogen- and carbon oxide-containing synthesis gas (12), b) synthesising ammonia by means of catalytic conversion of a second synthesis gas (25) constituting a hydrogen- and nitrogen-containing ammonia makeup gas, and c) synthesising carbon monoxide by means of oxidation of a methane-containing stream. At stage (a) thereof, a liquid methanol stream (13) and a gas stream (14) containing unreacted synthesis gas are generated; part (14a) of the gas stream containing unreacted synthesis gas is separated as a purge gas stream; said purge gas stream is directed to the hydrogen extraction stage, resulting in a hydrogen-containing stream (19) and methane- and CO-containing tail gas (20), and at least part of the hydrogen-containing stream (19) is a source of hydrogen for the ammonia makeup gas (25) undergoing conversion at stage (b), and at least part of the tail gas (20) is a source of methane for the methane-containing stream undergoing oxidation at stage (b).

EFFECT: more efficient recirculation of the stream of purge gas removed from the synthesis circuit of a methanol production plant.

15 cl, 1 dwg

Description

Technical field

The present invention relates to the field of methanol production. State of the art

The methanol synthesis process basically involves obtaining make-up synthesis gas containing oxides of carbon (CO, CO ₂ ) and hydrogen (H ₂ ) by reforming or partially oxidizing hydrocarbon feedstock in the head section, and converting this make-up synthesis gas to methanol in the loop synthesis.

The conversion of make-up gas to methanol is carried out at high temperature (200-300°C) and high pressure (70-100 bar) in the presence of an appropriate catalyst, and includes the following reactions of hydrogenation of carbon oxides (CO, CO ₂ ) and reversible water gas shift:

These reactions are characterized by unfavorable conditions of thermodynamic equilibrium, and only a part of the make-up synthesis gas is converted into methanol in one pass over the catalyst.

Therefore, the stream containing the unreacted gas is separated from the raw methanol stream obtained by the make-up gas shift.

Purification of the raw methanol stream is usually carried out in two stages: the first stage removes light ends, and the second stage removes heavier ends and water, and the result is purified methanol of the required quality.

The stream containing the unreacted gas is separated into a first part recycled (returned) to the synthesis loop for additional reaction, and a second part continuously withdrawn from the synthesis loop to prevent the accumulation of inert compounds, mainly including methane, argon and nitrogen. This second part is also called the purge gas stream and is used primarily as fuel in the burners of the reformer section.

The purge gas, however, is bled off at high pressure (approximately synthesis loop pressure), while the burners operate at low pressure. This means that the pressure energy of the purge gas is lost.

In addition, the purge gas contains converted components which are combusted as fuel for the reformer burners. This means that the energy used for reforming is wasted.

For these reasons, recycling the purge gas for use as burner fuel is not an acceptable and efficient solution.

US 2011/0236293 discloses an integrated syngas scrubber and process for simultaneously producing a hydrogen stream, hydrogen-rich syngas, hydrogen-depleted syngas, and optionally a carbon monoxide stream.

Disclosure of invention

The present invention is directed to overcoming the disadvantages of the prior art. In particular, the invention aims to provide more efficient recycling of the purge gas stream from the synthesis loop of the methanol plant.

The invention is based on the use of a purge gas stream as feedstock for the synthesis of ammonia and carbon monoxide, with the advantage of simultaneous production of methanol.

These problems are solved in the method of joint synthesis of methanol, ammonia and carbon monoxide according to paragraph 1 of the claims.

The proposed combined method includes:

a) synthesis of methanol by catalytic conversion of the first synthesis gas, namely a gas containing hydrogen and carbon oxides;

b) synthesizing ammonia by catalytic conversion of a second synthesis gas, namely an ammonia make-up gas comprising hydrogen and nitrogen, and

c) synthesis of carbon monoxide by oxidation of a methane-containing stream, wherein:

step (a) produces a liquid methanol stream and a gas stream containing unreacted synthesis gas;

a portion of this gas stream containing unreacted synthesis gas is separated as a purge gas stream;

the purge gas stream from the hydrogen recovery step produces a hydrogen-containing stream and a tail gas containing methane and CO, and

at least a portion of the hydrogen-containing stream is a source of hydrogen for the ammonia make-up gas being converted in step (b), and at least a portion of the tail gas is a source of methane for the methane-containing stream being oxidized in step (c).

Preferably, the first synthesis gas to be converted in step (a) is obtained by the conversion of a hydrocarbon feed. This conversion step may include partially oxidizing the hydrocarbon feed, or preferably reforming the hydrocarbon feed. The first synthesis gas thus obtained is then preferably compressed to a synthesis pressure in the range of 70-100 bar. Next, the first synthesis gas is converted into raw methanol under synthesis pressure inside the synthesis loop, and the raw methanol thus obtained is separated into a liquid methanol stream and a gas stream containing unreacted synthesis gas.

According to a preferred embodiment, the method includes the step of separating air in a dedicated air separation unit (ASU) generating a nitrogen stream and an oxygen stream. At least a portion of the nitrogen stream is a source of nitrogen for the ammonia make-up gas and at least a portion of the oxygen stream acts as an oxidant in the oxidation of the methane-containing stream. A portion of the oxygen stream may also be used as a source of oxygen in the aforementioned step of converting the hydrocarbon feed to first synthesis gas.

The gas stream containing unreacted synthesis gas is preferably separated into a purge synthesis gas stream and a stream recycled back to the conversion stage to raw methanol for further reaction.

According to a preferred embodiment, the hydrogen recovery step is carried out by means of separation membranes.

Preferably, prior to the hydrogen recovery step, the purge gas stream is subjected to an aqueous scrubbing where residual methanol is absorbed into the water and removed in the water stream, resulting in a methanol-free purge gas stream. An aqueous wash step is particularly preferred when separation membranes are used in a subsequent hydrogen recovery step, as methanol is removed, which can be problematic for the operation of these membranes.

Preferably, the pressure of this hydrogen-containing stream at the outlet of the separation membranes is about 25-30 bar less than the pressure of the purge gas stream at the inlet of these separation membranes. Accordingly, the pressure drop of the hydrogen-containing stream across the separation membranes is about 25-30 bar.

Preferably, the pressure of the tail gas at the outlet of the seals is 2-4 bar less than the pressure of the purge gas stream at the inlet of the seals. Accordingly, the pressure drop of this tail gas across the isolating diaphragms is approximately 2-4 bar.

Preferably, at least 90% of the hydrogen contained in the purge gas stream obtained by separating the gas stream containing unreacted synthesis gas is recovered in the hydrogen recovery step and collected in said hydrogen-containing stream. Preferably, the hydrogen-containing stream has at least 85 vol. % hydrogen.

Preferably, at least 70% of the methane contained in the purge gas stream and at least 60% of the carbon monoxide contained therein are contained in the tail gas withdrawn from the hydrogen recovery stage. This tail gas preferably contains about 60 vol. % methane and about 2 vol. % carbon monoxide.

According to an embodiment of the invention, the synthesis of ammonia comprises: the step of purifying a hydrogen-containing stream to obtain a purified hydrogen-containing stream; mixing this purified stream with an appropriate amount of nitrogen to form an ammonia make-up gas and compressing this make-up gas to synthesis pressure in a multi-stage compressor. This nitrogen is preferably produced in the aforementioned air separation unit (ASU) and introduced into said purified hydrogen stream at the suction inlet of the multi-stage compressor.

Preferably, the purification step is carried out in a pressure swing adsorption unit (PSA or PSA). Preferably, the pressure drop across this ADF is about 1 bar and the pressure of the purified hydrogen containing stream at the outlet of this ADF is in the range of 40-50 bar.

The APD unit preferably recovers from this purified stream about 80-90% of the hydrogen originally present in the stream prior to purification. The cleaned stream contains inert gases (ie, methane and argon) in an amount preferably not exceeding 2000 ppmv (parts per million by volume), more preferably in the range from 700 to 2000 ppmv. The cleaned stream contains less than 10 ppm of compounds such as CO, CO ₂ , O ₂ and water, which poison the ammonia synthesis catalyst.

Also, tail gas containing methane, hydrogen, nitrogen, CO, CO ₂ , Ar and water is removed from the APD unit. Preferably, the tail gas is at least partially recycled as fuel for the step of converting the hydrocarbon feed to the first synthesis gas, for example for the burners of the reformer section.

In accordance with an embodiment of the invention, in the synthesis of carbon monoxide: after the stage of extraction of hydrogen, the tail gas containing methane and carbon monoxide is subjected to partial oxidation in the corresponding PO unit, where methane is oxidized to CO, forming a synthesis gas containing CO; subjecting the synthesis gas to a CO ₂ removal step and then separating the CO to obtain a CO stream and another stream containing hydrogen.

Considering that the pressure drop in the PR block is 8-10 bar, the operating pressure of this block is chosen so as to optimize the operation of the next CO separation block. The CO separation unit can be an air separation column, or it can contain separation membranes, depending on the final requirements for the CO product.

Preferably, said other hydrogen-containing stream is mixed with the hydrogen-containing stream obtained from the above hydrogen recovery step. In another embodiment, the hydrogen-containing stream is recycled as fuel to the stage of conversion of the hydrocarbon feed to the first synthesis gas, for example, for the burners of the reformer section.

Another object of the present invention is the installation in accordance with the attached formula.

In particular, the installation is different in that it includes:

means for directing at least a portion of the hydrogen-containing stream from the methanol section to the ammonia section to provide at least a portion of the hydrogen needed to produce ammonia make-up gas, and

means for directing at least a portion of the tail gas from the methanol section to the carbon monoxide section to provide at least a portion of the methane for oxidation.

Installation may also include:

an air separation unit creating a nitrogen flow and an oxygen flow.

means for directing at least a portion of the nitrogen stream to the ammonia section to provide at least a portion of the nitrogen needed to produce ammonia make-up gas, and

means for directing at least a portion of the oxygen stream (30) to the carbon monoxide section to provide at least a portion of the oxidant for methane oxidation.

The means mentioned above may include piping and the necessary components for directing the respective gas flow, such as valves and a compressor or fan, if necessary.

The present invention has the following advantages.

The first benefit is the maximum energy utilization of the purge gas stream leaving the methanol synthesis loop as a high pressure stream containing valuable reforming components (e.g. hydrogen and carbon oxides), thereby reducing the overall energy consumption for methanol, ammonia and monoxide production. carbon.

Another advantage is the reduced capacity of the synthesis gas compressor in the ammonia section compared to existing technologies.

Moreover, the hydrocarbon purification sections normally present in existing ammonia and carbon monoxide plants are no longer used. In addition, there is also no hydrocarbon-to-synthesis gas conversion section and shift reactor typically found in existing ammonia plants.

In addition, the CO synthesis gas generation section is reduced in size (about 5%) due to the presence of CO in the purge gas. Moreover, the operation of the CO separation plant (air separation column or membranes) is optimized by selecting the most suitable operating pressure and technologically advanced equipment for it.

Another advantage is that the boiler process water (HWS) and the steam system are common to the production equipment for three different products.

The present invention also has a number of advantages from an industrial point of view, in particular: reduction of capital costs (material consumption) for co-production compared to three production lines for each product; a mutually beneficial interaction of capital costs for the APD unit, which can be used as an oxygen source for methanol production and CO production, and, at the same time, can be used as a nitrogen source for ammonia production.

The advantages of the invention will be better understood upon consideration of the following detailed description of the preferred embodiment shown in FIG. one.

Detailed description of the invention

Combined plant 1 shown in FIG. 1 includes a methanol synthesis section 100, an ammonia synthesis section 200, and a carbon monoxide synthesis section 300. This plant also includes a membrane-based hydrogen recovery unit (HRU) 400 and an air separation unit (ASU) 500.

The methanol synthesis section 100 mainly includes a natural gas purification unit 101, a reforming unit 102, a synthesis loop 103, a water washing column 104, and a distillation unit 105. According to this example, the reforming unit 102 performs reforming with pure steam in the presence of steam. The synthesis circuit 103 mainly contains a catalytic reactor, a condensation section and a separator.

The reformer 102 produces synthesis gas 12 which is compressed in a multi-stage compressor (not shown) and then reacted in the synthesis loop 103. Synthesis loop 103 produces a liquid methanol stream 13 and an unreacted synthesis gas gas stream 14. The liquid methanol stream 13 is sent to a distillation unit 105 and the unreacted gas stream 14 is washed with water in column 104.

The gas stream 17 discharged from the water washing column 104 is supplied to the hydrogen recovery unit 400 from the hydrogen-containing stream 19, and the tail gas 20 containing methane and CO is released.

The hydrogen-containing stream 19 enters the ammonia section 200, which mainly includes a purification unit 201, a multi-stage compressor 202, and an ammonia synthesis circuit 203. In accordance with the example shown in the figure, the purification unit 201 is a pressure swing adsorption (PSA) unit.

The tail gas 20 containing methane and CO enters a carbon monoxide section 300 mainly including a partial oxidation (PO) unit 301, a CO2 removal section 302, and a CO _separation unit 303. In accordance with the example shown in the figure, the CO separation unit 303 is an air separation column.

The following is a more detailed description of the operation of installation 1.

The natural gas stream 10 is divided into a first part 10a and a second part 10b.

The first part 10a is fed to the purification unit 101 of the methanol synthesis section 100, in which sulfur and other contaminants are removed to obtain a purified natural gas feedstock 11.

The second part 10b and the purified feedstock 11 to the reformer 102 where they are reformed to produce synthesis gas 12. Synthesis gas 12 contains oxides of carbon (CO, CO ₂ ), hydrogen and inert gases.

Synthesis gas 12 is compressed in a multi-stage compressor (not shown) to a synthesis pressure of about 70-100 bar. The resulting compressed gas is fed to the synthesis loop 103, where raw methanol is obtained, which is separated into the above-mentioned liquid methanol stream 13 and the gas stream of unreacted synthesis gas 14.

The liquid methanol stream 13 is purified in a distillation unit 105 .

The gas stream of unreacted synthesis gas 14 is divided into a first part 14a and a second part 14b. The first portion 14a (also referred to as the purge gas stream) enters the wash column 104, which also receives the wash water 15 and which removes residual methanol in the water stream 16, resulting in a methanol-free stream of unreacted synthesis gas 17. This second part 14b is sent back to the reformer 102 for further reaction.

The methanol-free stream 17 is fed to a hydrogen recovery unit 400 separating the already mentioned hydrogen-containing stream 19 from the tail gas 20 containing inert components such as methane and carbon monoxide.

The hydrogen-containing stream 19 is mixed with another hydrogen-containing stream 37 exiting the CO recovery unit 303 . The resulting hydrogen-containing stream 21 is sent to the APD unit 201 of the ammonia synthesis section 200, in which a purified hydrogen stream 22 and a tail gas 23 containing methane, CO, CO ₂ , H ₂ , Ar, N ₂ and water are obtained. This tail gas, for example, is used as fuel in the reformer unit 102 of the methanol section 100.

The pressure drop across the APD unit 201 is in the order of 1 bar, and the purified stream 22 enters the suction inlet of the synthesis gas compressor 202 at a pressure of 40-50 bar.

At the suction inlet of the compressor 202, this purified stream 22 is mixed with nitrogen 24 produced by the air separation unit (ASU) 500 to form ammonia make-up synthesis gas 25.

Make-up gas 25 is compressed to ammonia synthesis pressure in compressor 202 and fed to synthesis loop 203 producing ammonia 26 and a tail gas (not shown) that can be used as fuel for the reformer.

The tail gas 20 from the hydrogen recovery unit (HER) 400 is fed to the partial oxidation (PO) unit 301 of the carbon monoxide synthesis section 300, where the methane contained in the tail gas reacts with oxygen 30 and steam 31 to produce a CO rich synthesis gas 32, low CO ₂ content and low methane slip. Oxygen 30 is supplied from an air separation unit (ASU) 500 .

Saturated high pressure steam (HPS) or medium pressure steam (MPS) is produced in the heat recovery system of the CHO unit 301 (represented by flow 33 in the figure). This steam is used, for example, as a working medium in the turbines of the 500 BRV unit after overheating in the section of the methanol synthesis unit.

Synthesis gas 32 enters a CO ₂ removal section 302 which produces CO ₂ depleted synthesis gas 34 and a tail gas 35 containing predominantly CO ₂ . This tail gas 35 is vented to the atmosphere.

The CO ₂ -depleted synthesis gas 34 is sent to an air separation column 303, which produces a stream 36 containing CO and a tail gas 37 containing hydrogen. Stream 36 is withdrawn from the plant. Stream 37 is mixed with hydrogen containing effluent stream 19 BIV 400 with the formation of the input stream 21 unit 201 APD.

Example

When using a clean steam reforming methanol synthesis section sized for a capacity of 150 metric tons per day (mt/d) and an RDF sized for a capacity of 160 mt/d oxygen and 460 mt/d nitrogen, the above method is in accordance with with the invention makes it possible to obtain approximately 550 mt/d of ammonia and 150 mt/d of CO.

Claims (34)

1. A method for the joint synthesis of methanol, ammonia and carbon monoxide, including: a) synthesis of methanol by catalytic conversion of the first synthesis gas (12) containing hydrogen and carbon oxides, b) the synthesis of ammonia by catalytic conversion of the second synthesis gas (25), which is an ammonia make-up gas containing hydrogen and nitrogen, and c) synthesizing carbon monoxide by oxidizing a methane-containing stream, wherein in step (a) a liquid methanol stream (13) and a gas stream (14) containing unreacted synthesis gas are produced; part (14a) of the gas stream containing unreacted synthesis gas is separated as a purge gas stream; said purge gas stream is sent to a hydrogen recovery step to produce a hydrogen-containing stream (19) and a tail gas (20) containing methane and CO; and at least a portion of the hydrogen-containing stream (19) is a hydrogen source for the ammonia make-up gas (25) being converted in step (b), and at least a portion of the tail gas (20) is a methane source for the methane-containing stream being oxidized at stage (c). 2. The method according to p. 1, including the stage of air separation in a special air separation unit to obtain a stream (24) of nitrogen and a stream (30) of oxygen, and at least part of the stream (24) of nitrogen is a source of nitrogen for ammonia make-up gas (25 ), and at least a portion of the oxygen stream (30) acts as an oxidizer in the oxidation of the methane-containing stream. 3. Process according to claim 1 or 2, wherein the purge gas stream (14a) is sent to a water wash stage prior to the hydrogen recovery stage. 4. A process according to any one of the preceding claims, wherein the hydrogen recovery step is carried out by separating membranes. 5. Method according to claim 4, wherein the pressure of the hydrogen-containing stream (19) at the outlet of the separation membranes is 25-30 bar less than the pressure of the purge gas stream (17) at the inlet of the separation membranes. 6. Method according to claim 4 or 5, wherein the pressure of the tail gas (20) at the outlet of the separation diaphragms is 2-4 bar less than the pressure of the purge gas stream (17) at the inlet of the separation diaphragms. 7. Process according to any of the preceding claims, wherein at least 90% of the hydrogen contained in the purge gas stream is recovered into a hydrogen-containing stream (19), the hydrogen content of this stream (19) being preferably at least 85 vol. %. 8. The method according to any one of the preceding claims, wherein at least 70% of the methane contained in the purge gas stream (14a) and at least 60% of the carbon monoxide contained therein are recovered in the tail gas (20), wherein the methane content in the specified tail gas (20) is preferably about 60 vol.%, and the content of carbon monoxide is preferably about 2 vol.%. 9. The method according to any of the preceding paragraphs, in which the synthesis of ammonia includes the following stages: purification of the hydrogen-containing stream (19) to obtain a purified hydrogen-containing stream (22); mixing the purified stream with a suitable amount of nitrogen to produce an ammonia make-up gas (25); compressing said make-up gas (25) to synthesis pressure. 10. The method according to claim 9, wherein the purification is performed in a pressure swing adsorption unit (PSD), wherein the pressure drop across the SPD is about 1 bar and the pressure of the purified hydrogen-containing stream (22) at the outlet of the SPD is in the range of 40 -50 bar. 11. The method according to p. 10, in which the synthesis of methanol includes the stage of conversion of hydrocarbon feedstock (11) into the first synthesis gas (12), and the tail gas (23) containing methane, hydrogen, nitrogen, CO, CO ₂ , Ar and water is withdrawn from the APD unit and at least partially recycled as fuel to the conversion stage. 12. The method according to any one of the preceding paragraphs, in which the synthesis of carbon monoxide includes the following stages: partial oxidation of the tail gas (20) obtained in the stage of extraction of hydrogen, with the formation of CO-containing synthesis gas (32); the stage of extracting CO ₂ from the specified synthesis gas and subsequent separation of CO, obtaining stream (36) CO and additional containing hydrogen stream (37). 13. Process according to claim 12, wherein the additional hydrogen-containing stream (37) is combined with the hydrogen-containing stream (19) obtained from the hydrogen recovery step. 14. Installation, including section (100) for the synthesis of methanol, section (200) for the synthesis of ammonia and section (300) for the synthesis of carbon monoxide, wherein the methanol synthesis section contains: a reforming section (102) for reforming the hydrocarbon feedstock (11) to produce synthesis gas (12); a synthesis gas compressor for increasing the pressure of the synthesis gas (12) to the synthesis pressure; a synthesis loop (103) providing a liquid methanol stream (13) and a gas stream (14) containing unreacted synthesis gas; a hydrogen extraction unit configured to receive a part of said gas stream and produce a hydrogen-containing stream (19) and a tail gas (20) containing methane and CO, at the same time, the ammonia synthesis section (200) contains a synthesis circuit (203), in which the ammonia make-up gas (25), including hydrogen and nitrogen, is converted into ammonia, the carbon monoxide synthesis section (300) contains a partial oxidation reactor (301) in which methane is oxidized to carbon monoxide, characterized in that it includes: means for directing at least a portion of the hydrogen-containing stream (19) from the methanol section (100) to the ammonia section (200) to provide at least a portion of the hydrogen needed to produce the ammonia make-up gas (25), and means for directing at least a portion of the tail gas (20) from the methanol section (100) to the carbon monoxide section (200) to provide at least a portion of the methane for oxidation. 15. Installation according to claim 14, including an air separation unit (500), providing a stream (24) of nitrogen and a stream (30) of oxygen, and additionally including: means for directing at least a portion of the nitrogen stream (24) to the ammonia section (200) to provide at least a portion of the nitrogen needed to produce the ammonia make-up gas (25), and means for directing at least a portion of the oxygen stream (30) to the carbon monoxide section (200) to provide at least a portion of the oxidant for methane oxidation.

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