A system for methanol synthesis

Abstract

A reactive system for the synthesis of methanol from a make-up gas comprising hydrogen and carbon dioxide, comprising a catalytic converter and a steam system, wherein the converter includes at least one internal heat exchanger connected to the steam system and arranged to produce steam with heat of the methanol synthesis reaction, wherein the steam system includes a steam drum separated from the converter, the steam drum being connected to said heat exchanger via a feed line to feed water to the heat exchanger and a steam line to collect steam formed in the heat exchanger, wherein the steam system includes an electrical heater mounted internally in the steam drum and arranged to heat the feed water sent to the converter.

Description

[0001] A system for methanol synthesis

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of industrial production of methanol.

[0005] Prior art

[0006] Methanol is produced industrially by reacting a make-up synthesis gas (“makeup gas” abbreviated MUG, or “syngas”) containing hydrogen and carbon oxides.

[0007] A reactive system for conversion of said make-up gas typically comprises at least one catalytic converter in a synthesis loop. The synthesis loop includes the catalytic converter where methanol is formed, one or more heat exchangers where the hot effluent of the converter is cooled, a condenser where the reaction effluent, after cooling, is condensed to produce a methanol-containing liquid product, a separator where said product is separated from a gaseous fraction containing unreacted gas, a line arranged to return at least a portion of said gaseous fraction to the converter via a compressor.

[0008] The make-up gas is characterized by a stoichiometric number, which is a function of the moles of hydrogen, carbon monoxide and carbon dioxide, and by the molar ratio CO / CO2. The stoichiometric number is defined as SN = (H2-CO2) I (CO+CO2) and a make-up gas with SN equal to or greater than 2.0 is highly desirable for methanol synthesis. A high CO / CO2 ratio denotes a high reactivity of the gas meaning that less catalyst is required for producing a given amount of methanol.

[0009] In the converter, the considerable heat released by the exothermic conversion is generally removed by an internal heat exchanger connected to a steam system. The heat of reaction can be used to produce steam at a considerable pressure, which is an attractive way of recovering energy within the process.

[0010] Said heat exchanger can be directly immersed in the catalyst. A converter with a heat exchanger immersed in the catalyst is termed isothermal converter, because the temperature of the catalytic bed is maintained nearly constant within a narrow range. A converter wherein the heat of reaction is used to produce steam is termed steam rising converter.

[0011] The make-up gas can be produced by gasification of coal or by steam reforming or partial oxidation of a hydrocarbon such as natural gas. More recently, the production of hydrogen from renewable energy sources has emerged to reduce emissions and reduce the carbon intensity of the entire process. Conventionally, the sustainability of the process is identified by a colour. A coal-based process is named “brown”; a natural gas-based process is named “grey”; a process featuring carbon dioxide capture is named “blue” and a process where hydrogen is produced from biofuels or renewable electricity is named “green”. In recent years, the interest towards green hydrogen has increased dramatically.

[0012] Under certain circumstances, it may be desired to provide a heat input to the methanol converter. This is typically the case of start-up when the catalyst contained in the converter and the process streams must be heated to the operational temperature. Installing an additional heater in the process side (in direct contact with the make-up gas) is technically difficult, due to severe conditions of high partial pressure of hydrogen and CO.

[0013] It is a known practice to use the internal heat exchanger of the converter (steam side) for such purpose of heating the converter. Heat is provided by hot steam (“start-up steam”) produced elsewhere in the process. In a conventional coalbased or natural gas-based plant there is normally a large amount of sufficiently hot steam, coming for example from the reforming section. There are practical cases, however, wherein the availability of steam at a sufficient temperature for heating the converter is limited or not sufficient. Such lack of hot steam may arise when there are less hot process streams for heat recovery and steam production or heat for such purpose is unavailable for any reason. An example of considerable interest is given by the so-called green process where some or the totality of the hydrogen or syngas contained in the make-up gas is produced from renewable energy. A green process inherently has less hot streams from which steam can be generated, compared to a conventional fired reforming process.

[0014] Another problem which typically but not exclusively afflicts the green process concern the syngas production being dependent on intermittent power source. In such case, the fluctuations of the amount of hydrogen produced in the front-end cause noticeable variations of the load of the synthesis loop. The load may be defined as the amount of fresh make-up gas sent to the loop and converted to methanol, or the amount of methanol produced in the loop.

[0015] The problem of how to control the loop at a partial load arises whenever a plant is required to operate smoothly at a very low load such as 20% or 10% of nominal capacity. This problem is less common in coal-based or natural gas-based plants that can generally rely on a nearly constant input of fuel, and be designed to run almost constantly near 100% of the nominal capacity. In contrast, the problem is often faced in the green plants as most renewable sources like for example solar and wind are naturally subject to fluctuations.

[0016] WO 2023 / 110479 discloses a plant and process for the production of methanol wherein an electrical heater is arranged on the water feed line of the converter.

[0017] Summary of the invention

[0018] The invention addresses the above-mentioned problems in connection with a lack of hot steam for heating the converter when required (e.g. during start-up) and / or for control of the converter and the synthesis loop under load variations.

[0019] The above problems are solved with a reactive system for the synthesis of methanol and a method for controlling the reactive system according to the claims.

[0020] The invention uses an electrical heater to heat the boiling feed water which is sent to the internal heat exchanger of the converter. Accordingly, heat generated in the electrical heater is transferred to the reactive system and the invention compensates for a reduced or no availability of hot steam from the process. Said electrical heater is strategically positioned inside the pressure vessel of the steam drum connected to the converter. Said electrical heater can be fixed or removable according to embodiments of the invention. Furthermore, the electrical heater provides an additional means for controlling the reactive system during transients or when operation at reduced load is required.

[0021] The location of the electrical heater inside the steam drum has important advantages compared to the in-line electrical heater taught by WO 2023 110479. First, it does not require a separate vessel for the electrical heater. Second, the in-line electrical heater introduces a significant pressure drop which is avoided in the present invention. If the circulation from the steam drum to the converter is a forced circulation via a pump, this means that less power is absorbed by the pump. If the circulation is natural by gravity, the invention reduces the static head necessary to ensure circulation and, consequently, the elevation of the steam drum relative to the converter, so that the cost of the structure is greatly reduced.

[0022] Description of the invention

[0023] The invention concerns a reactive system for the synthesis of methanol comprising a catalytic converter and a steam system, wherein the converter includes at least one internal heat exchanger connected to the steam system and arranged to produce steam with heat of the methanol synthesis reaction, wherein the steam system includes a steam drum separated from the converter, the steam drum being connected to said heat exchanger via a water feed line, arranged to feed water from the steam drum to said heat exchanger, and a steam collection line adapted to collect steam formed in the heat exchanger, wherein the steam system includes an electrical heater mounted internally in the steam drum and arranged to heat the feed water sent to the converter.

[0024] In a preferred embodiment, said electrical heater includes a bundle of electrically heated tubes inside the pressure vessel of the steam drum.

[0025] The electrical heater may have a modular structure, for example being formed by a plurality of electrically heated elements.

[0026] In an embodiment, the system includes at least one inline electrical heater in addition to said electrical heater incorporated in the steam drum. Preferably, an in-line electrical heater is provided on a water inlet line and / or on a steam output line connected to said steam drum. Said inline heater may be added in a revamping procedure, according to some embodiments of the invention.

[0027] The invention is applicable to new methanol plants and revamping of existing plants. A revamping according to the invention includes the provision of the above-described electrical heater in the steam drum of the steam system. The invention is particularly interesting for revamping brown (coal-based) or grey (natural gas-based) methanol plants when, for example, part of the original hydrogen or syngas source is replaced by hydrogen or syngas produced from renewable energy. A revamping according to the invention may also be performed to allow smooth operation under variable loads, particularly when the plant is required to run at low load such as less than 50% of nominal capacity, for example 20% or less.

[0028] Another aspect of the invention is a method for controlling the above-mentioned reactive system for the synthesis of methanol, wherein the reactive system is controlled selectively according to a first mode, wherein the steam system removes heat from the converter or according to a second mode wherein the steam system supplies heat to the converter. In the first mode, heat is removed predominantly by means of evaporation of the feed water sent to the converter. In the second mode, heat is transferred to the converter by means of the feed water heated by said electrical heater in the steam drum.

[0029] Normally, the conversion of the make-up gas to methanol releases heat (exothermic reaction) and, accordingly, the system operates in the first mode. The second mode can be selected during transients wherein the converter requires a heat input. For example, the second mode is selected during start-up of the reactive system. The second mode may also be selected during a load variation of the reactive system.

[0030] In a highly preferred embodiment, the production of at least part of the make-up gas depends on a renewable energy. Particularly, the hydrogen in the make-up gas may be produced partially or entirely from a renewable source, such as electrolysis of water powered by renewable energy. Example of renewable sources include solar, wind, biomass, hydroelectric, and other sources that are naturally replenished. In such embodiments, periods of reduced availability of the renewable source (e.g. day / night for solar energy) decreases the production of hydrogen and the amount of make-up gas that can be sent to the reactive system. An intermittent input of renewable energy may also be dictated by variations of the price of energy. Said second mode of operation can be selected during said periods of temporary lack or reduced availability of said renewable energy and consequently to maintain a proper temperature in the converter.

[0031] The method of the invention is applicable to all embodiments of the plants described here. The revamping method of the invention may include the provision of a control system configured to operate with the inventive method.

[0032] The invention is now elucidated with the help of the diagram of Fig. 1 , showing an embodiment of the invention.

[0033] Fig. 1 is a simplified diagram of a reactive system for the production of methanol.

[0034] The figure shows the following items: 100 reactive system

[0035] 1 methanol converter

[0036] 2 internal heat exchanger (evaporator) of the converter

[0037] 3 fresh make-up gas containing hydrogen and carbon oxides

[0038] 4 product effluent of the converter

[0039] 5 processing of the converter effluent, including cooling / condensation and separation of the liquid fraction

[0040] 6 methanol-containing liquid product (crude methanol)

[0041] 7 unreacted gas (gaseous fraction separated from the effluent after cooling / condensation)

[0042] 8 circulator

[0043] 9 unreacted gas recycled to the converter 1

[0044] 10 steam system

[0045] 11 steam drum

[0046] 12 pump

[0047] 13 water feed to the heat exchanger 2

[0048] 14 steam line

[0049] 15 make-up water feed of the steam drum

[0050] 16 steam withdrawn from the steam drum

[0051] 20 electrical heater

[0052] The converter 1 is typically part of a synthesis loop. The block 5 of Fig. 1 denotes the processing of the product effluent 4 which typically comprises cooling, condensation of methanol and separation of a liquid fraction 6 containing methanol (crude methanol) from a gaseous fraction 7 containing unreacted gas. At least a portion of said unreacted gas is returned to the converter via a circulation compressor 8, thus forming a loop. A purge gas (not shown) is removed from the loop, for example upstream the compressor 8. The feed of the converter 1 contains fresh make-up gas from line 3 and recycle gas of line 9.

[0053] The fresh make-up gas 3 is a mixture of hydrogen and carbon oxides obtained in a suitable front-end of the methanol plant. In preferred embodiment, the hydrogen or syngas contained in the fresh gas is produced partially or entirely from renewable energy, for example solar-powered electrolysis of water. In some embodiments, carbon dioxide contained in the fresh gas may be obtained from a process of carbon dioxide capture, to reduce emissions. The heat exchanger 2 inside the converter 1 removes some of the heat of reaction to produce steam. The electrical heater 20 is housed in the pressure vessel of the steam drum 11 , for example being a bundle of electrically heated tubes. Said heater 20 can raise the temperature of water sent to the heat exchanger 2 via line 13, to provide a temporary heat input to the converter 1 when needed. In an embodiment, an additional inline electrical heater is provided on the water line 13 and / or on the steam line 14.

Claims

CLAIMS1 ) A reactive system (100) for the synthesis of methanol from a make-up gas , comprising a catalytic converter (1 ) and a steam system (10), wherein the converter (1 ) includes at least one internal heat exchanger (2) arranged to produce steam with heat of the methanol synthesis reaction, wherein the steam system includes a steam drum (11 ) separated from the converter, the steam drum being connected to said heat exchanger via a feed line (13) to feed water to the heat exchanger and a steam line (14) to collect steam formed in the heat exchanger, wherein the steam system includes an electrical heater (20) mounted internally in the steam drum (11 ) and arranged to heat the feed water sent to the converter.2) A system according to claim 1 , wherein said electrical heater includes electrically heated tubes arranged inside the pressure vessel of steam drum.3) A system according to claim 1 or 2, the electrical heater being integral with the steam drum or removable from the steam drum.4) A system according to any of claims 1 to 3 including at least one inline electrical heater in addition to said electrical heater (20) of the steam drum, said in-line electrical heater being provided on a water inlet line (13) and / or on a steam output line (14) connected to said steam drum.5) A method for controlling a reactive system (100) for the synthesis of methanol from a make-up gas, wherein: the reactive system comprises a catalytic converter (1 ) and a steam system (10), wherein the converter includes at least one internal heat exchanger (2) connected to the steam system and arranged to produce steam with heat of the methanol synthesis reaction, wherein the steam system includes a steam drum (11 ) separated from the converter, the steam drum being connected to said heat exchanger via a feed line to feed water to the heat exchanger anda steam line to collect steam formed in the heat exchanger, wherein the steam drum includes an electrical heater (20) mounted internally in the steam drum and arranged to heat the feed water sent to the converter, wherein the method includes: controlling the reactive system selectively according to a first mode wherein the steam system removes heat from the converter, by means of evaporation of the feed water sent to the converter, or according to a second mode wherein the steam system supplies heat to the converter by means of the feed water heated by said electrical heater in the steam drum.6) A method according to claim 5 wherein the second mode is selected at least during start-up of the reactive system.7) A method according to claim 5 or 6 wherein the second mode is selected at least during a load variation of the reactive system.8) A method according to claim 7 wherein said load variation includes operating the reactive system at a partial load less than 50% or less than 20% of the nominal load of the system.9) A method according to claim 7 or 8 wherein the production of at least part of the make-up gas depends on a renewable energy and said second mode is selected during a temporary lack or reduced availability of said renewable energy.10)A method according to claim 9 wherein at least some of the hydrogen in the make-up gas is produced by means of electrolysis of water running on a renewable power such as solar or wind or hydropower.11 )A method according to any of claims 5 to 10, wherein the reactive system includes also at least one inline electrical heater located on a water inlet line (13) and / or on a steam output line (14) connected to said steam drum, whereinin the second mode of operation heat is supplied to the converter by means of the heater (20) mounted in the steam drum and by means of said inline heater.12)A method of revamping a methanol plant wherein, in the original plant, a make-up gas for the synthesis of methanol is produced from a hydrocarbon, such as coal or natural gas, by means of any of steam reforming, partial oxidation or gasification, wherein the plant includes a steam rising methanol converter with an internal heat exchanger connected to a steam drum for the production of steam from heat released by the chemical reaction of methanol synthesis, the method including: modifying said steam drum by mounting an electrical heater inside the steam drum, said electrical heater being arranged to heat the water feed directed to said internal heat exchanger of the converter.13)A method according to claim 12 further including the step of replacing at least part of hydrogen or syngas contained in the make-up gas with hydrogen or syngas produced from renewable energy.14)A method according to claim 12 or 13 further including the step of adding an inline heater on a water line or steam line connected to said steam drum.15)A method according to any of claims 12 to 14, including the provision of s control system configured to operate with the method of any of claims 5 to 11 .

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