Method for revamping a methanol plant and process for synthesis of methanol

Abstract

A method for revamping a methanol plant where make-up gas is produced from reforming of natural gas, the method including the provision of a CO2 capture section processing a stream of combustion fumes produced in a fired equipment of the methanol plant, and the provision of a line arranged to add at least part of the captured CO2 to the make-up gas for the production of methanol; the provision of an additional hydrogen source arranged to add hydrogen to the make-up gas; the provision of a once-through reaction section before the existing methanol synthesis loop and a related bypass line; a process for production of methanol is also disclosed, wherein a portion of make-up gas is reacted in a once-through methanol converter and unreacted make-up gas separated from the effluent of said first converter is subsequently reacted in a methanol synthesis loop (12).

Description

[0001] Method for revamping a methanol plant and process for synthesis of methanol

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of methanol production and relates to a method for revamping a methanol synthesis plant wherein a make-up gas for the synthesis of methanol is produced by reforming of natural gas; the invention relates also to a process for synthesis of methanol.

[0005] Prior art

[0006] Methanol is produced industrially by reacting a make-up synthesis gas (“makeup gas” abbreviated MUG, or “syngas”) in a so-called synthesis loop. The synthesis loop includes basically a 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. Said compressor maintains the circulation in the loop and is called circulator. A common kind of converter is the so-called isothermal converter including a heat exchanger in contact with the catalyst to remove heat of reaction.

[0007] The loop receives the fresh make-up gas from a main compressor (MUG compressor). The fresh gas may enter at the suction side or delivery side of the circulator. Consequently, the feed of the converter is a mixture of fresh gas and recycled gas. Non-condensable gases may also be removed from the loop at a suitable location. The heat exchangers where the hot effluent is cooled may include evaporators to produce steam and a feed / effluent exchanger where the feed to the converter is preheated with heat removed from the product of reaction. The conversion of the make-up gas into methanol is carried out at high temperature (200-300 °C) and high pressure (70-100 bar), in the presence of an appropriate catalyst, and involves the following reactions of hydrogenation of carbon oxides (CO, CO2) and reversed water-gas shift:

[0008] The make-up gas is a mixture of hydrogen and carbon oxides. Noticeable parameters of the make-up gas are the stoichiometric number SN and the molar ratio CO / CO2. The stoichiometric number is defined as follows:

[0009] SN = (H2-CO2) / (CO+CO2).

[0010] For methanol synthesis, a make-up gas with SN equal to or greater than 2.0 is desirable. The CO / CO2 ratio is also an important feature, being representative of the reactivity of the gas and the amount of catalyst required in the converter. A high CO / CO2 ratio means that less catalyst is required for producing a given amount of methanol. Clearly, the quantity of catalyst in the converter is a major source of cost, because of cost of the catalyst as such and size of the converter.

[0011] The SN of the make-up gas produced by reforming of natural gas is generally slightly less than 2, and is adjusted with hydrogen recovered from a purge gas of the synthesis loop.

[0012] The reforming process involves typically a steam reforming stage in a fired furnace (“primary reforming”), optionally followed by a second reforming step. Said second reforming step is preferably autothermal reforming. In some embodiments a stand-alone autothermal reformer (“pure” autothermal reforming) is used. The fired furnace typically uses a portion of the available natural gas as a fuel. Other fired equipment of the methanol plant includes, typically, heaters of the make-up gas. Any fired equipment, using natural gas as a fuel, is a source of concern due to CO2 emissions and there is a growing interest in minimizing the carbon emissions (carbon intensity) of the process.

[0013] A process for synthesising methanol is described in WO 2022 / 238671 and WO 2022 / 238672 and a method for the preparation of methanol synthesis gas is described in WO 2020 / 148378. Further examples of a methanol process can be found in EP 3 402 772 and EP 3 402 773.

[0014] Summary of the invention

[0015] The invention addresses the problem of how to reduce the CO2 emissions of a natural gas-based methanol plant.

[0016] The problem is solved with a method of revamping according to claim 1 . Preferred options are the object of the dependent claims.

[0017] The invention is based on a combination of: using CO2 recovered from combustion fumes of the front-end as a feed for methanol production; the provision of an additional hydrogen source to correct the composition of the make-up gas balancing the added CO2; the addition of a new synthesis section including a new methanol converter, so that the conversion of the make-up gas into methanol is performed partly in the existing loop and partly in the new synthesis section.

[0018] The invention is based on the finding that CO2 recoverable from combustion fumes can be used for methanol production but would perturb the stoichiometric number of the gas. In order to balance the composition of the make-up gas and to keep the SN close to the desired value of 2.0, the invention provides the addition of hydrogen from a source external to the front-end. This added hydrogen can be, very preferably, a “green hydrogen” produced without emissions of CO2 but, on the other hand, production of green hydrogen is generally dependent on renewable energy and, for this reason, intrinsically variable. Then, the existing synthesis loop would be affected by a fluctuating input and may not be able to process the increased amount of make-up gas, and / or to properly follow the variable input.

[0019] The invention addresses the above issues in a brilliant manner by providing an added synthesis section, wherein the conversion of the make-up gas into methanol is performed partly in the existing loop and partly in the new synthesis section. With the addition of a new synthesis section, the synthesis capacity is greater and more flexible because the duty of conversion can be distributed between the existing loop and the new synthesis section. Accordingly, the invention provides the ability to follow a variable input of make-up gas, which is particularly useful in the presence of a variable source of green hydrogen.

[0020] The invention is based inter alia on the finding that, in view of the modified syngas composition due to the recovered CO2 and hydrogen used as feedstock, the existing methanol synthesis catalyst volume is no more adequate to convert the updated composition and, at the same time, the stable availability of hydrogen from renewable sources is not assured. The invention aims to maximizing the use of the available capacity in the existing equipment, which is achieved by installing a new reactor to directly process part of the make-up gas.

[0021] Another aspect of the invention is a process according to the claims.

[0022] Description of the invention

[0023] The invention includes the removal of CO2 form combustion fumes of one or more fired equipment of the methanol plant. Said items may include a fired furnace for reforming and / or auxiliary heaters or boilers of the front-end or other sections (e.g. of the distillation section). The so obtained CO2 stream can be added directly to the make-up gas together with added hydrogen from a source external to the front-end. A portion of CO2 may be added to the process line before reforming, when appropriate, e.g. to the feed of a steam methane reformer or to the feed of an autothermal reformer.

[0024] The new synthesis section is installed preferably in series and upstream of the existing synthesis loop. The new section is installed preferably in the make-up gas line from the front-end to the existing synthesis loop. The new synthesis section may include a converter, a cooler / condenser and a separator.

[0025] The methanol converter of the new section is preferably a once-through converter (also termed OTR, once-through reactor). The term once-through denotes a converter with no recycle of unreacted gas, in contrast with a converter of a synthesis loop where unreacted gas contained in the converter effluent is sent back to the same converter, together with fresh gas. The effluent of the once- through converter can be treated by cooling / condensation and separation of a liquid phase containing a methanol product and a gaseous phase containing unreacted gas. At least part of said gaseous phase is preferably sent to the main synthesis loop downstream.

[0026] Optionally, a line is arranged to feed the new synthesis section with at least a portion of the recycle gas from the synthesis loop. In such case, the converter of the new section can be named as upstream / parallel reactor.

[0027] Said recycle gas withdrawn from the synthesis loop contains unconverted methanol reagents (hydrogen and / or carbon oxides). Preferably, said recycle gas fed to the new synthesis section is separated from a methanol-containing product. Preferably, a stream containing unconverted reagents is separated in the synthesis loop, a portion of this stream is sent to the new synthesis section and a remainder portion is recycled internally in the loop, for example said remainder portion is sent to the suction of the circulation compressor of the loop.

[0028] An interesting embodiment includes the provision of a bypass line of the new synthesis section, where bypass line is arranged to send fresh make-up gas directly to the existing synthesis loop bypassing the new synthesis section. The unreacted gas separated from the effluent of the once-through converter can be sent to said bypass line.

[0029] The crude methanol product obtained in the new synthesis section can be sent to distillation separately from the product of the main synthesis loop.

[0030] The bypass on the new synthesis section allows smoothly managing the possible fluctuations of hydrogen from renewable sources and recovered CO2. The load of the existing synthesis loop load can be kept stable by opening and closing the bypass, so increasing and decreasing the load of the converter in the new synthesis section in view of the available feedstock.

[0031] In an embodiment, the revamped plant is configured so that the make-up gas is passed entirely through the new synthesis section, obtaining a first crude methanol stream, and the unreacted gas separated from said first crude methanol form the entire feed of the synthesis loop. In this embodiment the operation of the new synthesis section may be termed in series with the synthesis loop because the entire feed is processed in the new section, for example in the once-through converter of said section, and then in the synthesis loop.

[0032] In another embodiment, the revamped plant is configured so that the make-up gas is passed entirely through the new synthesis section together with a portion of recycle gas from the synthesis loop, and the unreacted gas separated from the first crude methanol form the entire feed of the synthesis loop.

[0033] In another embodiment only part of the make-up gas is passed through the new synthesis section, and unreacted gas separated from the first crude methanol together with the remainder of make-up gas form the feed of the synthesis loop. This operation may be termed series / parallel because the bypass fraction is processed only in the synthesis loop, in parallel with the new synthesis section.

[0034] In another embodiment only part of the make-up gas added with hydrogen is passed through the new synthesis section together with a portion of the recycle gas from the synthesis loop, and unreacted gas separated from the first crude methanol together with the remainder of make-up gas form the feed of the synthesis loop. The modified synthesis loop operates downstream of the new section in relation to the fraction of make-up gas sent to said converter. This fraction is reacted first in the new synthesis section and, after separation of the liquid methanol product, is further reacted in the loop. The bypass fraction, if any, goes directly to the loop.

[0035] In all the embodiments, the new synthesis section includes preferably a once- through converter (OTR). Accordingly, the OTR may operate in series or series / parallel with the synthesis loop, based on the embodiments described above.

[0036] Preferably, the additional hydrogen source includes the production of hydrogen by splitting of water, such as electrolysis of water. Preferably, hydrogen is produced from electrolysis of water which is powered in part or entirely by renewable energy. The term renewable energy denotes energy from a source which is naturally replenished and includes among others solar, wind, biomass, hydro power.

[0037] An embodiment includes the provision of a hydrogen recovery unit arranged to recover hydrogen from a purge gas of the synthesis loop, wherein at least part of the recovered hydrogen is added to the make-up gas directed to the new synthesis section and / or to the existing loop.

[0038] Depending on the existing plant configuration for syngas generation section, such as pure steam reforming, pure ATR or combined reforming, the existing main loop can be equipped with a new hydrogen recovery unit (HRU) or the existing HRU can be upgraded in order to maximize the recovery of H2 from purge and, consequently minimize the import of H2 from renewable sources import. An advantage of recovering H2 from the purge of the loop is to reduce the dependency on the variability of the H2 import, particularly when said import depend on intermittent source of energy. Preferred embodiments of the HRU are PSA or membrane-type.

[0039] An embodiment includes the step of modifying the converter of the existing loop into an isothermal steam rising reactor, preferably plate-cooled and more preferably with axial or axial-radial gas path through the catalyst.

[0040] An isothermal steam rising reactor is preferred because it is well suitable for the updated syngas composition and it is adequately flexible to manage feed flowrate variations thanks to the possibility of varying the steam generation pressure. The axial radial design is particularly preferred for the revamping of the present invention, having a very low pressure drop and, consequently, avoiding the need of revamping the make-up gas compressor or, in case, reducing the related revamping activities and cost.

[0041] A preferred embodiment is a methanol converter including a catalytic bed with annular configuration; an effluent heat exchanger arranged to transfer heat from a hot methanol-containing gas effluent from the catalytic bed to a cooling medium; said effluent heat exchanger being located in a central cavity of the annular catalytic bed, so that said catalytic bed is arranged concentrically around said effluent heat exchanger.

[0042] An interesting embodiment includes the provision of a control system configured to adjust the amount of make-up gas sent to the new synthesis section and / or the amount of added CO2 as a function of the amount of hydrogen available from the additional hydrogen source. The added CO2 denoted the CO2 removed from combustion fumes of the front-end and added to the make-up gas. Accordingly, the control system governs the load of the new converter and of the existing synthesis loop, as well as the composition of the make-up gas (hydrogen and CO2 content) following the availability of the hydrogen source, for example when the source depends on renewable energy (e.g. solar energy).

[0043] In a preferred embodiment, the invention provides the synergy of the following features: a) carbon capture from fumes of one or more fired equipment, preferably from a reforming fired furnace; b) the generation of green hydrogen and addition of said green hydrogen to the make-up gas, to balance the composition in view of the added CO2; c) the provision of a once-through conversion section arranged for preprocessing a portion of the make-up gas directed to the synthesis loop, the partially reformed effluent of said section being sent to said loop for completion of the reforming process; d) the provision of a bypass of said once-through conversion section, which is controlled according to variations of load; e) hydrogen recovery from a purge stream of the synthesis loop.

[0044] An advantage of the invention is that the newly added synthesis section, with once-through reactor, provides the additional catalyst volume required to process the full amount of make-up gas, when the added hydrogen is fully available, as well as to compensate for the different composition of the synthesis gas.

[0045] A process for the synthesis of methanol, according to the invention, includes: producing a make-up gas, suitable for the synthesis of methanol, from reforming of natural gas in a front-end; adding to the make-up gas a stream of CO2 removed from combustion fumes generated in a reforming fired furnace and / or a fired heater of the front-end; adding to the make-up gas a stream of hydrogen imported from a hydrogen source other than said reforming process; reacting the make-up gas in a once-through methanol converter followed by a methanol synthesis loop.

[0046] The once-through converter may be located on the make-up gas line feeding the main synthesis loop. Unreacted make-up gas separated from the effluent of said once-through converter, optionally with fresh make-up gas bypassing said converter, is subsequently reacted in the methanol synthesis loop downstream. An amount of make-up gas which bypasses the first converter and is sent directly to the synthesis loop, and / or the amount of CO2 which is added to the make-up gas, are controlled on the basis of the amount of hydrogen imported from said source. Said hydrogen source is preferably powered by renewable energy and preferably includes electrolysis of water.

[0047] Description of the figures

[0048] The invention is now elucidated with the help of the figures wherein:

[0049] Figs. 1 -4 are simplified diagrams of a methanol synthesis plant and process according to embodiments of the invention. Fig. 1 illustrates an embodiment of steam methane reforming (SMR) with the following main items:

[0050] Front-end 100

[0051] Steam methane reforming furnace 10

[0052] Once-through methanol reactor 11

[0053] Methanol synthesis loop 12

[0054] Distillation section 13

[0055] CO2 removal section 14

[0056] Water electrolysis unit 15

[0057] Hydrogen recovery unit 16.

[0058] Bypass line 17 of the reactor 11 , controlled by the bypass valve 18.

[0059] Line 19 denotes the main input of natural gas. Line 20 denotes the process gas reformed in the furnace 10, whereas line 21 denotes a fuel portion sent to burners of said furnace. Combustion fumes 23 of the furnace 10 are sent to the CO2 removal section 14 and CO2 removed from the fumes is added to the process line 20 or to the reformed gas 22.

[0060] The reformed gas 22 receives also additional hydrogen form line 26 of hydrogen produced in the electrolysis unit 15 and line 27 of hydrogen recovered from the purge gas 31 of the loop 12. The so obtained conditioned make-up gas is sent partly to the reactor 11 and partly to the loop 12 via the bypass line 17. The unit 15 produced hydrogen from water W and power E, preferably from a renewable source e.g. from a solar or wind installation.

[0061] The effluent 29 of the loop 12 is purified in the distillation section 13 obtaining methanol 30 of a desired purity. The purge stream 31 of the loop is sent to the hydrogen recovery unit 16 to obtain the hydrogen stream 27 and a tail gas 32 used as additional fuel in the furnace 10.

[0062] The fuel of line 21 and line 32 may be also directed to one or more auxiliary fired heaters (not shown). The fumes collected from said heaters are preferably treated in the CO2 removal unit 14.

[0063] In operation, the valve 18 is controlled on the basis of the availability of hydrogen 26, which follows the fluctuation of the power E. In condition of low or no power to the unit 15, the added CO2 of line 25 may be correspondingly reduced to maintain a balanced composition of the make-up gas, and the valve 18 opens to allow bypass of the reactor 11 . If the valve is fully open, almost the entire makeup gas is sent directly to the loop 12 and the plant operates similar to the original layout. In condition of h igh / fu 11 power to the unit 15, the valve 18 is almost closed or fully closed, to exploit the added conversion capacity of the reactor 11 . When the CO2 added via line 25 is reduced, the CO2 removed from the fumes may be sent back to reforming via line 24 or, if possible, stored or sent to a different process use. When possible, it is preferred not to discharge CO2 to atmosphere.

[0064] Fig. 2 illustrates a variant with two-step reforming wherein the steam reforming furnace 10 is followed by an autothermal reformer ATR fired with oxygen or enriched air (stream 33) produced by an air separation unit ASU.

[0065] Fig. 3 illustrates a variant wherein the reforming process is performed in a standalone autothermal reformer ATR. The item FH denotes a fired heater to preheat the process gas feed of the reformer.

[0066] Fig. 4 illustrates a variant embodiment of Fig. 1 wherein a line 34 is arranged to add a portion of recycle gas from the loop 12 to the feed of the once-through reactor 11 . Preferably the recycle gas is added downstream of the bypass line 17. The feed of the reactor 11 accordingly includes the entire flow of make-up gas, or only a portion thereof in case of bypass, plus said portion of recycle gas taken from the loop 12. Typically, said recycle gas 34 contains unconverted reagents separated from the methanol-containing product in a separator of the synthesis loop 12. In an embodiment, a stream containing unreacted gas from a separator of the loop 12 is sent partially to the circulator of the loop 12 and partially to the feed of the reactor 11 via said line 34.

[0067] The variant of Fig. 4 may be implemented also in the other embodiments of the invention, such as the embodiments of Fig. 2 and Fig. 3.

Claims

CLAIMS1 ) A method for revamping a methanol plant, wherein: said methanol plant originally includes a front-end (100) for production of a make-up gas comprising hydrogen and carbon oxides and a synthesis loop (12) including at least one methanol converter where the make-up gas is reacted to obtain a methanol product, and the make-up gas is produced in the front-end by reforming of natural gas; wherein the method includes: the provision of a CO2 capture section (14) arranged to process at least one stream of combustion fumes (23) produced in one or more fired equipment of the methanol plant, and the provision of a line (25) arranged to add at least part of the CO2 captured by said CO2 capture section to the make-up gas (22) for the production of methanol; the provision of an additional hydrogen source (15) arranged to add hydrogen (26) to the make-up gas; so that the amount of make-up gas for production of methanol is increased, and the method further includes the addition of a new synthesis section (11 ) including a methanol converter, so that the conversion of the make-up gas into methanol is performed partly in the existing loop and partly in the new synthesis section.2) A method according to claim 1 wherein the new synthesis section is installed in series and upstream of the existing synthesis loop.3) A method according to claim 2, including the provision of a bypass line (17) of the new synthesis section, said bypass line being arranged to send make-up gas directly to the existing synthesis loop (12) bypassing the new synthesis section (11 ).4) A method according to any of the previous claims, wherein the new synthesis section includes a once-through converter and unreacted gas contained in the effluent of said converter is separated and sent to the existing synthesis loop.5) A method according to claims 3 and 4, wherein said unreacted gas is sent to the fresh make-up gas bypass line.6) A method according to any of the previous claims wherein a line (34) is arranged to feed the new synthesis section with at least a portion of a recycle gas withdrawn from the synthesis loop.7) A method according to any of the previous claims, further including the provision of a hydrogen recovery unit (16) arranged to recover hydrogen from a purge gas (31 ) of the synthesis loop, wherein at least part of the recovered hydrogen (27) is added to the make-up gas feed line of the new synthesis section and / or to the feed of the existing loop.8) A method according to any of the previous claims, wherein the new synthesis section (11 ) includes: a methanol converter, a cooler / condenser arranged to cool the effluent of the converter to obtain condensation of a methanolcontaining liquid product, a separator arranged to process the effluent of the cooler / condenser and to separate said liquid methanol product from unreacted gas.9) A method according to any of the previous claims wherein the production of hydrogen in the additional hydrogen source (15) is powered in part or entirely by renewable energy.10)A method according to any of the previous claims wherein hydrogen is produced in the additional hydrogen source by electrolysis of water.11 )A method according to any of the previous claims further including the step of modifying the converter of the existing loop (12) into an isothermal steam rising reactor, preferably plate-cooled and more preferably with axial or axial- radial gas path through one or more catalytic beds of the converter.)A method according to any of the previous claims further including the provision of a control system configured to adjust the amount of make-up gas sent to the new synthesis section, and / or the amount of CO2 added to the make-up gas, as a function of the amount of hydrogen available from the additional hydrogen source. )A process for the synthesis of methanol including: producing a make-up gas (22), suitable for the synthesis of methanol, from reforming of natural gas in a front-end (100); adding to the make-up gas a stream of carbon dioxide (25) removed from combustion fumes (23) of at least one fired equipment (10); adding to the make-up gas a stream of hydrogen (26) imported from a hydrogen source (15); wherein at least a portion of the so obtained conditioned make-up gas, including the added CO2 and added hydrogen, is reacted in a first methanol converter (11 ) which is preferably a once-through methanol converter; wherein unreacted make-up gas separated from the effluent of said first converter, optionally with fresh make-up gas (17) bypassing said first converter, is subsequently reacted in a methanol synthesis loop (12) downstream of said first converter, wherein an amount of make-up gas (17) which bypasses the first converter and is sent directly to the synthesis loop, and / or the amount of carbon dioxide (25) which is added to the make-up gas, are controlled on the basis of the amount of hydrogen (26) imported from said hydrogen source (15). )A process according to claim 13 wherein said hydrogen source is configured to produce hydrogen using renewable energy, preferably by electrolysis of water powered at least in part by renewable energy.)A process according to claim 14, wherein the amount of make-up gas (17) which bypasses the first converter (11 ) and is sent directly to the synthesis loop, and / or the amount of carbon dioxide which is added to the make-up gas, are controlled on the basis of the availability and power of the source of said renewable energy. )A process according to any of claims 13 to 15, wherein a stream of recycle gas (34), withdrawn from said synthesis loop, is added to the feed stream of said first methanol converter.

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