Method for producing methanol from natural gas from natural wells containing high amounts of carbon dioxide

Abstract

The present invention relates to a method for exploiting natural gas fields rich in carbon dioxide to produce methanol.

Description

[0001] " Method for producing methanol from natural gas from natural wells containing high amounts of carbon dioxide" DESCRIPTION

[0002] Many natural gas reserves, not yet or little exploited, contain large quantities of pollutants such as H2S and CO2.

[0003] The former can be transformed into sulfur and used by the chemical industry, for example, for the production of sulfuric acid, while the latter poses serious environmental problems if it is released into the atmosphere.

[0004] Pollutants, such as H2S and CO2, are typically removed by means of purification technologies, such as amines, membranes, or combinations of these.

[0005] The purification plants are often equipped with units for transforming H2S into sulfur, while CO2 is released into the atmosphere or re-injected into the wells, where, however, it dilutes the hydrocarbons that make up the natural gas, making it increasingly difficult to exploit the wells.

[0006] As for hydrogen production, the possibility of replacing part of the natural gas normally transported in the pipelines with hydrogen, generated by electrolysis and, therefore, in principle, generated by renewable sources, is currently being considered.

[0007] Electrolytic hydrogen is produced by oxidation / reduction in an electrolytic cell, then it is compressed, dehydrated, and mixed with natural gas.

[0008] To date, various types of electrolytic cells have been developed, but the most promising for large-scale use are the Solid Oxydes Electrolitical Cells (SOEC), which hydrolyze water vapor at high temperature, with high efficiencies and large conversion factors. Hydrogen and oxygen are obtained from the hydrolyzed steam; the latter is often considered a waste product, to the point that the electrode at which it is generated is flushed with air, in order to lower its partial pressure and reduce the voltage of the electrolytic cell, with consequent saving of electrical energy.

[0009] Alternatively, the flushing can be carried out with steam which, being easily condensable, allows for the recovery of oxygen at a high degree of purity.

[0010] The transformation of CO2 into methanol by means of hydrogen is typically carried out in a methanol synthesis plant, where the following reactions are performed:

[0011] CO2+ H2→ CO + H2O

[0012] CO + 2H2→ CH3OH

[0013] which transform CO2 and H2 into methanol and water (in the working conditions of the reactors, albeit to a very limited extent, heavier hydrocarbons are also obtained).

[0014] In a typical plant:

[0015] 1 ) a stream of reagents, consisting mainly of CO2 and H2, is combined with a stream of compressed recirculation gases.

[0016] 2 ) The resulting stream is heated.

[0017] 3) The resulting heated stream is fed to a synthesis reactor (Rl ), where it undergoes the reactions

[0018] -CO2+ H2→ CO + H2O

[0019] -CO + 2H2→ CH3OH.

[0020] The reactions are overall exothermic, therefore heat develops in the reactor, which is removed with a heat exchanger (Reactor- exchanger) for the production of low thermal level steam (about 20 barg).

[0021] 4 ) A gas stream emerges from the R1 reactor, mainly containing CO2, H2, CH3OH and H2O.

[0022] 5) The current emerging from R1 is partially cooled by heat exchange with a reagent gas stream, obtaining the partially cooled gas stream emerging from Rl, recovering heat to heat the gas stream entering the reactor.

[0023] The methanol synthesis reactor requires an almost continuous supply; in fact, frequent switching on and off reduces the life of the catalyst and the heat exchange trains, as well as requiring rather long times to develop the thermal profiles, which guarantee the correct conversion of the reagents.

[0024] This is avoided by producing and consuming hydrogen, in the methanol synthesis, even in periods of energy scarcity, trying to reduce the hydrogen requirement of the reactors by bringing them to the Turndown.

[0025] Alternatively, it is possible to accumulate that hydrogen that should not be used in the form of compressed gas (possibly at low temperature to increase its density), or in liquid form, which is much more suitable for large accumulations.

[0026] To date, the state of the art of liquefaction (non-magnetic) of hydrogen requires from 10 to 13 KWh / kg (source DOE), where the lowest values are obtained by using volumetric compressors with a low number of revolutions ( 90% efficiency, as reported in " Large scale hydrogen liquefaction under the aspect of economic viability". Still for the synthesis of methanol, the presence of a high quantity of methane can lead to a reduction of the partial pressure of the reaction products (CO2 and H2) with problems related to the conversion to methanol.

[0027] This entails the possibility of having a high quantity of unreacted gas ( off gas) and with high quantities of inert and methane to be recycled or burned.

[0028] Even the SOEC-type electrolyzers cannot be switched on and off cyclically, except at the cost of long waiting times, which follow the heating curves of the equipment, in order to limit its degradation.

[0029] It is therefore typical to burn fuel, when renewable energies are not available (e. g. at night for a solar field) to keep both the SOEC and the methanol synthesis reactors running with the hydrogen it generates, albeit at the lower limit ( Turndown).

[0030] Since the use of a fuel implies both the production of a nonrenewable resource and the production of carbon dioxide, it is clear that this is detrimental to the benefits of using renewable energy.

[0031] Summary of the Invention

[0032] The inventors of the present patent application have developed a method that surprisingly allows a methanol synthesis plant to be kept running continuously from a carbon dioxide-rich natural gas flow using electrolytic hydrogen.

[0033] In particular, electrolytic hydrogen is produced discontinuously and without storage.

[0034] This method involves the use of an oxy-combustion system, which feeds the electrolysis process in the Turndown phases, where said oxy-combustion system uses a natural gas rich in carbon dioxide obtained from a well as fuel.

[0035] The method of the invention is particularly suitable for the exploitation of natural gas wells rich in carbon dioxide; moreover, it allows the use of such wells that are not yet exhausted as storage of carbon dioxide imported from other sources.

[0036] The use of natural gas rich in CO2 allows the methanol reactor to function optimally thanks to the correct partial pressure of the CO2 and has a sufficient methane content to obtain an adequate heat flow from the oxy-combustion in the Turndown phase.

[0037] Brief Description of the Figures

[0038] Figure 1 shows the diagram of an embodiment according to the present invention in the so-called Full Run conditions.

[0039] Figure 2 shows the diagram of an embodiment according to the present invention in the so-called Turndown conditions.

[0040] Figure 3 shows the diagram of a particular embodiment according to the present invention in the so-called Full Run conditions.

[0041] Figure 4 shows the diagram of a particular embodiment according to the present invention in the so-called Turndown conditions.

[0042] Obj ect of the invention

[0043] In accordance with a first obj ect of the present invention, a method for the production of methanol in conditions of abundance of electrical energy (Full Run phase) is described.

[0044] According to a preferred aspect, said method is carried out using a natural gas obtained from a natural gas well rich in CO2. In accordance with a second object of the present invention, a method for the production of methanol in conditions of scarcity of electrical energy ( Turndown phase) is described.

[0045] According to a preferred aspect, said Turndown phase is carried out using electrical energy produced by an oxy-combustion system fed with a portion of the natural gas obtained from a natural gas well rich in CO2 and exploiting the oxygen produced by the electrolyzers during the Full Run phase and a flow of carbon dioxide produced by the same oxy-combustion system.

[0046] The first and second methods described, in the context of the present invention, can be considered, respectively, as a first and a second phase of a method for the continuous maintenance of methanol synthesis reactors.

[0047] Detailed description of the invention

[0048] In accordance with a first obj ect, a method for the production of methanol in conditions of abundance of electrical energy, or at least corresponding to the needs of the method itself, is described.

[0049] The energy required to perform the water electrolysis phase, according to a preferred aspect of the present invention, is represented by an excess of electrical energy that is not used by other utilities, for example the electrical network for civil uses, and is obtained from renewable (and discontinuous) energy sources such as solar and wind energy.

[0050] For the purposes of the present invention, these conditions are the conditions in which a methanol plant is in the Full Run phase. For the purposes of the present invention, such method is carried out starting from a flow of natural gas obtained from natural gas wells rich in CO2.

[0051] For the purposes of the present invention, such natural gas flow comprises CO₂ in a quantity of approximately 1-80% (v / v) and preferably of approximately 30-70% (v / v).

[0052] For the purposes of the present invention, such natural gas flow also comprises C₁₋₅ light hydrocarbons.

[0053] In the continuation of the description, such flow of natural gas will be referred to as the "initial flow of natural gas".

[0054] According to an aspect of the present invention, such initial natural gas flow also comprises hydrogen sulfide and possibly other sulfur compounds.

[0055] The method of the invention is carried out in conditions called Full Run, i. e. of abundance of electricity, i. e. in the presence of an availability of electricity superior to the needs of the other utilities.

[0056] According to a preferred aspect of the invention, such electricity is produced from renewable energy sources and is, for example, represented by solar, wind, or nuclear energy.

[0057] In fact, as in the following description, it is low-cost electricity linked to the misalignment between production and consumption and is linked to overproduction with respect to consumption.

[0058] The method above is schematically represented in Figure 1. In particular, such method for the production of methanol includes the following steps: I ) obtaining a flow of methanol 2, and heat, from an initial flow of natural gas 1 subjected to methanolation,

[0059] II ) obtaining a hydrogen gas flow 5 and an oxygen gas flow 8 by electrolysis.

[0060] For the purposes of the present invention, the initial natural gas flow 1 of step I ) is obtained from a natural gas well D.

[0061] In particular, as described above, said well D is a natural gas well rich in CO2.

[0062] More specifically, said natural gas comprises CO2 in a quantity of about 1-80% (v / v) and preferably of about 30-70% (v / v).

[0063] For the purposes of the present invention, phase II ) is carried out on a vapor flow 7 obtained from a first water flow 6 vaporized by the heat developed in the methanolation phase I ).

[0064] For the purposes of the present invention, the hydrogen gas flow 5 obtained in step II ) is sent to the methanolation step I ).

[0065] A portion 10 of the hydrogen gas flow can be destined for injection into a well.

[0066] According to an embodiment of the invention, before being subj ected to step I ) of methanolation, the initial natural gas flow 1 can be subjected to a purification step in a purifier P.

[0067] In particular, this phase 0) is a purification phase from sulfur compounds.

[0068] According to a preferred aspect, the purification phase 0) eliminates the sulfur compounds present in the initial natural gas flow 1. In particular, said purification removes these sulfur compounds up to a concentration suitable for the production of methanol, which is less than about 50 ppm.

[0069] According to an embodiment of the invention, the desulphurization can be carried out by means of an absorption column.

[0070] According to an embodiment of the invention, the desulphurization can be carried out using an aqueous solution of a tertiary amine.

[0071] If particular sulfur compound contents are required, downstream of the absorption with amines, it is possible to use adsorption with zinc oxides such as ZnO2and ZnS formation.

[0072] According to an aspect of the present invention, the purification phase may also include a phase of removal of heavy metals, in order to prevent the so-called poisoning of the methanol synthesis catalysts.

[0073] According to an embodiment of the invention, the removal of heavy metals can be carried out by means of activated carbon filters.

[0074] The initial purified natural gas flow 1 ' thus obtained is then sent to the methanol synthesis phase I ).

[0075] For the purposes of the present invention, the methanolation phase I ) is carried out in a methanolation reactor R1.

[0076] Therefore, the initial natural gas flow of phase I ) can be a purified initial natural gas flow 1 '.

[0077] For the purposes of the present invention, the electrolysis phase II ) is carried out in an electrolyzer E (or water splitter).

[0078] According to an aspect of the present invention, step II ) of electrolysis is carried out using a first energy source el. According to a preferred aspect of the present invention, said first energy source el is represented by a renewable (discontinuous) energy source, such as, for example, solar energy, wind energy or nuclear energy.

[0079] According to an aspect of the present invention, the methanolation phase I ) can also be carried out in the presence of a flow of liquefied carbon dioxide 4, possibly pumped by a liquefied carbon dioxide pump PCO2l, obtaining a flow of pumped liquefied carbon dioxide 4 ' (not shown in the figure).

[0080] This flow of liquefied carbon dioxide 4 can be obtained from a storage tank TCO2l.

[0081] The gaseous oxygen flow 8 obtained from the electrolysis phase II ) is liquefied in a phase III ) in an oxygen liquefaction unit OL, obtaining a liquid oxygen flow 9, which is stored in a liquid oxygen tank TO2l.

[0082] In particular, the liquefaction phase is carried out using a second energy source e2.

[0083] According to a preferred aspect of the present invention, said second energy source e2 is represented by a renewable (discontinuous) energy source, such as, for example, solar energy, wind energy or nuclear energy.

[0084] For the purposes of the present invention, the obtained methanol flow 2 is sent to storage in a special storage tank TMeOH(not shown in the figure).

[0085] According to an embodiment of the invention, before being accumulated, said methanol flow 2 may be subj ected to a dehydration phase by distillation. In particular, this dehydration reduces the water content below the limits suitable for distillation.

[0086] For the purposes of the present invention, from the methanolation phase I) an off-gas flow 3 is also obtained, comprising CO₂, hydrogen and methane.

[0087] Said off-gas flow 3 can be destined for injection into a well D with the possibility of avoiding its partial recirculation to the methanol synthesis reactor; in this way, the accumulation of methane and inerts in the reactor is avoided.

[0088] In accordance with a second object of the present invention, a method for the production of methanol in conditions of scarcity of electrical energy is described, i.e. in the presence of a reduced availability of electrical energy, i.e. lower than the needs of the other utilities with respect to the needs of the method of the invention.

[0089] The need for the method is represented by the energy required to conduct the water electrolysis phase, which, according to a preferred aspect of the present invention, is obtained from discontinuous renewable energy sources such as solar, wind, or nuclear electricity.

[0090] For the purposes of the present invention, these conditions are the conditions in which a methanol plant is in the Turndown phase.

[0091] According to a preferred aspect, said method is therefore carried out using electrical energy produced in an oxycombustion phase fed with a portion of the initial natural gas flow. For the purposes of the present invention, this method is carried out starting from a flow of natural gas rich in CO₂ obtained from natural gas wells.

[0092] For the purposes of the present invention, this natural gas flow comprises carbon dioxide in a quantity of approximately 1-80% (v / v) and preferably of approximately 30-70% (v / v).

[0093] For the purposes of the present invention, this natural gas flow also comprises C₁₋₅ light hydrocarbons.

[0094] For the purposes of the present invention, such natural gas flow also comprises hydrogen sulphide and any other sulphur compounds.

[0095] The above method is schematically represented in Figure 2. In particular, such method for the production of methanol in conditions of scarcity of electricity from renewable sources includes the step of:

[0096] IV) conducting an oxycombustion step obtaining a third energy source e3, a steam flow from oxycombustion 27 and one or more of the flows selected from: a first flow of carbon dioxide from oxycombustion 28 and a second flow of carbon dioxide from oxycombustion 29 and a third flow of carbon dioxide from oxycombustion 30.

[0097] According to an aspect of the present invention, the oxycombustion step IV) is carried out in the presence of a portion of the initial natural gas flow 21'' or of a portion of the initial purified natural gas flow 21''' obtained from step 0) described below or of both portions. According to a preferred aspect of the present invention, the oxycombustion step IV) can be carried out in the presence of a liquid oxygen flow 26, which is obtained from a liquid oxygen storage tank TO2land which is obtained according to the method of the present invention described above, carried out in conditions of abundance of electrical energy (so-called Full Run).

[0098] The products of step IV) of oxycombustion and represented by: a third source of energy e3, a flow of steam from oxycombustion 27 and one or more of the flows chosen from: a first flow of carbon dioxide from oxycombustion 28, a second flow of carbon dioxide from oxycombustion 29 and a third flow of carbon dioxide from oxycombustion 30 can be used depending on the activity carried out in the Turndown step.

[0099] According to a first aspect of the invention, in the Turndown phase, methanolation is carried out at a minimum rate and electrolysis is also carried out at a minimum rate.

[0100] In these conditions, therefore, the method comprises the additional steps of:

[0101] Va) obtaining methanol 22 and heat from an initial natural gas flow 21 subjected to methanolation,

[0102] Via) obtaining a flow of gaseous oxygen 25 and a flow of gaseous hydrogen 24 by electrolysis.

[0103] In such situation, the resumption of the reactor and electrolyzer at full capacity in the Full Run phase will be advantageously faster.

[0104] As described above, according to an aspect of the present invention, prior to step Va), the initial natural gas flow 21 may be subjected to a purification step 0) in a purifier P, obtaining a purified initial natural gas flow 21 '.

[0105] In particular, this phase 0) is a phase of purification from sulfur compounds.

[0106] In particular, this purification removes these sulfur compounds up to a concentration suitable for the production of methanol, and less than 50 ppm.

[0107] According to an embodiment of the invention, the desulphurization can be carried out through an absorption column.

[0108] According to an embodiment of the invention, desulphurization can be carried out using an aqueous solution of a tertiary amine.

[0109] If specific sulfur compound contents are required, it is possible to use zinc oxide adsorption, such as ZnO2, with the formation of ZnS, downstream of the absorption with amines.

[0110] According to an aspect of the present invention, the purification phase 0) may also include a phase of removing heavy metals, in order to prevent the so-called poisoning of the methanol synthesis catalysts.

[0111] According to an embodiment of the invention, the removal of heavy metals can be carried out by means of activated carbon filters.

[0112] The purified natural gas flow 21 ' thus obtained is then sent to the methanolation phase Va).

[0113] Therefore, the initial natural gas flow subj ected to the methanolation phase Va) can be an initial natural gas flow 21 or a purified initial natural gas flow 21 '. For the purposes of the present invention, step Via) is carried out on a vapor flow 23 obtained from a first water flow 6 ' vaporized by the heat developed in the methanolation step Va).

[0114] For the purposes of the present invention, said hydrogen gas flow 24 obtained from step Via) can be sent to the methanolation step Va).

[0115] For the purposes of the present invention, said flow of gaseous oxygen 25 obtained from the electrolysis step Via) can be sent to the oxy-combustion step IV).

[0116] For the purposes of the present invention, the electrolysis step Via) is carried out using the third energy source e3 obtained from the oxycombustion step IV).

[0117] The flow of steam from oxy-combustion 27 obtained from step IV) can be sent to the electrolysis step Via) in the form of one or more flows.

[0118] The first flow of carbon dioxide from oxy-combustion 28, partly in liquid form and partly in gaseous form obtained by cooling and liquefaction by heat exchange with the flows inside the oxy-combustion unit, can be sent to a tank of liquefied carbon dioxide T_CO21.

[0119] The second flow of carbon dioxide from oxy-combustion 29 can be re-injected into a suitable well.

[0120] The third flow of carbon dioxide from oxy-combustion 30 can be sent to the methanolation phase Va).

[0121] In a second aspect of the invention, in the Turndown phase, the methanolation is suspended and possibly also the electrolysis and the methanolation reactor R1 and the electrolyzer E are in stand by. In these conditions, therefore, instead of steps Va) and Via), the method instead comprises the steps of:

[0122] Vb) flushing the methanolation reactor R1 with the third flow of carbon dioxide from oxy-combustion 30,

[0123] VIb) flushing the electrolyzer E with the flow of steam from oxycombustion 27.

[0124] Advantageously, this situation allows for a faster restart of the reactor and the electrolyzer in the Full Run phase.

[0125] A particular embodiment of the method of the present invention for the production of methanol is schematized in Figure 3.

[0126] As described above, these conditions can be indicated as Full Run conditions of a methanolation plant.

[0127] For the purposes of the present invention, this method is carried out starting from a flow of natural gas obtained from natural gas wells.

[0128] For the purposes of the present invention, this natural gas flow comprises CO₂ in a quantity of approximately 1-80% (v / v) and preferably of approximately 30-70% (v / v).

[0129] For the purposes of the present invention, such natural gas flow may also comprise light hydrocarbons, normally C₁₋₅.

[0130] According to an aspect of the present invention, such natural gas flow may also comprise hydrogen sulphide and possibly other sulphur compounds.

[0131] The method of the invention is carried out in conditions of abundance of electrical energy, i. e. in the presence of an availability of electrical energy superior to the needs of the method itself. According to a preferred aspect of the invention, this electrical energy is produced from renewable energy sources and is, for example, represented by solar, wind, or nuclear energy.

[0132] In particular, a natural gas flow is obtained from a natural gas well D, which can be preliminarily compressed by a natural gas compressor K_GN, obtaining an initial natural gas flow 51.

[0133] In a Purification Unit P, said initial natural gas flow 51 can optionally be purified of sulfur compounds, obtaining a purified initial natural gas flow 52.

[0134] Said initial natural gas flow 51 or said initial purified natural gas flow 52 is heated in a first heat exchanger EX1, obtaining an initial heated natural gas flow 53, which is destined for the methanolization phase (methanol synthesis) in the reactor R1.

[0135] For the purposes of the present invention, said initial natural gas flow 51 (possibly purified) is heated to a temperature that guarantees sufficient reaction kinetics and a thermal profile that is not too high in the methanolation reactor; typically 250°C.

[0136] The following exothermic reaction takes place in the methanol synthesis reactor R1:

[0137] CO2+ 3 H2→ CH₃OH + H2O

[0138] The heat developed during the reaction is partially removed in the exchanger reactor with the production of steam (about 20 barg) ( flows 60 and 61 ).

[0139] A gaseous flow of methanol 54 is obtained from the methanolization. For the purposes of the present invention, said gaseous flow of methanol 54 is a flow with a prevalent content of methanol; in fact, it also includes water, unreacted natural gas, carbon dioxide and hydrogen.

[0140] Said gaseous flow of methanol 54 is cooled in the first heat recovery unit EX1 by heat exchange with the initial natural gas flow 51, possibly purified 52, which is heated, obtaining a cooled gaseous flow of methanol 55.

[0141] Said cooled methanol gaseous flow 55 is further cooled in a first heat exchanger TE1, obtaining the condensation of the further cooled methanol flow in a mixed phase flow 56.

[0142] In particular, the gaseous methanol flow is cooled to a temperature of approximately 50-30°C.

[0143] For the purposes of the present invention, said first heat exchanger TE1 operates by means of a refrigerant fluid represented by air or water.

[0144] In a first separator S1, said mixed flow 56 is separated into an overhead flow 58 containing mainly unreacted carbon dioxide and hydrogen and unreacted natural gas, which is intended for reinjection into a well.

[0145] For the purposes of the present invention, in fact, said overhead flow 58 is not recirculated to the methanol synthesis; advantageously, this allows for better conversion yields.

[0146] From the same first separator S1, a final flow of methanol 57 is also obtained, containing methanol and water, which is stored in a methanol tank TMeOH, in order to be possibly subjected to distillation to enrich it in methanol. For the purposes of the present invention, a flow of hydrogen gas from methanolation h10 is also sent to the methanolation phase described above.

[0147] For the purposes of the present invention, said hydrogen gas flow from methanolation h10 is obtained by electrolysis.

[0148] According to a preferred aspect of the invention, this electrolysis is carried out using energy obtained from renewable (discontinuous) sources, such as, for example: solar energy, wind energy, or nuclear energy.

[0149] For this purpose, a first flow of water wl, represented for example by a flow of boiler feed water, is vaporized in the methanol exchanger reactor r, obtaining a first flow of saturated steam w2.

[0150] Said first saturated steam flow w2 is superheated in a second heat exchanger EX2, obtaining a steam flow w3 to be subjected to electrolysis.

[0151] For the purposes of the present invention, said steam flow w3 to be subjected to electrolysis has a temperature of approximately 700°C.

[0152] According to an embodiment of the present invention, a portion w4 of the steam flow to be subjected to electrolysis is superheated in a third heat exchanger EX3, obtaining a portion w5 of the first superheated saturated steam flow, which is combined with the steam flow w3 to be subjected to electrolysis.

[0153] Said steam flow w3 to be subjected to electrolysis is sent to an electrolyzer El for an electrolysis phase.

[0154] For the purposes of this invention, the electrolyzer El can be represented by a Solid Oxide Electrolytic Cell (SOEC) electrolyzer. The electrolysis phase produces a first flow of hydrogen gas hl and a first flow of oxygen gas ol.

[0155] For the purposes of the present invention, said hydrogen gas flow hl comprises a water content of approximately 50% (v / v).

[0156] For the purposes of the present invention, said flow of gaseous oxygen ol comprises oxygen in a concentration of approximately 99% (v / v).

[0157] For the purposes of this invention, said hydrogen gas flow hl is cooled, obtaining a first cooled hydrogen gas flow h2.

[0158] In particular, the hydrogen gas flow hl is cooled from approximately 800°C to approximately 440°C.

[0159] In particular, said cooling is obtained in the second heat exchanger EX2 for heat exchange with the first saturated steam flow w2.

[0160] The cooled hydrogen gas flow h2 is further cooled in a fourth heat exchanger EX4, obtaining a further cooled hydrogen gas flow h3.

[0161] In particular, the further cooled hydrogen gas flow h3 has a temperature of approximately 300°C.

[0162] Said further cooled hydrogen gas flow h3 is subj ected to one or more compression and cooling phases, obtaining a compressed and further dehydrated hydrogen flow h8.

[0163] In particular, said further cooled hydrogen gas flow h3 is cooled in a second heat exchanger TE2, obtaining a hydrogen stream at a first cooling level h4 from which a first portion of dehydration water wwl is separated in a second separator S2, obtaining a hydrogen gas flow at a first dehydration level h5. For the purposes of the present invention, said second heat exchanger TE2 operates by means of a refrigerant fluid represented by air or water.

[0164] Subsequently, said hydrogen gas flow at a first level of dehydration h5 is compressed in a first compressor KI, obtaining a hydrogen gas flow at a first level of dehydration and at a first level of compression h6.

[0165] Said hydrogen gas flow at a first level of dehydration and at a first level of compression h6 is further cooled in a third heat exchanger TE3, obtaining a hydrogen gas flow at a first level of dehydration and at a first level of compression h7, from which, in a third separator S3, a second portion of dehydration water ww2 is separated, obtaining the compressed and further dehydrated hydrogen flow h8.

[0166] For the purposes of the present invention, the cooling, separation and compression phases can be repeated n times in function of the operational needs.

[0167] Said flow of compressed and further dehydrated hydrogen h8 is compressed in a second compressor K2, obtaining a flow of further dehydrated and further compressed hydrogen h9.

[0168] Said further dehydrated and further compressed hydrogen flow h9 is heated in the fourth heat exchanger EX4 by heat exchange with said cooled hydrogen gas flow h2, obtaining a methanolation hydrogen flow hlO.

[0169] For the purposes of the present invention, said flow of hydrogen from methanolation preferably has a temperature of about 250°C. The gaseous oxygen flow ol is first subj ected to a heat exchange phase in a third recuperator EX3, obtaining a cooled gaseous oxygen flow o2, which is subsequently liquefied in an oxygen liquefier, obtaining a first liquefied oxygen flow o3.

[0170] According to an aspect of the present invention, in the third heat exchanger EX3, a portion w4 of the first saturated steam flow is heated, obtaining a portion w5 of the first superheated saturated steam flow, which is combined with the steam flow w3 to be subjected to electrolysis.

[0171] According to an aspect of the present invention, a second flow of liquid carbon dioxide CO2I2 can also be sent to the methanolation, obtained by pumping a first flow of liquid CO2 obtained from a tank of liquid CO2 TCO2lby means of a pump of liquid CO2 Pco2i.

[0172] A particular embodiment of the method of the present invention for the production of methanol is schematized in Figure 4.

[0173] In particular, said method is carried out in conditions of scarcity of electrical energy, i. e. in the presence of a reduced availability of electrical energy, as it is lower than the needs of the other utilities.

[0174] As described above, these conditions can be referred to as Turndown conditions of a methanol plant.

[0175] According to a preferred aspect of the invention, this electrical energy is scarcely available, as it is produced from renewable (discontinuous) energy sources such as solar, wind, or nuclear energy.

[0176] These conditions can be referred to as Full Run conditions of a metanation plant. For the purposes of the present invention, this method is carried out starting from a natural gas flow obtained from natural gas wells D.

[0177] For the purposes of the present invention, such natural gas flow comprises CO₂ in a quantity of approximately 1-80% (v / v) and preferably of approximately 30-70% (v / v).

[0178] For the purposes of the present invention, such natural gas flow may also comprise light hydrocarbons, normally Cl-5.

[0179] According to an aspect of the present invention, such natural gas flow can also include hydrogen sulphide and possibly other sulphur compounds.

[0180] In particular, an initial natural gas flow 51 is obtained from a natural gas well D, which can be preliminarily compressed by a natural gas compressor KGN.

[0181] In a Purification Unit P, said flow 51 can optionally be purified from sulfur compounds, obtaining the initial purified natural gas flow 52.

[0182] Said purified initial natural gas flow 52 is heated in a first heat exchanger EX1, obtaining an initial (possibly purified) and heated natural gas flow 53, which is destined for the methanoling phase.

[0183] For the purposes of the present invention, said purified initial natural gas flow 52 can be heated to a temperature that ensures sufficient reaction kinetics and a thermal profile that is not too high in the methanoling reactor; typically around 250°C.

[0184] The methanolation produces a final flow of methanol 57, as described below. More specifically, a gaseous flow of methanol 54 is obtained from the initial purified and heated natural gas flow 53 sent to a methanolation reactor R.

[0185] As described above, said gaseous methanol flow 54 is a flow with a prevalent methanol content; in fact, it also comprises water, unreacted natural gas, carbon dioxide and hydrogen.

[0186] Said gaseous flow of methanol 54 is cooled by heat exchange with the initial flow of natural gas 51, or possibly purified 52, in the first heat recovery unit EXI, obtaining a cooled gaseous flow of methanol 55.

[0187] Said cooled methanol gaseous flow 55 is further cooled in a first heat exchanger TE1, obtaining a mixed flow 56.

[0188] In a first separator SI, from said mixed flow 56, an overhead flow 58 is obtained containing mainly unreacted water, carbon dioxide and hydrogen and unreacted natural gas, which is intended for reinjection into a well, and a final methanol flow 57 from the bottom, which is stored in a methanol tank lAeon.

[0189] For the purposes of the present invention, in fact, said overhead flow 58 is not recirculated to the methanol synthesis; advantageously, this allows for better conversion yields.

[0190] A hydrogen flow from methanolation hlO is sent to the methanolation reactor R as described below.

[0191] For the purposes of the present invention, said hydrogen gas flow from methanolation h10 is obtained by electrolysis.

[0192] According to a preferred aspect of the invention, this electrolysis is carried out using an energy source obtained from an oxycombustion phase. As for the production of hydrogen gas, a first flow of water wl represented by a flow of boiler feed water is vaporized in a reboiler r of the methanolation reactor R, obtaining a first flow of saturated steam w2.

[0193] Said first saturated steam flow w2 is superheated in a second heat exchanger EX2, obtaining a steam flow to be subjected to electrolysis w3.

[0194] For the purposes of the present invention, said flow of steam to be hydrolyzed has a temperature of approximately 700°C.

[0195] In one embodiment of the present invention, a portion w4 of the first saturated steam flow is cooled in a third heat exchanger EX3, obtaining a portion w5 of the first superheated saturated steam flow, which is combined with the steam flow to be subjected to electrolysis w3.

[0196] For the purposes of the present invention, the electrolyzer El can be represented by a Solid Oxides Electrolytic Cell (SOEC) type electrolyzer.

[0197] The electrolysis phase produces a flow of hydrogen gas hl and a flow of oxygen gas ol.

[0198] For the purposes of the present invention, said hydrogen gas flow hl comprises a water content of approximately 50% (v / v).

[0199] For the purposes of the present invention, said flow of gaseous oxygen ol comprises oxygen in a concentration of approximately 99% (v / v).

[0200] For the purposes of the present invention, said hydrogen gas flow hl is cooled, obtaining a cooled hydrogen gas flow h2. In particular, the hydrogen gas flow hl is cooled from approximately 800°C to approximately 440°C.

[0201] In particular, said cooling is obtained in the second heat exchanger EX2 for heat exchange with the first saturated steam flow w2.

[0202] The cooled hydrogen gas flow h2 is further cooled in a fourth heat exchanger EX4, obtaining a further cooled hydrogen gas flow h3.

[0203] In particular, the further cooled hydrogen gas flow h3 has a temperature of approximately 300°C.

[0204] Said further cooled hydrogen gas flow h3 is subj ected to one or more compression and cooling phases, obtaining a fully dehydrated hydrogen flow h8.

[0205] In particular, said further cooled hydrogen gas flow h3 is cooled in a second heat exchanger TE2, obtaining a further cooled hydrogen flow h4, from which a first portion of dehydration water wwl is separated in a second separator S2, obtaining a hydrogen gas flow at a first dehydration level h5.

[0206] For the purposes of the present invention, said second heat exchanger TE2 operates by means of a refrigerant fluid represented by air or water.

[0207] Subsequently, said hydrogen gas flow at a first level of dehydration h5 is compressed in a first compressor KI, obtaining a hydrogen gas flow at a first level of dehydration and a first level of compression h6.

[0208] Said hydrogen gas flow at a first level of dehydration and at a first level of compression h6 is further cooled in a fourth heat exchanger TE4, obtaining a cooled hydrogen gas flow at a first level of dehydration and at a first level of compression h7, from which, in a third separator S3, a second portion of dehydration water ww2 is separated, obtaining a fully dehydrated hydrogen flow h8.

[0209] For the purposes of the present invention, the cooling, separation and compression phases can be repeated n times in function of the operational needs.

[0210] Said flow of fully dehydrated hydrogen h8 is compressed in a second compressor K2, obtaining a flow of fully dehydrated and compressed hydrogen h9.

[0211] Said flow of totally dehydrated and compressed hydrogen h9 is heated in the fourth heat exchanger EX4 by heat exchange with said cooled hydrogen gas flow h2, obtaining a flow of hydrogen from methanolization hlO.

[0212] For the purposes of the present invention, said flow of hydrogen from methanolation preferably has a temperature of approximately 250°C.

[0213] The gaseous oxygen flow ol can be subsequently used in an oxycombustion phase.

[0214] In particular, the gaseous oxygen flow ol obtained from the electrolysis phase can first be cooled in the third heat exchanger EX3 for heat exchange with the first saturated steam flow portion w4, obtaining a cooled gaseous oxygen flow o2.

[0215] Said cooled gaseous oxygen flow o2 can be subsequently liquefied in a Liquefaction Unit (LU) obtaining a liquefied oxygen flow to be stored o3, which is stored in a liquid oxygen tank TO2l. For the purposes of the present invention, a flow of liquefied oxygen o4 can be pumped by a liquefied oxygen pump PO2I, obtaining a pumped liquefied oxygen flow o5.

[0216] Said pumped liquefied oxygen flow o5 is sent to an Oxycombustion Unit (OPP) for the oxycombustion phase.

[0217] In particular, said flow of liquefied oxygen o4 is obtained from the liquefied oxygen storage tank TO2I.

[0218] According to a preferred aspect of the present invention, the liquefied and pumped oxygen flow o5 obtained according to the method described above of the present invention, carried out in conditions of abundance of electrical energy (Full Run), can also be sent to the oxy-combustion stage.

[0219] For the purposes of the present invention, the oxy-combustion phase can be carried out in the presence of a portion 51 ' of the initial natural gas flow, possibly compressed by a natural gas compressor KGN, or a portion of the initial purified natural gas flow 52 ', as described above in relation to the method of the present invention.

[0220] A first flow of methanol MeOHl obtained from the methanol tank TMeoH can also be sent to the oxy-combustion stage.

[0221] One or more of the following are therefore obtained from the oxy-combustion phase:

[0222] - a first flow of steam from oxy-combustion w7, which can be sent to the electrolysis phase upstream or downstream of the direct withdrawal to the third heat exchanger EX3. Alternatively, the first flow of steam from oxygen combustion w7 can be used to flush the electrolyzer E, so as to maintain its heat and make its restart faster;

[0223] - a second flow of steam from oxy-combustion w8, which can be sent to the electrolysis stage downstream of the second heat exchanger EX2 and upstream or downstream of the reintroduction of the portion w5 of the first superheated saturated steam flow. Alternatively, said second flow of steam from oxy-combustion w8 can be used to flush the electrolyzer E in stand by so as to maintain its heat and make its restart faster;

[0224] - a flow of hydrogen from oxycombustion hll, which can be sent to the methanolation reactor R together with the flow of hydrogen from methanolation hlO;

[0225] - a first flow of carbon dioxide from oxy-combustion CO2OXI, partly in liquid form and partly in gaseous form and obtained by cooling and liquefaction by heat exchange with the flows inside the oxy-combustion unit, which can be sent to the liquefied carbon dioxide tank TCO2l;

[0226] - a second flow of carbon dioxide from oxy-combustion C02ox2, which can be re-inj ected into a well;

[0227] - a third flow of carbon dioxide from oxy-combustion that can be used to be sent to the methanolation reactor R1 upstream or downstream of the first heat exchanger EXI to flush the methanolation reactor R1 in stand by so as to maintain its heat and make its restart faster or for the synthesis of methanol.

Claims

CLAIMS1. A method for the production o f methanol ( 3, 57 ) comprising the phases of:I ) obtaining a flow of methanol ( 3, 57 ), and heat, from an initial flow of natural gas ( 1, 21, 51 ) subjected to methanolation in a methanolation reactor Rl,I I ) obtaining a flow of hydrogen gas ( 5, hl ) and a flow of oxygen gas ( 8, ol ) by electrolysis in an electrolyzer E, wherein said phase I I ) of electrolysis is carried out on a flow of steam to be subjected to electrolysis ( 7, 23, w3 ) obtained from a first flow of water ( 6, wl ) vaporized by the heat produced in phase I ) of methanolation andwherein said initial natural gas flow ( 1, 21, 51 ) has a carbon dioxide content of about 30-70% (v / v).

2. The method for producing methanol ( 3, 57 ) according to the preceding claim, wherein an additional flow of liquefied carbon dioxide ( 4, CO212 ) is subj ected to the methanolation phase I ).

3. The method for producing methanol ( 3, 57 ) according to claim 1 or 2, wherein said electrolysis phase I I ) is carried out using a first available energy source ( el ).

4. The method for producing methanol ( 3, 57 ) according to any one of the preceding claims, comprising the phase I I I ) of liquefying said gaseous oxygen flow ( 8, 25, ol ) obtaining a liquid oxygen flow to be stored ( 9, 26, o3 ).

5. The method for producing methanol ( 3, 57 ) according to any one of the preceding claims, wherein said electrolysis phase I I ) is carried out using a first available renewable energy source ( el ).

6. A method for the production of methanol ( 22 ) comprising the steps of:IV) carrying out an oxycombustion phase obtaining a third energy source ( e3 ), a first flow of steam from oxycombustion ( 27 ), a first flow of carbon dioxide from oxycombustion ( 28, C02oxl ), a second flow of carbon dioxide from oxycombustion ( 29, C02ox2 ) and a third flow of carbon dioxide from oxycombustion ( 30 ),Va ) obtaining methanol ( 22 ) and heat from an initial natural gas flow ( 21, 51 ) subjected to methanolation in a methanolation reactor Rl,Via ) obtaining by electrolysis in an electrolyzer E a flow of gaseous oxygen ( 25 ) and a flow of gaseous hydrogen ( 24 ), wherein said step Via ) of electrolysis is carried out on a flow of steam to be subjected to electrolysis ( 7, 23, w3 ) obtained from a first flow of water ( 6, wl ) vaporized by the heat produced in step Va ), by means of said third energy source ( e3 ) obtained in step IV) and whereinsaid oxy-combustion phase is carried out by means of a portion ( 21 ', 51 ' ) of an initial natural gas flow ( 21, 51 ) having a carbon dioxide content of about 30-70% (v / v),7. The method for producing methanol according to claim 6, wherein said first flow of carbon dioxide from oxy-combustion ( 28, C02oxl ) obtained in step IV) is sent to step Va ).

8. The method for producing methanol ( 3, 57 ) according to any one of claims 6 or 7, wherein the liquid oxygen flow ( 9, 26, o3 ) obtained according to the method of claim 4 is sent to said oxycombustion step IV).

9. The method for producing methanol (3, 57 ) according to any one of claims 6 to 8, wherein to said oxy-combustion step IV) a first flow of methanol (MeOH1) obtained from step I ) of any one of claims 1 to 5.

10. The method for producing methanol (3, 57 ) according to any one of claims 6 to 9, wherein a flow of hydrogen from oxy-combustion (hll ) is also obtained from said oxy-combustion step IV) and is sent to step Va).

11. The method for producing methanol (3, 57 ) according to any one of claims 6 to 10, wherein said steps Va) and Via) are replaced respectively by the steps of:Vb) flushing said methanolation reactor R1 with said third flow of carbon dioxide from oxycombustion (30),VIb) flushing said electrolyzer E with said first flow of steam from oxy-combustion (27 ).

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