EA047470B1 - METHOD OF METHANOL SYNTHESIS - Google Patents

Description

The present invention relates to a method for synthesizing methanol.

Methanol synthesis is essentially carried out by feeding a synthesis gas containing hydrogen and carbon monoxide and/or carbon dioxide at elevated temperature and pressure into a synthesis reactor through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition. Raw methanol is essentially recovered by cooling the gaseous product stream to a temperature below the dew point and separating the product in liquid form. The raw methanol is typically purified by distillation. The process is often implemented using a loop: thus, unreacted gas can be returned to the synthesis reactor together with the feed gas via a circulation pump. Fresh synthesis gas, called make-up gas, is added to the recirculated unreacted gas to form a feed gas stream. To prevent the accumulation of inert gases in the loop, a purge stream is often taken from the cycle gas stream.

The synthesis of methanol can be described by the two equations below:

_H2 + _CO2 CH3OH + _H2O

_H2 + COCH3OH

There are two stoichiometric quantities that are commonly used to describe the proportions of reactants fed to a methanol synthesis reactor. They are designated by the letters R and Z and can be determined from the molar concentrations of the components in the synthesis gas as follows:

R = ([H ₂ ] - [CO ₂ ]) / ([CO] + [CO ₂ ])

Z = [H ₂ ] / (2 [CO] + 3 [CO ₂ ])

In addition, for the methanol synthesis reaction it is often useful to determine the value of S; which is the sum of ^Nm3 /h _H2 + ^Nm3 /h CO in the synthesis gas. Subsequently, S, Z and R can be related using the equation:

Maximum yield of methanol ( ^Hm3 /4)=ZxS/(R+1) for Z < 1.

Maximum yield of methanol ( ^Hm3 /4)=S/(R+1) for Z>1.

The ideal stoichiometric mixture is formed when there is enough hydrogen to convert all carbon oxides to methanol. This occurs when R=2 and Z=1. However, different technologies for producing synthesis gas produce different synthesis gases with different proportions of reactants.

WO2006126017 (A1) describes a method for synthesizing methanol comprising the following steps: (i) reforming a hydrocarbon feedstock and separating water from the resulting converted gas mixture to produce a make-up gas containing hydrogen and carbon oxides, said make-up gas mixture having a stoichiometric number R defined by the formula: R=([ _H2 ]-[ _CO2 ])/([ _CO2 ]+[CO]) less than 2.0, (ii) combining said make-up gas with unreacted synthesis gas to form a synthesis gas mixture, (iii) passing the synthesis gas mixture at elevated temperature and pressure through a bed of methanol synthesis catalyst to produce a product stream containing methanol and unreacted synthesis gas, (iv) cooling said product stream to recover a crude methanol stream from said unreacted synthesis gas, (v) removing a portion of said unreacted synthesis gas as a purge gas, and (vi) feeding the remaining unreacted synthesis gas to step (ii), characterized in that hydrogen is recovered from at least a portion of said purge gas and a portion of said make-up gas, and the recovered hydrogen is included in the synthesis gas mixture. Although effective for equilibrating the stoichiometry of the feed gas in this and other hydrogen recovery processes, the carbon-rich gas recovered after hydrogen separation often has a calorific value that exceeds the plant's fuel requirement. As a result, methanol production is reduced.

WO2016180812 (A1) describes a method for producing methanol from synthesis gas, comprising the steps of providing a make-up gas containing hydrogen and carbon monoxide, in which the carbon dioxide content is less than 0.1 mol%, mixing the make-up gas with a hydrogen-rich recycle gas and feeding the gas mixture to a methanol synthesis reactor, optionally through a sulfur protection system, and passing the effluent from the synthesis reactor to a separation step to thereby obtain raw methanol and a hydrogen-rich recycle gas, wherein the traditional addition of carbon dioxide to the make-up gas is replaced by the addition of water in an amount of from 0.1 to 5 mol%. In order to achieve an acceptable cycle efficiency, carbon dioxide is required; therefore, this method provides for the addition of water to the make-up gas instead of carbon dioxide in order to compensate for the low carbon dioxide content in it by promoting the water gas shift reaction that occurs over the methanol synthesis catalyst. The water gas shift reaction can be described by the following equation:

CO + H ₂ O CO ₂₊ H ₂

The applicant has found that the addition of water or steam to the make-up gas stimulates the water gas shift reaction on the methanol synthesis catalyst, resulting in an increase in the amount of dioxins

- 1,047,470 carbon and hydrogen in the methanol converter outlet stream, which in turn increases the amount of dissolved carbon dioxide in the crude methanol product. As a result, the purge gas extracted from the cycle has a higher concentration of hydrogen and carbon dioxide than carbon monoxide, which leads to greater hydrogen recirculation and the formation of such an amount of carbon-rich off-gas from the hydrogen recovery unit that does not exceed the fuel requirements of the process.

Accordingly, the invention provides a method for synthesizing methanol, comprising the following steps: (i) feeding a hydrocarbon feedstock to a synthesis gas production unit to form a synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and steam; (ii) cooling the synthesis gas in one or more heat exchange stages and extracting a process condensate from the cooled synthesis gas to form a make-up gas having a stoichiometric value R in the range from 1.70 to 1.94; (iii) feeding the feed gas containing the make-up gas to a methanol synthesis unit containing one or more methanol synthesis reactors containing a copper methanol synthesis catalyst; and (iv) recovering a purge gas and a crude methanol product from the methanol synthesis unit, wherein the hydrogen-rich gas is recovered from the purge gas and combined with the make-up gas and a stream of water or steam is added to the feed gas in the methanol synthesis unit.

The stoichiometric value R is defined as R=([ _H2 ]-[ _CO2 ])/([CO]+[ _CO2 ]) and can be determined by calculating or measuring the concentrations of hydrogen, carbon monoxide and carbon dioxide in the make-up gas.

The synthesis gas production plant may comprise a partial oxidation plant having one or more catalytic or non-catalytic partial oxidation vessels, or a gasification plant comprising one or more gasifiers, or a reforming plant comprising one or more catalytic steam reformers. Taking into account that any synthesis gas production plant produces a make-up gas having a stoichiometric R value in the range from 1.70 to 1.94, the present invention is of particular importance since the synthesis gas production plant comprises an autothermal reforming (ATR) unit. The synthesis gas production plant may comprise an autothermal reforming unit as the only reforming unit, which can be fed with hydrocarbon feedstock, optionally mixed with steam. However, in a preferred configuration, the synthesis gas production plant comprises an adiabatic pre-reforming unit and an autothermal reforming unit, connected in series. The hydrocarbon feedstock and steam mixture is fed to a prereformer to convert the ^C2+ hydrocarbons to methane and form a prereformed gas mixture containing hydrogen, steam, carbon monoxide, carbon dioxide, and methane, which is fed to an autothermal reformer. Including a prereformer upstream of the autothermal reformer allows more heat to be added to the process before the ATR and provides a higher R-value at the ATR outlet than using the ATR alone.

If the synthesis gas production plant includes a gasification plant, the hydrocarbon feedstock may be carbonaceous, such as coal, biomass or municipal waste. In the case where the synthesis gas production plant includes a reforming plant, the hydrocarbon feedstock may be any gaseous or low-boiling hydrocarbon-containing feedstock, such as natural gas, associated gas, LPG, petroleum distillate or naphtha. It is preferably methane, associated gas or natural gas containing a significant proportion, such as more than 85% v/v, of methane. A particularly preferred feedstock is natural gas. The feedstock may be available at a suitable pressure or may be compressed to a suitable pressure, typically in the range of 10-100 bar abs.

If the hydrocarbon feedstock contains sulphur compounds before or after compression, the feedstock may be subjected to desulphurisation, such as hydrodesulfurisation using Co or Ni catalysts, and hydrogen sulphide absorption using a suitable absorbent, such as a zinc oxide layer. To facilitate this and/or reduce the risk of soot formation in the synthesis gas production unit, hydrogen may be added to the hydrocarbon feedstock. The amount of hydrogen in the resulting mixed gas stream may be in the range of 1-20 vol.%, but is preferably in the range of 110%, more preferably in the range of 1-5%. In a preferred embodiment, a portion of the hydrogen-rich gas is mixed with the hydrocarbon-containing feedstock stream. The hydrogen stream may be combined with hydrocarbon upstream and/or downstream of any hydrodesulfurisation stage.

If necessary, in addition to adding hydrogen-rich gas to the make-up gas, hydrogen supplied from an external source can be added.

In the case where the synthesis gas production plant comprises a pre-reformer or other steam reformer, the hydrocarbon feedstock is mixed with steam: this introduction of steam can be carried out by direct steam injection and/or by saturating the hydrocarbon feedstock by contacting the latter with a stream of heated water in a saturator. One or more saturators can be used. If necessary, part of the hydrocarbon feedstock can bypass the addition of steam, for example, the saturator. The amount of steam introduced can be such as to obtain a steam content of

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0.3 to 3, i.e. from 0.3 to 3 moles of steam per gram atom of carbon in the hydrocarbon feedstock. Preferably, the steam to carbon ratio is < 1.5:1, more preferably in the range of 0.3-0.9:1. The hydrocarbon/steam mixture can subsequently be preheated before reforming, for example in a pre-reformer. This can be achieved by means of a fired heater. The fired heater can be heated by burning a portion of the hydrocarbon feedstock, typically with waste fuel gases separated from downstream processing, which preferably includes a portion of the carbon-rich gas obtained after recovery of the carbon-rich gas. The resulting hydrocarbon feedstock/steam mixture can subsequently be reformed in a synthesis gas production unit.

In a preferred configuration, the hydrocarbon feedstock is reformed in two stages sequentially, comprising a first adiabatic pre-reforming stage and a second autothermal reforming stage. In such a process, it is desirable for the hydrocarbon/steam mixture to be heated to a temperature in the range of 300-650°C and subsequently passed under adiabatic conditions over a bed of a suitable steam reforming catalyst, typically a catalyst having a high nickel content, such as above 40 wt%. During such an adiabatic steam reforming stage, any hydrocarbons above methane react with the steam to form a mixture of methane, carbon oxides and hydrogen. The use of such an adiabatic reforming stage, commonly referred to as pre-reforming, is desirable to ensure that the feed to the autothermal reformer does not contain hydrocarbons above methane and also contains a significant amount of hydrogen. This may be desirable in cases where feedstocks with a low steam to carbon ratio are used, in order to minimise the risk of soot formation in the autothermal reformer. The pre-reformed gas, which contains methane, hydrogen, steam and carbon oxides, is subsequently fed, optionally after addition of steam and/or a hydrogen-containing stream, to the autothermal reformer, where it is autothermally reformed. If necessary, the temperature and/or pressure of the pre-reformed gas can be adjusted before feeding it to the autothermal reformer. Steam reforming reactions are endothermic and therefore, particularly when using natural gas as the hydrocarbon feedstock, it may be desirable to reheat the pre-reformed gas mixture to the inlet temperature of the autothermal reformer. If the pre-reformed gas is heated, this can also be easily accomplished in a fired heater used to preheat the feedstock to the pre-reformer.

An autothermal reformer essentially comprises a burner located near the top of the reformer into which a hydrocarbon feedstock or pre-reformed gas and an oxygen-containing gas are fed, a combustion zone below the burner through which, as a rule, a flame passes over a fixed bed of steam reforming catalyst in the form of solid particles. Thus, in autothermal reforming, the heat for the endothermic steam reforming reactions is provided by burning a portion of the hydrocarbon and all of the hydrogen that are present in the feed gas. The hydrocarbon feedstock or pre-reformed gas is typically fed to the top of the reformer and the oxygen-containing gas is fed to the burner, mixing and combustion occur downstream of the burner to form a heated gas mixture that is brought to equilibrium as it passes over the steam reforming catalyst. While some steam may be added to the oxygen-containing gas, it is preferable not to add steam so that a low overall steam ratio is achieved for the reforming process. The autothermal reforming catalyst is typically nickel supported on a refractory support such as calcium aluminate rings or granules, magnesium aluminate, alpha alumina, titanium dioxide, zirconium dioxide and mixtures thereof. In a preferred embodiment, the autothermal reforming catalyst comprises a layer of a more active Rh catalyst on the support such as Rh on alpha alumina or Rh on stabilized zirconium compared to a traditional Ni catalyst on alumina to reduce volatilization of the catalyst support.

The oxygen-containing gas fed to the autothermal reformer preferably contains >95 vol.% O2, which may come from an air separation unit (ASU) or another source of oxygen.

The amount of oxygen-containing gas required in an autothermal reformer is determined by the desired composition of the product gas. Generally, increasing the amount of oxygen, and thus increasing the temperature of the synthesis gas leaving the autothermal reformer, results in a decrease in the [ _H2 ]/[CO] ratio and a decrease in the proportion of carbon dioxide.

Preferably, such an amount of oxygen-containing gas is added that 50 to 70 moles of oxygen per 100 g of carbon atoms contained in the feedstock are added in the pre-reforming and autothermal reforming stages. Preferably, an amount of oxygen is added in such a way that the synthesis gas leaves the autothermal reforming catalyst at a temperature in the range of 750-1100°C. For a given feed/steam mixture, the amount and composition of the oxygen-containing gas, and the reforming pressure, this temperature largely determines the composition of the synthesis gas. At a quantity

- 3 047470 methane content is affected by the ATR outlet temperature. High outlet temperatures reduce the methane content of the synthesis gas and also reduce the R value.

The gas extracted from the synthesis gas production unit is a synthesis gas containing hydrogen, carbon monoxide, carbon dioxide, methane and steam. The synthesis gas produced by the autothermal reforming unit may contain from 2.5 to 7 vol.% carbon dioxide on a wet basis, preferably from 3 to 5 vol.% carbon dioxide on a wet basis. On a dry gas basis, i.e. without steam, the carbon dioxide content may be in the range from 2.8 to 13 vol.%, preferably from 4 to 8 vol.%.

After leaving the autothermal reformer, the synthesis gas is subsequently cooled in one or more heat exchange stages, essentially comprising at least a first stage of steam production. After such steam production, the synthesis gas is preferably cooled by heat exchange with one or more of the following streams: hydrocarbon feedstock, water (including process condensate) used to generate steam that can be used for heating or used in the pre-reforming stage, a mixture of hydrocarbons and steam, a pre-reformed gas mixture and obtained by stripping raw methanol. For safety reasons, the synthesis gas is preferably not used to heat the oxygen-containing gas fed to the autothermal reformer.

Cooling is performed to reduce the temperature of the synthesis gas below the dew point so that the vapor is condensed. The liquid process condensate can be separated from the synthesis gas, which at this stage may be referred to as make-up gas, by standard gas-liquid separation equipment.

The process involves a cooling and condensate recovery step followed by a water or steam addition step, which may appear counterintuitive. However, it has been found that attempts to adjust the steam content of the synthesis gas at normal operating pressures for the process present significant technical challenges, which are likely to be expensive to resolve. For example, when operating at 30 bar abs., the make-up gas water content ranges from 0.25 to 3.44 mol.% over the temperature range of 40 to 100°C. At 40 bar abs., over the same temperature range, the water content ranges from 0.19 to 2.66 mol.%.

The make-up gas contains hydrogen, carbon monoxide, carbon dioxide and small amounts of unreacted methane, argon and nitrogen. In addition, a small amount of residual steam may be present. The make-up gas has an R value in the range of 1.70 to 1.94. The R value of the feed gas after the addition of the hydrogen-rich gas, excluding any recycle gas stream, is preferably in the optimum range for methanol synthesis from 1.95 to 2.05. In cases where a recycle or cycle gas stream is added to the make-up gas and hydrogen-rich gas mixture, the R value will be higher than 2.05, since the addition of water or steam to the feed gas results in an increase in the carbon dioxide content recovered as dissolved gas in the liquid raw methanol.

If necessary, part of the make-up gas can be transferred to external processes.

The make-up gas may be compressed in a synthesis gas compressor to achieve the required pressure before the make-up gas is fed to the methanol synthesis unit. Hydrogen-rich gas extracted from the purge gas is added to the make-up gas. Hydrogen-rich gas may be added to the make-up gas before or after compression in the synthesis gas compressor.

The feed gas entering the methanol synthesis unit, prior to the addition of water or steam, may consist of make-up gas and hydrogen-rich gas, or, when the first reactor in the methanol synthesis unit is operated cyclically, the feed gas directed to the methanol synthesis unit, prior to the addition of water or steam, may consist of a stream of make-up gas, hydrogen-rich gas and recycle gas containing unreacted gases extracted from the first methanol synthesis reactor and/or a subsequent methanol synthesis reactor in the methanol synthesis unit.

The methanol synthesis plant suitably comprises one or more methanol synthesis reactors, such as a first, second and optionally third methanol synthesis reactors, each of which comprises a bed of methanol synthesis catalyst, arranged in series and/or parallel so that each of them produces a gas product stream containing methanol. Thus, the methanol synthesis plant may comprise one, two or more methanol synthesis reactors, each of which comprises a bed of copper methanol synthesis catalyst, and wherein each of the reactors is fed with a feed gas containing hydrogen and carbon dioxide, each of which produces a gas product mixture containing methanol.

A crude methanol product stream is recovered from one or more gas product mixtures. This may be accomplished by cooling the one or more gas product mixtures to a temperature below the dew point, condensing the crude methanol product, and separating the crude liquid methanol product from the unreacted gases. After separating the crude liquid methanol product from the one or more methanol gas product streams, one or more unreacted gas mixtures are formed.

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The methanol synthesis unit typically operates in a cyclic manner. Accordingly, a portion of the unreacted gas mixture is returned as a recycle or cycle gas stream to one or more methanol synthesis reactors. The unreacted gas separated from the product gas mixture extracted from one methanol synthesis reactor may be returned to the same or another methanol synthesis reactor. The unreacted gas mixture contains hydrogen, carbon monoxide and carbon dioxide and can therefore be used to produce additional methanol. The recycle gas stream may be extracted from at least one of the methanol product gas streams and recycled to at least one of the methanol synthesis reactors. If multiple recycle gas streams are present, these streams may be recycled to one or more methanol synthesis reactors separately, or they may be combined and fed to one or more methanol synthesis reactors.

The methanol synthesis unit may comprise a first methanol synthesis reactor and a second methanol synthesis reactor connected in series. In such a configuration, the gas fed to the second methanol synthesis reactor may comprise at least a portion of the mixture of unreacted gases from the methanol product gas stream extracted from the first methanol synthesis reactor. While the gas fed to the second methanol synthesis reactor may comprise all of the unreacted gases from the methanol product gas stream from the first methanol synthesis reactor, if necessary, a portion of the unreacted gas stream not fed to the second methanol synthesis reactor may be recycled to the feed line to the first methanol synthesis reactor. In particular, preferred methanol synthesis units are described in US7790775, WO2017/121980 and WO2017/121981.

For example, a methanol synthesis plant may comprise a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein the first methanol synthesis reactor operates on a straight-through principle, and the gas supplied to the second methanol synthesis reactor consists of a stream of all unreacted gas extracted from the first methanol synthesis reactor and a stream of recycle gas extracted from the second methanol synthesis reactor.

Alternatively, the methanol synthesis unit may comprise a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein a portion of the unreacted gas stream extracted from the first methanol synthesis reactor is recycled to the first methanol synthesis reactor, and a portion of the unreacted gas stream extracted from the second methanol synthesis reactor is recycled to the second methanol synthesis reactor.

Alternatively, the methanol synthesis unit may comprise a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein a portion of the unreacted gas stream extracted from the second methanol synthesis reactor is recycled to the first methanol synthesis reactor.

In the method, a stream of water or steam is added to the feed gas sent to the methanol synthesis unit. The water may be tap water or demineralized water or a water stream recovered in the process, such as condensate, a water stream recovered by distillation from a raw methanol stream, or a water stream recovered from a purge gas scrubbing unit. The water or steam stream may consist of water or steam, although small amounts of methanol or other substances may be present when water or steam is recovered from the process. For example, the _H2O content of the water or steam may be >90 vol.%, preferably >95 vol.%, more preferably >98 vol.%.

Water or steam can be added to a feed gas fed to one or more methanol synthesis reactors in a methanol synthesis unit. For example, water or steam can be added to a feed gas fed to a methanol synthesis unit comprising one methanol synthesis reactor operated cyclically. Alternatively, water or steam can be added to a feed gas fed to a methanol synthesis unit comprising two or more methanol synthesis reactors operated in series or in parallel. In the case where the methanol synthesis unit comprises two or more methanol synthesis reactors operated in series, water or steam is preferably added to the feed gas before the first methanol synthesis reactor.

Water can be added by means of an evaporator, which may comprise a vessel through which the feed gas passes and into which liquid water is added, for example by spraying, causing it to evaporate. The vessel may comprise a structured packing or a bed of shaped inert material, such as alumina granules or extrudates, to provide a surface from which the water can evaporate more efficiently. The water may be boiler feed water or water obtained from process condensate. A particularly practical source of water is water recovered from the purge gas scrubber. The advantages of using this source are that this stream is already at high pressure and the small amount of methanol contained will assist in controlling the reactor peak temperature by a more stringent approach to equilibrium.

Steam can be added to the feed gas by direct addition using known methods. Steam can be obtained from boiler feed water or from process condensate recovered from synthesis gas.

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The addition of water or steam to the feed gas may be performed before or after preheating the feed gas upstream of the methanol synthesis unit, such as before or after preheating the feed gas in a gas-gas heat exchanger. The addition of water or steam before the gas-gas heat exchanger may provide improved mixing of water or steam before the methanol synthesis reactor.

Addition of water or steam to the feed to the methanol synthesis reactor promotes the water gas shift reaction over the methanol synthesis catalyst, whereby carbon monoxide reacts with water or steam to form carbon dioxide and hydrogen. Thus, the ratio of carbon dioxide to carbon monoxide in the product gas mixture is increased. The amount of water or steam added to the feed gas sent to the methanol synthesis unit can range from 0.1 to 6 mol % of the make-up gas. Addition of too much water or steam can result in an increase in the R value of the feed gas containing the recycle gas stream due to the removal of excess dissolved carbon dioxide in the crude methanol product. In addition, excess hydrogen can also be removed from the purge gas, resulting in a decrease in methanol production for a fixed amount of make-up gas. Too much water or steam can also decrease methanol production due to equilibrium limitations and a decrease in the forward reaction rate. Conversely, if too little water or steam is added, the R-value of the feed gas containing the recycle gas stream may drop below the required minimum, which may result in increased amounts of undesirable by-products.

If the methanol synthesis plant comprises a single methanol synthesis reactor, the reactor may be an uncooled adiabatic reactor. Alternatively, the reactor may be cooled by heat exchange with the methanol synthesis feed gas, such as in a quench reactor, a tubular cooled converter or a gas-cooled converter. Alternatively, the methanol synthesis reactor may be cooled with boiling water under pressure, such as in an axial steam upflow converter or a radial steam upflow converter. If the methanol synthesis plant comprises two or more methanol synthesis reactors, they may use any combination, although combinations of axial or radial steam upflow converters and tubular cooled or gas-cooled converters, or combinations of axial steam upflow converters followed by radial steam upflow converters are preferred.

In an adiabatic reactor, the methanol synthesis feed gas may pass axially, radially, or both axially and radially through a fixed bed of particulate methanol synthesis catalyst. The exothermic nature of the methanol synthesis reactions results in an increase in the temperature of the reaction gases. Thus, it is desirable to have a lower inlet temperature to the bed than in cooled reactor systems to avoid overheating the catalyst, which can adversely affect selectivity and catalyst life. An alternative is to use a cooled reactor, which can utilize heat exchange with a cooling medium within the reactor to minimize or control the temperature. There are a number of types of cooled reactors that can be used in this situation. In one configuration, the fixed bed of particulate catalyst is cooled by tubes or plates through which a cooling heat exchange medium passes. In another configuration, the catalyst is contained in tubes around which a cooling heat exchange medium passes. Methanol synthesis reactors can be cooled by a feed gas or boiling water, usually under pressure. For example, a methanol synthesis reactor can be an axial steam upflow converter, a radial steam upflow converter, a gas-cooled converter, or a tubular-cooled converter.

In an axial steam upflow converter (aSRC), the feed gas for methanol synthesis typically passes axially through vertical tubes filled with catalyst, which are cooled by heat exchange with boiling water under pressure flowing in the shell side. The catalyst may be provided in granular form directly in the tubes or may be provided in one or more cylindrical containers that direct the flow of synthesis gas in radial and axial directions to intensify heat transfer. Said contained catalysts and their use in methanol synthesis are described in U.S. Patent No. US8785506. A particularly suitable means for removing heat from the catalyst are upflow converters in which the catalyst is contained in tubes cooled by boiling water under pressure.

In a radial steam upflow converter (rSRC), the feed gas for methanol synthesis typically passes radially (inward or outward) through a bed of particulate catalyst, which is cooled by a plurality of tubes or plates through which pressurized boiling water is fed as a cooling medium. Such reactors are known and are described, for example, in US Patent No. US4321234. They provide lower pressure drop than aSRC converters, but have a more complex internal structure.

In a tube cooled converter (TCC), the catalyst bed is cooled by feeding

- 6 047 470 feed gas for methanol synthesis through open-ended tubes located inside the bed and releasing the heated gas into the space above the catalyst in the shell side of the reactor. Subsequently, the heated gas can be passed directly through the catalyst bed without leaving the converter. The TCC converter can provide sufficient cooling area for a number of synthesis gas compositions and can be used over a wide range of conditions. As an alternative to the TCC converter, a gas-cooled converter (GCC) can be used to cool the catalyst bed, in which the feed gas for methanol synthesis is passed through tubes or plates according to the heat exchanger principle. In this case, the heated synthesis gas is removed from the converter and then returned back to the catalyst bed. An example of a GCC converter is described in U.S. Patent No. US 5827901.

Alternatively, the methanol synthesis reactor may be a quench reactor in which one or more fixed beds of particulate methanol synthesis catalyst are cooled by a methanol synthesis feed gas mixture that is introduced into the reactor within or between the beds. Such reactors are described, for example, in U.S. Patent No. US4411877.

In a method comprising first and second methanol synthesis reactors, the first methanol synthesis reactor is preferably cooled with boiling water, for example in an axial steam upflow converter or a radial steam upflow converter, more preferably an axial steam upflow converter. The second methanol synthesis reactor may be a radial steam upflow converter. Such designs are particularly used for the present invention due to the performance characteristics of these reactors and their ability to handle various feed gas mixtures. Alternatively, the second methanol synthesis reactor may be a gas-cooled converter or a tubular-cooled converter.

The methanol synthesis catalysts are suitable copper-containing methanol synthesis catalysts that are commercially available. In particular, suitable methanol synthesis catalysts are copper oxide/zinc oxide/aluminum oxide catalysts in the form of solid particles, which may contain one or more promoters. The methanol synthesis catalysts in each of the methanol synthesis reactors may be the same or different. For example, the methanol synthesis catalyst in the methanol synthesis reactor to which the water- or steam-containing feed gas is directed may have a composition that is stable with water or steam and promotes the water gas shift reaction. The copper oxide content of the catalyst (calculated as CuO) may be in the range of from 30 to 70 wt.%. Within this range, a copper oxide content in the range of 50 to 70 wt.%, preferably 60 to 70 wt.%, finds general use in the synthesis of methanol, whereas for the water gas shift reaction a substantially lower copper oxide content can be used, for example in the range of 30 to 60 wt.%. The weight ratio of Cu:Zn (based on CuO:ZnO) can be 1:1 or more, but is preferably in the range of 2:1 to 3.5:1, in particular 2.5:1 to 2.75:1, for methanol synthesis catalysts and in the range of 1.4:1 to 2.0:1 for water gas shift catalysts. In the case of methanol synthesis catalysts, the catalyst preferably contains 20-30 wt.% zinc oxide. The catalyst typically contains aluminum oxide, which can be present in an amount in the range of 5 to 20 wt.%. Particularly suitable catalysts are silica-doped methanol synthesis catalysts as described in WO2020212681 (A1), which are surprisingly stable under the influence of feed gases containing water or steam.

Methanol synthesis can be carried out in methanol synthesis reactors at pressures in the range of 10 to 120 bar abs and at temperatures in the range of 130°C to 350°C. The pressure at the reactor inlets is preferably 50-100 bar abs, more preferably 70-90 bar abs. The temperature of the synthesis gas at the reactor inlets is preferably in the range of 200-250°C, and at the outlets is preferably in the range of 230-280°C.

Each methanol synthesis reactor produces a gas product mixture. In the present method, a liquid methanol-containing stream is preferably recovered from each gas product mixture prior to further use. This can be accomplished by cooling one or more gas product mixtures to a temperature below the dew point, condensing the crude methanol product, and separating the crude liquid methanol product from unreacted gases. Conventional heat exchange equipment and gas-liquid separation equipment can be used. In particular, a suitable heat exchange apparatus includes a gas-gas heat exchanger in which the feed gas mixture to the methanol synthesis reactor is used to cool the gaseous methanol product stream from that reactor. The use of a gas-gas heat exchanger typically allows for improved control of steam generation in updraft converters. In the presence of two or more methanol synthesis reactors, the gas product mixtures can be cooled separately or combined and cooled together to produce a crude methanol stream. Cooling of the gas product mixture is preferably performed to 50°C or less, preferably 45°C or less, so that the capture of dissolved carbon dioxide in the crude methanol product is maximized. However, cooling to a temperature below 40°C is not necessary.

The liquid crude methanol extracted from one or more gas product mixtures contains dissolved carbon dioxide and is therefore preferably treated first by

- 7 047470 reducing its pressure and/or increasing its temperature and separating the vaporous carbon dioxide using a flash tank.

The method may include a step of extracting a carbon dioxide stream from the raw methanol. The extracted carbon dioxide stream is not recycled to the process, but after optional purification, may be used in external chemical synthesis processes to enhance oil recovery or may be sequestered in a carbon collection and storage unit. The carbon dioxide stream may be used as a chemical feedstock, such as for the production of acetic acid or in the production of urea in an ammonia-methanol-urea co-production process.

Subsequently, the carbon dioxide-depleted crude methanol can be processed in a conventional manner by distillation to produce a purified methanol product. If necessary, a portion of the carbon dioxide-depleted crude methanol product can be advantageously recycled as a wash stream to the gas-liquid separator to improve the capture of carbon dioxide in the crude methanol product.

A portion of the unreacted gas mixture constituting the gas stream recycled to the cycle is compressed by one or more compressors or circulation pumps. The compression may occur before the stream is split, for example to provide a purge gas stream, or after its splitting, or after the recycle gas stream is combined with the feed gas. Recycle ratios for forming feed gas mixtures fed to one or more methanol synthesis reactors may be in the range from 0.5:1 or lower to 5:1, preferably from 1:1 to 3:1. The term recycle ratio means the molar ratio of the flow rate of the recycle stream of unreacted gas and the make-up gas, as a result of which the gas mixture fed to the methanol synthesis reactor is formed.

A portion of the unreacted gas mixture separated from the liquid raw methanol is removed from the cycle or recycle stream as a purge gas stream, which is used to prevent the accumulation of undesirable inerts in the process. The purge gas stream may be removed continuously or intermittently. The purge gas stream may be recovered from the separated unreacted gases before or after compression in the recirculating pump. Purge gas recovered downstream of the compressor may provide a higher driving force and assist in membrane separation.

In the method, at least a portion of the purge gas stream is separated into a hydrogen-rich gas stream, which is recycled into makeup gas for the method. As a result, a carbon-rich off-gas stream is obtained. The carbon-rich off-gas stream is understood to be a gas stream, which has a higher proportion of carbon-containing compounds (carbon monoxide, carbon dioxide and methane) than the purge gas. Although individual components can have the same or even a lower proportion than in the purge gas, the total amount of all carbon-containing components in the carbon-rich gas will be a higher proportion compared to the purge gas. Preferably, the entire purge gas stream undergoes a separation step. The separation of hydrogen-rich and carbon-rich gas streams can be carried out using known separation equipment, such as a hydrogen membrane separator, or a pressure swing adsorption unit, a separation system in a cold box, or any combination thereof. Using these methods, more than 50% of the hydrogen present in the purge gas stream can be recovered.

It should be understood that by adding a stream of hydrogen-rich gas to the make-up gas, the stoichiometric value R of the feed gas will be increased. Additional hydrogen from an external source can also be added if necessary.

In the present method, the carbon-rich off-gas, which contains inerts, is desirably fed as fuel, for example to a fired heater, such as a fired heater used for superheating steam, or preheating and/or reheating feedstock in a synthesis gas plant. Alternatively, the carbon-rich off-gas may be withdrawn from the process for use as fuel.

It is desirable that the hydrogen-rich gas recovered from the purge gas stream contains > 80 vol.% H2. The separated hydrogen, in addition to being recycled to the methanol loop, can also be used upstream in the hydrodesulfurization of the hydrocarbon feedstock and/or withdrawn from the process for another use. However, in a preferred embodiment, a major portion, such as at least 51 vol.% of the separated hydrogen, is fed to the methanol synthesis loop.

Carbon-rich gas, which will typically contain carbon oxides and methane, can be used as a fuel, for example in a fired heater. Carbon-rich gas can be effectively used as a fuel for a fired heater used to heat process feedstocks, such as feed streams to a pre-reformer and an autothermal reformer.

A CO2 removal unit may optionally be included in a system for recovering carbon dioxide from unreacted gas extracted from a gas product mixture, such as before

- 8 047 470 by recycling the unreacted gas or feeding it to one or more additional methanol synthesis reactors in the methanol synthesis unit. In cases where a CO2 removal unit is used, the resulting product gas stream, deprived of carbon dioxide, can be returned for inclusion in the feed gas. The CO2 removal unit can be any conventional CO2 removal unit, the operation of which is based on physical absorption, chemical absorption, adsorption in a porous material or uses a membrane to selectively separate _CO2 from a carbon-rich stream, thereby forming a methane-rich stream. A membrane CO2 removal unit is preferred. The recovered CO2 stream may contain small amounts of methane and inerts, and can therefore be used as a fuel, for example in a fired heater. Alternatively, the recovered CO2 stream, optionally after further purification, can be fed to an external chemical synthesis process used for enhanced oil recovery, or can be sequestered.

The purge gas stream mixture may contain methanol and therefore, if desired, methanol may be recovered from the purge gas stream upstream of the separation of the hydrogen-rich gas and the carbon-rich gas using a water wash. Preferably, at least a portion of the resulting methanol-containing wash water is added to the feed gas supplied to the methanol synthesis unit. Any methanol-containing purge gas wash water not added to the feed gas entering the methanol synthesis unit may be sent for purification together with the raw methanol.

The crude methanol stream recovered from the methanol production plant contains water as well as small amounts of higher alcohols and other impurities. The crude methanol can first be fed to a flash vessel or column where dissolved gases are liberated and separated from the liquid crude methanol stream. The crude liquid methanol can be subjected to one or more purification stages, including one or more, preferably two or three stripping stages in a methanol purification plant comprising one, two or more fractionating columns. The degassing stage and the stripping stages can be heated using heat recovered from the process, such as from cooling a product gas stream, a synthesis gas stream or from other sources. Typically, at least a portion of the crude methanol is purified by stripping to produce a purified methanol product.

The purified methanol product can be further processed, for example to produce derivatives such as dimethyl ether or formaldehyde. Alternatively, methanol can be used as a fuel.

The following is a description of the invention with references to the accompanying graphic materials, wherein:

Fig. 1 shows a method according to one embodiment of the invention;

Fig. 2 shows an additional method according to another embodiment of the invention;

Fig. 3 shows an additional method according to another embodiment of the invention; and Fig. 4 shows an additional method according to another embodiment of the invention.

In Fig. 1-4, the synthesis gas production plant comprises a pre-reforming plant and an autothermal reforming plant. In Fig. 1, the methanol synthesis plant comprises one stage, i.e., one methanol synthesis converter, or parallel converters of the same design, which operate cyclically. In Fig. 2 and 3, the methanol synthesis plant comprises two stages connected in series, wherein the first stage operates according to the straight-through principle, and the second stage operates cyclically. In Fig. 4, the methanol synthesis plant comprises two methanol stages connected in series, wherein both stages operate cyclically.

It will be understood by those skilled in the art that the graphical materials are schematic and that an industrial plant may require additional equipment items such as feedstock tanks, pumps, vacuum pumps, compressors, gas recirculation compressors, temperature sensors, pressure sensors, relief valves, control valves, flow controllers, level controllers, temporary storage tanks, storage tanks, etc. The provision of such additional equipment items is not part of the present invention and is carried out in accordance with normal practice in the design of chemical industry facilities.

In Fig. 1, a mixture of natural gas and steam fed through pipeline 10 is fed to a flame heater 12, where it is heated. The heated gas mixture is fed from the flame heater 12 through pipeline 14 to a pre-reformer 16 containing a fixed bed of steam reforming catalyst in the form of solid particles. The heated gas mixture is reformed under adiabatic conditions on the catalyst, thereby converting higher hydrocarbons present in the natural gas into methane, carbon oxides and hydrogen. The pre-reformed gas mixture is fed from the pre-reformer 16 through pipeline 18 to the flame heater 12, in

- 9 047470 in which it is heated to the temperature at the inlet of the autothermal reforming unit. The reheated pre-reformed gas mixture is fed from the flame heater 12 through the pipeline 20 to the autothermal reforming unit 22 with the supply of an oxygen stream 24. In the autothermal reforming unit, the pre-reformed gas mixture is partially burned, wherein the oxygen in the burner is installed near the upper part, and the resulting hot partially burned gas is brought to equilibrium through a layer of steam reforming catalyst located under the burner. The resulting autothermal reforming synthesis gas stream is fed from the autothermal reforming unit 22 through the pipeline 26 to the heat recovery unit 28, containing one or more heat exchangers, where it is cooled to a temperature below the dew point for condensing the steam. The process condensate 30 is removed from the cooled gas mixture using gas-liquid separation equipment into a heat recovery unit to produce make-up gas. A portion of the condensate 30 may be used to generate steam used to prepare feed gas 14 fed to the pre-reforming unit 16. The make-up gas is extracted from the heat recovery unit 28 via a pipeline 32 together with a hydrogen-enriched stream supplied via a pipeline 34, and the resulting mixture is fed via a pipeline 36 to a synthesis gas compressor 38, where it is compressed. The compressed gas mixture is extracted from the synthesis gas compressor 38 via a pipeline 40 and combined with the recirculation loop gas supplied via a pipeline 42 to form a feed gas mixture, which is fed via a pipeline 44 to a circulation compressor 46, in which the mixture is compressed to the loop pressure. Compressed feed gas is withdrawn from circulation compressor 46 via pipeline 48 and mixed with a stream of steam supplied via steam injection line 50. The resulting mixture of feed gas and steam is supplied via pipeline 52 to gas-gas heat exchanger 54 where it is heated to the temperature at the inlet of methanol converter and subsequently supplied via pipeline 56 to the inlet of methanol synthesis reactor 58. Although one converter is shown, it should be understood that this process flow diagram can operate with two or more converters in parallel. Methanol synthesis reactor 58 is a converter with an axial ascending steam flow, containing tubes 60 filled with a methanol synthesis catalyst, cooled by boiling water under pressure 62. The reactions of methanol synthesis and water gas conversion occur in reactor 58 to obtain a gas product mixture containing methanol, unreacted hydrogen and carbon dioxide. The gas product mixture is extracted from the reactor 58 via the pipeline 64, cooled in the heat exchanger 54 and fed via the pipeline 66 to one or more additional heat exchangers 68, where it is cooled to a temperature below the dew point for condensation of the liquid raw methanol. The cooled mixture is fed from one or more heat exchangers 68 via the pipeline 70 to the gas-liquid separator 72, where the unreacted gases are separated from the liquid raw methanol, which is extracted from the separator 72 via the pipeline 74. The raw methanol is sent to the distillation and purification unit (not shown), where it is degassed and distilled in two or three distillation stages to obtain a purified methanol product. The unreacted gases are extracted from the separator 72 and separated. A portion is fed through a pipeline 42 as a recirculation loop gas to form a portion of the feed gas fed to the methanol synthesis reactor 58. The remaining portion is fed through a pipeline 78 as a purge gas to a purge gas scrubbing unit 80, to which a water stream 82 is fed and which creates a purge gas scrubbing stream 84 containing a small amount of methanol. The purge gas wash stream is heated to generate steam, some of which may be fed into the feed gas mixture via pipeline 50. The washed purge gas is extracted from the purge gas wash unit 80 and fed via pipeline 86 to the membrane hydrogen recovery unit 88, where the hydrogen-enriched stream is separated from the washed purge gas and fed via pipeline 34 to the makeup gas stream in pipeline 32. The carbon-enriched exhaust gas is extracted from the hydrogen recovery unit 88 via pipeline 90 and sent as fuel for combustion to the flame heater 12.

In Fig. 2, the synthesis gas production plant is the same as shown in Fig. 1. However, while in Fig. 1 the methanol synthesis plant operates with one reactor in a cyclic manner, the methanol synthesis plant in Fig. 2 operates with two methanol synthesis reactors connected in series, with the first reactor operating on a straight-through principle, and the unreacted gases from the separator downstream of the first reactor are fed, unlike in the first embodiment, to an additional methanol synthesis reactor operating in a cyclic manner. Thus, in Fig. 2 compressed mixture of make-up gas and hydrogen-saturated gas in pipeline 40 is mixed with steam in pipeline 50, heated in gas-gas heat exchanger 54, passed through reactor 58, cooled in heat exchanger 54 and additional heat exchangers 68 and fed to gas-liquid separator 72, from which a first stream of liquid raw methanol 74 is extracted. First unreacted gas mixture 76 extracted from gas-liquid separator 72 is combined with second unreacted gas mixture from pipeline 110, and the combined feed gas is fed through line 112 to circulation compressor 114. Compressed feed gas is fed from compressor 114 through pipeline 116 to second gas-gas heat exchanger 118, where it is heated, and subsequently fed through pipeline 120 to the inlet of the converter 122 with a radial ascending flow of steam containing an annular layer of catalyst 124, cools

- 10 047470 tubes or plates containing boiling water under pressure 126. Although a radial updraft converter is shown, the method can equally be implemented using an axial updraft converter, a gas-cooled converter, or a tube-cooled converter. A second gas product mixture is withdrawn from the reactor 122 via a conduit 128 and cooled in a heat exchanger 118 and one or more additional heat exchangers 130 to a temperature below the dew point. The cooled mixture is then passed from the heat exchangers 130 to a second gas-liquid separator 132, from which a second stream of liquid raw methanol is withdrawn via a conduit 134. The first and second streams of liquid raw methanol can be combined if necessary and then sent for purification to obtain a purified methanol product, as described above. The unreacted gas mixture 136 is extracted from the second gas-liquid separator 132 and divided into a second stream 110 of unreacted gas and a purge stream 138. The purge stream 138 is fed to a purge gas scrubbing unit 140, which receives a water stream 142 and which creates a purge gas scrubbing stream 144 containing a small amount of methanol. The purge gas wash stream is heated to generate steam, some of which may be fed into the feed gas mixture via pipeline 50. The washed purge gas is extracted from the purge gas wash unit 140 and fed via pipeline 146 to the membrane hydrogen recovery unit 148, where the hydrogen-enriched stream is separated from the washed purge gas and fed via pipeline 34 to the makeup gas stream in pipeline 32. The carbon-enriched exhaust gas is extracted from the hydrogen recovery unit 148 via pipeline 150 and sent as fuel for combustion in the flame heater 12.

In Fig. 3, the synthesis gas production unit and the methanol synthesis unit are the same as those shown in Fig. 2, except that a portion of the gas fed to the radial updraft converter is recycled to add to the feed gas entering the axial updraft converter. Thus, the feed gas 116 entering the radial updraft converter 122 is divided into a first and a second portion. The first portion is heated in a heat exchanger 118 and fed to the radial updraft converter 122. The second portion is fed through a line 152 to the compressed feed gas in a line 40 upstream of the steam or water addition through a line 50, heated in a heat exchanger 54 and fed to the axial updraft converter 58.

Optionally, as shown in dotted lines, at least a portion of the unreacted gas mixture 76 recovered from the first gas-liquid separator 72 can be fed to a _CO2 removal unit 160 to remove a portion of the carbon dioxide from the first unreacted gas mixture. A suitable CO2 removal unit 160 can be a membrane unit that produces a CO2-free gas mixture 164 that is combined with the second unreacted gas mixture 110 to form a feed gas for a radial upflow vapor converter. The _CO2 removal unit also produces a _CO2 stream 162 that can be sent to an external process or sequestered in a CO2 production unit.

In Fig. 4, the synthesis gas plant and the methanol synthesis plant are the same as those shown in Fig. 2, except that the purge gas mixture 138 obtained from the unreacted gas mixture 136 is divided into portions. The first portion is fed to a purge gas scrubber 140 and a hydrogen separation unit 148 to form a hydrogen-rich stream 34. The second portion bypasses these units and is recycled to the compressed feed gas in conduit 40 upstream of the steam addition via conduit 50 to the feed gas for the axial steam riser converter 58. In this configuration, the second unreacted gas stream 110 is not combined with the unreacted gas stream in conduit 76 (or optionally with the CO ₂ -deprived stream 164), but is instead fed directly to the recycle compressor 114. Optionally, as shown in Fig. 3, at least a portion of the unreacted gas mixture 76 extracted from the first gas-liquid separator 72 can be fed to a CO2 removal unit 160 to remove a portion of the carbon dioxide from the first unreacted gas mixture to thereby produce a CO2 stream ₁₆₂ and a _CO2- free gas 164 that can be fed to a circulation compressor 114.

The present invention will be further described with reference to the following calculation examples obtained using conventional modeling software suitable for methanol synthesis processes. All these examples are based on the same amount of _H2 +CO in ^Nm3 /h at the outlet of the ATR, which was operated at a steam to carbon ratio of 0.6:1 and a pressure of 34 bar.

- 11 047470

Example 1.

Example 1 is an example of a process flow diagram in accordance with Fig. 1. The process conditions and compositions of the various streams are indicated below.

Stream number 20 24 26 30 32 34 36 Temperature (°C) 650 228 1050 92 45 55 46 Pressure (bar abs.) 35.4 39.5 34.0 30.9 30.9 30.9 30.9 Mass flow rate (tons/hour) 201.1 126.0 327.1 76.2 250.8 21.7 272.5 Molar flow rate (kgmol/h) 11 972 3960 25 666 4230 21 436 2789 24 225 Molecular weight 16.79 31.83 12.74 18.02 11.70 7.77 11.25 Composition (kgmol/h) Water 3986,6 55.5 4305.8 4228.2 77.4 9.5 86.9 Hydrogen 486.0 - 13,849.3 - 13,849.3 2339,5 16 188,8 Carbon monoxide 3.9 - 6110,3 0.7 6109,6 50.4 6160,1 Carbon dioxide 267.5 - 974.7 0.9 973.8 318.7 1292.5 Nitrogen 35.9 - 35.9 - 35.9 7.3 43.3 Argon - 11.7 And,7 - And,7 5.5 17.2 Methane 7192.2 - 378.3 - 378.2 58.0 436.2 Methanol - - - - - - - Oxygen - 3892,7 - - - - - Lungs - - - - - - - Heavy - - - - - - - Stream number 42 44 50 56 64 70 74 Temperature (°C) 45 88 303 230 251 45 45 Pressure (bar abs.) 76.8 76.3 84.2 83.0 80.0 77.1 77.0 Mass flow rate (tons/hour) 462.1 734.7 9.9 744.6 744.6 744.6 239.8 Molar flow rate (kgmol/h) 42 872 67 097 550 67 647 54 761 54 761 7936 Molecular weight 10.78 10.95 18.02 11.01 13.60 13.60 30.22 Composition (kgmol/h) Water 21.7 108.6 550,0 658.6 1195,5 1195,5 1171.9 Hydrogen 29 846,5 46,035.3 - 46,035.3 32,622.9 32,622.9 24.1 Carbon monoxide 2542.3 8702,3 - 8702,4 2788,6 2788,6 12.0 Carbon dioxide 5303.4 6595.8 - 6595.8 6063.8 6063.8 271.5 Nitrogen 453.4 496.7 - 496.7 496.6 496.6 1.5 Argon 177.6 194.8 - 194.8 194.8 194.8 0.9 Methane 4249.3 4685,5 - 4685,5 4685,5 4685,5 44.4 Methanol 272.8 272.8 - 272.8 6698.9 6698.9 6401,2 Oxygen - - - - - - - Lungs 5D 5.0 - 5.1 12.1 12.1 6.5 Heavy - - - - 2.1 2.1 2.1

- 12 047470

Stream number 76 78 82 84 86 90 Temperature (°C) 45 45 46 51 53 54 Pressure (bar abs.) 77.0 76.2 76.5 76.2 75.8 72.6 Mass flow rate (tons/hour) 504.7 42.6 7.0 7.7 41.9 20.2 Molar flow rate (kgmol/h) 46 825 3953 390 408 3935 1146 Molecular weight 10.78 10.78 18.02 18.96 10.65 17.64 Composition (kgmol/h) Water 23.7 2.0 390,0 381.5 10.4 0.9 Hydrogen 32,598.6 2752,0 - - 2752.1 412.7 Carbon monoxide 2776.7 234.4 - 0,0 234.4 183.9 Carbon dioxide 5792.3 489,0 - 1.3 487.7 169.0 Nitrogen 495.2 41.8 - - 41.8 34.5 Argon 193.9 16.4 - - 16.4 10.9 Methane 4641.1 391.8 - 0,1 391.7 333.6 Methanol 298,0 25.2 - 25.0 - 0,1 Oxygen - - - - - - Lungs 5.5 0.5 - - 0.5 0.5 Heavy - - - - - -

Example 2. Comparative.

Example 2 is similar to example 1 and is based on the method shown in Fig. 1, but does not contain the steam injection line 50 and additionally contains a make-up gas supply line that passes a portion of the make-up gas, designated below as line 41, from the compressed make-up gas mixture line into the washed purge gas 86, which enters the hydrogen recovery unit 88. This is an example of the method claimed in WO2016180812 A1. The process conditions and the compositions of the various streams are indicated below.

Stream number 20 24 26 30 32 34 36 Temperature (°C) 650 228 1050 93 45 101 51 Pressure (bar abs.) 35.4 39.5 34.0 30.9 30.9 30.9 30.9 Mass flow rate (tons/hour) 201.1 126.0 327.1 76.2 250.8 17.6 268.5 Molar flow rate (kgmol/h) 11 972 3959 25 665 4230 21 435 2502 23 938 Molecular weight 16.79 31.83 12.74 18.02 11.70 7.04 11.21 Composition (kgmol/h) Water 3986,6 55.5 4305.6 4228,1 77.4 10.5 87.9 Hydrogen 486.0 - 13,848.6 - 13,848.4 2103.7 15,952.5 Carbon monoxide 3.9 - 6109.7 0.7 6109,0 123.4 6232,5 Carbon dioxide 267.5 - 974.8 0.9 973.9 187.3 1161.2 Nitrogen 35.9 - 35.9 - 35.9 8.3 44.3 Argon - 11.7 And,7 - And,7 5.9 17.6 Methane 7192.1 - 378.8 - 378.7 63.3 442.0 Methanol - - - - - - - Oxygen - 3891.9 - - - - - Lungs - - - - - - - Heavy - - - - - - -

- 13 047470

Stream number 41 42 44 56 64 70 74 Temperature (°C) 175 45 88 230 251 45 45 Pressure (bar abs.) 77.0 76.8 76.3 83.0 80.0 77.1 77.0 Mass flow rate (tons/hour) 16.1 450.7 703.1 703.1 703.1 703.1 228.3 Molar flow rate (kgmol/h) 1436 42 871 65 372 65 372 52 595 52 595 7439 Molecular weight 11.21 10.51 10.76 10.76 13.37 13.37 30.69 Composition (kgmol/h) Water 5.3 15.9 98.5 98.5 786.2 786.2 769.5 Hydrogen 957.2 28,467.1 43,462.0 43,462.0 30,008.6 30,008.6 24.2 Carbon monoxide 374.0 2582.2 8440,6 8440,6 2731.9 2731.9 12.1 Carbon dioxide 69.7 3966,1 5057,6 5057,6 4374.9 4374.9 197.4 Nitrogen 2.7 735.4 776.9 776.9 776.9 776.9 2.4 Argon 1,1 282.0 298.6 298.6 298.6 298.6 1.5 Methane 26.5 6523.6 6939,0 6939,0 6939,0 6939,0 67.7 Methanol - 293.4 293.4 293.4 6663,6 6663,6 6354.6 Oxygen - - - - - - - Lungs - 5.4 5.4 5.4 13.0 13.0 7.4 Heavy - - - - 2,2 2,2 2,2 Stream number 76 78 82 84 86 90 Temperature (°C) 45 45 46 52 99 101 Pressure (bar abs.) 77.0 76.2 76.5 76.2 75.8 72.6 Mass flow rate (tons/hour) 474.8 24.0 4.0 4.4 39.7 22.1 Molar flow rate (kgmol/h) 45 156 2286 220 231 3711 1209 Molecular weight 10.51 10.51 18.02 19.02 10.70 18.28 Composition (kgmol/h) Water 16.7 0.8 220.1 214.7 And,5 1,1 Hydrogen 29,984.4 1517.8 - - 2474.9 371.2 Carbon monoxide 2719.8 137.7 - 0,0 511.6 388.2 Carbon dioxide 4177,5 211.5 - 0.6 280.6 93.2 Nitrogen 774.6 39.2 - - 41.9 33.5 Argon 297.1 15.0 - - 16.1 10.2 Methane 6871,3 347.8 - 0,1 374.2 310.9 Methanol 309,0 15.6 - 15.5 - OD Oxygen - - - - - - Lungs 5.6 0.3 - - 0.3 0.3 Heavy - - - - - -

Example 3.

Example 3 relates to the process according to Fig. 2, in which two stages of methanol synthesis are used, the first stage comprising an axial updraft converter operating on a straight-through principle, followed by a radial updraft converter operating cyclically. The process conditions and the compositions of the various streams are indicated below.

- 14 047470

Stream number 20 24 26 30 32 34 36 Temperature (°C) 650 228 1050 93 45 53 46 Pressure (bar abs.) 35.4 39.5 34.0 30.9 30.9 30.9 30.9 Mass flow (ton/h) 201.1 126.0 327.1 76.3 250.8 21.8 272.7 Molar flow (kgmol/h) 11,971 3,960 25,665 4,231 21,435 2,806 24,241 Molecular mass 16.79 31.83 12.74 18.02 11.70 7.78 11.25 Composition (kgmol/h) Water 3986.4 55.5 4306.4 4229.0 77.4 9.0 86.5 Hydrogen 485.8 - 13,848.0 - 13,848.4 2347.0 16,195.6 Carbon monoxide 3.8 - 6109.7 0.7 6109.2 58.0 6167.2 Dioxide Carbon 267.4 - 974.9 0.9 974.0 314.2 1288.3 Nitrogen 35.9 - 35.9 - 35.9 7.8 43.7 Argon - 11.7 I.7 - I.7 5.9 17.6 Methane 7191.6 - 378.4 - 378.3 63.7 442.0 Methanol - - - - - - - Oxygen - 3892.7 - - - - - Light - - - - - - Heavy - - - - - - - Number flow 50 56 64 70 74 76 110 Temperature (°C) 301 230 247 45 45 45 45 Pressure (bar abs.) 82.5 82.1 80 78.2 78.1 78.1 78.1 Mass flow (tonnes/hr) 10.8 283.5 283.5 283.5 107.3 176.2 440.0 Molar flow (kgmol/hr) 600 24,841 18,447 18,447 3,374 15,073 40,697 Molecular mass 18.02 11.41 15.37 15.37 31.80 11.69 10.81 Composition (kgmol/h) Water 600.0 686.5 133.6 133.6 131.5 2.1 31.4 Hydrogen - 16,195.6 10,361.9 10,361.9 13.6 10,348.3 28,102.1 Carbon monoxide - 6,167.3 2,412.1 2,412.1 16.0 2,396.0 2,645.1 Dioxide Carbon - 1288.3 1844.3 1844.3 121.8 1722.5 4874.5 Nitrogen - 43.7 43.7 43.7 0.2 43.5 435.5 Argon - 17.6 17.6 17.6 - 17.5 174.4 Methane - 442.0 442.0 4 42.0 6.5 435.5 4196.8 Methanol - - 3185.2 3185.2 3078.8 106.4 231.2 Oxygen - - - - - - Light - - 5.2 5.2 4.0 1.2 5.9 Heavy - - 1.4 1.4 1.3 -

Claims (19)

- - 15 047470 Stream number 120 128 134 136 138 142 144 Temperature (°C) 235 278 45 45 45 46 51 Pressure (bar abs.) 80.5 80.0 78.2 78.2 78.1 77.7 77.4 Mass flow (ton/h) Molar flow 616.2 616.2 133.0 483.3 43.2 7.1 7.8 (kgmol/h) 55,770 49,299 4,603 44,696 3,999 396,413 Molecular weight 11.05 12.50 28.88 10.81 10.81 18.02 18.86 Composition (kgmol/h) Water 33.5 1144.0 1109.6 34.5 3.1 396.0 389.2 Hydrogen 38,450.3 30,873.8 10.6 30,863.5 2761.3 - - Carbon monoxide 5041.2 2911.6 6.6 2905.1 259.9 - 0.0 Carbon dioxide 6597.0 5490.9 137.4 5353.5 479.0 - 1.3 Nitrogen 479.0 478.9 0.7 478.2 42.8 - - Argon 191.9 191.9 0.3 191.6 17.1 - - Methane 4632.3 4632.2 23.1 4609.1 412.4 - 0.1 Methanol 337.6 3564.7 3310.7 253.9 22.7 - 22.6 Oxygen - - - - - - - Light 7.1 8.4 1.9 6.5 0.6 - - Heavy - 2.3 2.3 - - - - Stream number 146 150 Temperature (°C) 52 53 Pressure (bar abs.) 76.3 73.8 Mass flow (ton/h) 42.6 20.8 Molar flow (kgmol/h) 3982 1176 Molecular weight 10.69 17.65 Composition (kgmol/h) Water 9.9 0.9 Hydrogen 2761.3 414.2 Carbon monoxide 259.9 201.8 Carbon dioxide 477.7 163.5 Nitrogen 42.8 35.0 Argon 17D H.2 Methane 412.2 348.5 Methanol - 0.1 Oxygen - - Light 0.6 0.6 Heavy - - A comparison of examples is given below. Example Methanol yield (kmol/h) 1 6426 2 comparative 6370 3 6412 In the examples, the total methanol yield consists of the methanol content of the raw methanol stream, plus the methanol recovered from the purge gas scrubber in the purge gas scrubber stream. Both examples 1 and 3 are superior to the comparative configuration in example 2. CLAUSE OF THE INVENTION 1. A method for synthesizing methanol, comprising the following steps: (i) feeding hydrocarbon feedstock to a synthesis gas production unit to form a synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and steam, wherein the synthesis gas contains from 2.5 to 7 vol.% carbon dioxide on a wet basis; (ii) cooling the synthesis gas in one or more heat exchange stages and extracting process condensate from the cooled synthesis gas to form make-up gas, - 16 047470 having a stoichiometric value R in the range from 1.70 to 1.94, where the stoichiometric value R is determined based on the molar concentrations of the components in the synthesis gas as R = ([H ₂ ] - [CO ₂ ]) / ([CO] + [CO ₂ ]); (iii) feeding a feed gas containing a make-up gas to a methanol synthesis unit comprising one or more methanol synthesis reactors containing a copper methanol synthesis catalyst; and (iv) recovering a purge gas and a crude methanol product from the methanol synthesis unit, wherein a hydrogen-rich gas is recovered from the purge gas and combined with the make-up gas, and a stream of water or steam is added to the feed gas in the methanol synthesis unit. 2. The method according to claim 1, wherein the synthesis gas production unit comprises a partial oxidation unit having one or more catalytic or non-catalytic partial oxidation vessels, or a gasification unit comprising one or more gasifiers, or a reforming unit comprising one or more catalytic steam reforming units. 3. The method according to claim 1 or 2, wherein the synthesis gas production unit comprises an autothermal reforming unit. 4. The method according to any one of claims 1 to 3, wherein the synthesis gas production unit comprises an adiabatic pre-reforming unit and an autothermal reforming unit connected in series. 5. The method according to any one of claims 1 to 4, wherein the hydrocarbon feedstock comprises natural gas. 6. The method according to claim 4 or 5, wherein the hydrocarbon feedstock is pre-reformed in an adiabatic pre-reforming unit upstream of the autothermal reforming unit with steam at a steam to carbon ratio in the range of 0.3 to 3, preferably < 1.5:1, more preferably in the range of 0.3 to 0.9:1. 7. The method according to any one of claims 1 to 6, wherein the synthesis gas contains from 3 to 5 vol.% carbon dioxide, calculated on a wet basis. 8. The method according to any one of claims 1 to 7, wherein the feed gas has a stoichiometric value R that is higher than that of the make-up gas. 9. The method according to any one of claims 1 to 8, wherein the amount of water or steam added to the feed gas sent to the methanol synthesis unit is in the range of 0.1 to 6 mol% of the make-up gas. 10. The method according to any one of claims 1 to 9, wherein at least part of the water added to the feed gas is recovered in the step of washing the purge gas. 11. The method according to any one of claims 1 to 10, wherein the methanol synthesis unit comprises one, two or more methanol synthesis reactors, each of which comprises a bed of methanol synthesis catalyst. 12. The method according to any one of claims 1 to 11, wherein the unreacted gas mixture separated from the gas product mixture extracted from one methanol synthesis reactor is returned to the same or another methanol synthesis reactor. 13. The method according to any one of claims 1 to 12, wherein the methanol synthesis unit comprises a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein the first methanol synthesis reactor operates on a once-through principle, and the gas fed to the second methanol synthesis reactor consists of a stream of all unreacted gas extracted from the first methanol synthesis reactor and a stream of recycle gas extracted from the second methanol synthesis reactor. 14. The method according to any one of claims 1 to 12, wherein the methanol synthesis unit comprises a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein a portion of the unreacted gas stream extracted from the first methanol synthesis reactor is recycled to the first methanol synthesis reactor, and a portion of the unreacted gas stream extracted from the second methanol synthesis reactor is recycled to the second methanol synthesis reactor. 15. The method according to any one of claims 1 to 12, wherein the methanol synthesis unit comprises a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein a portion of the unreacted gas stream extracted from the second methanol synthesis reactor is recycled to the first methanol synthesis reactor. 16. The method according to any one of claims 13 to 15, wherein the first methanol synthesis reactor is an axial upflow converter and the second methanol synthesis reactor is an axial upflow converter, a radial upflow converter, a gas-cooled converter or a tubular-cooled converter. 17. The method according to any one of claims 1 to 16, wherein the copper catalyst for methanol synthesis comprises copper, zinc oxide and aluminum oxide. 18. The method according to any one of claims 1 to 17, wherein the carbon-rich off-gas obtained by separating the hydrogen-rich gas from the purge gas is used as fuel in a combustion heater to heat one or more feed streams to a synthesis gas production unit or is discharged to a separate process. 19. The method of any one of claims 12 to 17, wherein a _CO2 removal unit is included for recovering carbon dioxide from the unreacted gas and releasing it for use in a separate process, or for purification and sequestration, or for use in enhanced oil recovery. -