A process and a plant for the production of methanol

Abstract

The process of producing methanol from synthesis gas through an equilibrium reaction is carried out in a methanol pre-converter within a specific operating window, said operating window is defined by the region under the approximately linear curve of partial pressure of carbon monoxide versus boiling water temperature at a water temperature of 210 to 270°C. By operating within a specific region of the operating window, different grades of methanol product can be obtained.

Description

Technology Field

[0001] The present invention relates to a process and a plant for the production of methanol. The present invention focuses particularly on the operation of a methanol reactor. More specifically, the main focus is on the operational window of the methanol reactor. Background Technology

[0002] The method of producing methanol by the catalytic conversion of synthesis gas containing hydrogen and carbon oxides has long been known to those skilled in the art. Accordingly, methanol is catalytically produced under high pressure and high temperature from a mixture of carbon monoxide, carbon dioxide, and hydrogen, namely methanol synthesis gas, and mostly uses copper-zinc oxide-alumina (Cu / ZnO / Al2O3) catalysts.

[0003] Methanol is produced from synthesis gas through an equilibrium reaction, which proceeds at an elevated temperature under elevated pressure. The synthesis reaction is as follows:

[0004] CO + 2H2<-> CH3OH + heat (1)

[0005] CO2 + 3H2<-> CH3OH + H2O + heat (2)

[0006] CO + H2O <-> CO2 + H2 + heat (3)

[0007] Since reactions (1) through (3) are exothermic reactions, the chemical equilibrium constant decreases as the temperature increases. Therefore, a lower reactor temperature can improve the conversion rate, provided that it is not so low that the specific reaction rate becomes too slow. For a given reactor size and a specific desired conversion rate, the recirculation flow rate increases as the reactor temperature decreases, which means more compressor work is required.

[0008] It has been found beneficial to operate the methanol synthesis reaction within an operating window limited by a curve describing the relationship between partial CO pressure and reactor temperature. More specifically, operation within a specific combination of partial CO pressure and temperature will result in rapid catalyst deactivation. This applies to all layouts surrounding methanol reactors, such as methanol loops with or without a pre-converter, regardless of whether the layout is a new design or a retrofit.

[0009] Depending on a specific combination of partial pressure of CO and boiler water temperature, different methanol grades, such as AA grade methanol or fuel grade methanol, can be obtained in the operating window.

[0010] A typical methanol plant operating on natural gas feedstock is divided into three main sections. In the first section of the plant, natural gas is converted into synthesis gas. In the second section, the synthesis gas reacts to produce methanol, and then, at the end section of the plant, the methanol is purified to the desired purity. In a standard synthesis loop, a methanol reactor, mostly a boiling water reactor (BWR), is used to convert a mixture of synthesis gas from the reformer / vaporizer unit and recirculated gas—that is, unconverted synthesis gas—into methanol.

[0011] It has been found that a specific combination range of partial CO pressure and reactor temperature (actually boiling water temperature) can be established, and within this range, operation is considered "safe" in that beneficial results are obtained. More specifically, a roughly linear curve can be derived at boiling water temperatures between 210°C and 270°C. In this specific temperature range, the partial CO pressure corresponding to a given temperature is 20 kg / cm² at 210°C. 2 From 32.5 kg / cm² at 270℃ 2 It shows an approximately linear increase up to. The area under this curve defines the "safe" operating region.

[0012] Accordingly, the present invention relates to a method for producing methanol from synthesis gas through an equilibrium reaction carried out at an elevated temperature under an elevated pressure according to the synthesis reactions (1) to (3), wherein the method is carried out in a methanol pre-converter within an operating window, and the operating window is:

[0013] - It is defined by the region under the approximately linear curve of partial pressure of carbon monoxide versus boiling water temperature at water temperatures between 210°C and 270°C, where the partial pressure of carbon monoxide is 20 kg / cm² at 210°C. 2 From 32.5 kg / cm² at 270℃ 2 Increases up to,

[0014] - It is divided into two regions by the estimated byproduct curve of partial pressure versus boiling water temperature of carbon monoxide, and these regions result in the production of methanol with different product qualities.

[0015] The above method is preferably performed in the area within the operating window to the left of the lower part of the estimated byproduct curve, which represents the upper limit for obtaining AA grade methanol or methanol of similar quality. The estimated byproduct curve is shown in FIG. 1, where the operating window that can be used in the method according to the present invention is identified.

[0016] It is well known in the art that synthesis gas derived from natural gas or heavy hydrocarbons and coal is highly reactive and detrimental to catalysts in direct methanol synthesis. Furthermore, the use of such highly reactive synthesis gas leads to the formation of large amounts of byproducts.

[0017] The reaction of carbon oxides and hydrogen to methanol is equilibrium-limited, and the conversion of synthesis gas to methanol is relatively low whenever it passes through a methanol catalyst, even when using highly reactive synthesis gas.

[0018] Due to the low methanol production yield in the once-through conversion process, the general practice in the industry is to recirculate the unconverted synthesis gas separated from the reaction effluent and dilute the fresh synthesis gas with this recirculated gas.

[0019] This typically results in a so-called methanol synthesis loop having one or more reactors connected in series, operated for fresh synthesis gas diluted with recirculated unconverted gas separated from the reactor effluent, or for reactor effluent containing methanol and unconverted synthesis gas. The recirculation ratio (recirculated gas to fresh synthesis feed gas) is typically from 2:1 to a maximum of 7:1.

[0020] When a methanol reactor in an existing methanol plant becomes a bottleneck in relation to a capacity upgrade project, the standard solution is to install redundant reactors in series or parallel, or to modify the existing reactor. This is typically all done within the loop. However, installing a one-time pre-switcher between the make-up gas compressor and the methanol loop has proven beneficial. This concept maintains the existing loop intact.

[0021] Accordingly, according to a preferred embodiment of the present invention, a one-time pre-converter is installed between the makeup gas compressor and the methanol loop, and said pre-converter operates within the operating window of the present invention, which is limited by a curve describing the relationship between the partial pressure of CO and the reactor temperature.

[0022] In relation to the prior art, the applicant’s WO 2015 / 193440 A1 describes a process for producing methanol in reactors connected in series, and one aspect of this provides a method for increasing the production capacity of an existing methanol plant by applying said process as part of an improvement.

[0023] The applicant’s WO 2014 / 012601 A1 describes a reaction system for the production of methanol, which comprises two reaction units, the first of which is operated on a mixture of fresh synthesis gas and unconverted synthesis gas, and the second unit is operated only on unconverted synthesis gas.

[0024] US 5.631.302 A describes the production of methanol from synthesis gas over a copper-containing catalyst at a temperature of 200–350°C under a pressure of 20 to 120 bar. Synthesis gas adiabatically passes through a first synthesis reactor containing a fixed bed of a copper-containing catalyst without synthesis gas recirculation. The unreacted gas mixture from the first synthesis reactor, along with the recirculated gas, passes through a second synthesis reactor containing a copper-containing catalyst placed in a tube and indirectly cooled by boiling water.

[0025] WO 2014 / 095978 A2 owned by the applicant contains high-grade (C 4+ The process of manufacturing alcohol is described, in which alcohol synthesis gas is selectively reacted first in a heterogeneous alcohol pre-converter to produce methanol, and then the effluent from the pre-converter (or synthesis gas when there is no pre-converter step) is reacted in a reactor for the synthesis of higher alcohols.

[0026] WO 2017 / 121981 A1 describes a methanol synthesis process comprising: (i) passing a first synthesis gas mixture containing makeup gas through a first synthesis reactor to form a first product gas stream; (ii) recovering methanol from the first product gas stream to form a first methanol-depletion gas mixture; (iii) combining the first methanol-depletion gas mixture with a loop recirculation stream to form a second synthesis gas mixture; (iv) passing the second synthesis gas mixture through a second synthesis gas reactor to form a second product gas stream; (v) recovering methanol from the second product gas stream to form a second methanol-depletion gas mixture; and (vi) using at least a portion of the second methanol-depletion gas mixture as a loop recirculation gas stream. In this process, the first synthesis reactor is a catalyst m than the second synthesis reactor 3 It is noted that the loop recirculation gas stream has a higher heat transfer and is not supplied to the first synthesis gas mixture, and the recirculation ratio of the loop recirculation gas stream to form the second synthesis gas mixture is in the range of 1.1:1 to 6:1. It is noted that the efficiency of multi-stage methanol synthesis can be improved by using different recirculation ratios in different types of reactors.

[0027] The best choice for a pre-converter that can be used according to the present invention is a boiling water reactor, due to the highly reactive synthesis gas. To limit the formation of byproducts, a lower boiling water temperature is typically required in the pre-converter than in a conventional reactor, which in turn requires a separate steam drum.

[0028] The pre-converter concept according to the present invention supplies the necessary extra catalyst to process excess makeup gas originating from an upstream unit. The pre-converter operates on fresh makeup gas. This is preferably a boiling water reactor (BWR) type and will require an additional cooling system and possibly also the individual cooling and separation of condensed methanol from the pre-converter. Fresh makeup gas is highly reactive toward byproduct formation. Therefore, a lower catalyst temperature is expected compared to a conventional reactor, thereby requiring an additional cooling system. Brief explanation of the drawing

[0029] The present invention is described in more detail with reference to the drawings. FIG. 1 shows an operating window that can be used in the process according to the present invention. FIG. 2 shows a specific example of the pre-transitioner concept according to the present invention. FIG. 3 shows a specific example of an alternative to the preliminary-transitioner concept according to the present invention. Specific details for implementing the invention

[0030] In FIG. 1, the operating window that can be used in the process according to the present invention is defined by the region under the approximately linear dashed curve (inactivation curve) of partial pressure of carbon monoxide versus boiling water temperature at water temperatures of 210 to 270°C, where the partial pressure of carbon monoxide is 20 kg / cm² at 210°C. 2 From 32.5 kg / cm² at 270℃ 2 It increases up to.

[0031] As previously mentioned, it is possible to obtain different methanol grades within the operating window depending on the combination of CO partial pressure and boiling water temperature. In Fig. 1, the solid curve (byproduct curve) indicates the upper limit for obtaining Grade AA methanol or similar high-quality methanol. Operating on this curve will shift the methanol product toward a methanol product containing more byproducts, but which is pure enough to be counted as fuel grade or methanol-olefin (MTO) grade methanol.

[0032] Furthermore, the dashed line deactivation curve indicates the limit for catalyst deactivation. Operation above this curve will result in rapid catalyst deactivation.

[0033] In FIG. 2, the compressed makeup synthesis gas (1) (compressor not shown) is heated in a feed / effluent heat exchanger (hex1) before entering a pre-converter (A). After passing through the pre-converter, the gas (2) is cooled in the feed / effluent heat exchanger (hex1) and sent to a condenser (c1), which is optionally sent to another condenser (c2) as a stream (9). As much methanol as possible is condensed in the condenser (c1) before the two-phase flow is separated in the first separation device (s1). Next, the gas (3) from the separation device (s1) is mixed with the gas (4) from the second separation device (s2) or optionally sent directly downstream of the recirculation device (R) as a stream (5).

[0034] After mixing, the gas is compressed in the recirculation device (R). The resulting raw material gas (6) to the reactor (B) is preheated in the raw material / effluent heat exchanger (hex2) before entering the reactor (B).

[0035] The effluent gas (7) is cooled in a raw material / effluent heat exchanger (hex2) and then cooled as much as possible in a condenser (c2), thereby condensing as much methanol as possible. Next, the 2-phase flow is separated in a second separation device (s2).

[0036] A small amount (8) of gas from the separator (s2) is sent to a purge to avoid the accumulation of inert components. The remaining gas flow from the separator (s2) is mixed with the gas from the separator (s1). Finally, the liquids from the two separators (s1 and s2) are mixed, and the mixture is sent to a low-pressure separator and then discharged from the methanol zone.

[0037] FIG. 3 illustrates a different embodiment from that shown in FIG. 2. Here, a makeup synthesis gas (1') is compressed (compressor not shown), the compressed gas is mixed with a portion of the recirculated gas from a recirculation unit (R'), and is heated in a feed / effluent heat exchanger (hex1') before entering a pre-converter (A'). After passing through the pre-converter, the gas (2') is cooled in the feed / effluent heat exchanger (hex1'). Next, the cooled gas from the pre-converter (A') is mixed with the cooled gas from the reactor (B'). After mixing, the two-phase flow is further cooled in a condenser (c1'), thereby condensing as much methanol as possible.

[0038] When the gas is cooled as much as possible, the two-phase flow is separated in the separator (s1'). A portion of the effluent gas from the separator is sent to a purge to avoid the accumulation of inert components. The remaining gas is sent to a recirculation unit (R') and used as feed gas to the reactor (B'). The feed gas to the reactor (B') is heated in a feed / effluent heat exchanger (hex2') before entering the reactor. After the reactor (B'), the gas is cooled in the feed / effluent heat exchanger (hex2') and mixed with the cooled gas from the pre-converter (A').

[0039] Optionally, the cooled gas from the pre-converter (A') is fed to another condenser (c2') and as much methanol as possible is condensed. After the gas is cooled, the two-phase flow is separated in another separator (s2'), from which the gaseous phase is sent to the inlet of the recirculation device (R') and the liquid phase is mixed with the liquid phase from the separator (s1').

[0040] Fresh makeup gas is highly reactive toward the formation of byproducts. Therefore, a lower catalyst temperature can be expected compared to conventional reactors; this necessitates an additional cooling system.

Claims

Claim 1 A plant for producing methanol from synthesis gas by a method for producing methanol through an equilibrium reaction proceeding at an elevated temperature under elevated pressure according to the following synthesis reactions in the presence of a catalyst, wherein CO + 2H2<-> CH3OH + heat (1) CO2 + 3H2<-> CH3OH + H2O + heat (2) CO + H2O<-> CO2 + H2 + heat (3) The above method is carried out in a methanol pre-converter within an operating window, said operating window is defined by the region under the linear curve of partial pressure of carbon monoxide versus boiling water temperature at a water temperature of -210°C to 270°C, where the partial pressure of carbon monoxide is 20 kg / cm² at 210°C. 2 From 32.5 kg / cm² at 270℃ 2 A method wherein the region under the linear curve of partial pressure of carbon monoxide versus boiling water temperature is divided into two by the byproduct curve of partial pressure of carbon monoxide versus boiling water temperature, said regions resulting in the production of Grade AA, fuel grade, or methanol-olefin (MTO) grade methanol, said byproduct being a compound other than methanol, said plant comprising a makeup gas compressor and a synthesis reactor of a methanol loop, said one-time pre-converter installed between the makeup gas compressor and the methanol loop, said pre-converter operates within an operating window defined by the region under the linear dashed curve of partial pressure of carbon monoxide versus boiling water temperature at a water temperature of 210°C to 270°C, said partial pressure of carbon monoxide is 20 kg / cm² at 210°C. 2 From 32.5 kg / cm² at 270℃ 2 A plant that increases up to. Claim 2 A plant according to claim 1, characterized in that the method is performed in the region below the linear curve of partial pressure of carbon monoxide versus boiling water temperature and below the byproduct curve within the operating window, which indicates an upper limit for obtaining Grade AA methanol. Claim 3 delete Claim 4 delete Claim 5 delete

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