Methanol synthesis plant with saturator

Abstract

A plant and a process for the synthesis of methanol from a hydrocarbon feedstock are provided. The plant comprises, inter alia, a saturation section. The plant is arranged to mix at least a portion of the recycle liquid stream from the saturation section with at least a portion of the excess water stream from the methanol distillation section, and to provide a mixed recycle stream. The plant is further arranged to heat the mixed recycle stream by means of heat exchange with a steam stream generated elsewhere in the plant.

Description

TECHNICAL FIELD A plant and a process for the synthesis of methanol from a hydrocarbon feedstock are provided. The plant comprises, inter alia, a saturation section. The plant is arranged to mix at least a portion of the recycle liquid stream from the saturation section with at least a portion of the excess water stream from the methanol distillation section, and to provide a mixed recycle stream. The plant is further arranged to heat the mixed recycle stream by means of heat exchange with a steam stream generated elsewhere in the plant. BACKGROUND A typical process for the synthesis of methanol from a hydrocarbon feedstock involves steam reforming a gaseous hydrocarbon feedstock in a front-end section, obtaining a synthesis gas; exothermically reacting the resulting synthesis gas in the presence of a catalyst in a methanol synthesis section, and using heat from the synthesis section to produce steam; using said steam as heat input to the steam reforming process. An important component of such a process / plant is a saturation section (typically a saturation tower) in which the hydrocarbon feedstock is mixed with water. The saturation tower assures a portion of the steam necessary for a safe and efficient steam reforming. A standard solution is to design the methanol synthesis section to provide steam at a pressure corresponding to process steam. This, however, fixes the minimum allowable boiling water pressure in the methanol synthesis reactor, which limits the flexibility of the synthesis operation and design. EP3583068A1 describes a process comprising exothermal catalytic reaction of a synthesis gas and a related plant. It is an object of embodiments of the invention to provide less by-product in the MeOH product, lower emissions, and improved reliability in operation. Additional aims of the present technology include separating the operation of the frontend saturator from the MeOH synthesis loop, maintaining partial use of steam generated in the methanol synthesis as indirect process steam for the frontend reforming step and handling excess water from a methanol distillation section. SUMMARY It has been found that such goals can be achieved with a methanol plant and process described herein. So, a plant for the synthesis of methanol from a hydrocarbon feedstock is provided, said plant comprising: - a hydrocarbon feed, - a steam feed, - a saturation section, said saturation section being arranged to receive at a first portion of said hydrocarbon feed, a mixed recycle stream, and a process condensate stream, and to output a first hydrocarbon stream and a recycle liquid stream, - said plant being arranged to mix said first hydrocarbon stream with a second portion of said hydrocarbon feed and said steam feed to provide a mixed hydrocarbon stream, - a reforming section arranged to convert the mixed hydrocarbon stream to a first synthesis gas stream, - a cooling section arranged to remove water from the first synthesis gas stream and to generate a second, water-depleted synthesis gas stream and a process condensate stream and wherein at least part of said process condensate stream is arranged to be fed to the saturation section, - a methanol synthesis section arranged to react at least a portion of said second synthesis gas stream to a raw methanol stream, - a methanol distillation section arranged to receive said raw methanol stream and to output a purified methanol stream and an excess water stream, - said plant being arranged to mix at least a portion of the recycle liquid stream from the saturation section with at least a portion of the excess water stream from the methanol distillation section, and to provide a mixed recycle stream, - wherein said plant is further arranged to heat the mixed recycle stream by means of heat exchange with a steam stream generated elsewhere in the plant. A process is also provided for the synthesis of methanol from a hydrocarbon feedstock, in a plant as defined herein. The process comprises the steps of: - providing a hydrocarbon feed, - providing a steam feed, - feeding a first portion of said hydrocarbon feed, a mixed recycle stream and a process condensate stream to the saturation section, and outputting a first hydrocarbon stream and a recycle liquid stream, - mixing the first hydrocarbon stream with a second portion of said hydrocarbon feed and said steam feed to provide a mixed hydrocarbon stream, - converting the mixed hydrocarbon stream to a synthesis gas stream in said reforming section, - removing water from the first synthesis gas stream in the cooling section to generate a second, water-depleted synthesis gas stream and a process condensate stream and feeding at least part of said process condensate stream to the saturation section, - reacting at least a portion of said second synthesis gas stream to a raw methanol stream in said methanol synthesis section, - feeding the raw methanol stream to the methanol distillation section and outputting a purified methanol stream and an excess water stream, - mixing at least a portion of the recycle liquid stream from the saturation section with at least a portion of the excess water stream from the methanol distillation section, to provide a mixed recycle stream, - heating the mixed recycle stream by means of heat exchange with a steam stream generated elsewhere in the plant. Further details are provided in the following description text, the claims and the appended figures. LEGENDS Fig. 1 illustrates a general layout of a plant / process according to the invention. Fig. 2 illustrates a further layout of a plant / process according to the invention. Fig. 3 illustrates a further layout of a plant / process according to the invention. DETAILED DISCLOSURE Unless otherwise specified, any given percentages for gas content are % by volume. All feeds are preheated as required. A plant is provided for the synthesis of methanol (in the form of a purified methanol stream) from a hydrocarbon feedstock. The purified methanol stream from the plant / process typically has a high purity (e.g. over 98 wt %, such as over 99 wt % or over 99.9 wt % methanol). In general terms, the plant comprises: - a hydrocarbon feed, - a steam feed, -   a saturation section, -   a reforming section, - a cooling section, -   a methanol synthesis section, and -   a methanol distillation section The plant comprises a hydrocarbon feed. The hydrocarbon feed typically comprises a hydrocarbon gas, such as CH4, and optionally also higher hydrocarbons, often in relatively small amounts, in addition to small amounts of other gasses. Higher hydrocarbons are components with two or more carbon atoms such as ethane and propane. Examples of "hydrocarbon feed" may be natural gas, town gas, naphtha or a mixture of methane and higher hydrocarbons, biogas or LPG. The hydrocarbon feed is typically a pre-reformed feed; i.e. one in which a higher hydrocarbons in the feed have been reacted with steam to provide a methane-rich feed suitable for further downstream reforming. Alternatively, the hydrocarbon feed in the present technology may be a not prereformed natural gas feed. The plant comprises a steam feed. The steam feed is a high-purity gaseous water feed, e.g. having a purity of above 95%, preferably above 99%. The plant comprises a saturation section. The saturation section is arranged to receive at a first portion of the hydrocarbon feed, a mixed recycle stream, and a process condensate stream, and to output a first hydrocarbon stream and a recycle liquid stream. The term "saturating" denotes that, after passage through the saturation section equipment, the hydrocarbon feed becomes enriched with steam (as the first hydrocarbon stream). In one aspect, the saturation section comprises a two-stage saturation tower, a first stage in which hydrocarbon feed is arranged to be saturated with the heated mixed recycle stream and a second stage in which the hydrocarbon gas from the first stage is saturated with at least a portion of the process condensate, to thereby output a hydrocarbon gas stream from the saturation tower after the second stage. In one aspect, a portion of the recycle liquid stream is arranged to be purged from the plant. This can reduce the build-up of undesired components in this stream. The plant is arranged to mix the first hydrocarbon stream with a second portion of the hydrocarbon feed and the steam feed to provide a mixed hydrocarbon stream. The plant comprises a reforming section arranged to convert the mixed hydrocarbon stream to a first synthesis gas stream. The reforming section may comprise one or more reformers. The reforming section may - for instance - comprise a pre-reformer, a primary steam reformer and a secondary reformer. The primary and / or the secondary reformers may for example be an auto-thermal reformer (ATR). The term "synthesis gas stream" is meant to denote a gas comprising hydrogen, carbon monoxide and carbon dioxide and small amounts of water and other gasses, such as argon, nitrogen, methane, etc. The first synthesis gas stream suitably has the following composition (by volume): -  0.5-5% methane (dry) -  40-80% H2 (dry) -  0-40% CO (dry) -  0-25% CO2 (dry) The plant comprises a cooling section arranged to remove water from the first synthesis gas stream and to generate a second, water-depleted synthesis gas stream and a process condensate stream and wherein at least part of said process condensate stream is arranged to be fed to the saturation section. The second synthesis gas stream thus has a lower water content than the first synthesis gas stream. For instance, the second synthesis gas stream may have a water content of less than 5%, preferably less than 2%, or less than 1% by volume. In one aspect the cooling section comprise a waste heat boiler wherein the cooling of the synthesis gas stream generates a steam stream. Methanol synthesis section The plant comprises a methanol synthesis section arranged to react at least a portion of said second synthesis gas stream to a raw methanol stream. The methanol synthesis reactor in the methanol synthesis section accommodates the following two reactions: CO2 + H2 < = > CO + H2O CO + 2H2 < = > CH3OH The process can occur, for example by sending the syngas stream through a boiling water reactor, where at least a portion of the syngas stream is converted to methanol followed by condensation and separation of the methanol as a liquid phase. Off-gas stream is produced in this process. The off-gas stream from the methanol synthesis section typically comprises: 8590% H2, 5-10% CO2, 0-3% CO. The raw methanol stream comprises a major portion of methanol, e.g., 75-98 wt% methanol and 2-25 wt% H2O by weight. Other minor components of this stream include but not limited to, higher alcohols, ketones, aldehydes, dimethyl ether (DME), organic acids and dissolved gases. To obtain an optimized yield in the methanol production, the stoichiometry of H2, CO and CO2 needs to be considered. In a preferred embodiment, the stoichiometry of H2, CO and CO2 in the syngas stream falls within an interval such that it has a module between 1.8 and 2.3, preferably between 1.9 and 2.1, where the module is defined in terms of molar content: M = (H2-CO2) / (CO+CO2). The module of the syngas stream may be adjusted by addition of a (further) hydrogen-rich stream, which is optionally arranged to be admixed with the syngas stream. The hydrogenrich stream can be provided by an external feed of hydrogen. Methanol distillation section A methanol distillation section is arranged to receive the raw methanol stream and to output a purified methanol stream and an excess water stream. The distillation section is arranged to upgrade the raw methanol stream to a purified methanol stream of the required grade, e.g. >95 wt%, >98 wt% or >99 wt% methanol. The methanol distillation section may comprise one or more, e.g. two or more distillation columns. The purified methanol stream outputted from the distillation section consists almost entirely of methanol, e.g. more than 95 wt% methanol, such as more than 98 wt% methanol, e.g. more than 99 wt% methanol. The excess water stream outputted from the distillation section originates from water formed when methanol is produced from the reaction CO2 + 3H2 = CH3OH + H2O. The water exits the methanol synthesis section as a component of the raw methanol stream. In the distillation section the water and other by-products are removed from the raw methanol to produce for example grade AA or similar quality methanol. The separated water stream is called excess water and the separated higher carbon components are named higher alcohols (HA). Therefore, in one aspect, at least a part of the excess water stream comprises a methanol distillation impurity stream, such as a higher alcohol stream. There is also a light component gas stream, which is typically used as fuel. The plant is arranged to mix at least a portion of the recycle liquid stream from the saturation section with at least a portion of the excess water stream from the methanol distillation section, and to provide a mixed recycle stream. The plant is further arranged to heat the mixed recycle stream by means of heat exchange with a steam stream generated elsewhere in the plant. In one aspect, the plant comprises a methanol synthesis section arranged to react at least a portion of said second synthesis gas stream to a raw methanol stream and to output a first steam stream. The plant is further arranged to heat the mixed recycle stream by means of heat exchange with at least a portion of the first steam stream from said methanol synthesis section. In one aspect, illustrated in Figure 2, the cooling section of the plant further comprises a waste heat boiler, said waste heat boiler being arranged to output a second steam stream, wherein said plant is further arranged to heat the mixed recycle stream by means of heat exchange with at least a portion of the second steam stream. In another aspect, illustrated in Figure 3, the plant further comprises an auxiliary boiler, said auxiliary boiler being arranged to output a third steam stream, wherein said plant is further arranged to heat the mixed recycle stream by means of heat exchange with at least a portion of the third steam stream. A process is provided for the synthesis of methanol from a hydrocarbon feedstock, in a plant disclosed herein. The process comprises the steps of: -   providing a hydrocarbon feed, -   providing a steam feed, - feeding a first portion of said hydrocarbon feed, a mixed recycle stream and a process condensate stream to the saturation section, and outputting a first hydrocarbon stream and a recycle liquid stream, - mixing the first hydrocarbon stream with a second portion of said hydrocarbon feed and said steam feed to provide a mixed hydrocarbon stream, - converting the mixed hydrocarbon stream to a synthesis gas stream in said reforming section, - removing water from the first synthesis gas stream in the cooling section to generate a second, water-depleted synthesis gas stream and a process condensate stream and feeding at least part of said process condensate stream to the saturation section, - reacting at least a portion of said second synthesis gas stream to a raw methanol stream in said methanol synthesis section, - feeding the raw methanol stream to the methanol distillation section and outputting a purified methanol stream and an excess water stream, - mixing at least a portion of the recycle liquid stream from the saturation section with at least a portion of the excess water stream from the methanol distillation section, to provide a mixed recycle stream, - heating the mixed recycle stream by means of heat exchange with a steam stream generated elsewhere in the plant. In one aspect of the process, the saturation section comprises a two-stage saturation tower, and the process further comprises saturating the hydrocarbon feed with the heated mixed recycle stream in a first stage, followed by saturating the hydrocarbon gas from the first stage in a second stage with at least a portion of the process condensate, to thereby output a hydrocarbon gas stream from the saturation tower after the second stage. In one aspect of the process, the content of caustic in the first hydrocarbon stream leaving the saturation stage is less than 0.1 ppb wt, preferably less than 0.005 ppb wt. In another aspect of the process, the process condensate flow is less than 50 % of the excess water stream. In one aspect, in which the saturation section comprises a two-stage saturation tower, there is a positive liquid flow outlet both saturation stage one and saturation stage two. In one aspect, at least 40 mol % of the required steam for the reforming step is added as direct steam via said steam feed stream. In one aspect of the process, in which the plant comprises a methanol synthesis section, the process comprises the steps of - reacting at least a portion of said second synthesis gas stream to a raw methanol stream in said methanol synthesis section and outputting a first steam stream, and - heating the mixed recycle stream by means of heat exchange with at least a portion of the first steam stream from said methanol synthesis section. Key advantages of the present plant / process include: - Ensuring that methanol in the excess water during process fluctuations is sent back into the process and thereby reducing the cost of downstream water treatment. - Caustic is added to the raw methanol to release light byproduct components. The caustic must be sent to water treatment. The saturator solution concentrates the caustic in a volumetric smaller purge stream and thereby reduces the cost of the water treatment section. - Caustic is a poison for the reforming section. The second saturator step using caustic free process condensate ensures that no caustic is passing into the reforming section - Recycling the higher alcohol byproduct stream HA as feed instead of using it as fuel or waste reduces energy consumption and remove possible caustic contamination of the fuel system. - Especially for low steam carbon reforming technologies it is important to have reliable steam control. By controlling the saturation capacity, ensuring sufficient direct steam secures reliable and safe steam control of the reforming process. EXAMPLE Reference is made to figure 1. Part of a hydrocarbon feed 1 is cooled in the feed / effluent exchanger El and sent to a bottom gas inlet of the two-stage saturator 50. Process condensate 121 obtained by cooling synthesis gas 5 is sent to the top of the two-stage saturator as liquid inlet for the second saturation stage. Methanol distillation excess water 142, optionally including the methanol distillation impurity stream, higher alcohols, is mixed with a recycle liquid stream 13 from the bottom of the saturator 50, after an optional purge stream 14 is separated from the recycle liquid stream. The mixed stream 151 is preheated in heat exchanger 60 before it is sent to the two-stage saturator 50 as liquid inlet for the first saturation stage. The hydrocarbon containing gas 3 leaving the top of the two-stage saturator 50 is preheated in heat exchanger 70 and mixed with the remaining hydrocarbon feed IB, before a steam stream 2 is added to obtain a mixed hydrocarbon stream 4. Mixed 5 hydrocarbon stream 4 is fed into a reforming section 110 with a steam / carbon (mol / mol) of 0.6 This ratio can range from 0.4 to 3.0 depending on reforming step design. Table 1: Stream 1 1A 3 142 121 14 132 2 Temp C 365 210 313 160 218 138 229 305 Pressure bar 42 38 36,8 43 41 38 26.5 37 Flow kg / h 17800 6000 1500 18800 47100 Flow Nm3 / h 169000 110000 255400 Na Ppm wt 0 0 <0.005 ppb wt 500 0 5900 0 0 10 The present invention has been described with reference to a number of embodiments and figures. However, the skilled person is able to select and combine various embodiments within the scope of the invention, which is defined by the appended claims. All documents referenced herein are incorporated by reference. 15

Claims

1. A plant (100) for the synthesis of methanol from a hydrocarbon feedstock, said plant comprising:- a hydrocarbon feed (1),- a steam feed (2)- a saturation section (50), said saturation section (50) being arranged to receive a first portion (1A) of said hydrocarbon feed (1), a mixed recycle stream (151), and a process condensate stream (121), and to output a first hydrocarbon stream (3) and a recycle liquid stream (13),- said plant being arranged to mix said first hydrocarbon stream (3) with a second portion (IB) of said hydrocarbon feed (1) and said steam feed (2) to provide a mixed hydrocarbon stream (4),- a reforming section (110) arranged to convert the mixed hydrocarbon stream (4) to a first synthesis gas stream (5),- a cooling section (120) arranged to remove water from the first synthesis gas stream (5) and to generate a second, water-depleted synthesis gas stream (5') and a process condensate stream (121) and wherein at least part of said process condensate stream (121) is arranged to be fed to the saturation section (50),- a methanol synthesis section (130) arranged to react at least a portion of said second synthesis gas stream (5') to a raw methanol stream (131),- a methanol distillation section (140) arranged to receive said raw methanol stream (131) and to output a purified methanol stream (141) and an excess water stream (142),- said plant (100) being arranged to mix at least a portion of the recycle liquid stream (13) from the saturation section (50) with at least a portion of the excess water stream (142) from the methanol distillation section (140), and to provide a mixed recycle stream (151),- wherein said plant (100) is further arranged to heat the mixed recycle stream (151) by means of heat exchange with a steam stream generated elsewhere in the plant.

2. The plant (100) according to claim 1, comprising a methanol synthesis section (130) arranged to react at least a portion of said second synthesis gas stream (5') to a raw methanol stream (131) and to output a first steam stream (132), wherein said plant (100) is further arranged to heat the mixed recycle stream (151) by means of heat exchange with at least a portion of the first steam stream (132) from said methanol synthesis section (130).

3. The plant (100) according to any one of the preceding claims, wherein the cooling section (120) further comprises a waste heat boiler, said waste heat boiler being arranged to output a second steam stream (133), wherein said plant (100) is further arranged to heat the mixed recycle stream (151) by means of heat exchange with at least a portion of the second steam stream (133).

4. The plant (100) according to any one of the preceding claims, further comprising an auxiliary boiler (150), said auxiliary boiler (150) being arranged to output a third steam stream (134), wherein said plant (100) is further arranged to heat the mixed recycle stream (151) by means of heat exchange with at least a portion of the third steam stream (134).

5. The plant (100) according to any one of the preceding claims, wherein the saturation section (50) comprises a two-stage saturation tower, a first stage in which hydrocarbon feed (1A) is arranged to be saturated with the heated mixed recycle stream (151) and a second stage in which the hydrocarbon gas from the first stage is saturated with at least a portion of the process condensate (121), to thereby output a hydrocarbon gas stream (3) from the saturation tower after the second stage.

6. The plant (100) according to any one of the preceding claims, wherein a portion of the recycle liquid stream (13) is arranged to be purged from the plant.

7. The plant (100) according to any one of the preceding claims, wherein at least a part of the excess water stream (142) comprises a methanol distillation impurity stream, such as a higher alcohol stream.

8. A process for the synthesis of methanol from a hydrocarbon feedstock, in a plant (100) according to any one of the preceding claims, said process comprising the steps of:- providing a hydrocarbon feed (1),- providing a steam feed (2),- feeding a first portion (1A) of said hydrocarbon feed (1), a mixed recycle stream (151) and a process condensate stream (121) to the saturation section (50), and outputting a first hydrocarbon stream (3) and a recycle liquid stream (13),- mixing the first hydrocarbon stream (3) with a second portion (IB) of said hydrocarbon feed (1) and said steam feed (2) to provide a mixed hydrocarbon stream (4),- converting the mixed hydrocarbon stream (4) to a synthesis gas stream (5) in said reforming section (110),- removing water from the first synthesis gas stream (5) in the cooling section (120) to generate a second, water-depleted synthesis gas stream (5') and a process condensate stream (121) and feeding at least part of said process condensate stream (121) to the saturation section (50),- reacting at least a portion of said second synthesis gas stream (5') to a raw methanol stream (131) in said methanol synthesis section (130),- feeding the raw methanol stream (131) to the methanol distillation section (140) and outputting a purified methanol stream (141) and an excess water stream (142),- mixing at least a portion of the recycle liquid stream (13) from the saturation section (50) with at least a portion of the excess water stream (142) from the methanol distillation section (140), to provide a mixed recycle stream (151),- heating the mixed recycle stream (151) by means of heat exchange with a steam stream generated elsewhere in the plant.

9. The process according to claim 8, wherein the saturation section (50) comprises a two-stage saturation tower, said process further comprising saturating the hydrocarbon feed (1A) with the heated mixed recycle stream (151) in a first stage, followed by saturating the hydrocarbon gas from the first stage in a second stage with at least a portion of the process condensate (121), to thereby output a hydrocarbon gas stream (3) from the saturation tower after the second stage.

10. The process according to claim 9, wherein the content of caustic in the first hydrocarbon stream (3) leaving the saturation stage is less than 0.1 ppb wt, preferably less than 0.005 ppb wt.

11. The process according to any one of claims 8-10, wherein the process condensate flow (121) is less than 50 % of the excess water stream (142)12. The process according to any one of claims 8-11, wherein the saturation section (50) comprises a two-stage saturation tower, and wherein there is a positive liquid flow outlet both saturation stage one and saturation stage two.

13. The process according to any one of the preceding claims wherein at least 40 mol % of the required steam for the reforming step is added as direct steam via steam feed (2).

14. The process according to any one of claims 8-13, wherein said plant (100) comprises a methanol synthesis section (130), wherein said process comprises the steps of14- reacting at least a portion of said second synthesis gas stream (5') to a raw methanol stream (131) in said methanol synthesis section (130) and outputting a first steam stream (132), and- heating the mixed recycle stream (151) by means of heat exchange with at least a5           portion of the first steam stream (132) from said methanol synthesis section (130).

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