WO2003097523A2

WO2003097523A2 - Integrated process for making acetic acid and methanol

Info

Publication number: WO2003097523A2
Application number: PCT/CY2003/000002
Authority: WO
Inventors: Daniel Marcel Thiebaut
Original assignee: Acetex (Cyprus) Limited
Priority date: 2002-05-20
Filing date: 2003-05-20
Publication date: 2003-11-27
Also published as: EA200401498A1; EA200600482A1; WO2003097523A3; EA007455B1; AU2003232578A1; NO20045381L; EA008283B1; AU2003232578B2; CN1656012A; CN1310826C; OA12861A

Abstract

The present invention provides a plant and a process that produces both methanol and acetic acid under substantially stoichiometric conditions, wherein an unadjusted syngas having an R ratio less than 2.0 is provided. All or part of the unadjusted syngas is supplied to a separator unit to recover CO2, CO and hydrogen. At least a portion of any one or combination of the recovered CO2 , CO and hydrogen is added to any remaining syngas not so treated or alternatively combines in the absence of any remaining unadjusted syngas to yield an adjusted syngas with an R ratio of 2.0 to 2.9 which is used to produce methanol. Any recovered CO2 not used to adjust the R ratio of the unadjusted syngas can be supplied to the reformer to enhance CO production. At least a portion of the recovered CO is reacted in the acetic acid reactor with at least a portion of the produced methanol to produce acetic acid or an acetic acid precursor by a conventional process.

Description

INTEGRATED PROCESS FOR MAKING ACETIC ACID AND METHANOL FIELD OF THE INVENTION

The present invention is directed generally to a process for making acetic acid and methanol from synthesis gas. BACKGROUND OF THE INVENTION

The manufacture of acetic acid from carbon monoxide and methanol using a carbonylation catalyst is well known in the art. Representative references disclosing this and similar processes include U.S. Patent Nos. 1 ,961 ,736 to Carlin et al (Tennessee Products); 3,769,329 to Paulik et al (Monsanto); 5,155,261 to Marston et al (Reilly Industries); 5,672,743 to Garland et al (BP Chemicals); 5,728,871 to Joensen et al (Haldor Topsoe); 5,773,642 to Denis et al (Acetex Chimie); 5,817,869 to Hinnenkamp et al (Quantum Chemical Corporation); 5,877,347 and 5,877,348 to Ditzel et al (BP Chemicals); 5,883,289 to Denis et al (Acetex Chimie); and 5,883,295 to Sunley et al (BP Chemicals), each of which is hereby incorporated herein by reference.

The primary raw materials for acetic acid manufacture are, of course, carbon monoxide and methanol. In the typical acetic acid plant, methanol is imported and carbon monoxide, because of difficulties associated with the transport and storage thereof, is generated in situ, usually by reforming natural gas or another hydrocarbon with steam and/or carbon dioxide. For this reason, attention has recently focused on the construction of integrated plants producing both methanol and acetic acid. A significant expense for new acetic acid production capacity is the capital cost of the equipment necessary for the carbon monoxide generation. It would be extremely desirable if this capital cost could be largely eliminated or significantly reduced.

Market conditions, from time to time in various localities, can result in relatively low methanol prices (an oversupply) and/or high natural gas prices (a shortage) that can make methanol manufacture unprofitable. Operators of existing methanol manufacturing facilities can be faced with the decision of whether or not to continue the unprofitable manufacture of methanol in the hope that product prices will eventually rebound and/or raw material prices will drop to profitable levels. The present invention also addresses a method of modifying an existing unprofitable methanol plant to make it more profitable when methanol prices are low and/or gas prices are high.

The following references disclose the manufacture of synthesis gas: Mayland (U.S. 2,622,089); Moe (U.S. 3,859,230); Steinberg et al (U.S. 5,767,165); Park et al. (U.S. 5,855,815); Lee, et al. (US 5,180,570); McShea, III, et al. (US 4,927,857); and Banquy (US 4,888,130 and 4,999,133). It seems well established that for a large capacity plant, pure autothermal reforming could be the more economic process leading to synthesis gas, since large capital costs are saved by not constructing large primary reformers. Nevertheless, the drawback is not being able to have a full usage of all carbon molecules, resulting in the venting of large quantities of CO₂, which is undesirable. As far as applicant is aware, there is no disclosure in the prior art for adjusting the R ratio, where R = [(H2 - CO2)/(CO + CO2)], of a syngas from an ATR for methanol production and to also supply stoichiometric MeOH and CO for manufacturing acetic acid, for example, with the flexibility to control both methanol production and acetic acid production, particularly when the R ratio is less than 2.0. SUMMARY OF THE INVENTION

The present invention is directed in one embodiment to adjusting the R ratio of the synthesis gas used to make methanol by separating at least a portion of the unadjusted syngas into streams substantially rich in carbon monoxide, hydrogen and carbon dioxide, then (1 ) adding as necessary carbon dioxide, carbon monoxide and/or hydrogen from these streams to the remaining unadjusted syngas or (2) combining such streams in the absence of any remaining syngas to produce an adjusted syngas having an R ratio [ R = (H2 - CO2)/(CO + CO2) ] from 2.0 to 2.9 and using the adjusted syngas to produce methanol, of which a portion is then reacted with at least a portion of the carbon monoxide stream in approximately stoichiometric proportions to directly or indirectly manufacture acetic acid.

Embodiments of the present invention provide a plant and a process that produces both methanol and acetic acid under substantially stoichiometric conditions, wherein an unadjusted syngas having an R ratio less than 2.0. All or part of the unadjusted syngas is supplied to a separator unit to recover CO₂, CO and hydrogen. At least a portion of any one or combination of the recovered CO₂, CO and hydrogen is added to any remaining syngas not so treated or alternatively combined in the absence of any remaining unadjusted syngas to yield an adjusted syngas with an R ratio of 2.0 to 2.9 which is used to produce methanol. Preferably, the adjusted syngas has an R ratio of between 2.00 and 2.05. Any recovered CO₂ not used to adjust the R ratio of the unadjusted syngas can be supplied to the reformer to enhance CO production. At least a portion of the recovered CO is reacted in the acetic acid reactor with at least a portion of the produced methanol to produce acetic acid or an acetic acid precursor by a conventional process. As noted above, the recovered hydrogen can be supplied to the MeOH synthesis unit for methanol production. To the extent the hydrogen produced is in excess of the requirements for methanol synthesis in the present invention, it can also be used for the manufacture of ammonia or other products, burned as a fuel, or exported. Any excess methanol beyond that needed for acetic acid production can be used, for example, as an intermediate to produce other products, such as methylamines, or sold as a product.

Any excess carbon dioxide can be fed into a reformer to which natural gas and steam (water) are fed to produce adjusted syngas. Syngas is formed in the reformer wherein both the natural gas and the carbon dioxide are reformed to produce syngas with a large proportion of carbon monoxide relative to reforming without added carbon dioxide.

Separated hydrogen, to the extent it is produced in excess beyond that required for methanol synthesis in the present process, can also be reacted with nitrogen, in a conventional manner, to produce ammonia. In addition, a portion of acetic acid that is produced can be reacted in a conventional manner with oxygen and ethylene to form vinyl acetate monomer. Oxygen can be supplied to an autothermal reformer to provide syngas. The nitrogen for the ammonia process and the oxygen for the vinyl acetate monomer process and/or autothermal reformer can be obtained from a conventional air separation unit.

Broadly, the present invention provides, in one aspect, a method for adjusting the R ratio of a syngas for use in methanol production alone or in combination with acetic acid production. The method adjusts the R ratio of an unadjusted syngas by separating at least a portion of the unadjusted syngas into streams rich in CO₂, CO and hydrogen and then adding to the remaining unadjusted syngas a sufficient quantity of one or more of the CO₂, CO and hydrogen streams, or alternatively combining a sufficient quantity of these streams when there is no remaining unadjusted syngas, to produce an adjusted syngas having an R ratio from 2.0 to 2.9.

In another aspect, at least a portion of the CO stream is reacted with at least a portion of the produced methanol in a stoichiometric manner to produce acetic acid or an acetic acid precursor by a conventional process. Based on the relative economics of acetic acid and methanol, the production quantities of each can be controlled. For example, all the methanol may be used for acetic acid production, or less acetic acid can be produced by using more of the recovered CO to make methanol rather than acetic acid resulting in methanol production in excess of that required for acetic acid production. Alternatively, the recovered CO can used for methanol production or exported for use nearby, resulting in no acetic acid being produced.

In one embodiment, this invention is directed to large capacity methanol plants, in which a synthesis gas is produced from an autothermal catalytic reforming of natural gas with oxygen, and in which an adequate stream of carbon monoxide is diverted to make acetic acid; the remaining synthesis gas being mixed with recycled gases and sent to a methanol synthesis loop in approximately stoichiometric proportions.

The proposed process eliminates these drawbacks mentioned in the background above by combining an acetic acid plant with a large capacity methanol plant. The syngas is produced by an autothermal reformer, after preheating all the feeds, then a portion of this syngas is sent to a separation unit, consisting of a CO₂ removal and a cryogenic separation unit in order to obtain a carbon monoxide stream, which is used in an acetic acid carbonylation plant. All the other streams from the separation unit are all recycled with the remaining portion of the unadjusted syngas, producing an adjusted syngas that is then sent to the methanol synthesis loop. The carbon monoxide stream is adequately diverted from the syngas so that the adjusted syngas is close to stoichiometric proportions for methanol manufacture. This process eliminates the large capital costs associated with the erection of primary reformers. And another advantage is that the methanol plant in itself, is a "green" plant since the carbon emissions are reduced to nearly zero.

Another aspect of the present invention is retrofitting an original methanol plant which has at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen, carbon monoxide and optionally carbon dioxide, and a methanol synthesis unit for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The method converts the methanol plant into a retrofitted plant for manufacturing methanol and a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof, wherein the R ratio of the unadjusted syngas produced by the retrofitted plant is less than 2.0.

Advantages of the retrofit compared to a completely new CO/MeOH plant are the use of existing units and equipment, such as desulfurization, reforming including waste heat recovery, synthesis gas compressor and circulator, etc. An additional advantage is provided by the use of the existing offsite and infrastructure such as steam generation, water treatment, cooling water system, control room and product loading facilities.

The reaction step can include the direct catalytic reaction of methanol and carbon monoxide to form acetic acid as in the Monsanto-BP process, for example, or alternatively can comprise the intermediate formation of methyl formate and isomerization of the methyl formate to acetic acid, the intermediate reaction of CO and two moles of methyl alcohol to form methyl acetate and hydrolysis of the methyl acetate to acetic acid and methanol, or the carbonylation of the methyl acetate to form acetic anhydride.

In one preferred embodiment of the retrofitting method, the present invention provides a method for retrofitting an original methanol plant that has at least one steam reformer for converting a hydrocarbon/steam feed to a syngas stream containing hydrogen and carbon monoxide, a heat recovery section for cooling the syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The retrofitted plant can manufacture a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The retrofitting method comprises the steps of: (a) modifying the reformer to produce an unadjusted syngas having an R ratio less than 2.0, (b) diverting at least a portion of the unadjusted syngas from at least one steam reformer to a separation unit; (c) installing the separation unit to separate the diverted syngas into a carbon dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; (d) provide for controllably recycling one or more of the carbon dioxide-rich stream, the carbon monoxide-rich stream and the hydrogen-rich stream from the separation unit to the methanol synthesis unit such that any remaining unadjusted syngas combined therewith yields an adjusted syngas fed thereto having an R ratio ranging from 2.0 to 2.9; and (e) installing an acetic acid reactor for reacting at least a portion of the carbon monoxide-rich stream from the separation unit with at least a portion of the methanol from the methanol synthesis unit to form the product. The separation unit can include a solvent absorber and stripper for carbon dioxide recovery, and a cryogenic distillation unit for carbon monoxide and hydrogen recovery.

The method can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen to make ammonia. In a retrofit embodiment, where the original methanol plant produces a hydrogen-rich stream comprising a loop purge from the methanol synthesis unit that was reacted with nitrogen to make ammonia, the retrofitted plant can use any excess amount of the hydrogen-rich stream from the separation unit as a primary hydrogen source for the ammonia production. In some cases, additional ammonia may be produced relative to the original plant.

The method can further comprise installing a vinyl acetate monomer unit for reacting a portion of the acetic acid with ethylene and oxygen to make vinyl acetate monomer. An air separation unit can be installed to make the oxygen for the vinyl acetate monomer unit and also for an autothermal reformer if incorporated into the new or retrofitted plant, and the nitrogen produced from the air separation unit preferably matches the nitrogen required for the ammonia production.

The process preferably has a molar ratio of carbon dioxide to hydrocarbon comprising natural gas in feed to the reforming step from about 0.1 to 0.5 and a ratio of steam to natural gas from about 2 to 6. The process can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen in an ammonia synthesis reactor to make ammonia. The process can also comprise the step of separating air into a nitrogen stream and an oxygen stream and supplying the nitrogen stream to the ammonia synthesis reactor. Where the product comprises acetic acid or an acetic acid precursor which is converted to acetic acid, the process can further comprise the step of supplying the oxygen stream from the air separation unit to a vinyl acetate synthesis reactor, along with a portion of the acetic acid from the carbon monoxide- methanol reaction step, and ethylene, to produce a vinyl acetate monomer stream. When an autothermal reformer is present, the process can further comprise the step of supplying oxygen from the air separation unit to the autothermal reformer.

In yet another aspect, the present invention provides a process for making hydrogen, a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof, from a hydrocarbon via intermediate methanol, carbon monoxide, and carbon dioxide, and optionally additional methanol beyond that required to manufacture the product. The process includes (1 ) reforming the hydrocarbon with steam to form an unadjusted syngas containing hydrogen, carbon monoxide, and carbon dioxide and having an R ratio less than 2.0, (2) recovering heat from the unadjusted syngas to form a cooled unadjusted syngas stream, (3) compressing at least a portion of the cooled unadjusted syngas stream to a separation pressure, (4) separating the compressed unadjusted syngas in a separation unit into streams rich in carbon monoxide, hydrogen and carbon dioxide, (5) feeding to the methanol synthesis unit any remaining unadjusted syngas, and a sufficient quantity of one or more of the streams rich in carbon monoxide, hydrogen and carbon dioxide, and optionally carbon dioxide from an another source to form an adjusted syngas such that the adjusted syngas (that is, the overall feed) fed to the methanol synthesis unit has an R ratio ranging from 2.0 to 2.9, (6) operating a methanol synthesis unit to react the hydrogen with the carbon monoxide and carbon dioxide in the adjusted syngas in a stoichiometric manner to obtain a methanol stream, and (7) reacting at least a portion of the carbon monoxide-rich stream from the separation unit with at least a portion of the methanol stream from the methanol synthesis unit in essentially stoichiometric proportions to form a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof.

Regardless of whether the plant is a retrofit or a new plant, where the product comprises acetic acid, the reaction step preferably comprises reacting methanol, methyl formate, or a combination thereof in the presence of a reaction mixture comprising carbon monoxide, water, a solvent and a catalyst system comprising at least one halogenated promoter and at least one compound of rhodium, iridium or a combination thereof. The reaction mixture preferably has a water content up to 20 weight percent. Where the reaction step comprises simple carbonylation, the water content in the reaction mixture is more preferably from about 14 to about 15 weight percent. Where the reaction step comprises low-water carbonylation, the water content in the reaction mixture is more preferably from about 2 to about 8 weight percent. Where the reaction step comprises methyl formate isomerization or a combination of isomerization and methanol carbonylation, the reaction mixture more preferably contains a nonzero quantity of water up to 2 weight percent. The reaction step is preferably continuous.

Another embodiment of the invention provides a process of pretreating the feed stream to allow a lower steam to carbon ratio to be employed while avoiding soot formation in the auto-thermal reformer, and the corresponding process facility. In this method, a hydrogen-rich stream is added to a feed gas stream containing higher hydrocarbons (2 or more carbon atoms), the resulting mixture is contacted with a hydrogenation catalyst at a hydrogenation temperature, and the hydrogenated mixture is fed to an autothermal reformer with steam and oxygen to form syngas. The hydrogen-rich stream is preferably a purge gas or fraction thereof from a methanol synthesis loop receiving syngas or a portion or fraction thereof. The hydrogen-rich stream is preferably added at a rate to provide at least a stoichiometric amount of hydrogen for hydrogenation of the higher hydrocarbons to methane. The hydrogenation temperature can preferably be from 300°C to 550°C. The process facility in this embodiment includes a feed gas supply comprising higher hydrocarbons; a pre-hydrogenation reactor comprising hydrogenation catalyst for converting the higher hydrocarbons to form a higher-hydrocarbon-lean stream (base metals such as Platinum, Palladium, Cobalt, Molybdenum, Nickel or Tungsten, supported on alumina or a zeolite are commonly used as catalyst); an autothermal reformer for reacting the higher-hydrocarbon-lean stream with steam and oxygen to form a syngas stream; a methanol synthesis loop for reacting hydrogen and carbon monoxide from the syngas stream to form methanol; a purge gas stream from the methanol synthesis loop; and a line for supplying a portion of the purge gas stream to the pre-hydrogenation reactor.

Because the reaction is exothermic, the hydrogenation process can be done in one or several reactors, with intermediate coolers if it is necessary. This hydrogenation step is particularly well adapted for use with auto-thermal reformers having a low steam to carbon ratio in the feed. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 (prior art) is an overall block flow diagram of a typical methanol/ammonia plant using hydrogen from the methanol synthesis loop purge to make ammonia, which can be retrofitted according to the present invention for acetic acid manufacture.

Fig. 2 is an overall block flow diagram of the plant of Fig. 1 after it has been retrofitted according to the present invention to make acetic acid, vinyl acetate monomer and additional ammonia. Fig. 3 is a simplified schematic process flow diagram of an integrated plant producing both methanol and acetic acid according to the present invention.

Fig. 4 is a simplified schematic process flow diagram of the integrated plant of Fig. 3 including a pre-hydrogenator according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1 , an original plant which can be retrofitted according to one embodiment of the present invention has an existing conventional steam reformer unit 10, methanol (MeOH) synthesis unit 12 and preferably ammonia synthesis unit 14 wherein hydrogen for the ammonia synthesis unit 14 is taken as purge stream 16 from the methanol synthesis loop.

The retrofit of the present invention is generally applicable to any plant that generates and uses synthesis gas to make methanol. As used in the present specification and claims, reference to "original plant" shall mean the plant as built and including any intervening modifications prior to the retrofit of the present invention.

The reformer unit 10 is typically a fired furnace containing parallel tube banks filled with conventional reforming catalyst such as alumina-supported nickel oxide, for example. The feed to the reformer(s) is any conventional reformer feed such as a lower hydrocarbon, typically naphtha or natural gas.

The reformer can be a single-pass reformer or a two-stage reformer, or any other commercially available reformer, such as, for example, a KRES unit available from Kellogg, Brown & Root, as is known to those skilled in the art. The reformer effluent of the original methanol plant can contain any conventional

H₂:CO ratio, but is normally close to 2.0 in plants making solely methanol, and substantially higher, e.g. 3.0 and above, in plants producing a separate hydrogen product or intermediate hydrogen-containing stream, e.g. for ammonia synthesis.

The hydrogen-containing stream is typically obtained as purge stream 16 from the methanol synthesis unit 12 loop which is necessary to keep the level of hydrogen and inerts from building up in the synthesis gas being recirculated through the methanol synthesis unit 12. According to the present invention, the original plant of Fig. 1 is retrofitted (or a new plant is built) to produce acetic acid (HAC) using reformer 10 and methanol synthesis unit 12, and keeping any ammonia synthesis unit 14, as shown in Fig. 2. An oxygen stream 66 from the new air separation unit 50 is fed to the reformer 10, in which case in this embodiment the reformer 10 can include an autothermal reformer (ATR) in parallel with the existing steam reformer or an ATR replacing the steam reformer. A portion of the effluent 18 from the reformer 10 is diverted from the methanol synthesis unit 12 via line 20 to a new CO₂ removal unit 22. The effluent 18 has an R ratio less than 2 for an ATR alone, and between 2 and 3 for a combination of ATR and classical steam reformer. The CO₂ removal unit 22 separates the stream from line 20 into a CO₂-rich stream and a CO₂-lean stream 26 using conventional CO₂ separation equipment and methodology such as, for example, absorption-stripping with a solvent such as water, methanol, generally aqueous alkanolamines such as ethanolamine, diethanolamine, methyldiethanolamine and the like, aqueous alkali carbonates such as sodium and potassium carbonates, and the like. Such CO₂ absorption- stripping processes are commercially available under the trade designations Girbotol, Sulfinol, Rectisol, Purisol, Fluor, BASF (aMDEA) and the like. The CO₂-rich stream can be fed via line 24 to the feed to the reformer 10 and/or to the methanol synthesis loop 12 via line 60.

The CO₂ recovered from the CO₂ removal unit 22 or from another source can be supplied to the methanol synthesis loop 12 to adjust the R ratio of the feed thereto.

The CO₂ can also or alternatively be supplied to the reformer 10. Increasing the CO₂ in the feed to the reformer 10 increases the CO content of the effluent 18. Analogous to steam reforming where a hydrocarbon reacts with steam to form synthesis gas, the reaction of the hydrocarbon with carbon dioxide is often called CO₂ reforming. As the carbon dioxide content of the reformer feed is increased, the share of the carbon in the carbon monoxide in the product synthesis gas 18 that is supplied from the carbon dioxide increases in relative proportion and the share originating from the hydrocarbon decreases. So, for a given CO production rate, the hydrocarbon feed gas requirement is reduced. During the early stage of reforming, heavier hydrocarbons are converted to methane:

HC + H₂O => CH₄ + CO₂ The main steam and CO₂ reforming reactions convert methane to hydrogen and carbon monoxide:

CH₄ + H₂O o 3 H₂ + CO CH₄ + CO₂ <-> 3 H₂ + 2 CO The shift reaction converts carbon monoxide to carbon dioxide and more hydrogen: CO + H₂O o CO₂ + H₂

The conversion of the heavier hydrocarbons goes to completion. The steam reforming, CO₂ reforming, and shift reaction are equilibrium-restricted. The overall reaction is strongly endothermic. The reformer 10 can, if desired, be modified for additional heat input for supplemental CO₂ reforming and additional heat recovery.

As noted above, the effluent 18 from the modified or autothermal reformer 10 has a molar ratio of hydrogen minus CO₂ to CO plus CO₂ (referred to in the present specification and claims as the "R ratio" (H₂ - CO₂)/(CO + CO₂)), which is less than 2. As explained later, the R ratio can be adjusted and optimized for methanol synthesis, preferably within the range from 2.0 to 2.9, by adding CO2 via line 60, CO via line 64 and/or hydrogen via line 62 to any remaining unadjusted syngas to achieve an adjusted syngas 38 the desired R ratio.

The CO₂-lean stream 26 contains primarily CO and hydrogen and can be separated in CO separation unit 28 into CO-rich streams 30 and 64 and a hydrogen-rich stream 32. The separation unit 28 can comprise any equipment and/or methodologies for separating the CO/hydrogen mixture into relatively pure CO and hydrogen streams, such as, for example, semi-permeable membranes, cryogenic fractionation, or the like. Cryogenic fractional distillation is preferred, and can include simple partial condensation without any columns, partial condensation with columns, optionally with a pressure swing absorption (PSA) unit and a hydrogen recycle compressor, or methane wash. Normally, partial condensation with columns is sufficient for obtaining CO and hydrogen of sufficient purity for acetic acid and ammonia production, respectively, keeping equipment and operating costs to a minimum, although the PSA unit and hydrogen recycle compressor can be added for increasing the hydrogen purity and CO production rate. For acetic acid manufacture, the CO stream 30 preferably contains less than 1000 ppm hydrogen and less than 2 mole percent nitrogen plus methane. For ammonia production, the hydrogen stream 32 which is sent to a nitrogen wash unit (not shown) preferably contains at least 80 mol% hydrogen, more preferably at least 95 mol% hydrogen.

A portion of the hydrogen stream 32 is supplied to the existing ammonia synthesis unit 14 in place of the methanol loop purge stream 16. The quantity of hydrogen produced in the stream 32 is generally much larger than the amount previously supplied via line 16. This is due in large part to the fact that less methanol is made in the retrofitted plant, and thus less hydrogen is consumed for methanol synthesis. The additional hydrogen capacity can be used as a fuel supply, or as a raw hydrogen source for another process, such as, for example, increased ammonia conversion. Additional ammonia can be made by supplying a portion of the additional hydrogen to the existing ammonia synthesis reactor 14 where the ammonia conversion capacity can be increased, and/or by installing additional ammonia synthesis unit 33. The increased ammonia capacity can be complemented by the presence of existing ammonia handling, storage and transport facilities which may be able to accommodate the additional ammonia capacity with little or no modification.

The methanol synthesis unit 12 is a conventional methanol conversion unit such as, for example, an ICI reactor. The methanol synthesis unit 12 of the retrofitted plant shown in Fig. 2 is essentially the same as in the original plant prior to the retrofit, except that the quantity of methanol produced is substantially lower, preferably about half of that of the original plant. Concomitantly, the loop recycle compressor (not shown) is operated at a lower capacity and the purge stream 16 is considerably reduced in quantity. As mentioned above, the purge stream 16 is no longer needed for supplying the hydrogen to the ammonia converter 14, since this is now supplied in the retrofitted plant from the hydrogen stream 32 separated directly from the portion of the reformer 10 effluent 18 diverted from the feed to the methanol synthesis unit 12 via line 20. If desired, the purge stream 16 can now be used for fuel and/or as a hydrogen source for hydrodesulfurization of the feed to the reformer 10. Since there is no longer any need to pass the excess hydrogen through the methanol synthesis unit 12 for use in the ammonia unit 14, the feed to the methanol synthesis unit 12, i.e. the effluent 18, can be compositionally optimized for more efficient methanol conversion, as described above. It can also be desirable to modify the methanol synthesis unit 12, if desired during the retrofit, to include any other modifications which are not present in the original plant but have become conventional and have been developed for methanol synthesis loops since the construction of the original plant and have not previously been incorporated therein.

The amount of syngas in the effluent 18 from the reformer 10 which is diverted to CO₂/CO/H₂ separation is preferably balanced to provide a stoichiometric ratio of methanol and CO to produce acetic acid therefrom in acetic acid synthesis unit 34. Preferably, the ratio of CO in line 30 and methanol in line 36 is about equal or the methanol is produced at a 10-20% molar excess, e.g. a molar ratio from 1.0 to about 1.2. To produce this ratio of methanol and CO, a relatively larger quantity (total kg/hr) of the effluent 18 is diverted into line 20, and the remaining minor portion is fed in line 38 to the methanol synthesis unit 12.

The acetic acid synthesis unit 34 employs conventional acetic acid manufacturing equipment and methodology well known and/or commercially available to those skilled in the art, such as, for example, from one or more of the acetic acid manufacturing patents mentioned above. For example, a conventional BP/Monsanto process can be employed, or an improved BP/Monsanto process employing BP-Cativa technology (iridium catalyst), Celanese low water technology (rhodium-lithium acetate catalyst), Millenium low water technology (rhodium-phosphor oxides catalyst), Acetex technology (rhodium-iridium catalyst) and/or dual process methanol carbonylation-methyl formate isomerization. The reaction generally comprises reacting methanol, methyl formate, or a combination thereof in the presence of a reaction mixture comprising carbon monoxide, water, a solvent and a catalyst system comprising at least one halogenated promoter and at least one compound of rhodium, iridium or a combination thereof. The reaction mixture preferably has a water content up to 20 weight percent. Where the reaction comprises simple carbonylation, the water content in the reaction mixture is preferably from about 14 to about 15 weight percent. Where the reaction comprises low-water carbonylation, the water content in the reaction mixture is preferably from about 2 to about 8 weight percent. Where the reaction comprises methyl formate isomerization or a combination of isomerization and methanol carbonylation, the reaction mixture preferably contains a nonzero quantity of water up to 2 weight percent. The reaction is typically continuous. An acetic acid product is obtained via line 40.

If desired, a portion of the acetic acid from line 40 can be fed to a conventional vinyl acetate monomer synthesis unit 42 where it is reacted with ethylene via line 44 and at least a portion of the oxygen from line 46 to obtain monomer product stream 48. The oxygen in line 46 can be obtained, for example, using a conventional (preferably cryogenic) air separation unit 50 which also produces a nitrogen stream 52 corresponding to the amount of air from line 54 needed for the oxygen in line 46 to supply the oxygen for the vinyl acetate monomer synthesis unit 42 and the reformer 10. The amount of air separated can be matched to produce the nitrogen required via line 52 for the additional ammonia capacity added by ammonia synthesis unit 33 as mentioned above.

Referring now to Figure 3, there is shown a schematic of a plant 100 according to one embodiment of an integrated process for making acetic acid and methanol according to the present invention. A cogeneration unit 110 is fed by natural gas 102 and air 104 to produce both electricity 108 and steam 106. The power produced supplies one or two air separation units 112 which produces oxygen and nitrogen (not shown). Only a small portion of nitrogen will be used in the other units (instrumentation and safety fluid), unless an ammonia plant is incorporated into the plant 100.

The oxygen stream will be combined with desulfurized natural gas and steam and preheated in a preheater 114 at a temperature sufficiently high (between 400 to 750 °C) to initiate catalytic oxidation of the feed of an autothermal reformer 116. The autothermal reformer 116 will be operated at a high pressure between 20 and 80 bars, and a high temperature (between 800 and 1250 °C). The main advantages of this process are: (1 ) to get a syngas 118 at a high pressure involves a simpler syngas compressor (not shown) with only one or two stages; and (2) the high output temperature gives a very low methane slip (concentration of methane in the syngas). The unadjusted syngas 118 will typically have an R ratio less than 2. The high temperature level is recovered through heat exchangers (not shown) to preheat the feeds and/or to generate steam.

A portion 120 of the cooled syngas is sent to a CO₂ removal unit 122 operated with ethanolamines to produce a CO₂ rich stream 124 and a mixed H₂/CO stream 126. Then, the separation unit 128 produces a CO rich stream 130 and an H₂ rich stream 132. The separation unit 128 can be, for example, a partial condensation cold box with two columns.

In this embodiment, streams 124 and 132 can be mixed with the other portion 134 of the syngas to yield an adjusted syngas 136 with an R ratio of 2.0 to 2.9, preferably about 2, before entering the methanol synthesis loop 138. The adjusted syngas 136 with an R ratio of about 2 yields a hydrogen purge stream 140 flowrate that is low. Preferably, the adjusted syngas R ratio is between 2.00 and 2.05.

The methanol synthesis loop 138 is a low pressure synthesis to produce a methanol stream 146, with steam generation. Depending on economics, the purge stream 140 could be used to recover its thermic value or separated in membranes or a PSA unit 142 to extract hydrogen 144 and recycle it to the main synthesis loop 138.

A part 148 of the methanol produced is sent to the acetic acid unit 150 for carbonylation with the CO stream 130 from the separation unit 128 to produce an acetic acid stream 152. The other part 154 of the methanol stream 146 is a methanol product. Referring to Fig. 4, the embodiment of the process 200 is similar to Fig. 3 but includes a pre-hydrogenator 202 and purge gas recycle line 204. The pre- hydrogenator 202 contains a hydrogenation catalyst, e.g. a nickel-cobalt- molybdenum catalyst typically used for steam reforming and is operated at a suitable hydrogenation temperature, e.g. 300-550°C.

The purge gas recycle supplies a hydrogen-rich stream to the pre- hydrogenator to convert any higher hydrocarbons in the feed gas stream to lower hydrocarbons, e.g. methane. A sufficient supply of hydrogen from the recycle stream should be available for stoichiometrically hydrogenating or cracking the higher hydrocarbons in the feed natural gas to lower hydrocarbons, preferably methane.

The remainder of the purge gas can be fed to the ATR to recover all the carbon molecules. The buildup of inert compounds, such as nitrogen or argon, is prevented in the methanol loop because a portion of the syngas goes to the CO separation unit, where these inert compounds usually follow the CO stream.

The feed natural gas, preferably consisting mainly of methane, is desulfurized upstream from the pre-hydrogenator and is preferably low in water vapor content. The pre-hydrogenator preferably operates under generally low water conditions, i.e. without a steam supply, which can be mixed with the pre- hydrogenator effluent for preheating in the conventional manner, except that a higher pre-heat temperature can be used due to the lower high-hydrocarbon content. Many natural gas sources such as associated gas contain appreciable quantities of ethane, propane, butane and C5+ hydrocarbons that can cause soot formation in the autothermal reformer 116 as mentioned in US Patent 6,375,916. The pre-hydrogenator converts the higher hydrocarbons to lower hydrocarbons, i.e. methane and allows the reformer to be operated at a lower steam to carbon ratio without soot formation. Using a substream of the purge gas can allow the size or capacity of the membrane/PSA unit to be proportionately reduced and/or eliminated entirely. Since the hydrogen recycle is an internal loop, the overall material balance of the facility is essentially unchanged.

Claims

CLAIMS: 1. A method for the production of syngas for the manufacturing of methanol or methanol derivatives, the method comprising the steps of: supplying hydrocarbon, steam, and oxygen to an autothermal reformer to produce an adjusted syngas having at least hydrogen, carbon monoxide and optionally carbon dioxide, wherein the unadjusted syngas has a molar R ratio less than 2, wherein the R ratio is [H₂ - CO₂]/[CO + CO₂]; separating at least a portion of the unadjusted syngas into a hydrogen- rich stream, a carbon monoxide-rich stream, and a carbon dioxide- rich stream; preparing an adjusted syngas having an R ratio from 2.0 to 2.9 by combining at least a portion of at least two streams selected from the group selected from any remaining unadjusted syngas, the hydrogen-rich stream, the carbon monoxide-rich stream, the carbon dioxide-rich stream and an additional source of carbon dioxide; and feeding the adjusted syngas to a methanol synthesis loop for the production of methanol. 2. A method for production of a syngas for the manufacturing of methanol or methanol derivatives, the method comprising the steps of: combining a hydrogen-containing stream with a feed natural gas containing higher hydrocarbons to form a hydrogen containing feed stream; contacting said feed stream with a hydrogenation catalyst at hydrogenation temperatures to produce a pretreated stream lean in higher hydrocarbons; supplying said pretreated stream, steam, and oxygen to an autothermal reformer to produce an unadjusted syngas having at least hydrogen, carbon monoxide and optionally carbon dioxide. 3. The method of claim 2, wherein the unadjusted syngas has an R ratio less than 2, further comprising: separating at least a portion of the unadjusted syngas into a hydrogen- rich stream, a carbon monoxide rich stream, and a carbon dioxide- rich stream; and preparing an adjusted syngas having an R ratio from 2.0 to 2.9 by combining at least a portion of at least two streams selected from the group selected from any remaining unadjusted syngas, hydrogen-rich stream, carbon monoxide-rich stream, carbon dioxide-rich stream and an additional source of carbon dioxide; feeding the adjusted syngas to a methanol synthesis loop for the production of methanol. 4. The method of any one of claims 1-3 further comprising: producing a product selected from acetic acid and a precursor of acetic acid by reacting at least a portion of the methanol produced with at least a portion of the carbon monoxide-rich stream in stoichiometric portions. 5. The method of claim 4, wherein the product comprises the precursor of acetic acid; and further comprising the step of converting the acetic acid precursor to acetic acid. 6. The method of any one of claims 2-5, wherein said feed stream in said contacting step is free of added steam to maintain low water conditions for hydrogenation. 7. The method of claim 3 further comprising recovering a purge gas stream from the methanol synthesis loop, and recycling at least a portion of the purge gas stream to the contacting step as the hydrogen-containing stream. 8. The method of claim 7, further comprising: recovering hydrogen gas from said hydrogen gas purge stream; and recycling the hydrogen gas to the unadjusted syngas stream. 9. The method of claim 7 comprising recycling said hydrogen gas purge upstream of the autothermal reformer. 10. The method of any one of claims 1 or 3-9, further comprising: reacting at least a portion of the carbon monoxide-rich stream from the separation unit with at least a portion of the methanol stream from the methanol synthesis unit in essentially stoichiometric proportions to form a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. 11. The method of claim 10, wherein the product is selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. 12. The method of any preceding claim, wherein the reformer is an autothermal reformer operated at a pressure of between 20-80 bars and a temperature of between 800 and 1250°C. 13. The method of any one of claims 1 or 3-12, wherein a portion of the carbon dioxide-rich stream is supplied to the reformer to enhance carbon monoxide production. 14. The method of any one of claims 1or 3-13, wherein the R ratio of the adjusted syngas is from 2.00 to 2.05. 15. A method for converting an original methanol plant to a converted plant producing methanol and a product selected from acetic acid and an acetic acid precursor which is converted to acetic acid, the method comprising: providing an original methanol plant having at least one reformer for converting a hydrocarbon to form an unadjusted syngas containing at least hydrogen, carbon monoxide and optionally carbon dioxide, a heat recovery section for cooling the syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis unit for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol; diverting at least a portion of the unadjusted syngas from the at least one reformer to a syngas separation unit; operating the separation unit to separate the diverted syngas into a carbon dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; controllably recycling one or more of the carbon dioxide-rich stream, the carbon monoxide-rich stream and the hydrogen-rich stream from the separation unit to the methanol synthesis unit such that any remaining unadjusted syngas combined therewith yields an adjusted syngas having an R ratio ranging from 2.0 to 2.9, and installing an acetic acid reactor for reacting at least a portion of the carbon monoxide-rich stream from the separation unit with at least a portion of the methanol from the methanol synthesis unit to form the product. 16. The method of claim 15 further comprising modifying the reformer to reduce the R ratio of the unadjusted syngas. 17. The method of claim 15 or 16, wherein the R ratio of the unadjusted syngas is less than 2. 18. The method of any one of claims 15-17, wherein the reformer is an autothermal reformer and further comprising installing an air separation unit to produce oxygen for the autothermal reformer. 19. The method of any one of claims 15-18, wherein the syngas separation unit includes a solvent absorber and a stripper for carbon dioxide recovery and a cryogenic distillation unit for carbon monoxide and hydrogen recovery. 20. The method of any one of claims 15-19, wherein the reformer is a steam reformer and an autothermal reformer is installed in parallel therewith. 21. The method of any one of claims 15-20, wherein the R ratio of the adjusted syngas is between 2.00 and 2.05.