Method for producing acetic acid

Abstract

The present disclosure generally relates to methods and systems for producing acetic acid. In one aspect, the present disclosure provides a method for producing acetic acid, which includes contacting carbon monoxide and one or more of methanol and its reactive derivatives with a carbonylation catalyst in a first reaction zone to produce a product stream, and transferring at least a portion of the product stream to a second reaction zone; introducing a first vapor stream (90-100% CO) into the second reaction zone at a first position between the reaction zone inlet and the outlet of the second reaction zone; introducing a second vapor stream at a second position between the reaction zone inlet and the outlet, the second vapor stream containing less than 90 wt.% of carbon monoxide; and discharging the effluent of the second reaction zone through the outlet.

Description

[0001] Disclosed Background 1. Technical Field

[0003] The present disclosure generally relates to methods and apparatuses for producing acetic acid. In particular, the present disclosure relates to a method for producing acetic acid from methanol and carbon monoxide using a carbonylation catalyst, and a system that can be used to implement such a method. 2. Background Art

[0005] Commercially, acetic acid has been manufactured for many years by carbonylating methanol with carbon monoxide in the presence of a Group VIII carbonylation catalyst. Generally, carbon monoxide and methanol are contacted in one or more reaction zones in the presence of a homogeneous or heterogeneous carbonylation catalyst of rhodium or iridium, methyl iodide, and water. Typically, the crude acetic acid product can be recovered by discharging it from the reactor and separating the acetic acid product from other components such as the Group VIII metal carbonylation catalyst, methyl iodide, methyl acetate, and water in one or more flash and / or distillation stages.

[0006] The carbonylation reaction is typically carried out in two reaction zones in series, namely a first reaction zone and a second reaction zone. The first reaction zone is typically a stirred tank reactor, while the second reaction zone is typically a plug flow reactor, where the effluent from the first reaction zone is directed to a separation zone. In the second reaction zone, entrained carbon monoxide can further react with methanol to provide more acetic acid product. Typically, additional carbon monoxide is added in the second reaction zone with the aim of increasing the temperature of the effluent from the second reaction zone, for example, by at least a certain number of degrees Celsius higher than the temperature of the effluent from the first reaction zone. This not only provides an increase in acetic acid conversion; it also enables improved separation of acetic acid and other condensable components from the carbonylation catalyst in subsequent separation operations. Thus, the vapor fraction from the separation will be more enriched in acetic acid, resulting in a higher yield.

[0007] However, there is still a desire for improvement, particularly in terms of increasing the temperature difference between the effluent from the first reaction zone and the effluent from the second reaction zone. Summary of the Invention

[0008] The scope of the present disclosure is not affected to any extent by what is stated in the summary of the invention.

[0009] In one aspect, the present disclosure provides a method for producing acetic acid. The method includes:

[0010] contacting carbon monoxide and one or more of methanol and its reactive derivatives with a carbonylation catalyst in a first reaction zone to produce a product stream and a waste stream, the product stream comprising water, acetic acid, carbon monoxide, the carbonylation catalyst, and one or more of methanol and its reactive derivatives;

[0011] Transfer at least a portion of the product stream into a second reaction zone through the reaction zone inlet of the second reaction zone;

[0012] At a first location between the reaction zone inlet and the outlet of the second reaction zone, introduce a first vapor stream into the second reaction zone, the first vapor stream comprising carbon monoxide and having a carbon monoxide concentration;

[0013] At a second location between the reaction zone inlet and the outlet of the second reaction zone, introduce a second vapor stream into the second reaction zone, the second vapor stream comprising carbon monoxide and having a carbon monoxide concentration that is lower than the carbon monoxide concentration of the first vapor stream; and

[0014] Discharge the effluent of the second reaction zone through the outlet.

[0015] On the other hand, the present disclosure provides a system for producing acetic acid, such as using the method described herein. The system includes:

[0016] A first reaction zone having one or more inlets in communication with a methanol source and a carbon monoxide source, an off-gas outlet, and a product stream outlet;

[0017] A second reaction zone having a reaction zone inlet and an outlet, the reaction zone inlet being in fluid communication with the product stream outlet of the first reaction zone, the second reaction zone comprising

[0018] A first vapor stream inlet at a first location between the reaction zone inlet and the outlet, the vapor stream inlet being in communication with a first vapor stream source rich in carbon monoxide, and

[0019] A second vapor stream inlet at a second location between the reaction zone inlet and the outlet, the second vapor stream inlet being in communication with a second vapor stream source poor in carbon monoxide.

[0020] Other aspects of the present disclosure will be apparent to those skilled in the art in view of the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram showing a method and system for producing acetic acid according to an embodiment of the present disclosure.

[0022] Figure 2 A graph showing the relationship between the pure CO flow rate and the acetic acid production (as a ratio to the production using only the second vapor stream) for the method described herein.

[0023] Figure 3 A graph showing the relationship between the pure CO flow rate and the acetic acid production (as a ratio to the production using only the second vapor stream) for the method described herein.

[0024] Figure 4A graph showing the relationship between the pure CO flow rate and the acetic acid production (as a ratio to the production using only the second vapor stream) of the method described herein. Detailed Description

[0025] In various aspects, the methods of the present disclosure provide for the production of acetic acid by carbonylating methanol or its reactive derivatives with carbon monoxide over a carbonylation catalyst. Specifically, one aspect of the present disclosure is a method for producing acetic acid. The method includes:

[0026] Contacting carbon monoxide and one or more of methanol and its reactive derivatives with a carbonylation catalyst in a first reaction zone to produce a product stream and a waste stream, the product stream comprising water, acetic acid, carbon monoxide, a carbonylation catalyst, and one or more of methanol and its reactive derivatives;

[0027] Transferring at least a portion of the product stream into a second reaction zone through an inlet of the second reaction zone;

[0028] Introducing a first vapor stream into the second reaction zone at a first location between the inlet and the outlet of the second reaction zone, the first vapor stream comprising carbon monoxide and having a carbon monoxide concentration (e.g., in the range of 90 - 100%);

[0029] Introducing a second vapor stream into the second reaction zone at a second location between the inlet and the outlet of the second reaction zone, the second vapor stream comprising carbon monoxide having a carbon monoxide concentration lower than that of the first vapor stream (e.g., less than 90%); and

[0030] Discharging the effluent of the second reaction zone through an outlet.

[0031] The inventors have noted that in conventional methods, the reaction in the second reaction zone can help react the carbon monoxide entrained and / or dissolved in the effluent from the first reaction zone to improve feedstock efficiency. Carbon monoxide can react not only with methanol but also with methyl acetate and methyl iodide, each of which can provide additional acetic acid. However, the carbonylation in the second reaction zone can also have another important role: the exothermic nature of the carbonylation reaction can increase the temperature to improve the efficiency of flashing in the flash separation zone.

[0032] Introducing additional carbon monoxide into the second reaction zone can further drive the carbonylation in the second reaction zone to produce more acetic acid and further increase the temperature. The increased carbonylation in the second reaction zone itself has many advantages in addition to the temperature increase. In particular, due to the production of additional acetic acid, the vapor fraction in the flash separation zone will be even further enriched in acetic acid. Further, since methyl acetate and water may also be consumed, separating the product acetic acid from the light components (which include methyl acetate and water) will require less energy than otherwise. Alternatively, since methyl acetate and water can be consumed in the second reaction zone, the first reaction zone can be operated at higher concentrations of methyl acetate and water without adversely affecting the composition that ultimately exits the second reaction zone (e.g., enters the flash separation zone). And since the formation of by-products during the methanol carbonylation process tends to decrease with increasing concentrations of methyl acetate and water, operating the first reaction zone at higher concentrations of methyl acetate and water can result in an overall reduction in by-products.

[0033] However, the inventors have noted that increasing the CO feed to the second reaction zone only raises the effluent temperature to a point beyond which further addition of CO provides little further temperature increase; instead, the additional CO simply passes through unreacted. If the effluent temperature is to be further increased, further benefits in flash efficiency are possible, so it is desirable to further increase the temperature rise caused by the carbonylation in the second reaction zone.

[0034] The inventors have determined that the reaction of CO in the liquid solution to the product is very slow compared to the mass transfer of CO from the gas to the liquid. Thus, the inventors have noted that it is highly desirable to maintain a high liquid residence time to achieve a high overall reaction rate.

[0035] The inventors have also determined that although the partial pressure of CO increases with increasing CO feed, the volume fraction of vapor in the second reaction zone associated with the gas stream (plus the light components that vaporize into vapor bubbles) also increases at the expense of the liquid fraction value. The inventors have determined that, advantageously, feeding a relatively impure CO stream (e.g., the off-gas from the first reaction zone, which contains CO in addition to inert gases and vaporized light components) to the second reaction zone can be used to recover carbon monoxide from the off-gas. Thus, in certain desirable embodiments, the second vapor further comprises one or more of water, acetic acid, and methanol and their reactive derivatives. This can advantageously help to keep the reactive species in the liquid phase in the second reaction zone; since the carbonylation reaction is primarily a liquid-phase process, keeping the reactants in the liquid phase is advantageous. A higher purity CO stream can be added to "fine-tune" the reaction to provide a desirably high effluent temperature and a low effluent CO concentration.

[0036] Thus, referring to Figure 1Describe an embodiment of the present disclosure. One or more of carbon monoxide and methanol and their reactive derivatives are contacted with a carbonylation catalyst in a first reaction zone. In Figure 1 the embodiment, carbon monoxide 2 and methanol 4 are introduced into the first reaction zone 10, which may include, for example, a stirred reaction tank. The contacting produces a product stream 20 and a waste gas stream 25. The product stream 20 includes water, acetic acid, carbon monoxide, the carbonylation catalyst, and one or more of methanol and its reactive derivatives. At least a portion of the product stream is directed through the reaction zone inlet into a second reaction zone. The second reaction zone desirably includes a flow reactor, i.e., a reactor in which reaction occurs as the material is directed from its inlet to its outlet. The flow reactor may be, for example, a plug flow reactor, such as in tubular form. In Figure 1 the embodiment, the product stream 20 is directed through the reaction zone inlet 32 into the second reaction zone 30. At a first location between the reaction zone inlet and outlet of the second reaction zone, a first vapor stream is introduced into the second reaction zone. The first vapor stream includes carbon monoxide. And at a second location between the reaction zone inlet and outlet of the second reaction zone, a second vapor stream is introduced into the second reaction zone. The second vapor stream also includes carbon monoxide, and its concentration is lower than the concentration of carbon monoxide in the first vapor stream. In Figure 1 the embodiment, the first vapor stream 40 is introduced into the second reaction zone 30 at a first location 36 between the reaction zone inlet 32 and the outlet 34, while the second vapor stream 45 is introduced into the second reaction zone 30 at a second location 38 between the reaction zone inlet 32 and the outlet 34. The effluent 50 of the second reaction zone is discharged through the outlet 34. As described above, the inventors have noted that introducing carbon monoxide into the second reaction zone using two different vapor streams can provide many advantages. Using a first vapor stream with a relatively high CO concentration and a relatively impure second vapor stream can advantageously allow a person of ordinary skill in the art to adjust the effluent temperature and CO concentration.

[0037] In addition, the inventors have determined that the order of introducing the first vapor stream and the second vapor stream into the second reaction zone can have an important impact on the carbonylation efficiency. As described in the following examples, the inventors have determined that introducing the first vapor stream upstream of the second vapor stream can provide a higher CO conversion rate than introducing them in the reverse order. This is true regardless of whether the second vapor stream is added near the reaction zone inlet of the second reaction zone, near the second reaction zone outlet, or at an intermediate location between them.

[0038] Thus, in certain embodiments as further described herein, the first location (i.e., the location where the first vapor stream with a relatively high CO concentration is introduced into the second reaction zone) is between the reaction zone inlet and the second location (the location where the second vapor stream with a relatively low CO concentration is introduced into the second reaction zone). As described above, this arrangement can provide improved CO efficiency compared to other arrangements.

[0039] In certain embodiments as further described herein, the distance between the reaction zone inlet and the first location is not greater than 25% of the distance between the reaction zone inlet and the outlet of the second reaction zone. For example, in certain embodiments, the distance between the reaction zone inlet and the first location is not greater than 15% of the distance between the reaction zone inlet and the outlet of the second reaction zone, or even not greater than 10%.

[0040] In certain embodiments as further described herein, the distance between the reaction zone inlet and the second location is at least 50% of the distance between the reaction zone inlet and the outlet of the second reaction zone. For example, in certain embodiments, the distance between the reaction zone inlet and the second location is at least 60% of the distance between the reaction zone inlet and the outlet of the second reaction zone, or even at least 70%.

[0041] Based on the disclosure herein, one of ordinary skill in the art can select the relative positioning of the first and second locations and the flow rates and concentrations of the first and second vapor streams to provide a desired high effluent temperature and a desired low effluent CO concentration.

[0042] In the methods described herein, methanol and / or its reactive derivatives can be introduced as liquid reactants into the first reaction zone (i.e., a liquid reaction composition is formed in the first reaction zone). For example, in certain desired embodiments of the methods as further described herein, methanol is introduced as a reactant into the first reaction zone. In other methods as further described herein, one or more reactive derivatives are introduced as reactants into the first reaction zone, or a combination of methanol and one or more reactive derivatives of methanol is introduced as a reactant into the first reaction zone. As used herein, "reactive derivatives" of methanol are methyl acetate, dimethyl ether, and methyl iodide. In certain embodiments as further described herein, methanol and / or methyl acetate are used as liquid reactants. In one embodiment as further described herein, methanol is used as a reactant; in another embodiment as further described herein, methyl acetate is used as a reactant; and in still another embodiment as further described herein, a mixture of methanol and methyl acetate is used as a reactant.

[0043] The methods described herein can use a variety of carbonylation catalysts, such as Group VIII noble metal carbonylation catalysts. The catalyst can include a Group VIII material supported on an inert carrier such as a carbon carrier. In a desirable embodiment as further described herein, the carbonylation catalyst comprises rhodium, iridium, or a mixture thereof. In a specific embodiment of the present invention, the carbonylation catalyst is iridium. In another specific embodiment as further described herein, the carbonylation catalyst is a rhodium catalyst. A cocatalyst may optionally be present, such as selected from alkali metal iodides (such as lithium iodide), alkaline earth metal iodides, Group III metal iodides, organic iodide salts, ruthenium, osmium, rhenium, and mixtures thereof. In the case where the catalyst is a rhodium catalyst, the optional carbonylation cocatalyst may desirably be selected from alkali metal iodides (such as lithium iodide), alkaline earth metal iodides, Group III metal iodides, and / or organic iodide salts and mixtures thereof. In the case where the catalyst is an iridium catalyst, the optional carbonylation cocatalyst may desirably be selected from ruthenium, osmium, rhenium, and mixtures thereof.

[0044] In the case where the carbonylation catalyst is an iridium catalyst, the iridium catalyst can include any iridium-containing compound that is substantially soluble in the liquid reaction composition. The iridium catalyst can be added to the liquid reaction composition in any suitable form that is substantially dissolved in the liquid reaction composition or can be converted to a soluble form. The iridium catalyst desirably is used as a chloride-free compound, such as an acetate, that is soluble in one or more liquid reaction composition components (such as water and / or acetic acid) and can thus be added to the reaction as a solution therein. Examples of suitable iridium-containing compounds that can be added to the liquid reaction composition include IrCl3, IrI3, IrBr3, [Ir(CO)2I]2, [Ir(CO)2Cl]2, [Ir(CO)2Br]2, [Ir(CO)4I2] - H + 、[Ir(CO)2Br2] - H + 、[Ir(CO)2I2] - H + 、[Ir(CH3)I3(CO)2] - H + 、Ir4(CO) 12 、IrCl3.4H2O、IrBr3.4H2O、Ir3(CO) 12 、iridium metal, Ir2O3, IrO2, Ir(acac)(CO)2, Ir(acac)3, iridium acetate, [Ir3O(OAc)6(H2O)3][OAc], and chloroiridic acid H2[IrCl6], desirably chloride-free iridium complexes such as acetates, oxalates, and acetoacetates.

[0045] When present, the iridium catalyst concentration in the liquid reaction composition in the first and second reaction zones can, for example, independently be in the range of from 100 to 6000 ppm by weight of iridium.

[0046] When the carbonylation catalyst is an iridium catalyst, the carbonylation cocatalyst is desirably ruthenium. The promoter can include any ruthenium-containing compound that is substantially soluble in the liquid reaction composition. The ruthenium promoter can be added to the liquid reaction composition in any suitable form that is substantially soluble in the liquid reaction composition or can be converted to a soluble form. The ruthenium promoter compound is desirably used as a chloride-free compound, such as an acetate, which is soluble in one or more liquid reaction composition components (such as water and / or acetic acid) and can thus be added to the reaction as a solution therein.

[0047] Examples of suitable ruthenium-containing compounds that can be used include ruthenium(III) chloride, ruthenium(III) chloride trihydrate, ruthenium(IV) chloride, ruthenium(III) bromide, ruthenium(III) iodide, metallic ruthenium, ruthenium oxide, ruthenium(III) formate, [Ru(CO)3I3] - H + , ruthenium(II,III) tetrakis(acetate) chloride, ruthenium(III) acetate, ruthenium(III) propionate, ruthenium(III) butyrate, ruthenium pentacarbonyl, dodecacarbonyltriruthenium, and mixed halo carbonyl rutheniums such as dichlorotricarbonylruthenium(II) dimer, dibromotricarbonylruthenium dimer, and other organo ruthenium complexes such as tetrachlorobis(4-cymene) diruthenium(II), tetrachlorobis(benzene) diruthenium(II), dichloro(cycloocta-1,5-diene) ruthenium(II) polymer, and ruthenium(III) tris(acetylacetonate). Desirably, the ruthenium-containing compound is free of impurities that in situ provide or generate ionic iodides that can inhibit the reaction, such as alkali metal or alkaline earth metal or other metal salts.

[0048] The ruthenium promoter can be present in any effective amount up to its solubility limit in the liquid reaction composition, the liquid fractionation, and / or any liquid process stream recycled to the carbonylation reaction zone. For example, the ruthenium promoter is present in the liquid reaction composition in a molar ratio of each ruthenium promoter:iridium in the range of [0.1 - 100]:1, preferably [greater than 0.5]:1, more preferably [greater than 1]:1, and preferably [up to 20]:1, more preferably [up to 15]:1, and still more preferably [up to 10]:1. The ruthenium promoter concentration in the liquid reaction composition in each of the first and second reaction zones is generally independently less than 6000 ppm. Suitable promoter concentrations are in the range of, for example, 400 - 5000 ppm, such as 2000 - 4000 ppm.

[0049] Suitable rhodium carbonylation catalysts are described, for example, in EP-A-0 161 874, US 6,211,405 and EP-A-0728727, each of which is hereby incorporated by reference in its entirety into this text. In the case where the carbonylation catalyst is a rhodium catalyst, the concentration of the rhodium catalyst in the liquid reaction composition is preferably in the range of 50 - 5000 ppm by weight, preferably 100 - 1500 ppm of rhodium. In the case where rhodium is used as the catalyst, an alkali metal iodide such as lithium iodide is preferably used as a promoter, as described, for example, in the above references.

[0050] Thus, in certain embodiments, the homogeneous carbonylation of carbon monoxide with methanol and / or its reactive derivatives is catalyzed by a soluble Group VIII metal carbonylation catalyst (e.g., comprising rhodium and / or iridium) in a liquid reaction composition comprising methanol and / or its reactive derivatives. In certain embodiments, the liquid reaction composition comprises water and one or more of methyl iodide and methyl acetate. In certain such embodiments, the liquid reaction composition further comprises a propionic acid by-product.

[0051] As described above, water may be present in the liquid reaction composition. One of ordinary skill in the art should appreciate that water is formed in situ in the liquid reaction composition by the esterification reaction between methanol and / or its reactive derivatives and the acetic acid product. In certain embodiments, water may also be introduced into the first reaction zone (e.g., together with or separately from other components of the liquid reaction composition). In certain desirable embodiments, water is present in the liquid reaction composition in an amount in the range of 0.1 wt.% - 15 wt.%, such as in the range of 1 wt.% - 15 wt.% or in the range of 1 wt.% - 8 wt.%.

[0052] Methyl acetate can be formed in situ in the liquid reaction composition by the reaction of methanol and / or its reactive derivatives with the acetic acid product or the solvent. In certain embodiments as further described herein, the concentration of methyl acetate in the liquid reaction composition in the first reaction zone is in the range of 2 wt.% - 50 wt.%, such as 3 wt.% - 35 wt.%.

[0053] As described above, a propionic acid by-product may also be present in the liquid reaction composition. In certain embodiments, propionic acid is present in the liquid reaction composition in an amount in the range of 200 ppmw - 2500 ppmw, such as in the range of 400 ppmw - 2000 ppmw or in the range of 600 ppmw - 1400 ppmw.

[0054] In certain desirable embodiments, methyl iodide is present in the liquid reaction composition in an amount in the range of 1 wt.% - 20 wt.%. For example, in certain such embodiments, methyl iodide is present in the liquid reaction composition in an amount in the range of 2 wt.% - 16 wt.%.

[0055] In certain embodiments as further described herein, the liquid reaction composition comprises a solvent. For example, in certain such embodiments, the liquid reaction composition comprises an acetic acid solvent (e.g., recycled from the separation zone of an acetic acid production unit).

[0056] As described above, acetic acid can be produced in the reaction zone by the carbonylation of methanol and / or its reactive derivatives with carbon monoxide. In certain embodiments as further described herein, the carbon monoxide provided to the first reaction zone is substantially pure. In other embodiments, the carbon monoxide provided to the first reaction zone contains one or more impurities such as carbon dioxide, methane, nitrogen, hydrogen, or noble gases. In certain embodiments as further described herein, the partial pressure of carbon monoxide (e.g., in the reactor of the first reaction zone) is in the range of 1 bar - 70 bar, for example in the range of 1 bar - 35 bar.

[0057] In certain embodiments as further described herein, the carbonylation reaction in the first reaction zone is carried out at a total pressure in the range of 10 barg - 100 barg (e.g., in the reactor of the reaction zone).

[0058] The inventors have noted that the temperature in the first and second reaction zones can have an important impact on product yield and overall process efficiency. In certain embodiments as further described herein, the carbonylation reaction is carried out in the first reaction zone (e.g., in the reactor of the first reaction zone) at a temperature in the range of 170 - 215 °C (e.g., 170 - 210 °C, or 170 - 205 °C, or 170 - 200 °C, or 170 - 195 °C, or 170 - 190 °C, or 175 - 215 °C, or 175 - 210 °C, or 175 - 205 °C, 175 - 200 °C, or 175 - 195 °C, or 175 - 190 °C, or 180 - 215 °C, or 180 - 210 °C, or 180 - 205 °C, 180 - 200 °C, or 180 - 195 °C, or 180 - 190 °C, or 185 - 215 °C, or 185 - 210 °C, or 185 - 205 °C, 185 - 200 °C, or 185 - 195 °C).

[0059] As described above, at least a portion of the product stream from the first reaction zone (e.g., consisting of a liquid reaction mixture) is directed into the second reaction zone through the reaction zone inlet of the second reaction zone. This can be at approximately the same temperature as the liquid reaction composition in the first reaction zone. In certain desirable embodiments as further described herein, the temperature of the product stream at the reaction zone inlet of the second reaction zone is in the range of 170 - 215 °C (e.g., 170 - 210 °C, or 170 - 205 °C, or 170 - 200 °C, or 170 - 195 °C, or 170 - 190 °C, or 175 - 215 °C, or 175 - 210 °C, or 175 - 205 °C, 175 - 200 °C, or 175 - 195 °C, or 175 - 190 °C, or 180 - 215 °C, or 180 - 210 °C, or 180 - 205 °C, 180 - 200 °C, or 180 - 195 °C, or 180 - 190 °C, or 185 - 215 °C, or 185 - 210 °C, or 185 - 205 °C, 185 - 200 °C, or 185 - 195 °C).

[0060] Notably, providing carbon monoxide in the second reaction zone can provide further reactions, raising the temperature of the liquid reaction composition therein. In certain embodiments as further described herein, the temperature of the effluent at the outlet of the second reaction zone is in the range of 175 - 215 °C (e.g., 170 - 210 °C, or 170 - 205 °C, or 175 - 200 °C, or 175 - 195 °C, or 175 - 190 °C, or 180 - 215 °C, or 180 - 210 °C, or 180 - 205 °C, 180 - 200 °C, or 180 - 195 °C, or 180 - 190 °C, or 185 - 215 °C, or 185 - 210 °C, or 185 - 205 °C, 185 - 200 °C, or 185 - 195 °C). Providing a high-temperature effluent can contribute to the separation efficiency in subsequent separation operations.

[0061] In certain desirable embodiments, the temperature of the effluent at the outlet of the second reaction zone is at least 3 °C higher than the temperature of the product stream at the reaction zone inlet of the second reaction zone. For example, in certain embodiments, the temperature of the effluent at the outlet of the second reaction zone is at least 5 °C or at least 7 °C higher than the temperature of the product stream at the reaction zone inlet of the second reaction zone. In certain such embodiments, the temperature of the effluent at the outlet of the second reaction zone is not more than 40 °C higher than the temperature of the product stream at the reaction zone inlet of the second reaction zone (e.g., not more than 35 °C, or not more than 30 °C, or not more than 25 °C). For example, in certain embodiments as further described herein, the temperature of the effluent at the outlet of the second reaction zone is not more than 20 °C higher than the temperature of the product stream at the reaction zone inlet of the second reaction zone (e.g., not more than 18 °C, or not more than 15 °C).

[0062] This increase in temperature enables improved separation of acetic acid and other condensable components from the carbonylation catalyst and optional carbonylation cocatalyst in the flash separation zone. Accordingly, the vapor fraction from the flash separation zone will be more enriched in acetic acid, enabling a higher acetic acid yield to be achieved. Further, the volume and flow rate of the liquid fraction will be reduced.

[0063] At least a portion, and desirably substantially all, of the temperature increase between the product composition of the first reaction zone and the effluent of the second reaction zone is provided by the carbon monoxide reaction in the second reaction zone (i.e., the carbon monoxide in the first and second vapor streams and any carbon monoxide dissolved and / or entrained in the product composition provided by the first reaction zone). Heat may optionally be applied to the second reaction zone to further increase the temperature increase of the liquid reaction composition. Accordingly, in one embodiment as further described herein, heat is applied to the second reaction zone. However, in certain desirable embodiments, no heat is applied to the second reaction zone and substantially all of the temperature increase in the second reaction zone is provided by the carbon monoxide reaction therein.

[0064] One of ordinary skill in the art can determine the flow rates of the first and second vapor streams based on the disclosure herein to provide the desired temperature increase. For example, in certain embodiments as further described herein, the weight ratio of the amount of the first vapor stream introduced at the first location to the amount of the product stream directed to the second reaction zone is no greater than 1:5, such as 1:1000 - 1:5, or 1:750 - 1:5, or 1:500 - 1:5, or 1:200 - 1:5. Similarly, in certain embodiments as further described herein, the weight ratio of the amount of the second vapor stream introduced at the second location to the amount of the product stream directed to the second reaction zone is 1:200 - 1:5. In certain embodiments as further described herein, the weight ratio of the amount of the first vapor stream introduced at the first location to the amount of the second vapor stream introduced at the second location is 5:1 - 1:5. Of course, one of ordinary skill in the art will recognize that in some cases, the various amounts may deviate from these general ranges.

[0065] The first vapor stream has a higher carbon monoxide concentration than the second vapor stream. For example, in certain embodiments as further described herein, the first vapor stream is carbon monoxide-rich, such as having at least 70 wt.% or even at least 80 wt.% carbon monoxide. For example, in certain such embodiments, the first vapor stream comprises at least 90 wt.% carbon monoxide (such as at least 95 wt.%, or at least 98 wt.%, or at least 99 wt.%). A variety of carbon monoxide sources can be used to provide the desired purity of carbon monoxide, including the same source used to introduce carbon monoxide into the first reaction zone.

[0066] The second vapor stream has a carbon monoxide concentration lower than that of the first vapor stream. Thus, its carbon monoxide content may be relatively low, such as 10 - 70 wt.% carbon monoxide. In certain embodiments as further described herein, the second vapor stream comprises 10 - 60 wt.%, or 10 - 50 wt.%, or 10 - 40 wt.%, or 10 - 30 wt.%, or 20 - 70 wt.%, or 20 - 60 wt.%, or 20 - 50 wt.%, or 30 - 70 wt.% or 30 - 60 wt.% carbon monoxide. For example, in certain embodiments, the second vapor stream comprises one or more of water, acetic acid, and methanol and their reactive derivatives. Advantageously, the second vapor stream can be provided by a vapor process stream from another part of the process or even from a different process. In certain embodiments, the second vapor stream comprises at least a portion of the effluent stream from the first reaction zone and comprises carbon monoxide, water, and acetic acid. In certain embodiments, at least 50 wt.%, at least 75 wt.% or even at least 90 wt.% of the second vapor stream can be provided by the effluent stream from the first reaction zone.

[0067] The carbon monoxide concentrations of the first and second vapor streams can be selected based on the disclosure herein. For example, in certain embodiments, the carbon monoxide concentration of the first vapor stream is at least 5 wt.% higher (such as at least 10 wt.%, at least 15 wt.%) than the carbon monoxide concentration of the second vapor stream. In certain embodiments as further described herein, the carbon monoxide concentration of the first vapor stream is at least 20 wt.% or at least 25 wt.% higher than the carbon monoxide concentration of the second vapor stream.

[0068] The effluent from the second reaction zone contains water and acetic acid. In certain embodiments as further described herein, the effluent from the second reaction zone comprises 70 - 95 wt.% (such as 70 - 90 wt.% or 75 - 95 wt.%) acetic acid. It may be desirable for carbon monoxide to react substantially in the first and second reaction zones; in certain desirable embodiments as further described herein, the effluent from the second reaction zone comprises no more than 2 wt.% (such as no more than 1 wt.% or even no more than 0.5 wt.%) carbon monoxide. However, it may be desirable to have a certain amount of CO remaining in the effluent to reduce the amount of solid deposits. For example, in certain embodiments, the effluent comprises at least 0.1 wt.% carbon monoxide. One of ordinary skill in the art will select the effluent CO concentration for the particular system and method at hand and should recognize that the amount of carbon monoxide remaining in the effluent will depend on, for example, the amount of carbon monoxide in the first and second vapor streams and the reaction rate of carbon monoxide in the second reaction zone. Thus, a variety of other effluent carbon monoxide concentrations are possible.

[0069] In certain embodiments as further described herein, the effluent from the second reaction zone further comprises one or more of methyl iodide, methyl acetate, and a Group VII metal carbonylation catalyst. For example, in certain such embodiments, the effluent from the second reaction zone comprises 5 - 95 wt.% (such as 70 - 95 wt.%, or 70 - 90 wt.%, or 75 - 95 wt.%) acetic acid, 0.1 - 70 wt.% (such as 0.1 - 25 wt.% or 0.1 - 20 wt.%) methyl acetate, 0.1 - 15 wt.% water, 10 - 6000 ppmw of a Group VIII metal carbonylation catalyst, up to 20 wt.% (such as 1 - 20 wt.%) methyl iodide, and 200 - 2500 ppmw propionic acid.

[0070] The effluent from the second reaction zone can be directed to a flash separation zone (e.g., via a flash valve 62), where it is separated into an acetic acid-rich vapor stream and an acetic acid-lean liquid stream. The liquid stream can be introduced (i.e., recycled) into the first reaction zone. For example, referring to Figure 1 , the effluent 50 is directed to a flash separation zone 60 to form a vapor fraction 70 and a liquid fraction 75. At least a portion of the liquid fraction 75 can be introduced into the first reaction zone 10.

[0071] Flash separation zones are known in the art. In certain embodiments, the flash separation zone can include an adiabatic flash vessel. Alternatively or additionally, the flash separation zone can be heated, for example, by a heater. The flash separation zone can generally be operated at a pressure in the range of 0 - 10 barg, preferably 0 - 3 barg.

[0072] As described above, improved separation in the flash separation zone can result in a reduced volume and flow rate of the liquid fraction. Thus, in the case of recycling at least a portion of the liquid fraction to the first reaction zone, the reduced flow rate of the liquid fraction results in less cooling in the first reaction zone, thereby reducing the energy requirements of the process. Additionally, due to the reduced flow rate of the liquid fraction, the flow rates of the liquid reaction composition flowing from the first reaction zone to the second reaction zone and the liquid reaction composition flowing from the second reaction zone to the flash separation zone can also be reduced. As a result, the amount of the carbonylation catalyst and optionally the carbonylation cocatalyst flowing to the flash separation zone per unit time can be reduced; and, since the vapor fraction is rich in acetic acid, the amount of the catalyst and optionally the promoter flowing to the flash separation zone per unit of acetic acid produced can also be reduced.

[0073] The carbonylation process as described herein can be carried out as a batch process or as a continuous process. In certain desirable embodiments, the carbonylation process is carried out as a continuous process.

[0074] In certain embodiments, at least a portion of the vapor fraction from the flash separation zone is directed to the light fraction recovery zone of the acetic acid production unit. In certain embodiments, the light fraction recovery zone of the acetic acid production unit is configured to separate at least the more volatile components than acetic acid from acetic acid (e.g., the vapor fraction discharged from the tank as further described herein). For example, in certain embodiments, acetic acid is produced in a reaction zone by carbonylation of methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the light fraction recovery zone of the acetic acid production unit is configured to separate acetic acid and further separate methyl iodide and methyl acetate, which can be recycled to the reaction zone.

[0075] In certain embodiments as further described herein, the light fraction recovery zone includes a distillation column that separates a crude acetic acid product containing acetic acid and propionic acid from a light fraction distillate including methyl iodide and methyl acetate. In certain such embodiments, the light fraction recovery zone further includes a drying column. For example, in certain embodiments as further described herein, the light fraction recovery zone includes a combined light fraction and drying column, where water is removed from the crude acetic acid product to form a dry acetic acid product containing acetic acid and propionic acid. As used herein, a "dry" or "dried" stream containing acetic acid (e.g., and optionally propionic acid) contains up to 1500 ppmw of water.

[0076] Suitable columns and their configurations for use in the light fraction recovery zone are generally known in the art. Generally, at least a first fraction containing acetic acid and propionic acid and a top vapor fraction containing methyl acetate, water, acetic acid, carbon monoxide, and methyl iodide are formed in the light fraction recovery zone. In certain embodiments, the separated water can be recycled to the reaction zone or removed from the acetic acid production unit.

[0077] In certain embodiments as further described herein, the light fraction recovery zone further includes one or more condensers and / or coolers to condense the top vapor fraction and form a liquid fraction. Those of ordinary skill in the art will appreciate that any suitable method known in the art for condensing the top vapor fraction to a liquid phase can be used. For example, in certain embodiments, the fraction is condensed using at least one heat exchanger (e.g., supplying water as a cooling medium). The components of the uncondensed top fraction (e.g., carbon monoxide, carbon dioxide, inert gases, reaction by-product gases) are removed as a waste stream from the light fraction recovery zone. In certain embodiments, acetic acid is produced in a reaction zone by carbonylation of methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the waste stream removed from the light fraction recovery zone further includes methyl iodide (e.g., present as entrained and / or vaporized methyl iodide), methyl acetate, and water.

[0078] In certain embodiments, the liquid fraction formed in the light fraction recovery zone comprises methyl acetate, water, and acetic acid. In certain embodiments, acetic acid is produced in the reaction zone by carbonylation of methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the liquid fraction further comprises methyl iodide. In certain embodiments, the liquid fraction further comprises entrained or dissolved gas components (e.g., carbon monoxide, carbon dioxide, inert gases).

[0079] In certain embodiments as further described herein, the light fraction recovery zone comprises a decanter in which the liquid fraction separates into two layers: a lower (e.g., organic) layer comprising methyl acetate and an upper (e.g., aqueous) layer comprising water. In certain embodiments, acetic acid is produced in the reaction zone by carbonylation of methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the lower layer further comprises methyl iodide. In certain embodiments, at least a portion (e.g., all) of the upper layer from the decanter is returned as reflux to the distillation column of the light fraction recovery zone. In certain embodiments, at least a portion (e.g., all) of the upper layer from the decanter is recycled to the reaction zone. In certain embodiments, off-gas is vented from the decanter and transferred to an off-gas scrubbing unit (e.g., prior to disposal).

[0080] In certain embodiments, a stream comprising acetic acid and propionic acid from the light fraction recovery section (e.g., the first fraction formed in the light fraction recovery section) is transferred to the heavy fraction column through a feed inlet located at a midpoint of the column. In such embodiments, the stream comprising propionic acid is discharged from the heavy fraction column through a heavy product outlet, and acetic acid is removed as a product stream at one or more outlets of the column (e.g., as an overhead stream from the top outlet of the column, as a side draw stream from an outlet located above the feed inlet). In certain embodiments, the product stream substantially comprises acetic acid and comprises less than 400 ppmw, or less than 300 ppmw, or less than 250 ppmw of propionic acid. In certain embodiments, the product stream substantially comprises acetic acid and comprises less than 1500 ppmw of water. In certain desirable embodiments, the product stream substantially comprises acetic acid and comprises less than 1,500 ppmw of the sum of propionic acid and water. Suitable columns and their configurations for use as the heavy fraction column are generally known in the art. For example, in certain embodiments, the heavy fraction column is connected to a condenser. In another example, in certain embodiments, a reboiler is connected to the bottom of the heavy fraction column.

[0081] Another aspect of the present disclosure is a system for producing acetic acid, such as using the methods described herein. Referring to Figure 1Describe an embodiment of the system. The system includes a first reaction zone 10 having one or more inlets 12 and 14 respectively communicating with a carbon monoxide source 2 and a methanol source 4, an exhaust gas outlet 16, and a product stream outlet 18; a second reaction zone 30 having a reaction zone inlet 32 and an outlet 34, the reaction zone inlet being in fluid communication with the product stream outlet 18 of the first reaction zone, the second reaction zone including a first vapor stream inlet at a first position 36 between the reaction zone inlet and the outlet (the vapor stream inlet communicating with a first vapor stream source 40 rich in carbon monoxide) and a second vapor stream inlet at a second position 38 between the reaction zone inlet and the outlet (the second vapor stream inlet communicating with a second vapor stream source 45 poor in carbon monoxide).

[0082] The system of this aspect of the present disclosure may be substantially as described above in terms of the method of the present disclosure. For example, in certain embodiments as further described herein, the first position is between the reaction zone inlet and the second position. In certain embodiments as further described herein, the distance between the reaction zone inlet and the second position is at least 50% (such as at least 60% or at least 70%) of the distance between the reaction zone inlet and the outlet of the second reaction zone. In certain embodiments as further described herein, the distance between the inlet and the first position is not greater than 25% (such as not greater than 15% or not greater than 10%) of the distance between the reaction zone inlet and the outlet of the reaction zone.

[0083] In the following examples, acetic acid is produced by carbonylation of methanol with carbon monoxide in the presence of an iridium catalyst and a ruthenium promoter, using a system similar to Figure 1 (although the introduction positions of the first vapor stream and the second vapor stream are different), and simulations are performed using a rigorously kinetic-driven ASPEN PLUS (trademark) (version 7.3) computer model. In the simulation, the first reaction zone includes a primary carbonylation tank reactor, the second reaction zone includes a secondary plug flow reactor with a reduced volume compared to the primary carbonylation stirred tank reactor, the volume being such that the residence time of the second reaction zone (based on the liquid fed only to the secondary reaction zone) is maintained at a predetermined residence time, and the flash separation zone includes an adiabatic flash vessel. The liquid reaction composition in the primary reactor is 2 wt.% catalyst plus promoter (modeled as an inert heavy substance), 4 wt.% water, 7 wt.% methyl iodide, and 10 wt.% methyl acetate. The operating pressure of the primary reactor is 3 × 10 6 Nm -2 ,and the temperature of the primary reactor is maintained at about 189 °C. Carbon monoxide is modeled as being supplied to the primary reactor via a spray at the bottom. The adiabatic flash vessel is at 2.38 × 10 5 Nm -2Operate under pressure. High-pressure waste gas is removed from the top of the primary reactor. The model includes kinetic expressions to provide reaction rates that depend on the CO composition. For example, under the conditions of this simulation process, when pure CO is used, the initial reaction rate at the first increment of the first part of the second reaction zone equipment provides a reaction rate of approximately 24 mol / l / hr based on the total available volume.

[0084] A series of simulations are performed using the above parameters to determine the conversion of CO relative to pure CO dosed as the first vapor stream (as kg / hr production of acetic acid product), where a fixed flow of impure CO (approx. 30 mol% CO, the remainder being a mixture of condensable and non-condensable typical of high-pressure waste gas streams) is used as the second vapor stream, and a variable flow of pure CO is used as the first vapor stream (to reflect actual operation). Figures 2-4 The graph shows the relationship between the pure CO flow rate and the acetic acid production as a ratio to the production using only the second vapor stream (i.e., without introducing additional pure CO).

[0085] Figure 2 Shows the effect of one vapor stream introduced at the feed and another vapor stream introduced at 83% of the reactor length from the main feed (i.e., near the exit of the second reaction zone), where one set of data has the first vapor stream introduced at the feed and the second vapor stream introduced at the 83% point; and one point has the opposite configuration. The data is plotted as a function of the first vapor stream dosing amount. Figure 3 Similar to Figure 2 , but one vapor stream is introduced at the 50% position along the length of the second reaction zone. And Figure 4 is similar, but one vapor stream is introduced at the 17% position along the length of the second reaction zone (i.e., near the entrance of the second reaction zone). For all of the above examples, the impure CO contains approximately 30 mol% CO, the remainder being a mixture of condensable and non-condensable typical of high-pressure waste gas streams. In all cases, the modeling shows that adding the impure CO second results in a relatively high acetic acid yield, and introducing the purer CO first vapor stream at an earlier point in the second reaction zone provides better results.

[0086] The entire content of each patent and non-patent disclosure cited herein is hereby incorporated by reference, and shall be considered to be in accordance with the disclosure or definition herein, unless inconsistent with any disclosure or definition in this specification.

[0087] The foregoing specific embodiments and drawings have been provided by way of illustration and explanation, and are not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments described herein will be apparent to those of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0088] It should be understood that the elements and features recited in the appended claims may be combined in different ways to yield new claims that also fall within the scope of the present disclosure. Accordingly, although the following appended dependent claims depend only on a single independent or dependent claim, it should be understood that these dependent claims may alternatively depend on any preceding claim—whether independent or dependent—and that such new combinations should be understood to form part of this specification.

Claims

1. A method for producing acetic acid, which comprises contacting carbon monoxide with one or more of methanol, methyl acetate, dimethyl ether, and methyl iodide in a first reaction zone in the presence of a carbonylation catalyst to produce a product stream and a waste stream, the product stream comprising water, acetic acid, carbon monoxide, the carbonylation catalyst, and one or more of methanol, methyl acetate, dimethyl ether, and methyl iodide; transferring at least a portion of the product stream through an inlet of a second reaction zone into the second reaction zone; introducing a first vapor stream into the second reaction zone at a first location between the inlet and the outlet of the second reaction zone, the first vapor stream comprising carbon monoxide and having a carbon monoxide concentration; introducing a second vapor stream into the second reaction zone at a second location between the inlet and the outlet of the second reaction zone, the second vapor stream comprising carbon monoxide and having a carbon monoxide concentration lower than that of the first vapor stream; and discharging the effluent of the second reaction zone through the outlet, and further, wherein the first location is between the inlet of the reaction zone and the second location.

2. The method of claim 1, wherein the distance between the inlet of the reaction zone and the second location is at least 50% of the distance between the inlet and the outlet of the second reaction zone.

3. The method of claim 1 or 2, wherein the distance between the inlet of the reaction zone and the first location is not greater than 25% of the distance between the inlet and the outlet of the second reaction zone.

4. The method of claim 1 or 2, wherein the temperature of the product stream at the inlet of the second reaction zone is in the range of 170 - 215 °C.

5. The method of claim 1 or 2, wherein the temperature of the effluent at the outlet of the second reaction zone is in the range of 175 - 215 °C.

6. The method of claim 1 or 2, wherein the temperature of the effluent at the outlet of the second reaction zone is at least 3 °C higher than the temperature of the product stream at the inlet of the second reaction zone.

7. The method of claim 1 or 2, wherein the weight ratio of the amount of the first vapor stream introduced at the first location to the amount of the product stream introduced into the second reaction zone is 1:1000 - 1:

5.

8. The method of claim 1 or 2, wherein the weight ratio of the amount of the second vapor stream introduced at the second location to the amount of the product stream introduced into the second reaction zone is 1:200 - 1:

5.

9. The method of claim 1 or 2, wherein the weight ratio of the amount of the first vapor stream introduced at the first location to the amount of the second vapor stream introduced at the second location is 5:1 - 1:

5.

10. The method of claim 1 or 2, wherein the first vapor stream comprises at least 90 wt.% carbon monoxide.

11. The method of claim 1 or 2, wherein the second vapor stream comprises 10 - 70 wt.% carbon monoxide.

12. The method of claim 1 or 2, wherein the carbon monoxide concentration in the first vapor stream is at least 5 wt.% higher than the carbon monoxide concentration in the second vapor stream.

13. The method of claim 1 or 2, wherein the carbon monoxide concentration in the first vapor stream is at least 15 wt.% higher than the carbon monoxide concentration in the second vapor stream.

14. The method of claim 1 or 2, wherein the second vapor stream further comprises water, acetic acid, and one or more of methanol, methyl acetate, dimethyl ether, and methyl iodide.

15. The method of claim 1 or 2, wherein the second vapor stream comprises at least a portion of an exhaust gas stream comprising carbon monoxide, water, and acetic acid.

16. The method of claim 1 or 2, wherein the effluent from the second reaction zone comprises 70 - 95 wt.% acetic acid.

17. The method of claim 1 or 2, wherein the effluent from the second reaction zone comprises no more than 1 wt.% carbon monoxide.

18. The method of claim 1 or 2, which comprises separating at least a portion of the effluent from the second reaction zone in a flash separation zone into a vapor stream rich in acetic acid and a liquid stream poor in acetic acid.

19. The method of claim 18, which further comprises introducing at least a portion of the liquid stream into the first reaction zone.

20. A system for producing acetic acid, which comprises a first reaction zone having one or more inlets in communication with a methanol source and a carbon monoxide source, an exhaust gas outlet, and a product stream outlet; a second reaction zone having a reaction zone inlet and an outlet, the reaction zone inlet being in fluid communication with the product stream outlet of the first reaction zone, the second reaction zone comprising a first vapor stream inlet at a first position between the reaction zone inlet and the outlet, the vapor stream inlet being in communication with a first vapor stream source rich in carbon monoxide, and a second vapor stream inlet at a second position between the reaction zone inlet and the outlet, the second vapor stream inlet being in communication with a second vapor stream source poor in carbon monoxide, and further wherein the first position is between the reaction zone inlet and the second position.

21. The system of claim 20, wherein the distance between the reaction zone inlet and the second position is at least 50% of the distance between the reaction zone inlet and the outlet of the second reaction zone.

22. The system of claim 20 or 21, wherein the distance between the inlet and the first position is not greater than 25% of the distance between the reaction zone inlet and the outlet of the reaction zone.

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