Methods for treating waste gas from acetic acid production units

Abstract

This disclosure generally relates to a method for washing a waste gas stream from an acetic acid production unit. In one aspect, the method includes introducing the waste gas stream into an absorption tower; introducing a methanol stream into the absorption tower at a first flow rate, the methanol stream having a first temperature at a liquid inlet, the first temperature being at least 18°C ​​(e.g., at least 20°C or at least 22°C); contacting the waste gas stream with the methanol stream in the absorption tower; removing a first liquid effluent from the absorption tower through a liquid outlet of one or more liquid outlets, the first liquid effluent comprising methanol and methyl iodine; and removing a vapor effluent from the absorption tower through a vapor outlet.

Description

[0001] Public background

[0002] 1. Field

[0003] This disclosure generally relates to waste gas treatment methods that can be used in acetic acid production. 2. Technical Background

[0005] For many years, acetic acid has been commercially prepared by carbonylating methanol with carbon monoxide in the presence of a Group VIII carbonylation catalyst. Typically, carbon monoxide and methanol are contacted in one or more reaction zones in the presence of a homogeneous or heterogeneous rhodium or iridium carbonylation catalyst, methyl iodine, and water. Generally, the acetic acid product can be recovered by removing crude acetic acid product from the reactor and separating it from other components such as the Group VIII metal carbonylation catalyst, methyl iodine, methyl acetate, and water in one or more flash evaporation and / or distillation stages. Acetic anhydride is typically provided as a byproduct.

[0006] In methods for preparing acetic acid and / or jointly preparing acetic acid and acetic anhydride, waste gases are typically removed at several stages of the process, such as from one or more stages in the reactor and distillation phases. Waste gas removal is necessary to maintain the constant concentrations of undesirable gaseous reaction byproducts and inert gases at acceptable levels.

[0007] The exact composition of the exhaust gas will vary depending on the specific carbonylation process conditions used, but it typically contains carbon monoxide, inert gases and reaction byproduct gases, iodides (mainly methyl iodine), and may also contain low amounts of methyl acetate, acetic acid, and water.

[0008] Exhaust gases are typically treated by washing with a suitable scrubbing solvent to recover valuable components, such as methyl iodine, which can eventually be returned to the reactor. The scrubbing exhaust gas, containing inert and byproduct gases, is usually combusted. Various scrubbing solvents can be used, such as acetic acid or methanol. In such a scrubbing process, methyl iodine is absorbed in the scrubbing solvent, and the exhaust gas containing a reduced amount of methyl iodine is removed from the scrubbing unit, typically as overhead. A conventional process, described in International Patent Application Publication No. 2015 / 193328, uses a methanol absorber and an acetic acid absorber positioned in series. In such a conventional system, these towers are used to ensure that the methyl iodine content in the overhead stream fed to the flare remains at a low level (e.g., 35 parts per million (ppmv) by volume). The acetic acid absorber can be used during start-up and shutdown processes while the equipment awaits methanol availability. The methanol absorber performs most of the methyl iodine washing during normal operation; during such normal operation, the liquid flow through the acetic acid absorber can be reduced to a low "tick over" flow rate for availability when needed, while allowing most of the acetic acid product stream to enter the light fractionation column.

[0009] However, it remains desirable to provide an alternative method for washing the waste gas streams generated during the production of acetic acid, and especially for the waste gas streams generated during the production of acetic acid via methanol carbonylation.

[0010] Overview

[0011] In one aspect, this disclosure provides a method for washing a waste gas stream containing carbon monoxide and methyl iodine in an absorption tower of an acetic acid production unit, the absorption tower comprising...

[0012] The bottom portion includes a feed inlet and one or more liquid outlets; and

[0013] The top portion, above the bottom portion, includes one or more liquid inlets and vapor outlets;

[0014] The method includes:

[0015] The waste gas flow is introduced into the absorption tower through the feed inlet;

[0016] A methanol feed stream is introduced at a first flow rate through one or more liquid inlets, the methanol feed stream having a first temperature at the liquid inlet, the first temperature being at least 18°C ​​(e.g., at least 20°C or at least 22°C).

[0017] In the absorption tower, the waste gas stream is brought into contact with the methanol stream;

[0018] A first liquid effluent, comprising methanol and methyl iodine, is extracted from the absorber through one or more liquid outlets.

[0019] The vapor effluent is removed from the absorption tower through the vapor outlet.

[0020] The vapor effluent preferably contains up to 500 ppmv (parts per million by volume) methyl iodine (e.g., up to 350 ppmv, or up to 200 ppmv, or up to 100 ppmv).

[0021] Other aspects of this disclosure will be apparent to those skilled in the art from the following description. Brief description of the attached diagram

[0023] Figure 1 This is a schematic diagram of the method disclosed herein.

[0024] Detailed description

[0025] The inventors have noted that under non-steady-state conditions, such as during plant start-up, plant shutdown, or plant trip / failure, the supply of methanol to the reactor and / or scrubbing unit often becomes limited and may even cease completely in acetic acid production processes. This is disadvantageous for acetic acid production processes that use methanol as a waste gas scrubbing solvent. Switching systems exist, such as those described in International Patent Application 2009 / 134332, which provide the use of different scrubbing solvents in a single scrubbing tower, which can reduce capital requirements and operating costs.

[0026] Typically, the efficiency of solvent-based scrubbing of the exhaust gas is maximized by using a scrubbing solvent that has been cooled before use. The scrubbing solvent can be cooled, for example, by passing it through a heat exchange unit configured to lower the solvent's temperature before it is used in the scrubbing unit. Typically, the temperatures of both acetic acid and methanol are lowered before scrubbing the exhaust gas to improve their scrubbing efficiency. For example, when acetic acid freezes at 16.7°C, a slightly higher temperature (e.g., ~24°C) is often used as the setpoint for feeding acetic acid into the absorber to address the risk of freezing while ensuring sufficient scrubbing efficiency to ensure the removal of methyl iodine under various conditions. In contrast, methanol has a freezing point close to -100°C, so freezing is not a major concern. Instead, the setpoint for feeding methanol into the absorber is typically set to a low but practical value (e.g., ~5°C) based on other system parameters (e.g., the temperature of the cooling system).

[0027] The inventors have noted that in systems using a single absorber for both methanol and acetic acid as washing solvents, when switching from methanol to acetic acid, it is necessary to wait for the absorber to heat up above the freezing point of acetic acid to prevent it from freezing within the tower. Acetic acid can freeze within the tower due to the packing and materials of the tower itself, which have already been cooled by contact with the colder methanol washing solvent. Freezing of acetic acid in the absorber is highly undesirable because it reduces washing efficiency or even leads to complete loss of washing, thereby releasing methyl iodine into the atmosphere.

[0028] The inventors have unexpectedly determined that methanol washing can be performed at temperatures significantly higher than those conventionally used. Notably, methanol washing can be carried out at temperatures above the freezing point of acetic acid, meaning there is no risk of acetic acid freezing in the column during solvent switching.

[0029] In various aspects, the methods disclosed herein provide for washing waste gas streams in acetic acid production units. Specifically, one aspect of this disclosure is a method for washing a waste gas stream containing carbon monoxide and methyl iodine in an absorption tower of an acetic acid production unit. The absorption tower includes:

[0030] The bottom portion includes a feed inlet and a liquid outlet; and

[0031] The top portion, above the bottom portion, includes one or more liquid inlets and vapor outlets.

[0032] The method includes

[0033] The waste gas flow is introduced into the absorption tower through the feed inlet;

[0034] A methanol feed stream is introduced at a first flow rate through one or more liquid inlets, the methanol feed stream having a first temperature at the inlet, the first temperature being at least 18°C ​​(e.g., at least 20°C, or at least 22°C, or at least 24°C).

[0035] In the absorption tower, the waste gas stream is brought into contact with the methanol stream;

[0036] A first liquid effluent, comprising methanol and methyl iodine, is extracted from the absorber through one or more liquid outlets.

[0037] The vapor effluent is removed from the absorption tower through the vapor outlet.

[0038] The vapor effluent preferably contains up to 500 ppmv (e.g., up to 350 ppmv, or up to 200 ppmv, or up to 100 ppmv) of methyl iodine. (The vapor effluent is a scrubbed waste gas and typically contains carbon monoxide as well as carbon dioxide and nitrogen.)

[0039] Therefore, one embodiment of this disclosure is referenced. Figure 1 The acetic acid production unit 100 includes an absorption tower 110, comprising a bottom portion 120 having a feed inlet (or waste gas inlet) 122 and one or more liquid outlets (here, liquid outlets 124 and 126); and a top portion 130 above the bottom portion having one or more liquid inlets (here, liquid inlets 132 and 134) and a vapor outlet 136. Waste gas stream 142 is introduced into the absorption tower through the feed inlet. Methanol stream 152, having a first temperature at the liquid inlet, is introduced through one of the one or more liquid inlets, here through liquid inlet 132. Notably, the first temperature is at least 18°C. In the absorption tower, the waste gas stream contacts the methanol stream, thereby washing the waste gas stream with methanol. A first effluent 144, containing methanol and methyl iodine, is withdrawn from the absorption tower through one of the one or more liquid outlets, here liquid outlet 124. Vapor effluent 156, i.e., the washed waste gas, is withdrawn through vapor outlet 136.

[0040] The waste gas stream can be provided by any of the various operations within the acetic acid production unit. For example, in some embodiments further described herein, the waste gas stream includes at least a portion of one or more gaseous effluents from the separation zone, the light fraction recovery zone of the acetic acid production unit, and the reaction zone of the acetic acid production unit. This is in Figure 1 The implementation scheme shows that: the gaseous effluent 186 of the flash separation zone 185; the gaseous effluent 191 of the light fraction recovery zone 190; and the gaseous effluent 161 of the reaction zone 160 are provided to the waste gas stream 142 introduced into the absorption tower 110.

[0041] The first temperature (i.e., the temperature of the methanol feed at the liquid inlet) is higher than the freezing point of acetic acid. Therefore, even at its coldest point, the column packing and other structures will be above the freezing point of acetic acid, and thus, if acetic acid is allowed to enter the column as a washing solvent (e.g., in the absence of a sufficient methanol feed), it will not freeze in the column. In some embodiments further described herein, the first temperature is at least 20°C, for example at least 22°C, or at least 24°C. In some embodiments further described herein, the first temperature is from 18°C ​​to 35°C, for example, 20°C to 35°C, or 22°C to 35°C, or 24°C to 35°C. In some embodiments further described herein, the first temperature is from 18°C ​​to 30°C, for example, 20°C to 30°C, or 22°C to 30°C, or 24°C to 30°C. In some embodiments further described herein, the first temperature is from 18°C ​​to 28°C, for example, 20°C to 28°C, or 22°C to 28°C.

[0042] The first liquid effluent includes methanol and methyl iodine (washed from the waste gas stream). This stream can be fed to the reaction zone of the acetic acid production unit as a reactant in the carbonylation reaction of methanol with carbon monoxide over a zeolite catalyst. For example, in Figure 1 In one embodiment, the first liquid effluent 144 is conveyed to the reaction zone 160. In another embodiment, the first liquid effluent conveyed to the reaction zone of the acetic acid production unit may include up to 250 ppmv (e.g., up to 200 ppmv, or up to 150 ppmv, or up to 100 ppmv, or up to 50 ppmv, or up to 25 ppmv) of acetic acid. For example, in some desirable embodiments, the first liquid effluent conveyed to the reaction zone of the acetic acid product unit is substantially free of acetic acid.

[0043] Surprisingly, the inventors have determined that methanol can be used to effectively wash methyl iodine from the waste gas stream in an acetic acid production unit, even at temperatures much higher than typically expected. In some embodiments further described herein, the vapor effluent (removed when the methanol stream is introduced into the absorber) has up to 500 ppmv methyl iodine, or up to 350 ppmv methyl iodine, or up to 200 ppmv methyl iodine, or up to 100 ppmv methyl iodine, or even up to 50 ppmv methyl iodine.

[0044] However, the higher temperatures of the methanol washing solvent can result in a relatively higher methanol concentration in the vapor effluent than usual. Heat exchangers (e.g., overhead condensers) can be used to separate the vapor effluent, providing a vapor fraction and a liquid fraction including methanol. Figure 1 In one embodiment, the vapor effluent 156 is condensed in the overhead condenser 165, providing a vapor stage 166 and a liquid stage 167 comprising methanol. Figure 1 As shown, the liquid fraction can be a portion of the methanol feed stream introduced into the absorber, such that the methanol feed stream includes at least a portion of a liquid fraction comprising methanol. Figure 1 As shown, the vapor fraction may be fed to a combustion system (180) and combusted therein (e.g., as a flare). In some desirable embodiments, the vapor fraction contains up to 2.5% by weight (e.g., up to 2.25% by weight, or up to 2.0% by weight, or up to 1.75% by weight, or up to 1.5% by weight) of methanol.

[0045] In many cases, it would be desirable to wash with acetic acid before washing with methanol. This is typically the case during process start-up, when there may not be enough methanol available for washing. Therefore, in some embodiments further described herein, the method further includes, before introducing the methanol feed stream into the absorber,

[0046] The waste gas stream is conveyed to the bottom part of the absorption tower through the feed inlet;

[0047] An acetic acid stream, having a second flow rate, is conveyed to the top portion of the absorption tower through one or more liquid inlets, the acetic acid stream having a second temperature at the inlet; and

[0048] In the absorption tower, the waste gas stream is brought into contact with the acetic acid stream.

[0049] The method further includes, after the first time period of contact between the waste gas stream and the acetic acid stream,

[0050] Reduce the flow rate of the acetic acid feed (e.g., to approximately zero flow rate) and

[0051] The methanol feed stream is introduced into the absorption tower to transition from acetic acid to methanol.

[0052] Preferably, the transition ensures that the weight ratio of methanol to acetic acid in the absorption tower is at least 500:1, while simultaneously bringing the waste gas stream into contact with the methanol stream. The second temperature is at least 18°C.

[0053] Therefore, in some other embodiments described herein (and in Figure 1 In the system identified, acetic acid is used as a washing solvent before methanol is introduced into the tower. Waste gas stream 142 is introduced into the bottom portion 120 of the absorber 110 through feed inlet 122. Acetic acid stream is introduced into the top portion of the absorber at a second flow rate through a liquid inlet (which may be the same or different from the liquid inlet through which the methanol stream is introduced later). Figure 1 In one embodiment, acetic acid stream 154 is introduced into the top portion 130 of absorber 110 through liquid inlet 134. The acetic acid stream has a second temperature at the liquid inlet; notably, this second temperature is at least 18°C. The waste gas stream comes into contact with the acetic acid stream in absorber 110. After a first time period of contact between the waste gas stream and the acetic acid stream, the washing solvent can be switched from acetic acid to methanol. Thus, the flow rate of the acetic acid stream can be reduced (e.g., to about zero), and the methanol stream can be introduced into the tower as described above. With the switching of the solvent introduced into the tower, the liquid contents of the absorber can be changed to be predominantly methanol, for example, such that after the transition between acetic acid and methanol, the weight ratio of methanol to acetic acid in the absorber is at least 500:1 (e.g., at least 1,500:1, or at least 4,000:1) while the waste gas stream is in contact with the methanol stream. This transition time is preferably relatively fast; those skilled in the art can switch between introducing the acetic acid stream and introducing the methanol stream within minutes or even seconds. However, depending on the volume of the absorber, it may take a little longer to establish a new high methanol / acetic acid ratio.

[0054] The second temperature (i.e., the temperature of the acetic acid feed at the liquid inlet) is higher than the freezing point of acetic acid. In some embodiments further described herein, the second temperature is at least 20°C, for example at least 22°C, or at least 24°C. In some embodiments further described herein, the second temperature is from 18°C ​​to 50°C, for example 18°C ​​to 45°C, or 18°C ​​to 40°C, or 20°C to 50°C, or 20°C to 45°C, or 20°C to 40°C, or 22°C to 50°C, or 22°C to 45°C, or 22°C to 40°C, or 24°C to 50°C, or 24°C to 45°C, or 24°C to 40°C.

[0055] The second temperature (i.e., the temperature of the acetic acid feed stream) can be conveniently set to be approximately the same as or higher than the first temperature (i.e., the temperature of the methanol feed stream). For example, in some embodiments further described herein, the second temperature differs from the first temperature by at most 5°C (e.g., at most 3.5°C, or at most 2°C). Of course, the process can also advantageously be operated at a higher temperature difference; in other embodiments, the second temperature is 5-25°C higher than the first temperature (e.g., 7.5-25°C, or 10-25°C, or 15-25°C, or 5-15°C, or 7.5-15°C, or 10-15°C).

[0056] In some embodiments, as further described herein, while the acetic acid stream is introduced into the absorber, a second liquid effluent is withdrawn from the absorber through one or more liquid outlets. This liquid outlet may be the same as or different from the liquid outlet through which the first liquid effluent is withdrawn; when the two are the same, those skilled in the art can provide the system with valves to allow the second liquid effluent to be directed to a portion of the system different from the first liquid effluent. The second liquid effluent includes acetic acid and methyl iodine (i.e., washed from the waste gas stream). Figure 1 In the system, the second liquid effluent 146 is withdrawn from the liquid outlet 126. The second liquid effluent can be conveyed to the light fraction recovery area of ​​the acetic acid production unit. Figure 1 (190 in the example), wherein acetic acid can be recovered as a product. In some such embodiments, the second liquid effluent delivered to the light fraction recovery zone of the acetic acid production unit has at least 25 ppmv (e.g., at least 50 ppmv, or at least 100 ppmv, or at least 150 ppmv, or at least 200 ppmv, or at least 250 ppmv) acetic acid.

[0057] In some embodiments further described herein, a vapor effluent is removed through a vapor outlet simultaneously with the introduction of the acetic acid feed stream into the absorber. In some desirable embodiments, this vapor effluent removed simultaneously with the introduction of the acetic acid feed stream into the absorber comprises up to 500 ppmv methyl iodine. In some desirable embodiments, it comprises up to 350 ppmv methyl iodine, up to 200 ppmv methyl iodine, up to 100 ppmv methyl iodine, or even up to 50 ppmv methyl iodine.

[0058] The acetic acid feed stream can be provided from a variety of sources. In some embodiments, the acetic acid feed stream includes at least a portion of the overhead feed stream from the heavy distillation column of the acetic acid production unit. For example, such as... Figure 1 As shown, the acetic acid feed stream 154 can be provided by the overhead feed stream 196 of the heavy distillation column 195.

[0059] In many cases, it would be desirable to wash with acetic acid after washing with methanol. This is typically the case in non-steady-state operating processes where there may not be enough methanol available for washing, such as during unit shutdowns or trips / failures during production. Therefore, in some embodiments further described herein, the method further includes, after introducing the methanol feed stream into the absorber,

[0060] Reduce the flow rate of methanol delivered to the absorption tower (e.g., to zero);

[0061] The waste gas stream is conveyed to the bottom part of the absorption tower through the feed inlet;

[0062] An acetic acid feed stream is delivered to the top portion of the column at a third flow rate through one or more liquid inlets, the acetic acid feed stream having a third temperature at the inlet; and

[0063] In the absorption tower, the waste gas stream is brought into contact with the acetic acid stream.

[0064] Preferably, after the transition from methanol to acetic acid, the weight ratio of methanol to acetic acid in the absorption tower does not exceed 1:500 (e.g., not exceeding 1:1500 or not exceeding 1:4000), while the waste gas stream is brought into contact with the acetic acid stream.

[0065] The third temperature is at least 18°C.

[0066] Therefore, in some other embodiments described herein (and in Figure 1 In the system identified, after methanol is introduced into the tower, acetic acid is used as a washing solvent. Waste gas stream 142 is introduced into the bottom portion 120 of the absorber 110 through feed inlet 122. Acetic acid stream is introduced into the top portion of the absorber at a third flow rate through a liquid inlet (which may be the same or different from the liquid inlet through which the methanol stream was introduced). Figure 1In one embodiment, acetic acid stream 154 is introduced into the top portion 130 of absorber 110 through liquid inlet 134. The acetic acid stream has a third temperature at the liquid inlet; notably, this third temperature is at least 18°C. The waste gas stream comes into contact with the acetic acid stream in absorber 110. After methanol is introduced into the absorber, the washing solvent can be switched from methanol to acetic acid. Thus, the flow rate of the methanol stream can be reduced, for example, to about zero, and the acetic acid stream can be introduced into the tower as described above. With the switching of the solvent introduced into the tower, the liquid contents of the absorber can be changed to be primarily acetic acid, for example, such that after the transition between acetic acid and methanol, the weight ratio of methanol to acetic acid in the absorber does not exceed 1:500 (e.g., not exceeding 1:1,500, or not exceeding 1:4,000) while the waste gas stream is contacted with the acetic acid stream. This transition time is preferably relatively fast; those skilled in the art can switch between introducing the methanol stream and introducing the acetic acid stream in a few minutes or even a few seconds. However, depending on the volume of the absorber, it may take a slightly longer time to establish a new low methanol / acetic acid ratio.

[0067] The third temperature (i.e., the temperature of the acetic acid feed at the liquid inlet) is higher than the freezing point of acetic acid. In some embodiments further described herein, the third temperature is at least 20°C, for example at least 22°C, or at least 24°C. In some embodiments further described herein, the third temperature is from 18°C ​​to 50°C, for example from 18°C ​​to 45°C, or from 18°C ​​to 40°C, or from 20°C to 50°C, or from 20°C to 45°C, or from 20°C to 40°C, or from 22°C to 50°C, or from 22°C to 45°C, or from 22°C to 40°C, or from 24°C to 50°C, or from 24°C to 45°C, or from 24°C to 40°C.

[0068] The third temperature (i.e., the temperature of the acetic acid feed stream) can conveniently be set to be approximately the same as or higher than the first temperature (i.e., the temperature of the methanol feed stream). For example, in some embodiments further described herein, the third temperature differs from the first temperature by at most 5°C (e.g., at most 3.5°C, or at most 2°C). Of course, the process can also advantageously be operated at a higher temperature difference; in other embodiments, the third temperature is 5-25°C higher than the first temperature (e.g., 7.5-25°C, or 10-25°C, or 15-25°C, or 5-15°C, or 7.5-15°C, or 10-15°C).

[0069] The waste gas generated in the reaction zone is typically referred to as high-pressure waste gas. Waste gas generated in the flash zone usually enters and is removed from the light fraction recovery section. The light fraction recovery section typically includes a light fractionation column, a condenser section including one or more condensers, and a phase separation vessel (decanter). Waste gas generated in the light fraction recovery section can be removed from the condenser section and / or from the decanter.

[0070] The waste gas generated from the light fraction recovery section is usually referred to as low-pressure waste gas.

[0071] High-pressure and low-pressure exhaust gas streams can be combined to produce low-pressure exhaust gas.

[0072] In some or all of the other embodiments described herein, the exhaust gas to be scrubbed is low-pressure exhaust gas.

[0073] Methyl iodine is commonly present in the waste gases to be scrubbed as described herein, typically as entrained and / or evaporated methyl iodine. The methyl iodine content in the waste gases to be scrubbed as described herein can vary widely; for example, methyl iodine can be present in the waste gases in amounts from about 1 mol% to about 20 mol%.

[0074] In addition to methyl iodine, the exhaust gas may contain one or more non-condensable components, such as carbon monoxide, inert gases such as nitrogen, and reaction byproduct gases such as hydrogen, carbon dioxide, and methane. The exhaust gas may also contain at least one of acetic acid, methyl acetate, and water.

[0075] The acetic acid production unit described herein is a unit that produces at least acetic acid, and includes units that co-produce acetic acid and acetic anhydride. Those skilled in the art will understand that various acetic acid production units can be configured to perform the washing methods described herein. Typically, an acetic acid production unit includes a reaction zone (e.g., including a reactor) and a light fraction recovery zone (e.g., including a light fractionation column) configured to recover acetic acid. Other reaction zones and recovery zones may be present. For example, a flash evaporation zone is typically used between the reaction zone and the light fraction recovery zone. Exhaust gas streams from any part of the acetic acid production unit can be washed as described herein. For example, production equipment for the preparation of acetic acid by the carbonylation reaction of methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst, and its operation, are familiar to those skilled in the art.

[0076] In the acetic acid production process described herein, methanol and / or its reactive derivatives may be introduced as liquid reactants into the reaction zone (i.e., to form a liquid reactive composition in the reaction zone). For example, in certain desirable embodiments of the methods further described herein, methanol is introduced into the reaction zone as a reactant. In other methods further described herein, one or more reactive derivatives are introduced into the reaction zone as reactants, or a combination of methanol and one or more reactive derivatives of methanol is introduced into the reaction zone as a reactant. As used herein, “reactive derivatives” of methanol are methyl acetate, dimethyl ether, and methyl iodide. In certain embodiments further described herein, methanol and / or methyl acetate are used as liquid reactants. In one embodiment further described herein, methanol is used as a reactant; in another embodiment further described herein, methyl acetate is used as a reactant; and in yet another embodiment further described herein, a mixture of methanol and methyl acetate is used as a reactant.

[0077] The methods described herein can utilize a variety of carbonylation catalysts, such as Group VIII noble metal carbonylation catalysts. The catalyst may include Group VIII compounds supported on an inert support, such as a carbon support. In some desirable embodiments further described herein, the carbonylation catalyst comprises rhodium, iridium, or mixtures thereof. In one specific embodiment of the invention, the carbonylation catalyst is iridium. In another specific embodiment further described herein, the carbonylation catalyst is a rhodium catalyst. A catalyst promoter may optionally be present, for example selected from alkali metal iodides (e.g., lithium iodide), alkaline earth metal iodides, aluminum group metal iodides, organoiodide salts, ruthenium, osmium, rhenium, and mixtures thereof. When the catalyst is a rhodium catalyst, the optional carbonylation catalyst promoter may be desirablely selected from alkali metal iodides, such as lithium iodide, alkaline earth metal iodides, aluminum group metal iodides and / or organoiodide salts, and mixtures thereof. When the catalyst is an iridium catalyst, the optional carbonylation catalyst promoter may be desirablely selected from ruthenium, osmium, rhenium, and mixtures thereof.

[0078] When the carbonylation catalyst is an iridium catalyst, the iridium catalyst may comprise any iridium-containing compound substantially soluble in the liquid reaction composition. The iridium catalyst may be added to the liquid reaction composition in any suitable form, either substantially soluble in the liquid reaction composition or convertible to a soluble form. The iridium catalyst is desirablely used as a chloride-free compound, such as an acetate, soluble in one or more components of the liquid reaction composition (e.g., water and / or acetic acid), and thus can be added to the reaction as a solution therein. Examples of suitable iridium-containing compounds that may be added to the liquid reaction composition include IrCl3, IrI3, IrBr3, [Ir(CO)2I]2, [Ir(CO)2Cl]2, [Ir(CO)2Br]2, and [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 hexachloroiridium acid H2[IrCl6], preferably chloride-free complexes of iridium, such as its acetate, oxalate and acetoacetate.

[0079] When present, the concentration of the iridium catalyst in the liquid reaction composition in the reaction zone can, for example, be independently 100 to 6000 ppm (by weight of iridium).

[0080] When the carbonylation catalyst is an iridium catalyst, the carbonylation catalyst promoter is preferably 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 desirablely used as a chloride-free compound, such as an acetate, which is soluble in one or more components of the liquid reaction composition (e.g., water and / or acetic acid) and can therefore be added to the reaction as a solution therein.

[0081] 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, ruthenium metal, ruthenium oxide, ruthenium(III) formate, and [Ru(CO)3I3]. - H + Ruthenium tetrachloro(II, III), ruthenium acetate(III), ruthenium propionate(III), ruthenium butyrate(III), ruthenium pentacarbonyl, ruthenium dodecyltriruthenium, and mixed halogenated carbonyl ruthenium such as dichlorotricarbonylruthenium(II) dimer, dibromotricarbonylruthenium(II) dimer, and other organorruthenium complexes such as tetrachlorobis(4-cymene)diruthenium(II), tetrachlorobis(benzene)diruthenium(II), dichloro(cyclooctyl-1,5-diene)ruthenium(II) polymers, and tri(acetylacetone)ruthenium(III). Preferably, the ruthenium-containing compounds do not contain impurities that provide or generate in-situ ionic iodides that inhibit the reaction, such as alkali metals or alkaline earth metals or other metal salts.

[0082] Ruthenium accelerators may be present in any effective amount up to their solubility limit in the liquid reaction composition, liquid fraction, and / or any liquid process stream recycled to the carbonylation reaction zone. For example, ruthenium accelerators are suitably present in the liquid reaction composition at a molar ratio of each ruthenium accelerator to iridium of [0.1 to 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 even more preferably [up to 10]:1. ​​The concentration of ruthenium accelerators in the liquid reaction composition in the reaction zone is typically less than 6000 ppm. Suitable accelerator concentrations are, for example, 400 to 5000 ppm, such as 2000 to 4000 ppm.

[0083] Suitable rhodium carbonylation catalysts are described, for example, in EP-A-0161874, US 6,211,405 and EP-A-0728727, each of which is incorporated herein by reference in its entirety. When the carbonylation catalyst is a rhodium catalyst, the concentration of the rhodium catalyst in the liquid reaction composition is preferably 50 to 5000 ppm, more preferably 100 to 1500 ppm by weight of rhodium. When using rhodium as a catalyst, it is preferable to use an alkali metal iodide such as lithium iodide as a promoter, for example as described in the above references.

[0084] Therefore, in some embodiments, methanol and / or its reactive derivatives are catalyzed by a homogeneous carbonylation reaction of carbon monoxide using a soluble Group VIII metal carbonylation catalyst (e.g., containing rhodium and / or iridium) in a liquid reaction composition comprising methanol and / or its reactive derivatives. In some embodiments, the liquid reaction composition comprises water and one or more of methyl iodide and methyl acetate. In some such embodiments, the liquid reaction composition also includes a propionic acid byproduct.

[0085] As described above, water can be present in the liquid reaction composition. Those skilled in the art will understand that water is formed in situ in the liquid reaction composition through an esterification reaction between methanol and / or its reactive derivatives with an acetic acid product. In some embodiments, water may also be introduced into the reaction zone (e.g., together with or separately from other components of the liquid reaction composition). In some desirable embodiments, water is present in the liquid reaction composition in an amount of 0.1% to 15% by weight, for example, 1% to 15% by weight, or 1% to 8% by weight.

[0086] Methyl acetate can be formed in situ in a liquid reaction composition by reacting methanol and / or its reactive derivatives with an acetic acid product or solvent. In some embodiments further described herein, the concentration of methyl acetate in the liquid reaction composition in the reaction zone is 2 to 50% by weight, for example, 3 to 35% by weight.

[0087] As described above, propionic acid byproducts may also be present in the liquid reaction composition. In some embodiments, propionic acid is present in the liquid reaction composition in amounts from 200 ppmw to 2,500 ppmw, for example from 400 ppmw to 2,000 ppmw, or from 600 ppmw to 1,400 ppmw.

[0088] In some desirable embodiments, methyl iodine is present in the liquid reaction composition in an amount of 1% to 20% by weight. For example, in some such embodiments, methyl iodine is present in the liquid reaction composition in an amount of 2% to 16% by weight.

[0089] In some embodiments further described herein, the liquid reaction composition comprises a solvent. For example, in some such embodiments, the liquid reaction composition comprises an acetic acid solvent (e.g., recycled from the separation zone of an acetic acid production unit).

[0090] As described above, acetic acid can be produced in the reaction zone by carbonylation of methanol and / or its reactive derivatives with carbon monoxide. In some embodiments further described herein, the carbon monoxide supplied to the reaction zone is substantially pure. In other embodiments, the carbon monoxide supplied to the reaction zone contains one or more impurities, such as carbon dioxide, methane, nitrogen, hydrogen, or an inert gas. In some embodiments further described herein, the partial pressure of carbon monoxide (e.g., in the reactor of the reaction zone) is from 1 bar to 70 bar, for example from 1 bar to 35 bar.

[0091] In some embodiments further described herein, the carbonylation reaction in the reaction zone is carried out at a total pressure of 10 barg (bar (gauge pressure)) to 100 barg (e.g., in a reactor in the reaction zone).

[0092] The effluent from the reaction zone can be directed to a flash separation zone (e.g., via flash valve 62), where it is separated into a gaseous stream rich in acetic acid and a liquid stream leaning in acetic acid. The liquid stream can be introduced (i.e., as recirculation) back to the reaction zone. For example, refer to... Figure 1 The effluent 50 is directed to the flash separation zone 60 to form a vapor fraction 70 and a liquid fraction 75. At least a portion of the liquid fraction 75 may be introduced into the reaction zone 10.

[0093] Flash separation zones are known in the art. In some embodiments, the flash separation zone may include an adiabatic flash vessel. Alternatively or additionally, the flash separation zone may be heated, for example, by a heater. Flash separation zones are typically operated at pressures of 0 to 10 barg, preferably 0 to 3 barg.

[0094] The carbonylation process described herein can be carried out in batch or continuous processes. In some desirable embodiments, the carbonylation process is carried out as a continuous process.

[0095] In some embodiments, at least a portion of the vapor stage from the flash separation zone is directed to the light fraction recovery zone of the acetic acid production unit. In some embodiments, the light fraction recovery zone of the acetic acid production unit is configured to separate at least components more volatile than acetic acid (e.g., components in the vapor stage removed from the tank, as further described herein) from acetic acid. For example, in some embodiments, acetic acid is produced in the reaction zone by the carbonylation reaction 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 back to the reaction zone.

[0096] In some embodiments as further described herein, the light fraction recovery zone includes a distillation column that separates the crude acetic acid product containing acetic acid and propionic acid from the light fraction containing methyl iodide and methyl acetate. In some such embodiments, the light fraction recovery zone also includes a drying column. For example, in some embodiments further described herein, the light fraction recovery zone includes a combined light fraction and drying column, wherein water is removed from the crude acetic acid product to form a dried acetic acid product containing acetic acid and propionic acid. As used herein, the “dried” or “pre-dried” feed containing acetic acid (e.g., and optionally propionic acid) contains up to 1,500 ppmw of water.

[0097] Suitable columns and their configurations for use in the light fraction recovery zone are generally known in the art. Typically, at least a first fraction containing acetic acid and propionic acid and an overhead vapor fraction containing methyl acetate, water, acetic acid, carbon monoxide, and methyl iodine are formed in the light fraction recovery zone. In some embodiments, the separated water can be recycled to the reaction zone or removed from the acetic acid production unit.

[0098] In some embodiments, as further described herein, the light fraction recovery zone also includes one or more condensers and / or coolers to condense the overhead vapor stage and form a liquid stage. Those skilled in the art will understand that any suitable method known in the art can be used to condense the overhead vapor stage into a liquid phase. For example, in some embodiments, at least one heat exchanger (e.g., supplied with water as a cooling medium) is used to condense the stage. Components of the uncondensed overhead stage (e.g., carbon monoxide, carbon dioxide, inert gases, reaction byproduct gases) are removed from the light fraction recovery zone as a waste stream. In some embodiments, acetic acid is generated in the reaction zone via a carbonylation reaction 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 also contains methyl iodine (e.g., present as entrained and / or evaporated methyl iodine), methyl acetate, and water.

[0099] In some embodiments, the liquid fraction formed in the light fraction recovery zone comprises methyl acetate, water, and acetic acid. In some 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 also comprises methyl iodine. In some embodiments, the liquid fraction further comprises entrained or dissolved gaseous components (e.g., carbon monoxide, carbon dioxide, inert gases).

[0100] In some embodiments further described herein, the light fraction recovery zone includes a decanter, wherein the liquid fraction is separated into two layers: a lower layer containing methyl acetate (e.g., an organic layer) and an upper layer containing water (e.g., an aqueous layer). In some 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 also contains methyl iodine. In some embodiments, at least a portion (e.g., all) of the upper layer from the decanter is returned as a reflux stream to the distillation column of the light fraction recovery zone. In some embodiments, at least a portion (e.g., all) of the upper layer from the decanter is recycled to the reaction zone. In some embodiments, exhaust gas is removed from the decanter and conveyed to an exhaust gas scrubbing unit (e.g., prior to disposal).

[0101] In some embodiments, a feed stream containing acetic acid and propionic acid from the light fraction recovery section (e.g., the first fraction formed in the light fraction recovery section) is fed to the heavy fraction column through a feed inlet located at the midpoint of the column. In such embodiments, the propionic acid-containing feed stream is withdrawn from the heavy fraction column through a heavy product outlet, and acetic acid is removed as a product feed stream at one or more outlets of the column (e.g., removed as a top stream from the top outlet, or as a side stream from an outlet above the feed inlet). In some embodiments, the product feed stream primarily contains acetic acid and contains less than 400 ppmw, or less than 300 ppmw, or less than 250 ppmw of propionic acid. In some embodiments, the product feed stream primarily contains acetic acid and contains less than 1,500 ppmw of water. In some desirable embodiments, the product feed stream primarily contains acetic acid and contains a combined total of less than 1,500 ppmw of propionic acid and water. Suitable columns that can be used as heavy fraction columns and their configurations are generally known in the art. For example, in some embodiments, the heavy fraction column is connected to a condenser. In another instance, in some implementations, the reboiler is connected to the bottom of the heavy distillation column.

[0102] Example

[0103] The invention can be illustrated by the following examples, in which the following has been simulated in Aspen Plus v10: Figure 1 The performance of the absorption tower shown is illustrated.

[0104] Example 1: Acetic acid

[0105] A waste gas composition 142, comprising carbon monoxide, carbon dioxide, nitrogen, and methyl iodine from the acetic acid production process, is introduced into an absorber 110, where it is contacted with the acetic acid feed stream. The acetic acid feed stream comprises a mixture of acetic acid from a heavy fractionation column 195 and an acetic acid storage container (not shown). The acetic acid feed stream is introduced through a liquid inlet 134 at an inlet temperature of 24°C. The liquid acetic acid containing the absorbed methyl iodine is removed from the column through a liquid outlet 126 and recycled to a light fraction recovery zone 190. Under these conditions, the overhead stream from the absorber contains carbon monoxide (as well as carbon dioxide and nitrogen) and approximately 90 ppmv of methyl iodine, representing a methyl iodine molar effluent ratio (the molar fraction of methyl iodine in the vapor outlet to that in the inlet waste gas) of 0.000430%.

[0106] Example 2: Methanol.

[0107] Example 1 was repeated, except that a liquid feed stream containing methanol was used instead of acetic acid. The feed stream was introduced through liquid inlet 132 at a temperature of 24°C (i.e., the same temperature as the liquid inlet in Example 1), and all other conditions (exhaust gas composition, pressure, temperature, and all flow rates) were kept the same. Liquid methanol containing absorbed methyl iodine was removed from the column through liquid outlet 124 and recycled to reaction zone 160. Under these conditions, the overhead feed stream from absorber 110 contained carbon monoxide and approximately 10 ppmv of methyl iodine, representing a molar effluent ratio (vapor effluent / vapor influent) of 0.000044%.

[0108] This example demonstrates that methyl iodine can be effectively removed from exhaust gas even at 24°C, which is much higher than the temperature at which methanol is conventionally used as an absorbent liquid.

[0109] Example 3 - Switching from acetic acid to methanol

[0110] In this embodiment, the absorber initially operates under the conditions of Example 1. This can represent, for example, "start-up," where the methanol initially present in the overhead condenser 165 is insufficient for the absorber. A switch from using acetic acid to using methanol is then required. In this embodiment, the liquid feed transitions from using only acetic acid (through inlet 134) to using only methanol (through inlet 132). During an intermediate time period, liquid is introduced through both inlets, thus the liquid stream in the absorber is a mixture of acetic acid and methanol.

[0111] During the transition, the total liquid flow rate remained constant, ensuring that the final conditions were the same as those in Example 2. In this case, the methyl iodine steadily decreased during the transition, as shown in Table 1 below.

[0112] Table 1

[0113]

[0114] This indicates that the transition can be performed without changing the temperature, and a low content of methyl iodine can be obtained throughout the waste gas stream. Once the liquid stream has transitioned to methanol, the liquid outlet of the tower is switched from outlet 126 to outlet 124.

[0115] Although not demonstrated in this or subsequent embodiments, it will be apparent that the liquid feed rate can be varied during the transition. For example, the results above indicate that methanol is more effective at removing methyl iodine, therefore its flow rate can be reduced during or after the transition while still achieving a low level of residual methyl iodine in the vapor.

[0116] Example 4 - Switching from methanol to acetic acid.

[0117] In this embodiment, the absorber is initially operated under the conditions of Example 2. (This could represent a transitional Example 2 or Example 3). Subsequently, a switch from using methanol to using acetic acid is required. In this embodiment, the acetic acid feed (to inlet 134) can be started immediately to transition the liquid feed from using only methanol to using only acetic acid. (At the start of the transition, the liquid outlet of the column is also switched from outlet 126 to outlet 124). Regarding the methyl iodine content of the absorber overhead distillate under different conditions, substantially the same results as in Example 3 are obtained, only the operation is reversed.

[0118] A particular advantage of the present invention, and illustrated in this embodiment, is that the temperature of the absorption tower does not need to be adjusted before the transition begins; therefore, the addition of acetic acid to initiate the transition can begin immediately.

[0119] Comparative Example 1 - Switching from methanol to acetic acid.

[0120] The process of Example 4 was repeated, except that, under the initial conditions, the methanol introduced into the absorber was introduced at 5°C. When it was necessary to switch from using methanol to using acetic acid, it was necessary to first raise the temperature of the absorber until the temperature at the liquid inlet (134) for acetic acid was above the freezing point of acetic acid. Therefore, the transition could not begin immediately and required a longer time.

[0121] All contents of each and every patent and non-patent publication cited herein are incorporated herein by reference, except where the disclosure or definitions herein shall prevail in the event of any inconsistency with the disclosures or definitions herein.

[0122] The foregoing detailed description and accompanying drawings have been provided by way of explanation and illustration 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 skilled in the art and remain within the scope of the appended claims and their equivalents.

[0123] It should be understood that the elements and features recited in the appended claims can be combined in different ways to produce new claims that also fall within the scope of this disclosure. Therefore, where the dependent claims appended below are subordinate to only a single independent or dependent claim, it should be understood that these dependent claims may alternatively be subordinate to any preceding claim—whether independent or dependent—and such new combinations should be understood as forming part of this specification.

Claims

1. A method for washing a waste gas stream containing carbon monoxide and methyl iodine in an absorption tower of an acetic acid production unit, said absorption tower comprising: The bottom portion includes a feed inlet and one or more liquid outlets; and The top portion above the bottom portion includes one or more liquid inlets and vapor outlets; The method includes: The waste gas stream is introduced into the absorption tower through the feed inlet; A methanol feed stream is introduced at a first flow rate through one or more of the liquid inlets, the methanol feed stream having a first temperature at the liquid inlet, the first temperature being at least 18°C; In the absorption tower, the waste gas stream is brought into contact with the methanol stream; A first liquid effluent, comprising methanol and methyl iodine, is extracted from the absorber through one or more of the liquid outlets. The vapor effluent is removed from the absorption tower through the vapor outlet. The method further includes separating the extracted vapor effluent in a heat exchanger to provide a vapor fraction and a liquid fraction containing methanol, wherein - The vapor fraction contains up to 2.5% by weight of methanol, and - The methanol feed stream comprises at least a portion of the methanol-containing liquid fraction; The method further includes, after introducing the methanol feed stream into the absorption tower, Reduce the flow rate of the methanol feed stream being transported to the absorption tower; An acetic acid stream is conveyed at a third flow rate to the top portion of the absorption tower through one or more of the liquid inlets, the acetic acid stream having a third temperature at the inlet; and In the absorption tower, the waste gas stream is brought into contact with the acetic acid stream; The third temperature is 18-40℃. Furthermore, the third temperature differs from the first temperature by at most 5°C.

2. The method of claim 1, wherein the vapor effluent extracted while introducing the methanol feed stream into the absorber contains up to 500 ppmv of methyl iodine.

3. The method of claim 2, wherein the vapor effluent extracted while introducing the methanol feed stream into the absorber contains up to 350 ppmv of methyl iodine.

4. The method of claim 2, wherein the vapor effluent extracted while introducing the methanol feed stream into the absorber contains up to 100 ppmv of methyl iodine.

5. The method of claim 1, further comprising conveying the first liquid effluent to the reaction zone of the acetic acid production unit.

6. The method of claim 5, wherein the first liquid effluent delivered to the reaction zone of the acetic acid production unit contains up to 250 ppmv of acetic acid.

7. The method of claim 6, wherein the first liquid effluent delivered to the reaction zone of the acetic acid production unit contains up to 100 ppmv of acetic acid.

8. The method of claim 6, wherein the first liquid effluent delivered to the reaction zone of the acetic acid production unit contains up to 50 ppmv of acetic acid.

9. The method of claim 1, further comprising, before introducing the methanol feed stream into the absorber, The waste gas stream is conveyed to the bottom part of the absorption tower through the feed inlet; An acetic acid feed stream is introduced into the top portion of the absorption tower at a second flow rate through one or more of the liquid inlets, the acetic acid feed stream having a second temperature at the liquid inlet; and In the absorption tower, the waste gas stream is brought into contact with the acetic acid stream; The method further includes, after a first time period, contacting the waste gas stream with the acetic acid stream. Reduce the flow rate of the acetic acid feed, and The methanol feed stream is introduced into the absorption tower to transition from acetic acid to methanol; And the second temperature is at least 18°C, and The second temperature differs from the first temperature by at most 5°C.

10. The method of claim 9, wherein the flow rate of the acetic acid stream is reduced to approximately zero.

11. The method of claim 9, wherein the second temperature differs from the first temperature by at most 3.5°C.

12. The method of claim 11, wherein the second temperature differs from the first temperature by at most 2°C.

13. The method of claim 9, further comprising, prior to introducing the methanol stream into the absorber, simultaneously introducing the acetic acid stream into the absorber, removing a second liquid effluent from the absorber through a liquid outlet of one or more of the liquid outlets, the second liquid effluent comprising acetic acid and methyl iodine.

14. The method of claim 13, wherein the second liquid effluent is conveyed to the light fraction recovery zone of the acetic acid production unit.

15. The method of claim 14, wherein the second liquid effluent delivered to the light fraction recovery zone of the acetic acid production unit contains at least 25 ppmv of acetic acid.

16. The method of claim 15, wherein the second liquid effluent conveyed to the light fraction recovery zone of the acetic acid production unit contains at least 100 ppmv of acetic acid.

17. The method of claim 9, further comprising, prior to introducing the methanol stream into the absorber, simultaneously introducing the acetic acid stream into the absorber, removing a vapor effluent from the absorber through the vapor outlet, wherein the vapor effluent removed simultaneously with the introduction of the acetic acid stream into the absorber contains up to 500 ppmv of methyl iodine.

18. The method of claim 17, wherein the vapor effluent contains up to 350 ppmv of methyl iodine.

19. The method of claim 17, wherein the vapor effluent contains up to 100 ppmv of methyl iodine.

20. The method of claim 9, wherein the acetic acid stream introduced before the methanol stream is introduced into the absorber comprises at least a portion of the overhead stream from the heavy distillation column of the acetic acid production unit.

21. The method according to claim 9, wherein after the transition, while contacting the waste gas stream with the methanol stream, the weight ratio of methanol to acetic acid in the absorption tower is at least 500:

1.

22. The method according to claim 9, wherein the second temperature is 18-30°C.

23. The method according to claim 1, wherein the first temperature is 18-30°C.

24. The method of claim 1, wherein the first temperature is at least 20°C.

25. The method according to claim 1, wherein the first temperature is 20-35°C.

26. The method according to claim 1, wherein the third temperature is 20-40°C.

27. The method of claim 1, wherein the first temperature is at least 22°C.

28. The method of claim 1, wherein the first temperature is at least 24°C.

29. The method of claim 1, wherein the waste gas stream comprises at least a portion of one or more gaseous effluents from the separation zone, light fraction recovery zone, and reaction zone of the acetic acid production unit.

30. The method of claim 1, wherein the vapor fraction comprises up to 2.0% by weight of methanol.

31. The method of claim 30, wherein the vapor fraction comprises up to 1.5% by weight of methanol.

32. The method of claim 1, further comprising delivering the vapor stage to a combustion system and burning the stage in the combustion system.

33. The method of claim 1, wherein the flow rate of the methanol stream to be conveyed to the absorption tower is reduced to approximately zero.

34. The method according to any one of claims 1-33, wherein the third temperature differs from the first temperature by at most 3.5°C.

35. The method of claim 34, wherein the third temperature differs from the first temperature by at most 2°C.

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