Process for treating off-gas from an acetic acid production unit

Abstract

The present disclosure generally relates to a process for scrubbing an off-gas stream from an acetic acid production unit. In one aspect, the process includes: introducing the off-gas stream into an absorber; introducing a methanol stream into the absorber 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 off-gas stream with the methanol stream in the absorber; withdrawing a first liquid effluent from the absorber through a liquid outlet of one or more liquid outlets, the first liquid effluent comprising methanol and methyl iodide; and withdrawing a vapor effluent from the absorber through a vapor outlet.

Description

[Technical Field]

[0001] The present disclosure relates generally to a process for treating off-gases useful in the production of acetic acid. [Background technology]

[0002] Commercially, acetic acid has been produced for many years by the carbonylation of methanol with carbon monoxide in the presence of a Group VIII carbonylation catalyst. Typically, carbon monoxide is contacted with methanol in the presence of a rhodium or iridium homogeneous or heterogeneous carbonylation catalyst, methyl iodide, and water in one or more reaction zones. Generally, the acetic acid product can be recovered by withdrawing the crude acetic acid product from the reactor and separating it 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. Acetic anhydride is often provided as a by-product.

[0003] In processes for the production of acetic acid and / or the co-production of acetic acid and acetic anhydride, off-gases are typically withdrawn at some stage of the process, for example from one or more of the reactors and distillation stages, and are removed to maintain working concentrations of undesirable gaseous reaction by-products and inert gases at acceptable levels.

[0004] The exact composition of the off-gas will vary depending on the particular carbonylation process conditions used, but will generally contain carbon monoxide, inert and reaction by-product gases, iodide compounds, primarily methyl iodide, and may also contain low levels of methyl acetate, acetic acid, and water.

[0005] The off-gas is typically scrubbed with a suitable scrubbing solvent to recover valuable components, such as methyl iodide, which can then be returned to the reactor. The scrubbed off-gas, including inert and by-product gases, is typically flared. Various scrubbing solvents, such as acetic acid or methanol, can be used. In such scrubbing processes, methyl iodide is absorbed into the scrubbing solvent, and the off-gas, containing small amounts of methyl iodide, is typically removed from the scrubbing unit as overhead. One conventional process, described in WO 2015 / 193328, utilizes a methanol and acetic acid absorber tower in series. In such conventional systems, these towers can be used to ensure that the methyl iodide level in the overhead stream sent for combustion is maintained at a low level (e.g., 35 ppmv (parts per million by volume)). The acetic acid absorber tower can be used during startup and shutdown while the plant waits for a methanol supply. The methanol absorber tower performs the majority of the scrubbing of the methyl iodide, and during such normal operation, the liquid flow through the acetic acid absorber tower can be reduced to a low "tick over" flow rate so that it is available on demand, while allowing the majority of the acetic acid product stream to enter the light ends tower.

[0006] However, it remains desirable to provide an alternative method for scrubbing off-gas streams generated in manufacturing processes for the production of acetic acid, particularly off-gas streams generated in manufacturing processes for the production of acetic acid by the carbonylation of methanol. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2015 / 193328 Summary of the Invention [Means for solving the problem]

[0008] In one aspect, the present disclosure provides a process for scrubbing an off-gas stream comprising carbon monoxide and methyl iodide in an absorption tower of an acetic acid production unit, the absorption tower comprising: a lower section including a feed inlet and one or more liquid outlets; an upper section above the lower section, the upper section including one or more liquid inlets and a vapor outlet; Equipped with This process is introducing an off-gas stream into an absorber tower through a feed inlet; introducing a methanol stream at a first flow rate through a liquid inlet of one or more liquid inlets, wherein the methanol stream has 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); contacting the off-gas stream with a methanol stream in an absorber tower; withdrawing a first liquid effluent from the absorber tower through a liquid outlet(s) of one or more liquid outlets, the first liquid effluent comprising methanol and methyl iodide; withdrawing a vapor effluent from the absorber tower through a vapor outlet; Includes.

[0009] The vapor effluent preferably contains up to 500 ppmv (parts per million by volume) (e.g., up to 350 ppmv, or up to 200 ppmv, or up to 100 ppmv) of methyl iodide.

[0010] Other aspects of the present disclosure will become apparent to those skilled in the art in view of the following description. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of the process of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have noted that during unsteady conditions, such as during plant startup, shutdown, or trip / upset, the supply of methanol to the reactor and / or scrubbing unit in an acetic acid production process is frequently limited or may even be completely cut off. This is disadvantageous for an acetic acid production process that uses methanol as an off-gas scrubbing solvent. Switching systems exist, such as those described in WO 2009 / 134332, that can enable the use of different scrubbing solvents in a single scrubbing tower, thereby reducing capital requirements and operating costs.

[0013] Generally, the efficiency of off-gas scrubbing with a solvent is maximized by using a scrubbing solvent that is cooled before use. The scrubbing solvent can be cooled, for example, by passing it through a heat exchange unit configured to reduce the solvent's temperature before use in the scrubbing unit. Traditionally, the temperatures of acetic acid and methanol are reduced before scrubbing the off-gas to improve scrubbing efficiency. For example, acetic acid freezes at 16.7 °C, so a somewhat higher setpoint temperature (e.g., ~24 °C) is often used for the acetic acid feed to the absorber to address the risk of freezing while ensuring sufficient scrubbing efficiency to remove methyl iodide under various conditions. In contrast, methanol's freezing point is near -100 °C, making freezing less of a concern. Instead, the setpoint temperature of the methanol feed to the absorber is typically set at a low but practical value (e.g., ~5 °C) based on other system parameters, such as the temperature of the cooling system.

[0014] The present inventors have noted that in systems using both methanol and acetic acid as scrubbing solvents in a single absorber, when switching from methanol to acetic acid, it is necessary to wait for the absorber to warm up to the freezing point of acetic acid to prevent the acetic acid from freezing in the absorber. Acetic acid can freeze in the absorber because the absorber packing and materials themselves are cooled by contact with the cooler methanol scrubbing solvent. Freezing of acetic acid in the absorber is highly undesirable because it can reduce scrubbing efficiency or cause a complete loss of scrubbing, potentially resulting in the release of methyl iodide into the atmosphere.

[0015] The inventors have surprisingly determined that methanol scrubbing can be carried out at temperatures substantially higher than those previously used. In particular, methanol scrubbing can be carried out at temperatures above the freezing point of acetic acid, which means that there is no risk of acetic acid freezing in the column during solvent switchover.

[0016] In various aspects, the processes of the present disclosure allow for scrubbing off-gas streams in acetic acid production units. Specifically, one aspect of the present disclosure is a process for scrubbing an off-gas stream containing carbon monoxide and methyl iodide in an absorption tower of an acetic acid production unit. The absorption tower comprises: a lower section having a feed inlet and a liquid outlet; an upper section above the lower section, the upper section including one or more liquid inlets and a vapor outlet; Equipped with. This process is introducing an off-gas stream into an absorber tower through a feed inlet; introducing a methanol stream at a first flow rate through a liquid inlet of one or more liquid inlets, the methanol 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); contacting the off-gas stream with a methanol stream in an absorber tower; withdrawing a first liquid effluent from the absorber tower through a liquid outlet(s) of one or more liquid outlets, the first liquid effluent comprising methanol and methyl iodide; withdrawing a vapor effluent from the absorber tower through a vapor outlet; Includes. 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 iodide. The vapor effluent is the scrubbed off-gas and generally contains carbon monoxide, as well as carbon dioxide and nitrogen.

[0017] Accordingly, one embodiment of the present disclosure is described with reference to FIG. 1 . An acetic acid production unit 100 includes an absorber 110 having a lower section 120 with a feed inlet (or off-gas inlet) 122 and one or more liquid outlets (here, liquid outlets 124 and 126), and an upper section 130 with one or more liquid inlets (here, liquid inlets 132 and 134) on the lower section and a vapor outlet 136. An off-gas stream 142 is introduced into the absorber through the feed inlet. A methanol stream 152 having a first temperature at the liquid inlet is introduced through one of the one or more liquid inlets, here, liquid inlet 132. Specifically, the first temperature is at least 18° C. Within the absorber, the off-gas stream contacts the methanol stream, resulting in the off-gas stream being scrubbed with methanol. A first effluent 144 comprising methanol and methyl iodide is withdrawn from the absorber through one or more liquid outlets, here, liquid outlet 124. Through the vapor outlet 136, a vapor effluent 156, ie, scrubbed off-gas, is withdrawn.

[0018] The off-gas stream can be sourced from any of a variety of processes within the acetic acid production unit. For example, in certain embodiments described elsewhere herein, the off-gas stream comprises at least a portion of one or more of the vapor-phase effluents of the separation zone, light ends recovery zone, and reaction zone of the acetic acid production unit. This is shown in the embodiment of Figure 1, where vapor-phase effluent 186 of flash separation zone 185, vapor-phase effluent 191 of light ends recovery zone 190, and vapor-phase effluent 161 of reaction zone 160 provide off-gas stream 142, which is introduced into absorber tower 110.

[0019] The first temperature (i.e., the temperature of the methanol stream at the liquid inlet) is above the freezing point of acetic acid. Therefore, even the coldest packing and other structures of the column should be above the freezing point of acetic acid, and as a result, acetic acid will not solidify within the column when it enters the column for use as a scrubbing solvent (e.g., if sufficient methanol flow is not available). In certain embodiments described elsewhere herein, the first temperature is at least 20°C, e.g., at least 22°C, or at least 24°C. In certain embodiments described elsewhere herein, the first temperature is in the range of 18°C ​​to 35°C, e.g., in the range of 20°C to 35°C, or in the range of 22°C to 35°C, or in the range of 24°C to 35°C. In certain embodiments described elsewhere herein, the first temperature is in the range of 18°C ​​to 30°C, e.g., in the range of 20°C to 30°C, or in the range of 22°C to 30°C, or in the range of 24°C to 30°C. In certain embodiments described elsewhere herein, the first temperature ranges from 18°C ​​to 28°C, e.g., from 20°C to 28°C, or from 22°C to 28°C.

[0020] The first liquid effluent comprises methanol and methyl iodide (scrubbed from the off-gas stream). This stream can be transferred to the reaction zone of an acetic acid production unit for use as a reactant in the carbonylation of methanol with carbon monoxide over a zeolite catalyst. For example, in the embodiment of FIG. 1 , first liquid effluent 144 is transferred to reaction zone 160. The first liquid effluent transferred to the reaction zone of the acetic acid production unit can, in embodiments, contain 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 certain desirable embodiments, the first liquid effluent transferred to the reaction zone of the acetic acid production unit is substantially free of acetic acid.

[0021] The present inventors have surprisingly determined that methanol can be used to effectively scrub methyl iodide from off-gas streams in acetic acid production units, even at temperatures much higher than previously thought. In certain embodiments described elsewhere herein, the vapor effluent (withdrawn during introduction of the methanol stream into the absorber) has a maximum of 500 ppmv of methyl iodide, or a maximum of 350 ppmv of methyl iodide, or a maximum of 200 ppmv of methyl iodide, or a maximum of 100 ppmv of methyl iodide, or a maximum of 50 ppmv of methyl iodide.

[0022] However, due to the high temperature of the methanol scrubbing solvent, the concentration of methanol in the vapor effluent may be relatively higher than in the past. A heat exchanger (e.g., an overhead condenser) can be used to separate the vapor effluent, resulting in a vapor fraction and a liquid fraction containing methanol. In the embodiment of FIG. 1, the vapor effluent 156 is condensed in an overhead condenser 165 to produce a vapor fraction 166 and a liquid fraction containing methanol 167. As shown in FIG. 1, the liquid fraction can be part of the methanol stream introduced into the absorber, such that the methanol stream contains at least a portion of the liquid fraction containing methanol. As shown in FIG. 1, the vapor fraction can be transferred to a combustion system (180) and combusted therein (e.g., as a flame). In certain desirable embodiments, the vapor fraction contains up to 2.5 wt.% methanol (e.g., up to 2.25 wt.%, or up to 2.0 wt.%, or up to 1.75 wt.%, or up to 1.5 wt.%).

[0023] In many cases, it will be desirable to use acetic acid for scrubbing before using methanol for scrubbing. This is often the situation during process start-up, when there may not be enough methanol available for scrubbing. Therefore, in certain embodiments described elsewhere herein, the process may include: transferring the off-gas stream to a lower section of the absorber tower through a feed inlet; conveying the acetic acid stream at a second flow rate through a liquid inlet of the one or more liquid inlets to an upper section of the absorber, the acetic acid stream having a second temperature at the liquid inlet; contacting the off-gas stream with an acetic acid stream in an absorption tower; Includes. The method includes, after a first period of time of contacting the off-gas stream with the acetic acid stream: reducing the flow of the acetic acid stream (e.g., to about zero flow); introducing the methanol stream into an absorption tower to convert acetic acid to methanol; Further includes: Preferably, the transition is such that while the off-gas stream is contacted with the methanol stream, the weight ratio of methanol to acetic acid in the absorber is at least 500: 1. The second temperature is at least 18°C.

[0024] Thus, in certain embodiments described elsewhere herein (as identified in the system of FIG. 1 ), acetic acid is used as a scrubbing solvent before methanol is introduced into the column. Off-gas stream 142 is introduced into lower section 120 of absorber tower 110 through feed inlet 122. The acetic acid stream is introduced into the upper section of the absorber tower at a second flow rate through a liquid inlet (which can be the same as or different from the liquid inlet through which the methanol stream is subsequently introduced). In the embodiment of FIG. 1 , acetic acid stream 154 is introduced into upper section 130 of absorber tower 110 through liquid inlet 134. The acetic acid stream has a second temperature at the liquid inlet; specifically, this second temperature is at least 18° C. The off-gas stream contacts the acetic acid stream within absorber tower 110. After a first period of contacting the off-gas stream with the acetic acid stream, the scrubbing solvent can be switched from acetic acid to methanol. Thus, the flow of the acetic acid stream can be reduced (e.g., to about zero flow) and a methanol stream can be introduced into the column as described above. By switching the solvent introduced into the column, the liquid content of the absorber can be changed to be predominantly methanol; for example, after the transition between acetic acid and methanol, the weight ratio of methanol to acetic acid in the absorber while the off-gas stream is contacted with the methanol stream is at least 500:1 (e.g., at least 1,500:1, or at least 4,000:1). This transition time is desirably relatively fast, and one skilled in the art can switch between the introduction of an acetic acid stream and the introduction of a methanol stream within minutes or even seconds. However, depending on the volume of the absorber, it may take a somewhat longer time for the new high methanol to acetic acid ratio to be established.

[0025] The second temperature (i.e., the temperature of the acetic acid stream at the liquid inlet) is greater than the freezing point of acetic acid. In certain embodiments described elsewhere herein, the second temperature is at least 20°C, e.g., at least 22°C, or at least 24°C. In certain embodiments described elsewhere herein, the second temperature is in the range of 18°C ​​to 50°C, e.g., 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.

[0026] The second temperature (i.e., the temperature of the acetic acid stream) can be conveniently set to be about the same as or higher than the first temperature (i.e., the temperature of the methanol stream). For example, in certain embodiments described elsewhere 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 be advantageously carried out with a higher temperature difference, and in other embodiments, the second temperature is 5 to 25°C (e.g., 7.5 to 25°C, or 10 to 25°C, or 15 to 25°C, or 5 to 15°C, or 7.5 to 15°C, or 10 to 15°C) higher than the first temperature.

[0027] In certain embodiments described elsewhere herein, while the acetic acid stream is being introduced into the absorber, a second liquid effluent is withdrawn from the absorber through one or more liquid outlets. This liquid outlet can be the same as or different from the liquid outlet through which the first liquid effluent is withdrawn; if the liquid outlet is the same, one skilled in the art can provide a valve to allow the second liquid effluent to be directed to a different part of the system than the first liquid effluent. The second liquid effluent comprises acetic acid and methyl iodide (i.e., scrubbed from the off-gas stream). In the system of FIG. 1, second liquid effluent 146 is withdrawn through liquid outlet 126. The second liquid effluent can be transferred to the light ends recovery zone (190 in FIG. 1) of the acetic acid production unit, where acetic acid can be recovered as a product. In certain such embodiments, the second liquid effluent transferred to the light ends 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.

[0028] In certain embodiments described elsewhere herein, a vapor effluent is withdrawn through a vapor outlet while the acetic acid stream is introduced into the absorber. In certain desirable embodiments, the vapor effluent withdrawn while the acetic acid stream is introduced into the absorber comprises up to 500 ppmv of methyl iodide. In certain desirable embodiments, the vapor effluent comprises up to 350 ppmv of methyl iodide, up to 200 ppmv of methyl iodide, up to 100 ppmv of methyl iodide, or up to 50 ppmv of methyl iodide.

[0029] The acetic acid stream can be provided from a variety of sources. In certain embodiments, the acetic acid stream comprises at least a portion of an overhead stream from a heavy ends column of an acetic acid production unit. For example, as shown in FIG. 1, acetic acid stream 154 can be provided from overhead stream 196 of heavy ends column 195.

[0030] In many cases, it would be desirable to use acetic acid for scrubbing after methanol has been used for scrubbing. This is often the case during transient operating situations, such as during unit shutdowns or trips / upsets in the production process, when there may not be enough methanol available for scrubbing. Thus, in certain embodiments described elsewhere herein, the process may include introducing a methanol stream into an absorber tower followed by: reducing (e.g., to zero) the flow rate of methanol delivered to the absorber; transferring the off-gas stream to a lower section of the absorber tower through a feed inlet; conveying an acetic acid stream at a third flow rate through a liquid inlet of the one or more liquid inlets to an upper section of the tower, the acetic acid stream having a third temperature at the inlet; contacting the off-gas stream with an acetic acid stream in an absorption tower; Further includes: Preferably, after the transition from methanol to acetic acid, during contact of the off-gas stream with the acetic acid stream, the weight ratio of methanol to acetic acid in the absorber is less than 1:500 (e.g., less than 1:1500 or less than 1:4000). The third temperature is at least 18°C.

[0031] Thus, in certain embodiments described elsewhere herein (as identified in the system of FIG. 1 ), acetic acid is used as a scrubbing solvent after methanol is introduced into the column. Off-gas stream 142 is introduced into lower section 120 of absorber tower 110 through feed inlet 122. An acetic acid stream is introduced into the upper section of the absorber tower at a third flow rate through a liquid inlet (which may be the same as or different from the liquid inlet through which the methanol stream is introduced). In the embodiment of FIG. 1 , acetic acid stream 154 is introduced into upper section 130 of absorber tower 110 through liquid inlet 134. The acetic acid stream has a third temperature at the liquid inlet; specifically, this third temperature is at least 18° C. The off-gas stream contacts the acetic acid stream within absorber tower 110. After methanol is introduced into the absorber tower, the scrubbing solvent can be switched from methanol to acetic acid. Thus, the flow of the methanol stream can be reduced, for example, to about zero flow, and the acetic acid stream can be introduced into the tower as described above. By switching the solvent introduced into the column, the liquid content of the absorber can be changed to be mostly acetic acid; for example, after the transition between acetic acid and methanol, the weight ratio of methanol to acetic acid in the absorber while the off-gas stream is contacted with the acetic acid stream is less than 1:500 (e.g., less than 1:1,500, or less than 1:4,000). This transition time is desirably relatively fast, and one skilled in the art can switch between introducing a methanol stream and an acetic acid stream within minutes or even seconds. However, depending on the volume of the absorber, it may take some time for the new, lower ratio of methanol to acetic acid to be established.

[0032] The third temperature (i.e., the temperature of the acetic acid stream at the liquid inlet) is greater than the freezing point of acetic acid. In certain embodiments described elsewhere herein, the third temperature is at least 20°C, e.g., at least 22°C, or at least 24°C. In certain embodiments described elsewhere herein, the third temperature is in the range of 18°C ​​to 50°C, e.g., 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.

[0033] The third temperature (i.e., the temperature of the acetic acid stream) can be conveniently set to be about the same as or higher than the first temperature (i.e., the temperature of the methanol stream). For example, in certain embodiments described elsewhere 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 be advantageously carried out at higher temperature differences, and in other embodiments, the third temperature is 5 to 25°C (e.g., 7.5 to 25°C, or 10 to 25°C, or 15 to 25°C, or 5 to 15°C, or 7.5 to 15°C, or 10 to 15°C) higher than the first temperature.

[0034] The off-gas generated from the reaction zone is generally referred to as high-pressure off-gas. The off-gas generated in the flash zone generally flows to and is removed from the light ends recovery section. The light ends recovery section generally includes a light ends column, a condenser section containing one or more condensers, and a phase separation vessel (decanter). The off-gas generated in the light ends recovery section can be the off-gas withdrawn from the condenser section and / or the off-gas withdrawn from the decanter.

[0035] The off-gas produced in the light ends recovery section is commonly referred to as low pressure off-gas.

[0036] The high pressure off-gas and low pressure off-gas streams may be combined to produce a low pressure off-gas.

[0037] In some or all of the embodiments described elsewhere herein, the off-gas being scrubbed is a low-pressure off-gas.

[0038] Generally, methyl iodide is present in off-gases that are scrubbed as described herein, typically as entrained and / or vaporized methyl iodide. Off-gases that are scrubbed as described herein can vary widely in their methyl iodide content; for example, methyl iodide may be present in the off-gases in amounts from about 1 mol % to about 20 mol %.

[0039] In addition to methyl iodide, the off-gas may include one or more non-condensable components, such as inert gases such as carbon monoxide, nitrogen, and reaction by-product gases such as hydrogen, carbon dioxide, and methane. The off-gas may also include at least one of acetic acid, methyl acetate, and water.

[0040] The acetic acid production unit described herein is a unit that produces at least acetic acid, and includes a unit that co-produces acetic acid and acetic anhydride. Those skilled in the art will appreciate that a variety of acetic acid production units can be configured to implement the scrubbing methods described herein. Typically, an acetic acid production unit includes a reaction zone (e.g., including a reactor) configured to recover acetic acid and a light ends recovery zone (e.g., including a light ends column). Other reaction and recovery zones can be present. For example, a flash zone is typically used between the reaction zone and the light ends recovery zone. Off-gas streams from any portion of an acetic acid production unit can be scrubbed as described herein. For example, production equipment and operations for producing acetic acid by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst are well known to those skilled in the art.

[0041] In the processes for producing acetic acid described herein, methanol and / or its reactive derivatives can be introduced into the reaction zone as a liquid reactant (i.e., to form a liquid reaction composition in the reaction zone). For example, in certain desirable embodiments of the processes described elsewhere herein, methanol is introduced into the reaction zone as a reactant. In other processes described elsewhere 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 is introduced into the reaction zone as reactants. As used herein, "reactive derivatives" of methanol are methyl acetate, dimethyl ether, and methyl iodide. In certain embodiments described elsewhere herein, methanol and / or methyl acetate are used as liquid reactants. In one embodiment described elsewhere herein, methanol is used as a reactant, in another embodiment described elsewhere herein, methyl acetate is used as a reactant, and in yet another embodiment described elsewhere herein, a mixture of methanol and methyl acetate is used as a reactant.

[0042] The processes described herein can employ a variety of carbonylation catalysts, such as Group VIII noble metal carbonylation catalysts. The catalyst can include a Group VIII species supported on an inert support, such as a carbon support. In certain desirable embodiments described elsewhere herein, the carbonylation catalyst comprises rhodium, iridium, or a mixture thereof. In one particular embodiment of the present invention, the carbonylation catalyst is iridium. In another particular embodiment described elsewhere herein, the carbonylation catalyst is a rhodium catalyst. A catalyst promoter can optionally be present and is selected from, for example, alkali metal iodides (e.g., lithium iodide), alkaline earth metal iodides, aluminum group metal iodides, organic iodide salts, ruthenium, osmium, rhenium, and mixtures thereof. When the catalyst is a rhodium catalyst, the optional carbonylation catalyst promoter can desirably be selected from alkali metal iodides, such as lithium iodide, alkaline earth metal iodides, aluminum group metal iodides, and / or organic iodide salts, and mixtures thereof. When the catalyst is an iridium catalyst, the optional carbonylation catalyst promoter may desirably be selected from ruthenium, osmium, rhenium, and mixtures thereof.

[0043] When the carbonylation catalyst is an iridium catalyst, the iridium catalyst can comprise 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 soluble in the liquid reaction composition or that can be converted to a soluble form. The iridium catalyst is desirably used as a non-chlorine-based compound, such as an acetate salt, that is soluble in one or more of the liquid reaction composition components (e.g., water and / or acetic acid) and can therefore be added to the reaction composition as a solvent therein. Examples of suitable iridium-containing compounds that can be added to the liquid reaction composition include IrCl, IrI, IrBr, [Ir(CO)I], [Ir(CO)Cl], [Ir(CO)Br], [Ir(CO)I]. - H + , [Ir(CO)2Br2] - H+ , [Ir(CO)2II2] - H + , [Ir(CH3)I3(CO)2] - H + , Ir(CO) 12 , IrCl3.4H2O, IrBr3.4H2O, Ir3(CO) 12 , metallic iridium, Ir2O3, IrO2, Ir(acac)(CO)2, Ir(acac)3, iridium acetate, [Ir3O(OAc)6(H2O)3][OAc], and hexachloroidinic acid H2[IrCl6], preferably non-chlorinated complexes of iridium such as acetate, oxalate and acetoacetate.

[0044] When present, the concentration of iridium catalyst in the liquid reaction composition in the reaction zone may range, for example, from 100 to 6000 ppm by weight of iridium alone.

[0045] When the carbonylation catalyst is an iridium catalyst, the carbonylation catalyst promoter is preferably ruthenium. The promoter may comprise any ruthenium-containing compound that is substantially soluble in the liquid reaction composition. The ruthenium promoter may be added to the liquid reaction composition in any suitable form that is substantially soluble in the liquid reaction composition or that can be converted to a soluble form. The ruthenium promoter compound is preferably used as a non-chlorine-based compound, e.g., acetate, that is soluble in one or more of the liquid reaction composition components (e.g., water and / or acetic acid) and therefore may be added to the reaction composition as a solvent therein.

[0046] 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, [Ru(CO)3I3] - H +Examples of suitable ruthenium halocarbonyls include tetra(aceto)chlororuthenium(II,III), ruthenium(III) acetate, ruthenium(III) propionate, ruthenium(III) butyrate, ruthenium pentacarbonyl, dodecacarbonyltriruthenium, and mixed ruthenium halocarbonyls such as dichlorotricarbonylruthenium(II) dimer, dibromotricarbonylruthenium(II) dimer, and organic ruthenium complexes such as tetrachlorobis(4-cymene)diruthenium(II), tetrachlorobis(benzene)diruthenium(II), dichloro(cycloocta-1,5-diene)ruthenium(II) polymer, and tris(acetylacetonato)ruthenium(III). Desirably, the ruthenium-containing compound is free of impurities that form or generate in situ ionic iodides, such as alkali metal or alkaline earth metal or other metal salts, which may inhibit the reaction.

[0047] The ruthenium promoter can be present in any effective amount up to the limit of its solubility in the liquid reaction composition, liquid fraction, and / or any liquid process stream recycled to the carbonylation reaction zone. For example, the ruthenium promoter is suitably present in the liquid reaction composition in a molar ratio of ruthenium promoter to iridium of from [0.1 to 100]:1, preferably from [greater than 0.5]:1, more preferably from [greater than 1]:1, preferably up to 20]:1, more preferably from [greater than 15]:1, and even more preferably from [greater than 10]:1. ​​The concentration of ruthenium promoter in the liquid reaction composition in the reaction zone is typically less than 6000 ppm. Suitable promoter concentrations range, for example, from 400 to 5000 ppm, e.g., from 2000 to 4000 ppm.

[0048] Suitable rhodium carbonylation catalysts are described, for example, in EP Publication No. 0161874, U.S. Patent No. 6,211,405, and EP Publication No. 0728727, the entire disclosures of which are incorporated herein by reference. When the carbonylation catalyst is a rhodium catalyst, the rhodium catalyst concentration in the liquid reaction composition is preferably in the range of from 50 to 5000 ppm, preferably from 100 to 1500 ppm, by weight of rhodium. When 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-mentioned documents.

[0049] Thus, in certain embodiments, the homogeneous carbonylation of methanol and / or its reactive derivatives with carbon monoxide 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 such 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.

[0050] As noted above, water can be present in the liquid reaction composition. Those skilled in the art will appreciate that water is generated 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 can also be introduced into the reaction zone (e.g., together with the other components of the liquid reaction composition or separately). In certain desirable embodiments, water is present in the liquid reaction composition in an amount in the range of 0.1 wt.% to 15 wt.%, e.g., in the range of 1 wt.% to 15 wt.%, or in the range of 1 wt.% to 8 wt.%.

[0051] Methyl acetate may be formed in situ in the liquid reaction composition by reaction of methanol and / or its reactive derivatives with the acetic acid product or solvent. In certain embodiments described elsewhere herein, the concentration of methyl acetate in the liquid reaction composition in the reaction zone ranges from 2 to 50 wt%, for example, from 3 to 35 wt%.

[0052] As noted above, propionic acid by-product may also be present in the liquid reaction composition, hi certain embodiments, propionic acid is present in the liquid reaction composition in an amount in the range of from 200 ppmw to 2,500 ppmw, for example, in the range of from 400 ppmw to 2,000 ppmw, or in the range of from 600 ppmw to 1,400 ppmw.

[0053] In certain desirable embodiments, methyl iodide is present in the liquid reaction composition in an amount in the range of from 1 wt.% to 20 wt.% For example, in certain such embodiments, methyl iodide is present in the liquid reaction composition in an amount in the range of from 2 wt.% to 16 wt.%.

[0054] In certain embodiments described elsewhere herein, the liquid reaction composition comprises a solvent, for example, in certain such embodiments, the liquid reaction composition comprises acetic acid solvent (e.g., recycled from the separation zone of an acetic acid production unit).

[0055] As noted above, acetic acid can be produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide. In certain embodiments described elsewhere herein, the carbon monoxide fed to the reaction zone is essentially pure. In other embodiments, the carbon monoxide fed to the reaction zone contains one or more impurities, such as, for example, carbon dioxide, methane, nitrogen, hydrogen, or a noble gas. In certain embodiments described elsewhere herein, the partial pressure of carbon monoxide (e.g., within the reactor of the reaction zone) is in the range of 1 bar to 70 bar, e.g., 1 bar to 35 bar.

[0056] In certain embodiments described elsewhere herein, the carbonylation reaction in the reaction zone is conducted at a total pressure (eg, within the reactor of the reaction zone) in the range of from 10 bar to 100 bar.

[0057] The effluent from the reaction zone may be directed to a flash separation zone (e.g., by flash valve 62) where it may be separated into a gas stream rich in acetic acid and a liquid stream lean in acetic acid. The liquid stream may be introduced into the reaction zone (i.e., as a recycle). For example, with reference to Figure 1, effluent 50 may be directed to flash separation zone 60 to form vapor fraction 70 and liquid fraction 75. At least a portion of liquid fraction 75 may be introduced into reaction zone 10.

[0058] Flash separation zones are known in the art. In certain embodiments, the flash separation zone may comprise an adiabatic flash vessel. Alternatively or additionally, the flash separation zone may be heated, for example, by a heater. The flash separation zone may typically operate at a pressure ranging from 0 to 10 barg, preferably from 0 to 3 barg.

[0059] The carbonylation processes described herein can be carried out as batch processes or as continuous processes. In certain desirable embodiments, the carbonylation process is carried out as a continuous process.

[0060] In certain embodiments, at least a portion of the vapor fraction from the flash separation zone is directed to a light ends recovery zone of an acetic acid production unit. In certain embodiments, the light ends recovery zone of the acetic acid production unit is configured to separate at least components more volatile than acetic acid (e.g., vapor fractions withdrawn from vessels described elsewhere herein) from the acetic acid. For example, in certain embodiments, acetic acid is produced in a reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the light ends recovery zone of the acetic acid production unit is configured to separate acetic acid and further separate methyl iodide and methyl acetate for recycling to the reaction zone.

[0061] In certain embodiments described elsewhere herein, the light ends recovery zone comprises a distillation column that separates a crude acetic acid product comprising acetic acid and propionic acid from a light ends fraction comprising methyl iodide and methyl acetate. In certain such embodiments, the light ends recovery zone further comprises a drying column. For example, in certain embodiments described elsewhere herein, the light ends recovery zone comprises a combined light ends and drying column in which water is removed from the crude acetic acid product to form a dry acetic acid product comprising acetic acid and propionic acid. As used herein, a "dry" or "dried" stream comprising acetic acid (e.g., optionally propionic acid) comprises water in an amount of up to 1,500 ppmw.

[0062] Suitable towers for use in the light ends recovery zone and their configurations are known in the art. Typically, at least a first fraction comprising acetic acid and propionic acid and an overhead vapor fraction comprising methyl acetate, water, acetic acid, carbon monoxide, and methyl iodide are formed in the light ends recovery zone. In certain embodiments, the separated water can be recycled to the reaction zone or removed from the acetic acid production unit.

[0063] In certain embodiments described elsewhere herein, the light ends recovery zone further comprises one or more condensers and / or coolers for condensing the overhead vapor fraction to form a liquid fraction. Those skilled in the art will appreciate that any suitable method known in the art can be utilized to condense the overhead vapor fraction to a liquid phase. For example, in certain embodiments, the fraction is condensed using at least one heat exchanger (e.g., supplied with water as a cooling medium). Components of the overhead fraction that are not condensed (e.g., carbon monoxide, carbon dioxide, inert gases, reaction by-product gases) are removed from the light ends recovery zone as an off-gas stream. In certain embodiments, acetic acid is produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the off-gas stream removed from the light ends recovery zone further comprises methyl iodide (e.g., present as entrained and / or vaporized methyl iodide), methyl acetate, and water.

[0064] In certain embodiments, the liquid fraction formed in the light ends recovery zone comprises methyl acetate, water, and acetic acid. In certain embodiments, the acetic acid is produced in the reaction zone by carbonylating 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).

[0065] In certain embodiments described elsewhere herein, the light ends recovery zone includes a decanter in which the liquid fraction separates into two layers: a lower layer (e.g., an organic layer) comprising methyl acetate and an upper layer (e.g., an aqueous layer) comprising water. In certain embodiments, acetic acid is produced in the reaction zone by carbonylating 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 to the distillation column of the light ends recovery zone as reflux. 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 withdrawn from the decanter and transferred to an off-gas scrubbing treatment unit (e.g., before disposal).

[0066] In certain embodiments, a stream containing acetic acid and propionic acid from the light ends recovery section (e.g., the first fraction formed in the light ends recovery section) is transferred to the heavy ends column through a feed inlet located at an intermediate point within the column. In such embodiments, a stream containing propionic acid is withdrawn from the heavy ends 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 an outlet at the top of the column, as a side draw stream from an outlet located higher than the feed inlet). In certain embodiments, the product stream comprises essentially acetic acid and further comprises less than 400 ppmw, or less than 300 ppmw, or less than 250 ppmw of propionic acid. In certain embodiments, the product stream comprises essentially acetic acid and further comprises less than 1,500 ppmw of water. In certain desirable embodiments, the product stream comprises essentially acetic acid and further comprises less than 1,500 ppmw of combined propionic acid and water. Suitable columns that can be used as heavy ends columns and their configurations are known in the art. For example, in certain embodiments, the heavy ends column is connected to a condenser. As another example, in certain embodiments, a reboiler is connected to the base of the heavy ends column. [Example]

[0067] The present invention can be illustrated by the following example, in which the performance of an absorber tower such as that shown in FIG. 1 was modeled in AspenPlus v10.

[0068] Example 1 acetic acid An off-gas composition 142 from the acetic acid production process, comprising carbon monoxide, carbon dioxide, nitrogen, and methyl iodide, is introduced into absorber 110, where it is contacted with an acetic acid stream. The acetic acid stream comprises a mixture of acetic acid from heavy ends column 195 and acetic acid from an acetic acid storage vessel (not shown). The acetic acid stream is introduced through liquid inlet 134 at an inlet temperature of 24° C. Liquid acetic acid, containing absorbed methyl iodide, is removed from the column through liquid outlet 126 and recycled to light ends recovery zone 190. Under these conditions, the overhead stream from the absorber contains carbon monoxide (and carbon dioxide and nitrogen) and about 90 ppmv of methyl iodide, representing a methyl iodide bleed ratio (the ratio of the mole fraction of methyl iodide in the vapor outlet to the mole fraction of methyl iodide in the off-gas inlet) of 0.000430%.

[0069] Example 2 methanol Example 1 was repeated, except that instead of using acetic acid, a liquid stream containing methanol was used. This 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 (off-gas composition, pressure, temperature, and all flow rates) were the same. Liquid methanol containing absorbed methyl iodide was removed from the column through liquid outlet 124 and recycled to reaction zone 160. Under these conditions, the overhead stream from absorber column 110 contained carbon monoxide and approximately 10 ppmv of methyl iodide, representing a molar vapor ratio (vapour out / vapour in) of 0.000044%.

[0070] This example demonstrates that effective methyl iodide removal from the off-gas can be achieved at 24°C, a temperature much higher than that conventionally used with methanol as the absorption liquid.

[0071] Example 3 Switching from acetic acid to methanol In this example, the absorber is initially operated under the conditions of Example 1. This would correspond to, for example, a "start-up" where initially there is not enough methanol in overhead condenser 165 for use in the absorber. It is then desired to switch from using acetic acid to using methanol. In this example, the liquid feed transitions from using only acetic acid (through inlet 134) to using only methanol (through inlet 132). During the intermediate period, liquid was introduced through both inlets, so the liquid stream in the absorber was a mixture of acetic acid and methanol.

[0072] The total liquid flow rate was kept constant during the transfer, resulting in the same final conditions as in Example 2. In this case, the methyl iodide steadily decreased during the transfer, as shown in Table 1 below. (Table 1) TIFF0007799686000001.tif26117

[0073] This demonstrates that the transition can be performed without changing the temperature during the transition and that low levels of methyl iodide can be obtained throughout the off-gas stream. After the liquid stream was transitioned to methanol, the liquid outlet from the column was switched from outlet 126 to outlet 124.

[0074] Although not done in this or subsequent examples, it will be apparent that the liquid flow can be varied during the transition. For example, since the results above indicate that methanol is more efficient at removing methyl iodide, its flow rate can be reduced during or after the transition while still achieving low levels of residual methyl iodide in the vapor.

[0075] Example 4 Switching from methanol to acetic acid In this example, the absorber tower is initially operated under the conditions of Example 2. This can represent either Example 2 or Example 3 after the transition. It is then desired to switch from using methanol to using acetic acid. In this example, the acetic acid feed (to inlet 134) can be immediately started, transitioning the liquid feed from using only methanol to using only acetic acid. At the start of the transition, the liquid outlet from the tower is also switched from outlet 126 to outlet 124. Essentially the same results as in Example 3 are obtained with respect to the methyl iodide content of the absorber tower overhead under different conditions, but in reverse.

[0076] It is a particular advantage of the present invention as shown in this example that there is no need to adjust the temperature of the absorber tower before initiating the transition, and therefore the addition of acetic acid to initiate the transition can be started immediately.

[0077] (Comparative Example 1) Switching from methanol to acetic acid The process of Example 4 was repeated, except that the initial conditions were that methanol was introduced into the absorber at 5° C. If it was desired to switch from using methanol to using acetic acid, the temperature of the absorber had to be first increased until the temperature of the absorber at the liquid inlet of acetic acid (134) was above the freezing point of acetic acid. Thus, the transition could not be initiated immediately and required a longer period of time.

[0078] The entire disclosures of all patent and non-patent literature cited herein are hereby incorporated by reference, except that in the event of any disclosure or definition that contradicts this specification, the disclosure or definition of this specification shall be deemed to control.

[0079] The foregoing detailed description and accompanying drawings have been provided for purposes of explanation and illustration and are not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to those skilled in the art and remain within the scope of the appended claims and their equivalents.

[0080] It is understood that the elements and features recited in the appended claims may be combined in different ways to create new claims that also fall within the scope of the present disclosure. Thus, where the dependent claims appended below depend only on a single independent or dependent claim, it is understood that such dependent claims may alternatively depend on any preceding claim in any other way, whether independent or dependent, and that such new combinations form part of this specification. [Explanation of symbols]

[0081] 110 Absorption Tower 120 Lower Section 122 Supply inlet 124 Liquid outlet 126 Liquid outlet 130 Upper Section 132 Liquid inlet 134 Liquid inlet 136 Steam outlet

Claims

1. 1. A process for scrubbing an off-gas stream comprising carbon monoxide and methyl iodide in an absorption column of an acetic acid production unit, the absorption column comprising: a lower section including a feed inlet and one or more liquid outlets; an upper section above the lower section, the upper section including one or more liquid inlets and a vapor outlet; Equipped with The process comprises: introducing the off-gas stream into the absorber tower through the feed inlet; introducing a methanol stream at a first flow rate through one liquid inlet of the one or more liquid inlets, the methanol stream having a first temperature at the liquid inlet, the first temperature being at least 18°C; contacting the off-gas stream with the methanol stream in the absorber; withdrawing a first liquid effluent from the absorber tower through one liquid outlet of the one or more liquid outlets, the first liquid effluent comprising methanol and methyl iodide; withdrawing a vapor effluent from the absorber tower through the vapor outlet; Including, The process further includes separating the withdrawn vapor effluent in a heat exchanger to provide a vapor fraction and a liquid fraction comprising methanol; - the vapor fraction contains a maximum of 2.5 wt. % methanol; - said methanol stream comprises at least a portion of said liquid fraction comprising methanol; The process includes, after the methanol stream is introduced into the absorber, reducing the flow rate of the methanol transferred to the absorber; conveying an acetic acid stream at a third flow rate through one liquid inlet of the one or more liquid inlets to an upper section of the absorber, the acetic acid stream having a third temperature at the liquid inlet; contacting the off-gas stream with the acetic acid stream in the absorber; further comprising the third temperature is in the range of 18°C ​​to 40°C; The process wherein the third temperature differs from the first temperature by up to 5°C.

2. 10. The process of claim 1, wherein the vapor effluent withdrawn during introduction of the methanol stream into the absorber contains up to 500 ppmv of methyl iodide.

3. The process described in claim 1, wherein the vapor effluent withdrawn during the introduction of the methanol stream into the absorption tower contains up to 200 ppmv of methyl iodide.

4. 4. The process of claim 1, further comprising transferring the first liquid effluent to a reaction zone of an acetic acid production unit.

5. 5. The process of claim 4, wherein the liquid effluent transferred to the reaction zone of the acetic acid production unit contains up to 250 ppmv of acetic acid.

6. prior to introducing the methanol stream into the absorber; transferring the off-gas stream to a lower section of an absorber tower through the feed inlet; introducing an acetic acid stream at a second flow rate into the upper section of the absorber through one liquid inlet of the one or more liquid inlets, the acetic acid stream having a second temperature at the liquid inlet; contacting the off-gas stream with the acetic acid stream in the absorber; Including, The method includes, after a first period of time of contacting the off-gas stream with the acetic acid stream, reducing the flow rate of the acetic acid stream; introducing the methanol stream into the absorption tower to convert acetic acid to methanol; Including, the second temperature is at least 18°C; 6. The process of claim 1, wherein the second temperature differs from the first temperature by up to 5°C.

7. 7. The process of claim 1, wherein the first temperature is in the range of 18 to 30°C.

8. A process described in any of claims 1 to 7, wherein the first temperature is in the range of 20°C to 35°C.

9. A process described in any of claims 1 to 8, wherein the third temperature is in the range of 20°C to 40°C.

10. 10. The process of any of claims 1 to 9, wherein the off-gas stream comprises at least a portion of one or more of the vapor effluents of a separation zone, a recovery zone, and a reaction zone of the acetic acid production unit.

11. 10. The process of claim 1, wherein the vapor fraction comprises up to 1.5 wt. % methanol.

12. The process of claim 1 , further comprising transferring the vapor fraction to a combustion system and combusting the vapor fraction in the combustion system.

13. The process of claim 1, wherein the flow rate of the methanol transferred to the absorption tower is reduced to zero.

14. 14. The process of any of claims 1 to 13, wherein the third temperature differs from the first temperature by up to 3.5°C.

15. The process of claim 14, wherein the third temperature differs from the first temperature by up to 2°C.

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