MXPA00009792A - Process for the manufacture of acetic anhydride - Google Patents

Abstract

Disclosed is a process for the manufacture of acetic acid anhydride in the liquid phase which comprises the steps of:(1) continuously feeding to a first reaction zone (i) a reactant compound selected from methyl acetate, dimethyl ether or a mixture thereof, (ii) methyl iodide, (iii) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, and (v) carbon monoxide, wherein the carbon monoxide is fed below the surface of the liquid reaction mixture to produce a first liquid reaction comprising components (i), (ii), (iii) and (iv) as well as (v) dissolved carbon monoxide and (vi) acetic anhydride product;(2) feeding the first liquid reaction mixture to a second reaction zone in which is maintained a liquid phase and an overhead vapor space under a total pressure of 40 to 50 bar absolute, no carbon monoxide being fed below the surface of the liquid phase, and the residence time of the liquid phase in said second reaction zone being at least 2 minutes to produce a second liquid reaction mixture comprising components (i), (ii), (iii), (iv), (v) and (vi), and (3) removing the second liquid reaction mixture from the second reaction zone. The second liquid reaction mixture contains less dissolved carbon monoxide than the first liquid reaction mixture.

Description

PROCESS FOR THE MANUFACTURE OF ACETIC ANHYDRIDE DESCRIPTION OF THE INVENTION This invention pertains to an improved carbonylation process for the manufacture of acetic anhydride in which the use of carbon monoxide is increased. More specifically, this invention pertains to the liquid phase manufacture of acetic anhydride by contacting a mixture of (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (11) methyl iodide, (111) catalyst components comprising rhodium and one or more promoters, and (iv) carbon monoxide dissolved in a carbonylation zone in which the carbon monoxide is not fed below the surface of the mixture in a form to increase the concentration of carbon monoxide in the liquid reaction mixture. The methyl acetate and / or dimethyl ether, methyl iodide and carbon monoxide present in the mixture react to form acetic anhydride, thereby significantly lowering the concentration of the carbon monoxide in the mixture. When the resulting reaction mixture is subjected to conventional rapid distillation to remove and recover acetic anhydride, less carbon monoxide is lost from the production system. The preparation of acetic anhydride has been reported extensively in the patent literature contact in the liquid phase a mixture comprising methyl acetate and / or dimethyl ether and methyl iodide with carbon monoxide in the presence of a rhodium catalyst. See, for example, U.S. Patents 3,927,078; 4,046,807, 4,115,444, 4,252,741, 4,374,070; 4,430,273; 4,559,183, 5,003,104, and 5,292,948 and European Patents 8396, 87,869; and 87,870 These patents disclose that the reaction rate may be increased if the catalyst system includes a promoter such as certain amines and quaternary ammonium catalyst components, phosphones and phosphonium compounds and inorganic compounds such as alkali metal salts for example, iodide lithium. Normally, both the reaction mixture (process) and the unpurified product are substantially anhydrous, homogeneous liquids comprising a solution of the reactants and catalyst component in an inert solvent such as acetic acid. In this way, the unpurified liquid product obtained from such acetic anhydride processes typically comprises a mixture of acetic anhydride and acetic acid as a result of the use of acetic acid as a process solvent. Acetic acid can be co-produced in the process by feeding methanol and / or water to the production system, for example, by feeding methanol and / or water to a process recirculation stream containing acetic anhydride and / or to the carbonylation reactor.

The processes described above for the manufacture of acetic anhydride are carried out by feeding carbon monoxide to a reaction zone containing a liquid mixture of (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (11) Methyl iodide and (111) catalyst components comprising rhodium and one or more promoters The carbon monoxide is fed below the surface of the liquid mixture in a finely divided form, for example, by means of a spraying device of gas, to maximize the concentration of carbon monoxide in the reaction mixture Normally, the process is operated by continuously feeding the reactive compound, methyl iodide, an inert solvent such as acetic acid, and catalyst components dissolved in acetic anhydride and / or acetic acid and carbon monoxide to a reaction zone and continuously withdraw from the reaction zone a product mixture without acetic acid comprising acetic anhydride and acetic acid The mixture of the unpurified product also contains a reactive compound, methyl iodide, acetic acid solvent and catalyst components and carbon monoxide dissolved in the product without purification The product without purification Car is continuously fed to a first separation zone where the pressure is reduced and the unpurified product is rapidly distilled to produce (i) a vapor effluent comprising the reactive compound, methyl iodide, acetic acid solvent, acetic anhydride product and carbon monoxide and (11) a liquid effluent comprising the components of the catalyst dissolved in a mixture of acetic anhydride and acetic acid. steam effluent typically comprises about 30 to 90 weight percent of the unpurified product fed to the first separation zone. The liquid effluent is recirculated to the reaction zone and the steam effluent is fed to a recovery zone of the product where acetic anhydride (and any co-produced acetic acid) is separated and removed from the production system The other condensable components (methyl acetate, methyl iodide and acetic acid solvent) are recovered and recirculated to the reaction zone The carbon monoxide contained in the mixture of the unpurified product can be recovered only at great expense For example, the recovery The conversion of the carbon monoxide by recompressing and recirculating the purged gas is undesirable since the inert impurity components of the recovered gas, for example, hydrogen, methane, carbon dioxide, nitrogen and argon accumulate in the reaction system and reduce the Partial pressure of carbon monoxide The removal of impurities from carbon monoxide involves complex, large-capital processes such as cryogenic distillation, adsorption, or membrane separators followed by recompression. The capital cost of equipment for the purification and recompression of the discharged carbon monoxide exceeds the value of the carbon monoxide. Consequently, the carbon monoxide contained in the steam effluent from the first separation zone is typically not recovered and is lost from the process at significant expense. The carbon monoxide that is not recovered can constitute up to 10 percent of the volume of the total carbon monoxide fed to the reaction zone. If the carbon monoxide is not recovered, the gaseous effluent must either be treated or burned. In this way, an economic means for the use of the carbon monoxide present in the reaction product without purifying can improve the overall economy of the process. Now an acetic anhydride manufacturing process has been developed where the use of carbon monoxide is increased, thus increasing the total production and yield of acetic anhydride (and any co-produced acetic acid) produced per unit of carbon monoxide fed to the process of carbonylation. The present invention therefore provides a liquid phase process for the manufacture of acetic anhydride ba or substantially anhydrous conditions comprising the steps of (1) continuously feeding to a first reaction zone) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (11) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, and (v) carbon monoxide, wherein the carbon monoxide is fed below the surface of the liquid reaction mixture comprising components (i), (n) and (iv) and the reactive compound is converted to acetic anhydride to produce a first liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, (v) dissolved carbon monoxide and (vi) acetic anhydride product; (2) removing the first liquid reaction mixture from the first reaction zone and feeding it to a second reaction zone which it comprises at least one reaction vessel in which a liquid phase is maintained comprising the first liquid reaction mixture and a vapor phase of heads wherein the partial pressure of the carbon monoxide of about 10 to 30 absolute bar (bara) it keeps, carbon monoxide is not fed below the surface of the liquid phase, and the residence time of the liquid phase in the second reaction zone is at least 2 minutes, preferably from about 4 to 10 minutes, to produce a second liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (II) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, (v) dissolved carbon monoxide and (vi) acetic anhydride product, and (3) removing the second liquid reaction mixture from the second reaction zone, where the use of carbon monoxide in the process is increased, that is, the concentration of the carbon monoxide in the second liquid reaction mixture is lower than the concentration of the carbon monoxide in the first liquid reaction mixture. Typically, the concentration of the carbon monoxide in the second liquid reaction mixture is about 50 volume percent or less, preferably about 5 to 50 volume percent, of the concentration of the carbon monoxide in the first liquid reaction mixture. U.S. Patent 5,672,744 pertains to a process for the preparation of acetic acid comprising the steps of: (1) carbonylating methanol with carbon monoxide in a first reactor in the presence of a reaction fluid comprising a rhodium catalyst , methyl iodide, an iodide salt, methyl acetate and water; (2) extracting a reaction fluid having carbon monoxide dissolved therein from the first reactor and introducing it into a second reactor; (3) carbonyl methanol in the second reactor with the carbon monoxide dissolved in the reaction fluid in a residence time of 7 to 30 seconds and a temperature of 150 to 220 ° C and form a mixture of acetic acid without purify; and (4) introduce the unpurified acetic acid mixture in a rapid zone to separate it into a vapor phase and a liquid phase. U.S. Patent 5,672,744 does not contemplate a process for making ba-acetic anhydride or substantially anhydrous conditions using a residence time in the second reactor or reaction zone of at least 2 minutes. Additionally, the 744 patent does not disclose the importance of using the particular reaction system used in the second reaction zone of the process of the present invention. The present invention is an improvement of the rhodium-catalyzed carbonylation processes described in the literature such as the above-referenced patent publications. In this way, our novel process can be carried out by continuously feeding to a first reaction zone (i) a compound Selected reagent of methyl acetate, dimethyl ether or a mixture thereof, (n) iodide of m ethyl, (m) dissolved catalyst components comprising rhodium and one or more promoters, (vv) acetic acid solvent, and (v) carbon monoxide wherein the carbon monoxide is fed below the surface of the liquid reaction medium The feed to the carbonylation zone can also include methanol and / or water to co-produce acetic anhydride and acetic acid as described in Published European Patent Publications 87,869 and 87,870. The process of the present invention is operated under substantially anhydrous conditions, that is, under steady-state operating conditions, ie, the water either can not be detected or can only be detected in trace quantities. Any water fed into the process is fed in an amount that is significantly less than the amount needed to convert all acetic anhydride to acetic acid. The first reaction zone may comprise one or more pressure vessels that may be provided with a means for agitation. The design of the container can be a pipe, column, tank, stirred tank reactor or other design. It is preferred that the first reaction zone comprises at least one generally column vessel equipped with one or more internal baffles which, in combination with the carbon monoxide gas spraying device, create a highly recirculating reaction mixture. agitated The residence time of the reagent within the first reaction zone is usually at least 20 minutes and, preferably, is in the range of about 30 to 50 minutes. The reactive compound, methyl iodide and monoxide of carbon react in the first reaction zone to form acetic anhydride to produce a first liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (11) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, (v) dissolved carbon monoxide and ( vi) acetic anhydride product. This first reaction mixture is fed to a second reaction zone comprising at least one container containing a liquid phase comprising the first liquid reaction mixture and a vapor phase of heads. The container constituting the second reaction zone may be of a column or tank design and may be equipped with a means for agitating the liquid phase contained therein. A non-agitated plug flow reactor is preferred as the monoxide is exhausted of carbon plus for plug flow design versus a stirred tank reactor design By controlling the total pressure and liquid level in the second reaction zone, the amount of carbon monoxide that is exhausted from the first reaction mixture liquid can be controlled to a predetermined level in order to avoid total or near total depletion, resulting in possible harmful effects for the catalytically active rhodium complex and its solubility in the reaction mixture. It is an important practical design feature to maintain a level of reactor liquid with a head steam space such that the vapor space is approximately 5 to 30% of the total reactor volume. This design feature provides a compressible gas damper in the case that the reactor is "bottled". If the bottling occurs, the temperature will increase as soon as the reaction of the dissolved carbon monoxide reaches the termination and an expansion of the volume of the reactor liquid will occur. With the presence of a gas damper, the gas will become compressed rather than having a hydraulic release from the reactor. The total pressure within the second reaction zone is approximately equal to the total pressure in the first reaction zone and is preferably in the range of about 30 to 50 bara. The carbon monoxide can be fed into the vapor space of the second reaction zone to maintain the gas buffer and the desired total reaction pressure. This may be necessary since the liquid in the upper part of the reactor comprising the second reaction zone will be considerably below the saturation limit of the carbon monoxide (since the dissolved carbon monoxide will have reacted) and the carbon monoxide in the space of steam will diffuse from the gas into the liquid. Production can be achieved additional from the aggregate carbon monoxide as well as from the dissolved carbon monoxide. Carbon monoxide is not fed below the surface of the liquid phase in the second reaction zone. The residence time of the liquid phase in the second reaction zone is at least 2 minutes, preferably from about 4 to 10 minutes. The reactive compound, methyl iodide and carbon monoxide dissolved in the first reaction mixture react in the second reaction zone to produce additional acetic anhydride, thereby decreasing the concentration of carbon monoxide in the liquid phase. The first reaction mixture usually contains sufficient reactive compound to react with and consume carbon monoxide in the second reaction zone. The reactive compound typically is present in a concentration of at least 5, preferably in a concentration in the range of about 20 to 40, percent by weight based on the total weight of the first reaction mixture. If necessary or desired, additional reactive compound may be added to the second reaction zone. The second liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components that comprise rhodium and one or more promoters, (iv) acetic acid solvent, (v) dissolved carbon monoxide and (vi) formed acetic anhydride product is removed from the second reaction zone and fed to a separation zone where the acetic anhydride product and any co-produced acetic acid are recovered. As mentioned previously, the concentration of the carbon monoxide in the second liquid reaction mixture is about 50 volume percent or less, preferably about 5 to 50 volume percent, of the concentration of carbon monoxide in the first liquid reaction mixture In a preferred mode of operation, the conversion of the reactive compound and methyl iodide components of the first liquid reaction mixture can be maximized by feeding the mixture in a finely divided form, for example, as a dew or turbulent liquid, to the gaseous space of the second reaction zone. The second mixture of the reaction Liquid ion is removed from the opposite end (bottom) of the second reaction zone This mode of operation results in increased uptake or consumption of carbon monoxide in the gas space and thus increased production of acetic anhydride while achieving the primary objective of reduce the dissolved carbon monoxide in the first liquid reaction mixture. The reaction zones are maintained in temperature and high pressure (total) such as 100 to 300 ° C and 21.7 to 276.7 bara (300 to 4000 pounds per square inch gauge - psig) although temperatures and total pressures in the range of 175 to 220 ° C and 35.5 are more common at 104.4 bara (500 to 1500 psig). Typically, the reaction of the dissolved carbon monoxide in the second reaction zone is allowed to occur adiabatically at an increased light temperature, for example, to about 5 ° C. The gas fed to the carbonylation zone may consist of essentially carbon monoxide or a mixture of carbon monoxide and hydrogen, for example, a mixture of carbon monoxide and up to 7 volume percent of hydrogen. The rhodium component of the catalyst system can be provided to the process in various forms such as rhodium t-chloride or triiodide, rhodium hydrate, or rhodium carbonyl complexes, for example, [h (CO) 2I] 2 from which forms the catalytically active, rhodium complex, soluble. See, for example, the description of the catalyst in U.S. Patent 4,374,070 and Roth et al., Chem. Tech., 1971 p. 600. The concentration of rhodium in the liquid mixtures contained in the reaction zones can be from 250 to 1300 ppm although concentrations of 400 to 1000 ppm are typically used. The promoter component of the catalyst system can be (1) an inorganic iodide salt such as iodide of lithium or an iodide salt of a quaternary organophosphorous compound or organonitrogen compound or (2) an inorganic compound or an organophosphorus or organonitrogen compound which forms an iodide salt in the carbonylation zone. Organophosphorous or organonitrogen iodides may be selected from phosphonium iodides, ammonium iodides and heterocyclic aromatic compounds in which at least one heteroatom ring is a quaternary nitrogen atom. Examples of such phosphorus and nitrogen containing iodides include tetraiodides ( hydrocarbyl) phosphonium such as t-butyl (methyl) phosphonium iodide, tetrabutylphosphonium iodide, tetraoctyl phosphonium iodide, tpphenyl (methyl) phosphonium iodide, tetraphenylphosphonium iodide and the like, tetra (hydrocarbyl) ammonium iodides such as tetrabutylammonium iodide and iodide tpbutyl (methyl) ammonium; and heterocyclic aromatic compounds such as iodide of N-methylpyridinium, N, N -dimethylimidazolium iodide, iodide N-methyl? 3-p? Col? Mo, N-met? L-2, 4-l? T? D? N iodide, N-met? L-2, 4-lut iodide? d? n? o and N-methylquimyl iodide. Preferred iodide salt promoters comprise lithium iodide and tetraalkyl phosphide iodides, tpphenyl (alkyl) phosphonium iodides, tetraalkylammonium iodides and N, N-dialkylimidazolium iodides wherein the alkyl groups contain up to 8 carbon atoms. It can be fed a portion or all of promoter compound as a compound that forms an iodide salt in the carbomination zone. In this way, the promoter compounds can be fed micially in the form of their corresponding acetates, hydroxides, chlorides or bromides or the promoters containing the phosphorus and nitrogen can be fed as compounds in which the phosphorus or nitrogen atoms are trivalent, for example, tributylphosphine, tributylamine, pyridine, imidazole, N-methylimidazole and the like, which are quaternized by the methyl iodide present in the carbonylation zone. The amount of the iodide salt promoter present in the carbonylation zone can be varied substantially depending in a variety of factors, especially in the particular promoter used. For example, the concentration of the lithium iodide in the reaction mixture may be in the range of 175 to 5000 ppm of lithium, preferably 1500 to 3700 ppm of Lithium, while promoters containing phosphorus and nitrogen may be present in concentrations of 0.5 to 25 weight percent, calculated as their iodide salts and based on the total weight of the reaction mixture, i.e., the contents of the carbonylation zone. The amounts of other materials, eg, acetic acid, acetic anhydride. , methyl iodide, methyl acetate and / or dimethyl ether present in the reaction mixture are substantially dependent, for example, on the proportion of carbonylation, residence time and concentrations of the promoter of the iodide salt and acetic acid solvent. An effluent from the carbonylation zone is continuously removed and separated into a main steam fraction comprising methyl iodide, methyl acetate and / or dimethyl ether, acetic acid and acetic anhydride and a minor fraction comprising a solution of components of catalyst in a mixture of acetic acid and acetic anhydride. The smaller fraction is recirculated to the carbonylation zone and the main fraction is separated by a series of distillations in its component parts. The process of the present invention is further illustrated by the following example wherein the amounts of materials are given in part by weight unless otherwise stated. The percentages are by weight unless otherwise stated. The following materials are continuously fed in the proportions shown to a first reaction zone comprising a reinforced mixing reaction vessel, of one or more baffles, to which carbon monoxide is fed below the surface of the reaction mixture by medium of a gas spraying device in the proportion of 560 parts per minute: Methyl acetate 2466 parts per minute Methyl iodide 704 parts per minute Acetic acid 1269 parts per minute Acetic anhydride 585 parts per minute Rhodium and lithium catalyst components are fed as a solution in acetic acid / acetic anhydride and give a concentration of 400 to 1000 ppm [Rh] and 1000 to 2500 ppm [Ll] in the liquid mixture contained in the first reaction zone. The capacity of the first reaction zone is designed to give a residence time of approximately 40 minutes. A temperature of about 190 to 210 ° C and a total pressure of about 40 to 50 bara are maintained in the first reaction zone. A first liquid reaction mixture is withdrawn continuously from the first reaction zone at a rate of about 5570 parts per minute and is fed to the vapor space of a second reaction zone comprising a tank reaction vessel to which the reactor is fed. carbon monoxide to the vapor space at the top. The composition of the first liquid reaction mixture is approximately 30% methyl acetate, 12% methyl iodide, 22% acetic acid, 29% acetic anhydride and 0.7% volume of carbon monoxide. Approximately 7% of the first liquid reaction mixture consists of other components such as ethylidene diacetate, tar, acetone, inert materials, etc. HE it supplies carbon monoxide to the vapor space of the second reaction zone to maintain a total pressure of about 40 to 50 bara. The residence time within the second reaction zone is approximately 4 to 6 minutes. The amount of the carbon monoxide delivered to the second reaction zone is about 20 parts per minute. A temperature of about 190 to 210 ° C and a total pressure of about 40 to 50 bara are maintained in the second reaction zone. A second liquid reaction mixture is continuously withdrawn from the bottom of the second reaction zone at a rate of 5590 parts per minute and fed to a first separation zone where the pressure is reduced to 2 to 5 bara to rapidly distill approximately 80% of the mixture fed. The composition of the second liquid reaction mixture is approximately 29% methyl acetate, 12% methyl iodide, 22% acetic acid, 31% acetic anhydride, 0.2% carbon monoxide and approximately 7% other materials . That portion of the second non-vaporized liquid reaction mixture comprises about 37% acetic acid, 45% acetic anhydride, the rhodium and lithium catalyst components and tar and other high-boiling compounds. Approximately 70% of the carbon monoxide dissolved in the first liquid reaction mixture is reacted in the second reaction zone. This increases the total utilization of the carbon monoxide from about 90% to about 95%. The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications may be made within the spirit and scope of the invention.

Claims (5)

1. REV NDICATIONS 1. A process for the manufacture of acetic anhydride in the liquid phase under substantially anhydrous conditions characterized in that it comprises the steps of: (1) continuously feeding to a first reaction zone (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, and (v) carbon monoxide, wherein the carbon monoxide is fed below the surface of the liquid reaction mixture comprising the components (i), (n) and (iv) and the reactive compound is converted to acetic anhydride to produce a first liquid reaction mixture which comprises (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promo towers, (vv) acetic acid solvent, (v) dissolved carbon monoxide and (vi) anhydride product acetic, (2) remove the first liquid reaction mixture from the first reaction zone and feed it to a second reaction zone comprising at least one reaction vessel in which a liquid phase comprising the first reaction mixture is maintained liquid and a head steam space where a total pressure of approximately 40 to 50 bar absolute is maintained (bara), carbon monoxide is not fed below the surface of the liquid phase, and the residence time of the liquid phase in the second reaction zone is at least 2 minutes to produce a second liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether, or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid, (v) dissolved carbon monoxide, and (vi) acetic anhydride product, and (3) remove the second liquid reaction mixture from the second reaction zone; where the use of carbon monoxide in the process increases. The process according to claim 1, characterized in that the concentration of the carbon monoxide in the second liquid reaction mixture is about 5 to 50 volume percent of the concentration of the carbon monoxide in the first liquid reaction mixture. 3. The process according to claim 1, characterized in that the process is carried out at a temperature of approximately 100 to 300 ° C and a pressure (total) of approximately as much as 21.7 a 276. 7 and the residence time within the second reaction zone is approximately 4 to 20 minutes. The process according to claim 1, characterized in that the process is carried out at a temperature of about 175 to 220 ° C and a (total) pressure of about as much as 35.5 to 104.4 bara, the residence time within the second zone of reaction is about 4 to 10 minutes, and the second liquid reaction mixture fed to the second reaction zone comprises about 20 to 40 weight percent of methyl acetate. 5 Process for the manufacture of anhydride acetic in the liquid phase under substantially anhydrous conditions at a temperature of about 175 to 220 ° C and a (total) pressure of about as much as 35.5 to 104.4 bar absolute (bara) which is characterized in that it comprises- (1) continuously supplying a first reaction zone (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (i) v) acetic acid solvent, and (v) carbon monoxide, wherein the carbon monoxide is fed below the surface of the liquid reaction mixture comprising the components (i), (n), and (iv) and the reactive compound is converted to acetic anhydride to produce a first liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, ( n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (v) acetic acid solvent, (v) dissolved carbon monoxide and (vi) product of acetic anhydride; (2) removing the first liquid reaction mixture from the first reaction zone and feeding it in a finely divided form to the vapor space of a second reaction zone comprising at least one reaction vessel in which a liquid phase is maintained which comprises the first liquid reaction mixture and a head steam space wherein a total pressure of about 40 to 50 bara is maintained, the carbon monoxide is fed to the vapor space, no carbon monoxide is fed below the surface of the liquid phase, and the residence time of the liquid phase in the second reaction zone is at least 2 minutes to produce a second liquid reaction mixture comprising (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) components of dissolved catalyst comprising rhodium and one or more promoters, (iv) acetic acid solvent, (v) dissolved carbon dioxide and (vi) acetic anhydride product; and (3) remove the second liquid reaction mixture from the second reaction zone, wherein the concentration of the carbon monoxide in the second liquid reaction mixture is about 5 to 50 volume percent of the concentration of the carbon monoxide in the first liquid reaction mixture 6 The process according to claim 5, characterized in that the residence time within the second reaction zone is approximately 4 to 10 minutes and the second liquid reaction mixture fed to the second reaction zone comprises approximately 20 to 40 weight percent of methyl acetate SUMMARY OF THE INVENTION A process for the manufacture of acetic acid anhydride in the liquid phase is described which comprises the steps of: (1) continuously feeding to a first reaction zone (i) a reactive compound selected from methyl acetate, dimethyl ether or a mixture thereof, (n) methyl iodide, (m) dissolved catalyst components comprising rhodium and one or more promoters, (iv) acetic acid solvent, and (v) carbon monoxide wherein the Carbon monoxide is fed below the surface of the liquid reaction mixture to produce a first liquid reaction comprising components (i), (n), (m) and (v) as well as (v) carbon monoxide dissolved and (vi) acetic anhydride product; (2) feeding the first liquid reaction mixture to a second reaction zone in which a liquid phase and a head steam space, or a total pressure of 40 to 50 absolute bar are maintained, does not feed carbon monoxide underneath of the surface of the liquid phase, and the residence time of the liquid phase in the second reaction zone which is at least 2 minutes to produce a second liquid reaction mixture comprising the components (i), (n), (m), (iv), (v) and (vi) and (3) removing the second liquid reaction mixture from the second reaction zone. The second liquid reaction mixture contains less dissolved carbon monoxide than the first liquid reaction mixture.