GB2033385A - Dehydration of Methyl Acetate - Google Patents

Abstract

Methyl acetate is dehydrated by treatment with acetic anhydride and the acetic anhydride used for the dehydration is preferably produced from the dehydrated methyl acetate in a carbonylation reaction.

Description

SPECIFICATION Dehydration of Methyl Acetate This invention relates to the dehydration of alkyl esters of alkanoic acids and is more particularly concerned with the dehydration of methyl acetate, especially methyl acetate used as feed to a carbonylation reaction.

For many purposes, methyl acetate substantially free from water is required. For example, in the preparation of acetic anhydride by the carbonylation of methyl acetate as described for example, in British patent 1,468,940 in U.S.

patent 4,115,444 of September 19, 1978, and in co-pending application 50375/78, Sorial No 2013184 or in the preparation of other compounds, such as ethylidene diacetate, along with acetic anhydride, as described, for example, in Belgian patent 839,321, it is important to have a substantially anhydrous reaction environment and it is necessary, therefore, that water which might enter the system by the use of wet methyl acetate be excluded as much as possible. The dehydration of methyl acetate, however, is not an easy matter and much effort has been directed to providing processes for effecting this dehydration.

Procedures heretofore proposed for the dehydration of esters such as methyl acetate have included treatment of the ester with sodium sulphate, copper sulphate, potassium carbonate, caustic soda, calcium chloride, and the like. The use of such materials, however, has been found to be expensive, of limited effectiveness and generally involving some objectionable features.

Typically, processes of this type are described in Brooks U.S. patent 2,022,074 and in Reich 2,031,669. More recently, the use of extractive distillation with hydrocarbons has been proposed in Curtis U.S. patent 3,904,676. This process, while apparently an improvement over processes of the earlier type, nevertheless involves multiple distillations and repeated handling of various fractions and liquid streams.

It is accordingly an object of this invention to provide an improved process for the dehydration of esters, particularly methyl acetate.

It is a further object of the invention to provide an improvement of the character indicated which is readily adapted to be integrated with a process for the carbonylation of methyl acetate.

Other objects and features of the invention will be readily apparent from the following detailed description of the invention.

In accordance with the invention, a substantially completely anhydrous methyl acetate is readily prepared from a wet methyl acetate containing varous significant amounts of water by bringing the wet methyl acetate into contact with acetic anhydride. The amount of acetic anhydride employed should be at least that stoichiometrically required to react with the water contained in the wet methyl acetate, or at least the water which is to be removed from the wet methyl acetate in those cases where a completely anhydrous methyl acetate is not required, but an excess over the stoichiometric quantity of acetic anhydride can, of course, be employed. Ordinarily, the use of a slight excess over the stoichiometric quantity may be desirable in order to ensure maximum dehydration but the use of an excess is not critical or necessary in obtaining the benefits of the invention.The maximum excess of acetic anhydride that can be employed will be governed only be economic considerations. The reaction between the acetic anhydride and the water in the methyl acetate will, of course, lead to the formation of acetic acid. The presence of acetic acid in the methyl acetate may not be objectionable for some uses, but if it is objectionable, it can readily be removed by fractional distillation since there is more than a 600C. difference in the atmospheric boiling points of these two components and acetic acid and methyl acetate do not form an azeotropic mixture.

As previously indicated, while the process of this invention can be applied to any wet methyl acetate from any source, and intended for use for any purpose, the process is of particular effectiveness and suitability for the dehydration of methyl acetate to be used as feed to a carbonylation reaction where a substantially anhydrous methyl acetate is desired, e.g., for the preparation of acetic anhydride as described in the above-mentioned U.S. and British patents and U.S. application and for the preparation of acetic anhydride in combination with other carbonylation compounds such as ethylidene diacetate as described in the Belgian patent wherein the carbonylation is carried out in the presence of hydrogen, i.e. reductive carbonylation.

The process of the invention will thus now be more particularly described with reference to an integrated system wherein the dehydrated methyl acetate is supplied as feed to a carbonylation zone to produce at least some acetic anhydride and the acetic anhydride thus produced is used, at least in part, to dehydrate additional quantities of methyl acetate, i.e., the dehydrating agent is, in effect, regenerated in situ.

The invention will be more readily understood by reference to the accompanying drawing which shows, diagrammatically, and solely for purposes of facile exemplification, a typical reaction system for carrying out the process of the invention. Thus, referring to the drawing, the reference numeral 10 designates a carbonylation zone, which may comprise one or more pressure reactors of any convenient type, which is fed with carbon monoxide and methyl acetate along with recycle streams and which contains a suitable catalyst, typically one comprising a metal of Group VIII of the Periodic Table in combination with iodine or bromine moieties, generally in a liquid-phase reaction system. Thus, carbon monoxide, in pure or diluted form, is supplied via line 12 and the methyl acetate enters via line 14.

The catalyst and the heavier components of the reaction mixture are recyled, as will be described below, via line 16, and make-up catalyst components are supplied, as needed, via line 1 8.

The carbonylation can be effected batch-wise, if desired, but it will be apparent that it can be readily carried out continuously and, for commercial purposes, continuous operation is preferred. The same is true for the subsequent steps of the process of the invention which will be described below.

From carbonylation zone 10, the reaction mixture is separated into its principal components. For this purpose the mixtue is passed to a distillation zone 20 which is defined by one or more distillation units, e.g., flash and/or fractional devices, as will be apparent to persons skilled in the art. The low-boiling components of the mixture, including methyl acetate, methyl iodide, and the like are removed through line 22, and suitably at least partially recycled via line 26 to carbonylation zone 10. The high boiling components of the reaction mixture, including the essentially non-volatile catalyst components, are recycled to carbonylation zone 10 via previouslymentioned line 1 6 which communicates with line 18. The product acetic anhydride is withdrawn through line 28.A portion of the acetic anhydride thus separated is typically divert through line 29 to be used in the dehydration reaction to be described below to produce dry methyl acetate to be used as feed to the carbonylation reaction. If withdrawn acetic anhydride is used for the manufacture of cellulose acetate, its most common use, or for the preparation of vinyl acetate, there will be produced a substantial quantity of acetic acid which can be supplied to the esterification reaction to be described below to produce methyl acetate for the carbonylation reaction, if desired.

The methyl acetate to be supplied to carbonylation zone 10 via line 14 normally contains 5 to 18% of water as typically produced by distillation of the effluent from a conventional esterification reaction. In accordance with the invention, this wet methyl acetate is dehydrated to any desired extent, including essentially complete dehydration, by bringing it into contact with acetic anhydride, such as the acetic anhydride contained in line 29 produced by the carbonylation of methyl acetate in carbonylation zone 10. The dehydration in accordance with the invention typically takes place in dehydration zone 31. The wet methyl acetate to be dehydrated enters zone 31 via line 33. Zone 31 may be of any convenient form but is suitably a stirred vessel in which the wet methyl acetate and the acetic anhydride can be intimately admixed.Other means for ensuring intimate contact between the liquid acetic anhydride and the water in the methyl acetate can, however, be used. The dehydration is radily effected at any temperature at which both the acetic anhydride and the wet methanol are in the liquid phase but temperatures of 80 to 1 800C. are preferred. Pressure is not a parameter of the dehydration process of the invention and is chosen to maintain the liquid phase in the dehydration vessel.

The effluent from dehydration zone 31 comprises the dehydrated methyl acetate, acetic acid and any excess acetic anhydride which may have been fed to dehydration zone 31. If the methyl acetate is to be used for some purpose in which the co-present acetic acid and/or acetic anhydride are not desired, these components can be separated from the methyl acetate by ordinary fractional distillation as will be apparent to persons skilled in the art. One of the features of the integrated process of this invention, however, is that when the dehydrated methyl acetate is used for the preparation of acetic anhydride by carbonylation, including reductive carbonylation, the presence of the acetic acid and acetic anhydride presents no problem and the entire anhydrous mixture can be supplied to carbonylation zone 10 through line 14.In the above-mentioned distillation of the reaction mixture from carbonylation zone 10, the acetic acid is readily separated and can be used to react with methanol to produce wet methyl acetate to be supplied to the carbonylation reaction after passing through dehydration zone 31, the acetic acid being withdrawn from the distillation system via line 34. The acetic acid can, of course, also be withdrawn and used for any purpose.

It is an advantage of this invention, however, that the acetic acid produced as a result of the dehydration can be used in producing additional methyl acetate. Thus, referring to the drawing, the acetic acid withdrawn via line 34 is passed to esterification zone 70 to react with methanol which is supplied through line 72. Acetic acid will also be provided via line 64 in order to form the quantity of methyl acetate which is needed to maintain the carbonylation reaction. A suitable acid-reacting catalyst is provided to promote the esterification reaction. The esterification reaction mixture which comprises methyl acetate, water, acetic acid and methanol, is then separated using conventional distillation techniques.Such conventional distillation involves multiple distillations, as is well-known to persons skilled in the art, indicated diagrammatically in the drawing by a distillation zone 80. From zone 80 wet methyl acetate is withdrawn through line 82 which communicates via line 33 leading to dehydration zone 31 where it is dehydrated in accordance with the process of this invention. At the same time, acetic acid is withdrawn through line 84 which communicates with line 64 which leads to line 34 which supplies the acetic acid to the esterification reaction in zone 70. Unreacted methanol is similarly withdrawn via line 86 and recycled to esterification zone 70 via line 72. Byproduct water produced in the esterification is removed from the system through line 88.

Additional methyl acetate, if desired, is supplied to line 33 via line 90 or, if the additional methyl acetate is already dry, it may be suppplied directly to line 14.

The esterification of acetic acid with methanol is a well-known reaction which is catalyzed by varous known catalysts which are acidic in nature, such as sulfuric acid. i.e., Fisher esterification.

Preferably, however, a solid catalyst such as an acid-reacting ion exchange resin of known type is advantageously employed, the solid catalyst forming a bed in the esterification zone through which acetic acid and methanol are passed.

Esterification catalysts of this type are well known and representative catalysts are described, for example, in U.S, 2,980,731 and U.S. 3,278,585.

Typically, esterification temperatures of 50 to 1 600C. are employed, and preferably the esterification is effected at super-atmospheric pressures, e.g., 30 to 200 psig, although lower or higher pressures can be employed, if desired. For best results residence times of the order of 5 to 50 minutes are observed. The esterification reaction theroretically consumes equal molar parts of acetic acid and methanol but an excess of one of the reactants is desirably present, e.g., 50 to 400% excess based upon the other reactant.

Preferably, however, the acetic acid is in excess.

Providing this excess is readily accomplished merely by recycling it through the system, i.e., recovering it from the esterification effluent and returning it to the esterification feed inlet.

The esterification effluent will comprise product methyl acetate, co-product water, unreacted acetic acid and methanol. This mixture is readily separated in conventional manner by a series of fractional distillations. Typically the mixture is first distilled at a temperature of 400C.

to 1 300C. and at a pressure of 10 to 20 psia to separate as a distillate a methanol-methyl acetate azeotrope from the remaining methyl acetate and the acetic acid and water. In a subsequent distillation the balance of the methyl acetate is taken overhead by distilling the bottoms from the first distillation at a temperature of 250 C. to 1 500C. and at a pressure of 4 to 40 psia. The methanol-methyl acetate azeotrope is returned to the esterification step for reaction with acetic acid and the freed methyl acetate is, in accordance with the invention, supplied as feed to the carbonylation reaction previously described after dehydration to remove accompanying water.The wet acetic acid recovered as bottoms from the second distillation is then typically distilled at temperatures in the range of 40 to 1 600C. and at atmospheric pressure to dehydrate the acetic acid. The thus recovered acetic acid is then recycled to the esterification zone for conversion to methyl acetate.

The carbonylation reaction involving dry methyl acetate and carbon monoxide which takes place in carbonylation zone 10 is typically carried out at temperatures of 200 to 5000C., preferably 1000 to 3000C. under a carbon monoxide partial pressure of 0.1 to 15,000 psi, and is facilitated by the use of a catalyst, most suitably a Group VIII metal, for example, a Group VIII noble metal, i.e., rhodium, iridium, ruthenium, palladium, osmium and platinum, as disclosed in Belgian patents 819,455 and 839, 322, or a nickel catalyst as described in U.S. patents 4,002,677 and 4,022,678. The disclosures of these two U.S.

patents are incorporated herein by reference.

Thus, in the case of a Group VIII noble metal catalyst, the Group VIII noble metal can be employed in any convenient form, viz. in the zero valent state or in any higher valent form. For example, the catalyst may be the metal itself in finely-divided form, or as a metal carbonate, oxide, hydroxide, bromide, iodide, chloride, lower alkoxide (methoxide), phenoxide or metal carboxylate wherein the carboxylate ion is derived from an alkanoic acid of 1 to 20 carbon atoms. Complexes of the metals can be employed, e.g. the metal carbonyls, such as iridium and rhodium carbonyls, e.g. hexarhodium hexadecacarbonyl, or as other complexes such as the carbonyl halides, e.g. iridium tri-carbonyl chloride [Ir(CO)3CI]2 or chlorodicarbonyl rhodium dimer, or the acetylacetonates, e.g. rhodium acetylacetonate Rh(C5H702)3.It will be understood that the foregoing compounds and complexes and classes of compounds and complexes are merely illustrative of suitable forms of the Group VIII noble metal catalyst and are not intended to be limiting.

The metal employed may contain impurities normally associated with the commercially available metal or metal compounds, and need not be purified any further. Thus, the commercially available metal or metal compound is suitable employed.

The amount of Group VIII noble metal catalyst is in no way critical and is not a parameter of the process of the invention and can vary over a wide range. As is well known to persons skilled in the art, the amount of catalyst used is that which will provide the desired suitable and reasonable reaction rate since reaction rate is influenced by the amount of catalyst. However, essentially any amount of catalyst will facilitate the basic reaction and can be considered a catalytically-effective quantity. Typically, however, the catalyst is employed in the amount of 1 mol per 10 to 100,000 mols of ester, preferably 1 mol per 100 to 10,000 mols of ester, and most preferably 1 mol per 500 to 2,000 mols of ester.

The carbon monoxide is preferably employed in substantially pure form, as available commercially, but inert diluents such as carbon dioxide,nitrogen, methane, and noble gases can be present if desired. The presence of inert diluents does not affect the carbonylation reaction but their presence makes it necessary to increase the total pressure in order to maintain the desired CO partial pressure. The carbon monoxide, like the other reactants, should, however, be essentially dry, i.e., the CO and the other reactants should be reasonably free from water. The presence of minor amounts of water such as may be found in the commercial forms of the reactants is, however, acceptable. Hydrogen, which may be present in very small (trace) amounts as an impurity, is not objectionable and even may tend to stabilize the catalyst.

It has been previously found that the activity of the Group VIII noble metal catalysts described above can be significantly improved, particularly with respect to reaction rate and product concentration, by the concurrent use of a promoter. Effective promoters include the elements having atomic weights greater than 5 of Groups IA, IIA, IIIA, IVB, VIB, the non-noble metals of Group VIII and the metals of the lanthanide and actinide groups of the Periodic Table. Particularly preferred are the lower atomic weight metals of each of these groups, e.g. those having atomic weights lower than 100, and especially preferred are metals of Groups IA, IIA and IIIA as are metals of Group VIB and the nonnoble metals of Group VIII. In general, the most suitable elements are lithium, magnesium, calcium, titanium, chromium, iron, nickel and aluminum.Most preferred are lithium and chromium, especially chromium. The promoters may be used in their elemental form, e.g. as finely-divided or powdered metals, or they may be employed as compounds of varous types, both organic and inorganic, which are effective to introduce the element into the reaction system.

Thus, typical compounds of the promoter elements include oxides, hydroxides, halides, e.g.

bromides and iodides, oxyhalides, hydrides, alkoxides, and the like. Especially preferred organic compounds are the salts of organic mono-carboxylic acids, e.g. alkanoates such as acetates, butyrates, decanoates and laurates, benzoates, and the like. Other compounds include the metal alkyls, carbonyl compounds as well as chelates, association compounds and enol salts.

Particularly preferred are the elemental forms, compounds which are bromides or iodides, and organic salts, e.g. salts of the mono-carboxylic acid corresponding to the anhydride being produced.

Mixtures of promoters can be used, if desired, especially mixtures of elements from different Groups of the Periodic Table. The exact mechanism of the effect of the promoter, or the exact form in which the promoter acts, is not known but it has been noted that when the promoter a added in elemental form, e.g. as a finely-divided metal, a slight induction period is observed.

The quantity of promoter can vary widely but preferably it is used in the amount of 0.0001 mol to 100 mols per mol of Groups VIII noble metal catalyst, most preferably 0.001 to 10 mols per mol of catalyst.

In the working up of the reaction mixtures, e.g.

by distillation, as discussed above, the promoter generally remains with the Group VIII metal catalyst i.e. as one of the least volatile components, and is suitably recycled or otherwise handled along with the catalyst.

The activity of the Group VIII noble metal catalysts described above is also significantly improved, particularly with respect to reaction rate and product concentration catalyst stability and corrosion inhibition, by the use of an organic promoter, and particularly advantageous is the concurrent use of a promoter combination or copromoter system containing a metal component which is a metal of Groups IVB, VB and VIB, and the non-noble metals of Group VIII, in any of the forms described above, in association or combination with an organo-nitrogen compound or an organo-phosphorus compound wherein the nitrogen and the phosphorus are trivalent.

The organic promoter can, in its broader sense, be any organo-nitrogen or organo-phosphorus compound wherein the nitrogen and phosphorous are trivalent. Preferably, however, the organonitrogen promoter is an amine, especially a tertiary amine of the formula

wherein R', R2 and R3 are the same or different and are alkyl, cycloalkyl, aryl or acyl groups which may be substituted by non-interfering groups, preferably having up to 20 carbon atoms, such as trimethylamine, triethylamine, triphenylamine, ethylenediamine tetraacetic acid, and the like, or a heterocyclic amine such as pyridine, picoline, quinoline, methylquinoline, hydroxy quinoline, pyrrole, pyrrolidine, pyrrolidone, and the like, or an imidazole, such as imidazole, methyl imidazole and the like, or an imide of a carboxylic acid which may be monobasic or polybasic and which may be aliphatic or aromatic and preferably contains up to 20 carbon atoms, such as acetic acid, succinic acid, phthalic acid, pyromellitic acid, e.g., N, N-dimethylacetamide, succinimide, phthalimide and pyromellitic diimide, or a nitrile or amide which may be aliphatic or aromatic and preferably contain up to 20 carbon atoms, e.g., acetonitrile, hexamethyl phosphoric triamide, and like imides, nitriles, and amides, or an oxime such as cyclohexanone oxime, and the like. It will be understood, however, that higher molecular weight promoters, e.g. polymeric forms of the organo-nitrogen compounds, may be used such as polyvinylpyridine, polyvinyl pyrrolidone, and the like.

The organo-phosphorus promoter is preferably a phosphine of the formula

wherein R4, R5 and R6 may be the same or different and are alkyl, cycloalkyl, aryl groups, amide groups or halogen atoms, preferably containing 1 to 20 carbon atoms in the case of alkyl and cycloalkyl groups and 6 to 1 8 carbon atoms in the case of aryl groups. Typical phosphines include trimethylphosphine, tripropylphosphine, tricyclohexyphosphine, triphenylphosphine, and tributylphosphine.

Although it is preferred that the organic promoters be added separately to the catalyst system, it is possible to add them as complexes with the Group VIII noble metal such as the trichloro trispyridine rhodium, tris(triphenyl phosphine) rhodium, chlorotris (triphenyl phosphine) rhodium, and chlorocarbonyl bis (triphenyl phosphine) rhodium and like complexes. Both free organic promoters and complexed promoters can also be used. Indeed, when a complex of the organic promoter and the Group VIII noble metal is used, it is desirable to add free organic promoter as well. The amount of organic promoter will generally lie in the ranges referred to above for the metal promoter except that preferably up to 50 mols per mol of catalyst are employed.

The carbonylation step is readily carried out in a single reaction zone to which a halide source, e.g., a hydrocarbyl halide such as methyl iodide, and the methyl acetate are both charged and are heated together, preferably in the liquid phase, in the presence of carbon monoxide and in the presence of the Group Vlil metal catalyst. It will be understood that the hydrocarbyl halide may be formed in situ and the halide may thus be supplied to the system not only as the hydrocarbyl halide but the halogen moiety may also be supplied as another organic halide or as the hydrohalide or other inorganic halide, e.g. salts, such as the alkali metal or other metal salts, or even as elemental iodine or bromine.

As previously mentioned, in carrying out the carbonylation steps described above, a wide range of temperatures, e.g. 20 to 5000C are suitable but temperatures of 100 to 3000C are preferably employed and the more preferred temperatures generally lie in the range of 125 to 2500C. Temperatures lower than those mentioned can be used but they tend to lead to reduced reaction rates, and higher temperatures may also be employed but there is no particular advantage in their use. The time of reaction is also not a parameter of the process and depends largely upon the temperature employed, but typical residence times, by way of example, will generally fall in the range of 0.1 to 20 hours. The reaction is carried out under super-atmospheric pressure but excessively high pressures, which require special high-pressure equipment, are not necessary.In general, the reaction is effectively carried out by employing a carbon monoxide partial pressure which is preferably 5 to 2,000 psi, although carbon monoxide partial pressures of 0.1 to 15,000 psi can also be employed. The total pressure is that required to provide the desired CO partial pressure and preferably that required to maintain the liquid phase. Typically, total pressures up to about 3,000 psig are used but most preferably they are at most about 1 ,000 psig. The reaction can be advantageously carried out in an autoclave or similar apparatus.

The ratio of ester to the halide in the reaction system can vary over a wide range. Typically, there are used 1 to 500 equivalents of the ester per equivalent of halide, preferably 1 to 200 equivalents per equivalent. Thus, there are typically used 1 to 500 mols, preferably 1 to 20Q mols of ester per mol of halide reactant. By maintaining the partial pressure of carbon monoxide at the values specified, adequate amounts of the reactant are always present to react with the hydrocarbyl halide.

The effluent from the carbonylation step is treated, e.g. distilled, by conventional techniques to separate the product acetic anhydride from it and to recover streams containing unreacted methyl acetate, iodine moieties, catalyst components (and promoter components, if employed), all of which are recylced to the carbonylation reaction for reuse. As indicated above, the distillation of the carbonylation effluent is conveniently effected in one or more distillation units, e.g. flash and/or fractional distillation devices, represented in the drawing by distillation zone 20. In distillation zone 20 temperatures of 50O to 1800C and pressures of 0 to 60 psig typically prevail.

As mentioned previously, a small portion of the acetic anhydride produced from the dry methyl acetate supplied by the process of this invention is employed to dry or dehydrate additional amounts of wet methyl acetate but the large bulk of the acetic anhydride is a product available for any use. It can be, for example, used in the preparation of cellulose acetate, a well-known conventional operation or it may be used to produce vinyl acetate by processes such as described in Oxley et al U.S. patent 2,425,389, Perkins U.S. patent 2,021,698 and Sharp et al U.S. patent 2,860,259.These processes involve the formation of ethylidene diacetate from acetic anhydride and acetaldehyde and the decomposition or "cracking" of the ethylidene diacetate to form vinyl acetate and acetic acid, the- process being carried out in two steps wherein the ethylidene diacetate is first formed and than cracked as described in Sharp et al or in a single step wherein the vinyl acetate and acetic acid are produced in single reaction zone to which acetic anhydride and acetaldehyde are fed as described in Perkins.

Whether the acetic anhydride is used for the manufacture of cellulose acetate or for the preparation of vinyl acetate in the manner described above, there is produced as a byproduct a substantial amount of acetic acid and this acetic acid is readily employed as the acid fed to the above described esterification reaction to supplement the acetic acid produced in the dehydration step of the invention. The acetic acid can, of course, be obtained from any other source if desired.

A particularly attractive process for producing vinyl acetate and acetic acid from acetic anhydride and acetaldehyde is described in the above-mentioned U.S. application Serial No.

866,089. That process is, for convenience, illustrated in the drawing. Thus, shown in the drawing, acetic anhydride in line 28 is brought into contact in zone 35 with acetaldehyde to line 38. The acetic anhydride entering zone 35 through line 28 is supplemented by recycle acetic anhydride coming via line 40 from a subsequent step in the process, as will be described below, and the acetaldehyde entering zone 35 via line 38 is augmented by acetaldehyde supplied through line 42, also from a subsequent step of the process.

In zone 35 the acetic anhydride and the acetaldehyde are brought together in the presence of an acidic catalyst in order to convert at least some of the acetic anhydride and acetaldehyde to ethylidene diacetate (EDA). From zone 35 the reaction mixture passes to a decomposition zone 45 via line 48 wherein the EDA is decomposed or "cracked" to form vinyl acetate and acetic acid. In order to promote the formation of EDA and vinyl acetate, it is desirable to have present a large excess of acetic anhydride in zone 35 and in decomposition zone 45, e.g. 1 to 40 mols per mol of acetaldehyde fed to zone 35. This is readily accomplished by providing for a built-in circulating stream of acetic anhydride which flows from zone 35 to zone 45 and, after separation from the effluent from decomposition zone 45, is returned to zone 35.Thus, in zone 35 the acetaldehyde is in contact with the acetic anhydride coming from line 28 in the presence of the large excess of acetic anhydride coming through line 40 and EDA is produced by the interreaction.

Decomposition zone 45 is preferably operated as a boiling reactor at a temperature typically of 100 to 2000C and a pressure of 5 to 200 psia. In this way the vinyl acetate is removed from the decomposition zone 45 substantially as soon as it is formed not only to force the reaction in the desired direction but also to minimize the risk of polymerization or other undesired reaction.

Indeed, it is advantageous to have present in the decomposition zone 45 a compound which is effective to stabilize vinyl acetate against polymerization such as sodium acetate and the receiver in which the vinyl acetate is finally collected also contains a compound, e.g. an organic inhibitor such as hydroquinone. Also present in decomposition zone 45 is an acidic catalyst which is suitably the same as the catalyst in zone 35.

The vaporous effluent from the boiling reactor 45 primarily comprising vinyl acetate, acetaldehyde, acetic anhydride, acetic acid and undecomposed EDA is passed via line 50 to distillation zone 55 in which it is separated into its respective components. The liquid in reactor 45, which contains the catalyst, is withdrawn via line 46 which communicates with line 40 for recycling to zone 35. If desired, some or all of this liquid can be passed via line 52 to distillation zone 55.In zone 55, which may consist of one or more distillation columns and which is typically operated at a temperature in the range of 30C to 1 700C and under a pressure in the range of O to 40 psig, the lower boiling acetaldehyde is separated from the vinyl acetate and returned to zone 35 via line 42, whereas the product vinyl acetate is withdrawn through line 62, condensed and collected, the acetic acid fraction is withdrawn through line 64 and a less volatile fraction which contains acetic anhydride and undecomposed EDA is, as previously mentioned.

recyled via line 40 to EDA formation zone 35. This fraction contains the large excess of acetic anhydride which flows in circuit zone 35 to.

zone 45 to zone 55 and then back to zone 35.

The acetic acid in line 64 is supplied to esterification zone 70.

The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that these examples are given for illustrative purposes only and are not to be interpreted as being limitative of the invention. In the examples, all parts are on a molar basis, unless otherwise indicated.

Example I This example illustrates a continuous dehydration of methyl acetate integrated with a continuous formation of methyl acetate from acetic acid and methanol and a continuous carbonylation of the dehydrated methyl acetate to produce acetic anhydride, a portion of which is continuously suppled to the dehydration step and the remainder of which is optionally converted to vinyl acetate and acetic acid.

Using an apparatus system such as illustrated in the drawing, a carbonylation zone 10 in the form of a stirred pressure reactor is filled to the level of withdrawal line 28 with a mixture composed of approximately 93.5 mol percent methyl acetate, 2.25 ml percent methyl iodide, 4 mol percent lithium iodide and 0.25 mol percent rhodium acetate. This mixture is heated to about 1 700C. and carbon monoxide is introduced into the reactor to provide and maintain a partial pressure of carbon monoxide of 300 psi, resulting in a total pressure of about 500 psig.Continuous liquid feed to the reactor is thus begun and liquid reaction is withdrawn and distilled to separate a product acetic anhydride stream and to provide recycle streams containing some acetic anhydride as well as unreacted methyl acetate and iodine, lithium and rhodium values resulting from the methyl iodide, lithium iodide and rhodium acetate initially charged, the recycle streams bring continuously returned to the reactor. The reaction is carried out to provide a residence time in the reactor of about three hours.

Thus, there are continuously fed approximately 790 parts per hour of methyl acetate (including 490 parts per hour recycle methyl acetate) along with recycle of the iodine, lithium and rhodium values representing 1 8 parts per hour of methyl iodide, 32 parts per hour of lithium iodide and 2 parts per hour of rhodium acetate, together with recycle acetic anhydride, the recycle streams being obtained as described below, and 40 parts per hour of acetic acid produced in the dehydration step as pointed out below. The reaction mixture is continuously withdrawn at the rate of 1,080 parts per hour and passed into distillation zone 20.In distillation zone 20, the reactor effluent is first flashed at about 50 psia and 1 500C. The heavy liquid from the flash, which contains the catalyst components, some methyl acetate and some acetic anhydride is recycled to carbonylation zone 10 at the rate of approximately 300 parts per hour. The vapor from the flash is fractionally distilled at a pressure of about 50 psia and at a temperature in the range of SOC to 1 600C. to separate approximately 440 parts per hour of a "lights" fraction comprising methyl acetate and methyl iodide, which is recycled to carbonylation zone 10 via line 26, and 40 parts per hour of acetic acid. The bottoms from this distillation are comprised of product acetic an hydride.

As a result of such distillation there are obtained approximately 300 parts per hour of acetic anhydride, 280 parts per hour of which are supplied to the EDA formation zone and the remaining 20 parts per hour are fed via line 29 to dehydration zone 31 via line 29. At the same time the 40 parts per hour of acetic acid are fed to esterification zone 70 through line 34.

EDA formation zone 35 which is defined by a stirred reactor is operated at 800C. At the same time, 280 parts per hour of acetaldehyde are fed via line 38 along with 50 parts per hour of recycle acetaldehyde which are fed via line 42 and a recycle stram supplying approximately 300 parts per hour of EDA and about 3,000 parts per hour of acetic anhydride. There is maintained in the EDA formation zone 1.5 weight percent of benzene sulfonic acid. which is obtained in the recycle stream from decomposition zone 45 plus any needed make-up.From zone 35, an effluent of about 610 parts per hour of EDA and approximately 3,000 parts per hour of acetic anhydride is fed via line 48 to decomposition zone 45 which is defined by a stirred reactor and is operated under atmospheric pressure at a temperature of about 1 500 C. The decomposition zone effluent is fed to distillation zone 55. In this distillation zone, about 50 parts per hour of acetaldehyde are separated as overhead at 500C.

and 45 psia and this acetaldehyde is recycled via line 42 to zone 35 as previously indicated. Vinyl acetate and acetic acid are distilled out under pressures in the range of 1 5 to 30 psia and at temperatures in the range of SOC to 1 500C. From this distillation, vinyl acetate product is collected via line 62 and about 260-parts per hour of acetic acid are sent to esterification zone 70 via line 64.

The excess acetic anhydride and unreacted EDA plus catalyst obtained as bottoms are recycled back to EDA formation zone 35 via line 40, as previously mentioned, and acetic acid in additon to the amount sent to esterification is removed.

Esterification zone 70 is suitably defined by a tank which contains a bed of an acidic ionexchange resin (Dowex 50W, Registered Trade Mark) and is operated at about 700C. and 120 psia. It is fed with 300 parts per hour of acetic acid (260 parts via line 64 and 40 parts via line 34) and with 300 parts per hour-of methanol, and a recycle stream of 50 parts per hour of a methanol-methyl acetate azeotropic mixture is supplied via line 86. The reaction effluent from esterification zone 70 enters distillation zone 80 in which there is distilled as overhead the above mentioned methanol-methyl acetate azeotrope which is recycled to the esterification reaction via line 86 at the rate of 50 parts per hour.Then about 300 parts per hour of methyl acetate product (accompanied by 20 parts per hour of water) are taken overhead via line 82 in a further distillation which is carried out at sub atmospheric pressure of about 1 2 psia and at temperatures in the range of 30C to 1300C. The bottoms from this distillation comprising water and acetic acid are then separated by distillation at a pressure of about 30 psia and at temperatures in the range of 1200 to 1 500C. As a result of this latter distillation, the separated water is drained via line 88 at the rate of about 280 parts per hour and about 500 parts per hour of acetic acid are recycled to esterification zone 70 via line 84. The wet methyl acetate stream is then fed to dehydration zone 31 via lines 82 and 33.In dehydration zone 31, which is defined by a stirred reaction vessel, the 20 parts per hour of acetic anhydride react with the 20 parts per hour of water which accompany the 300 parts per hour of methyl acetate, thereby dehydrating the methyl acetate and forming 40 parts per hour of acetic acid which, along with the 300 parts per hour of dry methyl acetate, pass to the carbonylation zone 10 via line 14.

In the formation of vinyl acetate and byproduct acetic acid from acetic anhydride and acetaldehyde, under prolonged continuous operating conditions there may be some processing and related losses with the result that the amount of acetic acid available will be less than the theoretical amount so that there will be less than that needed to supply the total quantity of methyl acetate required for the carbonylation zone. This shortfall of acetic acid can, of course, be compensated for by supplying makeup acetic acid or methyl acetate- as required.It is the feature of the methyl acetate dehydration process of this invention, however; that the acetic acid inherently produced in the dehydration will more than make up any shortfall so that there can be a wholly integrated system as illustrated in the drawing without requiring the addition of acetic acid and/or methyl acetate from external sources.

Thus, in the example just described, the 280 parts per hour of acetic anhydride fed to the EDA formation zone would theoretically result in the production of 280 parts per hour of vinyl and 280 parts per hour of acetic acid.ln view of the 40 parts per hour of acetic acid produced in dehydration zone 31, however, only 260 additional parts per hour of acetic acid need to be supplied to the esterification reaction. Thus, it would be possible in this example to have a shortfall in the vinyl-acetate-forming step as much as 7.7% without presenting the need for an external supply of acetic acid to produce the 300 parts per hour of methyl acetate which are needed for the carbonylation reaction.

In the foregoing Example I, all of the acetic anhydride produced in the carbonylation reaction form the anhydrous methyl acetate fed to the carbonylation zone is shown as being used to produce vinyla cetate and to dehydrate the methyl acetate feed; In many cases, however, it is desired to produce additonal acetic anhydride to be withdrawn froni the system for some other use, so that additional amounts of dehydrated methyl acetate must be fed per hour to the carbonylation zone or, even when greater amounts of acetic anhydride are not desired, additional anhydrous methyl acetate may be needed to produce carbonylation products other than acetic anhydride such as ethylidene diacetate and the like.

It is a further feature of the invention that the above described dehydration process is adapted to be integrated with the carbonylation system to provide any desired quantity of anhydrous methyl acetate. The following example illustrates this aspect of the process of the invention in a situation in which additional acetic anhydride product is to be produced.

Example II Using the system described in Example I, carbonylation zone 10 is continuously fed with approximately 1300 parts per hour of methyl acetate, (500 parts per hour fresh feed and 800 parts per hour recycle), and sufficient quantities of methyl iodide, lithium iodide and rhodium acetate are initially supplied to provide the concentrations specified in Example I. The feed to the carbonylation zone also includes 140 parts per hour of acetic acid produced in the dehydration step as described below. The reaction mixture is maintained at a temperature of about 1700C. and carbon monoxide is continuously introduced to maintain a partial pressure of carbon monoxide of 300 psi and a total pressure of 500 psig as described in Example I.The reaction mixture containing acetic anhydride, acetic acid and methyl acetate along with the catalyst and iodine compounds is withdrawn at a rate of approximately 2000 parts per hour and passed into distillation zone 20. In distillation zone 20, the reactor effluent is first flashed at about 50 psia and 150 C. The heavy liquid from the flash, which contains the catalyst components, some methyl acetate and some acetic anhydride is recycled to carbonylation zone 10 at the rate of approximately 570 parts per hour.The vapor from the flash is fractionally distilled at a pressure of about 50 psia and at a temperature in the range of 500 to 1600C. to separate approximately 790 parts per hour of a "lights" fraction comprising methyl acetate and methyl iodide, which is recycled to carbonylation zone 10 via line 26, and 140 parts per hour of acetic acid. The bottoms from this distillation are composed of approximately 500 parts per hour of product acetic anhydride, one hundred parts per hour of this product are withdrawn via line 32, 280 parts per hour are fed via line 28 to EDA formation zone 35, 50 parts per hour enter line 30 to be fed to esterification zone 70 and 70 parts per hour enter line 29 to be fed to dehydration zone 31.The EDA formation and cracking steps to produce vinyl acetate and acetic acid are carried out as described in Example I and 260 parts per hour of acetic acid are sent to esterification zone 70 via line 64. Along with the acetic acid from line 64 and 50 parts per hour acetic anhydride from line 30, esterification zone 70 (700 C. and 1 20 psia) is fed with 140 parts per hour of acetic acid from the carbonylation step via line 34 and 500 parts per hour of methanol, and a recycle stream of 90 parts per hour of a methanol-methyl acetate azeotropic mixture is supplied via line 86. The reaction effluent from esterification zone 70 enters distillation zone 80 in which there is distilled as overhead the above-mentioned methanol-methyl acetate azeotrope.Then about 500 parts per hour of methyl acetate (accompanied by about 70 parts per hour of water) are taken overhead via line 82 in a further distillation which is carried out at a subatmospheric pressure of about 12 psia and at temperatures in the range of 500 to 1300C. The bottoms from this distillation comprising water and acetic acid are then separated by distillation at a pressure of about 30 psia and at temperatures in the range of 1200 to 1 500C. As a result of this distillation, the separated water is drained via line 88 at the rate of about 380 parts per hour and about 800 parts per hour of acetic acid are recycled to esterification zone 70 via line 84.

The wet methyl acetate in line 82 is then fed to dehydration zone 31 which is in the form of a stirrred reaction vesel, as in Example I, at the same time 70 parts per hour of the acetic anhydride in line 29 are also fed to dehydration zone 31 in which they are intimately mixed with the wet methyl acetate, thereby dehydrating the methyl acetate and forming 140 parts per hour of acetic acid which, along with the 500 parts per hour of dry methyl acetate, pass to carbonylation zone 10 via line 14.

The foregoing example shows that even though the carbonylation reaction is run to produce 100 parts per hour of acetic anhydride in addition to the 280 parts per hour which are supplied to form EDA, the dehydration of the wet methyl acetate is effectively carried out using acetic anhydride which is generated in the carbonylation reaction so that acetic anhydride from an external source is not required to carry out the dehydration step in accordance with this invention.

In some cases, the wet methyl acetate from the esterification operation may be accompanied by some methyl alcohol, as when the distillation operations are not carried out to the extent illustrated in Examples I and II. This, however, poses no problem and the dehydration step is carried out with sufficient acetic anhydride not only to react with the water contained in the wet methyl acetate fed to the dehydration zone but to react with methanol to produce additional quantities of methyl acetate. This embodiment of the invention is illustrated in the following example.

Example Ill Using the system described in Example I, carbonylation zone 10 is fed with approximately 1 300 parts per hour of methyl acetate (500 parts per hour fresh feed and 800 parts per hour recycle) and sufficient quantities of methyl iodide, lithium iodide and rhodium acetate are initially supplied to provide the concentrations specified in Example I. The feed to the carbonylation zone also includes 1 70 parts per hour of acetic acid produced in the dehydration step as described below. The carbonylation is then carried out as described in Example I and the reaction mixture is withdrawn at the rate of approximately 2040 parts per hour and distilled under the conditions specified in Example I.The heavy liquid from the flash is recycled to carbonylation zone 10 at the rate of approximately 580 parts per hour and the vapor from the flash is fractionally distilled at a pressure of about 50 psia and at a temperature in the range of 500 to 1 600 C. to separate approximately 790 parts per hour of a "lights" fracticn which is recycled to zone 10, and 1 70 parts per hour of acetic acid. The approximately 500 parts per hour of product acetic anhydride are divided into 100 parts per hour to be withdrawn via line 32, 280 parts per hour to be suppied for EDA formation, 20 parts per hour are to be fed to the esterification reaction via line 30 and 100 parts per hour for use in the dehydration reaction, passing via line 29 into the dehydrator 31.The EDA formation and cracking steps to provide the vinyl acetate and acetic acid are carried out as described in Example I and 260 parts per hour of acetic acid are sent to esterification zone 70 via line 64. Along with the acetic acid from line 64 and 20 parts per hour acetic anhydride from line 30, esterification zone 70 (700C. and 120 psia) is fed with 170 parts per hour of acetic acid from the carbonylation step via line 34 and 500 parts per hour of methanol, and a recycle stream of 90 parts per hour of methanolmethyl acetate azeotropic mixture is supplied via line 86. The reaction effluent from esterification zone 70 enters distillation zone 80 in which there is distilled as overhead the above-mentioned methanol-methyl acetate azeotrope.Then about 470 parts per hour of methyl acetate product (accompanied by about 70 parts per hour of water and 30 parts per hour of methanol) are taken overhead via line 82 in a futher distillation which is carried out at subatmosphedc pressure of about 12 psia and at temperatures in the range of 500 to 1 300C. The bottoms from this distillation comprising water and acetic acid are then separated by distillation at a pressure of about 30 psia and temperatures in the range of 1200 to 1 500C. As a result of this distillation, the separated water is drained via line 88 at the rate of about 380 parts per hour and about 800 parts per hour of acetic acid are recycled to esterification zone 70 via line 84.

The wet methyl acetate containing methanol in line 82 is then fed to dehydration zone 31 which is in the form of a stirred reaction vessel, as in Example I, at the same time 100 parts per hour of the acetic anhydride in line 29 are also fed to dehydration zone 31 in which they are intimately mixed with the wet methyl acetate, thereby dehydrating the methyl acetate, esterifying the methanol to form 30 parts per hour of methyl acetate and forming a total 170 parts per hour of acetic acid which, along with the 500 parts per hour of dry methyl acetate, pass to carbonylation zone 10 via line 14.

The foregoing example shows that even though the carbonylation reaction is run to produce, 100 parts per hour of acetic anhydride in addition to the 280 parts per hour which are supplied to form EDA, and even though the wet methyl acetate is accompanied by unreacted methanol, the dehydration of the wet methyl acetate is effectively carried out using acetic anhydride which is generated in the carbonylation reaction so that acetic anhydride from an external source is not required to carry out the dehydration step in accordance with this invention and there is no need to remove the methanol from the methyl acetate.

In the preceding examples only stoichiometric amounts of acetic anhydride are employed but in practice an excess of about 30 parts per hour of acetic anhydride is suitably used, the excess merely circulating in a loop from dehydration zone 31 to carbonylation zone 10 to distillation zone 20 to line 29 and back to dehydration zone 31.

Claims (6)

Claims

1. A process for the dehydration of wet methyl acetate which comprises bringing said wet methyl acetate into contact with acetic anhydride in an amount of at least substantially stoichiometrica;ly equivalent to the water present in said wet methyl acetate, and recovering substantially anhydrous methyl acetate.

2. A process of carbonylating substantially anhydrous methyl acetate to produce a reaction product containing acetic anhydride therefrom, and including the steps of feeding substantially anhydrous methyl acetate to a carbonylation zone, recovering acetic anhydride from said zone, directing a portion of said acetic anhydride into contact with wet methyl acetate, said portion of acetic anhydride being an amount at least substantially stoichiometrically equivalent to the water present in said wet methyl acetate, recovering substantially anhydrous methyl acetate, and feeding said substantially anhydrous methyl acetate to said carbonylation zone.

3. A process of producing methyl acetate by the esterification of acetic acid with methanol in an esterification zone and carbonylating the resultant wet methyl acetate, after dehydration, in a carbonylation zone to form acetic anhydride, and including the steps of reacting said wet methyl acetate with acetic anhydride in an amount at least substantially stoichiometric equivalent to the water present in said wet methyl acetate in a dehydration zone to produce substantially anhydrous methyl acetate and acetic acid, feeding said substantially anhydrous methyl acetate and said acetic acid to said carbonylation zone, recovering acetic anhydride and acetic acid from said carbonylation zone, separating said acetic anhydride and said acetic acid, directing a portion of said acetic anhydride to said dehydration zone, said portion being said amount at least substantially stoichiometrically equivalent to the water present in said methyl acetate fed to said dehydration zone, and feeding said acetic acid separated from said acetic anhydride to said esterification zone for reaction with methanol to produce said wet methyl acetate.

4. A process as claimed in Claim 1. Claim 2 or Claim 3, substantially as hereinbefore described with particular reference to the Examples.

5. A process as claimed in Claim 1, Claim 2 or Claim 3, substantially as illustrated in any one of the Examples.

6. Apparatus for the dehydration of wet methyl acetate, substantially as described herein with reference to the accompanying drawing.