Raw Materials production

Acetic Anhydride production

I- From Acetic Acid

Acetic anhydride is produced from glacial acetic acid via ketene (CAS no 463-51-4). Acetic acid is vaporized and fed together with a catalyst to a cracking furnace operating under vacuum where ketene is produced together with water at high temperature. The reaction mixture is rapidly cooled to avoid the reversing of the reaction. The condensed dilute acetic acid is separated and the gases then pass through two absorption towers in which they are scrubbed by acetic acid/ acetic anhydride of various concentrations.

Glacial acetic acid is added to the second absorption tower. Ketene is reacting with acetic acid to form acetic anhydride. In following washing towers the gases are further scrubbed before released to the atmosphere. In a distillation column the crude acetic anhydride is distilled and recovered as the bottom product. The top product is acetic acid which is recycled to the process.

Ref : IBI Chematur

II- production of acetic anhydride from carbon monoxide, acetic acid and methanol.

Acetic anhydride and hydrogen iodide can be prepared by thereaction of methyl iodide, methanol or acetic acid, and carbon monoxide under carbonylation conditions using a catalyst comprising a Group VIII metal and a Lewis base promoter. By reacting the hydrogen iodide with methanol to produce methyl iodide and recycling the methyl iodide an integrated process for the production of acetic anhydride can be devised which uses only methanol or methanol and acetic acid as organic feedstocks. The process has the advantage over existing technology that methyl iodide and water produced by the reaction of hydrogen iodide and methanol can be separated without distillation.

Accordingly, the present invention provides a process for the production of acetic anhydride from methyl iodide, acetic acid and carbon monoxide characterised by reacting the methyl iodide and acetic acid with carbon monoxide under carbonylation conditions, in the presence of an effective amount of a carbonylation catalyst.

The products of the reaction described herein are, according to the following equation,

acetic anhydride and hydrogen iodide. The two products are readily separated e.g. by distillation, and both may be recovered pure. It is a particularily important feature of this invention that the hydrogen iodide is recoverable since it is able to react quantitatively with methanol to form methyl iodide and water

The hydrogen iodide may therefore be used to produce fresh methyl iodide which may be, in turn, carbonylated along with more acetic acid. Since during the reaction all the iodide is recycled within the process, the overall reaction is

The reaction between methanol and hydrogen iodide

is in some respects similar to the esterification reaction used in the prior art to product methyl acetate

since both reactions require water removal from either methyl iodide or methyl acetate before carbonylation can occur. However, unlike the methyl acetate and water mixture which forms a single phase separable only by distillation, methyl iodide and water mixtures from two separate phases, easily separable by low energy methods such as decanting. Furthermore, although small amounts of methyl iodide can dissolve in water no azeotrope is formed and therefore separation by distillation is easy.

Accordingly an embodiment of the present invention provides a process for the production of acetic anhydride from methanol, acetic acid and carbon monoxide in a series of process steps characterised in that

1) in a carbonylation stage a mixture of methyl iodide and acetic acid is reacted, under carbonylation conditions, with carbon monoxide in the presence of the catalyst described previously to form acetic anhydride and hydrogen iodide

2) in a separation stage the hydrogen iodide and acetic anhydride are separated and

3) in an iodination stage the hydrogen iodide and methanol are reacted to produce methyl iodide and water, the methyl iodide being subsequently separated from the water and recycled to stage (1).

The product stream withdrawn from stage (1) can consist exclusively of acetic anhydride and hydrogen iodide but more generally consists of a mixture of acetic anhydride, hydrogen iodide and unreacted reactants. In this case, stage (2) can consist of several steps which allow
(a) the unreacted reactants to be isolated and recycled to stage (1);
(b) the hydrogen iodide to be isolated and fed to stage (3) and
(c) the acetic anhydride to be recovered for sale or secondary uses.

A typical process scheme for the operation of the embodiment is shown in the figure and operates as follows:

1) Carbon monoxide (via line 4) and acetic acid (via line 12) are fed to the carbonylation reactor 2 along with recycled methyl iodide and acetic acid (via line 8) where reaction occurs.

2) In the first part of the separation stage, which is conveniently integral with the carbonylation reactor 2 two streams are removed. The higher boiling stream 16, consisting of acetic anhydride and some acetic acid, is fed to a distillation column 18 where the residual acid is removed overhead.

The lower boiling stream 14, consisting of carbon monoxide, hydrogen iodide, methyl iodide and residual acetic acid is fed to a scrubber 10.

3) In the scrubber (10) the carbon monoxide is removed overhead and returned to the carbonylation reactor (via line 6) and the methyl iodide and resiudal acetic acid removed at the bottom and returned to the reactor 2 via line 8. The hydrogen iodide is removed at an intermediate point in the scrubber and fed to an iodination unit 22

4) in which it (hydrogen iodide) is reacted with methanol (via line 24) to produce methyl iodide and water.

5)The methyl iodide and water may be conveniently separated by decanting in unit 22 and

6) the methyl iodide returned to the scrubber. The water, which may contain small amounts of unreacted methanol and trace amounts of water may be fed to a stripper if desired to remove the methyl iodide and methanol as overheads.

Acetic acid which is used as feedstock can be that produced by any of the well known industrial processes or it can be produced by hydrolysis of part of the acetic anhydride product. In a further embodiment of the invention described however the acetic acid is generated in situ in the carbonylation stage from methanol and carbon monoxide using the carbonylation catalyst. Thus in a further embodiment of the invention a process is provided for the production of acetic anhydride from methyl iodide, methanol and carbon monoxide characterised by reacting the methyl iodide and acetic acid with carbon monoxide under carbonylation conditions in the presence of an effective amount of a carbonylation catalyst.

Furthermore on replacing acetic acid by methanol as feed to the carbonylation reactor 2 in the figure a process is obtained for producing acetic anhydride from methanol and carbon monoxide using the same series of integrated process steps as described earlier.

In addition to the extremes of pure methanol and pure acetic acid described above, it will be obvious to the skilled man that methanol/acetic acid mixtures of any ratio can also be used as feedstocks.

The carbonylation catalyst conveniently contains a Group VIII metal and the Group VIII metal carbonylation catalyst is preferably rhodium. It can be supplied in almost any chemical form such as the metal, a simple inorganic salt, for example the halide or nitrate, a simple organic salt, e.g. the acetate or an organometallic complex or carbonyl complex. The catalyst concentration can be from 100 to 5000 ppm and can conveniently be from 400 to 1200 ppm based on the total weight of reactants.

A reaction promoter, in the form of a Lewis base, can also be added. Suitable Lewis bases include amines, phosphines and arsines which are capable of undergoing a quaternisation reaction with methyl iodide to form a quaternary ammonium salt which is soluble in the reaction mixture. Suitably the Lewis base is selected from nitrogen heterocycles, e.g. 2-picoline, pyridine, quinoline pyrrole, pyrrolidine, imidazole and 2-methyl imidazole. Imidazole for example is readily alkylated under the reaction conditions to N-methyl imidazole which readily forms soluble quaternary salts. The Lewis base may suitably added in an amount up to 20 mole % of the methyl iodide reactant preferably 2 to 10 mole %.

The carbonylation reaction is preferably effected in the liquid phase. In the liquid phase the reaction is conveniently carried out at elevated temperatures up to 250°C. Preferably the temperature is in the range 140 to 200°C.

Conveniently the process is carried out under a superatmospheric pressure of carbon monoxide preferably at least 25 bar and for example between 25 and 100 bar. The carbon monoxide used should be substantially pure although up to 10% by volume can be used.

As regards the ratio of methyl iodide to methanol or acetic acid in the carbonylation reaction it is preferred to use a molar ratio in the range 5:1 to 1:5. However, it will be obvious to the skilled man that in integrated processes of the type described herein it is important that the molar ratio of reactants corresponds substantially to that required by the expected stoichiometry of the carbonylation stage in order to reduce the levels of by products. Thus for the integrated processes described herein the molar ratio of methyl iodide to acetic acid or methyl iodide to methanol in the carbonylation stage should be substantially 1:1. Likewise in the iodination stage it is preferred to use a molar ratio of methanol to hydrogen iodide of 1:1.

The invention is now illustrated by the following Examples.

Example 1

A 100 ml Hastelloy 'B2' autoclave was charged with 39.1 g methyl iodide, 30.0 g acetic acid, 5.0 g 1-methylimidazole and 0.200 g rhodium acetate dimer. The vessel was pressurized with carbon monoxide to 65 bar and heated to 180°C during 3 hours with stirring. After cooling, (gas chromatography) g.c. analysis of the reaction mixture showed it to contain 4.5 g of acetic anhydride, hydrogen iodide and 35.1 g of acetic acid.

Example 2

A 100 ml Hastelloy 'B2' autoclave was charged with 39.5 g methyl iodide, 30.1 g acetic acid, 10.2 g 1-methylimidazole and 0.200 g of rhodium acetate dimer. The vessel was pressurized with carbon monoxide to 65 bar and heated at 150°C during 3 hours. After cooling, the reaction mixture was analysed by g.c. and shown to contain 4.1 g of acetic anhydride, hydrogen iodide and 32.7 g of acetic acid.

Example 3

A 100 ml Hastelloy 'B2' autoclave was charged with 38.6 g of methyl iodide, 30.2 g acetic acid, 5.0 g triphenylphosphine and 0.200 g rhodium acetate dimer and pressurized with carbon monoxide to 65 bar. The vessel was then heated at 150°C during 3 hours with stirring. After cooling, g.c. analysis of the reaction mixture showed it to contain 3.1 g of acetic anhydride, hydrogen iodide and 31.2 g of acetic acid.

Claims

1. A process for the production of acetic anhydride from (a) methyl iodide, (b) acetic acid and/or methanol (c) and carbon monoxide characterised by reacting the methyl iodide, acetic acid and/or methanol with carbon monoxide under carbonylation conditions in the presence of an effective amount of a carbonylation catalyst.

2. A process as claimed in Claim 1 characterised in that the carbonylation catalyst comprises

(1) a Group VIII noble metal preferably rhodium

(2) a Lewis base promoter.

3. A process for the production of acetic anhydride from methanol, acetic acid and carbon monoxide in a series of process steps characterised in that;

1) in a carbonylation stage a mixture of (a) methyl iodide and (b) acetic acid and/or methanol is reacted under carbonylation conditions with carbon monoxide in the presence of the carbonylation catalyst claimed in Claim 1 to form acetic anhydride and hydrogen iodide,

2) in a separation stage the hydrogen iodide and acetic anhydride are separated, and

3) in an iodination stage the hydrogen iodide and methanol are reacted to produce methyl iodide and water, the methyl iodide being subsequently separated from the water and recycled to stage (1).

4. A process as claimed in Claim 3 characterised in that in the iodination stage the methyl iodide and water are allowed to separate into two layers and are withdrawn separately.

5. A process as claimed in Claim 1 characterised in that the reaction is carried out at a temperature in the range 140 to 200°C and at a pressure in the range 25 to 100 bar.

6. A process as claimed in Claim 1 characterised in that the carbon monoxide contains up to 10% by volume hydrogen gas.

7. A process as claimed in Claim 2 characterised in that the Lewis base promoter is a nitrogen heterocycle.

8. A process as claimed in Claim 7 characterised in that the nitrogen heterocycle is imidazole or 2-methyl imidazole.

Drawing

Ref : EPO - European publication server

Process for the production of acetic anhydride and acetic acid

A process for the production of acetic anhydride with or without the net co-production of acetic acid from methanol and carbon monoxide involves the following steps

1) reacting methanol with recycle acetic acid in an esterification step to form an esterification product containing predominantly methyl acetate, water and optionally unreacted methanol,

2) removing part of the water from the esterification product,

3) reacting the esterificaion product still containing water with carbon monoxide in a carbonylation step in the presence as catalyst of free or combined metallic carbonylation catalyst and as promoter of free or combined halogen to form a carbonylation product containing acetic acid and acetic anhydride,

4) separating the carbonylation product by fractional distillation into a low boiling fraction containing carbonylation feed and volatile carbonylation promoter components, acetic acid and acetic anhydride fractions, and a higher boiling fraction containing carbonylation catalyst components,

5) recycling the low boiling fraction containing carbonylation feed and carbonylation promoter components and the higher boiling fraction containing carbonylation catalyst components to the carbonylation step and,

6) recycling at least part of the acetic acid fraction to the esterification step.

It is an object of the present invention to provide an improved process for the production of acetic anhydride, with or without the net co-production of acetic acid, from methanol and carbon monoxide as the essential feedstocks in an integrated series of esterification, carbonylation and separation steps, and in which the product of the esterification reaction containing methyl acetate and water is not subjected to extensive purification or dehydration before feeding to the carbonylation stage.

According to the present invention the process for the production of acetic anhydride with or without the net co-production of acetic acid from methanol and carbon monoxide in a series of esterification, carbonylation and separation steps comprises:

1) reacting methanol with recycle acetic acid in an esterification step to form an esterification product containing predominantly methyl acetate, water and optionally unreacted methanol,

2) removing part of the water from the esterification product,

3) reacting the esterification product still containing water with carbon monoxide in a carbonylation step in the presence as catalyst of free or combined metallic carbonylation catalyst and as promoter of free or combined halogen to form a carbonylation product containing acetic acid and acetic anhydride,

6) recycling at least part of the acetic acid fraction to the esterification step.

With regard to the individual steps, in the esterification step (1) methanol is reacted with recycle acetic acid to form esterification product containing methyl acetate, water and unreacted methanol. In a preferred embodiment recycle acetic acid forms substantially the entire acetic acid in the feed to the esterification. It is an advantage of the present invention that neither the methanol nor the recycle acetic acid need be essentially pure. Thus the methanol may be contaminated with water, for example, and the recycle acetic acid may contain, for example, water, methyl acetate, acetic anhydride, and carbonylation promoter components, such as alkyl halides, eg methyl iodide. Though the esterification step may, if desired, be effected in the absence of a catalyst, it is preferred to employ an esterification catalyst, which may be any of the esterification catalysts conventionally employed. Suitable esterification catalysts include mineral acids such as sulphuric acid and organic acids such as toluene-para-sulphonic acid. Alternatively, other esterification catalysts, such as acid ion-exchange resins, may be employed if so desired. The esterification catalyst may suitably be present in an amount from 0.1 to 10%, preferably from 2 to 6X by weight of the reaction mixture.

The esterification step may be effected in a variety of ways. Thus a feed mixture comprising excess methanol and recycle acetic acid may be fed continuously to an esterifier containing aqueous acetic acid and a strong acid catalyst under reflux conditions. A mixture of methyl acetate, water and excess methanol may then be taken overhead as a distillate product. A typical ester distillate product obtained from 2:1 molar ratio feed of methanol:acetic acid comprises 57.5% w/w methyl acetate, 27.9% w/w methanol and 13.8% w/w water. Some of the water may be removed from the product by azeotropic drying and methanol may be removed for recycle, if desired, by hydroselection. Additional distillation steps may be employed to recover methanol from the hydroselection base product and methyl acetate from the decanter water phase/entrainer for recycle. Alternatively, the process described in our published European Application No 0060717 (BP Case No. 5076) may suitably be employed. In this process methanol is reacted at elevated temperature with acetic acid in the presence of an esterification catalyst and an entrainer which is sparingly soluble in water and which forms a minimum boiling point azeotrope therewith to form a product containing methyl acetate, water and entrainer, an overhead fraction comprising methyl acetate, entrainer and water is distilled from the product mixture in a first column, water is separated from the methyl acetate and entrainer and thereafter methyl acetate is recovered from the entrainer by distillation in a second column. Preferably the process described in our published European application No. 00607196 (BP Case No. 5124) is employed. In this process methanol is reacted at elevated temperature with acetic acid in the presence of an esterification catalyst and an entrainer which is sparingly soluble in water and which forms a minimum boiling point azeotrope therewith to form methyl acetate and water and in a distillation column methyl acetate is recovered as an overhead fraction and from an intermediate point in the column there is removed a liquid sidestream fraction comprising water and entrainer. Entrainers which may be used in the processes of the above referred to European applications Nos. 0060717 and 0060719 include hydrocarbons, ethers, esters and ketones, of which butyl acetate is preferred. The processes are more fully described in the aforesaid UK specifications, the disclosures of which are incorporated herein by reference. Another mode for effecting the esterification step is by counter current esterification. In this mode of operation a single distillation column is employed. To this column acetic acid is fed near the top, methanol is fed near the bottom, a small water stream is added to the reflux to enhance the methanol/methyl acetate separation if required, methyl acetate is removed as head product and water as base product. The column may either be packed with an acid ion-exchange resin catalyst or a liquid acid catalyst, such as for example sulphuric acid, may be fed to the column.

In step (2), part of the water is removed from the esterification product. In a preferred embodiment the removal of the water from the esterification is effected by removing the water from the reactor vessel in which the esterification takes place.

The reactions which are believed to take place are as follows:

(1) Esterification

(2) Carbonylation

(3) Hydration

The overall equation is therefore:

It will be seen from the last equation that, provided some water is removed, then acetic anhydride will be produced.

The greater the proportion of water removed then the higher will be the yield of acetic anhydride rather than acetic acid. Since the object of the present invention is to coproduce acetic acid and acetic anhydride it is important to remove some, but not all, of the water. Preferably water is removed to reduce the water content to below 14% w/w or the equivalent amount of water and methanol for example to below 12% w/w based on the weight of ester + water + alcohol if present. Preferably however at least 6X of water (as water) is present more preferably at least 7% w/w based on the amount of ester + water + alcohol.

In step (3), the esterification product containing predominantly methyl acetate, optionally unreacted methanol, and still containing water is reacted with carbon monoxide in a carbonylation step in the presence as catalyst of free or combined metallic carbonylation catalyst and as promoter free or combined halogen to form a carbonylation product containing acetic acid and acetic anhydride. Any of the known metallic carbonylation catalysts may be employed. Suitable metals include the metals of Group VIII of the Periodic Table of the elements, of which the noble metals iridium, osmium, platinum, palladium, rhodium and ruthenium are preferred. Particularly preferred is rhodium. It is preferred to employ the metal in the form of a soluble compound _such as a salt, eg a rhodium halide, or a complex of the metal, eg a carbonyl complex. As promoter there is used a halogen in free or combined form. Suitable forms include elementary halogen, a hydrogen halide, an inorganic salt, such as for example sodium halide, potassium halide or cobalt halide and quaternary ammonium or phosphonium halide, such as for example tetramethylammonium halide. Particularly preferred as promoters are organo-halides, such as alkyl, eg methyl iodide, or aryl halides. Methyl iodide is particularly preferred. Suitable reactants and conditions of operation are more fully described in the aforesaid UK Patents Nos. 1468940 and 1,523,346. It is particularly preferred to employ co-promoters. Suitable co-promoters include the metals in free or combined form such, as zirconium and those described in the aforesaid UK Patent No. 1468940, eg lithium, magnesium, calcium, titanium, chromium, iron, nickel and aluminium, alkyl or aryl 7 phosphines and organic nitrogen compounds as described in the aforesaid UK patent No. 1523346 and either heterocyclic aromatic compounds in which at least one heteroatom is a quaternary nitrogen atom as described in European Patent No. 8396 or the precursors of heterocyclic aromatic compounds in which at least one heteroatom is a quaternary-nitrogen atom, such as for example N-methyl imidazole. Other suitable promoter sytems are described for example in UK Patent No. 1538783 and UK published applications Nos 2059794, 2067557 and European published application No. 26280.

In one embodiment the carbonylation product from step (3) is separated in a first separation step by fractional distillation into an overhead fraction containing carbonylation feed and carbonylation promoter components, an intermediate fraction containing acetic acid and acetic anhydride, and a lower fraction containing carbonylation catalyst components, the overhead fraction and the lower fraction are recycled to the carbonylation step, the intermediate fraction is separated in a second separation step by fractional distillation into an acetic acid product fraction and an acetic anhydride product fraction, and part or all of the acetic acid product fraction is recycled to the esterification step.

In step (4), the carbonylation product is separated by fractional distillation into a low boiling fraction containing carbonylation feed and carbonylation promoter components, acetic acid and acetic anhydride fractions, and a higher boiling fraction containing carbonylation catalyst components.

In a preferred embodiment of steps (4),(5) and (6) the carbonylation product is separated in a first separation step by fractional distillation into an overhead fraction containing carbonylation feed and carbonylation promoter components, an intermediate fraction containing acetic acid and acetic anhydride, and a lower fraction containing carbonylation catalyst componerts, the overhead fraction and the lower fraction are recycled to the carbonylation step, the intermediate fraction is separated in a second separation step by fractional distillation into an acetic _?cid product fraction and an acetic anhydride product fraction, and at east part of the acetic acid product fraction is recycled to the esterification step.

Step (4) may suitably be effected by passing the carbonylation product to a tlash zone from which there is recovered a liquid product which is predominantly acetic acid and anhydride but also contains the metallic catalyst components and any co-promoter present in the product and a volatile product containing acetic acid, acetic anhydride and volatile carbonylation promoter and feed components, eg alkyl halides such as methyl iodide and methyl acetate. Optionally, a partial pressure of carbon monoxide and/or hydrogen may be applied in the flash zone to assist in maintaining catalyst activity. Further details may be found in UK patent application publication No. 2037276A which is hereby incorporated by reference. The volatile fraction may suitably be fed to a distillation column wherein volatile carbonylation promoter and feed components are removed overhead optionally together with some acetic acid and anhydride, and recycled to the carbonylation zone, a liquid sidestream comprising acetic acid and acetic anhydride is taken from an intermediate point and a liquid residue containing involatile carbonylation catalyst components carried over from the flash stage is removed from the base for recycle to the carbonylation zone with the liquid product from the flash zone. If the catalyst carry over from the flash stage is acceptable, the acetic acid/anhydride fraction may be taken from the base of the distillation column containing less low boilding impurities. The side or base stream containing acetic acid and acetic anhydride is then fed to a further distillation column from which pure acetic anhydride is removed as base product, and impure acetic acid is removed as overhead product. The impure acetic acid overhead fraction, which may contain some acetic anhydride and small amounts of methyl acetate and methyl iodide depending on whether the acetic acid/anhydride stream was taken as a side stream or a base product from the previous distillation column is then recycled directly to the esterification step. A part of this acetic acid stream may if desired be removed and purified in a further column to produce pure acetic acid.

The process of the present invention is further illustrated with reference to the accompanying drawing which is a simplified flow diagram of a process for the manufacture of acetic anhydride and acetic acid from methanol and carbon monoxide in a series of integrated esterification, carbonylation, and separation steps.

In this process the esterification step is carried out continuously in the kettle of a single distillation column 1, with n-butyl acetate as internal entrainer and side decantation to remove water by line 4. Internal entrainer is recycled to the column. Recycle acetic acid is fed to the kettle by line 2, together with an at least equimolar quantity of methanol by line 3, and a mixture of methyl acetate with some water and unreacted methanol is removed as head product by line 5 and passed directly to the carbonylation reactor 6. In the carbonylation reactor 6 the esterification product is reacted with carbon monoxide, fed to the reactor by line 7, in the presence of a rhodium carbonylation catalyst recycled by line 13, a promoter of methyl iodide recycled by line 12, and a co-promoter of N-methyl imidazole recycled as quaternary ammonium salt by line 13. The initial catalyst components are charged by line 8, which is also used for any subsequent make-up. The product of the carbonylation reaction consisting of predominantly acetic anhydride, acetic acid, unreacted methyl acetate and some methyl iodide is passed by line 10 to the separation zone 11 in which it is separated into a low boiling overhead fraction containing carbonylation feed and volatile promoter components which is recycled by line 12 to the carbonylation reactor 6, a high boiling base product containing carbonylation catalyst components which is recycled by line 13 to the carbonylation reactor 6, and a mixed acetic acid/acetic anhydride fraction which is withdrawn as a liquid sidestream by line 14 and passed to the separation zone 15. In zone 15 the acid/anhydride product is separated by fractional distillation into an impure acetic acid overhead fraction, which is recycled by line 16 to the esterification reactor 1, and an acetic anhydride product fraction which is withdrawn as a base product by line 17. The net acetic acid product from line 16 may be further purified is desired.

Example 1: Parts = weight or weight per hour

In this process the esterification step is carried out continuously in the kettle of a 31 plate Oldershaw column 1. Tray 16 comprises a chimney tray with liquid sidedraw. 3130 parts of methanol are fed to the kettle which contains a refluxing mixture comprising 520 parts of acetic acid, 235 parts of n-butyl acetate as internal entrainer, 10 parts of water and 40 parts of methane sulphonic acid catalyst. The temperature at tray 15 is maintained at 80 to 90°C. The sidestream from the chimney tray is cooled and separated into two phases in a decanter. The lower oil phase containing the internal entrainer is returned to tray 15. 1925 parts of aqueous phase is discharged by line 4. Recycle acetic acid (5450 parts) is fed to the kettle by line 2. A mixture of 6500 parts methyl acetate, 200 parts water and 150 parts methanol is removed as a head product by the line 5, after maintaining a 5:2 reflux to the column. The product in line 5 is made up with 220 parts of methyl acetate and 70 parts of methanol corresponding to material which could be recovered in a simple stripping stage from the waste water stream in line 4, together with 550 parts of water to give a total of 9.8% water in the feed to the carbonylation reator 6. The carbonylation reactor 6 comprises a stirred autoclave in corrosion resistant material in which the methyl acetate is reacted with carbon monoxide sparged by line 7. The reactor contains 4290 parts of a mixture containing 14 parts of rhodium expressed as rhodium diacetate, together with 40 parts of zirconyl diacetate and 215 parts of N,N'-dimethyl imidazolimium iodide. The balance comprises methyl acetate, acetic acid, acetic anhydride, and acetophenone as high boiling solvent. The carbonylation reactor 6 is maintained at a temperature of 183°C and a total pressure of 30 bar absolute. The initial catalyst components are charged by line 8, which is also used for any subsequent make-up. The product of the carbonylation reaction consisting of predominantly acetic anhydride, acetic acid, methyl iodide, solvent and unreacted methyl acetate is passed by line 10 to the separation zone 11 which is maintained at a pressure of 3 bar absolute. A high boiling base product containing all of the rhodium catalyst and zirconium and quaternary ammonium salt promoters is recycled by line 13 to the carbonylation reactor 6 together with the acetophenone and some acetic acid and acetic anhydriude. A low boiling overhead fraction containing most of the unreacted methyl acetate carbonylation feed and volatile unquaternarised methyl iodide promoter is recycled by line 12 to the carbonylation reactor 6. 10,490 parts of a mixture of acetic acid and acetic anhydride vapourised in the flash stage of separation zone 11 at a temperature of ca 135°C is recoverable as an intermediate fraction by a liquid sidestream and passed by line 14 to the separation zone 15. This comprises a 40 plate Oldershaw column with the feedpoint at plate 15 maintained at a temperature of 126°C to give a measured kettle temperature of 143°C and a heads temperature of 118°C 5030 parts of a base product are recovered by line 17 containing 98.5% of acetic anhydride. With a column reflux of 2:1 an overhead product of 5460 parts of acetic acid is obtained by line 16, containing trace levels of methyl acetate and methyl iodide. All the acetic acid is recycled to the esterification column 1 by line 2.

Example 2

The process is repeated as Example 1, save that the water in line 5 to the carbonylation stage is made up to 955 parts corresponding to 12.1% of the carbonylation feed. In this case 10,640 parts of mixed acetic acid and acetic anhydride is recovered from the separation zone 11 and fed by line 14 to separation zone 15. 3845 parts of acetic anhydride are recovered as a product by line 17 and a net acetic acid product of 1330 parts recovered by line 16 for use or further purification if required.

The above described process has the following advantages:

The acetic acid for the esterification step (or at least a significant portion thereof) is passed directly from the separation stage (in which it is separated from the acetic anhydride) without treatment. This means the esterification feed is not dependent on recovered acetic acid from subsequent operations involving the use of acetic anhydride.

The process offers flexibility in acid/anhydride product ratios without the need for a separate facility to hydrolyse anhydride to acid.

The esterification product is passed directly from the esterification reactor (from which water is removed) to the carbonylation reactor without any treatment such as purification or drying.

Claims

1. A process for the production of acetic anhydride with or without the net co-production of acetic acid from methanol and carbon monoxide in a series of esterification, carbonylation and separation steps comprising:

1) reacting methanol with recycle acetic acid in an esterification step to form an esterification product containing predominantly methyl acetate, water and optionally unreacted methanol,

2) removing part of the water from the esterification product,

6) recycling at least part of the acetic acid fraction to the esterification step.

2. A process as claimed in Claim 1 wherein the carbonylation product is separated in a first separation step by fractional distillation into an overhead fraction containing carbonylation feed and carbonylation promoter components, an intermediate fraction containing acetic acid and acetic anhydride, and a lower fraction containing carbonylation catalyst components, the overhead fraction and the lower fraction are recycled to the carbonylation step, the intermediate fraction is separated in a second separation step by fractional distillation into an acetic acid product fraction and an acetic anhydride product fraction, and part or all of the acetic acid product fraction is recycled to the esterification step.

3. A process as claimed in Claim 1 or Claim 2 wherein recycle acetic acid forms at least 50% of the acetic acid in the feed to the esterification.

4. A process as claimed in claim 3 wherein the amount of water in the feed to the carbonylation step is at least 6% w/w based on the weight of ester and methyl alcohol if present so that sufficient acetic acid is produced whereby the recycle acid forms at least 50X of the acetic acid in the feed to the esterification.

5. A process as claimed in claim 4 whereby the amount of water and alcohol (if present) is such that more acetic acid is produced in the carbonylation step than is required for the acetic acid in the feed to the esterification so that the process produces a net amount of acetic acid.

6. A process as claimed in any one of the preceding claims wherein the esterification product, after the removal of water in step (2) is passed directly to the carbonylation step (3).

7. A process as claimed in any one of the preceding claims wherein the recycle acetic acid to the esterification stage is passed directly from the carbonylation product separation stage in which the acetic acid and acetic anhydride are separated from each other.

Ref : EPO - European publication server

IV- The produczion of Acetic anhydride from Vinegar and baking soda

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