WO1991001960A1

WO1991001960A1 - Process

Info

Publication number: WO1991001960A1
Application number: PCT/GB1990/001165
Authority: WO
Inventors: Andrew George Hiles; Michael William Marshall Tuck
Original assignee: Davy Mckee (London) Limited
Priority date: 1989-08-04
Filing date: 1990-07-27
Publication date: 1991-02-21
Also published as: GB8917859D0; EP0491714A1; JPH04507411A; KR920703491A

Abstract

In production of butane-1,4-diol from a feedstock such as maleic anhydride by a route which involves hydrogenation of diethyl maleate two ethanol streams are produced. One ethanol stream is a relatively wet ethanol stream recovered from the esterification stage in which typically maleic anhydride is reacted with excess ethanol to form firstly monoethyl maleate and then diethyl maleate. The other ethanol stream is relatively dry and contains n-butanol and is recovered from the hydrogenation product mixture after separation of tetrahydrofuran therefrom. By distilling the two ethanol streams in the same column a significant reduction in the heat input required for recovering a stream of relatively low water content and containing ethanol for recycle to the esterification stage can be achieved, compared with a plant in which the two ethanol streams are distilled in separate distillation columns. The use of a single column in place of two such columns also represents a capital cost saving in construction of the plant.

Description

PROCESS

This invention relates to a process for the production of butane-l,4-diol from a C4 dicarboxylic acid or its anhydride, such as maleic anhydride.

It has been proposed to produce butane-1,4-diol by a continuous process in which maleic anhydride is reacted with ethanol to form diethyl maleate which is then subjected to hydrogenation to form a reaction mixture containing, in addition to butane-1,4-diol and ethanol, also qamma- butyrolactone, tetrahydrofuran, n-butanol, water, and "heavies" including ethyl 4-hydroxybutyl succinate and other components, some of which have not been identified and hence can be classed as "unknowns". For further teaching regarding this process and methods for recovering the various desired components from the hydrogenation mixture, including butane-1,4-diol, qamma-butyrolactone and tetrahydrofuran, reference may be made to US-A-4584419, US-A-4751334, WO-A-86/07358 , and WO-A-88/00937.

In this process ethanol is a co-product in the hydrogenation step, which proceeds according to the following equation:

HC.CO.O.CH₂.CH₃ CH₂.CH₂.OH

II + 5H₂ = | + 2 CH₃.CH₂.OH (1).

HC.CO.O.CH₂.CH₃ CH₂.CH₂.OH

The co-product ethanol can be recovered from the hydrogenation product mixture by distillation in several stages and can then be recycled for reaction with fresh male anhydride in order to form further diethyl maleate, according to the following equations: HC.CD -t HC.CO.O.CH₂.CH₃

|| 0 + CH₃.CH₂.OH = H (2)

HC.CO HC.CO.OH

HC.CO.O.CH₂.CH₃ HC.CO.O.CH₂.CH₃

|| = + CH₃.CH₂.OH = || + H₂0 (3).

HC.CO.OH HC.CO.O.CH₂.CH₃

If succinic anhydride is used in place of maleic anhydride similar equations can be written.

If maleic acid is used in place of maleic anhydridejr. then the first step, i.e. formation of monoethyl maleate, proceeds thus: HC.CO.OH HC.CO.O.CH₂.CH₃

+ CH₃.CH₂.OH = || +H₂0 (4)

HC.CO.OH HC.CO.OH

Fumaric acid can be used as feedstock or may be present in the feedstock used for production of diethyl maleate. THe equation for formation of monoethyl fumarate is analogous to equation (4) whilst the subsequent conversion of that mono-ethyl ester to the corresponding diester follows a course similar to that of equation (3) .

In practice the hydrogenation of diethyl maleate is more complicated than is suggested by equation (1) above. Although the exact reaction mechanisms have not been fully investigated, the product spectrum obtained upon hydrogenation of diethyl maleate is consistent with the following reaction scheme CH.C0₂Et +H- CH₂.C0₂Et

CH.C0₂Et CH₂.C0₂Et

As will be seen from the above reaction scheme water is a by-product of several of the reactions involved. Another byproduct is n-butanol, whilst another low boiling product is tetrahydrofuran. Due to problems caused inter alia by the ready formation of azeotropes between the low boiling components of the hydrogenation product mixture, it relatively difficult to recover the ethanol by-product for recycling in a form which is substantially free from n- butanol and from tetrahydrofuran.

Although it is possible to produce diethyl maleate from maleic anhydride by reaction in a single reactor, it will usually be preferable to conduct the reaction in two reaction stages in a commercial plant operating on a continuous basis. In the first esterification stage, maleic anhydride is reacted with excess ethanol, according to equation (2) above to form monoethyl maleate. This reaction can be conducted in the absence of a catalyst. In the second esterification stage monoethyl maleate is reacted to form diethyl maleate according to equation (3) above. This esterification reaction requires the use of a suitable esterification catalyst in a commercial plant. In US-A- 4795824 it is proposed to use as the esterification catalyst an acidic ion exchange resin catalyst containing sulphonic acid groups; the resin Amberlyst 16 is an example of a suitable resin catalyst. (The word Amberlyst is a trade mark) .

Equation (3) is a reversible reaction; hence, in order to achieve a suitable conversion of monoethyl maleate to diethyl maleate, it is desirable to supply a relatively dry form of ethanol as a reactant to the second esterification stage in order to drive the equilibrium as far as possible towards the production of diethyl maleate and thereby simplify subsequent purification of the resulting diethyl maleate. In addition it will normally be preferred to use an excess of ethanol in the secondary esterification stage, again for the purpose of driving the equilibrium as far as possible in the direction of diethyl maleate production.

The esterification mixture exiting the secondary esterification stage contains, in addition to diethyl maleate, also a minor amount of mono-ethyl maleate, besides excess ethanSl and by-product water. Separation from this esterification mixture of the "light ends", i.e. water, ethanol and a minor amount of diethyl ether which is produced in the secondary esterification stage, is readily accomplished by distillation.

The crude diethyl maleate is then purified to separate it from unconverted mono-ethyl maleate. For further teaching regarding a suitable method of purifying diethyl maleate reference may be made to US-A-4765869. An alternative scheme is proposed in US-A-4740272; also of relevance are GB-A-2193207 and GB-A-2193492.

The purified diethyl maleate is then subjected to hydrogenation, preferably in the vapour phase, according to the teachings of US-A-4584419, US-A-4751334 and WO-A- 86/07358.

The product mixture exiting the hydrogenation zone contains, in addition to butane-1,4-diol, also minor amounts of qamma-butyrolactone. tetrahydrofuran, water, a little n- butanol, diethyl succinate, and "heavies" including ethyl 4-hydroxybutyl succinate and "unknowns". Although separation of this mixture into a low boiling fraction including tetrahydrofuran, water, a little n-butanol and ethanol, and a heavy fraction including butane-1,4-diol, qamma-butyrolactone, diethyl succinate, and "heavies", is relatively simple, the further separation of these fractions is fraught with difficulty due to the formation of azeotropes between two or more components thereof. Separation of the light ends fraction has been described, for example, in GB-A-2207429, and in GB-A-2207431. Separation of components of the heavy fraction forms the subject of US-A-4767869, EP-A-0256813, GB-A-2207430, and EP-A-0301852.

Just as in the recovery of ethanol from the esterification stages, so also there is a problem in recovering from the light fraction of the hydrogenation product mixture an ethanol stream which is sufficiently free from n-butanol and tetrahydrofuran to be suitable for re¬ cycle to the esterification stages.* Because of the problems of azeotrope formation complex distillation procedures have to be adopted which are normally expensive in terms of utilities consumption, particularly steam consumption. Moreover the distillation columns for production of the required relatively dry ethanol streams have to be relatively large which has a significant impact upon the capital cost of constructing a butane-1,4-diol plant.

It would be desirable to provide as efficient a plant as possible, in terms of capital costs, maintenance costs ajι running costs, for recovering from the product streams from the esterification stages and from the hydrogenation stage an ethanol stream for recycle to the esterification stage which is of a suitable dryness and which is substantially free from n-butanol.

The present invention accordingly seeks to provide an improved form of butane-1,4-diol production plant in which the efficiency of ethanol recovery is optimised.

According to the present invention there is provided a continuous process for the production of butane- 1,4-diol frojp a C^ dicarboxylic starting material selected from maleic anhydride, maleic acid, fumaric acid, succinic acid, succinic anhydride, and mixtures thereof, which comprises:

(i) reacting the C^ dicarboxylic starting material with ethanol in one or more esterification stages to produce an esterification mixture containing a corresponding C^ dicarboxylic acid ester selected from diethyl maleate, diethyl fumerate, diethyl succinate, and mixtures thereof;

(ii) distilling the esterification mixture of step (i) to separate a wet ethanol stream from an ester stream containing said C^ dicarboxylic acid ester;

(iii) contacting said C^ dicarboxylic acid ester with hydrogen in a hydrogenation zone containing a charge of an ester hydrogenation catalyst and maintained under ester hydrogenation conditions;

(iv) recovering from the hydrogenation zone a hydrogenation product mixture containing butane-l,4-diol, tetrahydrofuran, ethanol, qamma-butyrolactone, n-butanol, water and "heavies";

(v) recovering from the hydrogenation product stream by distillation in one or more stages (a) a relatively dry ethanol stream that further contains n- butanol, (b) butane-1,4-diol, and (c) one or more other fractions;

(vi) recycling recovered ethanol to step (i) for production of further C^ dicarboxylic acid ester; characterised in that ethanol for recycle in step (vi) is recovered by distillation in a distillation column by steps which include:

(A), feeding the wet ethanol stream of step (ii) to an intermediate part of the distillation column;

(B) feeding the relatively dry ethanol stream containing n-butanol of step (v) to an upper part of the distillation column above said intermediate part;

(C) recovering from the top of the distillation column a stream of low water content and containing ethanol;

(D) recovering from a lower part of the distillation column above the sump thereof a stream of n- butanol; and

(E) recovering from the sump of the distillation column a water containing stream.

It will normally be preferred to distil the stream of low water content and containing ethanol of step (C) in a subsequent distillation column from which diethyl ether is recovered as an overhead product and from which an ethanol stream for recycle in step (vi) is recovered as a bottoms product.

In a preferred process the C^ dicarboxylic starting material is maleic anhydride which is typically produced by oxidation of a C^ hydrocarbon feedstock or of benzene. Maleic anhydride made in this way may contain a minor amount of maleic acid, fumaric acid, or a mixture thereof. Succinic acid or succinic anhydride may alternately be used as the C^ dicarboxylic starting material.

Esterification of the C4 dicarboxylic acid starting material involves two steps, the first step being formation o . a monoethyl ester of maleic acid, fumaric acid or succinic acid or a mixture thereof. In the second step the monoethyl ester reacts with further ethanol to produce the corresponding diethyl ester or mixture thereof. When using an acid feedstock, such as maleic acid, fumaric acid, or succinic acid, an esterification catalyst may be used to catalyse both steps. However, when the C4 dicarboxylic acid starting jnaterial is an anhydride, the reaction to form the monoethyl ester, as exemplified by equation (2) above, can be carried out in the absence of an added esterification catalyst. Normally an esterification catalyst, preferably an acidic ion exchange catalyst such as Amberlyst 16, is used as an esterification catalyst in the diesterification step, as exemplified by equation (3) above. Typical reaction conditions in both steps include use of a temperature in the range of from about 70°C to about 180°C, preferably in the range of from about 100°C to about 160°C, and of a pressure.sufficient to maintain ethanol as a liquid at the reaction temperature, e.g. a pressure in the range of from about 1 bar to about 20 bar. Further teaching regarding the production of diethyl maleate can be obtained from US-A-4795824.

Distillation of the esterification mixture to yield a wet ethanol stream in step (ii) of the process can be conducted under conventional distillation conditions at atmospheric, sub-atmospheric or elevated pressure. Typically this distillation step is conducted at a pressure in the range of from about 0.5 bar to about 3 bar. The overhead temperature ranges usually from about 50°C to about 105°C.

Hydrogenation of the C^ dicarboxylic acid ester in step (iii) can be effected in the liquid phase at a pressure of, for example, from about 50 bar to about 200 bar and at a temperature in the range of from about 200°C up to about 320°C. However, it will normally be preferred to effect hydrogenation in the vapour phase according to the teachings of EP-A-0143634, US-A-4584419, US-A-4751334 or WO-A- 86/07358. Typical reaction conditions include use of a temperature in the range of from about 150°C to about 240°C and a pressure of in the range of from about 25 bar to about 75 bar. Copper chromite and barium promoted copper chromite are examples of suitable hydrogenation catalysts.

The hydrogenation product mixture from step (iii) of the process contains butane-l,4-diol, tetrahydrofuran, ethanol, qamma-butyrolactone. diethyl succinate, n-butanol, water and "heavies" including ethyl 4-hydroxybutyl succinate and "unknowns".

This can be distilled using several distillation columns in order to separate various of the components from the hydrogenation product mixture. In particular a combination of distillation, typically at atmospheric pressure or a little above it (e.g. about 1.2 bar), and stripping under vacuum, can be used to separate the "light" materials, i.e. ethanol, n-butanol, tetrahydrofuran and water, from the "heavy" materials, i.e. butane-l,4-diol, qamma-butyrolactone, diethyl succinate, diethyl ethoxysuccinate etc. Further work up of the mixture of "heavy" materials can be effected using the teachings of one or more of US-A-4767869, EP-A-0256813, GB-A-2207430 and EP-A-0301852. For separation of the "light" materials from this hydrogenation product mixture, distillation of the crude mixture at atmospheric or more than atmospheric pressure (e.g. at a pressure of about 1.2 bar) can be used to separate a water/ethanol/tetrahydrofuran mixture as overhead product from the distillation column ("the crude column"). This overhead product from the crude column can then be condensed and redistilled in a further distillation column ("the THF column") at higher pressure (e.g. at a pressure in the range of from about 5 bar to about 20 bar, such as 7 bar) to yield tetrahydrofuran as a bottom product, whilst a water/ethanol/ tetrahydrofuran vapour mixture is taken overhead from the THF column and, after passage through a pressure let down valve, is condensed in the condenser for the crude column. In this way water and ethanol are returned to the crude column in the reflux stream and are recovered in the bottom product therefrom. The bottom product from the crude column is then preferably distilled under vacuum to strip water, n-butanol, and ethanol therefrom to form the relatively dry ethanol stream of step (v)-_*

The distillation column of steps (A) to (E) of the process of the invention is conveniently operated at atmospheric pressure or a little above. However, it can be operated under reduced pressure or elevated pressure, if desired.

When maleic anhydride is used as feedstock for production o_f diethyl maleate, the water content of the wet ethanol stream of step (ii) of the process of the invention arises from water of esterification released by esterification- of monoethyl maleate according to equation (3) above, as well as from water present in the ethanol supplied to the esterification stage or stages, whether present in the recycled ethanol of step (vi) of the process of the invention or in any make up ethanol required. If maleic acid or fumaric acid is wholly or partially substituted for maleic anhydride, then additional water will be present as a result of equation (4) above. If succinic anhydride or succinic acid is used to make diethyl succinate as feedstock for the hydrogenation step then the water present in the wet ethanol stream of step (ii) arises in an analogous manner. In a typical process the recycled ethanol of step (vi) contains about 16 mole % of water corresponding to an approximately 6:1 molar ethanol:water mixture. As explained above, it will usually be preferred to supply ethanol in excess to the esterification stage or stages so as to assist in driving equations (2) and (3) as far as possible towards completion. Since each mole of maleic anhydride requires two moles of ethanol to form each mole of diethyl maleate, it will be appreciated that the water concentration in the wet ethanol stream of step (ii) of the process will further depend upon the number of moles of excess ethanol that are supplied to the esterification stage or stages in order to react with each mole of maleic anhydride or maleic acid, as the case may be. When operating with maleic anhydride the water content of the wet ethanol stream is typically in the range from about 20 mole % to about 80 mole %, but more usually in the range of from about 25 mole % up to about 75 mole %.

The relatively dry ethanol stream of step (v) of the process of the invention also contains a minor amount of water. This arises principally as a co-product of formation of tetrahydrofuran in the hydrogenation zone. It may also arise as a co-product of formation of n-butanol in the hydrogenation zone. Typically the water content of the relatively dry ethanol stream ranges from about 1 mole % to about 15 mole %, but is more usually in the range of from about 2 mole % to about 10 mole %.

In order that the invention may be clearly understood and readily carried into effect a preferred process in accordance therewith will now be described, by way of example only, with reference to the accompanying diagrammatic drawing which is a flow sheet of a plant designed to produce continuously butane-1,4-diol from maleic anhydride by production of diethyl maleate, hydrogenation thereof and separation of the reaction products of the hydrogenation reactions.

It will be understood by those skilled in the art that the drawing is diagrammatic and that further items of equipment stich as column reboilers, condensers, reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers^, holding tanks, storage tanks, and the like would additionally be required in a commercial plant. The provision of such ancillary items of equipment is in accordance with conventional chemical engineering practice.

Referring to the drawing, molten maleic anhydride is supplied-in line 1 to a static mixer 2 together with a slight excess of ethanol fed in line 3 and recycled acidic materials in line 4. In passage through mixer 2 and in the exit line 5 tlierefrom the mixture undergoes reaction to form monoethyl maleate, according to equation (2) above. Further ethanol is added to the mono-esterification mixture by way of line 6. The resulting mono-esterification mixture flows on in line 7 through cooler 8 where the mono-esterification reaction is essentially completed. In this mono- esterification reaction approximately 85% of the maleic anhydride is converted to monoethyl maleate, with about 12% of the monoethyl maleate reacting further with ethanol to form diethyl maleate.

The resulting mixture of ethanol and monoethyl maleate is mixed with a small amount of recycled condensate in line 9 and flows on in line 10 to reactor 11 which contains a static bed of an acidic ion exchange resin, such as Amberlyst 16 resin. (The word "Amberlyst" is a trade mark) .

Reactor 11 is maintained at a temperature of from about 90°C to about 120°C so that in passage through reactor 11 the monoethyl maleate undergoes further esterification to yield an equilibrium mixture of diethyl maleate, monoethyl maleate, water and excess ethanol, by reaction according to equation (3) above. The composition of this equilibrium mixture is influenced by the water content of the ethanol supplied in lines 3 and 6.

The esterification mixture exits reactor 11 in line 12 and passes to an ethanol recovery column 13 from which crude diethyl maleate is recovered as a bottoms product in line 14, whilst a mixture of ethanol, water, and a minor amount of diethyl ether pass overhead in line 15. The water results from the esterification reaction of equation (3) above, whilst diethyl ether is a minor by¬ product produced as a result of passage of ethanol at elevated temperature through the acidic ion exchange resin bed of reactor 11.

The vaporous mixture in line 15 is fed to an intermediate tray of a further distillation column 16 ("the ethanol column") which is also fed in line 17 with a mixture of make-up ethanol supplied to the plant by way of line 18 and with a relatively dry recycled ethanol stream, which is admixed with a minor amount of n-butanol, from the downstream product recovery section (to be described in further detail below) .

Further operation of ethanol column 16 will be described below.

The crude diethyl maleate stream in line 14 contains also monoethyl maleate, besides minor amounts of "light ends", including ethanol and water. This passes to a crude stripper column 20 which is operated under vacuum. Reference numeral 21 indicates the vacuum pump for crude stripper column 20 and for the downstream columns 22 ("the MEM column") and 23 ("the MAH column").

Volatile materials, i.e. essentially water and ethanol, remaining in the diethyl maleate in line 14 pass overhead from crude stripper column- 20 in line 24 and are condensed by means of condenser 25. The resulting condensate collects in drum 26 and is recycled to reactor 11 by way of line 9. The vent gases from vacuum pump 21 pass in line 26a to-a vent scrubber column 27 which is supplied with water in line 28. The scrubbed vent gases pass to a flue stack y way of line 29. The wash water recovered from the bottom of vent scrubber column 27 is passed by way of line 30 to the upper end of ethanol recovery column 13.

The bottom product from crude stripper column 20 is passed by way of line 31 to MEM column 22 which is operated under vacuum. In MEM column 22 monoethyl maleate tends -to undergo thermal decomposition by the reverse reaction to equation (2) above to yield a mixture of maleic anhydride and ethanol. Thus the vapours recovered from the top of MEM column 22 in line 32 contain, in addition to diethyl maleate, also minor amounts of ethanol and maleic anhydride. The vaporous maleic anhydride and diethyl maleate are, condensed in passage through condenser 33; however, the temperature of condenser 33 is controlled so that b _t far the major part of the ethanol vapour passes through without condensation. The resulting condensate collects in drum 34, whilst the ethanol vapour passes via lines 35 and 36 to the upper part of crude stripper column 20. Coψiengate from drum 34 is returned to the top of MEM column 22 by means of line 37 and pump 38. Part of the condensate is passed to the MAH column 23 in line 39. As the mixture in line 39 is now substantially free from ethanol, mon ethyl maleate cannot be re-formed in any significant amount. In MAH column 23 maleic anhydride vapour passes overhead in line 40, is condensed by means of condenser 41 and collects in condensate drum 42, still in molten form. Line 43 connects condensate drum 42 via line 36 to crude stripper column 20 so that MAH column 23 can be operated under vacuum under the influence of pump 21. Part of the condensate is recycled to the top of MAH column 23 by means of pump 45 via line 46, whilst part is recycled to the mono-esterification reactor 2 via lines 47 and 4.

The bottoms product from MEM column 22 comprises a mixture of monoethyl maleate and diethyl maleate. This is recovered in line 48. Part is recycled by way of line 49 and reboiler 50 to column 22, part is recycled to the mono- esterification reactor 2 in line 51 and line 4 whilst a third stream is passed by line 52 to a downstream product recovery plant (to be described in further detail below) .

The bottoms product from MAH column 23 comprises diethyl maleate, possibly admixed with diethyl fumarate formed as a result of isomerisation, and is recovered in line 53. Part is recycled to MAH column 23 via line 54 and reboiler 55. The balance flows on in line 56 and is admixed with recycled material in line 57, the combined streams flowing on in line 58 to a hydrogenation zone 59. Hydrogenation zone 59 is also supplied with hydrogen in line 60.

Hydrogenation zone 59 is preferably operated under vapour phase conditions utilising, for example, a copper chromite catalyst according to the teachings of EP-A- 0143634. Hydrogenation zone 59 may comprise a single reactor or a pair of reactors connected in series and operated according to the teachings of US-A-4584419, US-A- 4751334, or WO-A-86/07358. Normally, however, it will be preferred to utilise a single hydrogenation reactor in hydrogenation zone 59.

The hydrogen:diethyl maleate molar ratio in the hydrogenation reactor or reactors is selected so that, at the operating temperature, the mixture is always at least about 5°C above its dew point.

In hydrogenation zone 59 diethyl maleate is hydrogenated to a mixture containing, besides product butane-1,4-diol and ethanol, also minor amounts of qamma- butyrolactone, tetrahydrofuran, diethyl succinate, n- butanol, water, and "heavies" including ethyl 4-hydroxybutyl succinate and "unknowns". This mixture is passed in line 61 to a further distillation column 62 ("the crude column"). A purge gas stream is taken from hydrogenation zone 59 in line 63.

Crude column 62 is typically operated at a pressure in the range from about 0.1 to about 5 bar, preferably not more than about 2 bar, e.g. about 1.1 bar. There is recovered overhead in line 64 a first vaporous mixture comprising water, ethanol and tetrahydrofuran. This is admixed with a further vaporous mixture in line 65 from a downstream distillation column (to be described further below) , the combined stream flowing on in line 66 to a condenser 67. The resulting condensate collects in condensate drum 68. Part is returned to the top of crude column 62 by means of line 69 and pump 70, whilst the remainder flows in line 71 to another distillation column ("the THF column") which is operated at a higher pressure than column 62, for example a pressure in the range of from about 5 bar to about 20 bar, preferably not more than about 10 bar, e.g. about 7 bar.

The vent line 73 from condensate drum 68 passes to a further condenser 74 supplied with chilled coolant. The resulting condensate flows into condensate drum 68 via line 75. Line 7J6 is a vent line.

A vaporous mixture recovered from the top of THF column 72 is also a vaporous mixture comprising water, ethanol, and tetrahydrofuran but contains a lower concentration of tetrahydrofuran than the mixture in line 64. This mixture is passed in line 77 through a pressure let down valve 78 to form the mixture in line 65.

The bottom product from column 72 in line 79 comprises substantially pure tetrahydrofuran. Part is recycled to column 72 by way of line 80 and reboiler 81, whilst the remainder is passed to storage in line 82.

The bottoms product from column 62 in line 83 is passed by way of line 84 to another distillation column 85 ("the lights column") which is operated under vacuum. A minor part of the bottoms product stream in line 83 is recycled to column 62 in line 86 through reboiler 87.

The overhead product from lights column 85 comprises a mixture of ethanol, water, and n-butanol. This is passed in line 88 to a condenser 89, the resulting condensate being collected in drum 90. Line 91 indicates a connection to a vacuum pump and vent system. Part of the condensate from drum 91 is returned to the top of lights column 85 by way of line 92 and pump 93. The remainder of the condensate from condensate drum 90 is fed in line 19 to join the make-up ethanol stream in line 18 to form a feed in line 17 to the top of ethanol column 16.

The bottoms product from lights column 85 in line 94 contains, besides butane-l,4-diol, also minor amounts of qamma-butyrolactone. diethyl succinate, and "heavies". Part of this is recycled by way of line 95 and column reboiler 96 to the bottom of lights column 85, whilst the remainder flows on in line 97 to a product recovery zone 98. Product recovery zone 98 comprises a number of distillation columns from which the following streams are recovered: butane-l,4-diol in line 99, "heavies" in line 100, and a mixture of qamma-butyrolactone and diethyl succinate in line 57 which is recycled to hydrogenation zone 59 via line 58. Zone 98 can be operated, for example, according to the teachings of one or more of US-A-4767869, EP-A-0256813, GB-A-2207430, and EP-A-0301852.

Ethanol column 16 is operated at about 1.1 bar and with a reflux ratio of 0.52. The stream in line 17 is significantly drier than the stream in line 15 and contains n-butanol and a trace of tetrahydrofuran. Most of the water - 18 -

appears as a bottoms product in line 101. Part of this stream is returned to ethanol column 16 by way of line 102, and reboiler 103, whilst the remainder passes on in line 104 to form the stream in line 28 and a stream 105 which is passed beyond battery limits to water treatment and disposal. n-butanol is recovered as a liquid stream in line 106 from a tray relatively low in ethanol column 16.

A vaporous product is recovered overhead in line 106 consisting of ethanol, some water and a trace of diethyl ether which is formed as a by-product in reactor 11. This vaporous mixture is condensed by means of condenser 107, the condensate being collected in condensate drum 108 from where part is recycled to the top of ethanol column 16 by way of line 109 and pump 110, whilst the remainder passes in line

111 to another distillation column ("the ether column") 112.

Reference number 113 indicates a vent line to condensate-drum 108.

Ether column 112 is fitted with an overhead reflux condenser 114 which is supplied with chilled coolant. Line 115 indicatjes a line leading to a vent stack. Diethyl ether is recovered from an upper tray of ether column 112 in line 116. Relatively dry ethanol is recovered from the bottom of ether colunto 112 in line 116. Part is recycled to column

112 in line 117 through column reboiler 118, whilst the remainder is recycled in line 119 to provide the streams in line 3 and 6.

Lh a typical plant the approximate compositions of significant streams expressed in mol % are as set out in Table 1.

Notes to Table 1:

THF = tetrahydrofuran DEE = diethyl ether DEM = diethyl maleate BuOH = n-butanol

In the plant of Figure 8 of WO-A-88/00937 make up ethanol is supplied in line 977 and- admixed with a mixture of "lights" in line 976 recovered from plant 9 including ethanol and a little n-butanol before being distilled in column 975. Ethanol recovered from the secondary esterification reactor 950 is condensed and recycled in line 912 to primary esterification reactor 914. A wet ethanol stream is recovered in line 918 from a downstream distillation column intermediate the primary and secondary esterification reactors, the water/ethanol mixture in line 918 then being distilled in column 919, to separate water which is recovered as a bottoms product in line 928, and in column 927, from which diethyl ether is recovered in line 939 whilst relatively dry ethanol is recovered in line 947 and is blended with ethanol in line 948 from column 975. Thus, in the arrangement of Figure 8 of WO-A-88/00937 two separate columns are provided for distilling the"lights" mixture recovered from the hydrogenation zone, on the one hand, and the wet ethanol resulting from esterification, on the other hand. For a plant of comparable capacity to that of the drawing of the present application the reflux ratio in column 919 in the plant of Figure 8 of WO-A-88/00937 is 1.23 and that in column 975 is 0.85. The heat capacity of the steam reboiler 103 of the plant of the drawing of the present application is substantially the same as that of column 919 of Figure 8 of WO-A-88/00937 and is somewhat smaller than that of the reboiler of column 975. Hence the total heat capacity required for production of a relatively dry ethanol stream for recycle to the esterification stages in the process of the present invention is less than 50% that of a plant of comparable capacity designed according to Figure 8 of WO-A-88/00937.

In the process of the present invention a single splitting column, i.e. the ethanol column 16, is used to distil both the wet ethanol recovered from the esterification steps and also the mixture of n-butanol, ethanol and a minor amount of water recovered from the hydrogenation product mixture purification steps. Not only is there a saving in capital costs and maintenance costs through utilising one distillation column in place of two but also there are very significant savings in operating costs.

Claims

1. A continuous process for -the production of butane-l,4-diol from a C^ dicarboxylic starting material selected from maleic anhydride, maleic acid, fumaric acid succinic Uci , succinic anhydride, and mixtures thereof which comprises:

(i) reacting the C^ dicarboxylic starting material with ethanol in one or more esterification stages to produce an esterification mixture containing a corresponding C^ dicarboxylic acid ester selected from diethyl maleate, diethyl fumarate, diethyl succinate, and mixtures thereof;

(iv) recovering from the hydrogenation zone a hydrogenation product mixture containing butane-l,4-diol, tetrahydro uran, ethanol, qamma-butyrolactone, n-butanol, water and "heavies";

(v) recovering from the hydrogenation product stream by distillation in one or more stages (a) a relatively dry ethanol stream that further contains n- butanol, (by butane-l,4-diol, and (c) one or more other fractions;

(vi) recycling recovered ethanol to step (i) for production o£ further C^ dicarboxylic acid ester; characterised in that ethanol for recycle in step (vi) is recovered by distillation in a distillation column by steps which include:

(A) feeding the wet ethanol stream of step (ii) to an intermediate part of the distillation column;

2. A process according to Claim 1, characterised in that the stream of low water content and containing ethanol of step (C) is further distilled in a subsequent distillation column from which diethyl ether is recovered as an overhead product and from which an ethanol stream for recycle to step (i) is recovered as a bottoms product.

3. A process according to claim 1 or claim 2, characterised in that the wet ethanol stream of step (ii) contains from about 25 mole % to about 75 mole % of water.

4. A process according to any one of claims 1 to 3, characterised in that the relatively dry ethanol stream of step (v) contains from about 2 mole % to about 10 mole % of water.

5. A process according to any one of claims 1 to 4, characterised in that make up ethanol for use in the esterification stage or stages is admixed with the relatively dry ethanol stream of step (v) prior to its being fed in step (B) to the upper part of the distillation column

6. A process according to any one of claims 1 to 5, characterised in that the stream of low water content of ste (C) contains about 16 mole % water.

7. A process according to any one of claims 1 to 6, characterised in that the distillation column is operated at a pressure of from about 0.5 bar to about 3 bar.