WO2017064470A1

WO2017064470A1 - Process for the co-production of dialkyl maleate and dialkyl succinate

Info

Publication number: WO2017064470A1
Application number: PCT/GB2016/053073
Authority: WO
Inventors: Ian Campbell; Christopher Ferguson
Original assignee: Johnson Matthey Davy Technologies Limited
Priority date: 2015-10-13
Filing date: 2016-10-03
Publication date: 2017-04-20
Also published as: GB201518101D0; GB2548936A; GB201616800D0; TW201728565A; ZA201800667B; GB2548936B; AR106368A1; MY185371A

Abstract

A process for the co-production of dialkyl maleate and dialkyl succinate in which a feed comprising monoalkyl maleate and a feed comprising monoalkyl succinate are supplied to an esterification reactor where esterification occurs and dialkyl maleate and dialkyl succinate are recovered from the esterification reactor, wherein at least a portion of the feed comprising monoalkyl maleate is supplied to the esterification reactor at a point above the point at which the feed comprising monoalkyl succinate is supplied.

Description

PROCESS FOR THE CO-PRODUCTION OF DIALKYL MALEATE

AND DIALKYL SUCCINATE

The present invention relates to a process for the co-production of dialkyl succinate and dialkyl maieate. More particularly, the present invention relates to a process for the co- production of dimethyl succinate and dimethyl maieate.

It is known to produce diols by reaction of dicarboxylic acids, anhydrides of dicarboxylic acids, mono- or di- alkyl esters of dicarboxylic acids, lactones, or mixtures thereof with hydrogen. Commercially, where the desired product is 1,4-butanediol, typically with the co- products tetrahydrofuran and γ-butyrolactone, the starting material is normally a dialkyl ester of maleic acid and/or anhydride, such as dimethyl maieate or diethyl maieate, which may contain minor amounts of dialkyl fumarate and/or dialkyl succinate.

Information relating to these processes can be found in, for example, US4584419, US4751334, WO86V03189, WO88/00937, US4767869, US4945173, US4919765, US5254758, US5310954 and W091/01960.

The dialkyl maleates which are used as feedstock in these conventional reaction processes may be produced by any suitable means. The production of dialkyl maleates for use in such processes is discussed in detail in US4584419, US4751334, WO88/00937, US4795824 and WO90/08127.

In one conventional process for the production of 1,4-butanediol and co-product tetrahydrofuran with optional production of γ-butyrolactone, a dialkyl ester, such as dimethyl maieate together with any residual methanol from the esterification reactor, is fed to a vaporiser where it is vaporised by a stream of hot cycle gas fed to the vaporiser which may be mixed with make-up hydrogen. The cycle gas will normally contain a high concentration of hydrogen gas but may also include other gases including hydrocarbons, carbon oxides, methane and nitrogen. Further, where the cycle gas includes recycled gases from downstream, condensables including product ether, methanol, water, co-products and byproducts may also be present.

The combined vaporous stream from the vaporiser is then passed to a reactor where it is reacted in the presence of a catalyst to form 1,4-butanediol, tetrahydrofuran and/or y- botyrolactone. The product stream is cooled and the reaction products are condensed and separated from the excess cycle gas before being passed into a refining zone. In the refining zone the various products are separated and the 1,4-butanediol and the tetrahydrofuran are removed. The γ-butyrolactone, together with the intermediate, dimethyl succinate, and some 1,4-butanediol may be recycled. In one arrangement the v- butyrolactone may be at least partially extracted in an optional refining zone and recovered. The methanol water stream separated from the product mix will be recycled upstream. In general, a significant portion of the 1,4-butanediol produced by this or other conventional methods is subsequently converted to tetrahydrofuran.

The overall reaction which occurs is a series of steps and may include a final dehydration step in which tetrahydrofuran is produced. A probable reaction path starting from the dimethyl maleate is set out in Scheme 1.

An alternative process is described in W099/35113 in which maleic anhydride esters are fed to a reaction process in which three different catalysts are used. First the maleate is converted to the succinate in the presence of the first catalyst which is a heterogeneous selective hydrogenation catalyst at a temperature of from 120°C to 170°C and a pressure of 3 to 40 bara. The succinate is then passed directly to the presence of the second catalyst where it is converted mainly into v-butyrolactone. The product of the reaction with the second catalyst is then fed directly to the presence of a third catalyst which is used to dehydrate the γ-butyrolactone to produce tetrahydrofuran. Some of the v-butyrolactone formed in the presence of the second catalyst is transferred to a second reaction loop operating at a higher pressure where it is converted to 1,4-butanediol.

As the first step in Scheme 1 and the first catalyst used in the alternative process described in W099/35113 relates to the hydrogenation of the dimethyl maleate to dimethyl succinate, it has been suggested that dimethyl succinate or diethyl succinate may be suitable starting materials for the reaction with hydrogen to form 1 ,4-butanediol, tetrahydrofuran and/or γ- butyrolactone.

One process in which dimethyl succinate is used in the production of tetrahydrofuran and 1,4-butanediol is described in US4656297. In this process, methanol is added to the ester feed to increase conversion and reduce transesterification. Another example of a process in which dimethyl succinate is suggested as a feed is W099/35136 in which reaction with hydrogen occurs over two different catalysts, to form a mixture of tetrahydrofuran and v- butyrolactone.

Recently, there have been significant advancements in processes to produce and recover succinic acid from the fermentation of sugars. Examples of such processes can be found in, for example, USS958744, US6265190 and US8246792. Currently demonstration plants have been constructed. It is . anticipated that in due course such processes will enable succinic acid to compete with maleic anhydride as an economic feedstock for the production of 1,4-butanediol.

For ease of reference, succinic acid derived from a renewable feedstock such as from carbohydrates, including sugars, cellulose and hemi-cellulose, from lignin or from other biomass sources such as algae will be referred to as 'bio-succinic acid' and the term should be construed accordingly. The bio-succinic acid may be formed by, for example, fermentation processes. As bio-succinic acid generally contains impurities such as fermentation residues and by-products, whilst bio-succinic acid can be used in conventional processes designed for succinic acid, particular advantages can be observed where processes are specifically tailored to handle these impurities. An example of one suitable process is that described in WO2015/082916. In this process the reaction is a counter- current reaction. An alternative arrangement is described in WO20157082915 in which co- current reaction occurs.

As bio-succinic acid becomes increasingly available, there will be an opportunity for the operators of existing facilities for the production of 1,4-butanediol, tetrahydrofuran and/or v- butyrolactone to adapt their facilities to utilise this new feedstock. This may offer benefits in terms of reduced costs. In addition, it is expected that there will be reduced exposure to volatility in the cost of feedstock where the feedstock is bio-succinic acid than is noted where the feedstock is succinic acid from conventional sources or maleic anhydride where the costs depend on oil prices.

In addition, the use of biosuccic acid will allow the producers to identify their products as 'bio-based' or 'renewable' thereby enabling them to meet existing client demands and/or to enter new markets, it is also possible that a premium price for these products may be attainable when compared to the prices achievable for products obtained from a petrochemical originating maleic anhydride feed.

The conversion of a facility which has been constructed to use maleic anhydride as the feedstock to use succinic acid will, in general, require only modest capital investment when compared to the cost of building a completely new facility since a significant portion of the process to produce 1,4-butanediol, tetrahydrofuran and/or γ-butyrolactone is the same whether the starting material is maleic anhydride or succinic acid.

Whilst the conversion of a facility to operate with a succinic acid feedstock offers various advantages, it is possible that the amounts of succinic acid, particularly of bio-succinic acid, will not be sufficient to enable the requirement for 1,4-butanediol, tetrahydrofuran and/or v- butyro!actone to be met. In this situation, operators may choose to co-feed the succinic acid, such as bio-succinic acid, with the conventional maleic anhydride feedstock such that the requirement for the product can be met. This will be particularly attractive to the operators of existing facilities. Whilst this arrangement will not allow them to identify their products as 'bio-based' or 'renewable', they may be able to refer to the partial renewable status of the product.

Using a feed comprising both succinic acid, winch may be bio-succinic acid, and maleic acid will be particularly attractive to the operators of existing facilities for producing 1 ,4- butanediol, tetrahydrofuran and/or γ-butyrolactone where such a facility is located with a facility for producing the conventional maleic anhydride feedstock as it will enable the maleic anhydride to be consumed. A further advantage is that the use of the succinic acid with the maleic anhydride feed can increase the 1,4-butanediol production capacity of the plant without the need for additional maleic capacity or to buy in additional maleic feed.

Further, where a facility for producing 1,4-butanediol, tetrahydrofuran and/or γ-butyroiactone is located together with a facility for producing the conventional maleic anhydride feedstock, they may be constructed such there is some integration of utilities and of Outside battery limits' facilities.

For example, maleic anhydride is conventionally formed by the oxidation of butane or benzene. This reaction is strongly exothermic and, in a combined facility, the heat generated in the reaction to form the maleic anhydride is used to raise steam which can then be utilised in the facility for the production of 1,4-butanediol, tetrahydrofuran and/or v- butyrolactone. If the reaction to produce the maleic anhydride from butane or benzene is not performed, there will be a need to provide additional energy to the facility for manufacturing the 1 ,4-butanediol, tetrahydrofuran and/or γ-butyrolactone to make-up for the loss of the use of the steam generated in the exothermic oxidation of butane or benzene. This will significantly impact on the process economics. Whether the starting material is maleic acid, maleic anhydride, succinic acid (including bio-succinic acid), or succinic anhydride, or monoalkyl esters thereof, the first step in the production of the 1,4-butanediol, tetrahydrofuran and/or γ-butyrolactone is the formation of the dialkyl ester. There are many processes known for the production of the dialkyl ester but conventionally the reaction is carried out in a reaction column in which the acid or anhydride is fed to a reaction column where it flows downwardly against an upward flow of alcohol. As the acid passes down the column it contacts progressively drier alcohol which assists to drive the equilibrium of the reaction towards completion.

Although esterification reactions can be autocatalysed, a catalyst will generally be used, particularly where a dialkyl ester is to be formed. The catalyst will generally be located on trays within the reaction column. In some cases, particularly where a dialkyl ester such as dialkyl succinate is to be formed, a pre-reaction to the monoalkyl ester will be carried out, and it is the monoalkyl ester which is fed to the reaction column. However, although the pre- reaction is primarily for the formation of the monoalkyl ester, it will be understood that some dialkyl ester will be formed.

As reaction progresses in the reaction column, the product dialkyl ester will be removed from at, or near, the bottom of the reaction column and the alcohol, which is used to an excess, together with the water formed during the reaction will be removed from the reaction column at or near the top. This stream will be referred to as the Overhead stream'. Where both maleic anhydride and succinic acid are to be used as feeds to the process for the production of 1 ,4-butanediol, tetrahydrofuran and/or γ-butyrolactone it has been found that there may be benefits achieved if the first part of the esterification reaction, i.e. the formation of the monoester of the maleic anhydride and of the succinic acid are carried out separately.

In this connection it will be understood that since the maleic feed is an anhydride, it may be partially esterified to the monoalkylmaleate, usually the monomethylmaleate in a non- equilibrium reaction with high conversion. In addition, there is little over conversion to the dialkylmaleate as this can be controlled by restricting the addition of the alkanol, usually the methanol, to only slightly more than that required for the reaction stoichiometry.

However, it is not possible to .carry out the partial esterification of succinic acid with high conversion. This is because acid esterification is an equilibrium reaction where the presence of the water by-product in the reaction mixture limits the amount of conversion which can be achieved. In order to push the equilibrium, a large stoichiometric excess of alkanol, such as methanol, is required to achieve high conversion of the succinic acid to the mono-alkyl ester. However, this large stoichiometric excess of alkanol creates favourable conditions for the production of the dialkyl ester.

This formation of the dialkly ester at this stage in the overall scheme is problematic since the diesters are more volatile than the monoalkyl esters and so their presence in the top of the esterification reactor will lead to their being removed in the column overheads which represent a loss to the system.

Thus, as indicated above, since the maleic anhydride and the succinic acid perform substantially differently in the production of the monoalkyl ester, various advantages can be expected where each feed is. esterified to the monoalkyl ester separately before being introduced into the reactor in which the dialkyl ester is formed. By this means, the reactions for producing the monoalkyl maleate and the monoalkyl succinate may each be optimised.

Further, as maleic anhydride is liquid at the temperatures at which the diesterification reaction is typically carried out, i.e. at temperatures of from about 80°C to about 150°C, whereas succinic acid is a solid at this temperature separate reactions to form the monoalkyl esters are also advantageous since different reaction conditions can be used. In addition, succinic acid has a low solubility in methanol and water at temperatures below their atmospheric boiling points such that the formation of a liquid feed having a high succinic acid concentration is problematic. Since there are different requirements for the production of the monoaikyl maieate and the monoalky! succinate, it will be understood that different types or designs of reactors are likely to be optimal for the monoalkyiation reaction of each feed.

In addition once the monoaikyl ester is formed, and the subsequent reaction to the dialkyi ester is carried out, it is known that the losses of dimethyl succinate in the overhead stream from the esterification reaction column are higher than those observed in the corresponding system for the production of dimethyl maieate from maleic anhydride due to the different vapour pressures. This differential can lead to problems if the monoaikyl maieate and the monoaikyl succinate are to be co-fed to the reactor in which the diesterification reaction is to be carried out. The different liquid vapour pressures of dimethyl succinate, dimethyl maieate, monomethyl succinate and monomethyl maieate are illustrated in Figure 1.

Once the monoesters have been fed to the diesterification reactor, further reaction is carried out to form the diester. Whilst the diester will generally be removed from at, or near, the bottom of the column, some ester will be lost into the aqueous overhead stream. This is particularly problematic where there has been the pre-reaction to the mono ester since, as discussed above, this will mean some dialkyi ester is introduced toward the upper portion of the reaction column close to the portion of the column from where the overhead stream is removed.

Since the dialkyi succinate is more volatile that the equivalent dialkyi maieate, the risk of diaikyiester being removed in the overhead is higher where succinic acid is used as the, or part of the feedstock.

Where the alcohol used in the production of the dialkyi ester of succinic acid is methanol, dimethyl succinate will be formed. Dimethyl succinate forms a low boiling azeotrope with water at approximately 2 mol% dimethyl succinate at a temperature just below the boiling point of pure water. Thus where the dimethyl succinate is carried over in the aqueous overhead stream, it can be difficult to recover using conventional distillation or phase separation techniques. In this connection, it is noted that the azeotrope composition appears to lie outside, or at the very limit of, the immiscible region when cooled to near ambient temperature which makes phase separation inefficient.

Whether the dialkyi ester is dialkyi succinate or dialkyi maieate or both, the failure to recover the diaikyi ester carried over in the overhead stream results in a loss to the reaction which has a substantia! negative impact on the overall process economics. A further problem is that the presence of the dialkyl ester in the overhead stream will mean that the aqueous effluent stream will have a high organic loading. Indeed, the loading may be as high as about 5 wt% where the ester is dialkyl succinate. This loading will increase the cost of treating the effluent stream before it can be released to the environment.

Further, the dialkyl ester present in the overhead stream may be hydrolysed back to the monoalkyl ester or the starting acid during any future treatment of the overhead stream. For example, where there is an alkanol column for separating the alkanol from water, the hydrolysis of any ester present in the overhead stream may occur in the bottom of the alkanol column due to the high water and low alkanol content which creates equilibrium conditions favouring the reverse reaction.

Where hydrolysis does take place, significant concentrations of monoalkyl ester, dicarboxylic acid or both monoalkyl ester and dicarboxylic acid may build up in the bottom of the alkanol column. Where this occurs, the risk of corrosion and fouling is increased.

The problems detailed above will occur with the production of any dialkyl maleate in a reaction column but they are particularly problematic where the dialkyl ester of succinic acid is being formed, particularly where dimethyl succinate is being formed.

Some proposals have been made relating to recovering ester carried in the overhead stream. In US5536856 a process for forming an ester is discussed. Whilst there is a suggestion that an alkanol wash may be used to remove ester from the overhead stream, there is no suggestion as to how removal can be affected where the ester forms a low boiling azeotrope with water.

Methanol is suggested as being useful as a wash to recover traces of fatty ester or acid in overheads in US5157168. However, again there is no suggestion as to how removal can be affected where the ester forms a low boiling azeotrope with water.

In PCT/GB2016/050829 it has been proposed that the dialkyl succinate or dialkyl maleate carried in the overhead stream form the esterification reactor may be recovered therefrom by washing the overhead stream with butanol. This enables the dialkyl succinate and/or dialkly maleate to be separated from the overhead stream and recovered.

Whilst this process offers an attractive solution to the problem associated with overhead loses of the dialkyl, it is desirable to provide a process which reduces the amount of dialkyl ester removed in the overhead from the diesterification reactor. It has now been found that where the feed to the diesterification comprises both monoalkyl maleate and monoalkyl succinate, the amount of dialkyl succinate and dialkyl maleate removed in the overhead from the reactor can be reduced where the monoalkyl maleate is fed to the diesterification reaction column at a point above where the monoalkyl succinate

Thus according to the present invention, there is provided a process for the co-production of dialkyl maleate and dialkyl succinate in which a feed comprising monoalkyl maleate and a feed comprising monoalkyl succinate are supplied to an esterification reactor where esterification occurs and dialkyl maleate and dialkyl succinate are recovered from the esterification reactor, wherein at least a portion of the feed comprising monoalkyl maleate is supplied to the esterification reactor at a point above the point at which the feed comprising monoalkyl succinate is supplied.

The dialkyl maleate and dialkyl succinate may be dimethyl or diethyl succinate or dimethyl or diethyl maleate respectively, with the dimethyl ester being particularly preferred. In this arrangement, the feeds to the esterification reactor will comprise monomethyl maleate and monomethyl succinate with the monomethyl maleate being fed to the esterification reactor at a point above the point at which the monoalkyl succinate is fed.

It has been found, that surprisingly by providing at least a portion of the monoalkyl maleate feed to the esterification reactor above the point at which the monoalkyl succinate is supplied, the amount of dialkyl ester which is lost in the stream removed from at or near the top of the esterification reaction column can be significantly reduced. The reduction in the loss of the more volatile dialkyl succinate is particularly noted.

In one arrangement, all of the monoalkyl maleate feed may be supplied to the esterification reactor at a point above the point at which the feed comprising monoalkyl succinate is supplied. In one alternative arrangement, a portion of the monoalkyl maleate may be supplied to the esterification reactor at a point below the point at which the monoalkyl succinate is fed. In this arrangement, at least about 30% of the monoalkyl maleate may be supplied above the monoalkyl succinate although at least 50% of the monoalkyl maleate may be supplied. Generally about 70% or more, such as about 80%, may be supplied above the monoalkyl succinate.

It has been found that the benefits of the present invention may be achieved when only a relatively small proportion of the monoalkyl maleate is supplied to the esterification above the point at which the monoalkyl succinate is supplied to the reactor and thus lower amounts to those detailed above may be used provided that at least some is added above the monoalkyl succinate.

Whilst the feeds have been described as monoalkyl maleate and monoalkyl succinate feeds it will be understood that other components will be present. Thus the monoalkyl maleate feed may comprise unreacted maleic anhydride, monoalkyl maleate and dialkyl maleate although as discussed above it is unlikely that significant amounts of the dialkyl maleate will be present. Similarly, the monoalkyl succinate feed may comprise unreacted succinic acid, monoalkyl succinate and dialkyl succinate. As discussed above, the amount of the dialkyl ester present in the monoalkyl succinate feed is likely to be higher than that present in the monoalkyl maleate feed. Both monoesterified feeds will also generally include water and the alkanol from the esterification reaction.

Once the feeds have been fed to the esterification reaction column, the reaction to the diester can be carried out according to conventional processes.

In one arrangement, an alkanol wash may be supplied to the esterification reactor at a point above the point at which both monoesterified feeds are supplied to the reactor. The presence of this wash will assist in the reduction of the loss overhead from the esterification reactor.

The esterification reactor in which the diesterification reaction occurs may be of any suitable design. In one arrangement it will be a reaction column. In particular, the reaction column will generally comprise reaction trays. In this arrangement, the monoalkyl maleate feed may be supplied to the esterification reactor at any suitable position. The benefit of the present invention may be noted where the monoalkyl maleate is supplied only one tray above the tray to which the monoalkyl succinate is supplied. However, it may be supplied at two, three, four or more trays above the tray to which the monoalkyl succinate is supplied.

In one alternative arrangement, the reactor may comprise a reaction column and a flash column, said reaction column being coupled with to the flash column. In this arrangement, the monoalkyl feeds may be supplied to any suitable position provided that at least a portion of the monoalkyl maleate is fed to the esterification reactor at a point above that at which the monoalkyl succinate. In one arrangement, both the monobikyl maleate and the monoalkyl succinate may be supplied to the flash column of the esterification reactor. In another arrangement, some of the monoalkyl maleate may be provided to both the flash column and to the reaction column. The flash column may be integrated with the reaction column. In this arrangement, the column will include discrete reaction and rectifying sections. This may therefore mean that the present invention may be retrofitted into a reaction column.

In an alternative arrangement, reaction stages may be included at or near the base of the flash column where present. Any suitable number of stages may be used. Generally, from 2 to 10 stages may be present. In one arrangement, from 3 to 8 stages, such as 5 or 6 stages may be present. This allows more feed to be converted or pre-converted than would otherwise be achievable. The arrangement in which the reaction stages are present in the flash column is particularly suitable in the situation where an existing unit for the reaction of maleic feeds is to be retrofitted. It is particularly suitable for increasing the capacity of the existing unit.

An alkanol wash may be provided to the flash column.

Additionally or alternatively, the esterification reaction column may include a scrubbing section above the point at which the monoalkyl maleate is fed to the esterification reactor. Alkanol, will be fed to this scrubbing section, where it will wash ester back to the esterification reaction column which would otherwise be removed in the stream removed from at or near the top of the esterification reaction column.

In one arrangement of the present invention, the stream recovered from at or near the top of the esterification reactor may be washed with to remove any dialkly ester, such as dialkyl succinate, which may be removed from the reactor in this stream. In one arrangement, the stream may be washed with methanol. Where the diethyl ester is being formed, ethanol may be a suitable wash. In an alternative arrangement, butanol may be used for the wash. Particular advantages may be noted where the wash is carried out with butanol.

This wash with butanol enables the product dialkyl succinate or maleate to be separated from the overhead stream. Since the wash stream will enable the product dialkyl succinate and dialkyl maleate to be separated from the overhead stream, it can be recovered and therefore the presence of product dialkyl succinate and maleate in the overhead stream from the reaction column does not represent a loss of product to the system.

In one arrangement, the butanol for use in the wash may be recovered from within the flowsheet as this will be more cost-effective than supplying a separate stream. In this connection, it will be understood that the flowsheet may include post-esterification reaction steps. Thus, for example, since dialkly succinate and dialkyl maleate are often used in the production of 1,4-butanediol, tetrahydrofuran, or v-butyrolactone, the butanol for use as the wash stream may be that recovered from the hydrogenation process in the manufacture of 1,4 butanediol, tetrahydrofuran, or γ-butyrolactone which occurs after the esterification reaction. This offers particular advantages as the product butanol is normally purged and therefore use is made of a stream which would generally be lost and no new components have to be added which could cause further side chemistry.

The wash may be carried out at any suitable place in the flowsheet. Where the esterification reactor includes the flash column, the butanol may be supplied to the flash column. In this arrangement, the product dialkyl succinate or maleate will be recovered from the bottom of the flash column before being returned to the reaction column.

Where the overhead from the flash column is passed to an alkanol column rather than being recycled, butanol and water may be removed from the alkanol column as a side draw. This side draw may be cooled. Although butanol does form an azeotrope with the water, the azeotrope composition lies within the immiscible region when the liquid is cooled so that the butanol and water will separate. The butanol may be recovered and used to provide reflux to the flash column while the aqueous phase may be recycled to the alkanol column.

Heat exchangers may be present to allow heat integration between various streams in the flowsheet.

Since the esterification of a dicarboxylic acid generates two moles of water when compared to the reaction carried out on an anhydride starting material, there will be a hydraulic limit to the proportion of succinic acid which can be used in combination with the maleic anhydride even when the monc-esterification reactions are carried out separately. Thus in one arrangement of the present invention, the stream of mono-esterified succinic acid may be passed through a separation column before being fed to the esterification reaction column such that the water of esterification from the mono esterification reaction can be removed before the stream is fed to the esterification reaction column of the present invention. In this arrangement, the water will generally be removed from at or near the top of the separation column and the feed comprising the monoalkyl succinate is removed from at or near the bottom of the column.

According to a second aspect of the present invention there is provided a process for the manufacture of 1,4-butanediol with optional co-products tetrahydrofuran and and/or γ- butyrolactone and by-product butanol comprising;

forming dialkyl succinate and dialkyl maleate in a reaction column in accordance with the above first aspect of the present invention; recovering the dialkyl succinate and dialkyl ma!eate from at or near the reaction column bottom and further treating the ester to form 1,4-butanediol with optional co-products tetrahydrofuran and and/or γ-butyrolactone and by-product butanol.

The present invention will now be described, by way of example, by reference to the accompanying drawings in which:

Figure 1 is a graph comparing the liquid vapour pressure of dimethyl succinate, dimethyl maleate, mono-methyl succinate and mono-methyl maleate;

Figure 2 is a schematic representation of a process in accordance with a first aspect of the present invention;

Figure 3 is a schematic representation of a process in accordance with a second aspect of the present invention;

Figure 4 is a schematic representation of a process in accordance with a third aspect of the present invention;

Figure 5 is a graph illustrating the benefits of the present invention;

Figure 6 is a graph illustrating the losses for a 20% monomethylsuccinate feed in the arrangement of Figure 3 with a range of monomethylmaleate and methanol wash rates;

Figure 7 is a graph illustrating the losses for a 30% monomethylsuccinate feed in the arrangement of Figure 3 with a range of monomethylmaleate and methanol wash rates;

Figure 8 is a graph illustrating the losses for a 50% monomethylsuccinate feed in the arrangement of Figure 3 with a range of monomethylmaleate and methanol wash rates; and

Figure 9 is a graph illustrating the losses for a 80% monomethylsuccinate feed in the arrangement of Figure 3 with a range of monomethylmaleate and methanol wash rates.

It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

The process of the present invention will be discussed with reference to the co-production of dimethyl maleate and dimethyl succinate which are used in the production of 1,4-butanediol. However, it is equally applicable to the co-production of other dialkyl esters including diethyl maleate and diethyl succinate.

A schematic illustration of the process of one embodiment of the present invention is illustrated in Figure 2. In this embodiment the monomethyl maleate feed is supplied to the reactor 1 in line 2. The reactor in this illustrative arrangement is a reaction column comprising trays on which catalyst will be located. The monomethyl succinate feed is fed in line 3 to the reaction column 1. Thus the monomethyl maleate is supplied to the reaction column 1 at a point above that at which the monomethyl succinate is supplied. The methanol is added in line 4 at a point towards the base of the reaction column 1 so that it travels upwardly through the reactor in counter-current to the downflowing monomethyl maleate and monomethyl succinate. Thus as the monomethyl maleate and monomethyl succinate flow downwardly they encounter progressively drier methanol such that reaction to the dimethyl maleate and dimethyl succinate occurs.

The dimethyl maleate and dimethyl succinate are then recovered from at or near the base of the reactor in line 5 where it can be passed to hydrogenation.

In this illustrated arrangement, the esterification reactor includes a scrubbing section 6 located on top of reaction column 1. Methanol is supplied in line 7, which serves to wash the overheads from the reaction column which will comprise methanol, water and some ester. This methanol wash will enable the ester to be removed from the overheads and returned to the reaction column 1. The washed overheads from the reaction column are removed in line 8. These may be passed to a methanol column for treatment.

An alternative arrangement is illustrated in Figure 3. In this arrangement, the esterification reactor comprises a reaction column 11a and a flash column 11b. The overhead from the reaction column is fed in line 11c to the bottom of the flash column and the bottoms recovered from the flash column are fed in line 11d to the top of the reaction column. In this arrangement, the monomethyl succinate fee* is fed to the flash column 11b in line 13. The monoalkyl maleate feed is split with a proportion being fed to the flash column 11b in line 12b and a proportion being fed to the reaction column 11a in line 12a. Where a maleate feed reactor is being retrofitted to operate the process of the present invention in accordance with this arrangement, the reaction column overheads may be fed directly to a column for treating the recovered methanol. The methanol is added in line 14 at a point towards the base of the reaction column 11a so that it travels upwardly through the reactor in counter-current to the downflowing monomethyl maleate and monomethyl succinate. Thus as the monomethyl maleate and monomethyl succinate flow downwardly they encounter progressively drier methanol such that reaction to the dimethyl maleate and dimethyl succinate occurs.

The dimethyl maleate and dimethyl succinate are then recovered from at or near the base of the reactor in line 15 where it can be passed to hydrogenation.

In this arrangement, wash methanol is supplied in line 17a to the top of the flash column 11b. Wash methanol may also be supplied in line 17b to the top of the reation column 11a. This methanol serves to wash the overheads from the reaction column which will comprise methanol, water and some ester. This methanol wash will enable the ester to be removed from the overheads and returned to the columns 11b and 11a. The washed overheads from the flash column 11b are removed in line 18. These may be passed to a methanol column for treatment.

A modification of the arrangement of Figure 3 is illustrated in Figure 4. In this arrangement, the esterification reactor comprises a reaction column 21a and a flash column 21b. The overhead from the reaction column is fed in line 21c to the bottom of the flash column and the bottoms recovered from the flash column are fed in line 21d to the top of the reaction column. In this arrangement, the monomethyl succinate feed is fed to the flash column 21b in line 23. The monoalkyl maleate feed is split with a proportion being fed to the flash column 21b in line 22b and a proportion being fed to the reaction column 21a in line 22a. The methanol is added in line 24 at a point towards the base of the reaction column 21a so that it travels upwardly through the reactor in counter-current to the downflowing monomethyl maleate and monomethyl succinate. Thus as the monomethyl maleate and monomethyl succinate flow downwardly they encounter progressively drier methanol such that reaction to the dimethyl maleate and dimethyl succinate occurs.

The dimethyl maleate and dimethyl succinate are then recovered from at or near the base of the reactor in line 25 where it can be passed to hydrogenation.

In this arrangement, a butanol wash is supplied to the flash column 21b in line 29. In practice it will comprise about 60 to 70 wt% butanol. The butanol will remove any ester from the water and methanol. The ester will then be returned to the reaction.

Water and butanol form a low boiling azeotrope at approximately 26 mol% butanol at an operating pressure of the flash column of about 1.6 bara. It will be understood that other operating pressures may be used. The butanol wash is used to approach the water/butanol azeotrope composition toward the top of the flash column 21 b, and generally in the top tray of the flash column, this will prevent the ester, particularly the dimethyl succinate from concentrating and leaving in the overhead from the flash column.

At a temperature of 102.6°C and a pressure of 1.6 bara, the water/butanol azeotrope is more volatile than the water/dimethyl succinate azeotrope at 111.7°C, which significantly, by over 90%, reduces the concentration of the dimethyl succinate in the column overheads.

The overhead from the flash column 21b which will comprise water, methanol and butanol will be passed in line 30, optionally through a partial condenser (not shown), to the methanol column 31 where separation occurs. The partial condenser will generally be used if the flash column is operated at elevated temperatures. Fed into this stream will be recycled methanol from refining which is added in line 32. It is in this stream that the butanol is introduced into the system. The separated methanol is removed in overhead stream 33 and may be recycled to the esterification reactor or to any pre-reactor in which the monoesters are formed. The separated water is removed from the methanol column bottom in line 34.

Methanol may be removed from towards the top of the methanol column 31 and recycled to the esterification reactor 21 to provide the methanol for the esterification.

Butanol is removed from the methanol column 31 in side draw 35. The butanol stream recovered from the methanol column 31 will be cooled in heat exchangers 36 and 37, generally to about 40°C such that some phase separation may occur. The heat exchange in exchanger 37 will be against cooling water supplied in line 38.

In heat exchanger 36 interchanges the hot water-butanol draw removed in line 35 with the cooled aqueous phase exiting the decanter 39 thus reheating the stream returned to the methanol column 31 in line 40 and thereby reducing the reboiler duty for the methanol column.

An additional heat exchanger 41 may be present in which a portion of the hot butanol/water draw, supplied in line 42, is interchanged against the cooled organic phase exiting the decanter 39, thereby reheating the stream before it is supplied as reflux in line 29 to the flash column 21b thereby reducing the flash column reboiler duty.

Thus in this arrangement, the butanol/water side draw is removed from the methanol column 31 in line 35 and passed to heat exchanger 36 where it is cooled against an aqueous stream recovered from the decanter 39. It is then further cooled against cooling water in heat exchanger 37 before being passed to decanter 39. The aqueous stream is recovered in line 43 which is passed in counter-current heat exchange in exchanger 36 before being passed in line 40 back to the methanol column 31.

The organic phase from the decanter 39 is removed in line 44. It is then passed through heat exchanger 41 before being fed in line 29 to the flash column 21b. The hot stream against which the organic phase from the decanter in line 44 is heated is taken from the side draw 35 in line 42. This cooled stream is passed back to the heat exchanger 37 in line 45. A purge 50 may be removed.

The benefits of the present invention will now be illustrated with reference to the accompanying examples.

Example 1

The loss of maleate and succinate feed where the monomethyl maleate and monomethyl succinate feed are fed to the same tray was measured and compared to that of where the monomethyl maleate is fed to a tray in the reaction column which is above the tray on which the monomethyl succinate is added. In this example, the feed comprised 20% monomethyl succinate and 80% maleic anhydride. As can be seen from the graph in Figure 5 the percentage of maleate and succinate lost is significantly less when the monomethyl maleate is fed above the succinate when compared to their being fed to the same tray or below. As shown in the right hand part of the graph the greatest amount of ester prevented from being lost was the succinate.

The losses noted for a 20%, 30%, 50% and 80% monomethylsuccinate feed in the arrangement of Figure 3 with a range of monomethylmaleate and methanol wash rates are illustrated in Figures 6 to 9 respectively.

Claims

1. A process for the co-production of dialkyl maleate and dialkyl succinate in which a feed comprising monoalkyl maleate and a feed comprising monoalkyl succinate are supplied to an esterification reactor where esterification occurs and dialkyl maleate and dialkyl succinate are recovered from the esterification reactor, wherein at least a portion of the feed comprising monoalkyl maleate is supplied to the esterification reactor at a point above the point at which the feed comprising monoalkyl succinate is supplied.

2. A process according to Claim 1 wherein the dialkyl maleate and dialkyl succinate are dimethyl maleate and dimethyl succinate respectively.

3. A process according to Claim 1 or Claim 2 wherein all of the monoalkyl maleate is supplied added to the esterification reactor at a point above the point at which the feed comprising monoalkyl succinate is supplied.

4. A process according to Claim 1 or Claim 2 wherein some of the monoalkyl maleate feed is supplied to the esterification reactor at a point below the point at which the monoalkyl succinate is supplied.

5. A process according to any one of Claims 1 to 4 wherein at least 30%, 50%, 70% or 80% of the monoalkyl maleate is supplied to the esterification reactor above the point at which the monoalkyl succinate is supplied.

6. A process according to any one of Claims 1 to 5 wherein the esterification reactor comprises a reaction column coupled with a flash column.

7. A process according to Claim 6 wherein reaction stages are located at or near the base of the flash column.

8. A process according to Claim 6 or 7 wherein the monoalkyl maleate feed and the monoalkyl succinate feed may be supplied to any suitable position provided that at least a portion of the monoalkyl maleate is supplied to the esterification reactor at a point above that at which the monoalkyl succinate is supplied.

9. A process according to any one of Claims 6 to 8 wherein both the monoalkyl maleate feed and the monoalkyl succinate feed are supplied to the flash column of the esterification reactor.

10. A process according to any one of Claims 6 to 9 wherein a portion of the monoalkyl maleate feed is provided to both the flash column and the reaction column.

11. A process according to any one of Claims 6 to 10 wherein an alkanol wash is supplied to the flash column.

12. A process according to any one of Claims 1 to 7 wherein the esterification reactor includes a scrubbing section above the point at which the monoalkyl maleate is supplied to the esterification reactor.

13. A process according to Claim 12 wherein an alkanol wash is supplied to the flash column.

14. A process according to Claim 11 or Claim 13 wherein the alkanol wash is carried out with butanol.

15. A process for the manufacture of 1,4-butanediol with optional co-products tetrahydrofuran and and/or v-butyrolactone and by-product butanol comprising;

forming dialkyl succinate and dialkyl maleate in a reaction column in accordance with the process of any one of Claims 1 to 14;

recovering the dialkyl succinate and dialkyl maleate from at or near the reaction column bottom and further treating the ester to form 1,4-butanediol with optional co-products tetrahydrofuran and and/or v-butyrolactone and by-product butanol.