WO2014045034A1

WO2014045034A1 - Process for the production of carboxylic acid esters

Info

Publication number: WO2014045034A1
Application number: PCT/GB2013/052447
Authority: WO
Inventors: Robert Wild; Rikard Umberto Andersson; Paul J. CASSIDY; Michael Anthony Wood; Simon Nicholas Tilley
Original assignee: Johnson Matthey Davy Technologies Limited
Priority date: 2012-09-18
Filing date: 2013-09-18
Publication date: 2014-03-27
Also published as: AR092611A1; GB201216653D0; GB201316604D0; GB2508077A

Abstract

A process for the production of carboxylic acid esters by reaction of a carboxylic acid component and an alcohol component, said process comprising (a) feeding a liquid carboxylic acid stream to an upper section of a first reaction zone maintained under esterification conditions; (b) feeding a wet alcohol vapour stream comprising from about 4.4 to about 8 weight % water to a lower section of the first reaction zone; (c) allowing the carboxylic acid stream to pass in countercurrent to the wet alcohol stream to form an intermediate liquid product stream comprising product ester and unreacted carboxyiic acid; (d) passing the intermediate liquid product stream to an upper section of a second reaction zone without passing through a distillation column; said second reaction zone being maintained under esterification conditions; (e) feeding a dry alcohol stream comprising less than about 1 weight % water to a lower section of the second reaction zone; (f) allowing the intermediate product stream to pass in countercurrent to the dry alcohol stream such that further carboxylic acid is reacted to product ester; (g) recovering the product ester stream; (h) withdrawing a first stream comprising unreacted alcohol and water from the first reaction zone, treating the stream to reduce the water content to form a wet alcohol stream and feeding said stream to the first reaction zone in step (b); and (i) withdrawing a second stream comprising unreacted alcohol and water and having a water content of from about 2 to about 6 mole% from the second reaction zone and supplying said stream to the first reaction zone in step (b).

Description

PROCESS FOR THE PRODUCTION OF CARBOXYLIC ACID ESTERS

The present invention rebates to a process for the production of carboxylic acid esters. In an alternative arrangement it relates to apparatus for the production of carboxylic acid esters. More particularly the present invention relates to a process and apparatus for the production of fatty acid esters. Still more particularly, the present invention relates to the production of ethyl esters of fatty acids.

Esterification is a well-known equilibrium-limited reaction involving the reaction of a mono-, di- or poiycarboxiic acid or, in suitable cases, an acid anhydride, with an alcohol. The alcohol may be a mono, di- or poiyhydric alcohol.

A process for carrying out esterification in a column reactor having a plurality of esterification charge is described in US5536856 the contents of which are incorporated herein by reference. Each esterification tray has a predetermined liquid hold-up and contains a charge of a solid esterification catalyst thereon. Examples of suitable catalysts include an ion exchange resin containing ~S0₃H and/or -COOH groups. A liquid phase containing the carboxylic acid component, such as a fatty acid mixture, flows down the column reactor from one esterification tray to the next downward one against an upflowing alcohol vapour stream. The alcohol vapour is preferably methanol. Relatively dry alcohol vapour s injected into the bottom of the column reactor. Water of esterification is removed from the top of the column reactor in the vapour stream and ester product is recovered from the sump of the reactor. As the liquid flows down the trays it encounters progressively drier alcohol and the esterification equilibrium is driven further and further towards 100% ester formation.

Whilst this process is very effective, if the desired ester is, for example, ethyl ester, economic problems are encountered in obtaining the relatively dry ethanoi required for the process. This is because ethanol forms an azeotrope with water and is difficult to separate therefrom. Azeotropic ethanol will generally comprise about 5 weight % water and about 95 weight % ethanol and is known as "wet" ethanol. More particularly, it may be 95.63 wt % ethanol and 4.37 wt % water. Suitable processes to achieve the dry ethanol required for the above process include material separation agent addition, pressure swing distillation and the use of molecular sieves. However, the processes avai!abie to obtain the required dry ethanoi are expensive and therefore the use of dry ethanol impacts on the operating costs of the esterification process. if wet azeotropic ethanoi were to be used in the above process, it is not sufficiently dry to enable complete, or near-complete, conversion of fatty acid to fatty acid ethyl ester to be achieved in prior art arrangements due to equilibrium constraints.

Interest in the production of fatty acid ethyl esters has increased as it has been suggested that fatty acid ethyl esters are a better biofuel in terms of performance and physical characteristics than the present commercial biofuels formed from fatty acid methyl esters. It is therefore desirable to provide a process which enables the wet azeotropic ethanol to be used in the production of the ethyl esters of fatty acids. Since the separation of alcohol from water is also problematic with other alcohols, the desired process will also offer advantages where the alcohol is other than ethanol, in addition as ethanoi can be sourced from a sustainable feedstock via fermentation, it provides a more environmentally friendly approach to the production of fuels than methanol which is generally sourced from fossil fuels such as from natural gas or from coal gasification.

However, very high conversions, typicai!y of the order of 99.7% to 99,8% of fatty acid, are required to meet the typical biodiesei specification of 0.5 acid value. This can be difficult to achieve as to esterification is an equilibrium reaction.

An alternative process for producing organic acid esters is described in US2Q06/0014977, In this process the reaction is carried out using continuous countercurrent reactive distillation using acid catalysts in a structured packing in a single column. In one arrangement, absolute ethanol or an ethanol/water mixture is fed near the bottom of the column and a lactic acid solution in water is fed near to the top of the column. The ethyl lactate product is removed at the bottom of the reactor, in one arrangement, the ethanoi-water azectropic mixture is recycled back to the reaction distillation column. Whilst this process does appear to allow the use of wet ethanol, it does not produce a product which has the conversions required for biodiesei production. A method of producing fatty acid alkyi esters from a feedstock is described in US6399800. The described method comprises saponifying the feedstock, removing water from the saponified feedstock to form a dried saponified feedstock containing no more than about 10½ water, esterifying the dried saponified feedstock with an alcohol in the presence of an inorganic acid cataiyst to form fatty acid alkyl esters and recovering the fatty acid alky! esters. The process allows the esters to be formed even with water present at levels up to about 3 weight %. This level of water is for the reaction mixture as a whoie namely the saponified feedstock, a!cohol and inorganic acid cata!yst. Since substantia! water is present in the feedstock, this process requires that the alcohol be dry so as not to add to the overall water content.

An alternative process is described in US2009/0294358 in which free fatty acids are reacted with alcohols at temperatures between 50 and 120"C using acidic, heterogeneous ion exchange resin catalysts. The water produced is optionally removed with alcohol and the remaining free fatty acids are reacted with alcohols at temperatures of from 60 and 120T using acidic, heterogeneous ion exchange resin catalysts.

A still further process is described in US2011/021S355 in which fatty acids are reacted with alcohols in counter-current reactions. Two reactors may be used in series and water/alcohoi from each reactor is passed through a respective distil!ation column. This requirement for a separate distiliation column for each reactor adds substantially to the capital and operating costs of the process.

Production of ester-based fuels from iigriocelSuiosic material or algae is disclosed in US2009/0056201. Pulping and saccharification of the starting materials produces carboxyiic acids which are esterified via gas sparged, slurry form heterogeneous reactive distiliation to yield ester-based fuels.

A process for the production of a dialkyi maieate is described in EP 0255399 in which maleic anhydride is reacted with an alkyl alcohol in a monoesterification zone to form the corresponding monoaikyi maieate, followed by a reaction of resulting monoaikyi maieate with further alkyl alcohol to form the corresponding dia!kyl maieate which includes steps of supplying a first liquid feed comprising said monoaikyi maieate to a secondary este ification zone containing a charge of a so!id esterification catalyst, supplying a second feed stream comprising said a ky! aicohoi to said secondary esterification zone, maintaining said secondary esterification zone at an e!evated temperature to form or to maintain therein a vaporous stream containing said aikyi alcohol, intimately contacting said first liquid feed in said secondary esterification zone in the presence of said catalyst with said vaporous feed stream, recovering from said secondary esterification zone a vaporous effluent stream containing, in addition to alkyl alcohol vapour, also water in vapour form, said water being produced in said secondary esterification zone by esterification of said monoalkyl maieate with said alkyl aicohoi, and recovering from said secondary esterification zone a liquid product stream containing said diaikyl maieate. in this process, a dehydration unit is required to reduce the water content of the alkyl aicohoi.

Despite these various proposed processes there is stii! a need for a process which enables a wet stream of aicohoi to be used successfully and economically.

According to the present invention there is provided a process for the production of carboxylic acid esters by reaction of a carboxylic acid component and an aicohoi component, said process comprising:

(a) feeding a liquid carboxylic acid stream to an upper section of a first reaction zone maintained under esterification conditions;

(b) feeding a wet alcohol vapour stream comprising from about 4.4 to about 8 weight % water to a lower section of the first reaction zone;

(c) allowing the carboxylic acid stream to pass in countercurrent to the wet alcohol stream to form an intermediate liquid product stream comprising product ester and unreacted carboxylic acid;

(d) passing the intermediate liquid product stream to an upper section of a second reaction zone without passing through a distillation column; said second reaction zone being maintained under esterification conditions;

(e) feeding a dry aicohoi stream comprising less than about 1 weight % water to a lower section of the second reaction zone;

ff) allowing the intermediate product stream to pass in countercurrent to the dry alcohol stream such that further carboxylic acid is reacted to product ester; (g) recovering the product ester stream; (h) withdrawing a first stream comprising unreacted alcohol and water from the first reaction zone, treating the stream to reduce the water content to form a wet alcohol stream and feeding said stream to the first reaction zone in step (b); and

(i) withdrawing a second stream comprising unreacted alcohol and water and having a water content of from about 2 to about S rnoie% from the second reaction zone and supplying said stream to the first reaction zone in step (b).

Since a wet alcohol stream is used in the first reaction zone the need for expensive drying of the aicohoi required for this stage is overcome whilst the presence of the second reaction zone having a dry aicohoi stream enables the esterification to go towards completion.

The amount of ethanoi added in step (e) is preferabiy about the amount of ethanoi which will be required in the reaction. That is to say, in this arrangement, approximately the stoichiometric required amount will be added.

By "wet" alcohol we mean aicohoi having an azeotropic amount of water or more and by "dry" alcohol we mean aicohoi having a water content that is less than azeotropic.

The process of the present invention may be for the production of a monoester. Examples of monoesterification reactions include the production of alkyl esters of aliphatic mono- carboxyiic acids from alcohols and aliphatic monocarboxyiic acids. Any suitable monocarboxyiic acid may be used but in one arrangement, it may contain from about 6 to about 26 carbon atoms and may include mixtures of two or more thereof. Exampies of monocarboxyiic acids include fatty acids such as decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic,acid, octadecanoic acid, octadecenoic acid, iinoleic acid, eicosanoic acid, isostearic acid and the like, as well as mixtures of two or more thereof,

Mixtures of fatty acids are produced commercially by hydrolysis of naturally occurring triglycerides of vegetable origin, such as coconut oil, rape seed oil, and palm oils, and triglycerides of animai origin, such as lard, tallow and fish oils. If desired, such mixtures of acids can be subjected to distillation to remove lower boiling acids having a Sower boiling point than a chosen temperature {e.g. C_g to C₁₀ acids) and thus produce a "topped" mixture of acids, or to remove higher boiling acids having a boiling point higher than a second chosen temperature (e.g. C₂2_÷ acids) and thus produce a "tailed" mixture of acids, or to remove both iower and higher boiling acids and thus produce a "topped and tailed" mixture of acids, The resultant mixture may be the liquid carboxyiic acid stream fed to the first reaction zone.

The fatty acid mixtures may contain ethylersicaily unsaturated acids such as oleic acid.

In one alternative, the process of the present invention may be used to carry out a dieste ification. Thus the process can be used to produce dialkyl esters of aliphatic and cycloaliphatic C to C_1B saturated and unsaturated dicarboxyiic acids. These can be produced by reaction of alcohols with the dicarboxyiic acids or anhydrides thereof, or with mixtures of the dicarboxyiic acid and its anhydride. Dialkyl oxalates, dialkyl maleates, dialkyl succinates, dialkyi fumarates, diaikyi giutarates, dtalkyl pimelates, and dialkyl azeiaates are examples of dicarboxyiic acid esters which may be produced by the process of the present invention. Another example of a suitable carboxyiic acid is tetrahydrophthaiic acid. The C_x to CIQ alkyl esters of these dicarboxyiic acids are of particular interest. Either the free dicarboxyiic acid or its anhydride (if such exists) or a mixture of dicarboxyiic acids and anhydride can be used as the carboxyiic acid component starting material for production of such dialkyl esters,

Alkyl esters of aromatic C₇ to C_J0 monocarboxylic acids and mixtures thereof can be made by a process of the invention. Benzoic acid and 1-naphthoic acid are examples of such acids.

Alkyl esters of aromatic Q to C_2(! dicarboxyiic acids can also be produced by the process of the invention from the acids, their anhydrides and mixtures thereof.

It is also possible to produce poiyalkyl esters of polycarboxylic acids by the process of the invention. Suitable polycarboxyiic acid moieties include, for example, citri acid, pyromel!itic dianhydride, and the like.

Carboxyiic acid esters of dihydric and polyhydric aicohois can be produced by the process of the invention. Examples of these esters include ethylene glycol diformate, ethylene glycol diacetate, propylene glycol diformate, propylene glycol diacetate, glyceryl triacetate, hexose acetates, and the acetate, propionate and n-butyrate esters of sorbitol, mannitoi and xyiitol, and the like. it will be understood that the carboxylsc acid stream supplied to the first reaction zone may be a stream comprising a mixture of carboxylic acids.

The carboxylic acid stream will be fed to the first reaction zone at any suitable position. However, as it is to flow downwardly against an upfiowing alcohol vapour, it will be understood that it is desirable that it is supplied as high as possible within the zone to maximise the contact with the upf!o ing alcohol

Any suitable alcohol may be used provided that it is a vapour at the reaction conditions of the este ification zones. Suitable alcohols include those having from I to 10 carbon atoms with those having 5 or less carbon atoms being interesting.

Etharsoi is a particularly preferred as the alcohol used in the present invention, Where ethanoi is used as the alcohol, the wet ethanol stream will desirably be azeotropic ethanol which generally comprises in the region of 95.63 weight % ethanol and 43? weight % water. The wet alcohol stream added to the first reaction zone will generally comprise water in an amount that is equal to or more than the a?.eoiropic water content.

The alcohol stream will be fed to the first reaction zone at any suitable position, in one arrangement it will be injected into the bottom of the reaction zone.

The process of the present invention is carried out in two reaction zones. These may be located in separate reaction vessels or may be separate zones within a single reaction vessel. Where the reaction zones are located within the same vessel, the second reaction zone will generally be located directly below the first reaction zone such that the step of withdrawing a second stream comprising unreacted alcohol and water and having a water content of from about 3 to about 6 moie¾ from the second reaction zone and supplying said stream to the first reaction zone in step (b) is achieved simply by allowing the unreacted alcohol to flow upwardly into the first reaction zone. The majority of the esterification wiH generaHy take place in the first reaction zone. In one arrangement, around 95 to 99% conversion of the carboxylic acid will be performed in the first reaction zone with the remainder being carried out in the second reaction zone.

Whether the second reaction zone is a separate reactor or a separate zone within the same vessel, an intermediate product stream is recovered from the first reaction zone and passed to the second reaction zone. The carboxylic acid stream recovered from the first reaction zone may be fed to the second reaction zone at any suitable position. However, as it is to flow downwardly against an upfSowing alcohoi vapour, it will be understood that it is desirable that it is suppiied as high as possible within the zone to maximise the contact with the upffowing alcohol.

There is a requirement for a normal make-up feed of alcohoi to be supplied to the second reaction zone to cover that which has been consumed in the esterification reaction in the first zone and that lost in by-product make, such as the production of di-ethers. The alcohol suppiied to the second reaction zone will be as dry as is practicable within economic constraints. The drier the alcohol the more the equilibrium will favour the formation of further ester in the second reaction zone. However, since the bulk of the esterification is carried out in the first reaction zone the amount of dry alcohol required will generally be substantially less than the amount of wet alcohol stream required for the first reaction zone, in one arrangement, the dry alcohol fed to the second reaction zone may be equimolar to the carboxylic acid feed. The excess wet alcohol can operate in the range of 0,1 to 20, or of 0,5 to 5, moles when compared to the carboxylic acid feed. In one arrangement, the reaction in the first and second reaction zones will be carried out in the presence of a catalyst. The catalysts used in the first and second reaction zones may be the same or different. Any suitable catalyst may be used. Suitable catalysts include acidic ion exchange resins containing -SQ₃H and/or -COOH groups. MacroreticuSar resins of this type may be useful, Examples of suitable resins are those sold under the trade marks "Amberlyst", "Dowex", "Dow" and "Puro!ite" such as Amberlyst 13, Amberlyst 66, Dow C351 and Puroiite C150,

In one arrangement of the present invention, the reaction in the first reaction zone may be carried out in the absence of a separate cataiyst. Thai is to say it is autocatalysed. in a further arrangement at least a portion of the first reaction zone will be free of catalyst. This catalyst free area in the first reaction zone will generally be located towards the top of the reaction zone.

The stream received from the first reaction zone may require cooling before it is fed to the second reaction zone. The requirement for cooling will generally be dictated by the temperature limitation of the resin, In one arrangement, the stream ma be cooled from a temperature of about 15CTC to about 2Q0°C to a temperature of from about 100°C to about 130°C

Any suitable arrangement for the first and second reaction zones may be used, in one arrangement, the reaction zones wi!i comprise a plurality of esterification trays, Although two or three trays may suffice in some cases, it will typically be necessary to provide at least about 5 up to about 20 or more esterification trays in each reaction zone. Typically each esterification tray is designed to provide a residence time for liquid on each tray of from about 1 minute up to about 120 minutes, preferably from about 5 minutes to about 60 minutes. In one arrangement, the first zone will have more trays than the second zone, in one arrangement, the first zone may have about 10 trays and the second zone may have about 5 trays.

Where a catalyst is used in a reaction zone, different catalysis may be used on different trays. Moreover different concentrations of catalyst can be used on different trays.

The charge of catalyst on each tray, where present, is typically sufficient to provide a catalyst:iiquid ratio on that tray corresponding to a resin concentration of at least 0.2% w/v, for example a resin concentration in the range of from about 2% w/v to about 20% w/v, preferably 5% w/v to 10% w/v, calculated as dry resin. Sufficient catalyst should be used to enable equilibrium or near equilibrium conditions to be established on the tray within the selected residence time at the relevant operating conditions. On the other hand not so much catalyst shou!d be used on each tray that it becomes difficult to maintain the catalyst in suspension in the liquid on the tray by the agitation produced by the upflowing vapour entering the tray from below,

The particle size of the catalyst should be large enough to facilitate retention of the catalyst on each tray by means of a screen or similar device, However, as the iarger the catalyst particle size is the more difficult it is to maintain in suspension and the lower the geometrical surface area per gram, it is expedient to use not too large a catalyst particle size. A suitable catalyst particle size is in the range of from about 0,1 mm to about 5 mm.

In one arrangement, a treatment bed may be located above any reaction zone which includes the catalyst to remove potential resin poisons such as metais which may be present in the acid stream. Such poisons may be present where natural acids or acids from, for example, cooking oils are used.

One or more wash trays may be provided above the esterification trays in order to prevent loss of product, soivent and/or reagents from reactor zones.

Water of esterification is removed from the top of the first reaction zone together with the water supplied with the wet alcohol, excess alcohol and the by-product dialkyl ether, where formed , As the carboxylic acid stream flows down through the first reactor it encounters progressively drier alcohol and the esterification equilibrium is driven further towards a low acid containing ester product. This appiies even in the first reactor where the wet alcohol is used.

The second reaction zone is operated to complete the reaction preferably by taking the ester product to a stream containing less than about 0.5mgKOH/g acids. The water of esterification from the second reaction zone is removed from the second reaction zone in the vapour stream together with any excess alcohol and any by-product of dialkyl ether. This stream may be fed to alcohol refining or in one embodiment may be condensed and used as part of the wet alcohol feed to the first reaction zone.

The reaction conditions required will depend on the carboxyiic acid feed and the alcohol selected for the reaction, in one arrangement, particularly where the alcohol is ethanoi, the esterification conditions will normally require a temperature in the range of from about 80X to about 160 or MOT, or from about 100°C to about 125°C, Where the reaction in the first reaction zone is an autocatalytic reaction, the temperature required may be of the order of about 150X to about 2Q0°C. The precise temperature will be determined by factors such as the thermal stability of the esterification catalyst, the kinetics of the esterification reaction and the vapour temperature of the vaporous component fed to the lower section of the reaction zone at the relevant inlet pressure.

Typical operating pressures at the vapour inlet of the reaction zones will generally range from about 1.0 bara to about 5 bara.

A liquid hourly space velocity through the reaction zones will generally be in the range of from about 0.1 hr^"1 to about 10 hr^"1, typically from about 0.2 hr^"1 to about 2 hr^'1.

The treatment of the vapour stream removed from the first reaction zone may be treated in a refining zone which is operated to recover azeotropic alcohol which can be supplied to the first reaction zone in step (b). Ethanoi boils at 78.4 , water boils at 100°C but the azeotrope boils at 78.2°C. Thus it is relatively easy to recover close to azeotropic ethanoi and thus the wet alcohol stream required for the first reaction zone can be readily obtained and there is no requirement for expensive further water removal.

The present invention wiil now be described by way of example with reference to the accompanying figures in which:

Figure 1 is a schematic representation of a process according to a first aspect of the present invention;

Figure 2 is a schematic representation of the process of the first invention;

Figure 3 is a schematic representation of the process according to a second aspect of the present invention; and

Figure 4 is a schematic representation of the process of the second aspect. 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. One arra ngement of the present invention is illustrated schematically in Figure 1. A stream comprising carboxyiic acid is added in line 1 to the first reaction zone 2. This fiows downwardly through the trays, not shown, of the first reaction zone, 2, As it flows it contacts increasingly dry alcohol stream which has been added into the reactor in Sine 3. The stream added in line 3 will be a wet alcohol stream in that it will have a significant water content, As it travels upwardly and reacts with the downfiowing carboxyiic acid to form the ester, water is produced and so the vapour travelling upwardly becomes wetter as it moves upwardly.

The vapour containing the water of esterification, the water in the initial alcohol stream, unreacted alcohol and any ether by-product is removed in line 8. This stream is fed to an alcohol refiner 9 where excess water is separated from the alcohol and removed in line 11. The remaining stream which will be a wet ethanol stream having an azeotropic or higher water content will be returned in line 10 to be fed as part of the alcohol stream feed in line 3 to reactor 2.

A liquid stream comprising the ester product and unreacted carboxyiic acid will be removed from the base of reactor 2 and fed to the second reaction zone 5. The stream fiows downwardly through the reaction zone encountering progressively dryer vaporous aicohoL The alcohol is added as a dry alcohol stream in line 6. Product ester is removed from the reactor in line 7.

The unreacted alcohol together with the water of esterification is removed from the top of the second reaction zone in line 12. This stream will have a water content which will enable it to be used as part of the wet alcohol stream to the first reaction zone. It will generally be combined with the stream 10 from the alcohol refiner 9 before being added to the first reaction zone.

A modification of the process illustrated in Figure 1 is set out in Figure 2. Components of the system which are the same as in Figure 1 have the same reference numerals and their function is as described in connection with Figure 1. The modification illustrated in Figure 2 will generally be used when the reaction carried out in the first reaction zone 2 is autocatalysed and will therefore generally be operated at a higher temperature than that used when a catalyst is present in the zone, in addition, the autocatalysed reaction is likely to be at higher pressures due to the higher temperatures. The stream removed from the first reaction zone 2 in line 4 is passed to a cooler 13 before either being passed directly in line 15 to the second reaction zone 5 or through an optional treatment bed 14 where impurities that may poison the catalyst in the second reaction zone 5 are removed. In one arrangement, it may be necessary to condense the stream removed from the first reaction zone and feed it as a liquid to the reboiler.

An alternative arrangement is illustrated in Figure 3. This is similar to the arrangement of Figure 1 except that the first 22 and second 25 reaction zones are combined in a single vessel. The wet alcohol stream is added to the first reaction zone in line 33 where it flows upwardly through the first reaction zone 22 where it contacts the carboxyiic acid stream which is added in line 21.

The partly reacted carboxyiic acid stream continues downwardly into the second reaction zone 25 where it contacts dry alcohol added in line 26. The final product is removed in line 27.

As the dry alcohol flows upwardly through the second reaction zone it gathers the water of esterification and so by the time it reaches the first reaction zone it becomes a wet alcohol stream.

The wet stream travels up through the first reaction zone 22 where it picks up additional water of esterification. This stream is taken off in line 28 and fed to an alcohol refiner 29 where excess water is separated from the alcohol and removed in line 31, The remaining stream which will be a wet stream having a relatively high water content wiil be returned in line 30 to be fed to the first reaction zone 22.

A modification of the process illustrated in Figure 3 is set out in Figure 4. Components of the system which are the same as in Figure 3 have the same reference numerals and their function is as described in connection with Figure 3. The modification illustrated in Figure 4 will generally be used when the reaction carried out in the first reaction zone 22 is autocataiysed and will therefore genera!iy be operated at a higher temperature than thai used when a catalyst is present in the zone. A higher pressure will generally also be noted. The stream removed from the first reaction zone 22 is passed in line 34 to a cooler 35 before either being passed directly in line 37 to the second reaction zone 25 or through an optional treatment bed 36 where impurities that may poison the cata!yst in the second reaction zone 25.

Since the wet stream added in lines 3 in the arrangement of Figures 1 and 2 and 33 in the arrangement of Figures 3 and 4 may contain an azeotropic or above amount of water, there is no requirement for the alcohol refiner 3, 29 to break the azeotrope.

The present invention will now be described with reference to the following examples, Whilst these are specifically directed to the esterification of lauric acid with ethane!, it will be understood that the methods described are equally applicable to other acids and alcohols.

Comparative Example I

200 g (1 mol) of lauric acid was charged to a 500 mL round-bottomed flask, fitted with a still- head and condenser for overheads take off, and heated this up to 115" C, when the acid was molten the stirrer was started at 200 rpm; a heating tape around the top part of the flask was heated to 125°C to prevent internal reflux. Once at temperature, 20g of resin was charged (equivalent to 10 wt%) and then dry ethanol feed commenced (T = 0 minutes) at 2 rnois per hour. Samples were taken from the flask to monitor acidity and overheads were analysed for water and diethyl ether (DEE). The test was stopped when acidity was <0,5 rngKOH/g (<0.19 wt ).

Example I

The test was repeated but wet (4 wt water) ethanol was used and the test was stopped once equilibrium was reached, It is assumed at this stage that wet ethanol will be used for the initial reaction in the first reaction zone before switching to dry ethanol to complete the reaction in the second reaction zone. Assuming the use of 3:1 aicoholracld molar ratio used currently for fatty acid esterification and two thirds of the reaction is completed with wet ethanol then the final esterification will be 1:1 aicohokadd molar using dry ethanol.

Examples 2 and 3

Two further tests were performed using 1:1 ethano acid with 10 wt% resin (Example 3) and 5 wt% (Exam le 4).

Details of the examples and the results are set out in the following tables.

 Table: 2

Table 4

The rates of esterification using dry and wet ethano! as we!i as the corresponding diethylester production for Comparative Example 1 and Example 2 are very similar (wet is slightly slower, 101 minutes to reach 5 wt acid, dry 98 minutes to reach 5 wt% acid ) up to 95 wt% conversion when the wet ethanoi slows the reaction and reaches an equilibrium acidity of 0.7 wt%. The level of diethylether produced by both reactions is also similar at up to 95 wt% conversion.

Thus these experiments support the principle of the present invention and that the process will allow wet alcohol to be used in the first part of the esterification and gives similar kinetics to a process carried out with dry ethanoi.

Claims

1. A process for the production of carboxyiic acid esters by reaction of a carboxyiic acid component and an alcohoi component, said process comprising:

(a) feeding a iiquid carboxyiic acid stream to an upper section of a first reaction zone maintained under esterification conditions;

(b) feeding a wet alcohoi vapour stream comprising from about 4.4 to about 8 weight % water to a iower section of the first reaction zone;

{c) allowing the carboxyiic acid stream to pass in countercurrent to the wet alcohoi stream to form an intermediate iiquid product stream comprising product ester and unreacted carboxyiic acid;

(e) feeding a dry alcohol stream comprising less than about 1 weight % water to a iower section of the second reaction zone;

(f) allowing the Intermediate product stream to pass in countercurrent to the dry alcohoi stream such that further carboxyiic acid is reacted to product ester;

(g) recovering the product ester stream;

(h) withdrawing a first stream comprising unreacted alcohol and water from the first reaction zone, treating the stream to reduce the water content to form a wet alcohol stream and feeding said stream to the first reaction zone In step (b); and

(i) withdrawing a second stream comprising unreacted alcohol and water and having a water content of from about 2 to about 6 mo!e% from the second reaction zone and supplying said stream to the first reaction zone in step b .

2. A process according to Claim 1 wherein the carboxyiic acid stream is selected from one or more of: monocarboxylic acids having from about 6 to about 26 carbon atoms or mixtures of two or more thereof; aliphatic and cycioaiiphatic C to C_1S saturated and unsaturated dicarboxyiic acids; aromatic C₇ to C_2Q monocarboxylic acids and mixtures thereof; aromatic C_g to C₂₀ dicarboxylic acids; poiycarboxyllc acids; and anhydrides of the foregoing.

3. A process according ΐο Ciaim 1 or 2 wherein the alcohoi stream comprises aicohol selected from those having 1 to 10 carbon atoms,

4. A process according to Claim 3 wherein the alcohoi stream comprises ethane?!.

5. A process according to Claim 4 wherein the wet alcohol stream added to the first reaction zone comprises water in an amount that is equal to or more than the azeotropic water content.

5, A process according to any one of Claims 1 to 5 wherein the first and second reaction zones are iocated in separate reaction vesseis.

7. A process according to any one of Claims 1 to 5 wherein the first and second reaction zones are iocated in a singie reaction vessel.

8. A process according to any one of Ciaims 1 to 6 wherein around 90% to about 99% conversion of the carboxyiic acid will be performed in the first reaction zone with the remainder being carried out in the second reaction zone.

9. A process according to any one of Claims 1 to 8 wherein the reaction in the first and second reaction zones is carried out in the presence of a catalyst.

10. A process according to any one of Claims 1 to 8 wherein the reaction in the first reaction zone is autocataiysed and the reaction in the second reaction zone is carried out in the presence of a catalyst.

11. A process according to Ciaim 10 wherein the intermediate product stream is cooled before being passed to the second reaction zone.

12. A process according to any one of Claims 9 to 11 wherein the catalyst is an acidic son exchange resin containing ~S0₃H and/o -CO OH groups.

13. A process according to any one of Claims 9 to 12 wherein a treatment bed is Iocated before a reaction zone including catalyst.