Field of the invention

The present invention relates to processes for the production of lactic acid and Ci-6 alkyl lactate from saccharide. The invention also relates to processes for producing related products such as oligomeric lactic acid, lactide, alkyl lactyllactate and/or poly-lactic acid.

Background of the invention

Lactic acid is an important industrial chemical, which finds use as a feedstock in the biopolymer industry. Today, virtually all large scale production of the lactic acid available commercially is manufactured by fermentation processes, see for example Strategic Analysis of the Worldwide Market for Biorenewable Chemicals M2F2-39, Frost and Sullivan, 2009. In a typical fermentation process, biomass is fermented with microorganisms to produce either D- or L-lactic acid, most commonly L-lactic acid. Companies such as Cargill and Corbion (formerly Purac) operate large-scale fermentation processes for the production of optically active lactic acid. Many patent publications relate to recovery of lactic acid from fermentation mixtures, which can be challenging, and a number of patent documents rely on the preparation of a complex between lactic acid and an amine for the recovery (see, for example, US 4,444,881, US 5,510,526).

Chemical processes for preparing lactic acid from carbohydrates are known. For example, GB 400,413, dating from 1933, describes an improved process for preparing lactic acid or lactates comprising reacting a carbohydrate-containing material with a strong alkali at a temperature of at least 200°C, preferably at a pressure of at least 20 atmospheres, and recovering the lactic acid so produced by adding sulfuric acid or zinc sulfate to the reaction mixture. Hydrocyanation of acetaldehyde has also been used as a synthetic route for accessing lactic acid.

WO 2012/052703 describes an improved process for the production of a complex of lactic acid and either ammonia or an amine, which does not involve production of lactic acid by fermentation. The process comprises reacting one or more saccharides with barium hydroxide to produce a first reaction mixture comprising barium lactate, and contacting at least part of the first reaction mixture with ammonia or an amine and with carbon dioxide, or with the carbonate and/or bicarbonate salt of ammonia or an amine, to produce a second reaction mixture comprising the complex and barium carbonate. This process, which involves preparation of barium salts, has significant advantages over prior art processes. It does, however, have some disadvantages: specifically, if it is required to recycle the barium, a barium carbonate calcination step is required. Calcination (also referred to as calcining) is a thermal treatment process in absence of air applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The process of calcination derives its name from its most common application, the decomposition of calcium carbonate (limestone) to calcium oxide (lime) and carbon dioxide. The terms calcination, and calcine (the product of calcination), are typically used regardless of the actual minerals undergoing thermal treatment. Whilst barium carbonate calcination is feasible, the technology is not currently widely operated at industrial scale.

In addition, carrying out the process described in WO2012/052703 on an industrial scale requires facilities adapted to handle and transport large quantities of barium salts.

Processing solutions for large scale use of barium salts exist. However, there remains a need for improved processes for generating lactic acid and related materials, which still provide acceptable yields but which avoid the disadvantages associated with use of barium salts.

Electrodialysis is a known technique for carrying out ion exchange. In an

electrodialysis cell, a feed stream including an ionic species flows over an ion exchange membrane and, under the influence of an electric field, either the anion or the cation passes through the membrane. Saxena et al, Ind. Eng. Chem. Res. 2007, 46, 1270-1276 describes the use of electrodialysis to convert lactate salts to lactic acid, using an anion-selective membrane. Experiments were carried out using synthetic solutions of sodium and ammonium lactates, and using fermentation broth. It was observed that ammonium lactate can be more efficiently and more rapidly converted than sodium lactate, and that the process provides an efficient and simple way of recovering lactates from uncharged molecules such as polysaccharides and carbohydrates. US 2013/0225787 discloses a multi-step process for preparing lactic acid by fermentation, involving a salt swap from magnesium to sodium. Electrodialysis is used to treat the sodium lactate. Other proposals to use electrodialysis for the treatment of fermentation broths containing lactic acid include Czech J. Food Sci. 19(2), 73-80, US 4,885,247, US 5,681,728 and EP 393,818, all of which describe carrying out two separate electrodialysis steps, the first being to remove water from the fermentation broth.

Lactic acid streams obtained by fermentation differ fundamentally from those obtained by chemical means. As fermentation bacteria are typically pH sensitive, the lactic acid is usually converted into a metal lactate inside the fermentation reactor. For example, calcium carbonate or calcium hydroxide may be added to form calcium lactate. The fermentation product streams are therefore rich in metal lactates, carbohydrates, proteins and cell mass. Depending upon the feedstock used, some polysaccharide or oligosaccharide species may also be present: for example, if the feedstock is an incompletely hydrolysed starch. The concentration of these species is such that it is necessary to carry out a pre- treatment step to concentrate the stream prior to treating with electrodialysis. As stated above, concentration may be achieved in a first electrodialysis cell, before salt-splitting in a second electrodialysis operation. In addition, it is necessary to carry out a clarification or filtration step, and when the metal lactate comprises a dication such as Ca ²⁺ or Mg ²⁺ , an ion exchange process will be required to form the lactate salt of a monocation such as Na ⁺ . In contrast, streams obtained by treating saccharides chemically with hydroxides contain a completely different mixture of organic compounds. Specifically, they contain significant quantities of Ci-6 organic carboxylates, of which lactate is one. For example, when saccharides are treated with sodium hydroxide, the streams obtained contain significant quantities of the sodium salts of the Ci-6 organic acids, of which sodium lactate is one. These streams are difficult to handle, because of the large number of similar charged organic species which are present. The presence of multiple charged species, particularly the anions of multiple organic acids in addition to lactic acid, would suggest that electrodialysis would not be a promising technique for handling these streams. We have now found an effective way of treating such streams using a specific electrodialysis process which recovers sodium from the system. Most surprisingly, only a single electrodialysis step is necessary, unlike the two steps which are necessary when treating fermentation broths. In addition, it is possible to carry out the process without the necessity of carrying out a clarification or filtration step before the electrodialysis step.

Summary of the invention

The present invention provides a process for the production of lactic acid comprising:

(a) reacting a stream rich in saccharide with sodium hydroxide in the presence of water to produce a reaction mixture comprising sodium lactate; and

(b) subjecting at least part of said reaction mixture to electrodialysis using an electrodialysis cell containing a cation-selective membrane to produce a first product stream comprising lactic acid and a second product stream comprising sodium hydroxide.

Preferably only a single electrodialysis step is carried out as part of the process of the present invention. Preferably step (b) is carried out after step (a) without carrying out any intervening step to remove water. Preferably step (b) is carried out after step (a) without carrying out any intervening clarification or filtration step.

Brief description of the drawings

Figure 1 shows a schematic view of an electrodialysis cell suitable for use in step (b) of the process of the invention.

Figures 2, 3 and 4 show the results of Examples 1, 2 and 3 respectively.

Figures 5 and 6 show the results of Example 4.

Detailed description of the invention

In step (a) of the process of the invention, a stream rich in saccharide is reacted with sodium hydroxide. The saccharide present in said stream may be a mono-, di-, tri-, oligo- or poly-saccharide, with disaccharides and, especially, monosaccharides, being preferred.

Preferably, the stream rich in saccharide is a stream rich in monosaccharide. In some preferred embodiments, at least 50 wt % of the saccharides, at least 60 wt% of the saccharides, at least 70 wt % of the saccharides, at least 80 wt % of the saccharides, at least 90 wt % of the saccharides, at least 95 wt % of the saccharides present in said stream rich in saccharides are monosaccharides. Suitable monosaccharides include for example hexose monosaccharides, for example glucose, fructose, psicose, galactose and mannose, and pentose monosaccharides, for example arabinose, xylose, ribose, xylulose and ribulose. In one embodiment, the stream rich in saccharide comprises glucose. In another embodiment, the stream rich in saccharide comprises fructose. In another embodiment, the stream rich in saccharide comprises mannose. In another embodiment, the stream rich in saccharide comprises xylose. Mixtures of saccharides may be present in the stream rich in saccharides. For example, a mixture of two or more monosaccharides, for example a mixture of glucose and fructose, may be present. For example, a mixture of glucose and xylose may be present. Monosaccharides may be obtained from any known monosaccharide source, for example a higher saccharide such as sucrose, starch or cellulose. In some embodiments, the stream rich in saccharide may contain a mixture of glucose and fructose (known as invert sugar) obtained from sucrose, for example by enzymatic hydrolysis using a sucrase or invertase, or by heating the disaccharide in the presence of an acidic catalyst such as sulphuric acid, citric acid or ascorbic acid. In some embodiments, the stream rich in saccharide may contain glucose obtained by enzymatic hydrolysis (e.g. using an amylase) of starch contained in biomass feedstocks, for example maize, rice or potatoes. In some embodiments, the stream rich in saccharide may contain glucose obtained by hydrolysis of cellulose (e.g. enzymatic hydrolysis using one or more cellulases) contained in biomass feedstocks. The stream rich in saccharides may contain components other than saccharides, for example it may include other components of biomass such as lignin or lignin-derived products. Spent chemicals from processing of biomass may, for example, also be present. Water will typically be present. In some embodiments, at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt% of the material other than water present in the stream rich in saccharide is saccharide (e.g. monosaccharide).

The reaction between saccharide and sodium hydroxide is carried out in the presence of water. As discussed above, some sources of saccharide contain water, and such feedstocks may readily be used in the process of the invention. In certain embodiments, the reaction between the saccharide and sodium hydroxide may take place in the presence of additional water (i.e. additional to that present in the starting materials). The reaction between the saccharide and sodium hydroxide may also, if desired, take place in the presence of one or more organic solvents, for example an oxygenate such as an alcohol, ester, ether, or ketone; and/or in the presence of one or more reactive extractants such as an amine. However, in a preferred embodiment, the reaction between the saccharide and sodium hydroxide does not take place in the presence of an organic solvent, i.e. water is the only solvent. In some embodiments the weight ratio of the total amount of water used in step (a) to saccharide is in the range of from 0.5: 1 to 9: 1, more preferably in the range of from 1.25 to 4: 1, still more preferably in the range of from 1.5: 1 to 4 : 1.

Sodium hydroxide reacts with saccharide to produce sodium lactate. Any form of sodium hydroxide may be used in the process of the invention. Sources of sodium hydroxide such as sodium oxide may be used in the process of the invention, sodium oxide being converted into sodium hydroxide in the presence of water. The sodium hydroxide generated in situ reacts with saccharide to produce sodium lactate. The ratio of sodium hydroxide to saccharide should be sufficient to effect good conversion of saccharide to sodium lactate. Preferably, the molar ratio of sodium hydroxide to saccharide (calculated as monosaccharide) is at least 2: 1. Excess quantities of sodium hydroxide may be used, for example the molar ratio of sodium hydroxide to saccharide (calculated as monosaccharide) may be up to 10: 1. In some preferred embodiments, the molar ratio of sodium hydroxide to saccharide

(calculated as monosaccharide) is in the range of from 2: 1 to 6: 1, more preferably 2: 1 to 4: 1, still more preferably 2: 1 to 3 : 1. The present invention also encompasses molar ratios of sodium hydroxide to saccharide (calculated as monosaccharide) that are lower than 2: 1 ; however use of sub-stoichiometric quantities of sodium hydroxide will generally lead to lower conversion of saccharide to sodium lactate. Thus in some embodiments, the molar ratio of sodium hydroxide to saccharide (calculated as monosaccharide) is at least 1.5: 1, for example in the range of from 1.5: 1 to 6: 1, more preferably in the range of from 1.6: 1 to 4: 1; still more preferably in the range of from 1.8: 1 to 2.5: 1, yet more preferably in the range of from 1.9: 1 to 2.1 : 1.

Conversion of saccharide to sodium lactate may be carried out at ambient temperature, although the reaction is preferably carried out at elevated temperature, for example at a temperature of up to 160°C, preferably up to 1 10°C. Preferably, saccharide is reacted with sodium hydroxide at a temperature in the range of from 10 to 160°C, for example from 60 to 130°C, especially from 100 to 130°C. In some embodiments, saccharide is reacted with sodium hydroxide in water at reflux.

In a preferred embodiment, saccharide (e.g. monosaccharide) in water and sodium hydroxide in water are admixed over a period of time at elevated temperature. For example, a mixture of saccharide (e.g. monosaccharide) and water may be added over a period of time to a mixture of sodium hydroxide and water that is at elevated temperature, for example at a temperature of from 60 to 110°C. Slow addition of saccharide (e.g. monosaccharide) generally leads to a reduction in the formation of side products during the process of the invention, and leads to an improved conversion of saccharide (e.g. monosaccharide) into sodium lactate. Preferably the saccharide (e.g. monosaccharide) in water is added over a period of at least 10 minutes, more preferably at least 30 minutes, still more preferably over at least 1 hour. Preferably the concentration of saccharide in water is at least 0.2 M. In some preferred embodiments the concentration of saccharide in water is in the range of from 0.2 to 4.0 M, more preferably in the range of from 0.5 to 4.0 M. The reaction of saccharide with sodium hydroxide produces a reaction mixture comprising sodium lactate. The process typically leads to the production of racemic sodium lactate. Typically sodium lactate is the major product, but sodium salts of other organic acids are formed as by products, including sodium gluco/xylometasaccharinate, sodium 2,4-dihydroxy butanoate, sodium 2- hydroxyethanoate, sodium 2-hydroxy butanoate, sodium acetate, sodium formate, sodium 2,3-dihydroxy-2-methyl propionate, and sodium 2-hydroxy-2-methyl propionate. Small amounts of other organic compounds including aromatic compounds may also be produced. If desired, the reaction mixture from step (a) may be pre-treated prior to carrying out step (b). For example, it may be desirable to treat the reaction mixture to remove any multivalent cations. It may also be desirable to treat the reaction mixture using a clarification or filtration step, for example to remove any lignin and/or lignin derivatives which may be present. However, in a preferred embodiment of the process, step (a) is carried out in such a way as to minimise the chances of such materials being present after step (b). For example, lignin and/or lignin derivatives may be removed from the feedstock for step (a) prior to carrying out step (a). Thus in a preferred embodiment of the invention, no clarification or filtration step is carried out between steps (a) and (b). In this embodiment of the invention, it is surprising that the membrane(s) used in step (b) does not become fouled, as would always occur when using a fermentation broth as feedstock without applying a clarification or filtration step prior to electrodialysis.

It is also possible to carry out a step between steps (a) and (b) to remove water. This step may itself be an electrodialysis step, in which case two (or more) electrodialysis steps are carried out successively as part of the process of the invention. Alternatively, it may be any other conventional dewatering method, for example evaporation. In general, known processes in which reaction streams resulting from fermentation methods are treated using electrodialysis require the fermentation broth to be concentrated before treatment by electrodialysis, typically by carrying out two separate electrodialysis steps, the first of which is designed to remove water. However, it has surprisingly been found that it is not necessary to concentrate the reaction mixture of step (a) prior to carrying out electrodialysis, even when the quantity of lactic acid salt present in the reaction mixture from step (a) of the present invention is similar to that found in a typical fermentation broth. Therefore, preferably only a single electrodialysis step is carried out as part of the process of the present invention. In a preferred embodiment of the invention, step (b) is carried out after step (a) without carrying out any intervening step to remove water.

In an especially preferred embodiment of the invention, step (b) is carried out after step (a) without carrying out any intervening step to remove water, and without carrying out any intervening clarification or filtration step.

The electrodialysis cell used in step (b) of the process of the invention contains a cation-selective membrane (i.e. a membrane which has specific functional groups on its surface which will only allow the passage of cations), in contrast to a number of prior art processes in which electrodialysis of lactate salts is carried out using an anion-selective membrane (i.e. a membrane which has specific functional groups on its surface which will only allow the passage of anions). The present inventors have found that if an anion-selective membrane is used instead of a cation- selective membrane, fouling of the membrane occurs, necessitating the implementation of one or more pre-treatment steps between steps (a) and (b).

During step (b), sodium ions migrate across the cation-selective membrane, forming a sodium hydroxide solution. In one preferred embodiment of the process of the invention, the cell also contains a pair of bipolar membranes, one on either side of the cation- selective membrane. A bipolar membrane has one anion-selective layer and one cation- selective layer, and water is split into hydrogen ions and hydroxyl ions in the interlayer. When using a pair of bipolar membranes, one on either side of a cation-selective membrane, the electrodialysis becomes a water- splitting electrodialysis, i.e. the process splits water into hydrogen ions and hydroxyl ions at the bipolar membranes. In this embodiment, the feed stream from step (a) is fed into the electrodialysis cell which contains both a cation- selective membrane and bipolar membranes. Under the influence of the electric field applied during the electrodialysis, sodium ions migrate across the cation- selective membrane, and meet the hydroxyl ions generated at one bipolar membrane to form sodium hydroxide, while the lactate and other organic acid anions bind hydrogen ions generated at the other bipolar membrane to form acids. This process is illustrated in Fig. 1, which shows a two-compartment electrodialysis cell containing a cation-selective membrane (1) and two bipolar membranes (2) and (3) which together form two compartments (4) and (5). In use, reaction mixture from step (a) of the process of the invention, containing a mix of sodium salts of organic acids including sodium lactate, is circulated via line (6) into compartment (4), while aqueous sodium hydroxide solution is circulated via line (7) into compartment (5). An electric field is applied via cathode (8) and anode (9). Under the action of the electric field, sodium ions migrate across cation- selective membrane (1) from compartment (4) to compartment (5) as shown, while hydrogen ions and hydroxyl ions are generated at bipolar membranes (2 and 3) as shown. A first product stream (10) comprising a mix of organic acids is taken off from compartment (4), while a second product stream (11) comprising aqueous sodium hydroxide of higher concentration than the original feed of aqueous sodium hydroxide introduced via line (7), is taken off from compartment (5). Optionally, product stream (11) is recycled back to step (a) of the process of the invention.

The electrodialysis cell may contain two or more cation-selective membranes, and three or more bipolar membranes in alternating fashion. Thus, in a preferred embodiment of the invention, an arrangement in which cation- selective membranes alternate with bipolar membranes is used. A stack of multiple electrodialysis cells may be used to increase throughput. The cell will also contain electrodes to supply the electric current required to drive the process.

The detailed conditions under which step (b) is carried out will depend on the exact nature of the reaction mixture produced in step (a). Provision will need to be made to control such parameters as applied voltage, temperature and flow rates; such parameters are within the normal knowledge of the skilled person. In general the duration of step (b) will be such as to optimise the quantity of sodium hydroxide obtained while minimising energy costs. For this reason, it may be the case that the duration of step (b) is chosen so that the conversion of sodium lactate into sodium hydroxide may not be complete. Step (b) may be carried out as a batch, semi-continuous or continuous process, with appropriate use of recycle streams.

The output from step (b) of the process of the present invention is two product streams: a first product stream including lactic acid, and a second product stream including sodium hydroxide. Suitably some or all of the sodium hydroxide present in the second product stream is recycled back to step (a) of the process where it is used to produce further sodium lactate from saccharide. Accordingly in a preferred embodiment of the invention, there is provided a process for the production of lactic acid comprising:

(a) reacting a stream rich in saccharide with sodium hydroxide in the presence of water to produce a reaction mixture comprising sodium lactate;

(b) subjecting at least part of said reaction mixture to electrodialysis using an electrodialysis cell containing a cation-selective membrane to produce a first product stream comprising lactic acid and a second product stream comprising sodium hydroxide; and

(c) recycling at least part of said second product stream back to step (a).

Preferably only a single electrodialysis step is carried out as part of this process; and/or step (b) is carried out after step (a) without carrying out any intervening step to remove water; and/or step (b) is carried out after step (a) without carrying out any intervening clarification or filtration step.

Once the first product stream from step (b), which will generally contain a mix of organic acids including lactic acid, has been taken from the electrodialysis cell, it may be further processed in any desired way. If any residual sodium remains in the stream, this may be removed for example by acidification, for example using sulfuric acid.

In a preferred embodiment, the lactic acid in the first product stream is converted into an ester of lactic acid by reaction with a Ci-6 alkyl alcohol, for example methanol, ethanol, propanol, i-propanol, or a butanol such as n-butanol. Conversion into a Ci-6 alkyl lactate may offer advantages in respect of recovery and/or purification of lactate/1 actic acid-containing species, alkyl lactates being less corrosive and less susceptible to oligomerisation than lactic acid, and so easier to handle and manipulate. The use of a Ci-6 alkyl alcohol results in an alkyl lactate having a boiling point which facilitates separation from other components present in the reaction mixture by distillation but which does not require excessive energy input for its recovery. Accordingly, in a preferred embodiment, the invention provides a process for the preparation of a Ci-6 alkyl lactate which comprises: (a) reacting a stream rich in saccharide with sodium hydroxide in the presence of water to produce a reaction mixture comprising sodium lactate; (b) subjecting at least part of said reaction mixture to

electrodialysis using an electrodialysis cell containing a cation- selective membrane to produce a first product stream comprising lactic acid and a second product stream comprising sodium hydroxide; (c) optionally recycling at least a portion of the sodium hydroxide in said second product stream to step (a); and (d) reacting at least a portion of the lactic acid in said first product stream with Ci-6 alkyl alcohol to produce the corresponding alkyl lactate.

Preferably only a single electrodialysis step is carried out as part of this process; and/or step (b) is carried out after step (a) without carrying out any intervening step to remove water; and/or step (b) is carried out after step (a) without carrying out any intervening clarification or filtration step.

The reaction between lactic acid and Ci-6 alkyl alcohol is typically catalysed by the presence of acid, for example HC1. The reaction is suitably carried out at a temperature in the range of from 50 to 150°C, preferably 50 to 125°C, for example 65 to 120°C, especially 95 to 115°C. In order for good yields of Ci-6 alkyl lactate to be obtained, it is necessary to remove water from the reaction mixture, for example by evaporation or distillation, e.g. distillation under reduced pressure. For example, the reaction may be carried under reflux with removal of water. If present any additional water remaining from an earlier step in the process will also typically be removed during the esterification process. The Ci-6 alkyl alcohol may be used as solvent as well as reactant. An additional organic solvent may be present if desired.

The volume of the first product stream obtained following step (b) may be reduced by removal of water (e.g. by distillation, evaporation or membrane separation), prior to reacting the lactic acid with a Ci-6 alkyl alcohol. For example, the volume of the first product stream may be reduced by at least 10%, at least 20%>, at least 30%>, at least 40%> or at least 50%>, prior to admixing with Ci-6 alkyl alcohol.

The processes of the invention may be carried out under ambient or inert atmospheres. For example, the process may be carried out using equipment that is open to the air, or may be carried out under a nitrogen or argon atmosphere. It may be carried out in a batch, fed- batch, semi-continuous or continuous process, and the various products of the process of the invention may be subject to any desired purification and/or additional processing steps.

As mentioned above, the first product stream obtained from step (b) will generally contain a mix of organic acids including lactic acid. A number of the organic acids will contain 4 or more carbons, and in cases where the carbons distant from the carboxylic acid group bear hydroxyl groups, then the organic acid may be present as a cyclic lactone form, or as an equilibrium mixture between the linear organic hydroxyacids and the lactone. In some cases, the presence of multiple hydroxy groups distant from the carboxylic acid group may result in more than one cyclic lactone being formed. Where the process of the invention includes an esterification step carried out on the first reaction mixture, Ci-6 alkyl esters or lactones of those additional organic acids will typically be produced together with Ci-6 alkyl lactate. Typically, some of the byproducts present in the mixture obtained from the esterification step will have low boiling points (i.e. lower than the boiling point of the Ci-6 alkyl lactate), and some of the byproducts will have high boiling points (i.e. higher than the boiling point of the Ci-6 alkyl lactate) or will be non-volatile. Accordingly, where the process of the invention includes an esterification step, the process preferably also includes a multistage distillation process in which:

1) the first reaction mixture containing lactate ester obtained following step (d) is introduced to a first distillation column and the mixture is distilled so as to obtain i) a first light fraction comprising Ci-6 alkyl alcohol and one or more byproducts having low boiling points, said byproducts including at least one Ci-6 alkyl ester of an organic acid other than lactic acid, and ii) a first heavy fraction comprising Ci-6 alkyl lactate and one or more byproducts which have high boiling points or which are non-volatile; and

2) the first heavy fraction is introduced to a second distillation column and the mixture is distilled so as to obtain i) a second light fraction comprising Ci-6 alkyl lactate; and ii) a second heavy fraction comprising one or more byproducts which have high boiling points or which are non-volatile.

In such a process, the alkyl alcohol present in the esters of other organic acids may also be recovered and recycled to the process. For example, where the first light fraction obtained from the first distillation column comprises Ci-6 alkyl alcohol and Ci-6 alkyl esters of organic acids other than lactic acid, the process may comprise a hydrolysis step (y) in which the first light fraction is admixed with excess acidic water (e.g. acidic water obtained from decanting step (x)) and heated so as to hydrolyse said Ci-6 alkyl esters, resulting in the production of organic acids and Ci-6 alkyl alcohol. During the hydrolysis step, a mixture of acidic water and alkyl alcohol is removed from the mixture by evaporation. A stream containing a mixture of acidic water and Ci-6 alkyl alcohol removed from a hydrolysis reaction mixture during hydrolysis step (y) may optionally be combined with a stream containing a mixture of mixture of water and Ci-6 alkyl alcohol removed from the

esterification step (d), the organic and aqueous layers separated by decanting as described above for step (x), and the Ci-6 alkyl alcohol and/or acidic water recycled to the process.

The lactic acid or Ci-6 alkyl lactate produced by the process of the invention may be converted into further useful downstream products by routine methods. Accordingly, the invention also provides a process for the production of oligomeric lactic acid, lactide, alkyl lactyllactate or poly-lactic acid comprising producing lactic acid or a Ci-6 alkyl lactate by a process according to the invention; and converting said lactic acid or Ci-6 alkyl lactate into oligomeric lactic acid, lactide, alkyl lactyllactate or poly-lactic acid. For example oligomeric lactic acid may be produced by heating lactic acid or Ci-6 alkyl lactate with removal of water or alkyl alcohol as required. Lactide may be produced for example by converting lactic acid or Ci-6 alkyl lactate into oligomeric lactic acid, and heating the oligomeric lactic acid in the presence of a transesterification catalyst. Alkyl lactyllactate may be produced for example by converting lactic acid or Ci-6 alkyl lactate into lactide, and by reacting lactide with an alkyl alcohol under appropriate conditions. Poly-lactic acid may for example be produced by converting lactic acid or Ci-6 alkyl lactate into lactide, and polymerising the lactide to produce poly-lactic acid. The lactic acid or Ci-6 alkyl lactate may also be converted to propylene glycol (1,2-propanediol), for example by hydrogenation using a metal catalyst (e.g. a ruthenium, rhenium or copper-containing catalyst). The lactic acid or Ci-6 alkyl lactate may also be converted to acrylic acid or alkyl acrylate respectively, using, for example phosphate, sulfate, zeolite or clay catalysts.

The following Examples illustrate the invention.

Example 1: Conversion of Glucose into lactic acid

Water (532.7 g) was charged to a 5 L round bottom glass flask equipped with an overhead stirrer, reflux condenser, temperature probe and inlet for sugar feed. Sodium hydroxide pellets (355.1 g, 8.88 moles) were added in portions to form a 40 %w/w NaOH solution. In a separate 5 L round bottom flask equipped with an overhead stirrer water (1200.0 g) was charged followed by addition of glucose (800 g, 4.44 moles, 0.5 equivalents with respect to NaOH) in portions to form a 40 %w/w solution.

The 40 %w/w caustic solution was heated to 100-105 °C and then the 40 %w/w glucose solution was fed into the hot caustic solution over a period of 60 minutes using a peristaltic pump. Upon completion of the addition the reaction was cooled to ambient and a sample (~5 mL) was analysed for lactic acid content by HPLC. In situ yield of lactic acid was 43-44%. The mixture contained 40% solids and 60% water, and sodium concentration was 93.6g/L.

500 mL of this solution was used as one feed into an electrodialysis cell stack comprised of three unit cells each cell being a two-compartment cell as illustrated in Fig. l . The other feed comprised 0.1M NaOH (500 mL). Electrodialysis was carried out for 1.75 hrs at 50°C with a constant 12V DC current applied.

After this period, the concentration of sodium hydroxide in the compartment to which sodium hydroxide was fed increased from 0.1M to 3.75M. The concentration of sodium in the other compartment decreased from 93.6 g/L to 0.8 g/L, corresponding to 99.1% recovery of sodium, and this decrease of sodium with time is shown in Fig. 2.

Example 2: Conversion of xylose into lactic acid

Water (640 g) was charged to a 5 L round bottom glass flask equipped with an overhead stirrer, reflux condenser, temperature probe and inlet for sugar feed. Sodium hydroxide pellets (426.70 g, 10.67 moles) were added in portions to form a 40 %w/w NaOH solution. In a separate 5 L round bottom flask equipped with an overhead stirrer water (1200.0 g) was charged followed by addition of xylose (800 g, 5.33 moles) in portions to form a 40 %w/w xylose solution.

The 40 %w/w caustic solution was heated to 100-105 °C and then the 40 %w/w xylose solution was fed into the hot caustic solution over a period of 60 minutes using a peristaltic pump. Upon completion of the addition the reaction was cooled to ambient and a sample (~5 mL) was analysed for lactic acid content by HPLC. In situ yields of lactic acid at this point were typically found to be 40-44%).

This reaction mixture was treated as described in Example 1, the experiment being carried out at 50 °C and constant 12V DC was applied for 1.5 hrs. After this period, the concentration of sodium hydroxide in the compartment to which sodium hydroxide was fed increased from 0.1 M to 4.1 M. The concentration of sodium in the other compartment decreased from 93.6 g/L to 5.3 g/L, and this decrease of sodium with time is shown in Fig. 3. Example 3: Single Cell Operation

Example 1 was repeated save that only a single electrodialysis cell was used instead of a stack of three cells. The power supplied was 13.9 amps. The sodium concentration in the compartment to which reaction mixture from glucose hydrolysis was fed decreased from 93.6 g/L to 0.19 g/L over 5 hrs, as shown in Figure 4.

Example 4: Conversion of xylose into lactic acid

2.265 kg Sodium hydroxide (56.6 moles) was dissolved in 2.265 kg of demineralised water (total mass: 4.530 kg) in a 20 litre jacketed reactor equipped with overhead stirrer, reflux condenser, temperature probe and an inlet port for addition of xylose solution. The jacket was connected to a circulating oil heater. The resultant 50% w/w solution of NaOH was then heated to an internal vessel temperature of 105°C.

5.000 kg xylose (33.3 moles) was dissolved in 7.500 kg demineralised water (total mass: 12.500 kg) in an identical reactor, and was warmed to an internal vessel temperature of 50°C. The xylose solution was then added over 60 minutes to the sodium hydroxide solution using a peristaltic pump, with regular and continuous addition controlled by the use of a peristaltic pump. Refluxing vapours were condensed (1.508 kg) but not returned to the reaction in order to maintain a high pH during the addition.

At the end of the addition, the reactor contents were cooled and the product mixture (15.405 kg) discharged via a valve at the base of the reactor. The total mass balance of aqueous distillate and final product mixture was 99.3% that of the starting solutions.

HPLC analysis of the product mixture indicated a 43.7% yield of sodium lactate.

2.000 litres (c. 2.450 kg) of the product mixture was then fed into an electrodialysis cell stack (Figure 1, stream 6) comprising ten cells each formed of a cation- selective membrane sandwiched between bipolar membranes. Aqueous sodium hydroxide (1 M) was used as the second feed (Figure 1, stream 7). Electrodialysis was carried out at with a constant DC voltage (24V) applied for 116 minutes. The power supplied was 20 A.

Figure 5 shows a plot of product conductivity (mS/cm) versus time: over the 116 minutes, the product conductivity was observed to decrease from 54.1 to 3.1 mS/cm, a reduction of 94.3%.

A decrease in product pH was also observed, falling from an initial value of pH 12.6 to pH 1.5 (Figure 6).