Method for producing lactic acid

Abstract

1. A method for preparing lactic acid, comprising the steps of: a) providing an aqueous medium containing magnesium lactate; b) adding a monovalent base to the aqueous medium containing magnesium lactate to form an aqueous medium containing a water-soluble monovalent lactate salt and a solid magnesium base; c) separating said solid magnesium base from said aqueous medium containing said water-soluble monovalent lactate salt; d) preparing an aqueous medium containing the water-soluble monovalent lactate at a concentration of more than 30% by weight and not more than 45% by weight; e) subjecting the aqueous medium containing the water-soluble monovalent lactate from step d) to water splitting electrodialysis to produce a first solution containing a monovalent base and a second solution containing lactic acid and a monovalent lactate, wherein the electrodialysis is carried out to a partial conversion of 40 to 99 mol %; f) recovering lactic acid from the second solution containing lactic acid and a monovalent lactate salt. The method comprising the steps of:

Description

[Technical field]

[0001] One aspect of the present invention relates to a method for producing high purity lactic acid in an economical manner. [Background technology]

[0002] Lactic acid is often produced through the fermentation of carbohydrates by microorganisms. A common characteristic for all fermentation processes is the need to neutralize the acid excreted by the microorganisms. Depending on the microorganism used in the process, a drop in pH below a critical value can damage the metabolic processes of the microorganisms and stop the fermentation process. It is therefore common to add a base to the fermentation medium to control the pH. This results in the produced lactic acid being present in the fermentation medium in the form of lactate salt.

[0003] Despite the longstanding practice of producing lactic acid via fermentation, one of the challenges in producing lactic acid is to obtain the lactic acid in a relatively pure form while at the same time carrying out the process in an economical manner on a commercially attractive scale.

[0004] Electrodialysis is one of the purification processes that can be used in the production of lactic acid via fermentation. In particular, water-splitting electrodialysis allows the lactate salt to be directly converted to lactic acid and base. In this type of electrodialysis, bipolar membranes generally separate the water into H + and OH - This causes the anions and cations of each lactate to combine, producing separate solutions of lactic acid and base.

[0005] The use of water splitting electrodialysis for aqueous media provided by fermentation, particularly for the production of organic acids, has been limited by the need to remove fermentation-derived products (e.g., sugars, proteins, and amino acids) from the feed. Such fermentation-derived materials adversely affect the water splitting electrodialysis process, for example, by fouling ion-permeable membranes and reducing output efficiency.

[0006] International Publication No. WO2011095631 describes a method for preparing lactic acid, comprising: a) providing an aqueous medium containing magnesium lactate; b) adding a monovalent base to the aqueous medium containing magnesium lactate to form an aqueous medium containing a water-soluble monovalent lactate salt and a solid magnesium base; c) separating the magnesium base from the aqueous medium containing the water-soluble monovalent lactate salt; d) adjusting the concentration of the monovalent lactate in the aqueous medium to a value between 10 and 30% by weight; e) subjecting the aqueous medium containing the monovalent lactate to water splitting electrodialysis to produce a first solution containing a monovalent base and a second solution containing lactic acid and the monovalent lactate, wherein the electrodialysis is carried out to a partial conversion of 40 to 98 mole percent; f) separating the second solution containing lactic acid and a monovalent lactate salt by vapor-liquid separation into a solution containing lactic acid and the monovalent lactate salt; g) recycling the solution of step f) containing the monovalent lactate to step d). The present invention relates to the above method, which comprises the steps of: Summary of the Invention [Problem to be solved by the invention]

[0007] There remains a need for a process for producing lactic acid that provides high yields of high purity lactic acid, minimizes the generation of significant amounts of non-recyclable components (i.e., waste by-products), and can be economically implemented with low power consumption. [Means for solving the problem]

[0008] An aspect of the present invention is a method for preparing lactic acid, comprising the steps of: a) providing an aqueous medium containing magnesium lactate; b) adding a monovalent base to the aqueous medium containing magnesium lactate to form an aqueous medium containing a water-soluble monovalent lactate salt and a solid magnesium base; c) separating said solid magnesium base from said aqueous medium containing said water-soluble monovalent lactate salt; d) preparing an aqueous medium containing the water-soluble monovalent lactate at a concentration of more than 30% by weight and not more than 45% by weight; e) subjecting the aqueous medium containing the water-soluble monovalent lactate from step d) to water splitting electrodialysis to produce a first solution containing a monovalent base and a second solution containing lactic acid and a monovalent lactate, wherein the electrodialysis is carried out to a partial conversion of 40 to 99 mol %; f) recovering lactic acid from the second solution containing lactic acid and a monovalent lactate salt. The present invention relates to the above method, which comprises the steps of: [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 shows that when the water splitting electrodialysis of a potassium lactate solution is compared to the water splitting electrodialysis of a sodium lactate solution under similar conditions, it was found from the experiments carried out and the theoretical conductivity of a monovalent lactate solution (data and theoretical model not shown) that the increase in energy consumption due to the conductivity of the solution is greater when starting from a sodium lactate solution. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] By carrying out the water splitting electrodialysis to a partial conversion of 40-99 mol %, in particular 40-98 mol %, and optionally subsequently recycling the remaining lactate to the electrodialysis step, an optimized process advantageously results, combining low power consumption with high yields.

[0011] Moreover, the methods described herein generate virtually no waste by-products, since all compounds formed and separated in the different steps can be recycled. The magnesium base separated in step c) can be used, for example, in a fermentation process, and the solution containing the monovalent base in step e) can be used in the salt exchange step b). The recovery step f) generally produces a solution containing monovalent lactate and optionally lactic acid. This solution can be at least partially recycled, if desired, to the steps preceding the water splitting electrodialysis, for example to one or more of steps b), c), d) and e).

[0012] In step d), an aqueous medium containing the water-soluble monovalent lactate at a concentration of more than 30% by weight to 45% by weight or less is provided. This step may include a concentration step if the product of step c) has an unattractively low concentration step. If a very high concentration is obtained in step c), this step may also include a dilution step. If the product from step c) already has a concentration in the desired range, no further concentration adjustment is required. In that case, step d) generally does not include anything other than providing the product of step c) to step e).

[0013] It is preferred that the concentration of the monovalent lactate in the aqueous medium is relatively high so that less water must be removed from the system to isolate lactic acid. In addition, it has been found that the use of a relatively high concentration of the monovalent lactate, particularly when the monovalent lactate is potassium lactate, improves the performance of the water splitting electrodialysis step.

[0014] Therefore, it is preferred that the aqueous medium containing the water-soluble monovalent lactate from step d) has a water-soluble monovalent lactate concentration of at least 32% by weight. If the concentration is too high, it may be difficult to perform the water splitting electrodialysis. In some embodiments, the current efficiency will be affected. Therefore, it may be preferred that the concentration of the monovalent lactate is 40% by weight or less. Further preferences will be described below.

[0015] The preferences set forth above apply to all monovalent lactate salts, but are particularly attractive when the monovalent lactate salt is potassium lactate, as described in more detail below.

[0016] The aqueous medium containing the magnesium lactate salt may preferably be provided by a fermentation process. The magnesium lactate salt is generally already present in the aqueous medium when it exits the fermentation. In such a process, a carbohydrate source is fermented to lactic acid by a lactic acid producing microorganism. During the fermentation, a magnesium base is added as a neutralizing agent. This results in the formation of an aqueous medium containing the corresponding magnesium lactate salt.

[0017] The basic anion of the magnesium base is preferably selected from at least one of hydroxide, carbonate and bicarbonate, more preferably hydroxide. It is preferred to use magnesium as the base cation, but other alkaline earth metal cations, such as calcium cations, can also be used. The amount of alkaline earth metal base is determined by the amount of lactic acid produced and can be determined via pH control of the fermentation medium.

[0018] Biomass (i.e., microbial cell material) may be removed from the fermentation broth prior to further processing of the lactate-containing medium. Removal of the biomass may be performed, for example, by conventional methods, such as those described above, including filtration, flotation, sedimentation, centrifugation, flocculation, and combinations thereof. Separation of the biomass from the solid magnesium lactate crystals may also be performed using a hydrocyclone. It is within the skill of the art to determine a suitable method. Other optional treatments prior to further processing include washing, filtration, (re)crystallization, concentration, and combinations thereof.

[0019] Magnesium is the preferred alkaline earth metal since the use of a magnesium base advantageously results in the formation of magnesium lactate in a suitable crystalline form that allows the separation of the crystalline material from the fermentation broth, e.g. the fermentation broth described above that includes biomass. The separation of magnesium lactate can be carried out by any known processing technique for solid / liquid separation. For example, it can be carried out via filtration using a filter with a suitable pore size to retain the magnesium lactate on the filter and to allow subsequent removal of impurities by washing the filter cake. The biomass separation described above is not necessary in principle unless there is a desire to further process or (re)use the remaining fermentation broth for a specific purpose.

[0020] Magnesium lactate purified in this way is particularly suitable for further processing as described herein, in particular when using water splitting electrodialysis, where the products derived from fermentation (e.g. sugars, proteins, amino acids) can be adversely affected, for example by increased power consumption and fouling of the on-permeable membrane.

[0021] The aqueous medium containing the alkaline earth metal lactate, preferably magnesium lactate, is subjected to a salt exchange reaction (step b) in which a monovalent base is added to the aqueous medium to form a monovalent lactate and a solid alkaline earth metal base.

[0022] See also WO 2005 / 123647, which describes the use of magnesium bases in lactic acid fermentation, as well as the salt exchange reaction between magnesium lactate and a monovalent base.

[0023] When the aqueous medium containing the alkaline earth metal lactate is provided by fermentation, the basic anion is generally selected to correspond to the basic anion used as a neutralizing agent during fermentation.

[0024] The monovalent base added is preferably one or more of hydroxide, carbonate or hydrogencarbonate of a monovalent cation, more preferably hydroxide, where the monovalent cation is sodium, potassium, lithium, ammonium, monoalkylammonium, dialkylammonium, trialkylammonium or tetraalkylammonium, preferably sodium or potassium, more preferably sodium. The use of sodium and potassium bases advantageously results in a higher conversion rate of alkaline earth metal lactate to monovalent lactate than when ammonium bases are used. This is relevant for preparing a product with a low alkaline earth metal ion content suitable for water splitting electrodialysis. Nevertheless, residual alkaline earth metal ions can be removed by methods known to those skilled in the art, such as by using ion exchange resins.

[0025] The use of potassium may be particularly preferred when combined with potassium lactate at a concentration of greater than 30% by weight, even more particularly greater than 32% by weight, and especially greater than 33% by weight, with respect to the advantages associated with subsequent water splitting electrodialysis of the resulting potassium lactate salt, as described in more detail below.

[0026] The amount of monovalent base is determined by stoichiometry and pH considerations. To obtain high conversion and ensure removal of substantially all alkaline earth metal ions from the lactate salt, it may be preferable to use an excess of base. Generally, it is preferred to carry out the salt exchange reaction in two stages, the first stage at a pH of 9-12, preferably 9.5-11, and the second stage at a slightly increased pH to a pH of 10.5-12.

[0027] The alkaline earth metal base formed in the salt exchange reaction is typically in solid form, while the monovalent lactate salt is dissolved in the aqueous medium, and therefore the two components can be separated by conventional solid-gas separation processes, such as one or more of filtration, precipitation, and centrifugation.

[0028] If necessary, the alkaline earth metal base obtained after separation can be recycled to the fermentation process, optionally after washing to remove residual alkaline salts.

[0029] Additional treatments, such as one or more of ion exchange treatment, activated carbon treatment, desalination electrodialysis, dilution, concentration (e.g., using heat or membranes) and filtration (e.g., nanofiltration) may be carried out prior to the water splitting electrodialysis. For example, as a safety measure to prevent alkaline earth metal levels in the aqueous medium containing the monovalent lactate from becoming too high, an ion exchange step may be carried out prior to electrodialysis to reduce their alkaline earth metal content.

[0030] However, in some embodiments, the methods described herein advantageously do not require such additional processing, particularly when lactate is provided via fermentation and magnesium base is added during the fermentation process to provide a magnesium lactate fermentation broth. In particular, it has been found that the use of a magnesium base for neutralization during fermentation to provide magnesium lactate in the appropriate crystalline form as described above eliminates the need for further purification steps typically required to remove fermentation-derived materials (e.g., sugars, proteins and amino acids) from the feed for use in water splitting electrodialysis. This advantageously reduces the complexity, power demands and costs typically associated with such electrodialysis processes.

[0031] The aqueous medium containing the monovalent lactate is then subjected to water splitting electrodialysis.

[0032] As indicated above, the aqueous medium containing the water-soluble monovalent lactate from step d) has a water-soluble monovalent lactate concentration of more than 30% to 45% by weight, in particular more than 32% by weight. This is the initial concentration in the aqueous medium (feed) subjected to electrodialysis. It may be preferred that the monovalent lactate concentration is 40% by weight or less. In one aspect, the initial concentration is preferably 32-38% by weight, even more preferably 34-36% by weight, and most preferably an initial concentration of about 35% by weight is mentioned.

[0033] Depending on the salt concentration, the aqueous medium containing monovalent lactate as obtained after the salt exchange reaction, i.e. after the separation step c), may be used directly as feed for electrodialysis or, if necessary, may be diluted or concentrated to adjust the salt concentration before the water splitting electrodialysis. Concentration may be carried out, for example, by evaporation or conventional electrodialysis. Concentration by evaporation may preferably be carried out by evaporation (e.g. evaporation of water). Concentration by evaporation may be particularly preferred when concentrating to monovalent lactate concentrations of more than 30 wt.%. Evaporation may preferably be carried out in an evaporator with mechanical vapour recompression. Alternatively, membrane concentration may also be used or a combination of evaporation and membrane concentration may be used.

[0034] A high concentration of monovalent lactate (eg, at least above 30% by weight) will also result in a high concentration of lactic acid in the resulting solution obtained after water splitting electrodialysis.

[0035] For example, the lactic acid concentration in the second solution obtained by water splitting electrodialysis may be 14% to 35% by weight. Depending on the operating conditions, the lactic acid concentration is preferably at least 20% by weight, in particular at least 22% by weight, more in particular at least 25% by weight. A preferred upper limit may be 30% by weight. In certain embodiments, the upper limit may be 29% by weight or less, or 28% by weight or less, or even 27% by weight or less. A range of 25 to 30% by weight is generally considered to be preferred.

[0036] This may be particularly advantageous when the purpose is the isolation of lactic acid. In such a situation, all water must be removed from the system. It has now been found that by starting with a concentration of monovalent lactate of at least 30% by weight, for example in the range of 30-40% by weight, water can be advantageously removed prior to water splitting electrodialysis, for example during a concentration adjustment step to increase the monovalent lactate concentration. This is achieved without detrimentally affecting the performance of the water splitting electrodialysis, or even while improving the water splitting electrodialysis process, as described in more detail below.

[0037] Removing water before the water splitting electrodialysis step means that less water needs to be removed after the water splitting electrodialysis. This advantageously results in an improved method, since removing water from a lactic acid-containing solution (e.g., the solution obtained after water splitting electrodialysis) is more costly and laborious than removing water from a monovalent lactate salt (e.g., the starting solution before water splitting electrodialysis). In particular, removing water from an aqueous lactic acid solution has many corrosion problems associated with it and requires more expensive equipment.

[0038] Thus, in some embodiments of the present invention, adjusting the concentration of monovalent lactate in the aqueous medium to a value of more than 30% by weight and not more than 45% by weight prior to performing water splitting electrodialysis in the methods described herein includes a water removal step, which may preferably be carried out in an evaporator associated with mechanical vapour recompression.

[0039] It has also been found that an initial concentration of the monovalent lactate salt within the above-mentioned range of more than 30% by weight and preferably less than 40% by weight results in improved performance of the water splitting electrodialysis process. This may be particularly true when the monovalent lactate salt is potassium lactate.

[0040] It has been found that water splitting electrodialysis can be carried out to a given degree of conversion (e.g., up to 99 mol%, or up to 98 mol%) while using less energy than is required for the same degree of conversion by starting with a monovalent lactate solution having the concentrations shown above, when starting from a less concentrated monovalent lactate solution.

[0041] Similarly, water splitting electrodialysis can be carried out to a higher degree of conversion with the same amount of energy required to achieve a lower degree of conversion by starting with a monolactate having the concentrations shown above when starting with a lower concentration of monolactate.

[0042] Starting solutions having monovalent lactate concentrations greater than 40% by weight can adversely affect the current efficiency of water splitting electrodialysis.

[0043] The concentration of monovalent lactate in an aqueous medium can be determined by methods known to those skilled in the art, such as, for example, conductivity measurement or Inductively Coupled Plasma mass spectrometry analysis, or methods such as titration or Brix (refractive index).

[0044] The water splitting electrodialysis is carried out to a partial conversion of 40 to 99 mol%. It may be preferred that the partial conversion is at least 50 mol%, particularly at least 60 mol%, more particularly at least 70 mol%, even more particularly at least 80 mol%, particularly at least 85 mol%, or at least 90 mol%. It may be preferred that the partial conversion is 98 mol% or less. Thus, in one embodiment, the water splitting electrodialysis is carried out to a partial conversion of 90 to 98 mol%, particularly 90 to 95 mol%. In another embodiment, the water splitting electrodialysis may be carried out to a conversion of 98 to 99 mol%, particularly more than 98 mol% and not more than 99 mol%.

[0045] A first solution containing a monovalent base and a second solution containing lactic acid and a monovalent lactate salt are produced in this process.

[0046] A partial conversion rate of 40 to 99 mol % means that 40 to 99 mol % of the monovalent lactate present in the feed solution is converted to lactic acid, resulting in a second solution produced by electrodialysis containing 40 to 99 mol % of lactic acid, calculated from the total molar amount of lactic acid and lactate present in the solution.

[0047] The extent of conversion can be monitored by measuring the conductivity of the second solution using methods known to those skilled in the art.

[0048] The conductivity of the second solution, as well as the conversion level and the initial salt concentration of the feed solution, will depend on the temperature of the electrodialysis process. The higher the temperature at which electrodialysis is carried out, the lower the power consumption. The temperature used is therefore selected to optimize the power consumption without compromising the performance and life of the ion-permeable membrane. Typically, temperatures between 25°C and 80°C may be mentioned. Conventionally, water splitting electrodialysis is carried out at temperatures between 25°C and 40°C. However, it is possible to carry out electrodialysis at higher temperatures, for example above 40°C, for example at least 45°C, or at least 50°C, for example between 60°C and 80°C, allowing for lower power consumption and the possibility of heat recovery.

[0049] The water splitting electrodialysis described herein can be carried out using conventional equipment and conventional methods. Preferably, the water splitting electrodialysis is carried out in an electrodialysis device comprising a cation exchange membrane and a bipolar membrane.

[0050] In one embodiment, the water splitting electrodialysis cell comprises a two compartment unit. An aqueous medium containing monovalent lactate is introduced into the salt / acid compartment (or feed compartment). Monovalent cations are transported from the salt / acid compartment to the base compartment through a cation exchange membrane to produce a first solution containing monovalent base. At the same time, H +Ions are transported to the salt / acid compartment to produce a second solution containing lactic acid and monovalent lactate.

[0051] In another embodiment, a water splitting electrodialysis cell comprises a three compartment unit, the three compartments comprising a salt compartment, a base compartment, and an acid compartment. An aqueous medium containing monovalent lactate is introduced into the salt compartment (or feed compartment). Monovalent cations are transported from the salt compartment to the base compartment through a cation exchange membrane to produce a first solution containing monovalent base. At the same time, lactate anions are transported from the salt compartment to the acid compartment through an anion exchange membrane to produce a second solution containing lactic acid. Concurrently with the transport of monovalent salt ions into their respective compartments, H + Ions and OH - Ions are transported across the bipolar membrane into the acid and base compartments, respectively.

[0052] It is preferred to apply water splitting electrodialysis to the monovalent lactate salts of sodium and potassium.When ammonium lactate is used, care must be taken to control the emission of toxic ammonia resulting from the production of ammonium hydroxide.

[0053] It is believed that monovalent lactate salts of potassium are particularly preferred, since potassium lactate has been found to further improve the water splitting electrodialysis process by requiring less energy consumption than other lactate salts, such as sodium lactate. This is believed to be due to the discovery that potassium lactate solutions have a higher conductivity than sodium lactate solutions. The maximum conductivity of potassium lactate solutions has been found at concentrations of more than 30% by weight to about 45% by weight, particularly 40% by weight, particularly about 35% by weight. Without being bound by any theory, this higher conductivity may explain the lower voltage drop and lower energy consumption when using potassium lactate. It has also been found that potassium lactate solutions have a lower viscosity than, for example, sodium lactate solutions, which advantageously results in lower pressure drop and lower pump energy requirements when using the apparatus in water splitting electrodialysis as described herein.

[0054] The methods described herein further include recovering lactic acid from a second solution containing lactic acid and monovalent lactate produced by the water splitting electrodialysis.

[0055] In some embodiments, the second solution produced by water splitting electrodialysis is separated into lactic acid and a solution containing monovalent lactate. The separation can be achieved by one or more of vapor-liquid separation, liquid-liquid separation, and solid-liquid separation. Ion exchange methods, such as the use of ion exchange resins, can also be used to recover lactic acid from the second solution produced by water splitting electrodialysis.

[0056] In one embodiment, the lactic acid is separated from the monovalent lactate salt by vapor-liquid separation.

[0057] The vapor-liquid separation can be carried out by distillation or evaporation. Distillation is preferred since it provides a substantially complete separation of the acid from the salt. The solution containing lactic acid and monovalent lactate may be concentrated prior to distillation. The distillation is preferably carried out in a vacuum distillation unit. Vacuum distillation has been found to be particularly suitable for separating lactic acid from lactate in situations where a mixture of lactic acid and lactate is obtained by water splitting electrodialysis carried out to partial conversion as described herein and where magnesium base is used for neutralization in the fermentation.

[0058] Suitable methods and / or apparatus for concentration and reduced pressure distillation are described in WO 01 / 38283. The distillation process may comprise two or more distillation steps, where the first distillation step is preferably carried out at a temperature of 80-150°C, preferably 100-140°C, and a pressure of 40-250 mbar, preferably 50-150 mbar, and the second distillation step is preferably carried out at a temperature of 80-200°C, preferably 100-200°C, and a pressure of 0.01-50 mbar, preferably 0.1-20 mbar.

[0059] The first distillation step comprises a combination of one or more film evaporators and a distillation column and is carried out with the purpose of concentrating the lactic acid / lactate product stream as highly as possible.

[0060] The second distillation step may consist of a combination of one or more film evaporators and one or more distillation units. In the second distillation step, most of the lactic acid in the product of the first distillation step is distilled, preferably under vacuum, to form a top fraction containing most of the lactic acid and a distillation residue (bottom fraction) containing monovalent lactates. The second distillation step may be carried out in one or more short path distillation (SPD) apparatuses with an internal condenser. However, in order to reduce contamination due to splashing of impurities into the condensed lactic acid, i.e. to minimize the alkali metal content in the distilled lactic acid product, the use of a vacuum distillation unit as described in Figures 5A and 5B of the above-mentioned International Publication WO01 / 38283 (see page 10, line 17 to page 11, line 7) is preferred, since this particular setup prevents splashing. Preferably, the second distillation step comprises a film evaporator (preferably falling film, wiped film or thin film evaporator) which is connected at its bottom directly or in particular via a U-shaped connection to a vacuum distillation unit equipped with a packing and preferably a cooling device, and which may therefore be operated under reflux. In the film evaporator, the lactic acid is conveyed in the vapour phase and then enters the vacuum still via a bottom connection and is subsequently distilled there.

[0061] It is further preferred that the product from the first distillation step (first bottom fraction) is subjected to a conditioning step (so-called "preflash") before passing through the second distillation step, where the pressure in this conditioning step is preferably the same as that used in the second distillation. This preferred embodiment has the advantage that residual amounts of water and dissolved gases are removed before the product is subjected to the second distillation step. By using a preflash, the lactic acid / monolactate content can be increased, and therefore a purer lactic acid product can be obtained in the second distillation step and a more stable operation can be achieved.

[0062] Liquid-liquid separation may include extraction and back-extraction, or other techniques, for recovery of lactic acid from the loaded solvent. Liquid-liquid separation may also include filtration, such as ultrafiltration, microfiltration, nanofiltration, reverse osmosis, decantation, diffusion dialysis, or Donnan dialysis.

[0063] The solid-liquid separation may include a crystallization step. For example, lactic acid may be crystallized by a static crystallization unit, by fractional crystallization, by suspension crystallization and / or by wash column crystallization. The crystals may then be separated from the liquid phase of the solution by filtration or centrifugation. The crystallization may include one or more of a concentration step (e.g., a water evaporation step), a cooling step, a seeding step, and one or more washing steps. Solid-liquid separation, especially crystallization, has the disadvantage that the recovery yield is generally low in order to ensure a high purity product. For example, in several steps after the first crystallization, the recovery rate of lactic acid is about 46%, with a purification factor of 15-20 (the ratio of the amount of impurities in the product before and after crystallization). To achieve a degree of purification of 100-160, a second crystallization step is required, and the overall yield drops to 22%.

[0064] Lactic acid may also be recovered from the second solution containing lactic acid and monovalent lactate by an ion exchange process, whereby the remaining monovalent cations in the second solution are replaced by protons, for example by using an ion exchange resin. This method may be particularly suitable, for example, when the water splitting electrodialysis is carried out at a high conversion, for example at least 90 mol%, in particular at least 95 mol%, for example 98-99 mol%.

[0065] The solution containing monovalent lactate obtained after separation can be treated as desired. If desired, it can be recycled to an earlier step in the process so that it is provided for water splitting electrodialysis. For example, it can be at least partially recycled to one or more of steps b), c), d) and e). This recycling step ensures that no substantial yield loss occurs as a result of partial conversion of lactate to lactic acid during water splitting electrodialysis.

[0066] The lactic acid product obtained after recovery step f) may be in the form of a solid, liquid or solution and generally comprises at least 95% by weight of lactic acid, preferably at least 97% by weight of lactic acid, more preferably at least 99% by weight of lactic acid, even more preferably at least 99.5% by weight of lactic acid, and most preferably at least 99.9% by weight of lactic acid. The lactic acid obtained by the process according to one aspect of the present invention is therefore of high purity and is suitable for direct use, for example, in synthetic processes, food applications and cosmetic applications.

[0067] The obtained lactic acid is particularly suitable for the preparation of lactide and / or polylactic acid, where the presence of impurities, such as lactate salts, during the polymerization of lactic acid can result in undesirable racemization of the lactic acid moiety, resulting in poor quality lactide and polylactic acid products. Generally, the amount of metal ions should be less than 5 ppm. For example, the presence of as much as 20 ppm of sodium sulfide in lactic acid will adversely affect the optical purity of the polylactic acid product.

[0068] Conventional methods known to those skilled in the art can be used for the above production of lactide and / or polylactic acid, provided that the starting material containing lactic acid is produced via the methods described herein.

[0069] The methods described herein advantageously involve low power consumption and ensure that no or substantially no waste by-products are generated.

[0070] Aspects of the present invention are further illustrated by the following examples, without being limited thereto or by them.

[0071] Example 1: Partial electrodialysis of sodium lactate solution

[0072] An Electrocell electrodialysis module (Sweden) was equipped with a Fumatech FBM bipolar membrane and a Neosepta CMB cation exchange membrane. A set-up with two electrode compartments and one feed compartment was used. The membrane area of ​​the bipolar and cation exchange membranes was 0.01 m. 2 The first compartment consisted of the anode and the cation exchange side of the bipolar membrane, the second feed compartment consisted of the anion exchange side of the bipolar membrane and the cation exchange membrane, and the third compartment consisted of the cation exchange membrane and the cathode. To ensure high conductivity, 2 wt% sulfuric acid in water was circulated through the anode compartment. A 20 wt% aqueous solution of sodium lactate was circulated through the middle compartment as a feed. An 8 wt% aqueous solution of sodium hydroxide was circulated through the cathode compartment to ensure high conductivity at the cathode side and the sodium hydroxide produced was collected. The three solutions were circulated from 500 ml of glass buffer on the electrodialysis module using a peristaltic pump at 250 ml / min. The glass buffer container was double-walled and the temperature throughout the three compartments was kept at 40-60 °C using a water bath. The sulfuric acid and sodium hydroxide were reagent grade and the Purac sodium lactate was of high purity food grade quality.

[0073] The electrodialysis experiments were carried out at a constant DC current of 7.5 A. In the experiments, the sodium lactate solution in the feed compartment of the module was batchwise acidified by removing sodium through a cation exchange membrane to form sodium hydroxide in the cathode compartment, while the protons generated by the bipolar membrane returned to form lactate ions and lactic acid.

[0074] The experiment lasted for approximately 230 minutes, at which point all of the lactate salt had been converted. The sodium lactate solution at the start of the experiment had a high conductivity, and the voltage was relatively constant at 9.5-10.5 V. After 180 minutes, 80% of the sodium lactate had been converted to lactic acid, the residual sodium content was 0.84 wt%, and the voltage had increased to 12 V. The pH of the solution had decreased from an initial pH of 6.0 to 3.1. After 210 minutes, the conversion had increased to 94%, the residual sodium content had decreased to 0.25 wt%, the voltage had increased to 16 V, and the pH had decreased to 2.56. After 225 minutes, the conversion had increased to 98%, the residual sodium content had decreased to 0.0 wt%, the voltage had increased to 22.4 V, and the pH had decreased to 2.56. The sudden increases in voltage at these time intervals are the result of a gradual decrease in the conductivity of the feed solution. This voltage increase results in a sharp increase in the power consumption to convert the residual sodium lactate. These results demonstrate that an economical process can be obtained by running electrodialysis up to a partial conversion of 98% or less.

[0075] However, under different conditions, especially when the initial concentration of monovalent lactate in the aqueous medium is at least 30% by weight or more and 40% by weight or less, electrodialysis up to 99% by weight may be performed while having an overall process that can be sufficiently economical. This may be the case, for example, when vacuum distillation is not performed to isolate lactic acid, and other methods are selected to recover lactic acid, such as using ion exchange resins. This may also be the case when a higher starting concentration is used and part of the water removal is performed before electrodialysis. In such a situation, the energy saved by not performing vacuum distillation and / or performing water evaporation before water splitting evaporation may compensate for the energy required to increase the conversion rate to more than 98% by weight, for example up to 99% by weight, and provide a more economically viable process.

[0076] In the time interval from 180 to 210 minutes, the current efficiency calculated using the theoretical mass flow rate based on the Faradays law and the actual mass flow rate based on the analytical data of lactate, lactate and sodium was 0.68. In the time interval from 210 to 225 minutes, the current efficiency decreased to 0.64, and at a conversion rate above 98%, the current efficiency decreased to below 0.36. This means that at high conversion rates, the energy input itself increases, but less and less of the consumed electrical energy is used for conversion.

[0077] Example 2. Distillation of lactic acid from a solution of lactic acid and sodium lactate

[0078] A solution of lactic acid and sodium lactate was prepared by adding 98.4 grams of a 60% by weight food grade sodium lactate solution to 4,310 grams of a 49% by weight food grade lactic acid in water. This solution was representative of a mixture of lactic acid and sodium lactate and was concentrated by evaporating the water after partial conversion by electrodialysis. A Glass Lab Short Path Distillation (SPD) unit was used to further concentrate the solution and subsequently distill the lactic acid.

[0079] The laboratory SPD unit used was a glass KDL4 type unit from UIC. The SPD unit is basically a double-walled column that can be heated to over 100°C with an oil bath. Liquid (i.e., lactate / lactate feed solution) can be fed to the top of the SPD unit by pumping the liquid with a peristaltic pump. The SPD unit is equipped with an upper agitator and wipers so that a liquid film can be generated on the inner wall of the SPD unit column from the liquid pumped from the top. The top of the SPD unit is also connected to a vacuum system that includes a glass column indirectly cooled through the double wall by a cold finger at -60°C, an oil-operated vacuum pump, and a pressure control unit equipped with a vacuum gauge and a pressure valve. By applying vacuum and high temperature, compounds with relatively high boiling points, such as lactic acid, can be evaporated from the feed. The evaporated lactic acid can then be condensed in an SPD unit equipped with a water-cooled internal condenser at 55° C. The lactic acid can be collected in a glass bulb located directly below the internal condenser of the SPD unit. A portion of the feed that is not evaporated can be recovered using an outlet on the side wall at the bottom of the double-walled SPD column. Vapors that are not captured on the internal condenser of the SPD unit are captured in a glass column in the vacuum section operated with a cold finger.

[0080] First, the feed was led to the SPD unit for dehydration. The oil temperature was 120° C., the vacuum pressure was 100 mbar, and the feed rate was 10 ml / min. The internal condenser of the SPD unit was operated with tap water. A fraction of 336 g of a dehydrated mixture of lactic acid and sodium lactate was collected and immediately passed through the SPD unit again, where the oil temperature was 130° C., the vacuum pressure was 6 mbar, the feed flow rate was 15 ml / min, and the internal condenser of the SPD unit was cooled to 55° C. In this second SPD step, at least half of the feed was distilled and recovered as purified lactic acid, with an overall recovery of lactic acid (relative to the total weight of lactic acid present in the original feed) of 65%. The feed to the SPD unit in this second step contained 5500-6000 ppm sodium, while the lactic acid product contained only 112 ppm sodium, i.e. a purification factor of 49-54. Thus, using a short path distillation (SPD) unit, the majority of the lactic acid can be recovered in pure form from the original mixture of lactic acid and sodium lactate.

[0081] Example 3 Partial electrodialysis of potassium lactate solution

[0082] A pilot electrodialysis unit equipped with multiple cell pairs of bipolar and cationic membranes was used in batch experiments to prepare lactic acid from a model potassium lactate solution, which was a food-grade 60 wt.% potassium lactate solution (PURASAL from Purac). 登録商標 This was obtained by diluting HiPure P) into potassium lactate at a concentration of 34% by weight by adding water.

[0083] In batch mode, 20 kg of a 34 wt% aqueous potassium lactate feed was converted to lactic acid via water splitting electrodialysis in the pilot electrodialysis unit. The experiment lasted 76 minutes, after which a conversion of 88 mole % was obtained. The resulting solution had a lactic acid concentration of 26 wt% and a residual potassium lactate concentration of 2.0 wt%. The average voltage during the first 70 minutes of the experiment was about 42 V, and during the last 6 minutes the voltage was increased to 49 V, indicating increased energy use at high conversion. The average current efficiency was 0.78.

[0084] This experiment shows the suitability of a 34 wt% monolactate solution as a starting solution for obtaining lactic acid via water splitting electrodialysis. Furthermore, starting with a 34 wt% monolactate concentration results in a 27 wt% lactic acid solution, which is more concentrated than the solution obtained when starting with a lower concentration of monolactate (e.g., when starting with a 21-23 wt% monolactate solution, a 17 wt% lactic acid solution can typically be obtained). This means that the starting monolactate solution has a relatively low concentration (e.g., less than 30 wt%, less than 25 wt%, or less than 10 wt%). In order to start with a higher monolactate concentration, e.g., a lactate concentration of at least 30 wt% or more (e.g., a 34 wt% potassium lactate solution), it may be preferable to remove water before electrodialysis. As a result, less water needs to be removed after performing water splitting electrodialysis. This may be the case, for example, when the monolactate comes from a fermentation process.

[0085] It was found from the experiments carried out and the theoretical conductivity of monovalent lactate solutions (data and theoretical model not shown) that when the hydrolysis electrodialysis of potassium lactate solutions is compared with that of sodium lactate solutions under similar conditions, the increase in energy consumption due to the conductivity of the solutions is greater when starting from sodium lactate solutions (Figure 1). This means that under the same conditions, lower energy consumption can be achieved when using potassium lactate solutions compared to when using sodium lactate solutions. Furthermore, as can be derived from Figure 1, the optimum low energy for potassium lactate is found when working with concentrations of 30-40% by weight (Figure 1), especially at concentrations above 30% by weight and up to 40% by weight.

Claims

1. 1. A method for preparing lactic acid, comprising the steps of: a) providing an aqueous medium containing magnesium lactate; b) adding a monovalent base to the aqueous medium containing magnesium lactate to form an aqueous medium containing a water-soluble monovalent lactate salt and a solid magnesium base; c) separating said solid magnesium base from said aqueous medium containing said water-soluble monovalent lactate salt; d) providing an aqueous medium containing the water-soluble monovalent lactate salt at a concentration of more than 30% by weight and not more than 45% by weight; e) subjecting the aqueous medium containing the water-soluble monovalent lactate from step d) to water splitting electrodialysis to produce a first solution containing a monovalent base and a second solution containing lactic acid and a monovalent lactate, wherein the electrodialysis is carried out to a partial conversion of 40 to 99 mole percent. f) recovering lactic acid from the second solution containing lactic acid and a monovalent lactate salt. The method comprising the steps of:

2. 2. The method according to claim 1, wherein in step d) an aqueous medium is provided comprising a water-soluble monovalent lactate salt in a concentration of at least 32% by weight, and / or up to 40% by weight, and / or from 32 to 38% by weight, even more preferably from 34 to 36% by weight, the most preferred concentration being an initial concentration of about 35% by weight.

3. 2. The process according to claim 1, wherein step d) is a concentration step, a dilution step or a step in which the concentration is not further adjusted, preferably a concentration step in which water is removed.

4. 2. The method of claim 1, wherein recovering lactic acid from step f) comprises separating the second solution comprising lactic acid and a monovalent lactate salt into a solution comprising lactic acid and a monovalent lactate salt, wherein the separation is performed via one or more of vapor-liquid separation, liquid-liquid separation, and solid-liquid separation.

5. 5. The method according to claim 4, wherein the solution containing monovalent lactate is at least partially recycled to an earlier step of the method, for example to be provided to water splitting electrodialysis, for example to one or more of steps b), c), d) and e).

6. 4. The process of claim 3, wherein said separation is performed through vapor-liquid separation, wherein said vapor-liquid separation comprises distillation, in particular distillation performed in a vacuum distillation unit.

7. 2. The process according to claim 1, wherein the water splitting electrodialysis is carried out to a partial conversion of at least 50 mol%, in particular at least 60 mol%, more in particular at least 70 mol%, even more in particular at least 80 mol%, in particular at least 85 mol%, or at least 90 mol% and / or up to 98 mol%, in particular from 90 to 95 mol, or more than 98 mol% and up to 99 mol%.

8. 2. The method according to claim 1, wherein the monovalent lactate is potassium lactate or sodium lactate, in particular potassium lactate.

9. 2. The method according to claim 1, wherein the monovalent lactate salt is potassium lactate and in step d) an aqueous medium is provided comprising potassium lactate in a concentration of at least 32% by weight, and / or up to 40% by weight, and / or from 32 to 38% by weight, even more preferably from 34 to 36% by weight, with the most preferred concentration being an initial concentration of about 35% by weight.

10. 2. The method of claim 1, wherein the water splitting electrodialysis is carried out in an electrodialysis device comprising a cation exchange membrane and a bipolar membrane.

11. 2. The method of claim 1, wherein the aqueous medium containing the magnesium lactate is provided by fermentation, wherein a carbohydrate source is fermented by a microorganism to form lactic acid, and wherein a magnesium base is added as a neutralizing agent during fermentation to provide the magnesium lactate.

12. 12. The method of claim 11 , wherein the magnesium base is magnesium hydroxide.

13. 12. The method of claim 11 , wherein the aqueous medium containing the magnesium lactate is subjected to a separation step to remove microbial cellular material prior to step b).

14. 2. The method according to claim 1, wherein the lactic acid obtained after recovery step f) comprises at least 95% by weight of lactic acid, in particular at least 97% by weight of lactic acid, more in particular at least 99% by weight of lactic acid, at least 99.5% by weight, most in particular at least 99.9% by weight of lactic acid.

15. 15. A method for the preparation of lactide and / or polylactic acid, the method comprising preparing lactic acid using the method of any one of claims 1 to 14, and reacting the lactic acid to form lactide and / or polylactic acid.

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