A process for improving the thermal stability of an aqueous lactic acid solution of a specific enantiomeric purity.

Abstract

A process for improving the thermal stability of an aqueous lactic acid composition is disclosed, which includes a heat treatment step and further comprises contacting the aqueous lactic acid composition with at least one adsorbent selected from the group consisting of activated carbon, polymeric adsorbents, ion exchange resins, and bleaching clay.

Description

Technical Field

[0001] Technical Field The present invention generally relates to a method for preparing an enantiomerically pure lactic acid monomer composition for the production of either poly-L-lactic acid (or poly-L-lactide, hereinafter simply PLLA) or poly-D-lactic acid (or poly-D-lactide, hereinafter PDLA), and more particularly to a method for improving the thermal stability of such monomer compositions.

Background Art

[0002] Background of the Invention PLLA and PDLA are biodegradable semi-crystalline to crystalline polymers that are ultimately prepared either via ring-opening polymerization of the corresponding enantiomers, i.e., lactide dimers of l-lactic acid and d-lactic acid, respectively, or via condensation of enantiomers with elimination of water. Both of these polymers are of commercial interest and, since they are derived from stereoisomers, generally have similar physical and mechanical properties, although they have some differences that are not substantial with respect to the problems of the present invention. Blends and copolymers of PLLA and PDLA have also attracted commercial interest.

[0003] Manufacturers of both PLLA and PDLA typically try to limit the extent to which other enantiomers are involved in the formation of the desired polymer from a selected enantiomer. For this reason, PLLA manufacturers will typically want to limit the extent to which d-lactic acid is involved in the production of PLLA, and PDLA manufacturers will typically want to limit the extent to which l-lactic acid is present and involved in the production of PDLA. The extent to which other non-selective enantiomers may be acceptable will naturally vary depending on the specific polymerization method used and / or the desired properties of the PLLA or PDLA polymer product to be produced for a particular application or end use. However, generally, achieving a lactic acid monomer composition with a certain degree of enantiomeric purity (and, as noted herein, “lactic acid monomer composition” and “enantiomerically pure lactic acid monomer composition” are used herein in reference to the improved thermal stability of the aqueous lactic acid composition prepared by the process of the present invention, regardless of whether these compositions are subsequently used in a polycondensation process or in the formation of lactide corresponding to a subsequent ring-opening polymerization method) is one of the needs of all manufacturers of PLLA and PDLA polymers.

[0004] At the same time, in many applications and end uses where the desired PLLA and PDLA polymers are to be supplied, avoiding discoloration during use and over time, and / or avoiding a tendency to develop color, is an independent need of these manufacturers.

[0005] Unfortunately, as described in U.S. Patent No. 5,488,156 ("'156 Patent") granted to Kulprathipanja et al., lactic acid formed through fermentation is generally prone to chromogenicity when heated to temperatures above approximately 180 degrees Celsius. The inventors of the '156 Patent describe the use of lactic acid as a monomer in the preparation of poly(lactic acid) (presumably referring here to a polymer of a racemic mixture of both d- and l-enantiomers (hereinafter referred to as PDLLA) without any distinction or specific mention of one stereochemistry (or more precisely) of the lactic acid) and copolymers of glycolic acid and methylglycolic acid. However, since the polymers are prepared at high temperatures where chromogenicity is a concern, it should be noted that the tendency of lactic acid to chromogenicity poses a problem for some large-scale commercial applications where PDLLA (or more precisely, PLLA or PDLA) may be well-suited, for example, to containers for the fast-food industry and consumer packaging.

[0006] The inventors of the 156 patent observed that when heated under specified conditions to determine thermal stability, i.e., heated at 180 degrees Celsius for 3 hours, the lactic acid invariably developed color, regardless of its purity or the method used to purify it.

[0007] Based on this observation, the inventors of the '156 patent hypothesized that, since the observed coloration was in all cases associated with trace amounts of carbohydrates in lactic acid that could undergo carbonization under polymerization conditions, it might be possible to prepare heat-stable lactic acid by subjecting an aqueous solution of lactic acid to sufficient heat pretreatment to form a colorant via carbonization of residual carbohydrates, and then removing the colorant before subjecting the thus treated aqueous lactic acid monomer composition to a polymerization method to produce poly(lactic acid), etc. More specifically, the '156 patent specifies heating a lactic acid solution containing 8-20 weight percent lactic acid to a temperature of 180-230 degrees Celsius for a time sufficient to carbonize the residual carbohydrates dissolved in the lactic acid solution, the time suggested by the '156 patent, which naturally depends on the temperature used (higher temperatures correspond to shorter processing times, and vice versa), but is generally in the range of 0.5 to 6 hours.

[0008] The proposed method for decarburizing the heat-treated aqueous lactic acid monomer composition is not described in any length, but the examples show that when a filtered 15 percent lactic acid fermentation broth was passed through a mixed bed of anion and cation exchange resin, then through a cation exchange resin (hydrogen form) and activated carbon column, the initial APHA (or Pt / Co or Hazen) color was 93 according to ASTM D1209, and when concentrated to 88 wt percent and heated at 180 degrees Celsius for 3 hours, it was thermally unstable. On the other hand, when a 15 percent lactic acid fermentation broth was heat-treated at 230 degrees Celsius for 4 hours, filtered, and passed through a mixed bed of anion and cation exchange resin, cation exchange resin (hydrogen form), anion exchange resin (hydroxy form), and activated carbon, the color was reduced to provide a product with an APHA color of 40. This product, when then concentrated to 88 wt percent and evaluated as before, became "pale yellow," which the inventors attributed to the carbonization and removal of the majority of carbohydrate impurities.

[0009] In the final example, the “pale yellow” product of the second example was first heat-treated, then passed through adsorbents in the referenced order, concentrated, and evaluated after heating at 180 degrees Celsius for 3 hours, diluted to a 20 wt percent solution, filtered, passed through another activated carbon column, concentrated to 88 wt percent (exhibiting an APHA color of 48), and heated again to 180 degrees Celsius for 3 hours. In this iteration, the inventors subjectively observed no color formation and presumably considered the resulting aqueous lactic acid monomer composition to be ultimately thermally stable.

[0010] Unfortunately, based on our experimental efforts, we have concluded that the range of colorants and precursors faced by manufacturers of either PLLA or PDLA extends beyond mere residual carbohydrates, which the inventors of the '156 patent themselves were concerned about in relation to the thermal stability (or, more precisely, lack thereof) of the aqueous lactic acid monomer composition.

[0011] Unfortunately, we have further found that a simple heat treatment, as defined by the inventors of the '156 patent' as sufficient to solve the problem of the lack of thermal stability of these monomer compositions across the entire range of lactic acid obtained by either method and purified (or unpurified), results in an aqueous lactic acid monomer composition that is not as suitable at the same time as satisfying the first needs described above, particularly for producing PLLA or PDLA (at least as intended in the future), while having at least a certain degree of enantiomeric purity for the selected enantiomeric compounds (the preceding fermentation and purification steps have already been designed with considerable expense and care). [Overview of the project] [Means for solving the problem]

[0012] Summary of the Invention This section provides a general overview of the invention and is not a comprehensive disclosure of all its features.

[0013] In a first aspect, the present invention relates more broadly to a process for providing an aqueous lactic acid monomer composition having improved thermal stability, for subsequent use in a method for producing polymers therefrom.

[0014] In more specific embodiments, the present invention relates to a process for providing an aqueous lactic acid monomer composition having improved thermal stability and, in parallel, an enantiomeric purity of at least a certain desired degree of l-lactic acid enantiomer or d-lactic acid enantiomer.

[0015] Another aspect of the present invention relates to a process for providing an aqueous lactic acid monomer composition having improved thermal stability by treatment or conditioning means that does not involve or cause a degree of isomerization of the selected enantiomer that deviates from the enantiomeric purity required by a subsequent polymerization method for the selected enantiomer, using a treated or conditioned aqueous lactic acid monomer composition for producing a copolymer of a PLLA or PDLA homopolymer or a selected enantiomer or its lactide with one or more other comonomers. [Modes for carrying out the invention]

[0016] Description of the Embodiment The embodiments and various features and advantageous details herein are intended to be illustrative of the invention in these various aspects or from these various viewpoints, and should be considered not as an implied limitation of the invention, but merely as a demonstration of various methods and options on how the principles and understanding behind the invention may be applied in carrying out the invention. Therefore, unless otherwise indicated, any definitions and embodiments described in this or other sections are intended to apply to all embodiments and aspects of the subject matter described herein that may be deemed suitable according to the understanding of those skilled in the art.

[0017] As used herein, the singular forms "a," "an," and "the" include multiple references unless otherwise clearly indicated by the context. The term "comprising" and its derivatives as used herein are similarly intended to be open-ended terms specifically indicating the presence of specified features, elements, components, groups, integers, and / or steps, but not excluding the presence of other unspecified features, elements, components, groups, integers, and / or steps. This understanding also applies to the terms "including," "having," and their derivatives. The term "consisting" and its derivatives as used herein are intended to be closed terms specifically indicating the presence of specified features, elements, components, groups, integers, and / or steps, but not excluding the presence of other unspecified features, elements, components, groups, integers, and / or steps. As used herein, the term "consisting essentially of" is intended to specifically indicate the presence of elements, components, groups, integers, and / or steps that do not substantially affect the basic and novel properties of the specified elements, components, groups, integers, and / or steps.

[0018] As described above, the preferred embodiments of the present invention specifically relate to providing aqueous lactic acid monomer compositions having, in parallel with improved thermal stability, at least a certain desired degree of enantimeric purity of the l-lactic acid enantiomer or d-lactic acid enantiomer. In this regard, in the case of lactic acid monomer compositions intended to produce PLLA, the presence of d-lactic acid enantiomers via isomerization of l-lactic acid at heating, and even more harmfully, their formation, imposes yield loss and separation costs on the manufacturer, and the same is true with respect to l-lactic acid if the aim is to produce PDLA. Therefore, although it can be expected that various known polymerization methods for producing PLLA or PDLA deal with the presence of some non-selective enantiomers (the acceptable amount may vary depending on the particular polymerization method used), excess amounts of non-selective enantiomers should be avoided.

[0019] Lactic acid is most commonly produced commercially by fermentation from various carbohydrate sources, but conventional catalytic synthesis methods from other starting materials are also known. In a preferred embodiment of the present invention, the aqueous lactic acid monomer composition, which is of greatest interest, would be produced by fermentation.

[0020] In this context, the production of lactic acid by fermentation using genetically engineered yeast has been a field of focus for many years as a sustainable method for producing products useful in a variety of materials, such as foods and polymers. Candidate host yeast strains for lactic acid production include Saccharomyces cerevisiae, Schizosaccharomyces pombe, and various species from the genera Kluyveromyces, Pichia, Candida, and Hansenula. Numerous publications describe various operations on these candidate host yeast strains, most of which aim to more selectively produce l-lactic acid enantiomers. Although the process of the present invention has been primarily developed using lactic acid compositions produced by fermentation aimed at the more selective production of l-lactic acid, the inventors have no reason to believe that the solutions developed to improve the thermal stability of aqueous lactic acid monomer compositions while emphasizing the required enantiomeric purity for the associated downstream polymerization methods are not also useful for aqueous lactic acid compositions prepared (by fermentation methods and other means known to those skilled in the art in the production of such lactic acid compositions) to allow for greater utilization of d-lactic acid enantiomers, for example, in consideration of the intended use in the production of PDLA.

[0021] More specifically, in a more preferred embodiment, the present invention relates to improving the thermal stability of lactic acid compositions produced by low pH-tolerant microorganisms and related fermentation methods. Both are international publications 2012 / 114979 and 2011 / 021629 (Schizosaccharomyces pombe) granted to Asahi Glass Co., Ltd. Lactic acid compositions produced by fermentation methods and engineered microorganisms, such as those described in (based on pombe), or in particular in International Publication No. 2023 / 004336 (published January 26, 2023) of the same assignee, and in International Application PCT / US2023 / 060789 (an application filed January 17, 2023, claiming priority based on the application published as International Publication No. 2023 / 004336), and in International Application PCT / US2023 / 061683 (an application filed January 31, 2023, claiming priority based on the same application published as International Publication No. 2023 / 004336), are of most interest.

[0022] As previously stated, we concluded that even with respect to the distilled lactic acid products from fermentation relating to the application of the same assignee referenced, these aqueous lactic acid compositions contain other species that are detrimental to the thermal stability of the composition and, consequently, to the commercial preparation of the polymer, particularly PLLA, to a greater extent than recognized by the inventors of the '156 patent.

[0023] The process of the present invention provides a method for effectively dealing with these other problematic species, as will be demonstrated by subsequent examples. Essentially, this is achieved by applying heat to the aqueous lactic acid composition over a period of time and contacting the aqueous lactic acid composition with one or more adsorbents described below in any combination and in any order (i.e., all are effective in removing problematic species, and no particular criticality arises regarding the order of use when two or more adsorbents are used in combination with another adsorbent (which may but may not necessarily be from different categories of adsorbents listed hereafter, or may simply be from the same shared general category of adsorbent materials)).

[0024] The heat treatment step is carried out in such a manner that, depending on the heat treatment applied (for example, by thermally catalyzed isomerization from preferred and desirable enantiomers to other enantiomers, for example, by thermal catalysis), taking into consideration the amount of heat applied and the length of time the aqueous lactic acid composition containing l-lactic acid or d-lactic acid enantiomers in a percentage less than or equal to the target maximum percentage is exposed to the heating. However, preferably, the amount of heat and the length of time the aqueous lactic acid composition is exposed to the heating are such that the heat-treated aqueous lactic acid composition contains less than 1.0 weight percent, preferably less than 0.8 weight percent, and more preferably less than 0.5 weight percent of undesirable l-lactic acid or d-lactic acid enantiomers.

[0025] The range of adsorbents that can be used to contact the aqueous lactic acid composition is broad and includes one or more of activated carbon, polymeric adsorbents, ion exchange resins, and bleaching clays. Specifically, treatment of the heated aqueous lactic acid composition by exposure to a strong acid cation exchange resin, followed by one or more adsorbents selected from non-functional polymeric adsorbents, activated carbon, and other strong acid cation exchange resins, either alone, in any combination, and in any order, is contemplated. The manner in which the adsorbent is contacted with the aqueous lactic acid composition (or the aqueous lactic acid composition is contacted with the adsorbent) is not considered critical, and one of ordinary skill in the use of various adsorbents should be able to adequately design and implement suitable means for efficiently contacting the aqueous lactic acid composition within the scope relevant to the process of the present invention and then provide the treated, more thermally stable aqueous lactic acid composition to the relevant polymerization methods for further use.

[0026] In view of the above, it should be observed that the heat treatment step and (broadly speaking) the contact step with one or more adsorbents are preferably carried out sequentially, preferably with the heat treatment preceding the contact step with the adsorbent. The heat treatment and the contact step with one or more adsorbents can also be carried out in parallel or substantially in parallel, for example, by heating the aqueous lactic acid composition to contact at least one adsorbent and then carrying out the contact of the aqueous lactic acid composition and at least one adsorbent at an elevated temperature (such that at least some heating of the aqueous lactic acid composition occurs prior to some exposure to the adsorbent), or by combining the aqueous lactic acid composition with an adsorbent such as a strong acid cation exchange resin and then heating the combined material (in parallel). However, the essential aspect of the object of the present invention is that both the exposure of the aqueous lactic acid composition to an elevated temperature and the exposure to one or more of the adsorbents described herein are present.

[0027] In one embodiment of the "parallel mode" which involves at least one adsorbent being contained in a heating column or other vessel and passing an aqueous lactic acid composition through from the inlet to the outlet of the column or vessel in a manner of bringing the aqueous lactic acid composition into contact with the at least one adsorbent, the corresponding manner of performing the heat treatment thus simply involves passing the aqueous lactic acid composition through the column or other vessel from the inlet to the outlet, and the duration of the heat treatment is only substantially the time required to transfer the lactic acid composition from the inlet to the outlet of the column or vessel.

[0028] It will of course be understood by those skilled in the art that the temperature and heating or time of temperature rise to which a given aqueous lactic acid composition can be exposed and the specific adsorbent or order of adsorbents required for contact with the given aqueous lactic acid composition to improve the thermal stability of the composition to a desired degree will vary for different lactic acid compositions from different sources - different fermentations and different purification methods - and that it is very reasonably predictable that the relevant polymerization method will have different amounts of both the l-lactic acid and d-lactic acid enantiomers in relation to the target maximum proportion of the acceptable enantiomer and further different color formers and / or different distributions of such color formers to be taken into account.

[0029] However, generally speaking, considering the subsequent examples and the guidance provided by the present application, it is considered to be well within the scope of the skills of those skilled in the art to apply the principles behind the present invention while maintaining the proportion of the d-lactic acid enantiomer below the target maximum value in relation to the preferred l-lactic acid enantiomer (for use in the relevant polymerization method for producing PLLA) or maintaining the proportion of the l-lactic acid enantiomer below the target maximum value in relation to the preferred d-lactic acid enantiomer (for use in the relevant polymerization method for producing PDLA) in order to improve the thermal stability of any given aqueous lactic acid composition.

Examples

[0030] Examples Examples 1 and 2 The feed material for these two examples was an aqueous refined lactic acid composition from Schizosaccaromyces pombe fermentation having an initial APHA color value of 91 as measured in accordance with the current ASTM D1209 standard method.

[0031] 30 mL of Dowex® Monosphere® 99 sulfonate-functionalized styrene-divinylbenzene gel matrix strong acid cation exchange resin was loaded onto a jacketed Ace Glass #11 chromatography column. The resin was conditioned with 5 bed volume (BV) of 5% HCl followed by 5 BV of deionized water.

[0032] After conditioning, the column was heated to 90°C via the column jacket. Feed was introduced at a rate of 1 mL / min. A fraction collector was set up to collect effluent in a 6 mL fraction. The selected fraction was collected and analyzed for its APHA color. The results are summarized in Table 1 below.

[0033] [Table 1]

[0034] Next, in Example 1, the effluent fractions after #10 were combined and used as feed for carbon treatment also performed at 90 degrees Celsius, and the combined effluent will hereafter be referred to as the 90°C SAC effluent. When combined, the color of the 90°C SAC effluent was measured using an APHA value of 30.

[0035] In Example 2, the same experiment was performed with the second part of the same aqueous lactic acid composition, except that the temperature used throughout was 60°C instead of 90°C. The measured color of the 60°C effluent before 60°C carbon treatment was measured at an APHA value of 25.

[0036] For the 90°C carbon treatment of the combination effluent from Example 1 at 90°C and the combination effluent from Example 2 at 60°C, 30 mL of Calgon CPG LF 12X40 acid-washed granular activated carbon (Calgon Carbon Corporation), which has low acid-soluble iron content, was loaded into a 250 mL beaker. The carbon was rinsed with DI water at a rate that removed the fine powder but left the granules in the beaker. The rinsed carbon was loaded onto a jacketed Ace Glass #11 chromatography column. The carbon was rinsed with an additional 5 BV of deionized water.

[0037] First, the column was heated to 90 degrees Celsius through its jacket. Then, the combined 90°C effluent was fed through a carbon column at a rate of 1 mL / min so that the combined residence time of the aqueous lactic acid composition through the 90°C SAC column and activated carbon column was approximately 60 minutes, and the estimated resin volume was within the range of 30–60 minutes as the true residence time. A 6 mL fraction was collected. The first 15 carbon effluents reported in Table 2 below were from contact with the combined 90°C effluent. Next, the heat to the carbon column was reduced to provide a column temperature of 60 degrees Celsius, and then the feed to the column was switched to the combined 60°C SAC effluent. Then, under reflux, the column was heated at 200 degrees Celsius for 2 hours, and two effluent fractions—one from the combined 90°C effluent treatment (#14) and one from the combined 60°C effluent treatment (#34)—were evaluated for their thermal stability by measuring the APHA color value exhibited by each. The results are shown in Table 2 below.

[0038] [Table 2]

[0039] Example 3 The feed material for this Example 3 was refined lactic acid produced from different fermentations having an initial APHA color value of 162.

[0040] The strong acid cation exchange column and carbon column were set up sequentially again, as in Examples 1 and 2, and the same procedure as in Examples 1 and 2 was followed, except that a different strong acid resin was used (Mitsubishi DIAION® PK216 sulfonic acid-functionalized porous styrene-divinylbenzene matrix strong acid cation exchange resin, sodium type). The same carbon used as in Examples 1 and 2 was used. Both columns were run at 60°C. A specific effluent sample was evaluated for its thermal stability in the same manner as in Examples 1 and 2.

[0041] The results are shown in Table 3 below.

[0042] [Table 3]

[0043] Example 4 The feed material for this Example 4 was refined lactic acid produced from yet another fermentation batch having an initial APHA color value of 204.

[0044] Using the same strong acid resin and carbon as used in Example 3, the cation exchange column and carbon column were set up sequentially in the same manner as in the previous example, and the columns were run in the same configuration as in Example 3. Both columns were run again at 60 degrees Celsius, however, a mild heat treatment was also performed to remove the color of the lactic acid composition and improve its thermal stability. The APHA color and the relative contributions of the strong acid cation exchange column and activated carbon column were evaluated. The results are shown in Table 4 below.

[0045] [Table 4]

[0046] Example 5 The feed material in this example was refined lactic acid produced from different fermentation batches having an initial APHA color value of 339.

[0047] The strong acid cation exchange column and carbon column were set up and operated in the same manner as in Examples 3 and 4, and the operation was repeated at 60 degrees Celsius using the same resin and carbon. The results for the treatment of this particular aqueous lactic acid composition are reported in Table 5 below.

[0048] [Table 5]

[0049] Example 5 Multiple SAC resins, activated carbon samples, and non-functional polymeric adsorbent resins were screened for comparison of their ability to remove color from different refined lactic acid compositions from two or three different fermentation broths. IX columns and carbon columns were set up and conditioned for evaluation with these different adsorbents and different lactic acid compositions, and operated at 60 degrees Celsius, the same as in the previous examples. The polymeric adsorbents were set up and conditioned in the same way as the carbon columns. Several aqueous lactic acid compositions produced from fermentation broths for general comparison of the various adsorbent materials all had an initial APHA color of 99–152, as specifically shown for each set of experiments (each table corresponds to the experiment run with the aqueous lactic acid composition and specific adsorbent having the specified initial APHA color value).

[0050] [Table 6]

[0051] [Table 7]

[0052] [Table 8]

[0053] [Table 9]

[0054] [Table 10]

[0055] [Table 11]

[0056] [Table 12]

[0057] Example 6 In this embodiment, non-functionalized adsorbent resins were evaluated in relation to further exposure to adsorbents following the use of SAC resin.

[0058] In the evaluation of various non-functional resins, 25 mL of a specifically indicated non-functional adsorbent resin and a previously SAC-treated (SK216, 60C) lactic product mixture were mixed in a 100 mL beaker at 60°C for 2 hours using a stirring bar and internal temperature monitoring. Subsequently, the lactic was removed from the resin by vacuum filtration using a 0.45 μm cellulose nitrate (CN) analysis vacuum filter unit.

[0059] All lactic feedstocks used in these trials initially had an iodine value of 5.6 (APHA / Hazen 638) and a heated iodine value of 50.1 (outside the APHA / Hazen range).

[0060] The non-functional resins evaluated were characterized as shown in Table 13.

[0061] [Table 13]

[0062] Based on the observed results reported in Table 14 below, the findings suggest that, in most cases, styrene polymer matrices with characteristics such as high surface area and / or large pore size, as well as pore volume of 1.10–1.5 mL / g, are desirable because their natural color and heating color are significantly reduced.

[0063] [Table 14]

[0064] Example 7 The non-functional adsorbent resin treatment step demonstrated the feasibility of color removal in lactic feedstock, and added value was found in the tests regardless of whether the resin is recyclable or not. If it is found to be recyclable, significant cost savings will be observed.

[0065] In this test, as in Example 6, the lactic product was first treated with SAC using SK216 ion exchange resin at 60°C. The feed was then pumped through a series of two Ace Glass #15 jacketed chromatography columns sequentially connected to a 60°C hot oil recirculation heater. The first column contained glass beads to act as a preheater, while the second column was packed with either Mitsubishi Sepabeads SP700 non-functional adsorbent resin (in one example) or Dowex Optipore L493 non-functional adsorbent resin (in another example). The material was supplied to the system at a rate of 1 bed volume (BV) / hour and collected using an autosampler set to collect approximately 6.8 mL of processed material over 26 minutes (the lactic feed pump was set to 0.26 mL / min).

[0066] After observing a stable state of the Hazen / APHA color, the test was stopped and the resin was treated with low-temperature steam at approximately 100°C under atmospheric pressure for 60 minutes. After this time, the resin was placed in a vacuum drying oven and dried overnight at 60°C. The following day, it was reloaded into a chromatography column and the test was repeated to compare the performance of the steam-cleaned resin with that of the initial trial.

[0067] A comparison of the color performance before and after regeneration revealed that both resins exhibited functionally similar color breakthrough with respect to floor volume, suggesting the feasibility of regenerating these non-functional adsorbent resins through mild steam cleaning.

[0068] Example 8 In this example, various pairings of non-functional adsorbent resins were evaluated by subsequent carbon treatment. In all cases, the aqueous lactic acid composition was treated with SAC, then treated in exactly the same manner as described in Example 6, and subsequently treated with the carbon adsorbent in a 100 mL beaker at 60°C for a further 2 hours using a stirring bar and internal temperature monitoring. Lactic acid was again removed from the carbon under evaluation by vacuum filtration using a 0.45 μm cellulose nitrate (CN) analysis vacuum filter unit, and any reduction in color was noted as reported in Table 15 below.

[0069] [Table 15]

[0070] The non-functional resins and carbons evaluated as reported in Table 15 are more fully characterized in Table 16 below.

[0071] [Table 16]

[0072] Example 9 In this example, SAC-treated lactic acid was treated in a two-column configuration using two Ace Glass #15 jacketed chromatography columns sequentially connected to a 60°C hot oil recirculation heater. The first column was packed with glass beads and acted as a preheater for the SAC-treated feed, while the second column contained Norit ROX 0.8 activated carbon. The material was supplied to the system at a rate of 1 bed volume / hour and collected using an autosampler set to collect approximately 6.8 mL of treated material over 26 minutes (the lactic feed pump was set to 0.26 mL / min). The SAC-treated feed entering the carbon bed had an untreated Hazen color value of 242 and a heated Hazen value of 939. Gradual stepwise changes were observed in the Hazen values ​​after carbon treatment, but no breakthrough was observed after treatment with 108 bed volumes (Hazen values ​​below 50 throughout), while the heated color broke through at approximately 66 bed volumes.

[0073] Initial attempts to regenerate spent carbon using high-temperature steam yielded unsatisfactory results, resulting in some removal of adsorbed impurities but reduced surface oxygen functionality. In the modified regeneration protocol, 20 mL of carbon was rinsed with water, then transferred to a beaker containing a stirring bar and 100 mL of 3 M sodium hydroxide solution. The combination was then heated to 80 degrees Celsius while gently agitating, held there for 1 hour, and then evaluated. After that time, the base wash was removed by vacuum filtration, and the carbon was reloaded into the chromatography column for evaluation. After flushing with deionized water until the effluent reached a pH of 5–7 (approximately 20 bed volumes of deionized water were required), we found that the thus regenerated carbon regained approximately 96% of its original color removal capacity.

Claims

1. A process for improving the thermal stability of an aqueous lactic acid composition, further comprising a heat treatment step, and further comprising contacting the aqueous lactic acid composition with at least one adsorbent selected from the group consisting of activated carbon, polymeric adsorbents, ion exchange resins, and bleaching clay.

2. The process according to claim 1, wherein at least one of the adsorbents comprises a strong acid cation exchange resin.

3. The process according to claim 1 or 2, wherein the heat treatment step and the step of contacting with at least one adsorbent are performed in parallel.

4. The process according to any one of claims 1 to 3, wherein the step of contacting the at least one adsorbent comprises contacting the aqueous lactic acid composition with a strong acid cation exchange resin, and subsequently with one or more adsorbents selected from the group consisting of non-functional polymeric adsorbents, activated carbon, and other strong acid cation exchange resins, either alone or in any order.

5. The process according to any one of claims 1 to 4, wherein the aqueous lactic acid composition comprises at least 50 percent by total weight of l-lactic acid and d-lactic acid enantiomers.

6. The process according to any one of claims 1 to 5, wherein the aqueous lactic acid composition comprises at least 75 percent by total weight of l-lactic acid and d-lactic acid enantiomers.

7. The process according to any one of claims 1 to 6, wherein the aqueous lactic acid composition comprises at least 90 percent by total weight of l-lactic acid and d-lactic acid enantiomers.

8. The process according to any one of claims 1 to 7, wherein one of the l-lactic acid and the d-lactic acid enantiomer is present in the composition at a concentration of less than 1.0 weight percent of the composition both initially and after the heat treatment step.

9. The process according to any one of claims 1 to 8, wherein one of the l-lactic acid and the d-lactic acid enantiomer is present in the composition in an amount of less than 0.8 weight percent both initially and after the heat treatment step.

10. The process according to any one of claims 1 to 9, wherein one of the l-lactic acid and the d-lactic acid enantiomer is present in the composition in an amount of less than 0.5 weight percent both initially and after the heat treatment step.

11. The process according to any one of claims 1 to 10, wherein the heat treatment step includes exposure of the aqueous lactic acid composition to one or more temperatures below 120 degrees Celsius for a total of one hour or less.

12. The process according to any one of claims 1 to 11, wherein the heat treatment step includes exposure of the aqueous lactic acid composition to one or more temperatures below 100 degrees Celsius for a total of 60 minutes or less.

13. The process according to any one of claims 1 to 12, wherein the heat treatment step includes exposure of the aqueous lactic acid composition to one or more temperatures below 90 degrees Celsius for a total of 60 minutes or less.

14. The process according to any one of claims 1, 2, and 4-13, wherein the heat treatment step and the step of contacting the aqueous lactic acid composition with at least one adsorbent are carried out substantially in parallel by heating the aqueous lactic acid composition to contact the at least one adsorbent, and then contacting the aqueous lactic acid composition with the at least one adsorbent under rising temperatures.

15. The process according to any one of claims 1 to 14, wherein the at least one adsorbent is contained in a heated column or other vessel, and the manner in which the aqueous lactic acid composition is brought into contact with the at least one adsorbent includes passing the aqueous lactic acid composition from the inlet to the outlet of the column or vessel, and further, the duration of the heat treatment is substantially the length of time required to allow the aqueous lactic acid composition to flow through from the inlet to the outlet of the column or other vessel.

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