Process for improving heat stability of aqueous lactic acid solutions of a certain enantiomeric purity

Abstract

A process is disclosed for improving the heat stability of an aqueous lactic acid composition, 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 carbons, polymeric adsorbents, ion exchange resins and bleaching clays.

Description

[0001] PROCESS FOR IMPROVING HEAT STABILITY OF AQUEOUS LACTIC ACID SOLUTIONS OF A CERTAIN ENANTIOMERIC PURITY

[0002] TECHNICAL FIELD

[0003] The present invention relates generally to methods for making enantiomerically pure lactic acid monomer compositions for the production of either poly-L-lactic acid (or poly-L-lactide, hereafter simply PLLA) or poly-D-lactic acid (or poly-D-lactide, hereafter PDLA), and more particularly relates to methods for improving the heat stability of such monomer compositions.

[0004] BACKGROUND OF THE INVENTION

[0005] PLLA and PDLA are biodegradable, semicrystalline to crystalline polymers prepared ultimately either through ring-opening polymerization of the lactide dimer of the corresponding enantiomer, namely, Llactic acid and d-lactic acid, respectively, or through condensation of the enantiomer with the elimination of water. Both polymers are of commercial interest, and because they derive from stereoisomers have generally similar physical and mechanical properties albeit with some differences that are not material to the issues with which the present invention is concerned. Blends and copolymers of PLLA and PDLA have also attracted commercial interest.

[0006] Manufacturers of both PLLA and PDLA typically seek in any event to limit the extent to which the other enantiomer participates in the formation of the desired polymer from a selected enantiomer. Thus, manufacturers of PLLA will typically wish to limit the extent to which d-lactic acid is involved in the making of PLLA, and manufacturers of PDLA will typically wish to limit the extent to which Llactic acid is present and involved in the making of PDLA. The degrees to which the other, nonselected enantiomer may be tolerated will understandably vary according to the particular polymerization method used and / or according to the desired properties of the PLLA or PDLA polymer product sought to be made for a particular application or end use, but in general, the attainment of lactic acid monomer compositions (and parenthetically it is noted here that “lactic acid monomer compositions” and “enantiomerically pure lactic acid monomer compositions” shall be used herein for referring to the improved heat stability aqueous lactic acid compositions prepared by the process of the present invention, whether these compositions are subsequently used in a polycondensation process or for the formation of the corresponding lactide(s) for a subsequent ring opening polymerization method) with a certain degree of enantiomeric purity is one need that all manufacturers of PLLA and PDLA polymers have.

[0007] At the same time, for many applications and end uses to which the desired PLLA and PDLA polymers are to be put, the avoidance of color and / or the avoidance of a propensity to develop color in use and over time is independently a need that these manufacturers have.

[0008] Unfortunately, as related for example in US Patent No. 5,488,156 to Kulprathipanja et al. (the “ ‘156 Patent”), lactic acid formed via fermentation generally is susceptible to the development of color bodies when heated to temperatures of approximately 180 degrees Celsius and above. The inventors in the ‘156 Patent note the use of lactic acid as a monomer in the preparation of poly(lactic acid) (presumably here referring to a polymer of a racemic mixture of both d- and 1-enantiomers (hereafter, PDLLA), in the absence of any differentiation or specific mention of the stereochemistry (or stereochemistries) of the lactic acid in question) and copolymers with glycolic acid and methylglycolic acid, but note that the susceptibility of lactic acid to form color bodies is problematic for a number of large-scale commercial applications for which PDLLA (or for that matter, PLLA or PDLA) might otherwise be well-suited, as for example, containers for the fast food industry and consumer packaging generally, since polymers are prepared at high temperatures wherein color body formation is a concern.

[0009] What the inventors of the ‘156 Patent observed in regard to the problem was that lactic acid invariably developed color when heated under the prescribed conditions for determining heat stability, namely, heating at 180 degrees Celsius for 3 hours, regardless of the lactic acid’s purity or the methods used to purify the lactic acid in question.

[0010] The inventors of the ‘ 156 Patent hypothesized based on this observation that the color development that was observed was in all instances associated with trace amounts of carbohydrates in the lactic acid that would undergo carbonization under polymerization conditions, so that a heat stable lactic acid could be prepared by subjecting an aqueous solution of lactic acid to a sufficient heat pretreatment to form the color bodies via the carbonization of the residual carbohydrates and then removing the color bodies prior to supplying the thus-treated aqueous lactic acid monomer composition to a polymerization method to be made, for instance, into poly(lactic acid). The ‘156 Patent more specifically prescribes heating a lactic acid solution containing between 8 and 20 weight percent lactic acid to a temperature between 180 and 230 degrees Celsius for a time sufficient to carbonize the residual carbohydrates dissolved in the lactic acid solution, which the ‘156 Patent indicates naturally depends on the temperature employed (higher temperatures in the range corresponding to a shorter needed treatment time and vice-versa) but as generally in the range of from 0.5 hrs to 6 hours.

[0011] The methods suggested for the decarbonization of the thus heat-treated aqueous lactic acid monomer composition are not described at any length, but the Examples show a filtered 15 percent lactic acid fermentation broth having an initial APHA (or Pt / Co or Hazen) color per ASTM DI 209 of 93 passed through a mixed bed of anion and cation exchange resins, then a cation exchange resin (hydrogen form) and an activated carbon column when concentrated to 88 weight percent and heated at 180 degrees Celsius for 3 hours was not heat stable, whereas a 15 percent lactic acid fermentation broth heat treated at 230 degrees Celsius for 4 hours, when filtered and passed through a mixed bed of anion and cation exchange resins, a cation exchange resin (hydrogen form), an anion exchange resin (hydroxy form) and activated carbon provided a reduced color product having an APHA color of 40. This product in turn, when concentrated to 88 wt percent and evaluated as before turned a “light yellow color” - which the inventors attributed to the majority of carbohydrate impurities being carbonized and removed.

[0012] In a final example, the “light yellow color” product of the second example - heat treated a first time, then having passed through the referenced sequence of adsorbents, concentrated and evaluated after heating at 180 degrees Celsius for 3 hours - was subjected to a further heat treatment at 200 degrees Celsius for 4 hours, diluted back down to a 20 weight percent solution, filtered and passed through another activated carbon column before being concentrated back up to 88 weight percent (showing an APHA color of 48) and heated again to 180 degrees Celsius for 3 hours. In this iteration, the inventors subjectively noticed no color formation and presumably considered the resultant aqueous lactic acid monomer composition as finally being heat stable.

[0013] Unfortunately, we have concluded based on our experimental efforts that the range of color formers and precursors confronting a manufacturer of either PLLA or PDLA extends beyond just the residual carbohydrates that the inventors of the ‘ 156 Patent concerned themselves with in relation to the heat stability of aqueous lactic acid monomer compositions (or the lack thereof, more precisely).

[0014] And unfortunately, we have further found that just the sort of heat treatments prescribed by the inventors of the ‘156 Patent as sufficient for solving the problem of the lack of heat stability of these monomer compositions across the full range of lactic acids obtained and purified (or not) in any manner result in aqueous lactic acid monomer compositions, and especially (at least intended-to-be) enantiomerically pure lactic acid monomer compositions for making PLLA or PDLA, that are simultaneously less suited for meeting the first need mentioned above - for aqueous lactic acid monomer compositions having at least a certain degree of enantiomeric purity in a selected enantiomer (which the preceding fermentation and purification steps have already been designed at great expense and with great care to provide).

[0015] SUMMARY OF THE INVENTION

[0016] This section provides a general summary of the present invention, and is not a comprehensive disclosure of the full scope of all of its features.

[0017] The present invention in a first aspect broadly concerns a process for providing an aqueous lactic acid monomer composition with improved heat stability, as for subsequent use in a method for making a polymer therefrom.

[0018] The present invention in a more particular aspect concerns a process for providing an aqueous lactic acid monomer composition with improved heat stability and concurrently at least a certain desired degree of enantiomeric purity in an 1-lactic acid enantiomer or a d-lactic acid enantiomer.

[0019] The present invention in yet another aspect concerns a process for providing an aqueous lactic acid monomer composition with improved heat stability by treatment or conditioning means which are not accompanied by, or do not occasion, a degree of isomerization of a selected enantiomer away from that enantiomeric purity required by a subsequent polymerization method in respect of the selected enantiomer, using the treated or conditioned aqueous lactic acid monomer composition for making a PLLA or PDLA homopolymer or a copolymer of the selected enantiomer or a lactide thereof with one or more other co-monomers.

[0020] DESCRIPTION OF EMBODIMENTS

[0021] The embodiments herein and the various features and advantageous details thereof are intended to be illustrative of the present invention in these various aspects or from these various perspectives, and should not be taken as impliedly limiting of the present invention but simply as demonstrative of various ways and options for how the principles and understandings behind the present invention may be applied in carrying out the invention. Thus, unless otherwise indicated, any definitions or embodiments described in this or in other sections are intended to be applicable to all embodiments and aspects of the subjects herein described for which they would be suitable according to the understanding of a person of ordinary skill in the art.

[0022] As used in this application, the singular forms “a”, “an” and “the” include plural references unless the context clearly indicates otherwise. The term “comprising” and its derivatives, as used herein, are similarly intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and / or steps, but do not exclude the presence of other unstated 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 that specify the presence of the stated features, elements, components, groups, integers, and / or steps, but exclude the presence of other unstated features, elements, components, groups, integers, and / or steps. The term “consisting essentially of’, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and / or steps, as well as those that do not materially affect the basic and novel characteristic(s) of stated features, elements, components, groups, integers, and / or steps.

[0023] As mentioned above, the present invention in its preferred embodiments is particularly concerned with providing an aqueous lactic acid monomer composition with improved heat stability, but also concurrently at least a certain desired degree of enantiomeric purity in an 1-lactic acid enantiomer or a d-lactic acid enantiomer. In this regard, where the lactic acid monomer composition is intended for producing PLLA, the presence of, and even more detrimentally the generation of d-lactic acid enantiomer through isomerization of 1-lactic acid at elevated temperatures, represents a yield loss and separation expense to the manufacturer - and the same is true in regard to 1-lactic acid where the objective is to produce PDLA. Consequently, while the various polymerization methods known for producing PLLA or PDLA, as the case may be, can be expected to accommodate the presence of some of the non-selected enantiomer (which permissible amounts may vary according to the particular polymerization method used), excessive amounts of the non-selected enantiomer are to be avoided. Lactic acid is most commonly commercially produced by fermentation from various carbohydrate sources, though conventionally catalyzed synthetic methods are known from other starting materials. In preferred embodiments of the present invention, then, the aqueous lactic acid monomer compositions of greatest interest will have been produced by fermentation.

[0024] In this context, the production of lactic acid by fermentation using genetically engineered yeasts has been a field of endeavor for a number of years as a sustainable method of producing a product useful in materials as diverse as foods and polymers. Candidate host yeast strains for the production of lactic acid include Saccharomyces cerevisiae, Schizosaccharomyces pombe, and various species from the genera Kluyveromyces, Pichia, Candida and Hansenula, and there are numerous publications describing various manipulations of these candidate host yeast strains - most for the purpose of more selectively producing the Llactic acid enantiomer. It is with the lactic acid compositions produced by fermentations directed to the more selective production of l-lactic acid that the process of the present invention has been primarily developed, but having said this, the inventors have no reason to believe that the solution they have developed - for improving the heat stability of an aqueous lactic acid monomer composition while respecting the needed enantiomeric purities of an associated downstream polymerization method - would not also be useful for aqueous lactic acid compositions which have been prepared (by fermentative and other means known to those of skill in the manufacture of such lactic acid compositions) to make available more of the d-lactic acid enantiomer in view of an intended use for making PDLA, for example.

[0025] Still more particularly, in more preferred embodiments, the present invention is concerned with improving the heat stability of lactic acid compositions produced by a low pH tolerant microorganism and associated fermentation method. Lactic acid compositions such as produced by fermentation methods and engineered microorganisms such as described in WO 2012 / 114979 and WO 2011 / 021629 both to Asahi Glass Co. Ltd. (based on Schizosaccharomyces pombe), or especially as described in commonly-assigned WO 2023 / 004336 (published Jan. 26, 2023) and International Application Numbers PCT / US2023 / 060789 (filed Jan. 17, 2023 and claiming priority from the application published as WO 2023 / 004336) and PCT / US2023 / 061683 (filed Jan. 31, 2023 and also claiming priority from the same application published as WO 2023 / 004336) are of most interest. As earlier related, we have concluded even as to distilled lactic acid products from fermentations according to the referenced commonly-assigned applications that these aqueous lactic acid compositions contain other species deleterious to the heat stability of the compositions and thus their commercial readiness for making polymers therefrom, in particular, PLLA, than appreciated by the inventors of the ‘156 Patent.

[0026] The process of the present invention provides a way to effectively address these other problematic species, as demonstrated by the examples which follow. Fundamentally, this is accomplished by means of the application of heat for a time to the aqueous lactic acid composition in question and contacting the aqueous lactic acid composition with one or more adsorbents as described below, in any combination and in any sequence (that is, all are effective for removing problematic species, and there does not appear to be any particular criticality to the order in which these are used if more than one adsorbent is used in combination with another (which may be but need not necessarily be from a different category of the adsorbents listed hereafter, or may simply be a different adsorbent from the same shared, general category of adsorbent materials)).

[0027] The heat treatment step is carried out in such a way, considering the amount of heat applied and the length of time during which an aqueous lactic acid composition containing not more than a targeted maximum proportion of an 1-lactic acid or d-lactic acid enantiomer is exposed to elevated temperatures by means of such heating, that the targeted maximum proportion of the 1-lactic acid or d-lactic acid enantiomer, as the case may be, is not caused to be exceeded by the heat treatment that is applied (for example, but without limitation thereto, by a heat-catalyzed isomerization of the preferred and desired enantiomer to the other enantiomer). Preferably, however, the amount of heat and length of time during which an aqueous lactic acid composition is exposed to elevated temperatures by means of such heating are such that the heat-treated aqueous lactic acid composition comprises less than 1.0 weight percent, preferably less than 0.8 weight percent and still more preferably less than 0.5 weight percent of the nonpreferred 1-lactic acid or d-lactic acid enantiomer.

[0028] The range of adsorbents that can be used for contacting the aqueous lactic acid compositions in question is extensive, and includes one or more of the activated carbons, polymeric adsorbents, ion exchange resins and bleaching clays. In particular, treatment of a heated aqueous lactic acid composition by exposure to a strong acid cation exchange resin, followed by one or more adsorbents selected from the nonfunctional polymeric adsorbents, activated carbons and other strong acid cation exchange resins in isolation, in any combination and in any sequence, is contemplated. The manners in which the adsorbents are brought into contact with the aqueous lactic acid compositions (or the aqueous lactic acid compositions brought into contact with the adsorbents) are not considered critical, and those of skill in the art and accustomed to the use of various adsorbents should be well able to design and implement suitable means for efficiently contacting the aqueous lactic acid compositions within the context of the inventive process and then providing a treated, more heat stable aqueous lactic acid composition to an associated polymerization method for further use.

[0029] It should be observed in view of the foregoing that the heat treatment step and the contacting with one or more adsorbents step (broadly speaking) are preferably conducted sequentially, with a heat treatment preferably preceding the contacting with adsorbents step. The heat treatment and contacting with one or more adsorbents steps may also be conducted concurrently or substantially concurrently, for example, by heating the aqueous lactic acid composition for contacting the at least one adsorbent and then carrying out the contacting of the aqueous lactic acid composition with the at least one adsorbent at an elevated temperature (substantially concurrently, in that at least some heating occurs of the aqueous lactic acid composition in advance of any exposure to an 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 materials (concurrently) - the essential aspects for purposes of the present invention, however, being that there is both an exposure of the aqueous lactic acid composition to an elevated temperature and an exposure to one or more of the adsorbents described herein.

[0030] In one “concurrent mode” embodiment, wherein the at least one adsorbent is contained in a heated column or other vessel and the manner of the aqueous lactic acid composition’ s contacting the at least one adsorbent involves causing the aqueous lactic acid composition to pass through the column or vessel from an inlet to an outlet thereof, the corresponding manner of carrying out the heat treatment thus merely involves causing the aqueous lactic acid composition to flow through the column or other vessel from an inlet to an outlet thereof and the duration of the heat treatment is substantially only the time required for the lactic acid composition to transit through the column or vessel from its inlet to its outlet. Those of skill in the art will recognize, of course, that the temperatures and times under heating or at elevated temperatures to which a given aqueous lactic acid composition can be exposed, and the particular adsorbents or sequence of adsorbents with which a given aqueous lactic acid composition will need to be contacted to improve the heat stability of the composition to a desired degree, will vary from one lactic acid composition to the next as different lactic acid compositions from different sources - different fermentations and different purification methods - can very reasonably be expected to have both different quantities of the 1-lactic acid and d-lactic acid enantiomers in relation to the targeted maximum proportion of one such enantiomer that an associated polymerization method can accept as well as different color formers and / or different distributions of those color formers that must be considered.

[0031] In general, though, it is considered that given the examples that follow and the guidance provided by the instant application, it will be well within the skill of those of ordinary skill in the art to apply the principles behind the present invention for improving the heat stability of any given aqueous lactic acid composition while maintaining the proportion of d-lactic acid enantiomer below a targeted maximum in relation to the preferred 1-lactic acid enantiomer (as for use in an associated polymerization method for making PLLA) or the proportion of 1-lactic acid enantiomer below a targeted maximum in relation to the preferred d-lactic acid enantiomer (as for use in an associated polymerization method for making PDLA).

[0032] EXAMPLES

[0033] Examples 1 and 2

[0034] The feed material for these two examples was an aqueous refined lactic acid composition from a Schizosaccaromyces pombe fermentation, having an initial APHA color value of 91 as measured according to the current ASTM D1209 standard method.

[0035] 30 mL of a Dowex™ Monosphere™ 99 sulfonate functionalized, styrene divinylbenzene gel matrix strong acid cation exchange resin was loaded into a jacketed Ace Glass #11 chromatography column. The resin was conditioned with 5 bed volumes (BV) of 5% HC1 followed by 5 BV of deionized water.

[0036] After conditioning, the column was heated to 90°C through the column jacket. Feed was introduced at a rate of ImL / minute. A fraction collector was set up to collect the effluent in 6 mL fractions. Select fractions were collected and analyzed for their

[0037] APHA color. The results are summarized below in Table 1.

[0038] Table 1

[0039] The effluent fractions after #10 were then combined for Example 1 to be used as the feed for a carbon treatment also conducted at 90 degrees Celsius, with the combined effluents being collectively referred to hereafter as the. 90°C SAC effluents. When combined, the color of the 90°C SAC effluents was measured at an APHA value of 30.

[0040] For Example 2, the same experiment was conducted with a second portion of the same aqueous lactic acid composition, except that the temperature employed throughout was 60°C rather than 90°C. The measured color of the 60°C effluents prior to the 60°C carbon treatment was measured at an APHA value of 25.

[0041] For the 90°C and 60°C carbon treatments of the combined 90°C effluents in Example 1 and of the combined 60°C effluents for Example 2, 30 mL of Calgon CPG LF 12X40 acid washed granular activated carbon with a low acid soluble iron content (Calgon Carbon Corporation) was loaded into a 250 mL beaker. The carbon was rinsed with DI water at a rate that removed the fines hut allowed the granules to remain in the beaker. The rinsed carbon was loaded into a jacketed Ace Glass #11 chromatography column. The carbon was rinsed with an additional 5 BV of deionized water.

[0042] The column was heated first to 90 degrees Celsius through its jacket. The combined 90°C effluents were then fed through the carbon column at a rate of 1 mL / minute and collected in 6 mL fractions, such that the combined residence time of the aqueous lactic acid composition through the SAC and activated carbon columns at 90 degrees Celsius would be superficially approximately 60 minutes, and accounting for the resin volume would be in the range of 30 to 60 minutes as a true residence time. The first 15 carbon effluents reported in Table 2 below were from contacting the combined 90°C effluents. The heat to the carbon column was then reduced to provide a column temperature of 60 degrees Celsius, and the feed to the column was then switched to the combined 60°C SAC effluents. Two of the effluent fractions, one from processing of the combined 90°C effluents (#14) and one from the processing of the combined 60°C effluents (#34) were evaluated then for their heat stability, by heating at 200 degrees Celsius for 2 hours under reflux and then measuring the APHA color value exhibited by each. The results are in Table 2 below.

[0043] Table 2

[0044] Example 3

[0045] The feed material for this Example 3 was refined lactic acid produced from a different fermentation, having an initial APHA color value of 162. Strong acid cation exchange and carbon columns were again set up in series as in Examples 1 and 2, and the same procedures were followed as in Examples 1 and 2, with the only difference being that a different strong acid resin was used (Mitsubishi DIAION™ PK216 sulfonic acid functionalized porous styrene divinylbenzene matrix strong acid cation exchange resin, sodium form). The same carbon was used as was used for Examples 1 and 2. Both columns were run at 60°C. Certain effluent samples were evaluated for their heat stability in the same manner as in Examples 1 and 2.

[0046] The results are in Table 3 below.

[0047] Table 3

[0048] Example 4

[0049] The feed material for this Example 4 was refined lactic acid produced from yet a different fermentation batch, having an initial APHA color value of 204. The cation exchange and carbon columns were set up in series as in prior Examples and were configured and run as in Example 3, using the same strong acid resin and same carbon as used in Example 3 and again running both columns at 60 degrees Celsius, but with evaluating the APHA colors and the relative contributions of the strong acid cation exchange column and the activated carbon column in conjunction with the mild heat treatment toward the removal of color and improvement of the heat stability of the lactic acid composition. The results are in Table 4 below.

[0050] Table 4

[0051] Example 5

[0052] The feed material for this Example was a refined lactic acid produced from a different fermentation batch with an initial APHA color value of 339.

[0053] The strong acid cation exchange and carbon columns were set up and operated in the same manner as in Examples 3 and 4, using the same resin and carbon and operating again at 60 degrees Celsius. The results with respect to the processing of this particular aqueous lactic acid composition are reported in Table 5 below.

[0054] Table 5

[0055] Example 5

[0056] Multiple SAC resins, activated carbon samples and non-functional polymeric adsorbent resins were screened for their comparative ability to remove color from different refined lactic acid compositions from a couple of different fermentation broths. The IX and carbon columns for the evaluations with these different adsorbents and different lactic acid compositions were set up, conditioned and operated the same as in previous examples, at 60 degrees Celsius. The polymeric adsorbents were set up and conditioned the same as the carbon columns. The several aqueous lactic acid compositions produced from the fermentation broths for generally comparing the various adsorbent materials were all initially between 99-152 for APHA color, as specified for each set of experiments (each Table corresponds to an experiment run with an aqueous lactic acid composition having the indicated initial APHA color value and with a particular adsorbent).

[0057] Table 6 Table 7 Table 8

[0058] Table 9

[0059] Table 10

[0060] Table 11

[0061] Example 6 In this example, nonfunctional adsorbent resins were evaluated for exposure to a further adsorbent following use of a SAC resin.

[0062] In the evaluations of various nonfunctional resins, 25mL of a specified nonfunctional adsorbent resin was admixed with previously S AC-treated (SK216, 60C) lactic product mixtures at 60°C for two hours in a lOOmL beaker with a stir bar and internal temperature monitoring. Lactic was subsequently removed from the resin by vacuum filtration using a 0.45 pm cellulose nitrate (CN) analytical vacuum filter unit.

[0063] All lactic feedstock used in these trials had as starting iodine value of 5.6 (APHA / Hazen 638), and a heated iodine value of 50.1 (APHA / Hazen out of range).

[0064] The nonfunctional resins evaluated were characterized as follows in Table 13: Table 13

[0065] Based on the results observed and reported in Table 14 following, the findings indicate that in most cases, a styrenic polymer matrix with a feature such as high surface area and / or large pore size is desirable for large reductions in as-is and heated color, as well as a pore volume of 1.10 - 1.5 mL / g. Table 14 Example 7

[0066] With the viability of a nonfunctional adsorbent resin treatment step demonstrated for color removal in a lactic feedstock, additional value was found in testing whether or not the resin could be regenerated - if found to be capable of regeneration, there would be a significant cost savings.

[0067] For this test, lactic product was first SAC treated using SK216 ion exchange resin at 60°C, as was the case in Example 6. Feed was next pumped through a series of two Ace Glass #15 jacketed chromatography columns connected to a 60°C hot-oil recirculating heater in series. The first column contained glass beads and served as a pre-heater; the second column was filled in one instance with Mitsubishi Sepabeads SP700 nonfunctional adsorbent resin and in another instance with Dowex Optipore L493 nonfunctional adsorbent resin. Material was fed into the system as a rate of 1 bed volume (BV) per hour and collected using an autosampler set to collect ~ 6.8 mL of treated material over the span of 26 minutes (lactic feed pump set at 0.26 mL / min).

[0068] After the Hazen / APHA color was observed to level off, the test was stopped and the resin was treated with low temperature steam at ~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 next day, it was reloaded in the chromatography column and the test was repeated to compare how the steam washed resin performed with the initial trial.

[0069] Comparing the color performance of both resins pre- and post-regeneration, breakthrough of color was observed to be functionally similar with regard to bed volumes on both resins, indicating the viability of regeneration of these nonfunctional adsorbent resins by a mild steam wash.

[0070] Example 8

[0071] In this example, various pairings of nonfunctional adsorbent resins were evaluated with a subsequent carbon treatment. In all instances, the aqueous lactic acid compositions had been SAC treated and were then processed identically as described in Example 6, before being treated with a carbon adsorbent at 60°C for a further two hours in a lOOmL beaker with a stir bar and internal temperature monitoring. Lactic was again removed from the carbon under evaluation by vacuum filtration using a 0.45 pm cellulose nitrate (CN) analytical vacuum filter unit, and any reductions in color noted as reported below in Table 15: Table 15

[0072] The nonfunctional resins and carbons evaluated as reported in Table 15 are more completely characterized in Table 16 as follows:

[0073] Table 16

[0074] Example 9

[0075] For this example, SAC treated lactic acid was treated in a two column arrangement, with two Ace Glass #15 jacketed chromatography columns in series connected to a 60 degree Celsius hot oil recirculating heater. The first column was filled with glass beads and served as a preheater for the SAC treated feed, while the second column contained Norit ROX 0.8 activated carbon. Material was fed into the system at a rate of 1 bed volume per hour and collected using an autosampler set to collect 6.8 mL of treated material over the span of 26 minutes (lactic feed pump set at 0.26 mL / minute). The SAC treated feed entering the carbon bed had an as-is Hazen color value of 242, and a heated Hazen value of 939. While gradual, stepwise changes were observed in the Hazen value following treatment with the carbon, no breakthrough was observed after 108 bed volumes had been processed (Hazen values of 50 or less throughout), while heated color broke through around 66 bed volumes.

[0076] Regeneration of the spent carbon was initially attempted with high temperature steam with mixed results, resulting in some removal of adsorbed impurities but also reducing surface oxygen functionality. A revised regeneration protocol, wherein 20 mL of carbon was rinsed with water, then transferred to a beaker containing a stir bar and 100 mL of 3M sodium hydroxide solution which combination was then heated to 80 degrees Celsius with gentle agitation and held there for 1 hour, was then evaluated. After the hour, the base wash liquid was removed by vacuum filtration, and the carbon re-loaded into a chromatography column for evaluation. After flushing with deionized water until the effluent reached a pH of from 5 to 7 (about 20 bed volumes of deionized water being necessary), we found that the carbon thus regenerated returned to about 96% of its original color removal capacity.

Claims

CLAIMS:

1. A process for improving the heat stability of an aqueous lactic acid composition, 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 carbons, polymeric adsorbents, ion exchange resins and bleaching clays.

2. The process of Claim 1 , wherein at least one adsorbent includes a strong acid cation exchange resin.

3. The process of any one of Claims 1 or 2, wherein the heat treatment step and contacting with at least one adsorbent step are carried out concurrently.

4. The process of any of Claims 1-3, wherein the contacting with the at least one adsorbent step 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 nonfunctional polymeric adsorbents, activated carbons and other strong acid cation exchange resins whether in isolation or in combination in any sequence.

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

6. The process of any of Claims 1 -5, wherein the aqueous lactic acid composition comprises at least 75 percent by total weight of Llactic acid and d-lactic acid enantiomers.

7. The process of any of Claims 1-6, 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 of any of Claims 1-7, wherein one of the l-lactic acid and d-lactic acid enantiomers is present in the composition both initially and following the heat treatment step at less than 1 .0 weight percent of the composition.

9. The process of any of Claims 1-8, wherein one of the 1-lactic and d-lactic acid enantiomers is present in the composition both initially and following the heat treatment step at less than 0.8 weight percent.

10. The process of any of Claims 1-9, wherein one of the 1-lactic and d-lactic acid enantiomers is present in the composition both initially and following the heat treatment step at less than 0.5 weight percent.

11. The process of any of Claims 1-10, wherein the heat treatment step involves exposure of the aqueous lactic acid composition to a temperature or temperatures of less than 120 degrees Celsius for not more than an hour in total.

12. The process of any of Claims 1-1 1, wherein the heat treatment step involves exposure of the aqueous lactic acid composition to a temperature or temperatures of less than 100 degrees Celsius for not more than 60 minutes in total.

13. The process of any of Claims 1-12, wherein the heat treatment step involves exposure of the aqueous lactic acid composition to a temperature or temperatures of less than 90 degrees Celsius for not more than 60 minutes in total.

14. The process of any of Claims 1, 2 and 4 through 13, wherein the heat treatment step and step of contacting the aqueous lactic acid composition with at least one adsorbent are carried out substantially concurrently, by heating the aqueous lactic acid composition for contacting the at least one adsorbent and then carrying out the contacting of the aqueous lactic acid composition with the at least one adsorbent at an elevated temperature.

15. The process of any of Claims 1-14, wherein the at least one adsorbent is contained in a heated column or other vessel and the manner of the aqueous lactic acid composition’s contacting the at least one adsorbent involves causing the aqueous lactic acid composition to pass through the column or vessel from an inlet to an outlet thereof, and further wherein the duration of the heat treatment is substantially only the length of time required for the aqueous lacticacid composition to flow through the column or other vessel from an inlet to an outlet thereof.

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