Robbins, Lanny A. (4101 Old Pine Trail Midland, MI, 48642, US)
Ryan, Christopher M. (9186 Quinwood Lane North Maple Grove, MN, 55369, US)
|1.||A method of processing lactic acid; said process including a step of : (a) providing an aqueous solution of lactic acid; and (b) conducting a vapor phase transfer of an aqueous solution of lactic acid comprising at least 25 wt. % lactic acid and not more than 88 wt. % lactic acid under the following conditions: (i) a system pressure within the range of 30200 mm Hg; and, (ii) a vapor temperature of not more than 200 °C.|
|2.||The method of claim 1, wherein the system pressure is within the range of 4060 mm Hg and the vapor temperature is no more than 180°C.|
|3.||A method according to claim 1 wherein: (a) said vapor phase transfer comprises vaporizing at least 70% by wt. of the lactic acid in the aqueous solution.|
|4.||A method according to claim 1 wherein: (a) said step of conducting a vapor phase transfer comprises transferring a vapor phase comprising 6075 wt. % lactic acid.|
|5.||A method according to claim 4 wherein: (a) said step of conducting a vapor phase transfer comprises using a system pressure during the vapor phase transfer within the range of 4060 mm Hg.|
|6.||A method according to claim 5 wherein: (a) said step of conducting a vapor phase transfer comprises providing a vapor temperature within the range of 120 to 180°C.|
|7.||A method according to claim 1 wherein: (a) said step of conducting a vapor phase transfer comprises the steps of : (i) generating a two phase vapor/liquid mixture comprising no more than about 15 wt. % liquid phase; and, (ii) separating a vapor phase or the two phase mixture from liquid phase of the two phase mixture.|
|8.||A method according to claim 7 wherein: (a) said step of generating a two phase vapor/liquid mixture is conducted by directing the aqueous solution of lactic acid into a rising film evaporator system.|
|9.||A method according to claim 8 wherein: (a) said step of generating a two phase water/liquid mixture comprises removing from the rising film evaporator system a vapor/liquid mixture comprising 210 wt. % liquid phase.|
|10.||A method according to claim 8 wherein: (a) said step of conducting a vapor phase transfer is conducted under conditions such that a chiral purity of lactic acid in the vapor phase separated from the liquid phase in step 6 (ii) has decreased no more than 1 % when compared to a chiral purity of the solution directed into the rising film evaporator system in step 7 (a).|
|11.||A method according to claim 1 including a step of : (a) directing the vapor phase resulting from the vapor phase transfer into a polylactic acid prepolymer reactor system; and, (b) condensing at least 90 wt. % of lactic acid in the vapor phase, which is directed into the polylactic acid prepolymer reactor system, to form lactic acid oligomer within the polylactic acid prepolymer reactor system.|
|12.||A method according to claim 11 wherein: (a) said step of condensing at least 90 wt. % of an lactic acid component is conducted within a polylactic acid prepolymer reactor system maintained at a pressure within the range of 4060 mm Hg.|
|13.||A method according to claim 11 wherein: (a) said step of condensing at least 90 wt. % of an lactic acid component is conducted within a polylactic acid prepolmer reactor system operated with a water vapor temperature within the range of 3442°C.|
|14.||A method according to claim 11 wherein: (a) said step of condensing at least 90 wt. % of an lactic acid component is conducted within a polylactic acid prepolymer reactor system operated with a bottoms temperature within the range of 100 to 150°C.|
|15.||A method according to claim 1 wherein: (a) said step of providing an aqueous lactic acid solution includes: (i) a step extracting a solution of lactic acid with an amine solvent; and (ii) stripping the lactic acid from the amine solvent with an aqueous phase at a temperature of at least about 70°C.|
|16.||A system for processing an aqueous lactic acid solution; said system including: (a) an evaporator constructed and arranged for vaporizing at least 75% of an aqueous solution containing at least 65 wt. % lactic acid; (b) a vapor/liquid separator constructed and arranged to receive a vapor/liquid mixture from the evaporator and to separate the phases; (c) a prepolymer reactor constructed and arranged to receive a vapor phase from the vapor/liquid separator after separation; and, (d) wherein the evaporator, separator and prepolymer reactor define an integrated system constructed and arranged for operation at an internal system pressure within the range of 30200 mm Hg.|
|17.||A system according to claim 16 wherein: (a) wherein the internal system pressure is within a range of 4060 mm Hg.|
|18.||A system according to claim 16 wherein: (a) said integrated system is constructed and arranged for operation under an N2 atmosphere.|
|19.||A vapor stream comprising: (a) a mixture of lactic acid and water having an lactic acid presence between about 6075 % by wt; (i) the lactic acid having a chiral purity of at least 95%; (ii) the vapor stream having a pressure within the range of 4060 mm Hg.|
|20.||A method of processing lactic acid; said process including a step of : (a) providing an aqueous solution of lactic acid; (b) conducting a vapor phase transfer of an aqueous solution of lactic acid comprising at least 25 wt. % lactic acid and not more than 88 wt. % lactic acid under the following conditions: (i) a system pressure within the range of 4060 mm Hg; and, (ii) a vapor temperature of not more than 180 °C ; (c) directing the vapor phase resulting from the vapor phase transfer into a polylactic acid prepolymer reactor system; and, (d) condensing at least 90 wt. % of lactic acid in the vapor phase, which is directed into the polylactic acid prepolymer reactor system, to form lactic acid oligomer within the polylactic acid prepolymer reactor system.|
Field of the Disclosure The present disclosure relates to lactic acid processing. It particularly concerns methods for purifying lactic acid/water mixtures under conditions selected to facilitate downstream processing and to control racemization.
Background The potential of lactic acid as a commodity chemical, for example for use in the production of various industrial polymers, is known. Such use of lactic acid has become of particular interest, because polymers formed from lactic acid, i. e. polylactic acid and many of its products, are hydrolyzable, biodegradable and/or compostable. Additionally, lactic acid can be readily produced by fermentation; thus providing the possibility of commodity plastics formed using renewable carbon sources.
Production of lactic acid is the subject of U. S. patent applications Serial Nos. (U. S. S. N.): 08/949,420, entitled LOW pH LACTIC ACID FERMENTATION, filed October 15,1997; 08/950,289, entitled LACTIC ACID PROCESSING; METHODS; ARRANGEMENTS; AND, PRODUCTS, filed OCTOBER 14,1997; 09/132,720, entitled LACTIC ACID PROCESSING, METHODS, ARRANGEMENTS, AND PRODUCTS, filed August 12, 1998; and 09/412,085, entitled PROCESS FOR PRODUCING A PURIFIED LACTIC ACID SOLUTION, filed October 4,1999; the complete disclosure of these four applications being incorporated herein by reference. Production and isolation of polylactic acid, lactide and polylactide polymers are the subjects of U. S. patent Nos.
5,142,023; 5,274,073; and U. S. S. N. 09/044,701 (entitled CONTINUOUS PROCESS FOR THE MANUFACTURE OF LACTIDE AND LACTIDE POLYMERS, filed March 19,1998); the complete disclosures of these two patents and the identified application being incorporated herein by reference.
Lactic acid has a chiral center and is found in both the D-and L- forms. Management and control of the chiral purity of the lactic acid used to form the polymer can be important for industrial applications of the polymer, see for example U. S. patents: 5,142,023; 5,338,822; 5,484,881; and, 5,536,807, these four patents being incorporated herein by reference.
In general, although the formation of lactic acid during the fermentation process can be managed under conditions which allow for production with a relatively high degree (for example at least 90%, typically at least 98%) of chiral purity, downstream processing with many conventional techniques can lead to undesirable amounts of racemization.
Summarv A preferred method of processing lactic acid is provided. The method generally comprises conducting a vapor based transfer of an aqueous solution of lactic acid, typically under a system pressure within the range of 30-200 mm Hg, more preferably 40-120 mm Hg, most preferably 40-60 mm Hg.
Preferably the vapor phase transfer is conducted at the temperature of not more than 200 °C, more preferably not more than 180°C. Typically and preferably the vapor phase transfer is conducted on an aqueous solution of lactic acid comprising at least 25 wt. % lactic acid (i. e., lactic acid monomer and oligomer), and not more than 88 wt. % lactic acid. More preferably, it is conducted on an aqueous solution comprising 40-80 wt. % lactic acid, at a pressure of about 40-60 mm Hg.
The preferred process is integrated with a downstream prepolymerization or oligomerization process, such that the transferred vapor phase is fed directly into the oligomerization process, as a vapor, for condensation within the oligomerization process. Preferably the integration is such that both the vapor phase transfer and the downstream oligomerization process are conducted at the same pressure (e. g., 30-200 mm Hg,) and under an inert atmosphere (e. g. nitrogen).
According to the disclosure, preferred techniques for conducting the processes are provided, as well as preferred definition of equipment. Also, a preferred purified vapor phase composition of lactic acid is defined.
Brief Description of the Drawings Fig. 1 is a schematic presentation of a lactic acid isolation and purification process;
Fig. 2 is a schematic presentation of a step of, and equipment for, vapor phase transfer of a lactic acid solution; and Fig. 3 is a schematic presentation of a vapor/liquid separator.
Detailed Description I. Summary of Certain Aspects of Lactic Acid Formation and Purification.
The present disclosure generally concerns lactic acid processing, and it particularly concerns a preferred processing step conducted in an overall processing scheme involving: (a) generation of lactic acid for example by fermentation, and, (b) eventual generation of a purified lactic acid product from that lactic acid. The lactic acid from the process would typically either: (1) be isolated for later use or, (2) for certain preferred applications, be introduced directly into a process for preferred generation of lactide. Techniques related to overall processing schemes which relate to the generation and isolation of lactic acid are characterized in U. S. S. N. 08/950,289, U. S. S. N. 09/132,720, and U. S. S. N. 09/412,085. These three applications each having previously been incorporated herein by reference.
A. A typical lactic acid process.
Various techniques for lactic acid processing are described in U. S. S. N. 08/950,289, U. S. S. N. 09/132,720, and U. S. S. N. 09/412,085. Except as characterized herein with respect to preferred processing, the techniques described in those three applications may be applied or adapted for use in the disclosed process.
In this section of the disclosure, a general characterization of these techniques are presented.
Attention is directed to Fig. 1, which summarizes certain of the techniques characterized in U. S. S. N. 09/412,085. Fig. 1 represents a typical approach to obtaining a lactic acid solution which can be processed, using techniques according to the present disclosure. Referring to Fig. 1, reference letter (F) indicates the lactic acid solution being removed from the processing operation, to be directed into a lactic acid purification and, if desired, a processing system, according to the present disclosure.
Typically, the source of lactate material includes a fermentation broth.
As used herein,"fermentation"refers to any metabolic process that produces a useful product by a mass culture of microorganisms. A variety of microorganisms are suitable for use in the fermentation process, for example, bacteria, yeast and fungi.
The term"lactate material"herein refers to 2-hydroxypropionate in either its free acid or salt form and also to lactic acid oligomers, such as lactoyl lactate, in their free acid and/or salt form. The terms"lactic acid"and"free lactic acid", abbreviated HLa, are employed interchangeably herein to refer to the acid form, e. g., 2- hydroxypropionic acid, also called the"undissociated"form and lactic acid oligomers in the acid form, such as lactoyl lactate. The term"lactic acid monomer" refers to 2-hydroxypropionic acid in the acid form. The salt or"dissociated"form of lactate is specifically referred to herein as a"lactate salt,"for example, as the sodium (or calcium) salt of lactic acid or sodium lactate (or calcium lactate) and the salt or "dissociated"form of lactic acid oligomers."Nutrient medium"refers to media in the form originally provided to the microorganism for fermentation and typically includes a carbon source, a nitrogen source and other nutrients. The term "fermentation broth"refers to a mixture that includes lactate material (e. g., free lactic acid and lactate salt) produced after some or all of the originally provided nutrients have been consumed and fermentation products including lactate material have been excreted into the media by the microorganism. The fermentation broth can include recycle streams from other processes, including the processes described herein. The fermentation broth is also referred to as a"source of lactate material." "Clarified solution"refers to the source of lactate material or fermentation broth after at least some impurities have been removed.
Herein the terms"polylactic acid"or"polylactate"are intended to refer to any polymer comprising at least 50% by wt. polymer units of lactic acid residue or lactide residue. Thus, the two terms include within their scope polylactides. The terms"polylactic acid"and'polylactate", without more, are not meant to specifically identify the polymerized monomer, for example whether the material polymerized was lactide (lactic acid dimer) or lactic acid itself.
By convention, the amount of lactate material in a solution, such as a fermentation broth, can be represented by the weight percent of lactate material present calculated as if it was all in the undissociated or acid form; or the weight percent of lactate material present in the solution calculated as if it was all in the dissociated or salt form. When the amount of lactate material in a solution (by wt.
%) is provided herein, it generally represents the weight percent of lactate material present calculated as if it was all in the undissociated or acid form, and as if it were all in the monomer (non-oligomer) form, unless otherwise noted.
In general, in aqueous solution, an equilibrium is established between lactic acid monomer and oligomers of lactic acid, which form as an adduct of lactic acid monomers with expulsion of water. The solution equilibrium strongly favors the monomeric form of lactic acid, i. e. less than about 2 wt% lactic acid oligomers, when the lactic material concentration is less than about 40 wt%. However, once the lactic acid concentration in an aqueous medium raises above about 40%, measurable and sometimes significant amounts of oligomers are observed. For example, at room temperature an aqueous solution containing lactic acid calculated on a wt. basis of about 70% lactic acid, would in fact contain about 57% by wt. non- oligomerized lactic acid and 13% by wt., lactic acid in the oligomer form. Herein, when a concentration of an aqueous lactic acid solution is reported, on a wt. % basis for lactic acid, the calculation is meant to be a theoretical lactic acid concentration by wt., without regard to the amount which is non-oligomerized lactic acid vs. the amount which has been incorporated into oligomers, due to the referenced equilibria, unless otherwise specified.
Herein, the term"chiral purity"is sometimes used to characterize the lactic acid solution. The term"95% chiral purity"means 95% of the lactic acid/lactate content is in one of the two possible enantiomers: i. e.; either D-or L-.
Such a composition could alternatively be characterized as 10% racemic or 90% optically pure.
As used herein, the pressure of the prepolymer formation system refers to the pressure of the vapor stream exiting the condenser of the prepolymer formation system, unless otherwise noted. Since differences in pressure are generally needed to obtain gas flow, the pressure in the liquid/vapor separator or at the feed inlet to the evaporator may be higher than the pressure indicated.
Engineering techniques for minimizing the pressure differences are known.
2. Sources of Lactate Material; Fig. 1 (A).
The process described herein provides a method for obtaining a purified lactic acid solution from a source of lactate material. Suitable sources of lactate material include, but are not limited to, a fermentation broth, a recycle stream from polylactic acid production which contains lactate material, or recycled polylactic acid (e. g., post-consumer waste or production scraps) that has been hydrolyzed to form a solution containing lactate material. Typically, the source of lactate material is a fermentation broth. (The term"fermentation broth"includes a fermentation broth which includes recycle streams from the process described herein
or other processes.) The techniques described herein are not limited in application to only those lactic acid compositions obtained from fermentation.
Typically, the source of lactate material includes compounds other than lactic acid as impurities. For example, fermentation broths may include both lactic acid and lactate salt, collectively referred to as lactate material, along with cellular debris, residual carbohydrates, amino acids, nutrients (or nutrient media) and other impurities. Generally, for commercial purposes, it is desirable to purify the fermentation broth to obtain an aqueous solution of lactic acid that includes lactic acid (monomer and/or oligomer) in an aqueous carrier with less than about 5.0 g/L impurities, more preferably less than about 1.0 g/L impurities, most preferably less than about 0.1 g/L impurities. The acceptable concentration of impurities can vary depending on the intended commercial use of the solution and the concentration of lactic acid within the solution. As used herein, the phrase"purified lactic acid solution", without further definition, refers to a solution which contains: (1) between about 5 wt% to about 90 wt% lactic acid; (2) an aqueous carrier; and, (3) no more than about 5. 0 g/L impurities. Typically, preferred such solutions will be those which contain no more than about 1.0 g/L, more typically no more than about 0.1 g/L impurities, and most typically no more than 0.05 g/L of impurities, such as proteins, carbohydrates, cellular debris, salts, etc. Typically, preferred such solutions will be those which contain between about 10 wt % to about 90 wt% lactic acid, more typically between about 25 wt. % to about 88 wt. % lactic acid, most typically about 60 wt% to about 75 wt% lactic acid.
3. Clarification; Fig. 1 (G).
When the source of the lactate material is a fermentation broth, a "clarification"process may optionally be performed to reduce the presence of suspended cell mass and other high molecular weight compounds (e. g., impurities having a molecular weight (MW) of about 5,000 Da and greater, typically and especially those having a MW of about 40,000 Da and greater) in the source of lactate material. (See FIG. 1 (G)). Preferably, the step of clarification includes cross-flow filtration. More preferably, a two-stage cross-flow filtration technique is employed. A discussion relating to clarification and approach is provided in U. S. S. N. 09/412,085 previously incorporated herein by reference.
4. Concentration of the Clarified Broth; Fig. 1 (H).
As indicated in Fig. 1, prior to an acidulation step, an optional concentration step is indicated at (H). In this step, if desired, the source of lactate
material (for example a clarified broth), is concentrated to provide for preferred handling in downstream processes. This is described in U. S. S. N. 09/412,085, previously incorporated herein by reference.
Generally, a typical concentration of a clarified broth from a fermentation process would be about 8 to 18 wt. % lactate material. Evaporation of water adequate to provide for a concentration of about 15 to 30 wt% lactate material, prior to an acidulation step, would typically be preferred. A variety of useable evaporation or concentration techniques can be used. In general, falling film evaporators or rising film evaporators can be used with a multi-effect evaporator or heating could be provided by mechanical vapor recompression, or thermal recompression.
5. Acidulation ; Fig. 1 (B).
In a typical process for preparing a lactic acid solution to be processed according to the techniques described herein, a strong acid, such as sulfuric acid, is added to the clarified solution in an amount sufficient to convert most of the lactate material in the broth (e. g., at least about 90 wt%, more preferably at least about 95 wt% of the lactate material) to an undissociated acid form. (FIG.
1 (B)) Preferably, the source of lactate material includes calcium lactate so that the acidulation process results in the formation of lactic acid and calcium sulfate (gypsum).
6. Calcium Sulfate (Gypsum) Removal; Fig. 1 (C).
The calcium sulfate or gypsum generated in the previous step of the acidulation is only slightly soluble in water. Thus, it can be readily separated from the aqueous lactic acid solution. For example, techniques such as rotary drum filtration, belt filtration, press filtration, centrifugal separation or decantation, or a combination of these techniques, can be used.
7. Amine Extraction; Stripping; Fig. 1 (D) and 1 (E).
Even after the sulfate salt is removed from the solution, some impurities (e. g., impurities having a molecular weight of about 100Da to about 500,000 Da, more typically about 100Da to about 300,000 Da, such as amino acids and carbohydrates), may remain suspended or dissolved in the solution. Also, residual levels of calcium salts will remain. To reduce the levels of certain of these contaminants, the lactic acid in the acidulated solution is extracted with an extractant containing a water insoluble amine (FIG. 1 (D)) and stripped from the amine solvent, for example by back extracting the lactic acid into a liquid phase
(typically an aqueous phase) that is immiscible with the extracting solvent (FIG.
1 (E)). Herein, the step of extracting lactic acid from the extractant back into an aqueous phase is sometimes referred to as the"aqueous back extraction". The operating temperature of an aqueous back extraction process is typically at least about 70°C. Sulfuric acid may be included in the amine extractant as an enhancer.
Also the amine extractant may include a hydrocarbon fraction (for example kerosene) and an organic enhancer (for example octanol). The sulfuric acid can be present in the amine extractant as residual sulfuric acid from the acidulation step; added during the amine extraction step; or added to the amine extractant prior to or during the extraction step.
Amine extraction processes and stripping processes are described in detail in U. S. S. N. 08/950,289, U. S. S. N. 09/132,720, and U. S. S. N. 09/412,085, each previously having been incorporated herein by reference.
B. Processing of Lactic Acid to Polylactic Acid, Lactide and Polylactide.
With respect to processing of the lactic acid to polylactic acid, lactide and polylactide, attention is directed to U. S. patent Nos. 5,142,023; 5,274,073; and U. S. S. N. Patent Application Serial No. 09/044,701 (entitled CONTINUOUS PROCESS FOR THE MANUFACTURE OF LACTIDE AND LACTIDE POLYMERS, filed March 19,1998); the complete disclosures being incorporated herein by reference. In general, the'023 patent describes a process that involves: (a) feeding concentrated lactic acid to a prepolymer reactor; (b) polymerizing the concentrated lactic acid to polylactic acid in the prepolymer reactor, by removal of water; (c) feeding the resulting polylactic acid prepolymer to a lactide reactor; (d) removing crude lactide as a vapor from the lactide reactor; (e) conducting purification of the lactide; and, (f) eventually, conducting polymerization of the lactide to polylactide.
Herein, the term"prepolymer reactor"and variants thereof is used to refer to equipment to which the purified lactic acid is provided, to be processed with removal of water to a prepolymer (oligomer) solution of lactic acid prior to lactide formation. The processing step conducted during this concentration of lactic acid to an oligomeric form of polylactic acid, prior to lactide generation, is generally referred to herein by the term"prepolymerization of lactic acid"and variants thereof.
The disclosure of the'023 patent particularly concerns steps involving prepolymerization of lactic acid and processing through to generation of a
purified lactide stream. Polylactide formation, from the resulting purified lactide, is described for example in U. S. patent Nos. 5,338,822; 5,525,706; 5,475,080; 5,484,881; 5,536,807; 5,594,095; and 5,714,573, each of which is incorporated herein by reference.
II. Factors of Concern With Respect to Lactic Acid Racemization.
In general, lactic acid racemization (i. e. loss of chiral purity) is strongly influenced by at least the following two conditions: 1. Presence of material which can catalyze racemization; and 2. Exposure of the lactic acid material to heat, especially for prolonged periods at temperatures above about 180°C.
Of course, the two conditions operate together, to facilitate racemization. That is, exposure of the lactate material to temperatures above about 100 °C in the presence of a racemization catalyst can cause significant racemization.
A. Racemization Catalysts.
The rate of lactic acid racemization is generally found to increase, with the presence of certain ionic salts in the lactic acid solution, even in relatively low amounts, i. e. on the order of 5 to 500 ppm. Included within such racemization salts or catalysts are the salts of the Group I and the Group II metal cations, for example sodium, potassium, calcium and magnesium salts.
Even the presence of relatively low amounts of these salts, for example on the order of 5 to 500 ppm, can influence racemization of an aqueous lactic acid composition in an undesirable manner. Thus, it would be desirable to conduct an overall lactic acid process which ensures the absence (i. e. < 5 ppm) of significant amounts of these salts, if possible.
As indicated above, however, an otherwise preferred approach to processing of lactic acid includes a step which involves formation of calcium salts.
Thus, the presence of calcium ions within the lactic acid solution would (preferably) not be avoided. A step of isolating the lactic acid from residual amounts of the calcium ions is desirable, provided the step does not involve undesirable amounts of exposure of the lactic acid to heat, which, as explained in the next section, generally exacerbates racemization of the lactic acid.
B. Prolonge Exposure to Heat.
Prolonged exposure of aqueous lactic acid solutions to heat, generates racemization. In general, depending in part upon the presence or absence of racemization catalysts, prolonged exposures (i. e. exposures of 5 hours) of the lactic acid to temperatures on the order of 180°C can readily generate undesirable levels of racemization. It should be noted that the amount of racemization depends on the combination of time and temperature, meaning that the same amount of racemization can occur by holding the lactic acid at a high temperature for a short time or holding the lactic acid at a lower temperature for a longer time.
It is noted that during typical processes for the generation of the purified lactide, for example as characterized in the'023 patent, there is a step of concentrating the lactic acid in a prepolymer reactor, to form oligomers (i. e. polylactic acid) and to drive off water. Such a process is generally conducted at a pressure of about 30-200 mm Hg, to facilitate removal and eventual condensation of the water vapor. Generally the temperature of the bottoms (oligomers) in the prepolymer reactor will be maintained at about 100 °C to 150°C, during this stage of processing, for periods of at least 5 minutes to 300 minutes.
It would be preferred to avoid introducing into the prepolymer reactor, along with the lactic acid solution, significant amounts of racemization catalysts, which would tend to concentrate in the reacted bottoms and to exacerbate racemization reactions during the oligomer formation.
In addition, after the polylactic acid is formed in the prepolymer reactor, it is treated, in a lactide formation step, with further exposure to heat to generate crude lactide which is removed from the solution as a vapor. Again, this is a step of further exposure to heat, with racemization exacerbated by the presence of any racemization catalysts in the reactor bottoms.
If the lactic acid is not sufficiently pure, prolonged heating of the lactic acid solution may cause the lactic acid solution to change from a virtually colorless solution to a slightly yellow, yellow, brown or black solution, depending on the temperature, the length of time the solution is exposed to that temperature and the impurity level. Such color formation may be undesirable if the color is not easily separated from the lactic acid monomers, oligomers and/or lactide because it could result in a colored polylactic acid product. Many applications for polylactic acid desire a colorless resin.
Based on Malliard chemistry, carbohydrates and amino acids are compounds that are likely to degrade and cause color formation. Therefore, it may be desirable to separate the carbohydrates and amino acids from the feed stream entering the prepolymer formation reactor.
III. Certain Properties of Aqueous Lactic Acid Compositions.
A. Vapor Temperature as a Function of Lactic Acid Concentration.
As part of the present investigation, consideration was given to utilizing a step of evaporation or vapor transfer to facilitate separation of the lactic acid composition from residual racemization catalyst and other impurities. A particular concern was whether or not such an added step of evaporation would involve sufficient exposure of the lactic acid composition to heat to exacerbate the problems of racemization.
As part of the investigation, a study was conducted evaluating the temperature of the vapor of a system involving the vapor transfer of a water/lactic acid mixture of various concentrations. The study was conducted in accord with the Rayleigh Distillation Method, (See Separation Processes (2nd Edition) by C. J. King, McGraw-Hill Inc., NewYork, NY 1980 pgs. 115 to 121), at a pressure of 100 mm Hg and varying concentrations of water and lactic acid. The data was fit to the Wilson Equation, (See e. g. Molecular Thermodyanmics of Fluid-Phase Equilibria (2"d Edition), by J. M Prauznitz, R. N. Lichtenthaler, and E. Gomes de Azevedo, Prentice-Hall Inc., Englewood Cliffs, NJ 1986 pgs. 234 to 237) and this thermodyanmic model was used to predict the temperature of the water/lactic acid vapor, at 50 mm Hg, as a function of the concentration of the lactic acid solution.
These results are reported below, in Table 1.
TABLE 1 % lactic acid (by wt.) T of Vapor (°C at 50 mm Hg) Aqueous Lactic Acid monomer composition 30% lactic acid 104°C 50% lactic acid 119°C 88% lactic acid 145°C
From the above table, it can be concluded that if a step of vapor transfer is to be conducted as a purification step to isolate the lactic acid from undesired residual components such as racemization catalysts (for example, residual Group I and Group II salts) and other impurities with low volatility, such as carbohydrates, amino acids, and color bodies, to some extent there are factors favoring that the vapor transfer occur with a solution of lactic acid and water which is relatively dilute, i. e. less than 88% lactic acid, with the advantage favoring, in general, the more dilute solutions (especially < 80%) due to lower temperatures being involved.
B. Lactic Acid Concentration and Oligomer Formation.
In general, as the concentration of lactic acid in an aqueous solution is increased, generation of lactic acid oligomers increases. As indicated above, at a concentration calculated on a theoretical percent basis for lactic acid monomer of 70%, an evaluation would in fact show about 57%, by wt., monomeric lactic acid, the remaining 13% being in the form of lactic acid oligomers. As the lactic acid concentration is increased above about 70%, the amount of lactic acid in the oligomer form increases significantly.
On the other hand, a lower concentration of lactic acid in the aqueous lactic acid solution, requires a greater amount of time for evaporation or vapor transfer and to obtain complete transfer of materials (other than residual catalysts, etc.).
In general, for aqueous solutions having a concentration above about 88% wt. lactic acid on a theoretical basis, significant losses of lactic acid may occur due to two mechanisms. First, when concentrating lactic acid above 88 wt%, lactic acid can be lost in the overhead stream. Second, a higher level of lactic acid in the oligomer form would be produced during the concentration, and as is discussed later, the oligomers have a lower vapor pressure than lactic acid and require a higher temperature in orrder to be distilled overhead during the lactic acid vaporization step. Aqueous solutions having a concentration of lactic acid of below about 25 wt.
% on a theoretical basis, require significantly increased times of exposure to heat during the vaporization vapor transfer process to achieve a vapor phase transfer of the lactic acid, and require large equipment sizes due to the large vapor flow through the system.
In general, it is foreseen that in a well designed process, if an amine extraction (as described in U. S. S. N. 08/950,289, U. S. S. N. 09/132,720, and U. S. S. N.
09/412,085), is used to isolate the lactic acid, and an aqueous back extraction is conducted to partially purify the isolated lactic acid, the aqueous back extraction will typically be conducted to provide a concentration of lactic acid on the order of about 15-50 wt. % at a temperature at least 70°C.
In general, it is also foreseen that an efficiently conducted vapor transfer of a lactic acid solution can be conducted using the techniques described herein, with a solution having a lactic acid concentration of < 88%, typically about 25-80 wt. %, preferably 40-80% and more preferably around 60-75 wt. % lactic acid. Such concentrations of lactic acid in aqueous solution can be readily obtained from the aqueous phase of the aqueous back extraction using conventional techniques for concentrating aqueous solutions. This can be conducted using conventional mechanical vapor recompression evaporators, multi-effect evaporators, or thermal recompression evaporators such as those available from Dedert Corporation, Olympia Fields, IL.
Attempts to achieve lactic acid concentrations above about 70-75 wt.
% will typically have involve some losses of lactic acid due to evaporation of lactic acid itself during the concentration step.
C. Preferred Levels of Maintenance of Chiral Purity.
Using the techniques according to the present disclosure, a transfer can be conducted such that the chiral purity of the lactic acid after vapor transfer is maintained within about 1.0%, more preferably within 0.5%, and most preferably within about 0.2% of the value measured after the back extraction. That is, preferably the concentration of the aqueous solution from the aqueous back extraction to a preferred level for vapor transfer, and the conduct of the vapor transfer for purification described below, are steps conducted with conditions selected such that the chiral purity of the lactic acid in the aqueous back extraction is not reduced significantly. For example, if the chiral purity from the aqueous back extraction is about 98%, preferably the material is concentrated and transferred in the vapor transfer step, into the prepolymer reactor, without a modification in the chiral purity such that the chiral purity is any less than about 97% more preferably not less than about 97.5%, and most preferably not any less than about 97.8% when the vapor phase is introduced into the prepolymer reactor.
Based upon experimentation, empirical observation and calculation, it is foreseen that using the techniques referenced herein, an aqueous back extraction containing lactic acid with a chiral purity of about 98 % can readily be introduced into processes according to the present invention, and can be concentrated to a solution having about 65-75 wt. % lactic acid using a low residence time evaporator, wherein the concentrated solution has a chiral purity of at least 97.5%. It is foreseen that, using the techniques described herein, such a concentrated solution (i. e. 65-75 wt. % lactic acid), can be converted into a vapor phase wherein: at least 80% of the lactic acid in the concentrated solution is vaporized; and, the chiral purity of the lactic acid in the vapor phase (measurable by condensing the vapor phase) is at least 97.0%.
The chiral purity of a lactic acid feed for some commercial polylactic acid products can be as low as 85 % chiral purity. However, the chiral purity of the lactic acid feed is more typically greater than 95 %, more preferably greater than 97%, most preferably greater than 98%.
IV. A Desirable, Integrated Process.
According to the techniques described herein, a desirable integrated process is provided for recovering lactic acid from an aqueous back extraction of a lactic acid purification step, and providing lactic acid into a prepolymer reactor for a step of polylactic acid formation. In general, the conditions are provided such that undesirable levels of racemization due to prolonged exposure to heat and/or racemization catalysts, are avoided. The conditions also tend to reduce the reaction of carbohydrates and amino acids, thus reducing color formation. In addition, the conditions are preferably such that undesirable levels of racemization catalysts are not directed into the polylactic acid reactor. Also, the conditions tend to reduce the amount of carbohydrates, amino acids, and/or other impurities that cause color formation directed into the polylactic acid reactor. Instead, these impurities are separated from the lactic acid vapor stream.
A. Useable Techniques and Conditions, Generally.
In general, according to the present disclosure, an aqueous lactic acid solution is vaporized, in a vaporization process, to separate lactic acid, as a vapor, from bottoms which typically include oligomers and concentrated amounts of residual racemization catalysts. Preferably the conditions in the vapor transfer are
conducted such that the pressure of the system is maintained at between about 30- 200 mm Hg, more typically 40-120 mm Hg, most typically less than 60 mm Hg, for example about 40-60 mm Hg. A reason for the preferred pressure is that this pressure will be the typical preferred pressure for the formation of polylactic acid in a prepolymer reactor. Preferably, the vaporized water/lactic acid solution from the vaporization step is fed directly into a downstream polylactic acid prepolymer reactor; i. e. the vapor transfer step is preferably integrated with the prepolymer formation step.
One reason it is preferred to conduct the prepolymer reactor under conditions of about 50 mm Hg relates to the fact that water vaporizes at about 38°C, at 50 mm Hg. When a ready supply of cooling water to the water condenser is available at about 25°-28°C, it will typically be preferred that the vapor be taken off the polylactic acid prepolymer formation reaction at about 38°C. This, then, generally dictates or defines the pressure of the prepolymer reactor as being about 50 mm Hg since lower pressures could result in undesirably low water vapor temperatures and higher pressures could involve undesirably high water vapor temperatures. This operating pressure allows the use of relatively inexpensive cooling tower water for condensing the overhead stream from the prepolymer reactor. Note that under certain circumstances, an inexpensive supply of cooling media may be available that would allow the system to be run at a pressure lower than 50 mm Hg. It is also noted that if a lower operating pressure was chosen, two condensers (the first using cooling tower water at about 25°-28°C, the second using a higher cost, but lower temperature cooling media) could be used for this process.
The use of the two condenser system as described may have some advantages as the majority of any lactic acid being evaporated overhead from the prepolymer reactor could be condensed in the first condenser, This may allow the recycle of this stream back into the prepolymer system and increase product yield. The condensate from the second, lower temperature condenser would have a significantly lower lactic acid concentration, and could be discarded or recycled back into the lactic acid separation processing.
In general, what is preferred is to introduce into the aqueous lactic acid vaporization process an aqueous lactic acid composition comprising at least about 25% by wt. lactic acid, typically at least 30% by wt. lactic acid. In general, the bottom figure on lactic acid concentration is defined by: (1) concentrations readily achievable in cost effectively designed aqueous back extraction or stripping
processes, from a typical amine extraction process; and (2) a preferred minimum level to reduce residence time in vapor transfer equipment.
In general, the upper level of preferred concentration for the aqueous lactic acid material fed into the vapor transfer process will be about 88 wt. % lactic acid. Typically, however, the transfer will be conducted with a material having a concentration of no greater than about 75 wt. % lactic acid. A reason for this is, because, as indicated previously, when an effort is made to achieve concentration above about 70-75%, the concentration step may have led to losses in lactic acid due to evaporation.
Typically, the concentration of lactic acid introduced into the vapor transfer process will be about 45-75%, preferably as concentrated as reasonably obtainable within that range, and thus most preferably it is about 65-75 wt. % lactic acid. This concentration can be readily achieved by water evaporation from a less concentrated solution, for example by using mechanical vapor recompression evaporator as previously described. In general, it is observed that such concentration processes can be conducted in such a manner that they do not lead to undesirable levels of racemization.
Considering the above, then, a preferred aqueous lactic acid vaporization and vapor transfer process according to the present disclosure is conducted by introducing into the vaporization system an aqueous lactic acid composition comprising about 65-75%, by wt. lactic acid; and, conducting the vapor transfer at about 30-200 mm Hg, typically about 40-120 mm Hg, more typically about 40-60 mm Hg. In general, this will involve heating the composition adequately to provide a vapor temperature of about 100 °C to 200 °C, more preferably 120 °C to 200 °C.
The temperature of the vapor stream depends greatly upon the operating pressure, the amount of lactic acid oligomers present in the lactic acid feed, and the fraction of the feed being vaporized. The increase in boiling point temperature with increasing pressure is known.
It has been found the presence of lactic acid oligomers increases the vapor temperature. For a fixed vaporization of lactic acid (both monomer and oligomer), as the amount of oligomer increases the vapor temperature required to reach that amount of vaporization increases. This is due to the lower volatility of the oligomers when compared to lactic acid which need to be vaporized to reach a fixed
vaporization rate. In other words, a higher temperature is required to evaporate more of a lower volatility compound.
For a given lactic acid solution that contains both monomer and oligomer, as the fraction of vaporized feed increases, the vapor temperature increases. Table 2 reports the temperature of the vapor for as the fraction of the feed vaporized increases using the Wilson Equation (mentioned earlier). A lactic acid solution which contains 57 wt% lactic acid monomer, 11 wt% lactoyllactate (DP2), and 2 wt% lactoyl (lactoyllactate) (DP3) was fed into an evaporator and the fraction of the feed stream vaporized was varied. The vapor temperature increased as the fraction of feed vaporized increased.
TABLE 2 Wt % Fraction Feed Vaporized T of Vapor (°C at 50 mm Hg) 0.8 122.9 0.9 136.7 0. 95 149.2 0.98 195.8 Knowing that the presence of the oligomers tends to increase the vapor temperature of the lactic acid stream, it is preferable to reduce the amount of oligomers in the lactic acid feed solution. This can be accomplished by at least two methods. In the first method, a relatively dilute lactic acid stream (e. g., < 40 wt%) is fed into the evaporation step. In the second method, a more concentrated lactic acid stream (e. g., > 40 wt%) that has not reached equilibrium with respect to oligomer formation is fed into the evaporation step.
A non-equilibrium, concentrated lactic acid solution can be produced by quickly evaporating water from a dilute solution to form a concentrated solution.
To obtain a non-equilibrium, concentrated lactic acid solution, the time required for the evaporation must be small compared to the time required for oligomer formation.
Generally, the time required for evaporation can be reduced by using equipment having a high surface area to volumetric hold up ratio, such as a mechanically wiped film evaporator, a falling film evaporator, or rising film evaporator. The time required for oligomer formation can be increased by operating the evaporation process at a low temperature and avoiding the presence of esterification catalysts.
Generally, evaporating at low temperatures results in operating at pressures below
atmospheric pressure (e. g., 50 to 500 mm Hg). The hold time between the water evaporation step (lactic acid concentration) and lactic acid vaporization can be reduced to provide a lactic acid feed stream with less oligomer.
In a preferred process, the oligomer content of the lactic acid solution fed to the evaporation step contains 70 wt%, more preferably 50 wt%, most preferably 20 wt% of the oligomers that would be present if the solution had reached equilibrium with respect to oligomer formation.
Herein, an aqueous solution of lactic acid will be considered vaporized by a vaporization process if at least 70%, by wt., of the solution is vaporized, i. e. if the vaporization process generates a two phase mixture (vapor/liquid) comprising at least 70% by wt. vapor phase. Typical vaporization processes as described herein will involve at least 80% vaporization, by wt., typically 90-98% vaporization, by wt.
Techniques for efficiently separating liquid and vapor streams are known, see e. g. Chemical Engineer's Handbook, (5th Edition) ed. By R. H. Perry and C. H. Chilton, McGraw-Hill, Inc., New York, NY 1973 Chapterl8.
With typical preferred applications of the techniques described herein, the separated vapor stream is transferred directly into a prepolymer reactor for polylactic acid, and is condensed within the reactor. Within that reactor, then, polylactic acid oligomer and polymer is formed, with removal of water. The reactor conditions will typically be about 50 mm Hg (i. e. 40 to 60), with a vapor temperature (H20 being removed) of about 38°C (i. e. 34 to 42).
B. Preferred Techniques for the Vapor Transfer Step.
Preferably the vapor transfer step is conducted with equipment that provides for effective vaporization (at a high rate) so as to provide a short length of time of exposure of the liquid composition to heat. Equipment useable includes conventional falling film evaporators and rising film evaporators, appropriately constructed and configured for operation under conditions of about 30-200 mm Hg, and vapor temperatures on the order of about 100 to 200°C. Such equipment is commercially available from Dedert Corporation (Olympia Fields, IL).
In general, processes and equipment configured for operation under an inert atmosphere, for example a nitrogen (N2) gas atmosphere, will be preferred.
This will tend to inhibit decolorization of the product (s).
A preferred system will be a rising film evaporator, configured and controlled to provide a two phase outflow comprising about 2-10% liquid, the remainder being vapor. This outward flow would be directed to a conventional vapor/liquid separator, for example, an impingment separator (discussed in Chemical Engineer's Handbook, referenced earlier). The separated vapor phase is then directed (directly, i. e. without modification) into a downstream polylactic acid prepolymer reactor.
An overall preferred system according to the present invention is generally indicated in Fig. 2. Generally, a liquid feed 1 containing lactic acid is fed into an evaporator 2. Heat input 3 into the evaporator produces a two phase liquid- vapor stream 4. The liquid-vapor stream 4 is fed into a liquid-vapor separator 5 to obtain a liquid stream 6 and a separated vapor stream 7. The vapor stream 7 is then fed into a prepolymer reactor 8. In the prepolymer reactor 8, water is removed 9 to form lactide oligomers 10 (bottoms).
Typically, the liquid feed 1 includes about 25 wt % to about 88 wt %, more preferably about 40-80 wt%, most preferably about 60-75 wt. % lactic acid in an aqueous carrier. Preferably the evaporator 2 is a rising film or a falling film evaporator, more preferably a rising film evaporator. In the evaporator, heat applied to the liquid feed causes bubbles of vapor to form in the liquid feed. Typically, heat is applied at a temperature about 10°C to about 25°C higher than the desired vapor temperature. Typically, the temperature of the vapor coming off the evaporator is between about 100°C and 200 °C. Preferably, enough heat is applied to the liquid feed to produce a liquid-vapor stream which contains about 2 to 10 wt. % liquid and 98 to 90 wt. % vapor.
A liquid-vapor separator 5 is shown schematically in Figure 3. As shown in Figure 3, the liquid-vapor stream 4 is fed into a liquid-vapor separator 5 such that the liquid-vapor stream 4 is directed at a wall 20 of the separator 5. Upon contacting the wall of the separator, the liquid phase 21 drains along the wall, for example, by the force of gravity, and exits from the base 22 of the separator 5. In contrast, the vapor phase 7 of the liquid-vapor stream rises and exits the separator 5 from the top 25. The vapor stream 7 is then fed into a prepolymer reactor 8, Fig. 2 is an example of a prepolymer reactor is a distillation column. In the prepolymer reactor 8, water is removed from the vapor stream 7 which causes the lactic acid monomers to polymerize to form lactic acid oligomers.
While it is preferred that the lactic acid vapor be fed into a prepolymer reactor, other possibilities exist for the use of the lactic acid in the vapor phase. For example, PCT 92/00292 describes a method for making lactide using fixed bed catalysts from a lactic acid vapor stream. U. S. Patent 5,023,349 also describes the formation of lactide from lactic acid vapor stream. The lactic acid vapor could be condensed to a liquid state and used in any application of lactic acid.
Feedstock This experiment was performed using lactic acid solutions containing 88 wt% (Experiment la) and 60 wt% (Experiment lb) total lactic acid. The lactic acid solutions may include lactic acid, lactoyllactic acid, and other intermolecular lactic acid esters. A lactic acid solution containing 88 wt% lactic acid and 12 wt% water is commercially available from Purac America (Lincolnshire, IL) and Archer Daniels Midland (Decater, IL) The 60 wt% lactic acid solution was prepared by combining water and lactic acid to obtain the correct lactic acid to water ratio. No hydrolysis of the lactic acid was performed. For each experiment, approximately 40 to 50 lb. of lactic acid solution was used.
Equipment The following equipment was used: a feed pump, a preheater, a boiling tube heat exchanger, a liquid/vapor separator, a condenser/receiver, a bottoms pump, a hot oil circulation system, and a vacuum pump.
The feed pump was a positive displacement pump capable of pumping 6.1 gal/hr at a discharge pressure of 1000 psig.
The preheater was a double-pipe heat exchanger constructed out of 316 stainless steel tubing. The inner jacket heat transfer area was 0.393 sq. ft. The lactic acid feed flowed inside the inner tube. Steam flowed in the outer jacket and was controlled to maintain the desired outlet temperature.
The boiling tube evaporator was a double pipe heat exchanger which included a 1"O. D. 316 stainless steel tube jacketed by a 1.5" Schedule 40 316 stainless steel pipe. The heat exchanger was 5 feet long and provided 1.57 sq. ft. of heat transfer area. The inner pipe had Kenics static mixing elements to aid in the heat transfer and keep the vapor and liquid mixed. The outlet stream from the
preheater passed through the 1"inner tube and was vaporized. Hot oil flowed in the outer jacket at a significantly higher temperature than the inner jacket. This boiling tube evaporator was mounted vertically with feed fluid rising up through the heat exchanger.
In the liquid/vapor separator, vapor was withdrawn from the separator via a 1"opening in the upper section of the vessel. Liquid was removed at the bottom of the separator. The liquid/vapor feed entered via a 1"O. D. tube that extended to the middle of the separator from the top flange of the separator. The bottom of the separator had a conical shape to facilitate removal of the liquid.
A simple coil-in-shell condenser was used to condense the vapor from the liquid/vapor separator. Additional cooling was provided by a cooling jacketing. The condenser was constructed using 4"Schedule 40 316 stainless steel pipe. The distillate was collected in a 4.5 gallon receiver. The cooling media entered the cooling coil at 10°F.
A bottoms pump was used to discharge higher viscosity liquid that collected at the bottom of the liquid/vapor separator.
The hot oil heating system included a Julabo HT6-C2 heater capable of producing 6.1 kW with Julabo H350 fluid circulating through the system. The system was designed to handle temperatures up to 350°C. Julabo fluid was only used to heat the boiling tube evaporator.
The vacuum pump was capable of pumping 500 L/min of free air and capable of reaching 0.0001 mm Hg. Nitrogen was added to the inlet of the vacuum pump to maintain the pressure of the liquid/vapor separator. A control valve was used to adjust gas flow into the vacuum pump.
Results Experimental results are shown for Examples 1 a and 1 b (Table 1).
The distillate product was clear and the color of the bottoms ranged from a yellow tinted material to a dark yellow-brown material.
Table 1: Experimental Results with Analytical Results Parameter Exam le la Exam le lb Feed Composition (wt %) Water12.0 39.0 Lactic Acid 50.5 38.0 Lactoyllactic Acid (DP2) 25.7 15.4 DP39.6 6.2 DP42.3 1.2 Chiral Purity 99. 0 99. 0 Feed Rate (lb/hr) 18.3 18.3 Li uidNa or Se arator Pressure (mm Hg) 25 50 Vapor Temperature inside Se arator °C 154 162 Liquid Temperature inside Se arator °C 155 166 Mass of Feed used (lb) 41.2 40.1 Mass of distillate recovered (lb) 32.5 32.6 Composition of distillate Water13.4 37.8 Lactic Acid 59.2 40.7 Lactoyllactic Acid (DP2) 24.2 16.6 DP33.6 4.2 DP40.2 0.7 Chiral Purity 99.0 99.0 Color % of Feed Evaporated 78. 9 81. 3 % of Lactic Acid recovered in distillate 78.0 92.0 Mass of Residual Recovered (lb) 6.8 7.1 Composition of Residual (wt%) Water1.5 6.6 Lactic Acid 13.5 14.8 Lactoyllactic Acid (DP2) 37.4 34.3 DP340.6 33.5 DP47.4 10.8 Mass Balance % Recovery of Feed 95. 3 99. 0
As shown in Table 1, the process disclosed herein is capable of providing a lactic acid vapor in which the chiral purity is not reduced as compared to the lactic acid feed and when the fraction of feed evaporated is about 80%. Thus, the process can provide a vapor from which impurities, such as carbohydrates, amino acids, color bodies and salts are removed, without racimization of the lactic acid.
The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.