Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2002-01-03
2003-11-04
Drodge, Joseph (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S669000, C435S139000, C562S580000, C562S589000, C562S608000
Reexamination Certificate
active
06641734
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to processes for producing organic acids, such as lactic acid.
Lactic acid has a number of commercial uses, for example in food manufacturing, pharmaceuticals, plastics, textiles, and as a starting material in various chemical processes. In addition, it is used in the manufacture of polylactic acid, a degradable plastic.
Although organic acids can be prepared by chemical synthesis, production by fermentation is generally less expensive. It is well known to produce lactic acid by fermentation using microorganisms such as
Lactobacillus delbrueckii
. The broth that results from fermentation contains unfermented sugars, carbohydrates, amino acids, proteins, and salts, as well as organic acids, such as lactic acid. Typically, the organic acid is recovered from the fermentation broth and undergoes further purification before it is used. Purified organic acids recovered from fermentation broths can comprise small amounts of impurities, such as strong acids or certain unknown compounds. Some of these impurities can cause an undesirable color or can interfere with downstream processing of the organic acid. For example, lactic acid as it is sold commercially typically comprises small amounts of impurities such as pyruvic acid and oxalic acid. Even though present in relatively small amounts, such impurities can have negative effects on polymers produced from the lactic acid. For example, when lactic acid is polymerized to produce polylactic acid (PLA), the presence of even small amounts of pyruvic acid can cause the polymer to have an undesirable yellow color. However, it is difficult to further purify lactic acid that contains only a small fraction of pyruvic acid in the first instance.
Thus, there is a need for improved processes for the production and recovery of relatively pure organic acids, particularly lactic acid.
SUMMARY OF THE INVENTION
One aspect of the present invention is a process for purifying an aqueous feed stream that comprises a desired product organic acid and at least one strong contaminant. In certain embodiments, the aqueous feed stream can comprise a fermentation broth or can be obtained from a fermentation broth. (Whenever an acid is referenced herein, either as the desired product or as a contaminant, it should be understood that some or all of the acid may be present in the form of salts.) The molar concentration of the product organic acid in the feed stream can be at least 10 times greater than the molar concentration of the strong contaminant, and more preferably the molar concentration of the product organic acid to the strong contaminant is at least 20. In certain embodiments the molar concentration of the product organic acid to the strong contaminant is at least 90, in certain embodiments it is at least 500 and in certain embodiments it is at least 1000. The aqueous feed stream is contacted with a first immiscible basic extractant that has a selectivity, under the existing process conditions (including the combination of acids, solvents, etc., that are present) for the strong contaminant relative to the product organic acid that is greater than 3. The selectivity, which is further defined below, is preferably greater than 15, more preferably greater than about 25, most preferably greater than about 100. Preferably the selectivity is greater than the ratio of product organic acid to strong contaminant in the feed.
The contacting step in which the aqueous feed stream is contacted with a first immiscible basic extractant is preferably performed with sufficient equilibrium or near equilibrium stages, and with sufficient quantity of the first immiscible basic extractant (such as a solid amine ion exchanger or liquid amine extractant) to remove the majority of the strong contaminant. In certain embodiments the first immiscible basic extractant has previously been used to treat a solution comprising the product organic acid and at least one weak contaminant (e.g., it is recycled).
As a result, the majority of the strong contaminant and less than about 33 wt % of the product organic acid become complexed with the first immiscible basic extractant. “Majority” as used herein means more than 50% by weight of the substance, in this case the strong contaminant, that is present. In other words, more than 50% by weight of the strong contaminant present in the feed complexes with the extractant. The complexed first immiscible basic extractant is separated from the aqueous stream, thereby producing a first effluent stream that comprises product organic acid and that has a greater ratio of product organic acid to strong contaminant than the aqueous feed stream did. The complexed first immiscible basic extractant is contacted with a displacing acid. The first immiscible basic extractant has a greater affinity for the displacing acid than it does for the strong contaminant or the product organic acid, and as a result, product organic acid and strong contaminant are displaced over a period of time from the complexed first immiscible basic extractant. This produces a second effluent stream that comprises a major amount of product organic acid (i.e., more than 50% by weight of the solids dissolved or suspended in the stream are the product organic acid) and a third effluent stream that comprises a major amount of strong contaminant. Preferably, the total amount of product organic acid present in the first effluent stream and in the second effluent stream is at least about 90% by weight of the product organic acid that was present in the feed stream. More preferably, at least about 98% by weight of the product organic acid is recovered in those streams.
In many embodiments of the process, the strong contaminant comprises an organic acid that has a pK
a
that is lower than the pK
a
of the product organic acid. If the desired product organic acid is lactic acid, the strong contaminant preferably has a pK
a
less than about 3.46. In certain specific embodiments of the process involving a basic extractant that is a solid ion exchange resin, the strong contaminant is selected from the group consisting of pyruvic acid, oxalic acid, citraconic acid, citric acid, and mixtures thereof.
In other embodiments, the strong contaminant can be a weaker acid (e.g., higher pK
a
) than the desired organic acid product, but can have greater hydrophobic and/or hydrogen bonding character than the product. The strong contaminant is selectively removed relative to the organic acid of interest by an immiscible basic extractant comprising a solvent or a solvent mixture, for example an amine mixture comprising 1 M trilaurylamine and 1 M dodecanol with dodecane as a diluent.
When the immiscible basic extractant comprises a solvent mixture, preferably the organic acid of interest has somewhat hydrophobic or strong hydrogen bonding characteristics, and the strong contaminant must either (1) be of similar H-bonding and/or hydrophobic character and lower pK
a
than the acid of interest (acidic low pK
a
species) or (2) have a sufficiently stronger H-bonding and/or hydrophobic character so that the strong contaminant can still be removed despite its having a higher relative pK
a
. Thus, if the strong contaminant has a lower pK
a
than the organic acid product, a solid ion exchange resin can be used as an extractant to remove the strong contaminant. For example, if the organic acid to be recovered is lactic acid, strong contaminants that can be removed by methods of the present invention involving ion exchange resins include HCl, H
2
SO
4
, pyruvic acid and oxalic acid, among others. Acetic acid and butyric acid do not, however, have significantly lower pK
a
values than lactic acid, and they are not as readily removed by ion exchange resins. However strong contaminants having either low pK
a
or high hydrophobicity/hydrogen bonding characteristics relative to the organic acid product, can be removed using extractants that are solvents or a solvent mixture. For example, pyruvic acid, H
2
SO
4
, and butyric acid can be removed from an aqueous
Cockrem Michael Charles Milner
Heidel David
Kovacs Istvan
Mohamednur Idris
A. E. Staley Manufacturing Co.
Drodge Joseph
Williams, Morgan and Amerson
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