Chemistry: molecular biology and microbiology – Micro-organism – tissue cell culture or enzyme using process... – Preparing oxygen-containing organic compound
Reexamination Certificate
2000-10-12
2003-01-21
Lilling, Herbert J. (Department: 1651)
Chemistry: molecular biology and microbiology
Micro-organism, tissue cell culture or enzyme using process...
Preparing oxygen-containing organic compound
Reexamination Certificate
active
06509179
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a continuous process for preparing lactic acid from an aqueous feed which has been obtained by fermentation and mainly contains lactate. The term “a feed which mainly contains lactate” refers to a feed which contains about 250 g of lactate ion/l or less, based on the total amount of the feed, at least 90 wt % of the lactate ion being present as free lactate ion and the remaining quantity of the lactate ion being present as undissociated acid.
BACKGROUND OF THE INVENTION
Many methods are known from the prior art. WO 99/19290, published on Apr. 22, 1999, describes various methods for purifying lactic acid from an aqueous composition of lactic acid and a salt of lactic acid (a lactate) dissolved therein, the aqueous composition having been obtained by fermentation of a suitable feed at a pH of 4.8 or less (claim 1). The pH of the aqueous composition is therefore moderately to strongly acidic, so that the composition contains a substantial amount of lactic acid and a substantial amount of lactate. According to page 4, lines 21 to 28 of WO 99/19290, this is actually advantageous, since the lactate after separation of lactic acid can be recycled to the fermentation step where it serves as a buffering agent for controlling the pH during the fermentation. On the other hand, page 14, lines 21 to 30 of WO 99/19290 states that the methods can be implemented using compositions having a pH of at least 0.86 to less than 6.0, which implies that at 25° C., the molar ratio of lactic acid and its salt in these compositions is between 1000: 1 and 0.007:1. Preferably, these compositions have a pH of from 1.98 to 5.00 (molar ratio of lactic acid and its salt is between 75:1 and 0.070:1) and in particular a pH of from 3.0 to 4.5 (molar ratio of lactic acid and its salt is between 7.0:1 and 0.23:1). The lactic acid obtained by the method according to WO 99/19290 is used, in particular, for preparing polylactic acid.
Page 40, line 23, to page 41, line 11, of WO 99/19290 describes a preferred embodiment, wherein a suitable culture medium is fermented to form a composition which contains lactic acid and a lactate. In a subsequent step, the solids are separated by means of, for example, filtration, flocculation, centrifuging or a combination of such techniques, to form an aqueous mixture of lactic acid and lactate. The aqueous mixture of lactic acid and lactate is passed directly to an extraction apparatus, although it is possible for the aqueous mixture to undergo an operation before being passed to the extraction apparatus. Such an operation, for example, is an adjustment of the pH by the addition of an acid such as sulphuric acid, lactate thereby being converted into lactic acid (p. 5, lines 8 to 14 of WO 99/19290). Such an operation would have the drawback, however, of leading to the formation of a waste product consisting, for example, of calcium sulphate (calcium sulphate is formed by the reaction of the lactate formed during the fermentation with sulphuric acid; in the course of the fermentation, for example in the start-up phase, calcium carbonate is then added to maintain the pH of the fermentation medium at a sufficiently high value). Moreover, p. 13, lines 33-36 of WO 99/19290 clearly states that the methods described in this reference are especially suitable for purifying lactic acid from relatively acidic culture media which need not be treated with an acid and which preferably have indeed not undergone such an operation.
According to the above-described preferred embodiment of WO 99/19290, the aqueous mixture of lactic acid and lactate is separated, by means of extraction, into an organic phase which mainly contains lactic acid and an aqueous phase which mainly contains lactate. This extraction can be carried out at a temperature of from 15° C. to 60° C. and atmospheric pressure, and according to p. 28, lines 29 to 32 of WO 99/19290, the organic extractant used comprises a tertiary amine, an oxygen-containing solvent and a hydrocarbon, preferably from 60 to 80% by weight of a tertiary alkylamine such as Alamine 336 (a mixture of trialkylamines having at least 18 carbon atoms, the trialkylamines containing decyl and octyl groups) which is marketed by Henkel Corp., from 5 to 20% by weight of methyl isobutyl ketone and from 10 to 30% by weight of a hydrocarbon such as Isopar K™. Then the organic phase which mainly contains lactic acid is subjected to a back extraction, the lactic acid again being transferred into a polar solvent phase, for example water. This back extraction takes place, for example, at a temperature of from 30° C. to 160° C. or more, and customarily at a temperature of from 90° C. to 160° C. (page 30 lines 26 to 28 of WO 99/19290). According to page 26, lines 22 and 23 of WO 99/19290, this back extraction can also be carried out at a temperature of at least 100° C., for example 150° C. or more, and at a pressure of at least 30 psi(g) (about 2.1 bar). The polar solvent phase which is obtained from the back extraction and which contains lactic acid is then concentrated by means of distillation, either the lactic acid or the polar solvent being separated as a vapour, this being dependent on the respective volatilities. P. 20, lines 15 to 21 of WO 99/19290, describes that such a distillation is preferably carried out at a pressure of less than 300 mmHg (about 0.4 bar), to prevent as far as possible the dimerization and oligomerization of lactic acid. The aqueous phase which is formed in the extraction and mainly contains lactate is recycled (optionally after purification) to the fermentation step. For this method to be feasible and effective, the aqueous mixture of lactic acid and lactate which is purified according to this method must necessarily contain a substantial amount of lactic acid and a substantial amount of lactate. It is therefore necessary for the aqueous mixture of the lactic acid and lactate to be moderately to strongly acidic, for example 4.8 or less. The pK
a
of lactic acid is about 3.86 at 25° C., and about 3.89 at 50° C.; see also FIG. 11 of WO 99/19290).
The above-described preferred embodiment of the method for purifying lactic acid has a number of drawbacks. For the purpose of large-scale purification or continuous purification, the extraction should be carried out under carbon dioxide pressure, as described in claim 30 of the U.S. Pat. No. 5,510,526, wherein the partial carbon dioxide pressure is at least 50 psi(g) (about 3.5 bar), relatively expensive equipment being required consequentially which is able to withstand a relatively high pressure. Furthermore, the kinetics of the extraction of the carbon dioxide pressure proves sluggish. It was found that for effective extraction, the contact between the organic phase and the water phase has to be maintained for about 100 hours to reach equilibrium in a stirred system.
Yet another drawback of the abovementioned preferred embodiment is that recycling of the mainly lactate-containing water phase to the fermentation results in accumulation of lactate and impurities in the fermentation. No mass balances are described in WO 99/19290, but it is necessary for part of the aqueous composition of lactic acid and lactate formed during the fermentation to be removed from the fermentation (via a so-called sink or “purge”), which is not described in WO 99/19290, however. The need for a sink or “purge” with a recycling step of this type is described, in contrast, in WO 98/15517 on page 5, last paragraph. WO 98/15517 and WO 99/19290, moreover, come from the same applicant and list the same inventors. The use of a sink or “purge” naturally reduces the yield of the purification. According to the preferred embodiment the throughput is therefore low, and the preferred embodiment has a relatively low yield per unit weight of feed per unit time. The lack of experimental data in WO 99/19290 moreover makes it very difficult for those skilled in the art to reproduce the method described therein.
The extraction under carbon dioxide pressure also gives rise to calcium carbonat
Breugel Jan Van
Dierdorp Brenda M.
Fisher Rod R.
Groot Wim J.
Malsam Jeff J.
Coudert Brothers LLP
Lilling Herbert J.
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