Microbial process for the preparation of acetic acid as well as solvent for its extraction from the fermentation broth

Abstract

A modified water-immiscible solvent useful in the extraction of acetic acid from aqueous streams is a substantially pure mixture of isomers of highly branched di-alkyl amines. This solvent is substantially devoid of mono-alkyl amines and alcohols. Solvent mixtures formed of such a modified solvent with a desired cosolvent, preferably a low boiling hydrocarbon which forms an azeotrope with water are useful in the extraction of acetic acid from aqueous gaseous streams. An anaerobic microbial fermentation process for the production of acetic acid employs such solvents, under conditions which limit amide formation by the solvent and thus increase the efficiency of acetic acid recovery. Methods for the direct extraction of acetic acid and the extractive fermentation of acetic acid also employ the modified solvents and increase efficiency of acetic acid production. Such increases in efficiency are also obtained where the energy source for the microbial fermentation contains carbon dioxide and the method includes a carbon dioxide stripping step prior to extraction of acetic acid in solvent.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This is a 371 of PCT / US99 / 20416, which claims the benefit of the priorities of U.S. patent application Ser. No. 60 / 099,438, filed Sep. 8, 1998; U.S. patent application Ser. No. 60 / 099,439, filed Sep. 8, 1998; U.S. patent application Ser. No. 60 / 099,440, filed Sep. 8, 1998; and U.S. patent application Ser. No. 60 / 099,475, filed Sep. 8, 1998.[0002]This invention has been partially supported by grants from the United States Department of Energy, Cooperative Agreement No. DE-FC02-90CE40939. The United States government has an interest in this invention.FIELD OF THE INVENTION[0003]The present invention relates generally to improved methods for the microbial production of acetic acid. More particularly, the invention relates to extraction of acetic acid from aqueous streams, and from the microbial fermentation of desirable chemical products from gaseous streams, such as waste gas streams, industrial gas streams, or from gas streams produced fro...

AI Technical Summary

Benefits of technology

[0065]Still another step in the process involves continuously distilling the acetic acid and water component away from the extraction product's solvent and water. To accomplish this step, the solvent / acid / water solution passes to a first distillation column, where this solution is heated to a temperature which reduces the conversion of the amines in the solvent to amides. As described above, the distillation temperature must range between 115° C., to a maximum of about 160° C. to permit acetic acid recovery, while limiting solvent degradation and amide formation. Preferably, the temperature of the distillation step does not exceed about 130° C., in order to prevent amide formation. A key advantage of the present invention is that the extracting and distilling steps occur without substantially degrading the solvent amine to an amide, and thus enhances the efficiency of acetic acid recovery from the broth.

[0066]Where the solvent / co-solvent mixture of this invention employs an azeotroping cosolvent, the distillation columns used in the process operate more efficiently. The formation of an azeotrope permits the cosolvent and the acid / water to move together (essentially as one) up and out the top of the first distillation column during the distillation step. In the liquid form, the cosolvent and the acetic acid / water separate. Once separated, the cosolvent can be reintroduced into the distillation column. The acetic acid and water (and some residual cosolvent) then pass to a second distillation column where the cosolvent again forms an azeotrope with water and acid, and the three components flow as a vapor out the top of the column. The vapor is condensed and most of the liquid is refluxed. Because the condensed liquid contains a small amount of cosolvent, a small stream is continuously returned to the solvent distillation column. The product acetic acid is pulled out just above the first theoretical stage, i.e., the portion of the column where the solvent and acid separate.

[0067]A preferred embodiment of this method involves performing the distillation step under an oxygen-free vacuum, which also serves to reduce the temperature and avoid oxidative degradation of the solvent of solvent / cosolvent mixture. The higher the vacuum (i.e., lower absolute pressure) the lower the temperature and the less amide formation and oxidative degradation. As described above, the vacuum is preferably less than 10 psia. Desirably a vacuum of between about 0.1 psia and 5 psia is useful in the distillation step. More preferably, a vacuum of 4 psia or less is useful in this distillation step to further reduce the boiling point of the solvent / acid / water mixture, further reducing amide formation and enhancing recovery of the acetic acid. As yet a further advantage of the use of the modified solvent / azeotroping cosolvent mixture of this invention is the use of two distillation columns to accomplish enhanced recovery of acetic acid from aqueous phases in comparison to processes of the prior art.

[0068]The control of distillation temperature in the processes of this invention to limit solvent degradation may be accomplished by a combination of factors, such as selection of the cosolvent, ratio of solvent to cosolvent and conditions of vacuum for the distillation step. Given the teachings of this specification, one of skill in the art may select the appropriate combination of factors to control the distillation temperature as required. For example, one of skill in the art may readily adjust the temperature and vacuum conditions of the distillation step within the above ranges to achieve a desired efficiency of acetic acid recovery according to this invention. Such modifications are encompassed within the appended claims.C. Extractive Fermentation and Direct Contact Extraction Method

[0069]According to yet another embodiment of this invention, the above-described novel modified solvent / cosolvent mixtures are useful in a process of “direct contact extraction” and “extractive fermentation”, i.e., modifications of the anaerobic fermentation production process for the recovery of acetic acid described above. The modifications of the process allow the production of acetic acid via microbial fermentation without the need for bacterial cell separation prior to extraction or distillation. Further, these solvent mixtures when used in microbial fermentation of acetic acid can eliminate the need for use of a separate extractor. In addition to reducing the complexity of the process, this invention reduces the capital, operating and maintenance costs of the equipment needed to perform the process of producing acetic acid, as well as the time to obtain the product.

[0070]Thus, the “extractive fermentation” method of the invention provides an anaerobic microbial fermentation process for the production of acetic acid, which is a modification of the process described above. As a first step, the bioreactor or fermenter containing anaerobic acetogenic bacteria in a suitable nutrient mixture necessary for growth of the bacteria is contacted with the modified solvent / cosolvent mixture described above at about 37° C. and at least about one atmosphere of pressure (i.e., 14.7 psia) for a time sufficient to acclimate the bacteria to the presence of the solvent, i.e. to permit the bacteria to grow in the presence of the solvent. The anaerobic bacteria may be one strain of bacteria or a mixed culture containing two or more strains of acetogenic bacteria; the bacterial strains listed above in Part B may also be used in this modification of the invention. As many solvents are toxic to bacterial growth, this aspect of the invention involving direct contact between the bacteria and the solvent reflects the acclimation of the cells to solvent mixture, which is obtained by gradually increasing contact between the cells and solvent mixture over time.

Problems solved by technology

Despite such knowledge and advances in the art of microbial fermentation of a variety of gas streams, acetic acid production is limited by the acetic acid loading potential of the solvent used, and by the degradation of the solvent as it travels through the production process, among other issues.

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