Ind. Eng. Chem. Res. 1999, 38, 4123-4124
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Extractive Condensation: A New Separation Process K. J. Zeitsch† Duerener Strasse 393, D-50935 Cologne, Germany
A new highly selective vapor-phase extraction process is described. Hydrogen bonding between a scavenging extractant and the substance to be extracted results in a high-boiling complex forming fog droplets readily separable from the remaining vapor. The process is exemplified by the extraction of acetic acid from the predominantly aqueous vapor stream of furfural reactors. Triethylamine is used as the extractant. In solvent extraction, use is made of weak intermolecular forces selectively attracting some molecules more than others. This concept of selective attraction can be extended to the point of using an “extractant” capable of forming a chemical bond with the molecules to be extracted, but not with other molecules. In this case, the “extractant” can be said to “react” with the partner molecules. In so doing, a new species is formed which will have different physical properties, and in a favorable case these new physical properties will be conducive in facilitating the separation intended. To the end of extracting acetic acid (AA) from water, it is possible to make use of the fact that AA reacts with triethylamine (TEA) in forming a high-boiling complex held together by hydrogen bonds. When four molecules of AA (bp 118 °C) react with one molecule of TEA (bp 89 °C), the resulting complex has a boiling point of 165 °C as shown in Figure 1, thus readily permitting its separation from water by distillation. However, when dealing with a very weak aqueous solution of AA, as in the case of the wastewater of furfural plants, a separation of the complex from the water by distillation would require evaporating a huge quantity of water, and although this distillation would be easy because of the great difference in boiling points, in terms of economy it would not be significantly better than distilling the water from the AA without complex formation. Consequently, to make use of the complex in an economic fashion, the separation of the complex from the water must be based on a different principle. In furfural plants, this happens to be possible as the reactors emit a mixture of AA and water in the vapor phase. When TEA vapor is injected into the gaseous product stream of furfural reactors, the ensuing formation of the TEA/ AA complex results in the appearance of a fog since the complex has a higher condensation point (dew point) than steam. This effect can be readily demonstrated by a laboratory setup as shown in Figure 2. TEA vapor is admitted into a vapor stream produced by evaporating a 1% aqueous solution of AA at slightly reduced pressure. When the two vapor streams meet in a REITMEIR head, a dense fog is observed, and the fog droplets coalesce on the wall of the REITMEIR head to form a small fraction of the complex and water. With such a simple setup, 72% of the AA evaporated is found in the “fog fraction”. Of course, in an industrial installation, the REITMEIR head must be replaced by a unit operation capable of separating the fog droplets from the remain†
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Figure 1. Ebullition diagram for the triethylamine/acetic acid system at 760 mmHg.
Figure 2. Laboratory demonstration setup for the “fog process”.
ing vapor mixture to a high degree of perfection. To this end, use can be made of a coalescence filter or of an electrostatic separator. The latter is based on the fact that fog droplets are electrically charged. The concentration of AA in the “fog fraction” is essentially identical to the concentration at the maximum boiling point as only the molecules of the TEA/ AA complex undergo condensation. There is no problem with the introduction of TEA in a furfural plant for the following reasons: (1) Furfural plants must be explosion-proof anyway, on account of highly flammable byproducts such as acetaldehyde, acetone, methanol, and the like. (2) There is no problem of furfural contamination, as in any furfural plant the low boilers are taken out by a special column. (3) TEA does not react with furfural. The complex, once recovered, can be readily split by reacting it with ethanol at an elevated temperature in the presence of an ion-exchange resin. This leads to
10.1021/ie990232a CCC: $18.00 © 1999 American Chemical Society Published on Web 09/18/1999
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
ethyl acetate, triethylamine, and water. It is noted that many furfural plants are parts of cane sugar mills since they use bagasse as the raw material. Given the fact that such mills make molasses as a byproduct, they usually have an ethanol plant to use the molasses, so that the ethanol needed for the esterification is an inhouse commodity. The ethyl acetate represents a profitable byproduct. The method of injecting a suitable reagent into a vapor mixture to selectively capture a target compound by forming a fog has been given the name of “extractive condensation”.1 It is a new separation process of unerring specificity. Systems potentially suitable for the new process are readily found by looking for partners producing distinct peak-type maximum azeotropes analogous to Figure 1. Broad, flat maximum azeotropes are not suitable as they lack selectivity, and as their boiling point rise is too small to effectively produce fog formation. The difference
between peak-type and flat maximum azeotropes lies in the strength of the hydrogen bonds involved. Depending on the structure of the partners, hydrogen bonds can range from 3 to 8 kcal/mol (12.552 to 33.472 kJ/ mol).2 Carboxylic acids are particularly well-scavenged by tertiary amines, pyridine, and picolines. Literature Cited (1) Zeitsch, K. J. Verfahren zur Gewinnung von Essigsaeure aus einem die Saeure in geringer Konzentration enthaltenden Wasserdampfstrom. German Patent Application P 40 25 128.4, Aug 8, 1990. (2) March, J. Advanced Organic Chemistry; John Wiley & Sons: New York, 1992.
Received for review March 29, 1999 Revised manuscript received August 2, 1999 Accepted August 2, 1999 IE990232A
