A Process of Industrial Significance
Acetic Acid/Water/Ethyl Acetate System:
An important industrial system concerns the treatment of acetic acid aqueous solutions of three types: 0 - 2%, 2 - 20%, and 20 - 40%. (Bart et al. 17-18) This process of recovering acetic acid from water is one of the oldest applications of solvent extraction having been proposed by T. Goring more than a century ago. (German Patent 28064 Dec. 18, 1883) The importance of such a separation technique is a result of the many manufacturing processes that yield aqueous waste or by-product streams containing acetic acid. Some such processes include the manufacture of aspirin, camphor and the explosive RDX as well as the manufacture of acetic acid itself that also involves a water separation step. (C. Judson King 567)
There are several competing processes for the recovery of acetic acid from water including liquid-liquid extraction, azeotropic distillation, and extractive distillation. The avoidance of simple distillation to perform this separation stems from the relative volatility between water and acetic acid being very close to unity and the higher volatility of water (meaning that all the water must be taken over head) leading to large energy costs per unit of acetic acid recovered. Other suitable processes include: freeze concentration (a backyard process for the concentration for vinegar; adsorption with carbon or anion exchangers, and chemical derivatization, followed by separation of the derivatives and reconversion.
Using cost factors appropriate for the times of these processes, Brown (Brown, W.V. Chem Eng Progr 59 (10) 65-68 (1963)). determined extraction to be the most favorable approach except for feeds above 80% acetic acid content weight for weight (w/w), in which case azeotropic distillation became preferable. Eaglesfield (Eaglesfield et al., Ind Chem. 29, 147, 243 (1953) on the other hand, concluded that extraction was preferred for feeds containing up to 35% acetic acid w/w, with azeotropic distillation being more attractive for more concentrated feeds. Brockhaus and Forster (Brockhaus, R.; Forster F. Ullman's Encyklopadie der technischen Chemie, 4th ed, Vol. 11, Verlag Chemie, Weinheim, 1972, 57-74) support the conclusion of Eaglesfield but also indicate that simple fractionation could be considered for feeds containing only a few percent of water. (C. Judson King 568)
In addition to the extractor itself, a method is also needed for the regeneration of the solvent and recovery of acetic acid. Since many solvents used tend to have substantial water solubility themselves, there is usually also some means made available to remove the residual solvent that is is in the raffinate.
seen in the diagram, solvent regeneration is accomplished by distillation.
Since the desired product is usually glacial acetic acid and since most
solvents coextract substantial amounts of water, it becomes necessary to
concentrate the extracted acid further. This is done after solvent
regeneration, but again the low volatility of water relative to acetic
acid makes it difficult to carry out by simple distillation and as such
azeotropic distillation is the usual means of removing water from the extracted
acid. This can be performed by adding a separate entrainer and carrying
the azeotropic distillation out in a separate tower following solvent regeneration
but if often makes sense to choose a solvent for extraction that itself
can serve effectively as the entrainer for the azeotropic distillation.
These two operations of solvent regeneration and acid concentration are
usually combined into one column as shown in the above diagram, with an
overhead phase separator drum serving to segregate water sufficiently from
the recycle solvent. This separated water is then fed to the solvent
recovery stripping column (column that purifies raffinate). (C. Judson
King Pg. 568)
An important decision regarding the choice of a solvent is whether to use a solvent with a higher or lower boiling point than acetic acid. The use of a higher boiling solvent usually leads to lower steam requirements for the reboiler of the solvent regenerator since the acetic acid and coextracted water, rather than all the solvent are taken overhead. So with a high boiling solvent, the acetic acid is recovered as a distillate product whereas with a low boiling solvent it comes as a still residue, which may contain heavy impurities. The high boiling solvent, however, can collect heavy impurities itself thereby leading to a need for a continual or periodic solvent purge. Use of a high boiling solvent also precludes the combination water removal and solvent regeneration into a single tower.
important property in solvent selection is the equilibrium distribution
which is expressed as the weight fraction of acetic acid in the solvent phase divided by the weight fraction of acetic acid in the aqueous phase at equilibrium. Alcohols, although they provide a high KD value, tend to esterify with acetic acid and hence are seldom used. Ketones too have a high KD value but do not give good azeotroping properties for the removal of water by the post-extraction azeotropic distillation. The solvents that are most commonly used are the acetates and ethers.
When choosing a solvent it is important to attain as high a KD value as possible to minimize the needed solvent flow rate. In a counter current extractor, KD* S/W (where S/W is the mass flow ratio of solvent to water) must be greater than one and is typically 1.3 - 2.5. With this, the smaller the value of KD the larger the value of S that is needed to maintain the relation. Aside from having a high KD value, the solvent should also have a high selectivity for the extraction of acetic acid over water so as to reduce the amount of water that must be removed from the acetic acid after extraction.
Other desirable properties for solvents include low solubility in water so as to reduce the need for raffinate purification, adequate volatility of the solvent (to facilitate the distillation of the solvent from acetic acid in the regeneration of solvent and the stripping of residual solvent from the raffinate in raffinate purification), density, interfacial tension, price, stability, toxicity, and compatibility with system contaminants.
Ethyl acetate was the recommended solvent, by Eaglesfield et al, for the extraction of feeds containing up to 16% w/w acetic acid. Above 16% acetic acid in the feed, it was found that the limit of the ability of the solvent-entrainer to carry all the coextracted water overhead became controlling and as such the group recommended the use of an ethyl acetate-benzene mix for feeds between 16 and 25% w/w acetic acid. Benzene is added to the solvent to suppress the coextraction of water. (C. Judson King 568-570)