Recovery and Purification of Chemicals
Apart from its application as a multifuctional reactor, RD can be looked upon as an efficient separator for the recovery or purification of chemicals. The reversible reactions such as esterification and acetalization can be exploited for this purpose. The component to be removed is allowed to react in the distillation column and the resultant product can be separated simultaneously. The esters or acetals formed can be easily hydrolyzed back to the original component under different conditions.
Dilute acetic acid is produced in large quantities in many processes such as manufacture of cellulose esters, terephthalic acid, and dimethyl terephthalate; and in reactions such as acetylation and nitration. The recovery of acetic acid from these streams is a daunting problem. The conventional methods of recovery are azeotropic distillation, simple distillation, and liquid-liquid extraction. With the advent of RD processes, esterification of acetic acid with methanol seems to be an attractive alternative. Neumann and Sasson  carried out laboratory experiments to recover acetic acid in an RD column through esterification with methanol. Commercially available ion-exchange resin particles were used along with Raschig rings in the column. The use of a solid acid catalyst offers non-corrosive conditions so that a less expensive material of construction can be used. Up to 84 % recovery of acetic acid as methyl acetate was achieved. Xu et al.  have recently performed detailed experimental and simulation work on recovery of acetic acid from about 10% (w/w) aqueous solutions using the same reaction with Amberlyst-15 in the form of catalyst baskets. More than 50% recovery was obtained in a 1.5 m high column. They also explained their experimental results with the help of a steady state simulator. Hoechst Celanese Corporation have described an RD process for the recovery of acetic acid from aqueous solutions as methyl acetate. With the use of acidic ion-exchange resin as catalyst, more than 90% recovery from 530% aqueous acetic acid is described. They also suggest the use of Koch Engineering's Katamax packing as the catalyst for this reaction . Apart from ester-ification with methanol, esterification with other alcohols may also be used for recovery. Saha et al.  have explored the possibility of esterifying acetic acid from aqueous solution with n-butanol in an RD column. In this case one gets an over head product of composition close to the ternary heterogeneous azeotrope of buta-nol, butyl acetate, and water. The high-molecular weight acids such as lactic acid and myristic acid can be successfully recovered through esterification with alcohols such as methanol and isopropanol respectively [16, 17]. The same logic may work for the other acids such as adipic acid, succinic acid, chloroacetic acid, trifluoroa-cetic acid, and glycolic acid.
RD can be applied for recovery of many other chemicals from their dilute streams. The polymer industry is often faced with the challenge of treatment of aqueous formaldehyde solutions, which are environmentally damaging but difficult to remove. RD with methanol, ethanol, or ethylene glycol not only brings down the formaldehyde concentration to ppm level but also yields useful acetal products [34, 35]. On the other hand, the high-boiling alcohols such as ethylene and propylene glycol from fermentation streams can possibly be recovered through acetalization with either formaldehyde or acetaldehyde in an RD column . Similarly, non-boiling chemicals such as glyoxal and glyoxylic acid can be recovered from their aqueous solutions through the formation of their corresponding acetals or esters, which can be separated by distillation [107, 108].
RD is reported to have been employed for purification of phenol as the raw material for the manufacture of bisphenol A of polycarbonate grade. The impurities, in the form of carbonyl compounds such as acetone, mesityl oxide, and hydrotro-paldehyde, are required to be reduced from about 3000 ppm to vanishing levels (< 10 ppm). A continuous RD column has been described as a versatile method to achieve this objective . Hydrogenative distillation has been demonstrated to be an attractive choice for the removal of the impurities of carcinogenic benzene from light reformate and a-methyl styrene from cumene [64, 65]. The acrylonitrile product stream has been successfully purified by facilitating the reaction of aldehyde (e. g., acrolein) impurities with suitable amine in RD . Similarly crude acetone may be purified by removing aldehyde (formaldehyde or acetaldehyde) impurities through the reaction with diamines in RD .
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