It is of great importance to remove and recover acetic acid or the other low carboxylic acids from their dilute aqueous solutions or wastewater streams, such as in furfural production, petroleum production from coal by Fischer-Tropsch synthesis, pharmaceutical and pesticide production. At present the extraction based on chemical complexation by using trialkylamines or trialkyl phosphine oxide has been generally employed to industrially separate acetic acid or the other lower carboxylic acids from dilute acid solutions. Complexation-based extraction gives a large distribution coefficient which thus leads to a much lower ratio of the volumes of organic and aqueous phases. However, the generation of the extractant by distillation needs an operation at high vacuum and high temperature. Thus, the thermal degradation during the distillation may occur. Furthermore, although the acid is retrieved, there is a great loss of extractant in the raffinate phase, which results from physical dissolution loss, chemical-complexation dissolution loss and emulsion loss, makes the chemical oxygen demand (COD) of raffinate increasing by 1200 ~ 3000mg/L. Accordingly a new pollution formed and it is difficult to solve this problem. Therefore removal/recovery of acetic acid or the other lower carboxylic acids in an economical and environmental-benign way is much needed.
A physical extraction-distillation process was thus developed. The extractant used was an ether with a low density, low boiling point, low viscosity and low surface tension. To ensure that the recovery of acetic acid or the other lower carboxylic acids was greater than 99wt%, a compound with strong affinity with acetic acid was selected as the modifier with which the distribution coefficient of acetic acid can be increased by 3-6 times. As a following step, the raffinate stream was sent to a distillation column to remove and recover extractant and modifier as a top product, which is actually an azeotropic mixturee of ether, modifier and water with a low boiling point. The bottom product was nearly solvent-free water which could be reused or discharged directly. More importantly, the distillation-based separation of extractant (ether and modifier), water and carboxylic acid is naturally an azeotropic distillation. In such a process, the extract stream to separate acetic acid and the extractant, the extractant (ether and modifier) were also used as the entrainer which bring water to the top stream and thus lead to a nearly complete pure product of acetic acid or the other low carboxylic acid in the bottom product stream. This resulted in a significant reduction of steam requirement, and use of steam with lower-temperature or -pressure. In comparison with the commercial technique of complexation extraction by using trioctylamine, this physical extraction-distillation process has better economic and environmental benefits.