Simulation of the acetic acid dehydration collected from terephthalic acid production

Abstract

Terephthalic acid is an important raw material for the production of polyethylene terephthalate (PET). It is obtained by the catalytic oxidation of p-xylene in air, in the presence of acetic acid (HAc) as solvent. Once the synthesis is done, the solvent is to be recycled and reused back to the chemical reaction media. In the reactor output residual stream, the solvent is mixed with other reaction products (great amount of water, traces of methyl acetate) and unreacted p-xylene. High investment and operating costs are required if conventional distillation is used in the HAc dehydration unit (solvent purification) due to a tangent pinch on the pure-water end in the HAc-water system. An entrainer is often introduced into the unit in order to make the separation easier by using a heterogeneous azeotropic distillation (HAD) column. Given the non-idealities, the phase splitting, the distillation boundaries present, and the possible existence of multiple steady states in such a system, such columns can be extremely difficult to simulate and to operate. This work proposes a simple methodology to simulate the described process of acetic acid recovery using a heterogeneous azeotropic distillation column. A preliminary study of the thermodynamic properties of the mixture is performed in order to understand the system behaviour and to predict possible results.