Simulation of the water-acetic acid separation via distillation using different entrainers : an economic comparison

Diluted solutions of acetic acid (AA) in water (W) are a typical side stream in several production processes including therephthalic acid synthesis, acetyl cellulose manufacture and biochemical processes. Since AA is typically in the range of 10 to 40 % by weight in W, for most companies it is profitable to recover and recycle it as a solvent (Dylewski, 1980). Among the separation processes, distillation is the most adopted because of its flexibility and the relatively low cost compared to other technologies such as membrane separation or pervaporation. The challenge in the separation of W and AA through simple distillation is to find in the volatility of AA in the lower range of concentration, which tends to the unit (Ito and Yoshida, 1963). In other words, even if the mixture is never azeotropic, the equilibrium diagram exhibits a pinch point on the pure water end. An entrainer, either with lower or higher boiling point compared to the two main components, alters the volatilities of W and AA, modifying their activity coefficients and improving the separation. However, the introduction of a third component usually requires a second distillation unit to recover the entrainer (Wang and Huang, 2012). Among the entrainers proposed in the literature, AA esters are common for their efficiency. For instance, Chien et al. (2004) report a bottom product composition with 99.9 % acetic acid for ethyl, iso-butyl and n-ethyl acetate for molar ratios AA/W of 1:1 with only one distillation step. In the context of the therephthalic acid production process, we recently demonstrated that the use of p-xylene is profitable because of its availability in-situ (Pirola et al., 2013). We study the W-AA separation with different entrainers (e.g. p-xylene, ethyl acetate, propyl acetate and butyl acetate) and survey alternative configurations including a pre-reactor to synthesize the entrainer in-situ, and an intensified reactive distillation. We selected an algorithm based on the Guthrie equations to calculate the cost of both CAPEX and OPEX (Guthrie, 1969) with the simulation software PRO/II 9.3. The data confirm that propyl acetate is a practical and economical alternative to methyl acetate for the acetic acid recovery from water, and the p-xylene avoids introducing an extraneous component as an entrainer.