Design and control of an energy-efficient alternative process for the separation of methanol/toluene/water ternary azeotropic mixture

Separation of a ternary mixture containing methanol/toluene/water is a tough task because toluene forms azeotropes with both methanol and water. A recent paper proposed using N-methyl-2-pyrrolidone as heavy entrainer and included a two-column extractive distillation system plus a decanter for the separation of this ternary mixture. In this paper, an energy-efficient simpler process composed of an extraction column and a regular distillation column is investigated in order to achieve the same separation task. Water is used as the solvent to extract methanol from toluene in the extraction column. Toluene is obtained as raffinate while extract phase containing water and methanol is sent to a distillation column for the purification of these two components. Because water itself is a component in the separation mixture, there is no need to introduce any foreign component into the system and hence avoid the possibility of contaminating the products. To compare the optimal result of this design flowsheet with the recently published separation method, our proposed design provides 56.3% savings in total annual cost and 57.1% savings in annual operating cost. Furthermore, a control structure with adjustable solvent flowrate is devised for our proposed process based on the results of open-loop and closed-loop sensitivity tests. Closed-loop dynamic responses show that all products can be maintained at high purity despite large throughput and feed composition disturbances.