Batch separation of tetrafluoroethylene, hexafluoropropylene and octafluorocyclobutane

Abstract

This dissertation details research aimed at designing a small batch distillation column to purify tetrafluoroethylene and hexafluoropropylene from a mixture containing tetrafluoroethylene, hexafluoropropylene and octafluorocyclobutane. As no vapour-liquid equilibrium data are available for these chemicals in this mixture, new vapour-liquid equilibrium data were experimentally generated and modelled for use in the design of the batch distillation column. The data were fitted to the Peng-Robinson equation of state, utilizing the Mathias-Copeman alpha function. The model was used with the Wong- Sandler mixing rules alongside the NRTL alpha function. The model was fitted with mean relative deviations lower than 1.2 %, indicating an acceptably accurate description of the VLE data gathered by the model. The experimental data and the model also passed the thermodynamic consistency test for all the systems and isotherms. The design simulations were completed by means of the Aspen Batch Distillation, a module of the Aspen Technologies package. The results show that the optimum design for recovering high-purity products requires six equilibrium stages in the column. The batch column should consist of a still pot, also functioning as a reboiler, a packed column section and a total condenser. The total condenser and the reboiler both count as equilibrium stages. Using this design, a TFE product purity of 99.999 % is predicted with a recovery of 96 %. An HFP product purity of 99 % is predicted at a recovery of 68 %. The recovery of the HFP product can be increased, but entails a significant loss of product purity. The minimum column diameter required to achieve the flow rates suggested in the simulation is 29 mm. The column diameter was selectedas 1¼ ″ (or 31.75 mm) on the basis of the standard pipe diameters available in the industry. Pall ring packing is suggested for use in the column, with an estimated maximum HETP of 0.5 m. As there are five equilibrium stages in the column itself, the column has to be at least 2.5 m high. Copyright