(326p) Design of Distillation Process for Separation of Hfp/tfe

Hexafluoropropylene (C3F6, HFP) is produced 5 to 20 wt% as a byproduct when poly(tetrafluoroethylene) is thermally decomposed to get tetrafluoroethylene (C2F4, TFE) which may be used as a feed gas for the synthesis of another fluoro-compund. Up to now, HFP has been supplied as an inert material together with TFE, resulting in the unstable reaction condition as well as the incomplete separation of the final products. If HFP can be separated from the mixed gases prior to their usage as a feed gas, not only it might help to keep the stable reaction condition but also the separated HFP may be utilized as a raw material to the synthesis of a novel interfacial monomer which could be used for the electronic and the chemical industries. In this study, the Aspen Plus 2004 software is used for the design of distillation process for separation of TFE from the HFP/TFE mixture gas.

Both the optimal theoretical number of plates and the optimal reflux ratio were looked for as the distillation tower was designed using the Aspen Plus 2004. There are several input specifications to be taken into account: the components of the feed gas and their concentrations, the concentration of TFE in the top and bottom products, the type of the condenser, and the thermodynamic equations to describe the states of the mixture. The two steps of computer simulation were carried out: the rigorous simulation by RadFrac following the shortcut calculation by DSTWU. The Peng-Robinson equation of state was used to describe the state of the C2 and C3 gas mixture. Both the complete condenser and the partial condenser were selected by considering the process following the distillation in which HFP is used as an input material. For both types of condenser, the optimal design of the distillation tower was performed in terms of the theoretical number of plates and the reflux ratio at the given operating temperature and the pressure.

For a typical feed gas composed of 6.9 mol% HFP and 93.1 mol% TFE, the input gas is fed to the 10th plate among 22 plates equipped with the partial condenser, which is operated at -10°C and 13.09 atm. With the reflux ratio of 0.6448, the TFE concentration in the top product is obtained to be 99.8 mol% while the HFP concentration in the bottom product is 99.8 mol%.