ATTAINABLE PRODUCTS FOR THE VAPOR-LIQUID SEPARATION OF HOMOGENEOUS TERNARY MIXTURES

An equilibrium vapour-liquid separation process for some feed has a se t of obtainable product compositions. Current research activity has fo cused on the feasible products from ternary homogeneous mixtures separ ated using distillation columns. The conclusions reached have been con tradictory and have been undermined by the simplifying assumptions use d to obtain them. This work considers the compositions of products att ainable from homogeneous ternary mixtures in processes that involve se paration and mixing. The discussion is not limited to column distillat ion, and considers separation by simple distillation and by one or two equilibrium flash stages. It is found that, for processes involving s imple distillation and mixing, the region of attainable products is th e entire composition space, so any product may be obtained for a given feed. It is concluded that separation is not inherently restricted bu t that the choice of separation equipment imposes limitations on the a chievable product compositions. The region of achievable products for a single flash stage and mixing - and possibly a recycle stream - is f ound. The product regions of three configurations of two flash stages, i.e. in parallel, in series and in series with reflux, are found. The third configuration is essentially a two-stage distillation column. I t is found that, of the arrangements of two flash stages in series, wi th or without reflux, neither consistently outperforms the other. In o ther words, both processes achieve products which may not be obtained by the other. Furthermore, some products may be obtained for both proc esses but with vastly different cost implications, in terms of capital and operating costs. It is striking that sometimes a two-stage distil lation column cannot achieve what a serial arrangement of two flash st ages may achieve. Therefore, the assumption that distillation columns should invariably be used for separation is called into question.