Possibilities to Model Extractive Distillation Columns.

In order to prevent considerable losses of the solvent dimethyl formamide if benzene of high purity was seperated from crude hydrocarbon fractions by extractive distillation versions, a feed of steam usually is added. Computative modeling of this procedure using different calculation programs on the base of 'local-composition concept' activity coefficient model equations exhibit a significant demixing in the liquid phase. Calculation results have been compared with those, obtained from practical experience in laboratories or plants. There must have been noted, that an increase of amounts of dimethyl formamide vanishing within the vapor phase at the top of the column due to the extent of steam addition, will be reflected when the NRTL equation has been applied. This effect does not agree with observations in the plant, because really very small losses of solvent do appear. Calculations with aid of UNIQUAC model equation have given rather better correspondence with experiments.