Decrease of energy demand of semi-batch distillation policies

Batch distillation (BD) has much higher energy demand than continuous distillation (CD). Different semi-batch distillation (SBD) policies, by which one part of the feed is continuously fed into a batch column, are investigated and optimised to minimise the energy demand. SBD has the advantage, that contrary to the batch distillation (BD), the liquid volume in the reboiler, can be kept constant, even at its maximal value. Usually, continuous feeding is performed into the reboiler (SBD1), but by shaping a feed plate in the batch column, it has not only a rectifying but also a stripping section. Several SBD policies differing in feeding location are studied by dynamic simulation and compared based on the specific energy demand (SED) of production. The different policies and effects of the variation of feed composition are studied for the separation of the mixture dichloromethane-acetone. The lowest SED is obtained by SBD with feeding at a variable location of the column (SBD3). However, feeding at a fixed location (SBD2) is much simpler to realise and only slightly inferior. Further reduction of SED is reached by using two SBD2 or one SBD2 and one BD step with different reflux ratios. The separation of the mixture n-hexane-n-heptane-n-octane is also studied by SBD1 and SBD2 policies with both direct and indirect sequences. The energy demand of SBD2 in indirect sequence is lower than that of the direct sequence by 25%. By applying SBD2, SED was reduced by 38% and 39% for binary and ternary mixtures, respectively, compared to BD.