Effect of Internal Heat Exchange Rate Distribution on Energy Saving in Heat Integrated Distillation Column (HIDiC)

The optimal allocation of the internal heat exchange rate to the stages of an internally heat-integrated distillation column (HIDiC) was investigated as a fundamental study for developing a column of simpler structure. In the HIDiCs hitherto proposed, which consists of a double pipe or plate-fin type heat exchanger, the internal heat exchange rate at each column section is determined by the temperature difference profile between the rectifying section and the stripping section. The effect of internal heat integration on the reduction of reboiler heat load depends strongly on the location of heat exchange stages. To quantify this relationship, the heat exchange between a rectifying stage and a stripping stage is first considered fot the constant number of stages and constant compressor power, and the reduction of reboiler heat load by the heat integration was calculated. The result showed that there exists an optimal combination of the rectifying stage and the stripping stage that minimizes the reboiler heat load. Next, the optimal allocation of heat exchange rate among multiple stages was derived for a fixed total heat transfer area. The result showed that the reboiler load can drastically be reduced by assigning the heat exchange rate to a few stages based on the "energy-saving indices" obtained from the simulation results of single heat exchange between a rectifying stage and a stripping stage. Furthermore, the optimization results showed that the assignment of the heat exchange rate to the stages based on the "energy-saving indices" agreed closely with that obtained by rigorous optimization.