Bottom flashing with interreboiling action in a transient batch rectifier: Economic feasibility, dynamics and control

This work aims at proposing a batch rectifier with bottom flashing (BRBF) scheme, in which, a fraction of reboiler content is dynamically flashed to lower its temperature so that the exiting liquid can be used as a heat sink against the overhead vapor (heat source) in the condenser. Receiving latent heat from that vapor, the flashed liquid gets vaporized and merges with the flashed vapor. Recovering pressure of this combined stream, it is used as a boil-up vapor in the reboiler, thus lowers the external heat duty. By this way, the proposed BRBF arrangement reduces the utility consumption in both the reboiler and condenser. To improve the energetic potential further, the boil-up vapor produced in the flashing loop is proposed to split into a couple of fractions and distributed in intermediate stage(s) along with the reboiler. The BRBF coupled with this interreboiling action is subjected to economic feasibility test, for which, the concerned column configurations need to be dynamically close, if not same. Since the batch processing is inherently a transient operation, it is not trivial to have their dynamics close. To make the comparison meaningful, we formulate an open-loop control scheme that makes sure that the BRBF follows the conventional batch rectifier (CBR) dynamics, which is used as a basis. Finally, a binary system is simulated to demonstrate the proposed BRBF with and without interreboiling. It is investigated that the BRBF configuration provides an about 68% energy savings with a payback time of 3 yr. Further, proposing interreboiling in the BRBF column increases the energy savings to 81%, reducing the payback time to 2.32 yr.