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Fig. 18. One new flowsheet of separating C4 mixture by extractive distillation. 3. HEAT PUMP AND MULTI-EFFECT DISTILLATIONS

There are few reports about heat pump distillation in the international journals, only one [28] found by title search in ISI Web of Science (http: //wos5.isiknowledge.com). It implies that this technique of heat pump distillation isn't attractive in theoretical and practical aspects, and gains no wide attention in application, especially in special distillation processes.

The thermodynamic efficiency of a distillation column is usually quite low while at the same time the energy consumption is high. This is due to the fact that energy separating agent in the distillation process depends directly on the amount of energy used and the recovery of this energy is often low. A brief survey has been carried out by Bjorn et al. [28] of the possibilities of improving the energy conservation quotient and increasing the thermodynamic efficiency for a single distillation column system utilizing various heat pump arrangements, paying attention to the various aspects of both open and closed types of systems. The use of more elaborate systems involving intermediate heat exchanger is, theoretically, necessary in order to achieve a higher thermodynamic efficiency; in practice, however, these systems don't always turn out to be economically viable when compared with simpler arrangements.

Heat pump distillation is promising in the case where the recovering energy is very large and the temperature difference between the top and the bottom is not distinct, often below 20 °C. For example, in the propane / propylene distillation column, the amount of feeding mixture to be dealt with is often so high, up to 10-100 t h"1, the temperature difference being about 10 °C. Hence, the absolute value of energy heated at the bottom is very large. In order to relieve the energy, heat pump is employed by using a part of bottom product as refrigerating agent to connect top and bottom of the distillation column. It is reported that if a heat pump of open type with adiabatic flash and compression can improve thermodynamic efficiency from 20% to 70% [29, 30],

Unlike heat pump distillation, multi-effect distillation is widely used in practice, especially in seawater desalination [31-37], Strictly speaking, multi-effect distillation is exactly multi-effect evaporation with only one theoretical stage, i.e. the reboiler. One theoretical stage is enough because salt doesn't appear in the vapor phase. The design and synthesis of various multi-effect distillations are present convincingly by Richardson and Harker [38], It is beyond our scope to extend this content. The interested reader can refer to it.

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