The Study Of Multi-effect Distillation And Heat Pump Technology Used For Low-concentration Wastewater Treatment

Energy is the basis and driving force for national economic development. Inrecent years, with the rapid development of economic, there has been a significantincrease in energy consumption, so domestic energy resources face a potentialaggregate shortage. Therefore, energy conservation is an important link of our county’senergy strategy. In this paper, to solve the problem of the high energy consumption forthe recovery of low-concentration wastewater recycling processing, the multi-effectdistillation and heat pump technology for low concentration wastewater treatmentwere studied according to thermodynamic energy conservation principle, in order todevelop new energy-efficient process and technology in low-concentrationwastewater recycling treatment.The main contents and conclusions in this work include the following aspects:（1） Taking DMF wastewater treatment which mass concentration was5%forexample，two-effect、three-effect、four-effect distillation without preheating system，and two-effect、three-effect、four-effect distillation with multi-stage feed preheatingsystem were simulated and analyzed using Aspen Plus process simulation software.The simulation results were that compared with ordinary single-column distillation,the energy saving rate of two-effect、three-effect、four-effect distillation withoutpreheating system were15.6%、37.1%、47.2%, respectively. And the energy savingrate of two-effect、three-effect、four-effect distillation with multi-stage feed preheatingsystem were22.0%、44.3%、55.2%, respectively，with an average increase of about7to8percent in comparison of the former. The results indicates that not only theincreasing number of effects has an effect on reducing energy consumption, butadding multi-stage feed preheaters could fully use the logistics of latent and sensibleheat, which significantly reduces energy consumption further. （2） Optimization design method for multi-effect distillation process withmulti-stage feed preheating system was studied. Material balance and energy balanceof the whole system, every column and preheating system were carried out, then anew design method was presented on the basis of mathematical model above.Calculation formula of the appropriate number of effects Ntis as follows：DEG wastewater treatment process which mass concentration was of1%、5%were simulated and calculated. The results showed that the appropriate number ofeffects was7and6under the two different conditions. That was in agreement with theappropriate number of effects calculated by the presented formula above, whichverified the correctness and feasibility of the new design method. Compared withmulti-effect distillation without preheating system，it could save43.3%、42.6%ofenergy consumption, and75.7%、74.6%in comparition of single-column distillation,respectively. The energy-saving effect is remarkable.（3） For some deficiencies of low-concentration wastewater treatment usingmulti-effect distillation, distillation with heat pump used for DMF wastewater of5%concentration was studied. Single-columnr heat pump distillation of topr steamdirectly compression style was simulated. And based on that process was improved,which raised small tower operating pressure to ennable the top steam to meet theauxiliary heat source steam reboiler temperature requirements. The results indicatedthat energy saving effect of the heat pump distillation was more significant than themulti-effect distillation. Energy saving effect of three-effect distillation with heatpump was not as good as single-column heat pump distillation, as well as their COPcoefficient. Improved single-column heat pump distillation raised the small toweroperating pressure to enable the top steam temperature to meet the T1tower reboilerheating demand, used as an auxiliary reboiler heat source steam, then increased the energy-saving rate from71.1%to76.8%.（4） Brine is also a kind of low-concentration wastewater. For high energyconsumption of distillation-crystallization for brine treatment process, the heat pumpcrystallization technology for brine recycling treatment was studied. This workanalysed crystallization law of salts from brine after desalinized through evaporationand concentration. Thereby a new heat pump crystallization process was presented,which designed two crystallizers. First CaSO₄solid crystallized out in the CaSO₄crystallizer, affter separating the crystals out, the mother liquor entered the NaClcrystallizer in which NaCl solid was crystallized from solution containing K⁺、Mg²⁺ions. The purify of NaCl by the new heat pump crystallization technology is high.After simulation analysis on different operating conditions, it determined the bestcrystallization temperature of CaSO₄and NaCl crystallizer was122°C and13°C,respectively. Dealing with1000kg of RO brine under this operating condition,75.2kW of heat and63kW of cooling capacity were needed to solve the pollutionproblem of the RO brine discharge. At the same time it can get pure CaSO₄solid2.062kg and pure NaCl solid40.32kg. So it has some certain economic and socialbenefits.