Energy Comprehensive Utilization And Economy Study On Seawater Desalination System

Presently, the growing demand for water resources, the scarcity of water resources gradually is coming out, therefore it is imperative to search for an energy-saving and efficient way to make fresh water. The low-temperature multiple-effect evaporation for seewater desalination (LT-MED) system is one of main forms of commercial desalination systems, in which multiple-effect distiller with thermal vapor compression ( MED-TVC ) system is used widely because the additional equipment TVC have some advantages such as simple structure, low investment cost, easy maintenance and improving ability to water production. Seawater desalination solves the lack of fresh water resources, at the same time, the vast thick seawater emissions has brought the impact on the surrounding environment. Hence, the research on zero emission of seawater has been paid attention.This study builds multi-objective and unified scale evaluation criterion that integrates thermodynamic performance and economic performance, mainly researches on energy utilization of forward feed MED-TVC under the high cycle of concentration, and reveals gain operation ratio (GOR) and exergy consumption variety rules with the number of the evaporator's effects, the top brine temperature and sea water salinity, and also compares economic performances between countercurrent feed and forward feed seawater desalination systems. Furthermore, energy utilization status of the process of multi-effect vacuum evaporation salt production from dense seawater is studied, and exergy consumption variety rules with sea water salinity and temperature of this system are revealed. Based on the theory and principle of system integration, a novel polygeneration system producing fresh water, salt and power is proposed. Through analysis and comparison with conventional system, result shows the new system realizes synthetically effective utilization of seawater.Forward feed MED-TVC system with high cycle of concentration is simulated, also exergy destruction of the system is analyzed by the use of EUD method. The results indicate: (1) the exergy destruction of ejector and 15 effects are 31.2% and 30.26%, respectively. They are main contributors to total exergy destruction of the system. The application of TVC causes the reduction on steam supply which consequently reduces exergy destruction. (2) GOR reduces as top temperature increase, while exergy consumption increases. Increasing the temperature of feed seawater can raise up GOR and reduce exergy consumption. As the salinity of feed water is up, GOR will reduce accordingly. Increasing the effect numbers of MED and MED-TVC can raise up GOR and reduce energy consumption. (3) The economic analysis on MED and MED-TVC shows that TVC application makes an energy cost lower obviously and also reduces water product cost. (4) Through the analysis of DEEP, the results show that: with the increase of the number of effects, the motive steam efficiency would be enhanced, and the equipment's productivity and the total water cost would decrease. But the sum of temperature differences loss would increase, so that the effective heat transfer temperature differences, and at the same time, it will increase the equipment investment. Thus it was appropriate to apply MED-TVC with 15 evaporator effects.The material and energy balance of the five effects multiple-effect vacuum evaporation salt production system are carried out. Then exergy distribution of the system was analyzed by the use of EUD method. Results show that: (1) exergy efficiency the system of is about 14.19%, which means that exergy destruction accounts for 85.81%. (2) The exergy destruction in the atmospheric type mix condenser shouldn't be neglected, which accountes for 11.71%. The energy level difference between motive steam and cooling water is big, which is the main reason for exergy destruction. Making use of steam from the last effect would improve exergy efficiency of the system. (3) After flash process, the temperature and pressure of condensate would become lower. The temperature difference between condensate and brine is small in the preheated section, so that the exergy destruction is small, about 4.23%. The amount of circulating water and condensed water is in a large quantity, but their energy level is too low to take a potential utilization. (4) With the increasing of the number of effect, the temprature and salinity of feed, the exergy destruction will decrease. The number of effect and the salinity of feed influence exergy destruction more, while the temperature of feed influences less.Based on the fundamental theory and principle of system integration, a novel seawater desalination polygeneration system producing fresh water and salt is proposed. The results of simulation and calculation show; (1) Compared with a direct discharge alternative, the temperature of cooling water is 1.88℃down and coal consumption is reduced 3.07g/kWh. (2) As there is no need to discharge the brine to sea from cooling tower, the ecological impact on ocean is eliminated. (3) As the temperature of feed water is increased, the GOR of desalination is 3% up on average with different numbers of desalination effects, while exergy consumption is 3% down. With the increase of feed salinity, the GOR is 3% down while exergy consumption is 3% up. In consideration of the same numbers of desalination effects, the increase of temperature contributes greatly to a lager GOR compared with salinity increase. Generally speaking, the direct flow configuration can give a better performance on desalination system than the direct discharge. (4) With the increase of feed temperature, exergy consumption of vacuum salt-making system reduces over 4%. Consequently, more desalination effects will make exergy consumption decrease rapidly. With the increase of feed salinity, exergy consumption of vacuum salt-making system will decrease over 4%. It results in a decrease of exergy consumption by adopting more desalination . The increase both feed temperature and salinity will contribute to reduction on exergy consumption which is impetus to vacuum salty production.