| Seawater desalination technology is an efficient way to solve the problem of water shortage, which produces some negative impacts to the environment, such as the pollution of brine discharge. Concentrated brine discharged directly would not only pose a threat to marine ecological environment, but also is an enormous waste of resources. In order to reduce the cost of desalination, solve the pollution problem and efficiently use the resource, comprehensive utilization of brine has been proved to be the most effective way. This paper works on the simulation and optimization of salt production from desalination brine, and includes several parts.1. Determination on the ion crystallization precipitation sequence in the salt production from desalination brine. Brine is bottled into flask and heated to boiling, at the same time, Baume hydrometer monitors the density change in the evaporation process and measuring cylinder measures the amount of water.8 samples were taken at every certain amount of water. Get the mother liquor out and equilibrate it at room temperature for 24h. The relative density and chemical composition of 8 samples supernatant were measured. Ca2+ and Mg2+ were measured by EDTA complexometric titration. SO42-was measured by barium sulfate gravimetric method. K+was measured by four benzene boric acid potassium weight method. Cl- was measured with silver nitrate precipitation titration. And Na+was calculated by the subtraction method. The experimental results:CaSO4 precipitates firstly, at point of 10.02 ° Be’, NaCl precipitates subsequently at point of 14.98 ° Be’ Experimental conclusions provide basis for the simulation of salt-production process.2. The brine salt-production process using evaporation crystallization were simulated, included single-salt, double-salt and multi-salt respectively. For single-salt crystallization process, assuming the brine contains only NaCl, the crystallization temperature is 108.6℃, crystallization pressure is 101.3kPa, the recovery rate is 100%, the energy consumption is 671.2kW. For double-salt crystallization process, CaSO4 precipitates firstly and NaCl precipitates subsequently. The crystallization temperature are 117℃ and 61 ℃, the crystallization pressure are 142kPa and 12kPa, the crystal purity are 100% and 99.5%, the recovery rate are 89.6% and 95.1%, the energy consumption is 683.2kW. For multi-salt crystallization process, three processes were established. Brine passes through the method of a plurality of evaporation crystallizer assisted by the reaction and some valuable inorganic salt products, such as CaSO4、NaCl、Mg(OH)2、CaCl2、CaCO3、Na2SO4·10H2O、 Na2SO4, are collected in a specific order. The zero discharge of brine is realized. The numbers of crystallizer in three processes are 5,4,6, and the numbers of reactor are 1,1,2. The energy consumption is 711.4kW,710.5kW,715.8kW, respectively. Considering the cost of equipment and operation, Process 2 is the optimal.3. The brine salt-production process was optimized, including single-salt, double-salt and multi-salt respectively. Based on the gradual optimization integration strategy, put forward two kinds of optimization process:(1) using the steam separated from the evaporation chamber for preheating brine; (2) using the heat pump crystallization technology, the steam separated from the evaporation chamber is compressed by the heat pump compressor, the pressure and temperature increase and is evacuated back to evaporator used as heating steam, so that the material liquid maintains the state of boiling and the heating steam itself is condensed into water. In single salt crystallization process, energy saving of raw material preheating process and heat pump crystallization process are up to 81.9% and 94.6%. In double salt crystallization process, energy saving of raw material preheating process and heat pump crystallization process are up to 84.7% and 83.8%. In multi-salt crystallization process, the energy consumption of the three raw material preheating processes are 162.6 kW,94.8 kW,173.8 kW and the energy saving are 77.1%,86.7%,75.4%, respectively. The energy consumption of the three heat pump crystallization processes are 133.2 kW,90.1 kW,147.6 kW and the energy saving are 81.3%,87.3%, 79.1%, respectively. The energy saving effect of heat pump crystallization process is better than raw material preheating process, in spite of the compressor equipment. So the heat pump crystallization of Process 2 is the optimal. |
