Research On The Performance Of Low Temperature Multi-effect Distillation System

At present, about 25% of the world's people lack the quality and quantity of freshwater supplies. More than 80% of the countries have serious water problems. Seawater desalination has emerged as an important source of fresh water. Low-temperature multi-effect distillation (LT-MED) is one of main methods of seawater desalination. LT-MED desalination system is commonly divided into forward feed configuration, backward feed configuration, parallel feed configuration and mixed feed configuration. The spray density is very important for operation. Taking the actual conditions into account, the processes with spray density of evaporation tube in the range of 0.03-0.08 kg/(m·s) were selected as feasibility ones.Based on the mass and energy conservation, the mathematical models of a mixed feed LT-MED desalination system and a mixed feed LT-MED desalination system with thermal vapor compressor (TVC) were established, in which included evaporator, condenser, preheater, brine cooler etc. The temperature losses were considered caused by the boiling point elevation (BPE) of seawater and the flow pressure drop when steam flow through tube bundle, demisters and pipelines. The property parameters of seawater were considered as the functions of temperature and concentration. And the property parameters of fresh water and saturated steam were considered as the functions of temperature. A computer program was carried out to solve the model.Different factors impacting on the mixed feed LT-MED desalination system without thermal vapor compressor (TVC) were analyzed. The calculation was based on the system with a rated water production 10000 t/d. When other relative parameters are unchanged, the total area of evaporators is more sensitive to the change of the heating steam temperature of the first effect than gained output ratio (GOR). As the number of effects increase, GOR rises significantly while within some range, the total area of evaporators decreases. It is benefit for the performance of the system that the position of preheater moves toward the first effect.For the mixed feed LT-MED desalination system with TVC, The calculation was based on the system with a rated water production of 3500 t/d. Many factors impacting on the system performance were analyzed. The results show that when the entrained steam position of TVC is at the effect just behind the mixed feed position, GOR reaches the maximum value and the total area of evaporators reaches the minimum value which is benefit to reduce the water cost. The impact of motive steam parameters of TVC on GOR is greater than on total area of evaporators and it is better to increase the parameters of motive steam for reducing the cost of water. For reducing the water cost it is also better to put preheater at the position before the entrained steam position of TVC.Comparative study on parallel feed and mixed feed LT-MED desalination systems without TVC was done under different factors impacting on the systems. When the other factors are unchanged, the more the number of evaporators, the better the performance of mixed feed system compared to the parallel feed system. The impact of the heating steam temperature of the first effect on the performance difference of the two systems is very little. The less the rated water production, the better the performance of mixed feed system compared to the parallel feed system.Comparative study on parallel feed and mixed feed LT-MED desalination systems with TVC was done under different factors impacting on the systems. Under the same conditions, GOR of the mixed feed LT-MED desalination system has a considerable advantage, but the total area of evaporators is also larger. The impact of the entrained steam position of TVC on the mixed feed LT-MED desalination system is greater. The influence of motive steam parameters of TVC on GOR is greater than on total area of evaporators and it is better to increase the parameters of motive steam for reducing the cost of water. It is benefit for the performance of the two systems that the position of preheater moves toward the first effect. And the impact of the position of preheater on the mixed feed LT-MED desalination system is more significant.