Research Of Seawater Desalination Technology With Lng Cold Energy Utilization

LNG has valuable cold energy because its low temperature. when it gasifies, there is a lot of cold energy which can be used. More and more countries pay attention to the research of LNG cold energy utilization, including low-temperature power generation, cold storage, air condition, air separation, cryogenic comminution, phase-change energy storage and so on. Fresh water is a kind of human survival resources. Lack of fresh water has become increasingly serious, so desalination technology has been widely appreciated, which is classified into three methods: distillation, membrane and freezing. In freezing desalination, cold energy is need for water crystallization, so LNG cold energy and freezing desalination method can be combined together. This paper just studies "freezing desalination with LNG cold energy utilization". In the paper, through theoretical analysis and experimental research,a energy-saving seawater desalination device with LNG cold energy utilization is designed. This article focuses on theoretical and experimental research and has some results.(1) Comparing three kinds of seawater freezing desalination methods: direct-contacting method, vacuum method and indirect-contacting method. Through calculation and analysis, direct-contacting method has many advantages such as high heat transfer efficiency, small equipment size, consuming no power, but its crystallization process is complex and lack theoretical knowledge in basic research. Vacuum method is a kind of heat and mass transfer process, requiring less flow rate of LNG. However, vacuum bump consumes much energy and vacuum condition is difficult to be controlled. It also lacks theoretical knowledge. Indirect freezing method is more mature, relatively simple, having product reference. Therefore, this experiment uses indirect-contacting method to the design seawater desalination experiment device.(2) The middle refrigerant is selected, which must meet the low temperature operating conditions and specific experiment requirements. R410A is finally selected as middle refrigerant which is a kind of environmentally friendly refrigerant. At the same time, two cycle schemes are presented in accordance with the phase of middle refrigerant, which are non-phase-change process and phase-change process. Through software simulation and analysis, the phase-change process has advantages: small flow rate and low power consumption. So the phase-change process is finally selected as experiment process.(3) Seawater crystallizers (ice-making buckets) are compared and selected, including plate ice machine and tube ice machine. Through analysis and comparison, the plate ice machine has advantages: high heat transfer efficiency, continuous crystallization, removing ice without heat source. So it is chose as seawater crystallizer. Otherwise, the key parameters of entire experimental process is simulated and calculated. The diagrams of heat transfer coefficient vs. ice and ice thickness vs. time are given. The diagrams show that heat transfer coefficient and ice growth rate always reduces.(4) The experiments study that different parameters affect rate of desalination, including refrigerant evaporation temperature, the number of seawater distribution nozzles and seawater flow rate. The experiments show that higher refrigerant evaporation temperature, more seawater distribution nozzles and bigger seawater flow rate will lead to higher purity of ice. In this study, 2kg ice melt water can be got by consuming 1kg LNG. Freshwater flow reaches 150L/h. Cold energy is been fully used and the requirements of energy-saving and emission-decreasing are achieved.