Original Title: Process Flow of Low Temperature Multi-Effect Distillation (MED) Multiple Effect Distillation (MED) is an evaporation technology developed on the basis of single-effect distillation, which is divided into low-temperature and high-temperature multi-effect distillation. Low-temperature multi-effect distillation refers to a seawater desalination project in which the maximum evaporation temperature TBT of the saline is not more than 70 deg C. It is characterized in that a series of horizontal tubes or vertical tubes are connected in series with a membrane evaporator and divided into several effect groups, and a certain amount of steam is used to evaporate and condense for many times to obtain a desalination process that is many times the amount of heating steam. 1. Process flow The process flow of horizontal tube low-temperature multi-effect seawater desalination is shown in Figure 1. Seawater is first preheated and degassed in a condenser and then split into two streams. One is discharged back to the sea as cooling water, and the other is used as feed for the distillation process. The feed seawater is introduced into the later several effects of the evaporator after being added with the scale inhibitor. The feed liquid is evenly distributed on the top tube of the evaporator through the nozzle,rotary vacuum evaporator, and then flows downward along the top tube in the form of a film, and part of the water absorbs the latent heat of the condensed steam in the tube and evaporates. The secondary steam is condensed into product water in the next effect, the remaining feed liquid is delivered to the next effect group of the evaporator by the pump,molecular distillation systems, the operating temperature of the group is slightly higher than that of the previous group, and the processes of spraying, evaporation and condensation are repeated in the new effect group. The remaining feed liquid is delivered to the high temperature effect group by a pump, rotovap distillation ,wiped film distillation, and finally leaves the device in the form of concentrated liquid in the effect group with the highest temperature. Fig. 1 Process Flow Diagram of Low-temperature Multi-effect Seawater Desalination II. Analysis of advantages and disadvantages The main advantage of low temperature multi-effect seawater desalination process is low operating temperature, which can make full use of low temperature waste heat from power plants and chemical plants. Low-grade steam at 50 ~ 70 ℃ can be used as an ideal heat source, which can greatly reduce the impact of extraction of back-pressure steam on power generation in power plants, and also slow down the corrosion and scaling of equipment; The heat efficiency is high, and the heat transfer effect number of more than 12 can be configured at a temperature difference of about 30 deg C, so that the water production ratio of about 10 is achieved; meanwhile, due to the use of low-cost heat transfer materials, more heat exchange areas can be configured for the same investment scale; the power consumption is low, which is only about 0.9 to 1.2k W · H/m3; and the operation flexibility is high. In the peak period, the desalination system can provide 110% of the design value of product water; in the trough period, the desalination system can provide 40% of the rated value of product water stably; the operation is reliable. However, the evaporation temperature of saline can not exceed 70 ℃, which also becomes a restrictive factor for further improving the thermal efficiency of the process technology. The heat transfer coefficient of condensation and evaporation process increases with the increase of operating temperature. In addition, due to the large specific volume of steam at low temperature,decarboxylation after extraction, the volume of equipment is larger, which virtually increases the investment of equipment. 3. Scope of application The low-temperature multi-effect seawater desalination process is suitable for large-scale seawater desalination projects (hydropower cogeneration projects) with low heat sources of back pressure turbines in thermal power plants or nuclear power plants; the steam turbine low-pressure extraction produces high-quality water to supply water for high and medium pressure boilers; the plant waste heat is used to produce high-quality fresh water; If the concentration of raw water is higher than that of seawater, the desalination project can combine with reverse osmosis to form a mixed flow to reduce the cost of water production. Share Button Responsible Editor: Zhang Yali Return to Sohu to see more Responsible Editor:. toptiontech.com