Study On Low Temperature Multiple Effect Process With High Recovery Rate

For a daily output of 25000 tons high recovery rate low temperature multi-effect system,four different types of technology respectively,for four-effect parallel flow,six-effect countercurrent flow,ten-effect countercurrent,and fourteen-effect countercurrent are designed.From material conservation,energy conservation and heat transfer these three aspects,mathematical models of evaporator,condenser,steam ejector and flash tank are established.After simulation and calculation,the performance ratio of these four types of technology is respectively 7.19,7.78,12.02 and 16.8;The specific heat transfer area is respectively 399.7,275,420.68 and 674.13 m~2/(kg/s);The motive steam rate is respectively 144.84,133.83,86.63 and 61.99 t/h.From these calculations,we can find that as the number of effect increase,the performance ratio increase gradually,and the motive steam rate decrease gradually.On the condition that the production of water is certain,the performance ratio of 14-effect is largest,the motive steam rate is least,but in the meantime,the specific heat transfer area is largest.According to the available energy balance,the utilization efficient of available energy in four different processes is less than 20%,the loss of steam ejector accounts for a large proportion,much available energy losses in steam ejector.In addition,the paper makes a study on the effect of operation conditions on process performance.Process parameters including the location of extracting vapor,concentration ratio,heat transfer temperature difference,and feed seawater salinity have great impact on process performance.When the location of extracting vapor moves back,the performance ratio firstly increases and then decreases.The fifth effect is the best location of entraining vapor in six-effect countercurrent flow,the eighth effect is the best location of entraining vapor in ten-effect countercurrent flow,twelfth effect is the best location of entraining vapor in fourteen-effect countercurrent flow.With the pressure of motive steam growing,the performance ratio increases,the specific cooling seawater decreases gradually,but the change of them is very small,the specific heat transfer area and the compressed steam rate is almostly constant;When production rate increases,the performance ratio is unchanged,the specific heat transfer area and the motive steam rate increase proportionally;As the feed seawater salinity increases,the specific heat transfer area also increases;The change of last effect temperature or the temperature of compressed vapor is equal with the change of temperature difference in the technology.When the temperature difference increases,the motive steam rate increases,the performance ratio decreases,the specific heat transfer area decreases;When concentration ratio increases,the performance ratio increases,the specific cooling seawater rate increases,the specific heat transfer area increases,the feed seawater rate decreases,the compressed steam rate,the entrained steam rate and the motive steam rate decrease.By the simulation and performance analysis of four kinds of LT-MEE,we can use these rules to guide the practical production and process optimization,and provide theoretical basis for technology selection of LT-MEE.