Study Of Polymer Hollow Fiber Evaporator

The traditional evaporators are easy to be corroded and facing with scaling problems when treating with acid or alkali solutions because the main heat transfer parts are metals.This is not conductive for the evaporators'usage.The polymer hollow fibers are characteristic as corrosion resistance and anti pollution,and the evaporators made of hollow fibers are smaller but more heat transfer area can be obtained because of the smaller fiber diameter.Hollow fiber evaporators have remarkable advantages when compared to the traditional metal evaporators.In our research,the home-made graphite modified PVDF hollow fibers were used to design and construct polymer evaporators.The heat transfer coefficient and the heat transfer resistance both on the fiber outside surface and the inner surface were discussed based on the testing experiments.The energy efficiency during the evaporation process was also included.The results show that when the feed is boiling bumped in the evaporator,about two thirds of heat transfer resistance is focus on the polymer layer.The percentage of the shell side heat transfer resistance in the non-boiling feed experiments increases when the feed temperature is low.The percentage of tube side heat transfer resistance is lower than 15.3%and can be remarkably lower than 5%when the feed velocity increased.The water production of single effect hollow fiber evaporator decrease with the feed temperature,and the best feed velocity is about 0.138m/s in our experiments.The water production can reach 18.61 kg?m^-2?hr^-1 and the total heat transfer coefficient can reach 1320 w?m^-2?k^-1 with boiling feed and at the best feed velocity.Based on the single-effect hollow fiber evaporation experiments,the four-effect evaporation system were built in our study.The vacuum pressure distribution and the temperature distribution in every effect and the water production ability were discussed.The vacuum pressure in the evaporators increased with the effect number but the temperature of the produced vapor in the evaporation chamber decreased with the effect number.With the increase of the vacuum pressure on the fourth effect,the temperature of condensed solution and the produced vapor both decreased remarkably.The GOR value in our research is varied from 1.31 to 1.85 and the value of GOR increases with the vacuum pressure of the last effect.The largest water production in our system can reach 21.9 kg?m^-2?hr^-1 when the feed inlet velocity was 0.14 m/s and the vacuum pressure on the fourth effect was 0.05MPa.Besides,the simulation software FLUENT was engaged to optimize the shell side vapor flow in order to improve the heat transfer coefficient of the shell side.The simulation results reveal that,in the shell side the flow can be optimized when the evaporator with both-side vapor inlet and outlet,the shell side heat transfer coefficient can be improved by 20.8%.When the barriers were added in the blank area of the evaporator,the vapor flowing in the hollow fiber zone can be optimized better and the shell side heat transfer coefficient can be improved above 50%.The study laid a good foundation for the research of.polymer hollow fiber evaporator.