Study On Falling Liquid Reboiler For Distillation Of Heat-sensitive Materials

At present, distillation is the main method for the industrial separation and purification of heat sensitive-materials but the vaporizing devices for the distillation of heat-sensitive materials are more prone to coking and blockage. In this paper, a new type of reboiler, which was called falling liquid reboiler, was proposed. The problems of flow distribution non-uniformity and heat transfer enhancement in falling liquid reboiler were investigated.First, flow distribution non-uniformity of the falling liquid reboiler and effects of cone-type distributors with different structure parameters on distribution non-uniformity coefficient ξ at different flow rates were studied experimentally. The experimental results showed that, when the head of the falling liquid reboiler was filled with liquid, putting proper cone-type distributor in head would improve the flow distribution uniformity. Among all the seven distributors, the distributor whose cone angle was 120 degrees, bottom diameter was 0.5 times the inner diameter of the head, the distance from the feed inlet to the top of the distributor was 0.1 times the height of the head, was recommended for its better flow distribution performance and less energy dissipation. The non-uniform coefficient was reduced by 54%~69%, the value of(qi-q)max/q was 3.21%, and pressure drop increased by 4.3%~16.7%.Then, in order to investigate heat transfer enhancement of single-downcomer, three-dimensional models of downcomer with full length typical twisted tape inserts and regularly-spaced twisted tape inserts were established with FLUENT software, flow and heat transfer characteristics were simulated. Consequently, the variation of Nusselt numbers, friction factors and overall enhancement ratios( PEC: Performance Evaluation Criteria) for downcomer with different twisted tape inserts were presented. The results indicated that, when water was used as working fluid and the flow rate of the tube was in a range of Reynolds number between 9952 and 59712, the full length typical twisted tape inserts offered higher heat transfer rate and friction factor compared with the bare tube, but PEC was less than 1 due to excessive flow resistance. Therefore, reducing fluid friction resistance was another effective way to enhance heat transfer.Compared to the full length twisted tape, regularly-spaced twisted tape performed lower heat transfer and friction factor values. However, it was apparent that PEC of the tube with the regularly-spaced twisted tape insert was found to be higher than that with the full-length one. The best overall enhancement(PEC=1.05~1.33) was achieved for which the free space is equal to two times half-pitch.