Numerical simulation and thermohydraulic performance analysis of tube-bundle in Multi-effect distillation systems to investigate the risk of precipitation

Multi-effect distillation is one of the thermal desalting systems. MEDs have recently come to notice more than other systems because of their high energy utilizing and performance. High complicity and possessing different heat transfer mechanisms have distinguished them from other desalination systems such as Multi-stage flash. In MEDs although formation of thin falling-film layer on horizontal rows of tube-bundle increases heat transfer, however the risk of precipitation will be high especially on lower rows where film thickness is the least. Falling-film evaporation is a self-compensation phenomenon; the more the evaporation, the thinner the film, and subsequently the more the evaporation. In present work, an applicable algorithm is proposed and applied for thermohydraulic design of tube bundle, and heat transfer surface. Flow and heat distribution on tube-bundle is numerically simulated with the advantage of given algorithm. Results show that more concentration and precipitation risk will occur on outer surfaces, near the entrance of last rows. Uniform distribution of feed on tubes will result in non-uniform vapor generation throughout the tube length. Steam quality increases almost linearly inside tubes, whereas its cross-section is occupied by gas phase predominantly; and condensate film will experience an annular regime. Steam quality reduction and non-uniform vapor generation decrease thermal performance of the last zone of the tube-bundle.