New MED plate desalination process: Thermal performances

A new multi-effect plate evaporator, EasyMED, is presented. This patented process [1] is applicable for seawater desalination using simple and modular technology. Experimental results from the hydrodynamics and thermal performances obtained with a one-effect laboratory unit with 0.4 m3/d distilled water production were analysed. The hydrodynamics study shows the importance of the grids used to improve salted water film stability and to promote turbulence. A film distribution system was designed to ensure good plate wetting for flow rates lower than 50 L/h. Baffles were added in the heating and cooling cells to increase the heat transfer coefficient. The influence of heat carrier fluid and salted water film flow rates on performance was studied. The production of distilled water ranged between 0.3 and 0.4 m3/d for one plate whose area is 0.72 m2. Distillate conductivity was very low. The evaporation ratio ranged between 10% and 50%. The mean heat exchange coefficient between heating cell and evaporating film was around 1100 W.m!2.K!1. These results were higher than those obtained with a first pilot in 2000 [2]. The results were modelled in order to distinguish the three thermal resistances implied in the transfer. It is shown that the film flow may be considered as turbulent even at low Reynolds numbers due to the influence of the grids.

L Lv Nu Pr P Q R Re T ∆Tml — — — — — — — — — — The results obtained with the EasyMED single-effect laboratory unit clearly demonstrate the feasibility of this process. It was especially shown that high-quality distilled water can be produced. The distillate low conductivity Greek — Thermal conductivity, W.m!1.K!1 : — Dynamic viscosity, Pa.s This relation leads to a partial heat transfer coefficient hh between 1400 to 2600 W.m!2.K!1 for a Reynolds number varying between 14,000 and 30,000. These values are slightly higher than predicted by Sieder and Tate [7] that is, applicable for Reh >10,000.