CFD analysis of the demister location impact on the thermal losses and the vapor uniformity within the MED desalination plant

Several multiple-effect distillation (MED) configurations have been used in the desalination industry. Some configurations classified according to the tube orientation, however, another classified according to the location of the demister whether in the evaporator side or the backside. A lack of computational fluid dynamics (CFD) platform tool to address the effect of the thermal losses within the evaporator and the effect of the vapor flow distribution among tubes on the evaporator performance motivated to development of such CFD tool. This work presents CFD modeling of two commercial MED configurations to identify the optimum tube bundle arrangement and demister location with respect to the evaporator bundle thermal losses, shell volume, footprint as well as minimal operating challenges. The system consists of 620 tubes arranged in a triangular tube pitch either 30° or 60°. Analysis of thermal losses showed that the 60° triangle pitch is optimum for the side demister (MED-SD) evaporator because the 60° arrangement creates a wide passage along the horizontal vapor route which minimizes the pressure drop within tube bundle. On the other hand, the 30° triangle pitch arrangement fits the back demister (MED-BD) evaporator; where a wide vapor passage is created in the downward direction heading to the BD in a lower pressure drop. Moreover, increasing the tube pitch creates wide passages and accordingly smooth flow, hence, less thermal losses. However, increasing the tube pitch will increase both the footprint and shell volume. Furthermore, the vapor velocity distribution at the tube sheet of the MED-BD configuration has a better uniform flow than that of the MED-SD, which indicates that an operational challenge would occur within the MED-SD configuration as some tubes will be admitted by the amount of vapor higher than the design value and accordingly will become overheated.