The impact of vapor box location on the performance of the multiple effect distillation for seawater desalination technology

Seawater desalination and water purification is a sustainable solution for arid areas, where the lack of natural potable water is limiting economic growth. Some of the commercial multi-effect distillation (MED) configurations are classified according to the vapor box location with reference to the evaporator tube bundle; whether it is side or back-attached. The lack of technical comparison between the two configurations motivated this work to identify the optimal vapor box location and demister orientation to minimize thermal losses and ensure uniform vapor flow to the next tube bundle. The aim of this study is to perform a computational fluid dynamics (CFD) simulation to compare between two conventional configurations namely the back vapor box (MED-BVB) and the side vapor box (MED-SVB) concerning the thermal losses and vapor flow uniformity as well as to propose internal design modifications specifically in a number of demisters and demister orientation to enhance system performance. MED evaporator of three effects with 25 m³/d is considered for the CFD computational domain and conducted by COMSOL multiphysics. The CFD results showed that, the conventional MED-BVB with one horizontal demister has 49% better vapor uniformity compared to the conventional MED-SVB with three horizontal demisters. However, the modified MED-SVB with three inclined demisters has 45% better vapor uniformity compared to the modified MED-BVB with one inclined demister due to the less swirl flow in the vapor box. Moreover, it was concluded that the modified MED-BVB has a better vapor uniformity and slightly lower thermal loss compared to conventional MED-SVB. Generally, inclining the demister showed better uniform vapor flow in both configurations. On the other hand, the MED-BVB has a 45% lower footprint compared to the MED-SVB. Preference between the two studied configurations would be subjected to the compromise between operational challenges (in case of uneven vapor flow, some tubes would be exposed to a higher amount of vapor flow rate than the design value and accordingly will become overheated) and footprint.