Performance comparison of flat sheet and hollow fibre air gap membrane distillation: A mathematical and simulation modelling approach

Membrane Distillation (MD) systems could address current challenges related to global water demand. Among MD systems, Air Gap Membrane Distillation (AGMD) systems play crucial roles in addressing this challenge. AGMD systems can be categorized into two main structural groups: flat sheet and hollow fibre forms. It is important to study, analyse, model and compare these two systems under the same operational conditions. In this study, a detailed mathematical model of flat sheet and hollow fibre AGMD modules under the same conditions (i.e., membrane type, membrane area, porosity, and pore radius of membrane) has been introduced. The simulation of system performance parameters, including flux, Temperature Polarization Coefficient (TPC), Specific Thermal Energy Consumption (STEC) and Gain Output Ratio (GOR) have also been conducted, based on various operational conditions and membrane properties. A comprehensive comparison has been then performed between flat sheet AGMD and hollow fibre AGMD systems in terms of flux, TPC, STEC and GOR. The results show that amongst all operational conditions and membrane properties, feed water temperature, air gap thickness, feed water Reynolds number, membrane porosity and pore radius significantly impact the flux of both systems. Furthermore, the operational conditions and membrane properties affected the STEC and GOR. Moreover, the comparison results showed that the hollow fibre system had higher flux, TPC and GOR than the flat sheet system both in laminar and turbulent state of feed water flow, while the flat sheet system showed a higher STEC value than the hollow fibre system in all operational conditions and membrane properties, indicating that a high energy requirement in the case of hollow fibre system for producing the same amount of flow.