A Dynamic Model of Direct Contact Membrane Distillation for Application to High Salinity Feed Solutions, Multiple Flow Regimes, and Multiple Flow Modes

A large portion of the direct contact membrane distillation (DCMD) literature is focused on theoretical mass and heat transfer modeling to predict the amount of water flux produced and energy consumed for particular operating conditions. Many of the models are over-simplified and applicable only to low salinity feed solutions, laminar flow conditions, and a single flow mode. The utility of DCMD lies within its application to desalination of seawater and higher salinity solutions, which may occur under co-current or countercurrent flow mode, and turbulent flow conditions are often promoted to reduce polarization effects. The current study focuses on the development of a dynamic two-dimensional mass and heat transfer model for DCMD that selects the appropriate equations depending on the operating conditions. The new model is based on energy and mass balances, and includes the effects of temperature and concentration polarization for consideration of high salinity solutions. Concentration polarization is an important consideration for high salinity solutions because it drastically reduces the vapor pressure driving force of the DCMD process. The model also considers the effect of spacers on flow regime, utilizing different heat transfer equations for different flow regimes. Finite difference techniques were used to transform partial differential equations into a system of ordinary differential equations that can be solved simultaneously using the Runge-Kutta Fehlberg method. The membrane distillation coefficient was derived from experimental data using bench-scale flat sheet membrane cells. The model will be used to determine optimal module geometries and identify areas for improvement in energy efficiency for the design of a pilot scale system. The poster will present the equations used for the model and how the model will be used to optimize operating temperatures, module geometry, flow regime, and flow mode to provide an energy efficient basis for design of a pilot scale system.