DesignClinic

Design of an Economically Competitive Membrane Distillation System

W. L. Gore

W. L. Gore

2013-2014: In collaboration with W. L. Gore and Associates this team designed a membrane distillation system to produce potable water from seawater.

Membrane distillation (MD) is a thermally-driven separation process in which one component of a hot liquid mixture preferentially evaporates and diffuses through a microporous membrane. Many investigations and analyses have been conducted on MD, but there are few, if any, commercial applications. W. L Gore’s ePTFE is a suitable, perhaps optimal, material to use as the membrane barrier in an MD desalination process because it is hydrophobic and has a relatively low thermal conductivity. The team developed a spreadsheet model of heat and mass transfer through the membrane to evaluate influences on water vapor flux and conductive heat loss. They concluded that the most effective membrane design would have high porosity (80%), small thickness (100 μm), and a pore size as large as possible, but small enough to prevent liquid water intrusion. Using flux estimates from the first model, they developed a second spreadsheet to simulate the entire separation and heat recovery process and develop estimates of capital investment and operating cost. They concluded that heat exchanger and membrane module costs are too high for MD to compete with desalination by reverse osmosis even if virtually free thermal energy is available.

Results of the design study were made available to W. L. Gore so that the company can conduct further analysis if more favorable economic conditions develop.