The potential of membrane distillation as a stand-alone desalination process

A mathematical model is developed to investigate the potential of the membrane distillation process to be a stand-alone process. The model includes internal heat recovery and relates the major cost parameters to the heat and mass transfer of the process. The cost parameters are the energy required for heating, the energy required for pumping, and the specific membrane area. It was found that the permeability of membrane distillation system (defined below) for the modules available in the literature is in the range of (1)10−8 to (1.5)10−6. Taking the upper limit, it was found that for membrane distillation system to become competitive for seawater desalination, the feed solution need to be discharged at temperature over 50°C and so a low-grade energy is needed. Moreover, it can be concluded that the process is closer to be economic when Reynolds number is in the range 3000–5000 (transition from laminar to turbulent flow).

By solving a set of equations relating the energy required for heating, the energy required for circulation, and the specific membrane area to the mass and heat transfer of the MD process utilizing internal heat recovery, the economy of the process is assessed as a function of the discharge temperature of the feed solution and the permeability of the membrane. A lumped parameter called the permeability of membrane distillation system is developed to determine the efficiency of the membrane modules available in literature. It was found that permeability of MD system ranges from (2.2)10−8 to (1.53)10−6. Taking the highest value of these, the process can be a stand-alone only if the discharge temperature of the hot solution is 50°C or higher and so a low-grade energy is needed. On the other hand, when operating at low velocity (laminar regime), the power consumption for pumping is economically acceptable even at membrane permeability as low as 5 × 10−7 s−1 but the capital cost of the system represented by the cost of the membrane material becomes significant. So it seems that the process is closer to be economic when Reynolds number is in the range 3000–5000 (transition from laminar to turbulent flow).