Economic feasibility study of membrane based separation process :
Membrane distillation (MD) is an emerging technology for brackish water desalination. MD is a thermal, vapor-driven transportation process through microporous and hydrophobic membranes. MD is applied as a nonisothermal membrane process in which the driving force is the partial pressure gradient across a membrane that is porous, not wetted by the process liquid. In this process, saline water is heated to increase its vapor pressure, which generates the difference between the partial pressure at both sides of the membrane. Hot water evaporates through nonwetted pores of hydrophobic membranes, which cannot be wetted by the aqueous solutions in contact with and only vapor and noncondensable gases should be present within the membrane pores. The passing vapor is then condensed on a cooler surface to produce fresh water. . The recovery of MD process is higher than the RO process for seawater desalination. Fouling and scaling are two important mechanisms that affect stability of the MD process and lead to reduce the overall efficiency. Membrane fouling increases the costs by increasing (1) energy consumption, (2) system down time, (3) necessary membrane area, and (4) construction, labor, time, and material costs for washing and cleaning processes. It is a general conclusion that pretreatment has an important positive influence on MD. In MD, desalination plant is operated in conjunction with a power plant or any other source of waste heat, the cost of energy for heating the feed water is negligible, hence thermally polluted water can be treated economically. Other sources of energy such as renewable solar or geothermal energy could be utilized to heat the feed water. As opposed to warm condenser water, use of renewable sources would involve higher capital investment. However, this investment may eventually be paid off by lower operating costs. MD could be convenient to utilize cheap heat sources such as solar energy, geothermal energy, and waste heat. Therefore, in combination with such cheap energy, MD was a process of phase transition, and utilization of heat energy could decrease due to latent heat of vaporization. It has some significant advantages over RO process, including lower operating temperature and pressure, and thus possible to use energy sources such as renewable solar heat or waste heat, product quality, and higher resistance to fouling.