Expressions for The Heat Transfer Resistances

Water treatment is any process that improves the quality of water to make it more acceptable for various uses. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired use. For the water treatment process different methods are used like filtration, zeolite process, ion-exchange, reverse osmosis, ozonation, ultraviolet light, activated carbon towersand membrane distillation. Waterbodies contains many harmful constrain like specifically heavy metals It is very important to identify the relationship between the presence of heavy metals in drinking water and the prevalence of renal failure, liver cirrhosis, hair loss, and chronic anemia diseases. The prevalence of these diseases were markedly increases in the last few years due to air pollution, water pollution, and hazards over uses of pesticides in agriculture.

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Trace amounts of metals are common in water, and these are normally not harmful to our health. In fact, some metals are essential to sustain life. Calcium, magnesium, potassium, and sodium must be present for normal body functions. Cobalt, copper, iron, manganese, molybdenum, selenium, and zinc are needed at low levels as catalysts for enzyme activities. Drinking water containing high levels of these essential metals, or toxic metals such as aluminum, arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver, may be hazardous to our health. For water treatment membrane distillation process is used, principle of membrane distillation is the art processes that separate mass flows by a membrane, mostly use a static pressure difference as the driving force between the two bounding surfaces, a difference in concentration or an electric field . Selectivity of a membrane is produced by, either its pore size in relation to the size of the substance to be retained, its diffusion coefficient or electrical polarity. However, the selectivity of membranes used for membrane distillation is based on the retention of liquid water with-at the same time-permeability for free water molecules and thus, for water vapour. These membranes are made of hydrophobic synthetic material and offer pores with a standard diameter between 0.1 and 0.5 µm. As water has strong dipole characteristics, whilst the membrane fabric is non-polar, the membrane material is not wetted by the liquid. Even though the pores are considerably larger than the molecules, the liquid phase does not enter the pores because of the high water surface tension. The driving force which delivers the vapour through the membrane, in order to collect it on the permeate side as product water, is the partial water vapour pressure difference between the two bounding surfaces. This partial pressure difference is the result of a temperature difference between the two bounding surfaces. As can be seen in the image, the membrane is charged with a hot feed flow on one side and a cooled permeate flow on the other side. The temperature difference through the membrane, usually between 5 and 20 K, conveys a partial pressure difference which ensures that the vapour developing at the membrane surface follows the pressure drop, permeating through the pores and condensing on the cooler side.

Expressions for the heat transfer resistances and mass transfer resistances of all the physical domains composing direct contact membrane distillation processes are developed and their absolute and relative effects are evaluated to improve the process understanding and identify promising ways for its improvement. The resistances are computed based on two-dimensional conjugate model in which a simultaneous numerical solution of the momentum, energy and diffusion equations of the feed and cold solutions have been carried out, and the results of which were validated in comparison with available experimental results.