Pervaporation For Treating Vo Cs In Groundwater

2610 words - 11 pages

Introduction
“Pervaporation is a process in which a liquid stream containing two or more components is placed in contact with one side of a nonporous polymeric membrane, while a vacuum or gas purge is applied to the other side. The components in the liquid stream sorb into the membrane, permeate through the membrane, and evaporate into the vapor phase” (Freeman 6.59). The term pervaporation is derived “from the two major operations, involved in the separation process, namely, permeation and evaporation” (Peng Ming 815-820). Pervaporation has become more common place in the recent years regarding environmental concerns; this is mostly due to its effectiveness in removing volatile organic compounds from large amounts of water. Conventionally, air stripping or activated carbon are used in these situations but pervaporation has certain advantages over these methods. “Air stripping is susceptible to fouling and merely turns a water pollution problem into an air pollution problem, and activated carbon treatment involves costly regeneration steps and may not be suitable for VOCs that are easily displaced by other organic compounds” (Freeman 6.59). There are also a number of other advantages to pervaporation. The process creates no fugitive emissions, so there is no air pollution created. On the permeate side of the membrane, volatile organic compounds are continuously removed, so there are no regeneration costs. Also, the process uses compact systems and there is potential to recycle or reuse the recovered volatile organic compounds. The theory behind pervaporation can be explained through mathematical equations and certain experimental parameters.

Theory
Membrane performance is evaluated by the experimental parameters of permeation flux and selectivity. “The flux denotes the amount of component permeating per unit membrane area and unit time for a given membrane material. The flux is proportional to the concentration difference between the two phases” (Freeman 7.62). The permeation flux can be defined by the following equation; j_i=〖k_i〗^ov p^' [C L/i-C V/i]. The first term in the equation is the overall mass transfer rate constant is represented by 〖k_i〗^ov, the molar density of the feed is the second term which is represented by p^', the bulk liquid phase concentration C L/i and finally the bulk vapor phase concentration C V/i . Several factors influence permeation flux. These factors include feed concentration, temperature, and cross-flow velocity. “The permeation flux decreases as the feed concentration increases” (“Membrane Operations”). The result of this effect is an earlier buildup of the polarization layer and an increase in viscosity of the solution. Increasing temperature however, causes an increase in the permeation flux. “This increase is because diffusivity increases with temperature, while viscosity decreases” (“Membrane Operations”). With cross-flow velocity, “higher fluid velocity gives a higher mass transfer coefficient and...

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