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Unformatted text preview: Chapter 14 Mass Transfer Chapter 14 MASS TRANSFER Mass Transfer and Analogy Between Heat and Mass Transfer 14-1C Bulk fluid flow refers to the transportation of a fluid on a macroscopic level from one location to another in a flow section by a mover such as a fan or a pump. Mass flow requires the presence of two regions at different chemical compositions, and it refers to the movement of a chemical species from a high concentration region towards a lower concentration one relative to the other chemical species present in the medium. Mass transfer cannot occur in a homogeneous medium. 14-2C The concentration of a commodity is defined as the amount of that commodity per unit volume. The concentration gradient dC/dx is defined as the change in the concentration C of a commodity per unit length in the direction of flow x . The diffusion rate of the commodity is expressed as diff Q k A dC dx = - where A is the area normal to the direction of flow and k diff is the diffusion coefficient of the medium , which is a measure of how fast a commodity diffuses in the medium. 14-3C Examples of different kinds of diffusion processes: (a) Liquid-to-gas : A gallon of gasoline left in an open area will eventually evaporate and diffuse into air. (b) Solid-to-liquid : A spoon of sugar in a cup of tea will eventually dissolve and move up. (c) Solid-to gas : A moth ball left in a closet will sublimate and diffuse into the air. (d) Gas-to-liquid : Air dissolves in water. 14-4C Although heat and mass can be converted to each other, there is no such a thing as “mass radiation”, and mass transfer cannot be studied using the laws of radiation transfer. Mass transfer is analogous to conduction, but it is not analogous to radiation. 14-5C ( a ) Temperature difference is the driving force for heat transfer, ( b ) voltage difference is the driving force for electric current flow, and ( c ) concentration difference is the driving force for mass transfer. 14-6C ( a ) Homogenous reactions in mass transfer represent the generation of a species within the medium. Such reactions are analogous to internal heat generation in heat transfer. ( b ) Heterogeneous reactions in mass transfer represent the generation of a species at the surface as a result of chemical reactions occurring at the surface. Such reactions are analogous to specified surface heat flux in heat transfer. 14-1 Chapter 14 Mass Transfer Mass Diffusion 14-7C In the relation ( / ) Q kA dT dx = - , the quantities Q , k, A, and T represent the following in heat conduction and mass diffusion: Q = Rate of heat transfer in heat conduction, and rate of mass transfer in mass diffusion. k = Thermal conductivity in heat conduction, and mass diffusivity in mass diffusion....
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- Spring '06