che309- M3 report - Abstract Diffusion of gases through a...

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Unformatted text preview: Abstract Diffusion of gases through a pours solid is an important phenomena in chemical engineering area. For example, in some chemical operation, such heterogeneous catalysis an important factor affecting the rate of reaction is the diffusion of a gaseous component through a porous solid. The effective diffusivity in the solid is educed below what it would be in a free fluid, for two reasons. First, the tortuous nature of the path increases the distance, which a molecule must travel to advance a given distance in the solid. Second, the free cross sectional area is restricted. For many catalyst pellets, the affectivity of a gaseous component is of the order of a tenth of its value in a free gas. [7:490] The objective of this experiment is to study the diffusion of gas in porous slid and to determine experimentally the effective diffusion coefficient by a single pellet apparatus. The experiment was performed by using two gases: Helium (He) as a carrier gas and Nitrogen (N 2 ) as a tracer gas. Th cylindrical single pellet was made from CaO powder by use of a press and the pellet does not absorb the gases used. Diffusion of nitrogen was examined at different flow rates of the carrier gas. The results were obtained as chromatographic curves, which were analyzed, by numerical integration. The absolute moment was calculated for each run and result in: Highest value = 5.3158 s for the lowest flow rate F Lowest value = 2.056 s for run # 4 The limiting value of the first absolute moment at high flow rate was found graphically to be 2.00s . The effective diffusion coefficient was calculated experimentally and theoretically and the error difference was 39.84% . Introduction The diffusion of gases through porous solids, such as catalysts, absorbents etc., are of practical importance in a variety of chemical engineering processing industries. In porous media, pores are filled with a fluid and the solute diffuses through the fluid. However, diffusion takes place only through the cramped and tortuous pores of the composite. Because the pores are not straight, the diffusion effectively takes place over a longer distance than it would in a homogeneous s material. Because the solid is impermeable, diffusion occurs over a smaller cross sectional area than that available in a homogeneous material. In this type of diffusion the solid may be form of a porous barrier or membrane separating two bodies of fluid, as in the case of gaseous diffusion [6:95]. However, I any discussion of molecular movement in the solid state, the explanation of mass transfer is automatically divided into two major filed of interest: the diffusion of gases or liquid into the pores of the solid and the interdiffusion of solid constituents by atomic movement [5:498]. The first class of diffusion plays a major role in catalysis and is important to the chemical engineer....
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This note was uploaded on 09/27/2011 for the course CHEMICAL E CHE 309 taught by Professor M.elgaily during the Fall '09 term at King Fahd University of Petroleum & Minerals.

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che309- M3 report - Abstract Diffusion of gases through a...

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