Selectivity Sherwood

Many authors contributed to the field of diffusion and chemical reaction. Crank (1975) dealt with the mathematics of diffusion, as did Frank-Kamenetskii (1961), and Aris (1975). The book of Sherwood and Satterfield (1963) and later Satterfield (1970) discussed the theme in detail. Most of the published papers deal with a single reaction case, but this has limited practical significance. In the 1960s, when the subject was in vogue, hundreds of papers were presented on this subject. A fraction of the presented papers dealt with the selectivity problem as influenced by diffitsion. This field was reviewed by Carberry (1976). Mears (1971) developed criteria for important practical cases. Most books on reaction engineering give a good summary of the literature and the important aspects of the interaction of diffusion and reaction. 

Traditionally, an average Sherwood number has been determined for different catalytic fixed-bed reactors assuming constant concentration or constant flux on the catalyst surface. In reality, the boundary condition on the surface has neither a constant concentration nor a constant flux. In addition, the Sh-number will vary locally around the catalyst particles and in time since mass transfer depends on both flow and concentration boundary layers. When external mass transfer becomes important at a high reaction rate, the concentration on the particle surface varies and affects both the reaction rate and selectivity, and consequently, the traditional models fail to predict this outcome. 

In Table 1.4, the characteristic time-scales for selected operations are listed. The rate constants for surface and volume reactions are denoted by and respectively. Furthermore, the Sherwood number Sh, a dimensionless mass-transfer coefficient and the analogue of the Nusselt number, appears in one of the expressions for the reaction time-scale. The last column highlights the dependence of z p on the channel diameter d. Apparently, the scale dependence of different operations varies from dy f to (d ). Owing to these different dependences, some op-... 

P. C. Hayes, Process Selection in Extractive Metallurgy, Hayes Publishing Co., Sherwood, Queensland, Australia, 1986. 

In the. cnnvective-diffusion-contiolled region, the rate should be independent of the value of A/kT. Selecting arbitrary values for the Peclet number and aspect ratio, the Sherwood number is first calculated for A/kT = 0. Then the value of the ratio A/kT Is gradually increased,... 

Pg pressure inside the gas plug pi pressure in the liquid at the interface AP pressure drop q volumetric flow rate r radial coordinate reaction rate Tc radius at chaimel comer Ri principal radius of curvature Ri principal radius of curvature Re Reynolds number S selectivity Sc Schmidt number Sh Sherwood number SR slurry reactor STYv space lime yield t time... 

The influence of diffusional limitations in gas phase reactions has been extensively treated by Wheeler and from a chemical engineering viewpoint by Hougen and Watson More recently a monograph by Satterfield and Sherwood has appeared. The problem of diffusion can be separated into two parts, the first is diffusion or mass transfer to the external surface of the catalyst and second, for those catalysts which are porous, diffusion within the catalyst pores. When diffusion is the rate limiting process, reaction rate, selectivity and activation energy are affected. 

In this table are a representative selection of diffusion coefficients. The subsection Prediction and Correlation of Physical Properties should be consulted for estimation techniques. As general references, the works by Hirschfelder, Curtiss, and Bird, Molecular Theory of Gases and Liquids, Wiley, New York, 1964 Chapman and Cowling, The Mathematical Theory of Non-Uniform Gases, Cambridge, New York, 1970 Reid and Sherwood, The Properties of Gases and Liquids,... 

Selective Crystal Dissolution. An effective method for distinguishing bulk (lattice trapped) versus surface impurities involves the selective dissolution of a crystal sample while testing the liquid and/or crystalline phases for relative purity. In this technique, a small sample of crystals of a narrow size fraction is washed with successive small amounts of clean solvent until most of the crystalline phase is dissolved. The filtrate and/or crystalline phase are analyzed after each washing to discern whether impurities reside predominantly at the surface, or are more evenly distributed throughout the crystalline phase. The general approach is described by Narang and Sherwood (1978) for quantifying caproic acid incorporation in adipic crystals, and by Addadi et al. (1982) for amino acid separations. 

This same type of toxic stress can be applied to any spore-producing fungus or colony-producing bacterium as a selection tool. Dr. John Sherwood, a plant pathologist at Montana State University, indicated that... 

Mass transfer rates attainable In menbrane separation devices, such as gas permeators or dlalyzers, can be limited by solute transport through the menbrane. The addition Into the menbrane of a mobile carrier species, which reacts rapidly and reversibly with the solute of Interest, can Increase the membrane s solute permeability and selectivity by carrier-facilitated transport. Mass separation is analyzed for the case of fully developed, one-dimensional, laminar flow of a Newtonian fluid in a parallel-plate separation device with reactive menbranes. The effect of the diffusion and reaction parameters on the separation is investigated. The advantage of using a carrier-facilitated membrane process is shown to depend on the wall Sherwood number, tfrien the wall Sherwood nunber Is below ten, the presence of a carrier-facilitated membrane system is desirable to Improve solute separation. 

In many cases of practical interest, the membrane s mass transfer resistance is significant, i.e., the wall Sherwood number is small, leading to relatively low mass transfer rates of the solute. The diffusive flux of the permeate through the membrane can be increased by introducing a carrier species into the membrane. The augmentation of the flux of a solute by a mobile carrier species, which reacts reversibly with the solute, is known as carrier-facilitated transport (25). The use of carrier-facilitated transport in industrial membrane separation processes is of considerable interest because of the increased mass transfer rates for the solute of interest and the improved selectivity over other solutes (26). 

Bagshawe KD, Springer CJ, Searle F, Antoniw P, Sharma SK, Melton RG, Sherwood RF. A cytotoxic agent can be generated selectively at cancer sites. Brit J Cancer 1988 58 700-703. 

Craik, K.J.W. 1966. The mechanism of human action. In Sherwood, S.L. (Ed.), The Nature of Psychology — A Selection of Papers, Essays, and Other Writings by the Late Kenneth J.W. Craik, Cambridge, Cambridge University Press. 

For a detailed treatment, reference should be made to books devoted exclusively to properties estimation. The book by Reid, Prausnitz, and Poling (1987), along with its earlier versions by Reid and Sherwood (1958, 1966) and Reid, Prausnitz, and Sherwood (1977), and the works of Janz (1958), Hansch and Leo (1979), and Lyman, Reehl, and Rosenblatt (1982) are noteworthy. The following methods selected for a few properties are based in part on the recommendations contained in these treatises. 

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