Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

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2 THE SOLID-GAS INTERFACE 227detected by fluctuating rates of gas transfer across the pellet. Some improvementcan be obtained by heating the specimen before placing it in the tubing. The gasflow is directed tangentially near the pellet face to eliminate kinetic energy effectsLWFlowmetersMControlvalvesSS Shut-OffvalveT.C.cellFig. 26. (a) Schematic diagram of diffusion apparatus. (b) Details of the diffusion cell. From ref.191.References pp. 270-278
228 EXPERIMENT A L MET H 0 D S F 0 R HE T E R 0 GENE 0 US RE ACT I0 N Sand the build up of a stagnant layer. The effective diffusion coefficient per catalystparticle D, is given by1D, = - FC,awhere 1 is the length of the pellet, a is the cross sectional area over which diffusiontakes place, F is the gas flow rateand C,is the concentration of diffusing gas in thereference gas. Thus the effective diffusivities of both helium and nitrogen can bemeasured. Variation of the total pressure will show whether bulk diffusion orKnudsen diffusion is occurring, since the former is inversely proportional topressure and the latter independent of pressure.(d) Eflectiveness factorsMass transfer inside a porous catalyst particle was first analysed mathematicallyby Thielelg4 and the analysis further developed byand by Weiszand PraterIg3. More recent work is reviewed by Satterfield and Sherwood"'.These analyses provide a quantitative description of the factors which determinethe effectiveness of a porous pellet. The effectiveness factor, denoted by $, isdefined as the ratio of the measured reaction rate to the intrinsic chemical reactionrate, i.e. the reaction rate obtained on a plane surface of the same area.For a first order reaction in a spherical particlewhereis the Thiele diffusion modulus:where R is the radius of the sphere and kv is the first order reaction rate coefficientper unit volume of catalyst particles. For particles of arbitrary shape, R/3 is replacedby the ratio of particle volume to external surface. For reactions which are notfirst order, kV is replaced by kvCn-', where C is the reactant concentration and nis the order of reaction. The effectiveness factor q for different values of 4, isplotted in Fig. 27. The curves for different reaction orders and particle shapes lievery close together. The above equation for the Thiele modulus has importantt Wheeler18S~1a4 uses the symbolf and calls it "fraction of surface available".t This definition is somewhat different from that originally proposed by Thielele4. It is equkilentto h as used by Wheeler18**18' and to 413 as used by Weisz and Prater1@*.
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COMPREHENSIVE CHEMICAL KINETICS
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P.O.ELSEVIER PUBLISHING COMPANY335
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Contributors to Volume 1L. BATTDepa
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XCONTENTS4.3 Gas chromatography ...
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XI1CONTENTS4.6 Magnetic measurement
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4 EXPERIMENTAL METHODS FOR SLOW REA
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Reaction Furnace CircuitI10 EXPERIM
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278 EXPERIMENTAL METHODS FOR HETERO
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280 DETECTION AND ESTIMATION OF INT
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344 TREATMENT OF EXPERIMENTAL DATAf
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IndexAabsorption cross section, che
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424 INDEXbenzyl chloride, matrix ph
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426 INDEXchlorine (contdj—, react
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428 INDEXdeuterium oxide, addition
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430 INDEXentropy change, in radical
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432 INDEXfrequency of oscillation,
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434 INDEXhydrogen atoms (contdj—,
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436 INDEXisotopes (contd.J—, use
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438 INDEXmethyl radicals, chain ter
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440 INDEXoctadiene, ESR determinati
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442 INDEXphotoelectric measurement,
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444 INDEXrad, definition of, 66radi
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446 INDEXSchmidt number, and mass t
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448 INDEXTtailing reducer, in gas c
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450 INDEXviscosity, and diffusion c
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