Films mass transfer

The concentration of gas over the active catalyst surface at location / in a pore is ai [). The pore diffusion model of Section 10.4.1 linked concentrations within the pore to the concentration at the pore mouth, a. The film resistance between the external surface of the catalyst (i.e., at the mouths of the pore) and the concentration in the bulk gas phase is frequently small. Thus, a, and the effectiveness factor depends only on diffusion within the particle. However, situations exist where the film resistance also makes a contribution to rj so that Steps 2 and 8 must be considered. This contribution can be determined using the principle of equal rates i.e., the overall reaction rate equals the rate of mass transfer across the stagnant film at the external surface of the particle. Assume A is consumed by a first-order reaction. The results of the previous section give the overall reaction rate as a function of the concentration at the external surface, a. [Pg.366]

The overall effectiveness factor for the first-order reaction is defined using the bulk gas concentration a. [Pg.366]

The concentrations and a are coupled by the rate of mass transfer across the film [Pg.366]

Equation (10.29) is the appropriate reaction rate to use in global models such as Equation (10.1). The reaction rate would be —ka if there were no mass transfer resistance. The effectiveness factor rj accounts for pore diffusion and him resistance so that the effective rate is — 7ka. [Pg.367]

Typically, the him resistance is important only when the internal pore resistance is also important. If the Thiele modulus is small, the him resistance will usually be negligible. This idea is explored in Problems 10.11 and 10.12. [Pg.367]

Equations 10.28, 10.29, and 10.30 are combined to eliminate and to obtain an expression for 3 a based on the bulk gas phase concentration a. The reaction rate has the form of Equation 10.29 with [Pg.372]

The rate of mass transfer,/, is then assumed to be proportional to the concentration differences existing within each phase, the surface area between the phases,, and a coefficient (the gas or Hquid film mass transfer coefficient, k or respectively) which relates the three. Thus... [Pg.332]

If no wave formations are present, analysis of behavior of the liquid-film mass transfer as developed by Hatta and Katori [J. Soc. Chem. Ind., 37, 280B (1934)] indicates that... [Pg.1402]

Liquid film mass transfer coefficient ( b moles) (cu ft)/(sec) (sq ft) (lb mole)... [Pg.339]

Figure 9-73. Fellinger s overall gas film mass transfer data for ammonia-water system. Used by permission of Leva, M. Tower Packings and Packed Tower Design, 2nd Ed. U.S. Stoneware Co. (now, Norton Chemical Process Products Corp.) (1953).

In this process, the two streams flow countercurrently through the column and undergo a continuous change in composition. At any location are in dynamic rather than thermodynamic equilibium. Such processes are frequently carried out in packed columns, in which the liquid (or one of the two liquids in the case of a liquid-liquid extraction process) wets die surface of the packing, thus increasing the interfacial area available for mass transfer and, in addition, promoting high film mass transfer coefficients within each phase. [Pg.622]

There are several correlations for estimating the film mass transfer coefficient, kf, in a batch system. In this work, we estimated kf from the initial concentration decay curve when the diffusion resistance does not prevail [3]. The value of kf obtained firom the initial concentration decay curve is given in Table 2. In this study, the pore diffusion coefficient. Dp, and surface diffusion coefficient, are estimated by pore diffusion model (PDM) and surface diffusion model (SDM) [4], The estimated values of kf. Dp, and A for the phenoxyacetic acids are listed in Table 2. [Pg.515]

Onda et al. (1968) published useful correlations for the film mass-transfer coefficients kG and ki and the effective wetted area of the packing aw, which can be used to calculate Hg and HL. [Pg.601]

Suppose, for simplicity, that gas-film mass transfer is rate controlling. From Table 9.1, in this case, for a sphere,... [Pg.236]

An important difference between a shrinking particle reacting to form only gaseous product(s) and a constant-size particle reacting so that a product layer surrounds a shrinking core is that, in the former case, there is no product or ash layer, and hence no ash-layer diffusion resistance for A. Thus, only two rate processes, gas-film mass transfer of A, and reaction of A and B, need to be taken into account. [Pg.237]

Corresponding equations for the two special cases of gas-film mass-transfer control and surface-reaction-rate control may be obtained from these results (they may also be derived individually). The results for the latter case are of the same form as those for reaction-rate control in the SCM (see Table 9.1, for a sphere) with R0 replacing (constant) R (and (variable) R replacing rc in the development). The footnote in Example 9-2 does not apply here (explain why). [Pg.239]

Two-Film Mass-Transfer Model for Gas-Liquid Systems... [Pg.240]

Mass transfer of A through gas film Mass transfer of A through liquid film Reaction of A and B in bulk liquid... [Pg.243]

Figure 9.7 shows concentration profiles schematically for A and B according to the two-film model. Initially, we ignore the presence of the gas film and consider material balances for A and B across a thin strip of width dx in the liquid film at a distance x from the gas-liquid interface. (Since the gas-film mass transfer is in series with combined diffusion and reaction in the liquid film, its effect can be added as a resistance in series.)... [Pg.247]

Assume that the particle is spherical and isothermal, that both gas-film mass transfer resistance and reaction resistance are significant, and that the Ranz-Marshall correlation for k g is applicable. Do not make an assumption about particle size, but assume the reaction is first-order. [Pg.258]

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