Mass transfer Gas film

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).

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

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. 

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). 

Mass transfer of A through gas film Mass transfer of A through liquid film Reaction of A and B in bulk liquid... 

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.)... 

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. 

The performance of a reactor for a gas-solid reaction (A(g) + bB(s) -> products) is to be analyzed based on the following model solids in BMF, uniform gas composition, and no overhead loss of solid as a result of entrainment. Calculate the fractional conversion of B (fB) based on the following information and assumptions T = 800 K, pA = 2 bar the particles are cylindrical with a radius of 0.5 mm from a batch-reactor study, the time for 100% conversion of 2-mm particles is 40 min at 600 K and pA = 1 bar. Compare results for /b assuming (a) gas-film (mass-transfer) control (b) surface-reaction control and (c) ash-layer diffusion control. The solid flow rate is 1000 kg min-1, and the solid holdup (WB) in the reactor is 20,000 kg. Assume also that the SCM is valid, and the surface reaction is first-order with respect to A. 

As shown in Example 22-3, for solid particles of the same size in BMF, the form of the reactor model resulting from equation 22.2-13 depends on the kinetics model used for a single particle. For the SCM, this, in turn, depends on particle shape and the relative magnitudes of gas-film mass transfer resistance, ash-layer diffusion resistance and surface reaction rate. In some cases, as illustrated for cylindrical particles in Example 22-3(a) and (b), the reactor model can be expressed in explicit analytical form additional results are given for spherical particles by Levenspiel(1972, pp. 384-5). In other f l cases, it is convenient or even necessary, as in Example 22-3(c), to use a numerical pro-... 

For gas-film mass transfer control, we use equation 22.2-16a for reaction control, we use equation 22.2-18 and for ash-layer diffusion control, we integrate equation 22.2-13 numerically in conjunction with 22.2-19, as described in Example 22-3(c). The results generated by the E-Z Solve software (file ex22-4.msp) are shown in Figure 22.4. 

The cases considered thus far have all been based upon the premise that one process, ash-layer diffusion, surface reaction, or gas-film mass transfer, is rate controlling. However, in some cases, more than one process affects the overall kinetics for the conversion of the solid. This has two implications ... 

Repeat problem 22-8(a), if the rate is controlled by gas-film mass transfer. 

In these equations, the unknown quantities are p out> cAout, and D. The flux of A into the liquid film, NA(z = 0), is obtained from equation 9.2-45 for the special case of no gas-film mass transfer resistance (kAg -> large), and with cAb = cA out and pA = pAi0Ul. ... 

Molstad, M. C., McKinney, J. F. and Abbey, R. G. Trans. Am. Inst. Chem. Eng. 39 (1943) 605. Performance of drip-point grid tower packings, III. Gas-film mass transfer coefficients additional liquid-film mass transfer coefficients. 

As stated in Section 2.6, two kinds of gas film mass transfer coefficients - Icq, based on the partial pressure driving potential, and based on the concentration driving potential - can be defined. However, hereafter in this text only the latter type is used in other words. 

As mentioned, from the point of view of practical application, impinging streams is not suitable for the systems given in Table 7.2. On the other hand, the absorption processes for which impinging streams is applicable normally involve fast reaction(s) in liquid and thus are controlled by gas-film diffusion. Therefore the most important should be the gas-film mass transfer coefficient, kti, which is absent in the table. 

Figure 7.16 Influence of S02 concentration in feed gas on gas-film mass transfer coefficient. (The experimental conditions are the same as for Fig. 7.15).

Using the model equations described above and from the experimental data yielding Fig. 7.13, the calculated data are given in Fig. 7.18 as the plot of kG versus nn. By regression, the experimental data are fitted to represent the relationship between gas-film mass transfer coefficient and impinging velocity by... 

The gas-film mass transfer coefficient, kG, was determined based on the Sauter mean diameter of spray droplets. The results show essentially no influence of initial concentration of SOz on kG, suggesting that the process is controlled by diffusion through gas film and that the method proposed for the determination of kG is feasible ... 

The data on the relationship between impinging velocity and gas-film mass transfer coefficient were fitted by kG = 2.9 xlO 4no75821, with the standard deviation SD = 2.45X10 4 m-s 1, implying u0 is a strong effecting variable, and thus a very important operation variable ... 

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