Ash diffusion control

A stream of particles of one size are 80% converted (SCM/ash diffusion control, uniform gas environment) on passing through a reactor. If the reactor is made twice the size but with the same gas environment, same feed rate, and same flow pattern of solids, what would be the conversion of solids The solids are in... 

Solve Example 26.3 with the following modification the kinetics of the reaction is ash diffusion controlled with t R = 100 pum) = 10 min. 

For combustion reactions Levenspiel (4) gives the constant temperature integration for reaction and gas or ash diffusion controlled cases. The integration of the pyrolysis kinetics will be demonstrated in the following section. 

To get supporting evidence for the earlier conclusion of the reaction-controlled operation, the fractional conversion of FeS2 (in coal) with time was studied, and the time required for complete conversion was also obtained. The following SCM relationships were used (see Section 11.3.2.1.1) to determine the controlling mechaiusm. For ash diffusion control. 

Each stage is assumed to give perfect mixing of solids. Expressions are tabulated for different governing kinetic rate expressions homogeneous reaction model and shrinking core model with external mass transfer control, ash diffusion control and chemical reaction control. Population balances are required to... 

When ash diffusion controls (case E), the mass transfer coefficient depends on the thickness of the ash layer. The usual assumption is that this coefficient is... 

For the reaction and assumptions in Example 22-1, except that reaction-rate control replaces ash-layer-diffusion control, suppose the feed contains 25% of particles of size R for which t = 1.5 h, 35% of particles of size 2R, and 40% of particles of size 3R. What residence time of solid particles, fB, is required for /B = 0.80 ... 

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. 

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. 

B = 0.80, t, which is a measure of the size of reactor, is about 1.7 min for ash-layer control, 9.5 min for reaction control, and 14.5 min for gas-film control. The relatively favorable behavior for ash-layer diffusion control in this example reflects primarily the low value of (1.67 min versus 6.67 min for the other two cases) imposed. 

Time. Figures 25.9 and 25.10 show the progressive conversion of spherical solids when chemical reaction, film diffusion, and ash diffusion in turn control. Results of kinetic runs compared with these predicted curves should indicate the ratecontrolling step. Unfortunately, the difference between ash diffusion and chemical reaction as controlling steps is not great and may be masked by the scatter in experimental data. 

Particles of constant size Gas film diffusion controls, Eq. 11 Chemical reaction controls, Eq. 23 Ash layer diffusion controls, Eq. 18 Shrinking particles Stokes regime, Eq. 30 Large, turbulent regime, Eq. 31 Reaction controls, Eq. 23... 

On the other hand, if ash diffusion already controls, then a rise in temperature should not cause it to shift to reaction control or film diffusion control. 

As the experimentally found diameter dependency lies between these two values, it is reasonable to expect that both these mechanisms offer resistance to conversion. Using in turn ash diffusion and chemical reaction as the controlling resistance should then give the upper and lower bound to the conversion expected. 

Additionally, another quite plausible model, i.e., an ash diffusion reaction control model, was tested (see Eq. 28) [321] for the imide pyrolysis. 

Design of Adsorption and Ion-Exchange Processes Ash (solid) diffusion control... 

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