Duct reactors

The advantages of duct reactors relative to fixed-bed packed catalytic tubular... 

Reactor performance is established by calculating the molar density of reactant A from a steady-state mass balance that accounts for axial convection and transverse diffusion. Chemical reaction only occurs on the well-defined catalytic surface which bounds fluid flow in the regular polygon channel. Hence, depletion of reactant A due to chemical reaction appears in the boundary conditions, but not in the mass balance which applies volumetrically throughout the homogeneous flow channel. The mass transfer equation for duct reactors is written in vector form ... 

Unlike porous pellets, it is mathematically feasible to account for chemical reaction on the well-defined catalytic surfaces that bound the flow regime in regular polygon duct reactors. A qualitative description of the boundary conditions is based on a steady-state mass balance over a differential surface element. Since convective transport vanishes on the stationary catalytic surface, the following contributions from diffusion and chemical reaction are equated, with units of moI/(areatime) ... 

Several shape functions are presented in Table 23-3. Duct reactor simulations based on these catalyst deposition profiles are discussed in Section 23-6.7. 

The objective here is to simulate duct reactor performance with nonuniform catalyst activity and identify optimal deposition strategies when reactant diffn-sion toward the active surface is hindered, particularly in the corners of the flow channel. Both types of power-function profiles, listed in Table 23-3, are evaluated for n = 1,2,4, 8. The delta-function distribution has been implemented by Varma (see Morbidelli et al., 1985) to predict optimum catalyst performance in porous pellets with exothermic chemical reaction. Nonuniform activity profiles for catalytic pellets in fixed-bed reactors, in which a single reaction occnrs, have been addressed by Sznkiewicz et al. (1995), and effectiveness factors for... 

Hence, two-dimensional numerical integration of the velocity-weighted microscopic concentration profile is required to evaluate the effectiveness of each shape function F y ). Duct reactor performance curves are presented as CAbuik vs. z in dimensionless form, as illustrated in Figure 23-2. 

Comparison with Exact Results. It is not unreasonable to suspect that truncation errors in the numerical approximation of first and second derivatives might accumulate in the computational scheme used to integrate the mass transfer equation. One check for accuracy involves a comparison between numerical results and exact analytical solutions. Of course, only a limited number of analytical solutions are available. For example, the following solutions have been obtained analytically for catalytic duct reactors ... 

Consider the governing equations that describe convection, diffusion, and chemical reaction in tube-wall duct reactors where expensive metal catalyst is coated on the inner walls of the flow channel. 

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