Porous Catalyst Particles

Reactants must diffuse through the network of pores of a catalyst particle to reach the internal area, and the products must diffuse back. The optimum porosity of a catalyst particle is deterrnined by tradeoffs making the pores smaller increases the surface area and thereby increases the activity of the catalyst, but this gain is offset by the increased resistance to transport in the smaller pores increasing the pore volume to create larger pores for faster transport is compensated by a loss of physical strength. A simple quantitative development (46—48) follows for a first-order, isothermal, irreversible catalytic reaction in a spherical, porous catalyst particle. 

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

Explain why, when applying the equation to reaction in a porous catalyst particle, it is necessary to replace the molecular diffusivity D by an effective diffusivity De. 

Weisz, P. B. and Hicks, J. S., The behaviour of porous catalyst particles in view of internal mass and heat diffusion effects, Chem. Eng. Sci., 17, 265-275 (1962). 

Most of the actual reactions involve a three-phase process gas, liquid, and solid catalysts are present. Internal and external mass transfer limitations in porous catalyst layers play a central role in three-phase processes. The governing phenomena are well known since the days of Thiele [43] and Frank-Kamenetskii [44], but transport phenomena coupled to chemical reactions are not frequently used for complex organic systems, but simple - often too simple - tests based on the use of first-order Thiele modulus and Biot number are used. Instead, complete numerical simulations are preferable to reveal the role of mass and heat transfer at the phase boundaries and inside the porous catalyst particles. 

Above we considered a porous catalyst particle, but we could similarly consider a single pore as shown in Fig. 5.36. This leads to rather similar results. The transport of reactant and product is now determined by diffusion in and out of the pores, since there is no net flow in this region. We consider the situation in which a reaction takes place on a particle inside a pore. The latter is modeled by a cylinder with diameter R and length L (Fig. 5.36). The gas concentration of the reactant is Cq at the entrance of the pore and the rate is given by... 

The initial reaction rate of a non-porous single-channel micro reactor, filled with porous catalyst particles, was 2.0 10 mol min [12]. The same value of the porous 10-channel micro reactor was about three times larger. When normalized for the metal content of the device, the reaction rates of the porous reactor and the particle-containing reactor become similar, 6.5 10 and 4.5 10 mol min m , respectively. 

Reactor performance with porous catalyst particles and porous walls that are catalyst coated... 

A catalytic fixed bed reactor is a (usually) cylindrical tube that is randomly filled with porous catalyst particles. These are frequently spheres or cylindrical pellets, but other shapes are also possible. The use of rings or other forms of particles with internal voids or external shaping is on the increase. During single-phase operation, a gas or liquid flows through the tube and over the catalyst particles, and reactions take place on the surfaces, both interior and exterior, of the particles. 

HETEROGENEOUS CATALYSIS KINETICS IN POROUS CATALYST PARTICLES... 

Heterogeneous Catalysis Kinetics in Porous Catalyst Particles 199... 

Figure 8.9 Concentration (cA) and temperature (Z ) gradients (schematic) in a porous catalyst particle (spherical or end-on cylindrical)...

Porous catalyst particles are complex devices with appreciable internal gradients of temperature and composition, but these factors can be taken account of by the concept of catalyst effectiveness which is sometimes calculable. 

Weisz, P. B. (1966). Combustion of carbonaceous deposits within porous catalyst particles. III. The CO2/CO product ratio. J. Catal. 6, 425. 

The existence of radial temperature and concentration gradients means that, in principle, one should include terms to account for these effects in the mass and heat conservation equations describing the reactor. Furthermore, there should be a distinction between the porous catalyst particles (within which reaction occurs) and the bulk gas phase. Thus, conservation equations should be written for the catalyst particles as well as for the bulk gas phase and coupled by boundary condition statements to the effect that the mass and heat fluxes at the periphery of particles are balanced by mass and heat transfer between catalyst particles and bulk gas phase. A full account of these principles may be found in a number of texts [23, 35, 36]. 

Consider a porous catalyst particle bathed by reactant A. The rate of reaction of A for the particle as a whole may depend on ... 

Rate Influencing Factor Porous Catalyst Particle Catalyst Coated Surface Burning of a Droplet of Fuel Cells and Simple Living Creatures... 

Although here we introduce all the phenomena which affect the rate, the real world is never so exciting that we have to concern ourselves with all five factors at any one time. In fact, in the majority of situations with porous catalyst particles we only have to consider factors and . 

Use whichever definition is convenient. However, for porous catalyst particles rates based on unit mass and on unit volume of particles, r and r " are the useful measures. Hence for nth order reactions... 

Performance Equations for Reactions Containing Porous Catalyst Particles 393... 

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