Nonporous particle reactions

Film dijfusion With a fast surface reaction on a nonporous particle, mass transfer limitations can arise in the fluid phase. 

It should be noted that in the case of the reaction of a fluid with a nonporous solid, the chemical reaction step and the mass transport step are connected in series. This makes the analysis much simpler as compared to the case of a porous solid. In reactions of nonporous particles there can essentially be two cases one which shows absence of a solid product layer, and the other which shows its presence. 

The system discussed is easily extended to a three-phase reactor with mass transfer and reaction in series, for example a gas is absorbed in a liquid in which nonporous particles are suspended. Reaction occurs at the surface of the particles. Examples are the hydrogenation of organic liquids with a solid catalyst and the alkylation of a liquid re-... 

Fumed silica is a highly dispersed silicon dioxide of large industrial importance and a wide spectrum of applications. Due to its production in a flame process fumed silica exhibits a smooth and nonporous particle surface. Additionally to its high surface area fumed silica bears isolated and statistically distributed surface silanol groups that render this product hydrophilic. A most important technical reaction, therefore, is the silylation and hydrophobization of the hydrophilic surface. 

The dependence of rates of adsorption and catalytic reactions on surface makes it imperative to have a reliable method of measuring surface area. Otherwise it would not be possible to compare different catalysts (whose areas are different) to ascertain the intrinsic activity per unit surface. For surface areas in the range of hundreds of square meters per gram-- a-porous materiahwith equivalent-cylindrical-pore-r-adii-(see SecT -8--5)Tn-the range of 10 to 100 A is needed. The following example shows that such areas are not possible with nonporous particles of the size which can be economically manufactured. 

Consider the unrmolecular, irreversible solid-catalyzed gas-phase reaction, A—yB, carried out in a fixed-bed reactor packed with completely nonporous particles. Assume that the chemical steps of adsorption-reaction-desorption are represented by first-order kinetics and that the bulk temperature, Tin and surface temperature, Ts, are the same around a particle located at any point along the length of the reactor. 

Let s examine the significance of Eqn. (9-47) by means of a numerical illustration. Suppose that a gas-phase reaction with ATad = 100 K is taking place on the external surface of a nonporous catalyst particle. We assume a nonporous particle in order to isolate the effect of external mass transfer, and to avoid having to analyze the effect of changes in Ts and Ca on the effectiveness factor. Catalysts that are essentially nonporous are used in several commercial processes. For example, a Pt/Rh metal gauze is used for ammonia oxidation to nitric oxide and for the reaction of NH3,02, and CH4 to produce HCN, and a promoted Ag on a-Al203 is used in many ethylene oxide plants. 

The simplest system in fluid-solid reactions is that of a shrinking nonporous particle forming no solid product layer ... 

The changing particle size, however, will alter the relative importance of the rates of chemical reaction and the transport of heat and mass. It is conceivable then that there may occur a switch from transport-controlled regime to kinetic-controlled regime during the reaction of nonporous particles even when conditions external to the particle remain constant throughout the reaction period. 

It is of interest to note that Eq. (4.3.73) is of the same form as the relationship between conversion and time for the shrinking-core system of initially nonporous particles, where the overall rate is controlled by both diffusion and chemical kinetics. For a first-order reaction this relationship, given by Eq. (3.3.33), may be put in the following form, which is valid for spheres, cylinders, and flat plates ... 

The result is shown in Figure 10, which is a plot of the dimensionless effectiveness factor as a function of the dimensionless Thiele modulus ( ), which is R.(k/Dwhere R is the radius of the catalyst particle and k is the reaction rate constant. The effectiveness factor is defined as the ratio of the rate of the reaction divided by the rate that would be observed in the absence of a mass transport influence. The effectiveness factor would be unity if the catalyst were nonporous. Therefore, the reaction rate is... 

For testing and optimizing catalysts, the temperature region just below that where pore diffusion starts to limit the intrinsic kinetics provides a desirable working point (unless equilibrium or selectivity considerations demand working at lower temperatures). In principle, we would like the rate to be as high as possible while also using the entire catalyst efficiently. For fast reactions such as oxidation we may have to accept that only the outside of the particles is used. Consequently, we may decide to use a nonporous or monolithic catalyst, or particles with the catalytic material only on the outside. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

CO2 increased from 50% to 80%. It would be highly desirable to investigate MOR on model nonporous substrates with DBMS in order to determine the influence of particle size on various reaction steps and the partitioning between the direct and indirect pathways. 

Adhesive force, non-Brownian particles, 549 Admicelle formation, 277 Adsorption flow rate, 514 mechanism, 646-647 on reservoir rocks, 224 patterns, on kaolinite, 231 process, kinetics, 487 reactions, nonporous surfaces, 646 surface area of sand, 251 surfactant on porous media, 510 Adsorption-desorption equilibria, dynamic, 279-239 Adsorption plateau, calcium concentration, 229... 

Figure 9.2(a) or (b) shows the essence of the SCM, as discussed in outline in Section 9.1.2.1, for a partially reacted particle. There is a sharp boundary (the reaction surface) between the nonporous unreacted core of solid B and the porous outer shell of solid product (sometimes referred to as the ash layer, even though the ash is desired product). Outside the particle, there is a gas film reflecting the resistance to mass transfer of A from the bulk gas to the exterior surface of the particle. As time increases, the reaction surface moves progressively toward the center of the particle that is, the unreacted core of B shrinks (hence the name). The SCM is an idealized model, since the boundary between reacted and unreacted zones would tend to be blurred, which could be revealed by slicing the particle and examining the cross-section. If this... 

The particle is nonporous, so that reaction occurs only on the exterior surface. 

Experiments were conducted with four different sizes of nonporous spherical particles with the temperature and partial pressure of A kept constant at 700 K and 200 kPa, respectively. The time to complete reaction, t, was measured in each case, and the following data were obtained, with p m = 2.0 X 105 mol rrT3 ... 

First-order reactions without internal mass transfer limitations A number of reactions carried out at high temperatures are potentially mass-transfer limited. The surface reaction is so fast that the global rate is limited by the transfer of the reactants from the bulk to the exterior surface of the catalyst. Moreover, the reactants do not have the chance to travel within catalyst particles due to the use of nonporous catalysts or veiy fast reaction on the exterior surface of catalyst pellets. Consider a first-order reaction A - B or a general reaction of the form a A - bB - products, which is of first order with respect to A. For the following analysis, a zero expansion factor and an effectiveness factor equal to 1 are considered. 

External mass transfer reduces the concentration of reactant gas close to the particle surface and thus reduces the overall process rate. Thus, consider gasification to be a first-order reaction. Then at steady state, the rate of gasification equals the rate of mass transfer. For a nonporous solid, the surface reaction (whose rate constant is k ) consumes the diffusing reactant ... 

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