Reactors for heterogeneous catalysis

Equation (2) (as an ordinary differential equation) and Eq. (3) apply now with Eq. (4). As already implied, a laboratory well-mixed reactor for heterogeneous catalysis is more difficult to realize than a PFR. Many versions have been used 12), and Froment and Bischoff 13) illustrate reactors with external recycle, with internal recycle 1,14), and with an internal spinning basket 15). When using these reactors for experiments in the transient regime, it is important to keep to a minimum the volume outside the bed of catalyst. Internal recycle reactors involve bearings exposed to hot reactive gases and require a magnetic drive system for leak-proof operation. Exter-... 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Steps 1 through 9 constitute a model for heterogeneous catalysis in a fixed-bed reactor. There are many variations, particularly for Steps 4 through 6. For example, the Eley-Rideal mechanism described in Problem 10.4 envisions an adsorbed molecule reacting directly with a molecule in the gas phase. Other models contemplate a mixture of surface sites that can have different catalytic activity. For example, the platinum and the alumina used for hydrocarbon reforming may catalyze different reactions. Alternative models lead to rate expressions that differ in the details, but the functional forms for the rate expressions are usually similar. 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

The various mathematical techniques used to obtain MWD with different combinations of mechanisms in different reactor types are surveyed. As Wei and Prater (57) stated for heterogeneous catalysis, and Benson (6) for kinetics in general, the chief difficulties are not the solutions of the kinetic equations once the mechanisms and constants are known. The real problem is the application of solutions to experimental data to determine fundamental mechanisms and constants which may be useful under other conditions. Too few investigators have noted this. 

The principal use of tubular reactors for kinetic studies is as catalytic fixed-bed reactors in heterogeneous catalysis. They are rarely used for quantitative studies of homogeneous reactions because these are difficult to confine sharply to reactors of this type (see farther below). 

For heterogeneous catalysis, the catalyst is packed in such reactors, which are easy to design and control, as the gases or liquids pass through the reactor and are analyzed. Such reactors are efficient for catalyst screening, especially when they are arranged in a parallel mode (Figure 1.21). 

For industrially relevant process the relaxation time is ca. 1-100 s. For construction of a kinetic model for nonstationary conditions, knowledge about the evolution of the concentrations of adsorbed species on the catalyst surface is needed. Under nonstationary conditions the changes of concentration fields in time, reactor space and catalyst surface (for heterogeneous catalysis) are interrelated by complex dependencies. Therefore, for experimental investigation under nonstationary conditions, knowledge about the gas and surface composition is required. 

Heterogeneous catalysis is of utmost significance in many fields of gas conversion and processing in chemical industries. Fixed packed bed reactors are widely used for heterogeneous catalysis of many of these processes. Accurate modeling of intra-particle heat and mass transport is a prerequisite for the design of many industrial processes and the interpretation of experiments. To describe the mass diffusion processes within... 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

These reactors were, and unfortunately still are, used in a few laboratories for process studies on heterogeneous catalysis, frequently with the... 

A good review of the transient response method in heterogeneous catalysis was published by Kobayashi and Kobayashi (1974). These authors credit Bermett (1967) for applying this previously microcatalytic research technique to recycle reactors and thereby, in view of this author, to engineering problems. 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

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