Reaction gas-solid noncatalytic

Structural Variations as a Tool to Analyze the Mechanism of Noncatalytic Solid-Gas Reactions... 

Ishida, M. and C.Y. Wen. Effect of Solid Mixing on Noncatalytic Solid-Gas Reactions in a Fluidized Bed. 

Nonbulking sludge, 25 899 Nonbulk packages, of hazardous materials, 25 338, 342-343 Noncatalytic cracking, 18 648 Noncatalytic gas-solid reactions, 21 331 Noncatalytic solid-fluid reactions, 21 343-344 categories of, 21 344... 

Fluidized bed noncatalytic reactors. Fluidized heds are also suited to gas-solid noncatalytic reactions. All the advantages described earlier for gas-solid catalytic reactions apply. As an example. 

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

A heterogeneous catalytic reaction occurs at or very near the fluid-solid interface. The principles that govern heterogeneous catalytic reactions can be applied to both catalytic and noncatalytic fluid-solid reactions. These two other types of heterogeneous reactions involve gas-liquid and gas-Hquid-solid systems. Reactions between gases and liquids are usually mass-transfer limited. 

FIG. 7-14 Concentration profiles with intraparticle diffusion control, = 70. ii absence of gas particle mass-transfer resistance. [From Wen, Noncatalytic Heterogeneous Solid-Fluid Reaction Models, Ind. Eng. Chem. 60(9) 34-54 (1968), Fig. 12.]... 

Costa, E. Smith, J. Kinetics of noncatalytic, nonisothermal gas-solid reactions hydrofluori-nation of uranium dioxide. AIChE J. 1971, 17, 947-958. 

This is a different kind of heterogeneous reaction—a gas-solid noncatalytic one. Let us examine the process at initial conditions (t 0), so that there has been no opportunity for a layer of UF4.(5) to be formed around the UO pellet The process is much like that for gas-solid catalytic reactions. Hydrogen fluoride gas is transferred from the bulk gas to the surface of the UO2 pellets and reacts at the pellet-gas interface, and H2O diffuses out into the bulk gas. If the pellet is nonporous, all the reaction occurs at the outer surface of the UO2 pellet, and only an external transport process is possible. Costa studied this system by suspending spherical pellets 2 cm in diameter in a stirred-tank reactor. In one run, at a bulk-gas temperature of 377°C, the surface temperature was 462°C and the observed rate was — Tuo = 6.9 x 10 mole U02/(sec) (cm reaction surface). At these conditions the concentrations of... 

Gas-solid heterogeneous reactions may be noncatalytic. An example is the hydrofluorination of uranium dioxide pellets referred to in Sec. 7-1. Since one reactant is in the solid phase and is consumed, the rate of reaction varies with time. Hence such processes are basically transient, in comparison with the steady-state operation of gas-solid catalytic reactors. The process for smelting ores such as zinc sulfide,... 

In many noncatalytic types a solid product builds up around the reacting core [for example, Na2S04(j) is deposited around the NaCl particles in the last illustration]. This introduces the additional physical processes of heat and mass transfer through a product layer around the solid reactant. A somewhat different form of noncatalytic gas-solid reaction is the regeneration of catalysts which have been deactivated by the deposition of a substance on the interior surface. The most common is the burning of carbon (with air) which has been gradually deposited on catalyst particles used in hydrocarbon reactions. Many of the physical and chemical steps involved here are. the same as those for gas-solid catalytic reactions. The chief difference is the transient nature of the noncatalytic reaction. This type of heterogeneous reaction will be considered in Chap. 14. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

Many industrially important reactions are characterized by an interface across which heat/mass transfer occurs. They are called heterogeneous reactions. They include fluid-fluid reactions, gas-solid catalytic reactions, gas-solid noncatalytic reactions, and solid-solid reactions. 

Laboratory reactors for fluid-solid and fluid-fluid reactions were described in Sections 3.1.6 and 3.3.2, respectively. The discussion in these sections is also useful for gas-liquid-solid reactions. A combination of the Carberry reactor (Eigure 11.7) and a stirred cell (Figure 11.14A) is useful for noncatalytic and catalytic reactions. Some discussion of these issues is presented in Case Studies CS8 and CSll as well as by Joshi et al. (1985) and Joglekar et al. (1991). 

We now consider 12 case studies that include simple homogeneous liquid-phase reactions, complex homogeneous gas-phase reactions, gas-solid catalytic and noncatalytic reactions, gas-liquid simple and complex reactions, gas-liquid-solid (noncatalytic) reactions, gas-liquid-solid (catalytic) reactions, and solid-solid reactions. The scope and coverage of each case study are summarized in Table 11.27. In the first, homogeneous reactions are considered. For these relatively simple reactions, the possibility of optimum design is discussed. 

A glib generalization is that the design equations for noncatalytic fluid-solid reactors can be obtained by combining the intrinsic kinetics with the appropriate transport equations. The experienced reader knows that this is not always possible even for the solid-catalyzed reactions considered in Chapter 10 and is much more difficult when the solid participates in the reaction. The solid surface is undergoing change. See Table 11.6. Measurements usually require transient experiments. As a practical matter, the measurements will normally include mass transfer effects and are often made in pilot-scale equipment intended to simulate a full-scale reactor. Consider a gas-solid reaction of the general form... 

If the catalyst consists of particles, one has a fixed or mobile stationary particles reactor. The flow is no longer nniform and the reaction is considered heterophasic. In this case, the catalyst consists of solid particles, but the reactants and products are gas or liquid and may flow through the particle bed in the concurrent or counter current direction. The reaction may be catalytic or noncatalytic and takes place on the particle surface and/or within the pores of the catalyst. In this case, the system is heterophasic and may be solid-gas type, solid-liquid, or solid-liquid-gas taking place in the respective reactors fixed or mobile-bed, trickle bed, and slurry bed. 

The most catalytic or noncatalytic processes involving reactions in multiphase systems. Such processes include heat and mass transfer and other diffusion phenomena. The applications of these processes are diverse and its reactors have their own characteristics, which depends on the type of process. For example, the hydrogenation of vegetable oils is conducted in a liquid phase slurry bed reactor, where the catalyst is in suspension, the flow of gaseous hydrogen keeps the particles in suspension. This type of reaction occurs in the gas-liquid-solid interface. 

Successful process developments rely on the understanding of process requirements and constraints. Fluidized bed reactors have been used in many chemical processes. The typical reactions can be grouped as catalytic reactions, gas-solid reactions, noncatalytic gas reactions, and polymerization. The following describes process requirements for these systems. 

Chemical conversion is the principal operation in a wide variety of processes, including catalytic and noncatalytic gas phase reactions and the reaction of gas phase conqionents with solids. The reaction of gaseous species with liquids and with solid particles suspended in liquids is considered to be a special case of absorption and is discussed under that subject A generalized treatment of chemical reactor design broad enough to cover all gas purification applications is beyond the scope of tins book however, specific design parameters, such as space velocity and required time at tenqiaatuie, are given, whoi available, fra chemical conversion processes described in subsequent chqiters. 

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