Gas-liquid reactions with solid catalysts

The number of commercial processes of this type is substantial. A list of 74 is given by Shah (1979). A briefer list arranged according to the kind of reactor is in Table 17.21. Depending on the 

Leading characteristics of five main kinds of reactors are described following. Stirred tanks, fixed beds, slurries, and three-phase fluidized beds are used. Catalyst particle sizes are a compromise between pressure drop, ease of separation from the fluids, and ease of fluidization. For particles above about 0.04 mm dia, diffusion of liquid into the pores and, consequently, accessibility of the internal surface of the catalyst have a minor effect on the overall conversion rate, so that catalysts with small specific surfaces, of the order of 1 m2/g, are adequate with liquid systems. Except in trickle beds the gas phase is the discontinuous one. Except in some operations of bubble towers, the catalyst remains in the vessel, although minor amounts of catalyst entrainment may occur. 

In ebullated (liquid fluidized) beds the particles are much larger (0.2-1 mm) than in gas fluidization (0-0.1 mm). Little 

Slurry reactors (bubble towers) are fluidized with continuous flow of gas. The particles are smaller (less than 0.1 mm) than in the liquid fluidized systems (0.2-1 mm). In some operations the liquid and solid phases are stationary, but in others they circulate through the vessel. Such equipment has been used in Frscher-Tropsch plants and for hydrogenation of fatty esters to alcohols, furfural to furfuryl alcohol, and of glucose to sorbitol. Hydrogenation of benzene to cyclohexane is done at 50 bar and 220-225°C with Raney nickel of 0.01-0.1 mm dia. The relations between gas velocities, solids 

TABLE 17.18. Formulas for the Heat Transfar Coefficient at the Walls of Packed Vessels  

Tube Forqiiila (Z p in m) Kange of tTRe 01 odrer variable  

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Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Phases Gas, liquid, gas-liquid interacting with solid catalyst or inert solid. Use if the order of the reaction is positive and > 95 % conversion is the target, and for consecutive reactions with an intermediate as the target product. Caution use for highly exothermic reactions. Not suitable for Arr (ATad ) > 10 or > 100 °C usually keep AT j < 50 °C. Provides large gas throughput. Related topics for GL, trickle reactor. Section 6.17, or bubble reactor. Section 6.13. 

Three-phase reactions comprise gas-liquid-solid and gas-liquid-liquid reactions. Gas-liquid reactions using solid catalysts represent a very important class of reactions. Conventionally, they are carried out in slurry reactors, (bubble columns, stirred tanks), fluidized beds, fixed bed reactors (trickle beds with cocurrent downflow or cocurrent upflow, segmented bed, and countercurrent gas-liquid arrangements) and structured (catalytic wall) reactors. 

Here, we will only examine the basic kinetic equations of gas-liquid reactions on solid catalysts, taking as example the simple reaction of a gaseous reactant A that reacts after dissolution in the liquid phase with a liquid reactant B (va = Vb = —1) to a liquid product P ... 

Gas-liquid reaction with catalytic solid. Include the catalyst with structured packing in a distillation column. A related topic is distillation (Section 16.11.4.2). 

Solid-eatalyzed reactions can occur in either the liquid or gas phase. Gas-phase reactions are not very common in the production of fine chemieals, beeause eom-plex molecules with limited volatility and thermal stability are usually involved, which makes operation at the high temperatures required for their vaporization impossible. Gas-liquid reactions with a solid catalyst probably encompass the largest number of applications in fine-chemical and pharmaceutical processes [1]. Two other classes of solid-eatalyzed reaction taking place in the liquid phase are liquid-solid reactions and liquid-liquid-solid reactions, but these are much less eommon. We shall, therefore, foeus on gas-liquid-solid reaetions, in which the solid is a heterogeneous catalyst. 

Hydrogenation reactions have also been studied with catalytic membrane reactors using porous membranes. In this case the membrane, in addition to being used as a contactor between the liquid and gaseous reactants, could, potentially, also act as a host for the catalyst, which is placed in the porous framework of the membrane. As previously noted a triple-point interface between the three different phases (gas, liquid, and the solid catalyst) is then created in the membrane. The first application was reported by Cini and Harold... 

Taking into account the elements discussed before, we may formulate some guidelines for reactor selection. The strategy may be applied also for heterogeneous reactions described by pseudo-homogeneous models, as some gas-solid catalyst reactors, or gas-liquid reactions with reaction in the liquid phase. 

In gas/liquid reactions with a solid catalyst, the latter is usually submerged in the liquid, or at least completely wetted by it. This means that the gaseous reactant has to dissolve in the liquid first, and there diffuse toward the catalyst. In such processes as catalytic hydrogenations, the solubility of the gaseous reactant is so poor, that its diffusion in the liquid, either at the gas/liquid interface, or at the solid surface, is often rate determining. 

In the example a gas-liquid reaction with particulate solids (e.g., a catalyst) operating in regime 11 in a stirred reactor with a Rushton turbine is to be scaled up. The primary process requirement is for the same degree of reaction conversion at each scale, which means the same number of moles of gas transferred per mole of liquid fed ... 

Heterogeneous reactions of industrial significance occur between all combinations of gas, liquid, and solid phases. The solids may be inert or reac tive or catalysts in granular form. Some noncatalytic examples are listed in Table 7-11, and processes with solid catalysts are listed under Catalysis in Sec. 23. Equipment and operating conditions of heterogeneous processes are covered at some length in Sec. 23 only some highlights will be pointed out here. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

In a fixed-bed catalytic reactor for a fluid-solid reaction, the solid catalyst is present as a bed of relatively small individual particles, randomly oriented and fixed in position. The fluid moves by convective flow through the spaces between the particles. There may also be diffusive flow or transport within the particles, as described in Chapter 8. The relevant kinetics of such reactions are treated in Section 8.5. The fluid may be either a gas or liquid, but we concentrate primarily on catalyzed gas-phase reactions, more common in this situation. We also focus on steady-state operation, thus ignoring any implications of catalyst deactivation with time (Section 8.6). The importance of fixed-bed catalytic reactors can be appreciated from their use in the manufacture of such large-tonnage products as sulfuric acid, ammonia, and methanol (see Figures 1.4,11.5, and 11.6, respectively). 

The general concept of phase transfer catalysis applies to the transfer of any species from one phase to another (not just anions as illustrated above), provided a suitable catalyst can be chosen, and provided suitable phase compositions and reaction conditions are used. Most published work using PTC deals only with the transfer of anionic reactants using either quaternary ammonium or phosphonium salts, or with crown ethers in liquid-liquid or liquid-solid systems. Examples of the transfer and reaction of other chemical species have been reported(24) but clearly some of the most innovative work in this area has been done by Alper and his co-workers, as described in Chapter 2. He illustrates that gas-liquid-liquid transfers with complex catalyst systems provide methods for catalytic hydrogenations with gaseous hydrogen. 

This reactor is used with solid or liquid catalysts and with liquid reactants, or for gas/liquid reactions in the presence of solid catalysts. Batch reactors are also frequently used, as closed systems with circulation of the gas, for reactions of gases catalysed by solids. Nowadays, relatively few detailed kinetic studies are performed using batch reactors. A common use of these reactors is for the rapid screening of catalysts particularily in high pressure/high temperature reactions. 

The mechanisms described above similarly apply to the case of desorption with reaction (i.e., where the product of a liquid-phase reaction is volatile and desorbs in the gas phase). The word absorption in the above discussion will be replaced by the word desorption for this case. In most practical situations, more than one reaction occurs simultaneously. Under these situations, the terms "slow, fast, and instantaneous are applied to each reaction individually. Although the terms slow, fast, and "instantaneous reactions (or diffusion-controlled and mass-transfer-controlled regimes) are discussed with respect to gas-liquid reactions, they can also be applied to gas liquid-solid reactions, where the solid is either a catalyst or a reactant. 

Gas liquid-solid reactions with solid as a catalyst. 

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