Reactors for gas-solid reactions

Average Conversion Xb of Solid Feed Containing Particles of Different Sizes 

Continuous-flow reactors used for gas-solid reactions are essentially gas-solid contacting equipment through which both solid particles and the gaseous stream are passed continuously in counter-flow or cross-flow directions. 

All the solid particles are treated in a constant environment for the same duration of time 9, which is the residence time of the solid particles in the reactor. 9 is also the time taken by the conveyor belt to move from the first drum to the second drum. 

A moving grate conveyer-type reactor is used for roasting iron ore particles in the presence of air. The feed to the reactor consists of 20% (by weight) of 1 mm particle, 30% of 2 mm particle, 30% of 4 mm particle and 20% of 6 mm particle. Conveyer speed is adjusted so that the solid particles reside in the reactor for a time duration of 10 min. SCM holds good and the reaction is rate controlling. The time taken for complete conversion of 4 mm particles is 4 h. Calculate 

As the reaction is rate controlling, the time x for complete conversion is proportional to R, xocR and 

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The most widely used reactors for gas-solid reactions in fine chemistry are fixed-bed tubular... 

Figure 6.16 Modular microchannel reactor for gas-solid reactions. (Adapted from Ref.

In heterogeneous non-catalytic reactors, the reaction medium is a two-phase medium. The two-phase reaction medium is composed of a gas phase and a solid phase for the reaction between a gaseous reactant and a solid reactant, whereas for the reaction between a gaseous reactant and a liquid reactant the reaction phase is a two-phase gas-liquid medium. The design of heterogeneous non-catalytic reactors for gas-solid reactions and gas-liquid reactions are discussed in this section. 

Figure 4.10.8 Reactors for gas-solid reactions, for example, coal gasification (a) moving bed, (b) fluidized-bed, and (c) entrained-flow. Adapted from Moulijn, Makkee, and Van Diepen (2004).

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

The fluidised bed is only one of the many reactors employed in industry for gas-solid reactions, as reported by Kunii and Levenspiel.25 Whenever a chemical reaction employing a particulate solid as a reactant or as a catalyst requires reliable temperature control, a fluidised bed reactor is often the choice for ensuring nearly isothermal conditions by a suitable selection of the operating conditions. The use of gas-solid fluidised beds... 

This paper describes the development and operation of a continuous rotating annular chromatographic reactor (CRACR) for gas-solid reaction systems at elevated temperatures. Experimental and numerical simulation results for the dehydrogenation of cyclohexane on a Pt/Al2C>3 catalyst are presented. 

If a transport parameter rc — CS/CL is defined, where Cs is the concentration of C at the catalyst surface, then Peterson134 showed that for gas-solid reactions t)c < rc, where c is the catalyst effectiveness factor for C. For three-phase slurry reactors, Reuther and Puri145 showed that rc could be less than t)C if the reaction order with respect to C is less than unity, the reaction occurs in the liquid phase, and the catalyst is finely divided. The effective diffusivity in the pores of the catalyst particle is considerably less if the pores are filled with liquid than if they are filled with gas. For finely divided catalyst, the Sherwood number for the liquid-solid mass-transfer coefficient based on catalyst particle diameter is two. 

The laboratory reactors can also be further divided into three sections (see Table 5-1) some reactors are presently used for gas-liquid-solid reactions, some reactors are largely used for gas-solid reactions, and these can potentially be used... 

REACTORS USED FOR GAS-SOLID REACTIONS THAT CAN BE ADAPTED TO THREE-PHASE SYSTEMS... 

Laboratory Reactors for Gas-Solid Catalytic Reactions Their Principal Features and Ratings... 

In Section 11.4.2, we discussed the three main categories of reactors that can be used for gas-solid reactions. Since the present system uses pellets of reasonable size, fluid-bed reactors can be ruled out. We shall therefore consider fixed- and moving-bed reactors. Two features of this system should first be noted ... 

Conversion Equations for Gas-Solid Reactions at Specified Reactor Conditions... 

As must be evident from a previous section on classification, gas-liquid reactions can be carried out in a large number of reactor types. This is also true of other multiphase reactions in which a liquid phase is involved. For other reactions such as gas-solid, catalytic or noncatalytic, the choice of reactor is confined to a lesser number of variations. Therefore, although reactor choice is an important consideration for all reactions, particularly heterogeneous reactions, it is more so for gas-liquid, liquid-liquid, and slurry systems, all of which are widely used in industrial organic synthesis. We discuss below the cost minimization criteria for a rational choice of reactors for gas-liquid reactions. 

Of the different types of reactors that can be used, the stirred basket reactor is the most amenable to manipulation in terms of regimes (see Kenney and Sedriks, 1972). Such a reactor for gas-solid (catalytic) reactions was considered in Chapter 7 (Figure 7.4). Typically, to operate under chemical control conditions, say in a fully baffled 15-cm diameter reactor provided with an 8-cm turbine agitator, the speed of agitation should be in the range of 1000-5000 rpm (corresponding to an impeller tip speed of 24,000-120,000 cm/min). 

This review puts its focus on high-throughput preparation of heterogeneous catalysts, that is, solid-state materials that are apphed in fixed-bed reactors for gas-phase reactions and in trickle-bed or stirred-tank reactors for liquid or gas-hquid reactions, respectively. Other fields of catalysis are not discussed since very different catalytic systems are used. We refer to the following reviews for homogeneous catalysis (2, 3), where combinatorial catalysis deals mainly with variation of ligands and for electrochemical catalysis [4,5], where catalysts are prepared as arrays of thin films in electrochemical cells. 

The Torbed is a toroidal fluidised bed (Figures 1.11, 5.47 and 5.48) used as a reactor (Groszek, 1990). As a fluidised bed there is efficient gas-solid mass and heat transfer. It has a low pressure drop, allowing process gas recirculation (Shu et al., 2000). Torbed is suitable for gas-solid reactions taking a maximum of a few minutes. 

Figure 10.6 Reactors for gas-solid noncatalytic reactions, (a) Packed bed and (b) moving bed.

Figure 4.10.6 Adiabatic fixed bed reactors for gas-solid catalytic reactions (a) simple fixed bed (b) rack type reactor with interstage injection of gas (c) rack type reactor with interstage cooling or heating (ErtI, Knoezinger, and Weitkamp, 1997).

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