Laboratory reactors, types

In this chapter the emphasis is on the methods to obtain data relevant for kinetic modeling and comparison of catalyst activities for the ultimate purpose of engineering applications. Some laboratory reactor types will be discussed, the procedure for kinetic modeling outlined together with the related parameter estimation and the determination of the rate constants in the rate expression. It should serve as a comprehensive reference to this type of activity in catalysis. Most of it comes from authoritive reviews [2-14],... 

Select a suitable laboratory reactor type and scale, and analytical tools for kinetic measurements. 

Selection of the laboratory reactor type and size, and associated feed and product handling, control, and analytical schemes depends on the type of reaction, reaction time scales, and type of analytical methods required. The criteria for selection include equipment cost, ease of operation, ease of data analysis, accuracy, versatility, temperature uniformity, and controllability, suitability for mixed phases, and scale-up... 

Table 7-11 summarizes laboratory reactor types that approach the three ideal concepts BR, CSTR and PFR, classified according to reaction types. 

For instance, Fig. 7-17 summarizes laboratory reactor types and hydrodynamics for gas-liquid reactions. 

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

Most reactors have evolved from concentrated efforts focused on one type of reactor. Some processes have emerged from parallel developments using markedly different reactor types. In most cases, the reactor selected for laboratory study has become the reactor type used industrially because further development usually favors extending this technology. Descriptions of some industrially important petrochemical processes and their reactors are available (74—76). Following are illustrative examples of reactor usage, classified according to reactor type. 

Many configurations of laboratory reactors have been employed. Rase (Chemical Reactor Design for Proce.s.s Plants, Wiley, 1977) and Shah (Ga.s-Liquid-Solid Reactor Design, McGraw-Hill, 1979) each have about 25 sketches, and Shah s bibliography has 145 items classified into 22 categories of reactor types. Jankowski et al. (Chemlsche Tech-nik, 30, 441 46 [1978]) illustrate 25 different lands of gradientless laboratory reactors for use with solid catalysts. 

Bubble columns and mechanically stirred reactors are the most common reactor types for slurry systems in laboratories, but they have many disadvantages from an industrialization perspective. Mechanically stirred reactors usually used for laboratorial studies are difficult to scale-up. In order to achieve good mixing and mass transfer between the gas and slurry phases, bubble column must be operated at a high space velocity, which leads to a relative low one-through conversion of the syngas. 

Figure 5.19 Conversion of the direct fluorination of toluene in different reactor types as a function of the molar ratio of fluorine to toluene (a) and efficiency of these reactors, defined as conversion normalized by the molar ratio of fluorine to toluene, as a function of the molar ratio of fluorine to toluene (b). Falling film micro reactor (FFMR) micro bubble column (MBC) laboratory bubble column (LBC) [38].

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

General guidelines concerning the choice of the most proper reactor type for the particular system to study, modified after Mills et al. (1992) are given in Table 5.4-10. The types, applications, advantages, and disadvantages of several laboratory reactors for kinetic measurements are given in Table 5.4-11. 

Identify the predominant rate-controlling mechanism kinetic, mass or heat transfer. Choose a suitable reactor type, based on experience with similar reactions, or from the laboratory and pilot plant work. 

A comparison between the two last correlations and experimental data for small laboratory reactors (25 to 300 cm3) equipped with a magnetic stirrer or a traditional six-blade impeller (not Rushton-type) and using a catalytic hydrogenation in organic solvent has been published (Fig. 45.5) [55]. 

In summary, it can be concluded that the monolithic stirrer reactor is a convenient reactor type both for the laboratory and the production plant. It is user-friendly and can be used to compare different catalysts in the kinetically limited regime or hydrodynamic behavior in the mass transfer controlled regime. Stirrers or monolith samples can be easily exchanged and reloaded to suit the desired enzyme and/or reaction conditions. 

In these types of laboratory reactor, the flow of the liquid is very carefully controlled so that, although the mass transfer step is coupled with the chemical reaction, the mass transfer characteristics can be disentangled from the reaction kinetics. For some reaction systems, absorption of the gas concerned may be studied as a purely physical mass transfer process in circumstances such that no reaction occurs. Thus, the rate of absorption of C02 in water, or in non-reactive electrolyte solutions, can be measured in the same laboratory contactor as that used when the absorption is accompanied by the reaction between C02 and OH ions from an NaOH solution. The experiments with purely physical absorption enable the diffusivity of the gas in the liquid phase DL to be calculated from the average rate of absorption per unit area of gas-liquid interface NA and the contact time te. As shown in Volume 1, Chapter 10, for the case where the incoming liquid contains none of the dissolved gas, the relationship is ... 

The chemical kinetics are the same for any type of reactor. Most of the kinetic information used to design CSTRs is obtained from batch laboratory reactors ... 

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