Scale stirred-tank reactor

Equation 8.3.4 may also be used in the analysis of kinetic data taken in laboratory scale stirred tank reactors. One may directly determine the reaction rate from a knowledge of the reactor volume, flow rate through the reactor, and stream compositions. The fact that one may determine the rate directly and without integration makes stirred tank reactors particularly attractive for use in studies of reactions with complex rate expressions (e.g., enzymatic or heterogeneous catalytic reactions) or of systems in which multiple reactions take place. 

Ring pipes with 0.1-1.0 mm i. d. holes are common types of diffusers in lab-scale stirred tank reactors. Fine porous plate diffusers (dP = 10-50 pm) have also often been used, in STRs (e. g. Beltran etal., 1995) as well as bubble columns (e. g. Stockinger, 1995 Saupe, 1997). 

Figure 7 A bench-scale stirred tank reactor system for subcritical and supercritical studies. (From SRI International.)...

Since commercial-scale stirred-tank reactors can usually not be operated at very high power input, the monolith reactor is an excellent alternative for all processes that benefit from good mass-transfer characteristics. This includes processes for which the catalyzed intrinsic kinetics are very fast, processes where mass-transfer limitations lead to a drop in selectivity and processes where the stability of the catalyst deteriorates at low (hydrogen) concentrations inside the catalyst. 

ILLUSTRATIVE EXAMPLE 12.7 A continuous bench-scale stirred-tank reactor was used to study the reaction... 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

Baldyga, J. and Bourne, J.R., 1992. Interactions between mixing on various scales in stirred tank reactors. Chemical Engineering Science, 47, 1839-1848. 

The research programme into n-butyl lithium initiated, anionic polymerization started at Leeds in 1972 and involved the construction of a pilot scale, continuous stirred tank reactor. This was operated isothermally, to obtain data under a typical range of industrial operating conditions. 

A pilot scale plant, incorporating a three litre continuous stirred tank reactor, was used for an investigation into the n-butyl lithium initiated, anionic polymerization of butadiene in n-hexane solvent. The rig was capable of being operated at elevated temperatures and pressures, comparable with industrial operating conditions. 

Suppose the pilot-scale stirred tank of Example 11.18 is at the ragged edge of acceptable operation so that (us)g cannot be increased upon scaleup. Neither can QgjQi be decreased. What can be done to avoid a scaleup hmitation Your proposed solution should utilize the existing pilot reactor for experimental confirmation. 

The main disadvantage of all these systems is the Hmitation of scale-up. Monoclonal antibodies are produced by multiplying the hollow fiber systems and stirred tank reactors with membrane aeration are known up to 100 liter. Small quantities of product can be produced by these systems but they are not suitable for real industrial scale-up. 

Crameri et al. (1997) have reported an asymmetric hydrogenation constituting an important step in the production of a new calcium antagonist, Mibefradil (POSICOR) (of Hoffmann-LaRoche). Pilot-scale synthesis of (S)-2-(4-flurophenyl)-3-methylbutanoic acid by the asymmetric hydrogenation of 2-(4-fluorophenyl)-3-methyl but-2-enoic acid with a [Ru (/ )-MeOBIPHEP)(OAc)2]-catalyst has been described. The hydrogenation was performed in a continuous mode in a cascade stirred-tank reactor system at a pressure of 270 bar. A large reduction in total reactor volume compared to the batch mode was realized. 

To illustrate the complexity of process optimization, suppose that we are to scale-up a semibatch stirred-tank reactor for carrying out the following consecutive reactions ... 

Factors re.sponsible for the occurrence of scale-up effects can be either material factors or size/shape factors. In addition, differences in the mode of operation (batch or semibatch reactor in the laboratory and continuous reactor on the full scale), or the type of equipment (e.g. stirred-tank reactor in the laboratory and packed- or plate- column reactor in commercial unit) can be causes of unexpected scale-up effects. A simple misuse of available tools and information also can lead to wrong effects. 

A typical stirred-tank reactor is shown in Fig. 5.4-3. It is a cylindrical vessel with elliptical or torospherical bottom and cover. It is equipped with an axially mounted stirrer rotating with a speed from 25 rpm (large scale) to 2000 rpm (laboratory). Fig. 5.4-4 shows the stirrers that are mostly used in fine chemicals manufacture, viz. the marine propeller, turbine, flat- or pitched-blade agitator, and anchor. Agitators move the fluid into axial and radial direction. Marine propellers and pitched-blade stirrers predominantly impose axial motion. 

Chakrabarti, T. and Subrahmanyam, P.V.R., Biological hydrolysis of urea in a continuous flow stirred tank reactor under laboratory conditions—a bench scale study, Proc. 36th Industrial Waste Conference, Purdue University, pp. 477, 1981. 

Graphical Approach to the Analysis of Batteries of Stirred Tank Reactors Operating at Steady State. Even in reaction systems where it is not possible to determine the algebraic form of the reaction rate expression, it is often possible to obtain kinetic data that permit one to express graphically the rate as a function of the concentration of one reactant. Laboratory scale CSTR s are particularly appropriate for generating this type of kinetic data for complex reaction... 

The following data were recorded using a laboratory scale continuous stirred tank reactor... 

The slow water removal is obvious within the synthesis of, for example, myristyl myristate determining the total reaction time. In a stirred-tank reactor it takes 24 h to reach a conversion of 99.6% and in a fixed-bed reactor 14 h. Therefore, a new synthesis platform (Figure 4.11) which also enables conversion of highly viscous polyols and fatty acids from renewable resources to ester-based surfactants was designed. It is used by Evonik on a pilot scale, outperforming conventional methods, such as stirred-tank or fixed-bed reactors. In contrast to the setups introduced before, conversion of >99.6% is already obtained after 5.5 h in the bubble column reactor [44-47]. 

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