Stirred cell laboratory reactors

Fio. 4.12. Stirred cell laboratory reactor. Note In tome designs a perforated plate is positioned in the interface to prevent excessive disturbance and to maintain a known area of contact between gas and liquid... 

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. 

If it is assumed that the values for kG, k l, and a have been measured for the commercial tower packing to be employed, the procedure for using the laboratory stirred-cell reactor is as follows ... 

The kinetic parameter can be estimated in laboratory reactors. For solid-fluid systems, this subject was described in Section 11.3.1.6. For fluid-fluid reactions, the commonly employed laboratory reactors include stirred cell, wetted wall column, rotating drum, laminar jet, stirred contractor, and others. These are schematically shown in Figure 11.14. In practically all of these reactors, the value of the fluid-fluid interfacial area is known. These reactors have been described by Treybal (1980) and Doraiswamy and Sharma (1984). As an illustration, the stirred cell will be described first, followed by a comparison with other laboratory reactors. The discussion of the stirred cell is restricted to gas-liquid systems, but it is also applicable (with minor variations) to liquid-liquid systems. 

In addition to the stirred cell, other laboratory reactors commonly used include rotating drum contactor, wetted wall column, wetted sphere column, laminar jet, and stirred contactor. These equipments are shown schematically in Figures 11.14b-f. AU have several common features, the principal one being a weU defined gas-liquid interfacial area and the ability to vary the area per unit reactor volume a). In the stirred cell, it is achieved by varying the liquid height. As an alternative way, a solid circular baffle is placed at the gas-liquid interface. Holes are drilled on the baffle plate so that the hole opening area becomes the interfacial area. For varying a, baffle plates are made with different free (hole) areas. 

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). 

The perfonnances of amine blends to absorb CO2 were evaluated for mixtures of DEA and MDEA using a laboratory stirred cell reactor. The DEA to MDEA ratios were selected so that DEA-CO2 reaction would not dominate the overall reaction rate. It was observed that the addition of small amounts of DEA to MDEA resulted in a significant increase in the CO2 absorption rates as shown in Fig.l which, can be attributed to the higher reactivity of DEA with CO2. 

Special reactors are required to conduct biochemical reactions for the transformation and production of chemical and biological substances involving the use of biocatalysts (enzymes, immobilised enzymes, microorganisms, plant and animal cells). These bioreactors have to be designed so that the enzymes or living organisms can be used under defined, optimal conditions. The bioreactors which are mainly used on laboratory scale and industrially are roller bottles, shake flasks, stirred tanks and bubble columns (see Table 1). 

For fast or moderately fast liquid phase reactions, the stirred-tank reactor can be very useful for establishing kinetic data in the laboratory. When a steady state has been reached, the composition of the reaction mixture may be determined by a physical method using a flow cell attached to the reactor outlet, as in the case of a tubular reactor. The stirred-tank reactor, however, has a number of further advantages in comparison with a tubular reactor. With an appropriate ratio of... 

All of the investigations with cell flotation presented in the cited literature were performed with small laboratory equipment. Gehle et al. [117] reported on the investigation on a pilot-scale apparatus consisting of a 300-1 stirred tank reactor provided with a foam separator (Fundafoam Chemap) and a flotation colunm, 3.6 m height, 10 cm internal diameter, which was directly connected to the reactor (Fig. 7). 

Classical electrochemical reactor designs invariably evolved from direct scale-up of simple laboratory electrolysis experiments. The most common example of this concept is the tank cell where an array of electrodes is immersed in a plastic or metal tank. More sophisticated versions involve a variety of approaches to enhancing convection, by rapid stirring, rotating or moving electrodes or improving geometry with plate and frame or filter-press-type cells. 

Typical results in the author s laboratory (37-39) with Protocol 7 are 2.75 x 10 ° viable cells per htre giving an average yield of 166 mg/litre/day (compared to stirred reactor and airlift cultures of the same hybridoma of 25.5 and 18.5 mg/ litre respectively). This method is a low investment introduction to high productivity production of mAh which is simple to use and reliahle with low maintenance, at least for the first 50 days of culture. 

The Verax system comprises a bioreactor (fluidization tube), a control system (for pH, oxygen, medium flow rates), gas and heat exchanger, and medium supply and harvest vessels. The system is run continuously for long periods (typically over 100 days). In the authors laboratory it produced 15 x 10 cells/ hire and 540 mg mAb/litre/day (compared to 166 in the fixed bed described above, 25.5 in a stirred reactor, and 18.5 mg in an airlift fermenter). Protocols for its operation come with the equipment and versions have been published (41). In summary it is probably the most productive system available giving the cells a very high specific production rate but does require some skill to operate to its maximum potential... 

Added productivity of lactic acid fermentations can be achieved by combining continuous systems with mechanisms that allow higher bacterial cell concentrationsResearch is concentrated on two mechanisms (1) membrane recycle bioreactors (MRBs) and (2) immobilized cell systems (ICSs). The MRB consists of a continuous stirred-tank reactor in a semiclosed loop with a hollow fiber, tubular, flat, or cross flow membrane unit that allows cell and lactic acid separation and recycle of cells back to the bioreactor. The results of a number of laboratory studies with various MRB systems demonstrate the effect of high cell concentrations on raising lactic acid productivity (Litchfield 1996). O Table 1.12 lists examples of published results employing various MRB systems. 

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