Semicontinuous Flow Reactors

In semicontinuous-flow reactors (Figure 9.2-2) [25] the dense gas is saturated with substrates in an autoclave and then fed continuously through the enzyme bed. The advantage of this type of reactor is that the pure gas should be compressed. The disadvantage of semicontinuous flow reactors is that the concentration of substrates in dense gases cannot be varied, and that with changes of pressure and temperature, precipitation of substrates or products in the reactor can occur. 

Numerous studies have shown that low hydrogen overpotential electrically conducting catalysts (e.g., Raney nickel, platinum and palladium on carbon powder, and Devarda copper) can be used to electrocatalyticaUy hydrogenate a variety of organic compounds including benzene and multiring aromatic compounds, phenol, ketones, nitro compounds, dinitriles, and glucose [45, 46, 54, 55, 67-71]. These reactions have been carried out in both batch and semicontinuous flow reactors in most cases, the reaction products were similar to those obtained from a traditional chemical catalytic scheme at elevated temperatures and pressures. 

The advantage of a semicontinuous flow reactor is that only pure SCF needs to be pumped. Standard HPLC pumps can be used without fear of clogging the check valves. As in all flow reactors it is necessary to sub-cool the fluid to avoid cavitation and loss of pumping capacity. With CO2 it is usually enough to cool to approximately -5 °C at the pump head. A cooling jacket around the pump head is needed experience suggests that the temperature of the cooling liquid should be about -20 °C. 

The disvantage of semicontinuous flow reactors is that substrate concentration in the SCF is always close to saturation. Therefore, substrate concentrations and reaction conditions cannot be chosen independently. Small changes of pressure or temperature of the saturated fluid may cause precipitation of substrates in the enzyme bed. This may distort the results. 

Continuous and semicontinuous flow reactors are sometimes used in fine chemical and pharmaceutical applications, primarily because some reactions require the high intensity of mixing that can only be achieved in in-line mixers (see Examples 13-3, 13-7, and 13-8a and discussion in Chapter 17). 

Batch-, stirred-tank-, extractive semibatch-, recirculating batch-, semicontinuous flow-, continuous packed-bed-, and continuous-membrane reactors have been used as enzyme reactors, with dense gases used as solvents. 

Figure 9.2-2. Operating principle of dense-gases enzymatic reactor types a), extractive semibatch b), recirculating batch c), semicontinuous flow.

A variation of continuous processing is sometimes called semicontinuous processing if the product from a continuous flow reactor is collected in a vessel and isolated at the end of the operation cycle. 

Batch, recirculating batch, extractive semibatch, semicontinuous flow, continuously stirred tank (CSTR) and continuous packed bed reactors have alt been succesfully tested as enzyme reactors for SCFs (Figure 4.9-1). References to helpful descriptions for designing small-scale reactors for enzymatic studies are collected in Table 4.9-1. 

Figure 6.57. Summary diagram of work flow in the systematic development of a bioprocess of the presented integrating strategy. The diagram is based on the interaction between kinetics (Chap. 5) and transport (Chap. 3) processes, which are clarified during a kinetic analysis (Chap. 4). As a special situation, the design and utilization of new types of reactors are shown (discontinuous stirred vessel, DCSTR bubble column, BC semicontinuous stirred vessel, SCSTR recycle reactor, RR continuous stirred vessel, CSTR continuous cascade, NCSTR tower reactor, TR continuous plug flow reactor, CPFR fixed and fluidized bed reactor, FBR).

In a batch reactor, the first two terms in equation 12.2-1 are absent. In a semibatch reactor, one of these two terms is usually absent. In a semicontinuous reactor for a multiphase system, both flow terms may be absent for one phase and present for another. In a continuous reactor, the two terms are required to account for the continuous inflow to and outflow from the reactor, whether the system is single-phase or multiphase. 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

A semicontinuous reactor for a fluid-solid reaction involves the axial flow of fluid downward through a fixed bed of solid particles, the same arrangement as for a fixed-bed catalytic reactor (see Figure 15.1(b)). The process is thus continuous with respect to the fluid and batch with respect to the solid (Section 12.4). 

The CSTR operator, Rc, has an identical term to describe accumulation under transient operation. The algebraic sum of the two other terms indicates the difference of in-flow and out-flow of that species. This operator also describes semibatch or semicontinuous operation in cases where the volume can be assumed to be essentially constant. In the more general case of variable volume, V must be included within the differential accumulation term. At steady state, it is a difference equation of the same form as the differential equation for a batch reactor. 

Semicontinuous emulsion polymerizations are characterized by the continued addition of monomer to the reaction vessel. This permits the production of latexes with high weight percentage solids while allowing the initial burst of nucleation to be achieved in substantially aqueous surroundings. The theory for semicontinuous systems is substantially that set forth for Interval III of batch polymerizations, except that the materials balance equations [Eq. (17)] must be modified to include the flow of new material into the reactor. The effect of the monomer input is twofold first, the mass of material present in the system is increased and seccmd, the concentration of other reagents may be reduced. 

In a semicontinuous or batch flow process, one or more ingredient is charged to the reactor, and the remaining components are added gradually. None of the reaction mixture is displaced through overflow. The total volume of a vessel usually increases as the reaction progresses, and the product is isolated at the end of the operation cycle. This type of operation is useful for exothermic reactions in vessels with limited heat transfer capacity. 

Application of New Types of Process Techniques. As will be further explored in Chap. 3, a great number of mixing and aeration systems have been developed that could be of industrial interest. All of these reactors can be operated in discontinuous, semicontinuous, or fully continuous modes. The semi-continuous mode of operation offers some advantages (Pickett, Topiwala, and Bazin, 1979). However, a fully continuous operation seems to be limited (infections, strain stability, continuous flow). 

Although batch processes are the workhorse in research laboratory environments, continuous (and semicontinuous) reactors predominate for commercial PE production. In a continuous polymerization reactor, all monomers and reagents are constantly fed into the reactor, and the polymer is isolated from the effluent. Flows are adjusted to achieve the desired steady-state conditions as measured by online analytical instruments and polymer analysis. 

In Fig. 2B a semicontinuous process is shown. It was performed by Hammond et al. [18], Miller et al. [74], and Randolph et al. [64]. SCCO2 flows through a saturation vessel in which substrates are stored. The substrates can be impregnated on glasswool. The CO2 dissolves the reactants and transports them into the reactor. It is possible to produce a monophasic and saturated mixture. At the outlet of the reactor SCCO2 is expanded and all dissolved components condense. The CO2 can be recycled. 

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