Semiflow reactor

Semibatch or Semiflow Reactors. Semibatch or semiflow operations usually take place in a single stirred tank using equipment extremely similar to that described for batch operations. Figure 8.1 indicates some of the many modes in which semibatch reactors may be operated. 

For semibatch or semiflow reactors all four of the terms in the basic material and energy balance relations (equations 8.0.1 and 8.0.3) can be significant. The feed and effluent streams may enter and leave at different rates so as to cause changes in both the composition and volume of the reaction mixture through their interaction with the chemical changes brought about by the reaction. Even in the case where the reactor operates isothermally, numerical methods must often be employed to solve the differential performance equations. 

The chapter is divided into four parts Strategy, which explains the rationale for carrying out these reactions in SCF solution Chemistry, which outlines what the reactions are Equipment, which describes some of the key components and introduces a modular approach to high pressure experiments and Synthesis, which gives details of how to make particular compounds, including the use of flow reactors and semiflow reactors. It concludes with a brief summary and outlook. 

Most synthetic chemistry involves small-scale exploratory experiments followed by scale-up of the more successful experiments. Conventionally, exploratory experiments might be carried out in NMR tubes, with scale-up in Schlenk tubes. In SCFs, the exploration has mostly been done by IR [5,12,13], largely because it is experimentally simpler than NMR [14-19], (Figure 4.2.-1). Scale-up has involved the use of miniature flow reactors or semiflow reactors, neither of which have obvious analogs in conventional synthesis. 

Figure 4.2-8 Layout of the semiflow reactor for the synthesis and isolation of Cp Mn(CO)2(tl -H2) from Cp Mn(CO)2 L ( L = labile ligand) and H2 in SCCO2. The reactor is very similar to that shown in Figure 4.2-7 but without the UV photolysis cell. Here the variable volume view-cell R is used as a thermal reactor. All other components are labeled as in Figures 4.2-6 and 4.2-7 (reproduced with permission from P. D. Lee, J. L. King, S. Seebald, M. Poliakoff, Organometallics 1998, 77, 524 American Chemical Society).

FIG. 7-4 Typ es of flow reactors (a) stirred tank battery, (h) vertically staged, (c) compartmented, (d) single-jacketed tube, (e) shell and tube, (f) semiflow stirred tank. 

Semibatch or semiflow processes are among the most difficult to analyze from the viewpoint of reactor design because one must deal with an open system under nonsteady-state conditions. Hence the differential equations governing energy and mass conservation are more complex than they would be for the same reaction carried out batchwise or in a continuous flow reactor operating at steady state. 

The second-phase reaction is heterogeneous and occurs at the surface of the particle. The reaction causes the reacting surface to shrink and to leave an ash layer as the particle moves through the reactor. Unlike the first-phase reaction, which is only slightly affected by temperature, the second-phase reaction is quite sensitive to variations in temperature for tests conducted in a semiflow system (10). Since a high gas flow rate was maintained in semiflow tests, gas diffusion probably does not affect the rate. At temperatures below 1700°F., the first-phase reaction rate is an order or two larger than the second-phase reaction rate, but as the temperature approaches 2000°F., the two rates become comparable. This is, of course, true only when the reaction is controlled by the chemical step. 

These are generally classified as either CSTRs, semiflow batch reactors (SFBR), or plain batch reactors, which we treated in the previous section. If the reactor is well-mixed, the liquid-phase mass balance is the same general form for all. For component j. 

The second important configuration is that of the semiflow batch reactor (SFBR), in which the liquid phase is contained and only the gas flows through. This is also envisioned in Figure 8.15, but with no liquid flow. Again, we will take the liquid phase to be well-mixed, with limiting gas-phase behavior either plug flow or well mixed. 

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