Large scale homogeneous reactors

The austenitic stainless steels and most other metals and alloys of practical importance in large-scale homogeneous reactors are thermodynamically unstable in aqueous solutions and depend on protective films for their corrosion resistance. Tortunately, when austenitic stainless steels are oxidized in high-temperature uranyl sulfate solutions, the steel oxidizes uniformly so that no element (or elements) is leached preferentially from the alloy. However, not all the alloying elements contribute to film formation. 

LARGE-SCALE HOMOGENEOUS REACTOR STUDIES [CHAP. 9... 

Maintenance considerations. A study of the problem of maintenance of a large-scale homogeneous reactor indicated the following. It appears impossible to accomplish some repair operations remotely in place and under 20 ft of water. Experience to date tends to indicate that the repair of radioactive eciuipmcnt may be so difhc ult that it will be uneconomical to repair anything except such small components as valves and pumps. The larger defective components must be removed from the. system and a replacement installed. The repairs, if possible, can then be made in a "hot machine shop after the system is back in operation. 

Compared to batch processes, continuous processes often show a higher space-time yield. Reaction conditions may be kept within certain limits more easily. For easier scale-up of some enzyme-catalyzed reactions, the Enzyme Membrane Reactor (EMR) has been developed. The principle is shown in Fig. 7-26 A. The difference in size between a biocatalyst and the reactants enables continuous homogeneous catalysis to be achieved while retaining the catalyst in the vessel. For this purpose, commercially available ultrafiltration membranes are used. When continuously operated, the EMR behaves as a continuous stirred tank reactor (CSTR) with complete backmixing. For large-scale membrane reactors, hollow-fiber membranes or stacked flat membranes are used 129. To prevent concentration polarization on the membrane, the reaction mixture is circulated along the membrane surface by a low-shear recirculation pump (Fig. 7-26 B). 

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

Many chemical reactions are performed on a batch basis, in which a reactor is filled with solvents, substrates, catalysts and anything else required to make the reaction proceed, the reaction is then performed and finally the reactor is emptied and the resultant mixture separated (Figure 11.2). Conceptually, a batch reactor is similar to a scaled up version of a reaction in a round-bottomed flask, although obviously the engineering required to realize a large scale reaction is much more complicated. Batch reactors are suitable for homogeneous reactions, and also for multiphasic reactions provided that efficient mixing between the phases may be achieved so that the reaction occurs at a useful rate. 

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