Recycle-loop reactor

A real continuous-flow stirred tank will approximate a perfectly mixed CSTR provided that tmix h/i and tmix i. Mixing time correlations are developed using batch vessels, but they can be applied to flow vessels provided the ratio of throughput to circulatory flow is small. This idea is explored in Section 4.5.3 where a recycle loop reactor is used as a model of an internally agitated vessel. 

Reaction occurs in the loop as well as in the stirred tank, and it is possible to eliminate the stirred tank so that the reactor volume consists of the heat exchanger and piping. This approach is used for very large reactors. In the limiting case where the loop becomes the CSTR without a separate agitated vessel, Equation (5.35) becomes q/Q > 10. This is similar to the rule-of-thumb discussed in Section 4.5.3 that a recycle loop reactor approximates a CSTR. The reader may wonder why the rule-of-thumb proposed a minimum recycle ratio of 8 in Chapter 4 but 10 here. Thumbs vary in size. More conservative designers have... 

Figure 5.27 A recycle-loop reactor for catalyst studies.

Fig. 1 B iofilm fluidised-bed recycle loop reactor for nitrification.

Except the recycle loop reactor, real reactors, either packed beds or monoliths, are neither plug flow nor mixed flow reactors. However, in small laboratory reactors, heat conduction tln-ough the solid phase probably makes the temperature to be unifonn as in mixed flow. Conversely, the concentration profiles are those of plug flow. Real LO curves are thus intennediate between tliose of plug and mixed flow. 

Figure 4 Light-off curves for a plug flow reactor and a recycle loop reactor. Left reaction and transfer are competing. Right no heat and mass transfer limitations.

Figure 5 Best fit of the leading part of the LO curve in a recycle loop reactor. Upper curves with transfer limitations. Lower curves no transfer limitations.

Experiments with laboratory monoliths of small cross-section area can lead to biased results due to an uneven flow distribution in the channels, especially close to the reactor wall. The wash-coat of the outer broken chaimels should be scraped away, and the void between the reactor wall and the monolith should be carefully plugged. To minimize wall effects, the diameter of the monolith should be ten tunes the chaimel diameter at least. Plug flow must prevail in a packed bed of crushed catalyst. The bed length and radius should be more than 50 and 10 particle diameters respectively, the flow resistance of the bed support must be unifonn throughout its cross-section, and the particle size distribution must be as narrow as possible. Otherwise, there can be oy-passes or dead vohunes. These hydrodynamic problems are overcome in a recycle loop reactor because the same physical and chemical conditions prevail everywhere. 

Note that this control structure has both of the flow rates in the recycle loop (reactor effluent F and distillate from the column D) set by level controllers. 

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