PFTR with recycle

Next we consider the PFTR with recycle in which a portion of the product stream is fed back into the feed stream as shown in Figure 8-10. A pump is needed to recycle the fluid back to the entrance and control R, the amount of recycle. 

Contact stabilization PFTR with recycle mean cell residence time = 5-15 d food/ microorganism ratio = 2.3-7 mg BOD5/S kg MLVSS volumetric loading = 1-1.2 kg BODj/m MLSS = 4-10 g/L residence time = 4-10 h recycle ratio = 0.25-1. 

A —> P fej C 2 > 1 A- W P - W 2 ikj c Combo CSTR + PFTR PFTR + CSTR PFTR with large recycle series of CSTR ... 

We could still use a PFTR reactor for this reaction if we seeded the reactant with product or if we used a recycle tubular reactor to feed a portion of the product back to the feed (Figure 3-13). 

Recycle will always give a lower conversion in a PFTR for positive-order kinetics because it produces backmixing, in which product mixes with reactant at the entrance to the plug-flow section. 

For R = 0 we have the pulse at T at the same height as the pulse injected, for the perfect PFTR. For R = 1 the first pulse containing one-half of the tracer will leave at time r/2, and the rest will recycle one-half of this wiU exit at time r, one-fourth will recycle, etc. Thus we will observe pulses at intervals t/2 with heights The sum of all of the infinite number of pulses must be the... 

With a reaction-limited deposition process, the film should have uniform thickness as long as the partial pressures ofreactants do not vary with position. In a tubular reactor, the conversion of reactants must be kept small or the film thickness will depend on the location of the sohd in the reactor, with upstream regions having a greater deposition rate. It is therefore common to use a gas recirculation reactor (a recycle PFTR) so that the composition of the reactants is independent of position in the reactor to assure uniform film thickness. 

The simplest flow-sheet for the reaction Aj o Aj is the RD column sequence with an external recycling loop shown in Fig. 5.1. The system as a whole is fed with pure Aj. According to the assumed relative volatility of the two components a > 1, the reaction product A2 is enriched in the column distillate product whereas the bottom product contains non-converted reactant Aj, which is recycled back to the reactor (continuous stirred tank reactor, CSTR, or plug flow tube reactor, PFTR). The process has two important operational variables the recycling ratio cp = B/F, that is the ratio of recycling flow B to feed flow rate F, and the reflux ratio of the distillation column R = L/D. At steady-state conditions, D = F since the total number of moles is assumed to be constant for the reaction Aj A2. As principal design variables, the Damkohler number. 

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