Recycle reactors residence time distribution

Tanks-in-series reactor configurations provide a means of approaching the conversion of a tubular reactor. In modelling, systems of tanks in series are employed for describing axial mixing in non-ideal tubular reactors. Residence time distributions, as measured by tracers, can be used to characterise reactors, to establish models and to calculate conversions for first-order reactions. The reactor in this example has a recycle loop to provide additional flexibility in modelling the mixing characteristics. 

Recycling to monomers, fuel oils or other valuable chemicals from the waste polymers has been attractive and sometimes the system has been commercially operated [1-4]. It has been understood that, in the thermal decomposition of polymers, the residence time distribution (RTD) of the vapor phase in the reactor has been one of the major factors in determining the products distribution and yield, since the products are usually generated as a vapor phase at a high temperature. The RTD of the vapor phase becomes more important in fluidized bed reactors where the residence time of the vapor phase is usually very short. The residence time of the vapor or gas phase has been controlled by generating a swirling flow motion in the reactor [5-8]. 

The time spent by reactants and intermediates at reaction conditions determines conversion (and perhaps selectivity). It is therefore often important to understand the residence time distribution (RTD) of reaction species in the reactor. This RTD could be considerably different from what is expected. Reasons for the deviation could be channeling of fluid, recycling of fluid, or creation of stagnant regions in the reactor, as illustrated in Fig. 19-6. 

The residence time distribution of the recycle reactor was determined by tracer experiments. This permitted the interpretation of the flow patterns in the reactor, so that the degree of mixing could be quantified. 

For high recycle ratios, the residence time distribution of a recycle reactor tends towards that of a completely mixed flow reactor, regardless of the flow mode inside the reactor vessel provided that there is no stagnant zone (Buffham and Naum an, 1984). 

For a plug flow reactor with effluent recycle, Fu et al. (1971) derived the residence time distribution ... 

Bypassing the reactor showed that the broadening of the pulse caused by the feed and effluent lines was negligible. Hence, the normalized pulse response is the residence time distribution (RTD) of the recycle reactor. 

Particles of polypropylene are continuously formed at low pressure in the reactor (1) in the presence of catalyst. Evaporated monomer is partially condensed and recycled. The liquid monomer with fresh propylene is sprayed onto the stirred powder bed to provide evaporative cooling. The powder is passed through a gas-lock system (2) to a second reactor (3). This acts in a similar manner to the first, except that ethylene as well as propylene is fed to the system for impact co-polymer production. The horizontal reactor makes the powder residence time distribution approach that of plug-flow. The stirred bed is well suited to handling some high ethylene co-polymers that may not flow or fluidize well. 

In fixed beds, cooling and partial recycling of the exit stream of the reactor is possible but this affects the residence time distribution. To overcome this difficulty and obtain a very high conversion level, a second reactor in series could be required. Cold injections of gas or liquid in a multibed reactor are another technique but it is thermodynamically very inefficient (Figures 1 and 2). 

Figure 3.2. Relation between degree of segregation J (Danckwerts, 1958) and recycle ratio r, in a recycle reactor with the variance a (proportional to Botot as a measure of residence time distribution) as parameter (From Dohan and Weinstein, 1973. With permission from Ind. Eng. Chem. Fundam., 12, 64. Copyright American Chemical Society.)...

In evaluating the residence time distributions in a continuous system oper> ating without recycling, for the case of the ideal discontinuous stirred vessel the curve with an exponential decay of the dilution process normally appears. But it appears in such a way that the pulse functions do not overlap (see Fig. 3.6 and also Blenke, 1979). This means that mixing and dilution processes are superimposed, and that in reactors that deviate from the ideal continuous... 

Fig. 8. Combined flow reactor models (a) parallel flow reactors with longitudinal diffusion (diffusivities can differ), (b) internal recycle—cross-flow reactor (the recycle can be in either direction), comprising two countercurrent plug-flow reactors with intercormecting distributed flows, (c) plug-flow and weU-mixed reactors in series, and (d) 2ero-interniixing model, in which plug-flow reactors are parallel and a distribution of residence times dupHcates that...

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