Laminar flow tracer data

Experimentally injecting and measuring the tracer in a laminar flow reactor can be a difficult task if not a nightmare. For example, if one uses as a tracer chemicals that are photo-activated as they enter the reactor, the analysis and interpretation of (f) from the data become much more involved. ... 

These interesting devices consist of a tube or duct within which static elements are installed to promote cross-channel flow. See Figure 8.5 and Section 8.7.2. Static mixers are quite effective in promoting radial mixing in laminar flow, but their geometry is too complex to allow solution of the convective diffusion equation on a routine basis. A review article by Thakur et al. (2003) provides some empirical correlations. The lack of published data prevents a priori designs that utilize static mixers, but the axial dispersion model is a reasonable way to correlate pilot plant data. Chapter 15 shows how Pe can be measured using inert tracers. 

Figure 8.9 Residence time distribution (RTD) for a pulsed tubular reactor. Average residence time = 60 min, with a pulsation rate of 99 spm. C is a dimensionless concentration of a non-reactive tracer solid line is experimental data dashed line is theoretical laminar-flow RTD. [23]...

Laminar flow the velocity gradient causes an increase in interfacial area between adjacent layers, which is a measure of mixing which is taking place. If a second material was added at a fixed time it would appear at the outlet with a changing concentration over a period of time as shown in Figure 8.12. Data to produce such curves are easily obtained by practical experiments using tracers. 

The most important use of residence time theory is its application to equipment that is already bnilt and operating. It is usually possible to find a tracer together with injection and detection methods that will be acceptable to a plant manager. The RTD is measnred and then analyzed to understand system performance. In this section we focns on such uses. The washout function is assumed to have an experimental basis. Calculations using it will be numerical in nature or will be analytical procednres applied to a model that reproduces the data accurately. Data fitting is best done by nonlinear least squares using untransformed experimental measurements of W(t), F(t), or f(t) versus time, t. Eddy diffusion in a turbulent system justifies exponential extrapolation of the integrals that define the moments in Table 1-2. For laminar flow systems, washout experiments should be continued until at least five times the estimated valne for t. The dimensionless variance has limited usefnlness in laminar flow systems. 

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