Tracer Studies to Determine Reactor Parameters

When we talk about tracers, we generally mean conservative tracers with no sources or sinks. This is opposed to gas tracers, with gas transfer to the atmosphere, and reactive tracers, with a reaction occurring. Tracer studies typically use a conservative tracer, input to the system in a highly unsteady manner, such as a pulse or a front. The pulse and front are typically a more stringent test of the model than a steady-state process with any variety of reactions. Thus, a model that properly simulates the output concentration curve of a pulse or front is assumed to be sufficient for most real conditions with reactions. 

When we use a tracer study to develop reactor parameters for an environmental system, we are inherently assuming that the details of the transport processes are not essential to us. All that we have is an input and an output, and any sets of reactors that will simulate the output for a given input are acceptable. What you can learn about the system from a reactor model depends on your understanding of the transport processes and how they are simulated by reactor models. 

The reactor combinations that are given here are those that have been found to best simulate tracer studies for environmental systems. A more complete reactor analysis is provided in reactor texts, such as Levenspiel (1962). 

separating variables, integrating, and applying boundary condition 1 results in 

The response of a single complete mix reactor to a conservative pulse of mass M at t = 0 would have boundary conditions  

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Since the RTD of a particular reactor type does not describe its performance uniquely for nonlinear reactions, it is customary to develop flow models for the reactor. The parameters of the model are then determined from tracer studies and the structure of the model is used to calculate reactor performance. 

Bubble columns. Tracers are used in bubble columns and gas-sparged slurry reactors mainly to determine the backmixing parameters of the liquid phase and/or gas-liquid or liquid-solid mass transfer parameters. They can be used for evaluation of holdup along the lines reviewed in the previous Section 6.2.1. However, there are simpler means of evaluating holdup in bubble columns, e.g. monitoring the difference in liquid level with gas and without gas flow. Numerous liquid phase tracer studies of backmixing have been conducted (132-149). Steady-state or continuous tracer inputs (132,134,140,142) as well as transient studies with pulse inputs (136,141,142,146) were used. Salts such as KC Jl or NaCil, sulfuric acid and dyes were employed as tracers. Electroconductivity detectors and spectrophotometers were used for tracer detection. The interpretation of results relied on the axial dispersion model. Various correlations for the dispersion... 

Other reactors and uses of tracers. Tracers are used extensively in all other two-phase (gas-liquid, gas-solid, liquid-solid) and three-phase reactor types (gas-liquid-solid, liquid-liquid-solid). Tracers confined to a single phase are used to determine the RTD of that phase and evaluate its flow pattern. Tracers that can be transported from one phase to another are frequently used for evaluation of various rate parameters and transport coefficients such as mass transfer coefficients, particle effective diffusivity, adsorption rate constants, kinetic rate constants, etc. The interpretation of tracer studies in evaluation of the above parameters is always dependent on the selected model for the system. We do not attempt to review this vast literature but will just cite a few examples as good starting points for the interested reader. 

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