Residence time ideal mixer

A practical method of predicting the molecular behavior within the flow system involves the RTD. A common experiment to test nonuniformities is the stimulus response experiment. A typical stimulus is a step-change in the concentration of some tracer material. The step-response is an instantaneous jump of a concentration to some new value, which is then maintained for an indefinite period. The tracer should be detectable and must not change or decompose as it passes through the mixer. Studies have shown that the flow characteristics of static mixers approach those of an ideal plug flow system. Figures 8-41 and 8-42, respectively, indicate the exit residence time distributions of the Kenics static mixer in comparison with other flow systems. 

Residence Time Distribution Inside the Ideal Mixer... 

The shape of the residence time or cumulative residence time distributions are used when optimizing the mixing ability of a system. Often, this shape is compared to the residence time in an ideal or perfect mixer. Such a mixer is a well stirred tank, as depicted in Fig. 6.51(a). Here, two components, a primary and secondary component, are fed to the tank at a total flow rate Q. The output can be regarded as a flow rate Q with a concentration (1 — Co) of... 

M 87] [P 78] The mixing performance of the Coanda micro mixer is nearly constant over a large range of volume flows, from 1 to 100 pi min-1 [55], The small deviation from ideal is maximum at a flow rate of about 10 pi min-1. The T-type mixer instead shows a notable decrease in mixing efficiency with flow rate, simply owing to the reduction in residence time. 

Allowable Spread in Residence Time. Other ways of stating the requirement of equal residence time of all parts of the reactant is that the flow through the reactor should approach plug flow or that the residence time distribution (RTD) should be equivalent to that in a large number of mixers in series. An often used rule of thumb is that this requirement is met when the equivalent number of mixers (N ) exceeds a certain value, say 5. However, this criterion is at best a semi-quantitative one, since the minimum value of is dependent upon the accepted deviation from the ideal reactor, and on the degree of conversion and the reaction order. 

In the model of the mixer, the mixing of the streams at the column inlet is taken into account. The residence-time behavior of the piping can be described as a plug-flow reactor. Backmixing effects outside the column can be described by an ideal stirrer tank (Fig. 9.9). 

Residence time distribution experiments have shown that the reactor behaves almost like a plug flow tubular reactor with a small dispersion [6]. The RTD can be described using a tanks in series model with 35 ideal mixers. As the simulated reactor behaviour based on the kinetic model is only slightly influenced by the number of ideal mixers for more than 8 tanks, this value was used for all simulations in order to reduce the calculation time needed for parameter optimisation. 

An important question for the design of continuous flow systems is When can the classic perfectly mixed assumption (ideal CSTR) be used in a continnons flow stirred tank reactor The blend time concept can be used here. If the blend time is small compared to the residence time in the reactor, the reactor can be considered to be well mixed. That is because the residence time is proportional to the characteristic chemical reaction time. A 1 10 ratio of blend time to reaction time is often used, but often, larger values result because the mixer must do other jobs, which lead to even smaller blend times. Frequently, residence time distributions are used to determine whether a reactor is well-mixed. It is usually easy to achieve well-mixed conditions in continuous flow, turbulent stirred vessels unless the reactions are very fast, such as acid-base neutralizations. Even in laminar systems the blend time can be made much less than the required residence time for the chemical reaction mainly because required residence times are so long for high viscosity reactants. For discussions of residence time distribution analysis, see Chapter 1, Levenspiel (1972), and Nauman (1982). 

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