Turbulent mixing blend time

Dimensionless pumping number and blend time are independent of Reynolds number under fully turbulent conditions. The magnitude of concentration fluctuations from the final well-mixed value in batch mixing decays exponentially with time. 

Blend time and ehemieal produet distribution in turbulent agitated vessels ean be predieted with the aid of Computational Fluid Mixing... 

In contrast to turbulent mixing, in the laminar range the microblend time is identical with the macroblend time. In the laminar range Ne Re, that is, N R = const applies in the laminar zone (see Equation 13.5), when scaling up with P/V — const. The same rotational frequency n must be used on a commercial scale as on a model scale. This also means, however, that the same blend time is achieved on both scales, and is therefore in agreement with Equation 13.15. 

Example 2-4a Blend Time. Now that we have ways to estimate s and the characteristic length scale Lg, we return to Corrsin s equations in Table 2-4. Probably the most important practical point is that the time constant of mixing scales with (Lg/e). All the rest of the terms in the equation for (Sc 1) are either constants or relatively minor effects of the Schmidt number. For mixing in a pipe, we take the radius of the feed pipe, ro, as the initial integral length scale, and the fluctuating velocity, u, as a measure of the turbulent energy. Thus we can write... 

Grenville, R. K., and J. N. Tilton (1996). A new theory improves the correlation of blend time data from turbulent jet mixed vessels, Trans. Inst. Chem. Eng., 74A, 390-396. 

The recommended correlations for design of agitators for blending in the turbulent and transitional regimes were developed at the Fluid Mixing Processes consortium at Cranfield in the U.K. The work is discussed in detail in Grenville (1992). Briefly, blend times were measured using a conductivity technique in vessels 0.30, 0.61, 1.83, and 2.97 m in diameter. The vessels had a standard torispherical base and were all fitted with standard baffles. The correlation is based on experiments carried out with one impeller located one-third of the liquid depth above the vessel base. 

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). 

Tubular jet mixers are used in turbulent flow either to blend two streams or to mix reactants prior to reaction. They are high-throughput continuous-flow devices and accurate flow control is, therefore, essential. Mixing times as low as a few milliseconds on the small scale or a few seconds on the large scale are... 

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