Chapter 18: Models for Nonideal Reactors
Learning Resources
Solved Problems - Example CD18-6: Two CSTRs with Interchange
The elementary first-order liquid-phase reaction | ||||
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is carried out in a nonideal CSTR with k = 0.03 min-1. The flow patterns seem to approximate two CSTRs with interchange (Figure CDE14-1.1). Species A enters the reactor at a rate of 25 dm3/min and a concentration of 0.02 mol/dm3 . The total reactor volume is 1000 dm3 . The results of a pulse tracer test are shown in Table CDE14-1.1 Using the results of these tests, determine the conversion. | ||||
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Solution |
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A tracer balance yields (mass added at t = 0) = (mass out over all time) |
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(CDE14-1.1) (CDE14-1.2) |
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We could also have evaluated Equation (CDE14-1.2) by taking the area under the curve of a plot of C() versus(Figure CDE14-1.2). | ||||
Figure CDE14-1.2 |
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We will now determine the decay constants, m 1 andm 2 , from which the fraction exchanged,, can be determined. The dimensionless concentration is obtained from Equation (CD14-27). | ||||
(CDE14-1.3) |
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Plotting the ratio C(t) /C 10 as a function ofon semilog coordinates, we get the graph shown in Figure CDE14-1.3. At long times, the first term containing m2 in the exponent is negligible with respect to the second term. Consequently, if we extrapolate the portion of the curve for long times back to= 0, we have | ||||
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(CDE14-1.4) | |||
Solving for, we obtain= 0.1. The two parameters for this model are then | ||||
= 0.8 and=
0.1 |
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Substituting for,, andin Equation (CD14-18) yields | ||||
(CDE14-1.5) (CDE14-1.6) |
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So X = 0.51. For a single ideal CSTR, | ||||
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(CDE14-1.7) | |||
For a single ideal plug-flow reactor, | ||||
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(CDE14-1.8) | |||
Figure CDE14-1.3 |