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Unformatted text preview: ChE 101 2012 Set 8 Solutions 1. (a) The PFR mass balance is given by Equation 5.22 as u dc A dz = k c A exp E RT , where we have explicitly used the Arrhenius temperature dependence of k . The energy balance is given in Equation 5.31, u dT dz = Δ H R ρC p k c A exp E RT Up w ρC p A t ( T T c ) Note that in our convention, Δ H R is negative for an exothermic reaction. Now, since we would like the independent variable z to range from 0 to 1, the obvious choice of nondimensional distance is to take ˜ z = z L The most appropriate dimensionless concentration variable is one we have been using all term, namely the fractional conversion X , since this precisely ranges from 0 to 1 over the course of reaction completion. Thus, we define X = 1 c A c A, The choice of nondimensional temperature is less obvious, since there are two temper ature scales in the system: T and T c . However, notice that as long as heat transfer is sufficiently fast, fluid will enter the reactor at temperature T and leave at temperature T c , so these two quantities provide the natural range of temperature variation within the system. Thus, in order to have the temperature variable range from 0 to 1, the natural combination to use is Θ = T T T c T , which should be very reminiscent of variables we often use in heat transport calculations. Notice that this temperature scale only works if T c and T are unequal, but this condition is often met in any cooled reactor. Substituting these expressions into the mass balance, we find that c A, u L dX d ˜ z = k c A, (1 X )exp E/R ( T c T )Θ + T 1 To clean this up, we introduce the following dimensionless groups: α = k L u = k V V = k τ β = T c T T = T c T 1 γ = E RT Thus, the mass balance nondimensionalizes as follows: dX d ˜ z = α (1 X )exp γ 1 + β Θ Now we work on the energy balance: ( T c T ) u L d Θ d ˜ z = k c A, Δ H R ρC p (1 X )exp γ 1 + β Θ Up w ρC p A t ( T c T )( θ 1) To massage this formula further, we define the following two dimensionless variables, ˜ J = c A, Δ H R ρC p ( T c T ) ˜ K = Up w L ρC p A t u Notice that we have incorporated a negative sign into our definition of ˜ J in anticipation of the fact that many reactions of interest are exothermic. With these additional definitions, we obtain the dimensionless energy balance for the system, d Θ d ˜ z = ˜ Jα (1 X )exp γ 1 + β Θ ˜ K ( θ 1) (b) The parameter α can be viewed as a dimensionless residence time in the reactor, weighted by the reaction rate constant. Hence, α provides a measure of the extent to which the reaction is allowed to proceed relative to its intrinsic rate. If α is large, then we will approach complete conversion at the end of the reactor. The interpretation of β is fairly simple: it is simply the relative magnitude of the characteristic temperature variation in the reactor induced by the coolant. The parameter γ is one of the most important determinants of the reactor dynamics. Intuitively,determinants of the reactor dynamics....
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This note was uploaded on 03/21/2012 for the course CHE 101 taught by Professor Arnold during the Winter '11 term at Caltech.
 Winter '11
 ARNOLD
 Mass Balance

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