Page 137 - Chemical process engineering design and economics

Process Circuit Analysis 121

h4,i = fCT4') (3.4.18)

114,2= TO (3.4.19)
h4,3 = flT 4') (3.4.20)

hM = TO (3.4.21)
Iks = TO (3.4.22)

Variables

Y3,i - Y3.2 - Y3,5 - Y4,i - Y4,2 - Y4,3 - Y4,4 - y4,5- m 3 - nx, - Q - h 3 - h4 - h 3>1 - h 3;2 - h 3>5 -114,,
- Il4 2 - h43 - h4 j4 - h4 ;5
Degrees of Freedom

F = 21-19 = 2

Equation 3.4.12, the energy balance for the reactor, requires some explana-
tion. We write the general energy equation, Equation 3.10 at the beginning of the
chapter, for the boundary that encloses the process stream, but not the coolant. We
can again neglect the kinetic and potential energy terms. Also, the reactor does no
work on the reacting gases so that Equation 3.10 for the reactor becomes

Ah = Q (3.4.2)

where Q is the heat transferred from the coolant to the process stream, and Ah is
the enthalpy change of the process stream across the reactor. Since enthalpy is a
state function, you can chose any path to evaluate Ah, starting from the state at the
entrance and ending at the state at the exit of the reactor. Because enthalpies of
reaction are given at 25 °C, select the path shown in Figure 3.4.2 for evaluating
Ah. First, cool the reactants to 25 °C, then let them react isothermally at 25 °C, and
finally heat the exit gases to the exit temperature. Thus, the enthalphy change
across the reactor becomes

Ah = Ahs m 3 + Ah R x t y 31 ma + Ari41114 (3.4.3)

Copyright © 2003 by Taylor & Francis Group LLC