Page 400 - Chemical process engineering design and economics

380 Chapter 7

of the reactor is greater than the rated capacity to allow for some headspace. If the
rated capacity from Table 7.3 is greater than the calculated reaction volume, calcu-
x
late the actual conversion, - The conversion will increase because of the in-
n> A
creased reaction volume and therefore residence time.
The first step is to calculate limits for the reaction volume. One CSTR will
give the maximum volume and a plug-flow reactor will give the minimum vol-
ume. The total reaction volume for multiple CSTRs will lie somewhere between
these two limits. After calculating the reaction volume, calculate the required heat
transfer and the heat-transfer area. Then, either select a jacket, a coil, jacket plus a
coil, or an external heat exchanger.
First, calculate the reaction volume. Assuming that the density does not
change significantly during reaction, the total volumetric flow rate at the reactor
exit,
3
3
V v = 800 + 800 + 4000 = 5600 ft /h (159 m /h)
In this case the subscript A in Table 7.4 refers to propylene oxide. The molar
flow rate of propylene oxide is

800
mi, A = —— 0.859 (62.43) = 738.7 Ibmol/h (335 kgmol/h)
58.08

After substituting the reaction parameters into Arhenius's equation, Equation
7.4.20, we obtain

T = 100 + 459.7 = 559.7 °R (311 K)

- 32400 1
k = 16.92 x 10 12 exp ————— ——— = 3.766 h' 1
1.987 559.7

For one CSTR, n = 1 in Equations 7.4.1 and 7.4.3 to 7.4.5.

nii A = m 2A + (x 2A - XIA) m 1A

- A V r = (x 2A - x, A)
r 2
T2A = - k C 2A

c 2A = m 2A /V v

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