Page 410 - Chemical process engineering design and economics

390 Chapter 7

6
Q R = 997,900 Btu/h (1.05xl0 kJ/h)
Next, select a heat-transfer fluid. If we select steam at 700 °F (371 °C) the
jacket pressure will be 3094 psia (213 bar), which is much too high. At this pres-
sure, the jacket and reactor-wall will be very thick, resulting in a costly reactor.
Also, it is not good practice to use high-pressure steam for heating. If we select
Dowtherm A vapor, the jacket pressure will be 106.8 psia (7.37 bar) at 700 °F (343
°C) [24]. The maximum temperature allowed for Dowtherm A is 750 °F (399 °C)
[24].
Next, estimate the overall heat-transfer coefficient for the jacket, Uj. Assum-
ing DO/D] « 1 and Do/D LM » 1, where D LM is the log-mean diameter. Then Uj is
approximately given by

1 1 1 1 1 1 x w
_ = _ + _ _[, _ .f. _ -|. _ _|_ _
Uj hj h w h fi h fo ho k
The heat transfer-coefficients and fouling factors are listed in Table 7.2.1.
Because of the acetic acid, select SS316 as the material of construction. The ther-
mal conductivity, k, of SS316 and the wall thickness of the reactor, x w, are given
in Table 7.2.1.
1 1 1 1 0 . 5
—— = —— + —— + —— + 0.001 + ——
Uj 250 1700 360 113

2
2
Uj = 78.18 Btu/h-ft -°F (444 W/m -K)
Using Equations 7.8.8 to 7.8.12, the heat transfer rate for the 1500 gal reactor
2
2
containing 155 ft (14.4 m ) of jacket area is
6
6
Qj = 78.18 (155.0) (700.0 - 572.0) = 1.551xl0 Btu/h (1.64xl0 kJ/h)
5
which is acceptable because the heat absorbed by the reaction is 9.979x10 Btu/h
6
(1.05xl0 kJ/h).
Finally, calculate the mixer power. Using Equation 7.8.17, we find that for a
reaction with heat transfer the power required varies from 1.5 to 5 hp/1000 gal.
The average is 3.25 hp/1000 gal (640 W/m 3). Thus, from Equation 7.8.16,
P = (3.25 /1000) (1500) = 4.875 hp (364 kW)
From Table 5.10, the nearest standard size electric motor is 5 hp (373 kW).
The safety factor for this selection will only be 2.5 %. Therefore, select the next
larger-size motor, which is 7.5 hp (559 kW). The safety factor for this selection is
53.8 %.

Copyright © 2003 by Taylor & Francis Group LLC