Page 363 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

From Table 11.14, we use the following heuristics:
3 0.5
Rule 2: F = uρ υ 0.5 = 1.2 → 1.5 m/s(kg/m )
s
Rule 3: ΔP = 0.007 bar
tray
Rule 4: ε tray = 60 – 90 %

α ) = 2.28
x oυhd = 0.9962, x oυhd = 0.0308, α oυhd = 2.44, α = 2.13, α geom aυe = (α oυhd bot 0.5
bot
N min = ln{ [0.9962/(1 – 0.9962)]/[0.0308/(1 – 0.0308)]} /ln (2.28) = 10.9
R min = {142.2/105.6}/(2.28 – 1) = 1.05
Range of R = (1.2 → 1.5)R min = 1.26 → 1.58

N theoretical ≈ (2)(10.9) = 21.8
ε tray = 0.6
N ≈ (21.6/0.6)(1.1) = 40 trays
actual
ρ = 6.1 kg/m 3
υ
0.5
u = (1.2 → 1.5)/6.1 = 0.49 → 0.60 m/s
Vapor flowrate (Stream 13) = 22,700 kg/h
3
Vol. flowrate, υ = 1.03 m /s
0.5
0.5
D tower = [4υ/πu] = [(4)(1.03)/(3.142)/(0.49 → 0.60)] = 1.64 – 1.48 m
ΔP tower = (N actual )(ΔP tray ) = (40)(0.007) = 0.28 bar

A comparison of the actual equipment design and the predictions of the heuristic methods are given
below.

f. H-101
From Table 11.11, we use the following heuristics:
Rule 13: Equal heat transfer in radiant and convective sections
2
radiant rate = 37.6 kW/m , convective rate = 12.5 kW/m 2

Duty = 27,040 MJ/h = 7511 kW
2
2
Area radiant section = (0.5)(7511)/(37.6) = 99.9 m (106.8 m in Table 1.7)
2
2
Area convective section = (0.5)(7511)/(12.5) = 300.4 m (320.2 m in Table 1.7)
From the earlier worked examples, it is clear that the sizing of the equipment in Table 1.7 agrees well
with the predictions of the heuristics presented in this chapter. Exact agreement is not to be expected.
Instead, the heuristics should be used to check calculations performed using more rigorous methods and to
flag any inconsistencies.

11.3 Summary

In this chapter, we have introduced a number of heuristics that allow us to check the reasonableness of the
results of engineering calculations. These heuristics or guidelines cannot be used to determine absolutely