Page 234 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:37 PM Page 197

Heat Transfer 197

The value of the process side coefficieint, h p 1,061 From Equation 10-187A:
Btu/hr (ft )(°F) is lower than the value calculated in a step- L g c F B C
2
p> L (10-187A)
wise fashion. A slightly higher value of t would correct this. 144 g L
p 45.011.02
Example 10-20. Cyclohexane Column Reboiler 45 0.312 psi>ft
L 144
A thermosiphon reboiler is to be designed for a fraction- 1neglecting friction sensible zone2
ator that separates cyclohexane as the bottoms product. The
h l is from Equation 10-187.
conditions below the bottom tray are 16.5 psia and 182°F. A
Then, by Equation 10-170
total of 13,700 lb/hr vapor is to be produced.
Physical data for cyclohexane: p B p 3.6
0.41
p B p A 3.6 5.2
L 45.0 lb/ft 3
g 0.200 lb/ft 3 Hence, the length of the sensible zone 0.41 (8.0) 3.3 ft.

L 0.0208 lb/(ft) (hr) or 0.0086 cp From Figures 10-114 and 10-116 and for 0.098, the

g 0.97 lb/(ft) (hr) or 0.40 cp
following values are obtained:
( t/ p) s 3.6 °F/psi
c L 0.45 Btu/(lb) (°F) x R L 2 tp
k L 0.086 Btu/(hr) (ft)(°F)
0.05 0.28 — 12.74 —
154 Btu/lb
3
1.24 (10 ) lb/ft 1.10 — 25 — —
0.15 0.16 40 7.37 9.53
0.098
The circulation rate is calculated:
Preliminary Design
2
2
Boiling is to be inside 1-in. 12 BWG steel tubes, 8 ft long. W T (32.2)(0.181) (45.0)(45.0 12.74)(4.7)/ { 0.009
Condensing steam at 50 psig is available for heating. An over- 100 0.181 2 3.3 4.7
2
a b 0.013 a b c 25 a b10.902 d
2
all coefficient U 300 Btu/(hr) (ft )(°F) is expected. For the 0.336 0.320 0.065 0.065
given duty and a total driving force of 45°F, the inside surface
required is 157 ft . This is equivalent to 96 tubes. 0.181 2 50 0.181 2 }
2
0.009 a b 1402 a b1.852 2 9.53 a b
0.548 0.85 0.548
Circulation Rate
7,200
The inlet line consists of 100 equivalent ft of 6-in. stan- 615
2.68 6.35 1.68 1.04
dard pipe. The exit line consists of 50 equivalent ft of 10-in.
standard pipe. No special flow restriction is in the inlet line. W T 24.8 lb>sec
For the first trial, assume 15% vaporization per pass. This
gives a circulation rate of 91,300 lb/hr or 25.4 lb/sec. For This value is in reasonable agreement with the assumed
this rate, the following apply: value of 25.4 lb/sec.
2
h L 161 Btu/(hr) (ft ) (°F) Heat Transfer Rate—Simplified Method
f L, i 0.0045
f L, t 0.0065 For this calculation, the following are obtained:
f L, E 0.0045
2
_ h L 160 Btu/(hr) (ft )(°F)
It will be assumed that the liquid level is maintained even h tp /h L 3.2 (at x 0.4 x E 0.060)
_
with the top tubesheet. Neglecting inlet line friction, the h tp 510 Btu/(hr) (ft )(°F)
2
sensible heating zone length may be estimated from Equa-
0.2 { Figure 10-118
tion 10-176:
E 0 G T 66.0 lb/sec ft 2
_

0.1
2
D t N t h L 1t w t L 2 h b 200 Btu/(hr) (ft )(°F) at t w t s 30°F
t> L (10-187) h v 0.10 (200) 510 530 Btu/(hr) (ft )(°F), from
2
3600 W T c L
Equation 10-186
t 3.1410.065212182116121302 13.3211602 14.7215302 2
L 3600125.4210.452 5.2°F>ft. h p 8.0 377 Btu>1hr2 1ft 21°F2 ,
1assuming t w t L 20°F2 from Equation 10-185