Page 175 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10E 5/30/2001 4:33 PM Page 138

138 Applied Process Design for Chemical and Petrochemical Plants

Example 10-12. Steam Heated Feed Preheater— Note for oil-free steam, it is usually quite safe to assume
Steam in Shell the steam film coefficient 1,500. Most calculated values
will be considerably greater than this.
Design a preheater for heating the feed to a distillation
2
column. The 54,180 lb/hr of feed consists primarily of ethyl U o 1/0.00559 179 Btu/hr (ft ) (°F)
benzene and styrene and is to be heated from 50°F to 207°F.
Steam is available at 10 psig. Log mean temperature difference:
The average physical properties of the feed have been cal-
culated over the temperature range, at 128°F: 239.3°F 239.3°F
207.0°F 50.0°F
Molecular weight: 104 32.3°F 189.3°F
Specific heat, c p : 0.428 Btu/lb (°F)
Viscosity: 0.4765 centipoise 189.3 32.3 157.0 157
2
Thermal conductivity: 0.0891 Btu/lb (ft ) (°F/ft) t 189.3 ln 5.8607 1.767 88.85
Density: 53.4 lb/ ft 3 ln
32.3
Heat duty Q (54,180) (0.428) (207 50) 3,640,000 Btu/hr
t correction 1.0, because this is total condensing on one side.
Heat Transfer Surface Required
13,640,0002
area required 229 ft 2
Try an 18-in. diameter shell unit with 82 1-in. O.D. 12 11792 188.852 11.02
BWG steel tubes 12 ft long 6 pass tubes. Area available 1822 111.752 10.26182 252 ft 2
1252211002
Tube I.D. 0.782 Safety factor based on fouling condition: 1.10
12292
Tubes/pass (82/6 ) 13.67
10%
2
10.7822 10.78542
cross-section>pass 13.67c d 0.0455 ft 2 Tube-Side Pressure Drop
11442
154,1802 R e 67,200
ft>sec tube velocity 6.20 ft>sec
153.4213,600210.04552
10.7822 From Figure 10-137,
D 0.06512 ft, tube I.D.
1122
f 0.000165
154,1802 2
2
G 1,190,000 lb>hr 1ft 2 G 1,190,000 lb/hr ft
10.04552 D 0.06512 ft
10.06512211,190,0002 L 12.00 ft
R e 67,200 n 6.00 passes
10.4765212.422
L 53.4 lb/ft 3
j H 182, from Figure 10-54.
2
10.000165211,190,0002 112.00216.02
k c p 1>3 0.14 p t
h i 11822a ba b a b 1221417,000,0002153.4210.065122
D k a w
5.80 psi, uncorrected for tube passes
1182210.08912 10.4281210.4765212.422 0.333
h i c d
10.065122 0.0891
1249215.552 0.333 For pass corrections use fluid flow expansion and con-
traction as an illustration of one approach to these pressure
drop calculations.
2
h i 440 Btu/hr (ft ) (°F)
1/h i 1/440 0.002270
1/h io (0.002270) (1.00/0.782) 0.00290 Assume channel diameter 17.25 in. (based on approximate
Assume inside fouling, r i 0.001 layout)
2
r io (0.0010) (1.00/0.782) 0.00128 Sectional area of channel (17.25) (0.7854) 233.5 in. 2
Tube resistance, 1/k 0.00024 17.25 in. diameter
2
Assume outside (steam) fouling: r o 0.00050 Sectional area of tubes (82) (0.782) (0.7854) 39.5 in. 2
Steam side film, h o , assumed: 7.1 in. diameter
1/h o 1/1,500 0.00067 d 7.1
0.410
r 0.00559 D 17.25