Page 289 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:40 PM Page 251

Heat Transfer 251

3
h g a volumetric gas phase coefficient, Btu/(hr) (ft )(°F)
H g,d height of a gas phase mass transfer unit, ft ∆T V
H l,d height of a liquid phase mass transfer coefficient, ft
k g gas phase mass transfer coefficient,
2
lb-mol/(hr)(ft )(atm) L
2
L superficial liquid mass velocity, lb/(hr) (ft ) C L
M molecular weight
m exponent in baffle tray columns
1.18, experimental value for system studied
n exponent in baffle tray columns 0.44
P pressure, atm
Pr Prandtl number, dimensionless
Q heat transfer duty, Btu/hr
Sc Schmidt number, dimensionless
U a volumetric overall heat transfer coefficient,
3
Btu/(hr) ( ft )(°F)
2
U overall heat transfer coefficient, Btu/(hr) (ft )(°F) V
Z height, ft C V
Z sp height of individual spray zone, ft
density, lb/ft 3
Subscripts
d diffusional ∆T L
g gas
l liquid Figure 10-173. Direct contact tray column for heat transfer. This could
be a baffle tray, sieve type tray, bubble or other contact device, or
Smith 248 presents a design for this type of tray direct con- open spray or random packed column. (Symbols only used by per-
tact column, summarized as shown in Figure 10-173. Also mission: Smith, J. H. Hydrocarbon Processing, Jan. 1979, p. 147.
©Gulf Publishing Company. All rights reserved.)
rd
see Vol. 2, 3 Ed., Chapter. 8, of this series for design details.
When vapor stream has lower heat capacity than liquid
stream ( T v T L ), use 248
(10-278)
H v T v / T L distillate is to be heated from 325°F to 475°F. Because T L
n 1 n 1
H v * (H v H v )/(H v 1) T v / T v,max (10-279) T v , use Equations 10-281 and 10-282.
n 1 n 1
H v * (H v H v )/(H v 1.0), solve for n,
number of equilibrium stages (10-280) H L (475 325)/(500 440) 2.50
H* L (475 325)/(500 325) 0.857
When liquid stream has lower heat capacity than vapor 0.857 (2.5 n 1 2.5)/(2.5 n 1 1.0)
stream ( T L T v ) use 248 n 1.665

(10-281) About 65% efficiency is to be expected in this service,
H L T L / T v
requiring three actual trays.
n 1 n 1
H L * (H L H L )/(H L 1) T L / T L,max (10-282)
n 1 n 1
H L * (H L H L )/(H L 1.0), solve for n. (10-283)
where H v heat transfer factor, vapor limiting
H L heat transfer factor, liquid limiting
H L * heat transfer efficiency, equals ratio of actual liquid
temperature rise to maximum possible rise
Example 10-26. Determine Contact Stages
H v * heat transfer efficiency, equals ratio of actual vapor
Actually Required for Direct Contact Heat Transfer
temperature decrease to maximum possible decrease
in Plate-Type Columns
n number of equilibrium contact stages
T v actual vapor temperature decrease
Used by permission: Smith, J. H. Hydrocarbon Processing, V. T v,max maximum possible vapor temperature decrease (to
58, No. 1, ©1979. liquid inlet temperature)
How many theoretical contact stages are required for a T L actual liquid temperature rise
side reflux system on an atmospheric crude tower? The T L,max maximum possible liquid temperature rise (to vapor
vapor is to be cooled from 500°F to 440°F; the circulating inlet temperature.)