Page 226 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 21 5

where a = interfacial area, f$/fts Gas thermal conductivity 0.012 Btu/hr-ft-"F
CI = constant in heat transfer equation = 0.0025 Gas Prandtl number 0.490
(English units)
C, = specific heat, Btu/lb-"F For an F-factor of 1.0 ft/s (lb/ft3)0.j, L = G = 2,100
C, = orifice coefficient, Equation 8-326 lb/hr-ft2. For Equation 8-328 a value of C1 is taken as
G = gas mass velocity, lb/hr-ft2 0.0025. Then, by Equation 8-329 and assuming that most,
h, = gas phase heat transfer coefficient, Btu/hr-ft2-"F if not all, of the resistance is in the gas phase,
hL = liquid phase heat transfer coefficient, Btu/hr-ft2-
"P 0.294
(0516)2'3 = 4.20 ft
HETP = height equivalent to a theoretical plate, ft (HWog = (0.0025) (2,100)O.~~ 0.490
HTU = height of a transfer unit, ft
L = liquid mass velocity, lb/hr-ft2 and
m = exponent = 1.0
n = exponent 0.44 HETP = 4.20 (In 1.21/0.21) = 3.81 ft
Pr = Prandtl number, dimensionless
Sc = Schmidt number dimensionless Thus, a 20-foot baffle tray section, with 50% cut baffles
U, = linear velociq; of gas based on total column on 24in. spacing can contain 10 elements and produce
crosssectional area, ft/sec 5.2 theoretical stages of separation. A corresponding
v, = linear velocity of gas based on window area, ft/sec crossflow sieve tray section, with 10 trays at 90% efficiency
(16) *, can produce 9 theoretical stages. This ratio is about
Subscripts as expected.
g = gas The pressure drop per baffle is:
L = liquid
og = overall (gas concentration basis)
Aptyet = 0.186 (3.43/0.42)2 (0.34/38.0) = 0.11 in. liquid
Greek Letters For the 20-ft section, total AP = 10 x 0.11 = 1.10 in. liq-
AP = pressure drop, in. liquid
h = slope ratio, slope equilibrium line/slope uid. The crossflow sieve tray would have a significantly
operating line, Equation 8-329 higher pressure drop.
p = density; Ib/ft3
Tower Specifications
Example 8-42: Mass Transfer Efficiency Calculation for
Baffle Tray Column (used by permission [211]) Performance calculations must be interpreted for
mechanical construction and for summary review by oth-
Data for example calculation ers concerned with the operation and selection of equip-
System ment. Typical specification sheets are given in Figures
Mixture 50-30 molar cyclohexane/ 8-156A and B for the tower and internal trays, respective-
n-heptane ly. Suggested manufacturing tolerances are given in Figure
Total reflux loperation 8-157. A composite cut-a-way view of tower trays assembled
Operating pressure 24 psia is shown in Figure 8-158. A Fractionation Research, Inc.
Temperature 238°F (FRI) suggested distillation tray data sheet is shown in Fig-
Relative volatility 1.57 ures 8-159.
Slope of equilibrium line 1.21 The calculation of nozzle connections has not been
demonstrated, but normally follows line sizing practice, or
Flow Rates some special velocity limitation, depending upon nozzle
Vapor F-factor 1 .O ft/sec (lb/ft3) o.3 purpose.
Gas mass velocity 2,100 lb/hr-f$
Liquid mass velocity 2,100 lb/hr-ft2 Tower shells may be ferrous, non-ferrous, stainless alloys
or clad (such as monel-clad-steel). The trays are usually
Properties light gage metal consistent with the corrosion and erosion
Liquid density 38.0 lb/ft3 problems of the system. The velocity action of vapors flow-
Liquid viscosity 0.56 lb/ft-hr ing through holes and slots accentuates the erosion-corro-
Liquid diffusion coefficient 2.40 x ft2/hr sion problems, and often a carbon steel tower will use
Gas density 0.34 lb/ft3
Gas viscosity 0.020 lb/ft-hr
Gas diffusion coefficient 0.114 ft2/hr *Note: References in ( ) are from original article.
Gas Schmidt number 0.316 ( text continued on page 21 8)
Gas specific heat 0.294 Btu/lb-'F