Page 53 - Applied Process Design For Chemical And Petrochemical Plants Volume II

42 Applied Process Design for Chemical and Petrochemical Plants

Example 8-13: Estimating Distillation Tray Efficiency by
Yi -Yo
Plate/Tray Efficiency : Ew0 = - (8 - 66) Equations 8-70A and 870B (used by permission of
~i -Ye"
McFmland et al. [86])
The plate/tray efficiency is the integrated effect of all Solving the problem defined in the following table will
the point efficiencies. show the equations for estimating system physical proper-
ties and their relation to the calculation of Murphree
vapor plate efficiencies:
-Yb
Yl
Point Efficiency : Em * = - (8 - 67)
~i -Ye
System properties* Acetone Benzene
--
Molecular weight, M, lb/lb mole 58.08 78.11
No. Theoretical Trays
Overall tray efficiency, E, = (8 - 68) Viscosity, p lb/hr-ft 0.5082 0.8155
No. of Actual Trays
Parachor, [PI 162.1 207.1
API specific gravity coeff [ 2401 :
where yi = average composition of vapor entering tray A 0.8726 0.9485
yo = average composition of vapor leaving tray B 0.00053 0.00053
ye* = composition of vapor in equilibrium with liquid C 21.6 18.0
flowing to plate below E 536.0 620.6
-- ~
yi' = vapor composition entering local region Operatiug data
yo' = vapor composition leaving local region Acetone mole fraction, x1 = 0.637
ye = vapor composition in equilibrium with the liquid in Benzene mole fraction, x2 = 0.363
the local region
Temperature, T, "F = 166
Superficial vapor mass velocity, G, lb/hr-sq ft = 3,820
The proposal for calculating column vapor plate effi- Vapor velocity, Uv, ft/hr = 24,096
ciencies by MacFarland, Sigmund, and Van Winkle [86] Weir height, h,, ft = 0.2082
correlates with the Murphree vapor plate efficiencies in Fraction free area, FA = 0.063
percent: ~ ~~
*Used by permission of McFarland et al. [86].
(8- 69) Iiquid densities for pure hydrocarbon are calculated
[240] as a function of temperature using the following
equation for specific gravity:
where yn = average light key mol fraction of vapor leaving
plate n
yn + 1 = average light key mol fraction of vapor entering SgL =A - BT - C/(E - T)
plate n
y* = light key mol fraction of vapor in perfect equilib The liquid density is then:
rium with liquid leaving plate n
pL= (62.32) (sa)
Data from bubble cap and perforated tray columns for For acetone,
the Murphree vapor plate efficiencies are correlated [86] :
pL,1 = (62.32) r0.8726 - 0.00053 (166) - 21.6/(536.0 - 166)]
= 45.3 lb/ft3

For benzene,

PL,~ = (62.32) [0.9485 - 0.00053 (166) - 18.0/(620.6- 166)l
= 51.2 lb/ft3
Referenced to 806 data points for binary systems, Equa-
tion 8-70A gives absolute deviation of 13.2%, which is Vapor densities are calculated from the ideal gas relation:
about as accurate, or perhaps more so, than other effi-
ciency equations. Equation 8-70B uses the same data and pv = MPt/555(T + 460)
has an absolute average deviation of 10.6%. See Example
8-1 3 for identification of dimensionless groups. where total pressure P, is given in millimeters of mercury.