Page 28 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 17

9. Ki = P~/x (8- 23D) Calculate the Bubble Point: Assume composition is liquid.
where x = total system pressure absolute K, at Assume
Pi = vapor pressure of individual component at Mol assumed T = lOO"F,
temperature, abs Composition Fraction T = 90°F -- K x K Kx
Xi = X = mols of a component, i, in vapor phase plus 0.15 3.1 0.465 3.4 0.51
C2H6
mols of same component in liquid divided by C3H8 0.15 1 .o 0.130 1.2 0.18
the total mols of feed (both liquid and vapor) n-C&10 0.30 0.35 0.105 0.39 0.117
xf = mol fraction of a component in feed i-C4H10 0.25 0.46 0.115 0.52 0.13
xf = mol fraction of any component in the feed, Ft n-C5H12 0.15 0.12 - 0.13 0.0195
0.018
where Xf = F xf/F, for all components in F; for 1.00
the non-condensable gases, xf = VJF, 0.853 0.956
F = mols of feed entering flash zone per unit time (Too low)
contains all components except noncondens- Assume T = 105°F
able gases - &
K
F, = F + V, 3.45 0.517
V, = V + V, mols of vapor at a specific temperature
and pressure, leaving flash zone per unit time 1.23 0.18'7
'V, = mols of non-condensable gases entering with 0.41 0.123
the feed, F, and leaving with the vapor, V, per 0.55 0.13'7
unit time 0.15 0.022
V = mols of vapor produced from F per unit time, 0.986
F=V+L
L = mols of liquid at a specific temperature and By interpolation:
pressure, from F, per unit time
i = specific individual component in mixture 0.986 - 0.956 1.000 - 0.986
- -
K, = equilibrium K values for a specific component 105 - 100 X
at a specific temperature and pressure, from
References 18, 65, 79, 99, 131, 235 x = 2.34"F
T = temperature, abs So, T = 105 + 2.34 = 107°F Bubble point at 165 psia
xi = mol fraction of a specific component in liquid
mixture as may be associated with feed, distil- Calculation of Dew Point
late, or bottoms, respectively
yi = mol fraction of a specific component in vapor Composition Mol Frac. Assume, T = 160°F
mixture as may be associated with the feed, Vapor in Vapor K (from charts) x = y/k
distillate or bottoms, respectively
CZH6 0.15 5.1 0.0294
10. For the simplified case of a mixture free of non-condens- C3H8 0.15 1.83 0.081
able gases, see Equation 8-23A, where Xf = xf. nC4H1o 0.30 0.80 0.375
i-C4H10 0.25 1.00 0.250
n-CjH12 0.15 0.32 0.469
Example 8-2: Bubble Point and Dew Point 1.00 1.204 z Xy/K = 1

From the hydrocarbon feed stock listed, calculate the Assume T Assume T
bubble point and dew point of the mixture at 165 psia, = 180°F. K x = v/K = 175°F. K x = p/K
and using K values as listed, which can be read from a 5.95 0.0232 5.6 0.0268
chart in 3rd edition Perry's, Chemical Engineer's Handbook. 2.25 0.0666 2.2 0.0682
0.98 0.3060 0.91 0.330
Feed Stock 1.30 0.1920 1.2 0.208
0.41 0.3660 0.39 0.385
ComDosition Mol Fraction 0.9558 # Ey/K = 1 1.018 E Xy/K= 1.0
C2H6 0.15
C3H8 0.15 Dew point is essentially 175°F at 165 psia
n-C4H10 0.30
tC4H10 0.25 Example 8-3: Flashing Composition
n-CFiH12 0.15
1 .oo If the mixture shown in Example 8-2 is flashed at a tem-
perature midway between the bubble point and dew point,