Vapor-Liquid Equilibrium Calculations Using K Values
The vapor-liquid equilibrium constant or distribution coefficient for component A
is defined as
y
KA = A
xA (3-1)
where yA = mole fraction of A in the vapor phase and xA = mole fraction of A in
the liquid phase
For light hydrocarbon systems (methane to decane), the K values have been
determined semi-emperically and can be evaluated from the equations given in Table
3.13. In general, K is a function of temperature, pressure, and composition.
Table 3.1. Equilibrium K values for light hydrocarbon systems
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(1) ln K = -A/T2 + B - C ln(P) + D/P2
(2) ln K = -A/T2 + B - C ln(P) + D/P
(3) ln K = -A/T + B - C ln(P) , where P is in psia, T is in oR
compound A B C D Form
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Methane 292860 8.2445 .8951 59.8465 (1)
Ethylene 600076.9 7.90595 .84677 42.94594 (1)
Ethane 687248.2 7.90694 .866 49.02654 (1)
Propylene 923484.7 7.71725 .87871 47.67624 (1)
Propane 970688.6 7.15059 .76984 6.90224 (2)
i-Butane 1166846 7.72668 .92213 0 (1)
n-Butane 1280557 7.94986 .96455 0 (1)
i-Pentane 1481583 7.58071 .93159 0 (1)
n-Pentane 1524891 7.33129 .89143 0 (1)
n-Hexane 1778901 6.96783 .84634 0 (1)
n-Heptane 2013803 6.52914 .79543 0 (1)
n-Octane 7646.816 12.48457 .73152 (3)
n-Nonane 2551040 5.69313 .67818 0 (1)
n-Decane 9760.457 13.80354 .7147 (3)
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The relative volatility
α
for each individual component in a multicomponent mixture is defined with respect to a
reference component C.
y /x
i = i i = Ki
yC/xC K C (3-2)
The values of i will be less dependent on temperature than the values of K i
since the Ki all increase with temperature in a similar manner.
Boiling (Bubble) point calculation using K values
The liquid composition xi of a mixture is given at a specified pressure P, the
temperature T and composition yi of the vapor in equilibrium with the liquid can be
calculated with the following procedure:
- Choose a component C to be the reference (base) component.
- Assume a temperature T ( T = xi Tbi )
Tbi = Boiling point of pure component i at pressure P. If an equation for K
is given as function of temperature and pressure, this temperature can be obtained by
setting K = 1.
Iteration steps:
1. Let Tsave = T. Determine Ki (From table 7.1). yi = Kixi
2. Evaluate Sumy =
3. Let KC = KC/Sumy
4. Determine T from KC and P
5. If abs(T - Tsave) > 1∆oR goto step 1 else yi = yi/Sumy
Dew point calculation using K values
The vapor composition yi of a mixture is given at a specified pressure P, the
temperature T and composition xi of the liquid in equilibrium with the vapor can be
calculated with the following procedure:
- Choose a component C to be the reference (base) component.
- Assume a temperature T (
)
Tbi = Boiling point of pure component i at pressure P. If an equation for K
is given as function of temperature and pressure, this temperature can be obtained by
setting K = 1.
Iteration steps:
1. Let Tsave = T. Determine Ki (From table 7.1). xi = yi/Ki
2. Evaluate Sumx =
3. Let KC = KC Sumx
4. Determine T from KC and P
5. If abs(T - Tsave) > 1∆oR goto step 1 else xi = xi/Sumy
Example 3.1
A mixture contains 35 mole % isobutane, 35 mole % isopentane, and 30 mole % n-hexane is at
30 psia. The K values for these co mpound can be obtained from
ln K = A/T2 + B + C ln P where T is in oR and P is in psia
Compound A B C
Isobutane -1,166,846 7.72668 -.92213
Isopentane -1,481,583 7.58071 -.93159
n-hexane -1,778,901 6.96783 -.84634
The boiling point of (pure) n-hexane at 30 psia is 659.6 oR
This mixture is flashed at 582.74 oR where 60 % of the feed is evaporated and at this conditions K iC4 =
3.1718, KiC5 = 1.051, KnC6 = 0.3169
Let isopentane be the reference compound and T = 582.74 oR be a guessed value for the bubble
point calculation (for the above mixture), the next calculated temperature T cal can be determined from
KiC5 (at Tcal ) = KiC5 (at 582.74oR) /
= 0.66814
Let isopentane be the reference compound and T = 582.74 oR be a guessed value for the dew
point calculation (for the above mixture), the next calculated temperature T cal can be determined from
KiC5 (at Tcal ) = KiC5 (at 582.74oR) *
= 1.4609
Flash distillation of multicomponent mixture using K values
A liquid mixture is partially vaporized and the vapor is allowed to come to
equilibrium with the liquid. The process flow diagram is shown in Fig. 3.1. The vapor
and liquid phases are then separated.
Fig. 3.1 Flash distillation.
Making a component i balance,
FxiF = Vyi + Lxi = Vyi + (F - V)xi (3-3)
Defining f = V/F, eq. (7.3) becomes
xiF = fyi + (1 - f)xi (3-4)
The above equation can be solved for yi,
(3-5)
or for xi,
(3-6)
The feed composition xiF and the fraction f of the feed vaporized are given at a
specified separator pressure P, the temperature T and compositions xi and yi can be
calculated with the following procedure:
- Assume a temperature T = fTd + (1- f)Tb
Tb, Td = Bubble point and dew point of mixture at pressure P.
Let T1 = T and Sumx1 =
Let T2 = T1
Sumx1 and Sumx2 =
Iteration steps:
1. T = (T1 - T2 - T1
Sumx2 + T2
Sumx1)/(Sumx1 - Sumx2)
2. Evaluate Sumx =
at T, P
3. Let T1 = T2, T2 = T, Sumx1 = Sumx2, and Sumx2 = Sumx
4. If abs(Sumx - 1) > .001
then goto step 1
else T = (T 1 - T2 - T1
Sumx2 + T2
Sumx1)/(Sumx1 - Sumx2)
EndIf
and
y
If the feed composition xiF, temperature T and pressure P of separator are given,
then the fraction of the feed vaporized V/F and compositions x i and yi can be calculated.
Eqs. (3-5) and (3-6) can be arranged so that f = V/F is the only unknown.
(3-7)
F =
(3-9)
Equation (3-9), which is known as the Rachford-Rice equation, has excellent convergent
properties and can be solved by Newton’s method. Take the derivative of the function F
with respect to V/F (or f),
(3-10)
The following procedure can be used to solve for V/F:
- Check to see if T is between Tb and Td.
- f = (T - Tb)/(Td - Tb)
Iteration steps:
1. Evaluate F =
2. Evaluate
3. Let ER = F/
dF
. f = f - ER
4. If abs(ER) > .001 goto step 1
and
y
