Computer Application in Chemical Engineering

Machine Problem No.
3
VAPOR-LIQUID EQUILIBRIUM
Objectives
The objective of the experiment is to perform vapor-liquid equilibrium calculations using MS Excel® and using
Aspen Plus®.
Summary of Results
The first vapor-liquid equilibrium problem was concerned about the mole fractions of the components in the
liquid and the vapor phase using MS Excel®. Utilizing the goal seek function of Microsoft Excel®, the fraction
of the feed that is in the vapor (v’) was obtained to be 0.707853 or 70.79%. The results show that the most propane
went to the vapor stream while the most n-Pentane was accumulated in the L stream. Table 3.1 shows the summary
of the fractions of the components in both vapor and liquid phase.
Table 3.1. Microsoft Excel(R) Financial Functions

Component xi yi
Propane 0.06870012 0.25419044
n-Butane 0.23380124 0.32732173
n-Pentane 0.69748744 0.41849246
The second problem presents data for the components with respective mole fraction of a volatile oil in a reservoir
in Northern Louisiana. The objective is to solve for the composition of the gas and liquid streams and the vapor
fraction using RK-Soave option. Table 3.2 shows the data simulated by the ASPEN.
Table 3.2. ASPEN Results for the Composition of the Gas and Liquid Streams and the Vapor Fraction
Component FEED VAPOR LIQUID
NITROGEN 0.0167 0.0162604 0.000439507
METHANE 0.6051 0.5578077 0.0472922
CARBON DIOXIDE 0.0218 0.0179021 0.00389781
ETHANE 0.0752 0.0519346 0.0232654
PROPANE 0.0474 0.0197489 0.027651
BUTANE 0.0412 0.00739119 0.0338088
PENTANE 0.0297 0.00206993 0.02763
HEXANE 0.0138 0.000315582 0.0134844
HEPTANE 0.1491 0.00125032 0.1478497
Mendoza, Ronel A. Page 1

Machine Problem No. 3
Discussion
The first problem presented a list of components with the respective flow rate and K-value as shown in Table 3.3.
The stream condition is at 100 psia and 178F.
Table 3.3. Given Data for Problem 1

Component lbmol/hr (feed) K-value
Propane 20 3.7
n-Butane 30 1.4
n-Pentane 50 0.6
The K-value is known as the distribution coefficient which is the ratio of the vapor phase concentration to the
liquid phase.
𝑦𝑖 = 𝐾𝑖 𝑥𝑖 (1)
Propane exerts higher K-value, thus it is more volatile than the other components. The distribution coefficient is
directly proportional to the vapor phase concentration, which explains the high volatility property of Propane.
From this theoretical framework, it can be predicted that most propane would be accumulated in the vapor stream.
Further derivation and summation of Equation 1 and overall material balance yield the Rachford-Rice equation.
This equation (Equation 2) was utilized for the determination of v’, the vapor fraction of the feed.
𝑁𝐶𝑂𝑀𝑃
(𝐾𝑖 − 1) 𝑧𝑖
∑ =0 (2)
1 + (𝐾𝑖 − 1) 𝑣′
𝑖=1
The goal seek function will help on the calculation of v’.
The mole fraction of the components in the liquid phase was determined using Equation 3.
𝑧𝑖
𝑥𝑖 = (3)
1 + (𝐾𝑖 − 1) 𝑣′
The mole fraction of the components in the vapor phase may then be obtained by using Equation 4.
𝑧𝑖 = 𝑦𝑖 𝑣′ + 𝑥𝑖 (1 − 𝑣 ′ ) (4)

The second problem listed the components of a volatile oil in a reservoir in Northern Louisiana with the respective
mole fraction. The reservoir conditions at discovery were 246F, 4800 psia. The gas-liquid separator at the surface
is at 500 psia, 65F. The problem is well presented by the simulated flash phase separator.
V
yi
F
zi Pout
Tout
Pin
Tin
L
xi
Figure 3.1. Flash Phase Separator
The mechanism of Flash 2 block is the division of the feed in single vapor phase and in single liquid phase that
are both in equilibrium when the conditions of the flash are programmed such that a multicomponent solution’s
state is in the two-phase region. Since the problem deals with a single liquid phase and a single vapor phase.
One useful feature of the Flash 2 block is the its ability to specify entrainment that is, a specified fraction of
the liquid phase is carried by the flashing vapor into the overhead. Using different block with this
problem would lead to the increase of required parameters which will complicate the solution for the problem.
The RK-Soave was used because it is the most modified and accurate equation of state for vapor-liquid
equilibrium. The deviation in each value arises because of the properties being considered in their formula.
Equation of state is continuously being modified; thus giving more complicated formula with additional terms
and coefficients being considered. For instance, the ideal gas law only directly used the measured physical
properties of the gas which are the temperature, volume and temperature. But in Soave-Redlich-Kwong
equation of state, parameters like Tc, Pc, acentric factors, constants a and b were considered.

Conclusion
The objective of the machine problem was accomplished. The student was able to simulate the Rachford-Rice
equation in its algebraic form in solving the problem in Microsoft Excel. The result was achieved by the
utilization of goal seek function. Also, the student was able to know the underlying principles and chemistry
behind the behavior of the components during vapor-liquid equilibrium. Furthermore, ASPEN is a reliable tool
for the calculation of vapor-liquid equilibrium. The tool will tell you all the needed parameters and value upon
the simulation.
Reference
Seader, J. D., & Henley, E. J. (2006). Separation Process Principles, Second Edition. John Wiley & Sons, Inc.
Schefflan, R. (2011). Teach yourself the basics of Aspen plus. Hoboken, NJ: Wiley.

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Machine Problem No.

of the fractions of the components in both vapor and liquid phase.

Table 3.1. Microsoft Excel(R) Financial Functions

Mendoza, Ronel A. Page 1

The stream condition is at 100 psia and 178F.

Table 3.3. Given Data for Problem 1

The goal seek function will help on the calculation of v’.

Mendoza, Ronel A. Page 2

Figure 3.1. Flash Phase Separator

Mendoza, Ronel A. Page 3

Mendoza, Ronel A. Page 4

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