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Distilation

This document provides an introduction to distillation processes, including definitions of key terms like distillation, fractionation, and light and heavy keys. It describes basic distillation unit components and sections. It also covers concepts like vapor-liquid equilibrium, equilibrium calculations, and properties relevant to distillation simulations and design.

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Distilation

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Engineering Encyclopedia

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Introduction To Distillation Process

Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramcos
employees. Any material contained in this document which is not
already in the public domain may not be copied, reproduced, sold, given,
or disclosed to third parties, or otherwise used in whole, or in part,
without the written permission of the Vice President, Engineering
Services, Saudi Aramco.

Chapter : Process
File Reference: CHE20501

For additional information on this subject, contact

D. J. Clarke on 873-9817

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Contents

Pages

Distillation, Fractionation, And Key Components .......................................... 1

Column Sections, Reflux............................................................................... 3
Major Equipment ........................................................................................... 4
Auxiliary Facilities.......................................................................................... 5
Calculating Vapor And Liquid Compositions In Ideal Mixtures...................... 7
Ideal And Nonideal Gases.................................................................. 7
Real-Gas Equations ........................................................................... 7
Ideal Mixtures - Dalton's, Raoult's Laws............................................. 9
Equilibrium K-Values ........................................................................ 10
Two Component Example...................................................... 12
Mixtures Approximated As Ideal ............................................ 13
Equilibrium Diagram ......................................................................... 14
Nomenclature .............................................................................................. 17
Subscripts......................................................................................... 17
Work Aid 1:

Formulas And Guidelines For Calculating Vapor And

Liquid Compositions In Ideal Mixtures ................................ 18

Formulas........................................................................................... 18
Ideal Gas Law ........................................................................ 18
Dalton's Law .......................................................................... 18
Ideal Mixture Relationship...................................................... 18
Guidelines......................................................................................... 18
Glossary ...................................................................................................... 20

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Addendum A................................................................................................ 28
Introduction ....................................................................................... 28
Vapor-Liquid Equilibrium (Vle) Relationships ................................... 28
Ideal And Nonideal Gases ..................................................... 28
Vapor Pressure...................................................................... 29
Ideal Mixtures - Dalton's, Raoult's Laws................................ 31
Real-Gas Equations............................................................... 33
Fugacity ................................................................................. 34
Equilibrium K-Values.............................................................. 37
Relative Volatility.................................................................... 37
Nonideal Liquids .................................................................... 39
Equations Of State................................................................. 39
Vle Calculations................................................................................ 41
Equilibrium Diagram............................................................... 41
Vapor-Liquid Phase Diagrams............................................... 43
Bubble Point And Dew Point.................................................. 44
Equilibrium Flash Separation ................................................. 45
Bubble And Dew Point Calculations ...................................... 50
Flash Calculations: One Main Component Plus Gas ........... 51
Physical Properties........................................................................... 53
Physical Property Sources .................................................... 53
Average Boiling Point............................................................. 54
Characterization Factor ......................................................... 55
Inspection Properties ............................................................. 56

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Petroleum Fraction Distillations ........................................................ 57

15/5 Distillation....................................................................... 57
Astm Distillations.................................................................... 59
Gas Chromatographic Distillation (Gcd) ................................ 60
Equilibrium Flash Vaporization (Efv)...................................... 60
Distillation Curve Relationships ............................................. 61
Crude Assays ........................................................................ 62
Computer Simulations............................................................ 64
Nomenclature ................................................................................... 68
Addendum B................................................................................................ 69
Example 1......................................................................................... 69
Example 2......................................................................................... 71
Example 3......................................................................................... 73
Example 4......................................................................................... 75
Addendum C ............................................................................................... 77
Addendum D ............................................................................................... 91
Addendum E.............................................................................................. 100

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DISTILLATION, FRACTIONATION, AND KEY COMPONENTS

Distillation is the separation of the constituents of a liquid mixture by partial vaporization of
the mixture, followed by separate recovery of the vapor and liquid. The more volatile (light)
constituents of the mixture are obtained in increased concentration in the vapor, while the
less volatile (heavy) are components concentrated in the liquid residue, also called the
bottoms. The vapor is most frequently condensed by cooling and is called the distillate or
overhead product.
Fractionation is a term commonly used in the petroleum industry for a distillation in which
the vapor is contacted continuously and countercurrently with a condensed portion of the
vapor. In most petroleum processing plants, continuous fractionation is the only type of
distillation used, so the terms distillation and fractionation are used interchangeably.
An example of a simple distillation unit, the Ras Tanura Plant 10 Depropanizer, is shown in
Figure 1. The feed to the unit is a mixture of light hydrocarbons, mostly C3 through C5
paraffins. The objective of the unit is to remove the propane and lighter components
(overhead product) while keeping the butanes and heavier components in the bottoms.
Frequently in multicomponent distillation, a light component that must be recovered in the
distillate is also present in the residue in important amounts, while components lighter than
this component are present only in small amounts. This component is called the light key. In
the case of the Ras Tanura depropanizer, the light key is the propane, which has a
concentration in the residue of 0.7%.
Similarly, a heavy component present in the distillate in important amounts is called the
heavy key. If more than one of the heavy components is present in the distillate in important
amounts, then the more volatile component is the heavy key. In the Ras Tanura
Depropanizer, where both the isobutane and the n-butane are present in the distillate in
important amounts, the heavy key is the isobutane. Key components are used in shortcut
distillation calculations and in some tray and packing efficiency calculations. Frequently
product specifications are based on the concentration of key components, for example, 0.7%
maximum propane in the depropanizer bottoms.

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C2
0.4 mole %
C3 97.2 mole %
C4 + 2.4 mole %

Sour Propane
to ADIP
Treaters
Condenser

270
psig

Reflux Drum
43

130F

4078 gpm

NG L

15 psig
STM
29
Depropanizer

C 2 0.2 mole %
C 3 44.7 mole %
i - C 4 6.7 mole %
n - C 4 21.5 mole %
i - C 5 6.7 mole %
n - C 5 9.7 mole %
C 6 6.2 mole %
C 7+ 4.3 mole %

Data from Dwg.

NA-637118 Sh. 1 Rev. 1 - Summer

Reboiler
150 psig
STM
Cond.

C3 0.7 mole %
276F C4+ 99.3 mole %

Bottoms to
Debutanizer

Ras Tanura Plant 10 Depropanizer Simplified Process Flow Diagram

Figure 1

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COLUMN SECTIONS, REFLUX

The feed to the Ras Tanura depropanizer is partially vaporized in a steam heater and fed to
the distillation tower (Figure 2). When the feed enters the depropanizer, the vapor portion of
the feed rises in the column while the liquid portion of the feed descends in the column. As
the vapor portion of the feed, along with vapor from the bottom section of the tower, rises in
the column, it contacts the descending liquid. The section of the distillation column above
the feed is called the rectifying (or enriching) section. In the rectifying section, the
concentration of the light components increases toward the top of the tower; that is, the light
products are enriched. The section of the column below the feed is the stripping section.
Here, the light components are stripped out of the liquid as they descend the column.
Part of the liquid condensed from the vapor leaving the column top is returned to the column
as reflux. The reflux provides the liquid for the contact with the vapor in the rectifying
section of the column. The important role of reflux in distillation will be discussed in later
sections.
Condenser
Reflux
Drum

Reflux
Rectifying
Section

Steam Heater

Sour Propane to
ADIP Treaters

Vapor

Liquid

Stripping
Section

Bottoms to Debutanizer

Distillation Process in the Ras Tanura Depropanizer

Figure 2

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MAJOR EQUIPMENT
The column or tower is the main piece of equipment in a distillation unit. It contains vaporliquid contacting devices, trays in most cases, packing less frequently. The Ras Tanura Plant
10 Depropanizer column contains 43 trays, 29 below the feed, 24 above the feed. In this
unit, the section below the feed has a larger diameter, which is needed to accommodate the
heavier liquid load below the feed.
Typical major auxiliary equipment of a column includes the condenser, the condenser
separator, and the reboiler. The condenser condenses the reflux and the part of the distillate
that is removed as liquid. The condenser separator separates any vapor distillate from the
liquid and provides surge capacity for the reflux and the distillate. The Ras Tanura Plant 10
Depropanizer has a total condenser; that is, all the column overhead vapor is condensed, and
the distillate product is completely liquid.
The reboiler vaporizes part of the liquid that leaves the bottom tray of the column. This
vapor, playing a role similar to that of the reflux, contacts the descending liquid and strips
the lighter components. The most common types of reboilers are thermosyphon and kettle.
In thermosyphon reboilers, the driving force for the circulation of the liquid is the difference
in hydrostatic head between the column of liquid feeding the reboiler and the column of
mixed liquid and vapor leaving the reboiler. In the kettle reboilers, only vapor returns to the
tower. The heat source of the reboiler may be steam, or a process fluid; the reboiler may also
be a gas- or fuel-fired furnace. Fired reboilers are generally forced circulation type, that is, a
pump is used to circulate the fluid through the furnace.

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AUXILIARY FACILITIES
Figure 3 is a more detailed process flow diagram of the Ras Tanura Plant 10 Depropanizer.
It provides more information on auxiliary facilities such as pumps, piping, valves,
instruments, and controls.
A typical distillation tower has pumps for feed, reflux/distillate, and, if required, bottoms. In
the depropanizer, bottoms pumps are not required because the downstream unit, the
debutanizer, is at a lower pressure.
Control valves are used to maintain stable rates (e.g., feed and reflux rates) and stable liquid
levels in the distillate and bottoms surge vessels. Instruments such as flow meters and
pressure, temperature, and level indicators are used to monitor and control the operation of
the tower. Instruments such as gas chromatography analyzers are used to monitor the
quality of separation by identifying the concentration of important components. In the
depropanizer, for example, an analyzer in the liquid distillate identifies the concentrations of
ethane, isobutane, and n-butane. These are components related to the propane product
specifications.
Various control schemes are used to achieve objectives such as stable feed rate, on-spec
products, low utility consumption, and stable operation. The Ras Tanura Plant 10
Depropanizer, for example uses the following three control schemes:

Feed temperature control to assure stable feed vaporization levels.

Reboiler steam rate control, related indirectly to the concentration of propane in the
bottoms via a tray temperature.
Tower pressure control which is necessary for the stable operation of the tower.

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10-C-1
Depropanizer
16'/24'-6" ID-99'-6"TT
DP = 380 psig
o
DT = 350 F

10-E-2A/B
Depropanizer
Reboiler
CAP. MM BTU/HR:
S: 200.0
W: 199.0

10-E-3
Depropanizer
Condenser
CAP. MM BTU/HR:
S: 140.1
W: 148.6

10-G-2 A/B/C
Deprop. Reflux Pump
5720 gpm
86 psi p
412 bhp.

10-D-3
Depropanizer
Reflux Drum
11'-0" ID x 32'-0" T-T
DP = 320 psig
DT = 370 oF
To FC at ADIP Unit
Dwg. 637118 Sh. 3

To ADIP Treating
Dwg. NA. 637118 Sh. 3
Chromatograph
C2, iC4, nC4
AR

Split Range

Condenser

134o F
S: 282 psia
W: 290 psia

FC
10-E-3

15 psig Steam

NNF

Depropanizer

To Fuel
Gas

TC
276 psia

Steam Heater

10-D-3

10-E-1
LC

Feed

Reflux Drum

To Sour Water
Stripper
in Plant 45

130 F
o

10-G-2
A/B/C
10-C-1
TC

Condensate
FC

S: 287 psia
W: 295 psia

10-E-2 A/B

150 psig Steam

Condensate

Feed From
Plants 25,45

Reboiler
FR

S : Summer Conditions
W: Winter Conditions
Data from Dwg. NA-637118. Sh. No. 8, Rev 1

o
276
F (S)
o
274
F (W)

To Debutanizer
10-C-2

Ras Tanura Plant 10 Depropanizer Process Flow Diagram

Figure 3

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CALCULATING VAPOR AND LIQUID COMPOSITIONS IN IDEAL MIXTURES

A vapor-liquid system is considered to be in equilibrium when there are no longer any
detectable changes occurring in the system. Generally, a system is assumed to be in
equilibrium when the mass, energy, and composition of each phase remain constant with time.
An example of a system in equilibrium is a mixture of water and air in a closed vessel. After
some time, there will be no change in temperature, in the amount of water in the vapor phase,
or in the number of gas molecules dissolved in the water. The system is in equilibrium.
Equilibrium also applies to systems that are not static. We may have equilibrium in an
overhead condenser separator of a distillation column. The vapor and liquid leaving the
separator are in equilibrium, and their compositions can be described by relationships for
systems in equilibrium.
Ideal and Nonideal Gases
Ideal gases are those whose behavior can be described by the ideal gas law, which is stated
mathematically as:
PV = nRT or

PV
= 1.0
nRT

The ideal gas law indicates that the product of pressure P times volume V is proportional to
the number of molecules of the component, n, times the absolute temperature T. R is an ideal
gas proportionality constant. The values of R in various units are given in Work Aid 1.
Gases tend to behave as ideal gases at temperatures higher than their critical temperature and
pressures well below their critical pressure.
Real-Gas Equations
If the ideal gas equation is applied to situations with elevated pressures, significant errors may
result. Deviations from the ideal gas law at high pressure can be attributed to the assumptions
inherent in the law's derivation, namely, that all molecules are hard spheres that do not
interact with one another and that occupy negligible volume. Therefore, the ideal gas law is
independent of the composition of the gas. For example, the ideal gas law implies that one
mole of any gas will occupy the same volume as one mole of any other gas at the same
temperature and pressure. In this sense, it implies that all gases are identical on a molar basis.
This assumption is not correct because different gases have radically different molecular and
chemical structures. As an example, take the specific volumes of hydrogen sulfide, propane,
and nitrogen at 400 psia and 180_F. From the ideal gas law and n = 1,
V = RT/P = [10.73 psia-ft3/lb-mole-R x (180 + 460)R]/400 psia

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= 17.17 ft3/lb-mole

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The experimental molar volumes for these three gases are as follows:
Component

Molar Volume

Propane
Hydrogen sulfide
Nitrogen

11.44 ft3/lb-mole
16.28 ft3/lb-mole
16.82 ft3/lb-mole

Thus, although the ideal gas law provides a qualitative measure of the behavior of gases, it
does not predict PVT behavior accurately for most gases and cannot be used for liquids.
The compressibility factor Z expresses the deviation from the ideal gas equation. It can be
used to predict real gas properties. The compressibility factor is the ratio of the real gas
volume to that of the ideal gas at the same temperature and pressure:
PV = ZnRT or PV = Z
nRT
For an ideal gas, the compressibility factor is 1.0. The compressibility factor Z can be
obtained from generalized graphs such as those in Maxwell, pages 148-153 or the GPSA
Engineering Data Book, Chapter 16.
Ideal Mixtures - Dalton's, Raoult's Laws
Ideal mixtures, gas or liquid, consist of components that do not interact with each other
chemically or physically. The concept of ideal mixtures has formed the basis for many
quantitative relationships describing equilibrium. Of particular interest are Dalton's law of
partial pressures and Raoult's law relating the pressure exerted by a component in the vapor
phase to its concentration in the liquid phase.
Dalton's law states that the total pressure of a mixture of gases is equal to the sum of the
partial pressures of the mixed gases. Thus,
PT = _PPi = PP1 + PP2 + PP3 + ...
Dalton also postulated that the partial pressure of an ideal gas in a gas mixture is proportional
to its mole fraction, that is, the relative number of molecules of that gas in the mixture. Thus,
PPi = yi PT

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Raoult's law, relating the partial pressure in the vapor phase to the liquid phase composition,
is expressed as:
PPi = xi VPi
Combining Dalton's and Raoult's laws results in an expression describing mixtures of ideal
vapors and liquids in equilibrium.
PT = _PPi = _yi PT = _xiVPi
and for component i,
yi = xi (VPi/PT)
Equilibrium K-Values
The definition of equilibrium K-value, also called K factor or distribution coefficient, of
component i in a mixture is given in the following equation:
Ki =

yi
xi

The K-value is simply the ratio of the vapor to the liquid mole fraction of i. This ratio has no
special thermodynamic significance, but has found extensive use in high-pressure VLE work.
For ideal systems where Raoult's law applies, it can be expressed as:
y VP
Ki = xi = i
PT
i
Equilibrium K values can be obtained from graphs or nomographs like the De Priester
nomograph, Figure 4. K values are a function of temperature and pressure. For nonideal
mixtures, K values are also a function of composition.

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De Priester Nomograph
Figure 4
"Light Hydrocarbon Vapor-Liquid Distribution Coefficients", C. L. De Priester,
Chemical Engineering Symposium Series Vol. 49, No. 7, pp 1-43 (1953)
Reproduced by permission of the American Institute of Chemical Engineers.

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Two Component Example

Let's assume that we have an ideal mixture of propane and n-butane at 100F. The vapor
pressures of the two components at 100F are:

Propane 13 atm = 191 psia (Component 1).

n-Butane 3.5 atm = 52 psia (Component 2).

The total pressure, PT, of the mixture can be calculated from,

PT = PP1 + PP2 = x1VP1 + x2VP2
PT = 191x1 + 52(1-x1) = 52 + 139x1
This last equation indicates that the total pressure of an ideal binary mixture is a linear
function of the composition. This relationship is illustrated in Figure 5, which shows that the
total pressure is the sum of partial pressures and is a straight line between the vapor pressure
of n-butane (x1 = 0) and propane (x1 = 1.0).
200

Vapor Pressure
of Propane, 191 psia

180
100o F
160
140
120
100
80
60
Vapor
Pressure
of n-Butane 40
52 psia
20
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mole Fraction Propane in Liquid, x 1

Ideal Mixtures
Propane-n-Butane System Pressure
Figure 5

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Mixtures Approximated as Ideal

The mixtures that can be approximated as ideal must satisfy the following requirements:

Total pressure of the system must be below 200 psia.

The components must be chemically similar, for example, butane and pentane, both
paraffins. A mixture of an aromatic component and a paraffin such as benzene and
hexane cannot be approximated as ideal.
The components must be close boiling, that is, they must have similar boiling points.
The system pressure and temperature must not be near the critical pressure and
temperature of the mixture.

Using ideal mixture correlations in calculations results in approximate compositions or

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Equilibrium Diagram
Figure 6 depicts a simple flash separation. The feed consists of two components, propane and
n-butane. The feed temperature and composition vary. The table in Figure 6 lists vapor and
liquid concentrations of propane and distribution coefficients (K1 and K2) for propane and nbutane, of the two components for five temperatures. Pressure is fixed at 100 psia.
At 70F, the mole fraction of propane in the liquid phase is 0.746. Its mole fraction in the
vapor phase is higher, 0.907, since propane is the more volatile of the two components. The
distribution coefficient K1 for propane is equal to the ratio y1/x1 = 0.907/0.746 = 1.22.
As the temperature increases, the K values increase by a factor greater than two. From 70F
to 140F, the value of the relative volatility, however, changes by only about 25%. The small
effect of temperature on relative volatility is the reason for using relative volatility in shortcut
distillation calculations. Relative volatility data for only two or three points in the column
provide results of acceptable accuracy.

F
V

x
1

K
x

1
1

Vapor-Liquid Equilibrium
Figure 6

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Figure 7 is an equilibrium diagram for the propane/n-butane system using the data from
Figure 6 at 100 psia. The horizontal axis indicates the mole fraction of the more volatile
component, propane, in the liquid phase. The vertical axis indicates its mole fraction in the
vapor phase. The equilibrium line connects all the (x1, y1) points. Given the mole fraction in
the liquid phase, the equilibrium line can be used to obtain the mole fraction in the vapor
phase. Given the mole fraction in the vapor phase, the mole fraction in the liquid phase can
be found.
Propane - n-Butane
1.0
C3 - nC4 at 100 psia

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mole Fraction Propane in Liquid, x1

Equilibrium Diagram
Figure 7
Figure 7 contains a second line, the reference line. It is simply the diagonal of the diagram:
for all the points on the reference line, x = y. The reference line makes it easier to see the
differences between the vapor and the liquid phase compositions. Since by convention the
horizontal axis represents the composition of the more volatile component, y1 is larger than
x1. Therefore, the equilibrium line is above the reference line. Large differences in y1, x1
mole fractions indicate large differences in the volatility of the two components.
Accordingly, equilibrium lines bulging away from the reference line are indicative of
mixtures that are easy to separate by successive vaporization and condensation steps, that is,
by multistage distillation.
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NOMENCLATURE
C1, C2 Hydrocarbons with 1, 2 number of carbons
K

Distribution coefficient (K = y/x), also called K factor or equilibrium K.

Liquid rate, mole/hr

Next to a hydrocarbon name, it indicates a normal (paraffin) isomer

Number of moles

Pressure, absolute

Partial pressure

Total pressure

Gas constant. For values, see Figure 9.

Temperature, absolute

Boiling point

Vapor rate, mole/hr

Volume

mole fraction in the liquid phase

Mole fraction in the vapor phase

Compressibility factor

Subscripts
1,2

Value refers to component 1, 2, ... or measurement 1, 2, ...

Value refers to component i

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WORK AID 1: FORMULAS AND GUIDELINES FOR CALCULATING VAPOR

AND LIQUID COMPOSITIONS IN IDEAL MIXTURES
This Work Aid is designed to assist the Participant in calculating vapor and liquid
compositions in ideal mixtures.
Formulas
Ideal Gas Law
PV = nRT
where P
V
n
R
T

=
=
=
=
=

the absolute pressure (psia)

the volumetric rate (ft3/hr)
the molar rate (lb-mole)/hr
the ideal gas constant (see Figure 9)
the absolute temperature (R), T (R) = T (R) + 459.7

Dalton's Law
PPi = yi PT
Ideal Mixture Relationship
yi = xi (VPi/PT)
Guidelines
a.

Use the Ideal Gas Law to find the feed and vapor molar rates (n = PV/RT). Select the
value of R (see Figure 9) that corresponds to the appropriate P, V, T units, e.g., (psia)
(ft3) / (lb-mole) (R). Calculate the liquid molar rate by subtracting the feed molar rate
from the vapor molar rate.

Calculate the partial pressure of each component in the vapor phase by using Dalton's
Law.

Solve for the Ideal Mixture Relationship for xi for both propane and n-butane.

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Ideal Gas Constant

Figure 9

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GLOSSARY
15/5 distillation

A standard distillation analysis using a column with 15 trays

and a 5:1 reflux ratio.

absolute pressure

Pressure measured with respect to zero pressure, as distinct

from pressure measured with respect to some standard
pressure.

API gravity

An arbitrary scale expressing the gravity or density of liquid

petroleum products. The measuring scale is calibrated in
degrees API. It is calculated by the following formula:
Deg API =

141.5
- 131.5
Sp Gr 60F/60F

assay

A procedure for determining the distillation characteristics and

other properties of a crude oil.

ASTM

American Society for Testing and Materials. Organization

standardizes specifications and methods of testing for
engineering materials. Most of the data that describe, identify,
or specify petroleum products are determined in accordance
with ASTM test methods.

ASTM distillation

A distillation made in accordance with an ASTM distillation

procedure.

average boiling point

For component mixtures see definitions in text. For

hydrocarbon fractions, unless otherwise indicated it is the sum
of the ASTM distillation temperatures from the 10% point to
the 90% point, inclusive, divided by 9. Sometimes half the
initial and half the maximum distillation temperatures are also
added, and the sum is then divided by 10.

azeotrope

Liquid mixture of two or more components that boils at a

temperature either higher or lower than the boiling point of any
of the individual components. In refining, if the components of
a solution are very close in boiling point and cannot be
separated by conventional distillation, a substance can be
added that forms an azeotrope with one component, modifying
its boiling point and making it separable by distillation.

barrel

Standard unit of measurement in the petroleum industry,

equivalent to 42 standard U.S. gallons.

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binary distillation

Distillation of a mixture containing two components.

boiling range

The range of temperature, usually determined at atmospheric

pressure in standard laboratory apparatus, over which the
boiling or distillation of an oil begins, proceeds, and finishes.

bottoms

The bottom (heavy) product of a distillation column. A

synonymous term is residue.

Btu

British thermal unit. The quantity of heat required to raise the

temperature of one pound of water one degree Fahrenheit, at
60F and at a pressure of 1 atmosphere.

bubble point

The temperature and pressure at which a liquid is in

equilibrium with an infinitesimal amount of vapor.

bubble point curve

The temperature and pressure conditions at which an

infinitesimal amount of vapor (first bubble of vapor) is in
equilibrium with vapor. For a pure component, this curve is
the same as the vapor pressure curve.

butane

Gaseous paraffinic hydrocarbon (C4H10), usually a mixture of

iso- and normal butane. Also called, along with propane,
liquefied petroleum gas (LPG).

cetane

Colorless liquid hydrocarbon, C15H34, used as a standard in

determining diesel fuel ignition performance. See Cetane
Number.

cetane number

Measure of the ignition quality of a diesel fuel, expressed as

the percentage of cetane that must be mixed with liquid
methylnaphthalene to produce the same ignition performance
as the diesel fuel being rated, as determined by the test method
ASTM D 613. A high cetane number indicates shorter ignition
lag and a cleaner burning fuel.

characterization factor

Factor that expresses variations in physical properties with

change in character of the stock. The ratio of the cube root of
the molal average boiling point, TB, in degrees Rankine (R =
F + 460), to the specific gravity at 60F/60F:
3

Kw = TB / Sp Gr
It ranges from 12.5 for paraffinic stocks to 10.0 for aromatic
stocks. Also called Watson factor or Watson K or UOP K.

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chromatography (gas)

A method of separation based on selective adsorption capable

of identifying individual compounds. An analytical technique
for separating mixtures of volatile substances. The mixture is
inserted into the chromatographic column and washed down
with an inert gas. The column is packed with absorbent
materials that selectively retard the components of the sample.

cloud point

Temperature at which a cloud or haze of wax crystals appears

at the bottom of a sample of lubricating oil in a test jar, when
cooled under conditions prescribed by test method ASTM D
2500. Cloud point is an indicator of the tendency of an oil to
plug filters or small orifices at cold operating temperatures.

column

A vertical vessel containing contacting devices such as trays or

packing, used to perform separations such as distillation or
extraction. A synonymous term is tower.

compressibility factor

The ratio of the actual volume occupied by a vapor to the

theoretical volume occupied by the same quantity of vapor
under identical conditions of temperature and pressure.

condenser

A cooler that condenses all (total condenser) or part (partial

condenser) of the overhead vapor of a column.

condenser separator

A vessel that separates any vapor distillate from the liquid and
provides surge capacity for the reflux and the distillate.

countercurrent flow

A system in which one fluid flows in one direction and another

fluid flows in the opposite direction.

critical point

See Critical State.

critical pressure

The pressure necessary to condense a gas at the critical

temperature. Above the critical temperature the gas cannot be
liquefied, no matter what pressure is applied.

critical state

The pressure and temperature at which liquid or gaseous

phases reverse at the slightest change in conditions.

critical temperature

The maximum temperature at which a gas can be liquefied by

pressure (critical pressure); above this temperature the gas
cannot be liquefied, no matter what pressure is applied.

crude oil

Unrefined or unprocessed liquid hydrocarbons.

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cut

A fraction obtained by a separation process. (See also

Fractional Distillation.)

dew-point

The temperature and pressure at which a vapor is in

equilibrium with an infinitesimal amount of liquid.

dew-point curve

The temperature and pressure conditions at which an

infinitesimal amount of liquid (first drop of liquid) is in
equilibrium with vapor. For a pure component, this curve is
the same as the vapor pressure curve.

DGA

Diglycolamine. An amine used in the removal of H2S and CO2

from gases.

diesel fuel

The portion of crude oil that distills out in the temperature

range 392F to 698F. Diesel fuel is close in boiling range and
composition to lighter heating oils.

distillate

The overhead (light) product of a distillation column. It may

be vapor, liquid, or both.

distillation

The separation of the constituents of a liquid mixture by partial

vaporization of the mixture, followed by separate recovery of
the vapor and liquid residue.

distillation curve

Curve plotting the percentage of petroleum products distilled

versus the temperature.

distillation test

Methods for determining the volatility characteristics of a

hydrocarbon liquid by progressively boiling off a sample under
controlled heating. If the boiling range is small, the fluid is
narrow cut, that is, having components with similar volatilities.
If the boiling range is wide, the fluid is wide cut.

dry point

In a distillation test, the temperature at which the last drop of

petroleum fluid evaporates.

end point (EP)

In the distillation of liquids, the maximum temperature that

occurs during the test. Also called final boiling point, FBP.

enriching section

The section of the distillation column above the feed.

equilibrium

The state of a system under a constant environment when the

intensive properties remain unchanged with time. Net fluxes of
mass, energy, and chemical reactions in the system are zero.

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flash point

Lowest temperature at which the vapor of a combustible liquid

can be made to ignite momentarily in air, as distinct from fire
point. Flash point is an important indicator of the fire and
explosion hazards associated with a petroleum product. There
are a number of ASTM tests for flash point, e.g., Cleveland
open cup, Pensky-Martens closed tester, Tag closed tester, Tag
open cup.

flooding

Overloading of the tray interspace with liquid.

fractionation

Distillation in which the vapor is contacted continuously and

countercurrently with a condensed portion of the vapors.

freezing point

A specific temperature that can be defined in two ways,

depending on the ASTM test used. In ASTM D 1015, which
measures the freezing point of high-purity petroleum products
(such as nitration-grade toluene), freezing point is the
temperature at which a liquid solidifies. In ASTM D 2386,
which measures the freezing point of aviation fuel, freezing
point is that temperature at which hydrocarbon crystals formed
on cooling disappear when the temperature of the fuel is
allowed to rise.

fuel oil

Term encompassing a broad range of distillate and residual

fuels identified by ASTM grades 1 though 6.

fugacity

The tendency of a substance to escape or disappear from the

phase in which it is present.

gas chromatography

An analytical technique that can identify concentrations of

components in a gas or vaporized liquid.

gauge pressure

The pressure as shown by a pressure-registering instrument

(gauge). The gauge pressure, in pounds per square inch, is
approximately equal to the absolute pressure minus 14.7.

GCD

Gas Chromatography Distillation. A chromatographic

technique that produces results approximating 15/5
distillations.

grids

Countercurrent contacting devices fabricated in panels and

installed in an ordered manner. In contrast to structured
packing, grids provide wide clearances. See the figures in the
text.

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heavy key

A heavy component that must be recovered with the residue

and is present in the distillate in important amounts.

heterogeneous system

One in which intensive properties are uniform from point to

point and thereby constitute a single phase.

initial boiling point (IBP)

In a distillation test, the fluid temperature at which the first

drop falls into a graduated cylinder.

jet fuel

Fuel meeting the required properties for use in jet engines and
aircraft turbine engines.

kerosene

Relatively colorless light distillate.

kettle reboiler

A type of reboiler acting as a vaporizer and a separator. The

kettle reboiler produces a vapor stream that is sent to the tower
and a liquid stream that is in equilibrium with the vapor. The
liquid is the tower bottom product.

light ends

Low-boiling-point hydrocarbons having up to five carbon

atoms; including butanes, butenes, pentanes, pentenes. Also,
any extraneous low-boiling fraction in a refinery process
stream.

light key

A light component that must be recovered with the distillate

and is present in the residue in important amounts.

mid-boiling point (MBP)

In a distillation test, the temperature at which 50% of the fluid

has collected in the cylinder.

middle distillate

One of the distillates obtained between kerosene and

lubricating oil fractions in the refining process. Includes light
fuel oils and diesel fuel.

multicomponent
distillation

Distillation of a mixture containing more than two components.

naphtha

Generic, loosely defined term covering a range of light

petroleum distillates with boiling range of 90 to 475F.
Includes gasoline blending stocks, mineral spirits, and a broad
selection of petroleum solvents.

NGL

Natural gas liquids.

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normal boiling point

Boiling point is the temperature at which a substance boils. It

varies with pressure. Normal boiling point is the boiling point
at atmospheric pressure.

overhead

The vapor leaving the top of the column.

overhead product

The top product of a distillation column. It may be vapor,

liquid or both.

overlap

In adjacent fractions, the temperature interval between the

initial boiling point of the higher boiling fractions and the end
point of the lower boiling fractions.

packing

Devices that provide countercurrent vapor-liquid contact in

distillation columns.

partial condenser

A condenser that condenses part of the vapor.

phase

A homogeneous portion of a system. A gas or a mixture of

gases, a liquid or a liquid solution, and a solid are examples of
phases.

plates

See stages.

pour point

The lowest temperature at which oil will pour or flow when it

is chilled without disturbance under definite conditions. These
conditions are prescribed in ASTM Method D 97.

reboiler

A heater vaporizing part of the liquid leaving the bottom of the

distillation column. The vapor that returns to the column
provides the stripping action in the bottom section.

recirculating reboiler

A type of reboiler that sends both the vapor and liquid phases
to the distillation tower. Recirculating reboilers operate either
by natural circulation (thermosyphon) or forced circulation.

rectifying section

The section of the distillation column above the feed.

reflux

Condensed overhead vapor that is returned to the top tray of

the distillation column.

Reid Vapor Pressure

(RVP)

A measure of the vapor pressure of a sample of gasoline at

100F. The results are reported in pounds. This test is usually
carried out in accordance with ASTM Method D 323.

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residue

The bottom (heavy) product of a distillation column. A

synonymous term is bottoms.

stages

Contact points of the vapor and liquid in a column, such as on

column trays. The term theoretical stage is used to indicate that
equilibrium is reached at the contact point between the vapor
and the liquid. The actual stages reflect the obtained tray
efficiency. A synonymous term is plates.

stripping section

The section of the distillation column below the feed.

TBP

True Boiling Point of a component.

total condenser

A condenser that condenses all the vapor.

tower

See column.

trays

Horizontal devices providing crossflow vapor-liquid contact in

distillation columns.

weeping

Undesirable liquid flow through the tray openings (sieve holes,

valves, etc.).

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ADDENDUM A
Introduction
In distillation, the separation of a mixture of materials to obtain one or more desired products
is achieved through a series of countercurrent vapor and liquid contacts called stages. In each
stage, a difference in the relative concentration of hte components in the two phases is
attained. When the two phases are in equilibrium, the difference in concentrations between
the two phases is at its maximum; therefore, it is desirable to reach equilibrium in each stage.
A stage in which equilibrium is reached is called a theoretical stage. Because equilibrium
represents a theoretical boundary that is universal and easy to define, most design methods are
based on calculations using theoretical stages. The deviation from equilibrium is then
considered by including a stage efficiency when the equivalent actual stages are calculated.
This module examines the concept of vapor-liquid equilibrium (VLE) and the basic
relationships that apply to VLE. The module provides guidance for selecting BLE methods in
computer simulations. It also includes a brief review of physical properties related to
distallation and distillation processes.
Vapor-Liquid Equilibrium (VLE) Relationships
A vapor-liquid system is considered to be in equilibrium when there are no longer any
detectable changes occurring in the system. Generally, a system is assumed to be in
equilibrium when the mass, energy, and composition of each phase remain constant with time.
An example of a system in equilibrium is a mixture of water and air in a closed vessel. After
some time, there will be no change in temperature, in the amount of water in the vapor phase,
or in the number of gas molecules dissolved in the water. The system is in equilibrium.
Equilibrium also applies to systems that are not static. We may have equilibrium in an
overhead condenser separator of a distillation column. The vapor and liquid leaving the
separator are in equilibrium, and their compositions can be described by relationships for
systems in equilibrium.
Ideal and Nonideal Gases
Ideal gases are those whose behavior can be described by the ideal gas law, which is stated
mathematically as:
PV = nRT or PV = 1.0
nRT
The ideal gas law indicates that the product of pressure P times volume V is proportional to
the number of molecules of the component times the absolute temperature T. R is an ideal
gas proportionality constant. The values of R in various units are given in Work Aid 1.

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Gases tend to behave as ideal gases at temperatures higher than their critical temperature and
pressures well below their critical pressure.
Vapor Pressure
The vapor pressure of a pure component at a given temperature is the pressure that is exerted
by the component when it is in the liquid phase. Vapor pressure is a unique property, and it is
a direct function of temperature. A material having a higher vapor pressure at the same
temperature than another is said to be more volatile.
Vapor pressure and temperature are often related by means of the Antoine equation:
Log (VP) =

A-

B
T+C

where A, B, C are constants for a particular compound over a relatively narrow temperature
range, usually not over 100C. Values of these constants for various compounds and the
temperature ranges for which the constants apply appear in a number of references. The
Antoine equation is often plotted in charts with the horizontal axis in a reverse absolute
temperature scale and a vertical axis in a logarithmic scale. Vapor pressures for various
components can be obtained from Maxwell, Data Book on Hydrocarbons, Section 4. An
example for propane and propylene is shown in Figure 10.

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Vapor Pressure of Propane And Propylene

Figure 10

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Ideal Mixtures - Dalton's, Raoult's Laws

Ideal mixtures, gas or liquid, consist of components that do not interact with each other
chemically or physically. The concept of ideal mixtures has formed the basis for many
quantitative relationships describing equilibrium. Of particular interest are Dalton's law of
partial pressures and Raoult's law relating the pressure exerted by a component in the vapor
phase to its concentration in the liquid phase.
Dalton's law states that the total pressure of a mixture of gases is equal to the sum of the
partial pressures of the mixed gases. Thus,
PT = PPi = PP1 + PP2 + PP3 + ...
Dalton also postulated that the partial pressure of an ideal gas in a gas mixture is proportional
to its mole fraction, that is, the relative number of molecules of that gas in the mixture. Thus,
PPi = yi PT
Raoult's law, relating the partial pressure in the vapor phase to the liquid phase composition,
is expressed as:
PPi = xi VPi
Combining Dalton's and Raoult's laws results in an expression describing mixtures of ideal
vapors and liquids in equilibrium.
PT = PPi = yi PT = xi VPi
and for component i,
yi = xi (VPi/PT)
Two Component Example - Let's assume that we have an ideal mixture of propane and nbutane at 100F. The vapor pressures of the two components at 100F (Maxwell, pages 29
and 30) are:

Propane 13 atm = 191 psia (Component 1).

n-Butane 3.5 atm = 52 psia (Component 2).

The total pressure, PT, of the mixture can be calculated from,

PT = PP1 + PP2 = x1VP1 + x2VP2
PT = 191x1 + 52(1-x1) = 52 + 139x1

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This last equation indicates that the total pressure of an ideal binary mixture is a linear
function of the composition. This relationship is illustrated in Figure 11, which shows that the
total pressure is the sum of partial pressures and is a straight line between the vapor pressure
of n-butane
(x1 = 0) and propane (x1 = 1.0).

Ideal Mixtures
Propane - N-Butane System Pressure
Figure 11

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Mixtures Approximated as Ideal - The mixtures that can be approximated as ideal must
satisfy the following requirements:

Total pressure of the system must be below 200 psia.

Using ideal mixture correlations in calculations results in approximate compositions or

conditions (P, T). The error may be acceptable for a simple operation, such as a flash drum
separation. The same correlations used in a superfractionator, where tray-to-tray calculations
compound the error, may produce unacceptable results.
Real-Gas Equations
If the ideal gas equation is applied to situations with elevated pressures, significant errors may
result. Deviations from the ideal gas law at high pressure can be attributed to the assumptions
inherent in the law's derivation, namely, that all molecules are hard spheres that do not
interact with one another and that occupy negligible volume. Therefore, the ideal gas law is
independent of the composition of the gas. For example, the ideal gas law implies that one
mole of any gas will occupy the same volume as one mole of any other gas at the same
temperature and pressure. In this sense, it implies that all gases are identical on a molar basis.
This assumption is not correct because different gases have radically different molecular and
chemical structures. As an example, take the specific volumes of hydrogen sulfide, propane,
and nitrogen at 400 psia and 180F. From the ideal gas law and n = 1,
V = RT/P = [10.73 psia-ft3/lb-mole-R x (180 + 460)R]/400 psia
= 17.17 ft3/lb-mole

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The experimental molar volumes for these three gases are as follows:
Component

Molar Volume

Propane
Hydrogen sulfide
Nitrogen

11.44 ft3/lb-mole
16.28 ft3/lb-mole
16.82 ft3/lb-mole

where fPP i
=
fVP i
=
fP T

Fugacity of i in either phase of the system.

Fugacity of i as a pure saturated liquid (or vapor) at its vapor pressure
corresponding to the equilibrium temperature of the system.
Fugacity of i as a pure vapor at the equilibrium temperature and total
pressure of the system.

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Generalized correlations have been developed for the ratio of fugacity to pressure for pure
hydrocarbons as a function of reduced temperature and reduced pressure. A correlation of
this type was used in conjunction with the vapor pressure charts to develop the fugacity
function charts for individual hydrocarbons. The fugacity function given by these charts is
defined as:
Fi = fVPi PT / fPT = PT yi /xi
The fugacity function Fi may be considered a corrected vapor pressure and used in place of
vapor pressure in any equation pertaining to liquid-vapor equilibrium.
Values for fugacity functions can be obtained from Maxwell, Section 5. An example for
propane is reproduced in Figure 12. Fugacities for petroleum fractions can be obtained from
a generalized graph on pages 62-63 of Maxwell, which uses the reduced pressure and
temperature of the mixture.
The values obtained from the fugacity graphs in Maxwell provide a correction for pressure
and temperature. They do not take into account interactions between components in the vapor
or liquid phase; in other words, they assume ideal mixtures.
The simple fugacity relations greatly extend the pressure range for which liquid-vapor
equilibria for hydrocarbon systems may be predicted with confidence; they can be used up to
equilibrium pressures of 20 to 25 atm with a fair degree of accuracy. Beyond these pressures
and especially as the critical point of the mixture is approached, serious deviations from true
equilibrium conditions are encountered. Under these circumstances, the assumptions of ideal
mixtures no longer hold, and the fugacities of the individual compounds depend upon the
compositions of the liquid and vapor phases as well as temperature and pressure.
If other gases such as air, H2, and CO2, are present in the vapor phase, in addition to
hydrocarbon vapors, an effective pressure should be used in determining the fugacities of
individual hydrocarbons. The effective pressure is equal to the total pressure multiplied by
the square root of the mole fraction of the entire hydrocarbon portion of the vapor, or
Peff = PT yHC

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Fugacity Function of Propane

Figure 12

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Equilibrium K-Values
The definition of equilibrium K-value, also called K factor or distribution coefficient, of
component i in a mixture is given in the following equation:
Ki =

yi
xi

yi VPi
xi = PT

It can also be expressed in terms of fugacities as:

Ki =

yi fVPi
=
xi fP
T

Equilibrium K values can be obtained from graphs or nomographs like the De Priester
nomograph, Figure 13. K values are a function of temperature and pressure. For nonideal
mixtures, K values are also a function of composition.
Relative Volatility
Relative volatility is a relation widely used in distillation. It is defined by:
ij =

yi /xi Ki
=
yj /xj Kj

Relative volatility is a measure of separability. The larger the value of ij, the easier the
separation. For close boiling components, such as pentane and isopentane, the relative
volatility approaches 1.0.
Because the value of relatively volatility is not as sensitive to temperature as other measures
of equilibrium, it is used in a number of shortcut distillation calculations. Relative volatility
graphs are available in Maxwell, Pages 64-66. For ideal mixtures (Raoult's law applies), the
relative volatility of two components is equal to the ratio of their vapor pressures.
ij = VPi
VPj

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De Priester Nomograph
Figure 13
"Light Hydrocarbon Vapor-Liquid Distribution Coefficients", C. L. De Priester,
Chemical Engineering Symposium Series Vol. 49, No. 7, pp 1-43 (1953)
Reproduced by permission of the American Institute of Chemical Engineers.

Nonideal Liquids
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In liquids and liquid mixtures, the distances between molecules are much smaller than in
gases, and the forces attracting molecules to each other are much greater. Nonideal behavior
of liquids is indicated by heat of mixing and nonadditivity of volumes when two liquids are
mixed. The deviation from ideality is greater for chemically dissimilar substances. The
activity coefficient, , measures the deviation from ideal liquid solution behavior. Using the
coefficient in Raoult's law results in:
yi PT = PPi = ixi VPi
Generally, is greater than 1.0. For very dissimilar systems, such as hydrocarbons and water,
can be much greater than 1.0, in the order of 1000. There are cases where two components
attract each other, leading to < 1.0
Activity coefficients are used in a number of VLE methods such as the Chao-Seader and the
Grayson-Streed correlations. The Chao-Seader correlation requires relatively short
computing times. It was used extensively in the '60s and '70s when computing was costly.
Hydrocarbon VLE methods using activity coefficients have been replaced by the more
rigorous equations of state.
Equations Of State
Equations of State (EOS) predict the PVT behavior of gases and liquids. The simplest
equation of state is the one for ideal gases (per mole).
P = RT/V
In general, real fluids deviate from ideal fluids in two ways: there are variations in the sizes
and shapes of the molecules, and specific interactions between molecules, such as polarity or
hydrogen bonding, must be considered. The large variations in size and shape of molecules
have a great effect on PVT behavior.
The Soave-Redlich-Kwong (SRK) and the Peng-Robinson (PR) equations of state are among
the best known.

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SRK Equation of State - The Soave-Redlich-Kwong equation of state is a two-parameter

equation of the following form:
a
P = RT V-b V(V+b)
where:
a = (1 - cij) ( ai aj ) (xi xj)
i

b = xi bi
i

The parameters a and b must be specified for each component in a mixture and then combined
as a function of composition. The a parameter is temperature-dependent. In addition, a
binary interaction parameter cij is used to calculate the aij term for mixtures, to improve
vapor-liquid equilibrium calculations.
Peng-Robinson Equation of State - The Peng-Robinson equation is similar to the SRK
equation of state, except that it has an expanded volume term:
a
P = RT V-b V(V+b) +b (V-b)
The a parameter varies with temperature. Both constants use the same mixing equations as
the SRK equation of state.

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VLE Calculations
Equilibrium Diagram
Figure 14 depicts a simple flash separation. The feed consists of two components, propane
and n-butane. The feed temperature and composition vary. The table in Figure 14 lists vapor
and liquid concentrations of propane, distribution coefficients (K1 and K2) for propane and nbutane, and the relative volatility of the two components for five temperatures. Pressure is
fixed at 100 psia.
At 70F, the mole fraction of propane in the liquid phase is 0.746. Its mole fraction in the
vapor phase is higher, 0.907, since propane is the more volatile of the two components. The
distribution coefficient K1 for propane is equal to the ratio y1/x1 = 0.907/0.746 = 1.22. The
relative volatility 12 = K1/K2 = 3.31.
As the temperature increases, the K values increase by a factor greater than two. From 70F
to 140F, the value of the relative volatility, however, changes by only about 25%. The small
effect of temperature on relative volatility is the reason for using relative volatility in shortcut
distillation calculations. Relative volatility data for only two or three points in the column
provide results of acceptable accuracy.

V , y1

Comp 1 = C 3
P = 100 psia
Comp 2 = nC4

L,x 1
x 1

Temp, F

K1= y1/ x1

K2= y2/ x2

0.746

0.907

1.22

0.37

0.607

0.832

1.37

0.43

100

0.376

0.644

1.71

0.57

120

0.191

0.398

2.09

0.74

140

0.035

0.087

2.49

0.95

Vapor-Liquid Equilibrium
Figure 14

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Figure 15 is an equilibrium diagram for the propane/n-butane system using the data from
Figure 14 at 100 psia. The horizontal axis indicates the mole fraction of the more volatile
component, propane, in the liquid phase. The vertical axis indicates its mole fraction in the
vapor phase. The equilibrium line connects all the (x1, y1) points. Given the mole fraction in
the liquid phase, the equilibrium line can be used to obtain the mole fraction in the vapor
phase. Given the mole fraction in the vapor phase, the mole fraction in the liquid phase can
be found.

Equilibrium Diagram
Figure 15
Figure 6 contains a second line, the reference line. It is simply the diagonal of the diagram:
for all the points on the reference line, x = y. The reference line makes it easier to see the
differences between the vapor and the liquid phase compositions. Since by convention the
horizontal axis represents the composition of the more volatile component, y1 is larger than
x1. Therefore, the equilibrium line is above the reference line. Large differences in y1, x1
mole fractions indicate large differences in the volatility of the two components.
Accordingly, equilibrium lines bulging away from the reference line are indicative of
mixtures that are easy to separate by successive vaporization and condensation steps, that is,
by multistage distillation.

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For some mixtures, there is a reversal in relative volatilities and the equilibrium line intersects
the
x = y reference line. Because the vapor and liquid fractions at that point are equal, these
mixtures cannot be separated by distillation. Such mixtures are called azeotropes.
Vapor-Liquid Phase Diagrams
Phase diagrams are used to describe two-phase systems by plotting two of the three
independent variables (composition, temperature, and pressure) at a constant value of the third
variable. Figure 16 is a phase diagram at constant pressure for the binary mixture of propane
and n-butane. The two lines indicate the temperatures at which a phase change takes place.
The temperatures and concentrations (at the diagram pressure) below the two lines correspond
to an all-liquid mixture. In the region between the two lines, the vapor and liquid phases are
present. Above the lines there is only a vapor phase.
The phase lines in Figure 16 were drawn from data in Figure 14. For example, at 120F, the
point on the liquid phase line corresponds to x1 = 0.191 and the point on the vapor line to y1
= 0.398 (see Figure 14, 120F, x1, y1 data). The phase diagram can be used to determine the
compositions of the vapor and liquid phases from the pressure and temperature at equilibrium.

Phase Diagram
Figure 16

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Bubble Point and Dew Point

The phase diagram can also be used to determine the phase transition points. Figure 17 is an
example for a mixture of 45% propane and 55% n-butane at 70F. The phase diagram in
Figure 8 indicates that the mixture is in the liquid phase. If the temperature is increased at
constant pressure, 100 psia, the mixture will be liquid to 92F, at which point vaporization
begins. This is the bubble point of the mixture, the temperature and pressure at which a liquid
is in equilibrium with an infinitesimal amount of vapor.

Bubble Point and Dew Point

Figure 17
Between 92F and 117F the mixture is in two phases, vapor and liquid. At 117F, all the
liquid is vaporized. This is the dew point, the temperature and pressure at which vapor is in
equilibrium with an infinitesimal amount of liquid. At temperatures above the dew point, this
is only a vapor phase.
The liquid phase line is the bubble point curve; the vapor phase line is the dew point curve.

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Equilibrium Flash Separation

The equilibrium flash separator is the simplest equilibrium process for engineers to consider.
The process involves the separation of a two-phase feed into vapor and liquid in a vessel. The
feed is at a desired temperature and pressure, achieved by heating, cooling, pumping, or
letting down with a control valve.
Calculations of the compositions and the relative amounts of the liquid and vapor phases at
any given pressure and temperature involve a tedious trial-and-error solution. Since flash
calculations can be performed easily by computer, manual methods for multicomponent flash
calculations are not discussed here. Instead, phase and equilibrium diagrams will be used in a
binary system to demonstrate and reinforce the concept of equilibrium.
One-Stage Flash - Figure 18 shows an equilibrium separation. A propane and n-butane
vapor mixture from a distillation column is cooled to 120F at 100 psia. The vapor and liquid
are separated and the vapor is condensed and collected in a second drum. Calculations are
done to find the composition of the liquid in the two drums and the minimum cooling required
to condense the vapor leaving the first drum. For simplicity, assume that the entire system is
at 100 psia. The system is a one-stage flash. The second drum merely collects the condensed
liquid. It is not an equilibrium stage.

One-Stage Flash
Figure 18

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The required compositions can be determined by using a phase diagram for the propane/nbutane system at 100 psia. The liquid in the flash drum is represented by a point
(T, x1) = (120F, 0.19) on the bubble point curve of the phase diagram at 120F (Figure 19).
Similarly, the vapor is represented by a point on the dew point curve at 120F (T, y1) =
(120F, 0.4). Thus, the propane mole fractions in the liquid and vapor phases are 0.19 and
0.4, respectively.

One-Stage Flash on a Phase Diagram

Figure 19
The minimum cooling required to condense the vapor leaving the first drum corresponds to its
bubble point temperature. This is the maximum temperature at which all the vapor from the
first drum can be condensed. Since the vapor from the first drum and the liquid from the
second drum have the same composition, the bubble point can be located by drawing a
vertical line between the dew point and the bubble point curves. The temperature obtained,
96F, is the minimum temperature required to condense the vapor from the first drum.

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The equilibrium in the flash drum is represented in the equilibrium diagram by a point, y1, x1
= 0.4, 0.19 (Figure 20). Condensation in the second drum is represented by a horizontal line
from y1, x1, to the reference line, y1 x'1, where x'1 is the mole fraction of the liquid of the first
drum, which is equal to y1. The equilibrium diagram does not provide temperature
information; therefore, it cannot be used to determine the equilibrium concentrations or the
temperature of the second drum. If the composition of one phase is known, however, it can
be used to determine the composition of the other phase.

One-Stage Flash on An Equilibrium Diagram

Figure 20
Two-Stage Flash - Figure 21 illustrates a two-stage flash. The vapor from the first drum is
partially condensed in a second drum at 105F. The vapor from the second drum is totally
condensed, and the liquid is collected in a third drum. Pressure is constant at 100 psia.

Two-Stage Flash
Figure 21

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Representing the operation in a phase diagram (Figure 22) is similar to the one-stage flash.
Compositions and temperatures can be determined from the diagram.

Two-Stage Flash on A Phase Diagram

Figure 22
Figure 23 represents the two-phase flash on an equilibrium diagram. As with the diagram for
one-stage flash, compositions of one of the phases in each equilibrium are required.

Two-Stage Flash On An Equilibrium Diagram

Figure 23

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Binary Flash Rates - The vapor and liquid compositions and rates for a binary system (see
Figure 24) at a given pressure and temperature, can be determined from the following
equations:
x1 = 1 - K2
K1 - K2
y1 = K1 K2-K1
K2 - K1
z (K - K2) / (1 - K2) -1
V= 1 1
F
K1 - 1
where K1, K2 are the distribution coefficients of the two components, V is the mole vapor
rate, F is the mole feed rate, and z1 is the concentration of component 1 in the feed.

Flash Separation
Figure 24

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Bubble and Dew Point Calculations

Bubble point and dew point calculations are performed when the liquid/vapor composition is
known, the temperature or pressure is given, and the corresponding pressure or temperature
needs to be determined.

At the bubble point:

xi = zi, therefore,
yi = xiKi = ziKi = 1.0

The equation to be satisfied is:

zi Ki = 1.0

At the dew point:

yi = zi, therefore,
xi = yi /Ki = zi /Ki = 1.0

The equation to be satisfied is:

zi/Ki = 1.0
Finding the bubble point and the dew point for a multicomponent system involves tedious
trail-and-error calculations. It is recommended that PRO/IITM or HYSIM be used for such
calculations.

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Flash Calculations: One Main Component Plus Gas

In many cases, a gas stream is in equilibrium with liquid that contains only one component or
a main component plus small amounts of heavier components. Such an equilibrium, for
example, exists in the overhead condenser separator of a refluxed deethanizer, where ethane
and other gases are in contact with the refluxed liquid. The liquid consists mainly of propane
with very small amounts of heavier hydrocarbons. In this case, the amount of propane loss in
the gas will need to be estimated. The calculation is simple and can be performed with a hand
calculator.
A similar type of equilibrium is found on the top tray of a gas scrubber, where water removes
impurities from a gas mixture. An example is in Figure 25.

140 F
Washed Gas
5 psig
H2 0

H2 0 +
Trace Impurities
Water Wash of a Gas
Figure 25

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A gas with a dry (water not included) volumetric rate of 1000 moles/hr is contacted with
water in a multistage tower. The pressure at the top of the tower is 5 psig and the temperature
of the overhead gas is 140F. The amount of water vapor in the overhead gas must be found.
The first step is to examine the equilibrium at the top tray. A basic assumption for the
solution of the problem is that the amount of gas in the liquid phase on the top tray is not
significant
(xw = 1.0). It is also safe to assume that the temperature of the top tray is equal to the
temperature of the overhead gas, 140F.
From Raoult's law, the mole fraction xw for water is 1.0:
PPw = xwVPw = 1.0 VPw = VPw

From steam tables:

PPw = VPw (140F) = 2.9 psia

In the vapor phase:

PPw = ywPT
yw = PPw/PT = 2.9/(14.7 + 5.0) = 0.147

The amount of water in the overhead gas is:

Vw = yw V/(1 - yw) = 0.147 x 1000/(1 - 0.147) = 172 mole/hr

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Physical Properties
This section briefly covers the physical properties related to distillation calculations and
distillation processes. Further information may be obtained from the course ChE 202,
Physical Properties, or from the sources listed below.
Physical Property Sources
The following four sources are easily accessible to Saudi Aramco engineers and contain data
suitable for distillation and VLE calculations.

Maxwell, Data Book on Hydrocarbons.

Provided with this course. Data and format aimed for petroleum engineers.
GPSA Engineering Data Book.
Data for petroleum and gas-processing engineers.
Perry's Chemical Engineers' Handbook.
Source for a wide variety of properties. Not as simple to use for petroleum engineers as
the previous two sources.
PRO/IITM, HYSIM.
The data libraries of these programs can be used directly for simulations and to develop
properties for manual calculations or other computer programs.

Other sources of property data are:

W. C. Edmister and B. I. Lee; Applied Hydrocarbon Thermodynamics.

Focuses on pure hydrocarbons and hydrocarbon mixtures.
R. C. Reid, J. M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids.
A classic reference for estimating properties. It contains extensive discussions on the
many methods that have been proposed for predicting physical, thermodynamic, and
transport properties. Reid's book has nothing on thermodynamics and focuses on
defined compounds; there is no mention of petroleum fractions.
American Petroleum Institute (API), Technical Data Book.
Data and prediction methods for most properties related to petroleum processing.

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Average Boiling Point

Many physical properties of pure hydrocarbons can be correlated with specific gravity and
normal boiling point as independent variables. However, for use in the petroleum industry,
these correlations must also be applicable to petroleum fractions, which are mixtures of many
components, that have a wide variation in boiling points.
In all correlations involving the boiling points of petroleum fractions, the correct average
boiling point should be used. For the following physical properties, these are:
Average Boiling Point

Physical Property

Volume Average

Viscosity
Liquid specific heat

Weight Average

True critical temperature

Molar Average

Pseudocritical temperature
Thermal expansion of liquids

Mean Average

Molecular weight
Characterization factor
Specific Gravity
Pseudocritical pressure
Heat of combustion

Maxwell's Data Book on Hydrocarbons includes calculation and interconversion graphs for
the different average boiling points.

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Characterization Factor
The characterization factor, also known as Watson K and UOP K, is an index of the chemical
character of pure hydrocarbons and petroleum fractions. The characterization factor of a
hydrocarbon is defined as the cube root of its absolute boiling point in R divided by its
specific gravity (60F), or
3

Characterization Factor, Kw = TB / Sp Gr
For hydrocarbon mixtures the mean average boiling point, MABP, is used in place of TB.
Characterization factors are available in Maxwell, as a function of gravity in API and boiling
point in F for hydrocarbons and petroleum fractions.
The characterization factor is only an approximate index of the chemical nature of
hydrocarbons, as indicated by its variation with boiling point for members of a homologous
series and for fractions from the same crude. However, it has considerable value because it
can be applied to the entire boiling range of a crude and has been generally accepted by the
petroleum industry. In other words, we assume that the Kw of fractions is equal to the Kw of
the entire crude.
Crudes with high Kw are paraffinic, while crudes with low Kw are more aromatic. Below are
average Kw for components with up to ten carbon atoms.
Paraffins
Naphthenes
Aromatics

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Kw ~ 12.7
Kw ~ 11.5
Kw ~ 10.5

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Inspection Properties
Inspection properties generally relate to the use of the products. For example, the cetane
number is a measure of the ignition quality of a diesel fuel. It is expressed as the percentage
of cetane (hexadecane) that must be mixed with liquid methylnaphthalene to produce the
same performance as the diesel fuel being rated. The cetane number and other inspection
properties are determined by standard tests. ASTM tests are generally accepted. Some of the
inspection properties related to products of distillation units are as follows:

RVP (Reid Vapor Pressure) -- Gasolines and light distillates.

Freezing Point -- Jet fuel.
Flash Point -- Most products.
Cetane Number -- Diesel fuel.
Cloud Point -- Middle distillates, fuels.
Pour Point -- Heavy fuels, heavy distillates, lubes.

The inspection properties measured in standardized tests are often correlated by predictive
methods with other properties, such as average boiling point, Watson K, specific gravity, and
distillation characteristics.

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Petroleum Fraction Distillations

A complete component-by-component analysis of crude oil or its fractions is not practical
because of the large number of components. For this reason, the composition and the
vaporization characteristics of petroleum fractions are represented by various distillation
methods. Of these, the ASTM and 15/5 or TBP (true boiling point) are the most widely
accepted and best standardized. The simple, inexpensive ASTM distillation is universally
preferred for routine product testing and refinery operation control. Although seldom
available, data on a 15/5 basis are required for refinery planning, engineering, designing
fractionator towers, and evaluating major refining processes.
15/5 Distillation
15/5 is a standardized, accurate laboratory batch distillation that is used for crude assays and
feed or product separations. The fractionator has 15 theoretical plates, calibrated under total
reflux conditions, and is operated adiabatically with automated reflux of total condensate.
The reflux ratio employed is 5:1 at atmospheric pressure and 2:1 at low pressures (2-10 mm
Hg absolute). A maximum vapor temperature of 430F is normal for atmospheric operations,
while 700F is the maximum atmospheric equivalent vapor temperature (F AET) for
operations at 10 mm pressure. The usual practical limit at 2 mm Hg absolute is 800F AET.
Generally, 15/5 vapor temperatures are approximations of true boiling points; they are not
necessarily equivalent to those from an efficient analytical distillation such as GCD.
The term true boiling point (TBP) is ambiguous. Theoretically, a TBP distillation utilizes a
distillation system that is able to make very close separations; each compound present in the
mixture will thus be separated at its own boiling point and in the quantity present in the
original mixture. The concept is illustrated in Figure 26 for two components A and B boiling
at TA and TB at the total pressure of the distillation. The stepwise plot (solid lines) represents
an ideal TBP distillation. Component A, boiling at a lower temperature, is recovered first.
Recovery of Component B starts after all of Component A is recovered. The distillation
temperature then increases to TB.

Stepwise Plot of an Ideal TBP Distillation

Figure 26
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The smooth curve in Figure 26 (broken line), represents an actual curve with imperfect
fractionation, such as results from a 15/5 distillation. Recovery of Component B starts before
the recovery of A is complete. As a result, the temperature of the distillation increases
gradually, reflecting the increasing concentration of B in the distillate.
Figure 27 shows similar curves for a mixture with seven components. If the mixture, like
most petroleum fractions, contains many components, the TBP or 15/5 fractionation will
produce a smooth curve (Figure 28).

TBP Curve for Seven Components

Figure 27

TBP Curve of a Complex Mixture

Figure 28

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ASTM Distillations
ASTM distillation procedures were developed by the American Society for Testing Materials.
These methods are rapid batch distillations that employ no trays or reflux between the stillpot
and the condenser. The only reflux is that generated by heat losses from the apparatus.
ASTM test methods are used in control laboratories throughout the world.
ASTM distillation data are considered to be roughly equivalent to those from a one-plate
batch distillation. Figure 29 lists common ASTM distillations for petroleum products.

Pressure

Maximum Vapor Reproducibility

Temp., F
, F

ASTM

Range

D-86
Group 1&2

Naphtha and
Kerosene

Atm

480

5 - 10

D-86
Group 3&4

Middle
Distillates

Atm

760

5 - 10

D-158

Distillates
and Gas Oil

Atm

760

Not Defined

D-1160

Heavy
Distillates &
Residua

Atm

620
under vacuum

15 - 20

D-216

Natural
Gasoline

Atm
ASTM Distillation Procedures
Figure 29

In ASTM distillation, the thermometer reading when the first drop is recovered is the initial
boiling point (IBP).
The amount of distillate collected in the graduate may be recorded at specified temperature
intervals, or the temperature may be recorded when the amount of distillate reaches specified
levels. The maximum temperature, when the last vapor comes off, is recorded as the end
point or final boiling point (FBP).
The total amount of distillate collected is recorded as the recovery, and the volume of material
(if any) remaining in the flask is recorded as the residue. The difference between the volume
of the initial sample and the sum of the recovery and residue, is the distillation loss.
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Gas Chromatographic Distillation (GCD)

Gas chromatographic analytical techniques are used to obtain a breakdown of components in
petroleum fractions. The results are automatically converted by a computer associated with
the instrument into distillations that approximate 15/5 results.
Equilibrium Flash Vaporization (EFV)
A flash curve indicates the relative amounts of feed vaporized as a function of the flash
temperature (Figure 30). Pressure is constant. The amount vaporized is usually expressed as
a fraction or percentage of the feed on a mole, weight, or volume basis.

Equilibrium Flash Vaporization

Figure 30
The separation between light and heavy components in a flash separation is relatively poor,
because there is only one equilibrium stage. The vapor product is in equilibrium with the
liquid product, and the flash curve is relatively flat when compared to the curves from
multistage distillation processes.
EFV curves are seldom run because of the time and expense involved. They are almost
always limited to crude oil or to reduced crude samples (atmospheric tower bottoms liquid)
that are being evaluated as vacuum tower charge stocks. The EFV initial boiling point is the
bubble point of the fraction under study, and the EFV final boiling point is its dew point.

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Distillation Curve Relationships

Figure 31 illustrates the difference in the shapes of 15/5, ASTM, and EFV curves. The
steepest curve is the 15/5 because it provides the best separation between the components.
EFV is relatively flat, reflecting the poor separation obtained from one-stage flash.
Techniques for converting the results of one method to another will not be covered here.
Conversion techniques can be found in the API Technical Data Book.

Constant Pressure

100

LV % Distilled

Distillation Curves
Figure 31

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Crude Assays
The complete and definitive analysis of a crude oil, usually called a crude assay, is
considerably more detailed than a TBP curve and a whole crude API gravity. A complete
crude assay will contain some or all of the following:

Properties such as whole crude gravity, viscosity, sulfur content, and pour point.
Plots of properties such as TBP curve, mid-volume plot of gravity, viscosity, sulfur.
Light-ends analysis through C8 and C9.
Properties of fractions (naphthas, middle distillates, gas oils, and residua) -- yield as
volume percent, gravity, sulfur, viscosity, octane number, diesel index, flash and fire
point, freeze point, smoke point, pour point, vapor pressure, etc.
Properties of lube distillates, if the crude is suitable for the manufacture of lube
basestocks.
Properties of asphalts, if the residua have suitable characteristics for preparation of
asphalts.
Detailed studies of fractions for various properties, such as octane number versus yield
for naphthas or viscosity versus yield for lubestocks.
EFV curve run at atmospheric pressure and/or phase diagram, although this is rarely
done.

A Saudi Aramco assay of Abqaiq GOSP 283 is given in Addendum D. Curves that provide
TBP, gravity, and sulfur content are reproduced in Figure 32.

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Crude Assay Curves

Figure 32

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Computer Simulations
Component Data - Computer simulations of distillation columns break the hydrocarbon
streams fractionated into their constituents. Generally, hydrocarbons with up to five or six
carbons are identified as individual components. Hydrocarbons with more than five or six
carbons are represented by narrow fractions. The narrow fractions are defined by their
volume average boiling point (VABP) and their average gravity. In other words, components
boiling within certain ranges are represented in the simulation as one component. Such a
component is called a pseudocomponent.
Figure 33 illustrates the division of a wide petroleum fraction into 11 pseudocomponents.
The fraction can be divided into pseudocomponents of equal volume or equal boiling range.
Alternatively, there can be an increased number of components in the region where the
distillation column will split the products.

Divide TBP Curve Into Pseudocomponents

TBP

1
0

2
5

3
10

4
20

5
30

6
40

7
60

8
80

9
90

95 98 100

Vol. %

Pseudocomponent Breakdown
Figure 33
PRO/IITM and HYSIM offer a variety of options for representing petroleum fractions and
determining their pseudocomponents.

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Thermodynamic Systems In PRO/IITM - Figure 34 is a table of the various thermodynamic

methods for calculating VLE and physical properties in PRO/IITM. Figure 35 contains specific
processing examples and the corresponding thermodynamic methods suggested by PRO/IITM.
Figure 36 lists the main Saudi Aramco distillation processes and the recommended
thermodynamic methods. Second-choice but acceptable options in Figure 36 are in
parentheses.
Applicability
K Value
Enthalpy

Method

Keyword

Correlation Type

Entropy

Braun K10

BK10

Empirical

Yes

B-W-R (Modified by
Twu)

BWRST

Equation of state

Yes

Curl-Pitzer

Corresponding state

Yes

Grayson-Streed

Semi-empirical

Yes

Johnson-Grayson

Empirical

Yes

K Delta

KDELTA

Corresponding state

Yes

Lee-Kesler

Corresponding state

Yes

Lee-Kesler-Plocker

LKP

Corresponding state

Yes

Peng-Robinson

Equation of state

Yes

Redlich-Kwong(1)

Equation of state

Yes

Rice

RICE

Corresponding state

Yes

Soave Redlich-Kwong

SRK

Equation of state

Yes

SRK (Kabadi Danner)

SRK (KD)

Equation of state

Yes

(1)For Vapor Only

Thermodynamic Methods In Process

Figure 34

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Thermodynamic
System

Comments

Process
Demethanizers
Deethanizers
Depropanizers

PR or SRK
PR or SRK
PR or GS or SRK

Expander plant simulation.

Chiller plant simulation.

Debutanizers or
Deisobutanizers
C2 or C3 Splitter
B-T-X column

GS or SRK
or PR
PR or SRK
BK10

Crude Units
Bubble Towers

BK10 or GS
BK10 or GS

FCC main
fractionators
Vacuum Columns

BK10 or GS

See above comment.

BK10

Resid representation is
critical to accuracy.

Reformer systems
Natural gas with
high H2S or CO2
Nitrogen rejection

GS or LKP

High H2.

PR or SRK
PR or SRK

Wellhead proc.

PR or SRK

Special data are built-in.

Cryogenic natural gas
towers.
High pressure flashes.

Special data are built-in.

Special data for
aromatics at low
pressures included.
More vapor with GA.
Component breakdown
is very important to
simulation.

Thermodynamic System Application Guidelines

Figure 35

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Distillation Process

Approx. Pressure, psia

Components

Crude Stabilization

BK10

Condensate Stripping

250-470

SRK (PR)

Crude Fractionation

Vacuum - 50

BK10

Demethanizer

160 (No Condenser)

SRK (PR)

Deethanizer

430 (50-210F)

SRK (PR)

Depropanizer

330

SRK (PR)

Debutanizer

140

SRK (PR)

NGL Fractionation

Saudi Aramco Distillation Processes

Suggested Thermodynamic Options
Figure 36

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Nomenclature

Relative volatility

Feed rate, mole/hr

Fugacity Factor

Fugacity

MABP

Mean average boiling point

Sp Gr

Specific gravity

Temperature

Vapor pressure

Mole fraction in the feed

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ADDENDUM B
Example 1
Sweet gas from a DGA contactor (Figure 37) is water washed in the top two trays of the
column.
a. Find the water weight rate in the gas leaving the column.
b. What is the water dew point of the saturated gas at the tower pressure?
c. The sweet gas is let down to 60 psig. Find the dew point at this pressure. Assume that
there are no hydrocarbons in the liquid phase.
Dry gas rate:
Temperature:
Pressure:
Vol%:

300 MMSCFD (Million Standard Cubic Feet per Day)

120F
150 psig
C1 = 60
C2 = 20
C3 = 15
C4 = 4

Assume that the gas is an ideal gas. The ideal gas volume at standard conditions (60 F, 1
atm) is 379.5 SCF/lb-mole),
Water vapor pressures are available in Addendum E.

Figure 37

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Answer
a.

Since water is the only component in the liquid phase, its partial pressure in the vapor
phase is its vapor pressure at the temperature of the top tray, 120F.
VPw = PPw = 1.7 psia (from steam tables)
Water content in vapor

1.7 psia
PPw =
= 0.0103
PT (150 + 14.7) psia

Molar rate of dry gas: 300 MMSCFD/(379.5 SCF/lb-mole) = 790,500 lb-mole/day

Water rate in gas: 790,500 lb-mole/day x 0.0103/(1 - 0.0103) = 8227 lb-mole/day
Weight rate: (8227 x 18) lb/day = 148,000 lb/day = 6200 lb/hr
b.

Since the gas is saturated with water, it is at its dew point, 120F.

Partial pressure of water at 60 psig = 0.0103 x (60 + 14.7) = 0.77. This is also the water
vapor pressure at the dew point. From the steam tables, the temperature that
corresponds to 0.77 psia water-vapor pressure is 93F.

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Example 2
Use the provided crude assay TBP curves (Figure 38) to obtain the gravity and sulfur content
for 450F and 600F TBP components. Find the fraction of the crude in a 450-600F cut.

Figure 38

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Answer
450F TBP fraction
Volume = 38%
API gravity = 44
Sulfur = 0.2 wt%
600F TBP fraction
Volume = 54%
API gravity = 34
Sulfur = 1.3 wt%
The 450-600F volume is:
54 - 38 = 16% of the crude volume

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Example 3
Use the ideal gas law to solve the following problems.
The material balance of a DGA unit indicates the following conditions for the contactor feed:
Pressure:
Temperature:
Rate:

160 psig
115F
53,963 lb-mole/hr

Find the following:

The actual volumetric rate of the gas in MMACFD.

The actual volumetric rate of the gas at 200 psig and 115F.

The actual volumetric rate of the gas at 200 psig and 250F.

The standard volumetric rate of the gas (60F, 1 atm) using the ideal gas molecular
volume.

Fundamental constants are in Addendum C.

Answer
a.

Ideal gas law: PV = nRT

n
P
T
R
V

=
=
=
=
=

53,963 lb-mole/hr
160 + 14.7 = 174.7 psia
115F = (115 + 459.7)R = 574.7R
10.732 (psia-ft3)/(lb-mole R)
nRT/P = [53,963 lb-mole/hr x 10.732 (psia-ft3)/(lb-mole R) x
574.7R]/174.7 psia

= 1.905 x 106 ft3/hr = 45.72 MMACFD

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P1V1 = nRT1
P2V2 nRT1
P1V1 = P2V2 and V = P1 T2
2
V2
T2
P2 T1
Since T1 = T2:
(160 + 14.7) psia
V2 = V1 P1 = 45.72 MMACFD
= 37.20 MMACFD
P2
(200 + 14.7) psia

174.7 psia (250 + 459.7)R

V2 = V1 P1 T2 = 45.72 MMACFD
P2 T1
14.7 psia (115 + 459.7)R

Normal volume (1 atm, 0C) of ideal gases: 359.039 ft3/lb-mole

Standard volume (1 atm 60F):
359.039

(459.67 + 60)R
(459.67 + 32)R

= 379.5 SCF/lb-mole

Rate = 53,963 lb-mole/hr x 379.5 SCF/lb-mole x 24 hr/day = 491.50 MMSCFD

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Process
Introduction to Distillation Process

Example 4
Represent the illustrated condenser sequence (Figure 39) in the provided propane/n-butane
phase and equilibrium diagrams (Figures 40 and 41).
Assume that the pressure is 100 psia for all drums.

F, z 1

V, y 1
T = 110F

V', y'1
T' = 100F

L, x 1

L', x'1

T" = ?
L", x"1

Figure 39

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Process
Introduction to Distillation Process

Propane - n-Butane
150
140
P = 100 psia
130
120

Vapor
110
100

x1'

Liquid

y1'=x1"

60
50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Mole Fraction Propane, x ,1y

0.8

0.9

0.8

0.9

1.0

V-L Phase Diagram

Figure 40
Propane - n-Butane
1.0
C 3- nC 4 at 100 psia

0.9
0.8
0.7

(x1',y1')

(y 1',x 1")

0.6
(x1,y1)
0.5
0.4
0.3
0.2
0.1
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1.0

Mole Fraction Propane in Liquid, x 1

Equilibrium Line
Figure 41

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Introduction to Distillation Process

ADDENDUM C
Pg. 1 of 14

Figure 42
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Introduction to Distillation Process

Pg. 2 of 14

Figure 43
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Introduction to Distillation Process

Pg. 3 of 14

Figure 44

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Introduction to Distillation Process

Pg. 4 of 14

Figure 45

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Introduction to Distillation Process

Pg. 5 of 14

Figure 46

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Introduction to Distillation Process

Pg. 6 of 14

Figure 47

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Introduction to Distillation Process

Pg. 7 of 14

Figure 48

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Introduction to Distillation Process

Pg. 8 of 14

Figure 49

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Introduction to Distillation Process

Pg. 9 of 14

Figure 50

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Introduction to Distillation Process

Pg. 10 of 14

Figure 51

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Introduction to Distillation Process

Pg. 11 of 14

Figure 52

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Introduction to Distillation Process

Pg. 12 of 14

Figure 53

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Introduction to Distillation Process

Pg. 13 of 14

Figure 54

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Introduction to Distillation Process

Pg. 14 of 14

Figure 55

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Introduction to Distillation Process

ADDENDUM D
Pg. 1 of 9

Figure 56
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Introduction to Distillation Process

Pg. 2 of 9

Figure 57
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Introduction to Distillation Process

Pg. 3 of 9

Figure 58

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Introduction to Distillation Process

Pg. 4 of 9

Figure 59

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Introduction to Distillation Process

Pg. 5 of 9

Figure 60

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Introduction to Distillation Process

Pg. 6 of 9

Figure 61
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Introduction to Distillation Process

Pg. 7 of 9

Figure 62

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Introduction to Distillation Process

Pg. 8 of 9

Figure 63
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Introduction to Distillation Process

Pg. 9 of 9

Figure 64
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Introduction to Distillation Process

ADDENDUM E
Pg 1 of 2

v = specific volume
h - enthalpy, BTU per lb
Source: Steam Tables. Reprinted with permission of Combustion Engineering, Inc.

s = entropy, Btu per R per lb

Figure 65
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100

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Introduction to Distillation Process

Pg 2 of 2

v = specific volume
h - enthalpy, BTU per lb
s = entropy, Btu per R per lb
Source: Steam Tables. Reprinted with permission of Combustion Engineering, Inc.

Figure 66

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