2. TYPES AND IMPORTANCE OF PHYSICAL PROPERTIES
On the basis of the production and refining processes described above it may be
said that the petroleum industry is involved with many types of equipment for
production, transportation, and storage of intermediate or final petroleum
products. Some of the most important units are listed below.
3. 1.Gravity decanter (to separate oil and water)
2. Separators to separate oil and gas
3. Pumps, compressors, pipes, and valves
4. Storage tanks 5. Distillation, absorption, and stripping columns
6. Boilers, evaporators, condensers, and heat exchangers
7. Flashers (to separate light gases from a liquid)
8. Mixers and agitators
9. Reactors (fixed and fluidized beds)
10. Online analyzers (to monitor the composition)
11. Flow and liquid level measurement devices
12. Control units and control valves
4. The above list shows some, but not all, of the units involved in the petroleum
industry.
Optimum design and operation of such units as well as manufacture of products
to meet market demands and government regulations require a complete
knowledge of properties and characteristics for hydrocarbons, petroleum
fractions/products, crude oils, and reservoir fluids.
They are divided into two groups of temperature-independent parameters and
temperature dependent properties.
5. DEFINITION OF BASIC PROPERTIES
Molecular Weight
Boiling Point
Density, Specific Gravity, and API Gravity
Refractive Index
Critical Constants (Tc, Pc, Vc, Zc)
Kinematic Viscosity
Freezing and Melting Points
Flash Point
Autoignition Temperature
Flammability Range
Octane Number
6. DEFINITION OF BASIC PROPERTIES
MOLECULAR WEIGHT
The molecular weight of apure compound is determined from its chemical formula and the atomic weights of its elements.
The atomic weights of the elements found in a petroleum fluid are
C = 12.011,
H = 1.008,
S = 32.065,
O = 16.0,
and N = 14.01, as given by the IUPAC standard
As an example, the molecular weight of methane (CH4) is calculated as 12.011 + 4 x 1.008 = 16.043 kg/kmol or
16.043 g/tool (0.01604 kg/mol) or 16.043 lb/lbmol.
Molecularweight is one of the characterization parameters for hydrocarbons.
7. BOILING POINT
The boiling point of a pure compound at a given pressure is the temperature at which vapor and liquid exist
together
at equilibrium.
If the pressure is 1 atm, the boiling point is called the normal boiling point.
However, usually the term boiling point, Tb, is used instead of normal boiling point and for other pressures the term
saturation temperature is used.
In some cases, especially for heavy hydrocarbons in which thermal cracking may occur at high temperatures,
boiling
points at pressures other than atmospheric is specified. Boiling points of heavy hydrocarbons are usually measured
at 1,10, or 50 mm Hg.
8. DENSITY, SPECIFIC GRAVITY, AND API GRAVITY
Density is defined as mass per unit volume of a fluid. Density is a state function and for a pure compound depends
on both temperature and pressure and is shown by p. Liquid densities decrease as temperature increases but the
effect of pressure on liquid densities at moderate pressures is usually negligible. At low and moderate pressures
(less than a few bars), saturated liquid density is nearly the same as actual density at the same temperature.
liquid density at the reference conditions of 20~ (293 K) and 1 atm is shown byd and it is used as a
characterization parameter
Liquid density for hydrocarbons is usually reported in terms of specific gravity (SG) or relative density defined as
density of liquid at temperature T /SG = density of water at temperature T
9. Since the standard conditions adopted by the petroleum industry are 60 ~ F (15.5 ~ C) and 1 atm, specific gravities
of liquid hydrocarbons are normally reported at these conditions. At a reference temperature of 60~ (15.5~ the
density of liquid water is 0.999 g/cm 3
(999 kg/m 3
) or 8.337 lb/gal(U.S.). Therefore, for a hydrocarbon or a
petroleum fraction, the specific gravity is defined as
SG (60o
F /60o
F) = density of liquid at 60o
F in g/cm3
/ 0.999 g/cm 3
Water density at 60~ is 0.999 or almost 1 g/cm 3
; therefore, values of specific gravities are nearly the same as the
density of liquid at 15.5~ (289 K) in g/cm 3
. The Society of Petroleum Engineers usually uses y for the specific
gravity and in some references it is designated by S
10. In the early years of the petroleum industry, the American Petroleum Institute (API) defined the API gravity (degrees
API) to quantify the quality of petroleum products and crude oils. The API gravity is defined as
API gravity= 141.5 / SG (at 60F) - 131.5
11. respectively. Liquid hydrocarbons with lower specific gravities have higher API gravity.
Aromatic hydrocarbons have higher specific gravity (lower API gravity) than do paraffinic hydrocarbons.
For example, benzene has SG of 0.8832 (API of 28.72)
while n-hexane with the same carbon number has SG of 0.6651 (API gravity of 81.25).
A liquid with SG of 1 has API gravity of 10.
Once Eq. (2.4) is reversed it can be used to calculate specific gravity from the API gravity.
SG = 141.5 / API gravity + 131.5
12. GAS SPECIFIC GRAVITY
The definition of specific gravity for gases is somewhat different. It is defined as relative density of gas to density of
air at standard conditions. In addition, density of gases is a strong function of pressure.
Since at the standard conditions (15.5 O
C and 1 atm)
the density of gases are estimated from the idealgas law
the specific gravity of a gas is proportional to the ratio of molecular weight of gas (Mg) to themolecular weight of air
(28.97).
SGg = Mg / 28.97
13. REFRACTIVE INDEX
Refractive index or refractivity for a substance is defined as the ratio of velocity of light in a vacuum to the velocity
of light in the substance (fluid) and is a dimensionless quantity shown by n:
n = velocity of light in the vacuum / velocity of light in the substance
In other words, when a light beam passes from one substance (air) to another (a liquid), it is bent or refracted
because of the difference in speed between the two substances.
In fact, refractive index indicates the degree of this refraction. Refractive index is a state function and depends on
the temperature and pressure of a fluid. Since the velocity of light in a fluid is less than the velocity of light in a
vacuum, its value for a fluid is greater than unity. Liquids have higher values of refractive index than that of gases.
For gases the values of refractive index are very close to unity.
14. All frequencies of electromagnetic radiation (light) travel at the same speed in vacuum (2.998 x l0 s m/s); however,
in a substance the velocity of light depends on the nature of the substance (molecular structure) as well as the
frequency of the light. For this reason, standard values of refractive index must be measured at a standard
frequency. Usually the refractive index of hydrocarbons is measured by the sodium D line at 20~ and 1 atm. The
instrument to measure the refractive index is called a refractometer
refractive index is a very useful characterization parameter for pure hydrocarbons and petroleum fractions,
especially in relation with molecular type composition. Values of n vary from about 1.3 for propane to 1.6 for some
aromatics. Aromatic hydrocarbons have generally higher n values than paraffinic compounds
15. CRITICAL CONSTANTS (TC, PC, VC, ZC)
The critical point is a point on the pressure-volume temperature diagram where
the saturated liquid and saturated vapor are identical and indistinguishable. The
temperature, pressure, and volume of a pure substance at the critical point are
called critical temperature (To), critical pressure (Pc), and critical volume (Vc),
respectively. In other words, the critical temperature and pressure for a pure
compound are the highest temperature and pressure at which the vapor and
liquid phase can coexist at equilibrium. In fact, for a pure compound at
temperatures above the critical temperature, it is impossible to liquefy a vapor no
matter how high the pressure is. A fluid whose temperature and pressure are
above the critical point is called supercritical fluid. For pure compounds, critical
temperature and pressure are also called true critical temperature and true
critical pressure
16. The critical compressibility factor, Zc, is defined from To, Pc, and Vc according to the
general definition of compressibility factor.
17. There are several international organizations that are known as standard organizations that
recommend specific characteristics or standard measuring techniques for various petroleum
products through their regular publications. Some of these organizations in different countries that
are known with their abbreviations are as follows:
1. ASTM (American Society for Testing and Materials) in the United States
2. ISO (International Organization for Standardization), which is at the international level
3. IP (Institute of Petroleum) in the United Kingdom
4. API (American Petroleum Institute) in the United States
5. AFNOR (Association Francaise de Normalisation), an official standard organization in France
6. Deutsche Institut fur Norrnung (DIN) in Germany
7. Japan Institute of Standards (J-IS) in Japan
18. VAPOR PRESSURE
In a closed container, the vapor pressure of a pure compound is the force exerted per unit area of walls by the
vaporized portion of the liquid. Vapor pressure, pvap, can also be defined as a pressure at which vapor and liquid
phases of a pure substance are in equilibrium with each other. The vapor pressure is also called saturation
pressure, psat, and the corresponding temperature is called saturation temperature. In an open air under
atmospheric pressure, a liquid at any temperature below its boiling point has its own vapor pressure that is less
than 1 atm. When vapor pressure reaches 1 atm the saturation temperature becomes the normal boiling point.
Vapor pressure increases with temperature and the highest value of vapor pressure for a substance is its critical
pressure (Pc) in which the corresponding temperature is the critical temperature (To). When a liquid is open to the
atmosphere at a temperature T in which the vapor pressure of liquid is pvap, vol% of the compound vapors in the
air is
19. Vapor pressure is a very important thermodynamic property of any substance and it is a measure of the volatility of
a fluid. Compounds with a higher tendency to vaporize have higher vapor pressures. More volatile compounds are
those that have lower boiling points and are called light compounds. For example, propane (C3) has boiling point
less than that of n-butane (nC4) and as a result it is more volatile. At a fixed temperature, vapor pressure of
propane is higher than that of butane. In this case, propane is called the light compound (more volatile) and butane
the heavy compound. Generally, more volatile compounds have higher critical pressure and lower critical
temperature, and lower density and lower boiling point than those of less volatile (heavier) compounds, although
this is not true for the case of some isomeric compounds. Vapor pressure is a useful parameter in calculations
related to hydrocarbon losses and flammability of hydrocarbon vapor in the air More volatile compounds are more
ignitable than heavier compounds. For example, n-butane is added to gasoline to improve its ignition
characteristics.
20. KINEMATIC VISCOSITY
Kinematic viscosity is defined as the ratio of absolute p(dynamic) viscosity/u to absolute density p at the same
temperature in the following form
v = µ/p
kinematic viscosity is expressed in cSt, SUS, and SFS units. Values of kinematic viscosity for pure liquid
hydrocarbons are usually measured and reported at two reference temperatures of 38~ (100~ and 99~ (210~ in
cSt. However, other reference temperatures of 40~ (104~ 50~ (122~ and 60~ (140~ are also used to report
kinematic viscosities of petroleum fractions. Liquid viscosity decreases with an increase in temperature (see
Section 2.7). Kinematic viscosity, as it is shown in Chapter 3, is a useful characterization parameter, especially for
heavy fractions in which the boiling point may not be available.
21. ASTM is composed of several committees in which the D-02 committee is
responsible for petroleum products and lubricants, and for this reason its test
methods for petroleum materials are designated by the prefix D. For example, the
test method ASTM D 2267 provides a standard procedure to determine the
benzene content of gasoline
22. In France this test method is designated by EN 238, which are documented in
AFNOR information document M 15-023. Most standard test methods in different
countries are very similar in practice and follow ASTM methods but they are
designated by different codes. For example the international standard ISO
6743/0, accepted as the French standard NF T 60-162, treats all the petroleum
lubricants, industrial oils, and related products. The abbreviation NF is used for
the French standard, while EN is used for European standard methods
23. Government regulations to protect the environment or to save energy, in many
cases, rely on the recommendations of official standard organizations. For
example, in France, AFNOR gives specifications and requirements for various
petroleum products. For diesel fuels it recommends (after 1996) that the sulfur
content should not exceed 0.05 wt% and the flash point should not be less than
55~
