Page 53 - Characterization and Properties of Petroleum Fractions

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2. CHARACTERIZATION AND PROPERTIES OF PURE HYDROCARBONS 33
Experimental values of critical properties have been re-
ported for a large number of pure substances. However, for the saturation temperature becomes the normal boiling point.
Vapor pressure increases with temperature and the highest
hydrocarbon compounds, because of thermal cracking that value of vapor pressure for a substance is its critical pressure
occurs at higher temperatures, critical properties have been (P c ) in which the corresponding temperature is the critical
measured up to C 18 [2]. Recently some data on critical proper- temperature (T c ). When a liquid is open to the atmosphere at
ties of n-alkanes from C 19 to C 36 have been reported [3]. How- a temperature T in which the vapor pressure of liquid is P vap ,
ever, such data have not yet been universally confirmed and vol% of the compound vapors in the air is
they are not included in major data sources. Reported data vap --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
on critical properties of such heavy compounds are generally (2.11) vol% = 100 × P
predicted values and vary from one source to another. For ex- P a
ample, the API-TDB [2] reports values of 768 K and 11.6 bar where P a is the atmospheric pressure. Derivation of Eq. (2.11)
for the critical temperature and pressure of n-eicosane, while is based on the fact that vapor pressure is equivalent to partial
these values are reported as 767 K and 11.1 bar by Poling pressure (mole fraction × total pressure) and in gases under
et al. [4]. Generally, as boiling point increases (toward heav- low-pressure conditions, mole fraction and volume fraction
ier compounds), critical temperature increases while critical are the same. At sea level, where P a = 1 atm, calculation of
pressure decreases. As shown in Section 2.2, aromatics have vol% of hydrocarbon vapor in the air from Eq. (2.11) is simply
higher T c and P c relative to those of paraffinic compounds 100 P vap ,if P vap is in atm.
with the same carbon atoms. Vapor pressure is a very important thermodynamic prop-
erty of any substance and it is a measure of the volatility of
2.1.6 Acentric Factor a fluid. Compounds with a higher tendency to vaporize have
higher vapor pressures. More volatile compounds are those
Acentric factor is a parameter that was originally defined by that have lower boiling points and are called light compounds.
Pitzer to improve accuracy of corresponding state correla- For example, propane (C 3 ) has boiling point less than that of
tions for heavier and more complex compounds [5, 6]. Acen- n-butane (nC 4 ) and as a result it is more volatile. At a fixed
tric factor is a defined parameter and not a measurable quan- temperature, vapor pressure of propane is higher than that
tity. It is a dimensionless parameter represented by ω and is of butane. In this case, propane is called the light compound
defined as (more volatile) and butane the heavy compound. Generally,
(2.10) ω =− log P vap − 1.0 more volatile compounds have higher critical pressure and
10 r
lower critical temperature, and lower density and lower boil-
where
vap vap ing point than those of less volatile (heavier) compounds, al-
P r = reduced vapor pressure, P /P c , dimensionless though this is not true for the case of some isomeric com-
P vap = vapor pressure at T = 0.7 T c (reduced temperature pounds. Vapor pressure is a useful parameter in calculations
of 0.7), bar related to hydrocarbon losses and flammability of hydrocar-
P c = critical pressure, bar bon vapor in the air (through Eq. 2.11). More volatile com-
T = absolute temperature, K pounds are more ignitable than heavier compounds. For ex-
T c = critical temperature, K ample, n-butane is added to gasoline to improve its ignition
Acentric factor is defined in a way that for simple fluids such characteristics. Low-vapor-pressure compounds reduce evap-
as argon and xenon it is zero and its value increases as the oration losses and chance of vapor lock. Therefore, for a fuel
size and shape of molecule changes. For methane ω = 0.001 there should be a compromise between low and high vapor
and for decane it is 0.489. Values reported for acentric fac- pressure. However, as will be seen in Chapter 6, one of the
tor of pure compounds are calculated based on Eq. (2.10), major applications of vapor pressure is in calculation of equi-
which depends on the values of vapor pressure. For this rea- librium ratios (K i values) for phase equilibrium calculations.
son values reported for the acentric factor of a compound may Methods of calculation of vapor pressure are given in detail in
slightly vary from one source to another depending on the re- Chapter 7. For pure hydrocarbons, values of vapor pressure at
lation used to estimate the vapor pressure. In addition, since the reference temperature of 100 F (38 C) are provided by the
◦
◦
calculation of the acentric factor requires values of critical API [2] and are given in Section 2.2. For petroleum fractions,
temperature and pressure, reported values for ω also depend as will be discussed in Chapter 3, method of Reid is used to
on the values of T c and P c used. measure vapor pressure at 100 F. Reid vapor pressure (RVP)
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is measured by the ASTM test method D 323 and it is approx-
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◦
2.1.7 Vapor Pressure imately equivalent to vapor pressure at 100 F (38 C). RVP is
a major characteristic of gasoline fuel and its prediction is
In a closed container, the vapor pressure of a pure compound discussed in Chapter 3.
is the force exerted per unit area of walls by the vaporized
portion of the liquid. Vapor pressure, P vap , can also be de-
fined as a pressure at which vapor and liquid phases of a 2.1.8 Kinematic Viscosity
pure substance are in equilibrium with each other. The vapor Kinematic viscosity is defined as the ratio of absolute (dy-
pressure is also called saturation pressure, P sat , and the cor- namic) viscosity μ to absolute density ρ at the same temper-
responding temperature is called saturation temperature. In ature in the following form:
an open air under atmospheric pressure, a liquid at any tem-
perature below its boiling point has its own vapor pressure (2.12) ν = μ
that is less than 1 atm. When vapor pressure reaches 1 atm, ρ

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