Page 32 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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12 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
3. Heat capacity (C p ) of a fluid is needed in design and oper-
ation of heat transfer units such as heat exchangers. 14:26 such as density, boiling point, molecular weight, and refrac-
tive index are called physical properties. Properties such as en-
4. Enthalpy (H) of a fluid is needed in energy balance cal- thalpy, heat capacity, heat of vaporization, equilibrium ratios,
culations, heat requirements needed in design and oper- and fugacity are called thermodynamic properties. Viscosity,
ation of distillation, absorption, stripping columns, and thermal conductivity, diffusion coefficient, and surface ten-
reactors. sion are in the category of physical properties but they are also
5. Heat of vaporization ( H vap ) is needed in calculation of called transport properties. In general all the thermodynamic
heat requirements in design and operation of reboilers or and physical properties are called thermophysical properties.
condensers. But they are commonly referred to as physical properties or
6. Heats of formation ( H f ), combustion ( H c ), and reaction simply properties, which is used in the title of this book.
( H r ) are used in calculation of heating values of fuels A property of a system depends on the thermodynamic state
and the heat required/generated in reactors and furnaces of the system that is determined by its temperature, pressure,
in refineries. Such information is essential in design and and composition. A process to experimentally determine var-
operations of burners, furnaces, and chemical reactors. ious properties for all the industrially important materials,
These properties together with the Gibbs free energy are especially complex mixtures such as crude oils or petroleum
used in calculation of equilibrium constants in chemical products, would be prohibitive in both cost and time, indeed
reactions to determine the optimum operating conditions it could probably never be completed. For these reasons ac-
in reactors for best conversion of feed stocks into the prod- curate methods for the estimation of these properties are be-
ucts. coming increasingly important. In some references the term
7. Viscosity (μ) is another useful property in petroleum pro- property prediction is used instead of property estimation;
duction, refining, and transportation. It is used in reser- however, in this book as generally adopted by most scientists
voir simulators to estimate the rate of oil or gas flow both terms are used for the same purpose.
and their production. It is needed in calculation of power
required in mixers or to transfer a fluid, the amount of
pressure drop in a pipe or column, flow measurement de- 1.3 IMPORTANCE OF PETROLEUM FLUIDS
vices, and design and operation of oil/water separators CHARACTERIZATION
[37, 38].
8. Thermal conductivity (k) is needed for design and opera- In the previous section, various basic characteristic para-
tion of heat transfer units such as condensers, heat ex- meters for petroleum fractions and crude oils were intro-
changers, as well as chemical reactors [39]. duced. These properties are important in design and oper-
9. Diffusivity or diffusion coefficient (D) is used in calcula- ation of almost every piece of equipment in the petroleum in-
tion of mass transfer rates and it is a useful property in dustry. Thermodynamic and physical properties of fluids are
design and operation of reactors in refineries where feed generally calculated through standard methods such as cor-
and products diffuse in catalyst pores. In petroleum pro- responding state correlations or equations of state and other
duction, a gas injection technique is used in improved oil pressure–volume–temperature (PVT) relations. These corre-
recovery where a gas diffuses into oil under reservoir con- lations and methods have a generally acceptable degree of ac-
ditions; therefore, diffusion coefficient is also required in curacy provided accurate input parameters are used. When
reservoir simulation and modeling [37, 40–42]. using cubic equation of state to estimate a thermodynamic
10. Surface tension (σ)or interfacial tension (IFT) is used property such as absolute density for a fluid at a known tem-
mainly by the reservoir engineers in calculation of cap- perature and pressure, the critical temperature (T c ), critical
illary pressure and rate of oil production and is needed pressure (P c ), acentric factor (ω), and molecular weight (M)
in reservoir simulators [37]. In refineries, IFT is a use- of the system are required. For most pure compounds and hy-
ful parameter to determine foaming characteristics of oils drocarbons these properties are known and reported in var-
and the possibility of having such problems in distillation, ious handbooks [36, 47–50]. If the system is a mixture such
absorption, or stripping columns [43]. It is also needed as a crude oil or a petroleum fraction then the pseudocritical
in calculation of the rate of oil dispersion on seawater properties are needed for the calculation of physical proper-
surface polluted by an oil spill [44]. ties. The pseudocritical properties cannot be measured but
11. Equilibrium ratios (K i ) and fugacity coefficients (ϕ i ) are have to be calculated through the composition of the mix-
the most important thermodynamic properties in all ture. Laboratory reports usually contain certain measured
phase behavior calculations. These calculations include properties such as distillation curve (i.e., ASTM D 2887) and
vapor–liquid equilibria, bubble and dew point pressure, the API gravity or specific gravity of the fraction. However,
pressure–temperature phase diagram, and GOR. Such cal- in some cases viscosity at a certain temperature, the per-
culations are important in design and operation of distilla- cent of paraffin, olefin, naphthene, and aromatic hydrocar-
tion, absorption and stripping units, gas-processing units, bon groups, and sulfur content of the fraction are measured
gas–oil separators at production fields, and to determine and reported. Petroleum fractions are mixtures of many com-
the type of a reservoir fluid [45, 46]. pounds in which the specific gravity can be directly measured
for the mixture, but the average boiling point cannot be mea-
Generally, the first set of properties introduced above sured. Calculation of average boiling point from distillation
(temperature-independent) are the basic parameters that are data, conversion of various distillation curves from one type
used to estimate physical and thermodynamic properties to another, estimation of molecular weight, and the PNA com-
given in the second set (temperature-dependent). Properties position of fractions are the initial steps in characterization of

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