Page 27 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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1. INTRODUCTION 7
heptane-plus fraction. For example, for the crude oil sample
oil. It should be realized that these ranges for the gravity and
in Table 1.2, the specific gravity of the whole crude oil is 0.871 content of a crude is less than 0.5 wt% it is called a sweet --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
or API gravity of 31. Details of such calculations are discussed sulfur content are relative and may vary from one source to
in Chapter 4. These compositions have been determined from another. For example, Favennec [15] classifies heavy crude as
recombination of the compositions of corresponding sepa- those with API less than 22 and light crude having API above
rator gas and stock tank liquid, which have been measured 33. Further classification of crude oils will be discussed in
through analytical tools (i.e., gas chromatography, mass spec- Chapter 4.
trometry, etc.). Composition of reservoir fluids varies with the
reservoir pressure and reservoir depth. Generally in a produc- 1.1.3 Petroleum Fractions and Products
ing oil field, the sulfur and amount of heavy compounds in-
crease versus production time [10]. However, it is important A crude oil produced after necessary field processing and
to note that within an oil field, the concentration of light hy- surface operations is transferred to a refinery where it is
drocarbons and the API gravity of the reservoir fluid increase processed and converted into various useful products. The
with the reservoir depth, while its sulfur and C 7+ contents de- refining process has evolved from simple batch distillation
crease with the depth [1]. The lumped C 7+ fraction in fact is in the late nineteenth century to today’s complex processes
through modern refineries. Refining processes can be gener-
a mixture of a very large number of hydrocarbons, up to C 40
or higher. As an example the number of pure hydrocarbons ally divided into three major types: (1) separation, (2) con-
from C 5 to C 9 detected by chromatography tools in a crude oil version, and (3) finishing. Separation is a physical process
from North Sea reservoir fluids was 70 compounds. Detailed where compounds are separated by different techniques. The
composition of various reservoir fluids from the North Sea most important separation process is distillation that occurs
fields is provided by Pedersen et al. [13]. As shown in Chapter in a distillation column; compounds are separated based on
9, using the knowledge of the composition of a reservoir fluid, the difference in their boiling points. Other major physical
one can determine a pressure–temperature (PT) diagram of separation processes are absorption, stripping, and extrac-
the fluid. And on the basis of the temperature and pressure tion. In a gas plant of a refinery that produces light gases,
of the reservoir, the exact type of the reservoir fluid can be the heavy hydrocarbons (C 5 and heavier) in the gas mixture
determined from the PT diagram. are separated through their absorption by a liquid oil sol-
Reservoir fluids from a producing well are conducted to vent. The solvent is then regenerated in a stripping unit. The
two- or three-stage separators which reduce the pressure and conversion process consists of chemical changes that occur
temperature of the stream to atmospheric pressure and tem- with hydrocarbons in reactors. The purpose of such reactions
perature. The liquid leaving the last stage is called stock tank is to convert hydrocarbon compounds from one type to an-
oil (STO) and the gas released in various stages is called as- other. The most important reaction in modern refineries is
sociated gas. The liquid oil after necessary field processing is the cracking in which heavy hydrocarbons are converted to
called crude oil. The main factor in operation and design of an lighter and more valuable hydrocarbons. Catalytic cracking
oil–gas separator is to find the optimum operating conditions and thermal cracking are commonly used for this purpose.
of temperature and pressure so that the amount of produced Other types of reactions such as isomerization or alkylation
liquid (oil) is maximized. Such conditions can be determined are used to produce high octane number gasoline. Finishing is
through phase behavior calculations, which are discussed in the purification of various product streams by processes such
detail in Chapter 9. Reservoir fluids from producing wells are as desulfurization or acid treatment of petroleum fractions to
mixed with free water. The water is separated through gravi- remove impurities from the product or to stabilize it.
tational separators based on the difference between densities After the desalting process in a refinery, the crude oil en-
of water and oil. Remaining water from the crude can be re- ters the atmospheric distillation column, where compounds
moved through dehydration processes. Another surface oper- are separated according to their boiling points. Hydrocarbons
ation is the desalting process that is necessary to remove the in a crude have boiling points ranging from −160 C (boil-
◦
salt content of crude oils. Separation of oil, gas, and water ing point of methane) to more than 600 C (1100 F), which
◦
◦
from each other and removal of water and salt from oil and is the boiling point of heavy compounds in the crude oil.
any other process that occurs at the surface are called surface However, the carbon–carbon bond in hydrocarbons breaks
production operations [14]. down at temperatures around 350 C (660 F). This process is
◦
◦
The crude oil produced from the atmospheric separator has called cracking and it is undesirable during the distillation
a composition different from the reservoir fluid from a pro- process since it changes the structure of hydrocarbons. For
ducing well. The light gases are separated and usually crude this reason, compounds having boiling points above 350 C
◦
oils have almost no methane and a small C 2 –C 3 content while (660+ F) called residuum are removed from the bottom of
◦
the C 7+ content is higher than the original reservoir fluid. As atmospheric distillation column and sent to a vacuum dis-
an example, the composition of a crude oil produced through tillation column. The pressure in a vacuum distillation col-
a three-stage separator from a reservoir fluid is also given in umn is about 50–100 mm Hg, where hydrocarbons are boiled
Table 1.2. Actually this crude is produced from a black oil at much lower temperatures. Since distillation cannot com-
reservoir fluid (composition given in Table 1.2). Two impor- pletely separate the compounds, there is no pure hydrocarbon
tant characterisitcs of a crude that determine its quality are as a product of a distillation column. A group of hydrocarbons
the API gravity (specific gravity) and the sulfur content. Gen- can be separated through distillation according to the boiling
erally, a crude with the API gravity of less than 20 (SG > 0.934) point of the lightest and heaviest compounds in the mixtures.
is called heavy crude and with API gravity of greater than 40 The lightest product of an atmospheric column is a mixture of
(SG < 0.825) is called light crude [1, 9]. Similarly, if the sulfur methane and ethane (but mainly ethane) that has the boiling

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