Page 41 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

QC: IML/FFX
T1: IML
P2: IML/FFX
P1: IML/FFX
14:26
June 22, 2007
AT029-Manual
AT029-Manual-v7.cls
AT029-01
1. INTRODUCTION 21
the conversion factors may be simplified without major
error in the calculations. For example, 62.4 instead of 62.4259 volume fraction can be obtained from the following equation:
or 7.48 instead of 7.4803 are used in practical calculations.
In expressing values of densities, similar to specific volumes, (1.16) x vi = N x wi /SG i
the SC must be specified. Generally densities of liquid hydro- i=1 x wi /SG i
carbons are reported either in the form of specific gravity at
15.5 C (60 F) or the absolute density at 20 C and 1 atm in in which x vi is the volume fraction and SG i is the specific grav-
◦
◦
◦
3
g/cm . ity of component i. In Eq. (1.16) density (d) can also be used
instead of specific gravity. If mole and weight fractions are
1.7.14 Units of Specific Gravity multiplied by 100, then composition is calculated on the per-
centage basis. In a similar way the conversion of composition
For liquid systems, the specific gravity (SG) is defined as the from volume to weight and then to mole fraction can be ob-
ratio of density of a liquid to that of water, and therefore, it is tained by reversing the above equations. The composition of
a dimensionless quantity. However, the temperature at which a component in a liquid mixture may also be presented by its
specific gravity is reported should be specified. The specific molar density, units of which were discussed in Section 1.7.13.
gravity is also called relative density versus absolute density. Generally, a solution with solute molarity of 1 has 1 mol of
For liquid petroleum fractions and crude oils, densities of solute per 1 L of solution (1 mol/L). Through use of both
both the oil and water are expressed at the SC of 60 F (15.5 C) molecular weight of solute and density of solution one can
◦
◦
and 1 atm, and they are usually indicated as SG at 60 F/60 F obtain weight fraction from molarity. Another unit to express
◦
◦
or simply SG at 60 F. Another unit for the specific gravity of concentration of a solute in a liquid solution is molality. A so-
◦
liquid hydrocarbons is defined by the American Petroleum lution with molality of 1 has 1 mol of solute per 1 kg of liquid
Institute (API) and is called API degree and is defined in terms solvent.
of SG at 60 F (API = 141.5/SG−131.5). For gases, the spe- Another unit for the composition in small quantities is the
◦
cific gravity is defined as the ratio of density of the gas to ppm (part per million), which is defined as the ratio of unit
that of the air at the SC, which is equivalent to the ratio of weight (or volume) of a component to 10 units of weight or
6
molecular weights. Further discussion on specific gravity, def- volume for the whole mixture. Therefore, ppm can be pre-
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
initions, and methods of calculation are given in Chapter 2 sented in terms of both volume or weight. Usually in gases
(Section 2.1.3). the ppm is presented in terms of volume and in liquids it is
expressed in terms of weight. When ppm is presented in terms
1.7.15 Units of Composition of weight, its relation with wt% is 1 ppm = 10 −4 wt%. For ex-
ample, the maximum allowable concentration of H 2 S in air
Composition is the most important characteristic of homoge- for prolonged exposure is 10 ppm or 0.001 wt%. There is an-
nous mixtures in which two or more components are uni- other smaller unit defined as part per billion known as ppb
formly mixed in a single phase. Because of the nature of (1 ppm = 1000 ppb). In the United States a gas is considered
petroleum fluids, accurate knowledge of composition is im- “sweet” if the amount of its H 2 S content is no more than one
portant. Generally composition is expressed as percent- quarter grain per 100 scf of gas. This is almost equivalent to
age (%) or as fraction (percent/100) in terms of weight, mole, 4 × 10 −4 mol fraction [88]. This is in turn equivalent to 4 ppm
and volume. Density of the components (or pseudocompo- on the gas volume basis. Gas composition may also be rep-
nents) constituting a mixture is required to convert composi- resented in terms of partial pressure where sum of all partial
tion from weight basis to volume basis or vice versa. Similarly pressures is equivalent to the total pressure.
conversion of composition from mole basis to weight basis In general, the composition of gases is presented in volume
or vice versa requires molecular weight of the constituting or mole fractions, while the liquid composition may be pre-
components (or pseudocomponents). Mole, weight, and vol- sented in any form of weight, mole, or volume. For gases at
ume fractions are shown by x m , x w , and x v , respectively. Mole, low pressures (≤1 atm where a gas may be considered an ideal
weight, and volume percentages are shown by mol%, wt%, gas) mole fraction and volume fractions are the same. How-
and vol%, respectively. Some references use mol/mol, wt/wt, ever, generally under any conditions, volume and mole frac-
and vol/vol to express fractional compositions. For normal- tions are considered the same for gases and vapor mixtures.
ized compositions, the sum of fractions for all components For narrow boiling range petroleum fractions with composi-
in a mixture is 1 ( x i = 1) and the sum of all percentages is tions presented in terms of PNA percentages, it is assumed
100. If the molecular weights of all components in a mixture that densities and molecular weights for all three representa-
are the same, then the mole fraction and weight fraction are tive pseudocompoents are nearly the same. Therefore, with
identical. Similarly, if the density (or specific gravity) of all a good degree of approximation, it is assumed that the PNA
components is the same, the weight and volume fractions are composition in all three unit systems are the same and for
identical. The formula to calculate weight fraction from mole this reason on many occasions the PNA composition is repre-
fraction is given as sented only in terms of percentage (%) or fraction without in-
dicating their weight or volume basis. However, this is not the
x mi M i
(1.15) case for the crude or reservoir fluid compositions where the
N
x wi =
i=1 x mi M i composition is presented in terms of boiling point (or carbon
where N is the total number of components, M i is the molec- number) and not in the form of molecular type. The following
ular weight, and x wi and x mi are the weight and mole fractions example shows conversion of composition from one type to
of component i, respectively. The conversion from weight to another for a crude sample.

Copyright ASTM International
Provided by IHS Markit under license with ASTM Licensee=International Dealers Demo/2222333001, User=Anggiansah, Erick
No reproduction or networking permitted without license from IHS Not for Resale, 08/26/2021 21:56:35 MDT