Petroleum products are mixtures of many compounds that differ widely in their physical and chemi-cal properties. Some of them may be vaporized in the absence of air at atmospheric pressure without leaving an appreciable residue. Other nonvolatile compounds leave a carbonaceous residue when destructively distilled under such conditions. This residue is known as carbon residue when deter-mined in accordance with prescribed procedure.
The carbon residue is a property that can be correlated with several other properties of petroleum (Speight, 2000; Figure 2.2); hence, it also presents indications of the volatility of the crude oil and
0
50 20.0
Asphaltenes (wt%) Nitrogen (wt%) Sulfur (wt%)
0.5 5.0
Viscosity, poises (×105)
100 40.0 1.0 10.0
0.0 0.0 0.0
0 10 20 30 40
Carbon residue (wt%) FIGURE 2.2 Relationship of carbon residue to other physical properties.
the coke-forming (or gasoline-producing) propensity. However, tests for carbon residue are some-times used to evaluate the carbonaceous depositing characteristics of fuels used in certain types of oil-burning equipment and internal combustion engines.
The mechanical design and operating conditions of such equipment have such a profound influ-ence on carbon deposition during service that comparison of carbon residues between oils should be considered as giving only a rough approximation of relative deposit-forming tendencies. A more precise relationship between carbon residue and hydrogen content, H/C atomic ratio, nitrogen con-tent, and sulfur content has been shown to exist. These data can provide more precise information about the anticipated behavior of a variety of feedstocks in thermal processes.
Because of the extremely small values of carbon residue obtained by the Conradson and Ramsbottom methods when applied to the lighter distillate fuel oils, it is customary to distill such products to 10% residual oil and determine the carbon residue thereof. Such values may be used directly in comparing fuel oils, as long as it is kept in mind that the value is that for a residuum oil and are not to be compared with the carbon residue of the whole feedstock.
There are two older methods for determining the carbon residue of a petroleum or petroleum product: the Conradson method (ASTM D189) and the Ramsbottom method (ASTM D524). Both are applicable to the relatively nonvolatile portion of petroleum and petroleum products, which partially decompose when distilled at a pressure of 1 atmosphere. However, crude oil that contains ash-forming constituents will have an erroneously high carbon residue by either method unless the ash is first removed from the oil; the degree of error is proportional to the amount of ash.
A third method, involving micropyrolysis of the sample, is also available as a standard test method (ASTM D4530). The method requires smaller sample amounts and was originally devel-oped as a thermogravimetric method. The carbon residue produced by this method is often referred to as the microcarbon residue (MCR). Agreements between the data from the three methods are good, making it possible to interrelate all of the data from carbon residue tests (Long and Speight, 1989).
Even though the three methods have their relative merits, there is a tendency to advocate the use of the more expedient microcarbon method to the exclusion of the Conradson and Ramsbottom methods because of the lesser amounts required in the microcarbon method, which is somewhat less precise in practical technique.
The mechanical design and operating conditions of such equipment have such a profound influ-ence on carbon deposition during service that comparison of carbon residues between oils should be considered as giving only a rough approximation of relative deposit-forming tendencies. Recent work has focused on the carbon residue of the different fractions of crude oils, especially the asphal-tene constituents (Figure 2.3). A more precise relationship between carbon residue and hydrogen content, H/C atomic ratio, nitrogen content, and sulfur content has been shown to exist. These data can provide more precise information about the anticipated behavior of a variety of feedstocks in thermal processes. Thus, there is a fairly universal linear correlation between the carbon residue (Conradson) and the H/C ratio:
H/C = 171 − 0.0115CR (Conradson)
This equation holds within two limits; at H/C values = 171, where the carbon residue is zero (no coke formation) and H/C = 0.5, where the carbon residue is 100 (all the material converts to coke under test conditions). There is a relationship between the carbon residue (Conradson) and the nitro-gen content.
Because of the extremely small values of carbon residue obtained by the Conradson and Ramsbottom methods when applied to the lighter distillate fuel oils, it is customary to distill such products to 10% residual oil and determine the carbon residue thereof. Such values may be used directly in comparing fuel oils, as long as it is kept in mind that the values are carbon residues on 10% residual oil and are not to be compared with straight carbon residues.
2.4.3 CritiCal ProPertieS
A study of the pressure, volume, and temperature relationships of a pure component reveals a particular unique state where the properties of a liquid and vapor become indistinguishable from each other. At that state, the latent heat of vaporization becomes zero and no volume change occurs when the liquid is vaporized. This state is called the critical state, and the appropriate parameters of the state are termed the critical pressure (PC), critical volume (VC), and critical temperature (TC). It is an important characteristic of the critical state for a pure component with values of P or T greater than either PC or TC, the vapor and liquid states cannot coexist at equi-librium, and thus, PC and TC represent the maximum values of P and T at which phase separation can occur.
Since the critical state of a component is unique, it is perhaps not surprising that knowledge of PC, TC, and VC allows many predictions to be made concerning the physical properties of sub-stances. These predictions are based on the law of corresponding states that states that substances behave in the same way when they are in the same state with reference to the critical state. The particular corresponding state is characterized by its reduced properties, that is, Tr = T/TC, Pr = P/PC, Vr = V/VC.
The use of this concept permits generalized plots in terms of reduced properties to be drawn that are then applicable to all substances (which obey the law) and can be of great value in determining thermodynamic relationships. It is rare in petroleum engineering to have to deal with pure sub-stances, and unfortunately, the application of the law of corresponding states to mixtures is compli-cated by the fact that the use of the true critical point for a mixture does not yield correct values of reduced properties for accurate prediction from generalized charts. For a mixture, the critical state no longer represents the maximum temperature and pressure at which a liquid and vapor phase can coexist and phase separation can occur under retrograde conditions.
For engineering purposes, this difficulty is resolved by the use of pseudocritical conditions, which are based on the molal average critical temperatures and pressures of the compounds of the mixture. Although the use of pseudo-reduced conditions for mixtures of hydrocarbons is generally satisfactory, this is not true for states near the true critical, nor, in general, for mixtures of vapor and liquid.
0 0.8 1.0
H/C ratio (atomic) 1.5
1.8
10 20 30 40
Asphaltenes Resins
Saturates and aromatics
Carbon residue (wt%)
50 60 70
FIGURE 2.3 Relationship of thermal coke yields to feedstock fractions.
The temperature, pressure, and volume at the critical state are of considerable interest in petro-leum physics, particularly in connection with modern high-pressure, high-temperature refinery operations and in correlating pressure–temperature–volume relationships for other states. Critical data are known for most of the lower-molecular-weight pure hydrocarbons, and standard methods are generally used for such determinations.
The critical point of a pure compound is the equilibrium state in which its gaseous and liq-uid phases are indistinguishable and coexistent; they have the same intensive properties. However, localized variations in these phase properties may be evident experimentally. The definition of the critical point of a mixture is the same. However, mixtures generally have a maximum temperature or pressure at other than the true critical point; maximum here denotes the greatest value at which two phases can coexist in equilibrium.
Thus, when a pure compound is heated at atmospheric pressure, it eventually reaches its boiling point and is completely vaporized at a constant temperature unless the pressure is increased. If the pressure is increased, the compound is completely condensed and cannot be vaporized again unless the temperature is also increased. This mechanism, alternately increasing the pressure and tempera-ture, functions until at some high temperature and pressure it is found that the material cannot be condensed regardless of the amount of pressure applied. This point is called the “critical point,” and the temperature and pressure at the critical point are called the “critical temperature” and “critical pressure,” respectively.
The liquid phase and vapor phase merge at the critical point so that one phase cannot be distin-guished from the other. No volume change occurs when a liquid is vaporized at the critical point, and no heat is required for vaporization but the coefficient of expansion has become large.
Limited information concerning the behavior of complex mixtures is required that the pseudo-critical temperature and pseudopseudo-critical pressure be used for many petroleum fractions and prod-ucts. The pseudocritical point is defined as the molal average critical temperature and pressure of the several constituents that make up a mixture. It may be used as the critical point of a mixture in computing reduced temperatures and pressures. However, in computing the pressure–volume–
temperature relations of mixtures by the use of the pseudocritical point, it must be recognized that the values are not accurate in the region of the critical point and that it cannot be applied to mixtures of gas and liquid.
In the correlation of many properties, reduced properties are useful. Reduced properties are defined as the ratio of the actual value of the property to its critical value. Thus, for volume, tem-perature, or pressure, the relationships are as follows:
Reduced volume V V
R V
c
, =
where
V is the volume at specified conditions Vc is the volume at the critical point Similarly,
Reduced temperature TR = T/Tc
Reduced volume PR = P/Pc
where
T and P are the temperature and volume, respectively, at specified conditions Tc and Vc are the temperature and volume, respectively, at the critical point