3.2 CONTEXTO LABORAL
3.2.3 Políticas educativas: Carta Magna, Plan Nacional de
Simulated distillation by gas chromatography is often applied in the petroleum industry to obtain true boiling point data for distillates and crude oils (Speight, 2001). Two standardized methods (ASTM D2887 and ASTM D3710) are available for the boiling point determination of petroleum fractions and gasoline, respectively. The ASTM D2887 method utilizes nonpolar, packed gas chro-matographic columns in conjunction with flame ionization detection. The upper limit of the boiling range covered by this method is approximately 540°C (1000°F) atmospheric equivalent boiling point. Recent efforts in which high-temperature gas chromatography was used have focused on extending the scope of the ASTM D2887 method for higher-boiling petroleum materials to 800°C (1470°F) atmospheric equivalent boiling point.
example, the normal hydrocarbons (from hexane up) have an electrical conductivity smaller than 10−16Ω/cm; benzene itself has an electrical conductivity of 4.4 × 10−17Ω/cm, and cyclohexane has an electrical conductivity of 7 × 10−18Ω/cm. It is generally recognized that hydrocarbons do not usu-ally have an electrical conductivity larger than 10−18Ω/cm. Thus, it is not surprising that the electri-cal conductivity of hydrocarbon oils is also exceedingly small—on the order of 10−19 to 10−12 Ω/cm.
Available data indicate that the observed conductivity is frequently more dependent on the method of measurement and the presence of trace impurities than on the chemical type of the oil.
Conduction through oils is not ohmic insofar as the current is not proportional to field strength: in some regions, it is observed to increase exponentially with the latter. Time effects are also observed, the current being at first relatively large and decreasing to a smaller steady value. This is partly because of electrode polarization and partly because of ions removed from the solution. Most oils increase in conductivity with rising temperatures.
2.5.2 dieleCtriC ConStant
The dielectric constant (ε) of a substance may be defined as the ratio of the capacity of a condenser with the material between the condenser plates C to that with the condenser empty and under vacuum C0:
e= C C0
The dielectric constant of petroleum and petroleum products may be used to indicate the presence of various constituents, such as asphaltene constituents, resin constituents, or oxidized materials.
TABLE 2.9
Distillation Profile of Bitumen (Athabasca, McMurray Formation, Upper Cretaceous, Alberta, Canada) and Selected Properties of the Fractions
Feedstock
Boiling Range
Wt%
Wt%
Cumulative
Specific Gravity
API Gravity
Sulfur Wt%
Carbon Residue (Conradson)
°C °F
Whole bitumen 100.0 1.030 5.9 5.8 19.6
Fractiona
1 0–50 0–122 0.0 0.0
2 50–75 122–167 0.0 0.0
3 75–100 167–212 0.0 0.0
4 100–125 212–257 0.0 0.0
5 125–150 257–302 0.9 0.9
6 150–175 302–347 0.8 1.7 0.809 43.4
7 175–200 347–392 1.1 2.8 0.823 40.4
8 200–225 392–437 1.1 3.9 0.848 35.4
9 225–250 437–482 4.1 8.0 0.866 31.8
10 250–275 482–527 11.9 19.9 0.867 31.7
11 <200 <392 1.6 21.5 0.878 29.7
12 200–225 392–437 3.2 24.7 0.929 20.8
13 225–250 437–482 6.1 30.8 0.947 17.9
14 250–275 482–527 6.4 37.2 0.958 16.2
15 275–300 527–572 10.6 47.8 0.972 14.1
Residuum >300 >572 49.5 97.3 39.6
a Distillation at 762 mm Hg and then at 40 mm Hg for fractions 11–15.
Furthermore, the dielectric constant of petroleum products that are used in equipment, such as condensers, may actually affect the electrical properties and performance of that equipment (ASTM D877).
The dielectric constant of hydrocarbons and hence most crude oils and their products is usually low and decreases with an increase in temperature. It is also noteworthy that for hydrocarbons, hydrocarbon fractions, and products the dielectric constant is approximately equal to the square of the refractive index. Polar materials have dielectric constants greater than the square of the refrac-tive index.
2.5.3 dieleCtriC StrengtH
The dielectric strength, or breakdown voltage (ASTM D877), is the greatest potential gradient or potential that an insulator can withstand without permitting an electric discharge. The property is, in the case of oils as well as other dielectric materials, somewhat dependent on the method of measurement, that is, on the length of path through which the breakdown occurs, the composi-tion, shape, and condition of the electrode surfaces, and the duration of the applied potential difference.
The standard test used in North America is applied to oils of petroleum origin for use in cables, transformers, oil circuit breakers, and similar apparatus. Oils of high purity and cleanliness show nearly the same value under standard conditions, generally ranging from 30 to 35 kV. For alkanes, dielectric strength has been shown to increase linearly with liquid density, and the value for a min-eral oil fits the data well. For n-heptane, a correlation was found between the dielectric strength and the density changes with temperature. There are many reasons that the dielectric strength of an insulator may fail. The most important appears to be the presence of some type of impurity, produced by corrosion, oxidation, thermal or electrical cracking, or gaseous discharge; invasion by water is a common trouble.
2.5.4 dieleCtriC loSSand Power faCtorS
A condenser insulated with an ideal dielectric shows no dissipation of energy when an alternating potential is applied. The charging current, technically termed the circulating current, lags exactly 90° in phase angle behind the applied potential, and the energy stored in the condenser during each half-cycle is completely recovered in the next. No real dielectric material exhibits this ideal behav-ior; that is, some energy is dissipated under alternating stress and appears as heat. Such a lack of efficiency is broadly termed dielectric loss.
Ordinary conduction comprises one component of dielectric loss. Here, the capacitance-held charge is partly lost by short circuit through the medium. Other effects in the presence of an alter-nating field occur, and a dielectric of zero conductivity may still exhibit losses. Suspended drop-lets of another phase undergo spheroidal oscillation by electrostatic induction effects and dissipate energy as heat as a consequence of the viscosity of the medium. Polar molecules oscillate as elec-trets and dissipate energy on collision with others. All such losses are of practical importance when insulation is used in connection with alternating current equipment.
The measure of the dielectric loss is the power factor. This is defined as the factor k in the fol-lowing relation:
k = W EI
where W is the power in watts dissipated by a circuit portion under voltage E and passing current, I.
From ac theory, the power factor is recognized as the cosine of the phase angle between the voltage and current where a pure sine wave form exists for both; it increases with a use in temperature. When an insulating material serves as the dielectric of a condenser the power factor is an intrinsic property of the dielectric. For practical electrical equipment, low-power factors for the insulation are of course always desirable; petroleum oils are generally excellent in this respect, having values of the order of 0.0005, comparable with fused quartz and poly-styrene resin constituents. The power factor of pure hydrocarbons is extremely small. Traces of polar impurities, however, cause a striking increase. All electrical oils, therefore, are drastically refined and handled with care to avoid contamination; insoluble oxidation products are particu-larly undesirable.
2.5.5 StatiC eleCtrifiCation
Dielectric liquids, particularly light naphtha, may acquire high static charges on flowing through or being sprayed from metal pipes. The effect seems to be associated with colloidally dispersed contaminants, such as oxidation products, which can be removed by drastic filtration or adsorption.
Since a considerable fire hazard is involved that a variety of methods have been studied for mini-mizing the danger.
For large-scale storage, avoidance of surface agitation and the use of floating metal roofs on tanks are beneficial. High humidity in the surrounding atmosphere is helpful in lowering the static charge, and radioactive materials have been used to try to induce discharge to ground. A variety of additives have been found, which increase the conductivity of petroleum liquids, thus lowering the degree of electrification; chromium salts of alkylated salicylic acids and other salts of alkylated sulfosuccinic acids are employed in low concentrations, say 0.005%.