These are poly-aromatic compounds, insoluble in n-heptane, with a number of carbon atoms greater than 50. The asphalthene content of a crude may be the cause of deposits in inter-changers and/or lines. In fact, the mix of a crude having a high asphalthene content with a paraffinic crude can displace the balance of the asphalthenes, precipitating them. A high asphalthene content ensures that the vacuum pitch will be suitable for producing asphalt.
ASTM D86 distillation is a test that measures the volatility of gasoline, kerosene and diesel.
Basic Sediment and Water (BSW)
The BSW relates to the content of free water (not dissolved) and sediments (mud, sand) in the crude. It is important that its reading is low in order to avoid dirtiness and difficulties during the crude processing, in which the steam produced by the free water can damage the oven. It is reported as a percentage in volume over the crude.
This is the weight of the residue remaining after the combustion of a fuel sample. It represents the facility of a heavy fuel to produce particles during combustion.
This is the measurement of the mass of a volume. It is expressed in kilograms per liter, or grams per cubic centimeter. Density depends on the temperature as this affects the volume of the substances.
Temperature at which a liquid stops flowing when cooled, through the precipitation of crystals of solid paraffin.
The draining temperature is very important as, in the unloading of paraffinic crudes using sea terminals with underwater pipelines of a certain length, the temperature of the crude can fall below the draining point, creating deposits of wax or solid paraffin in the pipelines, thus obstructing the flow.
This is the minimum temperature at which the vapors of a product flash or detonate momentarily when a flame is applied in controlled conditions. It represents the maximum temperature at which a product can be stored or transported in safe conditions.
This is the temperature at which the crystals formed during the cooling of a product sample disappear completely when the temperature rises in a controlled way
The metals content of a crude, vanadium and nickel, gives us an indication of their content in the heaviest products obtained in the refining. This is important because, for example, the metals in gas oil in vacuum are poison for the catalytic and hydro-cracking catalysts. A high vanadium content or metals in the combustible oil may cause oven and boiler tube breakage problems because they form corrosive products during combustion.
Number of cetane
This measures the ease with which the spontaneous ignition occurs of diesel oil using a standardized engine and a reference fuel.
The cetane rating is determined by making a comparison of the ignition time of a mix of cetane (C16)) and hepta-methyl-nonane (C 15), which has the same delay time in ignition as the fuel being examined. The cetane rating measured is the percentage of the cetane compound in the cetane/hepta-methyl-nonane mix.
The C16 has a cetane rating equal to 100 (it is an easily-ignited paraffin) and C15 has a cetane rating equal to 0 (as being a slow-combustion aromatic).
A high cetane rating represents a high ignition quality or a short delay time between the fuel injection and the start of combustion.
The diesel engine uses a high compression ratio to produce the spontaneous ignition of the diesel, instead of a spark as in the case of the internal combustion engine. The compressed air temperature in the diesel engine is sufficiently high to fire the diesel.
The lineal paraffins have a high cetane rating and therefore burn well; on the other hand, the aromatics are of a low cetane rating and burn badly, producing deposits of carbon and the production of black smoke. For that reason, high-quality diesel should have an aromatic content compatible with the specified cetane rating.
The cetane rating can be calculated based on the volatility (corresponding to the temperature of 50% distilled) and the density of the diesel and is called Calculated Cetane Rating. The reason for using the formula is the high cost of the cetane engine.
Octane number (NOR)
The RVP and NOR are the most important parameters of gasoline quality. The NOR measures the resistance of the gasoline to self-ignition or premature detonation in an engine's functioning conditions.
Self-ignition is noted for the hammering or noise produced when the gasoline self-ignites, detonating before the cylinder compresses all the gasoline and air mixture, losing power. The detonation produces sound waves that are detected using special microphones.
The octane rating is measured by comparing the noise of the detonation made by a reference fuel mixture in a standardized engine with that for which the fuel examined is made. The reference fuels are iso-octane (2, 2, 4 trimethyl pentane), with an octane rating equal to 100 (high resistance to hammering) and the n-heptane which has an octane rating of zero (very low resistance). The octane rating determined is the percentage in volume of iso-octane in the iso-octane/heptane mixture.
Fuels with a high octane rating have greater resistance to premature detonation than those of a lower octane rating. In addition, fuels with a high octane rating can be used in engines with a high compression ratio, which are more efficient.
There are two types of engine for the determination of the octane rating of gasoline. One uses the Research method and the other the Motor method.
The Research method represents the behavior of an engine in cities at low and moderate speeds. The Motor method represents situations with fast acceleration, like climbing gradients or overtaking.
There is another way of expressing the octane rating of a gasoline which is called Highway Octane. The Highway Octane rating is expressed as the sum of the Research octane and the Motor octane ratings divided by 2. The Highway Octane rating is used in the United States while the Research method is used in Chile.
Reid Vapor Pressure (RVP)
The Reid vapor pressure is an empirical test that measures the pressure in pounds per square inch (psi) exercised by the vapors or light components of the crude or of an oil product, in a closed container at a temperature of 100 °F (38 °C).
A high vapor pressure of the crude tells us that light products are present in it and that they will burn in the torch in the processing if there is no suitable recovery system. In the case of an internal combustion engine, excessive vapor pressure will cause a blockage which will impede the flow of gasoline.
Crude oil contains salt (NaCI) which comes from the oil fields or the sea water used as ballast by oil tankers. It is necessary to extract the salt with desalination equipment before the crude oil can enter the atmospheric distillation oven in order to avoid corrosion that is produced in the upper part of the atmospheric tower. The salt decomposes and produces chlorhydric acid. It is expressed in grams of salt per cubic meter of crude.
The temperature at which some products inflame spontaneously in contact with the air (without flame), probably due to the heat the show oxidation produces, which accumulates, raising the temperature to the inflammation point. Fortunately, the oil distillers have very high self-ignition temperatures and are therefore difficult to achieve; this is 450ºC in gasoline. Oily rags, on the other hand, self-ignite easily and cause fires and so should be suitably destroyed.
The ratio of the weight of a substance and the weight of an equal volume of water at the same temperature. In oil, the API specific gravity is used which is measured with hydrometers that float in the liquid. The API grades are read directly on the scale that stands above the liquid at the flotation line point. The API scale arose from the facility of graduating the hydrometer rod uniformly.
°API =141.5/(specific gravity) -131.5
The °API determines whether the crude or product is light or heavy and enables us to calculate the tons of this unloaded. A light crude has an API of 40-50 while a heavy one has 10-24.
Sulfur and the API are the properties with the greatest influence on the price of crude.
This is the resistance to degradation through heat or oxidation of an oil product. Products containing olefinic material are unstable and susceptible to degradation.
The sulfur content permits the foreseeing of difficulties in meeting product and atmospheric emission specifications, as treatment units are needed to meet these; it is also poison for some catalysts. It also enables us to see whether the plant metallurgy is the most suitable for processing it. It is expressed as a percentage in weight of sulfur.
Sulfuric acid (H2S)
A prior knowledge of the sulfuric acid content of the crude permits preventive actions and avoids accidents to people. The sulfhydric acid is very dangerous because it anesthetizes the olfactory nerve which prevents people from being aware of the situation and is mortal in small quantities. Personnel working in contact with the crude have therefore to wear protection equipment and personal sulfhydric acid sensors.
This is the degree of resistance of a liquid to flow. The greater the viscosity, the greater the resistance to flow. Viscosity is affected by the temperature, reducing it when the latter rises. It is measured by using special viscosimeters and is expressed in USS (Universal Saybolt Seconds), FSS (Furol Saybolt Seconds) and in centi-stokes.
Viscosity is important for fuel injection in engines and burners. It is also critical in the pumping of crude oil and products by pipeline. A higher viscosity than that designed for will reduce the desired flow and make a greater pump motor capacity necessary. The viscosity also affects measuring instrument factors, altering the readings.
The measurement of the facility with which a product vaporizes. Volatile products have high steam pressure and a low boiling point. It is measured through the ASTM D86 test and is expressed as the temperature at which certain volumes are distilled.