In addition to hydrocarbons, crude oil contains hetroatom (S, N, metals) species that need to be removed if their concentrations are higher than the specified thresholds. Other impurities in crude oil include salt and sediment and water. The acidity of crude oil is also important, particularly for concerns of corrosion in pipes or other process units. Carbon residue of a crude oil indicates the tendency to generate coke on heter tubes or rector surfaces. All of these contaminants and properties of crude oils are measured using standard methods, as described in this section.
Sulfur content of crude oils is the second most important property of crude oils, next to API gravity. Sulfur content is expressed as weight percent of sulfur in oil and typically varies in the range from 0.1 to 5.0%wt. The standard methods that are used to measure the sulfur content are ASTM D129, D1552, and D2622, depending on the sulfur level. Crude oils with more than 0.5%wt sulfur need to be treated extensively during petroleum refining. Using the sulfur content, crude oils can be classified as sweet (<0.5%wt S) and sour (>0.5% %wt S). The distillation process segregates sulfur species in higher concentrations into the higher-boiling fractions and distillation residua. Removing sulfur from petroleum products is one of the most important processes in a refinery to produce fuels compliant with environmental regulations.
Nitrogen content of crude oils is also expressed as weight percent of oil. Basic nitrogen compounds are particularly undesirable in crude oil fractions, as they deactivate the acidic sites on catalysts used in conversion processes. Some nitrogen compounds are also corrosive. Crude oils with nitrogen contents greater than 0.25%wt need treatment in refineries for nitrogen removal.
Most common metals that are found in crude oil are included in organometallic compounds like nickel, vanadium iron and copper, ranging in concentration from a few ppm up to 1000 ppm by weight, depending on the source of crude oil. Similar to sulfur species, the metallic compounds tend to concentrate in the higher-boiling fraction of crude oil. Higher metal contents also require treatment during petroleum refining because of the corrosion activity of some metals and their tendency to accumulate on catalyst surfaces, thus deactivating the catalysts in a number of refinery processes. Metal content can be measured using a standard EPA Method 3040.
Acidity of crude oil is measured by titration with potassium hydroxide (KOH), using the standard method ASTM D664. The measured acidity is expressed as the Total Acid Number (TAN) that is equivalent to milligrams of KOH required to neutralize 1 gram of oil. This number is particularly important to control corrosion in the distillation columns through selection of corrosion-resistant alloys for surfaces that come into contact with oil.
Carbon residue (as % wt of crude oil, or crude oil fraction) is determined as the weight of solid residue remaining after heating crude oil to coking temperatures (700-800°C). Two standard tests with slightly different procedures are used to measure carbon residue: ASTM D524 Ramsbottom Carbon Residue (RCR) and ASTM D189 Conradson Carbon Residue (CCR). Carbon residue relates to asphalt (or asphaltenes) content of oil and indicates the tendency of fouling in heater tubes and catalyst deactivation. The higher the carbon residue, the higher is the coking (fouling) propensity of crude oil.
The standard method ASTM D4007 is used to measure the amount of suspended inorganic solid particles and water (BS&W) in crude oils. These contaminants are mixed with the oil during production, and high concentration of BS&W causes operational problems in a refinery.
Salt content of crude oils can be measured using the standard method ASTM D3230 and reported as lb NaCl/1000 bbl. Desalting (removing the salt) is necessary when NaCl content is greater than 10 lbs/1000 bbl. Such high salt contents lead to corrosion in distillation towers and other equipment.