Refining Glossary


Absorption - The disappearance of one substance into another so that the absorbed substance loses its identifying characteristics, while the absorbing substance retains most of its original physical aspects. Used in refining to selectively remove specific componens from process streams.

Acid Treating - Process in which unfinished petroleum products, such as gasoline, naphthas, kerosine, diesel fuel, and lubricating-oil stocks, are contacted with sulfuric acid to improced their color, odor, and other properties.

Additives - Any materials incorporated in finished petroleum products for improving their performance in existing applications or for broadening the areas of their utility.

Adsorption - A process in which a solid substance concentrates or holds another substance (usually liquid or gas) upon its surface.

Aliphatic Hydrocarbons - Organic hydrocarbon compounds in which the carbon atoms are joined in open chains, as opposed to ring structures of aromatic and naphthenic compounds. Examples: ethane, propane, butane. Many are building blocks for petrochemicals.

Alkylation - A refinery process for chemically combining isoparaffin with olefin hydrocarbons. The product, alkylate, has high octane value and is blended with motor and aviation gasoline to improve the antiknock value of the fuel.

American Candle - A standard candle whose illuminating power is sometimes employed as a unit in determing this characteristic of kerosine. Other units are the International and Hefner candles.

A.M.P. - American Melting Point. used for paraffin wax, it is an arbitrary 3ºF above the ASTM D 87 test for melting point of paraffin wax.

Aniline Point - The minimum temperature for a complete mixing of equal volumes of aniline, a liquid which has selective solvent action on hydrocarbons, and the oil being tested. Used in some specifications to indicate the aromatic content of oils, also in calculating approximate heat of combustion (ASTM D 611).

Antiknock Fluid - Common parlance for the chemical mixture containing tetra-ethyl lead as the knock inhibiting ingredients and ethylene dichloride and/or ethylene dibromide as scavengers of the lead products formed in combustion. Used as knock suppressant in gasoline.

API Gravity - An arbitrary scale expressing the gravity, or density, of liquid petroleum products in terms of API degrees. Relationship of the API scale to specific gravity is expressed by the equation ºAPI = (441.5/sp gr 60/60F) - 131.5. ASTM D 287 is the standard method for determining API gravity by means of a hydrometer.

Aromatic Hydrocarbons - Hydrocarbon characterized by unsaturated ring structures of the carbon atoms. Commercial petroleum aromatics are benzene, toluene, and xylenes.

Ash - The amount of ash or nonvolatile, incombustible content left from heating petroleum oils to the point of complete burning of the oil. Ash content is expressed in weight per cent of the original sample.

Asphalt - A dark brown to black cementatious material - solid, semisolid, or liquid in consistency in which the predominating constituents are bitumens. Petroleum asphalt, as distinguished from asphalt occurring as such in nature, is refined from crude petroleum into commercial grades of widely varying consistency. Asphalt is a natural constituent of asphaltic-base crude oils, some of which are refined primarily for its recovery.

Asphalt Cement (A.C.) - A refined asphalt, or combination of refined asphalt and flux, of suitable consistency for paving purposes.

Asphaltic Concrete - A plant mix of asphalt cement with coarse graded mineral aggregate, used in the construction of asphalt bases, binder courses, and surface courses.

Auto-ignition - The spontaneous ignition and resulting rapid reaction of a portion of or all the fuel-air mixture in the combustion chamber of an internal combustion engine. The flame speed is many times greater than that following normal ignition.


Barrel - For statistical purposes, the petroleum industry uses a barrel containing 42 U.S. standard gallons. Where the price of fuel oils or other products is quoted by the barrel, it is the 42-gal barrel, regardless of the method of shipment.

Baumé Gravity (Be) - Gravity expressed on the Baumé scale for liquids lighter or heavier than water. The API gravity scale is now used for liquids by the petroleum industry instead of the Baumé scale.

BD or BCD - Barrels per day or barrels per calendar day.

Benzene, Benzol - An aromatic hydrocarbon present in small proportion in some crude oils and made commercially from petroleum by the catalytic reforming of naphthenes in petroleum naphtha. Also made from coal in the manufacture of coke. Used as solvent in manufacturing detergents, synthetic fibers, and petrochemicals and as a component of high-octane gasoline.

Bitumen - Mixtures of hydrocarbons of natural or pyrogenous origin or a combination of both; frequently accompanied by non-metallic derivatives which may be gaseous, liquid, semi-solid, or solid and which are completely soluble in carbon disulfide. Include natural and petroleum asphalt.

Bleeding - The tendency of a liquid component to separate from liquid-solid or semi-solid mixture, as oil from lubricating grease in storage.

Blending - The process of mixing two or more oils having different properties to obtain a product of intermediate properties.

Bloom - The fluorescent color of a lubricating oil as shown by reflected light when this color differs from that shown by transmitted light.

Boiling Range - The spread of temperatures, usually expressed in degrees Fahrenheit, over which an oil starts to boil or distill vapors and proceeds to complete evaporation. Boiling range is determined by ASTM test procedures for specific petroleum products.

Bottled Gas - Trade term for LPG or LP-gas.

Breathing - The movement of gas (oil vapors or air) in and out of the vent line of storage tanks, due to daily temperature changes. A cause of evaporation losses in light oil products in storage.

Bright Stocks - High viscosity, fully refined, and dewaxed lubricating oils produced by the treatment of residual stocks and used to compound motor oils.

BSD - Operating capacity of a refinery expressed in barrels per day the unit is operating or "on stream".

BS&W - Bottom settlings and water. The heavy material which collects in the bottom of storage tanks, usually composed of oil, water, and foreign matter.

Btu (British thermal units) - The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. Used in expressing the thermal value of heating oils.

Bulk Plant or Bulk Terminal - Wholesale receiving and distributing facilities for petroleum products. Commonly includes railroad sidings, truck loading racks, and sometimes water and product pipeline receiving facilities; storage tanks for gasoline, kerosine, and heating oils; warehouse for storing and handling lubricating oils, greases, and other products sold in drums and packages.

Bunker C Fuel Oil - A heavy residual fuel oil used by ships and industry and for large scale heating installations, similar in requirements to No. 6 grade fuel oil.

Burner Fuel Oils - Trade term for distillate and residual oils sold for heating homes and buildings.

Burning Oil - Kerosine, mineral seal, or other petroleum light oils of such gravity and other properties that they can be used for illuminating and similar purposes.

Burning Point - The lowest temperature at which a volatile oil in an open vessel will continue to burn when ignited by a flame held close to its surface. It determines the degree of safety with which kerosine and other illuminating oils can be used.

Byproduct - A secondary or additional petroleum product, not of primary importance in the conventional refinery operating schedule. Examples: residual fuel oil, coke, asphalt, and road oil.


Carbon Black - A solid or finely divided carbon recovered by burning natrual gas or oil in a deficient supply of air or by thermal decomposition. Used in compounding rubber and making inks, paints, etc.

Carbon Residue - The amount of carbonaceous material left after evaporation and pyrolysis of an oil y standard test methods ASTM D 189 and D 524.

Catalyst - A substance used to accelerate or retard a chemical reaction without itself undergoing significant chemical change or change in volume during the process.

Catalytic Reforming - A catalytic process to improve the antiknock quality of low grade naphthas and virgin gasolines by the conversion of naphthenes and paraffins into higher octane aromatics such as benzene, toluene and xylenes.

Cat Cracker - Trade term for the refinery operating unit where a catalytic cracking process is being carried out.

Cetane - A pure paraffin hydrocarbon used as standard reference fuel in determining the ignition qualities of diesel fuels. It is arbitrarily given a cetane number of 100.

Cetane Index - An empirical method for determing the cetane number of a diesel fuel by a formula based on API gravity and the mid-boiling point (ASTM D 975).

Cetane Number - A term for expressing the ignition quality of a diesel fuel.

CFR (Cooperative Fuel Research) Engine - A standard test engine used in determining the octane number of motor fuels.

Chromometer, Colorimeter - Instrument used in determining the color of petroleum oils and petrolatum.

Clay - Filtering medium, especially fuller's earth, used in refineries, for the purpose of absorbing the solids or colorizing materials in oils.

Cleveland Open-cup Tester - A standard laboratory apparatus used in determining flash and fire points of petroleum products.

Cloud Test - The method (ASTM D 97) for determining the temperature, known as cloud point, at which paraffin wax or other solid substances begin to crystallize out or separate from solution when an oil is chilled under specified conditions.

Coke - The solid residue remaining after the destructive distillation of crude petroleum or residual fractions. Used commercially as domestic and industrial fuel and, when purified, in various metallurgical and industrial processes.

Cold Settling - Process for removing petroleum wax from cylinder stock and high viscosity distillate by chilling a naphtha solution of the oi and allowing the wax to crystallize out of the solution and settle to the bottom of the pans.

Cold Test - The temperature at which an oil become solid. Generally considered to be 5ºF lower than the pour point.

Combustion Shock - Abnormal burning of fuel in an internal combustion enginer.

Condensate - (1) A highly gaseous liquid coming from gas condensate wells, from which the gas is separated, the liquid remaining being shipped with crude oil in pipelines to refineries. (2) Any liquid material coming from the condensers in a refinery.

Cracking - Process carried out in a refinery reactor in which the large molecules in the charge stock are broken up into smaller, lower-boiling, stable hydrocarbon molecules, which leave the vessel overhead as unfinished cracked gasoline, kerosines, and gas oils. At the same time, certain of the unstable or reactive molecules in the charge stock combine to form tar or coke bottoms. The cracking reaction may be carried out with heat and pressure (thermal cracking) or in the presence of a catalyst (catalytic cracking).

Crude Oil, Crude Petroleum - A naturally occurring mixture, consisting predominantly of hydrocarbons and/or of sulfur, nitrogen, and/or oxygen derivatives of hydrocarbons, which is capable of being removed from the earth in a liquid state. Basic types of crudes are asphaltic, naphthenic, or paraffinic, depending on the relative proportion of these types of hydrocarbons present.

Cutback Asphalt - Asphalt which has been softened or liquefied by blending with petroleum distillates.

Cutting Oil - Oil used to lubricate and cool metal-cutting tools; usually mineral oil blended with other substances to make them water-soluble or water-insoluble, as required.

Cycle Plant - Similar to a natural gasoline plant in that the liquid hydrocarbons are removed from natural gas. In a cycle plant the gas is then put back into the ground to maintain pressure on the oil reservoir.

Cycle Stock - Unfinished product taken from a stage of a refinery process and recharged to the process at an earlier period in the operation.

Cycling - A series of operations in petroleum refining or natural-gas processing so conducted that the steps are periodically repeated in the same sequence.


Deasphalting - Process for removing asphalt from petroleum fractions, such as reduced crude.

Debutanizer - The fractionating column in a natural gasoline plant in which the butane and lighter components are removed overhead. The gas stream remaining is referred to as debutanized.

Deethanizer - The fractionating column in a natural gasoline plant in which ethane and lighter components are removed overhead. The gas stream remaining is deethanized.

Dehydrogenation - A reaction in which hydrogen atoms are eliminated from a molecule. Dehydrogenation is used to convert ethane, propane, and butane into olefins.

Depropanizer - The fractionating column in a natural gasoline plant in which propane and lighter components are removed overhead. The gas stream remaining is depropanized.

Desalting - Removing calcium chloride, magnesium chloride, and sodium chloride from crude petroleum.

Destructive Distillation - Process of distillation in which a hydrocarbon or other organic compound or mixture is heated to a temperature high enough to cause its decomposition.

Desulfurization - A chemical treatment to remove sulfur or sulfur compounds from crude oil.

Detonation - In a gasoline engine, the phenomenon occurring when the last portion of the fuel-air mixture ignites spontaneously instead of burning in a normal manner. The resulting explosion is known as engine knock.

Dewaxed Oils - Lubricating oils from which a portion of the wax has been removed.

Dew Point - The temperature at which vaporized materials start to condense into liquid form.

Distillate - The product of distillation obtained by condensing the vapors from a refinery still, also known as overhead fractions, as distinguished from the non-vaporizing residual components left in the still.

Distillate Fuel Oils - A general classification for one of the overhead fractions produced from crude oil in conventional distillation operations. The so called light heating oils, diesel fuels, and gas oils come from this fraction.

Distillation - The general process of vaporizing liquids, crude oil, or one of its fractions in a closed vessel, collecting and condensing the vapors into liquids. Commercial forms of distillation in petroleum refining are crude, atmospheric, vacuum, rerun, steam, extractive, etc.

Downflow - Process in which the hydrocarbon stream flows from top to bottom.

Dropping Point - For lubricating greases, the temperature at which the grease passes from a semi-solid to a liquid state under prescribed test conditions (ASTM D 566).

Dry-Cleaning Fluid - A petroleum naphtha having narrow, carefully selected boiling points and other properties for dry cleaning.

Dry Gas - Natural gas, mainly from gas fields rather than oil fields, which does not contain appreciable quantities of the heavier hydrocarbons, such as propane and butane, which condense easily. Methane and ethane are principal components of dry gases.

Dry Point - In the standard distillation test, the temperature when the last drop of liquid evaporates from the bottom of the flask. For solvents and some other products considered to be more indicative of final boiling point than end point, which is the maximum temperature observed on the distillation thermometer when no more vapor can be driven from the flask by heating.


Emulsification - The phenomenon of fine dispersion of one liquid held in suspension in a second liquid in which it is partly or completely immiscible.

Emsulsification Test - Standard laboratory procedure for evaluating the resistance of insulating oils, turbine oils, and other lubricating oils to emulsification.

Emulsified Asphalt - An emulsion of asphalt cement and water, containing a small amount of an emulsifying agent.

End Point - In the distillation tests for gasoline and other products, the highest thermometer reading during the distillation, usually that when the sample has been entirely vaporized. Also the final boiling point.

Engine Oil - Generic term applied to oils used for the bearing lubrication of all types of engines, machines, and shafting and for cylinder lubrication other than steam engines. In internal combustion enginers synonymous with motor oils, crankcase oils.

Engine Sludge - The insoluble degradation product of lubricating oils and/or fuels formed during their use in internal combustion engines and deposited from the oils outside the combustion space. Water may or may not be present.

Epoxy Resins - Plastics materials, petrochemically derived, used as surface coatings, laminating adhesives, in paints, etc.

Ethyl Fluid - Gasoline antiknock compound.

Extract, Extract Oil - In solvent refining, the less desirable portion of the oil under process, which is dissolved in and removed by the selective solvent used.

Extractive Distillation - In the distillation process, the separation of different components of mixtures which have similar vapor pressures by flowing a solvent which is selective for some of the components in the feed down the distillation column as the operation proceeds. By this means, the less soluble component passes overhead while the soluble component is scrubbed from the vapors. The solvent with the dissolved component is deposited in the bottom of the column and withdrawn for separation.


Feed Preparation Unit - A fractionation unit in a refinery, the primary purposes of which is to prepare one or more close-boiling point cuts to be used as feed for subsequent processing.

Film Strength - The property of a lubricant which enables it to maintain an unbroken film over lubricated surfaces under operating conditions, thus avoiding scuffing or scoring of bearing surfaces.

Filter Press - In petroleum refining, the equipment used to separate wax and oil in paraffin-wax distillates. It consists of a series of canvas-covered plates separated by narrow iron rings. The distillate is run into a narrow bore extending the length of the press and is forced into the spaces between the plates formed by the rings. The oil penetrates the canvas covering the plates and drips into a trough beneath the press, the wax remaining in the plates.

Fire Point - The lowest temperature at which, under standard test conditions (ASTM D 92), a petroleum product vaporizes rapidly enough to form above its surface an air-vapor mixture which burns continuously when ignited by s small flame. Generally applies for all petroleum products except fuel oils and those having an open-cup flash point below 175ºF.

Flash Point - The lowest temperature at which vapors from an oil will ignite momentarily on application of a flame under standard test conditions. In the range of 90 - 150ºF, flash point is significant in determining safety conditions for the storage, handling, and use of petroleum products.

Floating Roof - Special type of steel tank roof which floats upon the surface of the oil in the tank, thereby eliminating tank breathing and reducing evaporation losses.

Fractional Distillation - Separation of the components of a liquid mixture by vaporizing it and collecting the fractions, which condense in different temperature ranges.

Fractions - Refiner's term for the portions of oils containing a number of hydrocarbon compounds but within certain boiling ranges, separated from other portions in fractional distillation. They are distinguished from pure compounds which have specified boiling temperatures.

Freezing Point - The temperature at which a substance freezes by standard test procedure (ASTM D 1015). Used in determining the degree of purity of high-purity hydrocarbon compounds.

Fuel Oils - Any liquid or liquefiable petroleum product burned for the generation of heat in a furnace or firebox or for the generation of power in an engine, exclusive of oils with a flash point below 100ºF and oils burned in cotton- or wool-wick burners.

Fuller's Earth - A clay having a high natural adsorption capacity, used in refineries to filter and decolorize oils.


Gas Oils - A fraction derived in refining petroleum with a boiling range between kerosine and lubricating oil. Derives its name from having originally been used in the manufacture of illuminating gas. Now supplies distillate-type fuel oils and diesel fuel, also cracked to produce gasoline.

Gasoline - A refined petroleum naphtha which by its composition is suitable for fuel in a reciprocating-type internal combustion engine. ASTM D 439 specifies three grades for various types of motor vehicle operations. Straight-run gasoline is the product of distillation; cracked gasoline that of a cracking process.

Girbitol Process - A process for removing hydrogen sulfide, carbon dioxide, and/or organic gases from petroleum gases and liquids.


Heart Cut - In refining, a narrow boiling range fraction, usually taken near the middle portion of the stock being processed.

Heating Oils - Trade term for the group of distillate fuel oils used in heating homes and buildings as distinguished from residual fuel oils used in heating and power installations. Both are burner fuel oils.

Heavy Ends - The highest boiling portion of a gasoline or other petroleum oil. The end point as determined by the distillation test reflects the amount and character of the heavy ends present in a gasoline.

Hydrodesulfurizing - A process for combining hydrogen with the sulfur in refinery petroleum streams to make hydrogen sulfide, which is removed from the oil as a gas.

Hydrofining - A process for treating petroleum fractions and unfinished oils in the presence of catalysts and substantial quantities of hydrogen to upgrade their quality.

Hydroforming - A commercial catalytic reforming process developed in 1939 to produce aromatic components for high octance aviation gasoline. One of the several catalytic reforming processes.

Hydrogenation - A refinery process in which hydrogen is added to the molecules of unsaturated hydrocarbon fractios.

Hydrogen Sulfide - An objectionable impurity present in some natural gas and crude oils and formed during the refining of sulfur-containing oils. It is removed from products by various treating methods at the refinery.


Induction Period - A period under given conditions in which a petroleum product does not absorb oxygen at a substantial rate to form gum.

Inhibitor - An additive substance which, when present in a petroleum product, prevents or retards undesirable changes taking place in the product, particurlary oxidation and corrosion.

Isomerization - A refining process which alters the fundamental arrangement of atoms in the molecule. Used to convert normal butane into isobutane, an alkylation process feedstock, and normal pentane and hexane into isopentane and isohexane, high-octane gasoline components.


Jet Fuel - Kerosine-type fuels or blends of gasoline, distillate, and residual oils which are used as fuels for gas turbine powered aircraft.


Knock - The sound associated with the auto-ignition in the combustion chamber of an automobile engine of a portion of the fuel-air mixture ahead of the advancing flame front.

Knockout Drum - A vessel wherein suspended liquid is separated from gas or vapor.


Light Ends - The lower boiling components of a mixture of hydrocarbons.

Lime Treatment - The process of introducing lime into the still during the distillation of petroleum to reduce the acidity of the distillate. Other forms of distillate treatment are often preferred.

Liquefied Petroleum Gas. LP-Gas, LPG, Bottled Gas - Industry terms for any material composed predominantly of the following hydrocarbons or mixture of them: Propane, propylene, butanes, and butylenes. Recovered from natural and refinery gases and kept under pressure in a liquid state, they are marketed in liquid form for industrial and domestic use as gas.

Low Line / Low Pressure Gas - Low pressure (5 psi) gas from atmospheric and vacuum distillation recovery systems that is collected in the gas plant for compression to higher pressures.

Lube Stocks - Refinery term for fractions of crude petroleum of suitable boiling range and viscosity to yield lubricating oils when further processed and treated.


Mercaptans - Compounds of sulfur having a strong, repulsive garlic-like odor. A contaminant of sour crude oil and products.

Motor Oils - Lubricating oils designed for used in the oil circulating systems of automotive, aircraft, and diesel engines.


Naphtha - Liquid hydrocarbon fractions, generally boiling within the gasoline range, recovered by the distillation of crude petroleum. Used as solvents, dry-cleaning agents, and charge stocks to reforming units to make high-octane gasoline.

Naphthenic Crudes - A type of crude petroleum containing a relatively large proportion of naphthenic-type hydrocarbon.

Neutralization Number - An indication of the acidity of a petroleum product or lubricant. ASTM D 664 and D 974 are standard procedures for establishing neutralization values.


Octane Number - The antiknock quality of motor and aviation gasoline below 100 octane is expressed by a numerical scale which is based on the knocking tendencies of two pure hydrocarbons. One, normal heptane, has an assigned value of zero in the knock rating scale. The second, iso-octane, has an assigned octance number of 100.

Oil Emulsion - A mixture of oil and water in which the oil is permanently suspended in the water in the form of very small droplets or vice versa.

Olefins - A class of unsaturated paraffinic hydrocarbons recovered from petroleum, of which butene, ethylene, and propylene are examples.

On Stream - When a refinery processing unit is in operation.

Overhead - In a distilling operation that portion of the charge which leaves the top of the distillation column as a vapor.


Paraffin - A white, tasteless, odorless, chemically inert, waxy substance obtained from some petroleum oils.

Paraffin, Liquid - Highly refined, colorless, heavy USP oil used for medicinal purposes and other applications.

Paraffin-base Crude - A type of crude oil containing predominantly paraffin hydrocarbons as distinguished from asphaltic and naphthenic-base crudes. It is a source of high quality lubricating oils.

Petrolatum - A semi-solid product, ranging from white to yellow in color, composed of heavy residual oils and paraffin wax produced by filtration of cylinder stocks. Has varied pharmaceutical and industrial uses.

Polyforming - A process charging C3 and C4 gases with naphtha or gas oil under thermal conditions to produce high quality gasoline and fuel oil. Catalytic reforming has mostly replaced polyforming.

Polymerization - The process of combining two or more simple molecules of the same type, called monomers, to form a single molecule having the same elements in the same proportion as in the original molecule but having different molecular weights. The product of the combination is a polymer.

Pour Point - The lowest temperature at which an oil will pour or flow when chilled, without disturbance, under test conditions in ASTM D 97.

Pour Stability - The ability of a pour-depressant treated oil to maintain its original pour point when in storage at low temperatures approximating winter conditions.

Precipitate - A substance separating in solid form from a liquid as the result of some physical or chemical change, differing from a substance held only mechanically in suspension, which is known as sediment.

Precipitation Number - The basis for classifying steam cylinder stocks and other classes of residual oils as to relative content of asphaltic constituents by a standard test method (ASTM D 91).

Pyrophoric Iron Sulfide - A substance typically formed inside tanks and processing units by the corrosive interaction of sulfur compounds in the hydrocarbons and the iron and steel in the equipment. On exposure to air (oxygen) it ignites spontaneously.


Quenching Oil - An oil introduced into high-temperature process streams during refining to cool them.


Raffinate - In solvent refining, that portion of the oil which remains undissolved and is not removed by the selective solvent. Also called "good" oil.

Reboiler - An auxiliary unit of a fractionating tower designed to supply additional heat to the lower portion of the tower.

Recycle Gas - High hydrogen-content gas returned to a unit for reprocessing.

Reduced Crude - A residual product remaining after the removal by distillation of an apprciable quantity of the more volative components of crude oil.

Refluxing - In fractional distillation, the return of part of the condensed vapor to the fractionating column to assist in making a more complete separation of the desired fractions. The material returned is reflux.

Reformed Gasoline - Gasoline made by a reforming process.

Reforming - The mild thermal cracking of naphthas to obtain more volative products, such as olefins, of higher octane values; or catalytic conversion of naphtha components to produce higher octance aromatic compounds.

Regeneration - In a catalytic process the reactivation of the catalyst, sometimes done by burning off the coke deposits under carefully controlled conditions of temperature and oxygen content of the regeneration gas stream.


Scrubbing - Purification of a gas or liquid by washing it in a tower.

Selective Solvent - A solvent which, at certain temperatures and ratios, will selectively dissolve more of one component of a mixture than of another and thereby provide partial separation.

Shale Oil - The liquid obtained from the destructive distillation of oil shale. Further processing is required to convert it into products similar to petroleum oils.

Slop Oil - A term designating the small amounts of oil lost in various refining operations which are collected and used as charge stocks.

Sludge - The residue left after treatment in the refinery of petroleum oils to remove impurities. Also an insoluble degradation product of crankcase oils.

Solvent Refined - Term for lubricating oils which have been solvent-treated during the refining process. Most motor, airplane, diesel-engine, and steam turbine oils and other high quality oils are solvent refined.

Solvent Refining - The process of mixing a petroleum stock with a selected solvent, which preferentially disolves undesired constituents, separating the resulting two layer and recovering the solvent from the raffinate (the purified fraction) and from the extract by distillation.

Sour - (1) Crude oils containing a large amount of sulfur and sulfur compounds, which break down in refining to liberate troublesome quantities of corrosive sulfur compounds. (2) Gasoline, naphthas, and other refined oils containing hydrogen sulfide or other sulfur compounds.

Sour Gas - A gas containing sulfur bearing compounds such as hydrogen sulfide and mercaptans and usually corrosive.

Stabilization - A process for separating the gaseous and more volatile liquid hydrocarbons from crude petroleum or gasoline and leaving a stable (less volatile) liquid so that it can be handled or stored with less change in composition.

Straight-run - Fractions derived from the straight distillation of crude oil and containing no cracked material. Also called virgin stock.

Straight-run Distillation - Continuous distillation of petroleum oils which separates the products in the order of their boiling points without cracking.

Stripping - In refining, the removal of the more volatile components from a cut or fraction in order to raise the flash point of kerosine, gas oil, or lubricating oil.

Sulfonates - A group of petroleum hydrocarbons resulting from treating oils with sulfuric acid.

Sweet Wax - A white, moisture-free wax with the oil removed by a sweating process in which the unrefined wax is heated in shallow pans. In a semi-refined state it is known as sweated-scale wax. It can be filtered or re-run to yield a completely refined commercial roduct.

Sweet Crude - Crude petroleum containing little sulfur, with no offensive odor.

Sweet Gas - A gas containing no corrosive components such as hydrogen sulfide and mercaptans.

Sweetening - The process of improving petroleum products in color and odor by converting the undesirable sulfur compounds into less objectionable disulfides with sodium plumbite or by removing them by contacting the petroleum stream with alkalies or other sweetening agents.

Synthetic Crudes - The total liquid, multicomponent mixture of hydrocarbons resulting from a process involving molecular rearrangement of charge stock. Commonly applied to such products from cracking, reforming, visbreaking, etc.

Switch Loading - The loading of a high static-charge retaining hydrocarbon (diesel fuel) into a tank truck, tank car, or other vessels that has previously contained a low flash hydrocarbon (gasoline) and may contain a flammable mixture of vapor and air.


Tail, Tail End - That portion of an oil which vaporizes near the end of distillation, the heavy end.

Tailings - Remains, residues, or final byproducts from refining crude petroleum or its fractions.

Tank Bottoms - The oil in a tank below the level of the outlet pipe.

Tank Farm - Land on which a number of large-capacity storage tanks are located, generally for crude oil.

Thermal Cracking - The breaking up of heavy oil molecules into lighter fractions by the use of high temperature without the aid of catalysts. See Cracking.

Thermal Reforming - See Reforming.

Thief - A standard device which permits taking a sample from a predetermined location in the body of oil to be sampled.

Topped Crude - A residual product remaining after the removal, by distillation or other artificial means, of an appreciable quantity of the more volatile components of crude petroleum.

Topping - The distillation of crude petroleum to remove the light fractions only. The unrefined distillate obtained is called tops.

Tower - A refinery apparatus used in connection with a still to increase the degree of separation of fractions obtained during the distillation of oil in the still. Also called a column.

Turnaround - With a refinery unit, the procedure of shutting the unit down after a normal run, doing the necessary maintenance and repair work and putting the unit back on stream.


Unsaturates - Hydrocarbon compounds of such molecular structure that they readily pick up additional hydrogen atoms. Olefins and diolefins, which occur in cracking, are of this type.

Unsulfonated Residue - That portion of an oil which is not acted upon when the oil is agitated with a definite amount of sulfuric acid under specified conditions.


Vacuum Distillation - Distillation under reduced pressure, which reduces the boiling temperature of the material being distilled sufficiently to prevent decomposition or cracking. See Distillation.

Vapor Lock - The displacement of liquid fuel in the feed line and the interruption of normal motor operation, caused by the vaporization of light ends in the gasoline. Vaporization occurs when the temperature at some point in the fuel system exceeds the boiling points of the volatile light ends.

Vapor Pressure - The pressure exerted by the vapors released from an oil at a given temperature when enclosed in an airtight container. For motor gasoline a criterion of vapor-lock tendencies, for light products generally an index of storage and handling requirements. ASTM D 323 is the standard method of measuring vapor pressure for volatile products except for liquefied petroleum gases, where D 1267 is used.

Vapor Recovery Unit - A refinery unit to which gases and vaporized gasoline from various processing operations are charged to separate the mixed charged into desired intermediate qualities for further processing.

Virgin Stock - Oil processed from crude oil which contains no cracked material. Also called Straight Run Stock.

Visbreaking - Viscosity breaking is a low temperature cracking process used to reduce the viscosity or pour point of straight run residuum.


Wash - In petroleum refining, to cleanse or purify oils by agitation with water or chemicals.

Weathered Crude - Crude petroleum which, owing to evaporation and other natural causes during storage and handling, has lost an appreciable quantity of its more volatile components.

Wet Gas - Mainly natural gas produced along with crude petroleum in oil fields. In addition to methane, ethane, propane and butane, wet gas contains some higher hydrocarbons such as pentane and hexane which, with propane and butane, are easily recoverable as liquids.


Propietary Chemicals