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Definitions for Petroleum Refinery ICR: 

Activated sludge biological treatment unit: A basin, tank or surface impoundment where wastewater is 

aerobically treated using a biological floc composed of bacteria and protozoans (referred to as “activated 

sludge”) and air (oxygen) supplied be mechanical and/or diffused aerators followed by clarification and 

activated sludge return. 

Aerated surface impoundment: A surface impoundment where a transfer of oxygen from air into the 

solution of the basin occurs by mechanical action and/or diffused aerators. See also Surface 

impoundment. 

Aerobic sludge digester: A mechanical wastewater treatment unit that provides secondary wastewater 

treatment by mixing air (oxygen) and aerobic and facultative microbes with the wastewater sludge. 

Alkylate: The product of an alkylation reaction. It usually refers to the high-octane product from 

alkylation units. Alkylate is used in blending high octane gasoline. 

Alkylation unit: A refining process for chemically combining isobutane with olefin hydrocarbons (e.g., 

propylene, butylene) through the control of temperature and pressure in the presence of an acid catalyst, 

usually sulfuric acid or hydrofluoric acid. The product, alkylate, is an isoparaffin that has high-octane 

value and is blended with motor and aviation gasoline to improve the antiknock value of the fuel. 

Alternative fuels: Non-conventional or advanced fuels, for transportation applications, including the 

following: methanol; denatured ethanol, and other alcohols; fuel mixtures containing 85 percent or more 

by volume of methanol, denatured ethanol, and other alcohols with gasoline or other fuels, such as natural 

gas; liquefied petroleum gas (propane and/or butane); hydrogen; coal-derived liquid fuels; fuels (other 

than alcohol) derived from biological materials (biofuels such as soy diesel fuel); electricity (including 

electricity from solar energy). The term alternative fuels does not include alcohol or other blended 

portions of primarily petroleum-based fuels used as oxygenates or extenders (i.e., MTBE, ETBE, other 

ethers, and the 10-percent ethanol portion of gasohol). 

Alternative method: Any method of sampling and analyzing for an air pollutant which is not a reference 

or equivalent method but which has been demonstrated to the Administrator’s satisfaction to, in specific 

cases, produce results adequate for his determination of compliance. 

Amine treatment: A means to remove sulfur-containing and other undesirable compounds from acid gas 

by contacting the gas with amine. It is a common process in an oil refinery or in natural gas production. 

The amine absorbs H2S and other compounds, and the “rich” amine is then regenerated by steam stripping 

to produce regenerated or “lean” amine, along with an acid gas with a higher H2S concentration, which is 

then usually processed by the Claus process to convert it into sulfur. 

Anaerobic sludge digester: A tank or vessel which excludes oxygen and in which a sludge is modified by 

the action of anaerobic bacteria. 

Analyzer vent: A vent to the atmosphere associated with an extractive monitoring system, analyzer shed, 

or other equipment used to collect and transport samples to an analyzer. The analyzer vent does not 

include the exhaust from a flame ionization detector or associated chromatography column, if applicable. 

Aromatic extraction & UV stabilization: A lubricant manufacturing process by which vacuum tower 

residue or deasphalted oil (or both) is distilled for the separation of aromatics and cycloalkanes 

(naphthenes) to produce lube oil base oils having high viscosity indices and other desirable lube oil 
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qualities, such as color and stability. Main solvent used in this process is furfural, although phenol and 

liquid SO2 can also be used. 

Aromatics: Hydrocarbons characterized by their uniform unsaturated carbon ring structure and their often 

pleasant aroma. Commercial petroleum aromatics include benzene, toluene, ethyl benzene, xylenes, 

cumene, and naphthalene. The acronym BTX is commonly used to refer collectively to benzene, toluene, 

and xylenes; the acronym BTEX is commonly used to refer collectively to benzene, toluene, ethyl 

benzene, and xylenes. These chemicals are used as high octane components in gasoline. 

Aromatics production: All equipment used to produce aromatics from various separation processes after 

catalytic reforming. 

Asphalt: A dark brown-to-black cement-like material obtained by petroleum processing and containing 

bitumens as the predominant component. It includes crude asphalt as well as the following finished 

products: cements, fluxes, the asphalt content of emulsions (exclusive of water), and petroleum distillates 

blended with asphalt to make cutback asphalts. Note: The conversion factor for asphalt is 5.5 barrels per 

short ton. 

Atmospheric crude distillation or Atmospheric crude distillation unit: The primary distillation unit at a 

refinery used to separate crude oil components (including mixtures of other hydrocarbons) at 

approximately atmospheric pressure conditions by continuously vaporizing and condensing the 

components. For the purposes of this ICR, the atmospheric crude distillation unit includes the desalter or 

other crude oil feed pretreatment devices, the pipe still for vaporizing the crude oil, and the fractionation 

tower for separating the vaporized hydrocarbon components in the crude oil into fractions with different 

boiling ranges, the bottoms reboiler, and the overhead reflux condenser. 

Authorized representative: Person responsible for the overall operation of a facility or an operational unit 

(i.e., part of a facility), e.g., the plant manager, superintendent or person of equivalent responsibility. 

Aviation gasoline (finished): A complex mixture of relatively volatile hydrocarbons with or without 

small quantities of additives, blended to form a fuel suitable for use in aviation reciprocating engines. 

Fuel specifications are provided in ASTM Specification D 910 and Military Specification MIL-G-5572. 

Aviation gasoline blending components: Naphthas which will be used for blending or compounding into 

finished aviation gasoline (e.g., straight-run gasoline, alkylate, reformate, benzene, toluene, and xylene). 

Excludes oxygenates (alcohols, ethers), butane, and pentanes plus. 

Baghouse: An air pollution control device that filters dust particles by removing them from an air stream 

of exhaust gases before the stream is discharged to the atmosphere. Fabric filters, which use filtration to 

separate dust particles from dusty gases. They are one of the most efficient and cost effective types of 

dust collectors available and can achieve a collection efficiency of more than 99% for very fine 

particulates. 

Barrel: A unit of volume equal to 42 U.S. gallons. Barrel is abbreviated as bbl. 

Beavon process or BSR (Beavon sulfur removal: A process for removing residual sulfur compounds from 

the effluent gases from the “Claus” process. Usually used in conjunction of other processes. This process 

uses catalytic hydrogenation over a cobalt/ molybdenum catalyst converting carbonyl sulfide, carbon 

disulfide, and other organic sulfur compounds to hydrogen sulfide, which is then removed by the 

“Stretford” process. A variation of this is BSR/MDEA, developed for small plants, which uses 

preliminary scrubbing with methyl diethanolamine. 
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Benzene (C6H6): 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 a solvent in the manufacture of detergents, synthetic 

fibers, petrochemicals, and as a component of high-octane gasoline. See Aromatics. 

Biodiesel: A fuel typically made from soybean, canola, or other vegetable oils; animal fats; and recycled 

grease. It can serve as a substitute for petroleum-derived diesel or distillate fuel. It is a fuel composed of 

mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, and conforming to 

ASTM D6751-08, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate 

Fuels. 

Biogenic waste: Waste made from materials that were produced by living organisms or biological 

processes. 

Biofuels: Liquid fuels and blending components produced from biomass (plant) feedstocks, used 

primarily for transportation. 

Biomass or Biomass fuels: Non-fossilized and biodegradable organic material originating from plants, 

animals or microorganisms, including products, by-products, residues and waste from agriculture, forestry 

and related industries as well as the non-fossilized and biodegradable organic fractions of industrial and 

municipal wastes, including gases and liquids recovered from the decomposition of non-fossilized and 

biodegradable organic material. 

Bitumen: A naturally occurring viscous mixture, mainly of hydrocarbons heavier than pentane, that may 

contain sulfur compounds and that, in its natural occurring viscous state, is not recoverable at a 

commercial rate through a well. 

Blendstocks: Petroleum products used for blending or compounding into finished motor gasoline. These 

include RBOB (reformulated blendstock for oxygenate blending) and CBOB (conventional blendstock for 

oxygenate blending), but exclude oxygenates, butane, and pentanes plus. 

Blowdown: Act of emptying or depressuring a vessel. This may also refer to the discarded material such 

as blowdown water from a boiler or cooling tower. 

Blowdown steam recovery system: Boiler water is blown down to control the amount of suspended and 

total dissolved solids (TDS) in the boiler. This water is pressurized, hot and dirty, creating large volumes 

of flash steam and possible disposal problems. A heat recovery system can reclaim large amounts of 

energy during this essential process. The continuous boiler blowdown is routed to a flash tank where the 

pressure is reduced and flash steam is produced. The heat recovery system uses the flash steam from the 

flash tank for deaeration. The blowdown from the flash tank is passed through an exchanger and used to 

preheat boiler makeup water. This low pressure steam can also be used for other applications. 

Boiler: Enclosed device using controlled flame combustion and having the primary purpose of 

recovering and exporting thermal energy in the form of steam or hot water. Heat from an external 

combustion source is transmitted to a fluid contained within the tubes found in the boiler shell. This fluid 

is delivered to an end-use at a desired pressure, temperature, and quality. 

Boiler feed water: Water used to supply (“feed”) a boiler to generate steam or hot water. At most plants 

the feed water is usually conditioned or treated, stored, and pre-heated in a feed water tank and forwarded 

into the boiler by a boiler feed water pump. 
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Bottoms: The heaviest components of petroleum product left in the refining process after the light ends 

have been removed. These products include asphalt, coke and residuals. 

Bottom loading: A type of organic liquid loading operation where the discharge opening is set at the 

bottom of the loaded vessel (below the liquid level), reducing velocity and pressure, which results in a 

much lower vapor generation. 

Breathing losses or Standing losses: Displacement of vapors within a storage vessel due to changes in the 

tank temperature and pressure throughout the day and throughout the year. 

British thermal unit (BTU): Quantity of heat required to raise the temperature of one pound of water by 

one degree Fahrenheit at about 39.2 degrees Fahrenheit; equal to 252 calories. British thermal unit is 

abbreviated as Btu. 

Bulk terminal: A facility used primarily for the storage and/or marketing of petroleum products which 

has a total bulk storage capacity of 50,000 bbls or more and/or receives petroleum products by tanker, 

barge or pipeline. 

Butane or n-butane (C4H10): Normally a gaseous paraffinic straight-chain or branch-chain hydrocarbon 

extracted from natural gas or refinery gas streams. 

Butylene or n-butylene (C4H8): Olefinic straight-chain hydrocarbon extracted from refinery gas streams. 

By-product: A chemical that is produced coincidentally during the production of another chemical. 

Calcination: The process of thermally treating minerals without fusing to decompose carbonates from 

ore, so that hydrates, and the volatile material is expelled. 

Calcium hypochlorite: A white crystalline solid, Ca(OCl)2•4H2O, used as a bactericide, fungicide, and 

bleaching agent. 

CanSolv®: A regenerable process to remove sulfur dioxide from flue gas (flue-gas desulfurization) in tail 

gas cleanup processes. This patented technology uses an aqueous amine solution to achieve high 

efficiency selective absorption of sulfur dioxide from a variety of gas streams. The scrubbing byproduct 

is pure water-saturated SO2 gas recovered by steam stripping, using low quality heat. 

Carbon adsorber: A pollution control device that uses activated carbon to absorb volatile organic 

compounds from a gas stream (The VOC are later recovered from the carbon). 

Carbonate: Compounds containing the radical CO3

-2. Upon calcination, the carbonate radical 

decomposes to evolve carbon dioxide (CO2). Common carbonates consumed in the mineral industry 

include calcium carbonate (CaCO3) or calcite; magnesium carbonate (MgCO3) or magnesite; and 

calcium-magnesium carbonate (CaMg(CO3)2) or dolomite. 

Carbonyl sulfide (COS) hydrolysis: A thermal catalytic process through which COS is converted to H2S 

and CO2 in presence of water to facilitate removal of acid gases from feed gas streams such as synthesis 

gas produced by gasification of coal, coke, or heavy hydrocarbon oils. This process is mainly performed 

in combination with an acid gas removal process such as amine and solvent scrubbing systems (e.g., 

MDEA, Rectisol®, Selexol®, Sulfinol®). 
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Catalyst coke: In many catalytic operations (e.g., catalytic cracking and catalytic reforming), carbon is 

deposited on the catalyst, thus deactivating the catalyst. The catalyst is reactivated by burning off the 

carbon, which is used as a fuel in the refining process. This carbon or coke is not recoverable in a 

concentrated form. The conversion is 5 barrels (of 42 U.S. gallons each) per short ton. Coke from 

petroleum has a heating value of 6.024 million Btu per barrel. See Petroleum coke. 

Catalytic cracking: The refining process in which petroleum derivatives are continuously charged and 

hydrocarbon molecules in the presence of a catalyst are fractured or react with a contact material into 

smaller molecules to improve feedstock quality for additional processing and the catalyst or contact 

material is continuously regenerated by burning off coke and other deposits. For the purposes of this 

ICR, the catalytic cracking process unit includes the riser, reactor, primary (full-range) distillation 

column, regenerator, air blowers, spent catalyst or contact material stripper, catalyst or contact material 

recovery equipment, and regenerator equipment for controlling air pollutant emissions and for heat 

recovery (e.g., CO boiler). The catalytic cracking process unit does not include feed pretreatment (e.g., 

feed hydrotreater) or additional light-ends distillation columns after the primary (full-range) distillation 

column. Separate catalytic cracking reactor/regenerator pairs that exhaust to a common stack should be 

considered separate process units for the purposes of this ICR. Catalytic cracking process units typically 

employ a fluidized bed reactor (i.e., “fluid catalytic cracking units”), but they may employ a moving bed 

reactor (commonly referred to as a “thermal catalytic cracking units”). 

Catalytic hydrocracking unit: A refining process that uses hydrogen and catalysts with relatively low 

temperatures and high pressures for converting middle boiling or residual material to high-octane 

gasoline, reformer charge stock, jet fuel, and/or high grade fuel oil. The process uses one or more 

catalysts, depending upon product output, and can handle high sulfur feedstocks without prior 

desulfurization. For the purposes of this ICR, the catalytic hydrocracking unit includes the reactor, 

primary (full-range) distillation column, and hydrogen recovery equipment. The catalytic hydrocracking 

unit does not include feed pretreatment (e.g., feed hydrotreater) or additional light-ends distillation 

columns after the primary (full-range) distillation column and hydrogen recovery equipment. 

Catalytic hydrotreating or Catalytic hydrorefining: A refining process for treating petroleum fractions 

from atmospheric or vacuum distillation units (e.g., naphthas, middle distillates, reformer feeds, residual 

fuel oil, and heavy gas oil) and other petroleum (e.g., cat cracked naphtha, coker naphtha, gas oil) in the 

presence of catalysts and substantial quantities of hydrogen. Hydrotreating includes desulfurization, 

removal of substances (e.g., nitrogen compounds) that deactivate catalysts, conversion of olefins to 

paraffins to reduce gum formation in gasoline, and other processes to upgrade the quality of the fractions. 

Catalytic incinerator or Catalytic oxidizer: A control device that is mainly used for destroying organic 

compounds gas streams through an oxidation process employing a bed of active material (catalyst) that 

promotes the overall combustion reaction. Although no direct fire burning of feed gas stream occurs, it 

must be preheated to a sufficient high temperature (300 to 900 °F) to initiate the oxidation reactions. 

Catalytic reforming: A refining process using controlled heat and pressure with catalysts to rearrange 

certain hydrocarbon molecules, thereby converting paraffinic and naphthenic type hydrocarbons (e.g., 

low-octane gasoline boiling range fractions) into petrochemical feedstocks and higher octane stocks 

suitable for blending into finished gasoline. For the purposes of this ICR, the catalytic reforming process 

unit includes the reactor or series of reactors, catalyst regeneration equipment, primary (full-range) 

distillation column, and hydrogen recovery equipment. The catalytic reforming process unit does not 

include feed pretreatment (e.g., feed hydrotreater) or additional light-ends distillation columns after the 

primary (full-range) distillation column and hydrogen recovery equipment. Catalytic reforming units 

(CRU) are reported in three categories. They are: 
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• Continuous regeneration: Regeneration process where small quantities of the catalyst are 

continuously removed from a moving bed reactor system, purged, and transported to a 

continuously operated regeneration system. The regenerated catalyst is then recycled to the 

moving bed reactor. 

• Cyclic regeneration: Regeneration process where the unit has an extra reactor vessel, so that one 

reactor vessel can be isolated from the unit for regeneration. After the first vessel is regenerated, 

it is brought back on-line and the second reactor vessel is then isolated and regenerated and so on 

until all vessels have been regenerated. 

• Semi-regenerative: Regeneration process where the entire reforming unit is taken off-line to 

regenerate the catalyst in the reactor vessels. Catalyst regeneration in a semi-regenerative CRU 

typically occurs once every 12 to 24 months (18 months is typical) and lasts approximately 1 to 2 

weeks. 

Catch basin: An open basin which serves as a single collection point for stormwater runoff received 

directly from refinery surfaces and for refinery wastewater from process drains. 

Chloramine: Any of several compounds containing nitrogen and chlorine, especially an unstable 

colorless liquid, NH2Cl, used to make hydrazine. It is produced when ammonia and chlorine (as 

hypochlorous acid) react with each other. It is widely used as a disinfectant in water treatment plants 

rather than chlorine, as chloramine is much more stable. 

ChlorsorbTM

: The trade name of an air pollution control system for reducing chloride emissions from 

various process units, including catalytic reforming units. This system uses spent catalyst to recover 

chlorides from effluent gas streams and recycle the chlorides back to the process, decreasing the HCl 

content of the reduction zone effluent gas by as much as 99%. 

Claus sulfur recovery plant: A series of process units which recover sulfur from hydrogen sulfide (H2S) 

by a vapor-phase catalytic reaction of sulfur dioxide and H2S. The Claus sulfur recovery plant includes 

the reactor furnace and waste heat boiler, catalytic reactors, sulfur pits, and, if present, oxidation or 

reduction control systems. One Claus sulfur recovery plant may consist of multiple trains. See also 

Multi-stage Claus. 

Coal: All solid fuels classified as anthracite, bituminous, sub-bituminous, or lignite by the American 

Society for Testing and Materials Designation ASTM D388–05 Standard Classification of Coals by Rank. 

Co-fired: The process of simultaneously using two substantially different fuels within a process heater or 

boiler via the main burners. A process heater or boiler can be co-fired by supplying each burner with the 

two substantially different fuels at the same time or by supplying a portion of the burners with one fuel 

type and the other burners with a second fuel type. Fuels are considered substantially different when the 

fuels are of different physical phases, e.g., gaseous fuels are substantially different from liquid or solid 

fuels and liquid fuels are substantially different from gaseous or solid fuels. Process heaters that are fired 

with two different gaseous fuels, such as natural gas and still gas, are not co-fired process heaters. 

Process heaters that have gas burners with emergency oil back-up burners that are seldom if ever used are 

not considered co-fired process heaters. 

Coke burn-off: Coke removed from the surface of a catalyst by combustion during catalyst regeneration. 

Coke burn-off also means the coke combusted in fluid coking unit burner. 
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Coking: Thermal refining processes used to produce fuel gas, gasoline blendstocks, distillates, and 

petroleum coke from the heavier products of atmospheric and vacuum distillation. The coke produced 

can be used either as a fuel or in other applications such as the manufacturing of steel or aluminum. 

There are three types of coking process units: 

• Delayed coking: A semi-batch process where coke is produced in a series of coke drum reactor 

vessels. When a coke drum becomes filled with coke, it is taken off-line to remove the coke. The 

process feed is then diverted to a second coke drum to maintain near continuous feed processing. 

This is the most prevalent type of coking process. See Delayed coking unit. 

• Fluid coking: A continuous process unit in which high molecular weight petroleum derivatives 

are thermally cracked and petroleum coke is produced in a fluidized bed system. A portion of the 

coke produced is burned to provide energy for the coking reaction; the remainder of the coke 

produced is separated as a product. See Fluid coking unit. 

• Flexicoking: Similar to fluid coking except that the produced coke is then gasified to produce a 

synthetic fuel gas. See Flexicoking unit. 

Combined heat and power unit: A unit using a common energy source to produce both electricity and 

steam for other uses, resulting in increased fuel efficiency. 

Combustion: Chemical oxidation accompanied by the generation of light and heat. 

Combustion device: An individual unit of equipment such as a flare, incinerator, process heater, or boiler 

used for the combustion of organic hazardous air pollutant vapors. 

Condenser: Any device that removes condensable vapors by a reduction in the temperature of captured 

gases. 

Condensate: Hydrocarbon liquid separated from natural gas which condenses due to changes in the 

temperature or pressure, or both, and remains liquid at standard conditions. 

Connector: Any type of joined fittings other than a flange that is used to connect pipe line segments, 

tubing, pipe components (such as elbows, reducers, “T’s” or valves) or a pipe line and a piece of 

equipment or an instrument to a pipe, tube or piece of equipment. A common connector is a screwed 

fitting. Joined fittings welded completely around the circumference of the interface are not considered 

connectors. 

Controlled guidepole: A slotted guidepole with one of the following control measures specified in 

Appendix I to the storage tank emission reduction partnership program (STERPP) at 72 FR 19891: pole 

float system, pole sleeve system, internal sleeve system, or flexible enclosure system. 

Cooling tower: A heat removal device used to remove by direct contact the heat absorbed in circulating 

cooling water systems by transferring the heat to the atmosphere using natural or mechanical draft. 

Conventional amine scrubber: An air pollution control device that employs a high-energy liquid amine 

spray such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), or 

diisopropylamine (DIPA) to remove acidic substances (such as hydrogen sulfide and carbon dioxide) 

from industrial off-gases. 

Crude distillation unit: See Atmospheric crude distillation unit. 
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Crude oil: A mixture of hydrocarbons that exists in liquid phase in natural underground reservoirs and 

remains liquid at atmospheric pressure after passing through surface separating facilities. Depending 

upon the characteristics of the crude stream, it may also include small amounts of non-hydrocarbons 

produced from oil, such as sulfur and various metals, drip gases, and liquid hydrocarbons produced from 

tar sands, gilsonite, and oil shale. Crude oil is refined to produce a wide array of petroleum products, 

including heating oils; gasoline, diesel and jet fuels; lubricants; wax; asphalt; ethane, propane, and butane; 

and many other products used for their energy or chemical content. Crude oil is considered as either 

domestic or foreign, according to the following: 

• Domestic: Crude oil produced in the United States or from its “outer continental shelf’ as defined 

in 43 USC 1331. 

• Foreign: Crude oil produced outside the United States. Imported Athabasca hydrocarbons (tar 

sands from Canada) are included. 

Cryogenic separation process: A low temperature hydrogen separation process which uses the difference 

in boiling temperatures (relative volatilities) of the feed components to effect the separation. Hydrogen 

has a high relative volatility compared to hydrocarbons. The simplest and most common version of the 

cryogenic process is the partial condensation process. The partial condensation process condenses the 

required amount of feed impurities by cooling the feed stream against the warming product and tail gas 

streams in brazed aluminum multipass heat exchangers. The refrigeration required for the process is 

obtained by Joule-Thomson refrigeration derived from throttling the condensed liquid hydrocarbons. 

Additional refrigeration, if required, can be obtained by external refrigeration packages or by 

turboexpansion of the hydrogen product. 

Cutting water: Water used in the delayed coking process to “cut” the coke from the coke drum. High 

pressure water jets are commonly used for coke cutting operations. 

Delayed coking unit: A refinery process unit in which high molecular weight petroleum derivatives are 

thermally cracked and petroleum coke is produced in a series of closed, batch system reactors. A delayed 

coking unit includes all of the coke drums associated with a single fractionator; the fractionator, including 

the bottoms receiver and the overhead condenser; the coke drum cutting water and quench system, 

including the jet pump and coker quench water tank; process piping and associated equipment such as 

pumps, valves and connectors; and the coke drum blowdown recovery compressor system. See Coking. 

DESOX catalyst (50/25 ppmv SO2 Limit): A self-supporting hydrotalcite SOX reducing catalyst used in 

catalytic cracking units that achieves a 50 ppmv 7-day rolling average and 25 ppmv annual average SO2

concentration (dry basis, corrected to 0% excess air) in the regenerator exhaust. 

Desulfurization: The removal of sulfur, as from molten metals, petroleum oil, or flue gases. Petroleum 

desulfurization is a process that removes sulfur and its compounds from various streams during the 

refining process. Desulfurization processes include catalytic hydrotreating and other chemical/physical 

processes such as adsorption. Desulfurization processes vary based on the type of stream treated (e.g., 

naphtha, distillate, heavy gas oil) and the amount of sulfur removed (e.g., sulfur reduction to 10 ppm). 

DIPE (Di-isopropyl ether) (CH3)2•CHOCH(CH3)2: See Oxygenates. 

Dissolved air flotation unit: A tank and the corresponding ancillary equipment where the separation of 

solids and oils from liquid effluents occurs by dissolving pressurized air into a liquid to form small 

bubbles that will float material to the surface. 
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Distillate fuel oil: A general classification for one of the petroleum fractions produced in conventional 

distillation operations and from crackers and hydrotreating process units. The generic term distillate fuel 

oil includes kerosene, kerosene-type jet fuel, diesel fuels (Diesel Fuels No. 1, No. 2, and No. 4), and fuel 

oils (Fuel Oils No. 1, No. 2, and No. 4). Products known as diesel fuel are used in on-highway diesel 

engines, such as those in trucks and automobiles, as well as off-highway engines, such as those in railroad 

locomotives and agricultural machinery. Products known as fuel oils are used primarily for space heating 

and electric power generation. 

Distillation column or Distillation unit: A device or vessel in which distillation operations occur, 

including all associated internals (such as trays or packing) and accessories (e.g., the reboiler, condenser, 

vacuum pump, steam jet), plus any associated recovery system. 

Drain: Pit, pipe or trench used to collect or intercept and divert stormwater, wastewater, groundwater or 

perched water away from the ground, soil or treatment area or other system component to an effective 

outlet. 

Drain-dry: A storage vessel that is designed with a cone-down or sloped bottom and a sump. The 

storage vessel’s withdrawal line is located in the sump so that the only liquid remaining after the vessel is 

emptied is that which clings to the vessel bottom and other wetted surfaces. No free-standing liquid 

remains in the sump after emptying. 

Electricity-generating unit: A turbine (or similar device) and other ancillary equipment that is used to 

produce, store, and transmit electricity for sale to the grid or for use at other locations within the facility. 

Electrostatic precipitator (ESP): A particulate collection device that removes particles from a flowing 

gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly 

efficient filtration devices that minimally impede the flow of gases through the device, and can easily 

remove fine particulate matter such as dust and smoke from the air stream. 

Empty or Emptying: The partial or complete removal of stored liquid from a storage vessel. Storage 

vessels that contain liquid only as wall or bottom clingage, or in pools due to bottom irregularities, are 

considered completely empty. 

Energy efficiency: Refers to all changes or activities (including technological, behavioral and economic) 

that are aimed at reducing the energy used by substituting more advanced equipment, typically without 

affecting the services provided. Examples include high-efficiency appliances, efficient lighting programs, 

efficient maintenance programs, high-efficiency heating, ventilating, insulating and cooling systems or 

control modifications, efficient building/equipment design, advanced electric motor drives, and heat 

recovery systems. 

Equalization basin: A holding tank or surface impoundment within which variations in sewage inflow 

rate and liquid nutrient concentrations are averaged. 

Equipment: A pump, compressor, pressure relief device, sampling connection system, open-ended valve 

or line, valve, or instrumentation system. Vents from wastewater collection and conveyance systems 

(including, but not limited to wastewater drains, sewer vents, and sump drains), tank mixers, and sample 

valves on storage vessels are not equipment. 

Equipment or component malfunction: Any pump failure, pipe clogging, line breakage, or other similar 

event that can be directly identified and that substantially and unexpectedly alters the operation of a 

process unit or emission control device associated with a specific emissions point. 
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ETBE (Ethyl tertiary butyl ether) (CH3)3COC2H5: See Oxygenates. 

Ethane (C2H6): A normally gaseous straight-chain hydrocarbon. It is a colorless paraffinic gas that boils 

at a temperature of -127.48 degrees Fahrenheit. It is extracted from natural gas and refinery gas streams. 

Ethanol: A colorless liquid that burns to produce water and carbon dioxide. The vapor forms an 

explosive mixture with air and may be used as a fuel in internal combustion engines. An alcohol which is 

most often derived from corn. Ethanol is designed to be blended with gasoline to produce a cleaner 

burning fuel, and is an accepted oxygenate component for the oxygenated seasons mandated by the EPA. 

Ether: A generic term applied to a group of organic chemical compounds composed of carbon, hydrogen, 

and oxygen, characterized by an oxygen atom attached to two carbon atoms (e.g., methyl tertiary butyl 

ether). 

Ethylene (C2H4): An olefinic hydrocarbon recovered from refinery processes or petrochemical processes. 

Ethylene is used as a petrochemical feedstock for numerous chemical applications and the production of 

consumer goods. 

Ethylene process unit: A chemical manufacturing process unit in which ethylene and/or propylene are 

produced by separation from petroleum refining process streams or by subjecting hydrocarbons to high 

temperatures in the presence of steam. The ethylene process unit includes the separation of ethylene 

and/or propylene from associated streams such as a C4 product, pyrolysis gasoline, and pyrolysis fuel oil. 

The ethylene process does not include the manufacture of SOCMI chemicals such as the production of 

butadiene from the C4 stream and aromatics from pyrolysis gasoline. 

EuroClaus®: This process has been developed to recover elemental sulfur from H2S containing gases 

originating from gas treating plants such as alkanolamine units or physical solvent plants. The 

EUROCLAUS® process is an improvement of the SUPERCLAUS® process. Yields up to 99.7% overall 

sulfur recovery without any further tail gas clean up are possible. 

External floating roof: A storage vessel cover in an open top tank consisting of a double deck or pontoon 

single deck which rests upon and is supported by the petroleum liquid being contained and is equipped 

with a closure seal or seals to close the space between the roof edge and tank shell. 

Facility: Any physical property, plant, building, structure, source, or stationary equipment located on one 

or more contiguous or adjacent properties in actual physical contact or separated solely by a public 

roadway or other public right-of-way and under common ownership or common control. 

Feedstock: Raw material inputs to a process that are transformed by reaction, oxidation, or other 

chemical or physical methods into products and by-products. Supplemental fuel burned to provide heat or 

thermal energy is not a feedstock. Crude oil, VGO, catfeed, naphtha, condensate and straight run residual 

fuel are commonly referred to as feedstocks. 

Fixed roof tank: A tank with a mounted cover roof in a stationary position that does not move with 

fluctuations in the level of the liquid managed in the tank. 

Flange: A type of joined fitting that has a projecting rim or edge, usually drilled with holes, to allow 

bolting to other flanged fittings. A flange can be used to connect pipe line segments, pipe components 

(such as elbows, reducers, “T’s” or valves), or a pipe line and a piece of equipment or an instrument to a 

pipe or piece of equipment. A flanged fitting welded completely around the circumference of the 

interface is not considered a flange. 
11 

Flare: A combustion device, whether at ground level or elevated, that uses an open flame to burn 

combustible gases with combustion air provided by uncontrolled ambient air around the flame. The flare

includes the foundation, flare tip, structural support, burner, igniter, flare controls including air injection 

or steam injection systems, flame arrestors, and the flare gas header system. 

Flare gas: Gas disposed of by burning in flares usually at the production sites or at gas processing plants. 

Flare gas header system: All piping and knockout pots used to collect and transport gas to a flare either 

from a process unit or a pressure relief valve from the fuel gas system, regardless of whether or not a flare 

gas recovery system draws gas from the flare gas header system. 

Flare gas recovery system: a system of one or more compressors, piping, and the associated water seal, 

rupture disk, or similar device used to divert gas from the flare and direct the gas to the fuel gas system or 

to directly to a process heater, boiler, turbine or similar combustion device. 

Flare minimization plan: A plan each refinery is required to submit and that incorporates new measures 

identified to reduce refinery flare emissions by reducing the frequency and magnitude of flaring events 

(“prevention measures”). Prevention measures identified must address flaring as a result of planned 

major maintenance, including startup and shutdown; flaring that may be reasonably expected to occur due 

to issues of gas quality or quantity; and flaring caused by recurrent failure of air pollution control 

equipment, process equipment, or processes. 

Flashing losses: Occur when a liquid with entrained gases goes from a higher-pressure to a lowerpressure. As the pressure on the liquid drops some of the lighter compounds dissolved in the liquid are 

released or “flashed” and some of the compounds that are liquids at the initial pressure/temperature 

transform from a liquid into a gas/vapor and are also released or “flashed” from the liquid. As these gases 

are released, some of the heavier compounds in the liquids may become entrained in these gases and will 

be emitted with them. 

Flexicoking unit: One or more coking units in which high molecular weight petroleum derivatives are 

thermally cracked and petroleum coke is continuously produced in a fluidized bed system and in which 

the produced coke is then gasified to generate a synthetic fuel gas (“syngas”) for use in other processes. 

A flexicoking unit includes the fluidized bed coking vessel, the distillation column receiving gas directly 

from the fluidized bed coking vessel, coke burners, and other equipment for producing and handling the 

syngas. See Coking. 

Flexsorb®: The registered trade name of a sterically hindered amine scrubbing technology that 

selectively removes H2S from process gas streams (i.e., Flexicoker Low Btu Gas (LBG) stream), even in 

presence of CO2, reducing overall air emissions, and improving sulfur recovery. 

Flue gas desulfurization (FGD): The current state-of-the art technology used for removing SO2 from the 

exhaust flue gases in fuel combustion devices such as boilers, heaters and furnaces. Primary types of 

FGD processes are wet scrubbers, dry scrubbers, and sorbent injection. Sorbents include lime, limestone, 

sodium-based compounds, and high-calcium coal fly ash. 

Fluid catalytic cracking unit: A catalytic cracking unit that utilizes a fluidized bed reactor system. See 

Catalytic cracking. 

Fluid coking unit: One or more coking units in which high molecular weight petroleum derivatives are 

thermally cracked and petroleum coke is continuously produced in a fluidized bed system and in which 

the fluid coking burner exhaust gas is continuously released to the atmosphere. The fluid coking unit
12 

includes fluidized bed coking vessel, the distillation column receiving gas directly from the fluidized bed 

coking vessel, coke burner, and equipment for controlling air pollutant emissions and for heat recovery on 

the fluid coking burner exhaust vent. Flexicoking units that use gasifiers to generate a synthetic fuel gas 

for use in other processes and that do not exhaust to the atmosphere are not considered fluid coking units. 

See Coking. 

F/M (Food to mass ratio): The ratio between food (from primary effluent) and microorganisms (bacteria) 

in the aeration basins. It is a process control number that helps to determine the proper number of 

microorganisms for the wastewater treatment system. 

Force draft cooling: A mechanical draft tower with a blower type fan at the intake. The fan forces air 

into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much 

more susceptible to recirculation. Suitable to work with high static pressure. They can be installed in 

more confined spaces and even in some indoor situations. 

Fossil fuels: Natural gas, petroleum, coal, or any form of solid, liquid, or gaseous fuel derived from such 

material that resulted from the compression of ancient plant and animal life formed over millions of years, 

which is combusted with the purpose of creating useful heat or work. 

Fractionation or Distillation: The process by which hydrocarbons are separated into distinct products or 

intermediates based on differences in boiling points of the different hydrocarbon compounds. 

Fresh feed: Any petroleum derivative feedstock stream charged directly into the riser or reactor of a 

FCCU except for petroleum derivatives recycled within the FCCU, fractionator, or gas recovery unit. 

Fuel gas: Any gas which is generated at a petroleum refinery and which is combusted separately or in 

any combination with any type of gas. Fuel gas also includes natural gas when the natural gas is 

combined and combusted in any proportion with a gas generated at a refinery. Fuel gas does not include 

gases generated by catalytic cracking unit catalyst regenerators and fluid coking burners, but does include 

gases from flexicoking unit gasifiers. See Still gas and Refinery fuel gas. 

Fuel gas system: A system of compressors, piping, knock-out pots, mix drums, and, if necessary, units 

used to remove sulfur contaminants from the fuel gas (e.g., amine scrubbers) that collects gaseous streams 

from one or more sources for treatment, as necessary, and transport to a stationary combustion unit. A 

fuel gas system may have an overpressure vent to a flare but the primary purpose for a fuel gas system is 

to provide fuel to the various combustion units at the refinery or petrochemical plant, such as heaters, 

furnaces, boilers, incinerators, gas turbines, and other combustion devices located within or outside of the 

facility. The fuel is piped directly to each individual combustion device, and the system typically 

operates at pressures over atmospheric. The gaseous streams can contain a mixture of methane, light 

hydrocarbons (e.g., ethane, butane, hydrogen) and other miscellaneous species. 

Fuel gas treatment unit: A unit used to remove sulfur compounds from fuel gas.

Fuels solvent deasphalting: A refining process for removing asphalt compounds from petroleum 

fractions, such as reduced crude oil. The recovered stream from this process is used to produce fuel 

products. 

Gallon: A measure of volume equal to 4 quarts (231 cubic inches). One barrel equals 42 gallons. 
13 

Gas oil: A liquid petroleum distillate having a viscosity intermediate between that of kerosene and 

lubricating oil. It derives its name from having originally been used in the manufacture of illuminating 

gas. It is now used to produce distillate fuel oils and gasoline. 

Gas plant/light-ends distillation/LPG production unit: A series of equipment in which petroleum gas 

mixtures are separated from natural gas, or in which petroleum gas mixtures are fractionated or otherwise 

separated into liquefied petroleum gas (LPG) products or both. Examples of this equipment include: 

glycol dehydration; deethanizer, depropanizer, and/or debutanizer distillation columns. 

Gas service: Unless defined otherwise in the regulation to which a process unit is subject, a piece of 

equipment that contains a gas or vapor at operating conditions, and the organic compound concentration 

in the process fluid is at least 5 percent by weight. 

Gasification: A process where coal, petroleum coke, biomass, wastes, or other carbon-containing 

materials is subjected to high temperatures and mixed with steam and oxygen to produce a synthesis gas 

or “syngas” and can be burned to generate power, steam, or process heat. The syngas typically contains 

hydrogen, carbon monoxide, and various impurities. For the production of hydrogen, the syngas is 

cleansed of impurities and mixed again with steam at lower temperatures to create pure H2 and CO2 gas. 

The hydrogen is separated for use and the CO2 is either vented or captured. 

Gasoline blending components: Naphthas which will be used for blending or compounding into finished 

aviation or motor gasoline (e.g., straight-run gasoline, alkylate, reformate, benzene, toluene, and xylene). 

Excludes oxygenates (alcohols, ethers), butane, and pentanes plus. 

Geodesic dome roof: A domed or vaulted structure of a tank roof composed of straight elements that 

form interlocking polygons. 

GTBA (Gasoline-grade tertiary butyl alcohol, or T-Butanol) (CH3)3COH: See Oxygenates. 

Hard-piping: Pipe or tubing that is manufactured and properly installed using good engineering 

judgment and standards, such as ANSI B313. 

Heat content: The amount of heat energy available to be released by the transformation or use of a 

specified physical unit of an energy form (e.g., a ton of coal, a barrel of oil, a kilowatt-hour of electricity, 

a cubic foot of natural gas, or a pound of steam). The amount of heat energy is commonly expressed in 

British thermal units (Btu). 

Heat exchanger: Any device that transfers heat from one fluid (liquid or gas) to another or to the 

environment. 

Heat exchange system: A device or series of devices used to transfer heat from process fluids to water 

without intentional direct contact of the process fluid with the water (i.e., non-contact heat exchanger) and 

to transport and/or cool the water in a closed-loop recirculation system (cooling tower system) or a oncethrough system (e.g., river or pond water). For closed-loop recirculation systems, the heat exchange 

system consists of a cooling tower, all heat exchangers that are serviced by that cooling tower, and all 

water lines to and from the heat exchanger(s). For once-through systems, the heat exchange system

consists of one or more heat exchangers servicing an individual process unit and all water lines to and 

from the heat exchanger(s). Intentional direct contact with process fluids results in the formation of a 

wastewater. 
14 

Heat input capacity: The maximum rated energy input, determined on a higher heating value basis, of a 

boiler or process heater when burning the primary fuel. Heat input from preheated air, pilot flames, or 

other sources should not be included. 

Heavy liquid service: Unless defined otherwise in the regulation to which a process unit is subject, a 

piece of equipment not in gas service or light liquid service that contains a process fluid with an organic 

compound concentration of at least 5 percent by weight. 

Heavy gas oil: Petroleum distillates with an approximate boiling range from 651 degrees Fahrenheit to 

1000 degrees Fahrenheit. Commonly, this is a mixture of a distillate that contains residual fuel oil. 

Heavy oil/asphalt refinery: A refinery that processes heavy crude slates to produce primarily road 

asphalt, roofing tars, and other asphalt products. These refineries are similar in complexity and 

equipment to topping refineries, except they typically also use vacuum distillation. See Topping refinery. 

High-efficiency regenerator: A regenerator unit consisting of an energy-efficient version that operates in 

a fast-fluidized flow followed by plug flow reactor. The burning takes place in a fast-fluidized 

combustor. By conducting regeneration in this manner at high temperature, regeneration can be 

accomplished in a fraction of the time required and unit inventory is reduced, allowing complete CO 

combustion with little excess air or no promoter. 

Higher heating value (HHV): The thermodynamic heat of combustion or enthalpy change for the 

oxidation reaction of a fuel assuming a common temperature of the compounds before and after 

combustion, in which case the water produced by combustion is liquid. The higher heating value takes 

into account the latent heat of vaporization of water in the combustion products. 

Hydrochlorofluorocarbons (HCFCs): Chemicals composed of one or more carbon atoms and varying 

numbers of hydrogen, chlorine, and fluorine atoms. 

Hydrogen (H2): A colorless, odorless, highly flammable gaseous element. It is the lightest of all gases 

and the most abundant element in the universe, occurring chiefly in combination with oxygen in water 

and also in acids, bases, alcohols, petroleum, and other hydrocarbons. 

Hydrogen plant: A collection of process equipment that are used collectively for the production and 

purification of hydrogen as a primary product. These processes include hydrogen conversion (e.g., steam 

reforming, coal gasification, partial oxidation, electrolysis, thermolysis) and hydrogen purification (e.g., 

pressure-swing adsorption). Hydrogen plants do not include catalytic reforming units where hydrogen is 

produced as a by-product during the production of reformate. 

Hydroskimming refinery: A refinery equipped with atmospheric distillation, naphtha reforming, and 

necessary hydrotreating processes; hydroskimming refineries also typically include vacuum distillation as 

well as tankage, recovery facilities for gases and light hydrocarbons, and the necessary utility systems. A 

hydroskimming refinery is more complex than a topping refinery and it produces finished gasoline, but 

lacks conversion (coking or cracking) processes so they generally produce a surplus of fuel with 

unattractive price and demand. See Topping refinery. 

Individual drain system: A stationary system used to convey regulated-material to a waste management 

unit or to discharge or disposal. The term includes hard-piping, all drains and junction boxes, together 

with their associated sewer lines and other junction boxes (e.g., manholes, sumps, and lift stations) 

conveying regulated-material. For the purpose of this subpart, an individual drain system is not a drain 
15 

and collection system that is designed and operated for the sole purpose of collecting rainfall runoff (e.g., 

stormwater sewer system) and is segregated from all other individual drain systems. 

Incinerator: An enclosed combustion device that is used for destroying organic compounds and does not 

extract energy in the form of steam or process heat. Auxiliary fuel may be used to heat waste gas to 

combustion temperatures. Any energy recovery section present is not physically formed into one 

manufactured or assembled unit with the combustion section; rather, the energy recovery section is a 

separate section following the combustion section and the two are joined by ducts or connections carrying 

flue gas. 

Internal floating roof: A cover or roof in a fixed roof tank which rests upon or is floated upon the 

petroleum liquid being contained (but not necessarily in complete contact with it), and is equipped with a 

closure seal or seals to close the space between the roof edge and tank shell. 

Isomerization: A refining process which alters the fundamental arrangement of atoms in the molecule 

without adding or removing anything from the original material. Used to convert normal butane into 

isobutane (C4), an alkylation process feedstock, and normal pentane and hexane into isopentane (C5) and 

isohexane (C6), high-octane gasoline components. 

Junction box: A manhole or access point to a wastewater sewer system line. 

Kerosene: A light petroleum distillate with a maximum distillation temperature of 400 °F at the 10 

percent recovery point, a final maximum boiling point of 572 °F, a minimum flash point of 100 °F, and a 

maximum freezing point of -22 °F. Included are No. 1-K and No. 2-K, distinguished by maximum sulfur 

content (0.04 and 0.30 percent of total mass, respectively), as well as all other grades of kerosene called 

range or stove oil. Excluded is kerosene-type jet fuel (see Kerosene-type jet fuel). 

Kerosene-type jet fuel: A kerosene-based product used in commercial and military turbojet and turboprop 

aircraft. The product has a maximum distillation temperature of 400 °F at the 10 percent recovery point 

and a final maximum boiling point of 572 °F. Included is Jet-A, Jet A-1 (JP-8), and JP-5. 

Kilowatt: A unit of power, usually used for electric power or to energy consumption (use). A kilowatt 

equals 1000 watts. 

Kilowatt hour (kWh): A measure of electricity defined as a unit of work or energy, measured as 1 

kilowatt (1,000 watts) of power expended for 1 hour. One kWh is equivalent to 3,412 Btu or 3.6 million 

joules. 

Knock-out pot or Vapor-liquid separator: A vertical vessel used in several industrial applications to 

separate a vapor-liquid mixture. Gravity causes the liquid to settle to the bottom of the vessel, where it is 

withdrawn. The vapor travels upward at a design velocity which minimizes the entrainment of any liquid 

droplets in the vapor as it exits the top of the vessel. The feed to a vapor-liquid separator may also be a 

liquid that is being partially or totally flashed into a vapor and liquid as it enters the separator. 

Lift station: An structure containing relatively large pumps and associated piping, valves, and other 

mechanical and electrical equipment for pumping liquid. 

Light liquid service: Unless defined otherwise in the regulation to which a process unit is subject, a piece 

of equipment not in gas service that contains a liquid that meets the following conditions: (1) The vapor 

pressure of one or more of the organic compounds is greater than 0.3 kilopascals at 20°C, (2) The total 

concentration of the pure organic constituents having a vapor pressure greater than 0.3 kilopascals at 
16 

20°C is equal to or greater than 20 percent by weight of the total process stream, and (3) The fluid is a 

liquid at operating conditions. Alternatively, a piece of equipment is in light liquid service if the percent 

evaporated is greater than 10 percent at 150°C as determined by ASTM Method D-86. 

Liquid-mounted seal: A foam or liquid-filled primary seal mounted in contact with the liquid between the 

tank wall and the floating roof continuously around the circumference of the tank. 

Liquefied petroleum gases (LPG): A group of hydrocarbon-based gases derived from crude oil refining 

or natural gas stream fractionation that are often liquefied, through pressurization, for ease of transport. 

They include: ethane, propane, normal butane, isobutane and isobutylene. Uses of these fuels include: 

home heating, industrial, automotive fuel, petrochemical feedstocks and for drying purposes in farming. 

Loading operations: A fixed location for loading products into tank trucks, railcars, barges, tank ships, or 

other vessels. Loading equipment and operations that are physically separate (i.e., do not share common 

piping, valves, and other equipment) are considered to be separate loading operations. 

Loading rack: A single system used to fill tank trucks and railcars at a single geographic site. Loading 

equipment and operations that are physically separate (i.e., do not share common piping, valves, and other 

equipment) are considered to be separate loading racks. 

LoCat: A liquid phase oxidation process for tail gas stream desulfurization based on a dilute solution of a 

proprietary, organically chealated iron in water that convert the H2S in water and elemental sulfur. The 

process is not reactive to CO2. A small portion of the chelating agent degrades in some side reactions and 

is lost with the precipitate sulfur. Normally, sulfur is separated by gravity, centrifuging or melting. 

LoTOX scrubber: A NOX emission control system based on the patented Low Temperature Oxidation 

(LTO) Process for removal of NOX emissions. This system uses oxygen to produce ozone as the primary 

treatment chemical using an ozone generator. The oxidation of NOX using ozone is a naturally occurring 

process in the atmosphere. This treatment method produces the treatment chemical, ozone, on demand 

from gaseous oxygen in the exact amount required for oxidation of the NOX. The ozone is injected into 

flue gas stream where it reacts with relatively insoluble NO and NO2 to form N2O3 and N2O5, which are 

highly water soluble, and are easily and efficiently removed and neutralized in a wet scrubbing system. 

Low NOX burners (LNB): Combustion modification NOX controls that reduce NOX formation by 

reducing peak flame temperatures. There are two basic types of LNB as described below: 

• Staged air LNB: Air staging reduces both thermal NOX and fuel NOX, however it is most 

effective in controlling fuel NOX and is, in fact, the best method for controlling the conversion of 

fuel bound nitrogen to fuel NOX emissions. In this case the primary and secondary air are used to 

create the fuel rich combustion zone and the tertiary air stream on the periphery of the burner is 

the “staged air” used to complete combustion. 

• Staged fuel LNB: Fuel staging is the reverse of air staging. Generally 30 to 50% of the fuel is 

injected into the combustion air to form a lean primary flame zone. Although excess oxygen is 

available, NOX is minimized by the low flame temperatures that are generated due to the lean 

combustion conditions. The remainder of the fuel is then injected downstream forming a 

secondary flame zone where combustion is completed. 

Low sulfur feed: Regulatory requirement for SO2 control that require to process in the fluid catalytic 

cracking unit fresh feed that has a total sulfur content no greater than 0.30 percent by weight (40 CFR 

§60.104(b)(3)). 
17 

Lubricants: Substances used to reduce friction between bearing surfaces or as process materials either 

incorporated into other materials used as processing aids in the manufacture of other products, or used as 

carriers of other materials. Petroleum lubricants may be produced either from distillates or residues. 

Lubricants include all grades of lubricating oils from spindle oil to cylinder oil and those used in greases. 

Lubricant dewaxing: Lubricant manufacturing process by which the wax content of the raffinate oil from 

the aromatic extraction unit or the residual oil from the catalytic conversion unit is removed by either 

solvent extraction or catalytic treatment. These processes are described as follows: 

• Solvent dewaxing: There are several solvent dewaxing processes in use today. They include the 

use of MEK, MIBK, liquid propane or liquid SO2/benzol (Edeleanu) combination as the solvent 

that will be mixed with the feed entering the unit to promote the separation of the wax from oil. 

All of these processes with the exception of Edeleanu have similar process configuration, where 

the wax extracted by the solvent is removed through cold filtration. 

• Catalytic dewaxing: Catalytic dewaxing is used to improve cold flow properties of diesel fuels 

and lube oils by selective hydroisomerization/ hydrocracking of normal and slightly branched 

paraffins with or without hydrogen presence. They are suited for use in the following 

applications: 

i. conventional diesel fuels dewaxing to deep dewaxing (Arctic grade production) in 1st and 

2nd stage applications, 

ii. in combination with high pressure hydrocracking or mild hydrocracking catalyst package, 

iii. in mild hydrocracker bottoms pour point reduction for storage and transportation and 

iv. to produce lubes of exceptional quality. 

Lubricants production: All processes associated to the lube oil manufacturing, including but not limiting 

to propane deasphalting, aromatic extraction and UV stabilization, catalytic hydrocracking/hydrotreating, 

solvent or catalytic dewaxing, and final lubricant polish (i.e., lubricant grade separation). 

Lubricating oil: See Lubricants. 

Make-up water or Top-up water or Refill water: The water used to replace water lost to evaporation, 

splash-out, leaks and backwashing. 

Manhole: An opening in a sewer component through which physical access is gained for service; 

incorporates a cover that can be secured. 

Marine vessel or Marine tank vessel: Any tank ship or tank barge that transports liquid product such as 

gasoline, crude oil or petroleum products in bulk through the ocean, river, lake or any other surface water 

body. 

Marketable coke: Those grades of coke produced in delayed or fluid cokers which may be recovered as 

relatively pure carbon. This “green” coke may be sold as is or further purified by calcining. The 

conversion is 5 barrels (of 42 U.S. gallons each) per short ton. Coke from petroleum has a heating value 

of 6.024 million Btu per barrel. See Petroleum coke. 
18 

Maximum rated heat input capacity: The hourly heat input to a unit (in MMBtu/hr), when it combusts the 

maximum amount of fuel per hour that it is capable of combusting on a steady state basis, as of the initial 

installation of the unit, as specified by the manufacturer. 

Maximum true vapor pressure means the equilibrium partial pressure exerted by the stored liquid (1) at 

the temperature equal to the highest calendar-month average of the liquid storage temperature for liquids 

stored above or below the ambient temperature or (2) at the local maximum monthly average temperature 

as reported by the National Weather Service for liquids stored at the ambient temperature. 

Megawatt: A unit of electrical power equal to 1000 kilowatts or one million watts. 

Membrane separation process: A hydrogen purification process consisting of two layers of membrane 

material; the first layer is nonporous, and the second layer is porous. A gas mixture under increased 

pressure is subjected to the surface, which allows a smaller molecule such as hydrogen to permeate faster 

than a larger molecule such as carbon dioxide. Typically, membrane separation involves hollow fiber 

membrane tubes that have extremely high surface areas. 

MeroxTM

: The trade name of a catalytic chemical process used in oil refineries and natural gas processing 

plants to remove mercaptans from LPG, propane, butanes, light naphtha, kerosene and jet fuel by 

converting them to liquid hydrocarbon disulfides. 

Metallic shoe seal: Includes but is not limited to a metal sheet held vertically against the tank wall by 

springs or weighted levers and is connected by braces to the floating roof. A flexible coated fabric 

(envelope) spans the annular space between the metal sheet and the floating roof. 

Methane (CH4): A colorless, flammable, odorless hydrocarbon gas which is the major component of 

natural gas. It is also an important source of hydrogen in various industrial processes. Methane is a 

greenhouse gas. 

MTBE (Methyl tertiary butyl ether) (CH3)3COCH3: See Oxygenates. 

Miscellaneous products: All refined petroleum products not defined elsewhere. These include, but are 

not limited to, naphtha-type jet fuel (Jet B and JP-4), petrolatum lube refining by-products (aromatic 

extracts and tars), absorption oils, ram-jet fuel, petroleum rocket fuels, synthetic natural gas feedstocks, 

waste feedstocks, and specialty oils. Organic waste sludges, tank bottoms, spent catalysts, and sulfuric 

acid are not miscellaneous products. 

Motor gasoline (finished): A complex mixture of relatively volatile hydrocarbons with or without small 

quantities of additives, blended to form a fuel suitable for use in spark-ignition engines. Motor gasoline, 

as defined in ASTM Specification D 4814 or Federal Specification VV-G-1690C, is characterized as 

having a boiling range of 122 to 158 degrees Fahrenheit at the 10 percent recovery point to 365 to 374 

degrees Fahrenheit at the 90 percent recovery point. Motor gasoline includes conventional gasoline; all 

types of oxygenated gasoline, including gasohol; and reformulated gasoline, but excludes aviation 

gasoline. Note: Volumetric data on blending components, such as oxygenates, are not counted in data on 

finished motor gasoline until the blending components are blended into the gasoline. 

• Conventional gasoline: Finished motor gasoline not included in the oxygenated or reformulated 

gasoline categories. Note: This category excludes reformulated gasoline blendstock for 

oxygenate blending (RBOB) as well as other blendstock. 
19 

• Reformulated gasoline: Finished gasoline formulated for use in motor vehicles, the composition 

and properties of which meet the requirements of the reformulated gasoline regulations 

promulgated by the U.S. Environmental Protection Agency under Section 211(k) of the Clean Air 

Act. It includes gasoline produced to meet or exceed emissions performance and benzene content 

standards of federal-program reformulated gasoline even though the gasoline may not meet all of 

the composition requirements (e.g., oxygen content) of federal-program reformulated gasoline. 

Note: This category includes Oxygenated Fuels Program Reformulated Gasoline (OPRG). 

Reformulated gasoline excludes Reformulated Blendstock for Oxygenate Blending (RBOB) and 

Gasoline Treated as Blendstock (GTAB). 

Multi-stage Claus unit: The Claus process usually consists of two steps, a thermal step and a catalytic 

step. Though substantial amounts of elemental sulfur are created in the thermal step, the catalytic step is 

used to increase conversion of H2S to elemental sulfur. In order to maximize the overall recovery of 

elemental sulfur, Claus processes are typically multi-stage operations. With each stage comes additional 

conversion of H2S and SO2 to elemental sulfur, which is usually equilibrium-limited. While the first stage 

typically produces more elemental sulfur than any other single stage, adding stages can increase overall 

recovery. The amount of increase in sulfur recovery that will be realized in additional stages decreases as 

the number of stages is increased. 

• 2-stage Claus unit: A Claus process train utilizing two catalytic reactors in series. 

• 3-stage Claus unit: A Claus process train utilizing three catalytic reactors in series. 

• 4-stage Claus unit: A Claus process train utilizing four catalytic reactors in series. 

Naphtha: A generic term applied to a petroleum fraction with an approximate boiling range between 122 

°F and 400 °F. The naphtha fraction of crude oil is composed largely of paraffinic hydrocarbons that are 

subsequently upgraded to make up the major constituent of gasoline. 

Natural draft cooling tower: A cooling tower that depends upon natural convection of air flowing 

upward and in contact with the water to be cooled. This tower utilizes buoyancy via a tall chimney. 

Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Warm moist 

air is less dense than drier air at the same pressure. This moist air buoyancy produces a current of air 

through the tower. 

Natural gas: A naturally occurring mixture of hydrocarbon and non-hydrocarbon gases found in geologic 

formations beneath the earth’s surface. The principal hydrocarbon constituent is methane that may be 

field quality or pipeline quality. Natural gas is composed of at least 70 percent methane by volume or 

has a high heat value between 910 and 1150 Btu per standard cubic foot. 

Neutralization basin: A upstream wastewater treatment unit where high contaminant concentration such 

as ammonia, phenols, or high pH is diverted and treated to be neutralized or reduced avoiding a shock 

load that affect the activated biological sludge. 

Next generation low NOX burner: A burner that achieves extremely low NOX emissions by merging 

technology advancements in ultra-low NOX burners including combinations of lean premix combustion, 

internal furnace gas recirculation, multiple stages of fuel injection, and lifted flame technology. See 

Ultra-low NOX burner. 

Non-biogenic waste: Waste made from fossil materials or materials of non-biological origin, such as 

plastics, and tire-derived fuels. 
20 

Non-routine emissions: Emissions that occur during periods of start-up, shutdown, or equipment or 

component malfunction. 

Normal operation: For the purposes of the emissions inventory reporting, normal operation is all 

operating periods for a specific emissions point other than periods associated with start-up, shutdown, or 

equipment malfunction. For cyclic processes, normal operation includes all periods of the cyclic process. 

For example, cycling coke drums on a delayed coking unit or regeneration cycles for catalytic reforming 

units are considered normal operations. Periods of low processing rates are considered normal 

operations unless they occur during a period of startup, shutdown, or equipment or component

malfunction. 

Offshore loading terminal: A location that has at least one loading berth that is 0.81 km (0.5 miles) or 

more from the shore that is used for mooring a marine tank vessel and loading liquids from shore. 

Oil-water separator: Wastewater treatment equipment used to separate oil from water consisting of a 

separation tank, which also includes the forebay and other separator basins, skimmers, weirs, grit 

chambers, and sludge hoppers. Slop oil facilities, including tanks, are included in this term along with 

storage vessels and auxiliary equipment located between individual drain systems and the oil-water 

separator. This term does not include storage vessels or auxiliary equipment which do not come in 

contact with or store oily wastewater. 

Oily wastewater: Wastewater generated during the refinery process which contains oil, emulsified oil, or 

other hydrocarbons. Oily wastewater originates from a variety of refinery processes including cooling 

water, condensed stripping steam, tank draw-off, and contact process water. 

Once-through cooling: A system designed to withdraw water from a natural or other water source, use it 

at the facility to support contact and/or noncontact cooling uses, and then discharge it to a waterbody 

without recirculation. Once-through cooling systems sometimes employ canals/channels, ponds, or nonrecirculating cooling towers to dissipate waste heat from the water before it is discharged. 

Open-ended valve or line: Any valve, except safety relief valves, having one side of the valve seat in 

contact with process fluid and one side open to the atmosphere, either directly or through open piping. 

Other atmospheric vent: Any continuous or intermittent process vent located at the facility and under 

common control with the potential to emit hydrocarbons, criteria pollutants, HAP, or greenhouse gases 

directly to the atmosphere during normal operation. Other atmospheric vents do not include those vents 

specifically covered in Sections 5 through 10 of Part II of this ICR, vents associated with process heater 

or boiler exhausts, and wastewater vents. Other atmospheric vents do include distillation tower vents; 

blowdown systems vents, knock-out pot vents, vacuum ejectors (hot well vents), analyzer vents as well as 

vents from MeroxTM treatment systems, fuel gas treatment units (if any), catalytic hydrocracking units (if 

any), asphalt blowing stills, and coke calcining units. Vents recycled to process units, vents directed to a 

fuel gas system, or vents directed to a flare are not considered other atmospheric vents. Other 

atmospheric vents also do not include pressure relief vents where venting occurs only during upset, 

startup, or shutdown events or vents associated with storage tanks. 

Oxidation control system: An emission control system which reduces emissions from sulfur recovery 

plants by converting these emissions to SO2 and recycling the SO2 to the reactor furnace or the first-stage 

catalytic reactor of the Claus sulfur recovery plant. 
21 

Oxygenates: Substances which, when added to gasoline, increase the amount of oxygen in that gasoline 

blend. Fuel ethanol, methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), tertiary amyl 

methyl ether (TAME), di-isopropyl ether (DIPE) and methanol are common oxygenates. 

• Fuel ethanol: Blends of up to 10% by volume anhydrous ethanol (200 proof) (commonly 

referred to as the “gasohol waiver”). 

• Methanol: Blends of methanol and gasoline-grade tertiary butyl alcohol (GTBA) such that the 

total oxygen content does not exceed 3.5% by weight and the ratio of methanol to GTBA is less 

than or equal to 1. It is also specified that this blended fuel must meet ASTM volatility 

specifications (commonly referred to as the “ARCO” waiver). Blends of up to 5.0% by volume 

methanol with a minimum of 2.5% by volume cosolvent alcohols having a carbon number of 4 or 

less (i.e., ethanol, propanol, butanol, and/or GTBA). The total oxygen must not exceed 3.7% by 

weight, and the blend must meet ASTM volatility specifications as well as phase separation and 

alcohol purity specifications (commonly referred to as the “DuPont” waiver). 

• MTBE (Methyl tertiary butyl ether) (CH3)3COCH3: Ether used in the blending of reformulated 

gasolines, affecting vapor pressure and octane level. Blends up to 15.0% by volume MTBE 

which must meet the ASTM D4814 specifications. Blenders must take precautions that the 

blends are not used as base gasolines for other oxygenated blends (commonly referred to as the 

“Sun” waiver). Commonly made from the catalytic etherfication of methanol (methyl alcohol) 

and isobutene. 

• ETBE (Ethyl tertiary butyl ether) (CH3)3COC2H5: An oxygenate blend stock formed by the 

catalytic etherfication of isobutylene with ethanol. 

• TAME (Tertiary amyl methyl ether) (CH3)2(C2H5)COCH3: An oxygenate blend stock formed by 

the catalytic etherification of isoamylene or isopentene with methanol in a catalytic cracking unit. 

• DIPE (Di-isopropyl ether) (CH3)2CHOCH(CH3)2: An oxygenate blend stock formed by the 

etherification of propylene through a catalytic hydration. 

Ozonation: The act of treating with ozone something with disinfection purposes, also, the act of 

converting into, or producing, ozone. 

Packed-bed wet scrubber: An air pollution control device that removes soluble chemicals, fumes, 

particulates and odors through a specially designed packing media that is wetted with recirculated liquid 

spray. The liquid solvent absorbs the gas pollutant by physical or chemical means. A blowdown from 

the tank with makeup water addition removes contaminant products before they precipitate. 

Perchloroethylene or Tetrachloroethylene (C2Cl4): A chloride solvent that is excellent in its use as a 

chloriding agent in the reforming catalyst regeneration process because of the amount of available 

chlorides. Also the primary solvent used in the industrial and commercial dry-cleaning of clothes. Its 

other major uses are as a metal cleaning and degreasing solvent, and as a chemical intermediate in the 

production of several fluorinated compounds. 

Petrochemical feedstocks: Chemical feedstocks derived from petroleum principally for the manufacture 

of chemicals, synthetic rubber, and a variety of plastics. The categories reported are “Naphtha Less Than 

401°F” and “Other Oils Equal To or Greater Than 401°F.” 
22 

• Naphtha Less Than 401°F: A naphtha with a boiling range of less than 401 degrees Fahrenheit 

that is intended for use as a petrochemical feedstock. 

• Other Oils Equal To or Greater Than 401°F: Oils with a boiling range equal to or greater than 

401 degrees Fahrenheit that are intended for use as a petrochemical feedstock. 

Petroleum: The crude oil removed from the earth and the oils derived from tar sands, shale, and coal. A 

broadly defined class of liquid hydrocarbon mixtures. Included are crude oil, lease condensate, 

unfinished oils, refined products obtained from the processing of crude oil, and natural gas plant liquids. 

Petroleum coke: A solid residue high in carbon content and low in hydrogen that is the final product of 

thermal decomposition in the condensation process in cracking. This product is reported as marketable 

coke or catalyst coke. The conversion is 5 barrels (of 42 U.S. gallons each) per short ton. Coke from 

petroleum has a heating value of 6.024 million Btu per barrel. 

Petroleum products: All refined and semi-refined products that are produced at a refinery by processing 

crude oil, natural gas, and other petroleum-based feedstocks, including petroleum products derived from 

co-processing biomass and petroleum feedstock together, but not including plastics or plastic products. 

Petroleum products include unfinished oils, liquefied petroleum gases, pentanes plus, aviation gasoline, 

motor gasoline, naphtha-type jet fuel, kerosene-type jet fuel, kerosene, distillate fuel oil, residual fuel oil, 

petrochemical feedstocks, special naphthas, lubricants, waxes, petroleum coke, asphalt, road oil, still gas, 

and miscellaneous products. 

Petroleum refinery: Any facility engaged in producing gasoline, kerosene, distillate fuel oils, residual 

fuel oils, lubricants, asphalt (bitumen) or other products through distillation of petroleum or through 

redistillation, cracking, or reforming of unfinished petroleum derivatives. A facility that produces only 

oil shale or tar sands-derived crude oil for further processing at a petroleum refinery using only solvent 

extraction and/or distillation to recover diluent is not a petroleum refinery. 

Pressure relief device (PRD): A safety device used to prevent operating pressures from exceeding the 

maximum allowable working pressure of the process equipment. A common pressure relief device is a 

spring-loaded pressure relief valve. It is characterized by a rapid-opening 'pop' action or by opening in a 

manner generally proportional to the increase in pressure over the opening pressure. Devices that are 

actuated either by a pressure of less than or equal to 2.5 psig or by a vacuum are not pressure relief 

devices. For purposes of this ICR, liquid expansion pressure relief valves and pressure relief valves on 

mobile units and containers (e.g., trucks, tank cars, totes, etc) are not considered pressure relief devices. 

Pressure swing adsorption (PSA): An adiabatic process that is applied for purification of gases by 

removing the accompanying impurities by adsorption through suitable adsorbents in fixed beds contained 

in pressure vessels under high pressure. Regeneration of adsorbents is accomplished by countercurrent 

depressurization and by purging at low pressure with previously recovered near product quality gas. 

Dependent on the types of impurity to be adsorbed and removed, adsorbents to be used comprise zeolitic 

molecular sieves, activated carbon, silica gel and activated alumina. Mostly, combinations of adsorbent 

beds are used on top of one another, so dividing the adsorber contents into a number of distinct zones. 

Monitoring and proper control of process parameters ensures a stable operation. 

Primary clarifier: A wastewater treatment plant component or tank that physically removes suspended 

solids particles from wastewater, typically by settling and flotation with or without the use of coagulant. 
23 

Primary fuel: The fuel that provides the principal heat input (i.e., more than 50 percent) to the device. To 

be considered primary, the fuel must be able to sustain operation of the device without the addition of 

other fuels. 

Process or recycled blowdown water: Water that has been used in any level of the manufacturing process 

or came from any recycled blowdown system. 

Process gas: See Still gas. 

Process heater: An enclosed combustion device used to transfer heat indirectly to process stream 

materials (liquids, gases, or solids) or to a heat transfer material for use in a process unit, instead of 

generating steam. Process heaters do not include units used for comfort heat or space heat, food 

preparation for on-site consumption, or autoclaves. Process heaters can be classified by the type of draft 

used as follow: 

• Natural draft process heater: A process heater in which the combustion air is supplied under 

ambient or negative pressure without the use of an inlet air (forced draft) fan or an exhaust outlet 

(induced draft) fan. 

• Induced draft process heater: A process heater in which the combustion air is supplied under 

negative pressure produced by a fan at any location in the outlet exhaust gas line or stack. 

Induced draft process heaters do not use an inlet air (forced draft) fan. 

• Forced draft process heater: A process heater in which the combustion air is supplied under 

positive pressure produced by a fan at any location in the inlet air line prior to the point where the 

combustion air enters the process heater. Forced draft process heaters do not use an outlet air 

(induced draft) fan. 

• Balanced draft process heater: A process heater in which the combustion air is supplied with the 

use of both an inlet air (forced draft) fan and an exhaust outlet (induced draft) fan. Balanced draft 

process heaters are commonly used for process heaters using combustion air preheat. 

Process tank: A tank or other vessel that is used within a process to collect material discharged from a 

feedstock storage vessel or a vessel in which crude oil or petroleum liquids are processed (e.g., distillation 

column, condenser, coker) before the material is transferred to another processing vessel or a product 

storage vessel. Process tanks may be used only to provide temporary holding of a material, or unit 

operations (e.g., blending) may be conducted in the tank. Examples of process tanks include surge 

control vessels and bottoms receivers. 

Process unit: For the purposes of this ICR, a process unit is any segment of the petroleum refinery in 

which a specific processing operation is conducted. Examples of such units include, but are not limited 

to, alkylation units, catalytic hydrotreating (or catalytic hydrorefining or desulfurization), catalytic 

hydrocracking, catalytic reforming, catalytic cracking, crude distillation, lube oil processing, hydrogen 

production, isomerization, polymerization, thermal (coking) processes, sulfur recovery, and blending, 

sweetening, and treating processes. 

Process upset gas: Any gas generated by a petroleum refinery process unit as a result of upset or 

malfunction. 

Process vent: A gas stream that is continuously or periodically discharged during normal operation of a 

process unit, including gas streams that are discharged directly to the atmosphere, gas streams that are 
24 

routed to a control device prior to discharge to the atmosphere, or gas streams that are diverted through a 

product recovery device line prior to control or discharge to the atmosphere. Process vent does not 

include safety device discharges, equipment leaks, gas streams routed to a fuel gas system or to a flare, or 

discharges from storage vessels. 

Promoter: Any component added to a catalyst to increase activity or selectivity. Examples are tin added 

to platinum reforming catalysts to improve selectivity to coke formation and chloride added to 

isomerization catalysts to increase activity. 

Propane (C3H8): A normally gaseous straight-chain hydrocarbon. It is a colorless paraffinic gas that 

boils at a temperature of -43.67 degrees Fahrenheit. It is extracted from natural gas or refinery gas 

streams. It includes all products designated in ASTM Specification D1835 and Gas Processors 

Association Specifications for commercial propane and HD-5 propane. 

Propane deasphalting: A process by which the vacuum tower residue is distilled along with liquid 

propane to remove the asphaltene portion of the residue to prepare a suitable feedstock for catalytic 

conversion units (i.e., lube hydrotreater) in the lubricants production. 

Pyrolysis: The thermal decomposition of biomass at high temperatures (greater than 400 °F, or 200 °C) 

in the absence of air. The end product of pyrolysis is a mixture of solids (char), liquids (oxygenated oils), 

and gases (methane, carbon monoxide, and carbon dioxide) with proportions determined by operating 

temperature, pressure, oxygen content, and other conditions. 

Pyrolysis/Combustion unit: A unit that produces gases, liquids, or solids through the heating of MSW, 

and the gases, liquids, or solids produced are combusted and emissions vented to the atmosphere. 

Reduction control system: An emission control system which reduces emissions from sulfur recovery 

plants by converting these emissions to H2S and recycling the H2S to the reactor furnace or the first-stage 

catalytic reactor of the Claus sulfur recovery plant. 

Rectisol®: The trade name for an acid gas removal solvent (refrigerated methanol) that can separate, at 

relatively high pressures, acid gases such as hydrogen sulfide and carbon dioxide from feed gas streams 

such as synthesis gas produced by gasification of coal, coke, or heavy hydrocarbon oils. 

Recycled blowdown water: An industrial wastewater that is routinely released from a boiler or any other 

equipment that require to remove water from the system to avoid the buildup of any impurity, sediment or 

contaminants such as dissolved/suspended minerals, heavy metals (iron, copper), corrosion inhibitors, oil, 

and/or algaecides. 

Recycled feeds: Feeds that are continuously fed back for additional processing. 

Refinery fuel gas: A gaseous mixture of gases produced at the refinery (i.e., still or process gas) and 

natural gas that is used as a fuel in boilers and process heaters throughout the refinery. Still gas is 

typically augmented with natural gas in a refinery’s fuel gas system; the proportion of heating value 

contributed by still gas and natural gas is variable based on the quantity of still gas produced and the 

energy needs of the process units. For the purposes of this ICR, any mixture of still gas and natural gas is 

considered to be refinery fuel gas. See Still gas, Natural gas, and Fuel gas system. 

Reformate: The liquid product stream of catalytic reforming units. Reformate is characterized by a high 

concentration of aromatics and high octane level, and it is used as a gasoline blending component or as 

feedstock for aromatics production. 
25 

Reid vapor pressure (RVP): The volatility or tendency of a petroleum product to evaporate. It is defined 

as the absolute vapor pressure exerted by a liquid at 100 °F (37.8 °C) as determined by the test method 

ASTM-D-323-82 or 94. The test method applies to volatile crude oil and volatile non-viscous petroleum 

liquids, except liquefied petroleum gases. 

Relief valve: A valve used only to release an unplanned, non-routine discharge. A relief valve discharge 

can result from an operator error, a malfunction such as a power failure or equipment failure, or other 

unexpected cause that requires immediate venting of gas from process equipment in order to avoid safety 

hazards or equipment damage. 

Residual fuel oil: A general classification for the heavier oils, known as No. 5 and No. 6 fuel oils, that 

remain after the distillate fuel oils and lighter hydrocarbons are distilled away in refinery operations. It 

conforms to ASTM Specifications D 396 and D 975 and Federal Specification VV-F-815C. No. 5, a 

residual fuel oil of medium viscosity, is also known as Navy Special. It is used in steam-powered vessels 

in government service and inshore power plants. No. 6 fuel oil includes Bunker C fuel oil and is used for 

the production of electric power, space heating, vessel bunkering, and various industrial purposes. 

Residuum: Residue from crude oil after distilling off all but the heaviest components, with a boiling 

range greater than 1000 degrees Fahrenheit. 

Road oil: Any heavy petroleum oil, including residual asphaltic oil used as a dust palliative and surface 

treatment on roads and highways. It is generally produced in six grades from 0, the most liquid, to 5, the 

most viscous. 

Rotating biological contactor: An aerobic treatment system that rotates plastic media with fixed 

microbial film alternately through liquid effluent and air to reduce the BOD value. 

SCOT process or SCOT unit: A Shell air pollution control process for treating the tail gas or off-gas from 

a Claus unit to increase the overall sulfur recovery to as high as 99.9% and thus minimize emissions of 

sulfur compounds. 

Secondary clarifier: A wastewater treatment plant component or tank that uses biological and chemical 

treatment processes designed to remove organic matter. 

Secondary fuel: A fuel fired to a process heater or boiler that provides supplementary heat in addition to 

the heat provided by the primary fuel, but provides less that 50% of the heat input to the process heater or 

boiler. The secondary fuel may be fed through the primary burner or a burner other than the primary fuel 

burner. Pilot gas is not considered to be secondary fuel. See Primary fuel. 

Selective catalytic reduction (SCR): A non-combustion control technology that destroys NOX by 

injecting a reducing agent (e.g., ammonia) into the flue gas that, in the presence of a catalyst (e.g., 

vanadium, titanium, or zeolite), converts NOX into molecular nitrogen and water. 

Selective non-catalytic reduction (SNCR): A non-combustion control technology that destroys NOX by 

injecting a reducing agent (e.g., ammonia, urea, or cyanuric acid) into the flue gas, downstream of the 

combustion zone that converts NOX to molecular nitrogen, water, and when urea or cyanuric acid are 

used, to CO2. 

Selexol®: The trade name for an acid gas removal solvent (a mixture of the dimethyl ethers of 

polyethylene glycol) that can separate at high pressures acid gases such as hydrogen sulfide and carbon 
26 

dioxide from feed gas streams such as synthesis gas produced by gasification of coal, coke, or heavy 

hydrocarbon oils. 

Semi-refined petroleum product: All oils requiring further processing. Included in this category are 

unfinished oils which are produced by the partial refining of crude oil and include the following: 

naphthas and lighter oils; kerosene and light gas oils; heavy gas oils; and residuum, and all products that 

require further processing or the addition of blendstocks. 

Sewer line: A lateral, trunk line, branch line, ditch, channel, or other conduit used to convey refinery 

wastewater to downstream components of a refinery wastewater treatment system. This term does not 

include buried, below-grade sewer lines. 

Shutdown: The process by which the complete cessation of operation of a processing unit is achieved. 

For the purposes of this ICR, cycling coke drums on a delayed coking unit or regeneration cycles for 

catalytic reforming units are not considered a complete cessation of operation of these processing units. 

Shutdown of a delayed coking unit entails the eliminating feed to the delayed coking unit fractionator 

reboiler and all coke drums associated with the fractionators; shutdown of a catalytic reforming unit 

entails the eliminating feed to all reforming reactors for reasons other than catalyst regeneration (e.g., to 

completely replace the catalyst or to perform maintenance on equipment). 

Slotted guidepole: Any gauge pole, gauge pipe or stilling well that has slots or holes through the wall of 

the pole. The slots or holes allow the stored liquid to flow into the pole at liquid levels above the lowest 

operating level. 

Sludge age (SA): A measure of the time biological solids are retained in a basin calculated by dividing 

the mass of volatile solids in the basin by the total mass of volatile solids wasted during a given time 

period. 

Smart LDAR: Name used to describe the latest enhancements of the LDAR program that include the 

efficient locating and repair of the relatively small number of leaking equipment in a large population of 

process equipment through use of remote sensing technology to quickly & economically identify 

significant leakers, scanning often enough to catch significant leakers before emissions accumulate. 

Soil remediation: The collective term for various strategies that are used to purify and revitalize soil. 

This process of soil cleanup is part of a broader effort known as environmental remediation, which can 

also include efforts to purify the air and other wise repair damage done to the ecological balance of the 

planet. Among the processes used in soil remediation, excavation and dredging are among the most 

common. This process involves extracting soil that is contaminated and deemed to be unrecoverable 

using current technology, and transporting it to a landfill set aside for this purpose. Often, purified soil is 

used to fill in the area where the extraction took place. Soil remediation is sometimes accomplished using 

a process known as pump and treat. Essentially, this approach involves the removal of contaminated 

ground water, then using various methods to purify the extracted liquid. While the water is purified, the 

soil is also extracted and filtered to remove various contaminants, then returned to its original position. 

The purified water is pumped back into the purified soil, effectively restoring the ecological balance of 

the area. 

Sodium hypochlorite: An unstable salt, NaOCl, usually stored in solution and used as a fungicide and an 

oxidizing bleach. A solution of sodium hypochlorite is frequently used as a disinfectant. 

Solid guidepoles: Any gauge pole, gauge pipe or stilling well that does not have slots or holes through 

the wall of the pole. Controls include a guidepole cover at the top of the guidepole and a well cover 
27 

positioned at the top of the guidepole well that seals any openings between the well cover and the 

guidepole (e.g., pole wiper), any openings between the well cover and any other objects that pass through 

the well cover, and any other openings in the top of the guidepole well. 

Solids retention time (SRT): The average time the activated-sludge solids are in the system. The SRT is 

an important design and operating parameter for the activated-sludge process and is usually expressed in 

days. 

Sour water: Process or waste water that contains sulfur compounds, most commonly hydrogen sulfide. 

Sour water stripper: A stream stripper used to treat sour water to remove hydrogen sulfide from the 

water prior to other wastewater treatment units or prior to reuse in a process unit. For the purposes of this 

ICR, the sour water stripper is considered a wastewater treatment unit regardless of the disposition or use 

of the treated water stream. 

Special naphthas: All finished products within the naphtha boiling range that are used as paint thinners, 

cleaners, or solvents. These products are refined to a specified flash point. Special naphthas include all 

commercial hexane and cleaning solvents conforming to ASTM Specification D1836 and D484, 

respectively. Naphthas to be blended or marketed as motor gasoline or aviation gasoline and naphthas 

that are to be used as petrochemical and synthetic natural gas (SNG) feedstocks are excluded. 

Spray dryer absorber (SDA): A pollution control system developed to remove acidic pollutants, SO2, 

heavy metals and dust through absorption into a finely atomized aqueous mist of hydrated lime from flue 

gases of various origins, primarily fossil fuelled power stations, waste incinerators and steel industries. 

Stabilized crude oil: Crude oil that does not meet the definition of unstabilized crude oil. 

Staged air low NOX burner: See Low NOX burner. 

Staged fuel low NOX burner: See Low NOX burner. 

Startup: The process by which a new process unit first begins operation and an existing process unit 

initiates operation after a shutdown. 

Steam: Water in vapor form; used as the working fluid in steam turbines and heating systems. 

Steam generating unit: A unit in which steam is generated for sale or for use at other processing locations 

within the facility. Steam generating units do not include process heaters, hot water heaters, or space 

heaters. 

Steam methane reforming: A catalytic process that involves a reaction between natural gas or methane 

and steam. The result is a mixture of hydrogen, carbon monoxide, carbon dioxide, and water. 

Steam stripper: A process that uses steam for the removal of volatile component from a liquid by 

vaporization. The stripping operation is an important step in many industrial processes which employ 

absorption to purify gases and to recover valuable components from the vapor phase. 

Sterically-hindered amines: Chemical compounds containing an amine functional group surrounded by a 

crowded steric environment. They have uses such as gas scrubbing, as stabilizers against light-induced 

degradation of polymers, and as reagents for organic synthesis. Sterically-hindered amines are found to 
28 

be superior to diethanolamine (DEA) as promoters for alkaline salts in the “hot pot” acid gas scrubbing 

process. 

Sterically-hindered amine scrubber: An air pollution control device that employs liquid hindered amines 

spray to remove acidic substances (such as sulfur dioxide or hydrogen sulfide) from industrial off-gases. 

Still gas or Process gas: Any form or mixture of gases produced in refineries by distillation, cracking, 

reforming, and other processes. The principal constituents are methane, ethane, ethylene, normal butane, 

butylenes, propane, propylene, etc. Still gas is used as a refinery fuel and a petrochemical feedstock. For 

the purpose of this ICR, still gas is also any gaseous fraction containing hydrogen gas and any 

combination of the above other gases. The conversion factor is 6 million BTU per fuel oil equivalent 

barrel. Still gas mixed with natural gas is considered refinery fuel gas. See also Refinery fuel gas. 

Stretford process®: A process developed to remove hydrogen sulfide (H2S) from town gas. It was the 

first liquid phase, oxidation process for converting H2S into sulfur to gain widespread commercial 

acceptance. Developed by Tom Nicklin of the North-Western Gas Board (NWGB) in 1940. The process 

uses reduction-oxidation (redox) chemistry to oxidize the H2S into elemental sulfur, in an alkaline 

solution containing vanadium as an oxygen carrier. 

Storage vessel: A tank or other vessel (excluding sumps) that is designed to contain an accumulation of 

liquids that contain hydrocarbons, has a capacity greater than 10,000 gal, holds material with a maximum 

true vapor pressure greater than 0.1 psi, and is constructed entirely of non-earthen materials (e.g., wood, 

concrete, steel, plastic) that provide structural support. Process tanks and vessels permanently attached to 

motor vehicles are not considered storage vessels. 

Stormwater sewer system: A drain and collection system designed and operated for the sole purpose of 

collecting stormwater and which is segregated from the process wastewater collection system. 

SubDewPoint MCRC SUPERCLAUS®: Process consists of two well-known processes in series: The 

sub-dewpoint MCRC process followed by the selective oxidation stage of SUPERCLAUS®, with the 

patented main combustion air control of SUPERCLAUS®. The sub-dewpoint part consists of a thermal 

stage followed by two or more catalytic reaction stages with sulfur removed between stages by 

condensers. The first reactor is always operated at normal Claus conditions, the second reactor (MCRC) 

is a sub-dewpoint reactor being regenerated on-line by increased inlet temperature, the third sub-dewpoint 

reactor operates below the dewpoint of sulfur. These reactors are filled with standard Claus catalyst while 

the last reactor (the SUPERCLAUS® stage) is filled with proprietary selective oxidation catalyst. In the 

thermal stage, the acid gas is burned with a substoichiometric amount of controlled combustion air such 

that the tail gas leaving the last sub-dewpoint reactor contains an increased H2S concentration of typically 

0.2 to 0.3 vol.%. Under these conditions, hardly any SO2 leaves the sub-dewpoint part. Because the 

proprietary catalyst neither oxidizes H2S to SO2 and H2O nor reverses the reaction of sulfur and water to 

H2S and SO2, a total sulfur recovery efficiency of 99.7% can be obtained. 

Submerged fill loading: A type of organic liquid loading operation where the fill pipe discharge opening 

is completely submerged when the liquid level above the bottom of the vessel is eight centimeters (3.2 

inches) or higher. 

Sulfinol®: Trade name for an acid gas removal solvent (a mixture of Sulfolane, DIPA or MDEA and 

water) that can separate acid gases such as H2S, CO2, COS and mercaptans from feed gas streams such as 

synthesis gas produced by gasification of coal, coke, heavy hydrocarbon oils or natural gas production. 
29 

Sulfur: A yellowish nonmetallic element, sometimes known as “brimstone.” It is present at various 

levels of concentration in many fossil fuels whose combustion releases sulfur compounds that are 

considered harmful to the environment. Some of the most commonly used fossil fuels are categorized 

according to their sulfur content, with lower sulfur fuels usually selling at a higher price. 

Sulfur recovery plant: All process units which recover sulfur or produce sulfuric acid from hydrogen 

sulfide (H2S) and/or sulfur dioxide (SO2) from a common source of sour gas at a petroleum refinery. The 

sulfur recovery plant also includes sulfur pits used to store the recovered sulfur product, but it does not 

include secondary sulfur storage vessels or loading facilities downstream of the sulfur pits. For example, 

a Claus sulfur recovery plant includes: reactor furnace and waste heat boiler, catalytic reactors, sulfur 

pits, and, if present, oxidation or reduction control systems, or incinerator, thermal oxidizer, or similar 

combustion device. Multiple sulfur recovery units are a single sulfur recovery plant only when the units 

share the same source of sour gas. Sulfur recovery units that receive source gas from completely 

segregated sour gas treatment systems are separate sulfur recovery plants. See also Claus sulfur recovery 

plant and Sulfur recovery unit. 

Sulfur recovery unit: A single train of process equipment used to recover sulfur or produce sulfuric acid 

from hydrogen sulfide (H2S) and/or sulfur dioxide (SO2). For example, a Claus sulfur recovery unit

includes a single reactor furnace, the series of catalytic reactors associated with the reactor furnace, sulfur 

pits associated with the series of catalytic reactors, and oxidation or reduction control systems, or 

incinerator, thermal oxidizer, or similar combustion device used to treat the tail gas from the series of 

catalytic reactors (associated with the single reactor furnace). See also Sulfur recovery plant. 

Sump: Any lined pit or reservoir that serves to collect liquids drained from a leachate collection and 

removal system or leak detection system for subsequent removal from the system. 

Surface impoundment: A waste management unit which is a natural topographic depression, manmade 

excavation, or diked area formed primarily of earthen materials (although it may be lined with manmade 

materials), which is designed to hold an accumulation of liquid wastes or waste containing free liquids. A 

surface impoundment is used for the purpose of treating, storing, or disposing of wastewater or residuals, 

and is not an injection well. Examples of surface impoundments are equalization, settling, and aeration 

pits, ponds, and lagoons. 

Super Claus®: A sulfur recovery unit developed to further improve the removal of sulfur-containing 

compounds from gas streams over a normal Claus unit. This process consists of a thermal stage followed 

by three or four catalytic reaction stages. The first two or three reactors are filled with Claus catalyst 

while the last reactor is filled with the selective oxidation catalyst. In the thermal stage, the acid gas is 

burnt with a sub-stoichiometric amount of controlled combustion air so that the tail gas leaving the 

second reactor contains 0.8% to 1.5% by volume of H2S. In practice, the SUPERCLAUS process in 

which the selective oxidation stage is implemented with gas which has passed three Claus reactors yields 

sulfur recovery percentages of about 99.3%. 

TAME (Tertiary amyl methyl ether) (CH3)2(C2H5)COCH3: See Oxygenates. 

Tap water: For the purposes of this ICR, any non-used clean water coming from municipal water 

systems, groundwater, surface water (e.g., river, lake, lagoon), or storm water pond. 

Thermal cracking: A refining process in which heat and pressure are used to break down, rearrange or 

combine hydrocarbon molecules. Thermal cracking includes visbreaking, fluid coking, delayed coking 

and other thermal (non-catalytic) cracking processes. See Coking. 
30 

Thermal incinerator or Thermal oxidizer: Any enclosed device that is mainly used for destroying organic 

compounds by heating the feed stream with a nozzle-stabilized flame to its ignition temperature during 

certain residence time. It is the temperature at which the combustion reaction rate (and consequently the 

energy production rate) exceeds the rate of heat losses, thereby raising the temperature of the gases to 

some higher value. 

Turndown: Processing at less than 50% of equipment capacity. For example, a process heater is 

considered to operate in turndown when the actual heat input to the process heater is less than 50% of the 

rated heat input capacity of the process heater. 

Toluene (C6H5CH3): Colorless liquid of the aromatic group of petroleum hydrocarbons, made by the 

catalytic reforming of petroleum naphthas containing methyl cyclohexane. A high-octane gasolineblending agent, solvent, and chemical intermediate, base for TNT. See Aromatics. 

Topping refinery: The simplest refinery configuration designed to prepare feedstocks for petrochemical 

manufacture or for production of industrial fuels in remote oil-production areas. It consists of tankage, a 

crude distillation unit, recovery facilities for gases and light hydrocarbons, and the necessary utility 

systems (steam, power, and water-treatment plants). Topping refineries produce large quantities of 

unfinished oils and lack hydrotreating and reforming units needed to produce finished gasoline. See 

Hydroskimming refinery and also Heavy oil/asphalt refinery. 

Tray tower wet scrubber: An air pollution control device that removes soluble chemicals, fumes, 

particulates and odors through a specially designed perforated plates in different configurations (e.g., 

impingement, valve, tubular slot, and sieve trays) upon which a layer of liquid is kept at a certain level by 

a weir using liquid recirculation. The liquid solvent absorbs the gas pollutant passing the perforated trays 

by physical or chemical means. 

Trench: A narrow excavation in the soil either lined (e.g., concrete, plastic) or unlined for drainage 

diversion. 

Trichloroethene or Trichloroethylene (C2HCl3): A solvent usually used for degreasing operations and in 

the production of pure 100% ethanol. 

Trickling filter: A bed of gravel or pebble media through which clarified effluent percolates and 

microbial activity removes BOD, suspended solids and ammoniacal nitrogen. 

True vapor pressure (TVP): The equilibrium partial pressure exerted by a petroleum liquid as a function 

of temperature as determined in accordance with methods described in Section 7.1 of AP-42 or by any 

other method approved by the Administrator. See also maximum true vapor pressure.

Ultra-low NOX burner (ULNB): A burner that achieves very low NOX emissions by advances in burner 

technology that allows a higher percentage of fuel gas to be staged than in low NOX burners. 

Unfinished oils: All oils requiring further processing, except those requiring only mechanical blending. 

Unfinished oils are produced by partial refining of crude oil and include naphthas and lighter oils, 

kerosene and light gas oils, heavy gas oils, and residuum. 

Unstabilized crude oil: For the purposes of this ICR, crude oil that is pumped from the well to a pipeline 

or pressurized storage vessel for transport to the refinery without intermediate storage in a storage vessel 

at atmospheric pressures. Unstabilized crude oil is characterized by having a true vapor pressure of 5 

pounds per square inch absolute (psia) or greater. 
31 

Upgrading refinery: A refinery equipped with cracking and/or coking processes, such as catalytic 

cracking units, catalytic hydrocracking units, and/or fluid or delayed coking units, to convert heavy fuel 

oil into light and middle distillates. Upgrading refineries are more complex than hydroskimming 

refineries and have more flexibility in the mix of products produced. See Hydroskimming refinery. 

UV disinfection: The process of killing (inactivating) harmful and objectionable bacteria and other 

microorganisms (pathogenic) by using ultraviolet light. UV radiation penetrates the outer cell-wall of the 

microorganism, passes through the cell-body, reaches the deoxyribonucleic acid (DNA) and alters the 

genetic material, thereby destroying the microorganism in a non-chemical manner. 

Vacuum distillation: Distillation under reduced pressure (less the atmospheric) which lowers the boiling 

temperature of the liquid being distilled. This technique with its relatively low temperatures prevents 

cracking or decomposition of the charge stock. 

Valve: Any device for halting or regulating the flow of a liquid or gas through a passage, pipeline, inlet, 

outlet, or orifice; including, but not limited to, gate, globe, plug, ball, butterfly and needle valves. 

Vapor balancing system: A vapor collection system or piping system that is designed to collect organic 

vapors displaced from marine tank vessels during marine tank vessel loading operations and that is 

designed to route the collected organic vapors to the storage vessel from which the liquid being loaded 

originated or to compress collected organic vapors and commingle with the raw feed of a process unit. 

Vapor collection system: Any equipment located at the source, i.e., at the terminal, that is not open to the 

atmosphere, that is composed of piping, connections, and flow inducing devices, and that is used for 

containing and transporting vapors displaced during the loading of marine tank vessels to a control device 

or for vapor balancing. This does not include the vapor collection system that is part of any marine vessel 

vapor collection manifold system. 

Vapor-mounted seal: A foam-filled primary seal mounted continuously around the circumference of the 

tank so there is an annular vapor space underneath the seal. The annular vapor space is bounded by the 

bottom of the primary seal, the tank wall, the liquid surface, and the floating roof. 

Venturi wet scrubber: A scrubber that is designed to effectively use the energy from the inlet gas stream 

to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of 

air pollution controls collectively referred to as wet scrubbers. 

Visbreaking: A thermal cracking process in which heavy atmospheric or vacuum-still bottoms are 

cracked at moderate temperatures to increase production of distillate products and reduce viscosity of the 

distillation residues. 

Volatile organic compounds (VOC): A combination of chemical pollutants that form ozone or smog. 

The Clean Air Act was designed in part to reduce VOC in gasoline in order to reduce ozone pollution 

from gasoline exhaust and emissions. 

Wastewater or Process wastewater: Water or wastewater that, during manufacturing production or 

processing, comes into direct contact with or results from the production or use of any raw material, 

intermediate product, finished product, byproduct, or waste product and is discharged into any individual 

drain system. Examples are feed tank drawdown; water formed during a chemical reaction or used as a 

reactant; water used to wash impurities from organic products or reactants; water used to cool or quench 

organic vapor streams through direct contact; and condensed steam from jet ejector systems pulling 

vacuum on vessels containing organics. 
32 

Wastewater collection system: Any grouping of components or pieces of equipment such as catch basins, 

drains, junction box, sewer line, pipeline, etc. used for the collection and routing of waste waters to a 

plant to be processed, treated or purified. 

Wastewater treatment system: Any grouping of component, pieces of equipment, or installations that 

receives, treats, or processes waste waters. 

Water seal controls: A seal pot, p-leg trap, or other type of trap filled with water that has a design 

capability to create a water barrier between the sewer and the atmosphere. 

Watt: The unit of electrical power equal to one ampere under a pressure of one volt. A Watt is equal to 

1/746 horse power. 

Wax: A solid or semi-solid material at 77 °F consisting of a mixture of hydrocarbons obtained or derived 

from petroleum fractions, or through a Fischer-Tropsch type process, in which the straight-chained 

paraffin series predominates. This includes all marketable wax, whether crude or refined, with a 

congealing point between 80 (or 85) and 240 °F and a maximum oil content of 50 weight percent. 

Weir: A device designed to measure or control flow; consists of a wall or obstruction of known 

geometric shape placed perpendicular to the direction of flow. 

Wellman-Lord process: A regenerative process to remove sulfur dioxide from flue gas (Flue-gas 

desulfurization) in tail gas cleanup processes without creating a throwaway sludge product. The process 

employs an aqueous alkali sulfite solution (usually sodium sulfite) which chemically combines with the 

sulfur dioxide gas in an absorption tower by forming sodium bisulfite. The process includes a separate 

regeneration facility to convert the bisulfite back to sulfite and recover SO2 gas which is compressed and 

bottled or converted sulfuric acid or to elemental sulfur. In the case of flue gases from coal fired plants, a 

separate unit for removal of fly ash and chlorides is included. 

Wet electrostatic precipitator (WESP): A control device that operates with saturated air streams (100% 

relative humidity). A WESP is commonly used to remove liquid droplets such as sulfuric acid mist from 

industrial process gas streams. A WESP is also commonly used where the gases are high in moisture 

content, contain combustible particulate, have particles that are sticky in nature. See ESP. 

Working losses: The displacement of the vapors within the storage vessel as a tank is filled or emptied. 

Xylene (C6H4(CH3)2): Colorless liquid of the aromatic group of hydrocarbons made the catalytic 

reforming of certain naphthenic petroleum fractions. Used as high-octane motor and aviation gasoline 

blending agents, solvents, chemical intermediates. Isomers are metaxylene, orthoxylene, paraxylene. See 

Aromatics.