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Chemical Engineering Dictionary

These are terms I collected from several sources. The terms below include chemical engineering terms, general engineering, project management, and EPC project. I hope you find it useful!

Absolute pressure. The measurement of gas or air pressure relative to the pressure in a total vacuum.

Absolute temperature. The kelvin temperature scale is equal to the Celcius degree but with zero being absolute zero (0 K).

Absolute zero. The lowest possible thermal energy state of a material. This corresponds to 0 K.

Absorption. A mass transfer process in which one or more gases in a gaseous mixture is transferred into a liquid solvent or a solid. It is the most common form of separation of low molecular weight materials. Absorption is often used to remove gases from gas streams that may be harmful downstream or when released from the process.

Absorption tower. A tall vertical column containing a packing material in which a gas is absorbed by intimate contact with a liquid flowing downwards under the influence of gravity. The gas can be admitted either countercurrent or cocurrent to the flow of liquid in which one or more the gaseous components are absorbed into the liquid.

Acid. A chemical compound or material containing hydrogen that has the tendency to lose protons and form hydrogen ions in solution. Solutions of acids have pH values less than 7.

Acid gas. Natural gas, which consists mainly of methane, but also contains significant amounts of carbon dioxide, hydrogen sulphide, and other acidic contaminants. Natural gas that contains these corrosive and toxic contaminants are required to be removed or reduced using an amine gas treating process.

Acid number. A measure of the acidity of oils such as crude oil, mineral oils, and biodiesels. It is expressed in the mass in milligrams of potassium hydroxide titrated in one gram oil required to neutralize it.

Acid rain. A precipitation of rain that has pH below of typical rain, which is around pH 5.6. Rainwater is naturally acidic due to the absorption of carbon dioxide from the air to form carbonic acid. However, rainwater will also absorb other gases such as sulphur dioxide and various oxides of nitrogen that have been released into the atmosphere as pollutant gases through processes such as the combustion of fossil fuels and from car exhausts. The dissolved gases form sulphuric and nitric acids with pH values of less than 5.0 and have an adverse effect on trees and plants. Acid rain causes damage to leaves and increases the acidity of soil preventing further growth. The water run-off into rivers and lakes also prevents freshwater fish from thriving, leaving the water sterile, and has a major impact on the ecosystem.

Activated carbon. A compound of powdered or granular amorphous carbon mainly made from coconut shells. It has a very high specific surface area used to absorb vapors and gases. With a surface area typically around 1000 m2 per gram, it is widely used to adsorb vapors and gases. The amount of substance that can be adsorbed is proportional to the absolute temperature and pressure. The activated carbon can be reactivated for reuse using steam to strip the adsorbents and recover the carbon. Activated carbon is used in water and air purification, and used in gas masks for the removal of harmful gases. It is also known as activated charcoal and active carbon.

Activated sludge process. A process used in the treatment of sewage and wastewater.

Adiabatic. A thermodynamic process that takes place without heat transfer to or from an external source. When a fluid is compressed adiabatically, there is an increase in temperature of fluid.

Adsorption. A process in which components in gases, liquids, or dissolved substances are selectively held on the surface of a solid. It is used to remove components that may otherwise be harmful if released into the environment or may cause process difficulties further downstream such as causing poisoning of a catalyst.

Air-to-close. A type of pneumatically operated control valve that automatically opens in the event of loss of instrument air pressure. Air-to-open valve is a pneumatically operated control valve that automatically closes in the event of a loss of instrument air pressure. For example, the fuel supply to a furnace should automatically shut in the event of air failure (air-to-open).

ALARA. An abbreviation for as low as reasonably achievable, it is a management tool used in the controlling of risks.

ALARP. An abbreviation for as low as reasonably practicable, it is a management tool used to determine the level to which risks are to be assessed and controlled.

Ambient Temperature. The temperature of the surrounding atmospheric air. Ambient air temperature can affect the operation of process equipment, instruments, and control. It is sometimes referred to as room temperature.

Amine Gas Treating Process. A post-combustion process used to remove acid gases such as carbon dioxide, hydrogen sulphide, and mercaptans from natural gas using an amine chemical solvent to react and form reversible compound.

Anaerobic Digester. A type of bioreactor used for the anaerobic digestion of organic waste liquids from domestic and industrial sources. The biological process involves the use of bacteria in the near absence of oxygen to produce a mixture of methane and carbon dioxide, known as biogas.

Analogue Signal. An electrical signal used in the control of processes as a current or a voltage representing temperature, pressure, level, etc. The commonly used electrical current signal has a range of 4-20 mA. The voltage range commonly used is 0-5 volts DC.

Analogue-to-digital converter (ADC). Electronic hardware used in the control of processes that converts analogue signals such as electrical voltage, current, temperature, and pressure into digital data that a computer can process.

API Gravity. A measure of the density of petroleum oils used in the US and related to specific gravity. The specific gravity and API gravity refer to the weight per unit volume at 15.6 deg C (60 deg F). Most crude oils range between 20 and 45 API.

Aspect ratio. The ratio of the height to width or diameter of an item of process plant equipment such as a column or storage tank.

Atomizer. A device used in the process of atomization to produce very small droplets of a liquid within a gas. Such small droplets can be produced by forcing a liquid through very small aperture under high pressure of by contacting liquid with a high-speed rotating plate or disc.

Autoignition temperature. The temperature at which a material ignites in air or some other oxidant at specified pressure without the aid of a spark or flame.


Backflashing. A cleaning process used to dislodge particualte material in a pipe, column, or filter, etc. It involves reversing the flow of fluids to the normal direction of flow.

Back pressure. The resistance to a moving fluid to its direction of flow caused by an obstruction, bend, or friction in a pipe or vessel. It is often used to describe the discharge pressure from a pump or compressor. The term often refers to a pressure greater than atmospheric.

Backwashing. A method used to clean a fixed bed reactor for reuse. Under normal operation, the flow of a fluid is down through the fixed bed that may act as a support but over time may have become fouled or blocked. Backwashing therefore deliberately reverses the flow of the fluid up through the bed causing fluidization, detaching, suspending, and washing out of undesirable particles. It is often used in sand filtration and ion exchange resin beds.

Ball mill. A mechanical device used to reduce ores to small particles and powder. It consists of a rotating cylindrical chamber mounted on its side containing steel balls and the material to be ground. The slow rotating action of the chamber allows the balls to tumble and cascade over one another therefore crushing the material. They can be operated as a batch or as a continuous operation.

Ball valve. A type of valve used to control the flow of fluid in a pipe. It consists of a sphere with a hole through it. When the ball is rotated by 90 degree, the hole is perpendicular to the pipe axis and the valve is shut preventing flow. With a comparatively low pressure drop, its action is fast and is suitable for dirty and viscous fluids. It is also capable of tight shut-off.

Barrels per calendar day (BPCD) or barrels per stream day (BPSD). The average rates based on a 365-day year, or on the number of days per year the refinery was actually operating, respectively.

Barrel of oil equivalent (BOE). A measure of the amount of a combustible material which when burnt releases the same amount of energy as the combustion of one barrel of crude oil.

Basis of design. A document that is prepared prior to the design and development of a process. It includes the rationale for the design, and includes assumptions and decisions on the options identified for the design as well as codes, standards, and regulations required in the design.

Batch process. A process using a fixed quantity of material that is placed within the process equipment and the operation carried out to completion.

Battery limits. The geographical perimeter that surrounds a processing area and includes process equipment, piping, and associated buildings and structures of a process plant. It excludes utilities and process services such as boiler houses and laboratories.

Bbl. An abbreviation for US barrel and is a volumetric unit for crude oil and petroleum products where one barrel is equivalent to 158.978 litres.

Best efficiency point. The most efficient operation of a centrifugal pump in terms of flow rate and delivery pressure or head. It is usually represented as an identifiable point on the characteristic curve for a centrifugal pump. The efficiency of the pump is the ratio of the power output (the product of pressure and flow rate) to the power input (electrical energy).

Black Box. A term used to describe a process or  system that is considered only in terms of its inputs and outputs, and with no knowledge or understanding of internal operations.

Blanketing. A process in which an inert gas such as a nitrogen is used to fill the vapor space of a vessel containing a liquid to control its composition. Carbon dioxide and argon are also used although carbon dioxide can tend to be reactive in some cases and argon is relatively expensive gas.

Bleed. The controlled removal of a small amount of material from a process as a side stream that prevents an accumulation of undesired materials. It is also known as a purge.

Block Flow Diagram. A schematic representation of an entire process or major part of a process in which unit operations are symbolically represented as blocks in which process material collectively enters for processing and products leave.

Block Valve. A type of valve used in a pipeline that provides a tight shut-off. It is typically used for isolation purposes.

Blowdown. 1. The removal of oil and gas from a pipeline. 2. The difference in pressure between that which opens a safety valve and that which closes it. 3. A technique used to remove periodically accumulated solids in process vessels, boilers in particular, which would otherwise cause problems of deposition on heat transfer surfaces, foaming, and adverse effects on process performance.

Blowoff valve. A pressure release valve system used to prevent problems associated with fluid surge. It is used on turbocharged engines to prevent compressor surge and is usually vented to atmosphere. It is also known as a dump valve. 

Boiling Liquid Expanding Vapor Explosion (BLEVE). The explosion that occurs when a pressure vessel with a superheated flammable liquid of a liquefied flammable gas is heated and ruptures.

Bottleneck. A term used to describe a process or system whose performance or throughput is limited by the action of an item of process plant such as the performance of a pump or volume of a vessel. The term originates from the phenomenon a liquid pouring freely from a bottle being limited by the restricted area at the bottleneck.

Brazing. A process used to join metal in which a filler metal is heated to a melting temperature and is then distributed between close-fitting parts to be joined together by capillary action. The filler metal is an alloy and is protected by a flux which is a chemical agent used to prevent oxides from forming while the work piece is heated.

Butterfly valve. A type of valve used to control the flow of a fluid in a pipeline; it has a circular body and rotary motion disc closure member which is pivotally supported by its stem. They offer a low pressure drop and are suitable for dirty and viscous fluids, however, they are prone to inducing cavitation and choked flow.


Cascade Control. Two or more controllers working together to control a process. The output of the main (or master controller) is used as the set point for the other (or slave controller).

Catalyst Poison. A substance that is absorbed more strongly than the reactants at the surface of a solid catalyst such that it prevents the catalyst from functioning.

Catalytic cracking. A process that breaks down complex petroleum hydrocarbon molecules by the action of temperature and pressure in the presence of a catalyst.

Catalytic Hydrocracking. A refining process in which hydrogen and catalysts are used at a relatively low temperature and pressure to produce naphta, diesel fuel, jet fuel, and high-grade fuel oil. It is used for feeedstocks that are particularly difficult to process by catalytic cracking or reforming because they may have a high sulphur content, or contain a high level of polycyclic aromatics or olefins.

Catalytic Reforming. A process used to produce aromatic hydrocarbon by reforming straight-chain hydrocarbon in the C6 to C8 range from naphta or gasoline fractions into compound containing benzene rings.

Cathodic protection. A method of preventing the rusting and corrosion of iron by using a reactive metal that makes the cathode of an electrochemical cell. Magnesium alloy bars are used for pipelines and iron structures. while zinc or an aluminium/zinc/indium alloy is used for the jacket legs of offshore platforms where the ions from the bars are formed in preference to the steel structure.

Cavitation. The destructive collapse of vapor in a liquid in localized regions of high pressure. Cavitation is more likely to occur with high-speed pumps and hot liquids. One way of ensuring that cavitation does not occur is to ensure that the available net positive suction head (NPSH) exceeds the required NPSH.

Cetane number. A number used as a measure of diesel fuels that represents the percentage of hexadecane (i.e. cetane) in a mixture with 1-methylnaphthalene that has the same ignition characteristics as a diesel fuel being tested in a standard diesel engine.

Characteristic curve. A diagram presented the delivered head or pressure, total power consumption, and efficiency against volumetric flow rate for a centrifugal pump.

Check valve. A non-return valve used to control the direction of flow of a fluid such as the entry to and exit from a reciprocating pump. It is designed to close automatically in the event of reversed flow.

Choked flow. A condition in which a fluid becomes limited in its flow or choked and is not able to be increased further. For a fluid flowing through an orifice or small hole in a pipe, the increase in velocity corresponds to a decrease in pressure, known as the venturi effect. The choking of gases occurs when the velocity leaving the orifice approaches sonic velocity i.e. at a Mach number of one. This results in shock waves that restrict flow causing the choking effect. For liquids, the decrease in pressure below the vapor pressure results in partial flashing and cavitation, with the formation of vapor effectively limiting flow. It is also known as the critical flow an is important in process safety, particularly in terms of the rate of release of material from a vessel or vent when depressurizing.

Christmas tree. An assembly of valves and fittings used to control the pressure and flow in oil and gas wells. It is located on top of the wellhead and provides the controlling mechanism for the isolation of wells. It has many other functions including permitting the injection of chemicals into wells and pressure relief. Playing an essential role in an emergency shutdown (ESD), a basic form of assembly has several manual gate valves with typically four or five valves being arranged in a crucifix arrangement.

Churn flow. A two-phase flow regime in a pipe or tube characterized by the oscillatory transition from a continuous liquid phase to a continuous and predominant gas phase.

Closed loop control. A system or process being controlled in which the controlled variable is measured and the result of this measurement is used to manipulated one of the process variables, such as steam flow to a heat exchanger, for example.

Combined heat and power (CHP). The use of a heat engine or power station to simultaneously generate both electricity and useful heat.

Commissioning. A final and thorough check of an installed process plant or item of equipment to ensure that it is fully operable as intended. All aspects of the process or equipment are tested individually and collectively. Prior to commissioning, a site acceptance test is carried out.

Containment. The prevention of a hazardous material from being released into the environment or beyond a defined boundary.

Control loop. A part of a process control system. Open loop control involves human operator intervention, whereas closed loop control system is an automated system in which the output signal to the process in compared to a defined set point.

Control valve. A device used to control the rate of flow of a process material through a pipe. It is actuated either electronically or pneumatically in which either a stem or diaphragm changes the position of a plug in a seat either restricting or opening the passage of flow.

Coriolis flow meter. A non-invasive type of flow meter used to measure the mass flow of a fluid through a pipeline. It is based on the Coriolis effect and involves diversionary loops of pipe through which the fluid passes. As the fluid moves through the loop, the fluid momentum changes and rotates, exerting a force on the loop causing it to twist and vibrate. The extent of the twisting and vibration effect gives an indication of the rate of mass flow.

Cracker. The chemical reactor in which the catalytic cracking of high molecular weight hydrocarbons to small molecules takes place.

Cracking. A process in which high molecular weight hydrocarbon are broken down into lower molecular weight products by the effect of high temperature in the presence of an alumina-silica catalyst. The process is used to produce gases such as methane, ethane, propylene, and propane that are the subsequent raw materials used for the manufacture of a wide range of products.

Critical Point. The temperature and pressure of a substance at equilibrium when two phases become identical and form a single phase. The critical state is the condition in which the density of both the liquid and vapor phases of a substance are the same as occurs at the critical point.

Critical Pressure. The minimum pressure required to liquefy a substance at its critical temperature.

Critical Temperature. The temperature above which a gas cannot be liquefied by pressure alone.

Critical Velocity. The velocity above which the flow of a fluid no longer continues to be streamline but becomes turbulent.

Crude Distillation. The separation or fractionation of crude oil into separate components or groups of components known as fractions in a distillation column called a fractionator operating at atmospheric pressure.


Debottlenecking. An analysis used to identify and find engineering solutions to the  limiting part or parts of a process that restrict the way it operates.

Debutanizer. A continuously operated distillation column used to remove butane as distillate from a mixed feed of hydrocarbons with the heavier components leaving the bottom of the column. Lighter hydrocarbons may also leave the top of the column with the butane. To ensure that the light hydrocarbons can be boiled and condensed in the liquid phase, the distillation is operated at a gauge pressure of around 10 barg.

Decommissioning. The procedure of closing down a chemical process to a point that permits the release of the property for demolition and site clearance.

De-ethanizer. A continuously operated distillation column used for the removal of ethane as distillate from a mixed feed of light hydrocarbons. Any methane also leaves the top of the column along with the ethane while heavier components leave the bottom. To ensure that the light hydrocarbons can be boiled and condensed in the liquid phase, the distillation is operated at a gauge pressure at around 14 bar.

Degassing. The release of dissolved, absorbed, or adsorbed gases from a liquid or solid. Degassing is often achieved in reduce pressure or vacuum systems.

Delivery pressure. The pressure of a fluid at the outlet of a pump.

De-methanizer. A continuously operated distillation column used in petrochemical refineries to remove methane as distillate from a mixed feed of light hydrocarbons. Any hydrogen will also leave from the top of the column while ethane and other heavier components leave from the bottom. To ensure that the hydrocarbons can be separated in the liquid phase, the distillation process is typically operated at a gauge pressure of around 24 to 28 bar and temperature of -60 deg C.

Demister. A device used to remove a fine dispersion of liquid droplets from a gas or vapor.

Dense Gas. A gas that has a density greater than that of air at the same temperature.

De-propanizerA continuously operated distillation column used to remove propane as distillate from a mixed feed of hydrocarbons with the heavier components leaving the bottom.

Desalting. The process of removing mineral salts from water-containing oil.

Desulphurization. A process used to remove sulphur from crude oil and refined petroleum products, and also the removal of hydrogen sulphide from natural gas.

Dew Point. The temperature at which drops of condensate first appear on cooling a condensing vapors.

Double Block and Bleed. The use of two block or isolation valves on a length of pipe for the purposes of isolation with a bleed valve between them. It is used to prevent a process fluid from reaching an item of equipment such as pump requiring maintenance. The pump can therefore be taken out of service without having to drain the entire length of pipe.

Downcomer. A duct in an item of process equipment, such as a distillation column or evaporator, used to channel liquid from one location, such as a plate, down to the one below.


Economizer. A type of heat exchanger used to raise the efficiency of a steam boiler. It involves first heating the feed water to a steam boiler by the hot flue gases from the boiler.

Ejector. A type of compressor in which gas or vapor is entrained by injecting a high velocity jet of gas or vapor. A steam jet ejector uses live steam as the entraining high velocity jet in which part of the kinetic energy is converted to pressure energy.

Elbow. A short section of curved pipework, usually at 90 degree.

Enhanced Oil Recovery. One of a number of methods used to increase the amount of crude oil that can be recovered from an oil reservoir.

Explosion Limit. The highest or lowest concentration of a flammable gas or vapor in air or oxygen that will propagate a flame when ignited.

Explosure-proof equipment. The equipment in an enclosure that is capable of withstanding an internal explosion of a specified gas or vapor, and of preventing possible ignition of a surrounding flammable atmosphere.

Explosion suppression. A method, device, or system to effectively extinguish an explosion.


Fail-safe. A type of control used in situations such that a malfunction or unsafe condition results in a device, item of equipment, or process safely shutting down. An example is a gas burner where the fuel supply is automatically shut off in the event that air (oxygen) supply should inadvertenty cease. In contrast, a fail-to-danger is a fault or failure of a device or item of equipment that renders the plant in a dangerous condition such that a control system in unable to respond to provide the necessary protection.

Fault Tree Analysis. A method used to calculate the probability of an event occurring based on the probabilities or frequencies of its component parts.

Feedback Control. A closed-loop method of controlling a process in which information about the controlled variable is fed back to the input and compared against a desired value. The difference between the two sinals is called the error or deviation.

Fireproof. A condition in which a structure, equipment, wiring, controls, or piping is capable of functioning under the most severe conditions of fire likely to occur at its location.

Fire Triangle. A way of illustrating the three “factors necessary for the process of combustion which are fuel, oxygen, and heat. All three are needed for combustion to occur. A fire can therefore be prevented or extinguished by removing one of the the factors.

Flame Speed. The velocity of a propagating flame relative to the observer.

Flame Trap. A device used to prevent a gas flame from entering a pipe or process vessel containing flammable material. It is typically made from a screen, mesh, or metal gauze, and located in the vent pipe to a storage vessel.

Flaring. The burning of unwanted gaseous hydrocarbon in a petroleum refinery or on an offshore installation.

Flare Boom. It is used on offshore platforms, angled away and extending a safe distance from the deck to permit the safe burning of the gases.

Flash Drum. A vessel used for the rapid separation of a mixture into a liquid and vapor by flash evaporation caused by a sharp drop in pressure. The liquid leaves cooled from the bottom and the vapor from the top at the saturation temperature of the liquid.

Flowline. A pipeline that carries materials from one place to another. In the offshore industry, a flowline is a pipeline that carries oil on the seabed from a well to a riser.

Flue Gas. A mixture of gases produced as a result of combustion that emerge from a stack or chimney. The gases contain smoke, particulates, carbon dioxide, water vapor, unburnt oxygen, nitrogen, etc.

Force Convection. The process by which heat is transported within a fluid by the movement of the fluid with the assistance of a fan, pump, or blower.

Fouling. A deposition of solid material on a surface causing resistance to heat and mass transfer.

Free Convetion. The movement of a fluid caused by localized difference in density due to differences in temperature. The heated fluid due to a hot surface is less dense than the cooler and more dense fluid, and therefore rises, creating natural circulating currents.

Fuel Tank Inerting. A method used to prevent fire and explosion in tanks and vessels that contain flammable liquids by the use of inert or non-combustible gases such as nitrogen. The addition of the inert gas is used to exclude air and prevent otherwise flammable vapor and air mixturees from building up.


Gas Sweetening. A process used to remove hydrogen sulphide and mercaptans from natural gas. Commonly used in petroleum refineries, the gas treatment uses amine solutions such as monoethanolamine. The process uses an absorber unit and a regenerator

Gate Valve. A widely used device that regulates the flow in a pipe. It consists of a vertical moving section across the flow area. They are useful for on-off type flow control operations and provide a low pressure frop when fully open.

Globe Valve. A device used to regulate the flow of a fluid in a pipe and consists of a flat disc that sits on a fixed ring seat. The disc is movable and allows flow through the valve.


HAZAN. An abbreviation for Hazard Analysis, it is a technique used to assess the probability of a hazard occuring and for determining the subsequent consequences.

HAZID. An abbreviation for Hazard Identification, it is a systematic and wide-ranging structured hazard analysis technique to identify the potential hazards a t an early stage in process design and development.

HAZOP. An abbreviation for Hazard and Operability, it is a systematic and structured hazard evaluation technique used to identify the potential failures of equipment or plant systems that may otherwise become hazards and present potential operating problems.

Heat Pump. A device used to transfer heat from a cold source to a place or reservoir at a higher temperature by expending mechancal energy, such as in air conditioning.

Heat Treatment. The process of applying heat to a material to bring about a physical, chemical, and biological change.

Hot Tapping. A technique used to form a connection between pipes or vessels without the need for first emptying or draining the contents. The pipe or vessel can therefore remain in operation at the time of the maintenance or plant modification. It involves temporarily plugging or sealing the pipe to allow the connection to be made.

Hot Work. A method of carrying out a task in a hazardous area using a tool or item of equipemt that can provide source of ignition, heat, or flame. Examples include welding or the use of a flame or electric arc.


Inert Gas. 1. A noble gas such as helium, neon, argon, krypton, xenon, and radon. 2. A gas such as nitrogen or carbon dioxide, known as an inerting agent or blanketing gas, used to protect a substance from coming into contact with air or oxygen.