Glossary of crude oil treatment and hydrogen purification terms
This glossary defines the terms engineers meet when specifying, operating or troubleshooting desalters, dehydrators, electrostatic coalescers and hydrogen adsorption units. Each entry opens with a definition of 40 words or fewer that stands on its own, followed on its own page by a fuller explanation, synonyms and related terms.
A
- Activated alumina
- Activated alumina is a porous aluminium oxide adsorbent with a strong affinity for water and good mechanical strength. It is used as the first, guard layer in hydrogen PSA beds and as a drying adsorbent in temperature swing adsorption units.
- Activated carbon
- Activated carbon is a porous carbon adsorbent with large internal surface and low affinity for water. In hydrogen PSA units it forms the middle layer of the bed, where it removes carbon dioxide, methane and heavier hydrocarbons from the feed.
- Adsorbent
- An adsorbent is a porous solid with a very large internal surface that holds selected gas molecules on it. In gas purification, beds of adsorbent remove impurities from a stream and are then regenerated by lowering pressure or raising temperature.
B
- Basic sediment and water (BS&W)
- Basic sediment and water (BS&W) is the combined volume of water and solid sediment in a crude oil sample, reported as a volume percentage. It is the usual measure of how well crude has been dehydrated before sale or refining.
- Breakthrough
- Breakthrough is the point in an adsorption step at which the impurity being removed first appears in the product above its allowed level. It shows that the usable capacity of the bed has been reached and it must be regenerated.
C
- Crude oil dehydrator
- A crude oil dehydrator is an electrostatic coalescer that removes produced water from crude oil to meet a water specification, usually stated as basic sediment and water. No wash water is added. It is the same equipment as a desalter.
- Crude oil desalter
- A crude oil desalter is an electrostatic coalescer operated with wash water mixed into the crude upstream. The water dilutes the salty droplets in the crude, and the coalescer then removes the water and most of the salt with it.
D
- Demulsifier
- A demulsifier is a surface-active chemical injected into crude oil at parts-per-million levels to displace the natural films that stabilise water droplets. With the films weakened, droplets can merge and settle. It works together with heat, time and electrostatic fields.
- DeOxo dryer
- A DeOxo dryer is a purification package for electrolyser hydrogen. A catalytic deoxidiser reacts trace oxygen with hydrogen to form water, and a temperature swing adsorption dryer then removes the water, raising purity from about 99.9 to 99.999 per cent.
E
- Electrostatic coalescence
- Electrostatic coalescence is the use of a high-voltage electric field to make small water droplets dispersed in oil collide and merge into larger droplets. Larger droplets settle much faster under gravity, so water separates from crude in a smaller vessel.
- Electrostatic coalescer
- An electrostatic coalescer is a pressure vessel with energised electrode grids, in which a high-voltage field coalesces water droplets in crude oil so they settle. Used without wash water it is a dehydrator; with wash water it is a desalter.
- Emulsion (water-in-oil)
- A water-in-oil emulsion is a dispersion of small water droplets in crude oil, held apart by natural surface-active films of asphaltenes, resins, waxes and fine solids. The droplets will not settle until those films are broken and the droplets merge.
- Entrance bushing
- An entrance bushing is the insulated, pressure-rated feedthrough that carries the high-voltage conductor from the external transformer through the vessel wall of an electrostatic coalescer to the electrode grid. It must hold vessel pressure and insulate against full operating voltage.
H
- Hydrogen recovery
- Hydrogen recovery is the fraction of the hydrogen entering a purification unit that leaves as product, stated as a percentage. In a pressure swing adsorption unit the remainder leaves in the tail gas. Recovery trades against product purity.
I
- Indirect fired heater
- An indirect fired heater warms a process fluid without exposing it to flame. A burner heats a bath of water or other liquid through a fire tube, and the process fluid flows through a coil immersed in the same bath.
- Interface level
- Interface level is the height of the boundary between settled water and the oil above it in a separator, dehydrator or desalter. Controlled by the water outlet valve, it must sit below the electrodes and above the water outlet.
M
- Mixing valve
- A mixing valve is a throttling valve upstream of a desalter that shears wash water into fine droplets so they meet the salty water in the crude. Its pressure drop balances mixing against making an emulsion too tight to break.
- Molecular sieve
- A molecular sieve is a synthetic zeolite adsorbent with pores of uniform molecular size. It adsorbs small polar molecules strongly. In hydrogen PSA units it removes carbon monoxide, nitrogen and methane; in TSA dryers it removes water to low levels.
P
- Pounds per thousand barrels (PTB)
- Pounds per thousand barrels (PTB) is the customary unit for the salt content of crude oil: pounds of salt, expressed as sodium chloride, in one thousand barrels of crude. One PTB is about 2.85 milligrams per litre.
- Pressure swing adsorption (PSA)
- Pressure swing adsorption (PSA) separates gases using beds of solid adsorbent that hold impurities at high pressure and release them when pressure is lowered. Several beds cycle in sequence, so purified gas, typically hydrogen, is produced continuously.
R
- Rag layer
- A rag layer is a band of emulsion and solids that collects at the oil-water interface in a separator or desalter. As it grows it reduces settling volume, confuses level instruments and may reach the electrodes or the water outlet.
S
- Stokes’ law
- Stokes' law gives the settling speed of a small droplet in a liquid. Speed rises with the square of droplet diameter and with the density difference between the two liquids, and falls as the viscosity of the surrounding liquid rises.
T
- Tail gas
- Tail gas is the low-pressure stream released from a pressure swing adsorption unit as its beds are depressurised and purged. It holds the removed impurities with the hydrogen that was not recovered, and is usually used as fuel.
- Temperature swing adsorption (TSA)
- Temperature swing adsorption (TSA) removes trace impurities, most often water, from a gas using adsorbent beds that are regenerated by heating. One bed dries the gas while another is heated and then cooled, over a cycle of several hours.
W
- Wash water
- Wash water is low-salinity water injected into crude oil upstream of a desalter to dilute the salty water droplets it carries. It is typically added at 3 to 10 per cent of the crude volume, then removed in the desalter.