Refinery: Separation units Distillation

2) Desalting Before crude oil enters the atmospheric distillation unit dissolved brine in crude oil is removed in a process referred to as desalting. The salt in the crude is in the form of dissolved or suspended salt crystals in water emulsified with the crude oil. Desalting is used if the crude oil salt content is more than 10 lb/1000 bbl. Now days, all crude oil is desalted.

Importance of desalting Why is desalting necessary? Salt deposition – causes fouling and corrosion on heat transfer surfaces. Acids formed by decomposition of salt. Other unwanted materials are also removed: - metals: deactivate catalyst - removal of other suspended solids (fine sand, clay, soil particles, iron oxide/sulfide)

How are salts removed? The basic principle of the desalting is to wash the salt from the crude oil with water. Crude is mixed with 10 vol% wash water at 100-150oC . Salts will dissolve in wash water and oil-water is separated in settling vessel.

How are salts removed? The basic principle of the desalting is to wash the salt from the crude oil with water. Crude is mixed with 10 vol% wash water at 100-150oC . Salts will dissolve in wash water and oil-water is separated in settling vessel. Two immiscible liquids (phases A and B) that are not emulsified An emulsion of Phase B dispersed in Phase A 5

Oil-water separation Chemicals to assist in breaking emulsion High potential electrical field across the settling vessel to coalesce droplets of salty water rapidly: could be AC (interface), DC (oil phase) or both fields

Efficiency of salt removal The pH, gravity, and viscosity of the crude oil, as well as the volume of wash water used per volume of crude, affect the separation ease and efficiency. 1 to 3 stages depending on salt concentration in oil Single stage : 90-95% Two stage : 99% Efficiencies increase when: - Heavy naphtanes used (stable emulsions) - Temperature above 140oC - pH range 6-8

Desalting system Single stage desalting unit

Desalting Unit

Vacuum distillation To recover additional heavy distillates from residue, crude oil is piped to a second distillation column where the process is repeated under vacuum, called vacuum distillation Vacuum distillation is the process of distillation carried out at reduced pressure of 25-40 mmHg This allows heavy hydrocarbons with boiling points of 450oC and higher to be separated without them partly cracking into unwanted products such as coke and gas. Thermal cracking results in loss of product and equipment fouling

Vacuum distillation Vacuum distillation is the process of distillation carried out at reduced pressure of 25-40 mmHg. To improve vaporization, the effective pressure is lowered even further (to 10 mmHg or less) by the addition of steam to the furnace inlet and at the bottom of the vacuum tower. The amount of steam used is a function of the boiling range of the feed and the fraction vaporized, but generally ranges from 10 to 50 lb/bbl feed.

Atmospheric tower Vacuum tower The lower operating pressures cause significant increases in the volume of vapor per barrel vaporized As a result, the vacuum distillation columns are much larger in diameter than atmospheric towers. It is not unusual to have vacuum towers up to 40 feet in diameter. Atmospheric tower Vacuum tower

Vacuum distillation The desired operating pressure is maintained by the use of steam ejectors, vacuum pumps and surface condensers. The size and number of ejectors and condensers used is determined by the vacuum needed and the quality of vapors handled. For a flash zone pressure of 25 mmHg, three ejector stages are usually required. The first stage condenses the steam and compresses the noncondensable gases, while the second and third stages remove the noncondensable gases from the condensers.