An oil refinery includes many unit operations and unit processes. A unit operation is a basic step in a process. Like evolution of our planet, life and technology, oil refining industry has been developing with increasing complexity since its foundation in 1859. Unit operations involve physicochemical processes. Physical transformations, such as desalting, distillation, filtration, and evaporation, relate to separation. Chemical transformations, such as isomerization, hydrogenation, oxidation, and polymerization, relate to chemical reactions. These unit operations are connected into process. The main goal of refinery units is to extract useful substances from crude oil. Oil refinery and petrochemical plant are two ‘living organisms’ which are tightly connected.
The dream of the creator of the Periodic Table of elements, Dmitri Mendeleev is, that any chemical plant should be wasteless and is realized in the refinery: a product obtained in one unit is the raw material for the other unit. Nearly all wastes are utilized. Excluding is some gases (mostly CO2) and water vapor emitted into the atmosphere. Even nowadays CO2 utilization can be applied in oil refining and petrochemical industries.
There are associated facilities, such as cooling water system, power station (with water treatment and steam providing), and units related to the protection of the environment and people (the utilization of hydrocarbon wastes, purification of wastewater and emitted gases, and deodorization). To sum up, any oil refinery is a very complicated alive “organism”. Each oil refinery has its own unique processing scheme which is determined by the process equipment available, crude oil characteristics, operating costs, and product demand. There are no refineries absolutely identical in their operations but most corrosion problems and solutions may be similar. We will describe shortly main units and processes where corrosion problems occur more often.
Crude Distillation Unit
Crude distillation unit is a “heart” of any oil refinery and consists of a preheat train, a desalter, a preflash drum, a furnace, an atmospheric and vacuum distillation columns. Not at once crude oil coming from a storage tank or a transportation pipeline is distilled.
Crude oil is prepared by means of settling, then is treated in desalters to remove dissolved salts. Then crude oil is heated in furnaces, and the resultant liquid-vapor mixture flows via a transfer line to the flash zone of the preflash drum, then passes through the furnace, and then liquid crude enters into atmospheric distillation column. This column is the “main organ” of any crude distillation unit where crude is distilled and various petroleum products are obtained. These products are sent to other units for further treatment to obtain useful fuels or other substances as raw materials for petrochemical, chemical and pharmaceutical plants. The heaviest product, residual bottom, flows to the vacuum distillation column where is distilled under vacuum to form also valuable petroleum products.
Crude oil is an emulsion which contains various small amounts of water and salts. These salts in the presence of water cause corrosion, fouling and poison the catalysts in processing units downstream of the crude distillation unit. Desalting is the process of diluting the salt amount with fresh water and applying electric fields and special chemicals (demulsifiers and surfactants). Thus, the function of the desalter is to extract the water soluble salts (more than 90 wt%), suspended solids (corrosion products, soil, silt, and sand) from the crude oil into the aqueous phase (lower layer in the desalter). The heavy aqueous phase containing dissolved salts, H2S and suspended solids is sent to a sour water stripper. The light crude oil phase (containing small amounts of H2S and traces of water and salts) is withdrawn to the intermediate section of the preheat train.
Preheat train is made up of many heat exchangers with the crude usually processed on the tube-side. Then crude enters the furnace. Following the changes in crude temperature, the preheat train is usually divided into cool section (upstream of the desalter), intermediate (between the desalter and the preflash), and hot (downstream of the preflash).
Furnace is a fired heater where the crude oil is brought at the desired inlet temperature of the preflash drum or the distillation column. Usually crude units contain furnaces before the preflash, the atmospheric and the vacuum columns. Fuel oil, gas oil or natural gas are used as fuels in furnaces for the heating crude oil.
Preflash drum (column)
The preflash separates lighter hydrocarbon components of the crude oil before it enters the furnace. This process was suggested by Brugma A. J. in 1941. The use of preflash reduces the risk of a two-phase flow in the hot section of the train. While the gas phase bypasses the furnaces and enters the distillation column directly, the liquid phase is further preheated in the hot end of the preheat train before entering the furnace. If the crude oil feed to an atmospheric column is heavy and has nearly no light hydrocarbons, it is possible to avoid using the preflash.
Atmospheric distillation column
Crude is fractionated at atmospheric pressure in this column into petroleum products (named also primary products, distillates, fractions, or cuts). Fractional continuous distillation (named also rectification) is a physicochemical process in which numerous hydrocarbons are separated according to their boiling temperatures, and new different chemical mixtures are obtained. These combinations of chemical compounds (petroleum products) are withdrawn from the column according to their boiling temperatures. Gases and naphtha are withdrawn from the distillation column head (overhead).
Heavier fractions gasoline, kerosene, light and heavy gas oil, and fuel oil are withdrawn from different sections of the column. These petroleum products from gases to viscous liquids are not finished fuels and have a different fate. Some of them are used for inner needs at refinery. Others are sent for further treatment to obtain finished products. The third group is the raw material for petrochemical industry.
If we ask a layman how many petroleum products he knows, he can say gasoline, kerosene, fuel oil. Really, over 2000 products have different specifications. Only about 40 types of gasoline are produced by refineries. The residual bottom after atmospheric distillation is sent to the next unit.
Vacuum distillation unit
Atmospheric residue is distilled in vacuum column to obtain useful petroleum products. By lowering the pressure the boiling point of hydrocarbons is decreased and their destruction is minimized. Thus, unconditioned atmospheric bottoms is transformed into light hydrocarbons, light vacuum gas oil and heavy vacuum gas oil. The lightest fraction together with steam are taken off the top of the distillation column. The vacuum residue is taken off the bottom of the column and sent to a visbreaker, cocker or deasphalting unit for further processing.
Reference: Corrosion Problems and Solutions in Oil Refining and Petrochemical Industry. Adrian V. Gheorghe, Old Dominion University, Norfolk, VA, USA. Paraphrase by Franklin P. Jones (1908–1980), an American journalist