Each refinery is uniquely designed to process specific crude oils into selected products. In order to meet the business objectives of the refinery, the process designer selects from an array of basic processing units. In general, these units perform one of three functions:
1) separating the many types of hydrocarbon present in crude oils into fractions of more closely related properties,
2) chemically converting the separated hydrocarbons into more desirable reaction products, and
3) purifying the products of unwanted elements and compounds.
The primary process for separating the hydrocarbon components of crude oil is fractional distillation. Crude oil distillers separate crude oil into fractions for subsequent processing in such units as catalytic reformers, cracking units, alkylation units, or cokers.
Crude oil is withdrawn from storage tanks at ambient temperature and pumped at a constant rate through a series of heat exchangers in order to reach a temperature of about 120° C. A controlled amount of fresh water is introduced, and the mixture is pumped into a desalting drum, where it passes through an electrical field and a saltwater phase is separated. The desalted crude oil passes in a furnace. There it is heated to a temperature between 315° and 400° C, depending on the type of crude oil and the end products desired. A mixture of vapour and unvaporized oil passes from the furnace into the fractionating column, a vertical cylindrical tower containing 20 to 40 fractionating trays spaced at regular intervals.
The oil vapours rise up through the column and are condensed to a liquid in a water- or air-cooled condenser at the top of the tower. A small amount of gas remains uncondensed and is piped into the refinery fuel-gas system. Part of the condensed liquid is pumped back into the top of the column and descends from tray to tray, contacting rising vapours as they pass through the slots in the trays. The liquid progressively absorbs heavier constituents from the vapour and gives up lighter constituents to the vapour phase. Condensation and reevaporation takes place on each tray.
Intermediate products, or “sidestreams,” are withdrawn at several points from the column. Typical boiling ranges for various streams are as follows: light straight-run naphtha (overhead), 20°–95° C ; heavy naphtha (top sidestream), 90°–165° C; crude kerosene (second sidestream), 150°–245° C; light gas oil (third sidestream), 215°–315° C.
The principles of vacuum distillation resemble those of fractional distillation, except that larger-diameter columns are used.
The primary advantage of vacuum distillation is that it allows distilling heavier materials at lower temperatures, thus avoiding thermal cracking of the components. Firing conditions in the furnace are adjusted so that oil temperatures usually do not exceed 425° C. The residue remaining after vacuum distillation, called bitumen, may be further blended to produce road asphalt or residual fuel oil, or it may be used as a feedstock for thermal cracking or coking units. Vacuum distillation units are essential parts of the many processing schemes designed to produce lubricants.
Absorption processes are employed to recover valuable light components such as propane and butane. These volatile gases are bubbled through an absorption fluid, such as kerosene or heavy naphtha, in equipment resembling a fractionating column. The light products dissolve in the oil while the dry gases (hydrogen, methane, ethane, and ethylene) pass through undissolved. The light product vapours are condensed for recovery as liquefied petroleum gas (LPG).
The crystallization of wax from lubricating oil fractions is essential to make oils suitable for use. A solvent is first added to the oil, and the solution is chilled to about −20° C. The function of the benzene is to keep the oil in solution and maintain its fluidity at low temperatures, whereas the methyl ethyl ketone acts as a wax precipitant. Rotary filters deposit the wax crystals on a special cloth stretched over a perforated cylindrical drum. A vacuum is maintained within the drum to draw the oil through the perforations. The wax crystals are removed from the cloth by metal scrapers.
The separation processes described above are based on differences in physical properties of the components of crude oil. All petroleum refineries throughout the world employ at least crude oil distillation units to separate naturally occurring fractions for further use, but those which employ distillation alone are limited in their yield of valuable transportation fuels. By adding more complex conversion processes that chemically change the molecular structure of naturally occurring components of crude oil, it is possible to increase the yield of valuable hydrocarbon compounds.
Before petroleum products can be marketed, certain impurities must be removed. The most common impurities are sulfur compounds. Apart from their foul odour, sulfur compounds are technically undesirable. In motor and aviation fuels they reduce the effectiveness of antiknock additives and interfere with the operation of exhaust-treatment systems. In diesel fuel they cause engine corrosion and complicate exhaust-treatment systems. Most crude oils contain small amounts of hydrogen sulfide, but these levels may be increased by the decomposition of heavier sulfur compounds during refinery processing. In order to minimize noxious emissions, most refinery fuel gases are desulfurized.
Other undesirable components include nitrogen compounds, which poison catalyst systems, and oxygenated compounds, which can lead to colour formation and product instability.
Exercise 3. Answer the following questions.
1. What is refinery designed for? 2. What are the main refinery processes? 3. What do crude oil distillers separate? 4. Tell about the processes after crude oil is withdrawn from storage tanks. 5. What happens with a small amount of uncondensed gas? 6. What distinguishes fractional distillation from simple distillation columns? 7. Where does unvaporized oil entering the column flow? 8. What is the difference between vacuum and fractional distillation? 9. How may bitumen be used? 10. What is the purpose of absorption? 11. What is the purpose of crystallization?
Exercise 4. Match these words with their definitions.
a) a liquid that can change a solid substance into liquid;
b) plant, designed to process specific crude oils into selected products;
ń) process employed to recover valuable light components from the vapors that leave the top of crude-oil or process-unit fractionating columns within the refinery;
d) the primary process for separating the hydrocarbon components;
e) intermediate products;
f) the residue remaining after vacuum distillation.
Exercise 5. Decide whether the following statements are true (T) or false (F) in relation to the information in the text or the information is not enough to make a decision (?). If you think a statement is false, change it to make it true.
1. ___ Fractional distillation units are much larger than modern crude oil distillation units employed in chemical and other industries.
2. ___ The liquid absorbs heavier constituents from the vapour and gives up lighter constituents to the vapour phase.
3. ___ In comparison with vacuum distillation fractional distillation allows for distilling heavier materials at lower temperatures.
4. ___ The separation processes are based on differences in physical properties of the components of crude oil.
5. ___ The crystallization of wax from lubricating oil fractions is essential to make oils suitable for use.
Exersice 6. Restore the order of the sentences to give the meaning to the text.