Basic Refinery Processes

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.

SEPARATION

Fractional distillation.

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. In turn, each of these more complex processing units also incorporates a fractional distillation tower to separate its own reaction products.

Modern crude oil distillation units operate continuously over long periods of time and are much larger than the fractional distillation units employed in chemical or other industries. Process rates are commonly delineated in American barrels; units capable of processing 100,000 barrels per day are commonplace, and the largest units are capable of charging more than 200,000 barrels per day.

Crude oil is withdrawn from storage tanks at ambient temperature and pumped at a constant rate through a series of heat exchangers in order to reach a temperature of about 120°C. A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. (If the salt were not removed at this stage, it would be deposited later on the tubes of the furnace and cause plugging.) The desalted crude oil passes through additional heat exchangers and then through steel alloy tubes 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 as much as 45 metres high containing 20 to 40 fractionating trays spaced at regular intervals. The most common fractionating trays are of the sieve or valve type.

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. A pressure control valve on the fuel-gas line maintains fractionating column pressure at the desired figure, usually near atmospheric pressure. Part of the condensed liquid, called reflux, 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, in turn, gives up lighter constituents to the vapour phase. Condensation and reevaporation takes place on each tray. Eventually an equilibrium is reached in which there is a continual gradation of temperature and oil properties throughout the column, with the lightest constituents on the top tray and the heaviest on the bottom. The use of reflux and vapour-liquid contacting trays distinguishes fractional distillation from simple distillation columns.

Intermediate products, or “sidestreams,” are withdrawn at several points from the column. In addition, modern crude distillation units employ intermediate reflux streams. Sidestreams are known as intermediate products because they have properties between those of the top or overhead product and those of products issuing from the base of 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.

Unvaporized oil entering the column flows downward over a similar set of trays in the lower part of the column, called stripping trays, which act to remove any light constituents remaining in the liquid. Steam is injected into the bottom of the column in order to reduce the partial pressure of the hydrocarbons and assist in the separation. Typically a single sidestream is withdrawn from the stripping section: heavy gas oil, with a boiling range of 285°–370° C. The residue that passes from the bottom of the column is suitable for blending into industrial fuels. Alternately, it may be further distilled under vacuum conditions to yield quantities of distilled oils for manufacture into lubricating oils or for use as a feedstock in a gas oil cracking process.

Vacuum distillation. The principles of vacuum distillation resemble those of fractional distillation (commonly called atmospheric distillation to distinguish it from the vacuum method), except that larger-diameter columns are used to maintain comparable vapour velocities at reduced operating pressures. A vacuum of 50 to 100 millimetres of mercury absolute is produced by a vacuum pump or steam ejector.

The primary advantage of vacuum distillation is that it allows for distilling heavier materials at lower temperatures than those that would be required at atmospheric pressure, 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. Absorption processes are employed to recover valuable light components such as propane/propylene and butane/ butylene from the vapours that leave the top of crude-oil or process-unit fractionating columns within the refinery. 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. Absorption is more effective under pressures of about 7 to 11 kilograms per square centimetre than it is at atmospheric pressure.

The enriched absorption fluid is heated and passed into a stripping column, where the light product vapours pass upward and are condensed for recovery as liquefied petroleum gas (LPG). The unvaporized absorption fluid passes from the base of the stripping column and is reused in the absorption tower.

Crystallization. The crystallization of wax from lubricating oil fractions is essential to make oils suitable for use. A solvent (often a mixture of benzene and methyl ethyl ketone) 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 specially woven 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, after washing with solvent to remove traces of oil. The solvents are later distilled from the oil and reused.