Separation into components

The primary refinery process is fractional distillation, which may be followed by other physical separation methods, such as solvent extraction, in which superior lubricating stocks are extracted by means of a solvent, usually furfural obtained from oat hulls.

Fractional distillation. Modern petroleum distillation units operate continuously over long periods of time and are large compared with those that carry out the same process in other industries. Units with 100,000-barrel-per day capacities are commonplace, and units of over 200,000 barrels per day are now in operation. The American barrel is the most widely used unit in the oil industry and comprises about 160 litres (42 U.S. gallons, or 35 imperial gallons).

Crude oil is pumped at a constant rate through steel alloy tubes in a furnace, fired by gas or oil, and heated to a temperature between 315° and 370 °C (600° and 700 °F), 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 (150 feet) high, provided with 30 or 40 perforated fractionating trays spaced at regular intervals. The bubble-cap tray is the most common type used, but the sieve tray (less expensive, but with a narrower operating range) is sometimes employed. It consists of a simple perforated plate with small holes about 0.48 to 0.64 centimetre (0.19 to 0.25 inch) in diameter.

The oil vapours rise up through the column and are condensed to a liquid in a water-cooled condenser at the top. A small amount of gas remains uncondensed; this is piped away to the refinery fuel-gas system. A pressure control valve on this line maintains fractionating-column pressure at the required figure, usually near atmospheric pressure.

Part of the condensed liquid, called reflux, is pumped back into the top of the column and runs down from tray to tray, contacting the rising vapours as they pass through the slots in the bubble caps. The liquid progressively absorbs heavier constituents from the vapours and in turn loses its lighter components. Condensation and reevaporation take place on each tray. Eventually, an equilibrium is reached in which there is a continuous gradation of temperature and of oil properties throughout the column, with the lightest constituent on the top tray and the heaviest at the bottom. The use of reflux and a column of this type distinguishes fractional distillation from simple distillation.

Fractions called sidestreams are withdrawn at several points. These products have properties intermediate from the top and base of the column. Typical boiling ranges for various products are as follows: light gasoline (overhead) 25° – 95 °C (75° – 200 °F); naphtha 95° – 150 °C (200° – 300 °F); kerosene 150° – 230 °C (300° – 450 °F); and gas oil 230° – 340 °C (450° – 650 °F).

In practice, the boiling ranges of these products can be varied within wide limits according to requirements. This is accomplished either by selecting different draw-off points in the column or by varying the quantity of oil withdrawn and has the effect of changing the equilibrium concentration of the liquid on the tray concerned.

The degree of fractionation, or sharpness of separation between hydrocarbons, depends on the number of trays and their efficiency in achieving equilibrium between vapour and liquid. It also depends on the reflux ratio – i.e., the volume of liquid pumped back divided by the volume of overhead product. Reflux ratios in crude oil distillation columns are usually between 1:1 and 3:1.

Unvaporized oil entering the column flows downward over another set of trays in the lower part of the column called stripping trays, which remove any light constituents remaining in the liquid. To assist in this, steam is injected at the bottom of the column. The residue that passes from the base of the fractionating column is suitable for blending into fuel oils. Alternatively, it may be distilled a second time under vacuum conditions and further quantities of distillate recovered for use as a starting material for manufacturing lubricating oil or as feedstock for catalytic cracking.