Oil Refining Technologies

    GLOSSARY

    Introduction to Oil Refineries

    Introduction
    An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful products such as petroleum naphtha, gasoline, diesel fuel, asphalt base, heating oil, kerosene and liquefied petroleum gas. Oil refineries are typically large, sprawling industrial complexes with extensive piping running throughout, carrying streams of fluids between large chemical processing units.

    PHOTOGRAPH OF AN OPERATING REFINERY

    PROCESS FLOW FOR A REFINERY


    The image below is a schematic flow diagram of a typical oil refinery that depicts the various unit processes and the flow of intermediate product streams that occurs between the inlet crude oil feedstock and the final end products. The diagram depicts only one of the literally hundreds of different oil refinery configurations. The diagram also does not include any of the usual refinery facilities providing utilities such as steam, cooling water, and electric power as well as storage tanks for crude oil feedstock and for intermediate products and end products.

    There are many process configurations other than those outlined below. For example, the vacuum distillation unit may also produce fractions that can be refined into end-products such as: spindle oil used in the textile industry, light machinery oil, motor oil, and various waxes.
    Refinery Flow

    An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful products such as petroleum naphtha, gasoline, diesel fuel, asphalt base, heating oil, kerosene and liquefied petroleum gas. Oil refineries are typically large, sprawling industrial complexes with extensive piping running throughout, carrying streams of fluids between large chemical processing units. In many ways, oil refineries use much of the technology of, and can be thought of, as types of chemical plants. The crude oil feedstock has typically been processed by an oil production plant. There is usually an oil depot (tank farm) at or near an oil refinery for the storage of incoming crude oil feedstock as well as bulk liquid products. An oil refinery is considered an essential part of the downstream side of the petroleum industry.

    OPERATION

    Raw or unprocessed crude oil is not generally useful in industrial applications, although “light, sweet” (low viscosity, low sulfur) crude oil has been used directly as a burner fuel to produce steam for the propulsion of seagoing vessels. The lighter elements, however, form explosive vapors in the fuel tanks and are therefore hazardous, especially in warships. Instead, the hundreds of different hydrocarbon molecules in crude oil are separated in a refinery into components which can be used as fuels, lubricants, and as feedstocks in petrochemical processes that manufacture such products as plastics, detergents, solvents, elastomers and fibers such as nylon and polyesters.

    Petroleum fossil fuels are burned in internal combustion engines to provide power for ships, automobiles, aircraft engines, lawn mowers, chainsaws, and other machines. Different boiling points allow the hydrocarbons to be separated by distillation. Since the lighter liquid products are in great demand for use in internal combustion engines, a modern refinery will convert heavy hydrocarbons and lighter gaseous elements into these higher value products.

    Oil can be used in a variety of ways because it contains hydrocarbons of varying molecular masses, forms and lengths such as paraffins, aromatics, naphthenes (or cycloalkanes), alkenes, dienes, and alkynes. While the molecules in crude oil include different atoms such as sulfur and nitrogen, the hydrocarbons are the most common form of molecules, which are molecules of varying lengths and complexity made of hydrogen and carbon atoms, and a small number of oxygen atoms. The differences in the structure of these molecules account for their varying physical and chemical properties, and it is this variety that makes crude oil useful in a broad range of applications.

    Once separated and purified of any contaminants and impurities, the fuel or lubricant can be sold without further processing. Smaller molecules such as isobutane and propylene or butylenes can be recombined to meet specific octane requirements by processes such as alkylation, or less commonly, dimerization. Octane grade of gasoline can also be improved by catalytic reforming, which involves removing hydrogen from hydrocarbons producing compounds with higher octane ratings such as aromatics. Intermediate products such as gasoils can even be reprocessed to break a heavy, long-chained oil into a lighter short-chained one, by various forms of cracking such as fluid catalytic cracking, thermal cracking, and hydrocracking. The final step in gasoline production is the blending of fuels with different octane ratings, vapor pressures, and other properties to meet product specifications.

    Oil refineries are large scale plants, processing about a hundred thousand to several hundred thousand barrels of crude oil a day. Because of the high capacity, many of the units operate continuously, as opposed to processing in batches, at steady state or nearly steady state for months to years. The high capacity also makes process optimization and advanced process control very desirable.

    MAJOR PRODUCTS
    Petroleum products are usually grouped into three categories: light distillates (LPG, gasoline, naphtha), middle distillates (kerosene, diesel), heavy distillates and residuum (heavy fuel oil, lubricating oils, wax, asphalt). This classification is based on the way crude oil is distilled and separated into fractions (called distillates)

    • Liquified petroleum gas (LPG)
    • Gasoline (also known as petrol)
    • Naphtha
    • Kerosene and related jet aircraft fuels
    • Diesel fuel
    • Fuel oils
    • Lubricating oils
    • Paraffin wax
    • Asphalt and tar
    • Petroleum coke
    • Sulfur

    Oil refineries also produce various intermediate products such as hydrogen, light hydrocarbons, reformate and pyrolysis gasoline. These are not usually transported but instead are blended or processed further on-site. Chemical plants are thus often adjacent to oil refineries. For example, light hydrocarbons are steam-cracked in an ethylene plant, and the produced ethylene is polymerized to produce polyethene.

    COMMON REFINERY PROCESSES

    • DESALTER UNIT: Desalter unit washes out salt from the crude oil before it enters the atmospheric distillation unit.

    • ATMOSPHERIC DISTILLATION UNIT: Crude distillation unit (CDU) is at the front-end of the refinery, also known as topping unit, or atmospheric distillation unit. It receives high flow rates hence its size and operating cost are the largest in the refinery. Many crude distillation units are designed to handle a variety of crude oil types.

    • VACUUM DISTILLATION UNIT: Vacuum distillation unit further distills residual bottoms after atmospheric distillation.


    • NAPHTA HYDROTREATER UNIT: Naphta hydrotreater unit uses hydrogen to desulfurize naphtha from atmospheric distillation. Must hydrotreate the naphtha before sending to a catalytic reformer unit.

    • CATALYTIC REFORMER UNIT:Catalytic reformer unit is used to convert the naphtha-boiling range molecules into higher octane reformate (reformer product). The reformate has higher content of aromatics and cyclic hydrocarbons). An important byproduct of a reformer is hydrogen released during the catalyst reaction. The hydrogen is used either in the hydrotreaters or the hydrocracker.


    • DISTILLATE HYDROTREATED: Distillate hydrotreater desulfurizes distillates (such as diesel) after atmospheric distillation.

    • FLUID CATALYTIC CRACKER: Fluid Catalytic Cracker (FCC) unit upgrades heavier fractions into lighter, more valuable products.

    • HYDROCRACKER UNIT: Hydrocracker unit uses hydrogen to upgrade heavier fractions into lighter, more valuable products.

    • VISBREAKING UNIT: Visbreaking unit upgrades heavy residual oils by thermally cracking them into lighter, more valuable reduced viscosity products.

    • MEROX UNIT: Merox unit treats LPG, kerosene or jet fuel by oxidizing mercaptans to organic disulfides.Alternative processes for removing mercaptans are known, e.g. doctor sweetening process and caustic washing.

    • COKING UNIT: Coking units (delayed coking, fluid coker, and flexicoker) process very heavy residual oils into gasoline and diesel fuel, leaving petroleum coke as a residual product.

    • ALKYLATION UNIT: Alkylation unit uses Sulfuric or Hydrofluoric acid to produce high-octane components for gasoline blending.

    • ISOMERIZATION UNIT: Isomerization unit converts linear molecules to higher-octane branched molecules for blending into gasoline or feed to alkylation units.

    • STEAM REFORMING UNIT: Steam reforming unit produces hydrogen for the hydrotreaters or hydrocracker.

    • AMINE GAS TREATER: Amine gas treater, Claus unit, and tail gas treatment convert hydrogen sulfide from hydrodesulfurization into elemental sulfur.

    • SOLVENT DEWAXING UNITS: Solvent dewaxing units remove the heavy waxy constituents petrolatum from vacuum distillation products.

    • LIQUIFIED GAS STORAGE: Liquified gas storage vessels store propane and similar gaseous fuels at pressure sufficient to maintain them in liquid form. These are usually spherical vessels or “bullets” (i.e., horizontal vessels with rounded ends).
    • STORAGE TANKS: Storage tanks store crude oil and finished products, usually cylindrical, with some sort of vapor emission control and surrounded by an earthen berm to contain spills.
    • UTILITY UNITS: Utility units such as cooling towers circulate cooling water, boiler plants generates steam, and instrument air systems include pneumatically operated control valves and an electrical substation.
    • WASTEWATER COLLECTION AND TREATING SYSTEM: Wastewater collection and treating systems consist of API separators, dissolved air flotation (DAF) units and further treatment units such as an activated sludge biotreater to make water suitable for reuse or for disposal.