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Equipment and processes in oil-refining industry 25.01.2020 14:11
Now oil is a basic raw material used for production of fuels of different types and feed for oil chemical and chemical industry. As oil is a liquid including many different components in various proportions depending on oil type and place of oil production, recovery of different components from oil, or in other words separation of oil to different components is a complex issue.
Oil refining purpose is production of high-quality fuels such as gasoline and diesel. Gasoline quality is determined firstly by octane number that is a conditional number defining gasoline detonation characteristic. Detonation characteristic of isooctane is accepted as 100 units and detonation characteristic of n-heptane is accepted as zero. Octane number scale was determined on the basis of mixtures of isooctane and n-heptane in different ratios and therefore octane number reflects detonation characteristic. Octane numbers that are more than 100 units are defined as detonation characteristic of isooctane including tetraethyl lead (it is a substance improving detonation characteristic). Detonation characteristic is a significant property for gasoline because high-quality gasoline shall not detonate while compressing as the detonation results in early engine wear and high fuel consumption.
In addition, now environmental requirements to gasoline quality are made. There are new standards Euro IV and Euro V that limit content of harmful substances in gasolines and diesel. Many countries set deadlines and all refineries shall meet new gasoline quality standards till the deadlines. Firstly the standards limit content of sulphur compounds in gasoline. Compounds including sulphur are transformed to sulphur oxide while burning. The sulphur oxide is a toxic and harmful gas, and it forms acids in case of mixing with water, therefore in case of emission it results in acid rains.
As everybody knows, the most applied method of liquid separation to components is fractionation or distillation. Therefore, oil fed to refinery after previous treatment from mechanical impurities and purification at oil demineralization unit is pumped to atmospheric distillation unit (ADU). The unit has such name because distillation is performed at atmospheric pressure. During atmospheric distillation oil is separated to light and heavy gasoline, kerosene, diesel and atmospheric distillation residue - fuel oil. Straight-run gasoline produced by atmospheric distillation unit has octane number from 40 to 60 and it is not a finished product, it is only a raw material for subsequent treatment.
Atmospheric distillation residue is pumped to vacuum distillation unit (VDU) where distillation is performed at vacuum. Application of vacuum makes it possible to reduce boiling temperature of components to temperature that is less than its thermal decomposition temperature. That makes it possible to separate components with high boiling points. Main products of vacuum distillation are light vacuum gas oil, heavy vacuum gas oil and vacuum distillation residue (tar) that can be used for bitumen manufacturing or as a raw material for further treatment. Particularly it can be sent to delayed coker unit (DCU) to produce more valuable products.
Vacuum gas oil is sent to catalytic cracking unit. There are several types of the catalytic cracking process, but widely applied type of the process is fluid catalytic cracking (FCC). Fluid catalytic cracking unit products are catalytic cracking gas containing propane and propylene fraction as well as butane and butylene fraction, fluid catalytic cracking gasoline with research octane number (RON) approximately from 80 to 95, FCC light gas oil used as a diesel blending component and heavy gas oil used as feed for delayed coker unit or as fuel oil.
It is an important fact that compounds including sulphur and contained in vacuum distillation gas oil are catalyst poisons for fluid catalytic cracking catalyst. Therefore, it is not advisable to send vacuum gas oil directly from vacuum distillation unit to fluid catalytic cracking unit. Before sending to fluid catalytic cracking unit vacuum gas oil is sent to hydrotreating. At the hydrotreating unit feed pumped to the unit interacts with hydrogen in high temperature and high pressure. Hydrogen connects sulphur, therefore oil components are purified from compounds containing sulphur. Hydrotreating is performed not only for vacuum gas oil but also for gasoline, kerosene, diesel because it makes it possible to significantly reduce content of sulphur compounds in fuels achieving conformance of finished fuels with strict environmental requirements (Euro IV and Euro V). Hydrogen sulfide formed during hydrotreating process is used for elemental sulphur production in Claus process. The elemental sulphur is another product of refinery.
Hydrotreated straight-run gasoline is sent to catalytic reforming unit. At the catalytic reforming unit gasoline and naphtha are processed to high octane gasolines. In addition, with the use of catalytic reforming content of aromatic hydrocarbons in the reformate (it is a product of the unit) is increased. Another advantage of application of reforming unit is formation of gas containing hydrogen that is used at hydrotreating unit, hydrocracking unit and isomerization unit of a refinery.
For processing of heavy oil components visbreaking is used. It is a type of thermal cracking that makes it possible to produce fuel oil, gasoline and gas from vacuum distillation residue. Gasoline from visbreaking unit is sent to hydrogenation and then to catalytic reformer unit. At the catalytic reformer unit it is processed to gasoline blending component. Gas from visbreaking unit is sent to gas fractionation unit.
In relation to increasing high-octane blending components production rate, isomerization unit is very important at a refinery. The unit makes it possible to produce high-octane components from low-octane ones.
Finished components of gasoline / diesel blending are mixed at gasoline / diesel blending stations. Blending is performed in defined ratio. Furthermore, different auxiliary substances called additives are added to finished gasoline. The additives improve specific physical and chemical properties of gasoline.
As production rates at refineries are very high, any refinery has a large tank yard. In tanks there are both finished products (finished gasoline with different octane number, finished diesel, jet fuel TS-1, etc.) and intermediate products that are products of one of the units and that are pumped as feed to other units (for example, vacuum gas oil, straight-run gasoline, reformate, isomerate, catalytic cracking gasoline, etc.). In addition, tanks have different structure: there are float-roof tanks, fixed-roof tanks, tanks with nitrogen blanket, etc.
In addition, any refinery is equipped with different auxiliary units and systems required for main production assistance. The units are utilities.
In regards to mechanical equipment of refineries, firstly we shall speak about column equipment, heaters and heat exchangers. Column equipment is different distillation columns, evaporators, gas generators, absorbers, desorbers, stabilizers and extraction columns, towers. The most applied heaters are fire heaters where medium is heated with the use of direct combustion of fuel gas in the heater. In addition, there are tube still heaters, or furnaces, that have radiant and convection sections. Burners are installed in radiant section and fuel gas is burned off there. Heat is transferred to process medium being heated in convection sections. There are following tube still heaters: vertical cylindrical heaters, box heaters, slab ovens, etc. There are following heat exchangers: heaters, coolers and condensers as well as tube heat exchangers, double-pipe heat exchangers, plate heat exchangers, etc. In addition, refineries use waste heat recovery boilers. They make it possible to produce steam because of recovery of discharged medium heat. In order to filtrate process medium filter presses, vacuum filters and filters of other types are used. Mixing and separation equipment is represented by mechanical agitators, mixers, settling tanks, filters, centrifuges, etc.
Specific place at refineries is used for units where finished products (gasoline, diesel and jet fuel) are loaded to vehicle tanks and railway tank cars. It is an individual system including many manifolds located on racks and loading arms to be submerged to tanks. Finished products are pumped from storage tanks directly to vehicle tanks and tank cars with the use of pumps.
We should emphasize that oil and oil products processed at refineries are inflammables and therefore it is required to meet special safety requirements. Particularly during refinery design engineering special attention shall be paid to explosion safety calculation, calculation of relative energy potential, air-blasts and air-blast radiuses. In addition, it is required to determine places where there is an explosion risk, calculate the explosion risk and fire risk. Specific requirements are set in relation to equipment arrangement. Particularly equipment applied at explosive units of refineries shall not form sparks (mechanical equipment) or shall be explosion-proof (electrical equipment). Outdoor units are widely applied because in case of explosion at the units buildings where process units are installed will not be destroyed. Specific requirements are set in relation to buildings and structures, their fire resistance levels, supporting and protective structures, relief panels (that are thrown off in case of explosion and therefore it makes it possible to direct air-blast to required location in case of explosion), fire resistance ratios for walls, location of operating personnel. Emergency shutdown system ensures transition of the process to safety mode in case of emergency.