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Chapter 4 b4

petroleum refining

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Chapter 4 b4

  2. 2. Learning outcome 2  Classify and differentiate the primary processing and secondary processing in petroleum refinery processing.  Outline and sketch the operating processes in petroleum refinery. - downstream processing - cracking - thermal cracking
  3. 3. Introduction 3  Primary refining process - the distillation of the feedstock into its basic fractions, and then the re-distillation of these in separate towers into highly concentrated intermediates. - In a simple refinery, the split would probably consist of six basic cuts: petroleum gas (refinery gas and LPG), gasoline, naphtha, kerosene, gas oil and residue.
  4. 4. Introduction (cont.) 4 Source: http://www.galpenergia.com/EN/agalpenergia/Os-nossos-negocios/Refinacao-Distribuicao/ARL/Refinacao/Paginas/Didatico-Refinacao.aspx
  5. 5. Introduction (cont) 5  Primary processing unit The purpose of primary unit is to separate the crude into different fractions by distillation Know as mother unit of the refinery, consist of •CRUDE DISTILLATION UNIT (CDU) •VACUUM DISTILLATION UNIT (VDU) Commonly referred as Atmospheric and Vacuum Distillation unit (AVU)
  6. 6. Introduction (cont.) 6  Secondary Processes (downstream) Scenario - When automobile industry was in its infancy, it become obvious that then production of large quantities of straight run gasoline from crude distillation would involve production of much larger quantities of residual fuels than the market could absorb. - The demand for gasoline thus spurred on scientists to look for means of obtaining a higher yield of this product from crude oil.
  7. 7. 7  Modern refining - Essentially involves 2 categories of processing: 1) The physical separation of the raw material into a range of homogeneous petroleum fractions, include distillation and blending. 2) The subsequent chemical conversion of certain fractions to alter the product yield and improve product quality, include cracking, coking, reforming, alkylation, polymerization, isomerization and hydrogen treatment. Introduction (cont.)
  8. 8. Physical and Chemical Processes 8 PHYSICAL Distillation Solvent extraction Propane deasphalting Solvent dewaxing Blending THERMAL Visbreaking Delayed coking Flexicoking CATALYTIC Hydrotreating Catalytic reforming Catalytic cracking Hydrocracking Catalytic dewaxing Alkylation Polymerization Isomerization CHEMICAL
  9. 9. Secondary Processing 9  Secondary processes (downstream) After crude oil is separated into its fractions, each stream is further converted by changing the size and structure of molecules through cracking, reforming and other conversion processes.
  10. 10. Significance of Secondary Processing 10 • The significance of secondary processing (1) to remove any impurities and undesirable constituents from the distilled fractions. (2) to convert some of the distilled hydrocarbons into different molecular forms. (3) to improve product quality. (4) to improve the refinery profit margins by converting low value heavy ends to high value products like LPG/ gasoline/ kerosene/ diesel.
  11. 11. 11  (1) to remove any impurities from the distilled fractions - All crudes contain organic sulphur compounds (eg. H2S- hydrogen sulfide, mercaptan) which will be carried out over from the column into the resulting gases, distillates and residues. - The higher the density of a crude, the greater its sulphur content. - Sour cuts are corrosive and possess an extremely objectionable odor. - The secondary process for the treatment of toxic, corrosive and evil-smelling sulphur-compound impurities is known as “sweetening”. - Caustic washing and hydrodesulphurization are finishing processes designed to remove H2S and mercaptan impurities. Significance of Secondary Processing
  12. 12. 12  (2) to convert some of the distilled hydrocarbons into different molecular forms. - in conversion processes, the structures of natural hydrocarbon molecules are changed. - cracking process, large hydrocarbon molecules are cracked or broken to form two or more smaller molecules. - cracking can be done by the action of heat and pressure alone (thermal cracking) or by heat in the presence of suitable catalyst (catalytic cracking). - the main purpose of cracking is to increase the yield of lighter, more valuable fractions from medium and residual cuts. Significance of Secondary Processing
  13. 13. 13 Classification of Secondary Processing Units
  14. 14. Factors for Selecting Secondary Processes for a Refinery 14  Type of Crude  Product Slate  Product specifications  Investment cost  Operating cost  Sophistication of technology
  15. 15. Chemical Processing 15  We can change one fraction into another by one of three methods: CRACKING • Breaking large hydrocarbons into smaller pieces UNIFICATION • Combining smaller pieces to make larger ones ALTERATION • Rearranging various pieces to make desired hydrocarbons
  16. 16. Cracking 16  Cracking takes large hydrocarbons and breaks them into smaller ones.  After various hydrocarbons are cracked into smaller hydrocarbons, the products go through another fractional distillation column to separate them. CRACKING THERMAL CRACKING CATALYTIC CRACKING
  17. 17. Chemical Processing - Cracking 17  There are several types of cracking: (1) THERMAL – heat large hydrocarbons at high temperature (sometimes high pressure as well) until they break apart. Steam – high temperature steam is used to break ethane, butane and naptha into ethylene and benzene, which are used to manufacture chemicals. Visbreaking – residual from distillation tower is heated, cooled with gas oil and rapidly flashed in a distillation tower. This process reduces the viscosity of heavy weight oils and produces tar. Coking – residual from the distillation tower is heated to temperatures above 482 degree C until it cracks into heavy oil, gasoline and naphtha. When the process is completed, a heavy and almost pure carbon residue is left (coke).
  18. 18. 18 Chemical Processing – Cracking (cont.) (2) CATALYTIC – use catalyst to speed up the cracking reaction. Catalysts include zeolite, aluminium hydrosilicate, bauxite and silica alumina. Fluid catalytic cracking – a hot, fluid catalyst at 538 degree C cracks heavy oil into diesel oils and gasoline. Hydrocracking – similar to fluid catalytic cracking, but uses a different catalyst, lower temperature, high pressure and hydrogen gas. It takes heavy oil and cracks it into gasoline and kerosene (jet fuel).
  19. 19. 19  Fluidized Catalytic Cracking Unit Objective: to convert heavy vacuum gas oil to valuable distillates like LPG, Gasoline, Diesel by catalytic cracking in fluidized bed.  Hydro Cracker Unit Objective: to convert heavy vacuum gas oil to valuable distillates like LPG, Naptha, Automatic transmission fluid (ATF), Kerosene and Diesel  Visbreaker Unit Objective: to reduce viscosity of heavy ends i.e. RCO/ Vacuum residue by thermal cracking.  Coking Unit Objective: to produce valuable distillate from heavy ends by thermal cracking Cracking Process Units
  20. 20. 20  Combine smaller hydrocarbons to make larger ones  Major unification process is called catalytic reforming, and catalyst as platinum, platinum- rhenium mix to combine low weight naptha into aromatics.  A significant by-product of this reaction is hydrogen gas, which is then either used for hydrocracking or sold. Chemical Processing - Unification
  21. 21. 21  The structures of molecules in one fraction are rearranged to produce another.  Alkylation – The process combines an unsaturated light hydrocarbon (such as olefins, propylene or butene) with isobutane to produce alkylate in the presence of a catalyst such as hydrofluoric acid or sulfuric acid ( a by product from removing impurities from many oil products).  Product of alkylation are high octane hydrocarbons. Chemical Processing - Alteration
  22. 22. THERMAL PROCESSES When a hydrocarbon is heated to a sufficiently high temperature thermal cracking occurs. This is sometimes referred to as pyrolysis (especially when coal is the feedstock). When steam is used it is called steam cracking. We will examine two thermal processes used in refineries. • Visbreaking • Delayed coking
  23. 23. Visbreaking • Visbreaking is a mild form of thermal cracking that lowers the viscosity of heavy crude-oil residues without affecting the boiling point range. • Residuum from the atmospheric distillation tower is heated (425- 510ºC) at atmospheric pressure and mildly cracked in a heater. • It is then quenched with cool gas oil to control over-cracking, and flashed in a distillation tower. • Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity of residues used for blending with lighter fuel oils. Middle distillates may also be produced, depending on product demand. • The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum-flashed in a stripper and the distillate recycled.
  24. 24. Alternatively, vacuum residue can be cracked. The severity of the visbreaking depends upon temperature and reaction time (1‐8 min). • Usually < 10 wt% of gasoline and lighter products are produced. The objective is to reduce the viscosity as much as possible without significantly affecting the fuel stability. Soaker Visbreaking Process The furnace operates at a lower outlet temperature and a soaker drum is provided at the outlet of the furnace to give adequate residence time to obtain the desired conversion while producing a stable residue product, thereby increasing the heater run and reducing the frequency of unit shut down for heater decoking . The products from soaker drum are quenched and distilled in the downstream fractionator.
  25. 25. 25
  26. 26. Delayed Coking • Coking is a severe method of thermal cracking used to upgrade heavy residuals into lighter products or distillates. • Coking produces straight‐run gasoline (Coker naphtha) and various middle‐distillate fractions used as catalytic cracking feedstock. • The process completely reduces hydrogen so that the residue is a form of carbon called "coke." • Three typical types of coke are obtained (sponge coke, honeycomb coke, and needle coke) depending upon the reaction mechanism, time, temperature, and the crude feedstock. • In delayed coking the heated charge (typically residuum from atmospheric distillation towers) is transferred to large coke drums which provide the long residence time needed to allow the cracking reactions to proceed to completion.