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Chapter 5a -_cracking

petroleum refining

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Chapter 5a -_cracking

  1. 1. http://padlet.com/wall/EP422Petro EP422 SHOUT BOARD
  2. 2. CHAPTER 5 C R A C K I N G
  3. 3. CHAPTER 5 OUTLINE • Cracking • Fluid catalytic cracking I. Principles II. Recent developments III. Feedstock IV. Product yields and qualities V. Catalyst and operating parameters • Hydrocracking I. Principles II. Process requirements III. Product yields and qualities IV. Residue cracking
  4. 4. OIL REFINING • Method by which crude oil converted to petroleum products – (I think that a barrel (42 gal—produces 44 gal of petroleum products) • Distillation (fractionation) – At high temperature the lightest fractions rise to the top of a tower, heavier fractions condense at bottom
  5. 5. OIL REFINING Typical Oil – Gasoline C4 to C10 27% – Kerosene C11 to C13 13% – Diesel C14 to C18 12% – Heavy gas oil C19 to C25 10% – Lubricating oil C26-C40 20% – Residue >C40 18%
  6. 6. OIL REFINING • Thermal Cracking • Catalytic Cracking
  7. 7. OIL REFINING • What we get out of oil now with modern refineries: – 50% gas – 30% fuel oil – 7.5% jet fuel HOW??
  8. 8. CRACKING • Crude oil contains many large molecules. If these are to be used as fuels or feedstock for the chemical industry then they have to be cracked into smaller molecules. • When hydrocarbons burn they are reacting with oxygen in the air. In general, the smaller the molecule the better it will mix and then react with the air. Fuel gas Naphtha Diesel Petrol Kerosine Fuel Oil and bitumen
  9. 9. CRACKING Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products • THERMAL proceeds via a free radical mechanism • CATALYTIC proceeds via a carbocation (carbonium ion) mechanism
  10. 10. • High Pressure ... 7000 kPa • High Temperature ... 400°C to 900°C • Free Radical Mechanism • Homolytic fission • Produces mostly alkenes ...e.g. ethene for making polymers and ethanol • Produces Hydrogen ... used in the Haber Process and in margarine manufacture • Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission THERMAL CRACKING
  11. 11. • Slight pressure • High Temperature • Use catalyst to Catalysts include speed up the cracking reaction. zeolite, aluminium hydrosilicate, bauxite and silica alumina. • Carbocation Mechanism • Heterolytic fission • Produces branched and cyclic alkanes, naromatic hydrocarbons used for motor fuels **ZEOLITES are crystalline aluminosilicates; clay like substances CATALYTIC CRACKING
  12. 12. •Catalytic cracking is similar to thermal cracking except that catalysts facilitate the conversion of the heavier molecules into lighter products. •Use of a catalyst (a material that assists a chemical reaction but does not take part in it) in the cracking reaction increases the yield of improved-quality products under much less severe operating conditions than in thermal cracking. •Typical temperatures are from 450°-510° C at much lower pressures of 10-20 psi. •The catalysts used in refinery cracking units are typically solid materials (zeolite, aluminum hydrosilicate, treated bentonite clay, fuller's earth, bauxite, and silica-alumina) that come in the form of powders, beads, pellets or shaped materials called extrudates. CATALYTIC CRACKING
  13. 13. There are three basic functions in the catalytic cracking process: I. Reaction: Feedstock reacts with catalyst and cracks into different hydrocarbons; II. Regeneration: Catalyst is reactivated by burning off coke; and III. Fractionation: Cracked hydrocarbon stream is separated into various products. BASIC FUNCTIONS IN CATALYTIC CRACKING
  14. 14. molecules• Large hydrocarbons are broken into smaller using heat and a catalyst. • This process is known as catalytic cracking. • The small molecules produced are then separated by distillation. CATALYTIC CRACKING PROCESS Catalytic crackerHeat to vaporise Distillation tower pressure Big Molecules Molecules break up
  15. 15. In the catalytic cracker long chain molecules are ‘cracked’. An example of such a reaction is: C8H1 8  C6H14 +C2H4 C C H H H H + ethene H H H H H H H H H C C C C C C C C H H H H H H H H H Octane H H H H H H H C C C C C C H H H H H H H hexane Ethene is used to make plastics Heat pressure catalyst Used as a fuel CATALYTIC CRACKING REACTION
  16. 16. CATALYTIC CRACKING REACTION • Products formed are the result of both primary and secondary reactions. • Primary rxns – involve the initial C-C bond scission and the immediate neutralization of the carbonium ion. • Primary rxns as below: Paraffin paraffin + olefin Alkyl napthene napthene + olefin Alkyl aromatic aromatic + olefin
  17. 17. CLASSIFICATION OF CATALYTIC CRACKING • Catalytic cracking processes can be classified as either moving-bed (Thermafor catalytic cracking - TCC) or fluidized-bed units (FCC). • Very few TCC units in operation today, FCC unit has taken over the field – where the major fraction of the cracking reaction occurs.
  18. 18. PROCESS FLOW OF CATALYTIC CRACKING • Process flows for FCC and TCC are similar. • The hot oil feed is contacted with catalyst in either the feed riser or the reactor. • The catalyst is progressively deactivated by the formation of coke on the surface of the catalyst. • Catalyst and hydrocarbon vapors are separated mechanically, oil remaining on the catalyst is removed by steam stripping before catalyst enters the regenerator. • The oil vapors are taken overhead to a fractionation tower for separation into streams having the desired boiling ranges.
  19. 19. CATALYST REGENERATED What happen to the catalyst then? • It flows into the regenerator and is activated by burning off the coke deposits with air. • Regenerator temperatures are carefully controlled to prevent catalyst deactivation by overheating and to provide the desired amount of carbon burn-off. – by manipulating the air flow in the exit flue gas. • Flue gas & catalyst are separated by cyclone separators and electrostatic precipitators. • Important to make sure the catalyst is steam-stripped as it leaves the generator, to remove the adsorbed oxygen before it is contacted with the oil feed.
  20. 20. FLUIDIZED –BED CATALYTIC CRACKING
  21. 21. Introduction - FCC • The fluidized catalytic cracking (FCC) unit is the heart of the refinery and is where heavy low-value petroleum stream such as vacuum gas oil (VGO) is upgraded into higher value products, mainly gasoline and C3/C4 olefins, which can be used in the alkylation unit for production of gasoline (C7– C8 alkylates). • Major developments have occurred in areas of new catalysts and new reactor and regenerator designs.
  22. 22. Role of FCC in the Refinery • The role of the FCC is to take heavy desulphurised feedstock and crack it into lighter, mainly high octane gasoline. • The FCC also produces olefins (C5 = and C4 =) and LPG.
  23. 23. FCC Process Flow Diagram
  24. 24. Introduction - FCC • FCC employs a catalyst in the form of very fine particles (70 microns), that can behave as a fluid when aerated with a vapor. • Two type of FCC units: I. Side-by –side type, where the reactor and regenerator are separate vessels adjacent to each other II. Orthoflow/stacked type, reactor is mounted on top of the regenerator.
  25. 25. Zeolite as Catalyst in FCC • Early attempts to increase production of light olefins from the FCC were based primarily on process variables. • Poor selectivity of this approach resulted in excess production of dry gas and coke. • By 1970s, researchers found that non-Y zeolites could also co-produce light olefins (C2= to C5=), often at the expense of gasoline.
  26. 26. Zeolite as Catalyst in FCC • The development chronology of for catalyst and additives the light to enhance production of olefins in FCCs.
  27. 27. Feedstock • The main feedstock used in a FCC unit is the gas oil , which can be considered mixtures of aromatic, naphthanic and paraffinic molecules. • There are also varying amounts of contaminants such as sulphur, nitrogen and metals. To protect the catalyst, required feed pre-treatment by hydrotreating is in order to remove contaminants (especially sulphur) and improve cracking characteristics and yields.
  28. 28. • Nitrogen tends to poison the catalyst by neutralising its acid sites. However, the FCC process is unaffected if the nitrogen content level is controlled below 0.2%. • The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts. • Some possible feedstocks atmospheric distillates, coking distillates, visbreaking distillates, VGO, atmospheric residue (desulphurised) and vacuum residue (desulphurised, deasphalted). Feedstock
  29. 29. FCC products

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