Chapter 5a -_cracking

petroleum refining

1. http://padlet.com/wall/EP422Petro EP422 SHOUT BOARD

2. CHAPTER 5 C R A C K I N G

3. CHAPTER 5 OUTLINE Cracking Fluid catalytic cracking Principles Recent developments Feedstock Product yields and qualities Catalyst and operating parameters Hydrocracking Principles Process requirements Product yields and qualities Residue cracking

4. OIL REFINING M e thod b y w hic h crude oi l c o n v er t ed 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. OIL REFINING Typical Oil – Gasoline C 4 to C 10 27 % – Kerosene C 11 to C 13 13 % – Diesel C 14 to C 18 12 % – Heavy gas oil C 19 to C 25 10 % – Lubricating oil C 26 -C 40 20 % – Residue >C 40 18 %

6. OIL REFINING Thermal Cracking Catalytic Cracking

7. OIL REFINING What we get out of oil now with modern refineries: 50% gas 30% fuel oil 7.5% jet fuel HOW??

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 N a ph t h a Diesel Petrol K e r o s i n e Fuel Oil and bitumen

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. High Pressure ... 7000 kPa High Temperature ... 400°C to 900°C Free Radical Mechanism Homolytic fission Pr o duc e s m o st l y al k e n e s .. . e .g. et h ene for making polymers and ethanol Produc e s H y d r og e n ... use d in the Hab e r 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. Slight pressure High Temperature Use c a t al y s t to Catalysts include s pee d u p t h e c r a c king r eactio n . z eol i t e, a l uminiu m h y d r o si l i c a t e , bauxite and silica alumina. Carbocation Mechanism Heterolytic fission P r o du c es b r an c he d a n d c y cl i c al k ane s , na r o m a tic hydrocarbons used for motor fuels **ZEOLITES are crystalline aluminosilicates; clay like substances CATALYTIC CRACKING

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. The r e a r e th r ee b a s i c func t ions i n t h e ca t alyt i c cracking process: R eac t i o n: F eed s t oc k r eacts with ca t al y s t and cracks into different hydrocarbons; R e g ene r a t io n : C a t al y s t i s r eacti v a t ed by burning off coke; and Fractionation: Cracked hydrocarbon stream is separated into various products. BASIC FUNCTIONS IN CATALYTIC CRACKING

14. mo le cules Large hydrocarb o ns are brok e n i n to sm a ll e r using heat and a catalyst. This process is known as catalytic cracking. • Th e small mol e cu l es prod u c ed are then se p arat e d by distillation. CATALYTIC CRACKING PROCESS Catalytic cracker Heat to vaporise Distillation tower pressure Big Molecules Smaller molecules Molecules break up

15. In the catalytic cracker long chain molecules are ‘cracked’. An example of such a reaction is: C 8 H 18  C 6 H 14 + C 2 H 4 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 p r es s u r e c at al y s t Used as a fuel CATALYTIC CRACKING REACTION

16. CATALYTIC CRACKING REACTION Products formed are the result of both primary and secondary reactions. Pr i ma r y rxn s – i n v ol v e t h e in i tial 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. 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. 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. Th e c a t al y s t is p r og r essi v ely d e acti v a t ed b y t h e formation of coke on the surface of the catalyst. C a t al y s t and h y d r o c ar bo n v apo r s a r e s e pa r a t ed mechanically, oil remaining on the catalyst is removed b y s t e am s t r ipping b e f o r e c a t al y s t e n t e r s t h e r e g ene r a t o r . Th e oi l v a po r s a r e t a k e n o v e r h e ad t o a f r actio n a t ion t o w er f o r se p a r a t i o n i n t o s t r e a ms h a ving the d e si r ed boiling ranges.

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. FLUIDIZED –BED CATALYTIC CRACKING

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. 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. FCC Process Flow Diagram

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: Side-by –side type , where the reactor and regenerator are separate vessels adjacent to each other Orthoflow/stacked type , reactor is mounted on top of the regenerator.

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. Zeolite as Catalyst in FCC The development ch r onolog y of f o r catalyst and additives the lig h t to enhance production of olefins in FCCs.

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, r equi r ed f eed p r e - t r e a tme n t b y h y d r ot r e a ti n g is i n o r de r t o r em o v e c o n t amin a n t s (especiall y s ulph u r) and i m p r o v e c r a c ki n g characteristics and yields .

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. FCC products