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Conversion Processes: Cracking

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1 Conversion Processes: Cracking

2 Cracking Cracking is the breakdown of heavy hydrocarbon molecules into lighter ones. Cracking can be done Catalytically and/or Thermally. Catalytic Cracking: catalyzed by the presence of defined catalyst. Examples of catalytic cracking are fluidized-bed catalytic cracking (FCC) and hydrocracking Thermal Cracking: Catalyzed by heat. Examples of thermal cracking processes are visbreaking and solvent deasphalting In this Chapter, the focus will be on Catalytic Cracking. 2

3 3

4 Fluidized Catalytic Cracking Fluidized Catalytic Cracking
Convert heavy hydrocarbon fractions from vacuum distillation into a lighter mixture of more useful products Feedstock undergoes a chemical breakdown, under controlled heat (450 – 500 oC) and pressure, in the presence of a catalyst Fluidized Catalytic Cracking Effective catalyst: small pellets of silica, alumina or magnesia and nowadays zeolite. 4

5 Fluidized Catalytic Cracking: Products
Primary goals: - To make gasoline & diesel - To minimize the production of heavy fuel oil - To produce large amounts of olefins, which are used as » Feedstocks for petrochemical industry » Production of butylene, propylene & ethylene » C5+ olefins can be alkylated to produce high-octane gasoline 5

6 Fluidized Catalytic Cracking
REGENERATOR REACTOR RISER Catalytic cracking is the most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products (e.g. LPG). The cracking process also produces carbon (coke) which remains on the catalyst particle and rapidly lowers its activity. To maintain the catalyst activity at a useful level, it is necessary to regenerate the catalyst by burning off this coke with air. As a result, the catalyst is continuously moved from reactor to regenerator and back to reactor. The cracking reaction is endothermic and the regeneration reaction exothermic. 6

7 Chemistry of Catalytic Cracking
The “Cracking” reaction is a composite of many reactions 1. Initiation --- making the carbenium ion 2. Isomerization --- manipulating the carbenium ion 3. ß-scission --- cutting the carbenium ion into two 4. Hydrogen ion transfer --- the engine that keeps things going 5. Termination --- removing the carbenium ion as an olefin

8 Initiation 1. Production of olefin through mild thermal cracking of paraffins, then these olefins attract a proton from the catalyst to form carbenium ions. 2. 8

9 Isomerization - Stabilizing Carbenium Ions
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10 Cracking: cutting C-C bond
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11 Hydrogen Ion Transfer The carbenium ions propagate the chain reaction by transferring a hydrogen ion from a n-paraffin to form a paraffin molecule and a new carbenium ion. Thus another carbenium ion is formed and the chain is ready to repeat itself. 11

12 Termination 12

13 Fluidized Catalytic Cracking Catalyst
The FCC process employs a catalyst in the form of very fine particles [average particle size about 70 micrometers (microns)] which behave as a fluid when aerated with a vapor (and this is why it is called fluidized catalytic cracking). The fluidized catalyst is circulated continuously between the reaction zone and the regeneration zone and acts as a vehicle to transfer heat from the regenerator to the oil feed and reactor. REGENERATOR REACTOR RISER 13

14 Fluidized Catalytic Cracking
To Fractionator The hot oil feed is contacted with the catalyst in he feed riser line and/or the reactor. As the cracking reaction progresses, the catalyst is progressively deactivated by the formation of coke on the surface of the catalyst. The catalyst and hydrocarbon vapors are separated mechanically, and oil remaining on the catalyst is removed by steam stripping before the catalyst enters the regenerator. The oil vapors are taken overhead to a fractionation tower for separation into streams having the desired boiling ranges. REGENERATOR REACTOR RISER Hot oil air 14

15 Fluidized Catalytic Cracking
To Fractionator REGENERATOR REACTOR RISER Flue gases The spent catalyst flows into the regenerator and is reactivated by burning off the coke deposits with air. Regenerator temperatures are carefully controlled to prevent catalyst deactivation by overheating. Hot oil air This is done by controlling the air flow to give a desired CO2/CO ratio in the exit flue gases or the desired temperature in the regenerator. The flue gas and catalyst are separated by cyclone separators and electrostatic precipitators. 15

16 Fluidized Catalytic cracking
Two basic types of FCC units in use today are the ‘‘side-by-side’’ type, where the reactor and regenerator are separate vessels adjacent to each other, and the stacked type, where the reactor is mounted on top of the regenerator. side-by-side FCC units stacked type FCC units 16

17 FCC Process FCC feed is preheated to 650oF and is fed into the riser
The ratio of catalyst: oil = 4:1 to 9:1 by weight. Residence time of the gas < 5 seconds The vapor generated by the cracking process lifts the catalyst up the riser. The vapor velocity at the base of the riser is about 6 m/s and increases to over 20 m/s at the riser exit. REACTOR REGENERATOR FCC feed is preheated to 650oF and is fed into the riser Which contains hot catalyst from the regenerator RISER 17

18 A spent catalyst is withdrawn from the bottom of reactors and stripped with steam to vaporize the hydrocarbons remaining on the surface Stripping removes most of the hydrocarbon vapors which are entrained between the particles of catalyst FCC Process REACTOR It is desirable to separate the vapor and catalyst as quickly as possible to prevent overcracking of the desired products. REGENERATOR RISER 18

19 Fluidized Catalytic Cracking (FCC) Flowsheet
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20 Hydrocracking

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22 What is Hydrocracking? Hydrocracking is the conversion process of higher boiling point petroleum fractions to light hydrocarbons (mainly gasoline and jet fuels) in the presence of a catalyst

23 Why Hydrocracking? The increasing demand for gasoline and jet fuel compared to diesel fuel and home heating oils was a dominant factor in the development of hydrocracking process Hydrogen as a byproduct of catalytic reforming process was available in large amounts and relatively cheap

24 Benefits of Hydrocracking
Hydrocracking is one of the most versatile process, which facilitate product balance with the market demand Other advantages are: Very high gasoline yield High octane numbers Production of large amount of isobutane Supplementing FCC (Fluid Catalytic Cracking) to upgrade heavy stocks, aromatics and coker oils

25 Hydrocracking vs FCC Fluid Catalytic Cracking (FCC) takes more easily cracked paraffinic atmospheric and vacuum gas oil Hydrocracking is capable of using aromatics and cycle oils and coker distillates as feed (these compounds resist FCC) Cycle oils and aromatics formed in catalytic cracking (FCC) are satisfactory feedstock for hydrocracking Middle distillate and even light crude oil can also be used as feedstock for hydrocracking

26 Hydrocracking processes
The fresh feed is mixed with hydrogen gas and recycle gas (high in hydrogen content) and passed through a heater to the first reactor If the feed is high in sulfur and nitrogen a guard reactor is employed to convert sulfur to hydrogen sulfide and nitrogen to ammonia to protect precious catalyst in the following reactor. Jet fuel Two-Stage Hydrocracking gasoline

27 Hydrocracking processes
Hydrocracking reactors are operated at high temperatures to produce materials with boiling point below 400 F The reactor gaseous effluent goes through heat exchangers and a high pressure separator where the hydrogen rich gases are separated and recycled to the first stage. Jet fuel Two-Stage Hydrocracking gasoline

28 Hydrocracking processes
The liquid product from the reactor is sent to a distillation column where C1-C4 and lighter gases are taken off and the gasoline, jet fuel, naphta and/or diesel fuel streams are removed as liquid side streams. The distillation bottom product is sent to the second hydrocracker Jet fuel Two-Stage Hydrocracking gasoline

29 Hydrocracking Reactions
There are hundreds of simultaneous chemical reactions occurring in hydrocracking It is assumed that the mechanism of hydrocracking is that of catalytic cracking with hydrogen superimposed In catalytic cracking the C – C bond is broken, while in hydrogenation, H2 is added to a carbon- carbon double bond Cracking is an endothermic reaction Hydrogenation is an exothermic reaction

30 Hydrocracking Reactions
Cracking and hydrogentation are complementary as shown below

31 Hydrocracking reactions
Aromatics which are difficult to process in FCC are converted to useful products in Hydrocrackers.

32 Hydrocracking Reactions
Cracking provides olefins for hydrogenation and Hydrogenation provides heat for cracking The overall reaction provides an excess of heat as hydrogenation produces much larger heat than the heat required for cracking operation Therefore the overall process is exothermic and quenching is achieved by injecting cold hydrogen into the reactor and apply other means of heat transfer, e.g. intermediate heat exchanger Isomerization is another type of reaction, which occurs in hydrocracking

33 Hydrocracking Reactions
As a result of isomerization, the olefinic products formed are rapidly hydrogenated which provides high octane isoparaffins The volumetric yield can be as high as 125% as the hydrogenated products have a higher API gravity Hydrocracking reactions are normally carried out at an average catalyst temperature between 550 and 750 F

34 Hydrocracking Reactions
The reactor pressure ranges from 8275 – kPa (1200 to 2000 psig) Large quantity of hydrogen is circulated in order to prevent excessive catalyst fouling Catalyst poisons are removed from the feedstock to enhance catalyst life The feedstock may be hydrotreated to reduce the sulfur and nitrogen levels as well as metals In recent designs, the first reactor in the reactor train may be used for sulfur and nitrogen removal

35 Reaction Kinetics Kinetic modeling of reactions in hydrocracking is difficult due to the complexity of feedstock and products In general, adsorption of hydrogen to the active sites on the catalyst surface can be shown as H2 + S H2.S kA k-A S = vacant adsorption site on the catalyst surface kA = adsorption rate constant k-A = desorption rate constant

36 Reaction Kinetics The rate of adsorption ra is proportional to the concentration of active sites Ca and the partial pressure of hydrogen ra = kA pH2 Ca The desorption rate would be rd = k-A CH2S The net rate of adsorption is ra - rd = kA pH2 Ca - k-A CH2S The total concentration of active sites is constant CT = Ca + CH2S

37 Reaction Kinetics At equilibrium, the net rate will be zero
CH2S = (KA PH2 CT )/(1 + KA PH2) Where KA = kA/k-A Once the molecules have been adsorbed, they undergo several possible types of surface reactions The rate constant is also related to activation energy of the reaction In general, the rate of hydrocracking follows a first order kinetics

38 Catalysts Characteristics of good catalyst:
- ability to produce desirable product and not coke - selective to valuable products (e.g. high octane gasoline). - stable so it does not deactivate at the high temperature levels in regenerators. - resistant to contamination

39 Catalysts Hydrocracking catalysts are dual functional, (having metallic and acidic sites) promoting cracking and hydrogenation The main reactions are: Cracking Hydrogenation of unsaturated hydrocarbons obtained from cracking Hydrogenation of aromatic compounds Hydrogenolysis (breaking C-C bond by the addition of hydrogen) of naphthenic structure

40 Catalysts Cracking is promoted by metallic sites of the catalysts
Acid sites transform the alkenes formed into ions Hydrogenation reactions are also occur on metallic sites Both metallic and acidic sites take part in the hydrogenolysis reactions To minimize coke formation a proper balance must be achieved with the two sites on the catalyst (depending on the conditions of the operation)

41 Catalysts High temperatures lead to more reactions on acidic sites while increase in hydrogen partial pressure enhances hydrogenation on metallic sites Conventional catalysts are composed of transition metals deposited on acidic sites. The metals are those from group VIII (e.g. molybdenum, cobalt, nickel,…)

42 Catalysts Three classes: acid-treated natural aluminosilicates
amorphous synthetic silica-alumina combinations crystalline synthetic silica-alumina catalyst called zeolite or molecular sieves

43 Catalysts Zeolite-based catalyst is one of the most common used catalysts in hydrocracking The use of zeolite catalyst minimizes coke formation and improves catalyst stability Zeolites have large concentration of Brunsted acid sites which enhances their hydrocracking activity Zeolites also need lower temperatures to achieve a specified conversion Amorphous -alumina is also widely applied as a catalyst support due to its mechanical and thermal stability and porous structure

44 Advantages of using Zeolite as Catalyst
Higher activity Higher gasoline yield Higher octane number Lower coke yield Increased isobutane production Higher conversion without overcracking

45 End of Chapter Six


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