my presentation003 catalyticcracking steamcracking

8/11/2019 My Presentation003 Catalyticcracking Steamcracking

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Catalytic cracking

Catalytic cracking

Catalytic cracking uses heat, pressure and a catalyst to breaklarger hydrocarbon molecules into smaller, lighter molecules.

Feed stocks are light and heavy oils from the crude oil distillation

unit which are processed primarily into gasoline as well as fuel

oil and light gases.

The catalytic cracking processes, and also other refinery catalytic

processing, produce coke which accumulates on the surface of

catalyst and causes the gradually losses of catalytic properties

(deactivation).

Therefore, the catalyst needs to be regenerated continuously or

periodically by burning the coke off the catalyst at high

temperatures. A fluidized-bed catalytic cracking units (FCCU)

are the most common reactor to use.


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Catalytic Cracking

Temp. : 520550 oC (from bottom to top---decreases)

Pressure : 23 atm

Cat/oil : 4.56

Contact time : 7080 %

Increasing temp. results in increase conversion, but decreasing

yield of gasoline, due to the secondary cracking to smaller

products.

Catalyst regenerator conditions :Temp. : 650760 oC

Pressure : ~ 3 atm.

Catalytic cracking

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Catalytic hydrocracking

Primarily used for cracking gas-oil that contains high percentage of polynuclear

aromatics, to give gasoline, diesel fuel, or jet fuel.

Catalysts require both an acidic component and a metal component

Acidic component : SiO2-Al2O3, Zeolites

Metal component : CO, Mo, Ni, W yield lubricating oils + middle or heavy distillate

fuels. Pt or Pd yield gasoline or diesel + jet fuels.

Reactions conditions :

Temperature : 300425 oC

Presssure : 100 170 atm

Reactor : Fixed bed

Catalytic hydrocracking normally utilize a fixed-bed catalytic cracking reactor in

presence of hydrogen under pressure (1,200 to 2,000 psig). Feedstocks are often thefraction that are most difficult to crack in the catalytic cracking units (FCCU). These

feed include middle distillates, cycle oils, residual fuel oils and reduced crudes. The

hydrogen suppresses the formation of heavy residual material and increases the yield

of gasoline by reacting with the cracked products. Because the heavy, sulfur and

nitrogen containing hydrocarbons are potentially poison the catalyst, they must be

removed. That is why, hydrocracking feedstocks are usually first hydrotreated.


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Steam Cracking

Ethylene+Propylene are the most important chemical feedstocks. But,

due to their relatively high reactivities, only very limited amounts of

olefins exist in natural gas + crude oil. Thus they must be produced bycracking processes.

Dominant steam cracking feedstocks are LPG (C3H8+C4H10) + NGL

(C2H6, LPG, light naphtha). Most C4olefins are obtained from catalytic

cracking, and < 10% from steam cracking.

Thermodynamics & Kinetics

All olefins are thermodynamically unstable with respect to H2 and

graphite (coke). Thus, distribution of desired H2products is controlled

by regulating kinetic parameters :

1. Temperatur

At 400 oC, HCs chains preferentially cracked in center of molecule.

With increasing temperature cracking shifts toward end of molecule,

leading to larger quantities of the preferred low M.W. olefin products.

Reaction rate also increases with temp., allowing shorter residencetimes.

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2. Residence time : short residence times result in more olefin formation.

Longer residence times increase secondary reactions, such as coke

formtion + oligomerization.

3. HC partial pressure : formation of low M.W. olefin products causes

pressure to increase. Thus, reaction is favored by low pressure.

Steam is added to decrease partial pressure of HC and to minimize

coke formation.

Process

1) HC feed heated with steam to ~ 1050 oC and Fed to Cr-Ni reactor

tubes.

2) Cracked poducts exit at ~ 850 oC and are rapidly quenched to ~ 300o

C to prevent secondary reactions.3) Products scrubbed to remove H2S and CO2then drift.

4) C2+ C3components separated by low temp. fractional distillation. C4

components must be separated by chemical means, because B.P.s

are too similar.

Steam Cracking

Steam Cracking


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my presentation003 catalyticcracking steamcracking

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