Catalytic cracking

Catalytic cracking

Catalytic cracking uses heat, pressure and a catalyst to break larger 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.

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic Cracking

Temp. : 520 – 550 o C (from bottom to top---decreases)

Pressure : 2 –3 atm

Cat/oil : 4.5 –6

Contact time : 70 –80 %

Increasing temp. results in increase conversion, but decreasing yield of gasoline, due to the secondary cracking to smaller products.

Catalyst regenerator conditions :

Temp. : 650 – 760 o C

Pressure : ~ 3 atm.

Catalytic cracking

Catalytic cracking

Catalytic cracking

Catalytic hydrocrackingPrimarily 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 : 300 – 425 o C

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 the fraction 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.

Catalytic hydrocracking

Catalytic hydrocracking

Catalytic hydrocracking

Catalytic hydrocracking

Steam CrackingEthylene+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 by cracking processes.

Dominant steam cracking feedstocks are LPG (C3H8+C4H10) + NGL (C2H6, LPG, light naphtha). Most C4 olefins 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 H2 products is controlled by regulating kinetic parameters :

1. Temperatur

At 400 oC, HC’s 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 residence times.

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 ~ 300 oC to prevent secondary reactions.

3) Products scrubbed to remove H2S and CO2 then drift.

4) C2 + C3 components 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

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking

Steam Cracking