Cairo University
Faculty of Engineering
Chemical Engineering Department
Give reason for questions
Catalytic cracking &vis-breaking
Prepared by :
Name
Sec.
May, 2012
B.N
Give reason
Visbreaking :
1-Visbreaking is applied to molecules which have long paraffinic side chains.
They are the primary cause of high pour point and viscosities for paraffinic base residue.
2- Visbreaking is carried out at certain conditions.
-In order to optimize the breaking off of these long side chains and their subsequent cracking to
shorter molecules and with lower viscosities and pour point.
3- The amount of cracking is limited
-Because if the operation is too severe, the resulting product becomes unstable and form
polymerization products during storage which cause filter plugging and sludge formation.
4- The residuum from the atmospheric tower after cracking in a heater is quenched
with cool gas oil.
-To control over-cracking, and flashed in a distillation tower.
5- Visbreaking is important.
-as it is used to reduce the viscosity of heavy crude oil residues without affecting the boiling point
range.
- It is used to reduce the pour point of waxy residues & reduce the viscosity of heavy residues
used for blending with lighter fuel oils.
Catalytic cracking:
1) Catalytic cracking has almost completely replaced thermal cracking.
-Due to the higher yield to gasoline and olefins and light gases.
2) It is necessary to regenerate the catalyst in catalytic cracking process.
-To maintain the catalyst activity at useful level and this done by burning off the produced coke
(which remains on the catalyst particles) by air.
3) Olefins are not preferred feedstock.
-As they form less than 5% of the feed, and result in formation of undesired products.
4) In FCC the catalyst is continuously moved from the reactor to the regenerator
and back to the reactor again.
-The cracking process produces coke which remains on the catalyst particles and quickly lowers
its activity. To maintain the catalyst at a useful activity level it is necessary to regenerate the
catalyst by burning off this coke with air.
5) Paraffins are desirable feed to F.C.C.U.
- They are easy to crack and have highest yield for gasoline.
6) Naphthenes are desirable feedstock.
-They produce high octane gasoline which contains aromatics.
7) Aromatics are not preferred feed stock.
- They contain at least one benzene ring which is very stable and doesn’t crack easily and they
produce excess gaseous products and have a low gasoline yield.
8) Some Catalytic cracking units are known as “Heat Balance Units”.
-As they are designed to use the regeneration heat (exothermic reaction) to supply that needed for
the reaction (endothermic) and to heat the feed up to reaction temperature.
9) The feed to Fluid Catalytic Cracking Unit undergoes hydro treating firstly.
- This is done to reduce the amounts of sulfur and nitrogen and reduce also their deposition on the
catalyst and poisoning of it.
10) Feed preheating is required in FCC.
-As the boiling point of the feed is high (350 oC) preheating is used to avoid solidification of the
feed in the pipelines delivering the feed to the preheating zone.
11) 15% zeolite and 85% amorphous S.S-A is generally applied as a catalyst for
F.C.C.U.
-High frequency zeolite permits short residence time required in riser cracking operation
(negligible back mixing) and also minimizes coke formation but it has a very high cost.
-Amorphous S.S-A is cheaper and resistant to erosion but it has lower activity.
So, these combination is the most economic & effective.
12) Zeolite catalysts are preferred more than synthetic amorphous catalysts .
- This due to:
a) Their higher activity
b) Their higher gasoline yield at a given conversion
c) Increased isobutane production
D) Their Lower coke yield
e) Their ability to go to higher conversions per pass without over cracking
f) Production of gasoline containing a larger percentage of paraffinic & aromatic hydrocarbons.
13) Formation of dry gas in F.C.C.U.
Due to:
- Increase in Temperature and residence time.
- Increase in metal content.
-Increase in aromatic content of the feed.
14) Light gas oil is widely used as a blending stock in heating oil and diesel fuel.
-It is used as a blending stock as it reduces viscosity and fire points(suitable for winter).
15) The use of down turn separators although they are not very efficient.
- As they reduce the cyclones load effectively.
16) Hydrocarbons adsorped on the catalyst surface should be removed before
sending to the regenerator.
- Because if they are not removed they will burn in the regenerator & increase the temperature
beyond the limits which will affect the operation seriously.
17) The presence of cyclones inside the regenerator dilute phase in FCC process.
-To separate the entrained catalyst back to the dens phase where the combustion takes place.
18) Some catalysts will leave the regenerator as fine particles.
-this due to the erosion in the fluidized region & cyclones.