Catalytic Cracking, Hydrocracking Flashcards
Catalytic Cracking
Process that breaks down the larger, heavier, and more complex hydrocarbon molecules into simpler and lighter molecules by heat and aided by catalyst
Feedstocks for catalytic cracking
atmospheric and vacuum crude unit gas oils and coker gas oil
How is catalyst regenerated? Describe the movement
Burning off coke with air. The catalyst is continuously moved from reactor to regenerator and back to reactor
The cracking reaction is ___
endothermic
The regeneration reaction is ___
Exothermic
What are heat balance units
Units that use the heat from regeneration to heat the preheat the feed going into the reaction
2 types of bed for catalytic cracking
moving-bed or fluidized bed
Define TCC and FCC. What are their differences
Themafor catalytic cracking (moving -bed) and Fluid catalytic cracker (fluidized bed).
In TCC the catalyst is moving while in FCC it is fluidized. Few TCC are left as FCC has taken over.
Describe main pathways in fluidized bed catalytic cracking
The hot oil feed makes contact with the catalyst in either the feed riser line (transfer line) 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.
The oil vapors are taken overhead to a fractionation tower for separation into streams having the desired boiling ranges.
What happens to the oil/hydrocarbons remaining on the spent catalyst?
It is removed by steam stripping before the catalyst enters the regenerator
What happens to the spent catalyst?
It flows into the regenerator and is reactivated by burning off the coke deposits with air
What happens to the flue gas and catalyst?
separated by cyclone separators and electrostatic precipitaors
Describe the catalyst used in FCC
They are very fine particles which behave as a fluid when aerated with vapor. When fluidized, they act as a vehicle to transfer heat from the regenerator to the oil feed and reactor.
Two types of FCC units
side-by-side and stacked
How is regenerator temp and coke burn-off controlled?
By varying the air flow rate
The heat of combustion raises the catalyst temperature from ___ to ____ . The regenerated catalyst contains ____ to ____ wt% residual coke.
1150 to 1550 F
0.01 to 0.4 wt%
what is burned coke in the form of?
CO and CO2 or completely CO2 depending on regenerator design.
Cracking Reaction (3)
Paraffin -> Paraffin + Olefin
Alkyl napthene -> Napthene + olefin
Alkyl aromatic -> aromatic + olefin
3 classes of commercial cracking catalysts. Which are most common?
(1) acid-treated natural aluminosilicates
(2) amorphous synthetic silica-alumina combinations
(3) crystalline synthetic silica-alumina catalyst (zeolites or molecular sieves)
Class 3 or a mixture of 2 and 3 are most common
Advantages of zeolite catalyst over amorphous ones. (6)
- Higher activity
- Higher gasoline yield at a given conversion
- Production of gasoline containing larger
percentage of paraffinic and aromatic hydrocarbons - Lower coke yield (= larger throughput at a given conversion level)
- Increased isobutene production
- Ability to go to higher conversions per pass without overcracking.
Which catalyst is cheaper? Amorphous or Zeolites
Amorphous
What are poison to cracking catalysts? Explain how
Nitrogen compounds, iron, nickel, vanadium, and copper.
Nitrogen reacts with the acid centers on the catalyst and lowers its activity.
The metals accumulate on the catalyst causing a reduction in throughput by increasing coke formation.
(T/F) In catalysts, small pores results in lower activities
False (it is higher)
Major operating variables affecting conversion and product distribution (6)
charge stock
cracking temperature
catalyst/oil ratio:
space velocity
catalyst type and activity
recycle ratio
Ability:
Ability to crack a gas oil to lower boiling fractions.
Catalyst/oil ratio (C/O)
lb catalyst/lb feed
Conversion
100
(volume of feed - volume of cycle stock)/(volume of feed)
Efficiency
% gasoline x conversion
Recycle Ratio:
Volume recycle / volume of fresh feed
Selectivity
Ratio of yield of desirable products to the yield of undesirable products
Space velocity
Can be defined on either volume (LHSV) or weight (WHSV) basis
LHSV (liquid hour space velocity): (volume feed)/ (volume catalyst )(hr)
WHSV (weight hour space velocity, hr-1): (lb feed)/ (lb catalyst) (t) where t is the catalyst residence time in hr.
What increases conversion?
Increase in rxn temp (up to a certain point), catalyst/oil ratio, catalyst activity, and contact time.
Decreasing space velocity
(T/F) increase in conversion = increase in gasoline yield
False
CC:
Effect of increasing pressure? What is typical pressure range
It increases coke yield and decreases gasoline octane.
15-20psig
With the trend to heavier feedstocks, the carbon-forming potential of catalytic cracker feeds is increasing. What is being done to offset this?
Some units with limited blower capacity are adding oxygen to the air to the regenerator.
Oxygen contents of the gases to the regenerator are being increased to 24 to 30% by volume (limited by regenerator temperature capacity and heat
removal capacity.)
What is cataly
