DF5 Cracking Flashcards
What is cracking?
A reaction where larger molecules are made into smaller ones.
What is cracking used?
To get highly branched shorter alkanes and alkenes which are used in petrol.
What is a benefit from cracking?
It solves the supply and demand problem.
What is an unsaturated alkane?
An alkane that doesn’t have as many hydrogen atoms as it could and has a double or triple bond between carbon atoms.
What is catalytic cracking?
Cracking done by heating heavy oils in the presence of a catalyst to produce petrol.
What can you crack alkanes into?
Branched alkanes and branched alkenes, or smaller alkanes and cycloalkanes.
What can you crack cycloalkanes into?
Alkenes and branched alkenes.
What can you crack alkenes into?
Smaller alkenes.
What are alkenes important for?
Starting materials for other parts of the petrochemical industry.
What does catalytic cracking take place in?
A riser reactor.
What catalyst is involved in catalytic cracking?
Zeolite.
Why is it called a riser reactor?
The hot vaporised hydrocarbons and zeolite catalyst are fed into the bottom of the tube and forced up by steam.
How long does it take the mixture to flow up the riser reactor?
Two seconds.
What is one problem with catalytic cracking?
Coke forms on the catalyst surface so that the catalyst eventually becomes inactive.
What is coke?
Carbon from decomposition of hydrocarbon molecules.
How do you overcome coke forming on a catalyst surface?
The powdery catalyst needs to be regenerated.
Where does the mixture go once it leaves the riser reactor?
A separator.
What happens in the separator?
Steam carries away the cracked products leaving behind solid catalyst.
What happens to the catalyst in the separator?
It goes into the regenerator where the coke is burnt off with hot air that is blown through.
What happens to the catalyst after the regenerator?
It’s reintroduced into the base of the reactor ready to repeat the cycle.
What does the energy released from the burning coke do?
Heats up the catalyst.
Where does the catalyst transfer its energy to?
The feedstock.
Why does the catalyst transfer its energy to the feedstock?
So that cracking can occur without additional heating.
What is a catalyst?
A substance that speeds up a reaction but is chemically unchanged.
What is catalysis?
The process of speeding up a chemical reaction using a catalyst.
How can catalysts undergo change?
Physically but not chemically.
What does a catalyst being unchanged chemically but changed physically suggest?
That the catalyst is taking some part in the reaction but is being regenerated.
How much of a catalyst is needed?
Only a small amount.
What does the catalyst not effect?
The amount of product formed.
Where does a catalyst not appear in?
As a reactant.
What is homogeneous catalysis?
Catalysis where the reactants and catalysts are in the same physical state.
What is an example of homogenous catalysis?
Enzyme catalysed reactions in cells.
What is heterogeneous catalysis?
Catalysis where the reactants and catalyst are in different physical states.
What does heterogeneous catalysis usually involve?
A mixture of gases and liquid reacting with a solid catalyst.
Where does the reaction occur when a solid catalyst is reacted with gases or liquids?
On the surface of the solid.
What do the reactant gases and liquids do with the surface of the catalyst?
Form bonds with the atoms on the surface and are adsorbed.
What happens as a result of adsorption?
Bonds in the reactant molecules are weakened and break and new bonds form between the reactants and surface of the catalyst to form products.
What does forming products on a catalyst do?
Weakens the bonds to the catalyst surface and the product molecules are released.
What must a catalyst have for contact with the reactants?
A large surface area.
What are most heterogeneous catalysts in industrial processes.
Transition metals.
What does a catalyst poison do?
Block an enzyme catalysed reaction so that they no longer function properly.
What happens with poison molecules in heterogeneous catalysis?
The poison molecules are adsorbed more strongly to the catalyst surface then the reactants.
Why does the catalyst become inactive when the poison is adsorbed?
Because the catalyst can’t catalyse a reaction of the poison.
How do the poison molecules inactivate the catalyst?
They block the active sites on its surface.
