DF5 - Cracking & catalysts

Question 1

Gasoline is produced by fractional distillation of crude oil. What two problems does this leave us with?

Answer 1

• ‘Straight run’ gasoline from the primary distillation makes poor petrol - some is used directly but most is treated further
• Supply + demand - crude oil contains a lot of high bp fractions e.g gas oil but doesn’t contain enough of the lower bp fractions e.g gasoline. Although, demand for gas oil is quite low, it can be cracked + used in car petrol so it increases the demand

Question 2

What is the job of the refinery?

Answer 2

The job of the refinery is to convert crude oil into useful components

Question 3

What is a benefit of the process of cracking?

Answer 3

Long chain hydrocarbons can be broken down into smaller chain hydrocarbons (which tend to be highly branched) - this helps to solve the supply + demand problem

Question 4

What does unsaturated mean?

Answer 4

An organic compound that has a double/triple bond between carbon atoms e.g alkenes

Question 5

How is cracking carried out?

Answer 5

By heating heavy oils e.g gas oil in the presence of a catalyst

Question 6

Give some of the types of reactions when cracking hydrocarbons

Answer 6

• Alkanes –> branched alkanes + branched alkenes
• Alkanes –> smaller alkanes + cycloalkanes
• Cycloalkanes –> alkenes + branched alkenes
• Alkenes –> smaller alkenes

Question 7

What can alkenes produced in cracking be used for?

Answer 7

As starting materials for other parts of the petrochemicals industry

Question 8

Give a brief description of what happens in a modern catalytic cracker and what is used for this process to occur

Answer 8

• A tube (vertical 60m with 2m diameter), known as a rise reactor is used in for the reaction
• Hot vaporised hydrocarbons + zeolite catalyst are fed from bottom of tube + forced upwards by steam

Question 9

Describe the mixture + its movement in the process of catalytic cracking

Answer 9

• The mixture is a moving fluidised bed where the solid particles flow like a liquid
• It takes mixture about 2 secs to flow from bottom to top of tube - so hydrocarbons are in contact with catalyst for a very short period of time

Question 10

What is one of the problems with catalytic cracking?

Answer 10

• In addition to all the reactions that have occurred in catalytic cracking, coke (carbon from the decomposition of hydrocarbons) forms on catalyst surface so catalyst eventually becomes inactive. The powder catalyst needs to be regenerated to overcome this problem

Question 11

What is coke which is produced in catalytic cracking?

Answer 11

Carbon from the decomposition of hydrocarbon molecules

Question 12

What happens after the hydrocarbons go through the rise reactor?

Answer 12

• The mixture passes into a separator where steam carries away the cracked products leaving behind the solid catalyst
• The catalyst goes into the regenerator

Question 13

How do you regenerate the catalyst?

Answer 13

• The catalyst goes into a regenerator - takes 10 mins for the coke to burn off in hot air that is blown through the regenerator
• The catalyst is then reintroduced into base of the reactor ready to repeat cycle

Question 14

What does the energy from the burning coke do to the catalyst?

Answer 14

• The energy from the burning coke heats up the catalyst

* The catalyst transfers the energy to the feedstock so that cracking can occur without additional heating

Question 15

What is so good about catalytic crackers?

Answer 15

• Have become very flexible + adaptable since made in late 1940s
• Can handle a wide range of different feedstocks
• Conditions + catalyst can be varied to give the max. amount of desired product

Question 16

Give a detailed definition of a catalyst

Answer 16

• A substance which increases the rate of a reaction but can be recovered + remains chemically unchanged at the end of a reaction
• It does not undergo any permanent change but can be changed physically e.g surface of solid catalyst may crumble/become roughened - suggests catalyst is taking some part in reaction but is being regenerated
• Provides an alternative reaction pathway with a lower activation enthalpy

Question 17

Give the definition of catalysis

Answer 17

The process of speeding up a chemical reaction using a catalyst

Question 18

How much of the catalyst is needed and does it affect the product formed?

Answer 18

• Only small amounts needed
• The catalyst does not affect the amount of product formed but increases rate at which it is formed
• Catalyst does not appear as a reactant in overall equation of the reaction

Question 19

What is homogeneous catalysis?

Answer 19

Catalysis when reactants and catalyst are in the same physical state

Question 20

Give an example of homogenous catalysis

Answer 20

Enzyme-catalysed reactions in cells happen in aqueous solution

Question 21

Describe what happens in heterogeneous catalysis when a solid catalyst is used

Answer 21

• Catalysts are in different physical states e.g X+Y –> Z
• Reactant forms bonds with atoms on the surface of catalyst - this is ‘adsorption’ of reactants
• As a result bonds in reactant molecules are weakened + break
• New bonds form between reactants, held close together on the surface + product bonds formed (or product is formed)
• This weakens bonds to the catalyst surface + product molecule is released - ‘deadsorption’ of product from catalyst surface

Question 22

Why is it important for catalysts to have a large surface area and what can this be used as?

Answer 22

• For contact with reactants
• For this reason, solid catalysis is used in a finely divided form (or as a fine wire mesh)
• Sometimes catalyst is supported on porous material to increase SA + prevent it from crumbling - this happens to catalytic converters fitted to car exhaust systems

Question 23

What are many heterogeneous catalysts used for?

Answer 23

• Used in industrial processes + many are transition metals/transition metal compounds
• Platinum + rhodium used in catalytic converters in cars
• Nickel powder used in hydrogenation of unsaturated oils to give unsaturated fats

Question 24

What is catalyst poisoning?

Answer 24

• Catalysts can be poisoned so they can no longer function properly
• They block an enzyme-catalysed reaction

Question 25

What happens when the catalyst is poisoned in heterogeneous catalysis?

Answer 25

• Poison molecules are adsorbed more strongly to catalyst surface that reactant molecules
• Catalyst cannot catalyse reaction of the poison so becomes inactive - poison molecule blocks active sites on its surface

Question 26

Give examples of catalyst poisoning in heterogeneous catalysis to do with petrol used

Answer 26

• Leaded petrol cannot be used in cars fitted with catalytic converter - lead strongly is strongly adsorbed to the surface of the catalyst

Question 27

Give examples of catalyst poisoning in heterogeneous catalysis to do with the metal used in a catalytic converter

Answer 27

• Catalyst poisoning is also the reason why it’s not possible to replace costly metals (e.g platinum + rhodium) in catalytic converters by cheaper metals (e.g copper + nickel)
• These metals are vulnerable to poisoning by the trace amounts of SO2 present in their car exhaust gases
• Once catalyst in converter becomes inactive it cannot be regenerated + is very expensive to replace

Question 28

Why is catalytic cracking a problem in industrial processes?

Answer 28

• In UK, nearly all hydrogen for Haber process is prepared by steam reforming of methane - methane reacts with steam in the presence of nickel catalyst
• If feedstock for process contains sulfur compounds, these must be removed first to prevent severe catalyst poisoning

Question 29

Give an example of regeneration of a catalyst

Answer 29

• When cracking long-chain hydrocarbons, carbon is produced + the surface of catalyst becomes coated in a layer of soot
• This blocks the adsorption of reactant molecules + activity of catalyst is reduced
• Catalyst is constantly recycled through a regenerator
• The oxygen (in hot air blown in regenerator) converts the carbon to carbon dioxide + cleans the catalyst surface