FCC

Question 1

Fluidised Bed Catalytic Cracking

Answer 1

FCC is prefered to thermal cracking as thermal racking can break down lighter streams hence causing undesired products. Cat cracking has the advantage of higher selectivity (hence not affecting lighter streams) and also produces a product with a higher octane number. Regeneration must continually occur because of coke deposition on catalyst surface which reduces its activity.

Question 2

The process

Answer 2

The hot oil feed is brought into contact with the fresh regenerated catalyst at a mixing point. The mixture then travels through the riser tto the reactor which also contains a bed of catalyst. As cracking occurs, there is deposition of coke on the catalyst surface which reduces the activity and hence yield of the catalyst.
Th oil, coke and catalysts mixture is then mechanically separated, with the oil going to a fractionation column where its is separated into its oil and gases. The spent catalysts is then sent to the regenerator.
Before entering the regenerator, the catalyst is stripped with steam in order to release the absorbed hydrocarbons on the catalyst before it come in contact with air. In the regenerator, a stream of air is sent int which burns off the coke to form flue gases. The flue gas and catalyst are then separated in a cyclone with the flue gas going for heat or energy recovery.
The regenerated catalyst the return to the feed. Sometimes the catalyst is stripped with steam to remove any absorbed oxygen before it comes in contact with try hydrocarbon.
Regenerator temperarute is controlled by controlling air flowrate.

Question 3

The old design

Answer 3

In old design of the real fluidised, it was seen that a high amount of cracking was taking place in the riser as this is the part where the zeolite catalysts had the highest activity and since zeolite is already very active, the design was change to allow the reactions take place in the riser and the reactor being used for only separation.

Question 4

main concepts for FCC

Answer 4

• fluidised bed with catalyst of about 70 microns
• catalyst bed is continuously transferred between reactor and regenerator.
• reaction control is performed by air flowrate in regenerator. The catalyst is what provides the heat required for cracking reactions.

Question 5

The new FCC

Answer 5

• fresh and recycled feed is preheated with a heat exchanger or furnace before coming in contact with the regenerated catalyst.
• Cracking begins immediately the hydrocarbons and catalyst come into contact and does not stop until the mixture of coke, oil vapours and catalyst is separated. Heat from the catalyst is vaporises the feed and increases the feed temperature to the required temperature.
• When separated, the product is take to the fractionation column for separation into liquid and gaseous products.
• The spent catalyst is then steam stripped before going into the regenerator.
• in the regenerator, air is added which burns off the coke to form flue gases and activate the catalyst. regeneration temp is controlled by controlling air flowrate as the combustion reaction is what increases temperature as it is an exothermic reaction.
• ideally want co2 to be formed in order to reduce carbon residue on the catalyst. The flue gasses can be used in a waste heat boiler

Question 6

Types of catalysts used

Answer 6

1. acid treated natural alumina silicates
2. amorphous synthetic silica- alumina
3. Crystalline synthetic silica-alumina ie zeolite

Advantage of zeolites are that

• higher activity
• forms product with higher amount of paraffins and aromatics
• increase yield for a certain conversion
• reduce coke formation
• increased amount of isobutene formation
• higher conversions per pass without overcracking

The catalysts is either class 3 or a mixture of 2 and 3

Question 7

The feed stream

Answer 7

Is the bottoms of the atmospheric distillation column. Can have metals like vanadium and nickel present.
The metals can cause
- reduced activity
-lower yield
-promote coke and hydrogen formation
- increased need for regeneration hence increasing air/oxygen demand
Bismuth or antimony can be added to reduce effectiveness of the metals.

Nitrogen present present cal also react with the acid centre of the catalyst

Question 8

Random points

Answer 8

• have to hydrotreat to remove some metals and also sulphur and nitrogen
• Increase in temperatire, cat/oil ratio, cat activity and contact time increase conversion
• Increase in space velocity reduces conversion
• pressure does not have a great effect
• possible to have increase conversion with reducing yield when increasing temperature.

Question 9

Energy Recovery

Answer 9

-Temprature control and energy recovery is easy when co2 is formed.
Can deal with heat by
- taki g heat to heat exchnager or wast reboiler
- If CO is present, take it to a CO burning waste boiler for combustion to CO2 than can be used to power a turbine
- Can take gas to turbine to produce electricity