Chemistry and technology of petroleum By Dr. Dang Saebea
Introduction • The fluidised catalytic cracking (FCC) unit is the heart of the refinery and is where heavy low-value petroleum streamsuch as vacuum gas oil (VGO) is upgraded into higher value products, mainly gasoline and C3/C4 olefins, which can be used in the alkylation unit for production of gasoline (C7–C8 alkylates). • Major developments have occurred in two areas: new catalysts and new reactor and regenerator designs.
Role of FCC in the Refinery • The role of the FCC is to take heavy desulphurised feedstock and crack it into lighter, mainly high octane gasoline. • The FCC also produces olefins (C5 =and C4 =) and LPG.
Feedstock • The main feedstock used in a FCC unit is the gas oil , which can be considered mixtures of aromatic, naphthanic and paraffinic molecules. • There are also varying amounts of contaminants such as sulphur, nitrogen and metals. To protect the catalyst, feed pre-treatment by hydrotreating is required in order to remove contaminants and improve crackingcharacteristics and yields.
Feedstock • Nitrogen tends to poison the catalyst by neutralising its acid sites. However, the FCC process is unaffected if the nitrogen content level is controlled below 0.2%. • Some possible feedstocks atmospheric distillates, coking distillates, visbreaking distillates, VGO, atmospheric residue (desulphurised) and vacuum residue (desulphurised, deasphalted).
Fluidisation • When a fluid flows upward through a packed bed of catalyst particles at low velocity, the particles remain stationary • As the fluid velocity increases, the pressure drop increases. Then the particles begin to move. This is the minimum fluidisation velocity. • As the velocity increases, the bed expands and bed porosity increases while the pressure drop remains practically unchanged. • The stable fluidisation starts at a certain pressure drop. Modes of fluidisation
FCC Reactions The main reaction in the FCC is the catalytic cracking of paraffin, olefins, naphthenes and side chains in aromatics. • The VGO undergoes the desired ‘primary cracking’ into gasoline and LCO. • A secondary reaction also occurs, which must be limited, such as a hydrogen transfer reaction which lowers the gasoline yield and causes the cycloaddition reaction. • The latter could lead to coke formation (needed to provide heat for catalyst regeneration).
Primary Reactions • Primary cracking occurs by the carbenium ion intermediates in the following steps: (a) Olefin is formed first by the mild thermal cracking of paraffin: (b) Proton shift:
Primary Reactions (c) Beta scission: Carbon–carbon scission takes place at the carbon in the position beta to the carbenium ions and olefins. • The newly formed carbenium ion reacts with another paraffin molecule and further propagates the reaction.
Primary Reactions Beside paraffins: 1. Olefins – smaller olefins 2. Alkylaromatics – Dealkylation 3. Alkylaromatics – Side chain cracking
Secondary Reactions Isomerisation The final product is the transformation of paraffins and olefins to isoparaffins. Cyclisation The final result would be the cyclisation of olefins to naphthenes and possibly further cyclisation to coke.
FCC Configuration • The basic configuration of the FCC unit is a reactor (riser) and a regenerator.
Process Description • endothermic reactions • residence time =2–10 s 649–760 ˚C 316–427 ˚C riser Steam and VGO heated are fed to the bottom of the riser. The regenerated hot catalyst at is also fed to the bottom of the riser.
Process Description CO, CO2 , H2O, Air 482–538 ˚C • Steam is injected into the stripper section, and the oil is removed from the catalyst with the help of some baffles installed in the stripper. • The oil is stripped in this way from the catalyst and the spent catalyst is sent to the regenerator. • The coke in the spent catalyst is burned off in the regenerator by introducing excess air
Process Description • Hydrocyclones are installed to catch any solid particles carried out in the overheated stream. • The product gases from the reactor are sent to the fractionator. • The light gases are sent to the gas concentration unit. • The filtered decant can be used as an aromatic solvent or recycled back to the riser with the HCO.