Fluid catalytic cracking Plant
Contents
Background
The process of the FCC unit consists of the feed injection system, riser, riser outlet separator system, disengager/stripper, regenerator, catalyst cooler (optional), catalyst withdrawal well, catalyst transfer lines, and control systems.
The feed mixture is pumped to the base of the riser and divided into equal flows, to each of the feed nozzles. The feed, which has been preheated, is finely atomized and mixed with dispersion steam in the feed nozzles and injected into the riser. The small droplets of feed contact hot regenerated catalyst in a counter-current way and vaporize immediately. The vaporized oil intimately mixes with the catalyst particles and cracks into lighter, more valuable products along with slurry oil, coke, and gas. The product vapors travel up the riser while carrying the catalyst. Residence time in the riser is approximately 2 seconds at design conditions. The specially designed feed injection system ensures the reaction is carried out efficiently to minimize the production of coke, gas, and slurry oil.
Feed injection zone
Oil feed to the riser is preheated before entering the reaction system. Preheat temperature along with regenerated catalyst temperature is controlled to result in an optimum catalyst to oil ratio. Passivator injection into the fresh feed just ahead of the feed nozzles acts to inhibit the undesirable effects
Dispersion steam is supplied to each of the feed nozzles to promote atomization and vaporization of the feed. The flow to each of the feed nozzles is adjusted by flow controllers.
Upstream the feed injection, stabilization steam is injected in the riser, through the stabilization steam injectors, in order to promote a smooth and homogeneous catalyst flow at the feed injection point. The flow to each of the injectors is adjusted by flow controllers
Riser/Reactor
The sensible heat, heat of vaporization, and heat of reaction required by the feed is supplied by the hot regenerated catalyst. The riser outlet temperature is controlled by the amount of regenerated catalyst admitted to the riser through the regenerated catalyst slide valve. In the wye section at the base of the riser, steam is injected via a steam ring to keep the regenerated catalyst in a fluid state at all times.
The cracking reactions take place during the two-second residence time in the riser as the reaction mixture accelerates toward the Riser Outlet Separator System (ROSS).
The catalyst is quickly separated from the hydrocarbon/steam vapors in the ROSS separator located at the end of the riser. This separation is necessary to discourage the undesirable continuation of reactions that produce gas at the expense of gasoline.
This system drastically reduces the post riser catalyst/vapors contact time. After exiting the ROSS separator, the vapors pass through high efficiency single stage cyclones to complete the separation of catalyst from vapors, thus minimizing the amount of catalyst lost into the product.
The reactor pressure "floats on" the main fractionator pressure and as such is not directly controlled at the converter section. The ROSS separator and disengager cyclones separate the product vapors from the spent catalyst and return the catalyst to the stripper bed. The cyclone diplegs are equipped with trickle-valves to prevent reverse flow of gas up the diplegs. Also, the ROSS separator is equipped with diplegs fitted on its pre-stripping chambers. These diplegs are sealed into the stripper catalyst bed in order to avoid any possibility of vapors back mixing.
Stripper
Catalyst exiting the ROSS separator is pre-stripped with steam from a steam ring located immediately at the exit of the separator diplegs. This is an important feature for reducing coke yield. The catalyst is further stripped by steam from the main steam ring as the catalyst flows down the stripper.
Two additional rings (upper and lower rings) are also provided in addition to the main ring. The upper ring achieves the second stage of pre-stripping of the catalyst before it enters the stripper. The lower ring is located in the bottom head of the stripper to achieve stable fluidization at the inlet of the spent catalyst standpipe.
The contact between catalyst and steam is enhanced by the presence of fluidized bed packing allowing for cross and counter-current flow of steam and catalyst. This highly efficient contacting displaces the volatile hydrocarbons contained on and in the catalyst particles before they enter the first stage regenerator. Coke remaining on the catalyst is burned off in the regenerators. The catalyst is aerated in the spent catalyst standpipe to the proper density for stable head gain.
Spent catalyst transfer
The stripped spent catalyst flows down the spent catalyst standpipe and through the spent catalyst slide valve. Aeration by fuel gas (or nitrogen) is added to the standpipe at several elevations to maintain proper density and fluid characteristics of the spent catalyst emulsion. The spent catalyst slide valve controls the level in the stripper by regulating the flow of spent catalyst from the stripper. The spent catalyst flows into the first stage regenerator through a distributor which ensures that the entering coke-laden catalyst is spread across the regenerator bed.
Regeneration system
The first stage regenerator burns 50 to 80% of the coke and the remainder is burned in the second stage regenerator. This two-stage approach to regeneration adds considerable flexibility to the process as potential heat is rejected in the first stage regenerator in the form of CO.
The heat of combustion released by the combustion of coke is transferred to the catalyst which will later supply the heat required to the reactor. The heat balance of the unit is much more flexible than in single-stage regeneration systems because potential energy in the form of carbon monoxide from the first stage regenerator can be adjusted while complete regeneration of the catalyst is accomplished in the second stage.
Air blower and air heaters
The combustion air required is supplied by an air blower, commonly driven by a steam turbine. The steam supply to the turbine is throttled on cascade air flow trim control/compressor speed. Atmospheric air is introduced to the air blower through an intake filter and silencer. The blower air is distributed to a header system providing combustion air to first regenerator rings, second regenerator ring, lift air (and catalyst cooler fluffing air in case it is installed).
failure. Combustion air to the first stage regenerator is split between two air rings. The outer air ring and inner air ring are designed to handle about 70% and 30% of the combustion air to the first stage regenerator respectively.
First stage regenerator
Spent catalyst containing roughly 1 to 1.5 wt % coke flows from the spent catalyst distributor and is spread across the bed in the first stage regenerator.
The regeneration conditions are mild to limit hydrothermal deactivation of the catalyst. First stage regenerator total combustion air is controlled to limit the temperature in the first stage to a maximum of 730°C. The partially regenerated catalyst flows down through the first stage regenerator bed to the entrance of the airlift. Aeration is supplied in this area to ensure the smooth flow of catalyst to the lift. A hollow stem plug valve regulates the flow of catalyst to the lift line and is controlled by the level in the first stage regenerator. Air injected through the hollow stem of the plug valve into the airlift is flow controlled at a rate sufficient to lift the catalyst in a dilute phase up to the second stage regenerator.
Two-stage cyclones separate the entrained catalyst from the flue gas exiting the first stage regenerator. At the exit of the regenerator, the flue gas pressure is reduced through a double-disc flue gas slide valve controlling the regenerator pressure. Incineration of the CO in the flue gas is then accomplished at the CO incinerator. A continuous catalyst withdrawal is necessary to maintain the unit catalyst inventory in the normal operating region.
Second stage regenerator
The partially regenerated catalyst flows up the lift and enters the second stage regenerator below the air ring. A distributor at the end of the lift provides for efficient distribution of catalyst and air from the lift. Catalyst is then completely regenerated to less than about 0.05% carbon at more severe conditions than in the first stage regenerator. Very little carbon monoxide is produced in the second stage and excess oxygen is controlled by flow control of the second regenerator combustion air for efficient and complete combustion. Because most of the hydrogen in coke was removed in the first stage, very little water vapor is produced in the second stage. This limits hydrothermal deactivation of the catalyst as higher regeneration temperatures are experienced.
cyclones are used on the second stage flue gas to remove the entrained catalyst. This design expands the operating envelope for regenerator temperatures which tend to be higher for residue type feed. The cyclone dip legs are external to the regenerator. Catalyst recovered in the cyclone are returned to the regenerator bed below the normal operating level by way of the diplegs. Aeration is supplied to the diplegs to provide for smooth fluidized catalyst flow and the dip legs outlets are equipped with flapper valves to prevent catalyst and gas backflow into the cyclones.
The second stage regenerator pressure is controlled by a flue gas double-disc slide valve, through differential pressure between the first and second stage regenerators.
Regenerated catalyst transfer
The hot regenerated catalyst flows from the second stage regenerator through a lateral into the withdrawal well. In the withdrawal well a quiescent bed is established at proper standpipe density by introduction of a controlled amount of fluidizing air from the withdrawal well ring. A smooth stable flow of catalyst down the standpipe is provided by injection of aeration air at several elevations on the regenerated catalyst standpipe. As the head pressure increases down the standpipe and the catalyst emulsion is compressed, these aeration points are used to replace the "lost" volume, thereby to ensure a continuity of fluid catalyst flow properties.
At the bottom of the regenerated catalyst standpipe the regenerated catalyst slide valve controls the flow of hot catalyst.
Catalyst handling
The catalyst handling system includes hoppers for storage of fresh catalyst and spent catalyst, loading devices for catalyst addition and a draw-off device for continuous equilibrium catalyst withdrawal. Three hoppers are installed: the fresh catalyst hopper, the spent catalyst hopper and the auxiliary catalyst hopper. The auxiliary catalyst hopper provides flexibility in the operation for different options:
- storage of imported equilibrium catalyst reused as part of the make-up catalyst,
- storage of excess spent catalyst,
- storage of the second grade of fresh catalyst for make-up.
The spent and fresh catalyst hoppers are sized to contain the entire unit inventory. Each hopper is provided with one cyclone and with aerations in the bottom cone to assist the catalyst circulation to the transfer lines.
Feed injection system
The principle of the feed injection system is to atomize the oil feed into very small droplets with a high surface area for rapid heat transfer with the hot regenerated catalyst. Vaporization of the feed is then rapidly promoted. This enhances vapor phase cracking reactions and minimizes liquid phase coking reactions.
