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. Vaporization of the feed is then rapidly promoted. This enhances vapor phase cracking reactions and minimizes liquid phase coking reactions. In catalytic cracking, the reaction takes place in the vapor phase, the importance of maximizing the vaporization in the quickest possible time is then critical. A larger oil droplet stays in the liquid phase longer and can surround the catalyst particle, effectively blocking the active surface area. Slow vaporization of the feed promotes the formation of coke, gas, and slurry oil.
A larger oil droplet stays in the liquid phase longer and can surround the catalyst particle, effectively blocking the active surface area. Slow vaporization of the feed promotes the formation of coke, gas, and slurry oil.
The thinks are to inject the oil downwards counter-current of the accelerated catalyst. This injection pattern promotes the contact between oil droplets and hot regenerated catalyst. The heat transfer is drastically enhanced.
Uniform feed distribution and rapid vaporization also have other benefits. In the bottom of the riser, the catalyst turns upward with aid of the bottom steam, then the catalyst flow is stabilized by means of stabilization steam and meets the oil/steam mist at the feed injection point.
A well-distributed, rapidly vaporizing, feed helps accelerate the reaction mixture to its final velocity in a smooth manner. Catalyst slippage and back mixing will promote undesirable side reactions. In this last situation, some feed vapor sees too much catalyst and some not enough.
The venturi like effected of the high energy feed injection system associated to the counter current injection results is a faster, more uniform acceleration of the catalyst in its upward path and more uniform catalyst densities along the cross section of the riser.
The feed injection nozzle atomize the oil feed by a high energy shearing action on a specially designed venturi type injector. High velocity steam jet shears the oil, further into a fine mist. The injector tip is designed to discharge a wedge shaped spray that fans out from the tip in a carefully determined angle. The action of the feed nozzles together provide uniform coverage of the riser cross section without impingement on the riser walls.
Mix feed Temperature Control
to improve the process capabilities and performances with respect to the cracking of increasingly heavier and more refractory feedstocks. It is designed to face the two main challenges encountered when processing very heavy, highly contaminated feeds.
- Achieve satisfactory vaporization of the feed so as to eliminate the unnecessary coke production resulting from incomplete vaporization.
- To keep the desired heat balance while maintaining the conversion at the optimum level.
basically consists of the injection of an appropriate stream into the riser, downstream of the feed injection point, at a location and under conditions selected to obtain the desired quenching effect and the best yield selectivity.
The optimum MTC flowrate depends on the feed quality (the heavier the feed the higher the recycle flowrate) and on the desired yields structure: to switch from maximum gasoline to a maximum distillate mode of operation for instance, operating conditions will have to be modified directionally towards a decrease of the conversion per pass, which will result mainly in :
- A lower Riser Outlet Temperature and subsequently a lower mix temperature in the fresh feed injection zone will tend to increase the coke make if the mix temperature becomes lower than the feed dew point.
- A higher slurry yield with a lower aromatic content and therefore a superior potential of this product for an additional conversion through recycle.
- A lower gas make leading to an increased margin on fractionation and wet gas compressor section.
Riser Outlet Separation System
The principle of the patented IFP Riser Outlet Separation System (ROSS) is the symmetrical structure of separation chambers and collecting chambers alternatively arranged around the top of the riser.
The separation chambers are connected in their upper section to the riser and the vapor + catalyst mixture follows a 45° turn downwards around an inlet baffle.
The vapor outlets to the collecting chambers are located underneath the inlet baffle in the perpendicular direction of the incoming stream. This geometry provides simultaneously both the centrifugal effect and the inertial effect which are key factors for catalyst separation. The catalyst separated is collected in a hopper prorogated by a dipleg which achieves the necessary seal between the riser and the stripper vessel. The vapor exiting the separation chamber is then mixed in the collecting chamber with the steam evolved from the stripper vessel. The collecting chambers are then connected to a center pipe collector which distributes the vapor to the disengager cyclones for final catalyst separation.
This system achieves in a single device an extra-short time of vapor disengagement of about 1 second (45° turn, no vortex) and an efficient catalyst separation in two chambers arranged in series. The main chamber is the separation chamber itself and the secondary chamber is the vapor collecting chamber providing an additional safety buffer volume for further catalyst disengagement in case of catalyst carry-over.
Two stage regeneration
Residue cracking differs from gas oil cracking in that the feed contains more asphaltenes, and is more hydrogen deficient. Coke making, which is usually dictated by heat balance requirements, can become a critical factor in the operability of the process. Coke make increases as residual oil is added to the feed and regenerator temperatures rise as more heat of combustion from the coke is released. A conventional single-stage regenerator is limited in regard to cracking residues because of metallurgical limits within the regenerator vessel.
The R2R technology splits the regeneration into two stages. The first stage burns part of the coke from the catalyst at mild conditions and completes the regeneration in the second stage. The first stage burns part of the coke from the catalyst at mild conditions and completes the regeneration in the second stage.
Several benefits are realized as a result of this configuration. Most of the hydrogen in coke is removed in the first stage at mild conditions, i.e. a maximum operating temperature limited at 730°C (usually 670 to 690°C). The resulting water vapor is in contact with the catalyst at lower temperatures than those necessary for complete regeneration, leading to much less catalyst deactivation.
The relatively dry atmosphere of the second regenerator allows raising the temperature while the regeneration is completed. Thus the R2R two stage regeneration results in less hydrothermal deactivation of the catalyst.
In the two stage regenerator configuration, potential energy, in the form of carbon monoxide, is rejected from the regenerator system by increasing the amount of total coke burned in the first stage regenerator. Complete regeneration of the catalyst is accomplished in the second stage regenerator. Excess oxygen is maintained to ensure complete regeneration to essentially carbon-free catalyst. Since the second stage regenerator cyclones are external to the vessel and lined with refractory, the metallurgical limitations previously encountered with residue feeds are circumvented (the maximum operating temperature can reach 810°C relatively to the catalyst while the mechanical design is 840°C).
Riser wye steam ring
Its function is to straighten out the catalyst flow pattern as it makes the transition at the "wye" from its downward flowing, partially deaerated state, to an upward flowing, evenly aerated state.
In aerating the catalyst, this ring also provides the reverse seal needed to protect against oil flow reversals. The vertical column of catalyst below the feed nozzles provides a seal against such upsets. This ring is designed for a normal flowrate that should be maintained at all times when the catalyst is circulating.
Aeration and fluidization systems
The catalyst fluidization and aeration systems play a vital role in the stability of catalyst circulation. Proper attention should be given to all fluidization and aeration flows to make sure that they are properly set at their specified rates.
The standpipe aeration systems on the unit are designed to handle a wide range of conditions and still provide the smooth, stable, catalyst flow required for proper operation. This is essential for stable and adequate catalyst slide valve differentials. The system includes aeration points located along the vertical portions of the standpipes with a rotor meter or flow orifice provided for each tap. The flows to the taps are initially set equal to replace the volume of interstitial gas compressed by head pressure.
It is important to note the difference between aeration and fluidization systems. Aeration is the process of replacing, with the injection of gas, the volume lost to compression by head pressure in a column of fluidized catalyst. Aeration medium is necessary to keep the catalyst from becoming non fluidized and developing unstable flow characteristics.
Fluidization taps are employed when the direction of catalyst flow changes. In the R2R unit, 45° angle changes are used to redirect catalyst flow whenever possible. As the catalyst flows into a 45° line the fluidization media is injected to assist in the turn. On the other hand, when catalyst flows from a 45° line, the fluidization media is added to smooth the turn and to penetrate the denser catalyst layer at the wall to allow easier entrance.
Catalyst
The proper selection of catalyst is very important to successful residue cracking operations. The importance of magnifies as the percentage of residual oil increases in the feedstock. Several properties of the catalyst should be examined for a particular feed. The properties are:
• Zeolite content
• Micro-activity
• Rare earths content,
• Unit cell size
• Coke selectivity
• Particle size distribution
• Bulk density
• Thermal stability
• Surface area
• Pore volume and pore distribution (strippability)
• Attrition resistance
• Metals resistance
• Gasoline octane properties
