Revamping FCC Units
Volume 5 Issue 2
The following article on FCCU revamps was presented by Jean-Louis Mauleon of Total Technique at the 1989 BASF FCC Seminar held in Brugge, Belgium.
The optimum Fluid Catalytic Clacking (FCC) performance is normally achieved with a new grass root design. It is possible, however to minimize the required investment in a refinery by revamping an existing FCC unit whether it be a pressure or slide valve controlled unit. Revamping an old FCCU is a cost effective approach to the processing of economically favorable residual feeds. This is the subject of this catalyst report.
A key to most revamps is improving catalyst flow by minimizing catalyst attrition and increasing fines retention. Irrespective of the catalyst type, density, and particle size distribution, this improves:
- stripping efficiency
- regeneration capability
- the range of catalyst circulation rates
- pressure profile
- riser temperature control
These improvements are essential to subsequent improvements in:
- unit conversion
- product selectivties
- catalyst stability in the presence of contaminants
And, they minimize:
- coke and gas production
- catalyst consumption and air pollution
This results in a more stable and flexible operation that is easier to operate close to unit constraints and achieve substantial gains in unit performance.
Cyclone separator systems are key equipment in the FCC process. They should offer retention efficiencies better than 99.995% to retain the fines necessary for good fluidization, and be able to withstand the combined effects of extreme erosion, high operating temperature and periodic upsets. Performance must be maintained over the entire processing period, without excessive wear, damage or attrition to catalyst.
Metallurgy of cyclone equipment in the regenerator has, in recent years, focused on the use of type 304-H 18/8 stainless steel.
The design basis is normally a 1% creep rate at 100,000 hours at a sustained operating temperature of 780°C (1436°F). Short term exposure at higher temperatures such as 1000 hours at 980°C (1800°F) is also within design parameters. A life span of about 10 years is expected.
On the reactor side, the metallurgical considerations are not as stringent, since operating temperatures are in the 480-560°C (900-1040°F) range and low alloy carbon steel can be safely used up to 590°C (1100°F). Recent improvements in refractories and anchorages, more resistant to erosion, thermal expansion and vibrations, have greatly contributed to increasing the length of time between turnarounds.
The use of smaller plenums and modified supports results in more favorable cyclone elevations. Installing the cyclones higher in the vessel permits the use of much longer, more efficient cyclones, often designed with an external scroll on the primary stage. Operating at substantially higher velocities allows increases in both capacity and efficiency.
With appropriate refractory linings for the pressurized shell and 304 stainless steel for the internals, a sustained operating temperature up to 760°C (1400°F) is feasible. To minimize hot spots and catalyst deactivation, a counter-current flow regime between the spent catalyst and regeneration air is preferable.
To eliminate afterburning, special attention is given to the air distribution system (plate grid, spider grid or ring), as well as spent catalyst distribution. Higher than conventional superficial velocities in the range of 1.2-2.4 m/s (3.9-7.8 ft/s) in the bottom, and 0.8-1.2 m/s (2.6-3.9 ft/s) at the top of the catalyst bed are preferred. This greatly enhances diffusion and regeneration efficiency despite a lower apparent gas contact time in the dense bed. As shown in Table 1, as the superficial velocity increases:
- the bed density decreases and the gas phase hold up expands
- the catalyst density in the transition zone above the bed level rises
- relative gas contact time decreases
- relative regenerator efficiency increases
In designs where the regenerated catalyst draw off is below the bed level (submerged well), the regenerator catalyst inventory does not change. The differential pressure level instrument will continue to give the same indication.
In contrast, when the regenerated catalyst is withdrawn through an overflow well, the excess inventory is transferred to the stripper as in Model IV units.
Because of the higher superficial velocities and higher catalyst entrainment, the cyclone design must be modified. New, high efficiency configurations are capable of very low losses. Despite the high inlet velocities of up to 28 m/s (92 ft/s) for the first stage, they do not wear or cause significant attrition to most commercial catalysts. In the event that the pressure balance is critical and the available dipleg height of the second stage cyclone is not sufficient, efficient single stage cyclone systems can be installed. Typically, the retention of fines below 40 microns can be as high as 10 to 12 wt% .
By improving regenerator efficiency, it is now feasible to fully regenerate the catalyst in a single stage regenerator with a minimum amount of air with:
- Less than 0.1 vol.% CO in the flue gas
- Less than 0.3 vol.% oxygen in the flue gas
- Less than 0.05 wt.% carbon on regenerated catalyst
With good stripping, air requirements can be as low as 13.0 kg of dry air per kg of coke, compared with a value of 14.5 often cited in literature. Table 2 compares the effect of reducing coke from 7.0 to 5.5 wt. % due to improved stripping. Note the lower air requirement and lower heat generation. Obvious benefits include lower utilities and lower regenerator temperature.
Regeneration is an important aspect to consider when upgrading cracking capacity and severity of an existing unit.
Regenerated Catalyst Circuit
If the unit in question is a pressure balance operated unit, the regenerated catalyst is withdrawn through an overflow well. On slide valve operated units, catalyst withdrawal is through submerged wells (see Figures 1 and 2). The objective is to provide a steady flow of catalyst for precise reaction temperature control. This catalyst should be as deaerated as possible, while maintaining fluidization to provide maximum static head and to minimize wet gas compressor loading with inert carryover. In the case of a slide valve operated unit, the stand pipe should preferably be vertical, with the slide valve located in the bottom section. Switching to a cold wall design may eliminate the need for any expansion joint. The somewhat troublesome "Y" section is advantageously replaced with a "J" bend as shown in Figure 3. The advantage of "U" and "J" bends is that catalyst flow is more smoothly accelerated up to the feed injection zone.
Proper fluidization and aeration taps can greatly improve the control of catalyst circulation over the wide range needed to accommodate changes in feed properties, and operating conditions.
Feed-Catalyst Contacting Zone:
Feed Injection System
The equilibrium temperature between feed and catalyst must be reached in the shortest possible time to ensure a rapid and homogeneous feed vaporization. This results in hot catalyst quenching where, ideally, all the vaporized feed components are now subjected to the same cracking severity. Additionally, the feed injection system should be designed to eliminate catalyst back-mixing. This is important as it reduces the catalyst contact time in the reaction zone, the deleterious effects of nickel contamination, and avoids the recontacting of cooler catalyst particles with fresh feed (which may cause the formation of sticky agglomerates with the risk of coke deposition in downstream low velocity zones).
This is achieved by efficiently mixing the liquid feed, finely atomized in small droplets, into a pre-accelerated dilute suspension of regenerated catalyst. Typically, feed vaporization takes place in a fraction of a second. Table 3 compares the performance of conventional to current state-of-the-art feed injection designs.
Ideally, the feed should be instantaneously vaporized and brought to a mix temperature close to or higher than its dew point.
Mix Temperature Control (MTC)
In most units, the oil feed and catalyst mix temperature is essentially dependent on riser outlet temperature. Raising the riser temperature to increase the mix temperature may not be desirable as it increases the thermal cracking reactions, and negatively affects the desired selectivity to transportation fuels.
Paradoxically, in some cases, maximizing distillate yields by lowering the cracking severity may not even be possible as too high a fraction of the feed does not vaporize, as shown in Figure 4.
To solve this problem and make the above objectives compatible with each other, riser and mix temperature may be independently adjusted. This is achieved through the "Mix Temperature Control" (MTC) technique.
The ability to independently control the mix temperature represents an innovative concept. The catalytic cracking severity, the optimum regeneration temperature and the target catalyst circulation can be separately adjusted. MTC is performed by recycling an appropriate liquid cycle oil cut downstream of the fresh feed injection zone. This can only be effective with an appropriate feed injection system. It separates the riser into two reaction zones:
- an upstream zone, characterized by a high mix temperature, a high catalyst-to-oil ratio and a very short contact time
- a downstream zone, operating under more conventional catalytic cracking conditions.
As shown in Figure 5, the riser temperature is maintained by controlling the regenerated catalyst circulation through the hot slide valve; it is independent from the mix temperature measured in the vaporization zone, the latter being controlled by the amount of liquid recycle. Increasing this recycle results in more catalyst cooling in the reaction zone due to vaporization and cracking. To maintain the desired riser temperature, the catalyst circulation rate is in turn raised by opening the regenerated slide valve. This increases the mix temperature, the catalyst-to-oil ratio, and, by adjusting the boiling range of the recycle stream, the regenerator temperature can be controlled.
Riser and Close Coupled Separator
To minimize any back-mixing in the feed injection zone and vapor over-cracking reactions, the riser should be vertical, at least for the initial length downstream of the feed injection, and should be equipped with an efficient close-coupled separator capable of operating at high velocity without causing attrition to the catalyst.
Since the efficiency of the close-coupled separator is generally better than 95%, single stage high efficiency cyclones may be used downstream. Another decisive advantage of this configuration is the elimination of potential problems usually associated with the dipleg operation of second stage cyclones: if they are sized in accordance with the catalyst traffic, they become too small and prone to bridging, if they are oversized, they do not seal properly and the cyclone system loses efficiency.
Figures 6 and 7 show the change in configuration of a Model IV from bed to riser cracking with MTC, "Y" or "J" bends and an external regenerated catalyst withdrawal well. These modifications reduce gas and coke make while increasing cracking severity. Riser superficial velocities of up to 27 m/s (89 ft/s ) and residence times as low as 0.5 seconds have been successfully attained.
Spent Catalyst Stripping
Stripping is the combination of several actions:
- Displacement and recovery of the hydrocarbon vapors entrained on the catalyst particles and in the void spaces
- Desorption and recovery of higher molecular weight and polar hydrocarbons adsorbed on the catalyst
- Hydrolysis of some sulfur compounds
Steam is used as the preferred stripping medium. Most unit designs can be easily retrofitted to an efficient two-stage counter-current stripper. The first stage is effected in the disengager dilute phase as the ballistic separator disperses the spent catalyst evenly over the cross sectional area. The catalyst falls and is collected into a second stage dense phase stripper prior to flowing to the regenerator. The dense phase stripper is of an annular type either due to the riser configuration or to the installation of a dummy pipe. In this configuration there are no internals, with the exception of the steam ring distributor and an optional coke catcher. High efficiency is offered due to it's high apparent height to diameter ratio. Typical hydrogen on coke can be reduced to 5.5 to 6.5 wt%.
Considerable efforts have been committed for many years to the continuous upgrading of existing FCC units to increase capacity, performance, flexibility and reliability while providing the ability to add substantial amounts of resid into the feed. Design and performance studies for full technology revamps have been performed on almost every conceivable FCC configuration. It is rewarding to note that even the old designs can withstand obsolescence and still be upgraded with the most advanced technologies and thus almost meet the performance of modern designs. In this respect, the revamping of a FCC unit can constitute a challenging task, providing significant improvements in understanding of the process and its technology.