As suggested in FIG. 3, after a refiner has taken the steps nec-essary to minimize the loss of straight-run diesel to the FCC feedstock, some FCC operating adjustments are commonly applied in the interest of increasing refinery diesel production.
These include the following:
• Lowering FCC naphtha endpoint
• Increasing FCC catalyst matrix activity and lowering rare earth/hydrogen (H2) transfer activity
• Maximizing LCO endpoint
• Hydroprocessing the LCO as required.
Beyond these commonly applied strategies, two divergent options remain for dealing with the diesel situation:
1. Reduce FCC cracking severity to maximize LCO pro-duction, and take action, if needed, to mitigate the associated loss of FCC naphtha octane and LPG production
2. Increase FCC cracking severity to maximize the pro-duction of lower-molecular-weight olefinic products from the FCC unit, and oligomerize these olefins to produce high-qual-ity synthetic diesel.
Can the FCC-based refinery increase diesel production? The answer to this question is “yes.” The more germane question to consider is whether or not the increased diesel production justi-fies the associated investment costs and operating tradeoffs.
Data generated on an FCC pilot plant are presented in TABLE 2 to show how changing the FCC reaction severity can impact FCC yields and product qualities. Three cases are included, all based on the same feedstock and catalyst system. With this as
background, the low-severity and high-severity routes to in-creasing refinery diesel production are contrasted.
Reducing FCC cracking severity. Low-severity FCC op-eration can be considered the traditional avenue for maximiz-ing diesel production from an FCC-centered refinery. As men-tioned in the introduction, the quality and the yield of LCO improves as cracking severity is lowered. At the same time, re-ducing cracking severity will generally cause a loss of both LPG production and FCC naphtha octane. It will also increase the production of low-value slurry oil. There are practical limits to the amount of LCO that can be produced by lowering reaction temperature and catalyst activity because the coke make will become insufficient to heat-balance the FCC unit at a sustain-able regenerator temperature.
On the positive side, reducing FCC severity will not be con-strained by regenerator coke burning or by vapor recovery unit (VRU) capacity. On the other hand, increasing LCO produc-tion increases the burden on other refining units to meet mod-ern diesel fuel specifications by upgrading the LCO.
Recycling slurry oil and using a fired feed furnace.
These operating strategies are commonly employed to increase LCO yield while directionally helping to maintain regenerator temperature. However, with severely hydroprocessed vacuum gas oil (VGO) feedstocks, recycling slurry oil and increasing feed temperature can still be insufficient to maintain adequate regenerator temperature.
Nontraditional tactics can be employed to address the yield, product quality and heat balance issues associated with low-se-verity FCC operations. Two of these tactics are described below:
• Use of a dedicated slurry oil stripping tower to recover in-cremental LCO from the slurry oil produced by the FCC main fractionator and, optionally, to recycle some of the stripped slurry oil to the FCC reactor
• Direct firing of the regenerator with fuel, such as fuel gas or slurry oil, to maintain regenerator temperature.
Slurry oil stripping tower. The fractionation between LCO and slurry oil in the bottom of an FCC main fractionator is very coarse because the reactor products feed the fractionator through the bottom of the tower, where the slurry oil product is withdrawn, and also because there are few fractionation trays be-tween the slurry product and the LCO product draws. There is, at most, a one-stage flash available to separate the slurry oil from its equilibrium with the rest of the FCC reactor product stream. For example, FIG. 4 presents simulated true boiling point distillations from an FCC main fractionator producing LCO, heavy cycle oil (HCO) and slurry oil. In this example, 36% of the FCC slurry
Low-severity FCC strategies
FIG. 3. Strategies for FCC diesel maximization.
15
FIG. 2. Results from typical LCO inspections at US refineries.
200300
Cumulative FCC product distillation, vol%
True boiling point,°F Gasoline
Slurry
LCO HCO
FIG. 4. FCC product distillation example.
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Refining Developments
oil and 50% of the HCO boils below the LCO product endpoint.
It is ironic that, in maximum LCO operations, the amount of LCO lost in the slurry oil increases significantly because of the higher volume of slurry oil produced. Another fundamental violation of the maximizing LCO objective is that the recycle of a typical slurry oil also carries LCO-boiling-range material back into the reactor, where the quality will be further degraded and some will be cracked into a non-LCO-boiling-range material.
Based on the above considerations, it is apparent that, to truly maximize the production of LCO from the FCC unit, a sharp fractionation between LCO and heavier liquid products must be achieved. A feature used to enhance this separation is the use of a dedicated LCO/slurry fractionation tower to re-cover LCO that would otherwise be lost in the slurry oil. In a traditional maximum-gasoline FCC operation, downstream recovery of LCO from slurry is not normally economic because
of the relatively low slurry oil yield. However, in a maximum-LCO operation where the slurry production is higher and the LCO is more valuable, the additional fractionation tower may be economically viable.
The LCO/slurry fractionation tower can be a steam strip-per or a tower ostrip-perated under vacuum to achieve maximum LCO recovery. In addition to the prevention of the direct loss of LCO with the slurry product and the loss of LCO through its recycle to the reactor, the slurry oil fractionation tower pro-vides a slurry oil that is a more effective recycle stream for sup-porting the FCC heat balance, due to its higher boiling range and higher Conradson Carbon Residue (CCR) content.
HCO recycle. In low-severity FCC operations where maintaining adequate regenerator temperature is not an is-sue (such as may be the case when processing residue), HCO may be preferred over slurry oil as a recycle stream, due to its
TABLE 2. FCC pilot plant data showing the impact of changing operating severity
Low conversion Medium conversion High conversion FCC feed properties
Gravity, °API 22.5 22.5 22.5
50 vol% boiling point, °F 851 851 851
Aniline point, °F 176 176 176
Sulfur, wt% 0.55 0.55 0.55
CCR, wt% 0.89 0.89 0.89
FCC pilot plant operating conditions
Riser temperature, °F 940 979 1,020
Feed temperature, °F 416 485 337
Catalyst-to-oil ratio, wt/wt 6.6 6.7 11.4
Micro Activity Test (MAT) 67 67 67
Rare-earth oxides, wt% (FCC E-Cat property) 0.6 0.6 0.6
FCC pilot plant yields
Dry gas, wt% 1.23 2.08 3.5
C3 LPG, wt% 2.97 4.26 7.27
C4 LPG, wt% 5.98 7.88 11.57
Gasoline (C5 at 430°F), wt% 43.21 46.98 46
LCO (430°F–680°F), wt% 27.42 24.47 16.01
Slurry oil (680°F+), wt% 13.6 9.06 7.66
Coke, wt% 5.59 5.27 7.99
Conversion, wt% 58.98 66.47 76.33
FCC pilot plant product qualities
C3 LPG olefi nicity, wt% 83.8 83.8 85.7
C4 LPG olefi nicity, wt% 66.7 68.5 67
Naphtha gravity, °API 56.6 57.2 55.9
Naphtha octane, RON/MON 91.7/81.1 92.9/81.6 95.6/84.4
Naphtha P/O/N/A, wt% 27.2/49.5/11.8/11.5 25.7/49.1/10.9/14.3 31.3/36.8/10.5/21.4
LCO gravity, °API 22.2 17 11.3
LCO H2 content, wt% 10.7 9.9 8.8
Slurry oil gravity, °API 6 –0.8 –7.4
Slurry oil H2 content, wt% 9 7.8 6.7
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Refining Developments
very low carbon residue content and higher H2 content.4 Ide-ally, the HCO would also have its LCO-boiling-range material distilled before recycling it, but the economic practicality of redistilling the HCO can be questioned if this requires yet an-other cycle oil fractionator.
Direct firing of regenerator with fuel. The continuous direct firing of the regenerator with fuel can be essential to the operation of a maximum LCO FCC operation when process-ing non-residue-containprocess-ing FCC feedstocks.
Continuous firing of the regenerator air heater has been utilized for heat balance support, but this practice can have an adverse impact on the velocities through the regenerator air distributors and on the practical issues associated with moni-toring the heater firing.
Continuous firing of torch oil, which is normally only used during startup, has been practiced. However, this has reportedly been the cause of accelerated catalyst attrition and deactivation.
One company has developed a system for distributing liquid fuel in the regenerator.5,6 The system is designed to mitigate the cata-lyst damage associated with conventional torch-oil firing. This technology has been adapted for use in conventional FCC units.
A patent-pending version is also available for use in conventional FCC operations. In addition to this system for liquid fuels, a system for firing the regenerator with fuel gas, which is often a lower-cost fuel, has been commercialized.
Increasing FCC cracking severity. Increasing cracking sever-ity reduces LCO yield and provides the immediate impact of hav-ing less LCO to blend into the diesel pool. This can be a net bene-fit to the diesel blending operation, even though the quality of the LCO is diminished. At the same time, the increased LPG olefins can be oligomerized to produce high-quality synthetic diesel.
Increasing cracking severity can be achieved by raising reactor temperature and/or boosting catalyst activity. However, unless FCC capacity is reduced, increasing FCC severity may be con-strained by regenerator coke burning or by VRU capacity. Even with adequate coke- and gas-handling capacity, increasing regen-erator temperature can pose a limitation to the severity increase.
The use of slurry recycle in high-conversion FCC opera-tions is usually counter-productive because it only exacerbates the coke-burning and regenerator-temperature limitations.
Furthermore, referring back to TABLE 2, the slurry oil from high-conversion FCC operations is H2-deficient and has little to offer in terms of potential cracking yield. Beyond simply in-creasing reaction temperature and catalyst activity, these three hardware-related upgrades warrant mention for their assis-tance in high-conversion FCC operations:
• Applying an advanced riser termination system to mini-mize dry gas and coke production at the increased severity
• Applying regenerator catalyst cooling to control the heat balance at the increased severity
• Recycling FCC C4s and FCC light naphtha to an ultra-high-severity FCC riser for the purpose of producing incre-mental C3/C4 olefins and aromatic, high-octane naphtha.
These options are further discussed below.
Riser termination system. An advanced riser termination system can minimize product vapor residence time between the riser outlet and the main fractionator, thereby reducing the formation of incremental dry gas from post-riser thermal crack-ing. In addition to the reduction in dry gas, the riser termination system reduces delta coke on units that previously employed low-catalyst-separation-efficiency riser termination devices.
Therefore, the system is especially appropriate for use when increasing FCC operating severity because it simultaneously relieves VRU capacity and regenerator operating temperature constraints. The advanced riser termination system also in-creases LCO production by minimizing the thermal condensa-tion reaccondensa-tions that create slurry oil from LCO-range material.7
Catalyst cooling. In an unconstrained environment, increas-ing FCC reactor temperature is easy. However, in most cases, FCC units are already operating against several physical and eco-nomical constraints. High regenerator temperature can emerge as a major constraint to increasing reactor temperature because of the impact of the higher temperature on the unit heat balance.
FCC operators can implement a reduction in equilibrium cata-lyst activity to mitigate the increasing regenerator temperature,
200100
Cumulative FCC product distillation, vol%
True boiling point, °F
LCO Gasoline or LCO
Gasoline
FIG. 5. FCC liquid product distribution example.
77
Gasoline final boiling point, °C
MON RON
Octane number
FIG. 6. FCC gasoline octane examples.
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Refining Developments
but reducing catalyst activity runs counter to the more basic objective of increasing reaction severity. In high-severity FCC operations, the catalyst cooler can maintain the regenerator tem-perature at the optimum value, which increases olefins produc-tion. In a recent study, the addition of a catalyst cooler to a re-generator-temperature-constrained, high-olefins FCC operation enabled a 25+% increase in the unit’s propylene production.8
C4 and light FCC naphtha recycle. The recycle of C4 LPG and light FCC naphtha for the purpose of producing propylene and higher-octane FCC gasoline fits in well here because it can achieve the goals of increasing propylene yield and naphtha octane without destroying LCO. Ultimately, the application of such catalyst and hardware technology can push propylene yields into a range of 10 wt% to 20 wt% or more.6
The recycle of light naphtha to a high-severity second riser can be practiced in both high-severity and low-severity primary riser operations to improve octane and produce additional LPG olefins without sacrificing LCO yield or quality. There is a syner-gy between the low-severity primary riser operation and a high-severity light feed recycle riser because the naphtha from the low-severity primary riser is more olefinic, making this primary riser product better feedstock for the high-severity second riser.
Commonalities in basic process strategy. Even as the chosen strategy drives the refinery down a selected avenue of either a high- or low-conversion FCC operation, there will be some commonalities among the two strategies.
FCC fractionator cutpoint adjustment. Adjusting the FCC naphtha endpoint would be considered standard practice in most refineries for making seasonal adjustments for swings in gasoline vs. distillate demand. Reducing the endpoint of the FCC naphtha product shifts heavy naphtha into the LCO product. The limitation to the adjustment can be gasoline oc-tane, the flashpoint specification of the LCO product, or pool-cetane considerations. Another possible limitation is the mini-mum FCC main fractionator overhead temperature, which can be practiced without condensing water and fouling or corrod-ing the top of the main fractionator or its overhead system.9 Typically, the FCC naphtha ASTM D86 endpoint would not be reduced to less than 300°F to stay above a minimum accept-able main fractionator overhead temperature.
FIG. 5 provides a typical example of how changes in the gaso-line endpoint impact the naphtha yield and the LCO yield by implication. In addition to changes in the FCC naphtha and LCO yields due to the cutpoint adjustments, there will be changes in the product distillations, gravities, octanes, sulfur contents and cetane values.
Due to the wide variation in heavy FCC naphtha molecu-lar composition from one FCC operation to the next, a rule of thumb is not provided for the impact of the cutpoint adjust-ments on octane, sulfur content or cetane. These effects are best taken from empirical observations on the operating unit.
As an example of the variability of product property trends with cutpoint, FIG. 6 shows the impact of the FCC gasoline
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