Global product trends favor increasing production of diesel over gasoline (Fig. 1).1 Consequently, many new refineries have utilized hydrocracking, rather than fluid catalytic cracking (FCC), as the main conversion unit due to the hydrocrackers higher diesel yield and superior diesel quality.
| Fig. 1. Projected gasoline and diesel |
demand to 2025.
For refineries that have already invested in an FCC unit as the main conversion vehicle, the question becomes, How can existing refinery assets be used to economically increase diesel production? The question is challenging because light cycle oil (LCO) from FCC operations has limited value as a component in modern diesel transportation fuel due to its aromatic, sulfurous character. Furthermore, quality virgin distillate included in the FCC feedstock is essentially destroyed during FCC processing.
This article presents methodologies for maximizing the production of high-quality diesel in a refinery that relies on FCC as its principal means of heavy oil conversion. Such methodologies include:
- Preserving straight-run distillates for use in diesel blending by preventing its loss to FCC feed
- Increasing yield, quality and recovery of LCO
- Upgrading LCO quality through hydrotreating or hydrocracking
- Preserving FCC naphtha octane and liquefied petroleum gas (LPG) during maximum LCO FCC operations
- Producing synthetic diesel through the oligomerization of lighter olefinic FCC products.
Improving LCO yield and quality
FCC units produce a significant quantity of high-sulfur, low-cetane-number aromatic distillatei.e., LCO. Modern diesel quality specifications dictate that LCO must be upgraded in hydrotreating or hydrocracking units to make it an attractive diesel blending component. The disconnect between LCO quality and the specifications demanded for modern automotive diesel can be seen in Table 1, which compares the quality of a typical LCO with increasingly stringent diesel specifications.2
Directionally, the yield and quality of the LCO can be improved by lowering FCC conversion and adjusting the FCC catalyst formulation, but the improvement in LCO quality is not sufficient for the LCO to be considered a desirable diesel blending component.
Fig. 2 illustrates US refinery LCO samples from the years 1967 and 1982. The decline in LCO quality over the period would, in part, be the natural result of increasing FCC operating severity and the completion of the industrys changeover from amorphous catalyst to highly rare-earth-exchanged zeolite catalyst targeting increased gasoline production.3 These data are included to provide perspective on the range of LCO qualities that have been produced from FCC operations. The reader can see that, while some of the samples have much better diesel qualities than others, they all fall well short of modern specifications.
| Fig. 2. Results from typical LCO inspections |
at US refineries.
Strategies for maximizing diesel production
What can be done to maximize FCC-based refinery diesel production while taking advantage of an existing FCC asset? The simple answer, depicted in Fig. 3, is to avoid the loss of virgin distillate to the FCC feedstock and to maximize the production of hydroprocessed LCO and diesel synthesized from the oligomerization of lower-boiling FCC olefins. The following sections of this article explore how this is accomplished.
As suggested in Fig. 3, after a refiner has taken the steps necessary 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.
| Fig. 3. Strategies for FCC diesel maximization.|
Beyond these commonly applied strategies, two divergent options remain for dealing with the diesel situation:
- Reduce FCC cracking severity to maximize LCO production, and take action, if needed, to mitigate the associated loss of FCC naphtha octane and LPG production
- Increase FCC cracking severity to maximize the production of lower-molecular-weight olefinic products from the FCC unit, and oligomerize these olefins to produce high-quality 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 justifies 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 increasing refinery diesel production are contrasted.
Reducing FCC cracking severity. Low-severity FCC operation can be considered the traditional avenue for maximizing diesel production from an FCC-centered refinery. As mentioned in the introduction, the quality and the yield of LCO improves as cracking severity is lowered. At the same time, reducing 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 sustainable regenerator temperature.
On the positive side, reducing FCC severity will not be constrained by regenerator coke burning or by vapor recovery unit (VRU) capacity. On the other hand, increasing LCO production increases the burden on other refining units to meet modern 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-severity FCC operations. Two of these tactics are described below:
- Use of a dedicated slurry oil stripping tower to recover incremental 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 between 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 oil and 50% of the HCO boils below the LCO product endpoint.
| Fig. 4. FCC product distillation example.|
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 recover 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 stripper or a tower operated 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 provides a slurry oil that is a more effective recycle stream for supporting 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 issue (such as may be the case when processing residue), HCO may be preferred over slurry oil as a recycle stream, due to its very low carbon residue content and higher H2 content.4 Ideally, 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 another 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 processing non-residue-containing 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 monitoring 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 catalyst 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 severity reduces LCO yield and provides the immediate impact of having less LCO to blend into the diesel pool. This can be a net benefit 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 constrained by regenerator coke burning or by VRU capacity. Even with adequate coke- and gas-handling capacity, increasing regenerator temperature can pose a limitation to the severity increase.
The use of slurry recycle in high-conversion FCC operations 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 increasing reaction temperature and catalyst activity, these three hardware-related upgrades warrant mention for their assistance in high-conversion FCC operations:
- Applying an advanced riser termination system to minimize 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 incremental 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 cracking. 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 increases LCO production by minimizing the thermal condensation reactions that create slurry oil from LCO-range material.7
Catalyst cooling. In an unconstrained environment, increasing FCC reactor temperature is easy. However, in most cases, FCC units are already operating against several physical and economical 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 catalyst activity to mitigate the increasing regenerator temperature, 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 temperature at the optimum value, which increases olefins production. In a recent study, the addition of a catalyst cooler to a regenerator-temperature-constrained, high-olefins FCC operation enabled a 25+% increase in the units 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 synergy 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 octane, the flashpoint specification of the LCO product, or pool-cetane considerations. Another possible limitation is the minimum FCC main fractionator overhead temperature, which can be practiced without condensing water and fouling or corroding 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 acceptable main fractionator overhead temperature.
Fig. 5 provides a typical example of how changes in the gasoline 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.
| Fig. 5. FCC liquid product distribution example.|
Due to the wide variation in heavy FCC naphtha molecular composition from one FCC operation to the next, a rule of thumb is not provided for the impact of the cutpoint adjustments 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 endpoint on octane, calculated from narrow-boiling-range octane data for three different FCC situations operating with a variety of feedstocks, catalysts and operating conditions.10
| Fig. 6. FCC gasoline octane examples.|
In many cases, seasonal demand swings are accommodated with changes in the FCC gasoline cutpoint, with no change to the true (430°F) FCC conversion level, as this strategy works to preserve the LPG production, octane and total liquid volume associated with the higher-conversion operations.
Crude distillation. Another common practice is maximizing diesel production from the crude distillation processes so that losses of potential diesel to the FCC feed are minimized. There are intermediate swing cuts from some crude distillation operations that can be routed to the FCC unit when gasoline is demanded, and routed to diesel production when the objective is maximizing diesel. As a side benefit, keeping the diesel out of the FCC feed also improves FCC gasoline octane.11
Pilot plant data have shown that, in moderate or high-severity FCC operations, most of the straight-run diesel will be converted to gasoline and lighter products with only 20%30% leaving the FCC in the LCO product. The data have also shown that the LCO made from the distillate will have cetane values 10 to 15 numbers below that of the distillate feed, but still higher than that of typical FCC LCO.
Beyond standard operating adjustments, there may be investment opportunities in crude distillation hardware that can achieve a sharper separation between the diesel product and FCC feed streams, reducing the loss of potential diesel to the FCC feed.
A survey of over 100 refineries indicated that FCC feed typically contains between 10 vol% and 15 vol% of material, mostly diesel, boiling below 650°F.12 In environments where gasoline production is maximized, the loss of diesel to the FCC unit has little negative impact. However, if the objective is diesel maximization, better crude fractionation efficiency between diesel and FCC feed can be economically justified.
There are a number of ways to reduce the loss of virgin diesel to the FCC feed.13 Some of these options are listed below:
- Revamp the atmospheric distillation column to increase the degree of fractionation between the diesel and atmospheric gasoil products
- Revamp the vacuum column to produce a diesel product
- Add a gasoil tower or a vacuum preflash tower between the atmospheric and vacuum distillation columns, and recover diesel from the vacuum tower feedstock
- Add a splitter column to process the light vacuum gasoil (LGVO) and produce a diesel stream.
Table 3 shows examples of calculated incremental diesel production that were reported for some of these options.13 The best options for a given refinery are a function of the site specifics of the application, but the data in Table 3 indicate the magnitude of diesel production increases that are possible.
Part 2 of this article, to be published in October, will explore the selection of FCC catalysts, methods for hydroprocessing LCO, and the production of diesel fuel from FCC byproducts, among other topics.
1 Eskew, B., The Diesel Challenge and Other Issues Facing US Refiners, NPRA Q&A and Technology Forum, Champions Gate, Florida, October 2008.
2 Flinn, N. and S. P. Torrisi Jr., LCO Upgrading Options: From Simple to Progressive Solutions, Russia and CIS Refining Technology Conference and Exhibition, Moscow, Russia, September 2008.
3 Unzelman, G. H., Potential Impact of Cracking on Diesel Fuel Quality, Katalistiks Fourth Annual Fluid Cat Cracking Symposium, Amsterdam, The Netherlands, May 1983.
4 Hunt, D., R. Hu, H. Ma, L. Langan and W.-C. Cheng, Recycle Strategies and MIDAS-300® for Maximizing FCC Light Cycle Oil, Catalagram 105, W. R. Grace & Co., Spring 2009.
5 Peterson, R. B., C. Santner and M. Tallman, US Patent No. 7,153,479.
6 Gilbert, M. F., M. J. Tallman, W. C. Petterson and P. K. Niccum, Light Olefin Production from SUPERFLEXSM and MAXOFINTM FCC Technologies, ARTC Petrochemical Conference, Malaysia, February 2001.
7 Miller, R., T. Johnson, C. Santner, A. Avidan and D. Johnson, FCC Reactor Product-Catalyst SeparationTen Years of Commercial Experience with Closed Cyclones, NPRA Annual Meeting, San Francisco, California, March 1995.
8 Pillai, R. and P. K. Niccum, FCC Catalyst Coolers Open Window to Increased Propylene, Grace Davison FCC Conference, Munich, Germany, September 2011.
9 Melin, M., C. Baillie and G. McElhiney, Salt Deposition in FCC Gas Concentration Units, Catalagram 107, W. R. Grace & Co., 2010.
10 Akbar, M., B. Claverin, M. Borley and H. Otto, Some Experiences with FCC Octane Enhancement, Ketjen Catalysts Symposium, Scheveningen, The Netherlands, May 1986.
11 Fletcher, R., Meeting Transcript, 1997 NPRA Q&A Session: Refining and Petrochemical Technology, New Orleans, Louisiana, October 1997.
12 Sloley, A. W., FCC Network News, Refinery Process Services Inc., Vol. 35, January 2010.
13 Sloley, A. W., Increase diesel recovery, Hydrocarbon Processing, June 2008.
|The author |
Phillip Niccum joined KBR Inc.s fluid catalytic cracking (FCC) team in 1989 following nine years of FCC-related work for a major oil company. Since that time, he has held various FCC-related positions at KBR Inc., including process engineering manager, technology manager, chief technology engineer of FCC, and now director of FCC technology. Mr. Niccums professional activities have included engineering management, process engineering, project engineering, marketing, and licensing. Areas of technical strength include FCC unit design, precommissioning and startup, troubleshooting and economic optimization of FCC unit operations.