Part 1 of this article, published in September, presented several methodologies for maximizing the production of high-quality diesel in a refinery that relies on fluid catalytic cracking (FCC) as its principal means of heavy oil conversion. Part 2 focuses on the selection of FCC catalysts, methods for hydroprocessing light cycle oil (LCO) from the FCC unit, and the production of diesel fuel from FCC byproducts, among other topics.
FCC catalyst selection
Some catalyst recommendations apply to both high-severity and low-severity FCC operations. Low-hydrogen-transfer FCC catalyst is recommended for maximizing refinery diesel production, as this type of catalyst will generally produce a higher-yield and higher-quality LCO that can be hydroprocessed, while increasing the yield of FCC olefins that can be oligomerized. Similarly, active matrix functionality improves LCO yield and quality.
H2 transfer reactions strip H2 from saturated LCO molecules (such as naphthenes) and transfer it into gasoline boiling-range olefins. The net impact of these H2 transfer reactions is that the LCO becomes more aromatic (lower cetane number and more dense), the gasoline becomes more saturated (lower olefin content and lower octane), naphtha yield increases, and LPG olefin yield declines.
In FCC operations intended to maximize gasoline production, the H2 transfer reactions provide a net benefit due to the increased gasoline volume resulting from the saturation of the gasoline olefins before they catalytically crack into LPG olefins. The negative impact of H2 transfer activity on LPG olefins, and on naphtha yields and naphtha octane, has been widely documented, while the negative impact on LCO quality has been less publicized.
In high-LCO-yield FCC operations where LCO quality, gasoline octane and LPG yield considerations are more important than sheer gasoline volume, H2 transfer reactions are counter-productive. Refer to Table 1 for an example of how the rare-earth content of FCC catalyst can impact FCC yields and product qualities.1
The base catalyst can also be used in combination with a ZSM-5-containing catalyst additive to further preserve the gasoline octane and C3/C4 olefins at low conversion levels. The ZSM-5 additive is applicable to maximizing olefins production from high-severity FCC operations.1, 5 The data in Table 2 provide an example of how a ZSM-5 additive can change the yields and product qualities in a moderate-severity FCC operation.2
In low-severity, high-LCO-yield FCC unit operations, ZSM-5 additives have also been shown to convert higher-boiling FCC products into both gasoline and LPG. Two examples of the impact of ZSM-5 additions in low-severity FCC operations are shown in Table 3. These data show that the cracking of heavier molecules in the low-severity FCC products by the ZSM-5 results in a loss of total cycle oil (302°F698°F) production, along with increases in both 302°F true-boiling-point (TBP) gasoline and LPG production.3
Based on a large sampling of pilot plant product from runs having an average conversion level of 40% and a 0.5-wt% ZSM-5 crystal addition, the average Research Octane Number (RON) changes were as follows:
- Increase of 2.4 numbers for the initial boiling point (IBP) to 302°F gasoline
- Increase of 3.3 numbers for the IBP to 410°F gasoline.
Low-equilibrium catalyst micro-activity testing (MAT) activity is often employed when maximizing LCO production. Active-matrix FCC catalysts are also recommended for LCO maximization, as they enable the cracking of LCO boiling-range aliphatic side chains from high-molecular-weight feed components. In addition to increasing LCO yield, the aliphatic side chains that report to the LCO boiling range improve LCO cetane. The active matrix also contributes to cetane improvements because matrix cracking does not possess the higher H2 transfer characteristic of a zeolite. Refer to Table 4 for representative data concerning the impact of changing the catalyst matrix activity.4
Maximize LCO endpoint
The maximization of LCO endpoint is a common operating strategy for increasing LCO production at the expense of low-value FCC slurry oil. In many FCC operations, concern for coking in the FCC main fractionator bottoms circuit limits the LCO endpoint. A number of FCC operating parameters influence the propensity of the bottoms circuit to suffer coking problems:
- Bottoms circuit temperature
- Bottoms circuit liquid residence time
- Concentration of unconverted paraffins in the slurry oil.
In high-conversion FCC operations, the slurry oil is more aromatic and can be held at higher temperatures and longer residence times without coking. Some of the slurry oil quality data that FCC operators monitor as indicators of coking tendency are gravity and viscosity. The more aromatic slurry oil produced by high-conversion FCC operations will allow the unit to operate with lower API gravities while respecting bottoms viscosity targets selected to avoid fractionator coking.
FCC product considerations
Changes in FCC cracking severity directly impact FCC product yield distribution and qualities. In the FCC pilot plant example presented in Table 5, the VGO is of average quality as an FCC feedstock, and the catalyst is a low-rare-earth catalyst with some matrix activity. The pilot plant runs covered reactor temperatures and conversion levels ranging from low to high, relative to industry norms.
The pilot plant data show the tradeoffs between LCO production and quality, and the production and quality of FCC naphtha. As shown in Table 5, even without adjusting the LCO cutpoints, the LCO yield changes by a factor of almost 2 by adjusting the FCC reaction severity. At the same time, among the runs presented in Table 5, the gravity of the LCO increases by about 11°API as the operating severity is lowered.
Fig. 1 summarizes the positive relationship between increasing LCO production rate and LCO quality, as observed in a larger sampling of the same pilot plant study data. Conversely, Fig. 2 and Fig. 3 show a very direct and negative correlation between LCO yield and FCC naphtha octane. Fig. 2 demonstrates that, irrespective of the indicated FCC reaction temperature, FCC naphtha motor octane will suffer as LCO yield increases. Fig. 3 shows that the negative impact of increasing LCO yield on the olefin-dependent RON can be mitigated to some extent, if a high FCC reaction temperature is maintained.
Fig. 1. Relationship between increasing LCO
production rate and LCO quality.
Fig. 2. Relationship between FCC naphtha
quality (MON) and LCO yield.
Fig. 3. Relationship between FCC naphtha
quality (RON) and LCO yield.
The data in Table 5 also provide examples of how changing FCC reaction severity can impact LPG yield and naphtha octane. Comparing the low-conversion and high-conversion cases, the data show that the low-conversion case produces less than one-half the LPG and 3 to 4 numbers lower octane than the high-conversion case.
Table 5 also provides an example of the degradation of LCO as a potential feedstock for upgrading into diesel as the FCC conversion is increased; the LCO H2 content decreases from 10.7 wt% to 8.8 wt% as the FCC conversion level is increased from 59 wt% to over 76 wt%.
Processes for the upgrading of LCO range from mild hydrodesulfurization to full-conversion hydrocracking. Fig. 4 depicts some of the chemistry responsible for improving the cetane, density and aromatics content of the LCO. For the purposes of this article, three upgrading processes (hydrotreating, aromatics saturation and mild hydrocracking) are described as representative examples of some of the processes being used today.5
Fig. 4. Three reactions to upgrade LCO quality.
LCO hydrotreating. Mild hydrotreating of LCO will reduce its sulfur content significantly, but this will only modestly improve the product qualities related to aromatic content. In examples presented in Table 6, LCO in a 10% concentration, in a mixture including straight-run gas oil (SRGO), is hydrotreated. Two options are presented, with the latter representing a higher degree of desulfurization and greater aromatics reduction. These examples demonstrate that it is possible to include about 10% LCO in the diesel pool by hydrotreating the LCO/SRGO mixture.
Aromatics saturation. To accommodate larger concentrations of LCO in the diesel pool, more complete aromatics saturation and cetane improvement are required. These goals can be achieved through varying degrees of ring saturation and ring opening, as shown in Fig. 4. Table 7 shows what is possible utilizing a two-stage aromatics saturation unit to process 100% LCO.5 The drawback of ring saturation is high H2 consumption.
Mild hydrocracking. Another alternative is to rely on ring opening with mild hydrocracking to move some of the aromatics out of the LCO boiling range into gasoline, as shown in Fig. 5.5 This approach can provide substantive LCO quality improvement with lower H2 consumption. Table 8 provides an example of coprocessing LCO along with straight-run distillate and other cracked products.5
Fig. 5. Hydrocarbon comoponents and cetane
Creating diesel from FCC byproducts
Two processing options with limited application to date are the creation of synthetic diesel from FCC olefins and the extraction of aromatics from FCC naphtha. These options can be integrated into the overall processing scheme, along with the other options described earlier.
Reprocessing of C3C9 olefins into distillate. Olefins can be used to produce good-quality diesel with oligomerization processes. For example, an oligomerization unit distillate yield from a C3C9 olefin feed was reported to be 78% distillate with a byproduct gasoline yield of 19%, based on a zeolite catalyst, as shown in Table 9. After hydrotreating to saturate the olefins, the distillate was reported to have a cetane number of 52 to 54, zero sulfur and less than 2% aromatics.6
Therefore, for FCC-based refineries working to maximize diesel production, oligomerization of olefins-containing FCC light gasoline and LPG may provide viable investment opportunities.
FCC naphtha extraction. Extractive techniques are available for separating a middle boiling fraction of FCC gasoline into a higher-octane, aromatics-rich fraction and an olefins- and paraffin-rich fraction.7 A recently granted patent describes a combined FCC/extraction process wherein an aromatics-rich, higher-octane fraction of FCC gasoline can be produced as a gasoline product, while a paraffinic/olefinic naphtha fraction can be produced for recycle to an FCC riser for the purpose of producing propylene and other olefins.8
This FCC naphtha extraction concept and oligomerization technology can be used together, as shown in Fig. 6, to maximize the production of synthetic diesel from FCC olefins. The combination can be especially useful in the context of a high-LCO-yield, low-severity FCC operation because the low-severity FCC naphtha will have a higher olefins content than the more aromatic, more paraffinic naphtha from a high-severity FCC operation. Thus, the non-aromatic naphtha raffinate from a low-severity FCC operation will make a better-quality oligomerization feedstockor a better-quality FCC recycle streamfor the purpose of increasing lighter FCC olefins production, as olefins are easier to crack than paraffins.
Fig. 6. Production of diesel from FCC LPG and
With all the processing options presented in this article, an obvious question is, How much can the refinery diesel production be increased if many of these options are applied in a retrofit of an existing refinery? The answer depends on the specifics of the application. Table 10 shows estimated results from isolated examples provided in this article, giving insight into the question.
Assuming demand for diesel continues to increase faster than growth in gasoline, a number of reactions can be expected from the refining industry:
- The loss of virgin diesel to the FCC unit will diminish through crude distillation unit improvements
- FCC gasoline endpoint will be minimized
- Hydrocracking and hydrotreating units designed to upgrade LCO quality will proliferate
- Low-H2-transfer, higher-matrix-surface-area FCC catalyst will be used to improve LCO yield and quality, while increasing LPG olefins production and naphtha octane
- In some cases, ZSM-5 catalyst additives will be used to further increase LPG olefins production and octane, but in low-severity FCC operations, this may come at the expense of some LCO yield.
For refiners that also place high value on propylene production, high-octane gasoline, and minimization of refinery bottoms production, the high-severity FCC route to making more diesel will gain favor through the oligomerization of C4 and higher FCC olefins while continuing to hydroprocess the LCO production.
If a refiner has a more singular focus on the production of diesel, the low-severity, traditional FCC route to increasing diesel can be optimized and economically favored, with some enhancements:
- The loss of LCO in slurry oil product or recycle will diminish through the use of dedicated slurry distillation hardware
- Some of the stripped slurry oil may be recycled to the FCC reactor to produce more LCO and help maintain FCC heat balance, while HCO recycle may also be advantageous
- Low-severity FCC operations will rely on increasing feed temperature and, in some cases, direct firing of the regenerator with a liquid or gaseous fuel using technology designed to minimize damage to the catalyst
- FCC-produced LPG and naphtha olefins will be converted into diesel blending stock using oligomerization processes.
An ultimate vision for maximizing diesel production in a specific FCC-centered refinery may also include a selective combination of elements:
- Extraction processes will separate aromatics-rich fractions of FCC gasoline from fractions enriched in olefins and paraffins. The aromatic fraction can be used for BTX production or high-octane motor fuel; the non-aromatic fraction can be recycled to the FCC reactor for the production of more olefins (diesel precursors), or the olefins in the non-aromatic fraction may be directly oligomerized into diesel.
- FCC C4s and FCC light naphtha can be recycled to an ultra-high-severity FCC riser to increase propylene and aromatic naphtha yields, without diminishing LCO production.
A case-by-case analysis based on refinery-specific data is needed to accurately contrast the costs and benefits associated with the application of various options for increasing diesel production from the FCC-centered refinery. The performance of the study requires both refinery-wide and FCC-specific experience and related modeling capabilities. In the final analysis, it is simply a question of economics; technologies are available to maximize diesel production from the FCC-centered refinery.
1 Ritter, R., D. Wallace and J. Maselli, Fluid Cracking Catalyst to Enhance Gasoline Octane, Catalagram 72, W. R. Grace & Co., 1985.
2 Thiel, P., Additive O-HS Davisons Octane/Olefin Enhancing Additive, Catalagram 81, W. R. Grace & Co., 1990.
3 Das, A. K., Y. V. Kumar, V. R. Lenin and S. Ghosh, Performance of ZSM-5 Additive in Distillate FCC Units, Akzo Catalyst Symposium 1991: Fluid Catalytic Cracking, Scheveningen, The Netherlands, June 1991.
4 Silverman, L. D., S. Winkler, J. A. Tiethof and A. Witoshkin, Matrix Effects in Catalytic Cracking, NPRA Annual Meeting, Los Angeles, California, March 1986.
5 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.
6 Kohler, E., M. De Pontes, F. Schmidt and H. J. Wernicke, Converting olefins to dieselthe COD process, Hydrocarbon Technology International, Summer 1995.
7 Gentry, J., and W. Jin, FCC gasoline and C4 streams for BTX production, PTQ, Q4 2009.
8 Subramanian, A. and A. Claude, US Patent No. 7,883,618.
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 manager, technology manager, chief technology engineer of FCC, director of FCC technology, and now process engineering manager. 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.