June 2017

Catalysts

Raising the propylene bar: Increase FCCU profitability in dynamic market conditions

Today’s refining strategies rely on a constant optimization of product slate to comply with market demand and economic drivers, refining/petrochemical integration, product specifications, environmental regulations and competition.

Today’s refining strategies rely on a constant optimization of product slate to comply with market demand and economic drivers, refining/petrochemical integration, product specifications, environmental regulations and competition. Propylene is a key product in Europe, the Middle East, Africa (EMEA) and Asia, whereas many refineries in the Americas produce butylenes at maximum butylene-to-propylene ratios.

Fluid catalytic cracking units (FCCUs) are at the heart of refineries, and tailored FCC catalysts contribute to improve refiners’ flexibility to adapt to market drivers. The economic drivers for liquefied petroleum gas (LPG) olefins, and how refiners can improve their profitability and leverage the optimal propylene and butylene yields from their FCCUs by using customized FCC catalyst formulations, are highlighted here. The catalyst features that promote residue conversion, metal tolerance and bottoms upgrading to drive refinery profitability are discussed, and a case study describing a commercial application of resid FCCU to maximum propylene is presented.

Economic drivers for LPG olefins from the refinery

Global propylene demand has historically trended upward, with a relatively stronger increase forecasted in growth regions (FIG. 1). Demand is driven by petrochemicals, including plastics, and other derivative products. Propylene can also be fed into an alkylation unit, and incremental yields from the FCCU can be monetized as higher gasoline volumes and/or higher gasoline octane. Global propylene capacity has increased—from 94 MMt in 2010 to approximately 120 MMt to date—and is projected to continue to rise. To satisfy this increasing appetite for propylene, additional capacity is needed to complement existing production. New projects, based on propane dehydrogenation units (PDH), methanol-to-olefins (MTO) from gas and coal, olefin cracking and metathesis are being proposed. For some markets, that gap presents an opportunity for refineries with FCCUs that are integrated with a chemical complex, or that have the ability to export propylene.

Regional disparities in propylene demand and mapping propylene imports with corresponding forecasts are shown in FIG. 2, which highlights Asia, Africa and Latin America as net importers. Propylene remains a key product, and optimizing its production contributes to refinery profitability.

Within a refinery, the FCCU is the main light olefins producer, and is well-positioned to monetize its inherent olefin selectivity by maximizing the feedstock for the alkylation unit and other downstream processes. Propylene yields can be increased by 3 wt%–5 wt% in conventional FCCUs, and 12 wt%–30 wt% in high-severity FCCUs,¹ including the HSFCC process. Optimizing propylene yield contributes to closing the supply gap caused by a global increase in demand. Market volatility requires FCCU operators to leverage operating flexibility to optimize product yields. Maximum unit profitability relies upon the constant optimization of the value of FCC products through high-performance catalytic solutions.

Relative propylene yields of equilibrium catalysts (Ecats) of different units tested under the same conditions and feedstock are presented in FIG. 3. Actual unit yields vary due to differences in severity and feedstock. Propylene yields obtained from FCCUs also vary by region: in Asia-Pacific, and to a lesser extent in the EMEA regions, propylene demand is high, and maximizing propylene is a primary asset for numerous refineries. Propylene yields exceed 9 wt% for several units. In the Americas, propylene yields are less than 8.5 wt%. Additionally, yields in the maximum propylene units tend to increase over time, as a reflection of catalyst technology improvement.

To maximize LPG olefins, refiners can consider optimizing the unit operating conditions, primarily the reactor outlet temperature. It is well understood that the higher the severity, the higher the olefins yield. However, most units are unable to increase temperature to the desired level, due to gas compressor limitations, downstream limitations or heat balance constraints. If the refinery is processing new feed sources, unusual or unexpected feed contaminants may also inhibit the ability to raise unit severity. Most units do not have the operational flexibility to achieve the desired targets without violating some unit constraints. By optimizing FCC catalyst formulation, the catalyst supplier can effectively alleviate constraints by reducing the yield of undesirable products. Fine-tuned catalyst formulations are necessary to expand the FCCU operating window.

Catalyst design—promoting higher LPG olefins yield

The FCCU’s flexibility to adapt to propylene and butylene market dynamics strongly depends on FCC catalyst formulations. To maximize LPG olefins, the FCC catalyst supplier can focus its formulations offered to the refinery in four ways:

• Adjust unit zeolite acidity and acid site strength to increase olefins. The optimization of rare earth exchange and zeolite stabilization constitutes the best approach to minimize hydrogen (H₂) transfer activity and promote isomerization reactions, while providing the required conversion. The catalyst unit cell size (UCS) can be utilized to monitor the corresponding catalyst fine-tuning.
• Optimize coke selectivity to release a constraint on coke combustion, resulting in a lower regenerator temperature or reduced air consumption. Improved coke selectivity enables the refinery to either increase the catalyst-to-oil (C/O) ratio or the operating severity.
• Reduce dry gas, specifically H₂ production, to create room against a compressor constraint. A reduction in dry gas, achieved with the application of integral metals traps, is particularly critical for residue processing units.
• Regulate matrix activity and selectivity to more effectively convert the bottom of the barrel. A high-diffusivity matrix provides excellent coke selectivity, metals tolerance and increased olefin yields. Feed molecules can readily enter the catalyst, and cracked products can easily diffuse out before undergoing H₂ transfer to paraffins or overcracking to coke and gas. While the zeolite-to-matrix ratio plays a role, the intrinsic activity, porosity, selectivity and composition of the matrix itself are crucial to catalyst and FCCU performance, particularly for residue-to-propylene operations.

Most refineries reformulate FCC catalyst as market conditions shift. With tailored catalyst formulations offered by catalyst suppliers,² refiners can widen the window of comfortable operation and choose how to best utilize the flexibility—either with severity or increased processing of residue. Most technology suppliers fully vet new catalyst innovations prior to commercialization using a rigorous stage-gate process that first qualifies and verifies performance on the pilot scale, and then with appropriate scale-up to commercial scale, presenting a lower risk optimization plan vs. only moving into an in-unit trial.

In addition, additives containing ZSM-5 crystals can preferentially increase LPG olefins at the expense of gasoline yield;³ and the resulting gasoline will have a higher octane number. At low levels of additive in the FCCU circulating inventory, both propylene and butylene yields increase. However, at higher levels of ZSM-5 additive in the inventory, the propylene yield increases preferentially due to the depletion of higher-carbon-number olefins in the riser. Utilizing customized FCC catalyst formulations with optimized UCS, rare-earth levels and specific matrix activity, in combination with a separate ZSM-5 additive, can achieve step-out, in-unit performance. These tailored catalytic solutions provide additional flexibility and allow refiners to adapt to seasonal and dynamic shifts in propylene and light olefins demand.

Resid processing to maximum LPG olefins

Despite the recent narrowing of the price differential between light and heavy crudes, the long-term market forecast is for continued reliance on heavier crude processing. This is particularly true in the Asia-Pacific region, where fuel supply imbalances act as a key driver for FCC products and refinery operating strategies. Multiple refineries are processing heavy crudes and residue feedstocks in their FCCUs to produce gasoline, distillates and light olefins, or propylene and aromatics for the petrochemical industry. With feed characteristics and gravity, it is more difficult for these refiners to achieve 10 wt% propylene yields with low API feeds.

Due to peculiar feed characteristics (high Concarbon, asphaltenes content, contaminant metals), high-intrinsic coke translates into high delta coke and an increase in regenerator temperature. In turn, higher regenerator temperatures lead to a decreased C/O ratio and conversion. Technology advances, such as catalyst coolers, help to lower regenerator temperature and maintain a high C/O ratio. Technology and hardware—such as cat coolers, two-staged regenerators and internals, feed injectors, etc., as well as optimized unit operations and propane-propylene super fractionation—have also contributed to improvements in residue processing. In spite of these hardware improvements, FCC catalyst plays a crucial role in determining product selectivity and unit profitability when processing residue.

Metals from the resid feedstock deposit on the FCC catalyst and participate in dehydrogenation reactions, potentially further increasing regenerator temperature. Higher catalyst additions are required to maintain activity in the presence of metals, such as sodium (Na) and vanadium (V). Some resid feeds contain high iron (Fe) content, which results in catalyst deactivation and can greatly reduce the bottoms cracking activity of the catalyst and influence FCCU fluidization properties. Depending on the catalyst resistance to iron poisoning and its specific textural properties—in terms of porosity, surface area and alumina content—the fresh catalyst addition rate can increase, or flushing Ecat may be necessary, which induces additional operating expense.

What are the key catalyst attributes, and what challenges do they face with resid FCC-to-propylene units?

Regardless of the desired product slate, a superior resid cracking catalyst is built upon a foundation of:

• Metals tolerance and stable activity
• Coke selectivity
• Bottom-of-the-barrel conversion.

When targeting maximum propylene yield, additional requirements for the cracking catalyst include:

• The catalyst must achieve high conversion, preferably beyond maximum gasoline into the “over-cracking” range, via tailored base catalyst activity and high reactor severity. The desirable operation mode for maximum propylene deals with higher reactor temperature and higher catalyst circulation, or C/O ratio. This operating mode requires an FCC catalyst with the optimum coke selectivity.
• The zeolite UCS must be optimized to promote the formation of olefins and limit H₂ transfer.
• The ZSM-5 additive must be highly stable, with high propylene activity and selectivity.^3,4
• The ZSM-5 additive level must be optimized in the circulating inventory to the desired propylene and gasoline production.
• Optimized porosity must be included in the catalyst design to balance contaminant metals tolerance, allow the diffusion of large-feed molecules to active sites, and minimize H₂ transfer by enabling easy egress of the olefinic cracking products from the catalyst particle.
• The catalyst must possess the appropriate physical properties to guarantee smooth catalyst circulation, even in the case of poisoning, and exhibit high physical integrity of the particles.

For resid-to-propylene operations, a tailor-made catalyst with finely-tuned chemical and physical properties results in additional light olefins yield, and maximizes value for the refinery.

Nickel (Ni) present in resid feed deposits on the catalyst exterior surface, and Ni mobility throughout the catalyst particle is unit- and catalyst-age dependent. Ni strongly contributes to increasing coke and H₂ production. Antimony (Sb) injection does passivate Ni, but resid catalysts can also be formulated with specialty aluminas designed for Ni trapping. With these incorporated Ni traps, FCCU operators can often minimize the use of antimony injection, and reduce chemical costs and slurry heat exchanger fouling.

V contained in residue feeds deposits onto, and migrates throughout, catalyst particles. V not only promotes dehydrogenation reactions and coke formation, but is mobile under FCC regenerator conditions. As vanadic acid (H3VO₄), V can migrate across catalyst particles and combine with sodium, resulting in a detrimental effect on the zeolite surface area. The impacts of V and Na are enhanced at high temperatures when exposed for an extended period of time in an oxygen (O₂)-rich environment. So, two-staged regenerators characterized by a first combustion step under a partial burn mode at lower O₂ concentrations and lower temperatures limit the negative impacts of V and Na.

Similarly, for Ni passivation, resid FCC catalysts can incorporate a V trap functionality. The most effective V traps are integral rare-earth traps that are finely dispersed in each catalyst particle. Unlike other trapping approaches, such as magnesium (Mg), barium (Ba) and calcium (Ca) compounds, rare-earth traps do not suffer from the competitive presence of sulfur, and do not form silicates or sulfates. This integral approach can be effective in both partial and full-burn regenerators, as the trapping efficiency is less dependent on V mobility. Integral rare-earth V traps have been proven to improve catalyst activity retention and coke selectivity when cracking residue.

The design of catalyst porosity is the main lever to improve the bottoms upgrading capability and the resistance to poisons, such as Fe and Ca. An open-pore structure and connectivity with an optimal pore size distribution and tuned matrix activity deliver a premium bottoms cracking level toward distillates. A catalyst design with a higher catalyst mesoporosity in the 100 Å–600 Å range⁵ can improve coke selectivity by enabling pre-cracking reactions, facilitating feed vaporization, and by converting coke precursors into liquid products. The intrinsic activity of the matrix, as well as its composition and porosity, are critical capabilities for selective bottoms upgrading.⁶

Using advanced catalytic evaluation (ACE) testing with a fixed fluidized bed catalytic Ecat benchmarking of the Asia-Pacific market (FIG. 4)—where most maximum propylene applications are located—shows two distinct areas with propylene yields above 8 wt%: one for light feed applications (low metals on Ecat), and the other for resid applications (high metals, above 4,000 wppm Ni equivalents). This trend, illustrated for the sake of clarity over the 2014–2016 time period, is confirmed for a 5-yr period. High propylene yields of ≥10 wt% can be achieved with resid feeds with the use of highly selective and active catalysts. The economic drivers for bottoms upgrading are evidenced by the chart of LCO/bottoms in FIG. 4b. The data for resid-to-propylene units does not deviate from this trend. Deeper conversion is required to reduce slurry production. The economic incentives to convert deep into the bottom of the barrel are mostly the result of a limited market outlet for slurry and solids specifications.

Similar to propylene, the demand for butylene is relatively higher in Asia-Pacific and for several units in EMEA. Higher butylene yields in many Asia-Pacific FCCUs are a result of high ZSM-5 additive usage to maximize propylene. Higher butylene yields are profitable for some units, but for those that do not favor butylenes, the catalyst manufacturer must make adjustments to minimize the C₄⁼/C₃⁼ ratio while maintaining maximum propylene yields. Despite the high demand for gasoline and octane, butylene yields in the Americas appear to be lower than other regions (FIG. 5). However, in the Americas, FCCUs are targeting to maximize butylenes without a corresponding increase in propylene, given the low value of refinery-grade propylene in the region.

Most of the maximum LPG olefins applications are in the Asia-Pacific and EMEA regions. Globally, a strong correlation exists between butylene and propylene yields (FIG. 6). As previously discussed, many refineries in the Americas are achieving maximum butylenes yields relative to propylene—as an optimal ratio of butylene-to-propylene—as much as 2 wt% lower than operations in other regions. FIG. 7 demonstrates the necessity to upgrade bottoms to drive FCC profitability for maximum butylene applications in Asia-Pacific. The light cycle oil (LCO)-to-bottoms ratio does not depend on butylene yields (FIG. 7), but is the direct result of a well-optimized FCC catalyst.

CASE STUDY: RESID-TO-PROPYLENE MAXIMIZATION

A commercial resid FCCU in Asia-Pacific is operating in maximum propylene mode, with actual propylene yields of more than 9 wt%–10 wt%, while maintaining gasoline and LCO yields at minimum slurry production. In addition to targeted yields, best-in-class (BIC) coke selectivity is a primary objective for this FCCU, which processes a difficult feedstock with a Concarbon level of approximately 4 wt% and a feed-specific gravity of 22 API. This refinery performed a catalyst selection using in-unit, back-to-back trials.

Extensive pilot plant riser testing was conducted prior to application by the catalyst supplier’s laboratories to minimize risk for the refiner and fully optimize the catalyst design for the targeted yields. It is important to note that pilot plant testing contributes to the supplier’s approach to risk minimization, and testing associated with modeling provides an efficient and proven methodology for catalyst selection. Commercial performance data was obtained in this resid-to-propylene FCCU and compared in terms of propylene, butylenes, distillates and bottoms-to-coke. Using a finely-tuned formulation that was first fully vetted at pilot plant scale, with an optimized matrix and mesoporosity in the 100 Å–600 Å range, the refinery observed a significant improvement in its targeted yields.

At isoconversion, the gain in propylene yield reached 0.4 wt% to 0.5 wt% (FIG. 8), as compared to the previous catalyst technology, a commercially available, high-accessibility catalyst provided by an alternate supplier.

When compared at similar conversion, the customized catalyst exhibited a noticeable difference in propylene yield at similar or even higher butylene levels than those of the high-accessibility competitive technology (FIG. 9). The increase in light olefins production translated into higher profitability. Additionally, the tailored catalyst formulation enabled the unit to achieve an incremental octane benefit of 0.3 to 0.5. The coke selectivity was superior for the customized solution, along with the H₂ and dry gas make, as illustrated in FIG. 10. Not only was coke selectivity improved with the customized catalyst, but also slurry or residual bottoms conversion. A better coke selectivity induced a lower delta coke, allowing the unit to operate at maximum severity (maximum C/O) without an H₂ or coke penalty. At maximum severity, the feed conversion increased and the slurry yields decreased by approximately 0.2 wt% in the commercial unit.

The customized catalyst provided the refinery’s operation team with a wider operating window and enhanced flexibility due to a lower bottoms-to-coke ratio (FIG. 11), allowing the FCCU to operate at a higher severity while remaining within its constraints. At isocoke (i.e., under similar regenerator conditions), the slurry yields were lower by almost 10%, which translated into greater unit profitability.

Catalytic solutions increase FCCU profitability

To quantify the gain in refinery’s profitability in the case study, a set of reference economics were used that are representative for the Asia-Pacific market conditions. TABLE 1 highlights the differences in yields between the best competitive offering and a unit-customized solution that integrated metals traps, an active matrix with tailored porosity including a specific mesoporosity in the 100 Å–600 Å range and the optimal zeolite acidity profile, as described in the catalyst design section. With lower dry gas, lower coke and higher activity at similar metals, the benefits of higher light olefins (+0.5 wt%) without sacrificing fuels production (+0.5 wt% gasoline and –0.3 wt% LCO) resulted in improved unit profitability of +$0.87/bbl of feed processed. For the resid FCCU in the case study, the yearly profitability benefits approach $22 MM.

Key discoveries

The most profitable refiners leverage their flexibility to capture market opportunities. As the most flexible process in the refinery, the FCCU can process a wide range of feedstocks of varying quality, from hydrotreated VGOs to heavy resids. Those feedstocks can be converted into an equally wide range of products, from transportation fuels to petrochemicals, such as propylene and butylene. The yield profile can be modified via an adjustment of operating conditions, but few units possess the operating window to meet yield targets with process moves alone. FCC catalyst optimization is, therefore, an effective tool to increase yield against the primary operating constraints.

Achieving high propylene yields from residue feedstock is possible via the application of a customized catalytic solution that leverages BIC coke selectivity with a metal-tolerant function, optimum H₂ transfer properties and zeolite acidity profile (acid density, acid strength and distribution) to drive deep conversion of the bottom of the barrel into the most valuable products. Specifically in Asia-Pacific and EMEA regions, the desired products are propylene, gasoline and distillate or LCO. In the Americas, butylenes at minimum propylene production are most desirable to meet strong regional gasoline demand. In every case, a rigorous product development process is deployed to bring new catalytic innovations to market, including riser pilot plant testing and verification prior to introduction to the commercial unit. The rigorous product introduction process minimizes risk for the refinery, and maximizes the economic rewards of the new innovations in catalysis. In the case study described here, those rewards can be in excess of $20 MM/yr of incremental refining margins. HP

NOTE

^a  The customized catalytic solution belongs to Grace’s ACHIEVE series.

LITERATURE CITED

1. Fu, A., D. Hunt, J. A. Bonilla and A. Batachari, “Deep catalytic cracking plant produces propylene in Thailand,” Oil & Gas Journal, Vol. 96, Iss. 2, December 1998.
2. Chau, C. and R. Schiller, “A great achievement,” Hydrocarbon Engineering, November 2015.
3.  Zhao, X. and T. G. Roberie, “ZSM-5 additive in fluid catalytic cracking—Effect of additive level and temperature on light olefins and gasoline olefins,” Industrial & Engineering Chemistry Research, Vol. 38, 1999.
4. Cheng, W. C., M. S. Krishnamoorthy, R. Kumar, X. Zhao and M. S. Ziebarth, “Catalyst for light olefins and LPG in fluidized catalytic units,” US Pat. 9,365,779B2, June 14, 2016.
5. Cher, Y. Y., R. Schiller and J. Koebel, “Enhanced bottoms cracking and process flexibility with MIDAS FCC catalyst,” Grace Catalysts Technologies Catalagram, 2012.
6. Zhao, X., W. C. Cheng and A. Budesill, “FCC bottoms cracking mechanisms and implications for catalyst design for resid applications,” NPRA Conference, AM-02-53, January 2002.

The Authors

From the Archive

Comments