June 2021

Catalysts

Maximize refinery profitability with novel RFCC technologies

The Atyrau Oil Refinery (AOR), the oldest refinery in Kazakhstan, was built in 1945.

The Atyrau Oil Refinery (AOR), the oldest refinery in Kazakhstan, was built in 1945. It is located in Atyrau, the capital of the Atyrau Region, at the mouth of the Ural River on the Caspian Sea, and is a base for modern oil and gas industries. AOR is operated by the state-owned KazMunayGas (KMG) and has successfully completed modernization, adding a deep oil conversion refining complex that has increased crude oil refining capacity up to 5.5 MMtpy and converted the site to one of the key refining complexes in the region.

AOR’s target is to produce an optimized combination between high-value clean transportation fuels and specialty precursors for chemicals production, such as benzene or paraxylene. AOR is very complex (the Nelson Index is 13.8) and includes a number of conversion units, such as a proprietary resid fluid catalytic cracking unita (RFCCU); proprietary catalystb for selective AMPD/butadiene hydrogenation; and various hydrotreatments, including a proprietary catalytic processc for FCC gasoline, etherification (TAME), an extractive sulfur removal processd for liquefied petroleum gas (LPG) sweetening and indirect alkylation processe oligomerization, among others. AOR is continuously upgrading its process units (FIG. 1) with state-of-the-art technologies; for example, the refinery is working to modernize its TAME unit and adapt its product slate to IMO 2020 and use a proprietary digital service platformf for remote monitoring.

FIG. 1. Final configuration of modernization of Atyrau refinery.

The FCCU is the primary hydrocarbon conversion unit in the modern petroleum refinery. It utilizes heat and catalyst to convert a variety of high molecular-weight feed types (e.g., gasoils and atmospheric/vacuum residues) into lighter, more valuable products such as transportation fuels and petrochemical feedstocks.1 Almost every option under consideration involves upgrading residue type feedstock through an FCCU to compete under current market conditions. Considering future demand for refined products, slow-to-flat growth is projected for transportation fuels over the next decade, developing in parallel with the high-margins opportunities available in the petrochemicals market.

To be successful in such a challenging environment, it is of paramount importance to develop strong partnerships between refiners and technology suppliers to ensure maximum conversion, product slate flexibility and unit availability, driving to the most profitable operations needed to stay competitive. This article describes a successful case of how a robust combination of operating expertise, state-of-the-art hardware and novel catalyst technologies have enabled AOR’s RFCCU to achieve the facility’s ambitious conversion objectives while processing one of the most challenging feedstocks found worldwide.

Residue conversion challenges

Residual streams from atmospheric or vacuum distillation fractionators are characterized as having a heavier nature than traditional gasoils used in FCCUs due to the presence of high molecular-weight polynuclear aromatic rings—such as resins or asphaltenes—which then result in a higher boiling point distribution, between 500°C–700°C (FIG. 2).

FIG. 2. Boiling point distribution of residue vs. typical FCC gasoils (top), and an asphaltene molecule (bottom).
FIG. 2. Boiling point distribution of residue vs. typical FCC gasoils (top), and an asphaltene molecule (bottom).

Residue streams possess high density and aromaticity, in addition to much higher Conradson carbon residue, as compared to vacuum gasoil (VGO). It is commonly accepted that the conditions in the FCCU do not allow the ring-opening cracking of aromatic rings; therefore, increasing density and/or aromaticity directly results in loss of feed conversion. The high CCR presence is believed to convert to coke within 30%–90%, depending on the feed nature, which heavily impacts the heat balance in FCC operations, as seen with higher regenerator temperature, lower cat/oil (C/O) ratio and reduced conversion.

Residue feeds also contain high levels of contaminants. The high metal “complexes” typical of residues like nickel (Ni) or vanadium (V) accelerate poisoning of conventional catalysts. Ni deposits on the catalyst surface present a strong dehydrogenation activity to form undesired hydrogen and coke precursors if not inhibited properly. Unlike Ni, the V complexes destroy the zeolite Y portion in the cracking catalyst and lower activity due to the formation of vanadic acid. Further, its mobility characteristics allow it to move from one catalyst particle to another catalyst particle, extending its destructive impact to multiple catalyst particles. V is most detrimental when the FCCU is operated in full burn mode.2 Therefore, an effective V trap dramatically reduces the FCCU’s ability to destroy the active components of FCC catalyst and, therefore, maintain high inherent cracking activity.

In addition, other metals can be present, such as iron (Fe) or sodium (Na), which remarkably enhance the detrimental impact of V on the catalyst activity when processing heavy residue feedstock. Fe and Na tolerances are crucial factors for Atyrau’s FCCU, as it presents the highest Ecat [Fe+Na] among 81 FCCUs benchmarked in the industry (FIG. 3).

FIG. 3. Industry Ecat benchmark of [Fe+Na] (wt%) content.

Typical feedstock properties of AOR’s RFCCU are shown in TABLE 1. Besides the mentioned extreme [Fe+Na] levels, the residue fraction has very high Ni, V and calcium (Ca) levels, along with a Conradson carbon content exceeding 5.5 wt% and a 30% distillation point > 480ºC. This very challenging residual stream is fed into the unit well above 50 wt% of the total feed, making the FCC processing in the AOR one of the most challenging worldwide.

The feed characteristics in TABLE 1 from residual streams may lead to the following FCCU limitations:

  • Main air blower (MAB) or wet gas compressor (WGC) capacities may be exceeded—in many FCC operations, the Ni content of Ecat systematically grows, making it necessary to use catalysts with improved Ni trapping to avoid running into WGC constraints, as Ni is known to produce hydrogen.
  • Upper limits for regenerator temperature and stripping efficiencies create bottlenecks and are strongly interrelated with the coke remaining on the spent catalyst. If the hardware in the regenerator or regenerator design cannot cope with the temperature increase or afterburn, this will also be a limiting factor.
  • Bottom-of-the-barrel products (e.g., FCCU bottoms product) have a limited market, either for sale to a coking refinery or as one of a few different grades for coking.

RFCCU

AOR uses a proprietary RFCCUa technology to convert heavy, high molecular-weight and metals content feedstock into higher value streams. This technology was originally developed as a cost-effective, flexible and reliable means to profit from market opportunities at times when it was crucial to leverage the intrinsic potential of residuum feedstocks. Various challenges posed by such feeds have been overcome as detailed here, and innovations have been kept up-to-date. In particular, the Atyrau proprietary RFCCUa is equipped with:

  • A leading resid feed injector technologyc that ensures appropriate feed distribution to promote vapor-phase cracking, resulting in low coke and dry gas make and superior liquid selectivity that limits the burden on the WGC
  • Mix temperature control (MTC) that ensures riser temperature control through quench to increase the C/O ratio and displace the equilibrium between catalytic and thermal cracking to achieve better selectivities
  • A riser termination deviceg offering high gas containment to limit products degradation to light ends, while being highly resilient to transient phases
  • Superior catalyst stripping with structured packing to preserve the valuable hydrocarbon material and reduce the regeneration temperature at low stripping steam rates.

More importantly, the RFCCU configuration incorporates a two-stage catalyst regeneration (FIG. 4) that minimizes catalyst deactivation in the presence of metals to maintain higher activity with more metals on the catalyst, thereby reducing catalyst replacement costs.

FIG. 4. Typical RFCCUa arrangement with dual-regeneration system.

The first regeneration stage operates in partial burn mode. High hydrogen content molecules are burned in this stage, yielding water. The extent of the regeneration is controlled to limit the temperature elevation in the presence of water, which would otherwise cause high hydrothermal deactivation of the catalyst. Moreover, in the absence of excess oxygen, V oxidation cannot occur. This is the first step in the mobility cycle of this molecule, which ultimately leads to the collapse of the catalyst structure and drastic reduction of its ability to convert the longer molecules inherently present in FCC feeds.

The second regeneration stage operates in full burn mode to restore the full catalyst potential. In this stage, the temperature is further elevated, which is less detrimental to catalyst activity than other solutions because water was removed with flue gas in the first stage. Furthermore, in the absence of water, the transformation of vanadium oxide to vanadic oxide is largely impaired—once more this limits the V mobility, thus inhibiting the noxious effects of this contaminant on catalyst.

It is worth noting that the two-stage scheme also rejects a substantial portion of the regeneration heat as carbon monoxide (CO) in the first stage, thus removing the necessity to install a catalyst cooler at Atyrau despite the high concarbon feedstock. This has a direct and positive impact on unit costs and complexity of operation, as well as removing a burden for maintenance. CO is subsequently oxidized to CO2 and noxious components are captured or neutralized to ensure full compliance with environmental norms. The expertise of various partnerships are leveraged to systematically develop tailor-made, highly energy efficient FCC flue gas treatments abiding with the most stringent regulations worldwide. In the case of AOR, an SCR DeNOx, followed by a flue gas scrubber, was retained.

Additionally, state-of-the-art thermal integration was developed specifically to accommodate changes in feed composition, as well as the different seasonal modes of operation required by KMG, while maintaining a high energy efficiency throughout all those cases.

For these reasons and owing to a strong experience due to more than 40 designs performed on feedstocks with concarbon higher than 3 wt%, the RFCCUa process provides yields and performances with the flexibility to process a wide range of feedstocks—from gasoil through residue—in the same unit to meet multiple product scenarios (max distillate, max gasoline or max light olefins).

Finally, thanks to a robust design in conjunction with a dedicated catalyst design, AOR’s FCCU succeeds in ensuring catalyst circulation and is delivering expected performances while processing high Fe content feeds.

RFCC catalyst design

For a given residue-containing feedstock to be processed in an FCCU, there is an optimized set of operating conditions to maximize unit conversion for a given FCC hardware design. The mastery of such operating variables handling, as well as continuous improvements in hardware technology, enable refiners to push profitability up to the FCCU or to downstream process constraints.

Nevertheless, another variable exists that can significantly improve unit profitability and reliability, resulting in substantial economic benefits: the FCC catalyst technology. The main advantages of catalyst technologies are:

  • Wide flexibility to change design to adapt performance to objectives
  • No need to stop the unit as with major hardware revamps, which enables changes between planned mean-time to application recovery (MTAR) dates
  • Minor costs only involved and no CAPEX needed compared to high CAPEX investments
  • Significant value creation for a refinery with optimized catalyst system selection.

A commercially well-proven catalysth specifically tailored for the very adverse conditions of AOR’s process was proposed. The main characteristics of this catalyst are:

  1. High-stability zeolite technology that yields superior activity retention and hydrothermal stability while exhibiting low hydrogen-transfer selectivity to minimize undesired bimolecular reactions, as well as increased low-value LPG and gasoline linear paraffins.
  2. Integration of proprietary metals traps, including an alumina-based matrix Ni passivatori and the rare earth-based vanadium trap (IVT). Research has been conducted using x-ray diffraction (XRD) to demonstrate the interaction between γ-Al2O3 and Ni. As can be seen in FIG. 5, this interaction is stronger with increasing Ni concentration. An IVT trap technologyj was also used to reduce the harmful
    effects of V. This well-established technology has shown excellent V inhibition, as can be seen using a cross-section SEM-EDX microscopy technique (FIG. 6) in contaminated Ecat samples, which demonstrates the co-existence of hot-spots of rare earth RE and V, thus trapping the V and preserving the zeolite integrity.
    FIG. 5. XRD diffraction patterns of γ-Al2O3 with Ni, from containing no Ni (black) up to maximum Ni content (green).
    FIG. 6. SEM mapping for rare earth (RE) and vanadium (V) of a contaminated Ecat containing proprietary IVT technologyj.
  3. Intracrystalline pore architecture is optimized to enable large molecules often encountered in a residual feedstock stream to enter the catalyst system and therefore be cracked into smaller, higher value molecules. This open pore volume is a characteristic signature of a proprietary catalyst system seriesk, and it improves molecular diffusivity and pre-cracking of heavy molecules that otherwise would interact only on the surface of the zeolitic part of the catalyst and, in that case, hinder both the extent and selectivity of the monomolecular cracking aimed at the zeolite section properly designed for residue cracking.3 This is clearly observed when comparing the pore size distribution (PoSD) of different commercially available catalysts (FIG. 7). The PoSD of the catalystk exhibits a wide peak between 300 Å and 1,000 Å, where molecular diffusion is much faster and in line with typical heavy molecular size contained in resid fractions.
    FIG. 7. Pore size distribution (Å) of optimized resid catalystk and alternative resid catalyst in the market.

KMG FCCU industry benchmarking

Thanks to the combination of state-of-the art hardware and catalyst technologies, AOR’s RFCCU is showing outstanding conversion results that position it among the top refiners in the industry processing heavy feeds. Typical yields achieved in the refinery’s RFCCU are indicated in TABLE 2.

These yields and conversion levels, when compared to EMEA refineries in general at normalized yields, further support the outstanding performance of the AOR RFCCU. As can be seen in FIGS. 8–13, Atyrau’s RFCCU shows very high conversion and superb gasoline yield, while excelling at bottoms cracking. This is enabled by an excellent coke and gas selectivity, as observed in the EMEA Ecat benchmarking monitored by pilot plant laboratoriesl to Ecat samples of all FCCUs in the region.

FIG. 8. Conversion (wt% FF) vs. cat/oil (wt/wt) industry benchmarking.
FIG. 9. Gasoline (wt% FF) vs. conversion (wt% FF) industry benchmarking.
FIG. 10. LCO (wt% FF) vs. conversion (wt% FF) industry benchmarking.
FIG. 11. FCC slurry (wt% FF) vs. conversion (wt% FF) industry benchmarking.
FIG. 12. Ecatl H2 yield vs. metals industry benchmarking.
FIG. 13. Ecatl coke yield vs. metals industry benchmarking.

Value creation through partnership

The coordinated efforts between the KMG’s refinery operating excellence and additional expertise from partners like the authors’ companies are reflected in faster, more efficient problem-solving that ultimately significantly increases the unit availability and reliability. As an example, in the beginning of 2019, the Atyrau FCCU faced a catalyst loss issue. Catalyst particles were observed both in the regenerator flue gas scrubber and the main fractionator bottoms line. A team analyzed pertinent catalyst fines samples and reviewed the unit’s operating mode to identify changes that may lead to the root cause. A proprietary advanced microscopy techniquem revealed that catalyst particle fracture was occurring in the unit (FIG. 14).

FIG. 14. Ecatm analysis revealing particulate fracture.

After extensive joint brainstorming to target potential causes, a battery of actions was developed that included checking each air and steam injection point. It appeared that the steam values to the feed distribution nozzles were far above the recommendation. Through a comprehensive set of experiments, the team discovered that one of the seven feed nozzles received four times more steam than expected, well beyond the unit’s recommended range of operation. While this did not severely impair the yield pattern due to a robust feed injector design, this was confirmed as being the root cause of the catalyst particles fracture episode.

In this way, a double benefit solution was achieved: the steam-to-nozzles amount was optimized with the corresponding savings, and a more evenly distributed steam injection resulted in a smoother mixing without damaging the quality of the catalyst circulating inventory. The improved circulation with reduced particulate losses delivered savings estimated at $1.3 MM/yr. The three-party cooperation delivered value by means of significant cost savings and much improved unit performance at Atyrau’s FCCU and is expected to continue to provide benefits in the future.

Takeaway

A combination of technologies using a three-pronged approach—AOR’s expertise and operations management, tailored FCC catalyst technology and the advanced hardware technology—enabled AOR to extract the maximum profitability from the very challenging operation at its RFCCU. Thanks to the close partnership between different parties, including the unit licensor and the catalyst supplier, it has been demonstrated that multidisciplinary teams can achieve excellent results and drive value for the refinery. The combination of the RFCCUa with the proprietary catalysth delivered best-in-class bottoms cracking functionality, while preserving superior metals tolerance, which is crucial to keep the maximum number of residual streams in the feedstock diet. HP

NOTES

            a R2R™ resid fluid catalytic cracking unit by Atyrau Oil
            b IFP Energies Nouvelles’ Alkyfining®
            c Axens’ Prime-G+®
            d IFP Energies Nouvelles’ SULFREX®
            e Axens’ Polynaphtha®
            f Axens’ Connect’In®
            g Axens’ RS2™
            h W. R. Grace & Co.’s MIDAS®-350
            i  W. R. Grace & Co.’s alumina-based matrix Ni passivator
            j W. R. Grace & Co.’s IVT trap technology
            k W. R. Grace & Co.’s MIDAS® series
            l W. R. Grace & Co.’s Grace’s ACE™
           m W. R. Grace & Co.’s SEM/EDX

LITERATURE CITED

  1.  Bryden, K., U. Singh, M. Berg, S. Brandt, R. Schiller and W.-C. Cheng, “Fluid catalytic cracking: Catalysts and additives,” Encyclopedia of Chemical Technology, June 2015.
  2. “Grace guide to fluid catalytic cracking,” 2nd Ed., W. R. Grace & Co, 2020.
  3. Zhao, X., W. Cheng and J. Rudesill, “FCC bottoms cracking mechanisms and implications for catalyst design for resid applications,” National Petrochemical & Refiners Association (NPRA), 2002.

The Authors

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