January 2022

Special Focus: Sustainability

Safe and sustainable alkylation for the forward-looking refiner

In a post-pandemic economy, global governments and industry participants are increasingly committing to meeting climate action targets.

In a post-pandemic economy, global governments and industry participants are increasingly committing to meeting climate action targets. Current large-scale decarbonization efforts are aimed at lowering the impact of combustion fuels on the environment.

A key pathway for meeting climate action targets entails the production of clean fuels—fuels that have lower relative carbon intensities and lower contaminant (sulfur) levels, and that are produced using pathways that result in fewer emissions or waste byproducts. Consequently, clean-burning, low-sulfur and high-performance blending stocks, such as alkylates, are becoming increasingly sought after.

This article discusses a commercial, safe and sustainable alkylation technology^a, including the performance of a 7,400-bpd brownfield unit commissioned in March 2019 at the Sinopec Jiujiang refinery.

Alkylation process

Alkylation is the transfer of an alkyl group from one molecule to another. In refining, alkylation refers to a catalytic process for producing high-value C₈ compounds, typically by reacting lower-value isobutane with C₄ olefins. Alkylate produced at the refinery is used as an octane-booster and is blended into the gasoline pool.

Traditional alkylation processes use either hydrogen fluoride (HF) or sulfuric acid (H₂SO₄) to catalyze the alkylation reaction. However, these traditional processes are inherently unsafe due to the toxic and corrosive nature of the strong acid catalysts. Refiners using acid-catalyzed alkylation technologies require exotic metallurgies for process equipment, along with costly safety systems to protect refinery personnel and the public. The disposal and regeneration of spent acid catalysts have negative impacts on the environment and may pose serious chronic human health issues over prolonged exposure.

The referenced alkylation technology^a is an inherently safe process that uses a proprietary composite ionic liquid (CIL) catalyst to facilitate the alkylation reaction. CIL catalyst is a non-volatile, non-aqueous liquid salt that is formulated by modifying a chloroaluminate ionic liquid platform with a transition metal. The composite nature of the CIL catalyst allows the process to enhance alkylate product selectivity and to overcome residual corrosion issues typically associated with ionic liquid catalysts. CIL catalyst is non-hazardous and non-corrosive, allowing all alkylation process equipment to be manufactured using carbon steel. This catalyst can also be regenerated in-situ under moderate operating conditions, which provides added benefits of safe handling and emissions reductions vs. alternative technologies.

Alkylation market outlook

Although world oil markets have rebounded from a sharp decline in demand caused by the global pandemic, experts believe that policy-driven factors may lead to peak oil demand sooner than previously thought.¹ The International Energy Agency (IEA) forecasts that total gasoline demand is unlikely to return to 2019 levels, due to efficiency gains and a shift toward renewables.²

In developed economies, the shift to electric vehicles is expected to significantly impact the long-term demand for gasoline. However, despite the macro changes in gasoline demand, the alkylate outlook remains stable, offset by an increased demand for higher-performance fuels for high-efficiency engines.

Existing acid-based alkylation infrastructure is aging and represents a growing safety and financial risk for the modern refiner. In the wake of a bankruptcy-causing 2019 HF alkylation unit explosion at a U.S. refinery,³ the rising costs of insurance and safety are driving refiners to consider inherently safer alternatives.⁴ In the U.S. and Europe, opportunities for implementing this CIL-catalyzed alkylation technology^a are in the replacement, modernization and safety enhancement of the legacy acid-based alkylation processes.

In emerging or transitioning economies, robust mobility growth in the populace is expected to continue to drive modest gasoline demand for the next decade. In developing markets, the opportunity for implementing the CIL-catalyzed alkylation technology^a is in greenfield projects or expansions of brownfield projects.

Commercialization and scale-up

This CIL-catalyzed alkylation technology^a has been under development for two decades. Beginning with a 0.5-bpd continuous pilot testing project in 2003, the technology has been scaled up to a commercial production capacity of 7,400 bpd.

In 2005, the first commercial field demonstration was successfully performed at the PetroChina Lanzhou refinery by retrofitting an existing H₂SO₄ alkylation unit with CIL catalysts.⁵ In 2013, Deyang Chemical Co. Ltd., an independent refiner, commissioned a greenfield 2,450-bpd unit.⁶ In 2018, PetroChina Harbin Petrochemical Co. Ltd. implemented the technology in a brownfield operation at a scale of 3,700 bpd.⁷

Between 2017 and 2018, the Sinopec group licensed the CIL-catalyzed alkylation technology for three installations (the Jiujiang, Wuhan and Anqing refineries), each at a scale of 7,400 bpd.⁸ Construction for the units was completed between 2018–2020. Notably, the Sinopec Wuhan unit was a revamp from an existing HF-based alkylation process—representing a major milestone and an industry first.⁹

Commercial process performance

The following section discusses the recently commissioned brownfield 7,400-bpd CIL-catalyzed alkylation unit at Sinopec Jiujiang Co.’s Jiangxi plant (FIG. 1), which has a process operating flexibility of 60%–110% and occupies a plot space of 126 m x 74 m. The unit is designed to handle mixed olefin feedstocks from both methyl tertiary-butyl ether (MTBE) and fluid catalytic cracking (FCC) units.

The turnkey capital cost for the unit (including engineering, procurement, construction, commissioning and inspections) was approximately $78 MM and included the installation of four key sections: a feed pretreatment system, a reaction system, a catalyst regeneration system and a product separation/purification system. Sinopec Engineering Construction Co. Ltd. led the engineering aspects of the project, while Sinopec Nanjing Engineering Co. undertook the construction works.

After 1 yr of construction, mechanical completion was achieved in 4Q 2018, and the unit was commissioned 1Q 2019. In 2Q 2019, the operator conducted a calibration test to benchmark and compare commercial process performance data against design specifications and to identify optimization opportunities. At the time of this publication, no safety-related incidents or concerns have been identified by the operator.

TABLE 1 compares the commercial and design feed compositions. Although the isobutane content of the commercial feed (39.7 wt%) was comparable to that of the design basis (38.5 wt%), the total C₄ olefin content of the commercial feed (32.7 wt%) was significantly lower than expected (46.3 wt%). Further, the non-reactive n-butane content in the commercial feed (25.6 wt%) was significantly higher than the design basis (14.8 wt%), and the commercial feed also contained contaminates, namely 1,3 butadiene and methanol.

The CIL-catalyzed alkylation technology^a is intended to stoichiometrically react isobutane with C₄ olefins. The design basis anticipated that C₄ olefins feedstock would be supplied in excess relative to isobutane. In commercial operation, however, the opposite occurred—isobutane was supplied in excess relative to the C₄ olefin content.

TABLE 2 compares the commercial and design mass balances and alkylate yield. The design basis required a supplementary isobutane stream (3 tph) to fully react with C₄ olefins that were in excess in the design feed. The commercial application did not require utilization of the supplementary isobutane stream. Due to the commercial feed containing relatively lower quantities of C₄ olefins, excess isobutane and relatively higher quantities of non-reactive n-butane, the alkylate production and yield underperformed compared to the design case. As a result, the effective alkylate production rate for the unit was 5,600 bpd, as compared to the designed 7,400 bpd.

In commercial operation, 0.86 tph of flare gas was produced, representing 2.2 wt% of the feed. The flare gas was attributed to unrecovered isobutane, which, under the design scenario, was intended to be captured and recycled using the compressor system flash tank. As isobutane in the commercial feed was supplied in excess, the original design load of the recovery system was insufficient to handle the additional throughput; therefore, flaring was required. At present, a corresponding retrofit scheme has been proposed to add a pumping system to increase the recovery capacity of flashed isobutane.

It should be noted that the deviation in feed composition of commercial feed from that of the design feed experienced by the operator is not unusual, since the typical feedstocks for the alkylation process are off-gases from upstream FCC or MTBE units. The off-gas compositions are dependent on FCC/MTBE feed and their operating conditions.

TABLE 3 compares the commercial and design alkylate product specifications. Key metrics—such as the research octane number (RON), motor octane number (MON) and endpoint distillation temperature—met the design basis specifications during commercial operations, indicating that the process produces superior-quality alkylate. The observance of a higher commercial product Reid vapor pressure (RVP) was due to the operator’s inclusion of up to 10 wt% n-butane alongside alkylate product to balance the low RVP in the plant’s existing gasoline pool. Modifying the n-butane content in the alkylate product was achieved by adjusting the process operating conditions of the n-butane extraction column. Complete removal of the n-butane fraction from the alkylate product would result in an alkylate RVP of less than 30 kPa.

TABLE 4 shows the composition of the commercial alkylate product. The process achieved 100% olefin conversion. No C₁₂⁺ or higher boiling point compounds were detected in the product stream. The C₈ content in the alkylate was 82%, which is comparable to, or exceeding the performance of, best-in-class competitive acid-based alkylation processes. The narrow distribution of the alkylate composition indicates that the process is highly selective for alkylate yield.

TABLE 5 shows the commercial utilities consumption for the unit. The absolute consumption rates for steam, electricity, instrument air and recycled water were in alignment with other operators using the CIL-catalyzed alkylation technology^a. However, when energy use was normalized to kilograms of oil equivalent (kgEO) per ton of alkylate produced, the energy consumption for this installation (154.51 kgEO/t alkylate) was 20%–30% higher than the peer group, as expected. The discrepancy was attributed to the lower-than-anticipated alkylate production due to feed quality issues (low olefins and high non-reactive n-butane contents). The operator has identified feed control as an area of optimization and is considering measures to increase the feed olefin content to maximize alkylate production.

TABLE 6 lists the licensor-recommended contaminant limits for the process, and TABLE 7 compares the commercial and design consumables for the process. While processing feed with higher contaminant levels does not impact the safety or operation of the unit, the catalyst consumption rate may increase. Therefore, a feed-dependent pretreatment configuration is recommended for each installation.

During regular operations, the majority of spent CIL catalysts are regenerated onsite in the regeneration unit, and catalyst activity is maintained through the addition of an organic chloride activator compound. A 30% sodium hydroxide (NaOH) solution is used to remove feed contaminant sulfur via caustic wash and to secondarily neutralize small quantities of spent catalyst byproduct prior to disposal. A roughly equal quantity of catalyst active reagent is added to offset spent catalyst removals.

Despite the methanol content in the feed being in excess of the licensor-recommended limits, the commercial operation required 43% fewer total consumables than the design basis. A key reason for the lower-than-anticipated consumables was the presence of higher quantities of non-reactive n-butane in the commercial feed alongside lower quantities of olefinic reaction materials. It is expected that future modifications to the feed aimed at increasing the alkylate production rate will bring the commercial and design estimates closer to alignment. The future addition of a pretreatment methanol removal unit may present an opportunity to further reduce catalyst consumption.

Takeaway

The CIL-catalyzed alkylation technology^a is a commercially proven high-performance alkylation process that meets the health, safety and environmental objectives of stakeholders. As with any newly implemented technology, a retrospective review of initial process performance indexed against the design basis provides an excellent opportunity to identify optimization and debottlenecking projects, and such a review should be undertaken within the first 6 mos of commissioning.

Lessons learned from commercial experience highlight the importance of selecting a robust alkylation technology that has a high tolerance for feed composition variability, accurately predicting feed characteristics during the design phase of the project, while also ensuring stable feed during commercial operations. Significant deviations and fluctuations in the feed may result in variations in the alkylate yield, energy consumption per quantity of alkylate produced and consumables required. Nonetheless, the CIL-catalyzed alkylation technology^a is demonstrated to consistently produce high-quality alkylate in a safe and sustainable manner. HP

ACKNOWLEDGMENTS

Sinopec Jiujiang provided the design and commercial operating data. The National Natural Science Foundation of China (grants 21425626, 21036008, 20976194 and 20206018) and Shell Global Solutions International B.V. provided exploratory and strategic research grants. The China State Council granted the National Invention Award for the CIL-catalyzed alkylation technology^a.

NOTE

^a Ionikylation is a CIL-catalyzed alkylation technology developed by the China University of Petroleum–Beijing and licensed worldwide by Well Resources Inc.

LITERATURE CITED

1.  IEA, “Oil Markets Face Uncertain Future After Rebound from Historic COVID-19 Shock,” March 21, 2021, online: https://www.iea.org/news/oil-markets-face-uncertain-future-after-rebound-from-historic-covid-19-shock
2. IEA, “Oil 2021: Analysis and Forecast to 2026,” March 2021, online: https://www.iea.org/reports/oil-2021
3. Kearney, L. and J. Renshaw, “Philadelphia Energy Solutions Files for Bankruptcy After Refinery Fire,” Reuters, July 22, 2019, online: https://www.reuters.com/article/us-pes-bankruptcy-idUSKCN1UH0O9
4. Sanicola, L., “U.S. Refiners, Chemical Makers Pare Insurance Coverage as Accidents Boost Costs,” Reuters, January 29, 2020, online: https://www.reuters.com/article/us-usa-refineries-insurance-idUSKBN1ZT0FB
5.  Liu, Z., R. Zhang, C. Xu and R. Xia, “Ionic Liquid Alkylation Process Produces High-Quality Gasoline,” Oil & Gas Journal, 2006.
6.  Liu, Z., R. Zhang, X. Meng, H. Liu, C. Xu, X. Zhang and W. Chung, “Composite ionic liquid alkylation technology gives high product yield and selectivity,” Hydrocarbon Processing, March 2018.
7. Chung, W., R. Zhang, X. Zhang and D. Song, “Safe and sustainable alkylation: Performance update on composite ionic liquid alkylation technology,” Hydrocarbon Processing, April 2020.
8.  Brelsford, R., “Sinopec Starts Up Composite Ionic Liquid Alkylation Unit,” Oil & Gas Journal, April 2, 2019, online: https://www.ogj.com/refining-processing/refining/capacities/article/17279209/sinopec-starts-up-composite-ionic-liquid-alkylation-unit
9.  Brelsford, R., “Sinopec Starts Up New Unit at Wuhan Refinery,” Oil & Gas Journal, April 16, 2020, online: https://www.ogj.com/refining-processing/refining/operations/article/14174241/sinopec-starts-up-new-unit-at-wuhan-refinery

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