April 2020

Special Focus: Clean Fuels

Safe and sustainable alkylation: Performance and update on composite ionic liquid alkylation technology

Market and regulatory factors are pressuring refiners to adopt safe and sustainable processes for the production of clean-burning and environmentally friendly fuels.

Chung, W., Well Resources Inc.; Song, D., PetroChina Harbin Petrochemical Co.

Market and regulatory factors are pressuring refiners to adopt safe and sustainable processes for the production of clean-burning and environmentally friendly fuels. Global, long-term demand for high-quality gasoline is being driven by macro factors, such as a steadily growing consumer base and the adoption of high-compression engines, which have low vapor pressure requirements.1 Concurrently, implementation of stringent fuel and emissions standards, including low-sulfur or ultra-low-sulfur content specifications, are increasing the reliance on octane-boosting blending stocks, namely alkylate.2

This article discusses the implementation of a novel, safe and sustainable alkylation technology,a including performance of a 150,000-metric-tpy brownfield unit commissioned in November 2018.

Alkylation technologies

Traditional acid-based alkylation processes use either hydrogen fluoride (HF) or sulfuric acid (H2SO4) to catalyze the reaction of mixed C4 olefins feedstocks into high-octane C8 alkylates for gasoline blending. Both HF and H2SO4 alkylation processes are inherently unsafe due to the highly corrosive nature of the strong acid catalysts. Refiners using acid-based alkylation technologies require exotic metallurgies for process equipment and may require costly safety systems to protect refinery personnel and the public. The disposal and regeneration of spent acid catalysts are also problematic from an environmental perspective.

Since 2015, a series of high-profile refinery explosions in California, Wisconsin and Pennsylvania have raised alarm over using HF in refinery operations near densely populated urban centers in the continental U.S.3 If accidentally released, HF has a propensity to become airborne as a dense, toxic vapor cloud. North American regulators and local stakeholder groups have called on refiners to examine their options for mitigating the risks of aging HF alkylation units.4

In China, the Environmental Protection Ministry has imposed heavy penalties on alkylation operators with inadequate safe practices and is closely monitoring practices relating to the treatment of spent H2SO4.5 The highly emissions-intensive process of regenerating large quantities of spent H2SO4 from alkylation operations may present a future risk to refiners if regulations are further tightened for industrial emitters.

The referenced alkylation technologya is an inherently safe, commercial process that uses a proprietary composite ionic liquid catalyst—a non-volatile, non-aqueous liquid salt—to facilitate the alkylation reaction. The alkylation catalyst is non-hazardous and non-corrosive, allowing all process equipment to be manufactured using low-cost carbon steel. The catalyst is regenerated onsite under moderate operating conditions, which provides the added benefits of emissions reduction compared to alternative technologies. A small quantity of chemically benign solid waste byproduct is removed from the unit every few days to two weeks.6

Commercial deployment

Recent safety and emissions policy changes in China have prompted refineries to consider the proprietary technologya as a cost-effective, safe and sustainable alternative for alkylation production. Table 1 summarizes the six commercial units presently in operation in China. Additional licensed units are either at the engineering stage or under construction.

The technology has been under development for 20 yr. Significant process improvements have been made in recent years, concurrent with widespread commercial adoption in Asia-Pacific. In 2005, the first commercial field demonstration was successfully performed at the PetroChina Lanzhou refinery by retrofitting an existing, 65,000-metric-tpy H2SO4 alkylation unit with the proprietary catalyst.7

In 2013, an independent refiner, Deyang Chemical Co. Ltd., commissioned a greenfield, 100,000-metric-tpy unit. The commercial process performance data from this operation were published in the March 2018 issue of Hydrocarbon Processing.6 From 2017–2018, 10 new units were licensed to both state-owned and private Chinese refiners.

Notably, the commissioning of the Sinopec Wuhan unit is the world’s first commercial-scale revamp from an HF-based alkylation process. This revamped unit was one of two remaining HF-based alkylation processes in operation in China.

Commercial process performance

The following sections depict the process performance of the brownfield, 150,000-metric-tpy unit at PetroChina Harbin Petrochemical Co. Ltd.’s Heilongjiang plant. The unit was commissioned in November 2018 (Fig. 1).

Fig. 1. A brownfield, 150,000-metric-tpy unit.

The process flow diagram for this installation is shown in Fig. 2. It consists of four key sections: feed pretreatment system, reaction system, catalyst regeneration system, and product separation and purification system.

Fig. 2. Process flow diagram for the alkylation technology.

In March 2019, the operator conducted a calibration test to benchmark and compare commercial process performance data against design specifications and identify optimization opportunities. To date, no safety-related incidents or concerns have been identified by the operator.

The process operating flexibility of the Harbin unit was 60%–110% of the designed processing capacity, and the feedstock originated from an upstream MTBE unit. For this operation, a feed pretreatment hydrogenation unit was required for the hydrogenation of methyl ethyl ketone. Caustic wash and dechlorinating agents were used for product treatment.

Table 2 compares the commercial process operating parameters for key equipment against the design process operating parameters. The inlet and outlet temperatures of the alkylation reactor were 15°C higher than the design values. Previous exploratory testing has indicated an inverse relationship between alkylation reaction temperature and alkylate research octane number (RON8); however, the luxury of producing higher RON alkylates are often offset by feed cooling cost considerations.

Table 3 compares the commercial and design feed compositions. The olefins content of the design feed (44 wt%) was greater than that of the commercial feed (34 wt%). The design feed assumed a high purity (95 wt%) n-butane stream, whereas the commercial feed was more heterogeneous and contained a significant quantity of isobutane (25 wt%). Conversely, the commercial isobutane stream had marginally higher purity (92 wt%) than the design value (89 wt%), but also contained trace olefins content (< 0.11 wt%).

MTBE will be phased out in China in 2020. The design of the subject unit was based on feed from fluid catalytic cracker (FCC) offgas bypassing the MTBE unit, which contains significant amounts of isobutene. Isobutene is known to have detrimental effects on H2SO4 catalyzed isobutane alkylation reaction systems: acid-soluble oil formation,9 reduced octane number10 and high catalyst consumption.11

Table 4 compares the commercial and design alkylate product specifications. Despite the aforementioned deviations from design values for both reaction temperature and feed quality, the robustness of the alkylation process was demonstrated through its ability to meet or exceed process performance measures for key product specifications, such as RON and vapor pressure. Additionally, the low alkylate chloride content indicated that the product treatment dechlorination agent was highly effective.

Table 5 shows the composition and carbon chain length of commercial alkylate product. The process achieved 100% olefins conversion, and C12+ compounds were not detected in the product stream. The C8 alkylate yield was 77%, which is comparable to or exceeds the performance of best-in-class, competitive, acid-based alkylation processes.

Table 6 compares the commercial and design utilities consumption (normalized to a kg-of-oil-equivalent standard). The commercial energy consumption was slightly higher than the design value, which was expected since the commercial processing throughput was lower than the design processing capacity as a result of feed availability. Nevertheless, utilities consumption was within 1.5% of the design specifications.

Table 7 compares the commercial and design catalyst consumption for the process. During operation, the majority of catalyst is regenerated onsite via the composite ionic liquid regeneration unit, and catalyst activity is maintained through the addition of an organic chloride activator compound. A small amount of spent catalyst is removed from the process as a chemically benign waste stream, and is made up through the addition of catalyst active reagents. The design catalyst consumption was based on performance of a previous commercial unit commissioned in 2013. However, due to recent process and catalyst manufacturing improvements, total catalyst consumables for the Harbin unit decreased by 46%, leading to significant ongoing cost savings for the operator.

Takeaway

The commercially proven, high-performance alkylation processa meets the safety and environmental objectives of stakeholders in the 21st century. The novel nature of this technology allows for continuous improvement to identify and realize optimization and cost-saving opportunities, which are beneficial for operators and their owners.

The robustness of the technology is demonstrated through its ability to produce alkylate that meets or exceeds design specifications, even during deviation from prescribed design feed specifications. Despite already exceeding alkylate RON expectations, if required, the Harbin Petrochemical unit is capable of producing even higher-quality alkylate products by operating at lower temperatures. HP

NOTE

a Ionikylation, a composite ionic liquid catalyzed alkylation technology developed by the China University of Petroleum–Beijing and licensed by Well Resources Inc.

ACKNOWLEDGEMENTS

Dr. Zhichang Liu provided useful discussion. The National Natural Science Foundation of China (Grants No. 21425626, No. 21036008, No. 20976194 and No. 20206018) and Shell Global Solutions International BV provided exploratory and strategic research grants. The China State Council granted the National Invention Award for Composite Ionic Liquid Catalyzed Alkylation technology.

LITERATURE CITED

  1. Pawloski, J., “Refining: Alkylation—What’s a refiner to do?” Hydrocarbon Processing, November 2017.
  2. Brelsford, R., “Chinese refiners ramp up alkylation capacity,” Oil & Gas Journal, January 2019.
  3. U.S. Chemical Safety and Hazard Investigation Board, “Factual update: Fire and explosions at Philadelphia Energy Solutions refinery hydrofluoric acid alkylation unit,” October 2019, online: https://www.csb.gov/assets/1/6/pes_factual_update_-_final.pdf
  4. Zhang, S., L. Wilkinson, L. Ogunde, R. Todd, C. Steves and S. Haydel, “Alkylation technology study final report—South Coast Air Quality Management District (SCAQMD),” Norton Engineering, September 2016.
  5. China Financial News, “Environmental crackdown on spent sulfuric acid from alkylate producers,” October 2016.
  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. Liu, Z., R. Zhang, C. Xu and R. Xia, “Ionic liquid alkylation process produces high-quality gasoline,” Oil & Gas Journal, Vol. 104, 2006.
  8. Hu, P., Y. Wang., X. Meng., R. Zhang., H. Liu, C. Xu and Z. Liu, “Isobutane alkylation with 2-butene catalyzed by amide-AlCl3-based ionic liquid analogues,” Fuel, Vol. 189, February 2017.
  9. Albright, L. F., K. E. Kranz and K. R. Masters, “Alkylation of isobutane with light olefins: Yields of alkylates with different olefins,” Ind. Eng. Chem. Res., Vol. 32, 1993.
  10. Albright, L. F., M. A. Spalding, J. A. Nowinski, R. M. Ybarra and R. E. Eckert, “Alkylation of isobutane with C4 olefins: First-step reaction using sulfuric acid catalyst”, Ind. Eng. Chem. Res., Vol. 27, 1988.
  11. Albright, L. F., M. A. Spalding, J. Faunce and R. E. Eckert, “Alkylation of isobutane with C4 olefins: Two-step process using sulfuric acid catalyst,” Ind. Eng. Chem. Res., Vol. 27, 1988.

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