M. W. Da Silva, Petrobras, São José dos Campos, Brazil
In June 2019, a process accident occurred at Philadelphia Energy Solutions’ refinery. This incident restarted the discussion about the security of naphtha alkylation units based on hydrofluoric acid (HF) as a homogeneous catalyst. In the past few years, some refiners have decided to stop the operation of HF alkylation units due to the unacceptable safety risks.
The high toxicity and corrosion risks associated with HF require a sizable amount of attention from inspection equipment teams to control the integrity of process equipment. When this management system fails, the consequences can be dire.
Over the last 20 yr, the following process accidents involving alkylation units have brought significant attention regarding the safety characteristics of these process units:
This short list of process accidents has sounded a warning to authorities of the significant safety risks associated with the presence of HF alkylation process units near urban centers. The release of HF can affect the population and cause severe damage and loss of life. According to the U.S. Chemical Safety and Hazard Investigation Board, 46 HF naphtha alkylation units are in operation in the U.S.
It should be noted that naphtha alkylation technology as an octane boosting route can be attractive to refiners in markets with high gasoline demand and availability of liquefied petroleum gas (LPG), such as in the U.S. This article provides information on available technologies that can ensure an adequate balance between process safety and gasoline quality.
An interesting case study is the Brazilian market, which has historically relied on imports to satisfy domestic LPG demand. In 2020, the internal production of LPG in Brazil was 9.86 MMm3/yr, with refineries supplying 74% (7.34 MMm3/yr) and petrochemical plants and natural gas processing units supplying the remaining percentage. In the same year, LPG imports in Brazil reached 3.62 MMm3/yr.
With the development of pre-salt crude oil reserves, Brazil’s natural gas production is increasing, and the production of associated LPG tends to reduce the external dependence of this derivative. According to the Brazilian Petroleum Agency (ANP), Brazil’s natural gas imports decreased by 20% in 2020.
This scenario has led Brazilian refiners to consider the naphtha alkylation route for octane boosting to produce low-sulfur and high-quality gasoline. Considering the growing propylene gap, another attractive route to monetize LPG surplus is building propane dehydrogenation (PDH) units.
Despite the risks related to the operation of naphtha alkylation units, alkylate is an important octane booster for gasoline. According to GlobalData, global alkylation capacity reached nearly 2.7 MMbpd in 2023 and is forecast to increase 2%/yr to 2028.1
Although the U.S. is the leader in total naphtha alkylation capacity, China represents the strongest growth in new capacity builds.
Gasoline production process: The relevance of octane boosters. Gasoline is produced by blending different streams (FIG. 1). The use of straight-run and reformed naphtha is normally minimized to direct these streams to the petrochemical intermediates market due to the higher added value of these streams.
Cracked naphtha [naphtha from fluid catalytic cracking units (FCCUs)] helps increase gasoline’s octane number during processing. However, due to restrictions related to sulfur content in gasoline [a maximum of 10 parts per million (ppm)], the use of cracked naphtha without a treatment step is limited. Refineries that have catalytic alkylation units in their refining scheme normally direct this stream to produce aviation gasoline, which has a higher market value when compared with automotive gasoline. For this reason, the amount of alkylation naphtha is minimized in the composition of this fuel.
Isomerization naphtha has a low contaminants content (sulfur and nitrogen) and a high octane number. For these reasons, the amount of this stream in the formulation of gasoline is maximized in refineries that have isomerization units in their refining scheme. In markets with high gasoline demand, refiners can add butanes to the gasoline pool; however, the amount is limited due to the high vapor pressure of this stream. Normally, butanes are added to the LPG pool, respecting the limits to avoid breaking the mixture’s quality.
Due to the environmental and quality regulations of gasoline, the hydrodesulfurization of cracked naphtha can be utilized to adhere to sulfur content maximums in produced gasoline—this can be as low as 10 ppm in many markets around the world. The alkylated naphtha is a primary octane booster in the gasoline pool and is a fundamental stream for refiners to achieve specification requirements.
Traditional technological routes to naphtha alkylation. Gasoline is one of the most consumed crude oil derivatives. It is normally produced through a mixture of naphtha from different refining processing steps. The streams normally involved in gasoline production are straight-run naphtha, cracked naphtha, coke naphtha (after hydrotreatment) and reforming naphtha.
One of the main parameters of gasoline quality is the octane number, which is a measure of combustion quality. One of the streams that contributes to raising the octane number is reformed naphtha produced in the catalytic reforming unit. However, due to stringent restrictions on carcinogenic aromatic emissions (primarily benzene), some refiners have avoided the application of this stream to formulate gasoline, directing the reformed naphtha to the production of petrochemical intermediates in aromatics complexes.
An alternative to reforming naphtha is the production of branched hydrocarbons (with high octane numbers) through the catalytic alkylation process, which involves the reactions between light olefins (C3–C5) and isoparaffinic hydrocarbons like isobutane. The reaction product—called alkylate—is a mixture of branched hydrocarbons with a high molecular weight and a high octane number. An example of a typical alkylation reaction is shown in Eq. 1:
C4H10 + C3H6→ C7H16 (2,3 Dimethylpentane) (1)
The reaction is catalyzed in a strongly acidic reaction environment. The acids normally employed in industrial-scale technologies are HF and sulfuric acid (H2SO4). The primary advantages of the alkylation process are high octane numbers, high levels of chemical stability, and streams nearly free of contaminants such as nitrogen and sulfur. These characteristics make alkylate an attractive component to gasoline formulations for the automotive and aviation industries.
Alkylation feed is generally obtained from LPG produced in deep-conversion units, mainly the FCCU and the delayed coker. The LPG produced in these processing units has a high olefins content, which is ideal for the alkylation process. The isobutane stream is normally obtained through the separation of LPG produced in the atmospheric distillation unit, the FCCU or the delayed coker in deisobutanizer towers.
The acids generally employed as the homogeneous catalysts to the alkylation process are HF and H2SO4. FIG. 2 presents a process flow diagram (PFD) for the alkylation process catalyzed by HF. The feed stream travels through a pretreatment step (generally molecular sieves or alumina) before being pumped to the reactor. The objective is to remove process contaminants such as water (H2O), diolefins, and sulfur and nitrogen compounds. H2O is especially damaging to the process, as it accelerates corrosion in piping and equipment.
After pretreatment, the hydrocarbon streams are put in contact with the HF in the reactor, and the hydrocarbons mixture and HF solution are separated through gravity in a settler vessel. The hydrocarbon phase is sent to the fractionating section, while the aqueous phase (containing most of the HF) is cooled and sent back to the reactor. As alkylation reactions are exothermic, the reactor is continuously refrigerated to maintain ideal conditions.
A portion of the HF is sent to the stripping column, where the acid is removed with isobutane. The top product is a mixture of HF and isobutene, which is sent back to the reactor. The bottom stream contains an azeotropic mixture of H2O and HF. This step keeps the HF free of contaminants.
After the separation columns, the butane and propane streams are treated with alumina to decompose organic fluorides with potassium hydroxide (KOH) to neutralize the remaining acidity. The alkylate stream is treated with sodium hydroxide (NaOH) to neutralize the remaining acidity.
The alkylate stream is normally directed to the refinery’s gasoline pool to produce high-octane automobile gasoline or aviation gasoline; however, in petrochemical plants, this stream can be used as an intermediate to produce ethylbenzene (to produce styrene), isopropylbenzene (to produce phenol and acetone) and dodecylbenzene (used to produce detergents). Propane and butane streams can be sent to the refinery’s LPG pool or commercialized separately.
The alkylation process using H2SO4 as a catalyst is similar to the HF process. However, the H2SO4 regeneration step is more complex, and involves the decomposition of H2SO4 into SO2 and SO3 and the subsequent condensation of concentrated H2SO4. This regeneration can be conducted onsite or in an external processing plant. Consequently, the H2SO4 consumption in the process is much higher than HF. Furthermore, the solubility of H2SO4 in hydrocarbons is lower, requiring greater agitation to maintain the contact between the phases.
Widely used alkylation technologies are Elessent Clean Technologies’ STRATCO® Effluent Refrigerated Alkylation™ process, ExxonMobil Chemical’s and Axens’ ALKEMAX™ process, and Lummus Technology’s CDAlky™ process. FIG. 3 shows a simplified PFD of an alkylation unit incorporating H2SO4 technology. The olefins feed stream goes to a coalescer to remove H2O. The isobutane recycle mixture is sent to the reactor. The mixture of hydrocarbons and acid head to a settler where the phase separations occur. The organic phase is sent back to the reactor, while a control valve provides the necessary pressure reduction to vaporize the lighter hydrocarbons and remove heat from the reactor. This helps control the equipment’s temperature, which increases from the exothermic reaction of the alkylation process.
The hydrocarbon blend is sent to a flash drum, where the lighter phase is directed to a compressor to condense in an accumulator vessel, and the propane is recovered in the depropanizer tower, while the heavier hydrocarbons (essentially isobutane) are recycled to the reactor. The stream containing the alkylate is directed to a caustic treatment and then to a deisobutanizer column where the alkylate is removed.
As mentioned, the need for catalyst replacement is higher in the H2SO4 process; however, the HF process needs a higher isobutane/olefins ratio, which means a greater separation system. Over the past few decades, refiners have opted to use HF alkylation technologies due to the greater simplicity of the process. It also requires less of a need for catalyst replacement, and this results in lower operational costs.
However, regulatory pressures have led some refiners to convert their HF alkylation units to operate with H2SO4. Due to the high volatility and higher risks presented by HF, some licensors have developed technologies to convert HF units to H2SO4 units. The principal process variables of the alkylation process are the isobutane/olefins ratio, reaction temperature, acid/hydrocarbon ratio, acid purity, residence time in the settler, and operational pressure.
Alkylation processes using HF have reaction temperatures from 20°C–40°C, while H2SO4 processes operate at lower temperatures—e.g., 4°C–10°C. At higher temperatures, H2SO4 can suffer decomposition to SO2 and SO3. The operating pressure is generally sufficient to keep the hydrocarbons in the liquid phase (normally < 5 bar). The residence time in the settler is important, as not enough time can lead to undesirable reactions and the production of organic fluorides. With HF, catalyst consumption is increased and alkylate production is decreased.
Acid purity must be maintained as high as possible through the removal of acid soluble oil (ASO), H2O and dissolved reactants in the HF case, and through fresh acid replacement in processes with H2SO4 as the catalyst.
The main disadvantages of the alkylation processes with homogeneous catalysts (HF or H2SO4) include greater process safety risks and higher maintenance costs (primarily related to corrosion in piping and equipment).
Alternatives and safer technologies to naphtha alkylation. With an aim to eliminate these risks, some licensors have dedicated their efforts to developing heterogeneous catalysts that can replace the strong acids used in alkylate production processes (e.g., Honeywell UOP, Lurgi, Süd-Chemie, Topsoe).
One attractive alkylation technology is Honeywell UOP’s ISOALKY™ process (FIG. 4), which is based on an ionic liquid catalyst. The heterogeneous catalyst applied in the process—in substitution of liquid acids—is based on zeolites and can offer higher operational safety for refiners and adjacent communities.
Lummus Technology has developed a solid catalyst naphtha alkylation process called AlkyClean® (FIG. 5)—the process was developed by CBI-Lummus and Albemarle. K-SAAT™—developed by KBR—is another commercial naphtha alkylation technology based on solid catalyst (FIG. 6).
Despite the effectiveness of naphtha alkylation processes based on strong acids, the sizable number of incidents and accidents involving these units has forced governments and regulators to increase pressure on refiners using these units. As previously noted, there are viable technical alternatives to producing alkylated naphtha in a safe manner.
Revamping brownfield naphtha alkylation units from HF or H2SO4 conventional homogeneous catalysts to solid catalysts can be an attractive area for capital investments. For example, the U.S. has more than 45 naphtha alkylation units that could ultimately be converted. As shown in FIG. 7, alkylates have a significant market share in North America’s gasoline pool. With the need to produce low-sulfur and high-octane gasoline, alkylation processing units are imperative to comply with high-quality, clean gasoline.
Takeaway. The catalytic alkylation process is very attractive to countries with high gasoline consumption and a sizable availability of LPG. This process can produce high-octane gasoline with low levels of contaminants, although there are high capital investment and operational costs associated with this technology.
Despite the risks associated with handling strong acids like HF and H2SO4, naphtha alkylation units are necessary in many markets to satisfy domestic gasoline demand. It is important to note that HF and H2SO4 naphtha alkylation processing units must be monitored closely to ensure the integrity of these critical pieces of equipment. The failure of these units can be catastrophic, as detailed in the Philadelphia Energy Solutions’ incident in 2019 and other refinery cases mentioned in the opening of this article. However, when monitored correctly, new naphtha alkylation technologies can ensure an adequate balance between gasoline quality and process safety requirements. HP
LITERATURE CITED
Marcio Wagner da Silva is a Process Engineering Manager at Petrobras. He has extensive experience in research, design and construction in the oil and gas industry, including developing and coordinating projects for operational improvements and debottlenecking bottom-barrel units. In Brazil, he earned a Bch degree in chemical engineering from the State University of Maringa, and a PhD in chemical engineering from the State University of Campinas (UNICAMP). In addition, he earned an MBA degree in project management from the Federal University of Rio de Janeiro, and second MBA in digital transformation at the Pontifical Catholic University of Rio Grande do Sul. Dr. Silva is also certified in business by the Getúlio Vargas Foundation.