Advances in alkylation technology

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I. Overview: The main components of alkylated oils are octane and its isomers; they feature a high octane rating and low vapor pressure, making them excellent components for gasoline blending. Statistics show that in Europe and the United States, the proportion of alkylated oils in gasoline is high. United States: 12.5%, Europe: 7%, China: less than 0.53%. With the advent of an era of high oil prices, efforts have been made to develop alternative energy sources; technologies such as methanol-to-gasoline and biomass-to-oil production have emerged one after another. At this point, it is necessary to re-evaluate the feasibility of utilizing liquefied gas resources to produce high-octane gasoline. As natural gas is increasingly used as a fuel for household purposes, the demand for liquefied petroleum gas for domestic use is declining, and the price gap between liquefied petroleum gas and gasoline is growing larger. This provides a profit margin for the alkylation unit. Producing gasoline from liquefied gas resources is simpler and more feasible than using other alternative energy sources. After alkylation of the C4 fraction, the reaction mixture consists only of isobutane, n-butane, and alkylated oil, making it easy to separate high-purity n-butane. N-butane is an excellent raw material for ethylene cracking as well as for the production of various chemical products. Gasoline standards are continuously rising, and the limits on the content of olefins and aromatics in gasoline are becoming increasingly strict. The reduction of these octane-contributing components will lead to insufficient octane levels in the refinery gasoline pool, and this effect is particularly pronounced in refineries that produce synthetic fibers. Euro IV gasoline requires a minimum of 20% (v/v) of distillate at 70°C (E70); it is difficult to meet this requirement without adding alkylated oil fractions. The excellent properties of alkylated oils make them an indispensable component in gasoline blending. High oil prices, strict gasoline standards, and the widespread use of natural gas provide favorable opportunities for the development of alkylation units. II. Advances in Alkylation Technology Alkylation technology was developed and refined during World War II to meet the demand for aviation gasoline during the war. After the war, it played an important role in the increase of gasoline octane ratings and the prohibition of lead in automotive gasoline. Before the 1990s, the representative alkylation technologies included UOP’s hydrofluoric acid alkylation process, PHILLIPS’ hydrofluoric acid alkylation process, and STRATCO’s sulfuric acid alkylation process. With the increasing demands for environmental protection and safe, clean production, the rapid development of alkylation technology has been facilitated. Traditional and mature alkylation techniques have been continuously improved to meet the demands of societal development, while new safe and clean production technologies such as solid acid alkylation and ionic acid alkylation have emerged. There are many types of alkylation technologies. Based on the phase state of the catalyst, they can be classified into two main categories: liquid acid alkylation and solid acid alkylation. Liquid acid alkylation techniques include three types: hydrofluoric acid, sulfuric acid, and ionic acids. Liquid acid alkylation yields alkylated oils with a high octane number, as it utilizes a catalyst with high acidity, operates at lower reaction temperatures, requires lower reaction pressures to maintain a liquid phase reaction; however, it presents safety and environmental risks ； In the solid acid alkylation technique, the acid strength of the catalyst is low, the reaction temperature is usually high, and the octane number of the resulting alkylated oil is low. Due to the increased tendency for olefin polymerization, the catalyst tends to become deactivated more easily, yet the production process is safe and environmentally friendly. 1. Liquid acid alkylation technology 1.1 Hydrofluoric acid alkylation The first companies to develop hydrofluoric acid alkylation technology were UOP and PHLLIPS. The reaction conditions for these two technologies are essentially the same; the main difference lies in the reaction system: UOP uses forced acid circulation and internal regeneration techniques ； PHLLIPS uses acid gravity circulation and acid recontact technology. The two technologies are of comparable performance; however, since the PHLLIPS technology uses gravity-driven catalyst circulation, it eliminates the need for rotating equipment in environments with high acid concentrations, thereby reducing acid leakage points and enhancing safety. In recent years, UOP acquired PHILLIPS’ alkylation technology and, by combining the advantages of both technologies, developed the AlkyPlus technology. This technology incorporates PHILLIPS’ original gravity acid circulation technique and design concepts for acid settlers, along with UOP’s isothermal reactor technology, to create a new reaction system. It reduces the acid inventory by 30%, enhancing the safety of the device. The weakness of the alkylation using hydrofluoric acid lies in the fact that the hydrofluoric acid catalysts used are corrosive, volatile, and toxic. To improve the safety and environmental compatibility of such systems, in addition to proactive preventive measures such as detailed engineering specifications for the design of the equipment and the selection of valves, passive measures have also been adopted, including the use of water curtains for isolation in the reaction area ； Use color-changing paint and industrial television at the areas prone to leakage ； A tool neutralization tank, a personal neutralization tank, eyewash stations, and full-body shower facilities are installed within the facility ； Provide operators with special protective clothing and equipment ； There are strict procedures for various operations of the device ； It specifies first-aid measures after contact with hydrofluoric acid and topical ointments for the skin. To meet the increasingly stringent environmental regulations, UOP and PHLLIPS developed technologies aimed at improving the safety and environmental friendliness of hydrofluoric acid alkylation. The AlkyPlus technology resulting from the merger of these two companies retains such technologies: the ReVAP technique for reducing vapor pressure. Hydrofluoric acid is a gas at room temperature, and this technique allows the volatility of hydrofluoric acid to be reduced by adding certain substances, thereby cutting the evaporation of leaked hydrofluoric acid by over 90%. The multi-point feeding technique involves dividing the olefins into multiple streams and feeding them from different locations in the reactor or riser. This allows for a reduction in the amount of isobutane that needs to be recycled, as well as a decrease in the amount of acid present. The IMP acid management technology refers to a procedure that, in the event of a rupture in the settler, rapidly transfers the hydrogen fluoride and hydrocarbons stored therein into intact storage containers, thereby preventing large amounts of hydrogen fluoride from leaking out. By adopting the aforementioned new technology, the risk of environmental pollution has been effectively reduced, bringing it to a risk level of 1.2, which is acceptable to the American public and environmental protection agencies. Sulfuric acid alkylation is an alkylation process that uses liquid sulfuric acid as a catalyst; due to the low reaction temperature and the high viscosity of sulfuric acid at such temperatures, mixed approaches are necessary to ensure good contact between the acid and the hydrocarbons. Different mixing methods have given rise to various patented technologies. Representative patent technologies include DuPont’s STRATCO technology and LUMMUS’s CDAlky technology. The STRATCO technology reactor is a horizontal tubular heat exchanger equipped with an impeller for mixing, utilizing mechanical stirring to achieve the mixing of acid and hydrocarbons ； The reaction temperature is 5–8°C, and indirect heat exchange is used to remove the heat generated during the reaction ； Before reaching the distillation unit, the reaction products must undergo acid washing, alkali washing, and water washing ； The processing capacity of a single reactor is 50,000–100,000 tons per year ； The technology is mature, with many industrial installations in operation. The CDAlky technology involves a vertical reactor equipped with special fillers; acid and hydrocarbon mixing is achieved through the combination of a distributor and these fillers, without the use of any rotating equipment ； The reaction temperature is -3°C, with the heat of reaction removed through hydrocarbon self-vaporization ； The reaction product does not require any washing before entering the fractionation unit. The processing capacity of a single reactor can reach 380,000 tons per year ； Laboratory installations have already been built; Ningbo Haiyue in China is using this technology and is currently in the design phase. Both technologies employ waste acid regeneration techniques to address the issue of large amounts of waste acid generated by the alkylation unit. The spent acid regeneration unit can produce liquid sulfuric acid directly without the use of sulfur, thereby changing the current situation in refineries where sulfur recovery is limited to the production of sulfur alone and increasing the flexibility of sulfur recovery. In this regard, influenced by the traditional domestic sulfuric acid production structure, it has not yet played a role. 1.3 Ionic acid alkylation This technology was developed by China University of Petroleum (Beijing). It utilizes a liquid-phase catalyst formed by a specially formulated non-aqueous solvent and aluminum chloride; this catalyst helps to avoid the corrosion issues associated with conventional aqueous solutions of aluminum chloride ； An adiabatic tubular reactor is used ； The separation of the catalyst from the liquid is achieved using a centrifugal separator ； 2. Solid acid alkylation technology: To reduce environmental pollution during the production process, many major oil companies and research institutions around the world have been working on the development of solid catalysts. Notable technologies include LUMMUS Corporation’s AlkyClean process, UOP Corporation’s Alkylene and Inalk processes, and TOPSOE Corporation’s FBA process. 2.1 The LUMMUS AlkyClean process utilizes multiple liquid-phase fixed-bed reactors (usually three reactors). When one reactor is taken out for regeneration, a standby reactor is brought into operation within the reaction system; one of the three reactors is used for gradual regeneration, another for alkylation reactions, and the third for high-temperature regeneration. These reactors operate in turn to ensure continuous operation of the reaction process. This process has been successfully demonstrated on an industrial scale at the refinery of Fortum Oil & Gas in Finland. 2.2 The UOP Alkylene process utilizes a single, continuously operating liquid-phase reactor. This reactor consists of a vessel along with a lift pipe located at its center; the catalyst and feed materials enter the lift pipe from its bottom where the alkylation reaction takes place. At the upper end of the lift pipe, the reaction products and catalyst flow into the space outside the vessel. Due to gravity, the catalyst flows downward to the lower part of the lift pipe, thereby enabling catalyst recycling. In the annular space between the lift pipe and the vessel, isobutane saturated with hydrogen is introduced, creating a regeneration and washing zone within the vessel. A stream of catalyst is drawn from the upper part of the regeneration wash zone and enters an external regeneration washer; the regeneration medium remains hydrogen-saturated isobutane, only the regeneration temperature is higher than that in the internal regenerator, and the regenerated catalyst also flows to the bottom of the lift pipe. 2.3 UOP Inalk process: This process is known as indirect alkylation technology. Strictly speaking, it is not an alkylation process; however, it also converts isobutane and all olefins present in C4 hydrocarbons into C8 gasoline fractions, using solid catalysts as well. The actual reactions that occur are isobutane dehydrogenation, tetraene oligomerization, and olefin saturation. This process can achieve the conversion of all C4 compounds into alkylation oil through a n-butane isomerization section, an isobutane dehydrogenation section, and a butene polymerization and hydrogenation section. Whether to use the n-butane isomeric fraction or the isobutane dehydrogenation fraction depends on practical needs. The octane number of the alkylated oil after hydrogenation is very high; the RON of resin catalysts can reach 99, while that of solid phosphoric acid catalysts can reach 101. This process is particularly suitable for retrofitting MTBE plants. 2.4 The reactor in the TOPSOE FBA process is of fixed-bed type, and it contains solid carriers that have absorbed liquid superacids; these are known as supported liquid phase catalysts. It is the liquid superacids that actually carry out the catalytic function. As the reaction fluid flows downward, an acid-containing zone known as an active pool is formed within the catalyst bed. Above the active zone is the acid-poor region, where olefins and acid combine to form esters. These esters are not easily adsorbed and can flow with the reaction stream to the next region. The middle part is the active zone, where the esters that have flowed in from above come into contact with isobutane; due to the high acid concentration, an ester exchange reaction occurs, resulting in the formation of alkylated oil and acid. The lower part is the acid-absorption region, where the acid is re-adsorbed onto the solid carrier, thus creating a new active zone. As the reaction proceeds, the active zone slowly moves downward. The acid carried away by the reaction stream is separated from the hydrocarbons and sent to the acid recovery system. The acid-soluble oils discharged intermittently from the reaction system also contribute to acid recovery within this system. All the acid recovered by the acid recovery system, along with any additional acid added, is returned to the reactor to maintain an equilibrium of acid levels therein. This reaction system **reduces the amount of liquid acid present, results in less acid-soluble oil, and exhibits extremely low acid consumption.** Since the acid is adsorbed on a solid carrier, it will not evaporate even if the reactor is damaged. Looking at the development of alkylation technology, improving the clean and safe production of alkylation products has been the main focus of technological advancement. Solid acids can address issues related to cleaning and safe production, but they are limited by the maturity of the technology ； The sulfuric acid alkylation process, although it solves the problem of waste acid through acid regeneration techniques, increases investment costs ； The regeneration of alkylated waste acid using hydrofluoric acid is straightforward, but it is limited by the volatility and corrosiveness of hydrofluoric acid. As long as safety procedures are strictly followed, proper maintenance of the equipment is ensured, and new technologies are employed. The hydrofluoric acid method for alkylation remains one of the available techniques for alkylation production.