Alkylation unit
An alkylation unit is one of the conversion processes used in petroleum refineries. It is used to convert isobutane and low-molecular-weight alkenes into alkylate, a high octane gasoline component. The process occurs in the presence of an acid such as sulfuric acid or hydrofluoric acid as catalyst. Depending on the acid used, the unit is called a sulfuric acid alkylation unit or hydrofluoric acid alkylation unit. In short, the alky produces a high-quality gasoline blending stock by combining two shorter hydrocarbon molecules into one longer chain gasoline-range molecule by mixing isobutane with a light olefin such as propylene or butylene from the refinery's fluid catalytic cracking unit in the presence of an acid catalyst.
Since crude oil generally contains only 10–40% of hydrocarbon constituents in the gasoline range, refineries typically use an FCCU to convert high molecular weight hydrocarbons into smaller and more volatile compounds, which are then converted into liquid gasoline-size hydrocarbons. Byproducts of the FCC process also include other low molecular-weight alkenes and iso-paraffin molecules which are not desirable. Alkylation transforms these byproducts into larger iso-paraffins molecules with a high octane number.
The product of the unit, the alkylate, is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons. Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. The octane number of the alkylate depends mainly upon the kind of alkenes used and upon operating conditions. For example, isooctane results from combining butylene with isobutane and has an octane rating of 100 by definition. There are other products in the alkylate effluent, however, so the octane rating will vary accordingly.
Capacity installed and available technologies
The first alkylation units entered in service in 1940. In 2009 around 1,600,000 barrels per day of capacity were installed worldwide, with an equal share of 800,000 barrels per day for SAAU and HFAU technologies. On the 1st January 2016 according to the Oil & Gas Journal the worldwide installed alkylation capacity was 2,056,035 barrels per day. Since 2009 over 90% of the additional installed capacity was based on SAAU technology.According to the Oil & Gas Journal on the 1st January 2016 there were 121 refineries operated in US with an overall capacity of 18,096,987 barrels per day. These refineries had 1,138,460 barrels per day of alkylation capacity.
Alkylate is a component of choice in gasoline, because it is free of aromatics and olefins. About 11% of the gasoline winter pool in the U.S. is made up of alkylate. In the gasoline summer pool, the content of alkylate can be as high as 15% because lower Reid vapor pressure reduces the possibility to blend butane.
For safety reasons, SAAU is the prevalent current technology of choice. Indeed, in 1996 around 60% of the installed capacity was based on HF, but since then this ratio has been reducing because during the last decade on 10 new alkylation units commissioned, more than 8 of them were SAAU.
The two major licensors of the HFAU process were UOP and ConocoPhillips, which have been combined as UOP under the ownership of Honeywell. The main technology used for the SAAU is the STRATCO process licensed by DuPont, recently divested into privately held Elessent Clean Technologies. This is followed by the EMRE technology owned by ExxonMobil. From the mid-2000s to the mid-2010s, in excess of 85% of the SAAU capacity added worldwide has utilized Elessent's STRATCO technology.
Catalysts
The availability of a suitable catalyst is also an important factor in deciding whether to build an alkylation plant.Sulfuric acid
In a sulfuric acid alky, significant volumes of the acid are used. Access to a suitable plant is required for the supply of fresh acid and the disposition of spent acid. Constructing a sulfuric acid plant specifically to support an alkylation unit has a significant impact on both the initial requirements for capital and ongoing costs of operation. It is possible to install a WSA Process unit to regenerate the spent acid. No drying of the gas takes place, which means no loss of acid, no acidic waste material, and no heat is lost in process gas reheating. The selective condensation in the WSA condenser ensures that the regenerated fresh acid will be 98% weight, even with the humid process gas. It is possible to combine spent acid regeneration with disposal of hydrogen sulfide by using the hydrogen sulfide as a fuel.Hydrofluoric acid
The typical hydrofluoric acid alkylation unit requires far less acid than a sulfuric acid unit to achieve the same volume of alkylate. The HF process only creates a small amount of organofluorine side products which are continuously removed from the reactor and the consumed HF is replenished. HF alky units are also capable of processing a wider range of light-end feedstocks with propylenes and butylenes, and produce alkylate with a higher octane rating than sulfuric plants. However, extreme caution is required when working with or around HF. Due to its hazardous nature, the acid is produced at very few locations and transportation is stringently managed and regulated.Solid acids
Research in the area of a solid catalyst for alkylation has been ongoing for many years. Numerous patents exist for different catalysts, catalyst supports, and processes. Lewis acids will catalyze the alkylation reaction. Several of the current preferred solid catalysts use a salt of HF: either boron trifluoride or antimony pentafluoride. Since every alkylation process produces heavy polymers, solid catalysts have the tendency to foul quickly. Therefore, solid catalyst processes have two major hurdles to overcome: catalyst life and catalyst regeneration.Solid alkylation catalyst technology was first commercialized on August 18, 2015, with the successful start-up of an alky unit at the Wonfull Refinery in Shandong Province, China. The unit uses the AlkyClean® process technology jointly developed by Albemarle Corporation, CB&I and Neste Oil, and has a capacity of 2,700 barrels per stream day of alkylate production. The AlkyClean process, together with Albemarle's AlkyStar catalyst produces high-quality alkylate product without the use of liquid acid catalysts in the alkylate manufacturing process.
Ionic liquids
An alternative to using HF and H2SO4 as alkylation catalysts is the use of ionic liquid. ILs are liquid salts that have melting points below 100 °C. They exhibit strong acid properties, so they can be used as acid catalysis without using conventional liquid acids. Ionic liquids are salts in liquid state, composed mostly of ions that convert C4 paraffins and other olefins into excellent gasoline-range blending products.Many parameters are available for fine-tuning IL properties for specific applications, and the choice of cation and anion affects the IL's physical properties, such as melting point, viscosity, density, water solubility, and reactivity. Chloroaluminate IL has been studied in the literature for its ability to catalyze the alkylation reaction. However, pure chloroaluminate IL exhibits low selectivity towards synthesizing high-octane isomers.
A composite ionic liquid alkylation technology called ionikylation has been developed by the China University of Petroleum that utilizes a chloroaluminate IL base and a proprietary mixture of additional IL additives to overcome high-octane isomer selectivity issues. The ionikylation technology is reported to produce alkylate with octane rating generally ranging from 94-96, and as high as 98. The CIL catalyst used in ionikylation is non-hazardous and non-corrosive, which allows entire operating system to be constructed using carbon steel. Three CIL alkylation units, each with 300,000 ton-per-year capacity, came online in China in 2019 at Sinopec’s refineries in Jiujiang City, Anqing City, and Wuhan City. As of 2022, there were three such units operated by PetroChina and seven in total, including a converted HF alkylation unit at Sinopec's Wuhan plant.
Feeds
The olefin feed to an alkylation unit generally originates from a FCCU and contains butene, isobutene, and possibly propene and/or amylenes. The olefin feed is also likely to contain diluents, noncondensables and contaminants. Diluents in principle have no effect on the reaction of alkylation but occupy a portion of the reactor and can influence the yield of secondary reactions of polymerisation and of undesired organofluorine side products. Incondensable are from a chemical perspective similar to diluents but they do not condense at the pressure and temperature of the process, and therefore they concentrate to a point that must be vented. Contaminants are compounds that react with and/or dilute the sulfuric acid catalyst. They increase acid consumption and contribute to produce undesirable reaction products and increase polymer formation. Common contaminants are water, methanol and ethanol.The isobutane feed to an alkylation unit can be either low or high purity. Low purity makeup isobutane feedstock usually originates from the refinery and need to be processed in the deisobutanizer. High purity feedstock normally originates from an external De-isobutanizer tower and is fed directly to the alkylation unit reaction zone. Such isobutane feed does not normally contain any significant level of contaminants.
Mechanism
The catalyst protonates the alkenes to produce reactive carbocations, which alkylate isobutane. The reaction is carried out at mild temperatures in a two-phase reaction. Because the reaction is exothermic, cooling is needed: SAAU plants require lower temperatures so the cooling medium needs to be chilled, for HFAU normal refinery cooling water will suffice. It is important to keep a high ratio of Isobutane to Alkene at the point of reaction to prevent side reactions which produces a lower octane product, so the plants have a high recycle of Isobutane back to feed. The phases separate spontaneously, so the acid phase is vigorously mixed with the hydrocarbon phase to create sufficient contact surface.Unfortunately, a number of secondary reactions take place and they reduce the quality of the Alkylate effluent.
Polymerization results from the addition of a second olefin to the C8 carbocation formed in the primary reaction. The resulting C12 carbocation can continue to react with an olefin to form a larger carbocation. As with the previously described mechanisms, the heavy carbocations may at some point undergo a hydride transfer from isobutane to yield a C12 – C16 isoparaffin and a t-butyl cation. These heavy molecules tend to lower the octane and raise the boiling end point of the alkylate effluent.