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An alkylation unit (alky) is one of the conversion unit process used in Oil industry. It is used to convert isobutane and low-molecular-weight (primarily a mixture of propene and butene) into alkylate, a high octane gasoline component. The process occurs in the presence of an acid such as sulfuric acid (H2SO4) or hydrofluoric acid (HF) as catalyst. Depending on the acid used, the unit is called a sulfuric acid alkylation unit (SAAU) or hydrofluoric acid alkylation unit (HFAU). 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 (FCCU) 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 Alkane (mostly isoheptane and isooctane). 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. (2016). 9780444563538, Elsevier. ISBN 9780444563538
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 (RVP) 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, Solid-acid alkylation process development is at crucial stage 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 (sharing a similar share of the market) of the HFAU process were UOP LLC and ConocoPhillips, which have been combined as UOP LLC 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.
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.
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 (CIL) 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'
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The isobutane feed to an alkylation unit can be either low or high purity. Low purity makeup isobutane feedstock (typically < 70% vol isobutane) usually originates from the refinery (mainly from the reformer) and need to be processed in the deisobutanizer (DIB). High purity feedstock (> 95% vol isobutane) normally originates from an external De-isobutanizer (DIB) tower and is fed directly to the alkylation unit reaction zone. Such isobutane feed does not normally contain any significant level of contaminants.
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.
The purpose of the unit is to react an olefin feed with isobutane in the reaction section in the presence of the HF acting as catalyst to produce alkylate. Prior to entering the reaction section, the olefin and isobutane feed are treated in a coalescer to remove water, sulfur and other contaminants.
Temperature is held at , which is convenient as it does not require refrigeration, and sufficient pressure is maintained so that the components are in the liquid state. (1977). 9780070571457, McGraw-Hill. ISBN 9780070571457
In the fractionation section, alkylate is separated from excess isobutene and acid catalyst through distillation. Unreacted isobutane is recovered and recycled back to the reaction section to mix with the olefin feed. Propane is a major product of the distillation process. Some amount of n-butane that has entered with the feed is also withdrawn as a side product.
Propane and butane that have not been separated from the treated olefin pass through the unit. Although they do not participate directly in the reactions, and adversely impact product quality, they provide an avenue for organic fluorides to leave the unit. The propane stream is removed (typically in a tower called the HF stripper) and are then processed in the defluorinating section to remove combined fluorides and any trace acid that may be present due to mis-operation. Many units also remove Butane, which typically gets treated in a separate defluorinating section.
In the reaction section the reacting hydrocarbons (olefin feed with both fresh and recycled isobutane) are brought into contact with sulfuric acid catalyst under controlled conditions and at a temperature of 15.6 °C (60 °F). The feeds are treated to remove impurities, especially water in order to reduce corrosion.
The heat of reaction is removed in the refrigeration section and the light hydrocarbons are purged from the unit. In the effluent treating Section the free acid, alkyl sulfates and di-alkyl sulfates are removed from the net effluent stream to avoid downstream corrosion and fouling using a settler.
The sulfuric acid present in the reaction zone serves as a catalyst to the alkylation reaction. Theoretically, a catalyst promotes a chemical reaction without being changed as a result of that reaction. In reality, however, the acid is diluted as a result of the side reactions and feed contaminants. To maintain the desired spent acid strength, a small amount of fresh acid is continuously charged to the acid recycle line from the acid settler to the reactor and an equivalent amount of spent acid is withdrawn from the acid settler. In the fractionation section the unreacted isobutane is recovered for recycle to the reaction section and remaining hydrocarbons are separated into the desired products.
The spent acid is degassed in an acid blowdown drum, waste water pH is adjusted and acid vent streams are neutralized with caustic in a scrubber before being flared. Spent acid goes to storage and periodically removed.
In addition to a suitable quantity of feedstock, the price spread between the value of alkylate product and alternate feedstock disposition value must be large enough to justify the installation. Alternative outlets for refinery alkylation feedstocks include sales as LPG, blending of C4 streams directly into gasoline and feedstocks for chemical plants. Local market conditions vary widely between plants. Variation in the RVP specification for gasoline between countries and between seasons dramatically impacts the amount of butane streams that can be blended directly into gasoline. The transportation of specific types of LPG streams can be expensive so local disparities in economic conditions are often not fully mitigated by cross market movements of alkylation feedstocks.
The common source of the C3 alkenes for the alkylation is made available from the gas recovery unit processing the effluents of the Fluid catalytic cracking Unit. Isobutane is partly made available from the catalytic reforming and from the atmospheric distillation, although the proportion of the isobutane produced in a refinery is rarely sufficient to run the unit at full capacity and additional isobutane needs therefore to be brought to the refinery. The economics of the international and local market of gasolines dictates the spread that a buyer need to pay for isobutane compared to standard commercial butane.
For all these reasons the margin of the process is very volatile but in spite of its volalitility during the early 21st century it has been on a growing trend. In 2015 alkylation gross margin (calculated according to the prices of alkylation feedstocks and effluents) on the US Gulf Coast market reached US$80/barrel of alkylate produced while in the decade preceding 2021, it averaged about $40/bbl. As of 2018, alkylation was considered one of the highest value added operations on a refinery.http://futurefuel.kstudiofx.com/wp-content/uploads/sites/7/2019/01/UAI_Final_Report_FIN.pdf
Gross margin however exclude variable and fixed operating costs and depreciation. Notably, variable costs greatly depend on the technology used, the factor making the difference being the acid consumption. Between 50 and 80 kg of H2SO4 is frequently required to produce 1 ton of alkylate. At preferred condition, the consumption of acid can be much lower, such as 10–30 kg of acid per ton of alkylate, assuming feedstocks without diene impurities leading to undesired side reactions. In a SAAU acid costs frequently account for about one third of the total operating costs of alkylation, hence there is considerable incentive to reduce H2SO4 consumption. HF required quantity is in the range of 10–35 kg per ton of alkylate but most of the acid is recovered and recycled, so only a make-up is necessary to replace the consumed HF. In practice acid consumption in a SAAU is more than 100 times bigger than in a HFAU.
Utility costs tend to favor the SAAU. Many HFAU units require isobutene-to-olefin ratios on the order of 13 - 15/1 to produce an acceptable octane product. Other HFAU and most of SAAU develop conditions of mixing and recycle optimization such that they produce similar octane products with isobutane to olefin ratios on the order of 7 - 9/1. Clearly the latter, better-designed units operate with significantly lower fractionation costs.
Currently, many HF units are operating below the design isobutene-to-olefin ratio, but to obtain the required octane, due to increasingly tight gasoline specifications, these ratios will need to be increased back to design ratios. The SAAU process employs either electric or turbine drives for the reactors and compressor to optimize refinery utilities. Horsepower input to the HF reaction zone is lower than to the H2SO4 reaction zone. In addition, the HF process does not require refrigeration. Therefore, power costs are less for HF units. Normally, the difference in fractionation costs outweighs this advantage when comparing overall utility costs. However, HF units may show a utility advantage if fuel cost is low relative to power cost.
Because of its low boiling point, spent HF is regenerated by fractionation within the HF alkylation unit. However, fresh HF must still be brought into the refinery to replace the HF consumed. The unloading and handling of fresh HF must be undertaken with great care since this operation carries the same risk to the refinery workers and surrounding community from an HF release as previously discussed. Perhaps the greatest transportation risk related to HF is the potential release during an accident while transporting fresh acid from the manufacturer to the refinery. Since no mitigation equipment would be available at an accident site, the consequences could be catastrophic.
Spent sulfuric acid is regenerated by thermal decomposition outside the battery limits of the sulfuric acid alkylation unit. This may be accomplished on the refinery site in sulfuric acid regeneration equipment operated by the refinery or in a commercial sulfuric acid regeneration plant which serves several refiners. The choice between these two options is site specific and usually depends on capital versus operating cost considerations and the proximity of the refinery to an existing commercial regeneration plant. As there is low risk from the sulfuric acid itself, the choice to regenerate on-site the acid or elsewhere is based on consideration of economic nature. Of course, even this relatively minor risk is eliminated with on-site sulfuric acid regeneration equipment.
The tanks containing the alkylate produced through a HFAU needs to be monitored continuously. Indeed, alkylate produced in such units contains small impurities of HF corrosion products. If the alkylate enters in contact with water (for example in the bottom of the tank), HF can re-form in the water and cause corrosion of the steel. For this reason, many refiners utilize a weak caustic "heel" of water in the bottom of their alkylate tanks, in order to neutralize any acid that may form. pH monitoring of the tank water is however necessary to assess if any HF is formed downstream.
Conversely in SAAU corrosion is a less dominant issue and can be mastered by minimizing the amount of water entering in the process.
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