Alkylation unit

Alkylation unit, is one of the conversion processes used in the petroleum refineries. It is used to convert isobutane and low-molecular-weight alkenes (primarily a mixture of propene and butene) into an alkylate, a high octane gasoline component. The process occurs in presence of a strong acting acid as catalyst. The acid can be either sulfuric acid or hydrofluoric acid (HF). Depending on the acid used as catalyst the unit takes the name of SAAU (Sulphuric Acid Alkylation Unit) or HFAU (Hydrofluoric Acid Alkylation Unit). Since crude oil generally contains only 10 to 40 percent of hydrocarbon constituents in the gasoline range, refineries use a fluid catalytic cracking unit (FCCU) process to convert high molecular weight hydrocarbons into smaller and more volatile compounds, which are then converted into liquid gasoline-size hydrocarbons. Alkylation processes transform low molecular-weight alkenes and iso-paraffin molecules into larger iso-paraffins with a high octane number. FCCU is a very common unit in a modern oil refinery, but it is not uncommon for a refinery not to have an alkylation unit. Indeed, by 2010 there are some countries in the world without any installed alkylation unit.

The product of the unit, the alkylate, is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons (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, so the octane rating will vary accordingly.

Capacity installed and available technologies

The first alkylation units entered in service in 1940. Since around 1,600,000 barrels per day of capacity have been installed world-wide (capacity installed in 2009),^[1] with an equal share of 800,000 barrels per day for SAAU and HFAU technologies.

About 11% of the gasoline pool in American refineries is produced by alkylating isobutane with C3-C5 olefins. According to the Oil & Gas Journal there were 129 U.S. refineries with a capacity of 17,869,189 barrels per day as of 1 January 2011. These refineries had 1,140,077 barrels per day of alkylation capacity.

For safety reasons SAAU is the prevalent current technology of choice. Indeed in 1996 around 60% of the installed capacity was based on HF,^[2] 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 HSAU.

Two major licensors (sharing a similar share of the market) of the HFAU process are UOP and Conoco Phillips. The main technology used for the HSAU is the STRATCO process licensed by DuPont, followed by the EMRE technology owned by ExxonMobil. In the last ten years, in excess of 85% of the HSAU capacity added worldwide has utilized STRATCO DuPont technology.

Catalysts

The availability of a suitable catalyst is also an important factor in deciding whether to build an alkylation plant. If sulfuric acid (H₂SO₄) is used, significant volumes are needed. Access to a suitable plant is required for the supply of fresh acid and the disposition of spent acid. If a sulfuric acid plant must be constructed specifically to support an alkylation unit, such construction will have a significant impact on both the initial requirements for capital and ongoing costs of operation. Alternatively it is possible to install a WSA Process unit to regenerate the spent acid. No drying of the gas takes place. This means that there will be 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.^[3]

The second main option is to use hydrofluoric acid (HF) as catalyst. In typical alkylation plants, rates of consumption for acid are much lower than for sulfuric acid. The small amount of organofluorine side products are continuously removed from the reactor and the consumed HF is replenished. HF acid plants can process a wider range of feedstocks with propylenes and butylenes. These plants also produce alkylate with better octane rating than do sulfuric plants. However, due to its hazardous nature, HF acid is produced at very few locations and transportation must be managed rigorously.

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 (alkylation of isobutane with olefins was discovered using aluminum chloride promoted with HCl). Several of the current preferred solid catalysts use a salt of HF: either boron trifluoride (BF₃) or antimony pentafluoride (SbF₅). 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. Several companies are engaged in active research in this area, but no one has yet commercialized a solid catalyst alkylation technology.

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 (such as propane, n-butane, and n-pentane), noncondensables (such as ethane and hydrogen) and contaminants. Diluants 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 (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 DIB and is fed directly to the alkylation unit reaction zone. The isobutane feed does not normally contain any significant level of contaminants.

Chemistry

The catalyst protonates the Alkenes (Propene, Butene) to produce reactive carbocations, which Alkylate Isobutane. The reaction is carried out at mild temperatures (0 and 30 °C) 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.

Process description

HFAU process description

The HFAU can be divided into three main sections: reaction, fractionation, and defluorinating / alumina treating.

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.

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 and butane product streams are processed in the deflourinating section to remove combined fluorides and any trace acid that may be present due to mis-operation. This last operation is typically conducted in a HF stripper.

HSAU process description

A HSAU can be divided into five major sections: reaction, refrigeration, effluent treating, fractionation and blowdown.

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.

Operating variables

Many variables impact the product quality and operating costs of an alkylation unit.

Isobutane Concentration

In order to promote the desired alkylation reactions, which are those involving isobutane and olefins, it is necessary to maintain a high concentration of isobutane in the reaction zone. Low isobutane-olefin ratios increase the likelihood of olefin-olefi polymerization that will result in lower octane. Polymerization reactions also have a higher rate of production of acid soluble oils, resulting in higher acid consumption.

Temperature

Typically, alkylation is carried out in the neighborhood of 20 °C. Higher reaction temperatures dramatically favor polymerization reactions that will dilute the acid. Equipment corrosion will also increase with higher reaction temperatures. Low reaction temperatures slow the settling rate of the acid from the alkylate. Lower temperature than ambient cannot be achieved as the coldest possible temperature is that of the cooling fluids (air and water). Seasonal factors influence the production of polymerization reactions, therefore in summer the consumption of acid is higher, especially in HFAU.

Acid strength

As the concentration of the acid catalyst is reduced, the rate of production of acid soluble polymers increases. Feeds that contain high amounts of propylene have a much higher rate of increase in acid consumption over the normal spending range. High acid concentration must be maintained in order to minimize polymerization and red oil production. When concentrations is too low catalyst activity is substantially decreased and polymerization enhanced to the point that it is difficult to maintain acid strength. This condition is known as acid runaway. In HSAU recent studies have found that both butylenes and amylenes can be spent to a lower acid concentration without entering into a runaway condition. While the economics of alkylating both butylenes and amylenes will benefit from lowering the acid spending strength, the acid consumption of amylenes has a greater response than that of butylenes. Also the expected decrease in octane of alkylates produced at lower acid concentrations is less for amylenes than butylenes.

Olefin space velocity

Olefin space velocity is defined as the volume of olefin charged per hour divided by the average volume of sulfuric acid in the contactor reactor. In general, higher olefin space velocities tend to increase sulfuric acid consumption rates and decrease alkylate octane.

Mixing

Mixing is an important parameter, especially in HSAU because the alkylation reaction depends on the emulsion of the hydrocarbon into the sulfuric acid. This is an acid continuous emulsion and it is presumed that the reaction occurs at the interface of acid and hydrocarbon. The better the emulsion, the finer the droplets and the better the reaction.

Economics

Refineries examine whether it makes sense economically to install alkylation units. Alkylation units are complex, with substantial economy of scale. HSAU and HFAU have comparable capital investment costs.^[4] It is not surprising that the two processes are competitive on a capital cost basis, when one considers the basic process differences. The HSAU has a more expensive reactor section and requires refrigeration. However, equal costs are realized in the HF unit by the need for feed driers, product treating, regeneration equipment and more exotic metallurgy. In addition, most refiners will require a dedicated cooling system for an HF unit, to remove the risk of site-wide corrosion in the case of an HF leak. It should be noted that these capital cost estimates do not account for the additional safety and mitigation equipment now required in HF units. Due to the possible hazardous aerosol formation when the HF catalyst is released as a superheated liquid, expensive mitigation systems are now required in many locations throughout the world where HF is used as an alkylation catalyst.

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 alklylation 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 alkylation margin is very volatile but in spite of its volalitility during the last 10 years it has been on a growing trend. In 2013 alkykation gross margin reached 70 USD/barrel of alkylate produced (value calculated accorded to the prices of alkylation feedstocks and effluents on the US Gulf Coast market.

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 frequently required to produce 1 ton of alkylate. At preferred condition, the consumption of acis can be much lower, such as 10–30 kg of acid per ton of alkylate. In a HSAU 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 HSAU is more than 100 times bigger than in a HFAU.

Utility costs tend to favor the HSAU. 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 HSAU 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 HSAU 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.

Market specifications of the alkylate

Alkylate is a blending component so opposed to a finished gasoline ready for consumption it has no specifications to be marketable. Nevertheless, independent provider of energy and petrochemicals information like Platt's reports trades for alkylate ready for blending in the gasoline pool, with RVP < 5.5 psi, (RON + MON)/2 > 92 and of course free of any aromatics and olefins.

Maintenance

Maintenance costs and data are difficult to obtain on a comparable basis. HFAU have much more peripheral equipment (feed driers, product treaters, acid regeneration column and an acid-soluble oil neutralizer), thus more pieces of equipment to operate and maintain. HSAU have larger pieces of equipment, such as the compressor and reactor, but maintenance costs are generally lower. Unit downtime to prepare for a full unit turnaround can take longer for HF units, since the reactor-settler system and all the fractionators must be neutralized before maintenance work can proceed. In H2SO4 units, only the reactor-settler system requires neutralization. In addition, extensive safety equipment (breathing apparatus, etc.) is required whenever maintenance is performed with a potential of HF release. Once work is completed, the maintenance worker must go through a neutralization chamber to cleanse the safety equipment. A face shield and gloves are the only typical requirements when performing maintenance on an HSAU.

Safety

Alkylation units have two primary process hazards: 1) The unit process large volumes of light hydrocarbons which are highly flammable and potentially explosive. 2) The acid catalyst is corrosive and toxic. Both HSAU and HFAU contain similar volumes of hydrocarbon with similar risks, but the risks associated with each acid are quite different. HF requires much stricter precautions because of its greater potential for harm (this is due to its lower boiling point and higher harmful potential). In light of this high risk, the American Petroleum Institute has issued a Recommended Practice specifically for HF alkylation units (API RP 751). This publication recommends that access to an HF alkylation unit be strictly limited due to the potential hazards of HF. No similar, specific safety document is required for sulfuric acid alkylation.

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 ot elsewhere is based on consideration of economic nature. Of course, even this relatively minor risk is eliminated with on-site sulfuric acid regeneration equipment.

Corrosion issues

Despite significant advances in process technology, there continue to be recurring corrosion problems that affect the safety and reliability of HFAU. Any section of the unit in contact with HF need to be built with material particularly resistant to corrosion, such as Monel and inspected on a standing basis.

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. If the alkylate enters in contact with water (for example in the bottom of the tank), the HF can migrate in the water and cause corrosion of the steel. For this reason the PH of the water from the purge of the tanks used to storage alkylate need to be monitored periodically.

Conversely in HSAU corrosion is a less dominant issue and can be mastered by minimizing the amount of water entering in the process.