4.3 Alkylation - Treccani, il portale del sapere Introduction The alkylation reaction is the...

4.3.1 Introduction

The alkylation reaction is the addition of an alkylgroup to any hydrocarbon. In the petroleumindustry, however, the term alkylation is used forthe reaction of low molecular weight olefins with alight isoparaffin to form a liquid hydrocarbon. Thealkylation process was commercialized during thesecond half of the 1930s to convert the lighthydrocarbons in the Fluid Catalytic Cracking(FCC) off-gases into more useful, liquid products.During the Second World War, it experienced atremendous growth due to the need for high-octaneaviation fuel. From 1950 to 1970, the worldcapacity remained relatively flat due to thecomparative cost of other gasoline blendingcomponents. The lead phase-down in manycountries and additional environmental regulationsafter the 1970s increased the demand for alkylateas a blending stock for motor gasoline.

In addition, the phase-out of MTBE (methyltert-butyl ether) in some US states has furtherincreased the need for high-octane ‘clean’ streams. Atthe beginning of the millennium, 13% of the USgasoline marked was alkylate. In fact, alkylate is ahigh-octane blend-stock (RON, Research OctaneNumber, 93-98; MON, Motor Octane Number, 90-95)free of undesirable components such as sulphur,benzene and other aromatics. It is mostly made up of

C7 to C9 highly branched paraffins and is producedprimarily by reacting isobutane with light olefins inthe presence of strong acid catalysts, such ashydrofluoric and sulphuric acid.

Due to safety and corrosion problems caused bythe use of liquid strong acids, a number of companieshave carried out research to commercialize a solidalkylation catalyst. In fact, though the process hasbeen a reliable and safe producer of a prime high-octane gasoline for many decades, in recent years ithas been the object of environmental concerns and ofresearch and development efforts.

4.3.2 Process chemistry and thermodynamics

The alkylation unit is traditionally fed with the FCCoff-gases and is normally installed in refineriesequipped with catalyting cracking units. The usualfeedstocks are isobutane and light olefins, mostly C3and C4; the olefins from cokers (if available) are alsosent as feeds to alkylation. Table 1 shows thecomposition of two typical olefinic feedstocks fromFCC; the alkylation process requires more isobutanefrom sources other than cracking. The processchemistry is extremely complex due to the largenumber of possible side reactions. The main product isa mixture of isoparaffins called alkylate.

181VOLUME II / REFINING AND PETROCHEMICALS

4.3

Alkylation

Table 1. Typical olefin feeds from FCC units (weight %)

C3� 1-C4

� 2-C4� i-C4

� C5� C3 n-C4 i-C4 i-C5 others

C4 cut 6.7 8.2 18.9 6.0 1.1 3.7 10.2 37.4 7.3 0.5Wide cut 17.7 9.3 18.5 7.3 5.2 7.8 7.8 19.3 6.5 0.6

The symbol � as superscript indicates that there is a double link.

Isoparaffins with tertiary carbon atoms react withthe olefins. Among isoparaffins, isobutane has beencommonly used, since isopentane is a valuable liquidhydrocarbon blended directly with commercialgasoline. However, gasoline reformulation has reducedthe acceptable vapour pressure, and isopentane hasbecome an interesting material for propylene alkylation(Detrick et al., 2004). Some typical reactions of thealkylation process include the following:

It should be recalled that 2,2,4-trimethylpentane(isoctane) is one of the two standard hydrocarbons foroctane number definition, and that its ON (OctaneNumber) is 100.

The butene isomerization to isobutene (in the alkylation feed) is an important reaction toproduce high-octane hydrocarbons from feedscontaining appreciable quantities of 1-butene.A number of alkylation units processing butenes have an upstream isomerization unit (Detrick et al., 2004).

The reaction proceeds via the carbocationmechanism. The initiation step (step 1) generates thecarbocation (initially C3

+ or C4+, depending on feed

composition) by protonation of the olefin. Thecatalytic solvents capable of transferring this proton tothe olefin are strong acids. The more stable tertiarybutyl cation is then generated by transfer of a hydrideion (step 2). The direct formation of a cation fromisobutane at roughly room temperature requires asolvent system with acidity similar to or higher thanH2SO4 (Marcilly, 2003). The most diffusedcommercial processes have traditionally used HF andH2SO4 (Table 2). The propagation reaction involves thetertiary butyl cation, which reacts with the olefin toform a larger cation (step 3), and then generates a newtertiary butyl cation and the alkylate product (step 4).This sequence is illustrated below using propylene andisobutene as an example reaction:

An important side reaction of the process is thehydrogen transfer reaction, most pronounced in HFcatalyzed processes fed with propylene (the symbol =as superscript indicates that there is a double link):

C3��i-C4�

� C3�i-C4�

i-C4��i-C4�

� i-C8 (trimethylpentane)

182 ENCYCLOPAEDIA OF HYDROCARBONS

PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES

CH3 CH3 CH3CH2

CH2

CH

CH3

C �

CH3

CH3

CH3

C

CH3

CH3CH

CH3

CH3 CH2CH2 �CH

CH3 CH2 CH2 CH2 CH3

CH3

CH3

C

CH3 CH3CH

CH3

CH3 CH

CH3

CH3

CH3

CCH2 CH2

CH3 CH3CH2 CH�

CH3

CH3

isobutene isobutane

2,2,4-trimethylpentane

1-butene isobutane

2,2-dimethylhexane

propylene isobutane

2,2-dimethylpentane

CH2

CH3

CH3

CH3

CH3

HCCH3 CH3

CH3CHH�

�

CH2

CH3 CH3CH�

CH3

CH3

� CH3CH3

�C

CH3 �CH2CH CH3

CH3

CH3

C CH3

CH3

CH3

CH3

HC

CH3

CH3

CH3

CH2 CH2 C

CH3 CH2

CH2

CH

CH3 CH3

CH3

CH3

C

CH�

CH3

CH3

�CH3

�

�

C

CH�

CH3

CH3

�CH3

�C

The overall reaction is:

The octane number of trimethylpentane is sensiblyhigher than that of dimethylpentane normally obtainedfrom propylene. However, in this reaction twomolecules of isobutane are required to produce onlyone molecule of alkylate.

A number of other side reactions may be involvedin the process; the most common include thepolymerization and cracking. The polymerization ofolefins results in the production of low-octane, highboiling point components, which are undesirable. Thisreaction is minimized by using high isobutane/olefinratios and choosing proper reaction conditions.

The heavier polymerization products are known asAcid Soluble Oils (ASOs) or ‘red oils’ and tend todeactivate the catalyst. ASOs are unsaturatedcompounds with more than about 10 carbon atoms permolecule that can react with H2SO4.

Alkylation reactions are higly exothermic (onaverage 75-96 kJ/mol); the reaction equilibrium is,therefore, shifted to the alkylate formation at lowtemperature and high pressure. Moreover, lowertemperatures minimize the formation of by-productsdue to polymerization and cracking reactions.

4.3.3 Process kinetics

The traditional alkylation reaction takes place in amedium in which the hydrocarbon drops are dispersed

in a continuous acid phase. Being that olefins are moresoluble in acid than the isoalkane, one may expect ahigh conversion to polymers; this, however, is not inagreement with industrial practice. An explanationcould be that the carbocations formed by theinteraction of the acid with the olefin, which initiatethe reaction chain, are found to a larger extent at theinterface between the two phases, with the carboniumions oriented towards the hydrocarbon phase (Raseev,2003). The isoalkane in the hydrocarbon phase canthen interact with the carbonium ion. This opinion isnot unanimous, but it allows the alkylation reaction tobe treated as a homogeneous process where thereaction rate is proportional to the interfacial area. Therate will then increase with the degree of dispersionand, therefore, with the decrease in size of thehydrocarbon droplets. This is confirmedexperimentally: in fact, the octane number and,generally, the alkylate quality increase by intensifyingthe mixing in the reactor (Li et al., 1970). With goodmixing and the proper operating conditions, alkylationoccurs almost instantaneously.

4.3.4 Catalysts and reactionconditions

Strong acids: HF and H2SO4In order to favour the thermodynamics and to

minimize the formation of by-products, the alkylationreaction is carried out at the lowest possibletemperature. This is kinetically possible by using largeamounts of strong acid catalysts; the world market haslong been split between H2SO4 and HF (see again Table2). The acid strength of the two compounds is similarwhen they carry traces of impurities (Marcilly, 2003).The catalysts must be used almost pure, since thealkylation reaction requires a strong acidity in order toattain kinetics that are economically acceptable.

In general, the HF alkylate has a higher octanenumber due to the hydrogen transfer reactions;however, the process economics should be analyzedwhile keeping in mind the higher isobutaneconsumption and the lower catalyst consumption whenusing HF. During operation, the acid is contaminatedby water and soluble organic matter, which decreasethe total acidity; in such conditions, the isobutanesolubility is higher (e.g. 0.4% by weight in H2SO4 and3.6% by weight in HF; Marcilly, 2003).

The process temperature depends on the acid type.The oxidizing properties of H2SO4 suggest atemperature generally less than 12°C. However, theacid viscosity increases rapidly by lowering thetemperature, which restricts the useful temperaturerange between about 2-12°C (5°C being a good


ALKYLATION

C3H6 C3H82 i-C4H10

CH3

CH3

CH3

C

CH3

CH CH2

� �

CH3�

propylene

propane

isobutane

trimethylpentane

Table 2. Typical properties of fresh alkylation acids(Marcilly, 2003)

PROPERTY HF H2SO4

Molecular weight 20.01 98.08Boiling temperature (°C) 19.4 290Melting temperature (°C) �82.8 �10.4Specific weight (d4

15) 0.99 1.84Viscosity (cP) 0.256 (0°C) 33 (15°C)Hammett acidity (�Ho)* 10 11.1

* Acidity of the industrial acids during operation.

compromise). HF is not an oxidant and thus the usefultemperature can be in the range of 20-50°C (normallybetween 30-40°C), which simplifies the reactorcooling systems.

The reaction pressure is fixed at a level capable ofkeeping the reaction media in the liquid phase. In bothcases, an excess of isobutane must be used to avoidolefin polymerization; the excess isobutane is recycledto the reactor after separation of the alkylate product.

The reaction medium is composed of two phases:the acid phase (continuous phase) and the hydrocarbonphase (dispersed phase). The reacting hydrocarbons arethose solubilized into the acid phase. The acid’s physicalcharacteristics at the process temperature impose amuch more effective stirring in the case of H2SO4. Infact, one of the key differences between HF and H2SO4alkylation is the handling of the acid catalyst.

The catalyst activity decreases with time due todilution, ASO formation and impurity build-up.The HF acid can be fractionated to remove waterand ASO. H2SO4 must be removed from the unitand regenerated by completely decomposing theacid to SO2-SO3 and condensing them back toH2SO4. This regeneration process can be done atthe site or, usually, outside of the refinery inremote locations. For the above-mentionedreasons, the H2SO4 consumption is normally muchhigher than HF consumption, in spite of the factthat HF can form an azeotrope with water (the so-called CBM, Constant Boiling Mixture),responsible for acid losses.

Tables 3 and 4 illustrate the influence that both thetype of acid catalyst and the olefin bear on alkylate

yield and quality (Joly, 2001). As already mentioned,the impurities of the feed greatly affect yield, alkylateformation and acid composition, especially in the caseof H2SO4 catalyst. For HF acid catalyst, consumptionis normally less than 1 kg/t since the catalyst isregenerated by simple distillation.

Mitigation of the risk due to acid use: solid catalystsAlthough extensive experience shows that

alkylation plants, regardless of acid catalyst choice,can be operated safely and with low process risks; theprocess acid catalysts have been subject to criticalattention in the last decades.

Hydrofluoric acid is very volatile (boiling point:19.5°C) and produces dangerous mists in the event ofan accidental release. Refiners with sulphuric acidalkylation units must ship large quantities of spentacid offsite for regeneration, thus creating potentialtransportation hazards. Both concentrated acids arecontained in carbon steel and become very corrosivewhen diluted with water.

The refining industry has developed a numberof mitigation strategies to face these problems:water curtain systems, rapid acid dump methods,remotely operated isolation systems, etc. At thesame time, catalyst producers and processlicensors have developed, and in some casescommercialized, solid-phase acid catalysts.Several pilot-size units are operating and anumber of processes have been presented(Refining processes, 2002, 2004; Meyers, 2004;D’Amico et al., 2006). Solid acid catalysts havelong been investigated; these include exchanged



Table 3. Yield and octane number of the product from H2SO4 alkylation process

Type of feed Propylene Butenes Amylenes

Yield (vol C5�/vol olefin) 1.45-1.78 1.74 1.57i-C4 consumption (vol/vol olefins) 1.27-1.32 1.14 1Catalyst consumption (kg/t C5�) 137-171 51-102 102-171MON 88-90 92-94 88-90RON 89-92 94-98 90-92

Table 4. Yield and octane number of the product from HF alkylation process

Type of feed C3� 1-C4

� 2-C4� i-C4

� C3��C4

� C5�

Yield (vol C5�/vol olefin) 1.76 1.73 1.77 1.78 1.79 1.63i-C4 consumption (vol/vol olefins) 1.36 1.1 1.14 1.28 1.28 1MON 92 94.4 97.8 95.9 93.7 91.5RON 90 91.6 94.6 93.4 90.8 90

The symbol � as superscript indicates that there is a double link.

zeolites, ion-exchange resins such as Amberlyst,perfluoropolymers with sulphonic acid groupsalong its backbone (Nafion), superacid solids(chlorinated alumina, sulphated zirconia) andliquid superacids immobilized on solids.

Examples of solid catalysts promoted by strongacids are: alumina (or zeolites)/BF3,silica/CF3SO3H, silica/SbF5. Most catalysts areproprietary and little information is normally givenregarding their composition. Solid catalysts canimprove safety and production costs, but tend todeactivate rapidly under alkylation conditions due tobuild up of coke and heavy compounds on thecatalyst surface. Burning off the heavy hydrocarbonsthrough high-temperature oxidation quickly destroysthe catalyst activity.

To solve the deactivation problem, somecompanies have developed new reactor types and newgeneration systems based on the desorption of theheavy hydrocarbons with the use of a hydrogen stream(Roeseler, 2004). Another approach proposes the useof supercritical fluids as the reaction media; as anexample, supercritical CO2 was found to be good atdissolving the heavy coke material on the catalystssurface (Subramaniam, 2001).

Some companies have proposed special additivesthat reduce the tendency of HF to form mists. On-sitesulphuric acid regeneration is available to eliminatethe shipment of spent and regenerate acid; althoughthis technology has been available for half a century,only few refineries operate on-site regeneration.

4.3.5 Sulphuric acid alkylationprocesses

Sulphuric acid alkylation was the first to bedeveloped, during the decade preceding the SecondWorld War. Essentially, the H2SO4 processes consistin a reaction section where an emulsion ofhydrocarbons and acid is formed (and the reactionoccurs), and in a settling section that separates andrecycles the acid. A fractionation section separates thealkylate from the excess isobutane, which is recycled

to the reactor. There are currently two majoralkylation processes using H2SO4 as catalyst: Stratcoeffluent refrigeration alkylation, and ExxonMobilcascade autorefrigeration process; each uses differentapproaches for the design of the reaction andrefrigeration sections.

Stratco processThe Stratco reactor is a horizontal pressure vessel

containing an inner tube bundle, which acts as anexchanger to remove the heat of reaction, and a mixingimpeller (Fig. 1). It operates at a pressure of about 3.5to 5.0 bar, sufficient to keep the two phases in theliquid state. The acid and hydrocarbon feed come intocontact and are vigorously stirred by the impellerblades. An emulsion is formed and the reaction takesplace almost instantly; the contact time is very shortand the side reactions are kept to a minimum. The highrecycle rate of the emulsion allows an efficient controlof the reaction temperature.

The general scheme of the process is shown inFig. 2. The dehydrated olefin feed is mixed with therecycle isobutane and cooled in the feed/effluentexchangers; water is removed in the coalescer beforeentering the reactor. A portion of the emulsion in thecontactor reactor is withdrawn from the dischargeside of the impeller and sent to the acid settler,which separates the reacted hydrocarbon phase fromthe acid emulsion. The settled acid is returned to thesuction side of the impeller. Acid is purged from theunit, usually on a continuous basis, and fresh acid isintroduced so that the acid strength is kept high. Thehydrocarbon phase, containing the alkylate productand the isobutene, is sent to the tube bundle in thereactor by reducing the pressure to about 0.4-0.6 bar,across a back pressure control valve. At thispressure, the lighter components of the effluentstream are vaporized, reducing the temperature tobelow 0°C. Additional vaporization occurs in thetube bundle as the net effluent stream removes theheat of reaction (Graves, 2004). The stream from thetube bundle is sent to the suction trap/flash drum toseparate the vapour and liquid phase. The liquidisobutene from the flash drum-side is directly


ALKYLATION

hydrocarbonfeed

pressurerelief

to settler

coolant

coolant

acid

drain

driver

Fig. 1. Stratco-typealkylation reactor.

recycled to the reactor, while the liquid from thesuction trap-side is transferred to the effluentfractionation section after caustic washing orpassing over a bauxite bed for the elimination ofsulphates. Isobutane is recycled back to the reactorsection. The vapour phase from the flash drum iscompressed, cooled and condensed. Propane iseliminated in the depropanizer, whose bottoms arerecycled to the reactor.

ExxonMobil processThe ExxonMobil cascade process uses the auto-

refrigeration system to remove the heat of reaction andto maintain the low reaction temperature (4-5°C)needed for alkylation. The reactor is a horizontal vesselcontaining a number of compartments with mixers ineach stage to emulsify the hydrocarbon-acid mixture.The reaction is held at low pressure and the heat ofreaction is eliminated by evaporating an isobutanestream directly fed into one end of the reactor. The acidis fed on the same end and moves together with theisobutane by overflowing from one compartment to theother. The olefin feed is split and added into eachcompartment. It is not necessary to maintain a highpressure in the reactor to prevent vaporization of lighthydrocarbons: the pressure varies from about 1.5 bar inthe first stage (richer in isobutane) to about 0.5 bar inthe last stage. Usually, the reactor contains a settling

zone at the end. A flow scheme of the process is shownin Fig. 3 (Lerner and Citarella, 1991).

Olefin feed is mixed with recycle isobutane fromthe deisobutanizer, cooled and fed to the reactor. Watercondensed at a lower temperature is removed in thecoalescer. The vapours leaving the reactor are routedto the refrigerator section where they are compressed,condensed and sent to the economizer (an intermediatepressure flash drum), which reduces the powerrequirements of the refrigeration compressor. Aslipstream of refrigerant (isobutane) is depropanizedafter being caustic and water washed. Propane isseparated overhead, while isobutane-rich bottoms arereturned to the process.

The reactor liquid product is sent to the settler,from where the settled acid is recycled back to thereactor. The hydrocarbon portion (containing alkylate,excess isobutane and n-butane) is caustic and waterwashed to remove any acid components, and is sent tothe deisobutanizer. Overhead from the tower is anisobutane-rich stream, which is recycled to the reactor,while the bottoms are sent to the debutanizer for theseparation of the alkylate product from butane.

Feed impurities and small amounts of polymerizedolefins that form ASO tend to accumulate in therecycle acid. Therefore, a spent acid purge is takenfrom the process to remove these oils and fresh make-up acid is added to maintain sufficient acid strength.



accumulator

olefin feed

propaneproduct

isobutane feed spent acidalkylateproduct

n-butane product

fresh acid

compressor

i-C4

economizer

Fig. 2. Simplified flow diagram of the H2SO4 Stratco process.

4.3.6 Fluoridic acid alkylationprocesses

In 1994, there were 127 HF units and 92 H2SO4units in the world refineries (Joly, 2001). In theRefining processes handbook (Refining processes,2004), the declared licences for the HF units were160, which almost doubled the declared H2SO4licences. Of course not all the licensed plantswere still working, but the data provide a roughidea of the diffusion of the processes. At normaldesign conditions, an HF alkylation unit requiresa higher ratio of isobutane to olefin (I/O) than aH2SO4 unit. Both processes fractionate theisobutane from the reactor effluent stream andrecycle it back to the reactor. Due to its higher I/Oratio, an HF alkylation unit is designed with alarger fractionation section. The low HF viscosityand better solubility of isobutane in the acid allowsimpler reactors to be used: it is sufficient to

inject the hydrocarbon feed into the acid phase toobtain a good emulsion. Therefore, the HF unitsdo not have mechanical stirring devices. Watercan be used to cool the reactor, given the higherreaction temperatures. The Conoco-Phillips andUOP (Universal Oil Products) technologiesshared the market at the beginning of the thirdmillennium.

The Conoco-Phillips processThe original Phillips process is characterized by its

very simple reactor, similar to that shown in Fig. 4(Gary and Handwerk, 1975). Essentially, it iscomposed of an acid cooler, a riser reactor and asettler. Acid circulation is by gravity differential andan acid circulation pump is not necessary. Theresidence time in the tubular reactor is in the order ofhalf a minute.

A basic flow scheme of the Phillips process isillustrated in Fig. 5. The more recent version converts


ALKYLATION

compressor

econ

omiz

er

caus

ticw

ashi

ng

spentcaustic

wastewater

propane

butane

alkylate

wastewater

water

freshcaustic

wat

erw

ashi

ng

caus

ticw

ashi

ng

spentcaustic

olefinfeed

recycle acid freshcaustic

spentacid

make-upacid

water make-upisobutane

wastewater

wat

erw

ashi

ng

KO

dru

m depr

opan

izer

coal

esce

r

reactor

olefin feed plusisobutane recycle

M M M M

settler

deis

obut

aniz

er

debu

tani

zer

i-C4recycle refrigerant

Fig. 3. Flow diagram of the H2SO4 ExxonMobil process.

propylene, amylene, butenes and isobutane to motorfuel using ReVAP (Reduced Volatility AlkylationProcess) alkylation.

Both the olefin and isobutane feeds are dehydratedby passing them through a solid bed desiccant. Gooddehydration is essential to minimize potentialcorrosion of process equipment, which results fromaddition of water to hydrofluoric acid. The olefin andisobutane feeds are then mixed with hydrofluoric acidat a sufficient pressure to maintain all components inthe liquid phase. The reaction mixture is allowed tosettle into two liquid phases. The acid is withdrawnfrom the bottom of the settler and passed through acooler to remove the reaction heat; it is then recycledand mixed with fresh feed.

A slipstream of acid is withdrawn from the settlerand fed by a pump to the acid rerun column to removedissolved water and polymerized hydrocarbons. Theoverhead product from the rerun column is mostlyhydrofluoric acid, which is condensed and returned to

the system. The bottom product from the rerun columnis a mixture of ASO and HF-water azeotrope. Thesecomponents are separated in a settler (not shown in theflow diagram). The ASO is used for fuel and the HF-water mixture is neutralized with lime or caustic.This rerun operation is added to maintain the activityof the hydrofluoric acid catalyst.

The hydrocarbon phase removed from the top ofthe acid settler is a mixture of propane, isobutane,normal butane, and alkylate, along with small amountsof hydrofluoric acid. These components are separatedby fractionation and the isobutane is recycled to thereactor. Propane and normal butane products arepassed through caustic treaters to remove tracequantities of hydrofluoric acid. The design of the acidsettler-cooler-reactor section is critical to goodconversion in a hydrofluoric acid alkylation unit.

The UOP processIn the UOP HF-alkylation process, a pump

provides the inlet pressure into the reactor nozzles,which allow the hydrocarbon phase to be dispersed inthe acid continuous phase. The dried olefin andisobutene are fed at different reactor heights, while theacid is fed at the reactor bottom. The mixing betweenhydrocarbons and acid phases is improved by thepumping design. The reactor heat is removed by meansof cooling water. A simplified flow scheme of atypical C4 HF alkylation unit is shown in Fig. 6.Similar schemes are available for the C3-C4 alkylationunits (Detrick et al., 2004).

The combined feed enters the shell of thereactor-heat exchanger through several nozzles,which maintain an even temperature in the reactor.The effluent from the reactor flows to the settler andthe acid is recycled to the reactor. The hydrocarbonphase (containing dissolved HF acid) is charged tothe iso-stripper. The alkylate and n-butane arerecovered from the bottom and as a side stream,respectively. Isobutane is also dotained as a side-cutand recycled to the reactor. The overhead, mainlyconsisting of isobutane, propane and acid, is in partcharged to the HF stripper. An HF alkylation unitfed with C3-C4 olefins is normally equipped with adepropanizer, which may be also required with C4olefins, if the propane quantity is too high. A smallslipstream of circulating HF acid is regeneratedinternally, to keep the purity at the requested level.The internal acid regeneration technique hasvirtually eliminated the need for an acidregeneration (Detrick et al., 2004). All effluentstreams and process vents, sewer effluents and acidregeneration bottoms are treated either with KOH oralumina. The KOH is regenerated on a periodic basisby using lime.



hydrocarbonfeed

riserreactor

settler

hydrocarbon product

acidaccumulator

coolingwater

acidcooler

acid

� h

ydro

carb

ons

(20-

27°C

)

acid

Fig. 4. Simplified scheme of the Phillips HF reactor.

Risk mitigation in the HF alkylation plantsIn order to reduce aerosol formation in the event of

an HF release, UOP proposes the Alkad process, to beused with HF alkylation technology. The Alkadprocess is a passive mitigation system that will reduceaerosol from any leak occurring in the unit. Thealkylation reactions take place in the presence of aliquid additive, which form a long chain of associatedHF molecules; in this form, HF acid loses its tendencyto form aerosols when released to the atmosphere.

4.3.7 Processes with solid acidcatalysts

Since the 1990s, a number of companies haveproposed alkylation processes based on solid catalysts,both using fixed-bed or riser reactors. At the moment,most solid catalyst processes are at the pilot stage andvery few commercial units are installed.

The Alkylene processThe UOP Alkylene process was developed during

the late 1990s, based on a liquid transport reactor (riserreactor) to promote fast and good contact between

hydrocarbons and solid catalyst, with in situregeneration capability. The liquid-phase operationminimizes abrasion problems. A novel catalyst (HAL-100) was developed with declared good performanceand long stability; it is easily regenerated without a hightemperature carbon burn. A simplified flow diagram ofthe process is shown in Fig. 7 (Roeseler, 2004).Reactants and catalyst flow up the reactor riser at a rateof about one foot per second as the reaction occurs. Thecatalyst is quickly separated from the hydrocarbon atthe top of the riser and falls by gravity into thereactivation zone. The catalyst flows slowly downwardand it is contacted with a recycle of cold isobutanesaturated with hydrogen. Heavy hydrocarbons arehydrogenated and desorbed from the catalyst. Thereactivated catalyst flows down and back into the riserbottom. A catalyst slipstream is reactivated at a highertemperature in a separate vessel to completely removethe small quantities of the residual heavy hydrocarbons.Alkylate from the reactor is sent to a fractionationsection similar to those of the liquid acid processes.

Other processesThe Topsoe process employs a catalyst system

of the supported liquid phase type. The superacid


ALKYLATION

recycle acid

butane toKOH treater

total alkylate to storage

depropanizer feed

acid fromdepropanizer section

mainfractionator

acid eductor

acidcooler

mixernozzle

olefinfeed

isobutane

acidtank

acidreruncolumn

recycle isobutane

strippingi-C4

ASO

steam

steam

reactor settler

acidmake-upreactor

riser

isobutane fromdepropanizer section

acid standpipe

coolingwater

acidrecontactor

Fig. 5. Typical scheme of the Phillips HF process.

liquid is adsorbed on a porous solid support. Thisenables the use of simple fixed-bed reactors.However, through proper operation of the reactorsystem, the liquid nature of the catalyst can be usedto achieve simple maintenance of catalytic activity.The AlkyClean process is licensed by ABB Global,Akzo Nobel and Fortum Oil and Gas, and uses asolid acid catalyst. The reactors are undergoing amild liquid-phase regeneration using isobutane andhydrogen. The process does not produce any ASO

or require post treatment of the reactor effluent orfinal product.

4.3.8 Hazards and corrosion problems

Among refinery processes, alkylation plants aresomewhat unique because they use strongaggressive acids and contain large volumes ofLPG. A key element of hazard management isdirected at preventing the release in the first place.Extensive experience demonstrates that alkylationunits can be operated safely, and with minimumprocess risk to employees or neighbouringcommunities (Scott, 1992). The plants areconstructed according to standards and withmaterials that are specifically designed to make theplant as safe as possible.

In case of liquid exposure, both acids willcause serious burns. HF also has the property ofpenetrating the tissues and reacting with thecalcium and magnesium in the body. In thepresence of water, HF forms an azeotrope thatcontains 36% HF and boils at 109°C. HF isvolatile, and when spilled it forms stable aerosolclouds. Vapour and aerosol cause seriousinhalation hazards (lung damage). Bothconcentrated acids can be contained in carbonsteel, but they become very corrosive when diluted



isos

trip

per

i-C4make-up

olefinfeed

i-C

4 re

cycl

e

coolingwater

ASO and CBMto neutralization

alkylate n-butaneto depropanizer

acidrecycle

settl

er

acid

rege

nera

tor

reac

tor

HF

stri

pper

alum

ina

trea

ter

KOHtreater

KOHtreaterKOH

treater

Fig. 6. Typical scheme of a UOP HF-alkylation unit fed with C4 cuts (Detrick et al., 2004).

Alkylenereactor

hotreactivationvessel

lightends

LPG

isobutane recycle

alkylate

products

olefinfeed

catalystreactivation

zone

i-C4/H2

H2

Fig. 7. Simplified flow diagram of the Alkylene process.

with water. The aggressiveness of both acids varieswith its concentration, temperature, nature ofcontaminants and velocity relative to exposedsurfaces. In H2SO4 alkylation, the hydrocarbonsare emulsified in concentrate acid and reacted atlow temperatures; the acid remains fairlyconcentrated, diluting to about 88%. Stainless steel(304 L or 316 L) should be preferred in areaswhere high velocity may be encountered, such aspumps, valves and bends (Schillmoller, 1998a).

Austenitic stainless steels are resistant toanydrous HF, but are unreliable in dilute acid.Alloy 400 (Monel) containing Ni and Cu andminor amounts of Fe and Mn has been thestandard material in alkylation plants for manyyears and has been used for heat exchangers,columns, reboilers and overhead condensers withacid concentrations between 85 and 95%. Alloy400 is resistant to all concentrations of HF,including anhydrous, over a wide range oftemperatures, in the absence of oxygen orsulphur dioxide contamination. HF contaminatedwith even small amounts of oxygen can causesevere pitting and cracking of steel and Monel(Schillmoller, 1998b). Special attention shouldbe given to the welding, which can be severelycorroded if not done properly. HF also attacks allmaterials containing silica (glass, china, etc.),asbestos and many plastics, except teflon. Inaddition to the usual refinery safety procedures,the operators of the ‘acid area’ in the alkylationunits must follow special ad hocprocedures.

4.3.9 Process and operatingvariables

The main process variables that influence the yieldsand quality of the alkylate product are: a) quality ofthe feed; b) acid strength and composition; c) isobutene/olefin ratio; d) temperature; e) mixingand space velocity.

In the case of the liquid catalysts, the properties ofthe two acids can explain the differences between therelated processes and the operating conditions. Thehigher viscosity and surface tension of sulphuric acidas opposed to hydrofluoric acid make it much moredifficult to obtain a fine dispersion of the hydrocarbonin the acid phase. The higher solubility of isobutane inHF acid leads to a higher ratio of isobutane to olefinsin the acid phase (especially at the interface) withrespect to H2SO4 processes; the secondary reactionsare then reduced and the quality of the alkylate isimproved.

Quality of the feedImpurities increase catalyst consumption. The

hydrocarbon feed should be dried and desulphurized,especially in HF units. Diolefins cause heavy losses ofsulphuric acid.

The olefin type in the feed, particularly the ratio ofbutylene to propylene, affects the product quality andacid consumption: in propylene alkylation, the octanenumber can be five units lower and acid consumptionalmost double (Parkash, 2003). The olefin type alsoinfluences the heat of reaction, isobutane consumptionand alkylate yield.

Acid strengthAcid composition at the equilibrium is an

important parameter that influences alkylate quality.There is a minimum acid strength required by theprocess, which varies depending on the acid andolefin type and spent acid composition. At loweracid concentrations polymerization becomespredominant. Water lowers the acid catalyticactivity three to five times faster than hydrocarbondiluents (Parkash, 2003). However, some water isnecessary to ionize the acid. The optimum andminimum water contents for H2SO4 units are closeto 99% and 90%, respectively. In HF processes, thebest alkylate quality is produced with a watercontent of about 2.8% (Joly, 2001). The impuritiespresent in the feed can be absorbed or react with thecatalyst, causing a decrease in strength and the needto increase the acid make-up.

Isobutane/olefin ratioIsobutane/Olefin (I/O) ratio is the most important

operating parameter: it controls alkylate yields andquality, as well as catalyst consumption.

Polymerization occurs in the acid phase and is themost important reaction competing with the alkylationreaction. During polymerization, two or more olefinmolecules react to form a polymer, which causes lowerproduct octane and increased acid consumption. Thesolubility in the acid phase is much higher for olefinsthan for isobutane; therefore, a large excess of thelatter must be maintained in the reaction zone toensure that enough isobutane is absorbed at the acidinterface. The usual I/O ratio ranges from 5 to 8 inH2SO4 units and from 10 to 15 in HF processes.

TemperatureThe alkylation process is thermodynamically

favoured by low temperatures. Reaction temperaturehas a greater effect in H2SO4 than in HF processes.Reducing the reaction temperature minimizes thepolymerization rate relative to the alkylation rate,resulting in a higher octane number and lower acid


ALKYLATION

consumption. In the case of H2SO4, temperaturesbelow 2-4°C are generally avoided because of the veryhigh viscosities of the acid phase. Also, very lowtemperatures slow down settling rates and favour acidcarryover. The temperature in the reactor depends onthe olefin feed rate, which influences the reaction heat.Efficient removal of the heat from the reactor isessential for all catalytic systems.

Mixing and space velocityIncreasing mixing produces a better and finer

dispersion of the hydrocarbon droplets in theemulsion, increasing the surface area, and thus thekinetics and product quality.

In the case of liquid acid catalysts, the spacevelocity can be a measure of the olefin concentrationin the acid phase and may be defined as follows:

olefin space velocity = olefin in the reactor (m3/h)/acid in the reactor (m3)

As the olefin space velocity increases, the octanenumber tends to decrease and the acid consumptionincreases. In general, the residence time of thereactants is not a limiting parameter because thealkylation reaction occurs almost instantaneously.

References

D’AMICO V. et al. (2006) Consider new methods to dedottleneckclean alkylate production, «Hydrocarbon Processing»,February, 65-70.

Detrick K.A. et al. (2004) UOP HF alkylation technology,in: Meyers R.A. (editor in chief) Handbook of petroleumrefining processes, New York, McGraw-Hill, Chapter 1.33.

Gary J.H., Handwerk G.E. (1975) Petroleum refining.Technology and economics, New York, Marcel Dekker, 152.

Graves D.C. (2004) Stratco effluent refrigerated H2SO4alkylation process, in: Meyers R.A. (editor in chief)Handbook of petroleum refining processes, New York,McGraw-Hill, Chapter 1.11.

Joly J.F. (2001) Aliphatic alkylation, in: Leprince P. (editedby) Conversion processes, Paris, Technip, 257-289.

Lerner H., Citarella V.A. (1991) Improve alkylationefficiency, «Hydrocarbon Processing», November, 89.

Li K.W. et al. (1970) Alkylation of isobutane with light olefinsusing sulfuric acid. Operating variables affecting physicalphenomena only, «Industrial & Engineering ChemistryProcess Design and Development», 9, 434-440.

Marcilly C. (2003) Catalyse acido-basique. Application auraffinage et à la pétrochemie, Paris, Technip, 2 v.; v. I, 201-203.

Meyers R.A. (editor in chief) (2004) Handbook of petroleumrefining processes, New York, McGraw-Hill.

Parkash S. (2003) Refining processes handbook, Amsterdam,Elsevier, 128-140.

Raseev S. (2003) Thermal and catalytic process in petroleumrefining, New York, Marcel Dekker, 556-585.

Refining processes handbook 2002 (2002) «HydrocarbonProcessing», November, 86-90.

Refining processes handbook 2004 (2004) «HydrocarbonProcessing», CD.

Ritter S.K. (2001) Alkylate rising, «Chemical and EngineeringNews», 11, 62-67.

Roeseler C. (2004) UOP alkylene process for motor fuelalkylation, in: Meyers R.A. (editor in chief) Handbook ofpetroleum refining processes, New York, McGraw-Hill,Chapter 1.25.

Schillmoller C.M. (1998a) Select alloys that perform well in sulfuric acid, «Chemical Engineering Progress», 2,38.

Schillmoller C.M. (1998b) Select the right alloys forhydrofluoric acid service, «Chemical Engineering Progress»,11, 49-54.

Scott B. (1992) Identify alkylation hazards, «HydrocarbonProcessing», 10, 77.

Subramaniam B. (2001) Enhancing the stability of porouscatalysts with supercritical reaction media, «AppliedCatalysis. A: General», 212, 199-213.

Carlo GiavariniDipartimento di Ingegneria Chimica, dei Materiali,

delle Materie Prime e MetallurgiaUniversità degli Studi di Roma ‘La Sapienza’

Roma, Italy



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