-
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):
C3i-C4 C3i-C4
i-C4i-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
CH3CH3C
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
CH3C
-
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 12C.
However, theacid viscosity increases rapidly by lowering
thetemperature, which restricts the useful temperaturerange between
about 2-12C (5C being a good
183VOLUME II / REFINING AND PETROCHEMICALS
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 (0C) 33 (15C)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 20-50C (normallybetween 30-40C), 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
acids 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:19.5C) 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;DAmico et
al., 2006). Solid acid catalysts havelong been investigated; these
include exchanged
184 ENCYCLOPAEDIA OF HYDROCARBONS
PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES
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
C3C4
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 0C. 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
185VOLUME II / REFINING AND PETROCHEMICALS
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 (4-5C)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.
186 ENCYCLOPAEDIA OF HYDROCARBONS
PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES
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
187VOLUME II / REFINING AND PETROCHEMICALS
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.
188 ENCYCLOPAEDIA OF HYDROCARBONS
PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES
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
189VOLUME II / REFINING AND PETROCHEMICALS
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 109C. 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
190 ENCYCLOPAEDIA OF HYDROCARBONS
PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES
isos
trippe
r
i-C4make-up
olefinfeed
i-C
4 recy
cle
coolingwater
ASO and CBMto neutralization
alkylate n-butaneto depropanizer
acidrecycle
settler
acid
rege
nerator
reac
tor
HF stripp
er
alum
ina
trea
ter
KOHtreater
KOHtreaterKOHtreater
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
191VOLUME II / REFINING AND PETROCHEMICALS
ALKYLATION
-
consumption. In the case of H2SO4, temperaturesbelow 2-4C 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
DAMICO 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 la ptrochemie, 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 MetallurgiaUniversit degli Studi di Roma
La Sapienza
Roma, Italy
192 ENCYCLOPAEDIA OF HYDROCARBONS
PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES