Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

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CHAPTER TEN

Alkylation

10.1. Introduction

Alkylation is the process of producing gasoline range material (alky-

lates) from olefins such as propylene (C

), butylenes (C

) and amylene

), and isobutane. Butylene is the most widely used olefin because of the

high quality of the alkylate produced. The current trend toward elimination

of methyl tertiary butyl ether (MTBE) has resulted in increased attention to

alkylation technology.

An alternative process is the polymerization process in which polymeric

materials from unreacted olefins are formed. Reformulated gasoline requires

a low olefin content. This makes polymer gasoline undesirable as a blending

stock. The motor octane number of a polymer gasoline is much lower than

the corresponding values obtained from alkylation. This has resulted in the

shutdown of the polymerization units in refineries using alkylation.

10.2. Role of Alkylation and Polymerization

Units in the Refinery

Refinery gases produced from different units are collected and sent to

the gas plant. Olefins and isobutanes are separated and used as a feed to the

alkylation plant (Figure 10.1). Sources of these two components from

different refinery units are shown in Table 10.1.

Olefins are sent to the polymerization unit as shown in Figure 10.1.

Both alkylation and polymerization units produce gasoline which can be

sent to the gasoline pool.

10.3. Alkylation Processes

Alkylation is catalysed by a strong acid, either sulphuric (H

)or

hydrofluoric (HF). In the absence of catalysts, alkylation between isobutane

and olefin must be run under severe conditions such as T ¼ 500



C (932



Fundamentals of Petroleum Refining

2010 Elsevier B.V.

263

and P ¼200–400 bars (2940–7080 psia). In the presence of an acid catalyst, the

reaction temperature will be lower than 50



C(122



F), and the pressure will

be lower than 30 bars (441 psia). The major difference in using either acid is

that isobutane is quite insoluble in H

but reasonably soluble in HF. This

requires the use of high isobutane/olefin ratios to compensate for low solubil-

ity in H

. Furthermore, the reaction must occur at low temperature.

The alkylation process consists of running the hydrocarbons in liquid form

(enough pressure is used to ensure that) and at low temperature and with a

high isobutane (iC

) to olefin (such as C

) ratio. The reaction products are

sent to an acid settler where the acid is recycled back to the reactor. Products

are then separated into gaseous LPG propane and n-butane and the desired

product of alkylate. A block diagram of the process is shown in Figure 10.2.

10.3.1. Sulphuric Acid Alkylation Process

Two sulphuric acid alkylation processes are commonly available. These are

the auto-refrigeration process licensed by Exxon and the effluent refrigera-

tion process licensed by Stratford. The major difference between the two

Table 10.1 Oleﬁns and isobutane production from different units

LV %

Isobutane Olefins

Hydrocracker 3 –

FCC 6 15

Coker 1 15

Hydrotreater 1 –

Reformer 2 –

Isomerization 1 –

Crude unit 0.5 –

Polymerization

Gas

Plant

Alkylation

Gasoline

Pool

Feed

Alkylate

Polymerized

Gasoline

Olefin

Gasoline

Figure 10.1 Role of alkylation and polymerization units in the refinery

264 Chapter 10

processes is in the reactor design. In the auto-refrigeration process, the

evaporation of iC

and C

induces cooling of the emulsion in the reactor.

In the effluent refrigeration process, a refrigeration unit provides cooling to

the reactor.

The auto-refrigeration unit is shown in Figure 10.3. The olefin is fed to

the first reactor in the cascades, together with the recycled acid and refriger-

ant. Recycled and make-up isobutanes are distributed to each reactor. Eva-

porated gases are compressed and fed back to the reactor along with the fresh

olefin feed which is also cooled by this stream (Gary and Handwerk, 1994).

The reactor operates at a pressure of 90 kPag (10 psig) and at a tempera-

ture of 5



C (40



F) for up to 40 min. In the Stratco process, the reactor is

operated at a higher pressure of 420 kPag (60 psig), to prevent evaporation

of hydrocarbon, and at a temperature of 10



C (50



F). A block diagram of

the Stratco effluent refrigerated process is shown in Figure 10.4. In this

diagram the ‘‘effluent treating’’ section is used to remove free acid and alkyl

Recycle Acid

Alkylate

Butane

Isobutane

Reactor

Refrigerant

Olefin Feed

Isobutane

Recycle

Hydrocarbon vapor

To Compressor

Hydrocarbon

Acid

Figure 10.3 Auto-refrigerated sulphuric acid a lkylation process

Acid Catalyst

Isobutane Recycle

LPG Propane

n-butane

Alkylate

Isobutane

Reactor

Acid

Settler

Product

Separation

Olefins:

Figure 10.2 Blockdiagram of alkylation process

Alkylation 265

sulphate to avoid corrosion and fouling. The ‘‘blowdown’’ section is used to

purge and neutralized spent acid.

The effluent refrigeration process uses a single Stratco reactor design as

shown in Figure 10.5. An impeller emulsifies the hydrocarbon–acid mixture

for about 20–35 min.

Refrigeration

Alkylation

Reaction

Effluent

Treating

Blowdown

Olefin Feed

Makeup Isobutane

Alkylate

Product

n-Butane

Product

Fresh

Acid

Process

Water

Refinery

Wastewater

Treatment

Recycle Isobutane

Acid

Spent Acid

Acid

Fractionation

Figure 10.4 Blockdiagram for Stratcoeffluent refrigerated sulphuric acid alkylationunit

Olefin and iC

Feed

Tube bundle

Recycle C

and C

Mixer

Driver

To Condenser

and Separator

Settling

Drum

Alkylate and

s + C

Figure 10.5 Stratco reactor

266 Chapter 10

10.3.2. Hydrofluoric Acid Alkylation

Two hydrofluoric acid (HF) alkylation processes are commonly available. These

are the Phillip process and the UOP process. The HF processes have no

mechanical stirring as in the sulphuric acid processes. The low viscosity of HF

and the high solubility of isobutane in the acid allow for a simpler design. The

emulsion is obtained by injecting the hydrocarbon feed into the continuous HF

phase through nozzles at the bottom of a tubular reactor. Reaction temperature

is about 30



C(86



F), allowing for the use of water as a coolant to the reactor.

The two processes are quite similar. The flow diagram of the Phillips

process is shown in Figure 10.6. The residence time in the reactor is 20–40 s.

The hydrocarbon phase is sent to the main fractionation column to obtain

stabilized alkylate. H

alkylation processes are favoured over the HF

processes because of the recent concern about the mitigation of HF vapour.

HF is a very hazardous material for humans because it can penetrate and

damage tissue and bone.

10.3.3. Solid Catalyst Alkylation

Alkylation processes based on solid acids are not yet operated on an indus-

trial scale. However, several companies have developed processes or already

offer technology for licensing. The overall process scheme is similar to

the liquid acid base process scheme, except for the regeneration section,

recycle

Separator

Reactor

Debutanizer

HF stripper

Main

fractionation

Stabilizer

alkylate

to caustic

washing

To H F

treatment

n-C

to caustic

washing

Dry C

feed

Dry

isobutane

From

regenerated

Figure 10.6 Simplified di agram of the Phill ip’s HF alkyation process

Alkylation 267

which is necessary for solid acid catalysts because of rapid deactivation.

Hydrogen has proven to be very effective for the regeneration of the catalysts.

Examples of solid acid alkylation technologies are shown in Table 10.2.

10.3.4. AlkyClean Process

Lummus technology has developed a solid acid catalyst gasoline alkylation

technology (Amico et al., 2006). The AlkyClean process employs a zeolite

catalyst coupled with a novel reactor processing to yield a high quality

alkylate product. The process shown in Figure 10.7 consists of four main

sections: feedstock pretreatment, reaction, catalyst regeneration and product

distillation. An olefin feed is preheated and fed with the isobutane recycle to

the reactor. The reactor operates at 50–90



C (122–194



F) with liquid

phase conditions. Multiple reactors are used to allow for the catalyst regen-

eration cycle. During regeneration, olefin addition is stopped and hydrogen

is added to achieve a low reactor concentration of dissolved hydrogen while

maintaining liquid phase alkylation reaction conditions. This minimizes

Table 10.2 Solid acid alkylation processes

Process

Reaction

temperature (



C) iC

/olefin Catalyst

UOP alkylene 10–40 6–15 HAL-100

Lurgi Eurofuel 50–100 6–12 Faujasite-derived

Haldor Topsoes FBA 0–20 CF

H/SiO

ABB Lummus

AlkyClean

50–90 8–15 Zeolite-derived

(SAC)

Pretreatment

Reactor System

Product

Distillation

Catalyst

Regeneration

Olefin Feed

Isobutane

Light Ends

Isobutane

Feed

n-butane

Alkylate

Product

Hydrogen

Figure 10.7 AlkyClean process

268 Chapter 10

energy consumption during the switching of the operation. The swing

reactor coupled with long catalyst life allows the refiner to work without the

need of taking the reactor off-line for moderate temperature regeneration

that restores the catalyst activity completely.

10.4. Kinetics and Thermodynamics of Alkylation

Alkylation is carried out in the liquid, the gas phase or in a mixed gas–

liquid system. Consider the simple liquid phase reaction of isobutene (A)

and isobutane (B) giving isooctane (A

A þ B⇆A

ð10:1Þ

If y is considered as the degree of conversion m

and m

are number moles

of A and B in the initial mixture, respectively, and d ¼ m

. The

number of moles in the reaction system can be expressed by:

ðm

 m

yÞþðm

 m

yÞþm

y ¼ m

ð1 þ d  yÞð10:2Þ

Accordingly, mole fractions can be expressed as:

1  y

1 þ d  y

; x

d  y

1 þ d  y

and x

1 þ d  y

ð10:3Þ

Thus, the reaction equilibrium constant can be expressed by:

xeq

ðx

Þðx

yð1 þ d  yÞ

ð1  yÞðd  yÞ

ð10:4Þ

And the degree of conversion, y, can be calculated as:

y ¼

ðd þ 1Þ

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

ðd þ 1Þ



xeq

þ1

ð10:5Þ

Assuming an ideal solution and applying Raoult’s law, the equilibrium

constant in the gas phase can be expressed as:

¼ K

xeq

ð10:6Þ

where K

and K

xeq

are the equilibrium constants of the reaction in the gas and

liquid phases, respectively, and K

is the standard gas equilibrium constant

which can be calculated using the saturated vapour pressures of the compounds

at the temperature of the reaction. K

can be calculated from the equation:

ð10:7Þ

The alkylation reactions considered in this chapter are between olefins (butene

or isobutene) and isobutane. Typical reactions and values of the equilibrium

constants, K

, at different operating temperatures are given in Table 10.3.

Alkylation 269

Table 10.3 Equilibrium constant K

for alkylation reactions at 1 bar in ideal gas phase (Zhorov, 1987)

Reaction

(MPa

1

)

Equations300 K 400 K 500 K 600 K 800 K

Ethylene þ isobutane $ 2,3-dimethylpentane 7.7  10

5.4  10

170.0 39 0.4 (10.8)

Propene þ isobutane $ 2,3-dimethylpentane 1.3  10

2.6  10

168.0 6.0 0.1 (10.9)

n-Butene þ isobutane $ 2,2,4-trimethylpentane 21.7  10

2.82  10

14.0 0.40 5.2  10

3

(10.10)

1-Pentene þ isobutane $ 2,2,5-trimethylhexane 55.5  10

2.9  10

85.0 2.0 1.7  10

2

(10.11)

Isobutene þ isobutane $ 2,2,4-trimethylpentane 0.11  10

76.0 1.0 0.06 1.7  10

3

(10.12)

cis-2-Butene þ isobutane $ 2,2,4-trimethylpentane 2.4  10

662.0 4.0 0.2 4.5  10

3

(10.13)

trans-2-Butene þ isobutane $ 2,2,4-trimethylpentane 0.77  10

303.0 3.0 0.1 3.0  10

3

(10.14)

2-Methyl-2-butene þ isobutane $ 2,2,5-trimethylhexane 0.23  10

105.0 1.0 0.06 1.9  10

3

(10.15)

270 Chapter 10

Example E10.1

Consider an alkylation reaction between isobutene (A) and isobutane (B) to give

isooctane (A1) at 300 K. The partial pressures of the reacting species are:

¼ 0:32 MPa; P

¼ 0:37 MPa and P

¼ 0:005 MPa, respectively. Plot the

conversion versus dilution ratio at different equilibrium constants.

Solution:

The standard gas equilibrium constant can be evaluated as:

0:005

ð0:32Þð0:37Þ

¼ 0:042 MPa

1

From Table 10.3 the equilibrium constant K

for isobutene–isobutane at 300 K

is 1:1  10

MPa

1

(reaction 10.12). Thus, the liquid phase equilibrium con-

stant can be evaluated as:

xeq

1:1  10

0:042

¼ 26:2  10

Similarly, for 400 K, we have P

= 2.94 MPa, P

= 3.16 MPa and

= 0.16 MPa, and K

is 76 MPa

1

(Table 10.3). Thus K

¼ 0:017 MPa

1

and K

xeq

¼ 4.5  10

Higher degrees of conversion (close to unity) are obtained when conducting

the alkylation reactions in th e liquid phase compared with the gas phase reac-

tions at moderate pressures. For this reason, industrial alkylation processes for C

and C

alkenes and isobutane are conducted in the liquid phase at temperatures

between 0–10



C (32–50



F) (for the sulphuric acid (H

) catalyst) or

30–40



C (68–104



F) (for the hydroﬂu oric acid (HF) catalyst). Figure E10.1

0.8

0.6

0.4

0.2

1.0

0.0

246

8010

=100

=10

=1.0

=0.1

Figure E10.1 Effect of dilution ratio (d) on conversion for different equilibrium

constants (K

xeq

)

Alkylation 271

shows the relationship of conversion with dilution ratio (d) for different

operating conditions, thus, different values of the equilibrium constants. It is

shown that y becomes close to unity at 300 K (540 R), where K

xeq

 100 and

d 1.0, for reaction (10.12) in Table 10.3.

A series of consecutive reactions leading to the formation of alkylate

bottoms and tar can also take place. If solid acid catalyst is used, it requires

higher temperatures to ensure favourable thermodynamic conditions. At

industrial conditions with temperatures greater than 10



C (283 K), about

17 side reactions will occur, and the simple model fails to predict the

kinetics in liquid phase.

10.4.1. Effect of Operating Conditions

The process conditions that influence the quality of alkylate product and

acid consumption rate are the olefin type, dilution ratio d (iC

/iC

mixing temperature, impeller speed, space velocity (or residence time)

and acid strength.

10.4.1.1. Olefin Type

The presence of propylene or pentene with butane will lower the octane

number and increase the acid consumption. The octane number of alkylates

produced from light olefins is given in Table 10.4.

Butene in sulphuric acid as a catalyst gives the best octane numbers as

shown in Table 10.4. The presence of propylene with butene increases

acid consumption and lowers the alkylate octane number. In the case of a

/iC

feed mixture, the trend is interesting since sulphuric acid con-

sumption decreases up to 82 vol% of the C

=i C

mixture. However, the

octane number also decreases. This might suggest that at lower acid con-

sumption, it is better to separate the C

=i C

mixture from C

and let it

react with iC

in a separate reactor (Kranz and Graves, 1998).

Table 10.4 Effect of type of oleﬁn on alkylate octane number

Types of Olefin

RO N M O N

HF H

Propylene 91–93 91–92 89–91 90–92

Butene-1 90–91 97–98 88–89 93–94

Butene-2 96–97 97–98 92–93 93–94

Isobutene 94–95 90–91 91–92 88–89

Amylene 90–92 91–92 88–89 89–91

272 Chapter 10