8 De h y d rog enation

Although dehydrogenation is the reverse of hydrogenation, and theoretically the same catalysts
should be operable in either direction, there are substantial practical differences between the two
processes. These differences become clear when the following facts are considered:

• Since dehydrogenation is endothermic, high temperatures and lower pressures favor

dehydrogenation.
• Low partial pressures of reactants favor dehydrogenation in the gas phase.
• Removal of hydrogen by purging from a liquid reactant system or reaction of hydrogen
with an acceptor reactant as it is formed favors dehydrogenation.
• Dehydrogenation catalysts are less sensitive to catalyst poisons, possibly because of the
lower chemisorption of potential poisons at the high operating temperatures. 1

8.1 D E HY D ROG E N AT I O N OF ETHYL B E N Z E N E

8.1 .1 ETHYLBENZENE � STYRENE

The dehydrogenation of ethylbenzene is, on a tonnage basis, the major catalytic dehydrogenation
process. Styrene is the fourth in the series of most used monomers.

ethylene > vinyl chloride > propylene > styrene

Polystyrene is the major product ( 63%) produced from styrene. Its uses are well known to most
consumers: protective packaging, electronic housings, toys, insulation, and disposable cups and
utensils. Copolymers have unusually valuable properties. Styrene-butadiene rubber (SBR) is the
most widely used synthetic rubber. It is a major component of tire treads for automobiles, where
it is blended with polybutadiene, yielding a tread surface of excellent grip. Natural rubber, because
of its better flexing properties, is blended with SBR for use in forming the carcass. 3 The liner is
made from butyl rubber because it is impermeable to air. Radial tires use a higher percentage of
natural rubber in the blends with SBR. 3 SBR rubber is also used for electrical wire insulation and
footwear. Styrene-butadiene latex is used for paper coatings and carpets. Styrene-acrylonitrile is
molded for the exterior cover of various home appliances and housewares, as is acrylonitrile­
butadiene-styrene, which is a also cast for automobile parts, pipe, electronic devices, and canoes. 2
Copolymers of styrene such as styrene-acrylonitrile (SAN) and acrylonitrile-butadiene-styrene
(ABS) can be produced with excellent crystal-like clarity and are easily dyed.

Chem istry

Dehydrogenation of ethylbenzene to styrene is an equilibrium-limited reaction. It is highly endo­

thermic (LlH = 29.83 kcaVg mole) and involves an increase in moles. Several side reactions occur

67
68 H a n d bo o k oi Com m e rc i a l Cata l ysts

that can be minimized by careful control of operating conditions and catalyst selectivity. The
suggested reaction scheme is as follows. 4 . 1 2
900 K
�H, kcal K
C6H�CH�CH> -t C6H5CHCH, + H� (I) 29.83 0.376
C6H5CH�CH, -t C6H. + C�H" (2 ) 25.22 0.238
C6H�CH,CH> + H � -t C6H�CH> + CH" (3) -15. 18 8 . 874 X 1 0--'"
C�H" + 2 H p -t 2CO + 4H� (4 ) 55 .43 4. 1 03 X 1 0-�
CH" + H�O -t CO + 3H� (5 ) 59.59 1 .29 1

co + HP -t co� + H, (6) - 8.53 2 . 1 93

Reaction 1 is the main reaction that accomplishes the selective dehydrogenation of the ethyl
side chain without hydrogenating the benzene ring with hydrogen that accumulates as the reaction
progresses. Figure 8 . 1 illustrates the effect of temperature, operating pressure, and steam to ethyl­
benzene ratio on the equilibrium conversion. Of course, the relevant pressures influencing reaction
rate are the partial pressures of the reactants. In addition to high-temperature operation, equilibrium
conversion is improved for a reaction with an increase in moles by lower partial pressures, by
reducing operating pressure, and by increasing the steam-to-ethylbenzene ratio in the feed. Steam,
however, represents a significant operating cost factor, and improvements in the process have
invariably been accomplished by lower steam rates and a commensurate lowering of total pressure
to counteract the higher reactant and product mole fractions. In fact, most adiabatic styrene reactors
now operate at sub-atmospheric pressures that allow steam rates as low as 4 or 6: 1 moles steam
per mole of ethylbenzene. Earlier reactors were operated in the range of a 1 4 - 1 8 steam-to­
ethylbenzene mole ratio.
There is a limit to how low the steam rate can be lowered, because steam serves several other
valuable purposes as follows:

• It provides the required heat to the reaction mix to replace the endothermic heat consumed
by the reaction.

1 .00 ,....----,

0.95

Equilibrium conversion
• 0.411 .2
• 1 .011 .2
0.412.0
& 1 .012.0
•

570 590 610 630 650

Temperature. • c
S/HC: steam t o hydrocarbon ratio
Pt : total pressure (kg/cm2 absolute)

F I G U RE 8. 1 Equilibrium Conversion of Ethylbenzene to Styrene [at Various Mass Ratios of Steam-to­

Ethylbenzene and Total Pressures (P, at 0.4 and 1 .0 kg/cm 2 and SIEB at 1 .2 and 2 .0) ] . Reprinted by permission:
Sundaram, K. M., Sardina, H., Femandez-Boujin, J. M. and Hildreth, J. M., Hydrocarbon Processing, p. 83,
Jan. 198 1 . P, = total pressure, S = steam, and EB = ethylbenzene.
Dehyd roge n a t i o n 69

• It maintains the iron oxide catalyst in the optimum oxidation state.

• It removes coke buildup on the catalyst via the water gas reaction.

Since preheating ethylbenzene to the desired operating temperature will cause extensive thermal
cracking, steam also serves conveniently as a means for rapidly heating ethylbenzene to the desired
inlet temperature for the requisite mixture. Steam is heated in a furnace to a higher temperature
(- 750°C or higher) and then combined with preheated ethyl benzene at a lower temperature
( -525°C).
Reactions 2 and 3 , which are respectively cracking and hydrocracking reactions, can occur
both catalytically and thermally. Reactions 4, 5, and 6 are typical water-gas reactions. All of these
side reactions can be minimized by selective catalysts and by avoiding excessively high reaction
temperatures (above 650°C ) . Other by-products that are produced in much smaller quantities include
phenylacetylene, cumene, n-propylbenzene, a-methylstyrene , biphenyl, stilbene, and phenan­
2
thrane. Thermal cracking is a significant contributor to the formation of these side-reaction aro­
matics, and, again, avoidance of excessively high temperatures will reduce their production.

Catalyst Types

The commercial catalyst of choice for most styrene plants is a mixture of iron oxide and potassium
carbonate with small amounts of one or more of the following promoters: Cr20 3 . Ce203 , Mo03 ,
CaO, MgO, and V 205 • 5 ·6 Pigment-grade iron oxide is kneaded with potassium carbonate and water
to produce a paste that can be extruded or further prepared for pilling.7 Heat treatment and calcining
at the catalyst plant causes decomposition of the potassium carbonate .

It has been suggested6 that potassium ferrite sites are formed on the surface of the iron oxide6 by
reacting with K20 to produce the active sites.

Others have suggested that the K20 increased catalytic activity by promoting electron transfer at
the solid-gas interface. 8 One issue, however, is clear. The K20 promoted catalyst exhibits an order
of magnitude greater activity than unpromoted iron oxide. This high activity, however, is reached
after three to four days of operation at atmospheric pressure, 600°C , and 2.0 weight ratio of steam­
to-hydrocarbon. The actual planned design operating conditions can then be imposed.
The Cr203 promoter in the range of 1 - 3 wt% acts as a structural stabilizer by preventing
sintering and the resulting loss of surface area. 8 Alternatively, other binders, such as cement, can
2
be used to assure structural strength. Other promoters are used including, respectively, MgO with
Cr2 03 and Ce 2 03 , Mo03 , and MgO without Cr 03 •
2
The K2 0 component acts as an effective water-gas shift catalyst and continuously removes
carbonaceous deposits as CO and C02• The long catalyst life of one to two years is attributed to
this property.

Catalyst Su rface A rea

Calcination of the extrudates or pills at the catalyst plant is done at a high enough temperature
(900-950°C) to reduce the surface area to 1 .5 to 3 m2/g. In addition, combustible fibrous materials
70 H a n d bo o k oi C o m m e rc i a l C a t a l ysts

can be added that. under calcining conditions. vaporize and leave macropores. The combination of
low surface area due to calcining, which tends to eliminate micropores. and the overt creation of
macropores serves to enhance selectivity. At the high temperatures required to assure favorable
equilibrium conditions for styrene formation, slower thermal and catalytic reactions including
cracking reactions are significant enough to cause increased unwanted side reaction products if
given enough time in the catalyst pores. By confining pore structure to macropores, diffusion in
and out of the pores becomes more rapid and thereby minimizes side reaction products and thus
improves selectivity.
The intrinsic reaction rate of the major reaction is high and, even with large pores, the actual
rate is affected by intraparticle diffusion. For any given catalyst composition and operating condi­
tions, higher conversions are obtained by using smaller diameter extrusions. This option is, of
course, limited by an acceptable maximum pressure drop.

Catalyst Suppl iers

The major suppliers of ethylbenzene dehydrogenation catalysts provide a variety of promoted iron­
oxide catalysts to serve the specific needs of each operating plant. The offerings can be divided
into two general categories, high-activity, high-selectively and high-stability catalysts. 5 ·2 0 The high
activity catalysts permit high styrene yields at lower operating temperature and lower steam ratios.
The high-selectivity catalysts require some temperature increase in exchange for much improved
selectivity. Other catalysts optimize high-activity and high selectivity characteristics. It should be
remembered that in all cases selectivity tends to decline at higher conversions approaching equi­
librium since the rate of dehydrogenation of ethylbenzene will decline while the rate of the side
reactions which are not equilibrium limited will increase. Potential users should seek the advice
of supplier's technical representatives to optimize the economics of their particular unit.

Suppliers
Criterion, United Catalysts, Engelhard

L icensors
ABB Lummus Global, UOP LLC, Fina/Raytheon, Lungi, BASF, Dow/Engelhard

Typical Catalyst Characteristi cs

Components Wt% (Ranges)

Extrudates (plain or ribbed for lower M') 1/8. 3/ 1 6. 1/4. 3/8 45 -75 27- 1 0 Various (see text )

Catalyst Deactivation

Poisons
Ethylbenzene dehydrogenation catalysts are subject to poisoning by halides (usually chlorides and
fluorides) and compounds of sulfur, phosphorous, and silica. Solids contained in steam caused by
low-quality boiler feed water can also deactivate the catalyst by coating the active sites. 5 Ethylben­
zene is produced either by Friedel-Crafts alkylation of benzene with ethylene using AlC13 in the
liquid phase along with co-catalysts such as ethyl chloride, BF3 , ZnC14 , SnC14 , H 3 P04 , or by zeolite
catalyzed alkylation. Although organic sulfur compounds at times can easily contaminate the
petrochemical feeds of benzene and ethylene, they are temporary poisons, and the catalyst will
return to previous activity once the sulfur compounds are no longer in the feed. The halides, by
contrast, act as permanent poisons. The feed to the reactor should have no more than 1 ppm of
Dehyd roge n a t i o n 71

chloride to perform satisfactorily over a long period o f operation. Halides react with K � O t o produce
KCI. which apparently lowers the promoting effect of K 1 0. 8 In addition, significant amounts of
KCl tend to be transferred as a vapor to the cooler part of the bed which is toward the outlet. Such
large quantities can totally destroy the activity of that portion of the bed as well as seriously affect
selectivity in the remainder of the bed because of K�O loss.

Potassium Migration
During prolonged use the potassium promoter tends to migrate from the periphery toward the center
of the catalyst pellet. In so doing, the highly concentrated potassium content in the center region,
and its absence in the peripheral region, ultimately leaves both regions inactive and confines the
active region to a narrow band between the center and the periphery. 8 At this condition, the catalyst
presents little or no diffusion resistance and can continue operation for some time until further loss
of activity occurs.
Steam treatment of used catalysts has been shown to cause chemical-vapor transport from
regions of high K � O concentration to those of low concentration.9

The KOH will migrate to the low concentration region containing smaller agglomerates of K�O.
Because of the lower surface area-to-mass ratio of the larger agglomerates, they will have a lower
temperature and drive the exothermic equilibrium to the right. Conversely, as the KOH moves to
the region of smaller agglomerates, the reverse reaction will occur, and K 2 0 is deposited. B ackflow
of steam through a bed removed from service can also cause redistribution of K�O from the lower­
temperature outlet region throughout the remainder of the bed. 10

Process U n its

Reactors used in dehydrogenation of ethylbenzene include both adiabatic and isothermal designs.
Adiabatic reactors are the most widely used, especially in the United States. Isothermal reactors
have found favor in Europe with several producers. With the newer catalysts conversions of 60 to
75 % and selectivities of 85 to 95 % are possible with either reactor type. �

Adiabatic Reactor System

Adiabatic reactors for styrene production, as is the case for many other adiabatic units, are staged
so that reheating of the reaction mix can be accomplished either by direct contact of additional
steam or by means of a superheated-steam exchanger. Two separate reactors or single reactors with
separate beds and the necessary internals for adding reheating steam may be used.
Superheated steam at around 830°C is mixed with preheated ethylbenzene ( -530°C). The
ethylbenzene can be heated to the lower temperature in a heat exchanger along with a portion of
the steam to be charged. The higher temperature superheated steam is heated in a direct-fired
furnace . It is important to avoid heating the ethylbenzene alone to higher temperatures and allowing
it to have excessive contact type in the transfer line to the reactor, for ethylbenzene will undergo
cracking reactions and reduce the yield of styrene.
Most adiabatic reactors are radial reactors, which are essential for low pressure-drop operation.
Since the volumetric flow increases as reaction proceeds due to the increase in moles, the flow is
directed from the inside of the annular bed radially outward.
Isothermal Reactor
Two major types of isothermal reactors for styrene production have been described. �·9 The Lurgi
reactor uses 20,000 to 30,000 tubes, 1 to 2- 112 in. (2.5 to 6.4 em) diameter and 8 to 1 0 ft (2.4 to
3 m) lengths packed with catalyst and surrounded by flowing molten salt solution of carbonates of
72 H a n d book oi C o m m e rc i a l C a t a l ysts

sodium. lithium. and potassium. The molten salt is circulated through an external heater to maintain
its temperature at about 630°C, 20° above the reaction temperature. which is held constant.
Steam is used for the same reasons given for the adiabatic system except it is not needed to
provide the heat energy to replace the endothermic reaction now being supplied by the heating
medium. Thus, less steam is consumed than for the adiabatic reactor.
The other major isothermal process is used by BASF in Europe. It differs from the Lurgi in that
it uses flue gas from a fired heater at 760°C ( 1 400°F) as the heating medium. This higher temperature
is necessary to compensate for the low heat capacity of the flue gas. The packed tubes are fewer in
number and larger; 4-8 in. ( 1 0-20 em) diameter and 8 . 2 - 1 3 . 1 ft (2.5-4 m) length.9
Both of these isothermal processes can claim some improvement in yield and. because of better
temperature control and optimization, savings in steam cost. However. the capital costs are higher
than the adiabatic units, and the maximum practical size of a single isothermal reactor limits the
total capacity to substantially less than a single modem adiabatic design.

Adiabatic Reactor with Oxygen A ddition (Oxida tive Dehydrogena tion)

A process has been described in which oxygen is added between stages in a separate bed containing
an oxidation catalyst reported to be composed of Pt, Sn. and Li on alumina. Sufficient oxygen is
added to combine with the hydrogen and form steam. Thus, the equilibrium is shifted further toward
styrene by removing the hydrogen. In addition, heat is supplied by the oxidation for the second
dehydrogenization bed. Concerns have been expressed about safety, formation of phenylacetylene
(may effect styrene polymer quality) , and extra cost for the Pt catalyst. One plant has been built
in Japan. The dehydrogenation catalyst is thought to be similar to that for standard existing
dehydrogenation-only units.

Operating Conditions
All of the more recent reactors just described are operated at subatmospheric pressure (0.5 atm
outlet) to take advantage of the improved equilibrium conditions. Also, lower pressures permit
lower steam rates that can yield substantial energy savings. Since temperatures above 6 1 ooc mark
the beginning of undesirable cracking reactions, 2 isothermal reactors are operated conveniently at
6 1 0°C. Adiabatic reactors, however, experience temperature decline along the bed, and it is nec­
essary to enter the beds with a steam-ethylbenzene mixture at a temperature as high as 630-640°C.9
Thus, some cracking does occur, but it is minimized by the relatively short contact time at the
entering temperature.
As the catalyst slowly deactivates over time, it becomes necessary to gradually raise the
operating temperature in all the reactor types to keep up production. This procedure ultimately
approaches the design temperature for the reactor, and continued operation can then occur only by
lowering the feed rate ( lowering the space velocity) . In this way, lower production may be acceptable
until the planned not-too-distant tum-around date.

Process Kinetics

Various rate expressions have been proposed for the main reaction and the side reactions} 1 - 1 6 In
general, they take the Langmuir- Hinshelwood, Hougen-Watson (LHHW) form or use simple
molecular (power law) forms as follows:

LHHW Form
Dehyd roge n a t i o n 73

where E = ethylbenzene
H = hydrogen
S = styrene
Side reactions 4 . 5, and 6 have often been written as simple power-law rate expressions a s
shown below; or i f one assumes these reactions to b e very rapid, they can b e combined with
reactions 2 and 3 to yield the following, assuming that half of the ethylene is converted to methane
( 1 /2 C 1 H4 + H1 � CH4):

An industrial model for simulation and optimization of a styrene unit has been described with
brief references to the kinetics. 1 8 Since most of the reactions, at the high temperatures required for
dehydrogenation, occur not only catalytically but to some degree thermally as free-radical reactions,
the model formulated the thermal reactions as simple molecular power-law equations. Catalytic
rates for the same apparent overall reactions were written in forms similar to LHHW equations.
Power-Law
Typical power-law forms have also been used. 15

r1 = k1 ( PE - P5PH/K 1 )
r1 = k1 PE
r3 = k3 PEPH
r4 = k4Pc 1 11 pw
r5 = ks PM Pw
r6 = �PcoPw
Complexity
As is true with so many commercial reactions, the reaction system is complex. Initially, the reactions
are strongly controlled by intraparticle diffusion. But because of changes that occur in the catalyst,
including K10 migration, the process gradually becomes reaction controlled while catalyst activity
is declining. To base any practical model on intrinsic kinetics and effectiveness factors seems futile.
Time would be better spent on obtaining a useful empirical expression for activity as a function
of time-on-stream and operating temperature history. The actual activity decline is related directly
to accumulated coke and the loss of active components such as K10 by agglomerationY

8.2 STYRENE DERIVATIVES FROM OTH E R A L KYL AROMATICS

Several styrene derivatives are commercially produced by dehydrogenation over iron oxide cata­
lysts. but in much smaller quantities. These products include the vinyltoluene isomers and divinyl­
benzene. These compounds are more reactive than styrene and thus cause more side reaction
products than styrene in the dehydrogenation reaction. 1 9
74 H a n d book oi Com m e rc i a l Cata l ysts

8.2.1 D I ETHYLBENZENE � D IVINYLB ENZENES

rA( CH = CH 3
� CH = CH 3
diethylbenzene divinylbenzene

The process is conducted above 600°C using superheated steam in a fixed-bed reactor.9 Diethyl­
benzene is a side reaction product in the production of ethyl benzene and occurs in all three isomeric
forms.
Dehydrogenation produces a number of side reaction products including benzene, toluene,
xylene, styrene, and toluene derivatives. All three isomers (ortho, meta, and para diethylbenzene )
are produced, but the ortho isomer converts to naphthalene, which must be separated from the other

isomers because it has no useful polymerizable functions and also causes an objectionable odor.9· 1 9
The refined product consists of meta and para divinylbenzene and ethylvinylbenzene with various
amounts of divinylbenzene, depending on the commercial grade.
Because of the two polymerizable groups, divinylbenzene is used in small amounts as a
copolymer with styrene to produce crosslinking and a product of low solubility, heat resistance ,
surface hardness, and high impact and tensile strengths. These characteristics are ideally suited for
ion exchange resins for water conditioning, which constitutes its major use.

Process K i netics

Detailed Langmuir-Hinshelwood rate equations have been developed for the two-step mechanism
of the formation of ethylvinylbenzene from diethylbenzene and then divinylbenzene from ethylvi­
nylbenzene. 2 1 The initial rates can be expressed rather simply.

k 1 K0 P 0
(1 + K EP E) 2

k2 Ko P o
(1 + K EPE) 2

where r 1 = rate of reaction of diethylbenzene to ethylvinylbenzene

r2 = rate of reaction of ethylvinylbenzene to divinylbenzene
subscripts D and E = diethylbenzene and ethylvinylbenzene

Equations involving higher conversions in which the reverse reaction and other adsorbed products
2
must be considered are provided, along with a suggested model for simulation. 1

8.2.2 ETHYLTOLUENE � VI NYLTOLU ENES

ethyltoluene vinyltoluene
Dehyd roge n a t i o n 75

Ethyltoluene is produced by the Friedel-Crafts or zeolite-catalyzed alkylation of toluene with ethylene

to form the isomers of ethyltoluene. Again. the ortho isomer must be removed prior to dehydroge­
nation. since it forms indene, which causes undesired characteristics in the final copolymers. 9
The meta and para ethyltoluenes are then dehydrogenated using iron oxide catalysts and steam.
The purified commercial product contains 60% meta and 40% p-vinylbenzene and is used in
producing a copolymer with styrene. These copolymers exhibit high heat resistances, good flow
properties. and increased solubility in aliphatic solvents.9· 1 9

8.2.3 I SOPROPYLBENZENE (CUMENE) � a-M ETHYLSTYRENE

isopropyl benzene
a-methylstyrene
cumene

The styrene derivative a-methylstyrene is used as a monomer in conjunction with styrene and other
monomers to produce specialized products. When copolymerized with acrylonitrile/butadiene/sty­
rene (AB S ) polymers, it increases the temperature at which heat distortion occurs. In copolymers
for coatings and resins, it lowers the reaction rates to a practical value and improves product
clarity.9. 1 9
Dehydrogenation of cumene is a viable process, similar to all other styrene dehydrogenations
to styrene or styrene derivatives, but it is conducted on a small scale because the quantities required
are modest. However, most of the alpha-methylstyrene used today is obtained as a by-product from
the production of phenol from cumene.

Catalyst Suppl iers

The catalyst used in dehydrogenation in the production of various styrene derivatives are essentially
the same as used in producing styrene. See page 70.

8.3 D E HY D ROGENAT I O N O F LOW E R A L KA N ES

8.3.1 G ENERAL BACKGROUND

Dehydrogenation of alkanes to olefins, like other dehydrogenations in the vapor phase, requires
high temperatures, low pressures, and a low hydrogen-to-hydrocarbon ratio. This latter requirement
impinges on the need for maintaining catalyst activity by hydrogen in sufficient amounts to slow
the formation of coke.

Thermodynam ics

Figures 8.2 and 8.3 present equilibrium plots for several lower alkanes. ' These highly endothermic
reactions, with an increase in moles and low values of K P , require relatively high temperatures and
either subatmospheric pressures or atmospheric pressures or above with high steam content in the
feed. Ethane requires very high temperatures, which enables thermal steam cracking to be the
preferred and cheaper route. Higher olefins, C 3 , C4 , and C 5 , can be conveniently recovered from
both thermal and catalytic cracking units. High demand, however, has driven the development of
separate catalyzed dehydrogenation processes for converting alkanes to ole:fins or diolefins.