This is advance technique of distillation

1. Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Article No : c22_c01
Reactive Distillation
MICHAEL SAKUTH, Sasol Solvents Germany GmbH, Moers, Germany
DIETER REUSCH, Degussa AG, Marl, Germany
RALF JANOWSKY, Degussa AG, Mobile, Alabama, United States
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 264
2. Mathematical Modeling of Reactive
Distillation Processes . . . . . . . . . . . . . . . . . . 265
2.1. Equilibrium-Based Models . . . . . . . . . . . . . 265
2.2. Rate-Based Models . . . . . . . . . . . . . . . . . . . 266
3. Design of Reactive Distillation Processes . . . 267
3.1. Procedures for Process Design Studies . . . . 267
3.2. Flow sheet for Process Development . . . . . . 269
4. Industrial Applications . . . . . . . . . . . . . . . . 270
4.1. Commercial Packing Structures . . . . . . . . . 270
4.2. Industrial Catalytic Distillation Processes . . 272
4.3. Novel Application of CD with regard to
Process Intensification . . . . . . . . . . . . . . . . . 275
References . . . . . . . . . . . . . . . . . . . . . . . . . . 275
Symbols and Abbreviations
aj: specific interfacial area, m2
/m3
ctj: mixture molar densities, kmol/m3
E: energy transfer rate, W
fij
L
: feed flow rate of component i to stage j in
the liquid phase, kmol/s
kj: matrix of mass transfer coefficients, m/s
kij: equilibrium ratio of component i on
stage j
Lj: liquid flow rate from stage j, kmol/s
LjÀ1: liquid flow rate to stage j, kmol/s
Nij
L
: liquid phase mass transfer rate of com-
ponent i, kmol/s
Nj: vector of mass transfer rates, kmol/s
Ntj: total mass transfer rate, kmol/s
NC: number of compounds
QL: liquid phase heat loss, W
QV: vapor phase heat loss, W
rij: reaction rate of component i on stage j,
kmol/s
Sj
L
: ratio of liquid sidewithdrawal
Vj: vapor flow rate from stage j, kmol/s
Vjþ1: vapor flow rate to stage j, kmol/s
Xi: transformed liquid phase composition of
component i
xij: mole fraction of component i in liquid
phase of stage j
xij
I
: liquid mole fraction of component i in
interface
xj: vector of liquid mole fractions
Yi: transformed vapor phase composition of
component i
yij: mole fraction of component i in vapor
phase of stage j
yij
I
: vapor mole fraction of component i in
interface
yj: vector of vapor mole fractions
DHr: heat of reaction, J/kmol
ni: stoichiometric coefficient for compo-
nent i
nT: sum of stoichiometric coefficients de-
fined by Equation 9
Superscripts
I: Interface
L: liquid phase
V: vapor phase
Subscripts
B: bottom
D: distillate
F: feed
P: product
R: reference component
i: component number
DOI: 10.1002/14356007.c22_c01.pub2

2. 1. Introduction
Reactive distillation (RD) is a process in which a
catalytic chemical reaction and distillation (frac-
tionation of reactants and products) occur simul-
taneously in one single apparatus. Reactive
distillation belongs to the so-called ‘‘process-
intensification technologies’’. From the reaction
engineering view point, the process setup can be
classified as a two-phase countercurrent fixed-
bed catalytic reactor.
In the literature this integrated reaction –
separation technique is also known as catalytic
distillation (CD) or reaction with distillation
(RWD). According to [1], CD is a process in
which a heterogeneous catalyst is localized in a
distinct zone of a distillation column. RD is the
more general term for this operation, which does
not distinguish between homogeneously or het-
erogeneously catalyzed reactions in distillation
columns. RWD is a trademark of the Koch
Engineering Company for reactive distillation
technology that uses their KataMax packing
structures. A brilliant overview on the current
status of RD technologies, modeling, industrial
applications, etc., can be found in [2].
The present article exclusively deals with RD
processes that operate with a heterogeneous cat-
alyst system, i.e., CD technology. The most
important advantage of CD technology for equi-
librium-controlled reactions is the elimination of
equilibrium limitation of conversion by continu-
ous removal of products from the reaction mix-
ture. It is the application of Le Chatelier’s prin-
ciple to displace the chemical equilibrium by
increasing the concentrations on the one side of
the reaction, i.e., the reactants, and decreasing it
on the other, i.e., the product side. The chemical
composition at this equilibrium point can be
calculated by means of the Gibbs energy of
reaction at a given temperature. Activities must
be used to recalculate the composition from the
equilibrium constant (i.e., the molar fractions of
the components).
Usually, a partially converted reaction mix-
ture, close to chemical equilibrium, leaves the
fixed-bed reactor section and enters the CD
column in the fractionating zone to ensure the
separation of products from feedstock compo-
nents. The fractionated unconverted feedstock
components enter the catalytic section in the CD
column for additional or total conversion. The
catalyst packing zone is installed in the upper or
lower-middle part of the column, with normal
distillation sections above and below.
CD technology has several advantages over
conventional operating methods, such as a fixed-
bed reactor connected to a fractionating column,
in which the distillate or bottoms have to be
recycled after further separation steps for a total
overall conversion.
Apart from increased conversion, the follow-
ing benefits can be obtained [1]:
. The most important benefit of CD technology
is the lower capital investment, because two
process steps can be combined and carried out
in the same device (so called ‘‘process intensi-
fication’’). Such integration leads to lower
costs for pumps, piping and electrical
instrumentation.
. If CD is applied to an exothermic reaction, the
reaction heat can be used to vaporize part of the
surrounding liquid, which represents three
j: stage number
t: total
Abbreviations and Acronyms
AIBN: Azobisisobutyronitrile
CD: Chemical distillation
DIPB: Diisopropylbenzene
ETBE: Ethyl tert-butyl ether
HB: High-boiling
HETP: Height equivalent to a theoretical plate
LB: Low-boiling
MB: Medium-boiling
MESH: Material balance/equilibrium condition/
summation equation/heat balance
MTBE: Methyl tert-butyl ether
RD: Reactive distillation
RWD: Reaction with distillation
TAME: tert-Amyl methyl ether
TAEE: tert-Amyl ethyl ether
TIPB: Triisopropylbenzene
264 Reactive Distillation Vol. 31

3. fundamental advantages: The maximum tem-
perature in the structured catalytic packing is
limited to the boiling point of the reaction
mixture, so that the danger of hot spots is
reduced significantly (so-called ‘‘Siedek€uh-
lung’’). Also, extremely simple and reliable
temperature control is achieved. In addition,
the integration of reaction heat in the distilla-
tion process leads to energy savings by reduc-
ing reboiler duty.
. Product selectivity can be improved owing to
fast removal of reactants or products from the
reaction zone. Thus, the probability of conse-
cutive reactions, which may occur in the con-
ventional operation mode, is generally
lowered.
. If the reaction zone in the CD column is located
above the feed point, poisoning of the catalyst
can be avoided. This leads to longer catalyst
lifetime compared to the conventional mode of
operation.
. The possibility to break azeotropes in the vapor
- liquid equilibrium, because reactants or pro-
ducts can act as entrainers or because the
azeotropes can simply disappear.
There are three important constraints for ap-
plying CD technology to catalytic chemical
reactions:
. The use of CD technology is only possible if the
temperaturewindowofthevapor – liquidequi-
librium is equivalent to the reaction tempera-
ture. By changing the column operating pres-
sure, this temperature window can be altered.
. The flexibility in the operating temperature of a
CD column is not only restricted by the fact
that two phases are required for the distillation
process. Also the thermal stability of the cata-
lyst can limit the upper operating temperature.
. Because of the necessity of wet catalyst pellets,
the chemical reaction must take place entirely
in the liquid phase.
. As it is very expensive to change the catalyst in
the structured packing of a CD column, only
catalysts with a long lifetime are suitable for
this process.
As a ‘‘fourth constraint’’ CD technology is
somehow difficult to model mathematically,
which complicates the scale-up from technical
plant scale to full production scale as well [2].
In the literature, it can be found that endother-
mic reactions are not suitable for the CD tech-
nology, because the reaction heat condenses part
of the vapor stream. Although endothermic reac-
tions require more reboiler duty and therefore
exhibit no large energy savings, there are no
restrictions with regard to the application of this
technology [3].
Chemical reactions, which may benefit from
CD technology, should fulfill the above-men-
tioned criteria in general. Reactions of this type
include, for example, etherifications, esterifica-
tions, transesterifications, hydrations, hydroly-
sis, condensations, hydroisomerizations, oligo-
merizations, alkylations, transalkylations, and
selective hydrogenations.
An excellent overview of the current status of
published applications is given in [2].
2. Mathematical Modeling of
Reactive Distillation Processes
2.1. Equilibrium-Based Models
Multicomponent separation processes, such as
normal distillation processes, have been modeled
by using the equilibrium-stage concept for a
century. Therefore, early works on reactive dis-
tillation also used the equilibrium-stage model to
simulate reactions with superimposed distillation.
The principal assumption of the equilibrium-
stage model is that the vapor and the liquid stream
that leave the stage are in thermodynamic equi-
librium. In most real distillation columns, of
course, the residence time is too short to reach
total equilibrium. For this reason, efficiencies
have been introduced into the model (e.g., Mur-
phree efficiency, vaporization efficiency, etc.) to
account for the nonideal behavior. MESH (i.e.,
Material balance, Equilibrium relationship,
Summation of all substances, and enthalpy bal-
ance H) equations are used to simulate conven-
tional distillation columns (! Distillation, 1.
Fundamentals, Section 4.3).
To introduce the chemical reaction superim-
posed to the distillation further equations are
needed to simulate reactive distillation processes
(! Reaction Columns, Chap. 2.). The simplest
way to consider chemical reactions is to use the
equilibrium constant Ki, but many reactions are
not fast enough to reach chemical equilibrium in
Vol. 31 Reactive Distillation 265

4. one theoretical stage. Therefore, it is often neces-
sary to use kinetic expressions, which describes
the reaction rate as a function of temperature and
concentration activities of each component of the
reaction scheme.
2.2. Rate-Based Models
To describe the real phenomena of mass transfer
in distillation processes in more detail, a second-
generation nonequilibrium model was developed
by KRISHNAMURTHY and TAYLOR [4]. A nonequi-
librium stage (Fig. 1) represents either a single
tray or a section of packing in a column.
Contrary to the equilibrium-based stage mod-
el, this rate-based model treats the vapor and
liquid phases separately and combines them by
means of a mass transfer rate and a heat transfer
rate through the interfacial area. The total mass
transfer rates are:
NV
j ¼ cV
tj kV
j aj yjÀyl
j
þNV
tj yj ð1Þ
NL
j ¼ cL
tjkL
j aj xl
jÀxj
þNL
tj xj ð2Þ
where Nj
V
or Nj
L
are the vectors of molar fluxes of
the vapor or liquid stream, respectively, ctj are the
molar densities, kj are the matrices of mass trans-
fer coefficients, aj is the specific interfacial area,
and Ntj
V
or Ntj
L
are the total mass transfer rate of
the vapor or liquid stream, respectively [5].
At the interface of each stage j phase equilib-
rium (E equation, see above):
Kijxl
ijÀyl
ij ¼ 0 ð3Þ
and summation equations (S equations, see
above) for each component i:
X
i
yI
ijÀ1 ¼ 0; ð4Þ
as well as
X
i
xI
ijÀ1 ¼ 0 ð5Þ
must be fulfilled.
To simulate the reactive distillation process,
an expression for the reaction rate rij ¼ f (tem-
perature, concentrations/activities) must be
added. For catalytic distillation the chemical
reaction is taken into account in the material
balance of the bulk liquid phase
ð1þsL
j ÞLjxi;jxi;jÀ1ÀLjÀ1ÀfL
ij ÀNL
ij Àrij ¼ 0 ð6Þ
where sj
L
is the liquid side-withdrawal and fij
L
is
the feed flow rate of component i to stage j in the
liquid phase. For many catalytic distillation pro-
cesses, liquid – catalyst mass transfer must also
be considered.
The reaction heat DHr which is produced
(exothermic reaction) or consumed (endothermic
reaction) is considered in the heat balance of the
liquid phase (see Fig. 1).
YUXIANG AND XIEN [6] used this nonequilibri-
um stage model with simplified expression of the
reaction kinetics based on concentrations to sim-
ulate the synthesis of MTBE, which they inves-
tigated experimentally in a column filled with
catalyst granules in cloth pockets. The results of
their simulation agreed fairly well with the ex-
periments. However, SUNDMACHER AND HOFF-
MANN [7] investigated MTBE formation over a
broad concentration range and found that the
macrokinetics could be correctly described only
by using liquid-phase activities for the intrinsic
reaction kinetics and liquid – solid mass
transfer.
The equilibrium-based stage model is an ex-
cellent pragmatic approach to simulate reactive
distillation processes, especially those systems
with homogeneous reactions. This model is part
of all available process simulators (e.g., RAD-
FRAC in ASPEN-PLUS, Aspen Technology,
Inc.). For a detailed study of the superimposed
distillationandinteractionsbetweenreactionwith
mass transfer a rate-based approach is usually
preferred (e.g., RATEFRAC in ASPEN-PLUS,
Figure 1. Nonequilibrium based stage model without feed
and side streams
266 Reactive Distillation Vol. 31

5. Aspen Technology, Inc.). Due to its complexity
and the fact that only limited data are available
on mass-transfer coefficients, this approach
does not find a broad usage. Furthermore, small
deviations in these coefficients can have a se-
vere negative impact on the simulation results
[8]. (see Fig. 2)
3. Design of Reactive Distillation
Processes
3.1. Procedures for Process Design
Studies
Only a few systematic methods are available for
the process design of multifunctional units such
as reactive distillation columns. One method is
the concept of reactive distillation lines with
reactive azeotropes by BUZAD AND DOHERTY
[9], BESSLING ET AL. [10] AND STICHLMAIER et al.
[11]. A brief description of this method is given
below. More details can be found in [2].
Figure 3 shows the transformation of coordi-
nates for the equilibrium reaction A þ B
C.
This transformation reduces the equilibrium con-
centration of component C to a single point on the
line of the transformed coordinates A and B.
Mathematically one can define a chemical equi-
librium reaction by Equation (7):
naAþnbB . . .
npPþ . . . ; ð7Þ
where ni are the stoichiometric coefficients of the
reactants A, B, etc., and the desired products P,
etc. The transformed liquid and vapor coordi-
nates for an arbitrary component i, i.e., Xi and Yi,
are given by Equation (8) [12].
Xi ¼
xiÀ nixR
vR
1À nTxR
nR
ð8Þ
Yi ¼
yiÀ niyR
vR
1À nTyR
nR
; ð9Þ
where R is a chosen reference component. R can
be reactant or a product component. The stoi-
chiometric coefficient nT in these equations is
defined as:
nT ¼
XNC
i¼1
ni ð10Þ
Both the vapor and liquid molar fractions of
component P are set to zero, i.e., XP ¼ YP ¼ 0.
According to Equation (8), the transformed com-
position coordinates fulfill the unity constraints:
XNC
i¼1
Xi ¼
XNC
i¼1
Yi ¼ 1 ð11Þ
By using these transformed coordinates for the
feed components A and B, the mass balance and
operating lines of a distillation column can be set
up in the same way as for a system without a
reaction. It reveals that feasible top and bottom
compositions have to meet the lever rule (!
Distillation, 1. Fundamentals, Section 4.1)
through the feed concentration and an appropri-
ate distillation line, as shown in Figure 4. This is
the same procedure as for conventional nonreac-
tive systems.
Figure 2. Model complexity in simulation of reactive distil-
lation systems
Figure 3. Transformation of coordinates for a equilibrium
reaction A þ B
C
Vol. 31 Reactive Distillation 267

6. Figure 5 shows reactive distillation lines for
the equilibrium reaction A þ B
C in the
presence of an inert component D. Note that
product C is no longer necessary, because of the
transformed-concentration coordinates between
A and B (see Eq. 8). For a given feed concentra-
tion F, two process layouts can be derived. In
design type I, component C decomposes, and B
can be separated from A and D. With design
type II, component C is formed, and the inert
componentD isseparatedfromamixtureofAand
B, which is in chemical equilibrium with C. If the
equilibrium constant of the reaction A þ B
C
is increased, a reactive azeotrope appears along
line AB (Fig. 6). According to UNG and DOHERTY
[13] this reactive azeotrope is characterized by
identical vapor and liquid transformed-concen-
tration coordinates (Eq. 12).
Xi ¼ Yi ð12Þ
In this case the reactive azeotrope and the
inert component D are nodes; components A and
B are saddles. Therefore, in contrast to Figure 5,
it is not possible to realize design type I (decom-
position of C). Only design type II, i.e., the
formation of C and separation of D at the top
and A, B, and C at the bottom of the RD column,
is feasible. The concept of reactive distillation
lines with reactive azeotropes can also be ex-
tended to several other reaction types and to
nonideal reactive distillation systems as well.
In collaboration between the chemical indus-
try and different European universities (EU-re-
search project ‘‘Brite-Euram’’), two computa-
tional tools have been developed for the design
of RD processes, namely SYNTHESIZER and
DESIGNER. SYNTHESIZER is based on the
above-mentioned analogy between normal dis-
tillation processes and RD processes. It investi-
gates the feasibility of RD for an existing process
during re-engineering or in the planning stage for
a new process. DESIGNER gives a more precise
Figure 4. Application of the concept of reactive distillation
lines. As mass balance must be fulfilled, the feed concentra-
tion XF is located on a straight line between the transformed
concentrations XD and XB.
Figure 5. Reactive distillation lines for an equilibrium reaction A þ B
C in the presence of an inert component D (LB ¼
low-boiling, MB ¼ medium-boiling, HB ¼ high-boiling)
268 Reactive Distillation Vol. 31

7. insight into the RD process with respect to col-
umn parameters (e.g., column diameter, etc.). It
contains calculation procedures with equilibri-
um-based stage models as well as rate-based
models for a specific process design [14].
3.2. Flow sheet for Process
Development
Figure 7 shows a flow sheet as a conceptual basis
for the development of a CD process.
The key feature of this flow sheet is the
feasibility study followed by pilot plant experi-
ments (optional) and a basic engineering
concept. After using the above-mentioned tools
for process synthesis and design (see Sec-
tion 3.1), the parameters of a kinetic model must
be determined in an appropriate experimental
setup.
Simulation studies with a simple equilibri-
um-based stage model, such as RADFRAC
(ASPEN-PLUS), should reveal the optimal feed
point to the CD column, the operating condi-
tions of the CD column, and the position of the
reaction zone in the CD column. As a first
approximation, these studies can be carried out
by using equilibrium constants to incorporate
the chemical reaction into the column (less
complex model simulation, see Fig. 2.). For a
more detailed investigation, it is recommended
to use an appropriate activity-based reaction
expression with the determined kinetic para-
meters in a conventional simulator.
In pilot plant experiments, which are carried
out in a CD column with an inner diameter of
50 – 100 mm, the simulation studies are exam-
ined. As the process is iterative, these experi-
ments are needed to check and to improve the
simulations.
As the last step of the feasibility study, all
obtained results are fed into a final simulation to
decide whether an RD or conventional setup
(e.g., reaction section followed by separation
steps with internal recycle streams) should be
applied. Preferably, a rate-based model, such as
for example RATEFRAC (ASPEN-PLUS), is
used to check whether any mass-transfer limita-
tions interfere with the removal of reactants.
Such limitations can cause undesired consecu-
tive reactions in the reaction zone.
As it is reported by some authors [2] it should
be mentioned that predictive scale-up procedures
from lab-scale experiments (50 – 80 mm inner
diameter column) to pilot-plant scale (%300 mm
inner diameter column) already reveal some
evident deviation in conversion, e.g., for methyl
acetate synthesis or MTBE decomposition in a
CD column. The origin of this effect is up to now
not fully understood.
Figure 6. Reactive distillation lines for an equilibrium reaction A þ B
C in the presence of an inert component D with a
reactive azeotropic point (LB ¼ low-boiling, MB ¼ medium-boiling, HB ¼ high-boiling).
Vol. 31 Reactive Distillation 269

8. The possible reasons for the observed devia-
tions, discussed in [2], could be:
. reduced reaction rates due to incomplete cata-
lyst wetting,
. mass-transfer limitations, or
. maldistribution, etc.
Sensitivity studies of different CD column pa-
rameter can help to understand the order of
magnitude of the deviations on the overall con-
version, if scale-up calculations are made. These
studies can facilitate improvement of the reliabil-
ity of such simulations.
4. Industrial Applications
4.1. Commercial Packing Structures
All commercial CD packing structures have in
common that the catalyst is located in a distinct
Figure 7. Flow sheet for the process development of a CD technology (CSTR ¼ continuous stirred tank reactor)
(*: see Chapter 1)
270 Reactive Distillation Vol. 31

9. zone of a distillation column. Apart from the
problem of positioning these packing structures,
adequate contact between catalyst surface and
liquid phase must be ensured.
Nowadays, in most industrial processes
macroporous acidic ion-exchange resins are
used, but any other solid catalyst pellet can be
placed in the pockets of a structured packing. As
the entire catalyst structure must be removed
from the distillation column to exchange the
catalyst, one evident prerequisite for the catalyst
is a long lifetime (more than about three years).
Catalyst replacement means shutting down
whole plants which results in a significant loss
of costs and operating time.
CD Tech Catalyst Bales. This type of spa-
tial configuration in a distillation column was
first developed and commercialized by CR L
(Chemical Research Licensing Company) in
1980 [15]. It is now licensed by CD Tech (Chem-
ical Distillation Technologies), a joint venture
between CR L and ABB Lummus Crest. In
this technology the catalyst is supported in a
fiberglass cloth, which is wrapped with stainless
steel wire mesh. The demister wire mesh has two
tasks. First, it stabilizes the fiberglass packing;
second, it provides the void space necessary for
distillation. These fiberglass cloths are rolled into
catalyst bales and stacked on sieve trays in the
column (Fig. 8).
KataMax and KataPak Technology.
Analogous catalytic packing configurations were
developed in 1991/1992 independently by Koch
Engineering Company as KataMax technology
[16] and by Gebr€uder Sulzer [17] as KataPak
technology. Here the catalytic section in the CD
column has a conventional corrugated structure,
wire-cloth distillation packing which contains
appropriate catalyst pellets for the desired reac-
tion. The solid catalyst is held in an envelope of
meshed and crimped screen, sealed at the edges.
These envelopes are stacked together in modular
blocks, so that the crimp of one envelope has a 90
degree angle of inclination to the crimp of the
next. In this architecture, channels are formed for
the rising vapor flow and descending liquid flow.
At the intersections, they provide intensive mix-
ing of vapor and liquid phases and radial distri-
bution in a block. It is reported [18] that the
performance parameters of the KataMax packing
(i.e., the hydraulic and mass transfer efficiencies)
are similar to those of standard distillation de-
vices (e.g., the Flexipac packing). At liquid
loadings of 10 – 50 m3
mÀ2
hÀ1
catalyst contact
efficiency exceeded 75 – 90 %. Catalyst contact
efficiency is the ratio of the rate constant of the
catalyst in the packing to the rate constant of
the catalyst in a stirred tank reactor for a simple
liquid-phase first-order reaction at the same
temperature.
Usually the catalyst content of KataMax or
KataPak lies between 20 and 25 vol. %. For
KataMax a HETP value (height equivalent to a
theoretical plate) between 0.4 and 0.6 m can be
used for simulation. For KataPak the HETP is
1.0 m at normal F factors (! Distillation, 2.
Equipment, Section 2.2). The specific surface
area of KataMax is 210 m2
/m3
and of KataPak,
85 or 125 m2
/m3
(KataPak-S 170.Y or KataPak-
S 250.Y, respectively).
The benefits of this type of packing are a very
good distribution of liquid and vapor phases at a
low pressure drop, efficient contact of the reactants
with catalyst pellets and instantaneous distillative
removal of reactants. Therefore, it combines ex-
cellent separation performance with an efficient
mass and heat transfer for chemical reaction.
MultiPak Technology. This structured cat-
alyst packing was developed by the company
Montz in 1998 in cooperation with the University
of Dortmund [19]. In MultiPak technology theFigure 8. CD Tech catalyst bales
Vol. 31 Reactive Distillation 271

10. stacks are built up of alternating layers of
meshed, but not crimped, catalyst envelopes and
crimped structured distillation sheets. From the
viewpoint of reaction and fractionating behavior,
it has the same performance as KataMax or
KataPak. The advantage of MultiPak is the iden-
tical geometric structure in an 80-mm laboratory
packing and a 300- or 1000-mm pilot plant
packing. Therefore, scaleup from pilot plant
experiments to industrial plants is much easier.
It is claimed that a defined flow pattern of vapor
and liquid is achieved by this construction. This
has advantages when resins are used as catalyst.
As there is only liquid phase in the catalyst
envelopes, the pellets are completely wetted
under process conditions, and there is no swelling
or shrinkage due to alternating contact with vapor
and liquid. The disadvantage is a higher pressure
drop along the structured packing.
Multichannel Packing [20]. Details on this
type of catalyst packing were published in 2003
by BASF. The point of departure to develop these
packings was to improve trickle-bed reactor per-
formance by installing corrugated distillation
packings inside the reactor and pouring catalyst
particles into them. The catalyst particles are
brought loose into the packing cavities, distrib-
uted under the influence of gravity. By alternat-
ing packing layers of high and low specific
surface areas (so called ‘‘catalyst barrier layer’’)
the catalyst could be held in a specific area of the
CD column. The advantage of these packings is
their simplicity and the ease of catalyst removal
in case of deactivated catalyst. As the way of
introducing the catalyst into a specific column
segment is similar to a normal trickle-bed reac-
tor, the authors describe also a simpler modeling
in case of scale-up studies.
However, if the catalyst size distribution is not
100 % uniform, some particles may enter the
lower part of the CD column. Especially with
reversible reactions, which can only be per-
formed in a specific segment of a CD column
setup (e.g., MTBE synthesis), the undesired re-
verse reaction could occur in the bottom, if
catalyst particles are present there.
Reactive Rings. The reactive rings of
VEBA Oel [21] are based on packing bodies
such as Raschig rings or Berl saddles. On the
outer and inner surface of these bodies, macro-
porous ion exchange resins are bonded chemi-
cally or physically. For example, the active
Raschig rings are prepared by using an open-
celled sintered glass, which is impregnated with
a mixture of styrene, a long chain alkane, p-
divinylbenzene, and a radical starter (AIBN:
azobisisobutyronitrile). After the polymeriza-
tion procedure, the resin is sulfonated with
chlorosulfonic acid to build up the active cata-
lytic sites.
These types of reactive rings have been used
in an etherification process in a pilot plant col-
umn [7, 22].
Other Technologies. Another interesting,
but as yet not commercialized, technology for
immobilizing catalyst pellets in a distinct zone of
a CD column is to incorporate a fixed-bed in the
downcomer of a conventional tray column [23].
4.2. Industrial Catalytic Distillation
Processes
Catalytic distillation can be used in catalytic
chemical reactions which are limited by a chem-
ical equilibrium. There are various reactions that
satisfy this criterion, but only for etherification,
esterification, and alkylation (synthesis of ethyl-
benzene or cumene) is this technology applied on
an industrial scale. An overview of further pos-
sible applications can be found in [1, 2, 22].
Etherification Processes (see also !
Methyl Tert-Butyl Ether). The application of CD
technology to etherification is limited to synthe-
sis of methyl tert-butyl ether (MTBE), ethyl tert-
butyl ether (ETBE), tert-amyl methyl ether
(TAME), and tert-amyl ethyl ether (TAEE).
Because of their good octane enhancing prop-
erties and low volatilities these ethers are excel-
lent gasoline blending compounds (so-called
oxygenates). Reduction of the tetraethyllead con-
tent in gasoline in the mid-1970s led to a dramatic
increase in the demand for octane enhancers,
with MTBE being used increasingly. Driven by
the U.S. Reformulated Gasoline Program, which
mandated a minimum oxygen content in gasoline
of 2 %, world MTBE production has reached ca.
19 Â 106
t/a in 1997 [24]. The successful use of
CD technology in MTBE production led to its
worldwide establishment.
272 Reactive Distillation Vol. 31

11. It should be mentioned that end of 1997
California started to ban MTBE from the gaso-
line pool due to its effect as a water contaminant
and its possible cause of health problems (see
also ! Methyl Tert-Butyl Ether). In mid-2006
the U.S. Environmental Protection Agency ter-
minated the legal requirement of using MTBE in
the gasoline pool [25]. The production capacities
of MTBE are therefore shrinking day-by-day.
Table 1 summarizes processes for ether pro-
duction based onCD technology. Asshown in the
simplified process flow diagram for the ETHER-
MAX process (Fig. 9), all processes have in
common a reactor section followed by a CD
column. There is no industrial process in which
the whole conversion is performed in a single CD
column. A partially converted mixture from the
reactor section, which is nearly in chemical
equilibrium, enters the CD column below the
catalyst packing zone to ensure the separation of
the ether from the feed stream. The catalyst
packing is installed in the upper middle of the
column, with normal distillation sections above
and below.
Usually, the reaction of the isoalkene with
methanol or ethanol is conducted in the presence
of a slight stoichiometric excess of the alcohol. In
addition to the advantages of a higher selectivity
for the ether (lower formation of C8 and C10
dimers) and shifting of the chemical equilibrium
to the product side, this leads to a secure process
control. In the absence of the alcohol in the
reaction zone of the CD column exothermic
dimerization and oligomerization of the C4 and
C5 alkenes takes place at high reaction rates. A
sharp and excessive temperature rise (hot spot)
Table 1. Overview of CD processes for ether production [27]
Process (licensor) Process description Catalyst
Market
share*
ETHERMAX (UOP) fixed-bed tubular reactor or adiabatic recycle reactor
followed by a reactive distillation column with KataMax Packing
sulfonated
ion-exchange resin
ca. 32 %
CDMTBE CDTAME
(CD-Tech)
adiabatic reactor operating at the boiling point
followed by a reactive distillation column with CD Tech Bales
acidic
ion-exchange resin
ca. 58 %
CDETHEROL
(CD-Tech)
adiabatic fixed-bed reactor operating at the boiling
point followed by a reactive distillation column with CD Tech Bales
trifunctional
ion-exchange resin
–
CATACOL (IFP) recycle reactor, fixed-bed reactor followed by a
reactive distillation column
acidic ion-exchange resin ca. 10 %
*
For MTBE, produced by CD technology in 1997
Figure 9. Simplified flow diagram for the ETHERMAX process
Vol. 31 Reactive Distillation 273

12. causes irreversible catalyst deactivation and cat-
alyst damage. The excess alcohol is collected in
the overhead of the CD column and separated
from the hydrocarbon stream by extraction with
water.
In the case of MTBE production, a conversion
of up to 99.9 % is possible by using this technol-
ogy, if sufficient ether is present at the bottom of
the column. Because of a minimum boiling azeo-
trope in the binary MTBE – methanol system,
the ether carries the alcohol into the reaction zone
of the CD column. This unusual vapor – liquid
behavior and the high difference in the heat of
vaporization between C4 hydrocarbons and
methanol are possibly responsible for the known
multiple steady states in CD column operation
[22, 26].
The TAME process usually gives a lower
conversion of 91 – 95 % only.
Esterification Processes. Esterification is a
good example for the beneficial use of the CD
technology. In conventional methyl acetate pro-
duction the recovery of methyl acetate from the
reactor outlet stream is complicated, because
methanol forms a low boiling azeotrope with
water and the separation of water from uncon-
verted acetic acid is difficult. Due to these sepa-
ration problems, the old Eastman Kodak process
used a reactor coupled with eight distillation
columns and an extraction column [28].
In the reactive distillation process, almost
pure methyl acetate can be collected in the
overhead of a single CD column at acetic acid
conversions of greater than 99 %. The Eastman
Kodak process uses sulfuric acid as catalyst [29],
but a heterogeneous catalyst system (acid ion-
exchange resin) can also be used successfully
[30].
The conceptual basis for the successful im-
plementation of reactive distillation in methyl
acetate synthesis is shown in Figure 10. There
are four zones in the column, which ensure the
fairly high conversion. Acetic acid is separated
from methyl acetate at the top of the column
(zone I). In zone II, an extractive distillation
section below the acetic acid feed extracts water
from methyl acetate. The reaction takes place in
the middle of the column (zone III). At the
bottom (zone IV) methanol is fed and stripped
from descending byproduct water.
Alkylation Processes. CD-technology is al-
so applied for alkylation reactions in the case of
ethylbenzene and cumene manufacture [1]. As
the two processes are fairly similar, only the
cumene process is discussed here.
In the CDCUMENE process, CD Tech cata-
lyst bales are filled with an acidic, wide-pore
zeolite catalyst, for example, zeolite Y, b or w
[31]. The bales are stacked in the middle of a
distillation column. Below the reaction zone
propene is fed; benzene is passed as reflux to
the top of the column. The propene concentration
in the reaction zone is held fairly low to slow
down a side reaction in which diisopropylben-
zene (DIPB) and triisopropylbenzene (TIPB) are
formed. The cumene product with impurities of
Figure 10. Methyl acetate synthesis with CD technology
274 Reactive Distillation Vol. 31

13. ethylbenzene and n-propylbenzene, is taken from
the bottom of the column. The overhead pressure
of the CD column is maintained at 5 bar.
The advantages of CD technology in cumene
synthesis are lower formation of oligo-isopro-
pylbenzenes (DIPB and TIPB), higher catalyst
lifetime, and a higher conversion of benzene
[32]. The internal benzene recycle to the reaction
zone, which is a result of the distillative cu-
mene – benzene separation in the lower portion
of the column, ensures the observed higher
conversion.
4.3. Novel Application of CD with
regard to Process Intensification
Distillation columns with dividing walls are used
with success in some chemical processes. An
EU-research project named INSERT has been
started to investigate if CD technology can also
be applied advantageously to such distillation
columns with divided wall internals [33].
The successful use of CD technology in a
divided wall column is reported in [34] for the
transesterification of methyl acetate to butyl
acetate (Fig. 11.). This process can be performed
in one column of this type instead of using one
CD column followed by two normal distillation
columns.
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Figure 11. Transesterification of methyl acetate to butyl
acetate with CD technology in a divided wall column
Vol. 31 Reactive Distillation 275

14. Further Reading
C. A. M. Afonso: Green Separation Processes, Wiley-VCH,
Weinheim 2005.
Z. Lei B. Chen Z. Ding: Special Distillation Processes, 1st
ed., Elsevier, Amsterdam 2005.
W. L. Luyben, C.-C. Yu: Reactive Distillation Design and
Control, Wiley, Hoboken, NJ 2008.
K. Sundmacher: Reactive Distillation, Wiley-VCH, Wein-
heim 2003.
276 Reactive Distillation Vol. 31