Chemical Engineering Science: Arnab Atta, Shantanu Roy, Faïçal Larachi, Krishna Deo Prasad Nigam
Chemical Engineering Science: Arnab Atta, Shantanu Roy, Faïçal Larachi, Krishna Deo Prasad Nigam
Chemical Engineering Science: Arnab Atta, Shantanu Roy, Faïçal Larachi, Krishna Deo Prasad Nigam
Arnab Atta
a,b
, Shantanu Roy
a
, Faal Larachi
a,c
, Krishna Deo Prasad Nigam
a,n
a
Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016, India
b
Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
c
Department of Chemical Engineering, Universit Laval, Qubec, Canada G1V 0A6
H I G H L I G H T S
Scopes for future research are outlined considering process intensication of TBR.
a r t i c l e i n f o
Article history:
Received 16 April 2013
Received in revised form
16 July 2013
Accepted 17 August 2013
Keywords:
Trickle bed reactor
Cyclic operation
Onoff
Minmax
Process intensication
Shock wave attenuation
a b s t r a c t
Familiarized with the steady state behavior and advantages of the trickle bed reactors (TBR), for the past
two decades, researchers and process engineers are continuously exploring the unsteady state hydro-
dynamics of periodically (or cyclic) operated trickle bed reactors to extract even more from its efciency
and performance. Despite its complicated nonlinear behavior that attributes to the process control safety
at the commercial scale, cyclic operation of TBR is a promising process intensication technology that has
immense potential for implementation on industrial TBR. In this paper, we have summarized and
reviewed the research and progresses made in recent years on cyclic operation of TBR and potential
applications of various mode of its operation. Several issues associated with its hydrodynamics scale-up
and designs have been discussed with an emphasis on shock wave attenuation that arises during ow
modulation.
& 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Trickle bed reactor (TBR) is one of the classical multiphase packed
bed reactor congurations that nds extensive application in petro-
leum industries for the purpose of hydrocracking, hydrotreating, and
alkylation (Saroha and Nigam, 1996). TBRs are also commercially
utilized in the petrochemical, and chemical industries for hydrogena-
tion of aldehydes, reactive amination, liquid-phase oxidation, etc.
Recognizing several benets of using TBR, including simplicity in
construction and large production volumes, there is signicant
amount of incentive and motivation involved for further improvement
of its performance from the economic as well as environmental
perspective (Nigam and Larachi, 2005; Charpentier, 2007). Optimiza-
tion of the performance can be accomplished by (a) evolving
process intensifying novel operational methods/techniques, and/or
(b) designing efcient equipment/reactor internals that are potentially
compact, safe, energy-efcient, as well as eco-friendly sustainable
(Stankiewicz and Moulijn, 2000). Cyclic (or periodic/unsteady state)
operation of TBR belongs to the rst category and has gained
considerable attention due to its substantial effect on improved
reaction rate and reactor operational life.
Most of the reactions, encountered in chemical process indus-
tries that are carried out in TBRs, can be broadly categorized as
either limited by liquid phase or by gas phase reactants. In any
case, gaseous reactants must dissolve into and then pass through
the liquid phase to reach the catalyst surface. For sparingly soluble
gaseous reactants, this transport resistance is one of the vital
parameters in inuencing the rector performance. During cyclic
mode of operation, a continuous ow of certain uid phase
contacts other uid stream that is forced to toggle periodically at
the reactor inlet between a low-level (base) and a high-level
(pulse) ow rate. Considerable decrease in transport resistance,
that is desirable for the gas phase limited reactions, can be
achieved by temporal variation of catalyst wetting due to such
liquid feed cycling operation (Boelhouwer et al., 2002b). For liquid
phase limited reactions, the operation of TBR in natural pulsing
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n
Corresponding author. Tel./fax: 91 11 2659 1020.
E-mail addresses: drkdpn@gmail.com, nigamkdp@gmail.com (K.D.P. Nigam).
Please cite this article as: Atta, A., et al., Cyclic operation of trickle bed reactors: A review. Chem. Eng. Sci. (2013), http://dx.doi.org/
10.1016/j.ces.2013.08.038i
Chemical Engineering Science ()
ow regime is most appropriate since complete catalyst wetting
and high solidliquid mass transfer can be achieved; however,
relatively higher supercial liquid/gas velocities (than trickle ow
regime) are associated with higher energy expenses and lesser
conversion due to shorter contact time between phases. As a
trade-off, forced cycling of gas phase feed can be a viable solution
(Xiao et al., 2001).
Despite potential advantages, cyclic operation in TBR at the
commercial scale is seldom exercised, mainly due to its compli-
cated nonlinear hydrodynamic behavior that endangers the pro-
cess control safety. However, advances in meticulous research
including rigorous experiments and comprehensive modeling are
capable of attaining further reliable predictions. Haure et al. (1989)
pioneered the concept of periodic ushing of the bed for the
removal of SO
2
in a TBR, packed with activated carbon. This
process intensication technique eventually resulted in increasing
oxidation rates up to 45%. Since then, several research groups have
mentioned the performance enhancement by practicing different
reactions with various modulation schemes (Silveston and Hanika,
2002; Tuka et al., 2007; Liu et al., 2008). The hydrogenation of
-methyl styrene (AMS) studied by researchers (Lange et al., 1994;
Castellari and Haure, 1995; Gabarain et al., 1997; Lange et al., 1999;
Banchero et al., 2004) has reported an increase in reaction rates
even up to 400% under appropriate condition. An increase in pilot
reactor productivity of styrene hydrogenation by 30% for periodic
operation in comparison with steady state operation has been
reported (Tuka et al., 2007) that can be considered as signicant
achievement for potential industrial applications of cyclic opera-
tion. A summary of few contemporary research works on periodic
operation of TBR is listed in Table 1.
Observing the plausible wide spread use and popularity of cyclic
operation as process intensication technique, this effort is to
summarize the characteristic studies contributed in understanding
this promising area and its present research status. There is a review
article available on periodic operation of all three-phase catalytic
reactors by Silveston and Hanika (2004), and therefore, this study has
been mainly focused on the efforts made in case of only TBR with a
special importance on the hydrodynamic features and applications
thereafter.
2. Flow modulation strategy
During liquid feed cyclic operation, a continuous ow of gas
phase interacts with a liquid stream which is enforced to toggle
periodically at the reactor inlet between a low-level (base) and a
high-level (pulse) liquid ow rate. Similarly, for gas feed cyclic
operation, the liquid phase feed is kept constant while the gas ow
rate is switched. In general, this mode of operation is called as
minmax or basepulse mode. However, when the base or mini-
mum ow rate value is set to zero then it is especially assigned as
onoff mode.
It has been observed that in almost all studies published on
periodically fed TBRs, the specic liquid feed rate is varied in a
square waved pattern and the feed cycle is characterized by four
feed parameters (Fig. 1). The low or base specic liquid feed rate,
u
Lb
, is applied for a duration t
b
, followed by a high or pulse specic
liquid feed rate, u
Lp
, applied for a duration t
p
. There are three key
parameters that are derived as: (i) the feed cycle period (p), (ii) the
split (S), and (iii) the average specic liquid feed rate (u
Lm
) which
can be dened by
p t
b
t
p
1a
S
t
p
t
b
t
p
t
p
p
1b
u
Lm
u
Lb
t
b
u
Lp
t
p
t
b
t
p
1c
It is interesting to note from most of the studies that the ow
regimes during both high and low liquid ow rates are in trickle-
ow regime. Based on the time period applied for the pulse/max
mode ow rate, there also exists slow mode operation, where the
Table 1
Summary of some recent studies on cyclic TBR.
Researchers Modulation
strategy
System studied Reactor dia. and packings Observations
Lange et al. (2004) Onoff, slow
mode
Hydrogenation of
-methyl styrene
ID: 0.02 m, 0.7% Pd=-Al
2
O
3
Reactor performance was signicantly improved by feed
liquid ow modulation
Massa et al.
(2005)
Onoff, slow
mode
Oxidation of phenols ID: 0.021 m, cylindrical
pellets of 2.6 mm size CuO/
Al
2
O
3
catalyst
Mild effect on phenol conversion due to liquid ow modulation was detected
but the ow modulation did not affect product distribution, in the range
of operating conditions studied
Fraguio et al.
(2004);
Muzen et al.
(2005)
Onoff, slow
mode
Catalytic oxidation of
ethyl and benzyl
alcohols
ID: 0.04 m, 3 mm spherical
1% Pd=-Al
2
O
3
Conversion was improved for different combinations of cycle period and
split. Depending on liquid reactant and operating variables, both positive
and detrimental effects of the liquid ow modulation were observed
Liu et al. (2005);
Liu and Mi
(2005)
Onoff, slow
and fast mode
Hydrogenation of
2-ethylanthraquinone
ID: 0.021 m, 1.9 mm spherical
0.5% Pd=-Al
2
O
3
Improvement in conversion and selectivity up to 21% and 12%, respectively,
was found. A transient model involving axial dispersion, partial wetting, and
drainage behaviour was developed
Liu et al. (2008) Onoff, min
max, hybrid
mode
Hydrogenation of
dicyclopentadiene
ID: 0.024 m, 1.9 mm 0.3%
Pd=-Al
2
O
3
egg-shell catalyst
A novel operation strategy of trickle bed reactor, hybrid modulation of liquid
ow rate and concentration was proposed. The performance enhancement
under the hybrid modulation was higher (15%) than the minmax
modulation of single parameter
Schubert et al.
(2010)
Onoff, slow
mode
Hydrodynamics of
airwater ow
ID: 0.10 m, 3.24 mm
spherical -alumina particles
Time-averaged liquid saturation decreased in slow mode operation,
particularly at small splits and long period length. Unlike previous studies,
improvement of the liquid distribution was not detected
Ayude et al.
(2012)
Onoff, slow
mode
Catalytic oxidation
of ethanol
ID: 0.0254 m, 2.7 mm 0.5%
Pd=-Al
2
O
3
egg-shell catalyst
Product distribution was signicantly adjusted by frequency tuning. High
frequency liquid ow modulation increased selectivity towards the
intermediate product, however, low frequency led to catalyst deactivation
Wongkia et al.
(2013)
Onoff, slow
and fast mode
Hydrogenation of
styrene
ID: 0.0191 m, 2 mm spherical
0.3 wt% Pd=-Al
2
O
3
Fast mode was found favourable. A maximum improvement of styrene
conversion of 18% was observed
A. Atta et al. / Chemical Engineering Science () 2
Please cite this article as: Atta, A., et al., Cyclic operation of trickle bed reactors: A review. Chem. Eng. Sci. (2013), http://dx.doi.org/
10.1016/j.ces.2013.08.038i
phase feed changes periodically over few-minutes time spans, and
fast mode operation, where the feed pulse incursion lasts for a
few-seconds (Aydin et al., 2006; Hamidipour et al., 2007).
Continuity shock waves may appear as a result of uid
accumulation in a ow channel depending on its ow rate
variation (Wallis, 1969). Boelhouwer et al. (2002a) reported that
liquid feed ow rate modulations result in formation of liquid-rich
continuity shock waves that propagate down the bed. It is
apparent from the experimental analysis (Boelhouwer et al.,
2002a) that these shock waves also diminish by leaving liquid
behind their tail while moving down the reactor. At sufciently
high gas ow rates, inception of pulses may occur within the
liquid-rich shock waves that can be referred as liquid-induced
pulsing ow. The frequency of the pulses during fast mode of
liquid-induced pulsing ow is usually less than 1 Hz. Giakoumakis
et al. (2005) redened a clear demarcation between various mode
of induced pulsing. Based on the measure of the existence of at least
one pulse in the packed bed, at any time, Giakoumakis et al. (2005)
mentioned that when the characteristic time period of a pulse is
bigger than the ratio of reactor height and pulse rapidity, it will be
referred as slow mode operation. Giakoumakis et al. (2005) also
mentioned that at substantially smaller frequencies compared to
limiting case, the bed can operate at a liquid-rich and a gas-rich
state, interchangeably. According to their study, apart from the
temporal and spatial holdup variations, a typical feature of slow
mode operation is noticeable pressure drop uctuations, whereas
in case of fast mode operation, the pressure drop over the bed
remains almost unchanged (Giakoumakis et al., 2005).
2.1. Onoff mode
There are numerous studies on the onoff mode operation to
unveil the transient behavior of cyclic TBR. Giakoumakis et al.
(2005) experimentally characterized several parameters of
induced pulses namely, its evolution, holdup, pressure drop, pulse
intensity and celerity for fast mode cycling operation in a cylind-
rical bed of 0.14 m inner diameter. With the help of conductivity
probes for measuring liquid holdup, they analyzed axial propaga-
tion and attenuation of the induced liquid pulses. In line with the
study of Boelhouwer (2001), their study also recognizes the
varying shape of induced pulses and its decay down the length
of the bed (Fig. 2).
In order to distinguish between slow and fast mode of operations,
Giakoumakis et al. (2005) highlighted that considerably larger
amount of liquid holdup than the static holdup of the packed bed
was found during fast mode. This phenomenon was ascribed to the
greater characteristic time of drainage of the bed compared to the
pulse periodicity at the inlet that results into liquid accumulation in
the bed in between two consecutive pulses. Such fast mode operation
was further examined by Trivizadakis et al. (2006) in an identical
experimental setup to study the particle shape and size effects on
pulse characteristics (Fig. 3). Considering widely used catalyst parti-
cles in TBR applications, Trivizadakis et al. (2006) selected two
different types of porous alumina particles (3 mm spheres and
1.5 mm dia. cylindrical extrudates) along with 6 mm glass beads for
the comparative study.
From Figs. 2 and 3, it can be observed that liquid holdup traces
displayed similar trends for same kind of particle shape (e.g.,
6 mm and 3 mm glass spheres). In the case of cylindrical extru-
dates, the liquid holdup traces completely faded even before
reaching the bottom of the bed. For both spherical and extrudate
particles, Trivizadakis et al. (2006) also indicated that during fast
mode of onoff liquid pulsing, total liquid holdup (i.e. sum of
dynamic and static holdup) was always higher than the static
liquid holdup. This, in turn, implies that chances of liquid dry-out
phenomena can be minimized by fast mode of operation as there
will be always ample liquid in the bed at all the time of pulses.
Time
L
i
q
u
i
d
f
e
e
d
r
a
t
e
t
p
t
b
u
Lp
u
Lb
Fig. 1. Schematic of the cyclic liquid feed. u
Lb
base liquid feed; u
Lp
pulse liquid
feed; t
b
duration of base liquid feed; t
p
duration of pulse liquid feed.
Fig. 2. Typical simultaneous liquid holdup traces (gure reprinted from Giakoumakis et al., 2005 with permission from the publisher, Elsevier). The packing material is glass
spheres of 6 mm diameter and the ow rates are G0.22 kg m
2
s
1
, L3.34 kg m
2
s
1
. Cycle frequency0.167 Hz (3 s on3 s off).
A. Atta et al. / Chemical Engineering Science () 3
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10.1016/j.ces.2013.08.038i
Trivizadakis et al. (2006) further claimed that the trend of liquid
holdup traces is similar for several operating conditions including,
liquid cyclic feed frequency and mean uid ow rates. However,
due to smaller pressure drop and rate of pulse attenuation,
spherical packings can hold signicant advantages over cylindrical
extrudates of comparable size.
To recognize the effect of porous packings on pulse attenuation due
to liquid ow modulation, Ayude et al. (2007b) extensively studied the
temporal variations of the liquid holdup at different axial positions in a
mini-pilot scale TBR (of inner diameter 0.07 m), packed with porous
beads of -Al
2
O
3
. Using a conductometric technique, Ayude et al.
(2007b) analyzed liquid holdup modulation for different supercial
velocities, bed depth and cycling parameters. For a square wave input
of liquid feed, slightly deformed structure of the wave was found
down the bed length during slowmode operation. These structures, as
it moved along the column length, had different trends of decay that
occurred during the dry period of the ow modulation (Ayude et al.,
2007b). Furthermore, a decrease in the width of plateau was also
realized during the wet period of the ow modulation. In case of fast
mode onoff operation, the structure of input pulse deteriorates
severely and eventually becomes at at the lower part of the bed
representing a pseudo steady state (Ayude et al., 2007b). Moreover, it
was realized that the mean liquid velocity determined the pulse
intensity in the bed. In case of lowmean liquid velocity, pulse intensity
can only be realized near the feed inlet and diminishes to zero down
the length of the bed. However, in case of a higher mean velocity,
there is a continuous decrease in pulse intensity along the bed length
and can be appreciated even at the bottomof the column (Ayude et al.,
2007b).
Banchero et al. (2004) experimented the efcacy of onoff fast
mode liquid modulation in a TBR, of 0.04 m inner diameter, packed
with Pd/C catalyst for -methyl styrene hydrogenation (AMS). This
study disclosed that cyclic operation can improve the conversion
rate up to 60% compared to the steady state operation for any
particular operating condition. Because the experiments were
executed in isothermal conditions, this conversion rate improve-
ment clearly advocates the utility of periodic operation that led to
increased mass transfer.
Ayude et al. (2008) discussed the effect of onoff liquid ow
modulation on the oxidation of ethanol in a lab-scale TBR having
internal diameter of 0.0254 m. For a varied set of uid ow rates,
cycle periods and splits, experiments were carried out to compare
the performance with the steady-state experiments. Fig. 4 shows
substantial improvements ( 30%) in catalyst activity with larger
split for any xed period of modulation. This study also revealed
that enhancement in reaction rate was favored during relatively
fast mode of liquid feed cycling (i.e. shorter off period).
It has been observed that experiments to unravel the hydro-
dynamics characteristics were scarcely reported in case of gas feed
modulation. In fact, there are very few literature available dealing
with gas feed modulation. One of the main reasons behind this can
ascribe to the fact that compared to the liquid phase, the inertia
effect due to gas phase modulation is considerably small which
does not help in damping of the oscillation (Banchero et al., 2004).
Nonetheless, with a different feed modulation strategy than the
conventional type operation, Larruy et al. (2007) tested gas feed
modulation instead of liquid ow modulation (Tuka et al., 2003;
Massa et al., 2005) in catalytic wet air oxidation (CWAO) of phenol.
They studied the possibilities of reducing active carbon burn-off
using gas ow and composition modulation in a small TBR of
0.011 m internal diameter. It was demonstrated that modulation of
gas feed composition was essentially advantageous for CWAO of
phenol due to considerable reduction in active carbon burn-off
compared to steady state operation. Larruy et al. (2007) further
showed that gas feed modulation with high splits was very
attractive for preserving catalyst activity in long term.
Later, Ayude et al. (2007a) added to this observation with
extended experimental study in a similar reactor setup to analyze
the impact of gas feed composition and ow rate modulation on
the short time activity of the catalyst through 50 h of operation.
Fig. 5 depicts the signicant result of this study. It shows that
nearly same conversion rates of phenol and total organic carbon
(TOC) can be achieved by either gas feed composition or ow rate
modulation. However, the temperature proles are different in
both cases (Ayude et al., 2007a). From the perspective of
Fig. 3. Typical, simultaneously recorded, liquid holdup traces along the packed bed
(gure reprinted from Trivizadakis et al., 2006 with permission from the publisher,
Elsevier). The ow rates are G0.12 kg m
2
s
1
, L3.34 kg m
2
s
1
. Cycle fre-
quency0.167 Hz (3 s on3 s off). (a) 3 mm spheres, (b) 1.5 mm dia. extrudates.
Fig. 4. Enhancement as a function of split for different periods (9, 6 and 3 min),
(gure reprinted from Ayude et al., 2008 with permission from the publisher,
Elsevier). V
L;ss
70 mL min
1
; V
G
200 mL min
1
.
A. Atta et al. / Chemical Engineering Science () 4
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10.1016/j.ces.2013.08.038i
commercial application, such results are extremely appealing
because consumption of nitrogen can be minimized which in turn
also reduces operating costs.
All these studies proved the effectiveness of periodic gas phase
modulation in CWAO of phenol, compared to steady state oxida-
tion with proper selection of cycling parameters, i.e. split and
period. Higher splits and cycle periods were found to be highly
benecial to the catalyst activity in CWAO of phenol. Ayude et al.
(2007a) disclosed that mean conversion rate in phenol CWAO can
signicantly be enhanced (from 35% to 60%) by gas feed cycling
with suitable split and period of modulation.
Researchers argue that periodic liquid feed modulation can
minimize liquid maldistribution in the TBR which is an inherent
advantage of the cyclic operation of TBR. With a state-of-art
noninvasive multiphase ow visualization technique, Electrical
Capacitance Tomography (ECT), Liu et al. (2009) demonstrated
this fact while analyzing the transient behaviors of the liquid
holdup of airkerosene system in a periodically operated TBR, of
0.14 m internal diameter, packed with spherical glass particles.
They captured instantaneous ECT images at several axial positions
of a TBR modulated with the slow mode onoff operation.
Furthermore, the transient radial liquid distributions and a mal-
distribution factor were computed from those ECT images to
estimate the liquid distribution under periodic modulation. Their
study revealed that liquid maldistribution generally occurred
during the draining period and more appreciable at the top section
of the bed. This study manifests further investigations using
noninvasive process tomography in cyclic TBR considering the
limitations of intrusive measurement techniques such as conduc-
tometric technique for determining local liquid-holdup and radial
distributions that are prevalent for industrial applications.
2.2. Minmax mode
There are relatively limited amount of literature available on
the peakbase or minmax mode of operation than onoff mode
of operation. Urseanu et al. (2004) examined the use of periodic
liquid feed modulation in a high pressure TBR which is very
relevant from the standpoint of industrial application of cyclic TBR.
They studied the effect of minmax periodic operation on the
reaction rate in hydrogenation of -methyl styrene (AMS) to
cumene in a TBR of 0.051 m diameter. Their results showed an
increase of at least 50% in reaction rate for the case of periodic
operation compared to steady state operation. Fig. 6 shows the
temperature prole of the liquid with the corresponding situation
of liquid feed velocity. It can be observed from Fig. 6 that during
the ushing of the bed with liquid, the temperature of liquid
increased as it effectively removed the heat from the catalyst.
In an attempt to compare and quantify the liquid maldistribu-
tion during onoff and minmax operation, Borremans et al.
(2004) came up with the remarks that for the very limited number
of operating conditions, periodic operation was not able to
improve the liquid distribution compared to steady state case in
a TBR with 0.3 m diameter. In these operating conditions, the
maldistribution factors had their minimum values in steady state
operation, signifying best possible distribution that could have
been achieved. They argued that for periodic operation, better
distribution can be achieved in cases of lower mean liquid ow
rates and by setting base liquid ow velocity to zero i.e. by onoff
operation. Borremans et al. (2004) also indicated that cyclic
modulation of liquid feed can ensure better liquid distribution in
the conditions that are close of pulsing regime under steady state
operation.
Considering the advantage of shock wave formation during ow
rate modulation, Hamidipour et al. (2007) applied minmax opera-
tion to reduce nes deposition that helped to prolong reactor
operational life under ltration conditions of a TBR. In order to
examine the efciency of the shock waves in delaying solids
re-deposition, Hamidipour et al. (2007) studied several modes of
operation, namely, slow-, fast- and semi-fast liquid cyclic operation,
fast-mode gas cyclic operation, alternating gas/liquid cyclic opera-
tion including the effects of cycle time, split ratio and bed height in
Fig. 5. Temperature, phenol and total organic carbon (TOC) conversion proles with time for different gas phase modulations. Filled and empty symbols represent phenol
and TOC conversion, respectively. (Figure reprinted from Ayude et al., 2007a with permission from the publisher, Elsevier.)
Fig. 6. Adiabatic temperature rise of the liquid phase and supercial liquid velocity
during periodic operation (period480 s; split0.25). (Figure reprinted from
Urseanu et al., 2004 with permission from the publisher, Elsevier.)
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a TBR of 0.057 m internal diameter. They concluded that slow- and
fast-mode liquid cyclic operation strategies were not capable of
minimizing deposition and pressure drop. However, a new scheme,
semi-fast mode liquid cyclic operation, dened as minutes-lasting
pulses and seconds-lasting base velocities, was benecial in reducing
bed deposition. Additionally, alternating gas/liquid cyclic operation
was also found to be efcient in reducing deposition.
In an interesting and industry relevant study, contrary to the
previous investigations focusing on liquid induced pulsing ows
that were performed only at atmospheric pressure and ambient
temperature, Aydin et al. (2006) illustrated the effects of tempera-
ture and pressure on the shock wave characteristics of both
Newtonian and non-Newtonian liquids in slow mode induced
pulsing of minmax operation. In a 0.048 m diameter TBR, they
showed reduced decay process in shock waves for increasing
temperature and pressure (Figs. 7 and 8). Aydin et al. (2006) also
demonstrated that the shock wave breakthrough and decay times
decreased with increasing temperature and pressure. Liquid
holdup was also found to decrease with temperature, especially
at the high liquid feed rates. This phenomenon may lead to
reduced reactor performance during liquid induced pulsing ow
at high temperature and pressure operations.
Most recently, Atta et al. (2010) paid attention on the propaga-
tion of a solitary square-wave liquid feed to visualize its behavior,
effect and attenuation in the bed during the minmax operation.
For analyzing the inherent ow dynamics during wave propaga-
tion in a cyclic TBR, experiments for multiphase ow visualization
and local variable measurements using ECT were carried out in
0.057 m diameter TBR. To unveil the behavior and effect of solitary
liquid-rich square-wave on the continuous mode of minmax
periodic operation, two different types of ow modulation had
been examined (slow mode of 60 s and fast mode of 10 s) for
different gas velocities. The effect of gas velocity on the shape of
solitary wave is shown in Figs. 9 and 10 for fast and slow modes,
respectively. The deformation in shape of solitary liquid rich wave
is apparent from these gures. Figs. 9 and 10 also depict the
propagation characteristics of the wave down the column length.
In both the cases, higher gas velocity restricted the upper limit of
liquid holdup and tries to atten the square shape of the wave.
It was evident from all the experiments that the solitary liquid
wave always restricted within the steady state holdups range of
base and pulse velocities in the bed. It had also been observed that
in case of slow mode minmax operation, the liquid rich wave
plateau upheld the steady state liquid holdup value (correspond-
ing to pulse velocity) for sufciently long period even at the
bottom of the column which in turn helped to envisage the
situation as a pseudo-steady state TBR operation. However, in case
of fast mode operation, this plateau just touched the upper limit of
liquid holdup at the bottom of reactor and then starts to decay
within very short duration. The duration of decaying period was
longer in cases of fast mode operation which got prolonged by the
increase in gas supercial velocity. It was also noticed that the rise
of the solitary wave got delayed with the reduction of the gas
supercial velocity. Therefore, in this scenario, it can be antici-
pated that if continuously short (in duration) pulse or liquid rich
wave comes in the bed then there may be chances of pulsing
inside the bed (even at trickle ow regime) which can be treated
as pseudo-natural pulsing during fast mode minmax cyclic
operation. Henceforth, that study suggested that slow mode
operation tried to keep the shock-wave identity intact thus is
benecial for the cases of exothermic gas phase limited reactions
where partial catalyst wetting is more desirable and simulta-
neously hot spot formation occurs. In case of fast mode operation,
0.20
0.15
0.05
0.10
T = 25C
L
0.00
Time (s)
T = 50C
T = 75C
0 50 100 150
Fig. 7. Effect of temperature on shock wave patterns measured 40 cm from bed top,
P0.3 MPa, u
G
0.2 m s
1
. Air-water system, u
Lb
0.0035 ms
1
, u
Lp
0.0105 m s
1
(Aydin et al., 2006).
0.16
0.08
0.12
0.04
L
L
0
Time (s)
P = 0.3MPa
P = 0.7MPa
0 50 100 150
Fig. 8. Effect of pressure on shock wave patterns measured 40 cm from bed top,
T75 1C, u
G
0.2 m s
1
. Airwater system, u
Lb
0.0035 m s
1
, u
Lp
0.0105 m s
1
(Aydin et al., 2006).
Fig. 9. Experimental visualization of solitary liquid wave propagation with time
along the bed length for (a) u
G
0.062 m s
1
and (b) u
G
0.25 m s
1
during fast
mode (10 s) cyclic operation (Atta et al., 2010).
A. Atta et al. / Chemical Engineering Science () 6
Please cite this article as: Atta, A., et al., Cyclic operation of trickle bed reactors: A review. Chem. Eng. Sci. (2013), http://dx.doi.org/
10.1016/j.ces.2013.08.038i
with shorter pulse ow rate time period and higher liquid ow
rate, pseudo-natural pulsing situation can be achieved at the
bottom of the reactor due to its longer decay time.
3. Modeling and simulation
Despite several advantages, commercial cyclic TBR is far from its
application mainly due to process control safety related with the scale
up issues. Highly non-linear hydrodynamics coupled with poor
understanding of the transport processes and reaction kinetics makes
the situation even more involved for unsteady operation of TBR.
In order to demystify the complex behavior, several modeling studies
have been reported in the literature, dealing with both transient and
pseudo-transient models (Lange et al., 1999; Khadilkar et al., 2005;
Dietrich et al., 2005; Ayude et al., 2005a, 2005b; Liu et al., 2008;
Ayude et al., 2009; Brzi et al., 2010). All models basically stem from
the variation of steady state approach and have corresponding
limitation that restricts their use to real unsteady state or periodic
systems. Khadilkar et al. (2005) had comprehensively pointed out the
general limitations of presented reaction models as follows: