cyclic operation of trickle bed reactors: a review

Cyclic operation of trickle bed reactors: A review

Arnab Atta a,b, Shantanu Roy a, Faïçal Larachi a,c, Krishna Deo Prasad Nigam a,n

a Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016, Indiab Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, Indiac Department of Chemical Engineering, Université Laval, Québec, Canada G1V 0A6

H I G H L I G H T S

� We present a concise review on cyclic operation of Trickle Bed Reactors.� Potential applications of its various modes of operation are discussed.� We emphasis on shock wave attenuation that originates during flow modulation.� Scopes for future research are outlined considering process intensification of TBR.

a r t i c l e i n f o

Article history:Received 16 April 2013Received in revised form16 July 2013Accepted 17 August 2013

Keywords:Trickle bed reactorCyclic operationOn–offMin–maxProcess intensificationShock 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 pasttwo 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 efficiencyand performance. Despite its complicated nonlinear behavior that attributes to the process control safetyat the commercial scale, cyclic operation of TBR is a promising process intensification technology that hasimmense potential for implementation on industrial TBR. In this paper, we have summarized andreviewed the research and progresses made in recent years on cyclic operation of TBR and potentialapplications of various mode of its operation. Several issues associated with its hydrodynamics scale-upand designs have been discussed with an emphasis on shock wave attenuation that arises during flowmodulation.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Trickle bed reactor (TBR) is one of the classical multiphase packedbed reactor configurations that finds extensive application in petro-leum industries for the purpose of hydrocracking, hydrotreating, andalkylation (Saroha and Nigam, 1996). TBRs are also commerciallyutilized in the petrochemical, and chemical industries for hydrogena-tion of aldehydes, reactive amination, liquid-phase oxidation, etc.Recognizing several benefits of using TBR, including simplicity inconstruction and large production volumes, there is significantamount of incentive and motivation involved for further improvementof its performance from the economic as well as environmentalperspective (Nigam and Larachi, 2005; Charpentier, 2007). Optimiza-tion of the performance can be accomplished by (a) evolvingprocess intensifying novel operational methods/techniques, and/or(b) designing efficient equipment/reactor internals that are potentially

compact, safe, energy-efficient, as well as eco-friendly sustainable(Stankiewicz and Moulijn, 2000). Cyclic (or periodic/unsteady state)operation of TBR belongs to the first category and has gainedconsiderable attention due to its substantial effect on improvedreaction 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 aseither limited by liquid phase or by gas phase reactants. In anycase, gaseous reactants must dissolve into and then pass throughthe liquid phase to reach the catalyst surface. For sparingly solublegaseous reactants, this transport resistance is one of the vitalparameters in influencing the rector performance. During cyclicmode of operation, a continuous flow of certain fluid phasecontacts other fluid stream that is forced to toggle periodically atthe reactor inlet between a low-level (base) and a high-level(pulse) flow rate. Considerable decrease in transport resistance,that is desirable for the gas phase limited reactions, can beachieved by temporal variation of catalyst wetting due to suchliquid feed cycling operation (Boelhouwer et al., 2002b). For liquidphase limited reactions, the operation of TBR in natural pulsing

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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

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flow regime is most appropriate since complete catalyst wettingand high solid–liquid mass transfer can be achieved; however,relatively higher superficial liquid/gas velocities (than trickle flowregime) are associated with higher energy expenses and lesserconversion due to shorter contact time between phases. As atrade-off, forced cycling of gas phase feed can be a viable solution(Xiao et al., 2001).

Despite potential advantages, cyclic operation in TBR at thecommercial scale is seldom exercised, mainly due to its compli-cated nonlinear hydrodynamic behavior that endangers the pro-cess control safety. However, advances in meticulous researchincluding rigorous experiments and comprehensive modeling arecapable of attaining further reliable predictions. Haure et al. (1989)pioneered the concept of periodic flushing of the bed for theremoval of SO2 in a TBR, packed with activated carbon. Thisprocess intensification technique eventually resulted in increasingoxidation rates up to 45%. Since then, several research groups havementioned the performance enhancement by practicing differentreactions with various modulation schemes (Silveston and Hanika,2002; Tukač et al., 2007; Liu et al., 2008). The hydrogenation ofα-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 rateseven up to 400% under appropriate condition. An increase in pilotreactor productivity of styrene hydrogenation by 30% for periodicoperation in comparison with steady state operation has beenreported (Tukač et al., 2007) that can be considered as significantachievement for potential industrial applications of cyclic opera-tion. A summary of few contemporary research works on periodicoperation of TBR is listed in Table 1.

Observing the plausible wide spread use and popularity of cyclicoperation as process intensification technique, this effort is tosummarize the characteristic studies contributed in understandingthis promising area and its present research status. There is a reviewarticle available on periodic operation of all three-phase catalyticreactors by Silveston and Hanika (2004), and therefore, this study has

been mainly focused on the efforts made in case of only TBR with aspecial importance on the hydrodynamic features and applicationsthereafter.

2. Flow modulation strategy

During liquid feed cyclic operation, a continuous flow of gasphase interacts with a liquid stream which is enforced to toggleperiodically at the reactor inlet between a low-level (base) and ahigh-level (pulse) liquid flow rate. Similarly, for gas feed cyclicoperation, the liquid phase feed is kept constant while the gas flowrate is switched. In general, this mode of operation is called asmin–max or base–pulse mode. However, when the base or mini-mum flow rate value is set to zero then it is especially assigned ason–off mode.

It has been observed that in almost all studies published onperiodically fed TBRs, the specific liquid feed rate is varied in asquare waved pattern and the feed cycle is characterized by fourfeed parameters (Fig. 1). The low or base specific liquid feed rate,uLb, is applied for a duration tb, followed by a high or pulse specificliquid feed rate, uLp, applied for a duration tp. There are three keyparameters that are derived as: (i) the feed cycle period (p), (ii) thesplit (S), and (iii) the average specific liquid feed rate (uLm) whichcan be defined by

p¼ tbþtp ð1aÞ

S¼ tptbþtp

¼ tpp

ð1bÞ

uLm ¼ uLbtbþuLptptbþtp

ð1cÞ

It is interesting to note from most of the studies that the flowregimes during both high and low liquid flow rates are in trickle-flow regime. Based on the time period applied for the pulse/maxmode flow rate, there also exists slow mode operation, where the

Table 1Summary of some recent studies on cyclic TBR.

Researchers Modulationstrategy

System studied Reactor dia. and packings Observations

Lange et al. (2004) On–off, slowmode

Hydrogenation ofα-methyl styrene

ID: 0.02 m, 0.7% Pd=γ-Al2O3 Reactor performance was significantly improved by feedliquid flow modulation

Massa et al.(2005)

On–off, slowmode

Oxidation of phenols ID: 0.021 m, cylindricalpellets of 2.6 mm size CuO/Al2O3 catalyst

Mild effect on phenol conversion due to liquid flowmodulation was detectedbut the flow modulation did not affect product distribution, in the rangeof operating conditions studied

Fraguio et al.(2004);Muzen et al.(2005)

On–off, slowmode

Catalytic oxidation ofethyl and benzylalcohols

ID: 0.04 m, 3 mm spherical1% Pd=γ-Al2O3

Conversion was improved for different combinations of cycle period andsplit. Depending on liquid reactant and operating variables, both positiveand detrimental effects of the liquid flow modulation were observed

Liu et al. (2005);Liu and Mi(2005)

On–off, slowand fast mode

Hydrogenation of2-ethylanthraquinone

ID: 0.021 m, 1.9 mm spherical0.5% Pd=γ-Al2O3

Improvement in conversion and selectivity up to 21% and 12%, respectively,was found. A transient model involving axial dispersion, partial wetting, anddrainage behaviour was developed

Liu et al. (2008) On–off, min–max, hybridmode

Hydrogenation ofdicyclopentadiene

ID: 0.024 m, 1.9 mm 0.3%Pd=γ-Al2O3 egg-shell catalyst

A novel operation strategy of trickle bed reactor, hybrid modulation of liquidflow rate and concentration was proposed. The performance enhancementunder the hybrid modulation was higher (15%) than the min–maxmodulation of single parameter

Schubert et al.(2010)

On–off, slowmode

Hydrodynamics ofair–water flow

ID: 0.10 m, 3.2–4 mmspherical α-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)

On–off, slowmode

Catalytic oxidationof ethanol

ID: 0.0254 m, 2.7 mm 0.5%Pd=γ-Al2O3 egg-shell catalyst

Product distribution was significantly adjusted by frequency tuning. Highfrequency liquid flow modulation increased selectivity towards theintermediate product, however, low frequency led to catalyst deactivation

Wongkia et al.(2013)

On–off, slowand fast mode

Hydrogenation ofstyrene

ID: 0.0191 m, 2 mm spherical0.3 wt% Pd=γ-Al2O3

Fast mode was found favourable. A maximum improvement of styreneconversion of 18% was observed

A. Atta et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎2






phase feed changes periodically over few-minutes time spans, andfast mode operation, where the feed pulse incursion lasts for afew-seconds (Aydin et al., 2006; Hamidipour et al., 2007).

Continuity shock waves may appear as a result of fluidaccumulation in a flow channel depending on its flow ratevariation (Wallis, 1969). Boelhouwer et al. (2002a) reported thatliquid feed flow rate modulations result in formation of liquid-richcontinuity shock waves that propagate down the bed. It isapparent from the experimental analysis (Boelhouwer et al.,2002a) that these shock waves also diminish by leaving liquidbehind their tail while moving down the reactor. At sufficientlyhigh gas flow rates, inception of pulses may occur within theliquid-rich shock waves that can be referred as liquid-inducedpulsing flow. The frequency of the pulses during fast mode ofliquid-induced pulsing flow is usually less than 1 Hz. Giakoumakiset al. (2005) redefined a clear demarcation between various modeof induced pulsing. Based on the measure of the existence of at leastone pulse in the packed bed, at any time, Giakoumakis et al. (2005)mentioned that when the characteristic time period of a pulse isbigger than the ratio of reactor height and pulse rapidity, it will bereferred as slow mode operation. Giakoumakis et al. (2005) alsomentioned that at substantially smaller frequencies compared tolimiting case, the bed can operate at a liquid-rich and a gas-richstate, interchangeably. According to their study, apart from thetemporal and spatial holdup variations, a typical feature of slow

mode operation is noticeable pressure drop fluctuations, whereasin case of fast mode operation, the pressure drop over the bedremains almost unchanged (Giakoumakis et al., 2005).

2.1. On–off mode

There are numerous studies on the on–off mode operation tounveil the transient behavior of cyclic TBR. Giakoumakis et al.(2005) experimentally characterized several parameters ofinduced pulses namely, its evolution, holdup, pressure drop, pulseintensity and celerity for fast mode cycling operation in a cylind-rical bed of 0.14 m inner diameter. With the help of conductivityprobes for measuring liquid holdup, they analyzed axial propaga-tion and attenuation of the induced liquid pulses. In line with thestudy of Boelhouwer (2001), their study also recognizes thevarying shape of induced pulses and its decay down the lengthof the bed (Fig. 2).

In order to distinguish between slow and fast mode of operations,Giakoumakis et al. (2005) highlighted that considerably largeramount of liquid holdup than the static holdup of the packed bedwas found during fast mode. This phenomenon was ascribed to thegreater characteristic time of drainage of the bed compared to thepulse periodicity at the inlet that results into liquid accumulation inthe bed in between two consecutive pulses. Such fast mode operationwas further examined by Trivizadakis et al. (2006) in an identicalexperimental setup to study the particle shape and size effects onpulse characteristics (Fig. 3). Considering widely used catalyst parti-cles in TBR applications, Trivizadakis et al. (2006) selected twodifferent types of porous alumina particles (3 mm spheres and1.5 mm dia. cylindrical extrudates) along with 6 mm glass beads forthe comparative study.

From Figs. 2 and 3, it can be observed that liquid holdup tracesdisplayed 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 beforereaching the bottom of the bed. For both spherical and extrudateparticles, Trivizadakis et al. (2006) also indicated that during fastmode of on–off liquid pulsing, total liquid holdup (i.e. sum ofdynamic and static holdup) was always higher than the staticliquid holdup. This, in turn, implies that chances of liquid dry-outphenomena can be minimized by fast mode of operation as therewill be always ample liquid in the bed at all the time of pulses.

Time

Liqu

idfe

edra

te

tp tb

uLp

uLb

Fig. 1. Schematic of the cyclic liquid feed. uLb¼base liquid feed; uLp¼pulse liquidfeed; tb¼duration of base liquid feed; tp¼duration of pulse liquid feed.

Fig. 2. Typical simultaneous liquid holdup traces (figure reprinted from Giakoumakis et al., 2005 with permission from the publisher, Elsevier). The packing material is glassspheres of 6 mm diameter and the flow rates are G¼0.22 kg m�2 s�1, L¼3.34 kg m�2 s�1. Cycle frequency¼0.167 Hz (3 s on–3 s off).

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Trivizadakis et al. (2006) further claimed that the trend of liquidholdup traces is similar for several operating conditions including,liquid cyclic feed frequency and mean fluid flow rates. However,due to smaller pressure drop and rate of pulse attenuation,spherical packings can hold significant advantages over cylindricalextrudates of comparable size.

To recognize the effect of porous packings on pulse attenuation dueto liquid flowmodulation, Ayude et al. (2007b) extensively studied thetemporal variations of the liquid holdup at different axial positions in amini-pilot scale TBR (of inner diameter 0.07 m), packed with porousbeads of γ-Al2O3. Using a conductometric technique, Ayude et al.(2007b) analyzed liquid holdup modulation for different superficialvelocities, bed depth and cycling parameters. For a square wave inputof liquid feed, slightly deformed structure of the wave was founddown the bed length during slowmode operation. These structures, asit moved along the column length, had different trends of decay thatoccurred during the dry period of the flow modulation (Ayude et al.,2007b). Furthermore, a decrease in the width of plateau was alsorealized during the wet period of the flow modulation. In case of fastmode on–off operation, the structure of input pulse deterioratesseverely and eventually becomes flat at the lower part of the bedrepresenting a pseudo steady state (Ayude et al., 2007b). Moreover, itwas realized that the mean liquid velocity determined the pulseintensity in the bed. In case of lowmean liquid velocity, pulse intensity

can only be realized near the feed inlet and diminishes to zero downthe length of the bed. However, in case of a higher mean velocity,there is a continuous decrease in pulse intensity along the bed lengthand can be appreciated even at the bottom of the column (Ayude et al.,2007b).

Banchero et al. (2004) experimented the efficacy of on–off fastmode liquid modulation in a TBR, of 0.04 m inner diameter, packedwith Pd/C catalyst for α-methyl styrene hydrogenation (AMS). Thisstudy disclosed that cyclic operation can improve the conversionrate up to 60% compared to the steady state operation for anyparticular operating condition. Because the experiments wereexecuted in isothermal conditions, this conversion rate improve-ment clearly advocates the utility of periodic operation that led toincreased mass transfer.

Ayude et al. (2008) discussed the effect of on–off liquid flowmodulation on the oxidation of ethanol in a lab-scale TBR havinginternal diameter of 0.0254 m. For a varied set of fluid flow rates,cycle periods and splits, experiments were carried out to comparethe performance with the steady-state experiments. Fig. 4 showssubstantial improvements (� 30%) in catalyst activity with largersplit for any fixed period of modulation. This study also revealedthat enhancement in reaction rate was favored during relativelyfast 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 feedmodulation. In fact, there are very few literature available dealingwith gas feed modulation. One of the main reasons behind this canascribe to the fact that compared to the liquid phase, the inertiaeffect due to gas phase modulation is considerably small whichdoes not help in damping of the oscillation (Banchero et al., 2004).Nonetheless, with a different feed modulation strategy than theconventional type operation, Larruy et al. (2007) tested gas feedmodulation instead of liquid flow modulation (Tukač et al., 2003;Massa et al., 2005) in catalytic wet air oxidation (CWAO) of phenol.They studied the possibilities of reducing active carbon burn-offusing gas flow and composition modulation in a small TBR of0.011 m internal diameter. It was demonstrated that modulation ofgas feed composition was essentially advantageous for CWAO ofphenol due to considerable reduction in active carbon burn-offcompared to steady state operation. Larruy et al. (2007) furthershowed that gas feed modulation with high splits was veryattractive for preserving catalyst activity in long term.

Later, Ayude et al. (2007a) added to this observation withextended experimental study in a similar reactor setup to analyzethe impact of gas feed composition and flow rate modulation onthe short time activity of the catalyst through 50 h of operation.Fig. 5 depicts the significant result of this study. It shows thatnearly same conversion rates of phenol and total organic carbon(TOC) can be achieved by either gas feed composition or flow ratemodulation. However, the temperature profiles are different inboth cases (Ayude et al., 2007a). From the perspective of

Fig. 3. Typical, simultaneously recorded, liquid holdup traces along the packed bed(figure reprinted from Trivizadakis et al., 2006 with permission from the publisher,Elsevier). The flow rates are G¼0.12 kg m�2 s�1, L¼3.34 kg m�2 s�1. Cycle fre-quency¼0.167 Hz (3 s on–3 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),(figure reprinted from Ayude et al., 2008 with permission from the publisher,Elsevier). VL;ss ¼ 70 mL min�1; VG ¼ 200 mL min�1.







commercial application, such results are extremely appealingbecause consumption of nitrogen can be minimized which in turnalso reduces operating costs.

All these studies proved the effectiveness of periodic gas phasemodulation in CWAO of phenol, compared to steady state oxida-tion with proper selection of cycling parameters, i.e. split andperiod. Higher splits and cycle periods were found to be highlybeneficial to the catalyst activity in CWAO of phenol. Ayude et al.(2007a) disclosed that mean conversion rate in phenol CWAO cansignificantly be enhanced (from 35% to 60%) by gas feed cyclingwith suitable split and period of modulation.

Researchers argue that periodic liquid feed modulation canminimize liquid maldistribution in the TBR which is an inherentadvantage of the cyclic operation of TBR. With a state-of-artnoninvasive multiphase flow visualization technique, ElectricalCapacitance Tomography (ECT), Liu et al. (2009) demonstratedthis fact while analyzing the transient behaviors of the liquidholdup of air–kerosene system in a periodically operated TBR, of0.14 m internal diameter, packed with spherical glass particles.They captured instantaneous ECT images at several axial positionsof a TBR modulated with the slow mode on–off operation.Furthermore, the transient radial liquid distributions and a mal-distribution factor were computed from those ECT images toestimate the liquid distribution under periodic modulation. Theirstudy revealed that liquid maldistribution generally occurredduring the draining period and more appreciable at the top sectionof the bed. This study manifests further investigations usingnoninvasive process tomography in cyclic TBR considering thelimitations of intrusive measurement techniques such as conduc-tometric technique for determining local liquid-holdup and radialdistributions that are prevalent for industrial applications.

2.2. Min–max mode

There are relatively limited amount of literature available onthe peak–base or min–max mode of operation than on–off modeof operation. Urseanu et al. (2004) examined the use of periodicliquid feed modulation in a high pressure TBR which is veryrelevant from the standpoint of industrial application of cyclic TBR.They studied the effect of min–max periodic operation on thereaction rate in hydrogenation of α-methyl styrene (AMS) tocumene in a TBR of 0.051 m diameter. Their results showed anincrease of at least 50% in reaction rate for the case of periodicoperation compared to steady state operation. Fig. 6 shows thetemperature profile of the liquid with the corresponding situation

of liquid feed velocity. It can be observed from Fig. 6 that duringthe flushing of the bed with liquid, the temperature of liquidincreased as it effectively removed the heat from the catalyst.

In an attempt to compare and quantify the liquid maldistribu-tion during on–off and min–max operation, Borremans et al.(2004) came up with the remarks that for the very limited numberof operating conditions, periodic operation was not able toimprove the liquid distribution compared to steady state case ina TBR with 0.3 m diameter. In these operating conditions, themaldistribution factors had their minimum values in steady stateoperation, signifying best possible distribution that could havebeen achieved. They argued that for periodic operation, betterdistribution can be achieved in cases of lower mean liquid flowrates and by setting base liquid flow velocity to zero i.e. by on–offoperation. Borremans et al. (2004) also indicated that cyclicmodulation of liquid feed can ensure better liquid distribution inthe conditions that are close of pulsing regime under steady stateoperation.

Considering the advantage of shock wave formation during flowrate modulation, Hamidipour et al. (2007) applied min–max opera-tion to reduce fines deposition that helped to prolong reactoroperational life under filtration conditions of a TBR. In order toexamine the efficiency of the shock waves in delaying solidsre-deposition, Hamidipour et al. (2007) studied several modes ofoperation, 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 profiles with time for different gas phase modulations. Filled and empty symbols represent phenoland 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 superficial liquid velocityduring periodic operation (period¼480 s; split¼0.25). (Figure reprinted fromUrseanu et al., 2004 with permission from the publisher, Elsevier.)







a TBR of 0.057 m internal diameter. They concluded that slow- andfast-mode liquid cyclic operation strategies were not capable ofminimizing deposition and pressure drop. However, a new scheme,semi-fast mode liquid cyclic operation, defined as minutes-lastingpulses and seconds-lasting base velocities, was beneficial in reducingbed deposition. Additionally, alternating gas/liquid cyclic operationwas also found to be efficient in reducing deposition.

In an interesting and industry relevant study, contrary to theprevious investigations focusing on liquid induced pulsing flowsthat were performed only at atmospheric pressure and ambienttemperature, Aydin et al. (2006) illustrated the effects of tempera-ture and pressure on the shock wave characteristics of bothNewtonian and non-Newtonian liquids in slow mode inducedpulsing of min–max operation. In a 0.048 m diameter TBR, theyshowed reduced decay process in shock waves for increasingtemperature and pressure (Figs. 7 and 8). Aydin et al. (2006) alsodemonstrated that the shock wave breakthrough and decay timesdecreased with increasing temperature and pressure. Liquidholdup was also found to decrease with temperature, especiallyat the high liquid feed rates. This phenomenon may lead toreduced reactor performance during liquid induced pulsing flowat 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 min–max operation.For analyzing the inherent flow dynamics during wave propaga-tion in a cyclic TBR, experiments for multiphase flow visualizationand local variable measurements using ECT were carried out in0.057 m diameter TBR. To unveil the behavior and effect of solitaryliquid-rich square-wave on the continuous mode of min–maxperiodic operation, two different types of flow modulation hadbeen examined (slow mode of 60 s and fast mode of 10 s) fordifferent gas velocities. The effect of gas velocity on the shape ofsolitary 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 figures. Figs. 9 and 10 also depict thepropagation characteristics of the wave down the column length.In both the cases, higher gas velocity restricted the upper limit ofliquid holdup and tries to flatten the square shape of the wave.

It was evident from all the experiments that the solitary liquidwave always restricted within the steady state holdups range ofbase and pulse velocities in the bed. It had also been observed thatin case of slow mode min–max operation, the liquid rich waveplateau upheld the steady state liquid holdup value (correspond-ing to pulse velocity) for sufficiently long period even at thebottom of the column which in turn helped to envisage thesituation as a pseudo-steady state TBR operation. However, in caseof fast mode operation, this plateau just touched the upper limit ofliquid holdup at the bottom of reactor and then starts to decaywithin very short duration. The duration of decaying period waslonger in cases of fast mode operation which got prolonged by theincrease in gas superficial velocity. It was also noticed that the riseof the solitary wave got delayed with the reduction of the gassuperficial velocity. Therefore, in this scenario, it can be antici-pated that if continuously short (in duration) pulse or liquid richwave comes in the bed then there may be chances of pulsinginside the bed (even at trickle flow regime) which can be treatedas pseudo-natural pulsing during fast mode min–max cyclicoperation. Henceforth, that study suggested that slow modeoperation tried to keep the shock-wave identity intact thus isbeneficial for the cases of exothermic gas phase limited reactionswhere 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

0.10T = 25C

Lε

0.00Time (s)

T = 50CT = 75C

0 50 100 150

Fig. 7. Effect of temperature on shock wave patterns measured 40 cm from bed top,P¼0.3 MPa, uG¼0.2 m s�1. Air-water system, uLb¼0.0035 m s�1, uLp¼0.0105 m s�1

(Aydin et al., 2006).

0.16

0.08

0.12

0.04

Lε L

0Time (s)

P = 0.3MPaP = 0.7MPa

0 50 100 150

Fig. 8. Effect of pressure on shock wave patterns measured 40 cm from bed top,T¼75 1C, uG¼0.2 m s�1. Air–water system, uLb¼0.0035 m s�1, uLp¼0.0105 m s�1

(Aydin et al., 2006).

Fig. 9. Experimental visualization of solitary liquid wave propagation with timealong the bed length for (a) uG¼0.062 m s�1 and (b) uG¼0.25 m s�1 during fastmode (10 s) cyclic operation (Atta et al., 2010).







with shorter pulse flow rate time period and higher liquid flowrate, pseudo-natural pulsing situation can be achieved at thebottom of the reactor due to its longer decay time.

3. Modeling and simulation

Despite several advantages, commercial cyclic TBR is far from itsapplication mainly due to process control safety related with the scaleup issues. Highly non-linear hydrodynamics coupled with poorunderstanding of the transport processes and reaction kinetics makesthe situation even more involved for unsteady operation of TBR.In order to demystify the complex behavior, several modeling studieshave been reported in the literature, dealing with both transient andpseudo-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; Brzić et al., 2010). All models basically stem fromthe variation of steady state approach and have correspondinglimitation that restricts their use to real unsteady state or periodicsystems. Khadilkar et al. (2005) had comprehensively pointed out thegeneral limitations of presented reaction models as follows:

� Most of the formulations assume plug flow condition in thebed, thus neglecting any spatial and temporal change in phasevelocity and holdup.

� For simplification, most of the models are based on equilibriumor pseudo-homogeneous condition between reacting phases.

Accumulation terms for non-limiting species have never beenconsidered.

� Thermodynamically ideal fluid conditions are assumed forcalculating single component mass transfer terms.

� Several models are based on computing reaction and transportphenomena of only the limiting species, considering constantconcentrations of other species.

� Most of the models do not incorporate multi-componenttransport between phases.

� In some cases, spatial terms on the catalyst level are disre-garded to simplify computation.

Furthermore, most of those models take partial wetting ofcatalyst pellets into account but neglect the role of static liquidholdup during off period of a cyclic TBR (Liu et al., 2008). Althoughnearly 20–30% of the total area of the catalyst may be exposed tostagnant liquid (Rajashekharam et al., 1998), however during offperiod of a cyclic operation such exposed area may rise to 70–90%(Liu et al., 2008). Considering aforementioned limitations in thepreviously proposed formulations, Liu et al. (2008) suggested amodel involving vapor–liquid equilibrium of a complex exother-mic reaction under unsteady-state operation. They chose hydro-genation of dicylcopentadiene (DCPD) in the presence of Pd/Al2O3

catalyst as the sample reaction and proposed an unsteady-stateTBR model with the following improvements over their steady-state model (Liu et al., 2006):

� New model incorporated time dependency of species concen-tration variation in gas phase.

� It also included dynamic exposure of the catalyst surface topulsing liquid, and temperature of liquid and solid phases.

� To estimate the axial liquid holdup profiles during liquid flowmodulation, Liu et al. (2008) utilized a nonlinear approxima-tion of liquid holdup and time.

� Moreover, periodic initial conditions were utilized in the newunsteady state model.

The model, thus formulated, demanded several parameters to beapproximated from the correlations reported in the literature.Thereafter, this model was solved by the method of lines forseveral operation strategies including modulation of liquid flowrate and/or concentration, and a hybrid modulation of liquid flowrate and concentration (Fig. 11). Liu et al. (2008) reported animprovement in hydrogenation rate up to 10–20% by the modula-tion of liquid flow rate in TBR of 0.024 m internal diameter.In cases of hybrid modulation of liquid flow rate and concentra-tion, the performance enhancement was higher than the min–max/peak–base modulation of a single parameter. Despite the factthat only few typical operating conditions for different modulationstrategies were examined, Liu et al. (2008) showcased a promisingmodulation strategy for the exothermic reactions through dynamicmodeling.

Regarding, the cold flow hydrodynamics during max/min andon/off periodic operation, Brkljac et al. (2007) proposed a modelbased on the relative permeability concept for predicting two-phase pressure drop and dynamic liquid holdup. They utilized thepermeability parameters determined under steady-state condi-tions to model dynamic operation of a TBR. Brkljac et al. (2007)demonstrated that two-phase pressure drop and dynamic liquidholdup during max/min periodical operation were better pre-dicted by the use of the permeability parameters estimated underthe decreasing liquid flow rate mode (i.e. upper branch of thehysteresis curve). Similarly, hydrodynamic conditions during on–off periodic operation were more closely predicted by the use ofthe permeability parameters corresponding to the lower hysteresisbranch. To exhibit the feasibility of process intensification by

Fig. 10. Experimental visualization of solitary liquid wave propagation with timealong the bed length for (a) uG¼0.062 m s�1 and (b) uG¼0.25 m s�1 during slowmode (60 s) cyclic operation (Atta et al., 2010).







increased gas–liquid mass transfer, Borren et al. (2010) presentedan interesting simulation study on gas-limited reaction in a cyclicTBR that predicted an enhancement in conversion of about 14%based on the appropriate selection of the operating mode. Withthe help of Magnetic Resonance Imaging (MRI) results, Dietrichet al. (2011) showed that application of actual local liquid dis-tribution, measured by NMR-tomography, as input to the simula-tion studies provided more reliable assessment of a cyclic TBR andits corresponding benefit.

Additionally, despite several successful numerical attempts tomodel the multiphase flow in TBR operating under steady statecondition, except the study of Janecki et al. (2008), a CFD basedmodel to track the shock wave attenuation in a cyclic TBR wasseldom reported (Wang et al., 2013). Giakoumakis et al. (2005) andKostoglou and Karabelas (2006) discussed the applicability of themulti-fluid model to correlate pulse decay. They concluded that fortheir studies (on–off mode), the lack of temporal stability of themulti-fluid closures makes it incapable for tracking the transientdynamics of the TBR. Janecki et al. (2008) employed this tradi-tional three-phase Eulerian model along with the closures devel-oped by Attou et al. (1999) for their studies on min–max operation.The results from this study encourage accessing the applicability ofdrag force models, developed for steady state situations, in case ofcyclic operation. However, the study was unable to provide thedetails of rise and falling (or tail) characteristics of a shock waveinside the bed. Recently, Atta et al. (2010) singled out a solitaryliquid-rich square-wave and studied its propagation phenomenainside the bed. A two-phase Eulerian model, based on porousmedia concept (Atta et al., 2007b, 2007a) had been successfullyemployed to examine the flexibility and temporal stability of theCFD modeling approach in case of unsteady operation. This study

also helped in understanding the underlying physics behind thepulse attenuation during its propagation across the bed. Consideringcatalytic wet oxidation as test cases, few studies (Lopes andQuinta-Ferreira, 2010, 2011; Lopes et al., 2012a, 2012b) reportedthe application of a CFD framework embedded with volume-of-fluid (VOF) model to predict the dynamic behavior of a pilot TBRand to demonstrate the efficacy of periodic operation over steadystate operation.

4. Summary

With the help of periodic operation in a TBR, the mass transferrate of the gaseous reactant can be considerably enhanced whichin turn results into higher solubility of gaseous phase in the liquid.Therefore, for a desired conversion, cyclic TBR can be operated atfairly lower pressures. Commercially, low pressure operatingconditions can substantially reduce capital and energy costs(Boelhouwer, 2001). For gas-limited reactions, partial wetting ofcatalysts is generally preferred (Boelhouwer, 2001). However, in asteady state TBR operation, partial wetting leads to severe liquidmaldistribution. To eliminate the risk of bed scale maldistributionthat strongly affects reactor performance, cyclic liquid feed canresult in temporal variations of the catalyst wetting efficiency.Such modulation also minimizes the possibilities of hot spotformation.

In cases of consecutive reactions, selectivity is the key para-meter during periodic operation. During the off/base period ofthe feed cycle, there are extreme probabilities of the side chainreactions to the undesired product. Therefore, a faster mode of

Fig. 11. Parameters characterization of periodic operation of TBR. (a) on–off modulation of liquid flow rate; (b) peak–base modulation of liquid flow rate; (c) peak–basemodulation of concentration; (d) hybrid modulation of liquid flow rate and concentration. (Figure reprinted from Liu et al., 2008 with permission from the publisher,Elsevier.)







feed modulation can prevent the conversion to undesired products(Boelhouwer, 2001).

For liquid-limited reactions, the ideal operating condition canarise when there is complete catalyst wetting and the maximumpossible particle–liquid mass transfer. Such optimum scenario canonly be achieved in pulse flow regime of any steady state TBRoperation. However, operating in pulse regime will eventuallydemand undesirable energy expenses and lesser conversion due toshorter contact time between phases. In this regard, fast mode ofliquid-induced pulsing flow can considerably decrease the need ofhigher liquid flow rates (Boelhouwer, 2001). Furthermore, withoutinduction of natural pulsing, a relatively slow mode operation mayreduce the risk of hot-spot formation in the bed. Since liquid feedmodulation induces continuity shock waves which try to retain theiridentity as it moves along the bed, can act as flushing of the bed.

The plausible advantages of periodic or cyclic operation havebeen established by intense exploitation of on–off mode operation.In this regard, the exploring potentials of min–max operation needmore extensive attention. Interesting characteristics of this modehave been disclosed in some recent studies (Urseanu et al., 2004;Borremans et al., 2004; Aydin et al., 2006; Hamidipour et al., 2007;Atta et al., 2010). Numerous attempts have been demonstratedfocusing the important aspect of pulse attenuation revelation inthe bed over the past 5 years. Utilization of non-invasive measure-ment technique, e.g. ECT, has considerably enhanced the knowl-edge behind fundamental physics of transport during severaldifferent modes of operations (Hamidipour et al., 2008, 2009;Liu et al., 2009; Atta et al., 2010; Hamidipour et al., 2013).

Several mathematical and few numerical models have beenproposed by researchers. Unsteady state model developmentincorporating the possible complicated parameters which wereneglected or assumed for model simplification in earlier years,have been observed over the years. Few CFD studies have beenreported in recent years for cold flow simulation and tracking ofthe shock wave attenuation.

5. Recommendations for future studies

� Despite having a notion of reduced liquid maldistributionduring cyclic operation, more flow visualization studies shouldbe directed to strongly establish this fact for different modes ofoperations.

� Since purposely used cyclic operation can enhance the reactorperformances, innovative hybrid cycling schemes can berecommended for specific processes.

� Although the effect of particle diameter on modes of cyclicoperation has been studied, the effect of larger scale beddiameters has to be tested thoroughly with an ambition ofcommercial utility of the results or predictions.

To realize the influence of all key parameters while purposefulusage and designing of a cyclic TBR only by extensive experimentsis emphatically cumbersome and sometimes exceedingly expen-sive. With the advent of sophisticated instrumentation, increasingcomputational power and development of CFD, a sensible andeffective alternative may be the use of CFD based model validatedby comprehensive experimental data set for further investigations.

� Considering the feasibility of numerical modeling in case ofonly min–max operation, unsteady state CFD models should bemeticulously developed incorporating all possible parametersfor extensive studies which will essentially lay the foundationof scale-up and design in industrial scale.

� Understanding the model instability limitations in cases ofnumerical modeling of on–off operation, the potentials of

min–max operation should be exploited for industrial purpo-sive use and recommendation should be identified by theCFD model.

� Studies on multidimensional CFD based model, particularlyincluding reaction kinetics, are necessary for better match withexperimental results reported in the literature and for furtherreliable predictions in view of attracting professionals forindustrial applications.

Nomenclature

Roman symbols

C molar concentration (mol L�1)G gas phase mass flow rate (kg m�2 s�1)L liquid phase mass flow rate (kg m�2 s�1)P pressure (MPa)t time (s)T temperature (1C)u velocity (m s�1)V flow rate (mL min�1)X conversion (–)

Greek symbol

ε holdup

Subscripts

A time averaged valueb baseG gas phaseL liquid phasep pulsess steady state

Acknowledgment

One of the authors (K.D.P. Nigam) thanks AvH Foundation forthe Humboldt Research Award during the preparation of thismanuscript.

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