Cyclic Operation Strategies in Trickle Beds and ... - ACS Publications

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Ind. Eng. Chem. Res. 2010, 49, 934–952

Cyclic Operation Strategies in Trickle Beds and Electrical Capacitance Tomography Imaging of Filtration Dynamics Mohsen Hamidipour,† Faı¨c¸al Larachi,*,† and Zbigniew Ring‡ Department of Chemical Engineering, LaVal UniVersity, Que´bec, QC, Canada, G1 V 0A6, and BPsResidue Hydrocracking, 150 West WarrenVille Road, NaperVille, Illinois

The dynamics of propagating liquid pulsations generated via various cyclic operation strategies in trickle beds was monitored through electrical capacitance tomography (ECT) for gas and liquid superficial velocities in the range of trickle flow regime. The characteristics of ON-OFF liquid, ON-OFF gas, and gas/liquid alternating cyclic operations were compared in terms of mean liquid holdup, pressure drop, pulsation intensity, pulsation propagation velocity, and spatial maldistribution maps of liquid holdup and liquid pulsation propagation velocity. The morphological features of liquid holdup pulsations as a function of cycle frequency were characterized in terms of breakthrough, plateau, and decay times. Gas/liquid alternating cyclic strategy was shown to produce long-lived liquid pulsations under the applied operating conditions and thus could be viewed as a new process intensification means to achieve uniform phase holdup and velocity distributions. In ON-OFF liquid cyclic operation, pulsation velocity did not increase along the bed unlike the ON-OFF gas and gas/liquid alternating cyclic modes where increased pulsation velocities were able to give rise to pulse flow regime. The gas/liquid alternating cyclic operation resulted in the shortest breakthrough and decay times and the longest plateau time, thus approaching the ideal square-shaped inlet pulsations for symmetrical splits. ECT imaging was also used to scrutinize the dynamics of local deposition of fines in trickle beds fed with kaolin suspensions under the three cyclic operations. Data revealed that applying ON-OFF gas and gas/ liquid alternating cyclic methods resulted in significant reduction of fines deposition. This suggests new practical solutions for possible industrial implementation of self-cleaning modulation strategies of trickle beds subject to unwanted filtration during suspension flows. 1. Introduction Any conversion process aims at maximizing the volumetric productivity for a target value-added product conditionally to minimization of (atom, material, and energy) resources utilization and waste production.1 Improvements, even minute ones, targeting reactor selection and/or its operating conditions are usually dubbed as process intensification. Often, prior to opting for radical measures such as replacing an existing reactor by “a would-be better one”, altering reactor operating modes by the process operators, when this is permitted, is the least troublesome of ways to seek improvements. Trickle beds are among the most widespread reactors in highthroughput production as in raw materials’ conversions, especially in the petroleum industry and hydroprocessing.2 In conventional operation, constant gas and liquid feed flow streams traverse cocurrently downward a packed bed of catalyst. Liquid cyclic operation modes have been studied extensively in recent years in the pursuit of a boosted reactor productivity under the flagship of process intensification.3,4 While a constant gas flow is fed into the reactor, liquid feed flow rate is periodically yoyoed; either between zero and a specified value (i.e., ON-OFF mode, Figure 1a) or between a low and high value (i.e., base-peak mode). This modulation is characterized by base and peak times (τb, τp), base and peak velocities (Ub, Up), split ratio (R ) τp/(τp + τb)), and also by a barycentric velocity (U ) (τp × Up + τb × Ub)/(τp+τb)) to which the constantthroughput (isoflow) mode, cyclic operation is compared to. The * To whom correspondence should be addressed. Tel.: +1 (418) 6563566. Fax: +1 (418) 656-5993. E-mail address: Faical.Larachi@ gch.ulaval.ca. † Laval University. ‡ BPsResidue Hydrocracking.

wavy shape of liquid holdup obtained through flow modulation is referred to liquid holdup pulsation or liquid pulsation (or in short pulsation). Hence pulsations should not be confused with the pulses forming in the pulse flow regime as triggered through hydrodynamic instabilities and which occur at relatively high liquid and gas throughputs. In the case of gas-limited reactions, ON-OFF liquid flow modulation is advantageous as it facilitates accessibility of the gaseous reactant to the catalyst wet surface during the weaning sequence, that is, absence of liquid flow.5 Several other advantages are claimed to be achievable through such liquid flow modulation, for example, heat removal role of the traveling pulsations prevents formation of hot spots and reaction products are swept away, therefore undesirable byproduct are avoided and the selectivity of the reaction can be enhanced.6-8 Liu et al.6 performed isothermal gas-limited hydrogenation of 2-ethylanthraquinone (EAQ) to 2-ethylanthrahydroquinone (EAHQ) under ON-OFF liquid flow modulation (EAQ H2 f Pd/Al2O3 EAHQ H2 f Pd/Al2O3 undesired products). For a 60 s cycle period, a symmetrical split (R ) 0.5) was observed to maximize EAQ conversion. In isothermal condition, direct access of gas reactant to the catalyst surface plays an important role in boosting reaction conversion. Brief gas periods overburden liquid reactant supplies and set extra resistance for the catalyst accessibility by the gaseous reactant thus lowering EAQ conversion. During large gas periods, on the contrary, availability of gas more than liquid occasions liquid starvation and thus lowers conversion. Adjusting the cycle time and split ratio could result in larger selectivity toward EAHQ. Up to 12% increase in selectivity was reported for different tested combinations of cycle time and split ratio (e.g., 40 s and 0.5). At high pressure, a reduction in EAQ conversion was observed due to the passage from gas to liquid limited conditions in which

10.1021/ie900605b  2010 American Chemical Society Published on Web 07/20/2009

Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010

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Figure 1. Cyclic operation strategies and their characterizing parameters: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating.

isoflow mode was more advantageous than liquid ON-OFF mode. Phenol flameless combustion in the aqueous phase (phenol f intermediate products f CO2 + H2O) was studied using the same flow modulation strategy by Massa et al.7 Oxygen was identified as the adverse reactant because of its limiting accessibility via internal and/or external mass transport. It was speculated that flow modulation would lead to more exposure of the catalyst surface to oxygen. Though this did not result in improved phenol conversion, a positive effect on the mineralization selectivity (CO2 production) was reported for the longer periods which entailed deeper conversion of the intermediate partially oxidized products. In trickle flow, the liquid structure consists of films on the surface of catalyst, droplets, and rivulets while the gas flows in the available void space.9 The long liquid residence time to prevail in trickle flow regime is sought-after for pushing conversions, whereas pulse flow regime which takes place at higher throughputs improves interfacial mass transfers and catalyst wetting though at the expense of shortened liquid residence time.10,11 The positive effects stemming from both flow regimes are claimed to be combinable in cyclic operation.12 Boelhouwer et al.8,13 and Giakoumakis et al.14 have reported the occurrence of pulse flow during liquid cyclic operation at lesser barycentric gas and liquid superficial velocities in comparison to the corresponding constant throughput operation. This has been considered as an enlargement of the pulse flow regime domain via the cycling operation. Aydin and Larachi,15 however, reported that higher (with respect to isoflow) superficial liquid velocities during the peak liquid sequence (i.e., τl) are required in cyclic operation for the inception of pulse flow regime, thus questioning the broadening of the pulse flow regime region in liquid cyclic operation. Xiao et al.16 have addressed the improvements of radial and axial distributions of liquid through performing gas cyclic operation. In this mode, a constant liquid flow rate is imposed while the gas flow rate is switched periodically between two levels, that is, either in base-peak or in ON-OFF mode (Figure 1b). A different cyclic operation mode has been recently proposed to mitigate adverse effects due to deposition of fines from the flow of suspensions and of foaming liquids in trickle-bed reactors.17,18 In this manner, a single-gas flow sequence is alternated with a single-liquid flow sequence as sketched in Figure 1c. Attempts to measure the time-varying liquid holdups, el, corresponding to this alternating flow modulation using classical el-Ul-Ug electrical conductance calibration protocols19 proved unsuccessful. This is because during the liquid weaning sequence (liquid OFF), it is almost impossible to relate the instantaneous residual liquid holdup to a de facto unknown liquid velocity whose value is determined, during the time lapse τg, by the gas flow field and liquid drainage dynamics.18 Therefore, the instantaneous liquid holdup variations need to be measured via more appropriate techniques. To the authors’ best knowledge, characterization of the dynamics of this flow

modulation in terms of its flow parameters (holdup, intensity, velocity, etc.) is still undocumented in the open literature. Unlike liquid cyclic operation, visual inspection of the gas/liquid alternating cyclic operation reveals the occurrence of pulse flow regime for certain conditions despite the gas and liquid velocities (during the ON sequences, Ugp, Ulp) are representative of trickle flow regime conditions. Tomographic imaging is helpful to capture, without interference with the actual hydrodynamics, the evolving topography at depths inside the bed which otherwise are inaccessible from mere wall scrutiny. Electrical capacitance tomography (ECT)20,21 is fit for measurements of electrically nonconductive organic liquids such as kerosene as used in the present study. Through appropriate sensors’ calibration, the instantaneous permittivity distribution of the materials in the sensed bed cross-section is obtained and then converted into instantaneous two-dimensional liquid saturation maps. In addition, owing to the ECT high temporal resolution (up to 100 Hz), real time liquid holdup maps under fast modulation can be easily captured. Reinecke and Mewes22 applied ECT technique to investigate the mechanism and characteristics of pulse flow regime in trickle-bed reactors. ECT imaging has also proven its ability to unveil some mechanistic and structural deposition phenomena in isoflow filtration tests.25 The deposition of fine particles in trickle beds is particularly crucial in the Canadian context of hydrorefining operations. Inorganic fines, present in some bitumen heavy fractions, cause plugging which is an undesirable epiphenomenon taking place in trickle beds during some hydrotreating operations, such as hydrodesulfurization. The hydrodynamics of trickle beds can indeed be dramatically altered by the deposition of fines contained in the dilute liquid feed suspensions.23 Due to bed porosity reduction, the two phase pressure drop starts to rise which could shift reactor operation from nominal conditions it was designed and rated for. This could result in profit losses occasioned by frequent shutdowns and catalyst replacements.24 Implementation of cyclic operation can be proposed as a potential means to thwart this problem.17 The idea consists in generating and in keeping alive, as much as possible, liquid pulsations along most of the bed length to promote reentrainment (and evacuation) of the deposits thus delaying bed plugging. The aim of this work is to use ECT imaging to evaluate the performance of liquid, gas, and gas/liquid alternating cyclic operations. The latter case consists of combining ON-OFF mode for both phases; therefore, the functioning of alternating strategy inside the bed is compared to ON-OFF liquid and ON-OFF gas cyclic operations tied altogether. Gas and liquid superficial velocities (i.e., during the ON parts, Ugp, Ulp, Ug or l) are kept within the range of trickle flow regime. The instantaneous cross-sectionally averaged liquid holdup values, intensity of the liquid pulsation as a function of reactor length, and gas and liquid superficial velocities, the morphological features of

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liquid holdup pulsation under cyclic operation (breakthrough, plateau, and decay times), the liquid pulsation propagation velocity along the bed as well as the per-pixel pulsation propagation velocity map will be obtained to enable objective assessments of the merits of the different cyclic strategies. ECT imaging will also be performed to demonstrate the ability of the different cyclic strategies to mitigate the extent of deposition in trickle beds. 2. Experimental Section The experiments with an air-kerosene system were carried out at room temperature and atmospheric pressure using a 5.7 cm (ID) diameter Plexiglas column, packed up to 160 cm bed height with 3 mm spherical glass bead particles (bed porosity, e ) 0.4). The packing was maintained by means of a rigid stainless steel screen placed at the column bottom. The screen’s mesh opening (rhombic shape 0.65 × 1.3 mm2) was set to prevent the packing elements from crossing over, and to barely block the exiting flow of suspension. The liquid feed was pumped to the column top and distributed through a spray nozzle to ensure even distribution. Since the nozzle opening was ca. 0.5 mm, special attention was paid to prevent introduction of solid impurities in the feed tank which would easily block the nozzle. Gas feed (i.e., air) was fed through several small holes on the top of the bed. The pressure drop signal across the whole bed was recorded at a frequency of 1 Hz. The ON-OFF liquid modulation was performed with the help of a computerautomated pump to adjust the base and peak times (τb,τp) and velocities (Ub,Up). To perform ON-OFF gas flow modulation, the air line was equipped with a computer-controlled on-off valve to assign the ON and OFF time intervals. The gas flow rate was tuned prior to the experiment to provide the desired velocity during the ON (i.e., peak) sequences. In the case of gas/liquid alternating, the pump and the on-off valve were synchronized to send either liquid or gas flow to the reactor for desired time intervals. It is important to note that the experiments were conducted at superficial velocities (i.e., during the ON periods, Ugp, Ulp) of air and kerosene coinciding with trickle flow regime. Therefore, any observation of pulse flow could be interpreted as an intensification of the process. The equipment used for tomography measurements is a PTL300E type consisting of a twin-plane 12-electrode-per-plane vertically sliding sensor with a DAM200E data acquisition system from Process Tomography Ltd. ECT is suitable for imaging electrically nonconducting organic liquids such as kerosene as tested in this study. Details about the ECT method, calculation procedure, and its applications could be found elsewhere.26,27 The measured capacitances will be used to reconstruct the distribution of permittivity and hence of the materials distribution per pixel (32 × 32 pixels per image). In an earlier study,28 different reconstruction algorithms were reviewed and simulations were performed to compare their capability to capture qualitative (shape) and quantitative (size) image features of phantoms with known characteristics. The Tikhonov algorithm was found to be one of the most successful approaches. Moreover, application of ECT along with this reconstruction algorithm to determine liquid holdups in tricklebed reactors was validated in an earlier study.25 The same method is used here to generate normalized permittivity images from the measured capacitance data. For any specific operating condition, online tomographic images (50 Hz) of liquid saturation profile were obtained at three different vertical positions along the bed length (10, 75, and 145 cm) and postprocessed to extract the instantaneous liquid holdup values and their

characteristic parameters for each cyclic mode and as a function of bed length. The residual liquid holdup (elres), used to calculate the liquid holdup values, was determined experimentally through bed drainage tests. For the filtration tests, kaolin-kerosene suspension and air were the test fluids. Kaolin, as a major component of the clay in Athabasca oil sand,29 was added to kerosene which was used as a model hydrocarbon liquid.24 Kaolin powder was heated to remove any moisture before addition to kerosene. Direct addition of kaolin to kerosene with no further preparation would result in suspensions with fines typically several hundred micrometers in size. Therefore, kaolin suspensions were preventively sonicated in an ultrasonic cleaner before addition to kerosene to keep their sizes around ca. 8 µm.23 The inlet concentration of fines was 1 g/L in all experiments. Thorough mixing and a conically shaped reservoir bottom deposition was prevented in the feed tank. The connected tubes between reservoir, pump, and reactor were kept as vertically as possible to avoid untimely deposition of kaolin particles. The bed outlet kaolin suspension in cyclic operation was collected each 2 h using very fine fabrics to evaluate the overall bed specific deposit which is compared to the one determined in isoflow experiment through turbidity measurements. In isoflow mode, the suspension outlet concentration was measured using a turbidimeter and sampling at the bed exit at regular time intervals. The specific deposit was determined from a mass balance around the bed.17 After each experiment the column was dismantled and the particles were carefully washed with water before subsequent uses. 3. Results and Discussion In the following discussion, liquid pulsations generated via various cyclic operations should not be confused with the liquid pulses that are initiated in pulse flow regime due to the high interaction between liquid and gas. 3.1. Clean Bed Experiments. The ECT sensor was calibrated at drained prewetted-bed (0% reference) and at floodedbed conditions (100% reference) whose corresponding per-pixel mixture permittivities are, respectively, Drained-bed permittivity res ε[0] ) (1 - e)εs + eres l εl + (e - el )εg

(1)

Flooded-bed permittivity ε[1] ) (1 - e)εs + eεl

(2)

Similarly, in two-phase flow operation, the corresponding permittivity writes as fd res fd ε[gl] ) (1 - e)εs + (eres l + el )εl + (e - el - el )εg

(3) fd In eqs 1-3, e, eres l , and el , represent, respectively, the bed porosity, the residual liquid holdup due to capillary forces, and the free-draining liquid holdup, whereas, εs, εl, εg, are, respectively, the packing (bed element), liquid, and gas electrical permittivities. Therefore, the normalized permittivity, NP, formed as [(3) - (1)]/[(2) - (1)], gives rise to the free-draining liquid holdup normalized by the “effective” void space after resting the residual liquid holdup:

NP )

efd l ε[gl] - ε[0] ) ) βfd ε[1] - ε[0] e - eres l

(4)


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Figure 2. Instantaneous liquid holdup traces along the bed for different cyclic strategies: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating. Barycentric velocities, Ul ) 0.001 m/s, Ug ) 0.062 m/s; cycle frequency ) 0.167 Hz; R ) 0.5.

Here, βfd is called the free-draining liquid saturation. All the 32 × 32 pixels of the ECT image will provide instantaneous local free-draining saturation values from which the crosssectionally average free draining liquid holdup, elfd, could be calculated from eq 4 and will be referred to, for short, as liquid holdup in the course of this manuscript. The cyclic operation parameters to be used later will refer either to the fractional period feed flow is ON and OFF or to the reciprocal of the total period (or frequency). For example, a mode of 3 s ON-3 s OFF means that either gas or liquid (depending on operational mode) flow for 3 s followed by 3 s of flow interruption. The cycle frequency is 1/(3 + 3) ) 0.167 Hz. Analysis of the ECT time series under various operating conditions indicates that the frequency of the traveling liquid pulsations does not change along the bed and maintains at the preimposed value. Only results obtained for symmetric splits (equal ON and OFF time durations) will be presented in this work. For symmetric splits, the barycentric superficial velocity, U ) (τp × Up + τb × Ub)/(τp + τb), whether for liquid or gas, counts for half the actually imposed superficial velocity during the ON (peak) sequence. Propagation along the bed main axis of the imposed liquid pulsations was examined via liquid holdup time series at different bed elevations. The sliding ECT sensor facilitates measurements at as many as desired vertical positions. To avoid any unwanted experimental biases due to local external wall asperities and irregularities, the calibration procedure was repeated for each interrogated vertical location. The results for three heights (10, 75, and 145 cm) are presented below. 3.1.1. Liquid Holdup and Pressure Drop. Figure 2 illustrates liquid holdup time-series for ON-OFF liquid, ON-OFF gas, and gas/liquid alternating cyclic operations where barycentric liquid and gas velocities are Ul ) 0.001 m/s and Ug ) 0.062 m/s, respectively. The overall cycle frequency is 0.167 Hz (3 s-3 s). In the case of ON-OFF liquid cyclic operation

(Figure 2a), the attenuation of liquid pulsations is evident from the liquid holdup time series. The liquid pulsations close to top exhibit sharp fronts and gradually become blunt as they travel downward. Aydin et al.12 attributed this phenomenon to the nonisolated nature of the traveling liquid pulsations. The pulsations decay by leaving, through a dispersive/convective mass transfer process, part of their liquid content behind their tail. In this way, a fraction of the liquid payload is transferred from peak to base. The base liquid holdup increases therefore at the expense of a loss of peak amplitude. Eventually, pulsations totally collapse after traveling some depth in the bed. Since the operation is performed in fast mode, the liquid drainage time (i.e., τb) is too short to favor bed dryness. In ON-OFF gas and gas/liquid alternating strategy, though the pulsations near the bed entrance experience some attenuation, they nonetheless are able to conserve a sawtoothed morphology at the approach of the bed exit (Figure 2b,c). This stresses out the role of gas phase flow in isolating liquid slugs. Resumptions of the gas flow reduce or prevent leakage of liquid from the tail of the moving liquid pulsation thus preserving the wavy nature of liquid flow. Therefore, more advantages could be expected from ON-OFF gas and gas/liquid alternating cyclic operations. Although all cyclic operations operate at the same barycentric velocities (gas and liquid), each exhibits its own instantaneous velocity pattern depending on the chosen mode. One first question to arise relates to how the mean liquid holdup varies for the three cyclic operations? Figure 3 shows the variation of the mean liquid holdup as a function of bed height for two gas barycentric velocities (0.062, 0.154 m/s) and a liquid barycentric velocity Ul ) 0.001 m/s (6 s-6 s, 0.083 Hz). Each strategy shows quite constant mean liquid holdup along the bed confirming the accuracy and reproducibility of the ECT sensor calibration method. For a constant gas and liquid velocity, the

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the bed. Resumption of gas flow may eventually lead to some bed occlusions challenging the gas flow paths through an opposing liquid leading to the formation of overshoots (Figure 4c). Long τg lapses enable most of the liquid to be evacuated so that pressure drop can approach that in single-phase flow. It is worthy of notice that the initial overshoot in pressure drop depends on τl and gas superficial velocity. The larger the τl values are, the sharper is the pressure drop overshoot. Such pressure drop overshoots translate into highly liquid rich regions in the bed where trickle flow regime is no more sustainable and the excess liquid must be transferred via pulse flow. However, the depth of penetration of pulses in the bed is a function of (τl,Ul) and (τg,Ug). It will be demonstrated later that implementation of slow mode gas/liquid alternating cyclic operation is conducive of pulse flow even for very low barycentric gas and liquid superficial velocities. 3.1.2. Liquid Pulsation Intensity. Pulsation attenuation measures the decrease in liquid holdup increment (between base and peak) along the bed, and depends on operating and cyclic mode conditions. To characterize the different cyclic modes, a liquid pulsation intensity, Ielfd, is defined:14 Figure 3. Mean liquid holdup versus bed height: liquid barycentric velocity, Ul ) 0.001 m/s; cycle frequency ) 0.083 Hz; R ) 0.5.

three cyclic operations present close liquid holdup values to the isoflow condition; however, it is observed that a systematically decreasing trend exists from liquid to gas/liquid alternating to gas cyclic operation. In gas/liquid alternating, the bed experiences during the gas ON sequence a gas superficial velocity which is twice the constant gas velocity in liquid cyclic operation. This expectedly causes more liquid to leave the bed (i.e., lesser liquid holdup). With the same gas feeding strategy as in the alternating mode, gas cyclic operation includes a constant liquid velocity that is half that in action in liquid and alternating gas/liquid modes (during the peak periods). Consequently, the gas phase has more effect on the accumulation of liquid inside the bed leading to somehow lesser liquid holdup. As one would expect from steady-state liquid holdup trends, increasing gas superficial velocity (hollow symbols, Figure 3) results in lower liquid holdup; however, the trend of the average values remains unchanged. Pressure drop has been measured concomitantly to liquid holdup under different cyclic operations. Representative pressure drop time series are presented in Figure 4a-c for ON-OFF liquid, ON-OFF gas, and gas/liquid alternating cyclic operations, respectively (barycentric velocities: Ul ) 0.001 m/s and Ug ) 0.154 m/s, cycle frequency ) 0.056 Hz). Fast modulation of liquid flow in the presence of constant gas flow reveals mild variation of pressure drop (Figure 4a) between single-phase (liquid OFF, low limit) and two-phase (liquid ON, high limit) values. In ON-OFF gas cyclic operation, liquid holdup rises along the bed when gas flow is disabled (OFF portion). Turning the gas ON causes a surge in the flow resistance because of the existing liquid and an abrupt rise of pressure drop occurs (Figure 4b). For gas ON sequences long enough, the liquid that built up can be evacuated completely from the bed. The pressure drop, following a declining trend, will approach a steady-state two-phase flow value at the corresponding gas and liquid superficial velocities. Gas cut off is equivalent to zero pressure drop for the flowing liquid phase. In fact, at zero gas velocity, the stagnant gas levels off pressure from top to bottom to the atmospheric pressure. In gas/liquid alternating strategy, the liquid velocity during the ON part (Ulp) is twice that with respect to the gas cyclic operation. Therefore, more liquid will be held in

Iefd ) l

σefd l

〈efd l 〉

(5)

where σelfd and 〈efd l 〉 are, respectively, the standard deviation and time-average obtained from liquid holdup time series as shown in Figure 2. Bed axial profiles of Ielfd for ON-OFF liquid, ON-OFF gas, and gas/liquid alternating modes are shown in Figure 5a-c, respectively. The three runs were carried out using the same barycentric velocities, Ul ) 0.0013 m/s and Ug ) 0.062 m/s. In ON-OFF liquid mode, the liquid pulsations gradually fade away while traveling downstream (Figure 5a). Increasing the cycle frequency exacerbated this effect causing even nearly complete pulsation collapse near the bed exit. The intensity, Ielfd, of the liquid pulsations generated in ON-OFF gas mode (Figure 5b) shows a weak dependence with respect to cycle frequency, though in similar qualitative trends with respect to ON-OFF liquid mode. Yet liquid pulsations undergo minor attenuation. It appears that the low (but constant) liquid velocity (Ul ) 0.0013 m/s, Figure 5b) is not sufficient to result in sizable liquid accumulations during the Ug ) 0 sequences, even at low cycle frequency. This could be a reason for the much less pronounced liquid pulsation intensities obtained in ON-OFF gas mode compared to ON-OFF liquid mode. In the gas/liquid alternating mode, a burst of liquid and gas slices, the thickness of which is determined by τl and τg, starts the journey at the bed entrance. The intensity, Iefdl , of the liquid pulsations generated in this mode (Figure 5c) undergoes an initial attenuation but then stabilizes around a constant value regardless of bed height and prescribed cycle frequency. This suggests that the liquid pulsations, regarded as packets of various lengths sliced by the alternating gas flow, are more or less liquid-tight thus preserving some amplitude along the bed. The low initial pulsation intensity characterizing ON-OFF gas mode emphasizes the importance of liquid superficial velocity. Figure 6 displays a series of Ielfd runs, realized in ON-OFF gas mode at identical barycentric gas velocities (Ug ) 0.062 m/s), as a function of liquid velocity for two cycle frequencies at two bed heights. Inception of liquid pulsations is driven by the OFF to ON transition of gas flow which signals that liquid holdup has to be lowered in the bed. Larger liquid velocities translate into larger liquid holdup surplus that should


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Figure 4. Time evolution of overall pressure drop signal: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating. Barycentric velocities, Ul ) 0.001 m/s and Ug ) 0.154 m/s; cycle frequency ) 0.056 Hz; R ) 0.5.

be cleared out from the bed causing more prominent pulsations to arise. Similarly, lower frequencies are conducive to higher pulsation intensities as more liquid is allowed to accumulate along the bed during the OFF gas sequence (hollow symbols, Figure 6). As previously pinpointed, pulsation intensities near the bed exit are lower due to pulsation attenuation. The effect of (barycentric) Ug on pulsation intensity is depicted in Figure 7a-c,f for ON-OFF liquid, ON-OFF gas, and gas/liquid alternating modes at two bed heights and for two frequencies. The same liquid barycentric velocity, Ul ) 0.0013 m/s, was imposed. For the duration of the ON lapse in liquid cyclic mode (Figure 7a), increasing Ug entails lower peak liquid holdup as expected from conventional isoflow runs. Increasing Ug also promotes faster liquid drainage after permuting to the OFF half-period resulting in lower base liquid holdups too. Consequently the pulsation intensity keeps barely indifferent to gas superficial velocity changes irrespective of axial position in the bed and cycle frequency. The role of gas velocity in ON-OFF gas mode is displayed in Figure 7b. Increasing Ug is

tantamount to decreasing liquid holdup. This calls for further liquid to be chased out of the bed once gas flow is enabled (ON sequence) prompting larger pulsation intensities near the entrance, the larger the gas velocity (Figure 7b) is. The resulting increased gas-liquid interactions, on the contrary, tend to level off the holdup variations further in the bed causing pulsation intensity to diminish there with increasing Ug (Figure 7b). In gas/liquid cyclic mode, Figure 7c shows that Ielfd near the bed entrance increases as Ug is increased at low-cycle frequency. In this instance more liquid is entrained by the gas flow from the bed top. As shown earlier, after some length (see Figure 5c) liquid pulsation intensity becomes independent of frequency, and so it does regarding gas velocity (Figure 7c) in the range explored in this study. 3.1.3. Liquid Pulsation Morphological Features. Figure 8a depicts some morphological traits of a liquid holdup pulsation according to the definitions proposed by Aydin et al.12 The breakthrough time, τB, measures the transient duration for the

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Figure 5. Pulsation intensity as a function of bed height and cycle frequency: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating. Barycentric velocities, Ul ) 0.0013 m/s and Ug ) 0.062 m/s; R ) 0.5.

liquid holdup to rise from base to peak. Conversely, the time interval it takes for the liquid holdup to decline from peak to base is called decay time, τD. The time duration intertwined in-between τB and τD, in which the peak liquid holdup nearly plateaus, is referred to as plateau time, τP. “Mathematical” pulsations with symmetrical split would showcase uncorrupted square shapes regardless of bed axial position, while verifying τB ) τD ) 0 everywhere. However, imperfection in generating such pristine square pulsations and/or deviation from plug flow undergone by the liquid flow inside the bed cause actual pulsation shapes to deviate from the ideal one. It is reasonable to assume hence that cyclic modes achieving the shortest τB and τD time intervals are those approaching the ideal plug-flow the closest. In the restrictiVe assertion meant here, plug flow refers to a flow context where externally triggered holdup and velocity perturbations conserve their characteristics in translationally indifferent manner along the bed.

The evolution of τB, τP, and τD as a function of cycle frequency, barycentric gas velocity and operational modes are shown in Figure 8b-d at a near-exit axial bed position H ) 145 cm. An identical barycentric liquid velocity, Ul ) 0.0013 m/s, is used. The evolution of breakthrough times as a function of cycle frequency and Ug is illustrated in Figure 8b for ON-OFF liquid, ON-OFF gas, and gas/liquid alternating modes. Irrespective of cyclic mode, τB becomes smaller the larger the cycle frequency is when Ug is low. This presumably is ascribable to strongly smoothed-out base-peak contrasts near the bed exit where these data are taken. Regardless of frequency, the longest breakthrough times occur in gas cyclic mode at low gas velocity (Ug ) 0.062 m/s). This latter underlies low interaction between phases preventing sizable expulsions of liquid holdup excesses to occur, whether gas flow is enabled or not. Higher gas velocity (Ug ) 0.154 m/s), on the contrary, promotes gas-liquid


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duration of the cycle period. Only in ON-OFF liquid mode does increasing Ug tends to increase τD. Furthermore, regardless of frequency and gas velocity, the slowest decay times are also achieved therefrom. In summary, this strategy, exhibiting the slimmest plateau times and slowest decay times, is the worst to preserve square pulsations. With minimum τB and τD, and maximum τP, typification of ideal square pulsation shape (or plug-flow) is approached by the gas/liquid alternating mode. As a matter of fact, interspersing gas and liquid flows in a slicewise manner is the key to prevent squandering, through dispersive-advective effects, the content of liquid pulsations while they travel along the bed. This special type of flow is reminiscent of a Taylor flow regime encountered in monolith channels with small opening. Deviation indices have already been defined for slow mode base-peak liquid cyclic operation to demonstrate the nonisolated nature of the propagating pulses due to liquid transfer between base and peak liquid holdups:12 Peak deviation index: ∆elp Figure 6. Pulsation intensity of ON-OFF gas cyclic operation as a function of liquid velocity, bed height, and cycle frequency: gas barycentric velocity, Ug ) 0.062 m/s; R ) 0.5.

interactions and favors sharper liquid holdup changes in the bed. This results in breakthrough times that are systematically lower than at Ug ) 0.062 m/s for all three cyclic modes (Figure 8b). Breakthrough transitions accomplished in gas cyclic mode are faster than in liquid cyclic mode, despite both modes being run under equal barycentric gas and liquid velocities. Note the trend reversal obtained at lower Ug. The strongest gas-liquid interactions are achieved under a gas/liquid alternating policy which results, regardless of Ug, in the fastest breakthrough transitions (Figure 8b). Unlike liquid cyclic mode with its Ug-indifferent declining trend of τB versus frequency, gas cyclic and gas/liquid alternating modes develop monotonically increasing τB versus frequency at higher Ug. Lower frequencies for these latter modes appear to be more favorable to an approach of liquid plug-flow. The liquid phase having more time to replenish in the bed due to longer gas OFF time (gas cyclic mode) or longer liquid ON time (gas/liquid alternating mode) makes the subsequent transition to gas flow meet further resistance. Such forced occlusion has been observed to shift the flow pattern from trickle flow to pulse flow regime as will be discussed later. Figure 8c shows the plateau time as a function of frequency and gas velocity for ON-OFF liquid, ON-OFF gas, and gas/ liquid alternating modes. At given frequency, and barycentric gas and liquid velocities, the largest plateau time is attained through gas/liquid alternating mode. The increase of frequency results, ceteris paribus, in lowering τP for ON-OFF gas and gas/liquid alternating cyclic modes. However, the opposite is observed for the ON-OFF liquid cyclic strategy. In this latter case, low frequencies (or long liquid ON sequences) result in sawtoothed eroding pulsations (such as at H ) 75 cm in Figure 2a) in which liquid holdup approaches its isoflow equivalent at the same constant throughput Ug and Ul. Increasing Ug leads, regardless of the prevailing frequency, to increased plateau times for gas cyclic and gas/liquid alternating modes, whereas in liquid cyclic mode τP is barely affected by gas velocity. For all three cyclic operations, increased cycle frequency shortens the decay time (Figure 8d). It was verified that the sum τB + τP + τD is constant and expectably restores the

e0lp

)

e0lp - elp

)

elb - e0lb

e0lp

(6)

Base deviation index: ∆elb e0lb

e0lb

(7)

For ON-OFF liquid cyclic operation, according to Figure 9a, ∆elp is the difference between peak liquid holdup during cyclic operation and the liquid holdup under isoflow condition at Ul equal to peak velocity (i.e., Ulp), and ∆elb is the difference between base liquid holdup during cyclic operation and residual liquid holdup had the bed been let to be drained completely. Thus e0lp is the liquid holdup at constant Ulp, and e0lb is the residual liquid holdup. Positive deviation indices reveal a loss in peak liquid holdup with respect to isoflow mirroring a gain for the baseline resulting in liquid holdup exceeding the residual holdup. Values approaching zero signify liquid pulsations are isolated thus with a configuration closer to square shape (or plug-flow). Figure 9b illustrates the deviation indices for ON-OFF liquid cyclic operation as a function of frequency and barycentric Ug (barycentric Ul ) 0.001 m/s). Decreased cycle frequencies increase the likelihood for the peak holdup, elp, to get eroded 0 , during the and to evolve toward the isoflow liquid holdup, elp ON time. However, the baseline (liquid OFF) portion will be longer too, and more liquid will be allowed to drain so that liquid holdup, elb, gets closer to the residual value, e0lb. Therefore, the deviation indices given by eqs 6, 7 are ascending functions of cycle frequency. At constant frequency, increasing Ug reduces deviations of liquid behavior from plug-flow, thus lowering the deviation indices. It is worthy of notice that the peak deviation index (eq 6) is 1 order of magnitude smaller than the base deviation index (eq 7). This trend is logical since liquid drainage, even under Ug + 0, is a slow process. This implies an important consequence that beds under fast ON-OFF liquid modulation will unlikely experience dry conditions if completely prewetted and if nonisothermal effects corner, through evaporation, only a marginal portion of the liquid. 3.1.4. Pulsation Structure and Pulsation Propagation Velocity. Each ECT image (32 × 32 pixels) consists of a twodimensional tomogram which is averaged over 5 cm high volumes in the reactor corresponding to the electrodes’ height. An Eulerian slice representation of ECT images is used to

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Figure 7. Pulsation intensity as a function of gas velocity, bed height, and cycle frequency: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating. Liquid barycentric velocities, Ul ) 0.0013 m/s; R ) 0.5.

visualize the axial development of the liquid flow field.22 Pixelized liquid saturations (eq 4) reconstructed along a selected diametrical line (e.g., A-A line in Figure 10) are shot in a volley at 50 Hz pace and plotted one after the other from individual images recorded in cyclic operation. Evolving this line, timewise, from bottom to top, would be tantamount to picking up events for the flow direction from top to bottom but delayed in time until they hit the tomograph sensing plane. This gives, in an approximate sense, a virtually local axial tomogram representation of the liquid flow field. The measurements were obtained at H ) 145 cm. The images were registered to encompass a complete 9 s-9 s (or 0.056 Hz) cycle [including some moments before and some after] where the cycle’s breakthrough is easily recognizable from the sensor’s response, which is followed by the remainder of the pulsation. Different flow structures emerge. A statistical parameter, referred to as a degree of uniformity χβ, is used to express the extent of

uniformity of liquid saturation over the trickle-bed crosssection25 under different cyclic operations as a function of time: χβ )

1 n

n

∑ i)1

( ) βi - β¯ β¯

2

(8)

where n denotes the number of pixels per image, βi and βj are the local and cross-sectionally averaged liquid saturations, respectively. χβ approaches zero as uniformity in distribution improves. In liquid cyclic operation (Figure 10a), a flow with constant Ug is permanently enabled across the bed. The gas flow is persistent near the wall consistent with the larger bed porosity thereabouts.16 While competing for space with gas flow, the inflowing liquid is naturally routed toward regions with lesser resistance on account of the gas flow “fait accompli”. Consequently, as Figure 10a shows, the main portion of the liquid


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Figure 8. Variation of morphological parameters of cyclic operation as a function of cycle frequency and gas velocity: (a) illustration of morphological parameters, (b) ON-OFF liquid, (c) ON-OFF gas, (d) gas/liquid alternating. Liquid barycentric velocities, Ul ) 0.0013; R ) 0.5.

chooses to flow core-wise in the bed. The front (or nose) liquidrich part (between 0 and 9 s, Figure 10a) tries to spread over the whole cross-section. The dashed line at midheight indicates the moment when liquid flow is disabled (OFF). A liquid tail, slightly shifted in the right, keeps draining subsequently. The areas alongside the wall drain faster in agreement with more gas bypass flowing nearby the wall. Figure 10a also illustrates that the best (minimum lack of uniformity) and the worst (maximum lack of uniformity) distributions occur when liquid saturation is at its maximum and minimum values, respectively. In gas cyclic operation, a liquid flow with constant Ul is maintained enduringly. The gas flow, facing a resistance from the liquid, spreads out this latter in axial and radial directions accomplishing a more uniform liquid distribution as revealed via ECT in the specified area near the bottom of Figure 10b. When the resistance due to extra liquid ceases, the gas naturally migrates nearby the wall region. Therefore, the wall area shows lower liquid saturation values. The dashed line shows the moment when the gas is turned OFF. Immediately afterward the liquid-poor areas start to grow (Figure 10b) while the liquid

flow having already “dredged” its preferential region will continue to go through it. Here also it is shown that high liquid saturation improves distribution, whereas low liquid saturation degrades it (Figure 10b). When the amount of liquid held in the bed locally exceeds the maximum allowed liquid holdup conveyable under trickle flow regime, the excess liquid is transported via a different mechanism, that is, through the emergence of pulses in the pulse flow regime. In gas/liquid alternating mode, both gas and liquid actual velocities (Ugp, Ulp) are larger compared to the other two cyclic modes. Therefore, depending on cycle frequency, the amount of liquid to build up during the liquid ON portion could reach a level that subsequent resumption of gas flow during the gas ON portion results in pulse flow regime. Figure 10c shows for the selected operating conditions that a passage of a pulse has been detected by ECT. The pulse is brief and covers the whole bed cross-section, whereas the remainder of liquid past the pulse evolves in trickle flow regime. Since Ug is relatively large and the remainder of liquid is sizable, a uniform two-phase distribution is observed in the bed between

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Figure 9. (a) Definition of deviation indices; (b) variation of deviation indices of ON-OFF liquid cyclic operation as a function of cycle frequency and gas velocity, H ) 145 cm, R ) 0.5.

the passage of pulse and the dashed line, that is, the moment that gas flow is disabled (Figure 10c). The region alongside the walls reveals lower liquid saturation values due to the uneven radial gas flow distribution discussed above (gas ON portion). The subsequent liquid ON portion is able to spread almost uniformly the liquid flow across the bed. The gas/liquid alternating mode can generate pulses along the bed to ensure better approach to uniform liquid distribution during the liquid and gas portions of the cycle. Figure 10c shows that in both gas and liquid sequences, the degree of uniformity remains almost constant which proves the ability of this strategy to establish good distribution in the bed. Pixel-based pulsation propagation velocity (32 × 32 pixels) along the bed is a function of cyclic operation mode, bed length, and operating conditions. The double-plane ECT proves particularly powerful in capturing these pixel-based pulsation propagation velocities. They are computed by dividing the double-plane distance (50 mm) by the pulsation time-of-flight. This pulsation time-of-flight is computed as the maximum crosscorrelation time between the two liquid saturation time-series for the two pixels sitting, correspondingly, on the upper (x,y,z) and lower (x,y,z + 5 cm) planes of the ECT. The crosscorrelation function allows locating the time required for a liquid holdup disturbance to travel from the upper to the lower ECT plane. The assumption for performing such a determination is that such a disturbance does not have the time, while migrating over the short distance between the two electrode planes, to exchange mass with its environment, neither axially nor radially (negligible dispersions). Figure 11 panels a1-c2 illustrate such pulsation propagation velocity maps for different cyclic modes (barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.154 m/s; cycle frequency, 0.056 Hz). The pulsation velocities are expressed in mm/s and each cyclic strategy has its own velocity color bar range. Figure 11 panels a1,a2 show the velocity maps obtained, respectively, at H ) 10 cm and H ) 145 cm during ON-OFF liquid cyclic operation. A quite uniform pulsation velocity distribution is observed near the top of the bed where the spray nozzle appears to distribute the liquid in a relatively uniform manner (Figure 11a1) under constant Ug. Deeper in the bed

where distributor memory effects have long been erased, as shown in Figure 10a above, more liquid holdup is present corewise in the bed mirroring more gas holdup to flow faster nearby the higher-porosity wall area. Therefore, higher liquid velocity spots are observed near the wall (Figure 11a2). Figure 11 panels b1,b2 show the velocity maps obtained, respectively, at H ) 10 cm and H ) 145 cm bed heights under ON-OFF gas cyclic operation. Though the nozzle distributes the liquid flow in a relatively uniform manner on the bed top, due to the fact that the (constant) Ul is low, some areas could receive lower liquid holdup because of fortuitous preferential paths. Hence, Figure 11b1 velocity map reveals few spots of higher liquid velocity, recognized as having lower liquid holdup, that are associated with high-velocity gas flow in the wall region. Liquid preferential paths can develop along the bed as mentioned earlier. Despite the fact that the gas flow during the ON portions improves liquid radial distribution, shortly after, the trend deteriorates and poor radial distribution reinstalls anew. Figure 11b confirms that part of the bed cross-section experiences higher velocities almost in a segregated manner in accordance with the structure depicted in Figure 10b. These are the lower liquid containing spots which offer lesser resistance to flow. Figure 11panels c1,c2 show the velocity maps obtained at the same bed elevations in gas/liquid alternating mode. Since both liquid and gas velocities are large, resumption of gas flow after the liquid ON portion will result in a high interaction regime due to the forcing gas flow. The velocity pattern nearby the bed top (Figure 11c1) is more or less uniform with some high liquid pulsation velocity incursions close to the wall presumably due to the higher local porosity and lesser resistance. Under the chosen gas and liquid velocities, pulse flow regime develops along the bed as detected with ECT in Figure 10c. The velocity map at the bed bottom seems nearly uniform; albeit some areas of somehow lower or higher pulsation velocities manifest themselves, which could be attributed to some jetting phenomena generated by the passage of pulses. The cross-sectionally averaged velocities of cyclic operations are shown in Figure 12a as a function of bed height (barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.154 m/s; cycle frequency, 0.056 Hz). No velocity change is observed


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Figure 10. ECT tomographic plot, liquid saturation, and degree of uniformity of different cyclic operations: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.154 m/s; cycle frequency ) 0.056 Hz; H ) 145 cm.

under ON-OFF liquid cyclic operation along the bed. ON-OFF gas cyclic operation reveals small increase of velocity as a function of bed height. As mentioned above, pulse flow regime occurs under gas/liquid alternating mode under the selected operating conditions. A certain bed length is necessary for these pulses to form which is also in agreement with the formation of natural pulse flow.30 After pulse flow regime generation; the pulsation velocity remains practically constant along the bed remaining portion. The increase of pulsation velocity along the bed for ON-OFF gas (though tepid) and gas/liquid cyclic operation suggests that it is difficult for the gas phase to dominate the liquidbed friction on the bed top. Then the pulsation starts to accelerate along the bed. Consequently, it becomes more difficult for the gas phase to move larger liquid holdups. This

concept can be verified in Figure 12b where the pulsation velocity near the bed top (10 cm) is plotted as function of cycle frequency for different cyclic operations. In ON-OFF liquid cyclic strategy the pulsation velocity is independent of cycle frequency because a constant gas flow rate is present through the bed. In the case of ON-OFF gas and gas/liquid alternating cyclic operations, the pulsation velocity is an increasing function of frequency. At higher frequency, the liquid ON portion being increasingly brief, the gas phase faces lower resistance so that the pulsation velocity rises. 3.1.5. Slow Mode. The slow mode cyclic operation has been defined as a procedure in which an externally imposed hydrodynamic disturbance (i.e., propagating pulse) should leave the bed before resumption of the subsequent disturbance.14 The

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Figure 11. Pulsation propagation velocity map (in mm/s) for different cyclic operations: (a1,2) ON-OFF liquid, (b1,2) ON-OFF gas, (c1,2) gas/liquid alternating. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.154 m/s; cycle frequency ) 0.056 Hz; H ) 10, 145 cm; R ) 0.5.

ON-OFF liquid slow mode cyclic operation is known as a very useful means to increase chemical yields of gas-limited catalytic reactions by minimizing the liquid resistance from the catalyst surface. Inception of pulse flow under liquid cyclic operation has been claimed to achieve an enlargement of the pulse flow regime domain because the barycentric liquid and gas superficial velocities would lie in the trickle flow regime range.8,13,14 According to Aydin and Larachi,15 these pulses are observed simply because the imposed peak liquid velocities exceed the trickle-to-pulse flow transition isoflow liquid throughput. Figure 10c above showed that gas/liquid alternating fast mode indeed generates pulse flow regime despite the fact that gas and liquid actual superficial velocities are captive of the trickle flow regime domain. Examination of different combinations of gas and liquid velocities shows that the penetration depth of pulses is a function of cycle period and split ratio. The aim of this section is to show that even by applying very low velocities belonging to the trickle flow regime domain in slow mode operation, it could be possible to induce a stable pulse flow regime. Figure 13a-c illustrates instantaneous liquid holdup time-series for ON-OFF liquid, ON-OFF gas, and gas/liquid alternating modes, respectively. The barycentric liquid and gas velocities are Ul ) 0.001 m/s and Ug ) 0.062 m/s using 100 s cycle times with symmetrical splits, R ) 0.5. The measurements were obtained for H ) 145 cm. Figure 13a shows that in slow ON-OFF liquid cyclic operation, liquid holdup plateaus in trickle flow regime during the liquid ON portion followed by its draining in the liquid OFF portion. No pulse is detected as expected. In the slow ON-OFF gas cyclic (Figure 13b), liquid flows discontinu-

ously along the bed. The entering gas phase can remove some extra liquid; however, the gas-liquid interaction is insufficient to bring pulse flow. Figure 13c reveals that alternating gas and liquid flows in slow mode with oscillating Ug and Ul, respectively, between 0 and 2 × 0.062 m/s, and 0 and 2 × 0.001 m/s, intensifies the interactions to an extent that induced pulses are generated. Gas/liquid alternating mode truly combines pulse and trickle flow features while exhibiting fast liquid draining dynamics in which the mass-transfer resistance of liquid layer from the catalyst surface can be minimized to boost even further the catalytic performances of gas-limited reactions. This contention could indeed be tested experimentally in future works. 3.2. Filtration Experiments. For trickle beds experiencing deposition and plugging, the goal behind implementing cyclic operation strategies would be to generate instabilities inside the bed to counter the deposition of fines by controlling the deposits growth kinetics. It was shown elsewhere that overall bed deposition can be reduced and the level of pressure drop can be controlled provided selection of adequate cyclic parameters is made.17 ON-OFF gas and gas/liquid alternating cyclic operations showed indeed improved mitigation of deposition. However, if the local structure and mechanism of deposition inside the bed was studied using ECT, it only concerned constant-throughput experiments.25 ECT imaging has not been implemented, to the best of the authors’ knowledge, to make sense of the deposition dynamics and patterns under cyclic operation. Noninvasive ECT time-resolved interrogation of local bed scale permittivities, liquid saturations, and pulsation velocities is expected to unveil valuable information regarding the comparative performances of the three cyclic strategies being investigated in this study. As constant-flux two-phase flow is nonexistent under cyclic operation and the operating conditions are different for each strategy, one unique zero reference was assigned to the drained prewetted clean bed as in the above deposition-free experiments. Further increase in permittivity was attributed to the accumulation of fine particles in the bed. Another difficulty stemming in ECT calibration was to specify a valid 100% reference to reconstruct quantitatiVe fines holdup fields. Setting such reference was elusive as explained elsewhere.25 The flooded bed state with fines-free kerosene (and no air flow) was used instead, and the discussion focused on the qualitative deposition structure. However, by using one single scale it would be possible to compare permittivity variations for all the experiments. Figure 14a-d shows the variation of normalized permittivity for isoflow, ON-OFF liquid, ON-OFF gas, and gas/liquid alternating cyclic operations (barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.062 m/s; cycle frequency, 0.056 Hz), respectively. The normalized permittivities averaged on the ECT plane are shown in Figure 14e and the measured overall bed specific deposits are, correspondingly, depicted in Figure 14f. As explained elsewhere,23,25 deposition begins near the bed entrance and then expands progressively downstream in the bed as usual in depth filters. Therefore, the bed entrance layer determines to a large extent the initial increase of pressure drop. Hence ECT measurements were performed at H ) 15 cm. The normalized permittivity values when filtration was just initiated (t ) 0 h) correspond to the liquid saturation in the two-phase flow clean bed state and further increase in permittivity is unambiguously attributed to deposition of kaolin particles. The increase of permittivity is fast at the beginning of filtration under isoflow conditions (Figure 14a). Details about this deposition process were already discussed elsewhere and are skipped here.23,25 The rise of base and peak permittivities


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Figure 12. (a) Average pulsation propagation velocity for different cyclic operations as a function of bed height: cycle frequency ) 0.056 Hz. (b) Average pulsation propagation velocity of different cyclic operations as a function of cycle frequency: H ) 10 cm. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.154 m/s; R ) 0.5.

during the course of ON-OFF liquid mode (Figure 14b) looks similar to the isoflow case. During τl intervals, Ulp is double the Ul value in isoflow, thus deposition at Ulp is more prominent in addition to fast liquid modulation which would compress further the deposition layer. The increase in average normalized permittivity in liquid modulation is fast (Figure 14e, filled circles) and may even surpass that in isoflow. In addition, overall specific deposit measurements (Figure 14f, filled circles) reveal that ON-OFF liquid mode results in the highest overall specific deposits. It can be concluded that this mode is inadequate as a deposition mitigating method. The variation of permittivity in the course of ON-OFF gas mode is illustrated in Figure 14c. The corresponding average values in Figure 14e (filled squares) show that after 3 h the rate change of normalized permittivity slows down significantly yielding normalized permittivity that are lesser than for isoflow conditions. This is reinforced by the measured overall specific deposits which are also lesser in ON-OFF gas mode (filled squares, Figure 14f). Figure 14c also shows that jitters in permittivity, triggered by the sudden passages of suspension; appear during filtration. The reduction of porosity mirrored by deposition is likely to trigger pulse flow to which the jitters are linked. The instantaneous normalized permittivity time-series for gas/ liquid alternating mode under filtration is shown in Figure 14d. The corresponding average values in Figure 14e (filled triangles) show an almost similar trend with regards to the ON-OFF gas mode, except that in the beginning, because of exposure to higher incoming fines flux in alternating mode, the permittivity slope is slightly steeper. The overall specific deposit values in this case (filled triangle, Figure 14f) show a significant reduction of deposition along the bed. Visual observations reveal that in the relatively tall bed being tested, pulses (i.e., pulse flow) generated in gas modulation start to form from the bed top but fade away before reaching the bed exit, whereas those generated in gas/liquid alternating mode need some depth to develop but remain stable and influential through the remainder of the bed, as previously discussed for clean flow. The detached fines are

successfully expelled from the bed due to the frequent passages of pulses; thus, lesser deposition occurs under gas/liquid alternating. The developing deposition patterns obtained from ECT under isoflow, ON-OFF liquid, ON-OFF gas, and gas/liquid alternating modes are illustrated in Figure 15 panels a-c, respectively. These ECT images were taken at the end of each run after the liquid was drained to emphasize only the deposits fingerprint in the bed. The configurations of deposition in the case of isoflow and ON-OFF liquid cyclic operation (Figure 15a,b) are very similar. As a matter of fact, liquid modulation is inefficient at washing-out kaolin deposited fines and reveals the same structure as in isoflow. On the other hand, ON-OFF gas and gas/liquid alternating strategies share similar deposition patterns (Figure 15c,d) and show slight kaolin accumulations in the middle area. Locally, gas modulation yields somehow lesser deposition, in agreement with the average permittivity values in Figure 14e that could be ascribed to pulse formation near the bed top. However, its overall bed specific deposit is higher presumably because of die-off of pulses which cannot propagate along the whole bed to expel the fines re-entrained upstream. Figure 14b shows that the morphological features of propagating liquid pulsations in the course of filtration are altered in ON-OFF liquid cyclic operation. The breakthrough, plateau, and decay times are shown in Figure 16a. τB and τD decrease during the run and τP increases. Because of porosity reduction, pulsation propagation velocity increases causing shorter τB. Similarly, the decay time reduces resulting in durable plateau segments. The evolution of the corresponding pulsation propagation velocity is shown in Figure 16b (filled circles). As time elapses, pulsation propagation velocity accelerates until ca. 4 h and then almost plateaus in accordance with the trend of local values of permittivity (Figure 14e, filled circles). The local deposition pattern has reached a quasi-steady state configuration with minor subsequent changes. A statistical parameter measuring the degree of uniformity, χU, of pulsation propagation velocities has been determined based on pixel-based permittivities on the two ECT planes as a

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Figure 13. Instantaneous liquid holdup signals for different cyclic strategies: (a) ON-OFF liquid, (b) ON-OFF gas, (c) gas/liquid alternating. Barycentric velocities, Ul ) 0.001 m/s, Ug ) 0.062 m/s; cycle time ) 100 s; R ) 0.5; H ) 145 cm.

quality criterion for liquid Velocity distribution.25,31,32 For cyclic operation, as shown in Figure 11, it is possible to compute pulsation propagation velocities based on the time lag between permittivity signals from the two ECT planes. A degree of uniformity can be defined to determine the evolution of velocity uniformity in the course of filtration under cyclic operation: χU )

1 n

n

∑ i)1

(

j Ui - U j U

)

2

(9)

where, n denotes the number of pixels per image (32 × 32), Ui j are the local and the cross-sectionally averaged pulsation and U propagation velocity, respectively. Hence, χU approaches zero as uniformity in velocity distribution improves. Figure 16b shows that in the clean bed state (t ) 0 h, filled circles), liquid cyclic mode achieves quite a uniform velocity

distribution at H ) 15 cm. Subsequently during deposition, pulsation propagation velocities increase but the uniformity of their distribution deteriorates. Maldistribution keeps deteriorating up until the deposition pattern plateaus after 4 h. The variation of pulsation propagation velocity and degree of uniformity under simultaneous gas/liquid alternating mode and filtration are also shown in Figure 16b. Here on the contrary, deposition affects neither the pulsation propagation velocity nor the uniformity of the pulsation propagation velocity distribution. In ON-OFF gas cyclic operation at same barycentric gas and liquid velocities, liquid accumulation is less significant and therefore deposition can help raise liquid holdup, through increased pressure drop, to the level of pulse flow formation on the bed top. However, these pulses are not prevailing enough to travel along the whole bed. Figure 16c shows the evolution of pulsation propagation velocity and degree of uniformity for


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Figure 14. Time evolution of normalized permittivity under (a) isoflow, (b) ON-OFF liquid, (c) ON-OFF gas, (d) gas/liquid alternating, (e) average normalized permittivity for different cyclic operations as a function of time, (f) specific deposit versus time for isoflow and cyclic strategies. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.062 m/s; cycle frequency ) 0.056 Hz; H ) 15 cm.

ON-OFF gas cyclic operation during the course of filtration. The occurrence of deposition near the bed top intensifies phase interactions and gives rise to pulse flow regime. Consequently, the pulsation propagation velocity increases and approaches a constant value as the deposition pattern plateaus in turn. Turbulence at the onset of pulse flow significantly affects the uniform distribution. Increased deposition promotes an upsurge

in two-phase interactions leading to a more prevailing pulse flow pattern mirroring with faster pulsation propagation. These in turn will spread over the bed cross-section, and the velocity pattern becomes more uniform. Computed pulsation propagation velocity maps under filtration and different cyclic operations are shown in Figure 17. It should be noted that each row (from 1 to 5), representing a

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Figure 15. ECT normalized permittivity images after the filtration run: (a) isoflow, (b) ON-OFF liquid, (c) ON-OFF gas, (d) gas/liquid alternating. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.062 m/s; cycle frequency ) 0.056 Hz, H ) 15 cm.

Figure 16. (a) Time evolution of morphological parameters under simultaneous ON-OFF liquid cyclic operation and filtration, (b) pulsation propagation velocity and degree of uniformity in ON-OFF liquid and gas/liquid alternating cyclic operation versus time, (c) time evolution of pulsation propagation velocity and degree of uniformity in ON-OFF gas cyclic operation. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.062 m/s; cycle frequency ) 0.056 Hz; H ) 15 cm.

cyclic mode, has its own color-bar scale. The pulsation propagation velocity maps are obtained for the first 4 h of experiments, after that the changes are insignificant. As time elapses (Figure 17a1-5) in ON-OFF liquid cyclic mode, the

pulsation propagation velocity generally increases while developing a low-velocity region in the center. The deposition layer in the middle (see Figure 15b) reduces indeed the accessibility of the incoming suspension flow (liquid ON portion) redirecting


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Figure 17. Pulsation propagation velocity maps (in mm/s) in the course of filtration: (a1-5) ON-OFF liquid, (b1-5) ON-OFF gas, (c1-5) gas/liquid alternating. Barycentric velocities, Ul ) 0.0013 m/s, Ug ) 0.062 m/s; cycle frequency ) 0.056 Hz; H ) 15 cm; time interval ) 1 h.

it to the immediately adjacent areas with still higher permeability. Therefore, high velocity spots are observed right beside the low velocity region in the center (Figure 17a5). In ON-OFF gas cyclic mode, porosity reductions augment gas-liquid interactions so that resuming gas flow promotes the carriage of liquid in the form of pulses. The bed core undergoes more deposition and directs the liquid outwardly. The gas flows through areas offering lesser hydraulic resistance, mainly in the middle where the highest pulsation propagation velocities take place. Pulses could vanish at depths where the bed permeability has not been reduced by the depositing fines and the gas-liquid interaction diminishes again. The pulsation propagation velocity pattern becomes more uniform as pulse formation expands radially due to radial enlargement of deposition. In gas/liquid alternating cyclic mode also two-phase interactions increase due to deposition. However, for the same reasons explained above, the incoming gas flow requires longer distances to generate pulse flow. Thus the velocity pattern on the top remains almost unchanged in the course of filtration experiment (Figure 17c1-5). 4. Conclusion Electrical capacitance tomography (ECT) was used to study the hydrodynamic behavior of ON-OFF liquid, ON-OFF gas, and gas/liquid alternating cyclic operations in a trickle-bed reactor. Using the ECT data, the mean liquid holdup values, pulsation intensity, and propagation velocity were calculated for each cyclic operation. Additionally, the morphological features of liquid pulsations (breakthrough, plateau, and decay times) were estimated. Each cyclic operation was applied to a trickle bed experiencing filtration to examine the local evolution of deposition and benefits/drawbacks of each approach. The following remarks could be drawn from this study: (i) Gas periodicity helps liquid slugs preserve their content. Therefore, ON-OFF gas and gas/liquid alternating cyclic strategies resulted in long-lived pulsations.

(ii) The three cyclic operations produce comparable mean liquid holdup; however, larger actual gas superficial velocity (gas ON portion) and lower actual liquid velocity yield somehow lower values in ON-OFF gas cyclic mode. (iii) Because of higher gas and liquid actual velocities (Ulp, Ugp) and the alternated nature of feeds, a more uniform distribution of phases was observed in gas/liquid alternating cyclic mode. (iv) Using superficial velocities falling in the trickle flow range, gas/liquid alternating mode was able to initiate pulse flow regime in fast mode. The combination of pulse flow (high mass transfer), trickle flow (high residence time), and reduction of liquid phase resistance (liquid OFF portions) could be explored further as a new process intensification in TBR. (v) Gas/liquid alternating cyclic operation achieved the closest shapes to ideal square pulses because of the shortest breakthrough and decay times and the longest plateau time. (vi) Under filtration conditions, ON-OFF gas and gas/liquid cyclic operations can successfully reduce the extent of deposition on the bed top; however, the overall specific deposit values reveal that gas/liquid alternating mode is more efficient in tall beds due to the prevalence of the generated pulses which sweep the whole bed length. (vii) In ON-OFF liquid cyclic mode, preferential deposition in the bed core redirects liquid toward the periphery and generates regions of low and high pulsation velocities. In ON-OFF gas cyclic mode, the two-phase interaction increases to the point of pulse flow regime due to porosity reduction. The high turbulence at the place of pulse generation reduces flow uniformity. Acknowledgment Financial supports from the Natural Sciences and Engineering Research Council of Canada and Natural Resources Canada is gratefully acknowledged.

952


Nomenclature e ) porosity el ) liquid holdup I ) pulsation intensity, unitless R ) split ratio, unitless U ) superficial velocity, m/s ) time average Greek Letters β ) liquid saturation ε ) electrical permittivity, F/m τ ) time, s χ ) degree of uniformity, unitless σ ) standard deviation Superscripts 0 ) drained state of the bed 1 ) flooded state of the bed fd ) free-draining res ) residual Subscripts b ) base B ) breakthrough D ) decay g ) gas l ) liquid p ) peak P ) plateau

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ReceiVed for reView April 15, 2009 ReVised manuscript receiVed June 15, 2009 Accepted June 30, 2009 IE900605B

Cyclic Operation Strategies in Trickle Beds and ... - ACS Publications

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