Monitoring Filtration in Trickle Beds Using Electrical Capacitance

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Ind. Eng. Chem. Res. 2009, 48, 1140–1153

Monitoring Filtration in Trickle Beds Using Electrical Capacitance Tomography Mohsen Hamidipour,† Faı¨çal Larachi,*,† and Zbigniew Ring‡ Department of Chemical Engineering, LaVal UniVersity, Que´bec, QC, Canada G1K 7P4, and BP, Residue Hydrocracking, 150 West WarrenVille Road, NaperVille, Illinois 60563

Experiments were carried out to monitor the evolution of the deposition of fine particles in trickle-bed reactors during the flow of nonpolar hydrocarbon oil-like liquid suspensions using electrical capacitance tomography (ECT) imaging. The accuracy of the ECT rendition was validated in the pristine (i.e., deposit-free) bed state by comparing the liquid holdup measurements from ECT with the liquid holdup from residence time distribution (RTD) measurements. The pulse-flow characteristics (pulse velocity and frequency) estimated from the ECT signals were in agreement with existing literature data. For filtration experiments, the effects of the initial liquid suspension distribution, the gas and liquid superficial velocities, and single-phase flow (i.e., zero gas velocity) on the structure of the deposition in the bed were studied. ECT imaging successfully tracked the unsteady-state progression of bed plugging throughout the trickle bed. It was found that increasing the liquid or gas superficial velocity resulted in increased local deposition. The transition, due to deposition, from trickle to pulse flow was also determined from ECT. In the case of stagnant gas, a filter cake formed on top of the bed. 1. Introduction Until recently, modern living standards have relied on cheap and abundant fossil energy. This might be incompatible with the breakneck escalation in energy demand resulting from current world demography and increasing economic development. Accelerating growth and the proliferation of powerintensive machinery, the auto and manufacturing sectors, towering urbanization, and low energy efficiency could lead to widespread energy shortages. Despite technological achievements in oil reservoir discovery, Kuwait′s large oil field, Burgan, which provides 3% of world demands, was found in the late 1930s, and the discovery of Ghawar, the world′s largest conventional oil field in Saudi Arabia, which still supplies 5% of the world′s oil, dates back to 1948.1 Future trends regarding conventional oil indicate that the discovery of new oil fields has been decreasing. Eventually, supplies of easy-to-access oil and gas are foreseen to no longer be able to keep up with demand. North America is especially exposed to a lack of energy because it has only a small fraction of the world′s oil and natural gas. Therefore, Canadian tar sands or bitumen, one of the world′s largest unconventional (or extra-heavy) oil resources, could become an important source of energy, particularly in North America. The molasses-like viscous oil extracted from oil sands, before being turned into sweet bitumen-derived fuels, requires more steps and processing to reach the market than conventional oil. One of the pretreatment steps carried out in hydrotreating tricklebed reactors (TBRs) concerns the sulfur content of the fractions, which should be significantly decreased to satisfy sulfur regulation standards. In this regard, hydrodesulfurization is performed in TBRs where hydrogen and bitumen-derived heavy gas oil streams are contacted cocurrently downward. Some of the fine particles, percolating into the mineral processing units upstream or escaping from the cokers, carry over to the trickle beds because, being so fine, they cannot be completely stopped by the upstream filters. Hence, their intrusion into the catalytic * To whom correspondence should be addressed. Tel.: +1 (418) 6563566. Fax: +1 (418) 656-5993. E-mail: [email protected]. † Laval University. ‡ BP.

beds can disrupt stable operation of the reactor in the medium and long term. Specifically, the deposition of fine particles in the catalytic beds reduces bed porosity and results in extra hydraulic resistances on the two-phase flow, translating into increased bed pressure drops. Eventually, the risk of physical exposure to pressure would go beyond a critical level, forcing unit shutdown and catalyst replacement independently of the chemical activity left in the plugged catalyst bed. Systematic research on trickle-bed plugging due to the deposition of fines was initiated by Gray’s group2 at the University of Alberta, who monitored the time evolution of deposition through increasing pressure drops. We recently reported some results on the effect of gas flow on the extent of deposition by raising the gas superficial velocity.3 A decreasing trend of specific deposition (i.e., mass of deposited fines per unit vessel volume) was found, and a correlation was established to relate the filter coefficient to the liquid holdup in trickle beds. Later, an operating window was identified on the gas versus liquid superficial velocity map specifying the conditions under which a changeover from trickle flow to pulse flow is possible during the course of filtration.4 Plateaus of pressure drops under severe plugging conditions were ascribed to fluids shortcircuiting the wall region, which consisted of deposition islands and plug-free corridors. Various cyclic operation strategies5 and monolithic reactor configurations6 were also tested as potential options for minimizing the deposition in fixed-bed reactors. Table 1. Chemical Composition of RTD Tracer (SR 1795) component

composition (wt %)

toluene dodecylbenzene sulfonic acid kerosene solvent naphtha polysulfone amine polymer

10-20 2-8 60-70 2-7 2-7 2-8

Table 2. Comparison between Physical Properties of Pure Kerosene and RTD Tracer property

kerosene

tracer

density (kg/m3) viscosity (mPa s) surface tension (mN/m)

789 1.05 25.32

798 1.11 25.28

10.1021/ie800810t CCC: $40.75  2009 American Chemical Society Published on Web 10/10/2008

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1141

Figure 1. (a) Example of experimental inlet and outlet electrical conductance response curves along with the fit of the outlet response using a two-parameter ADM RTD model (Ul ) 0.0026 m/s, Ug ) 0.185 m/s). (b) Parity plot of liquid holdups obtained with the ECT and RTD methods (Ul ) 0.0017, 0.0026 m/s; Ug ) 0.062, 0.0123, 0.185 m/s).

The past mechanistic modeling efforts in two-phase flow filtration focusing on the microscopic level showed limited success in relating some simulated local quantities, such as local specific deposits, to macroscale measurements, such as bed pressure gradients and overall specific deposition.7 Filtration effects in trickle-bed reactors, such as local deposition rates of fines, are difficult to measure without altering the process. To date, the overall trend of deposition has been assessed by measuring the pressure drop and the particle concentrations in the suspension streams entering and leaving the reactor, as well as through visual observations through the wall of the reactor. Powerful noninvasive imaging techniques, such as tomography, would enable the visualization of the fines accumulation deep inside porous media. Process tomography is a technique that involves the use of imaging methods to obtain detailed topographical information over thin slices deep in the catalyst bed at locations that are otherwise inaccessible. Electrical capacitance tomography (ECT) is a powerful tomography technique that, in addition to being fast and suitable for small and large vessels,8,9 enables measurements with electrically nonconducting nonpolar hydrocarbon oil-like liquid suspensions, such as kerosene suspensions, and air as the gas phase. With the help of suitable reconstruction algorithms, ECT measurements enable reconstruction of the permittivity distribution and, hence, the voxel distribution of materials present in the sensed cross section of the catalyst bed. The high time resolution (100 image frames per second) of ECT provides the ability to characterize the pulse-flow regime, which could arise during the course of filtration due to the intensification of the interactions between gas and liquid phases. Electrical capacitance tomography has been already used in trickle beds to investigate pulse-flow-regime mechanisms.10-13 Although the passage of pulses appears one-dimensional from wall observations, it was shown that they indeed are threedimensional in nature. The increase of gas velocity produces pulses with more round-shaped fronts and flatter rears. The aim of this research was to further unveil the hydrodynamic behavior of trickle beds undergoing plugging by systematically exploring the length of the bed under gas-liquid

filtration using ECT imaging. The effects of different gas and liquid superficial velocities on the structure of deposition along the bed were examined, and the occurrence of maldistribution was monitored. Analysis of the deposition behavior was paralleled by monitoring of the pressure drop and the buildup of specific deposition. Time series of captured permittivity images were studied to extract useful information on the flow regime transition. 2. Experimental Section Single-pass experiments were carried out at room temperature and atmospheric pressure using a 5.7-cm-i.d. Plexiglas column packed up to 160-cm bed heights with 2.7-mm spherical γ-alumina catalyst particles (bed porosity, ) 0.4). The packing was maintained by means of a rigid stainless steel screen placed at the column bottom. The screen′s mesh opening (0.65 × 1.3 mm2 rhombic shape) was set to prevent the packing elements from passing and to barely block the exiting flow of suspension. Kaolin-kerosene suspensions and air were the test fluids. Kaolin, as a major component of the clay in Athabasca oil sand,7 was added to kerosene, which was used as a model hydrocarbon liquid.10 Kaolin powder was heated to remove any moisture before being added to the kerosene. The direct addition of kaolin to kerosene with no further preparation would result in suspensions with fines typically several hundred microns in size. Therefore, kaolin suspensions were preventively sonicated in an ultrasonic cleaner before addition to kerosene to keep their sizes at ca. 8 µm.4 Prior to experiments, the catalyst was immersed in kerosene overnight to achieve full wetting of the bed and liquid imbibition of the intraparticle porosity. The suspension was stirred in a feed tank, pumped to the top of the column, and distributed through a spray nozzle to ensure even distribution of the suspension. Because the nozzle opening was ca. 0.5 mm, special attention was paid to prevent the introduction of solid impurities into the feed tank, which would easily block the nozzle. The inlet concentration of fines was 1 g/L in all experiments. Thorough mixing and a conically shaped reservoir bottom

1142 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

Figure 2. Schematic view of different liquid distribution conditions with the corresponding ECT normalized permittivity images and effect of gas superficial velocity on radial liquid distribution: (a) uniform liquid distribution, (b) distant spray nozzle with respect to bed uppermost layer, (c) point distribution.

prevented deposition in the feed tank. The outlet concentration was measured by sampling at the bed exit at regular time intervals using turbidimetry-concentration calibrations for the range of experiments. Air entered through several small holes on the top of the bed. The pressure drop across the whole bed was recorded every 30 s. The experiments were conducted at superficial velocities of air and suspension coinciding with the trickle-flow regime in clean bed tests. After each experiment, the column was dismantled, and the particles were carefully washed with water and dried overnight before subsequent use. The amount of deposit in the bed is expressed in terms of the specific deposit, σ, defined as the mass of kaolin particles trapped per unit vessel volume (empty-reactor basis). In singlepass mode, the bed specific deposit is calculated from a mass balance around the bed σ(t) )

∫

t

0

Ql[CIn(t) - COut(t)] dt VR

(1)

where Ql is the liquid volumetric flow rate; CIn and COut are the fines concentrations in the suspension at the inlet and outlet, respectively; and VR is the reactor volume. The equipment used for tomography measurements was a PTL300E instrument consisting of a twin-plane 12-electrodeper-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 suspensions as tested in our study. The two rows of measurement electrodes were 50 mm in height with 38-mm-high guard electrodes placed immediately above and below. The role of the guard electrodes was to adjust the electric field measured by the active electrodes to a horizontal section of the reactor. There were 66 combinations of independent capacitance measurements between electrode pairs of each plane. The 24 electrodes were mounted peripherally on the trickle bed, so that the ECT system allowed noninvasive visualization of permittivity at different cross-sectional and longitudinal positions


controlled three-way valve to control the period of tracer injections. The two electrical conductivity probes were installed 10 cm below the top of the bed and 10 cm above the bed bottom. 3. Results and Discussion

Figure 3. Effect of gas superficial velocity on the distribution of liquid along the bed.

of electrodes at a sampling frequency of up to 100 Hz. Details about the ECT method, the calculation procedure, and the application of ECT can be found elsewhere.14,15 The measured capacitances were used to reconstruct the distribution of permittivity and, hence, the material distribution per pixel (32 × 32 pixels per image), in particular, that of the fines that accumulate in the bed as a result of deposition. The tomography images obtained at different positions allowed for the detection of differences in the deposition profiles that might vary with the height of the catalyst bed. In an earlier study,16 different reconstruction algorithms were reviewed, and simulations were performed to compare their capability to create a qualitative (shape) and quantitative (size) image of an assumed object. The Tikhonov algorithm was found to be one of the most successful approaches. Hence, this method was chosen to generate normalized permittivity images from the measured capacitance data. The tracer-injection method was performed in an 80-cmhigh Plexiglas column packed with 2-mm glass beads to study the accuracy of global liquid holdup measured with ECT (Tikhonov reconstruction algorithm) and the residence time distribution (RTD) method using electrical conductance probes. Kerosene and air were the feeding fluids. Because kerosene is a nonpolar liquid, selection of the tracer to increase its electrical conductivity was a nontrivial task. To the best of our knowledge, this is the first time that the conductivity method has been applied for nonpolar hydrocarbon oil-like liquids to obtain residence time distributions. The composition of the tracer (SR 1795) obtained from Dorf Ketal Chemicals LLC (Stafford, TX) is listed in Table 1. It is a conductivity improver that is added to the fuels to reduce the risk of building up electrostatic charge, especially during transportation. It contains originally 60-70 wt % kerosene and is soluble in kerosene in all proportions. The characteristics of a tracer should be very similar to those of the main liquid flow in order to reliably mimic the liquid residence time in the reactor. Therefore, the original tracer was diluted by a factor of 6 in kerosene. Table 2 reveals very slight differences between the properties of the tracer and kerosene. A separate reservoir was used for the tracer that was connected to the pump inlet by an automated computer-

3.1. Clean Bed Experiments. In the clean bed, calibration for tomography measurements was performed for a drained prewetted bed as the 0% reference point (blue) and a bed flooded with kerosene as the 100% reference point (red) to compute normalized permittivities. Intermediate permittivity values appear with different green-colored intensities. During the two-phase flow measurements, the normalized permittivity measurements provide access to the volume fraction of liquid in the empty space of the bed, i.e., liquid saturation. Provided that the local porosity law in the bed is known, the associated local liquid holdup can be estimated. Because the distribution of void space in the bed could not be determined experimentally, when we refer to the pixel values, we are reporting the local liquid saturation instead of the local liquid holdup. To calculate the global liquid holdup, the mean of volumeaveraged cross-sectional values (measured over a height of 5 cm, i.e., the electrode height) of saturation were used in conjunction with the bed-average porosity ( ) 0.373) for 2-mm glass beads. Six measurements of liquid holdup were obtained for liquid superficial velocities of Ul ) 0.0017 and 0.0026 m/s and and gas superficial velocities of Ug ) 0.062, 0.123, and 0.185 m/s, corresponding to the trickle-flow regime. To determine the liquid holdup using the residence time distribution (RTD), the Aris double-detection tracer response method was implemented.17 The RTD curves were acquired using the imperfect-pulse Aris method in which the inlet and outlet tracer response conductivity signals are used to fit a two-parameter impulse-response RTD model. The plug flow with axial dispersion model (ADM) was used to describe liquid backmixing. The space time (τ) and the axial-dispersion Pećlet number (Pe) were determined using a nonlinear leastsquares fitting and employing the convolution method for a time-domain analysis of the nonideal pulse tracer response data.17 Liquid holdup (l) was then calculated from the space time according to the equation εl )

τQl VR

(2)

where Ql is the liquid volumetric flow rate and VR is the reactor volume. Figure 1a shows an example of the agreement between the measured and ADM-predicted outlet curve (Ul ) 0.0026 m/s, Ug ) 0.185 m/s). Only for this part of the study, a bed of nonporous particles (glass beads) was used to reduce the number of parameters identified by RTD models. The parity plot obtained with liquid holdups measured by the two methods (Figure 1b) shows very close results, confirming that electrical capacitance tomography can be used for determining liquid holdup in trickle beds. Additionally, for all of the average liquid saturation values obtained from ECT measurements, the average bed porosity () was obtained from the corresponding space time (τ) as ε)

τQl βlVR

(3)

where Ql is the liquid volumetric flow rate, βl is the liquid saturation, and VR is the reactor volume. Using this equation,


Figure 4. (a) Time series of pulse- and trickle-flow holdups obtained from ECT permittivity images. (b) Typical power spectral density to extract dominant frequency of pulse flow. (c) Typical cross-correlation function to estimate time delay between sensors located at two successive ECT planes (Ul ) 0.003 m/s, Ug ) 0.062 m/s).

the porosity was calculated to be 0.372 ( 0.005, which is very close to the experimental value ( ) 0.373). Previous applications of ECT to trickle-bed reactors have been limited to studying the three-dimensional nature of the pulseflow regime inside the bed. Therefore, before performing the filtration tests, experiments on clean beds were carried out to evaluate the ability of ECT to observe the effect of the liquid distributor on the pattern of liquid distribution in the bed (eventually the pattern of deposition) and also the ability of the twin-plane electrodes to detect and quantify pulse-flow regime dynamics, i.e., pulse frequency and velocity, which would also occur during the course of filtration. 3.1.1. Initial Liquid Distribution. Uniform initial distribution (spray nozzle) and nonuniform centered point-entry distributor were examined. The ECT measurements were taken 10 cm below the top of the bed. Figure 2a,b shows the

application of the spray nozzle with different distances between liquid source and bed uppermost layer. Liquid saturation curves in Figure 2a indicate that uniform radial distribution could be attained with careful adjustment of the position of distributor with respect to the bed uppermost layer. In the case the distance (between nozzle and the top of the bed) is too large (Figure 2b) a significant part of the liquid stream encounters the wall and flow from the wall region. Thus, lesser irrigation with liquid could be experienced by the center of the bed as revealed by the dip in holdup (saturation) profiles. The ECT image in Figure 2b reveals a central area with lower normalized permittivity values. The ECT image and corresponding liquid saturation curves in Figure 2c show more persistence of liquid in the middle of the bed cross section. A statistical parameter, referred to as a degree of uniformity χ, is used to express the extent of


Figure 5. Pulse characteristics obtained from ECT data: (a) pulse frequency, (b) pulse velocity.

Figure 6. Filtration test in the presence of 2.5-cm-diameter nylon rod: (a) cross-sectional view of the position of the rod inside the bed, (b) ECT normalized permittivity image for t ) 0 h, (c) ECT normalized permittivity image for t ) 3 h.

uniformity of liquid saturation over the trickle-bed cross section:18,19 χ)

( )

n βi - βi 1 n i)1 βi

∑

2

(4)

where n denotes the number of pixels per image and βi and jβi are the local and cross-sectionally averaged liquid saturations, respectively. χ approaches zero as the uniformity in distribution improves. Figure 3 shows χ versus Ug for point distributor and spray nozzle (Ul ) 0.002 m/s). With point distributor, the highest nonuniformity occurs in the upper bed part (Figure 3, open circles). Because liquid streams are shredded, reshuffled and again reassembled by the packed bed layers, these latter act as internal distributors, more or less repairing liquid distribution while fluids progress downward (Figure 3, open triangles). The values for χ at the bed bottom with the two types of distributors are close to each other suggesting a self-repairing distribution inside the trickle bed (Figure 3, open and solid triangles); however, differences still persist that could be attributed to initial distribution. Depth location subsequent to which good distribution is arrived at with initial point

distribution depends on bed diameter due to the outward broadening of liquid plume. The best distribution was obtained on the upper part of the bed with spray nozzle (Figure 3, solid circles). Although the initial distribution can be adjusted precisely, it is worthy of notice that after a certain height, the bed affects this process and slightly lesser uniform distribution is obtained in the lower part of the bed (Figure 3, solid triangles and circles). Increasing gas velocity at constant liquid velocity (Ul ) 0.002 m/s) results in a better distribution. It seems that for the lower velocities, gas is able to easily flow through the available void space, whereas for higher velocities, a competition sets on between gas and liquid to occupy space. Gas flow exerts higher shear force on the liquid to flow also through the space originally occupied by the liquid, resulting in a more uniform radial distribution as also reported in the literature.20-22 3.1.2. Pulse-Flow Characteristics. Depending on the gas and liquid throughputs, several flow regimes arise in TBRs. We focus here on the trickle-flow regime at lower liquid and gas flow rates and the pulse-flow regime at higher flow rates, which can be recognized when the liquid and gas reach zones


Figure 7. (a1-a7,b1-b7) ECT normalized permittivity images and (c,d) corresponding radial distributions in the course of filtration with distant spray nozzle with respect to bed uppermost layer (Ul ) 0.002 m/s, Ug ) 0.062 m/s). Measurements were made (a1-a7) 10 and (b1-b7) 70 cm below the top of the bed.

traversing the column in alternating pulses. Figure 4a shows an example of liquid holdups obtained by ECT 1.40 m below the top of the bed in both trickle- and pulse-flow regimes. The dominant pulse frequency was extracted from estimated spectral power densities (Figure 4b) of the liquid holdup time series acquired at 50 Hz sampling frequency and assembled into more than 24 packets of 29 data points each. The time delay in the pulse-flow regime corresponds to the time interval required for a pulse to travel between the upper and lower planes of the ECT instrument. This time delay was estimated from the crosscorrelation function (Figure 4c) of the liquid holdup time series from the two successive planes 50 mm apart. The pulse velocity was estimated by dividing the two-plane distance by the time delay. Figure 5a shows that the pulse frequency increases as a function of the gas and liquid superficial velocities. This trend, which is consistent with literature observations,23-26 is justified in the following manner: For a given gas flow rate, any excess liquid above the transition liquid flow rate contributes to pulses, with more excess liquid generating more pulses. For a given liquid flow rate, increasing the gas flow rate decreases mean liquid holdup, and the excess liquid requires an increasing number of pulses. Figure 5b shows that pulse velocity increases with increasing Ug, whereas the influence of Ul seems negligible. This behavior has already been reported in the literature, where

it was claimed that the pulse velocity during natural pulsing flow is independent of the liquid flow rate.24-26 Tsochatzidis and Karabelas27 noticed a minor effect of the liquid on the pulse velocity at high flow rates. 3.2. Filtration Experiments. To distinguish the evolving patterns of deposition, the 0% reference point (blue) in the tomography measurements corresponded to the pristine twophase flow before filtration took place. Subsequently, any increase in permittivity was attributable to kaolin deposition in the bed. The difficulty of setting quantitative permittivity-holdup calibrations resides in the impossibility of setting a 100% reference point that remains valid for all filtration conditions. In the strict sense, one must achieve at this upper limit a deposit that completely fills the extragranular pores of the bed while being impregnated with kerosene to reproduce deposits with representative porosities. Considering that the porosity of the deposited layer varies with the operating conditions28 and is, itself, unknown a priori, it became impossible to make the ECT measurements fully quantitative in terms of liquid and deposit saturations under filtration. Instead, a 100% reference point was defined as the bed flooded with fines-free kerosene, which had a lower permittivity than the kaolin-kerosene suspension. Because quantitative data on the amount of deposited solid (i.e., solid holdup) could not be obtained, the discussion in the


Figure 8. (a1-a7,b1-b7) ECT normalized permittivity images and (c,d) corresponding radial distributions in the course of filtration with point distribution (Ul ) 0.002 m/s, Ug ) 0.062 m/s). Measurements were made (a1-a7) 10 and (b1-b7) 70 cm below the top of the bed.

following subsections focuses on the structure and mechanism of deposition inside the bed. 3.2.1. Preliminary Test. To assess the ability of the ECT reconstruction algorithm to analyze the filtration tests, a phantom consisting of a 40-cm-high, 2.5-cm-diameter nylon rod was

immersed flush to the top of the bed before filtration tests were started. ECT measurements were carried out 10 cm below the top of the bed. Figure 6a illustrates the position of the nylon rod between the axis and the wall of the column. The 0% (relative permittivity) calibration was set in the presence of the


Figure 9. (a) Two-phase pressure drop and (b) specific deposition for different filtration tests (uniform and point distributions).

nylon rod (Figure 6b). Figure 6c shows that, in the course of a filtration run (Ul ) 0.002 m/s, Ug ) 0.062 m/s), a 2.3-cmdiameter disk with 0% relative permittivity appeared after 3 h. As will be explained later, more deposition occurred in the center because of the lower bed permeability; however, the area corresponding to the rod remained deposit-free. It can be seen that the reconstruction algorithm captured the reconstructed size of the nylon rod with acceptable accuracy. The diameter of the circle consisted of 8 pixels, yielding an estimated pixel resolution of ca. 3 mm. 3.2.2. Initial Liquid Distribution. The initial liquid distribution is illustrated in Figure 2b (Ul ) 0.002 m/s, Ug ) 0.062 m/s). The permittivity of two-phase flow before the resumption of filtration defines the 0% reference. Therefore, at t ) 0 h, a uniform pattern (blue) was observed (Figure 7a1) 10 cm below the top of the bed. The ECT normalized permittivity image after 1 h (Figure 7a2) showed a structure similar to the liquid distribution of Figure 2b. The chances for deposition are higher in areas with more suspension flow. For the next 2 h, a donutshaped deposit (Figure 7a3,a4) formed in the middle. The corresponding radial distributions of deposition were monitored in terms of relative permittivities, as shown in Figure 7c. Deposition reduces bed porosity and poses extra resistance against two-phase flow. Consequently, the flow was redirected toward areas with more available void space, i.e., in the center and near the walls. Bed permeability was lower in the center than near the walls, thus resulting in more deposition in the center (Figure 7a5-7a7). With porosity decreasing more rapidly in the center, the suspension flow was redirected outward toward the walls, in agreement with wall visual observations. Initially, almost no flow was seen near the wall. This situation changed during the course of filtration with the gradual appearance of small rivulets along the wall. In severe deposition, liquid flow with very high interstitial velocity could be seen from the wall which could even turn into pulse flow as will be discussed later. It appears that the high velocities near the wall exerted sufficient “sweeping” shear stress to prevent deposition from taking place in amounts comparable to those in the bed core regions. As will be shown later, in the absence of gas flow, this mechanism cannot be observed. Figure 7b1-b7 shows the ECT normalized permittivity images at a depth of 70 cm, and their corresponding

radial distributions are shown in Figure 7d. These figures depict similar evolutions except for a time lag. Deposition spread axially downward. During the first 2-3 h, the deposition rate at 70 cm was low and then accelerated to reach a pattern similar to that established earlier at a depth of 10 cm (Figure 7b7,d). The radial structure of deposition 10 cm deep in the bed evolved slowly after 4 h (Figure 7c), whereas at 70 cm, most of the deposition occurred between 4 and 5 h and slowed afterward. Such local observations are consistent with the increasing trend of exit suspension concentrations, which are mirrored with declining bed capture efficiencies with time.4 When the efficiency of the upper layers weakens, the lower (cleaner) layers take over by receiving more incoming flux of fines, thus allowing more capture to take place. A decrease of capture efficiency at each axial location causes migration of deposition to nearby layers downstream. At each axial location, when the radial profile of deposits starts evolving slowly, most of the two-phase flow is rerouted toward the wall through more or less cleared corridors, as explained earlier. Short-circuiting is established near the walls, which translates into pressure-drop plateaus even though the overall specific deposit continues to rise.4,6 Figure 7c confirms this trend at the local level, where permittivity (and deposition) kept increasing, albeit slowly, at each radial position. The effect of a point distributor (Figure 2c) on the structure of deposition in the bed was examined next. Figure 8a1-a7,b1-b7 shows the ECT normalized permittivity images at 10 and 70 cm from the top of the bed, respectively. The corresponding radial distributions of normalized permittivity are shown in Figure 8c,d. Obviously, deposition starts in the core for the regions close to the distributor (Figure 8a1-a7). A reduction of bed porosity in the center causes the flow to spread to nearby areas. The area of maximum deposition (red spot) grows outwardly to the wall. At a depth of 70 cm (Figure 8b1-b7,d), the deposition pattern is akin to the initial distribution for a spray nozzle (Figure 7b1-b7,d). It seems that, at this depth, the bed has already distributed the suspension flow in a relatively uniform manner, erasing the effects of the initial distribution. The average normalized permittivity values for different bed heights are shown in Figure 8e. Deposition seems to occur over a broader area in the bed, being irrigated by the suspension flow,


Figure 10. Evolution of ECT average normalized permittivity in the course of filtration runs: (a) Ul ) 0.002 m/s, Ug ) 0.062; (b) Ul ) 0.0027 m/s, Ug ) 0.062; (c) Ul ) 0.002 m/s, Ug ) 0.25 m/s.

at a depth of 70 cm than at the bed depth of 10 cm. Although the center of the bed at 10 cm encounters larger fines fluxes, its ability to filter them is limited. The lower sections of the bed, because they have a better distribution, have the opportunity to collect the particles escaping from top. Following the same observation of the distribution farther from the bed entrance being better, the ECT plane located at 15 cm exhibits higher deposition than that at 10 cm. The trend is reversed for the plane at 75 cm compared to that at 70. Because a more uniform distribution exists there, regions in contact with upstream suspensions have a chance to filter them sooner than the regions downstream. The net increases in pressure drop and the corresponding overall specific deposit values for different filtration runs are shown in Figure 9a,b for point and uniform distributions. For the same operating conditions (Ul ) 0.002 m/s, Ug ) 0.062 m/s), the point distribution revealed a lower initial pressure drop than the uniform distribution (Figure 9a)

because the bed upper layers were relatively deposition-free and did not impose a resistance to flow. As filtration progressed in time, the structures of the two beds became increasingly similar, and the same path of pressure drop increase was obtained. Figure 9b shows that, for the same conditions, the point distribution (open and solid circles) gave a slightly lower specific deposit. This could be attributed to a lower activity of the upper part of the bed with respect to fines capture compared to the uniform distribution (Figure 2a). 3.2.3. Gas and Liquid Superficial Velocities. Figure 10a shows the evolution of the average normalized permittivity (i.e., deposition) for Ul ) 0.002 m/s and Ug ) 0.062 m/s at different vertical bed positions. As can be seen, there exists a time delay for each depth to experience deposition. Increasing Ul at constant Ug (Ul ) 0.0027 m/s, Ug ) 0.062 m/s) gives larger deposits (Figure 9b, solid triangles) with expectedly higher pressure drops


Figure 11. (a) Evolution of trickle-to-pulse transition point as a function of overall specific deposit as assessed in terms of minimum standard deviation of ECT signals. (b) Sensitivity of pulse velocity and frequency to specific deposition in the pulse-flow regime (Ul ) 0.0027 m/s, Ug ) 0.062 m/s).

(Figure 9a). Although a higher liquid velocity means a shorter suspension residence time, it also means a higher fines flux impinging on the collector surface; the net outcome is increased capture with higher liquid velocity for the explored filtration conditions. This is illustrated through the steeper rise in relative permittivity with time at higher Ul values (Figure 10a,b). Moreover, two adjacent planes reveal very similar values, which is an indication that each one rapidly approaches its final capture capacity. Figure 10c shows the evolution of the average normalized permittivity for increased gas superficial velocity at constant liquid velocity (Ul ) 0.002 m/s, Ug ) 0.25 m/s). Figure 9a depicts the maximum pressure drop increase (Ul ) 0.002 m/s, Ug ) 0.25 m/s versus Ul ) 0.002 m/s, Ug ) 0. 062 m/s) for this case. The corresponding global specific deposit values are shown in Figure 9b (compare solid squares and solid circles). This was interpreted4 considering compression effects of the deposition layer caused by increased gas velocities, making more room available for fines. Therefore, at each vertical position, more deposition would occur at higher values of Ug, as was also confirmed through ECT measurements. Data for the two pairs of adjacent planes (10 and 15 cm; 70 and 75 cm) in Figure 10c show very similar values, indicating high local capture efficiency. The escaping fines from the first plane are trapped in the second one. Therefore, as reported elsewhere,4 the outlet concentration remains close to zero for a longer time than for the case with a lower value of Ug. Moreover, until the end of the run, the upper bed section reveals higher permittivity values with increasing slope in time, confirming that more local deposition is occurring. 3.2.4. Trickle-to-Pulse-Flow Regime Changeover. Bed porosity decreased during filtration when fluid velocities were constant and were chosen to correspond to trickle flow before resumption of filtration. However, the interaction between phases increases during bed plugging, bringing about the inception of pulse flow. Not all combinations of gas and liquid superficial velocities guaranteed a shift from trickle flow to pulse flow during deposition.4 The operating point (Ul ) 0.0027 m/s, Ug ) 0.062 m/s) for which this shift was possible was studied using ECT (placed at a depth of 70 cm) to determine the transition point and to characterize the pulse-flow regime (i.e., pulse

velocity and frequency) as a function of the overall specific deposit. The coefficient of variation of time series signals obtained from ring conductivity electrodes was used29 as a criterion to detect the transition from trickle to pulse flow as the flow rates were increased. In the filtration experiments, the standard deviation of ECT measured signals was used instead, as it was also affected by the extent of deposition. The trickleto-pulse-flow transition point corresponds to the dip in the standard deviation in Figure 11a. The experiment was initiated in the partially wetted trickle-flow regime where the standard deviation of the signal was dictated by the lack of regularity of the flow over the bed elements. As the transition to pulse flow was approached, full wetting was achieved (because of the high interstitial velocity of the fluids), yielding very regular signals with minimum dispersion around their mean. The occurrence of pulse flow can be recognized thereafter by the inception of large deviations in the signal, which, in turn, increased the dispersion around the mean. Using this criterion, it is possible to accurately determine the specific deposit value at which transition takes place. It is important to realize that, in tall beds, it is the upper part that sees the initiation of pulses because deposition affects the bed entry first. Hence, pulses can be observed in the upper part of the bed while the lower part is still trickling. In the clean state of a trickle bed, pulse flow due to increased flow rates is initiated from the bottom of the bed and can expand upward as flow rates are raised. Because over the time span, the whole column did not turn into pulse flow, fluctuations in the pressure drop signals were not observed (Figure 9a). For shorter beds, pressure fluctuations have been already observed.6 Figure 11b presents the pulse-flow properties, where it is seen that the pulse velocity increases with deposition in the beginning and then stabilizes in accordance with the observed stabilized standard deviations (Figure 11a). As the bed porosity decreases, increasing fractions of liquid velocity are in excess of what trickling flow is capable of handling, and the number of pulses per unit time increases. 3.2.5. Suspension Flow with Stagnant Gas (Ug ) 0 m/s). To compare filtration mechanisms in two-phase and single-phase flow, a test was performed without gas flow in the trickle-flow regime (Ul ) 0.002 m/s, Ug ) 0 m/s). ECT normalized


Figure 12. (a1-a5, b1-b5) ECT normalized permittivity images and (c,d) corresponding radial distributions in the course of filtration in stagnant gas, (e) formation of filter cake, and (f) evolution of average normalized permittivity (Ul ) 0.002 m/s, Ug ) 0.062 m/s).

permittivity images at positions 10 and 70 cm below the top of the bed are shown in Figure 12a1-a5,b1-b5, respectively. The corresponding normalized permittivity radial distributions are presented in Figure 12c,d. These plots show that, eventually, a uniform distribution of deposition is achieved over the bed cross section at each height. It is believed that, in the absence of shear forces exerted by the gas phase, the deposition layer develops

a high porosity. Under such conditions, after the initial deposition of fines around the catalyst particles is completed, a plausible mechanism of deposition to appear would be straining (or sieving),30 which is described as trapping of fines larger than the pore constrictions. Sieving could be responsible for the formation of a filter cake and for the dramatic surge in pressure drops. Figure 12e is a photograph of the top of the


bed after 3 h of filtration. It can be seen that a filter cake has formed on top of the bed, resulting in the accumulation of a liquid layer. Figure 12f shows that the time for deposition to reach a depth of 70 cm is much longer than in the presence of gas-phase flow. This indicates that more deposition is occurring in the upper part of the bed (due to sieving), and Figure 9a shows a significant increase in pressure drop because of the blocking filter cake. 4. Conclusions Electrical capacitance tomography (ECT) imaging was used as a noninvasive method to study the mechanism and structure of deposition of fine particles inside trickle-bed reactors. Liquid holdups obtained by ECT for a clean bed were very close to those measured by tracer injection. For the filtration tests, the effects of the initial liquid distribution, gas and liquid superficial velocities, and single-phase flow with stagnant gas (suspension flow with zero gas velocity) on the structure of deposition were studied. The transition from trickle to pulse flow due to deposition was characterized according to the ECT signals. The following conclusions were drawn: (1) With a point distributor, part of the upper section of the bed is less active in the capture process. With the progress of filtration, deposits spread radially because of the lower available porosity in the center. (2) Increasing the liquid flow rate resulted in increased local specific deposits because of the higher flux of incoming fines, whereas increasing the gas superficial velocity (over a threshold) revealed increased local deposition attributable to a compressed deposition layer. (3) Sieving deposition was observed in the case of a stagnant gas phase, and a filter cake formed on top of the bed. (4) A decrease in bed porosity enhanced the interaction between the liquid and gas flows, causing a changeover from trickle flow to the pulse-flow regime. The properties of the obtained pulses were determined from the ECT signals. The pulse velocity increased after the transition and then leveled off, whereas the pulse frequency increased with the progress of filtration. Acknowledgment Dorf Ketal Chemicals LLC (Stafford, TX) is acknowledged for providing the conductivity improver (SR 1795). Financial support from the Natural Sciences and Engineering Research Council of Canada and Natural Resources Canada is gratefully acknowledged. Prof. Bernard Riedl (Department of Wood Sciences, Laval University) and Prof. Denis Rodrigue (Department of Chemical Engineering, Laval University) are acknowledged for providing access to the surface tension and viscosity measurements. Nomenclature C ) fines concentration in suspension, kg/m3 Pe ) Pećlet number Q ) volumetric flow rate, m3/s t ) time, s U ) superficial velocity, m/s V ) volume, m3 Greek Letters β ) liquid saturation ) bed porosity

l ) liquid holdup τ ) mean residence time, s σ ) specific deposit, kg/m3 Subscripts g ) gas In ) inlet condition l ) liquid Out ) outlet condition R ) reactor

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ReceiVed for reView May 21, 2008 ReVised manuscript receiVed September 1, 2008 Accepted September 3, 2008 IE800810T

Monitoring Filtration in Trickle Beds Using Electrical Capacitance

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