8126
Ind. Eng. Chem. Res. 2008, 47, 8126–8135
Effect of Particle Porosity on Hysteresis in Trickle-Bed Reactors Rabindranath Maiti, Arnab Atta, and K. D. P. Nigam* Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
Hydrodynamics in trickle-bed reactors (TBRs) is quite complex because of the coexistence of gas-liquid-solid phases. Recent past hysteresis have been the subject of investigation to improve the understanding of the flow features at the microlevel, aiming to demystify the complex hydrodynamics. The purpose of the present study is to identify the role of particle porosity on hysteresis by choosing particles of different pore density (nonporous, semiporous, porous) but prepared from same material with identical shape and sizes. Experiments were carried out with industrial relevant-sized alumina extrudates in a 150 mm ID column using both a dryand a wet-bed startup procedure. Comprehensive pressure drop hysteresis data were generated in increasing and decreasing modes of water flow in the presence of a constant flow of air at ambient condition. Pronounced but different magnitudes of pressure drop hysteresis were observed with all three types of particles at first cycle as well at subsequent cycle of operation. A deviation in pressure drop up to 90% was found between increasing and decreasing modes of operation, even after prewetting the bed. The same amount of hysteresis was observed for all the subsequent cycles, but the value is higher for particles with higher porosity. This confirms that particle porosity plays a major role in the existence of different flow texture at the microlevel in the trickle flow regime. This observation is reported here for the first time, and we believe that there is no such experimental data available in the literature. The genesis of this different hysteretic behavior of porous particles lies in the different ways liquid spreads/retracts over porous and nonporous particles. A conceptual framework of hysteresis proposed by Maiti et al. (2005), which is based on the concept of participating and nonparticipating particles and principles of liquid spreading on porous and nonporous substrates, is found to explain successfully the various features of hysteresis observed with all three types of particles. This study is expected to be useful to the TBR researcher and practitioner in enhancing the understanding further to demystify the complex hydrodynamic phenomena in TBRs. 1. Introduction Trickle bed reactors (TBRs) are catalytic gas-liquid-solid reactors in which the fluids are fed cocurrently downward over a packed bed of stationary particles. Its use is widespread in the oil refinery and petrochemical industry, but they are also used in fine chemistry, water treatment, and electrochemical processes.1-5 Among multiphase reactors, TBRs have probably the most complex hydrodynamics as trickling flow exhibits the unique feature of solid partial wetting under some conditions, mainly at a low liquid flow rate. In general, the reaction occurs between the dissolved gas and liquid phase reactant at the interior surface of the catalyst. The external surfaces of the catalyst happen to be partially or fully covered with liquid for maximum utilization of catalyst based on mass transfer limitation with gas or liquid.6 In the macrolevel, the existence of gas or liquid phase is well recognized as various flow regimes such as trickling flow, pulsing flow, spray flow, and bubble (Figure 1).7-11 At the microlevel or particle level, the liquid flow textures in a bed consist of a number of features: liquid flows as films, rivulets over the particles, pendulum structures, liquidfilled channels, and liquid-filled pockets (Figure 2).12 The relative amounts of these features are expected to result in various hydrodynamic parameters observed at the macrolevel. The phenomenon of hydrodynamics in TBRs have been investigated for several decades, but it is still poorly predicted, while for various reasons it has growing importance in the oil industry. The main industrial applications of TBRs concern catalytic fixed-bed reactors implemented in oil refining for hydrotreatments such as hydrodesulfurization and hydrocracking. * To whom correspondence should be addressed. E-mail: drkdpn@ gmail.com. Tel.: (011) 26591020. Fax: (011) 26591020.
In the development and industrial operation of these reactors, partial wetting is a major issue due to three main reasons: (1) The continuous drastic reduction of sulfur in transportation fuels required by regulations for environment protection involves higher residence times and, then, lower liquid flow rates. (2) Similarly, heavier oils have to be more and more converted which requires longer residence times to reach the standards of purity. (3) New catalysts and operating conditions are investigated in pilot scale reactors in order to improve catalysts efficiency and stability. These laboratory scale reactors are to be operated at the same liquid hourly space velocity (LHSV) as in commercial plants, and then at very low liquid flow rates for small volumes of catalyst tested. As the volume of catalyst tested has to be decreased for practical and economical reasons, partial wetting becomes a major issue for the correct operation of the small size pilot plant. The important design hydrodynamic parameters, such as pressure drop, liquid holdup, and wetting efficiency are mostly predicted by using experimentally developed empirical/semiempirical correlations.1,4,6,13-20 Predictions of these correlations and actual performance of TBR differ significantly in many cases, and there is no general consensus about the correlations developed so far. One of the major contributors to these differences is thought to be the existence of multiple hydrodynamic states, referred to as hysteresis. It refers to the phenomena that the values of bed average hydrodynamic parameters (∆P, eL) are not uniquely determined by steady state operating condition. Several authors have reported the phenomena of hysteresis in TBRs over decades, 21-38 with experimental studies investigating the cause, behavior, and implications of hysteresis and with mathematical modeling, incorporating the history of
10.1021/ie8003539 CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8127
Figure 1. Flow features at the macrolevel in trickle-bed reactors.
Figure 2. Flow textures at microlevel in trickle-bed reactors.
operation into hydrodynamic models.22,23,33,38 It is revealed that the flow history experienced by the bed determined the specific hydrodynamic state. Differences in pressure drop up to 700% and in liquid holdup up to 130% due to hysteresis were reported. It has been reported that this hysteresis depends mainly on startup conditions, the mode of operations, and types of packing material and sizes of the particles. It is also affected by several other parameters, such as diameter of column, addition of surface wetting agents, distribution of liquid at inlet, flow rate of gas, operating flow ranges, etc.38 However the impact of later parameters are not so intense on hysteresis. Most of the studies were with nonporous glass particles and limited to first cycle of operation.39 A close scrutiny of the literature revealed that very few studies reported the hysteretic behavior of porous alumina particles in cold conditions34,37 as well as in a reacting system.38 One of the studies34 generated comparative hysteresis data for porous and nonporous particles
in identical flow situations and indicated strikingly different features of hysteresis with porous particles as compared to the nonporous particles. In all the studies, the phenomena of hysteresis were attributed to the difference between advancing and receding contact angle, rivulet and film flow, change in tortuosity of the gas flow channels, etc. But all the above perception of hysteresis could not explain some of the features of hysteresis, that is, hysteresis in subsequent cycles, hysteresis in wet bed, occurrence of close loop/open loop hysteresis, and specifically different hysteretic behavior of porous particles as compared to nonporous particles. The observations have encouraged several researchers to study the different flow textures at the microlevel and to look closely into the effects of porosity on existence of multiple flow texture at same operating flow arrived in different way, that is, hysteresis. Some of the international school are engaged in developing and using direct visualization techniques to find the appearance of different flow textures at various operating situations such as colorimetry,26,27 computed tomography,40,41 MRI,42,43 X-ray radiography,44 etc. It is revealed from these studies that spatial location of the liquid and distribution of dry zones are strongly influenced by local geometry of the pore space, that is, radial position of the pore, pore diameter, and relative amount of interfacial surface area adjoining neighboring pores. It is also reported that depending on different prewetting procedures the liquid is improperly distributed and this phenomenon persists even at high liquid velocities. However these techniques suffer from various constraints like versatility that make them unsuitable for investigation of hysteresis using the controlling parameters in a systematic way (where it may be required to study online, use a fairly large diameter column and a nonspherical porous particle, and where it may be required to process a very large number of images). Some mathematical approaches have been tried to characterize the history of the process by including several criteria/additional parameters and proposed correlations for quantifying hysteresis
8128 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Figure 4. Experimental setup: (1) air saturation tank; (2) pressure gauge; (3) air rotameter; (4) liquid tank; (5) liquid feed pump; (6, 7) liquid rotameter; (8) rough liquid distributor; (9) final liquid distributor; (10) packed bed; (11) liquid separator.
Figure 3. Change in wetting with movement of contact line (Khanna and Nigam43).
by means of the physical model using correlations or the fundamental approach using computational fluid dynamics (CFD). The models were able to capture the trend of hysteresis but with a large deviation between predicted and experimental data. Also it could not predict different hysteresis for subsequent cycles of operation, porous particles, different maximum operating flow range, different startup conditions, etc. The models are limited by the number of fitting parameters, and some have to be supplied from prevailing experimental condition. Therefore the understanding and investigation of liquid flow behavior in a packed bed has great merit. Recently some attempts have been made by the school of Khanna and Nigam to enhance the understanding of the physics of hysteresis phenomena in TBRs.45-48 They predicted several features of liquid spreading over the porous surface on the basis of thought experiments. They modeled the porous surface (Figure 3) as alternating patches of saturated pore (of width m) and solid surface (of width n). When the wettability is increased, the wetting efficiency increases as the contact line advances on the solid surface, merging with neighboring rivulets (see from stage A to stage C in Figure 3). When it reaches the edge of the pore, it accelerates across the patch to form a contact with other side of the patch, showing enhanced spreading (see from stage C to stage D in Figure 3). When the catalyst wettability is decreased, the contact line starts to retract, which results in a reduction in wetting efficiency. The retracting contact line retracts on a wetted surface differently than on a dry surface during increased wettability. After reaching the edge of the liquid filled pore, it becomes stranded there, thus making any more retraction very difficult (see from stage D to stage E in Figure 3), that is, the pinning effects of pores. If nonhomogeneities are present (in the form of completely wettable saturated pores and partially wettable solid patches), film rupture occurs, showing a sudden dip in wetting (see stage F in Figure 3). Experimentally they verified the above concept of liquid
spreading using a model substrate with millimeter-sized pores and observed that, depending on whether the drop edge is moving toward the pore or away from the pore, the pore acts as accelerator or brake for the drop edge. On the basis of the experimental observation and heuristic argument they proposed a framework of hysteresis to explain the different hysteresis with nonporous and porous particles.48 The porous/nonporous nature of the packing, though being taken as one of the control parameters for study, has not been invoked to explain the observed hysteresis due to lack of systematic data generation concerning the porosity effect. In the present work an attempt has been made to study the impact of porosity of the particles on multiplicity in hydrodynamic states in a relatively larger diameter column (D/dp > 50) using industrial relevant smaller-sized alumina extrudates with different porosity levels. The basic focus of the experiment is to understand the possible role of pores toward the existence of possible flow physics and phenomena of hysteresis especially in porous particles. We believe that there is no such experimental data available in the literature. 2. Experimental Setup A schematic diagram of the experimental apparatus is shown in Figure 4. The setup consists of a Perspex column with an internal diameter of 15 cm and with packing height of 50 cm. The column consists of a gas/liquid distributor, a packed section, and a gas-liquid separator section. In the distributor section a baffle plate with six trapezoidal cuts was used for rough distribution of liquid followed by a capillary-type final liquid distributor. The final distributor (Figure 4) consists of 129 capillaries that are 2 mm i.d. and 6 cm in length at a 12 mm triangular pitch. The capillaries are placed between two plates. The bottom plate had circular holes around the capillaries that have a diameter of 6 mm slightly larger than the outer diameter of the capillary. The air enters the chamber formed between these two plates through 1/4 in. tube and passes to the packed section through annular space around the capillary tubes. The liquid enters at the top and gets distributed roughly first by a baffle-plate distributor and then enters the packed section through the capillaries. The top of the packed section of the column rested about 15 cm below the distributor. Three types of alumina extrudates with different pore density but having same size and shape (supplied by SUD Chemie, India) were used as the packing material. It is expected that particles with more specific surface will have a greater number of pores, that
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8129 Table 1. Details of Packing Properties packing surface type shape dp, mm Av length, mm Av pore radius, A Specific surface area, m2/g Adsorption pore vol, cm3/g
alumina nonporous cylindrical extrudates 1.43 4.2
alumina semiporous cylindrical extrudates 1.43 4.5
alumina porous cylindrical extrudates 1.43 4.3
0
51
61
0.3
125
220
0
0.32
0.66
Table 2. Details of Bed Properties and Operating Conditions column properties operating conditions
diameter, cm height, cm distribution temp pressure G L
15 50 uniform 30 °C 1 atm 0.05 kg/m2s 0.5-5.5 kg/m2s
is, higher particle porosity. The porosity was checked using a BET apparatus. Details of the bed properties are shown in Table 1. From the average pore diameter and pore volume data it can be calculated that the pore density of the porous alumina particles is ∼2 times that of the semiporous particles (assuming the same pore length for all pores in both types of particles). The novelties of the present data are that they have been carried out on a same material. The packing material was supported on a wire mesh screen at the bottom. Small spherical ceramic balls of sizes 5-6 mm diameter were used at the bottom and top of the packed bed for better uniform distribution of the liquid. At the outlet of the column, the gas and the liquid were separated while passing through separation sections. The column was equipped with pressure taping at the inlet and outlet of the packed section and a U-tube manometer was used to monitor the pressure drop. The liquid and gas flow rate were measured using rotameters. The operating flow rates were chosen so that the trickle flow regime was maintained, and it was checked further using the ANN model.11 Details of operating parameters are shown in Table 2. Experimental Procedure. It is reported that different startup procedures also produce beds with different wetting efficiency (Maiti et al.39 and Loudon et al.49) In the present study two types of startup procedures (which produce minimum and maximum wetting efficiency) were used, namely, dry bed and wet bed. In one water was introduced in a packed bed of dry particles at a low flow rate and then it was increased to the desired flow rate. Then airflow was initiated. The state of bed was referred to as a nonprewetted bed or dry bed. In the other case, water was introduced at a low flow rate and allowed to fill the bed by closing the outlets. Once the water flow reached the top of the packing, water flow was set to the desired value and then the outlet was gradually opened to drain the excess liquid. The airflow was initiated thereafter and the state of the bed was referred to as a prewetted bed or wet bed. The pressure drop was monitored after fixing the gas and liquid flow rate at the desired level for each experiment. The run was continued until the steady state was attained. It was observed that the pressure drop increased with time and achieved a steady value in a time period ranging from 20-120 min on the basis of the startup procedure. Normally dry bed start-up procedures take
a longer time to achieve steady state. After achieving steady state for each step, the pressure drop was recorded and then liquid flow was increased stepwise to get the lower branch of the pressure-drop curve. After achieving a higher flow rate as predecided (for trickle regime), it was gradually reduced to a lower starting flow rate after recording the pressure drop for each step, and it was marked as the first cycle of operation. In the next cycles of operation, the liquid flow was gradually increased from the end point of the first cycle (without draining the bed) to complete the second cycle of operation the same way as it was done for first cycle. The experiment was continued further to the third cycle while recording the pressure drop. The major observations are classified according to the two control parameters of the experiments, namely, startup conditions (wet/dry bed) and porosity of the particles (porous, semiporous, and nonporous) as shown in Table 3. To compare and analyze the experimental observation in a more systematic way, a method of estimating the extent of hysteresis is formulated. The extent of hysteresis can be characterized by the average pressure-drop difference between the increasing and decreasing flow rate branches or paths of any given cycle. The amount of hysteresis has been estimated by measuring the area of pressure drop loop as shown in Figure 5a-c. No normalization is required as the ranges of flow rates are the same for all cases. To check the impact of the repacking of the bed, the particles from the earlier study were dried overnight to drive out moisture and repacked following the same shock loading procedure as earlier. Similar sets of data were collected in dry-bed and wetbed conditions for each type of particle. The major observations are classified according to the two control parameters of the experiments, namely, startup conditions (wet/dry bed) and porosity of the particles (porous, semiporous, and nonporous) as shown in Table 4 and Figure 5a-c. Although there are differences in a few data points, the reproducibility of the trend in change of hysteresis with variation in porosity of the particles was observed as discussed in the following sections. 3. Results and Discussions Figure 5, panels a-c show the different hysteretic behavior of three types of alumina extrudates with the same shape and sizes but of different particle porosity in both wet-bed and drybed startup condition. In both cases, the first cycle was started with a very low liquid mass flow rate of about 0.5 kg/(m2 · s), which was gradually increased (following the path 1-i) to a higher value of about 5.5 kg/(m2 · s), and the pressure-drop line obtained was marked as the lower branch of hysteresis loop. The first cycle was completed by gradually decreasing the liquid flow rate to the starting value (following path 1-d), and the pressure-drop line obtained was marked as the upper branch of hysteresis loop. In the next cycle (second cycle) of operation, the liquid flow was gradually increased from the end point of the first cycle (without draining the bed) to complete the second cycle of operation the same way as it was done for first cycle (2-if2-d). The third cycle was also operated between the same flow ranges following the second cycle (3-if3-d), which was identical to second cycle. In both the dry-bed and wet-bed startup condition, the upper branches were same for all the cycles (1-d, 2-d, 3-d), but the pressure drop in the lower branch of first cycle (1-i) was much less than that of second and third cycle (2-i, 3-i). Several features of hysteresis such as variation in gap between beginning and end point of operating cycles, that is, existence of close or open loop hysteresis and variation in loop area between first cycle and subsequent cycles of
8130 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 3. Hysteresisa in Pressure Drop and Deviation in Starting Point in First Cycle and in Subsequent Cycles (dp ) 1.43 mm) dry hysteresis (cmwc kg/(m2 · s)) end point shifted from starting point second cycle onward hysteresis (cmwc kg/(m2 · s)) deviation in end point pressure drop variation (cmwc) in the flow range first cycle
a
wet
nonporous
semi porous
porous
nonporous
semi porous
porous
2913 × 10-2 yes 738 × 10-2 no 3.2-20.8
2838 × 10-2 yes 933 × 10-2 no 5.8-29.0
2588 × 10-2 yes 1308 × 10-2 no 7.0-32.0
2380 × 10-2 yes 1855 × 10-2 no 10.0-30.0
2928 × 10-2 yes 2553 × 10-2 no 10.2-38
31788 × 10-2 no 3178 × 10-2 no 12.3-42.0
Area of the pressure-drop hysteresis loop or area between increasing and decreasing operating branch.
operation for each type of particles and between different particles in dry-bed and wet-bed startup conditions were tabulated (Table 3) and discussed in the following sections. 3.1. Dry-Bed Start-up Condition. In the case of nonporous particles (Figure 5a), the pressure drop in the beginning of the first cycle was very low (∼3.2 cmwc). As liquid flow rate was gradually increased (following the path 1-i), the pressure drop also increased and reached a value of about 20.8 cmwc at the end of the increasing liquid flow branch (∼5.5.0 kg/(m2 · s)). At the end of the decreasing liquid flow branch (path 1-d), the pressure drop was higher than that in the beginning of the cycle. The cycle did not close and pronounced hysteresis amount of ∼29.13 cmwc · kg/(m2 · s) (by our characterization scheme) was obtained. In the case of semiporous particles (Figure 5b), the pressure drop in the beginning of the first cycle was a little more (∼5.8 cmwc), and the end point of the first cycle was closer to the starting point as compared to nonporous particles. Thus an appreciable amount of open loop hysteresis was found (∼28.38 cmwc · kg/(m2 · s)) but it was 2.5% lower than the corresponding value for nonporous particles. For porous particles the same trend of change in hysteresis features were found. As observed from Figure 5c, the pressure drop in the beginning of the first cycle was again little more (∼7.0 cmwc) compared to semiporous particles and the gap between beginning and end point of the cycle reduced. The observed open loop hysteresis (∼25.88 cmwc · kg/(m2 · s)) was reduced further by 8.1% from the corresponding value for semiporous particles and which was also 11.1% lower than the value for nonporous particles. In the subsequent second and third cycle, constant close loop hystesis was observed with each type of particles but it was much lower than the first cycle, for example, for nonporous particles the amount of close loop hysteresis was ∼7.38 cmwc · kg/(m2 · s) (by our characterization scheme) which is found to be 75% lower than first cycle. In the case of semiporous particles the hysteresis loop area increased to ∼9.33 cmwc · kg/ (m2 · s), which is 26% more than the corresponding value for nonporous particles, and for porous particles this area further increases to ∼13.08 cmwc · kg/(m2 · s) which is 40.2% more than the corresponding value for semiporous particles and 77.2% more than the value for nonporous particles So the important observation of the above analysis is that there was a substantial increase in hysteresis loop area with the increase of porosity of the particles in subsequent cycles of operation, whereas it showed marginal decrease in the case of the first cycle of operation. A similar trend was found with the repeat test data upon repacking the bed with all three types of particles (Table 4). The measured loop areas were found reproducible within 1.25% in the first cycle of operation and 20.6% in subsequent cycles. Although there is a little larger error in the reproducibility of data in subsequent cycles, it can be easily seen that the trend in the change of hysteresis features with porosity of the particles is maintained even if three data sets (with increasing particle porosity) are chosen randomly
either from the first experiment or from the second experiment (repeat test). 3.2. Wet-Bed Condition. In the case of nonporous particles (Figure 5a), the pressure drop in the beginning of the first cycle was low (∼10.0 cmwc). As liquid flow rate was gradually increased (following the path 1-i), the pressure drop also increased and reached a value of about ∼30 cmwc at the end of the increasing liquid flow branch (∼5.5.0 kg/(m2 · s)). At the end of the decreasing liquid flow branch (path 1-d), the pressure drop was higher than that in the beginning of the cycle. The cycle did not close, and pronounced hysteresis in the amount of ∼23.8 cmwc · kg/(m2 · s) (by our characterization scheme) was obtained. In the case of the semiporous particles (Figure 5b), the pressure drop in the beginning of the first cycle was a little more (∼10.2 cmwc) and the gap between the beginning and end point of the loop was decreased further. The observed open loop hysteresis was ∼29.28 cmwc · kg/(m2 · s) which is 23% higher than the corresponding value for nonporous particles. In the case of the porous particles (Figure 5c), the pressure drop in the beginning of the first cycle was more (∼12.3 cmwc) compared to the semiporous particles and the end point of the loop returns back to the starting point. The observed close loop hysteresis was ∼31.78 cmwc · kg/(m2 · s) which is 8.54% more than the corresponding value for semiporous particles and 33.5% more than the value for nonporous particles. In the subsequent second and third cycle, constant close loop hystesis was observed with each type of particles, and the difference in loop area with the first cycle is much less compared to drybed conditions; for example, observed nonporous particles close-loop hysteresis is ∼18.55 cmwc · kg/(m2 · s) which is 22% lower than the first cycles, whereas it is 75% of that in a drybed situation. But there is a substantial increase in hysteresis loop area in subsequent cycles with an increase in porosity of the particles. In the case of semiporous particles the hysteresis loop area increased to ∼25.53 cmwc · kg/(m2 · s) which is 37.6% more than the corresponding value for nonporous particles. For porous particles this area further increases to ∼31.78 cmwc · kg/ (m2 · s) which is 24.5% more than the corresponding value for the semiporous particles and 71.3% more than the value for nonporous particles. It was also observed that maximum deviation in pressure drop between two branches of liquid flow (∼90%) occurred in subsequent cycles of operation with porous particles. The main observation of the above analysis is that there is substantial increase in the hysteresis loop area with the increase of porosity of the particles in the first cycle, and it is more pronounced in subsequent cycles. The similar trend was found with the repeat test data also (Table 4). The measured loop areas were found to be reproducible within 3.1% in case of first cycle and within 12.0% in subsequent cycles.
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8131
Figure 5. Hysteresis loops in dry-bed and wet-bed conditions (air-water system) with 1.4 mm alumina extrudates of type (a) nonporous, (b) semiporous, and (c) porous (1,2,3-i indicates increasing operating mode in 1st, 2nd, and 3rd cycle, 1,2,3-d indicates decreasing operating mode in 1st, 2nd, and 3rd cycle).
4. Current Perceptions and Unresolved Issues It is observed from the results and discussions in the previous section that at identical wetting policy and packing geometry the hysteretic features change with the change in porosity of the particle which is less understood within general perceptions
of hysteresis. So in the following section these observations were explained on the basis of general perceptions of hysteresis, and underlying differences were identified. Then an attempt is made to explain all the features using the framework of hysteresis proposed by Maiti et al.48 4.1. Dry Bed. General perception is that starting from zero liquid flow rates in a dry bed, as the liquid flow is commenced, the liquid flows in rivulets or channels (Maiti et al.48). The cross sectional area or number of particles covered in a channel is small. At this stage the pressure drop is equivalent to the pressure drop in a dry bed and the liquid flows as rivulet. With increasing liquid flow, the rivulet will grow in size; enlargement of existing channels or formation of additional channels also takes place. Therefore the pressure drop increases because of lower effective porosity and larger liquid-solid and liquid-gas interactions. Also, it is understood that with the increasing flow rate the three-phase contact line has to advance over the dry surface and will spread less because of a higher contact angle; that is, rivulet-type flow persists. The cross sectional area of the channel remains unchanged even as the flow rate is decreased. The liquid retracts from the peripheral regions of the channel leaving a thin film of liquid over the packings and gas flowing in the resultant voids. The central core of the channel remains in filament mode. Even though the actual liquid coverage of the solid remains about the same, the effective porosity is higher and the gas/liquid interfacial area becomes larger because of the simultaneous presence of the central core and the surrounding film in place of liquid filament. This combined effect of increased solid-liquid and gas-liquid interactions more than offsets the decrease due to increased voidage. This results in more of an increased pressure drop during the decreasing mode than during the increasing mode at the same flow rate and, consequently, a higher pressure drop toward the end of the cycle. Thus, significant hysteresis is expected for porous, semiporous, and nonporous particles in the first cycle. The experimental observations also confirm this (Table 3, Table 4). In subsequent cycles liquid will spread over the film already formed over the wetted particles, and the three phase contact line also will retract over the wetted surface thus bringing the values of advancing and receding contact angles closer to each other. Thus, the voidage, gas-liquid, and solid-liquid interactions remain the same for increasing and decreasing branches. It is, therefore, expected that there should be negligible or no hysteresis in pressure drop in subsequent cycles. This is confirmed in the case of the nonporous particles (Figure 5a and Table 3, Table 4) where very less amount of hysteresis is observed compared to semiporous and porous. To reconcile these anomalies and get more focused one should try to find answers to the following question: (1) (First cycle) How to account for the decreasing amount of open-loop hysteresis with the increase of pore density of the particles? (2) (Subsequent cycles) How to account for reduced hysteresis in the subsequent cycles for each type of particles compared to the first cycle and how to account for a substantial increasing amount of hysteresis with the increase of pore density of the particles? 4.2. Wet Bed. In wet bed liquid pockets, a thin layer of film exists over the packing at the start of the first cycle. Thus, increased gas-liquid, solid-liquid interactions and lower effective voidage will contribute to the increase in pressure drop. Even at low liquid flow, more pressure drop is expected as compared to the dry bed. This is also observed experimentally (Figure 5a-c). It should be noted that wet bed would become similar to a dry bed of subsequent cycles. Therefore, along the
8132 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 4. Hysteresisa in Pressure Drop and Deviation in Starting Point in First Cycles and in Subsequent Cycles (dp ) 1.43 mm) with Repacking of Bed dry nonporous hysteresis (cmwc kg/(m · s)) end point shifted from starting point second cycle onward hysteresis (cmwc kg/(m2 · s)) deviation in end point pressure drop variation (cmwc) in the flow range first cycle
a
2
-2
2888 × 10 yes 588 × 10-2 no 3.2-20.8
wet
semi porous -2
2788 × 10 yes 750 × 10-2 no 5.8-29.0
porous
nonporous -2
2555 × 10 yes 1038 × 10-2 no 7.0-32.0
-2
2353 × 10 yes 2078 × 10-2 no 10.0-30.0
semi porous -2
2978 × 10 yes 2603 × 10-2 no 10.2-38
porous 3078 × 10-2 no 3078 × 10-2 no 12.3-42.0
Area of the pressure-drop hysteresis loop or area between increasing and decreasing operating branch.
Figure 6. Liquid flow in the proposed favorable cluster (a, a1) when flow started, (b, b1) at increased flow, (c, c1) when it decreased back to initial flow (Maiti et al.48).
lines of the argument presented for subsequent cycles for the dry case (section 4.1), hysteresis is not expected to be present in any of the cycles in the wet bed including the first one. But, all three types of particles showed a large amount of hysteresis in pressure drop (Figure 5a-c; Table 3, Table 4) in the first cycle. Moreover, porous particles showed closed-loop hysteresis in the first cycle itself in contrast to the nonporous and semiporous particles. A large amount of constant closed-loop hysteresis with all three types of particles are shown in subsequent cycles (Figure 5a-c; Table 3, Table 4). Also the hystereses in the first cycle as well as in subsequent cycles were found to increase with pore density of the particles (Table 3 and Table 4). Once again, the observations do not conform to the expectations and to reconcile this one needs to find answer to these additional questions: (3) (First cycle) Why should there be hysteresis in a wet bed and how to account for the reduced gap between the end point and starting point of the hysteresis loop with increasing pore density of the particles? (4) (Subsequent cycles) Why should there be constant closed-loop hysteresis for porous particles only compared to first cycle and how to account for the increasing amount of hysteresis with the increase of pore density of the particles?
4.3. Verification and Resolution with Conceptual Models. Maiti et al.47reported a framework of hysteresis, based on the pore level concept of liquid spreading by Khanna and Nigam44 and incorporated it with the concept of participating and nonparticipating particles along with the current understanding of fluid flow in TBRs. As per the framework (Figure 6), with the commencement of liquid, a liquid flow pattern is established through favorable particle clusters. With the increase of flow, the liquid spreads laterally covering more particles in the periphery of the core channel, that is, channel diameter increases. During decreasing flow, the liquid filled core or filaments retract leaving the surrounding region with the film. On a further decrease of the flow, a stage will come when peripheral particles are dried up and the core of the channel reduces to film flow. Thus, peripheral particles of the channel are the major contributors to the hysteretic behavior. These particles are called seasoned particles or participating particles. The behavior of these peripheral (seasoned or participating) particles during the spreading and retraction of flow in increasing and decreasing flow modes was tested to explain the different hysteretic behavior of nonporous, semiporous, and porous particles.
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Dry Bed. Starting with the dry bed conditions, the liquid will start to flow as rivulet or film in the channel. With the increase of flow rate, the rivulet will grow in size and the enlargement of existing channels or the formation of additional channels takes place, covering more particles or the thickness of the rivulets increases. Therefore the pressure drop increases because of the lower effective bed porosity and larger liquid-solid and liquid-gas interactions. Also, it is understood that with the increasing flow rate, the three-phase contact line has to advance over the dry surface and will spread less due to higher contact angle, that is, rivulet-type flow persists. Liquid will spread more on porous particles because of the combined effect of wettability and capillary action of the pore, and film flow will be there in peripheral particles. So as per the farmework, this effect of enhanced liquid spreading will be more pronounced for porous particles having a higher pore density as compared to semiporous particles. In contrast, the liquid will spread less on nonporous particles, and mostly rivulet flow will dominate. During decreasing flow, liquid retracts over previously wetted surfaces. For a nonporous surface, this retraction is contactangle guided, and film flow will be there because of a lower contact angle. In porous surfaces, a combination of the following scenarios is likely to happen: (1) the retracting liquid is held up because of a pinning force provided by the pores, which will result in greater liquid coverage and higher pressure drops, (2) the retracting film breaks free of this pinning either by rupture due to the presence of pore as nonhomogeneity or by a drying up by the gas flow which will result in lower liquid coverage and lower pressure drop. So this effect will be more pronounced for porous particles having higher pore density compared to semiporous particles. This dry section may be at the particles in the periphery of the channel or part of the surface in a particle, called participating particles or seasoned particle as discussed earlier. So the enhanced liquid spreading during the increasing mode and film flow during the decreasing mode becomes more pronounced with increasing pore density, and the average difference of pressure drop between the two modes decreases. This answers the first question; that is, how to account for the decreasing amount of open-loop hysteresis with the increase of pore density of the particles? After the first cycle is over, a greater number of dry particles or seasoned particles will reappear in the channels for semiporous particles compared to nonporous particles due to rupture of the film. This number will be even greater for porous particles because of the greater pore density of the particles. In the subsequent cycles the liquid will spread and retract over these particles, in addition to the nonparticipating particles, and a constant closed-loop hysteresis will be observed. As the participating particles remain same, the descending branch of these cycles is expected to be the same as that of the first cycle, and this is observed in Figure 5a-c. In contrast the ascending branch of the second cycle is different as the participating particles are identified only after the first retraction. This is confirmed by the experimental observations also (Figures 5a-c). Because of the presence of a very less number of drier or seasoned particles the hysteresis effect is less pronounced in nonporous particles. Also, the bed is likely to come back to the same state as the one after the end of first cycle because of the loss of the residual film. This implies that the hysteresis loop should close. As the numbers of participating particles are greater for porous particles as compared to semiporous and nonporous particles, the hysteresis amount will be also greater. This answers the 2nd question; that is, how to account for reduced hysteresis in the subsequent cycles for each type of
particles compared to the first cycle, and how to account for an increasing amount of hysteresis with the increase of pore density of the particles? This is confirmed by the experimental observations for all three types of particles as shown in Table 3 and Table 4. Wet Bed. Even though the bed was wet, there is a possibility of some dry particles for nonporous particles as discussed earlier. These may be very less for porous or semiporous particles due to enhanced liquid spreading.43 There will be a greater number of favorable clusters or channels which will be generated in a wet bed than in dry bed, as the liquid has spread more in this case by way of prewetting. As stated earlier, each channel will consist of some nonparticipating and some participating particles. With an increasing pronounced effect of pores on liquid spreading, the participating particles will be greater for porous particles compared to semiporous and nonporous particle. Again the total number of such participating particles will be greater here as compared to dry bed (identified during the descending branch of the first cycle) due to a greater number of favorable clusters. During first cycle of operation these participating particles justify the existence of hysteresis even after prewetting the bed. This answers the first part of the third question; that is, why should there be hysteresis in a wet bed? In nonporous particles some of the dry particles, which are present, will be covered by liquid and may not be identified as participating particles during the descending branch. The numbers of these type of particle will be reduced from nonporous to porous because of the enhanced liquid spreading by pores. Thus the hysteresis will be open loop for nonporous particles during the first cycle and mostly closed loop for semiporous and porous particles. This answers the second part of the third question; that is, how to account for the reduced gap between the end point and the starting point from nonporous and semiporous to porous particle? For nonporous particles the ascending branch of the first cycle is affected by the presence of dry portions, whereas, due to enhanced liquid spreading, this is absent for porous particles. As these will not contribute to the hysteresis in subsequent cycles, the extent of hysteresis is expected to be less during subsequent cycles than the first cycle, whereas for porous it is expected to be absent showing closed-loop hysteresis. This answers the first part of the fourth question raised earlier; that is, why should there be constant closed-loop hysteresis for porous particles in the subsequent cycles? As the number of participating particles or seasoned particles increases from semiporous to porous due to the increasing impact of pore density, an increase in the amount of hysteresis is expected as per the framework of hysteresis, and this is observed as shown in Table 3 and Table 4. This answers the second part of our fourth question; that is, how to account for a substantial and different amount of closed-loop hysteresis in subsequent cycles for both porous and semiporous particles? 5. Conclusions Experiments were conducted to study the hysteretic behavior of industrial relevant-sized alumina extrudates with varying porosity level (i.e., porous, semiporous, and mostly nonporous), but made of same material. Comprehensive pressure-drop hysteresis data were generated with all three types of particles using dry-bed as well as wet-bed starting procedures in an increasing and decreasing mode of operation. Pronounced but different magnitudes of pressure-drop hysteresis were observed with porous, semiporous, and nonporous alumina extrudates. For comparison purposes, the hysteresis amount was quantified,
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and differences were compared with respect to startup procedure and porosity level of the particles. It can be concluded that pores play a major role in the hysteresis of TBRs. In a dry-bed startup procedure, the first cycle of operation showed open-loop hysteresis, and the amount of hysteresis also decreased with the increase of pore density of the particles. In a wet-bed startup procedure, the first cycle showed marginally open-loop hysteresis, but the amount of hysteresis increased with an increase of pore density of the particles. In the subsequent cycles, all cycles were closed loop, and amount of hysteresis increased with an increase of particle pore density. It is also observed that the maximum amount of hysteresis occurred with porous particles in a wet-bed startup procedure. The genesis of this difference lies in the different ways liquid spreads/retracts over porous, semiporous, and nonporous particles. Conceptual models of Khanna and Nigam (2002), based on the concept of participating and nonparticipating particles and principles of liquid spreading on porous and nonporous substrates, explained successfully the various features of hysteresis with particles of different pore density qualitatively. This pore-level concept may be exploited in the understanding of hydrodynamic states in prevailing operating conditions with backup from flow history and to optimize operating conditions for better performance. This study will also be expected to be useful to TBR researchers and practitioners in enhancing the understanding further to demystify the complex hydrodynamic phenomena in TBRs. However larger data banks are missing in the open literature for exact quantification of the effect of pores on hysteresis. Acknowledgment We wish to acknowledge the support of Center for High Technology, Ministry of Petroleum and Natural Gas, Government of India, for providing research facilities in the area of trickle-bed reactors. Nomenclature dp ) packing diameter, mm D ) diameter of bed, cm G ) gas mass flux, kg/m2s H ) height of the packing, m L ) liquid mass flux, kg/m2s P ) operating pressure, atm ∆P ) pressure drop, cmwc (cm of water column) or mmwc(mm of water column) eL ) liquid holdup, dimensionless η ) wetting efficiency, dimensionless
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ReceiVed for reView March 4, 2008 ReVised manuscript receiVed August 19, 2008 Accepted August 21, 2008 IE8003539
