- Email: [email protected]
Trickle Flow Multiplicity
TRICKLE FLOW MULTIPLICITY The Influence of the Prewetting Procedure on Flow Hysteresis I. van der Westhuizen, E. Du Toit and W. Nicol Department of Chemical Engineering, University of Pretoria, South Africa.
Abstract: The existence of multiple hydrodynamic studies (MHS) in trickle flow is a well-known phenomenon. It is also known that different prewetting procedures result in major differences in MHS when the hydrodynamic variables pressure drop, liquid holdup and gas–liquid mass transfer are considered. Given a certain prewetting procedure one still has the option to perform flow hysteresis cycles to achieve an even wider variety of MHS. Although numerous studies have been performed on trickle flow hysteresis, none have attempted to decouple the hysteresis behaviour from the prewetting procedure followed. Accordingly there are numerous hysteresis possibilities that have not been investigated. In this work a single liquid and gas cycle were performed for four distinct prewetting procedures described here as a dry bed, a Levec type prewetted bed, Kan prewetted bed (achieved by increasing either the liquid or the gas flow rate until the pulsing flow regime is reached) and a Super prewetted bed. Pressure drop, liquid holdup and gas–liquid mass transfer are the hydrodynamic parameters studied to quantify the various MHS. It is shown that the shape and extent of the hysteresis cycle are strongly dependant on the prewetting procedure. In terms of flow structure, similar hysteresis trends on the Kan Liquid and Super prewetting modes indicate that these modes are hydrodynamically similar. The additional measurement of the hysteresis behaviour of gas –liquid mass transfer proofs that neither holdup nor pressure drop can be used as an indicator of the distribution uniformity. Keywords: trickle flow; multiple hydrodynamic states; hysteresis.
INTRODUCTION
Corresponding to: Dr W. Nicol, Department of Chemical Engineering, University of Pretoria, South Africa, 0002. E-mail: [email protected]
DOI: 10.1205/cherd07080 0263–8762/07/ $30.00 þ 0.00 Chemical Engineering Research and Design Trans IChemE, Part A, December 2007 # 2007 Institution of Chemical Engineers
found a significant amount of variation in the trickle bed hydrodynamic parameters as a result of the various prewetting procedures. They proposed that the prewetting procedures define the extremes in MHS, which provide boundaries for an envelope of operation possibilities in trickle bed reactors. They also proposed that each trickle bed reaction might have an optimum hydrodynamic condition within the envelope defined by these extremes. The prewetting modes investigated by Loudon were:
Gravity driven liquid flow accompanied by a down flowing gas over a packed bed of particles (trickle flow) is a commonly employed method for liquid –gas –solid contacting. The complexity of this flow type has been extensively studied over the past five decades, but up to date no unified theory exists that encapsulates all the observed phenomena of trickle flow. This has not discouraged the industrial use of trickle flow, typically employed in trickle bed reactors, due to its relative simplicity as a multiphase reactor type in terms of construction and operation. The greatest contributing factor to the complexity of trickle flow is the existence of hydrodynamic multiplicity. Hydrodynamic multiplicity implies that a given gas and liquid velocity does not guarantee a single hydrodynamic state, even for a thoroughly prewetted bed. The extent of variation in multiple hydrodynamic states (MHS) was recently quantified by Loudon et al. (2006), for pressure drop, liquid holdup and gas–liquid mass transfer. They proposed specifically defined prewetting procedures as limiting cases of MHS. Loudon
. Dry mode: a bed packed with completely dry particles. The liquid and gas are introduced into the system at the desired flow rates. . Levec mode: The bed is prewetted by flooding the packed bed. Once prewetted, the liquid flow is shut off and the liquid is allowed to drain under gravity for 20 min. Liquid is then introduced into the system. . Kan liquid mode: The bed is prewetted by operating in the pulsing regime by increasing the liquid flow rate to the point of pulsing. The flow rates are subsequently decreased to their operating set points. 1604
Vol 85 (A12) 1604– 1610
TRICKLE FLOW MULTIPLICITY . Super mode: The bed is flooded completely with liquid. The flow rates are adjusted to the specific operation points without allowing the column to drain (the bed drains under irrigation) . Kan gas mode: The bed is prewetted by operating in the pulsing regime by increasing the gas flow rate to the point of pulsing. The flow rates are subsequently decreased to their operating set points. In their investigation it was found that specific regions were created for each parameter on each prewetting mode. Liquid holdup for instance, indicated four distinct regions as a result of the different prewetting procedures (Kan liquid, Super, Levec and the Dry bed). Liquid holdup in the Kan liquid bed was up to four times greater than the liquid holdup in the Dry bed, and thirty percent greater than the holdup in the Levec bed. Pressure drops in the Kan liquid and Super beds were as much as seven times greater than the pressure drop in the Dry and Levec beds. The volumetric gas-liquid mass transfer coefficients in the Kan liquid and Super beds were up to six times greater than the mass transfer coefficient in the Dry bed and two and a half times greater than the mass transfer coefficient in the Levec bed. The original work on MHS was based on the flow hysteresis behaviour of bed pressure drop and holdup. Here, a single operating point reached in one instance by increasing the flow, is compared to the other instance where it was reached by decreasing the flow. Various authors such as
1605
Kan and Greenfield (1978), Levec et al. (1988), Christensen et al. (1986), Lazzaroni et al. (1989) and Wang et al. (1995), investigated hysteresis behaviour. In these studies it was shown that these multiple steady states are stable and the difference in obtained steady states are substantial; Christensen et al. (1986), found that the pressure drop on the upper curve (decreasing path), could be as much as 100% greater than those on the lower curve (increasing path). However, discrepancies exist between the findings of the different studies—not only in terms of the extent of variation, but also the observed trends of the measured hydrodynamic parameters. These differences can be explained if the hysteretic behaviour is considered together with the prewetting mode extremes found by Loudon et al. (2006). Ravindra et al. (1997), and Lazzaroni et al. (1989), started their flow cycles on a dry bed while Christensen et al. (1986), Levec et al. (1988), Wammes et al. (1991) and Wang et al. (1995), performed their flow cycling on a Levec wetted bed. Kan and Greenfield (1978), first increased the flow rates to reach the high interaction (pulsing) regime before studying hysteresis behaviour. In addition, some authors like Levec et al. (1986, 1988), increased the flow rate on the upward leg of the flow cycle to such a degree that the pulsing regime was reached. Effectively changing the prewetting mode of the bed and therefore resulting in significantly enhanced hysteresis behaviour. It is therefore important to decouple the MHS obtained through flow hysteresis from the extreme cases, which result as a function of the prewetting mode.
Figure 1. The experimental setup enables dynamic monitoring of hydrodynamic parameters pressure drop, liquid holdup and gas– liquld mass transfer. Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
1606
VAN DER WESTHUIZEN et al.
Figure 2. Hysteretic behaviour of the hydrodynamic parameters pressure drop (a)–(e), Liquid holdup (f) –(j) and gas– liquid mass transfer (k)– (o), on different prewetting modes obtained by cycling the liquid flow rate at a constant gas superficial velocity of 20 mm s21.
In this work flow hysteresis is investigated as a function of the four different prewetting modes suggested by Loudon et al. (2006). Liquid and gas flow cycles were performed on all the modes while ensuring that the maximum increase in velocities is such that the high interaction regime is never reached on the upward leg of the flow cycle—i.e., ensuring that only one prewetting mode is considered during each loop. In addition to the popular hydrodynamic variables, pressure drop and liquid holdup, the variation of gas –liquid mass transfer is also measured. Previously this parameter was only considered by Wammes
et al. (1991) who only investigated liquid cycles on Levec prewetted beds. The resultant hysteresis cycles may provide insight into the flow morphology of the various prewetting modes and enable comparison of the hydrodynamic behaviour of the different modes. In addition it may shed additional light on the analysis of trickle flow hydrodynamics.
EXPERIMENTAL The same experimental set-up (shown in Figure 1), prewetting procedures and measurement techniques described
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
TRICKLE FLOW MULTIPLICITY in detail in the work by Loudon et al. (2006) were used in this work. All experiments were performed in a Perspex column, 85 cm long and 67 mm in diameter, at atmospheric pressure and ambient temperature. The bed was packed with 3 mm glass beads, while a constant bed porosity of 0.36 was maintained. Water was used as the liquid phase in all the experiments with nitrogen as the gas phase. The gas mass flow rate was controlled with a Brooks Smart Mass Flow Model 5851A and the liquid flow was measured by a PROline promag electromagnetic measuring instrument from Endress þ Hauser. The gas and liquid were fed concurrently into the bed—the liquid through a perforated plate-type distributor with 0.5 mm diameter holes spaced 8 mm apart in a square pitch arrangement. This results in a drip-point density of 16 000 points/m2, which according to Burghardt et al. (1995) is sufficient for uniform liquid distribution. The gas was distributed via three 1/4 in stainless steel tubes in order to ensure that the liquid distribution remained uniform even at high gas flow rates.
Measurement of Hydrodynamic Parameters The experimental set-up allowed for dynamic monitoring of the bed average hydrodynamic parameters: pressure drop, liquid holdup and gas–liquid mass transfer. Pressure drop was measured with a differential pressure transmitter, with pressure taps 650 mm apart, along the length of the bed. The liquid holdup was measured by mounting the column on a high accuracy load cell using the weighting technique described by Ellman et al. (1990). A desorption technique was used to determine the gas–liquid mass transfer coefficient in conjunction with a water–nitrogen system; oxygen is transferred from water, pre-saturated with air, to nitrogen, which is fed co-currently into the column with the oxygen rich water. The gas–liquid mass transfer coefficient is determined by the amount of oxygen stripping that occurs and is calculated using equation (1) (Goto and Smith, 1975). L (CL,O2 )e0 (CL,O2 )f kL a ¼ ln (1) Z (CL,O2 )e (CL,O2 )f 0 At each operating point, the feed (CLO2f) and exit (CLO2 e) dissolved oxygen concentrations were measured with a 499ADO Dissolved Oxygen Sensor from Rosemount Analytical. Each experiment was mimicked in a small (10 cm) glass column (of the same diameter) to determine the end effects rendering the terms (CLO2f’) and (CLO2e’). A mass balance over both of the columns, assuming plug flow of the liquid phase, is the basis for the derivation of equation (1).
Procedure Flow cycles were performed on all the prewetting modes described by Loudon et al. (2006), in order to investigate the possible effect of prewetting mode on the hysteresis behaviour of the hydrodynamic parameters. After the prewetting procedure was followed, the column was operated at each set point for ten minutes—after which no change in any of the measured variables was observed. Previous work by Van der Merwe et al. (2007), where X-ray radiography was used to study the flow distribution and stability in trickle bed columns of similar size, also confirms that this is sufficient to ensure steady state. The flow was then incrementally increased to
1607
the highest flow rate, and subsequently decreased back to the lowest in order to complete a flow cycle. Cycles where either the gas flow rate or the liquid flow rate was kept constant were performed. Liquid flow cycles were created by following the flow path UL ¼ 5, 7, 9, 7, 5 mm s21 (superficial liquid velocity) at a constant gas superficial velocity (UG) of 20 mm s21 except on the Kan Gas prewetting mode. Here a minimum liquid superficial velocity of 9 mm s21 was required to induce pulsing during the initial prewetting procedure and the cycle path was subsequently adjusted to UL ¼ 9, 9.7, 10.5, 9.7, 9 mm s21. Gas flow cycles were created by following the superficial velocity flow path UG ¼ 20, 40, 70, 40, 20 mm s21 at a constant liquid superficial velocity of 9 mm s21. The highest flow rate on each cycle was always well below pulsing velocity to ensure that the mode of operation did not change during the cycles. The prewetting modes were used in the following order: . . . . .
Dry Levec Kan liquid Super Kan gas mode
RESULTS AND DISCUSSION Almost all of the prewetting modes exhibit hysteresis behaviour as a result of liquid flow loops, for most of the hydrodynamic parameters (Figure 2). In order to facilitate comparison of the effect of the different prewetting modes, the variations in all the parameters are quantified in Table 1. The maximum observed variation of a parameter between identical flow rate values in a single loop is reported as a percentage of the absolute value of the total variation between the lowest and highest liquid flow rate in the cycle. All the cycles were repeated at least twice on all the prewetting modes. Similar hysteresis trends to those shown in Figure 2 were obtained for all the repeat experiments. Repeatability was such that on the values reported in Table 1 pressure drop variation varied with +6.7% on average, holdup with +2.9% and gas –liquid mass transfer with +2.5% Pressures drop hysteresis trends on all the prewetting modes [Figure 2 (a) –(e)] conform to the behaviour expected from literature where a higher pressure drop is observed for the decreasing leg than the increasing leg. A significant increase in the gas liquid mass transfer is observed for the Dry and Levec prewetted beds. On the Levec mode, an increase in liquid flow rate increases the gas liquid mass transfer coefficient. Once the Levec bed is drained, stagnant liquid pockets are left in the bed. With an increase in flow rate, it will be easier for the liquid to spread over these liquid pockets than it will be to spread through the completely Table 1. Scaled maximum variations observed for hydrodynamic parameters during a liquid flow cycle on different prewetting modes. Prewetting mode
dP/Z
Ht/1
kLa
Dry Levec Kan liquid Super Kan gas
33.9% 60.4% 23.5% 43.1% 61.4%
59.7% 37.1% 2.3% 3.0% 47.4%
37.0% 31.2% 223.2% 216.8% 277.3%
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
1608
VAN DER WESTHUIZEN et al.
Figure 3. Hysteretic behaviour of the hydrodynamic parameters on different prewetting modes obtained by cycling the gas flow rate.
dry particles. On both these modes, the newly formed liquid channels add gas –liquid interfacial area, which remains stable when the flow rate is decreased again. The gas – liquid mass transfer is therefore higher on the decreasing leg. The increase, though substantial, does not nearly approach the mass transfer coefficients measured on the more uniformly wetted Kan liquid and Super modes. In all probability, the same high mass transfer coefficients will only be achieved on the Dry/Levec modes when the maximum liquid flow rate in the flow cycle is within the high interaction (pulsing) regime—actually then resulting in a change in the prewetting procedure. This confirms the observation by Loudon et al. (2006) that the extremes of MHS are actually
set by the prewetting modes. It is interesting to note that the loops are either open ended or closed for all three measurements on each mode, with the exception of the pressure drop loop on the dry mode. The reason for this single exemption is not clear but can be attributed to the well known erratic behaviour of dry beds (Louden et al., 2006). On both the Kan liquid and the Super modes the mass transfer coefficient have lower values on the descending leg of the flow cycle while the pressure drop shows an increase in pressure along the return leg. However, on both these modes the cycle trends show that the hydrodynamic parameters practically return to their initial values at the beginning of the cycle. This suggests that the Kan liquid
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
TRICKLE FLOW MULTIPLICITY and Super prewetting mode have a similar, relatively stable flow structure, which is not permanently modified during a liquid flow cycle. While the Kan gas mode hysteresis cycles also show lower values for the mass transfer coefficient on the return leg with increased pressure drop measurements, these variations continue to increase as the starting point of the cycle is approached. The increase in liquid flow rate may therefore block some of the channels that were initially blown open by the pulsing gas flow—resulting in an irreversible change in flow structure. This will also explain the significant increase in liquid holdup. This phenomenon implies that the flow distribution is significantly different than on the Kan liquid and Super modes—although similar mass transfer coefficients at the start of each cycle may suggest relatively equal initial gas–liquid interfacial areas. In Figure 3 the trends observed with gas cycling at a constant liquid flow rate are shown. The variations obtained, as quantified in Table 2, are much less significant than with liquid flow cycles. Repeat experiments again confirmed the trends observed in Figure 3 and for the gas flow cycles the pressure drop variation varied with +7% on average, holdup with +7,8% and gas–liquid mass transfer with +4.7%. Specifically the gas liquid mass transfer shows very little hysteretic behaviour on all the modes, despite the fact that in some instances a more notable variation in either holdup or pressure drop is observed. Especially on the Dry and Levec modes much higher holdup values are observed on the decreasing path and although the pressure drop and gas–liquid mass transfer coefficient also increase, it is to a much lesser degree than the holdup. Wang et al. (1995), also report increased pressure drop values on the return leg of gas flow cycles. According to Wang a lower pressure drop is related to less uniform distribution of gas–liquid flow in the trickle-bed. The increased gas flow rate on these modes should therefore serve to promote more uniform liquid distribution. However, the trends observed on the Kan liquid and Super modes show that this argument can not be generalized. On both these modes the pressure drop is lower on the decreasing path, while correspondingly a slight increase in mass transfer coefficient and holdup is measured. Overall, these results show that neither pressure drop nor holdup can be used as an indicator of the distribution uniformity. It rather indicates that the quantification of the flow morphology is more intricate than merely distribution uniformity. In order to explain the decrease in pressure drop on the return leg of some of the cycles, it has been proposed by Kan and Greenfield (1978), that the gas may orientate the liquid bridges in a manner that the gas tortuosity is decreased. On the decreasing leg the lower tortuosity will accordingly result in a lower pressure drop. As with the
1609
liquid flow cycles the hysteresis trends and extent of the hydrodynamic parameters on the Kan liquid and Super modes are similar. This confirms the fact that the flow structure on these two modes is hydrodynamically equivalent. The Kan gas mode behaves in a completely different manner. Here practically no hysteresis is observed in all the hydrodynamic parameters. The initial orientation gained by increasing the gas flow rate to the point of pulsing (during prewetting), seems to remain stable throughout the cycles.
CONCLUSIONS This work again confirms that variation in the hydrodynamic parameters of trickle bed reactors can be achieved by performing both gas and liquid flow cycles. In addition it is shown that the trend and extent of such hysteretic behaviour depends on the prewetting procedure followed before each flow cycle. Specifically, the different trends observed with flow cycling can be used to compare the flow structures, which result from different prewetting procedures. In this manner it can be concluded that the Kan liquid and Super modes are hydrodynamically equivalent, while the Kan gas mode illustrates significantly different hydrodynamic behaviour. Comparison of the observed trends of the different measured hydrodynamic parameters—pressure drop, liquid holdup and volumetric gas liquid mass transfer coefficient— clearly shows that neither pressure drop nor liquid holdup can be used as indicator of the distribution uniformity. Finally, the inclusion of an additional hydrodynamic parameter (gas – liquid mass transfer), combined with prewetting dependent hysteresis cycles, highlights the complexity of trickle flow morphology.
NOMENCLATURE
L dP/Z U Z
oxygen concentration in the liquid, kg m23 total liquid holdup volumetric gas–liquid mass transfer coefficient, l s21 liquid flux, kg m2 s21 pressure drop per unit bed length, kPa m21 superficial velocity, mm s21 bed length, m
Greek symbol 1
porosity
Subscripts e f G L
entrance final or exit gas liquid
CL,O2 Ht kLa
REFERENCES Table 2. Scaled maximum variations observed for hydrodynamic parameters during a gas flow cycle on different prewetting modes. Prewetting mode
dP/Z
Ht/1
kLa
Dry Levec Kan liquid Super Kan gas
11.6% 4.0% 236.1% 27.5% 1.13%
148.0% 301.3% 39.8% 13.0% 5.5%
11.1% 19.8% 13.9% 15.8% 10%
Burghardt, A., Bartelmus, G., Jaroszynski, M. and Kolodziej, A., 1995, Hydrodynamics and mass transfer in a three-phase fixedbed reactor with cocurrent gas-liquid downflow, Chem Eng J, 58: 83–99. Christensen, G., McGovern, S.J. and Sundaresan, S., 1986, Cocurrent downflow of air and water in a two-dimensional packed column, AIChE J, 32(10): 1677– 1689. Ellman, M.J., Midoux, N., Wild, G., Laurent, A. and Charpentier, J.C., 1990, A new, improved liquid holdup correlation for trickle-bed reactors, Chem Eng Sci, 45: 1677–1684. Goto, S. and Smith, J.M., 1975, Trickle-bed reactor performance— Part 1. Holdup and mass transfer, AIChE J, 21: 706 –713.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
1610
VAN DER WESTHUIZEN et al.
Kan, K.M. and Greenfield, P.F., 1978, Multiple hydrodynamic states in cocurrent two-phase down-flow through packed beds, Ind Eng Chem Process Des Dev, 17: 482– 485. Lazzaroni, C.L., Keselman, H.R. and Figoli, N.S., 1989, Trickle bed reactors. multiplicity of hydrodynamic states, relation between the pressure drop and the liquid holdup, Ind Eng Chem Res, 28: 119– 121. Levec, J., Grosser, K. and Carbonell, R.G., 1988, The hysteretic behaviour of pressure drop and liquid holdup in trickle beds, AIChE J, 34: 1027– 1030. Levec, J., Saez, A.E. and Carbonell, R.G., 1986, The hydrodynamics of trickling flow in packed beds. Part I: Conduit models, AIChE J, 32: 515–523. Loudon, D., Van der Merwe, W. and Nicol, W., 2006, Multiple hydrodynamic states in trickle flow: Quantifying the extent of pressure drop, liquid holdup and gas-liquid mass transfer variation, Chem Eng Sci, 61: 7551– 7562.
Ravindra, P.V., Rao, D.P. and Rao, M.S., 1997, Liquid flow texture in trickle-bed reactors: an experimental study, Ind Eng Chem Res, 36: 5133– 5145. Van der Merwe, W., Nicol, W. and De Beer, F., 2007, Trickle flow distribution and stability by X-ray radiography, Chem Eng J, 132: 47–59. Wammes, W.J.A., Middelkamp, J., Huisman, W.J., de Baas, C.M. and Westerterp, K.R., 1991, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, AIChE J, 37: 1849 – 1862. Wang, R., Mao, Z. and Chen, J., 1995, Experimental and theoretical studies of pressure drop hysteresis in trickle bed reactors, Chem Eng Sci, 50(14): 2321–2328. The manuscript was received 30 May 2007 and accepted for publication after revision 5 September 2007.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
Abstract: The existence of multiple hydrodynamic studies (MHS) in trickle flow is a well-known phenomenon. It is also known that different prewetting procedures result in major differences in MHS when the hydrodynamic variables pressure drop, liquid holdup and gas–liquid mass transfer are considered. Given a certain prewetting procedure one still has the option to perform flow hysteresis cycles to achieve an even wider variety of MHS. Although numerous studies have been performed on trickle flow hysteresis, none have attempted to decouple the hysteresis behaviour from the prewetting procedure followed. Accordingly there are numerous hysteresis possibilities that have not been investigated. In this work a single liquid and gas cycle were performed for four distinct prewetting procedures described here as a dry bed, a Levec type prewetted bed, Kan prewetted bed (achieved by increasing either the liquid or the gas flow rate until the pulsing flow regime is reached) and a Super prewetted bed. Pressure drop, liquid holdup and gas–liquid mass transfer are the hydrodynamic parameters studied to quantify the various MHS. It is shown that the shape and extent of the hysteresis cycle are strongly dependant on the prewetting procedure. In terms of flow structure, similar hysteresis trends on the Kan Liquid and Super prewetting modes indicate that these modes are hydrodynamically similar. The additional measurement of the hysteresis behaviour of gas –liquid mass transfer proofs that neither holdup nor pressure drop can be used as an indicator of the distribution uniformity. Keywords: trickle flow; multiple hydrodynamic states; hysteresis.
INTRODUCTION
Corresponding to: Dr W. Nicol, Department of Chemical Engineering, University of Pretoria, South Africa, 0002. E-mail: [email protected]
DOI: 10.1205/cherd07080 0263–8762/07/ $30.00 þ 0.00 Chemical Engineering Research and Design Trans IChemE, Part A, December 2007 # 2007 Institution of Chemical Engineers
found a significant amount of variation in the trickle bed hydrodynamic parameters as a result of the various prewetting procedures. They proposed that the prewetting procedures define the extremes in MHS, which provide boundaries for an envelope of operation possibilities in trickle bed reactors. They also proposed that each trickle bed reaction might have an optimum hydrodynamic condition within the envelope defined by these extremes. The prewetting modes investigated by Loudon were:
Gravity driven liquid flow accompanied by a down flowing gas over a packed bed of particles (trickle flow) is a commonly employed method for liquid –gas –solid contacting. The complexity of this flow type has been extensively studied over the past five decades, but up to date no unified theory exists that encapsulates all the observed phenomena of trickle flow. This has not discouraged the industrial use of trickle flow, typically employed in trickle bed reactors, due to its relative simplicity as a multiphase reactor type in terms of construction and operation. The greatest contributing factor to the complexity of trickle flow is the existence of hydrodynamic multiplicity. Hydrodynamic multiplicity implies that a given gas and liquid velocity does not guarantee a single hydrodynamic state, even for a thoroughly prewetted bed. The extent of variation in multiple hydrodynamic states (MHS) was recently quantified by Loudon et al. (2006), for pressure drop, liquid holdup and gas–liquid mass transfer. They proposed specifically defined prewetting procedures as limiting cases of MHS. Loudon
. Dry mode: a bed packed with completely dry particles. The liquid and gas are introduced into the system at the desired flow rates. . Levec mode: The bed is prewetted by flooding the packed bed. Once prewetted, the liquid flow is shut off and the liquid is allowed to drain under gravity for 20 min. Liquid is then introduced into the system. . Kan liquid mode: The bed is prewetted by operating in the pulsing regime by increasing the liquid flow rate to the point of pulsing. The flow rates are subsequently decreased to their operating set points. 1604
Vol 85 (A12) 1604– 1610
TRICKLE FLOW MULTIPLICITY . Super mode: The bed is flooded completely with liquid. The flow rates are adjusted to the specific operation points without allowing the column to drain (the bed drains under irrigation) . Kan gas mode: The bed is prewetted by operating in the pulsing regime by increasing the gas flow rate to the point of pulsing. The flow rates are subsequently decreased to their operating set points. In their investigation it was found that specific regions were created for each parameter on each prewetting mode. Liquid holdup for instance, indicated four distinct regions as a result of the different prewetting procedures (Kan liquid, Super, Levec and the Dry bed). Liquid holdup in the Kan liquid bed was up to four times greater than the liquid holdup in the Dry bed, and thirty percent greater than the holdup in the Levec bed. Pressure drops in the Kan liquid and Super beds were as much as seven times greater than the pressure drop in the Dry and Levec beds. The volumetric gas-liquid mass transfer coefficients in the Kan liquid and Super beds were up to six times greater than the mass transfer coefficient in the Dry bed and two and a half times greater than the mass transfer coefficient in the Levec bed. The original work on MHS was based on the flow hysteresis behaviour of bed pressure drop and holdup. Here, a single operating point reached in one instance by increasing the flow, is compared to the other instance where it was reached by decreasing the flow. Various authors such as
1605
Kan and Greenfield (1978), Levec et al. (1988), Christensen et al. (1986), Lazzaroni et al. (1989) and Wang et al. (1995), investigated hysteresis behaviour. In these studies it was shown that these multiple steady states are stable and the difference in obtained steady states are substantial; Christensen et al. (1986), found that the pressure drop on the upper curve (decreasing path), could be as much as 100% greater than those on the lower curve (increasing path). However, discrepancies exist between the findings of the different studies—not only in terms of the extent of variation, but also the observed trends of the measured hydrodynamic parameters. These differences can be explained if the hysteretic behaviour is considered together with the prewetting mode extremes found by Loudon et al. (2006). Ravindra et al. (1997), and Lazzaroni et al. (1989), started their flow cycles on a dry bed while Christensen et al. (1986), Levec et al. (1988), Wammes et al. (1991) and Wang et al. (1995), performed their flow cycling on a Levec wetted bed. Kan and Greenfield (1978), first increased the flow rates to reach the high interaction (pulsing) regime before studying hysteresis behaviour. In addition, some authors like Levec et al. (1986, 1988), increased the flow rate on the upward leg of the flow cycle to such a degree that the pulsing regime was reached. Effectively changing the prewetting mode of the bed and therefore resulting in significantly enhanced hysteresis behaviour. It is therefore important to decouple the MHS obtained through flow hysteresis from the extreme cases, which result as a function of the prewetting mode.
Figure 1. The experimental setup enables dynamic monitoring of hydrodynamic parameters pressure drop, liquid holdup and gas– liquld mass transfer. Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
1606
VAN DER WESTHUIZEN et al.
Figure 2. Hysteretic behaviour of the hydrodynamic parameters pressure drop (a)–(e), Liquid holdup (f) –(j) and gas– liquid mass transfer (k)– (o), on different prewetting modes obtained by cycling the liquid flow rate at a constant gas superficial velocity of 20 mm s21.
In this work flow hysteresis is investigated as a function of the four different prewetting modes suggested by Loudon et al. (2006). Liquid and gas flow cycles were performed on all the modes while ensuring that the maximum increase in velocities is such that the high interaction regime is never reached on the upward leg of the flow cycle—i.e., ensuring that only one prewetting mode is considered during each loop. In addition to the popular hydrodynamic variables, pressure drop and liquid holdup, the variation of gas –liquid mass transfer is also measured. Previously this parameter was only considered by Wammes
et al. (1991) who only investigated liquid cycles on Levec prewetted beds. The resultant hysteresis cycles may provide insight into the flow morphology of the various prewetting modes and enable comparison of the hydrodynamic behaviour of the different modes. In addition it may shed additional light on the analysis of trickle flow hydrodynamics.
EXPERIMENTAL The same experimental set-up (shown in Figure 1), prewetting procedures and measurement techniques described
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
TRICKLE FLOW MULTIPLICITY in detail in the work by Loudon et al. (2006) were used in this work. All experiments were performed in a Perspex column, 85 cm long and 67 mm in diameter, at atmospheric pressure and ambient temperature. The bed was packed with 3 mm glass beads, while a constant bed porosity of 0.36 was maintained. Water was used as the liquid phase in all the experiments with nitrogen as the gas phase. The gas mass flow rate was controlled with a Brooks Smart Mass Flow Model 5851A and the liquid flow was measured by a PROline promag electromagnetic measuring instrument from Endress þ Hauser. The gas and liquid were fed concurrently into the bed—the liquid through a perforated plate-type distributor with 0.5 mm diameter holes spaced 8 mm apart in a square pitch arrangement. This results in a drip-point density of 16 000 points/m2, which according to Burghardt et al. (1995) is sufficient for uniform liquid distribution. The gas was distributed via three 1/4 in stainless steel tubes in order to ensure that the liquid distribution remained uniform even at high gas flow rates.
Measurement of Hydrodynamic Parameters The experimental set-up allowed for dynamic monitoring of the bed average hydrodynamic parameters: pressure drop, liquid holdup and gas–liquid mass transfer. Pressure drop was measured with a differential pressure transmitter, with pressure taps 650 mm apart, along the length of the bed. The liquid holdup was measured by mounting the column on a high accuracy load cell using the weighting technique described by Ellman et al. (1990). A desorption technique was used to determine the gas–liquid mass transfer coefficient in conjunction with a water–nitrogen system; oxygen is transferred from water, pre-saturated with air, to nitrogen, which is fed co-currently into the column with the oxygen rich water. The gas–liquid mass transfer coefficient is determined by the amount of oxygen stripping that occurs and is calculated using equation (1) (Goto and Smith, 1975). L (CL,O2 )e0 (CL,O2 )f kL a ¼ ln (1) Z (CL,O2 )e (CL,O2 )f 0 At each operating point, the feed (CLO2f) and exit (CLO2 e) dissolved oxygen concentrations were measured with a 499ADO Dissolved Oxygen Sensor from Rosemount Analytical. Each experiment was mimicked in a small (10 cm) glass column (of the same diameter) to determine the end effects rendering the terms (CLO2f’) and (CLO2e’). A mass balance over both of the columns, assuming plug flow of the liquid phase, is the basis for the derivation of equation (1).
Procedure Flow cycles were performed on all the prewetting modes described by Loudon et al. (2006), in order to investigate the possible effect of prewetting mode on the hysteresis behaviour of the hydrodynamic parameters. After the prewetting procedure was followed, the column was operated at each set point for ten minutes—after which no change in any of the measured variables was observed. Previous work by Van der Merwe et al. (2007), where X-ray radiography was used to study the flow distribution and stability in trickle bed columns of similar size, also confirms that this is sufficient to ensure steady state. The flow was then incrementally increased to
1607
the highest flow rate, and subsequently decreased back to the lowest in order to complete a flow cycle. Cycles where either the gas flow rate or the liquid flow rate was kept constant were performed. Liquid flow cycles were created by following the flow path UL ¼ 5, 7, 9, 7, 5 mm s21 (superficial liquid velocity) at a constant gas superficial velocity (UG) of 20 mm s21 except on the Kan Gas prewetting mode. Here a minimum liquid superficial velocity of 9 mm s21 was required to induce pulsing during the initial prewetting procedure and the cycle path was subsequently adjusted to UL ¼ 9, 9.7, 10.5, 9.7, 9 mm s21. Gas flow cycles were created by following the superficial velocity flow path UG ¼ 20, 40, 70, 40, 20 mm s21 at a constant liquid superficial velocity of 9 mm s21. The highest flow rate on each cycle was always well below pulsing velocity to ensure that the mode of operation did not change during the cycles. The prewetting modes were used in the following order: . . . . .
Dry Levec Kan liquid Super Kan gas mode
RESULTS AND DISCUSSION Almost all of the prewetting modes exhibit hysteresis behaviour as a result of liquid flow loops, for most of the hydrodynamic parameters (Figure 2). In order to facilitate comparison of the effect of the different prewetting modes, the variations in all the parameters are quantified in Table 1. The maximum observed variation of a parameter between identical flow rate values in a single loop is reported as a percentage of the absolute value of the total variation between the lowest and highest liquid flow rate in the cycle. All the cycles were repeated at least twice on all the prewetting modes. Similar hysteresis trends to those shown in Figure 2 were obtained for all the repeat experiments. Repeatability was such that on the values reported in Table 1 pressure drop variation varied with +6.7% on average, holdup with +2.9% and gas –liquid mass transfer with +2.5% Pressures drop hysteresis trends on all the prewetting modes [Figure 2 (a) –(e)] conform to the behaviour expected from literature where a higher pressure drop is observed for the decreasing leg than the increasing leg. A significant increase in the gas liquid mass transfer is observed for the Dry and Levec prewetted beds. On the Levec mode, an increase in liquid flow rate increases the gas liquid mass transfer coefficient. Once the Levec bed is drained, stagnant liquid pockets are left in the bed. With an increase in flow rate, it will be easier for the liquid to spread over these liquid pockets than it will be to spread through the completely Table 1. Scaled maximum variations observed for hydrodynamic parameters during a liquid flow cycle on different prewetting modes. Prewetting mode
dP/Z
Ht/1
kLa
Dry Levec Kan liquid Super Kan gas
33.9% 60.4% 23.5% 43.1% 61.4%
59.7% 37.1% 2.3% 3.0% 47.4%
37.0% 31.2% 223.2% 216.8% 277.3%
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
1608
VAN DER WESTHUIZEN et al.
Figure 3. Hysteretic behaviour of the hydrodynamic parameters on different prewetting modes obtained by cycling the gas flow rate.
dry particles. On both these modes, the newly formed liquid channels add gas –liquid interfacial area, which remains stable when the flow rate is decreased again. The gas – liquid mass transfer is therefore higher on the decreasing leg. The increase, though substantial, does not nearly approach the mass transfer coefficients measured on the more uniformly wetted Kan liquid and Super modes. In all probability, the same high mass transfer coefficients will only be achieved on the Dry/Levec modes when the maximum liquid flow rate in the flow cycle is within the high interaction (pulsing) regime—actually then resulting in a change in the prewetting procedure. This confirms the observation by Loudon et al. (2006) that the extremes of MHS are actually
set by the prewetting modes. It is interesting to note that the loops are either open ended or closed for all three measurements on each mode, with the exception of the pressure drop loop on the dry mode. The reason for this single exemption is not clear but can be attributed to the well known erratic behaviour of dry beds (Louden et al., 2006). On both the Kan liquid and the Super modes the mass transfer coefficient have lower values on the descending leg of the flow cycle while the pressure drop shows an increase in pressure along the return leg. However, on both these modes the cycle trends show that the hydrodynamic parameters practically return to their initial values at the beginning of the cycle. This suggests that the Kan liquid
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
TRICKLE FLOW MULTIPLICITY and Super prewetting mode have a similar, relatively stable flow structure, which is not permanently modified during a liquid flow cycle. While the Kan gas mode hysteresis cycles also show lower values for the mass transfer coefficient on the return leg with increased pressure drop measurements, these variations continue to increase as the starting point of the cycle is approached. The increase in liquid flow rate may therefore block some of the channels that were initially blown open by the pulsing gas flow—resulting in an irreversible change in flow structure. This will also explain the significant increase in liquid holdup. This phenomenon implies that the flow distribution is significantly different than on the Kan liquid and Super modes—although similar mass transfer coefficients at the start of each cycle may suggest relatively equal initial gas–liquid interfacial areas. In Figure 3 the trends observed with gas cycling at a constant liquid flow rate are shown. The variations obtained, as quantified in Table 2, are much less significant than with liquid flow cycles. Repeat experiments again confirmed the trends observed in Figure 3 and for the gas flow cycles the pressure drop variation varied with +7% on average, holdup with +7,8% and gas–liquid mass transfer with +4.7%. Specifically the gas liquid mass transfer shows very little hysteretic behaviour on all the modes, despite the fact that in some instances a more notable variation in either holdup or pressure drop is observed. Especially on the Dry and Levec modes much higher holdup values are observed on the decreasing path and although the pressure drop and gas–liquid mass transfer coefficient also increase, it is to a much lesser degree than the holdup. Wang et al. (1995), also report increased pressure drop values on the return leg of gas flow cycles. According to Wang a lower pressure drop is related to less uniform distribution of gas–liquid flow in the trickle-bed. The increased gas flow rate on these modes should therefore serve to promote more uniform liquid distribution. However, the trends observed on the Kan liquid and Super modes show that this argument can not be generalized. On both these modes the pressure drop is lower on the decreasing path, while correspondingly a slight increase in mass transfer coefficient and holdup is measured. Overall, these results show that neither pressure drop nor holdup can be used as an indicator of the distribution uniformity. It rather indicates that the quantification of the flow morphology is more intricate than merely distribution uniformity. In order to explain the decrease in pressure drop on the return leg of some of the cycles, it has been proposed by Kan and Greenfield (1978), that the gas may orientate the liquid bridges in a manner that the gas tortuosity is decreased. On the decreasing leg the lower tortuosity will accordingly result in a lower pressure drop. As with the
1609
liquid flow cycles the hysteresis trends and extent of the hydrodynamic parameters on the Kan liquid and Super modes are similar. This confirms the fact that the flow structure on these two modes is hydrodynamically equivalent. The Kan gas mode behaves in a completely different manner. Here practically no hysteresis is observed in all the hydrodynamic parameters. The initial orientation gained by increasing the gas flow rate to the point of pulsing (during prewetting), seems to remain stable throughout the cycles.
CONCLUSIONS This work again confirms that variation in the hydrodynamic parameters of trickle bed reactors can be achieved by performing both gas and liquid flow cycles. In addition it is shown that the trend and extent of such hysteretic behaviour depends on the prewetting procedure followed before each flow cycle. Specifically, the different trends observed with flow cycling can be used to compare the flow structures, which result from different prewetting procedures. In this manner it can be concluded that the Kan liquid and Super modes are hydrodynamically equivalent, while the Kan gas mode illustrates significantly different hydrodynamic behaviour. Comparison of the observed trends of the different measured hydrodynamic parameters—pressure drop, liquid holdup and volumetric gas liquid mass transfer coefficient— clearly shows that neither pressure drop nor liquid holdup can be used as indicator of the distribution uniformity. Finally, the inclusion of an additional hydrodynamic parameter (gas – liquid mass transfer), combined with prewetting dependent hysteresis cycles, highlights the complexity of trickle flow morphology.
NOMENCLATURE
L dP/Z U Z
oxygen concentration in the liquid, kg m23 total liquid holdup volumetric gas–liquid mass transfer coefficient, l s21 liquid flux, kg m2 s21 pressure drop per unit bed length, kPa m21 superficial velocity, mm s21 bed length, m
Greek symbol 1
porosity
Subscripts e f G L
entrance final or exit gas liquid
CL,O2 Ht kLa
REFERENCES Table 2. Scaled maximum variations observed for hydrodynamic parameters during a gas flow cycle on different prewetting modes. Prewetting mode
dP/Z
Ht/1
kLa
Dry Levec Kan liquid Super Kan gas
11.6% 4.0% 236.1% 27.5% 1.13%
148.0% 301.3% 39.8% 13.0% 5.5%
11.1% 19.8% 13.9% 15.8% 10%
Burghardt, A., Bartelmus, G., Jaroszynski, M. and Kolodziej, A., 1995, Hydrodynamics and mass transfer in a three-phase fixedbed reactor with cocurrent gas-liquid downflow, Chem Eng J, 58: 83–99. Christensen, G., McGovern, S.J. and Sundaresan, S., 1986, Cocurrent downflow of air and water in a two-dimensional packed column, AIChE J, 32(10): 1677– 1689. Ellman, M.J., Midoux, N., Wild, G., Laurent, A. and Charpentier, J.C., 1990, A new, improved liquid holdup correlation for trickle-bed reactors, Chem Eng Sci, 45: 1677–1684. Goto, S. and Smith, J.M., 1975, Trickle-bed reactor performance— Part 1. Holdup and mass transfer, AIChE J, 21: 706 –713.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610
1610
VAN DER WESTHUIZEN et al.
Kan, K.M. and Greenfield, P.F., 1978, Multiple hydrodynamic states in cocurrent two-phase down-flow through packed beds, Ind Eng Chem Process Des Dev, 17: 482– 485. Lazzaroni, C.L., Keselman, H.R. and Figoli, N.S., 1989, Trickle bed reactors. multiplicity of hydrodynamic states, relation between the pressure drop and the liquid holdup, Ind Eng Chem Res, 28: 119– 121. Levec, J., Grosser, K. and Carbonell, R.G., 1988, The hysteretic behaviour of pressure drop and liquid holdup in trickle beds, AIChE J, 34: 1027– 1030. Levec, J., Saez, A.E. and Carbonell, R.G., 1986, The hydrodynamics of trickling flow in packed beds. Part I: Conduit models, AIChE J, 32: 515–523. Loudon, D., Van der Merwe, W. and Nicol, W., 2006, Multiple hydrodynamic states in trickle flow: Quantifying the extent of pressure drop, liquid holdup and gas-liquid mass transfer variation, Chem Eng Sci, 61: 7551– 7562.
Ravindra, P.V., Rao, D.P. and Rao, M.S., 1997, Liquid flow texture in trickle-bed reactors: an experimental study, Ind Eng Chem Res, 36: 5133– 5145. Van der Merwe, W., Nicol, W. and De Beer, F., 2007, Trickle flow distribution and stability by X-ray radiography, Chem Eng J, 132: 47–59. Wammes, W.J.A., Middelkamp, J., Huisman, W.J., de Baas, C.M. and Westerterp, K.R., 1991, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, AIChE J, 37: 1849 – 1862. Wang, R., Mao, Z. and Chen, J., 1995, Experimental and theoretical studies of pressure drop hysteresis in trickle bed reactors, Chem Eng Sci, 50(14): 2321–2328. The manuscript was received 30 May 2007 and accepted for publication after revision 5 September 2007.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2007, 85(A12): 1604–1610