Energy & Fuels 1994,8, 531-535 53 1

Liquid Distribution in Trickle Bed Reactors Sankaran Sundaresan

Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544

Received September 3, 1993. Revised Manuscript Received March 7, 1994'

The quality of liquid distribution at the top of the bed and the manner in which the flows are established can affect the liquid flow behavior in trickle beds profoundly. This, in turn, can impact the rates of chemical reactions. Some recent experimental results highlighting these points are reviewed.

Introduction

Trickle bed reactors in which a gas and a liquid flow cocurrently downward over a solid packing are widely used in the petroleum and chemical industries. At low liquid and gas flow rates, the trickling regime, in which the liquid trickles over the packing and the gas phase is continuous, is obtained. At high gas and liquid flow rates, a time- dependent flow pattern referred to as pulsing is obtained. Both of these regimes are relevant in industrial practice.' This brief review is concerned with liquid distribution and its effect on chemical reactions.

The extent to which the catalyst particles are wetted by the liquid is determined by the quality of liquid distribu- tion. This, in turn, affects rates of chemical reaction. Consequently, the wetting behavior has been a subject of many studies and reviews (for example, see refs 2 and 3). Recent experiments illustrating this point are reviewed in this brief communication.

Trickling Regime

The first key point which has emerged in the past decade is the multiplicity of hydrodynamic states that is observed over the entire trickling regime.4-e*20 The multiplicity in pressure gradient observed in a constant gas flow rate experiment (for air-water flow through a bed of 3-mm spherical glass beads) is illustrated in Figures 1 and 2. In these experiments! the bed was completely wetted at first and then allowed to drain before any series of experiments were started. Starting from zero liquid flow rate, as the liquid flow rate is increased, the lower curve shown in these figures is obtained. Starting from the liquid flow rate corresponding to the onset of pulsing (shown by X in these figures), if the liquid flow rate is decreased to zero, the upper curve is obtained. Only the two end points are common for the two curves. Note that the pressure gradient for the upper curve at intermediate liquid flow rates are as much as 100 % larger than those for the lower curve. In Figure 1, starting from zero liquid flow rate, the

e Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Satterfield, C. N. AIChE J. 1975,21, 209. (2) Shah, Y. T. Gas-Liquid-Solid Reactor Design; McGraw-Hill: New

York, 1979. (3) Herskowitz, M.; Smith, J. M. AIChE J. 1978, 24, 439. (4) Kan, K. F.; Greenfield, P. F. Ind. Eng. Chem. Process. Dev. Des.

( 5 ) Levec, J.; Saez, A. E.; Carbonell, R. G. AIChE J. 1986, 32, 369. (6) Chrietensen, G.; McGovernm S, J.; Sundareean, 5. AZChEJ. 1986,

(7) Goto, S.; Gaepillo, P. D. Ind. Eng. Chem. Res. 1992,31, 629,

1978,17,4a2,1979,18,760.

32, 1677.

MAX. LIQUID FLOWRATE ( kg/m2 hr )

1 . 4 7 x l b

8.0 - . 2.45 X l d

3.42 x 1 d

A A 0 9 8 X 1 d

0 1.96 x l d *

6.0 -

Q

0.0 1 , 1 # 1 # 1 ) 0.0 1 .o 2.0 3.0 4.0

LIQUID FLOWRATE ( X I $ kplm'hr)

Figure 1. Effect of decreasing liquid flow rate from various maximum values. Gas maas flux = 756 kg/(m*.h). Reprinted with permission from ref 6. Copyright 1987 American Institute of Chemical Engineers.

liquid flow rate was increased to various maximum values (as described in the figure) and then decreased back to zero. The lower limiting curve was followed every time as the liquid flow rate was increased. However, the maximum liquid flow rate imposed affected the return path. In Figure 2, starting from the liquid flow rate corresponding to pulsing (,E,*), the liquid flow rate was decreased to various minimum values, and then increased back to L'. The upper limiting curve was followed every time the liquid flow rate was decreased, while the return path depended on the minimum liquid flow attained.6

The liquid holdup variations corresponding to Figures 1 and 2 were measured at various locations in the bed using a microwave probe that sampled roughly 30 cm3 of bed volume,6 and it was concluded that no large-scale nonuniformities could be detected. Therefore, Figures 1 and 2 should be explained on the basis of differences in flow patterns a t the level of individual particles.

0887-0624/94/2508-0531$04.50/0 0 1994 American Chemical Society

532 Energy & Fuels, Vol. 8, No. 3,1994 Sundaresan

8.0 CI

5 4 Y

5 6.0 w n a a (3 W

v)

a n.

5 4.0

3

2.0

MIN. LIWID FLOWRATE ( kgm2 hr )

4 0 . 2 4 ~ 1 8

o 0 . 4 9 ~ 1 3 A 0.98xld o 1.47 x l d

v 2.44 x l d

- 1.96xld

- (a) RIVULET FLOW

Corresponding to the multiple hydrodynamic states observed in pressure gradient, a difference was seen in the liquid holdup between increasing and decreasing liquid flow rates. The liquid holdup for increasing liquid flow rate was slightly larger than that for decreasing flow rate. However, the difference in holdup between increasing and decreasing flow rates was less than the uncertainty associated with the microwave technique.

Visual observation of the column during a constant gas flow rate experiment suggests the following picture. Upon starting the liquid flow rate in a drained bed, the liquid was seen to move down the column in a few large rivulets. These were approximately 1 cm wide and were roughly 4 cm apart. These rivulets meandered down the packing, coalescing and splitting as they went. The rivulets were

Figure 4. Rivulet flow in trickle beds, as captured by computer- assisted tomography. Reprinted from ref 7.

stable, manifesting no change in shape over a long time. Increasing the liquid flow rate a t this point led to growth in the size of the rivulets, but not their number or flow path. Just prior to pulsing, rippling was observed on the rivulets, indicating enhanced gas-liquid interactions. A t this stage, rivulets split, forming several smaller rivulets. When pulsing started, the liquid was spread evenly over the packing and no persistent rivulets could be observed. On reducing the liquid flow rate, this evenly spread distribution was apparently maintained and no coalescence of these thin films into rivulets was observed. Eventually a state was reached where the flow rate has been decreased so much that the liquid supply was insufficient to maintain all of the thin films and dry sections could be observed.6 The two liquid distributions are shown schematically in Figure 3. A rationalization of the multiple hydrodynamic states in terms of these two distributions has been presented by Christensen et al.6 I t should be noted that visual observation is confined to the immediate proximity

Delikat' have recently studied liquid distribution in trickle beds using computer-assisted tomography, which allowed them to monitor the distribution in the interior of the bed directly. Their study demonstrates conclusively the existence of rivulets in the interior of the bed as well. See Figure 4. Mathematical analyses of trickling flow by Chu and Ng21 and Melli and Scrivenn have also established unequivocally the hysteresis as difference in flow patterns a t the level of packing particle.

It is readily apparent from the schematic shown in Figure 3 that the wetting of packing particles by the liquid will be impacted by the manner in which the flows are established. Most chemical reactions that are carried out in trickle beds involve one or more species which enter the reactor in the liquid phase and one that enters through the gas phase. The gas-phase reactant typically dissolves into the liquid phase and diffuses into the catalyst pores (which are completely or partially filled with liquid). The

of the walls and that an extrapolation of this observation (8) Lutran, P. G.; Ng, K. M.; Delikat, E. P. Id. Eng. Chem. Res. 1991, to interior of the bed is only a conjecture. Lutran, Ng, and 30,1270.

Liquid Distribution in Trickle Bed Reactors

a Psrd : PUR AIMS : HzGas Phase A Ustd : he AMS : H2Gas Phase

Pstd : PUIC AIMS : N G ~ s P h a ~

Energy & Fuels, Vol. 8, No. 3, 1994 533

0.0 0.2 0.3 0.6 0.8 1.0

E, Figure 5. Dependence of reaction rate on wetting efficiency. Reprintedwith permission from ref 13. Copyright 1991 American Institute of Chemical Engineers.

total path length for this transport process will vary significantly with flow pattern. In a similar manner, the availability of the liquid-phase reactant a t the catalytic sites will also change with flow pattern. Therefore, i t seems inevitable that the observed reaction rate in the trickling flow regime may depend on the manner in which the flows are established (i.e., the start-up procedure). However, to the best of my knowledge, there does not appear to be any direct documentation of this effect.

The liquid flux in the immediate vicinity of a catalyst particle controls the fraction of external surface covered by liquid, often referred to as wetting efficiency. The relationship between wetting efficiency and observed reaction rate has been a subject of many studies (for example, see refs 3 and 8-15). The possibility of effective- ness enhancement by partial wetting has been demon- strated both theoretically and experimentally.12-14 Funk et al.13 studied the hydrogenation of a-methylstyrene to cumene over a Pd/AlzOa catalyst in a single pellet reactor, where both the wetting efficiency and reaction rate can be measured. One set of results reported by these authors is shown in Figure 5. This figure shows the observed reaction rate as a function of wetting efficiency for various combinations of gas - and liquid-phase compositions. Pure a-methylstyrene (AMS) which has been presaturated (Pstd) with hydrogen or unsaturated (Ustd) containing little dissolved hydrogen was used as liquid feed. Experi- ments performed with presaturated liquid and in the absence of gas-phase hydrogen (filled circles) reveal a monotonic increase in reaction rate with wetting efficiency. Increasing the wetting efficiency increases availability of both reactants and hence the trend is to be expected. In contrast, when hydrogen is available in the gas phase, it can dissolve into the liquid inside the catalyst pores through the unwetted region on the catalyst surface, thereby increasing the availability of hydrogen. This leads to higher reaction rates. Note, however, that an increase in wetting efficiency results in a decrease in the unwetted

(9) Leung, P. C.; Recasens, F.; Smith, J. M. AIChE J. 1987,33,996. (10) Ring, 2. E.; Missen, R. W. AIChE J. 1989,35, 1821. (11) Capra, V.; Sicardi, S.; Gianetto, A.; Smith, J. M. Can. J. Chem.

Enc. 1982.60. 282

, M. P.; Ng, K. M. AZChE J. 1$ (16) Yentekakis, I. V.; Vayenas, G. C. Chem. Eng. Sci. 1987,42,1323.

i a) LOW Flux b) Hlgh Flux -1

0 0 3 O b 0 9 I 2 0 0 3 O b 0 9 12

H ” a I P m m m a t d P w m + l m

Figure 6. Cold flow experimental results on transverse mixing in the liquid phase. Reprinted with permission from ref 16. Copyright 1991 American Institute of Chemical Engineers.

area and hence a decrease in the hydrogen availability. Consequently, the observed rate of reaction can and does decline (filled triangles and squares). The solid lines in this figure represent approximate fit of the data, while the broken lines denote extrapolation of the data to origin. Note that the reaction rate on a partially wetted catalyst can differ from that on a fully wetted catalyst by a factor of 2 or more. (The apparent convergence of the reaction rates for “unsaturated AMS-hydrogen gas phase’ and “presaturated AMs-nitrogen gas phase” at complete wetting is a mere coincidence. The difference between the two presaturated cases at complete wetting conditions is a likely consequence of evaporation of hydrogen into nitrogen gas phase.)

Figures land 2 were obtained in a column where great care was taken to maintain the distribution of liquid at the top as close to uniform as possible. In commercial practice, such a meticulous distribution is not commonly employed. Liquid rains through nozzles that are typically a foot apart. I t is natural to wonder if there is adequate mixing or spreading of the liquid in the transverse direction so that the catalyst utilization is maximized. Experiments by Anderson and Sapre’6 in a two-dimensional trickle bed reveal that mixing in the transverse direction does not occur to a significant degree in both the trickling and pulsing regime of flow. This is illustrated in Figure 6, taken from their publication. The dark stripes in the figure were produced by injecting colored dye in the liquid inlet a t three locations. Low flux (Figure 6a) corresponds to operation in trickling regime.

In a recent study, McManus et al.17 have examined the effect of the quality of liquid distribution at the top of a trickle bed reactor on the rate of hydrogenation of a-methylstyrene over Pd/A1203. By varying the number of inlet tubes used to distribute the liquid feed, they have conclusively demonstrated a relationship between catalyst utilization and quality of liquid distribution at the top of the bed.

It is clear from the above examples that, in trickle bed reactors operating in the trickling regime of flow, both the quality of liquid distribution at the top of the bed and the start-up procedure for establishing the flows can impact the catalyst utilization. The quality of liquid distribution at the top of the bed dictates the macroscopic details of liquid flow through the reactor; i.e., it determines if large regions of bed are left dry, etc. The start-up procedure

~ ~ _ _ _ _

(17) Anderson, D. H.; Sapre, A. V. AZChE J. 1991,37, 377. (18) McManus, R. L.; Funk, G. A.; Harold, M. P.; Ng, K. M. Ind. Eng.

Chem Res. 1993, 32, 670.

S34 Energy & Fuels, Vol. 8, No. 3,1994 Sundaresan

(a) UNEVEN LIQUID DISTRIBUTION (b) EVEN LIQUID DISTRIBUTION ; GAS ABOVE PACKING

(C) UNEVEN SOLID PACKING (d) EVEN UOUlD DISTRIBUTION ;

Figure 7. Effect of different distributions at the top of the column on position of pulses. Reprinted with permission from ref 6. Copyright 1987 American Institute of Chemical Engineers.

GAS INTO PACKING

affects microscopic details such as extent of wetting (rivulet vs film flow) in a given zone.

A comprehensive model for a trickle bed reactor performance in the trickling regime of flow should account for both levels of detail. Such a model has been described and analyzed recently by Funk, et al.,B to bring forth the coupling between flow patterns and wetting on individual particles on catalyst utilization.

Pulsing Regime

The available (albeit limited) evidence suggests that, in the pulsing regime of flow, the start-up procedure for establishing the flows is not an important consideration. The high degree of interaction which exists between the gas and liquid in this regime of flow, with superimposed time-dependent pulses, leads to frequent and good bathing of the catalyst particles in the regions irrigated by liquid. The strong influence of the quality of liquid distribution at the top of the bed still remains.

The importance of the quality of liquid and gas distribution a t the top of the bed on the details of pulsing flow has been addressed by Christensen et al.6 Figure 7 summarizes their findings. In their experiments in a trickle bed of rectangular cross section, air was admitted into the column through five tubes, while the liquid rained through several tens of smaller tubes. Figure 7a shows schemati- cally the uneven liquid distribution which results if the liquid is forced to rain through only a few holes (as shown). The irrigation of the bed is limited in this case to a region directly below the liquid inlet, with regions far away (in the transverse direction) seeing little liquid flow. In such a case, pulsing occurs only directly below the liquid inlet (Figure 7a). Even when great care was taken to distribute the liquid uniformly at the top, the pulsing was observed to occur a t some preferential location as shown in Figure 7b. It was found that unevenness in the top surface of the packed bed was responsible for this effect. To illustrate

Figure 8. Representation of gas by passing the liquid-rich pulse when the pulse does not span the column cross section. Reprinted with permission from ref 6. Copyright 1987 American Institute of Chemical Engineers.

this, the bed was packed intentionally wit h a nonuniformity a t the top surface as shown in Figure 7c. In this case, it was found that pulsing tended to occur directly beneath this mound. Through experiments in a refractive-index- matched system, Christensen et showed that when a mound is present as shown in Figure 7c, the gas tended to enter the bed preferentially in the vicinity of the mound. By inserting the gas entry tubes into the packed bed as shown in Figure 7d, Christensen et al.6 were able to achieve uniform gas distribution, which led to pulsing occurring uniformly over the cross section of the bed.

Christensen et aL6 found that the nature of pulses which formed in a trickle bed of rectangular cross section (Figure 7d) was very different from that observed in cylindrical beds which are typically used in most laboratory studies on hydrodynamics. The pulses observed in cylindrical columns are toroidal in shape, and they span the cross section of the column.18 In contrast, the pulses observed in a column of rectangular cross section are localized, with a width much smaller than the column width. Anderson and Saprel6 have also reported similar findings.

Christensen et aL6 found that the pressure gradient required to maintain given fluxes of gas and liquid in the column of rectangular cross section was considerably smaller than that required in a cylindrical column of same cross-sectional area. Thus it is clear that the nature of pulsing flow is dependent on the geometry of the column, even when the gas and liquid distribution at the top of the column are uniform. This must be contrasted with flow in the trickling regime, where the geometry of the column per se does not exert any significant influence on the flow pattern (provided the distribution a t the top is uniform and the flows are established in the same manner). The geometry dependence of pulsing flow may be rationalized as follows (refs 6 and 19): Under most operating conditions of practical interest, the interstitial velocity of gas is higher than the pulse velocity. This implies that the gas behind a pulse (of liquid) will necessarily try to overtake the pulse, either by bypassing it as shown schematically in Figure 8

(19) W e e k ” , V. W., Jr., J. E. Myers, AIChE J. 1964,20,951. (20) Sundaresan, S. AIChE J. 1987,33,455. (21) Chu, C. F.; Ng, K. M. AIChE J. 1989,35,1365. (22) Melli, T. E, Scriven, L. E. Id. Eng. Chem. Res. 1991,30,951. (23) Funk, G. A.; Harold, M. P.; Ng, K. M. I d . Eng. Chem. Res. 1990,

29,738.

Liquid Distribution in Trickle Bed Reactors

or by ripping apart the pulse rendering it porous for gas flow. In small-diameter cylindrical columns where the pulses nearly span the entire cross section, there is little room for the gas to bypass the pulse, and therefore the gas is forced to percolate through the pulse, which is a rather resistive path. In contrast, in columns with a large width (or diameter) where the pulses do not span the cross section, the gas will take the less resistive path of flowing around the pulses.

It has been reported in many laboratory studies on gas- liquid mass transfer in laboratory trickle beds that the mass-transfer coefficient kLa (where a is the interfacial area per unit bed volume and k L is the mass-transfer coefficient per unit interfacial area) increases rapidly upon going from operation in trickling regime to pulsing regime. This has been attributed to the high interaction between gas and liquid which is brought about by the gas percolation through the pulses. It appears now, however, that this rapid increase in kLa upon going from trickling regime to pulsing regime may not be realized in beds of large cross section where the gas flows around the pulses.

Anderson and Sapre16 have studied the extent of transverse mixing in the liquid phase in pulsing regime. They found very little difference in the transverse mixing behavior between the trickling and pulsing regimes (compare Figure 6, a and b). One can conclude from this study that the motion of liquid in the pulses is primarily in the vertical direction, with negligible transverse com- ponent . This was indeed found to be the case in a theoretical modeling study by the present author.lg This has direct implications to the operation of trickle bed reactors. I t is not uncommon to encounter obstacles for flow in trickle bed reactors. These may be regions of low porosity formed by agglomerated catalysts (resulting from overheating of the catalyst pellets) or inserts such as quench tubes. As the liquid flows around these obstacles, liquid maldistribution evolves. Regions directly under these obstacles will not be properly irrigated by the liquid, leading to either poor utilization of the catalyst if the liquid- phase reactant is nonvolatile or extremely high reaction rates and heat generation in these pellets if the liquid- phase reactant is volatile (which can potentially damage the catalyst pellets and initiate thermal runaway of the reactor),

Energy & Fuels, Vol. 8, No. 3,1994 535

While one can readily visualize why poor liquid irrigation can lead to localized hot spots, it is hardly obvious that the occurrence of localized hot spots is always caused by poor liquid irrigation. Recent studies have revealed that fixed bed reactors operating in a downflow configuration are prone to hot spot formation if the reaction involved is highly exothermic. A detailed report on these studies is beyond the scope of this brief review. Instead, let us refer briefly to the work of Stroh and Balakotaiah26-2s who have carried out bifurcation analysis of models for downflow packed bed reactor. It is clear from their studies that the changes in physical properties resulting from large temperature (and concentration) variations along the reactor (in the vertical direction) can induce large trans- verse nonuniformities, giving rise to stationary as well as moving hot spots. They have considered the case of a single-phase flow, so their results do not apply directly to trickle bed reactors with two-phase flow. Nevertheless, it is reasonable to expect that the complexities and insta- bilities which have been brought forth in their work may be translated qualitatively for the trickle bed reactors. We are then led to conclude that some of the hot spot problems in a trickle bed reactor may be caused by inherent instabilities, and that these will persist even with good liquid distribution at the top of the column and absence of obstacles.

Hot Spots in Trickle Bed Reactors

Most chemical reactions carried out in trickle bed reactors involve volatile liquid-phase reactants. These reactions are frequently very exothermic. Formation of hot spots in these reactors, which can lead to thermal runaway of the reactors, is a major concern in the operation of these reactors. Frequently, the occurrence of a hot spot is accompanied by poor liquid irrigation in the neighbor- hood of the hot spot. It is generally believed that the hot spot is a consequence of the poor liquid irrigation and that the liquid maldistribution is a result of poor liquid distribution at the top of the column or obstacles in the bed (as mentioned in the previous p a r a g r a ~ h ) . ~ ~ ~ ~ ~ Con- sequently, one strives to achieve good liquid distribution at the top of the column, and also make provisions to redistribute the liquid at one or more heights in the bed, in order to mitigate the problems associated with hot spot formation.

(24) Jaffe, S. B. Ind. Eng. Chem. Process Des. Deu. 1976, 15, 411. (25) Barkelew, C. H.; Gambhir, B. S. ACS Symp. Ser. 1984,237, 61.

Summary

In both trickling and pulsing regimes, the liquid flow is predominantly vertically downward, with very little transverse mixing. Therefore, liquid distribution at the top of the bed is of critical importance in both regimes. Careful design of liquid injectors to accomplish uniform liquid distribution at the top of the bed is perhaps the most important consideration in trickle bed reactors.

In the trickling regime of flow, the manner in which the flows are established (i.e., the start-up procedure) can have a significant impact on the quality of wetting (even when we have good liquid distribution a t the top). The size of the column (i.e., scale-up) itself is not a serious consid- eration, provided the quality of liquid distribution a t the top of the column is not adversely affected by the scale- UP.

It seems to be desirable to inject the gas directly into the bed through uniformly spaced nozzles when the reactor is operating in the pulsing regime. This will promote the formation of pulses uniformly over the cross section of the bed. The start-up procedure for establishing the flows does not appear to be important in the pulsing regime. The details of pulsing hydrodynamics will be significantly altered upon scaleup.

In both regimes of flow, any impediments to liquid flow which tend to laterallyredistribute the liquid in an adverse way will lead to an essentially irreversible maldistribution. I t is well-known that liquid maldistributions can promote formation of hot spots if the reaction is exothermic. It has become clear from recent studies that, even with good liquid distribution at the top of the column and absence of obstacles in the bed, localized hot spots can evolve spontaneously as a result of the physical property variation with temperature and composition.

(26) Stroh, F.; Balakotaiah, V. AZChE J. 1991,37,1035. (27) Stroh, F.; Balakotaiah, V. Chem. Eng. Sci. 1992,47,593. (28) Stroh, F.; Balakotaiah, V. Chem. Eng. Sci. 1993,48, 1629.