Energy & Fuels 1994,8, 531-535
531
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 timedependent 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 distribution. 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.
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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
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.
e Abstract
1978,17,4a2,1979,18,760.
( 5 )Levec, J.; Saez, A. E.; Carbonell, R. G. AIChE J. 1986, 32, 369.
(6) Chrietensen, G.; McGovernm S, J.; Sundareean, 5. AZChEJ. 1986,
32, 1677.
(7) Goto, S.; Gaepillo, P. D. Ind. Eng. Chem. Res. 1992,31, 629,
1
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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$04.50/00 1994 American Chemical Society
532 Energy & Fuels, Vol. 8, No. 3,1994
Sundaresan
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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 increasingand
decreasing flow rates was less than the uncertainty
associated with the microwave technique.
Visual observation of the column during a constant gas
flow rate experimentsuggeststhe 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
stable, manifesting no change in shape over a long time.
Increasing the liquid flow rate at 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 apparentlymaintained 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 supplywas insufficientto maintain
all of the thin films and dry sections could be observed.6
The two liquid distributions are shown schematically in
Figure 3. A rationalizationof the multiple hydrodynamic
states in terms of these two distributions has been
presented by Christensen et al.6 It should be noted that
visual observation is confined to the immediate proximity
of the walls and that an extrapolation of this observation
to interior of the bed is only a conjecture. Lutran, Ng, and
Figure4. Rivulet flow in trickle beds,as captured by computerassisted tomography. Reprinted from ref 7.
Delikat' have recently studied liquid distribution in trickle
beds using computer-assistedtomography, 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 schematicshown 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
(8) Lutran, P.G.;Ng,K.M.;Delikat, E.P.I d .Eng. Chem. Res. 1991,
30,1270.
Liquid Distribution in Trickle Bed Reactors
Energy & Fuels, Vol. 8, No. 3, 1994 533
i
a Psrd : PUR AIMS : HzGas Phase
A Ustd : h e AMS :H2Gas Phase
Pstd : PUICAIMS : N G ~ Psh a ~
a) LOW Flux
0
03
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Figure 5. Dependence of reaction rate on wetting efficiency.
Reprintedwith permission from ref 13.Copyright 1991American
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, it 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 effectiveness enhancement by partial wetting has been demonstrated 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. Experiments 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
b) Hlgh Flux
09
m
I2
0
03
Ob
1-
09
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. It 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
conclusivelydemonstrated 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
~
, M. P.; Ng,K. M. AZChE J. 1$
(16)Yentekakis, I. V.; Vayenas, G. C. Chem. Eng. Sci. 1987,42,1323.
12
matdPwm+l m
~____
(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
(a) UNEVEN LIQUID DISTRIBUTION
Sundaresan
(b) EVEN LIQUIDDISTRIBUTION ;
GAS ABOVE PACKING
Figure 8. Representationof gas by passing the liquid-rich pulse
when the pulse does not spanthe column cross section. Reprinted
with permission from ref 6. Copyright 1987 American Institute
of Chemical Engineers.
(C)
UNEVEN SOLID PACKING
(d) EVEN UOUlDDISTRIBUTION ;
GAS INTO PACKING
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 Instituteof Chemical Engineers.
affectsmicroscopicdetailssuch asextent of wetting (rivulet
vs film flow) in a given zone.
A comprehensive model for a trickle bed reactor
performancein 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
summarizestheir findings. In their experimentsin 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 schematically 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 at 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
this, the bed was packed intentionallywith 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-indexmatched system, Christensen et
showed that when a
mound is present as shown in Figure 7c, the gas tended
to enter the bed preferentiallyin the vicinity of the mound.
By inserting the gas entry tubes into the packed bed as
shown in Figure 7d, Christensenet 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 significantinfluence 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 (refs6 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 gasliquid 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
coefficientper 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 component . 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. It 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,
leadingto either poor utilization of the catalyst if the liquidphase reactant is nonvolatile or extremely high reaction
rates and heat generation in these pellets if the liquidphase 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 transverse 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 instabilities 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.
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 consideration, provided the quality of liquid distribution a t the
top of the column is not adversely affected by the scaleUP.
It seems to be desirable to inject the gas directly into
Hot Spots in Trickle Bed Reactors
the bed through uniformly spaced nozzles when the reactor
is operating in the pulsing regime. This will promote the
Most chemical reactions carried out in trickle bed
formation of pulses uniformly over the cross section of the
reactors involve volatile liquid-phase reactants. These
bed. The start-up procedure for establishing the flows
reactions are frequently very exothermic. Formation of
does
not appear to be important in the pulsing regime.
hot spots in these reactors, which can lead to thermal
The details of pulsing hydrodynamics will be significantly
runaway of the reactors, is a major concern in the operation
altered upon scaleup.
of these reactors. Frequently, the occurrence of a hot spot
is accompanied by poor liquid irrigation in the neighborIn both regimes of flow, any impediments to liquid flow
hood of the hot spot. It is generally believed that the hot
which tend to laterallyredistribute the liquid in an adverse
spot is a consequence of the poor liquid irrigation and that
way will lead to an essentially irreversible maldistribution.
the liquid maldistribution is a result of poor liquid
It is well-known that liquid maldistributions can promote
distribution at the top of the column or obstacles in the
formation of hot spots if the reaction is exothermic. It
bed (as mentioned in the previous p a r a g r a ~ h )Con. ~ ~ ~ ~ ~has become clear from recent studies that, even with good
sequently, one strives to achieve good liquid distribution
liquid distribution at the top of the column and absence
at the top of the column, and also make provisions to
of obstacles in the bed, localized hot spots can evolve
redistribute the liquid at one or more heights in the bed,
spontaneously as a result of the physical property variation
in order to mitigate the problems associated with hot spot
with temperature and composition.
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.
(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.