Ind. Eng. Chem. Res. 2005, 44, 9727-9738
9727
Critical Phenomena in Trickle-Bed Reactors
Valery A. Kirillov*,† and Igor V. Koptyug‡
Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia, and International Tomography Center,
630090 Novosibirsk, Russia
Thermocouple probing and nuclear magnetic resonance imaging (MRI) method were used to
study both heat regimes and liquid distribution under conditions of exothermal hydrogenation
reactions proceeding on a catalyst particle and in the trickle bed. The experiments were performed
using the model reactions of hydrogenation of R-methylstyrene, octene, and heptene. It was
shown that critical phenomena, such as “hot spots” in trickle bed and catalyst particles,
multiplicity of steady-state regimes, and hysteresis phenomena, are generated by liquid
evaporation and transition of the reaction to the gas-phase mode. The transition is promoted by
a number of factors, such as exothermicity of the hydrogenation reaction, presence of dry and
partially wetted catalyst particles for liquid superficial velocities lower than 5-6 mm/s,
nonuniform distribution of the liquid within the reactor cross section, and phase nonequilibrium
in the trickle bed. Multiplicity of the steady-state regimes, hysteresis phenomena, and impact
of liquid superficial velocity and catalyst particle size on the onset of critical phenomena were
studied experimentally.
1. Introduction
Multiphase reactors with cocurrent upward or downward gas-liquid flows are widely used in the petrochemical and oil processing technologies. The reactors
usually operate in a stable steady-state regime. Unfortunately, both practical experience and research experimental data accumulated during the recent three
decades revealed the formation of hot spots in the
trickle-bed reactor,1-3 hysteresis phenomena,4,5 temperature oscillations6,7 resulting in the side reactions,
and consequent decreases in the reactor productivity.2,8,9
Of particular risk are the operation regimes facilitating
the formation of runaways in the catalyst bed.8,10 In this
work, all the above phenomena will be referred to as
critical phenomena formed in the trickle-bed reactor.
High gas holdup, liquid maldistribution, and low liquid
superficial velocity11,12 are responsible for the apperance
of completely wetted (liquid-filled) or completely unwetted or dry particles in the trickle bed. Partially wetted
catalyst particles, exothermic hydrogenation reactions,
resulting in the formation of “hot spots”, and reagents
evaporation facilitate the occurrence of the gas-phase
hydrogenation on dry catalyst particles in parallel to
the liquid-phase reaction. This fact was experimentally
verified for the hydrogenation of benzene,13-15 croton
aldehyde,16 R-methylstyrene,1,17-21 cyclooctadiene,22 and
cyclohexene.4
Analysis of the current hypothesis of critical phenomena in the porous active medium suggests the following
explanation of their appearance. First, critical phenomena are caused by the interaction between chemical and
phase-conversion rates, transition of a chemical reaction
from the liquid phase to the gas phase. A motive is that
large volumes of gases provide a simaltaneous existence
of completely wetted, partially wetted, and dry particles
in the trickle bed. Both partial wetting of a catalyst
* Corresponding author. Tel./Fax: 7-3832 306187. E-mail:
[email protected].
† Boreskov Institute of Catalysis.
‡ International Tomography Center.
particle and exothermicity of a reaction, resulting in the
catalyst heating and reacting components’ evaporation,
generate the conditions for the simultaneous occurrence
of liquid-phase and gas-phase reactions on the unwetted
particles. A transition of the reaction from the liquidphase to the gas-phase hydrogenation regime is accompanied by a decrease of the coefficients of heat
transfer between the gas-liquid flow and the catalyst
particles and an increase of the coefficients of mass
transfer. As a result, the apparent reaction rate and
heat generation increase, whereas heat removal becomes inhibited. This provides the runaway conditions.
A very important aspect in this hypothesis is the
suggestion that an external wetting efficiency of the
catalyst particle depends on its temperature. Note that
an increase in the catalyst temperature should decrease
this value. The existence of such dependence may result
in the appearance of the feedback between temperature
and liquid distributions in the reactor, which provides
oscillation regimes. The experimental data on the presence of temperature oscillations5-7 indirectly confirm
the above dependence. This effect will be studied in
detail using a thermocouple probing method and MRI.
The second hypothesis has a chemical basis. This
suggests that runaways are associated with side reactions between the evaporated components, especially if
a heat effect of the side reaction exceeds that of the main
reaction. The hypothesis is usually realized upon performance of selective hydrogenation reactions. An increase of heat generation caused by side reactions in
the gas phase during constant heat removal results in
the increase of temperature in the reactor and transfers
the system to a new high-temperature regime. This
effect was experimentally observed in refs 2, 8, 9, and
15.
The third hypothesis has a hydrodynamics origin and
is associated with nonhomogeneous liquid distributions
in the trickle bed caused by the effect of phase distribution at the catalyst bed inlet and random geometry of a
trickle bed. A natural solution of the problem is an
increase of liquid irrigation of the catalyst bed up to the
10.1021/ie050276l CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/11/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005
Figure 2. Principal scheme of the experimental setup.
Figure 1. Plausible hypotheses of the runaway in trickle-bed
reactor.
level of complete wetting and filling of particles. According to refs 11 and 23, the catalyst particles are
completely wetted when the critical liquid irrigation is
2-3 kg/m2‚s, which approximately corresponds to the
superficial liquid velocity of 3-4 mm/s. The experimental data suggest5,15 that the liquid superficial velocity
required to provide complete particle wetting is 4-5
mm/s. For a number of reasons, the increase of the
liquid irrigation cannot be a universal tool for suppression of hydrodynamic reasons of runaways.
Liquid distribution in the trickle-bed reactor can
considerably depend on the temperature of catalyst
particles. This hypothesis was first suggested in ref 24
and required experimental verification. Thus, the zones
with lower temperatures are preferred for liquid spreading. Moreover, overheating of the particle, caused by the
gas-phase reaction, hampers liquid flow in its neighborhood because of intense evaporation and an increase in
the local hydraulic resistance, which provides appearance and growth of “hot spots” on the length scale of
several particles. Of particular importance is the suggestion that there is no phase equilibrium between the
liquid and the gas.
Actually, the above-mentioned factors exhibit themselves simultaneously, thus promoting nonlinear interactions in the trickle catalyst bed. Figure 1 suggests the
mechanisms describing the appearance of critical phenomena in a trickle bed.
To investigate the above phenomena, we used a
method of the nuclear magnetic resonance imaging
(MRI), providing observation of in situ liquid distribution in the porous structure during the chemical reaction. MRI was successfully applied to study liquid-phase
distribution in the porous structure and to determine
liquid holdup.25,26 It should be noted that the above
experiments were performed in the absence of the
exothermic reactions and in “cold” conditions. The
fundamental restrictions of this method for studying the
exothermal reactions in the porous, catalytically active
media were overcome in refs 27-29. At present, the MRI
is used to study critical phenomena occurring on a single
porous catalyst particle, in a catalyst bed with a regular
structure formed by several particles, and directly in a
trickle-bed reactor during hydrogenation.
The goal of this work is to study the mechanism of
critical phenomena onset, the nature of chemical and
phase interactions, and the effect of critical states at
the microlevel (a catalyst particle) on the generation and
development of critical states at the macrolevel (a trickle
catalyst bed). This work summarizes the MRI investigations of the critical phenomena in a trickle-bed reactor,
which were performed at the Institute of Catalysis and
International Tomography Center (Novosibirsk, Russia).
2. Runaway Problem in a Catalyst Particle
A catalyst particle is that microlevel which exhibits
the formation of critical phenomena associated with the
interaction of chemical and phase transitions. For this
reason, it is principally important to investigate the
processes on the catalyst particle in order to understand
the mechanism of critical phenomena generation at the
catalyst bed level. The goal of this section is to experimentally study heat regimes on the dry, partially
wetted, and liquid-filled catalyst particles in the exothermal hydrocarbon hydrogenation and to elucidate the
impact of both the exothermal chemical reaction and
the phase transition on the steady state and multiplicity
of heat regimes on the catalyst particles. For this
purpose, the methods of thermocouple probing of heat
regimes and MRI will be used to study the liquid
distribution inside a catalyst particle during the hydrogenation of R-methylstyrene (AMS). The methods were
described in detail in refs 20, 21, 28, and 29.
2.1. Heat Regimes on the Partially Wetted and
Filled Catalyst Particles. In these runs, a liquid
reagent was fed to the catalyst particle top through a
stainless steel tube fitted with a glass capillary at one
end (see Figure 2). Two thermocouples, 0.2 mm in
diameter, were carefully implanted into the particle.
The cylindrically shaped particle is 4.5 mm in diameter
and 6 mm long. One thermocouple measures temperature T1 in the particle center; the other measures
subsurface temperature T2 at a certain distance from
the top. Gas temperature (T0) was measured with a
moveable thermocouple, providing measurements at any
point in the reactor. Two series of steady-state experiments were performed. During the first series, the
catalyst particle was blown with a hydrogen flow
saturated with AMS to measure temperature distribution along the particle axis. During the second series,
the particle was blown with dry hydrogen to study
temperature variations in the paricle core and at a
distance of 0.8 mm from the top, where liquid was fed.
Heat regimes were investigated using the reactions
of exothermal hydrogenation of R-methylstyrene (AMS)
to cumene (C9H10 + H2 ) C9H12 + 109 kJ/mol) and
octene to octane (C8H16 + H2 ) C8H18 + 125 kJ/mol) on
the Pt/γ-Al2O3 catalyst. As catalysts, we used 15% Pt/
γ-Al2O3 and 3.5% Pd/γ-Al2O3 with uniform and egg-shell
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9729
Figure 3. Impact of AMS liquid feeding on the axial temperature
distribution in the catalyst particle, catalyst 15% Pt/Al2O3 with
uniform active component distribution.
distributions of the active component along the particle
radius. The detailed information about the supports and
catalysts was given elsewhere.20,21
Figure 3 presents the experimental data on the axial
temperature distribution in a cylindrical catalyst particle during the AMS hydrogenation. In the experiments, we used several particles of 15% Pt/γ-Al2O3 with
uniform distribution of the active component in the
catalyst particle. As Figure 3 suggests, the catalyst
particle is practically isothermal in the gas-phase
hydrogenation, if liquid AMS is not fed on its top
surface. An insignificant temperature decrease near the
top of the particle may be attributed to heat losses
through the thermocouples. If AMS is fed at 3.9 × 10-4
g/s, the top surface temperature sharply drops from 230
°C (the gas-phase regime) to 157 °C, i.e., below the AMS
boiling point. Because of the low heat conductivity of
the particle, the center temperature is much higher than
that of the wetted surface. A further increase in the rate
of AMS feeding decreases the wetted surface temperature to T0. If the AMS mass flow rate is <12.4 × 10-4
g/s, the liquid boiling point is reached at a distance of
0.2-1.2 mm from the top face. This means that the
external surface of the particle is partially wetted and
filled with liquid to a thickness of 0.2-1.2 mm and the
remaining particle volume is filled with gas. However,
an insignificant increase in the AMS mass flow rate
(12.4 × 10-4 to 14.4 × 10-4 g/s) provides a complete
filling of the particle.
Figure 4 illustrates the multiplicity of steady-state
regimes on the catalyst particle at gas temperatures of
125, 132, and 143 °C for x0AMS ) 0.30 and at T0 ) 132
°C for x0AMS ) 0.41. For GAMS ) 0, an increase in the gas
temperature at a constant molar fraction of AMS does
not affect the temperature difference. This indicates the
external diffusion limitation of the AMS gas-phase
hydrogenation in the high-temperature steady-state
regime. When the catalysts are in the upper steady state
and the liquid flow rates do not exceed the critical value,
the catalyst temperatures gradually decrease with
increasing liquid supply. A slow decrease in the temperatures along the upper branches can be attributed
to heat consumptions required for heating and evaporation of the increasing amounts of fed liquid. When the
rates of liquid feeding exceed the critical value, G×
AMS ,
the particle temperature sharply decreases to the value
which is practically equal to the gas temperature. The
Figure 4. Impact of the AMS liquid mass flow rate and hydrogen
saturation on the heat steady-state regimes of the catalyst particle.
0
Continuous line (equilibrium condition): T0 ) 125 °C (xAMS
)
0
0
0.30), T0 ) 132 °C (xAMS
) 0.41), T0 ) 143 °C (xAMS
) 0.30).
0
Dotted line (nonequilibrium condition): T0 ) 132 °C (xAMS
)
0.30). Hydrogen flow rate is 18.5 cm3/s.
catalyst particle steady state changes from the gas-filled
state to the completely liquid-filled state. A further
increase (or decrease) in the liquid flow rate under the
gas-liquid equilibrium conditions does not influence the
catalyst temperature. The catalysts remain internally
fully filled. Even if the liquid supply is reduced to zero
(GAMS ) 0), the particle temperature does not change
for at least 30 min. This fact suggests that, if the gas
phase is completely saturated with AMS, the particle
pores are filled with liquid even in the absence of liquid
feeding. For the nonequilibrium condition (dashed line
in Figure 4), liquid evaporates from the external wetted
surface of the particle to the unsaturated gas phase. The
liquid critical flow rate increases with decreasing hydrogen saturation. For the low-temperature steady-state
regime, the temperature of the particle is lower than
that of gas by 10 °C. Such a steady-state regime is stable
till the AMS flow rate decreases to 3 × 10-4 g/s. In
contrast to the saturated gas, as GAMS continues to
decrease, the temperature increases by 130 °C, and the
reaction shifts to the high-temperature regime.
The experiments with dry hydrogen were similar to
those with the presaturated hydrogen. Figure 5 shows
the impact of the liquid AMS mass flow rate on the
temperature difference T1 - T0. As in the case with the
presaturated hydrogen, two steady states exist within
the certain ranges of the liquid flow rates. The region
between the upper and lower branches corresponds to
the unstable steady-state regimes and hysteresis phenomena. As follows from Figure 5, a change from the
pure hydrogen to the presaturated hydrogen is responsible for significant changes in the steady-state behavior
of the particles. The temperature of the particles exceeds
that of gas by 20-60 °C and is much lower than the
temperature of the same particles in the experiments
with presaturated hydrogen. The temperature difference
is probably caused by evaporation of AMS from the
particles, because the gas fed to the reactor does not
contain AMS and hydrogen carries away the evaporated
AMS.
When the liquid feed rate is reduced, liquid evaporation provides favorable conditions for partial wetting of
the catalyst particle. The gas-phase reaction starts on
the formed unwetted surface and promotes intensive
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Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005
Figure 5. Hysteresis phenomena for the filled and partially
wetted catalyst particles. Catalyst: 15% Pt/Al2O3 with uniform
and egg-shell active component distributions. The shaded area is
the wetted surface temperature hysteresis; the hydrogen flow rate
is 18.5 cm3/s.
Figure 6. Liquid distribution in the catalyst particle. (a) The
upper hysteresis branch: bulk gas temperature T0 ) 67 °C,
temperature of particle 120 °C, dry hydrogen flow rate 18.5 cm3/
s, AMS mass flow rate (2.9-3.9) × 10-4 g/s. (b) The low hysteresis
branch: bulk gas temperature T0 ) 67 °C, temperature of particle
54 °C, dry hydrogen flow rate 18.5 cm3/s, AMS mass flow rate
increases to 6.5 × 10-4 g/s. Straight lines denote the particle’s
overall dimensions.
particle drying and temperature increasing. The particle
temperature reaches the ignition point, then abruptly
shifts to the dry, high-temperature hysteresis branch.
Since the method of thermocouple probing permits only
indirect investigations of the wetting and submerged
states of the porous catalyst particle, it is expeditious
to make use of MRI to obtain additional information.
2.2. MRI Experiments on a Single Catalyst Particle. MRI experiments were performed on a Bruker
DRX-300 NMR spectrometer equipped with a vertical
bore superconducting magnet and a microimaging accessory.27,28 In the experiments with a single particle,
a cylindrical catalyst, 4.5 mm in diameter and 7 mm in
length, was used (in the Figure 6 particles, overall
dimensions are denoted by straight lines). The experiments were performed in a dry hydrogen flow, and an
AMS liquid flow was directed to the upper part of the
dry particle of catalyst 15%Pt/γ-Al2O3 (see Figure 2).
We simulated the conditions of the catalyst particle
states on the upper hysteresis branch (Figure 6a) and
in the lower hysteresis branch (Figure 6b). Lighter
shades in Figure 6 correspond to the higher MRI signal
in the regions with higher concentrations of the liquid
phase. A vivid defect in the left part of the particle
(approximately at a half-height of it) is caused by
suppression of the MRI signal in the immediate vicinity
of the thermocouple, which was introduced into the
particle to measure its temperature during the experiment run. The image in Figure 6a suggests that the
upper part of the particle is impregnated with liquid;
moreover, the liquid is nonuniformly distributed in the
particle. The lower part of the particle is almost unwetted and, hence, overheated compared to the rest of
the particle because of the AMS evaporation caused by
the gas-phase hydrogenation. A weak MRI signal in the
lower part of the particle confirms this observation. An
evaporation front is situated between the abovementioned zones. The experimental results suggest that
the catalyst particle is not completely dry; it is partially
filled with the liquid phase on the high-temperature
branch of the hysteresis curve. As the liquid mass flow
rate increases to the critical value, the liquid begins to
gradually fill the particle and leads to the heat regime
corresponding to the low-temperature hysteresis branch
(see Figure 6b). When the catalyst particle is almost
completely filled with liquid, the AMS mass flow rate
was decreased to 6.5 × 10-4 g/s. In this state, the
temperature of the particle is 54 °C, which is lower than
the temperature of the hydrogen flow. This fact is
associated with the intense liquid evaporation from the
particle surface and confirmed by the low intensity of
the MRI signal at the pellet periphery and the uneven
edges of the image near the outer catalyst surface.
Therefore, the MRI experiments show that the impregnation of the porous catalyst with a liquid reagent
during concurrent endothermal reagent evaporation and
exothermal hydrogenation of its vapor can result in the
formation of large temperature and liquid-phase gradients inside the catalyst particle. It was established
that the catalyst particle on the upper hysteresis branch
contains two zones markedly different in the liquidphase fractions: the upper part is filled with liquid and
the lower part is almost dry and filled with a vapor-gas
mixture, in which the vapor-phase hydrogenation occurs. The process of evaporation proceeds on the boundary between the above zones, inside the catalyst particle.
The location of the boundary depends on the quantity
of the liquid supplied to the catalyst particle, its heat
conductivity, and a ratio of evaporation and hydrogenation rates. As follows from Figure 6b, the porous particle
structure is completely filled with liquid and the particle
is isothermal on the lower branch of the hysteresis
curve. On the basis of this observation and the data in
Figure 5, one can determine the value of the external
wetting efficiency of the catalyst particle under the
conditions of the chemical reaction.
2.3. Estimation of External Wetting Efficiency
of the Catalyst Particle. Let us assume that a particle
is isothermal, with a partially wetted external surface
and with the inner porous volume completely filled with
liquid; the external wetting efficiency is characterized
by factor f, and hydrogenation in the liquid phase is
negligible. The gas-phase reaction proceeds over the dry
catalyst surface between hydrogen and the AMS vapor,
diffusing after evaporation from the gas-liquid surface
into the porous structure. According to ref 30, the
activation energy of AMS hydrogenation is E ≈ 43 500
J/mol; the rate of gas-phase reaction depends on hydrogen concentration to the power 0.8 and is almost
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9731
Figure 7. Impact of the liquid AMS mass flow rate and the gas
temperature on the wetted area fraction on the low-temperature
hysteresis branch. Parameters for calculations of eq 3 in accordance with ref 30 are as follows: Q ) 116 000 J/mol, E ) 43 500
J/mol, and E/H ) 1.1.
insensitive to the AMS vapor concentration.1 Assuming
that there is no mass transfer limitation of the gasphase reaction with respect to hydrogen, the heat and
mass balance equations can be written as
(
K0η exp -
E 0 0.8
(x ) (Q - H)(1 - f) +
RT H
P
fHxvap
R(T0 - T) - βAMS
AMS ) 0 (1)
RT0
)
P
SMAMSfxvap
GAMS ) βAMS
AMS +
RT0
E 0 0.8
K0η exp (x ) (1 - f)SMAMS (2)
RT H
(
)
From eqs 1 and 2, one can obtain
f)
GAMS
R(T0 - T) + (Q - H)
SMAMS
Qxvap
AMSβAMSP/RT0
(3)
Using eq 3 and experimental data for GAMS and (T0 T) for the low-temperature hysteresis branch from
Figure 5 and calculated values of R and βAMS from ref
31, one can estimate wetting efficiency f. The obtained
results are presented in Figure 7. The dots correspond
to the external wetting efficiency calculated by eq 3 from
the experimental data presented in Figure 5. A dotted
line is calculated from the results of ref 21 and corresponds to the conditions of transition from the lower
branch to the upper branch of the hysteresis curve. It
is evident that the external wetting efficiency depends
on the mass flow rate of the liquid supplied to the
catalyst particle, gas temperature, and catalyst types.
3. MRI Study of Runaway in the Regular
Catalyst Bed
On studying the critical phenomena onset in the
trickle catalyst bed, it is particularly important to
elucidate how a critical phenomenon on the microlevel
(catalyst particle) can initiate critical phenomena on the
macrolevel (catalyst bed). For this purpose, we prepared
a catalyst bed containing a number of rows of catalyst
particles, 4.2 mm in diameter; 3-4 particles were placed
in the reactor cross section. The reactor was placed
inside an MRI probe. Dry hydrogen and liquid AMS or
Figure 8. Effect of the liquid superficial velocity on the particles
runaway in the regularly packed catalyst bed. A catalyst bed
containing a number of rows of catalyst particles 4.2 mm in
diameter, where 3-4 particles were placed in the reactor cross
section; dry hydrogen and liquid AMS were supplied to the reactor
inlet using the central one-point feeding. Catalyst: 1% Pd/γ-Al2O3.
heptene were supplied to the reactor inlet using the
central one-point feeding. During the experiment runs,
the conversion of AMS into cumene and heptene into
heptane were determined from the chromatographic
analysis of the composition of the liquid phase condensed at the reactor outlet. Temperature of the liquid
at the trickle-bed inlet and temperature of the phases
at the trickle catalyst bed outlet were also measured.
The experiments started from the state of the preliminary filled catalyst bed by gradual reduction of the
superficial liquid velocity. This gradual reduction resulted in the runaway of catalyst particles followed by
their complete drying. In this case, dry particles became
invisible. The procedure permits one to follow the impact
of the catalyst particle (where runaway takes place) on
the neighboring particles.
The images in Figure 8 illustrate the effect of liquid
flow rate on the runaway of four catalyst particles.
These particles were placed in the central part of the
fixed bed formed by three catalyst particle layers and
two inert layers. In all experiments, the hydrogen flow
rate was constant at 39.73 cm3/s, and the AMS superficial velocity was varied within U2 ) 0-6.1 mm/s.
Figure 8(1) shows completely wetted catalyst particles.
Note that, because the liquid AMS superficial velocity
was 5.7 mm/s, Figure 8(2) showed that U2 decreased to
2.9 mm/s and the state of the catalyst particles did not
practically change. A further decrease of U2 to 1.6 mm/s
resulted in the runaway and drying of one catalyst
particle, since it disappeared from the image in Figure
8(3). As the liquid velocity decreased to 0.5 mm/s,
runaway appeared on the next particle, which disappeared with time, Figure 8(5).
According to the images in Figure 8 parts (6)-(8), an
increase in the liquid superficial velocity provides
gradual appearance of particles within the field of view
due to their rewetting. A comparison of the figures with
respect to the submergence factor (color) of the particles
upon gradual decrease in the liquid superficial velocity
indicates differences caused by hysteresis phenomena.
Thus, it was shown experimentally that, even if the rate
of liquid feeding is 5.7 mm/s, the degree of pellet filling
is different. The experiments show that a catalyst bed
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Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005
Figure 9. Distribution of the liquid phase during hydrogenation
of AMS inside catalyst particles 4.2 mm in diameter; the catalyst
bed is regularly packed (catalyst: 1% Pd/γ-Al2O3). The images
correspond to (1) partially filled catalyst particle, the superficial
liquid velocity is U2 ) 3.7 mm/s, the hydrogen flow rate is 20 cm3/
s, the inlet gas and liquid temperatures are 85 °C; (2) and (3)
transport of liquid to the dry catalyst particle from its liquid-filled
neighbors.
involves dry particles, the liquid reagent filling of which
follows the mechanism of capillary impregnation from
the neighboring wetted particles. The reagent quickly
evaporates near the outer surface and reacts inside the
particle in the gas phase. Thus, such dry particles act
as minireactors, operating by the principle of reactive
evaporation and providing performance of the reaction
in the gas phase.
For lower flow rates of the liquid, we observed wetted,
filled, and dry catalyst particles in the catalyst bed cross
section. Dry particles are open for the gas-phase reaction
and overheated with respect to the wetted and filled
particles. It should be noted that the particle runaway
follows an individual mechanism, and the thermal
interaction of the neighboring particles is apparently
insignificant. The interaction of particles manifests itself
through the effect of capillary transport on the degree
of wetting and distribution of the liquid inside the
particle.
Despite the fact that the mechanism of liquid transfer
in the stagnant zones around the particle contacts was
already described in detail in the literature,32 our
experimental data permitted us to establish a new
mechanism of liquid transition along the external dry
particle surface without wetting of the porous structure;
see Figure 9. In the experiment run, the hydrogen
temperature was 85 °C, its flow rate was 20 cm3/s, and
the liquid superficial velocity was 3.7 mm/s. Under the
above conditions, the catalyst bed exhibits the presence
of dry, overheated catalyst particles which can draw
liquid from their liquid-filled neighbors, Figures 8 parts
(4)-(6) and 9 parts (2)-(3). At the moment it is difficult
to say whether the liquid moves over the surface of the
dry particle without wetting its internal porous structure or whether it is imbibed by the catalyst particle.
Nevertheless, it is likely that the partially absorbed
reagent quickly evaporates and reacts in the gas phase
near the outer surface, thereby maintaining the high
particle temperature. Therefore, such dry particles act
as minireactors following the principle of reactive
evaporation.
4. Trickle-Bed Experiments
Compared to the known literature data, our experiments are aimed at studying the interaction of phase
transitions and chemical conversions in the trickle bed
and its impact on critical phenomena. For this reason,
we had to develop an experimental method for determination of the phase fraction, the concentrations of the
reacting components and reaction products in the liquid
and gas phases.24,33 The experiments were performed
using a model reaction of the R-methylstyrene hydrogenation. The experimental setup contained a thermostating quartz reactor with a catalyst bed. The setup
was equipped with a feeding system of liquid AMS and
hydrogen, a vaporizer of liquid AMS to saturate hydrogen with AMS vapor at the reactor inlet, a separator
for phase separation at the reactor outlet, a condenser
of AMS and cumene vapor, and a unit for chromatographic analysis of the AMS and cumene mixtures.
Hydrogen from a cylinder and liquid AMS were
supplied through a system of gas pressure regulators
and flow meters to a vaporizer. The vaporizer is
designed as a heat exchanger (annular tube type) heated
by the silicone oil from a thermostat. The hydrogen
saturated with AMS vapor to the equilibrium state at
the inlet temperature (Tin) was fed together with nonevaporated liquid AMS to the catalytic reactor. Because
of the exothermal hydrogenation occurring in the reactor, the temperature increased, which resulted in further evaporation of the liquid. At the reactor outlet, the
phases were separated, the AMS and cumene vapors
were condensed, and the compositions of the liquid and
condensed vapor phases were chromatographically measured.
The experimental conditions were as follows: reactor
diameter 11 mm, average size of spherical catalyst
particles 1.7 mm, catalyst 4% Pd/Al2O3, length of the
bed L ) 5/25 mm, inlet temperature Tin ) 90/140 °C,
hydrogen flow rate 25 cm3/s, superficial liquid velocity
U2 ) 0/5 mm/s, and pressure in the reactor 1 atm. The
experiments provided the information on the effect of
liquid supply rate, inlet temperature, mass content of
vapor at the bed inlet, mole fraction of the AMS vapor,
and catalyst bed length on the temperature, phase
composition, and a vapor-gas/liquid-phase ratio at the
catalyst bed outlet. Using the experimental data and
the formulas given below, we calculated the following
parameters:
(i) mass flow rate of AMS and cumene vapor at the
reactor outlet, Fout ) Gvap/(Gvap + Gliq);
(ii) overall AMS conversion in the liquid and vapor
phases,
vap
YA ) 1 - [Foutxvap
AMC + (1 - Fout)yAMS] ≈ FoutxCUM +
(1 - Fout)yCUM;
(iii) AMS hydrogenation apparent rate, Wreac )
GAMSYA;
(iv) mass flow rate of AMS vapor at the trickle-bed
inlet,
in
Gin
AMS ) [PAMS(Tin)/(P - PAMS(Tin))]NH MAMS, PAMS )
10
3
1.3367 × 10 exp[-(( 43 × 10 )/RTin)] H/m2;
(v) molar fraction of AMS vapor in a mixture with
cumene and hydrogen at the reactor inlet, xAMS )
PAMS(T)/P;
(vi) AMS mass fraction in the gas phase at the reactor
inlet, Fin ) Gin
AMS/GAMS;
(vii) apparent rate of the evaporation in trickle-bed
total
cond
) GAMS(Fout - Fin) ) Wev
reactor, Wevap
AMS - WCUM;
(viii) AMS evaporation apparent rate, Wev
AMS )
GAMS(Foutxvap
AMS - Fin) + Wreac;
(ix) cumene condensation apparent rate, Wcond
CUM )
GAMS(1 - Fout)yCUM;
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9733
Figure 10. The impact of AMS mass flow rate and inlet vapor mass fraction on (a) the trickle-bed temperature and (b) the AMS mole
fraction.
Figure 11. The impact of superficial liquid velocity and inlet AMS mass vapor fraction on the apparent reaction rate, phase-transition
rate, and AMS and cumene mole fractions in liquid and gas phases.
(x) molar flow rate of the AMS and cumene
out
) Gvap(xvap
vapors at the reactor outlet, Nvap
AMS/MAMS +
vap
xCUM/MCUM);
(xi) hydrogen molar flow rate at the reactor outlet,
in
Nout
H ) NH - Wreac/MAMS;
(xii) molar fraction of vapors of AMS and cumene in
vap
the gas phase at the reactor outlet: xvap
AMS ) NAMS/Nout,
vap
vap
out
out
xCUM ) NCUM/Nout, Nout ) Nvap + NH , Nvap
AMS )
vap
vap
Gvapxvap
AMS/MAMS, NCUM ) GvapxCUM/MCUM.
Figure 10 shows the profiles of concentrations and
temperatures of AMS along the catalyst bed. The figure
suggests that the presence of liquid at the reactor inlet
decreases the temperature in the reactor as compared
to a one-phase flow. Note that, because of considerable
evaporation of liquid in the trickle bed, the concentration of AMS in the gas phase is higher in the case of a
two-phase flow. If the temperature of the liquid in the
reactor is near the AMS boiling point, there are tem-
perature oscillations (error bars in the figure) near the
AMS boiling point.
Figure 11 shows the concentrations of AMS and
cumene in the gas and liquid phases (experimental
conditions are similar to those in Figure 10). It is evident
that the AMS concentration in the gas phase is lower
than that in the liquid phase because of the AMS
evaporation in the course of reaction. However, the
concentration of cumene in the gas phase exceeds that
in the liquid phase under similar conditions. Such a
situation may occur if the rate of the gas-phase reaction
is significantly higher than that of the liquid-phase
reaction.
The apparent rates of AMS hydrogenation and evaporation and cumene condensation are also shown in
Figure 11. If a vapor mass fraction in the inlet gas liquid
flow is low, the rate of AMS evaporation is higher than
that of AMS hydrogenation, which results in an increase
of the AMS concentration at the reactor outlet (Figure
9734
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005
Figure 12. Temperature hysteresis phenomena in trickle catalyst bed.
10). Increasing the inlet vapor mass fraction, one
increases the rate of hydrogenation compared to the rate
of evaporation. This observation is probably associated
with an increasing effect of the reaction in the gas phase
caused by the decrease of the catalyst external wetting
efficiency.
On varying the rate of liquid feeding, we observed
hysteresis of temperature at the catalyst bed outlet,
which is shown in Figure 12 for different Tin. These
results confirm the earlier assumption that, at such
conditions, the process follows the mechanism of liquid
AMS evaporation and subsequent hydrogenation in the
gas phase. Thus, if the liquid flow rate is 0.5-0.8 mm/
s, the liquid AMS fed into the reactor completely
evaporates inside the trickle bed; moreover, most of the
catalyst particles remain dry. In Figure 12, this observation corresponds to the maximum temperature values
at the reactor outlet.
As the liquid velocity increases, the external wetting
efficiency and the fraction of wetted and completely
filled particles increases, resulting in a decrease of the
apparent reaction rate, Tout, Wev
AMS, and Wreact. If the
liquid superficial velocity is >5-6 mm/s, most of the
catalyst particles are filled with liquid.
Since a decrease in the liquid velocity to 1.5-2 mm/s
is accompanied by an increase in the temperature of the
catalyst bed and, consequently, by its ignition; it is
reasonable to suggest that, at U2 ≈ 5 mm/s, dry particles
(not completelly filled with liquid) still remain in the
trickle catalyst bed. The most probable reason of different trickling conditions is the nonuniform liquid
distribution generated by the fixed bed. The above
suggestions will be tested by MRI.
The earlier experimental data suggest that the initial
conditions in the trickle bed are responsible for the
generation of different hydrodynamic regimes in reactor,
which results in different wetting efficiencies of particles
and pressure drop hysteresis in the trickle bed.34
Besides, the liquid evaporation and the exothermal
chemical reaction in the catalyst bed, as follows from
the experimental data, are also responsible for hysteresis phenomena. This is associated with the process of
the drying of the particles (which proceeds at a lower
rate than their impregnation) and different gas-liquid
hydrodynamic regimes. An increase of temperature to
140 °C at the reactor inlet, caused by an increase of the
gas-phase reaction rate and almost complete drying of
the catalyst bed, leads to a collapse of the hysteresis
cycles.
Data on the composition of liquid and vapor phases
at the reactor outlet permit one to quantitatively
estimate phase nonequilibrium between vapor and
liquid. To characterize this phenomenon, we used the
following dimensionless criterion:
χ)
yCUM xvap
AMS
xvap yAMS
(4)
CUM
This expression can be derived by the following reasoning. Under the condition of phase equilibrium in the
reactor outlet, the component concentrations in the gas
and liquid phases of the ideal mixture are interrelated
by the Raoult-Dalton law:
xvap
AMS ) yAMPAMS(Tout)
(5)
xvap
CUM ) yCUMPCUM(Tout)
(6)
As follows from eqs 5 and 6, parameter χ defined by eq
4 depends only on the temperature of the equilibrium
two-phase mixture; therefore,
χeq )
PAMS(Tout)
PCUM(Tout)
(7)
For AMS and cumene, the equilibrium temperature
dependence χ(T) is rather weak. In Figure 13, this
dependence is shown as a bundle of curves. As the flowtemperature varies from 90 to 150 °C (0 < YA < 30%),
the equilibrium value χeq holds within a narrow range
of 0.6-0.7. The χ values derived from the experimental
data according to eq 4 (0.1 < χ < 0.2) fall well-below
the calculated lines. In other terms, the concentration
of the product (cumene) exceeds the equilibrium values
in the gas phase and is lower in the liquid phase. The
ratio of these concentrations weakly depends on the
liquid flow rate and AMS conversion.
A comparison of the data calculated by the phaseequilibrium model and experimental results suggests
that there is a marked disequilibrium between concentration and temperature in the gas-liquid flow at the
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9735
Figure 13. Parameter χ versus conversion YA. Experimental
conditions: Tin ) 90 °C; (1-3) L ) 10 mm and (l′-3′) L ) 25 mm;
in
and GAMS
) (1, 1′) 73-74, (2, 2′) 137-146, and (3, 3′′) 191-206
mg/s. The lines are calculated by the phase-equilibrium model (7).
The points were calculated from experimental data using eq 4.
conditions stimulating the appearance of “hot spots” in
the trickle bed. This observation is inconsistent with the
conventional models of trickle-bed catalytic processes
accompanied by rather intensive heat and mass transfer, which easily compensate a shift from the phase
equilibrium. We present the following explanation of our
results. Some particles are filled with liquid because of
capillary forces; moreover, their outer surface may be
completely or partially wetted by a flowing liquid film.
At the normal pressure and low hydrogen solubility in
the liquid, the rate of liquid-phase hydrogenation is
negligible as compared to the rate of possible AMS
evaporation from the surface of these particles into the
gas phase. Both the AMS vapor fed into the reactor and
the AMS vapor yielded by evaporation of the liquid AMS
on the wetted particles undergo the reaction on the dry
particles. The adiabatic reaction of a stoichiometric AMS
+ hydrogen mixture heats the system to a temperature
of 450 °C, while the same reaction controlled by external
diffusion increases the temperature to ∼180 °C.35
Therefore, both the dry particles and gas flows around
them may be much overheated compared to the wetted
particles and the liquid. The concentration of the
product in the gas phase (cumene vapor) is considerably
higher than in the liquid, because the rate of the
reaction on the dry particles exceeds that of cumene
condensation at the liquid-gas interface.
Therefore, the experimental results suggest the existence of significant phase disequilibrium in the trickle
bed, which is caused by the reaction occurrence on the
partially wetted and dry catalyst particles in the gas
phase. This disequilibrium is mainly responsible for
critical phenomena in trickle-bed reactor.
5. MRI Experimental Results on the Liquid
Distribution in a Trickle Bed
MRI experiments were aimed at studying the liquid
distribution versus particle diameters and the superficial liquid velocity in the trickle bed during the chemical
reaction with phase transitions, as well as at ascertaining data on the critical conditions at which both drying
and runaway of the trickle bed begin and experimentally
verifying the hypothesis for the effect of catalyst particle
temperature on the liquid distribution in the tricklebed reactor.
Figure 14. Distribution of liquid in trickle bed during hydrogenation of heptene. Diameter of particles 2-3 mm; reactor diameter
10 mm; catalyst bed length L ) 46 mm (1% Pd-γAl2O3 + 0.1% Mn
catalyst).
The MRI permits a direct observation of liquid
distribution and presence of wetted catalyst particles.
If a part of the trickle bed or catalyst particles are dried
during the experiment, they disappear from the detector’s field of view and become invisible. The experiments
were performed using the model reactions of hydrogenation of R-methylstyrene to cumene and heptene to
heptane. The experimental conditions were as follows:
reactor diameter 10 mm; catalyst bed length L ) 46 mm;
catalyst (1% Pd-γAl2O3 + 0.1% Mn); particle diameter
d ) 0.5/1 mm and 2.5/3 mm; superficial liquid velocity
U2 ) 0/6.5 mm/s; superficial hydrogen flow velocity 10/
50 cm/s; inlet flow temperature 60/90 °C; and pressure
in the reactor atmospheric. Hydrogen (no vapor) and
liquid AMS or heptene were supplied to the reactor inlet
using the central one-point feeding. During the experiment, the conversions of AMS into cumene and heptene
into heptane and their concentrations were determined
by the chromatographic analysis of the liquid and
condensed vapor phases at the reactor outlet. These
data permitted us to calculate conversion and output
of the catalyst bed (W) vs experimental conditions. The
experiments were performed on the dry and preliminary
wetted catalyst particles. Figures 14 and 15 show the
images of the trickle catalyst beds formed by particles
2-3 and 0.5-1 mm in diameter, respectively.
In Figure 14 parts (1)-(4), the experimental conditions correspond to the trickle bed formed by catalyst
particles 2-3 mm in diameter (preliminary filled); the
liquid superficial velocity is decreased from 4.2 to 0.93
mm/s. Figure 14(1) suggests that, as the liquid velocity
decreases, the fraction of wetted particles also decreases.
Liquid moves in the trickling regime over the surface
of the wetted catalyst particles. Despite the fact that
the liquid velocity decreases, the gas-liquid interface
increases. This results in the growth of evaporation rate
and, correspondingly, of the hydrogenation rate in the
gas phase. The liquid superficial velocity of 0.93 mm/s
is a critical value, and its subsequent slight reduction
provides drying and runaway of the trickle bed.
9736
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005
on the catalyst bed filling is shown in Figure 15 parts
(5)-(8). As in Figure 14 parts (6)-(8), the liquid spreads
predominantly into a low-temperature, wetted part of
the catalyst bed, whereas unwetted catalyst particles
are filled because of radial expansion of the wetted
catalyst part; see Figure 15 parts (5)-(8). A comparison
of the experimental data in Figures 12, 14, and 15
suggests that the size of a catalyst particle significantly
affects liquid distribution in the trickle bed and the
value of the critical rate at which both drying and
runaway of the bed occur.
6. Conclusion
Figure 15. Distribution of liquid in the trickle bed formed by
particles 0.5-1 mm in diameter during the reaction of heptene
hydrogenation. Reactor diameter 10 mm; catalyst bed length L )
46 mm (1% Pd-γAl2O3 + 0.1% Mn catalyst).
To compare the effect of initial conditions, the images
in parts (5)-(8) of Figure 14 simulate the situation when
liquid is fed to a preliminary dried trickle bed with a
subsequent increase of the liquid superficial velocity
from 0.93 to 4.2 mm/s. As liquid is fed to a dry fixed
bed, it is accumulated in the areas nearing the reactor
inlet; moreover, the main part of the particles in Figure
14 parts (6) and (7) remains unwetted and open for
hydrogenation of heptene vapors. This regime provides
much higher efficiency of the trickle bed than that
presented in Figure 15 parts (1-4). As the superficial
velocity increases to 4.2 mm/s, the liquid spreading in
the unwetted part of the fixed bed proceeds by expansion
of a filled part of the trickle bed, increase of the number
of wetted particles, and concurrent decrease of the
apparent output of the trickle bed; see Figure 14 parts
(7) and (8). Therefore, liquid preferably flows over the
wetted catalyst particles with lower temperature. According to Figure 14 parts (1) and (8), the apparent
catalyst bed output depends on the initial state of the
trickle bed. This result qualitatively correlates with
Figure 12.
On the basis of the MRI data on the heptene hydrogenation (see Figure 15), we suggest the following
mechanism of the onset of critical phenomena. When
superficial liquid velocity is ∼6 mm/s, the catalyst bed
is completely filled; Figure 15(1). A decrease in the
liquid superficial velocity or worsening of its distribution
in the reactor, as well as heat generation during the
exothermal reaction accompanied by the reagent evaporation, result in drying of part of the trickle bed; see
Figure 15(2). Because of the gas-phase hydrogenation,
the temperature sharply increases to 282 °C. As a result,
no liquid is detected at the catalyst bed outlet. As the
liquid velocity decreases to 0.6-0.5 mm/s (Figure 15
parts (3) and (4)), the volume of the wetted part of the
bed decreases. Liquid is detected as a narrow zone
situated near the trickle catalyst bed inlet. Note that
the apparent output of the trickle bed increases. The
impact of an increase of the liquid superficial velocity
We considered a number of hypotheses for the mechanism of appearance of critical phenomena. It was
established that critical phenomena, such as overheating of the trickle bed and catalyst particles, multiplicity
of the steady-state regimes, and hysteresis phenomena,
are caused by liquid evaporation and transition of the
reaction to the gas-phase hydrogenation regime. The
factors promoting this transition are as follows: the
exothermicity of the hydrogenation reaction, the existence of dry and partially wetted catalyst particles at a
superficial liquid velocity of 5-6 mm/s, the nonuniform
liquid distribution across the reactor cross section, and
the phase nonequilibrium in the trickle bed.
The method of thermocouple probing and MRI were
used to experimentally study both heat regimes and
liquid distribution under conditions of the exothermal
hydrogenation occurring on the catalyst particle and
catalyst bed. In the experiments, we studied the multiplicity of steady-state regimes, hysteresis phenomena,
and the effect of both liquid flow rate and particle size
on the appearance of critical phenomena. It was shown
that the liquid distribution in the trickle bed depends
on the particle temperature. Using MRI, we studied the
mechanism of interactions between the critical phenomena on the microlevel (a catalyst particle) and the
critical phenomena on the macrolevel (a trickle bed). In
a regular structured bed, each catalyst particle is ignited
individually, and the effect of the neighboring particles
is revealed through their impact on the wettability of
the porous structure of this particular particle. Liquid
reactant drawn by the dry particle from its liquid-filled
neighbors promptly evaporates because of the high
particle temperature and readily undergoes hydrogenation in the gas phase.
Acknowledgment
This work was supported by Grant No. 047.014.004
of Russian-Dutch Research Cooperation (NWO) and
CRDF Grant No. RU-C1-2581-NO-04 of Russian-USA
Research Cooperation.
Notation
d ) diameter of the catalyst particle, m
E ) activation energy, J/mol
f ) external wetting efficiency of the particle
F ) mass vapor fraction
H ) heat of evaporation, J/mol
G ) mass flow rate, g/s
K0 ) constant of the reaction, mol/m2s
L ) length of the catalyst bed, mm
M ) molar mass, g/mol
N ) molar flow rate, mol/s
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9737
P ) atmospheric pressure, atm
PAMS ) partial pressure of AMS, atm
PCUM ) partial pressure of cumene, atm
T ) temperature, °C
T1 ) temperature in the center of a catalyst particle, °C
T2 ) axial temperature of the catalyst particle, °C
T0 ) bulk gas temperature, °C
U2 ) superficial liquid velocity, mm/s
Q ) heat of the reaction, J/mol
R ) universal gas constant, 8.31 J/mol K
S ) external surface of the particle, m2
ev
WAMS
) AMS evaporation rate, g/(cm3 s)
Wreac ) total apparent rate of reaction, g/(cm3 s)
cond
WCUM
) cumene condensation apparent rate, g/(cm3 s)
W ) apparent output of trickle bed, mg/s
x ) mole fraction in gas flow
y ) mole concentration in liquid phase
Greek Letters
R ) heat transfer coefficient, W/(m2 K)
β ) mass transfer coefficient, m/s
η ) efficiency of the catalyst particle
Subscripts
0 ) bulk
AMS ) R-methylstyrene
CUM ) cumene
H ) hydrogen
in ) inlet condition
out ) outlet condition
vap ) gas phase
liq ) liquid phase
× ) critical value
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Received for review March 1, 2005
Revised manuscript received August 15, 2005
Accepted September 23, 2005
IE050276L