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MASS TRANSFER IN SMALL CHANNELS WITH SLUG-FLOW UNDER REACTING CONDITIONS
Stefan Haase, Tobias Bauer, Robert Langsch, Rüdiger Lange
Dresden University of Technology, Department of Chemical Engineering, 01062 Dresden, Germany
Abstract: In a reacting environment, overall mass transfer rates of a gas-liquid slug-flow in a
small catalytically active channel with square cross section and a hydraulic diameter of 1 mm
were determined by extensive reaction experiments on the well known hydrogenation of
alpha-methylstyrene on a supported Pd/Al2O3 catalyst under gas-liquid mass transfer limited
conditions (pressures up to 3∙106 Pa and temperatures up to 420 K). The experiments were
carried out in a new developed reaction-apparatus which allowed the simultaneous
determination of the gas bubble and liquid slug length, the two-phase velocity, the liquid
holdup at the entrance and the exit of the reaction channel and the overall reaction rates.
Furthermore, the setup could handle different gas/liquid mixing systems to vary the gas and
liquid slug lengths. The comparison between observed mass transfer/reaction rates and
reactor model predictions, based on published data on the gas-liquid, liquid-solid and gassolid mass transfer rates under non-reacting conditions, aimed on a survey on the state of the
art and on identification of crucial parameters for future research.
Keywords: Multiphase Reactor, Mass Transfer, Slug-Flow, Single-Channel, Hydrogenation
1. INTRODUCTION
Gas-liquid two-phase flow in straight mini-channels and micro-channels is used in an emerging way in process
engineering and chemical reaction engineering. The specific properties of such devices make them well suited for
mixing, heat and mass transfer as well as for chemical multiphase reactions processes. In particular, the large
surface-to-volume ratio and very short diffusion lengths inside the channels lead to higher performances in heat and
mass transfer compared to conventional chemical devices. In the last decade, especially micro reactors and
monolithic reactors showed several benefits in space-time-yield and selectivity over conventional reactors such as
stirred tank reactors and randomly packed bed reactors (Pangarkar et al., 2008; Watts and Wiles, 2007; Dudukovic,
2007; Hessel et al., 2005; Kiwi-Minsker and Renken, 2005; Roy et al., 2004).
If gas and liquid flow in small channels, different flow patterns occur and the boundaries from flow regime to
flow regime vary with channel geometry, pressure and fluid properties as demonstrated by Bauer (2007). Typically
at low gas and liquid velocities (below 1 m s-1), slug-flow, also known as Taylor flow or bubble-train flow, is
dominant. This flow regime affect high gas-liquid mass transfer rates as demonstrated by Bercic et al. (1997) for the
solubility of methane in water and by Vandu et al. (2005) for the solubility of oxygen in water; both studies were
performed at ambient conditions. The high gas to liquid mass transfer rates via the ends of the gas bubbles are
supported by circulation vortices within the liquid slugs. Therefore, a large number of small gas bubbles, which
produce a high gas-liquid interfacial area, is favorable for high gas-liquid mass transfer rates. Reaction studies in
small catalyzed single-channels have demonstrated high overall mass transfer rates from gas and liquid to the
catalyst surface, which was indirectly measured by the overall reaction rates (Tsoglikas et al., 2007a,b; Natividad et
al., 2007; Enache et al., 2005; Bercic et al., 2001). It is believed that this phenomenon is mainly due to the diffusion
of gas through a very thin liquid film between the cylindrical part of the gas bubbles and the catalyst surface. With
other words, long gas bubbles favor the gas-solid mass transfer via the film and enhance the overall reactor
performance if the gas is the limiting reaction-component.
Nevertheless, modeling and simulations have revealed a big discrepancy between experimental reaction data
and model predictions. This is mainly attributed to the mass transfer correlations which are derived for non-reactive
systems at ambient conditions. Furthermore, the mass transfer correlations require information on the gas bubble
length, the liquid slug length and the specific gas-liquid interfacial area, which is unknown for reacting systems in
opaque channels.
This paper focuses on advancing the fundamental understanding of mass transfer in slug flow with
simultaneous chemical reaction at elevated temperature and pressure. A new setup was used which allowed studying
hydrodynamic parameters of the slug-flow regime at the entrance and the exit of a reactive channel at pressures up
to 3∙106 Pa and temperatures up to 420 K. Furthermore, the hydrogen solubility was measured in a non-reacting
environment for a broad range of operating conditions. Reaction experiments under mass transfer limitations were
used to evaluate overall mass transfer rates. The new experimental data and the derived mass transfer information
were used to improve the reactor modeling further. In summary, the paper provides a review on the state of the art
on mass transfer in slug flow regime and gives new information on mass transfer with simultaneous chemical
reaction.
2. EXPERIMENTAL SECTION
2.1 Single-Channel Reactor
A reactive single-channel with square cross section, with a hydraulic diameter of 1 mm and with a length of
600 mm (details in section 2.2) was used in the hydrogenation experiments. This reactor was operated in single-pass
mode for gas and liquid phases and two transparent sections above and below the catalytic part of the channel, with
identical dimensions, allowed visual flow observations by a high-speed camera (VDS-Vosskühler, model HCC
1000), see Fig.1.
Gas-Liquid mixing sections (see Fig. 2)
Catalytically
active channel
Set of high-speed
cameras
Fig. 1. Experimental setup of the high-pressure single-channel reactor.
Fig. 2. Applied gas/liquid mixing sections
to adjust gas and liquid slug lengths
(A, B: T-shape, C: Y-shape).
Before the reaction studies, alpha-methylstyrene (Sigma Aldrich, purity > 99%) was treated with activated
alumina oxide for 8 h in a stirred tank to remove the polymerization inhibitor p-tert-butylcatechol because this
substance affects the catalyst activity as proven by own experiments. After this procedure, alpha-methylstyrene was
mixed with Cumene (Sigma Aldrich, purity > 99%) to a concentration of 4 mol l-1 and fed via a pre-saturator and via
the mixing section to the channel using a non-pulsative pump (AlphaCrom, model SD1). Three different gas/liquid
mixing sections were used to adjust different gas bubble and liquid slug lengths, see Fig.2. The hydrogen (Praxair,
purity > 99.99%) was supplied via a cylinder and its flow rate was adjusted by a mass flow controller (Brooks,
model 5850 E). The pressure in the reactor was maintained by a pressure controller in the vent (Bronkhorst, model
P-702CV). Thermocouples were accomplished to measure the temperature at gas and liquid inlet as well at the
reactor outlet, to regulate electrical preheating of liquid and reactor and to control isothermal conditions. After
reaching steady state regarding temperature and pressure, the liquid phase composition in the reservoir and in the
separator was analyzed 3 times with a delay of 15 minutes in between by a GC-MS-system (HP, model 6890).
2.2 Reactive Channel Preparation
The reactive channels were manufactured by anchoring a porous layer of gamma-alumina (washcoat) on the
monolith substrate using a modified sol-gel dip-coating method. The carrier was subsequently impregnated with
palladium(II)-acetate (Sigma Aldrich, purity > 99.9%) and finally reduced in situ in flowing hydrogen at 513 K for
5 h. The monolith substrate (Corning) was made of cordierite and had a length of 50 mm and a cell density of 400
channels per square inch. The washcoat layer was characterized by a BET-surface area of 236 m² g-1 and an average
loading of 6.5 wt.% based on the weight of the bare monolith. The palladium loading was 1.1 wt.% based on the
weight of the washcoat. The reproducibility of both steps was high (variation less than 3 %).
2.3 Experimental Procedure
The mass transfer studies with and without chemical reaction were carried out in the slug-flow operation. An
overview about the main experimental parameters is given in Table 1.
Table 1 Parameters of Experimental Studies
Liquid
Gas
Alpha-Methylstyrene/Cumene
Hydrogen
Pressure
[105 Pa]
1…30
Temperature
[K]
298…413
Superficial gas
velocity [m s-1]
0.005…0.30
Superficial liquid
velocity [m s-1]
0.005…0.30
In all experiments, the desired temperatures, the gas and liquid flow rates as well as the system pressure were
adjusted and the system was put into steady-state before the measurement was started. The gas bubble and liquid
slug length as well as the velocities were measured simultaneously at the channel entrance and the exit. Hydrogen to
alpha-methylstyrene mass transfer was investigated in a catalytically non-active channel using a dissolved-hydrogen
analyzer (Fugatron HYD-100) in dependence on gas and liquid flow rates, pressure and temperature. Furthermore,
overall mass transfer was measured using the hydrogenation of alpha-methylstyrene. In these experiments, the
reaction rate per channel pass was determined by the change in concentration between channel inlet and outlet. If the
reaction temperature is high enough (T > 393 K), the reaction rate is not a function of temperature anymore and,
consequently, the reaction was only external mass transfer limited which means that the transport of the gaseous
reactant limits the reaction rate. In this case, the overall hydrogen mass transfer rate was calculated by the superficial
velocity and the change in liquid-phase composition between inlet and outlet.
3. RESULTS AND DISCUSSION
In each mass transfer process, the mass transfer rate is correlated to mass transfer coefficient [m s-1], to the
specific interfacial area [m2 m-3] and to the driving concentration gradient [mol l-1]. In slug flow mode, overall mass
transfer for the gaseous component is divided in three separated processes: (a) gas-solid mass transfer via a thin
liquid film between the gas bubble and the catalyst surface, (b) gas-liquid mass transfer via the semi-spherical ends
mass transfer rate [mol m-3 s-1]
mass transfer-area coefficient [s-1]
of the bubbles and (c) liquid-solid mass transfer from the liquid slug to the solid catalyst. The literature offers
several empirical correlations to describe each process mathematically. However, the validity especially for square
channels and reacting conditions is still in discussion. For further understanding, a process model of the reactor was
developed which took into consideration data and correlations from the literature, the change of gas bubble size
along reactor length as well as new results on gas-liquid mass transfer at elevated conditions.
This model was used to analyze the mass transfer rates and mass transfer-area coefficients of hydrogen in
alpha-methylstyrene inside the catalyzed channel under reaction conditions, see Fig. 4 and Table 1. At the reactor
entrance no gas-liquid mass transfer occurs since the liquid enters fully saturated with hydrogen. Contrary, liquidsolid mass transfer is high because of the maximum possible concentration gradient. With increasing reactor length,
more hydrogen is taken from the liquid slug than can be refreshed into the liquid slug. Consequently, the hydrogen
concentration inside the liquid slug is constantly declining. As this concentration determines the driving force
between gas-liquid and liquid-solid mass transfer, the change in concentration affects a rising gas-liquid mass
transfer rate and a reducing liquid-solid mass transfer rate until both processes will be equalized after around 0.3 m.
Moreover, the gas bubble size and the two-phase velocity are decreasing due to the consumption of hydrogen by the
chemical reaction. This leads to a deflating gas-solid and gas-liquid mass transfer-area coefficient. The liquid-solid
mass transfer-area coefficient increases because of the rising liquid holdup in the unit cell (single gas bubble and
liquid slug).
reactor length [m]
Fig. 4. Axial profile of mass transfer rate and mass transfer-area coefficient inside the square channel at 393 K and
at 106 Pa, superficial gas velocity: 0.2 m, superficial liquid velocity: 0.2 m, hydraulic channel diameter: 1mm.
Based on the presented reactor model the conversion of alpha-methylstyrene per pass was calculated and
compared to the experimental results, see Fig. 5. In the experiment, mass transfer limitations occurred above 393 K
reaction temperature as already reported by Kreutzer et al. (2001) - visible in the diagram by a stagnating
conversion. Although all available correlations (for all mass transfer steps and parameters) have been studied, this
trend is not described by any set of the equation-based reactor model. This reveals that the state of the art in mass
transfer modeling still needs improvements especially in the field of square channels (micro-channels are included)
and under reaction conditions. Such work is currently ongoing.
Furthermore in this paper, affects of liquid film thickness, gas-bubble and liquid slug lengths, superficial liquid
and gas velocity and material properties such as diffusion coefficients, dynamic viscosity, surface tension, Henry
coefficients and density will be discussed based on experimental and theoretical investigations.
temperature [K]
Fig. 5. Experimental vs. simulated conversion of alpha-methylstyrene (AMS) at 106 Pa with different gas-liquid
mass transfer correlations found in the literature; square channel with 1 mm hydraulic channel diameter and a
length of 0.6 m, superficial gas velocity: 0.2 m, superficial liquid velocity: 0.2 m.
4. SUMMARY AND CONCLUSIONS
Detailed theoretical and experimental investigations on mass transfer phenomena in slug flow mode in a small
catalyzed channel with simultaneous chemical reaction at elevated pressure and temperature were performed. The
mass transfer phenomena were directly related to hydrodynamic studies of the slug flow regime. This work
contributes to a better understanding of mass transfer of reactive gas/liquid two-phase flow. Based on sensitivity
analysis and aimed experiments, suggestions for further research will be given and ordered by priority.
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