Taylor flow hydrodynamics in gas-liquid-solid micro reactors

Chemical reactions in which a gas phase component reacts with a liquid phase omponent at the surface of a solid catalyst are often encountered in chemical industry. The rate of such a gas-liquid-solid reaction is often limited by the mass transfer rate of the gas phase component, which depends on the hydrodynamics of the gas-liquid flow. Furthermore, the efficiency of a gas-liquid-solid reactor further depends on the pressure drop, which is also determined by the hydrodynamics. Therefore, the trend in chemical industry towards more sustainable production methods has led to developments aimed at improving the performance of gas-liquid-solid reactors by tailoring the hydrodynamics. Two examples of these efforts are monolith reactors and microreactors, in which the gas and liquid are forced to flow through channels with diameters in the order of 10-4 to 10-3 m. At these length scales, the hydrodynamics differ from those in conventional reactors and the Taylor flow regime is the main flow regime of interest. It consists of an alternating sequence of gas bubbles and liquid slugs. The length of the gas bubbles is larger than the channel diameter and athin liquid film separates the gas bubbles from the channel walls, where the catalyst is located. The liquid at the interface of the gas bubble and this film is saturated with the gas component and, for a fast reaction, the concentration at the catalyst surface is very small. The large concentration gradient in the liquid film results in a high diffusion rate. Furthermore, due to the small dimensions of the channel, the specific interfacial surface area between the gas bubbles and the liquid film is large, which further increases gas component mass transfer rates compared to conventional reactors. It is, therefore, important to understand the relation between gas-liquid Taylor low hydrodynamics and gas component mass transfer. Additionally, it is required to understand how the pressure drop depends on the various hydrodynamic parameters, since the pressure drop partly determines the efficiency of a reactor. Furthermore, to be able to fully optimize the reactor design, it has to be understood how to manipulate the Taylor flow hydrodynamics by varying parameters that can be controlled directly, e.g. the gas and liquid feed velocities, the geometry of the gas-liquid contactor and the geometry of the channel. The thickness of the liquid film is a key parameter for gas component mass transfer in small channels, but it also determines the excess velocity of the gas bubbles with respect to the average velocity in the channel, which, in turn, determines the gas and liquid hold-up in the channel. The behaviour of liquid film thickness is well understood for channels with a circular cross-sectional area and for negligible inertial and gravitational forces. In microreactors and monoliths, these conditions are not necessarily met and channels with a square or rectangular cross-sectional area are often used. Therefore, an experimental study was performed regarding the fraction of channel cross-sectional area occupied by the liquid film and the gas hold-up. Experiments were done for nitrogen-water Taylor flow in rectangular micro channels under conditions where inertial forces were significant. The results are presented in chapter 2 and show that the gas hold-up as a function of the superficial gas and liquid velocities follows the well known Armand correlation. The model shows that the validity of the Armand correlation implies that the fraction of cross-sectional channel area occupied by the liquid film does not depend on the gas bubble velocity. From comparison of these results with literature data it was also shown that, when inertial effects are significant, the liquid film thickness is not only independent of the bubble velocity, but also occupies a fixed fraction of the channel cross-section independent of the channel diameter. Pressure drop models for gas-liquid Taylor flow in capillaries are hardly available in literature, with the notable exception of one semi-empirical model for channels with a circular cross-section. In this work, a new pressure drop model was developed for gas-liquid Taylor flow with a non-negligible film thickness in small channels with a circular cross-section. The model takes two sources of pressure drop into account: (i) frictional pressure drop caused by laminar flow in the liquid slugs, and (ii) an additional pressure drop over a single gas bubble due to the gas bubble disturbing the otherwise parabolic velocity profile in the liquid slugs. The model includes the effects of the liquid slug length on the pressure drop, similar to the semi-empirical model available in literature. Additionally, the model developed in this work includes the effect of the gas bubble velocity on the pressure drop over a single gas bubble. Data were obtained from experiments with nitrogen-water Taylor flow in a round glass channel with an inner diameter of 250 µm. The model described the experimental results with an accuracy of ± 4% of the measured values. This work is described in chapter 3. Although the understanding of the pressure drop of gas-liquid Taylor flow in channels with a circular cross-sectional area is growing, no models are available for non-circular channels and detailed data sets are lacking. One complicating factor is that accurately measuring the pressure drop in microfluidic chips and in channels with a non-circular cross-sectional area is not straightforward. In chapter 4, a method is presented for estimating the pressure of a gas-liquid Taylor flow in a microchannel by combining results obtained from image analysis with a mass balance based Taylor flow model. The method was applied to nitrogen-water Taylor flow in channels with a square or rectangular cross-sectional area, as well as to nitrogen-isopropanol Taylor flow in a channel with a rectangular cross-sectional area. It was shown that the method developed in this chapter yields realistic values for the pressure drop of gas-liquid Taylor flow in micro-channels with a non-circular cross-section. It, therefore, appears to be a viable method for determining the pressure drop of gas-liquid Taylor flow in micro-channels. However, a more firm validation of the method by comparison with data obtained by another measurement technique still needs to be done. For proper design of a gas-liquid-solid reactor in the Taylor flow regime, it is important to know for what combinations of gas and liquid velocities this regime occurs and how this range of combinations varies with various parameters. Flow maps were, therefore, determined while varying the liquid phase, the mixer design and the dimensions of the microfluidic channel. The mixer design was found to be mainly of influence on the regime transitions occurring at higher superficial velocities of one or both phases, where inertial effects are significant. On the other hand, varying the liquid phase between isopropanol and water affected all regime transitions, except those at high superficial gas and liquid velocities. When decreasing the dimensions of both the channel and the mixer, annular flow was no longer observed and Taylor flow could be obtained at higher gas velocities. This work is described in chapter 5 The mass transfer rate of the gas component depends on the lengths of the gas bubbles and liquid slugs. Furthermore, the liquid slug length partly determines the pressure drop of gas-liquid Taylor flow. Therefore, the length of a gas bubble was studied as a function of the gas and liquid flow rates, the liquid phase, and the dimensions of the mixer. Experiments were performed for nitrogen-water and nitrogen-isopropanol Taylor in cross-mixers with either square or rectangular channels. This work is described in chapter 5. All the results could be described by a simple correlation, and they showed showed that, for a given mixer and channel, the gas bubble and liquid slug lengths can not be varied independently from each other. However, if the geometry of the mixer can be varied separately from the dimensions of the downstream channel, then the gas bubble length, liquid slug length and total flow rate, can each be controlled. The rate of a gas-liquid-solid reaction per unit of reactor volume depends on the rate of external mass transfer, on the diffusion rate in the catalyst layer, and on the amount of catalyst per unit of reactor volume. The latter is determined by the ratio of the thickness of the catalyst layer and the channel diameter. The channel diameter also affects the external mass transfer rate, while the thickness of the catalyst layer determines, amongst others, the rate of diffusion in the catalyst layer. It is, therefore, important to choose the right combination of catalyst layer thickness and channel diameter system in order to optimize the performance of the microreactor for a given gas-liquid-solid reaction. Other researchers recently developed a new method for applying a thin film of mesoporous titania to the wall of a capillary, and the film served as a catalyst support for Pd nanoparticles. This type of capillaries was further tested in this work, and the resulting data set was used as input for an optimization study of the channel diameter and the thickness of the catalyst layer. This work is described in chapter 6. The hydrogenation of phenylacetylene in isopropanol was performed over Pd supported on mesoporous titania films with a thickness of 120.10-9 mcoating coated on the walls of a glass capillary with an inner diameter of 250 µmc. Two such capillaries were used, containing 1 wt%coating and 2 wt%coating Pd, respectively. The reaction was performed in the Taylor flow and Taylor-ring-annular regimes at temperatures varying from 313 to 343 K. The phenylacetylene conversions were smaller than 0.2 and styrene selectivities were higher than 0.92. An optimization study based on these data showed that, for this catalyst: • External mass transfer limitations can be avoided for channel diameters less than approximately 650 µmc, regardless of the thickness of the catalyst layer. • Internal mass transfer limitation can be avoided if the thickness of the catalyst layer is less than 4 µmcoating. • Both internal and external mass transfer limitations are avoided if the thickness of the catalyst layer is less than 4 µm and the channel diameter is smaller than 1.47.10-3 mc. At these limiting values, the overall reaction rate coefficient has a value of 0.10 ml3mc-3s-1. The amount of catalyst per unit of capillary volume is then 16.9 kgcoating/mc3. • For a fixed bubble velocity, fixed catalyst coating thickness and a relatively thin liquid film and catalyst coating, the ratio of the volumetric reaction rate coefficient of the catalyst and the volumetric mass transfer coefficient scales linearly with the channel diameter. Furthermore, it was shown that, for channel diameters small enough to avoid external mass transfer limitations, further reducing the channel diameter at a constant catalyst layer thickness results in a nearly linear increase of the overall volumetric reaction rate, solely due to increasing the amount of catalyst per unit reactor volume. However, the pressure drop, and thus the frictional energy dissipation per unit of reactor volume, then increases quadratically. All other things being equal, the net result of decreasing the channel diameter, when external mass transfer limitations are no longer significant, is that the reactor efficiency decreases with decreasing channel diameter. While microreactor technology enables the use of channels with a diameter an order of magnitude smaller than currently used in monolith reactors, its potential for gas-liquid-solid reactions does not lie in further increasing the reaction rate per unit of reactor volume, unless significant progress in catalyst development is made