Taylor Flow Regime Research Articles

CFD modelling of flow and heat transfer in the Taylor flow regime

Transport phenomena in the Taylor flow regime for gas–liquid flows in microchannels have received significant attention in recent years. Whilst the hydrodynamics and mass transfer rate in the Taylor flow regime have been studied extensively using experimental and numerical techniques, studies of heat transfer in Taylor flow have been neglected. In this work, the flow and heat transfer in this regime is studied using the volume of fluid (VOF) and level-set techniques to capture the gas–liquid interface, as implemented in the ANSYS Fluent and TransAT codes, respectively. The results obtained from the two different codes are found to match very closely. Fully-developed flow and heat transfer are studied using the VOF method for a Reynolds number ( Re) of 280, Capillary number ( Ca) of 0.006 and homogeneous void fraction ( β) of 0.51 for constant wall heat flux (H) and constant wall temperature (T) boundary conditions. The Nusselt numbers obtained for both cases are 2.5 times higher than those for liquid-only flow. The effects of the mixture velocity and the homogeneous void fraction on flow and heat transfer are also studied.

Monolith loop reactor for hydrogenation of glucose

Monolithic and powder catalysts based on Ru on γ-Al 2O 3 for hydrogenation of glucose to sorbitol were prepared and shown to have comparable physico-chemical properties. Monolithic catalysts were investigated in a loop reactor in the Taylor Flow regime. For this purpose an experimental setup was built, where gas and liquid can be recycled independently. For comparison of monolithic and powder catalysts, a stirred tank reactor is integrated in the setup. Analysis of hydrogenation experiments revealed that external mass transfer limitations occur in case of the powder catalyst, whereas the monolithic catalyst used in the present study suffers from internal mass transfer resistances. Monolithic catalysts with sufficiently thin catalyst should be a promising alternative to suspended powder catalysts as they allow for an improvement of overall reaction rates.

Pressure drop and liquid hold-up in multiphase monolithic reactor with different distributors

Total pressure drop and liquid hold-up in two different monolith packings have been investigated experimentally in the co-current downward flow mode with a reactor diameter of 0.1 m and 0.6 m high monolith packing operating in the Taylor flow regime. The effect of distributor design on the flow distribution was investigated using two different types of distributor (nozzle distributor and packed bed distributor with 1 mm glass beads). A close relation of the distributor design to the hydrodynamics in monolith beds was observed to exist, evidenced by the larger pressure drop and liquid hold-up values for the monolith packings with an accompanying use of the packed bed distributor, as compared with the use of the nozzle distributor. An explanation for this phenomenon is the better gas and liquid distribution capability the packed bed has. By taking account of the effect of the liquid slug lengths, correlations were derived for predicting the two-phase friction factor and liquid hold-up and satisfactorily describe the experimental data. Finally an analysis of the unstable flow phenomenon characterized by negative pressure drops within bed shows that the unstable region is not only dependent on the operating conditions and properties of monolith but also on the distributor design.

Simulation of an industrial trickle bed hydrogenation reactor in the pulsing flow regime

An industrial trickle bed reactor was simulated for pyrolysis gasoline selective hydrogenation. The operation was recognised in the upper limit of pulsing flow regime by the Charpentier-Tosun flow pattern diagram at a very low hydrogen to oil volumetric ratio. Hydrodynamic parameters in the pulsing flow regime were calculated from the literature correlations which apply to the hydrogen-paraffin system. The external wetting efficiency which should be no higher than 100% was found to be 147% according to the correlation of Al-Dahhan and Dudukovic (1995), due to the undefined upper limit of their equation. The reactor simulation shows the gas-liquid and liquid-solid mass transfer rates were large enough for the active Pd hydrogenation catalyst. By comparing with the mass transfer rate in the Taylor flow regime in a monolithic structure, it shows the hydrogen concentration in the liquid phase in an industrial trickle reactor is more than two times of that in the monolith, which implies the substitution of the conventional catalyst pellet by the monolith is not required.

Influence of orientation upon the hydrodynamics of gas–liquid flow for square channels in monolith supports

Monoliths are being used increasingly as catalyst supports for two-phase gas–liquid reactions, yet substantial differences in the mass transfer performance between different configurations have not been thoroughly explained using either mass transfer or hydrodynamic arguments. In this paper, investigations of the differences in hydrodynamics between up-flow and down-flow have been made in a single channel using square glass capillaries of either 1.5 or 2 mm section. The fluids used were either water or 30 % v / v isopropanol/water mixture and air. Predictive flow maps are presented for down-flow: annular, Taylor (slug) flow, bubbly and churn flow were observed. In the Taylor flow regime, slug velocities and lengths measured using an optoelectronic technique were found to be in good agreement with the drift flux model [Zuber, N., Findlay, J.A., 1965. Average volumetric concentration in two-phase flow systems. Journal of Heat Transfer 87, 453–468]. Non-zero drift velocities were obtained. Particle image velocimetry (PIV) was used to investigate the velocity fields within the liquid slugs. For short slugs (slug length less than the tube hydraulic diameter), a flow is developed where the axial velocity component is only a function of position in the tube cross-section. The velocity profile is relatively flat, with the maximum observed velocity at the axis of the tube, V max , being 0.8–1 times the bubble velocity, V B . For long slugs, the axial velocity component depends on both the axial position and the position in the tube cross-section. Close to parabolic profiles are developed with V max / V B ≈ 1.1 – 1.7 . The location of the centre of the recirculation vortices produced in long slugs was found to be closer to the tube centre in down-flow compared with up-flow. Recirculation times in up-flow were 3 times faster: this has implications for the models used to predict rates of mass transfer and residence time distribution.

Measuring gas–liquid distribution in a pilot scale monolith reactor via an Industrial Tomography Scanner (ITS)

An Industrial Tomography Scanner (ITS) was designed and developed to study and quantify the phase distribution in a two-phase flow pilot scale monolith reactor that was 24 in. (0.60 m) in diameter and 192 in. (4.9 m) in height. The monolith reactor was operated co-current up-flow in the Taylor flow regime with water as the liquid phase and air as the gas phase. The cross-sectional holdup distributions were measured at three axial elevations. The operating conditions were selected to bracket commercial operating conditions for fixed bed monolithic reactor systems. The results show that ITS can capture the flow features in a large diameter column. Also the findings suggest the need for careful design of the internals of the reactor. Spatial resolution down to 1.5 cm was obtained so that gross phase maldistribution could be reliably observed. However, improvement is needed for the ITS to be effectively utilized in industry.

Simulation of Taylor Flow in Capillaries Based on the Volume-of-Fluid Technique

Taylor flow, a flow regime characterized by Taylor bubbles separated by liquid slugs that do not contain entrained micro bubbles, is a predominant gas−liquid two-phase flow regime in capillaries and minichannels (channels with hydraulic diameters in the 0.1−1 mm range), and it occurs in monolithic catalytic converters and other multiphase reactors. Taylor flow regime is morphologically relatively simple and has been modeled in the past using computational fluid dynamics (CFD) methods. However, most of the past CFD models have either assumed a fixed gas−liquid interfacial geometry or have modeled the gas−liquid interphase movement based on the method of spines, which imposes some restrictions on the free movement of the interface. In this study, we examine the feasibility of CFD modeling of the Taylor flow regime in capillaries by using the volume-of-fluid (VOF) technique for the motion of the gas−liquid interphase. It is shown that such a model predicts well the experimental data and empirical correlation...

Selective Oxidation of 1-Butene by Molecular Oxygen in a Porous Membrane Taylor Flow Reactor

A Taylor flow membrane reactor was developed and tested in the reaction of selective oxidation of 1-butene to methyl ethyl ketone by a Pd2+−heteropoly anion aqueous catalyst. The reactor enables macroscopic separation of oxygen and hydrocarbon streams by a liquid layer at the membrane interface, thus avoiding the formation of potentially explosive vapor mixtures even when pure reagents are used at elevated pressures. Gas−liquid Taylor flow in the tube side of the membrane was studied using glass capillaries and carbon membranes. Experimental mass transfer data were used to validate the reactor model. On the basis of the comparison of experimental results and model prediction, the hydrophilic carbon membrane is believed to be fully wetted by the aqueous catalyst. The gas−liquid mass transfer of 1-butene in the Taylor flow regime was shown to be the rate-limiting step. Both the experimental data and the reactor simulation confirmed that under the mass transfer limiting regime the concentration of reactants ...

Flow distribution characteristics of a gas–liquid monolith reactor

Flow distribution characteristics in a 0.05 m diameter monolith reactor, operated co-current downward in the Taylor flow regime, was investigated and quantified using gamma ray computed tomography (CT). The results indicate that the flow distribution is a strong function of liquid distributor design, and gas and liquid superficial velocities. Additionally, within the range of gas and liquid superficial velocities investigated, a window of operating condition (gas and liquid velocities) was identified which resulted in a close to uniform phase distribution.

Gas–liquid mass transfer of aqueous Taylor flow in monoliths

The gas–liquid mass transfer of a monolith operating in the Taylor flow regime is presented. Mass transfer measurements are compared with a literature model derived for single capillaries. The comparison resulted in a prediction of the unit cell length (gasbubble+liquidslug). Independent measurements of the liquid slug length showed that the predicted unit cell length is close to the measured ones. This leads to the conclusion that mass transfer models for single capillaries may indeed be used for monoliths. Additionally, it is shown that the liquid slug length may also be estimated from pressure drop measurements.

Hydrogenation of dimethyl succinate over monolithic catalysts

Hydrogenation of dimethyl succinate over monolithic copper and nickel catalysts was studied using a semibatch reactor in which liquid was circulated, while hydrogen was continuously passed through the mixture. Flow rates of both phases were adjusted, so the system operated in the Taylor flow regime. The process was investigated at 150–240°C and 3–7MPa. A nickel catalyst was active, but was insufficiently selective. Higher loaded copper catalysts were active and selective with respect to γ-butyrolactone at relatively low conversions. Catalyst activity (initial rates) was similar to that for the liquid phase hydrogenation of maleic anhydride over conventional catalysts previously reported. The catalysts deactivated, but unlike the nickel catalyst, activity of copper catalyst was easily restored by high temperature treatment in hydrogen.

Influence of operating conditions on the observed reaction rate in the single channel monolith reactor

The catalytic hydrogenation of nitrobenzoic acid (NBA) to the aminobenzoic acid was used as a model reaction for a quantitative study of influences of the operating conditions on the observed reaction rate in a single channel monolith reactor operated in Taylor flow regime. A simple mathematical model was derived and used for the analysis of hydrogenation experiments carried out in batch mode. Results showed that in the investigated concentration range of NBA, i.e. 0.0005–0.02mol/l and under the hydrogen pressure of 1bar, the observed reaction rate is considerably limited by mass transport. At higher concentrations of NBA, the reaction is controlled by the hydrogen mass transport while at lower concentrations the mass transport of NBA is dominant. The analysis of experimental results, which were obtained when the length of gas bubbles and liquid slugs were varied, showed that the reaction took place in the thin liquid film surrounding the gas bubble. The liquid slug serves as exchanger of reactants and reaction products between bulk liquid slug and liquid film surrounding the catalyst surface.

The role of gas bubbles and liquid slug lengths on mass transport in the Taylor flow through capillaries

Gas-liquid and liquid-solid mass transfer were studied in capillaries under Taylor flow regime. The influence of the capillary diameter, unit cell length and gas hold-up on measured kLa and kSa coefficients was correlated by simple correlation, which showed that in both cases the mass transport is mostly determined by the liquid slug length and velocity. The results demonstrated that the contribution of the mass transferred in the thin liquid film surrounding the gas bubble is not dominant. The conclusions agreed with the predictions made on the basis of a model developed by means of RTD measurements which were carried out in the same capillaries. The results obtained during catalytic hydrogenation of nitrite ions can also be explained in view of new findings.

Finite‐element analysis of Taylor flow

AbstractA finite‐element analysis of Taylor flow in a cylindrical capillary was performed using a commercial FEM program (FIDAP) to solve the fundamental fluid dynamics equations together with the capillary forces at the gas–liquid interface. A moving‐surface formulation was used to calculate the bubble shape. The thickness of the liquid film surrounding the gas bubble, the degree of mixing in the liquid phase, and the slip velocity between the two phases were calculated. These parameters influence the performance of monolith reactors operating in the Taylor flow regime. On comparison with experimental results it was found that the FEM calculation generally predicts a thinner liquid film, which can possibly be explained in terms of a peripheral variation in surface tension. Moreover, the wavelength of the wiggles predicted in the liquid film near the tail end of the bubble was compared to those arising from a simplified mathematical analysis available in the literature. Good agreement was found for Ca < 0.005, while for higher Ca the FEM predicts significantly shorter wavelengths, indicating that the lubrication theory is not valid here.