Liquid Phase Residence Time Distribution Research Articles

Liquid-phase backmixing in bubble columns, structured by introduction of partition plates

Installation of perforated sieve plates into a bubble column has the effect of introducing structure into an otherwise chaotic hydrodynamic behaviour. In this study, we focus on the reduction of backmixing of the liquid phase in compartmentalised bubble columns. Liquid-phase residence time distribution (RTD) measurements were carried out in bubble columns with diameters DT=0.10, 0.15 and 0.38m with air–water system operating at superficial gas velocities of UG=0.05–0.4m/s. Partition sieve plates with open areas of 18.6 and 30.7% were used in the studies. The measured data on RTD were interpreted in terms of an axial dispersion model extended to allow for liquid interchange between compartments. The interchange velocity was found to be strongly dependent on the free area of the plates but practically independent of the column diameter.

Experimental Study on a Co-current Gas-Liquid Down Flow Contactor with Gas Entrainment by a Liquid Jet

Two-phase flow co-current vertical downflow reactor with gas entrainment by a liquid jet is investigated in an air–water system. Experiments are carried out in order to clarify the flow behavior of the reactor under various conditions. Gas entrainment flow rates and gas holdup are quantified experimentally and their dependency on the liquid jet flow rates are shown. The experimental program also included determination of liquid phase residence time distribution (RTD) characteristics for different liquid jet flow rates. The result of the analysis of the liquid phase RTD curves justified the tank-in-series model flow for the liquid phase. On the basis of these analyses, the reactor hydrodynamics are modeled by the tank-in-series model including dead zones. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. The volumetric mass transfer coefficient kLaLis determined experimentally by a “gasing out” method. The interfacial area is deduced from the bubble diameter measurements which are determined by visualization experiments.

Gas- and liquid-phase residence time distribution in the three-levels-of-porosity reactor

Gas- and liquid-phase residence time distribution (RTD) were measured in different three-levels-of-porosity reactor configurations. Both the gas and the liquid residence time distribution can be predicted with a tanks-in-series model. For the gas phase, the number of stirred tanks proved to be equal to the number of catalyst layers. For the liquid phase, the number of tanks depends on the height of the catalyst layers and on the character of the liquid flow. The influence of the distance between the layers on the RTD is adequately described by plug flow for the jets that fall from layer to layer. The measurements indicated a uniform axial liquid velocity profile.

Counter-current operation of a structured catalytically packed-bed reactor: : Liquid phase mixing and mass transfer

The liquid-phase residence time distribution has been measured in two structured packed column configurations, of 0.1 and 0.24 m diameter, in which the catalyst particles are enclosed within wire gauze envelopes (“sandwiches”). In order to interpret these results Computational Fluid Dynamics (CFD) has been used to model the liquid flow within the packed sandwich structures. A representative sandwich structure, containing catalyst particles, is modeled as a set of triangular tubes (“Toblerones”), intersecting at 90° angles. The liquid flowing in a tube has the possibility of maintaining its flow direction or taking a sharp 90° turn. Using CFD, the dispersion characteristics of the “cross-over” junction can be determined as a sum of two components: straight-through and 90°-turn flow. The dispersion characteristics of the entire sandwich can be estimated reasonably well from information on the number of cross-over junctions along the flow direction. Comparison of the liquid-phase RTD measured in the two columns with those determined from CFD lead to the conclusion that there is channeling of liquid through the open channels, with good interchange of liquid between the open and packed channels. Liquid-phase mass transfer within the packed channels is studied also by means of CFD techniques. Due to the “upheaval” caused by the flow splitting at the crossovers, the mass transfer coefficient is about 2–3 times larger than for fully developed laminar flow in a circular tube.

Counter-current operation of structured catalytically packed distillation columns: pressure drop, holdup and mixing

Structured packed columns, in which the catalyst particles are enclosed within wire gauze envelopes (“sandwiches”) are promising reactor configurations for reactive distillation and hydroconversions. By allowing preferential channels for the gas and liquid phases, counter-current operation is achieved even for millimeter sized catalyst particles without the problem of flooding. This paper reports the results of a comprehensive experimental study of the hydrodynamics of structured packed columns of 0.1 and 0.24 m diameter. The pressure drop is found to increase sharply when the superficial liquid velocity exceeds a certain threshold value. This threshold corresponds to the situation in which a maximum flow of liquid in the packed channels is realized and the excess liquid flows through the “open” channels. The liquid flow in the open channel causes a sharp rise in the pressure drop. A model is developed to describe the holdup of liquid in the open channels. With increasing liquid flow rate, the pressure drop is found to increase exponentially with the liquid holdup within the open channels. Liquid phase residence time distribution studies lead to the conclusion that there is a good exchange of the liquid phase inside and outside the packed channels. The residence time distribution can be described by an axial dispersion model. Compared with a trickle bed reactor, the results of this study show that a structured packed column has a much larger operating window at a much lower pressure drop.

Liquid phase residence time distribution for two phase flow in coiled tubes

AbstractThe residence time distribution (RTD) of the liquid phase in air‐water flow through helical coils has been studied. Upward and downward cocurrent flows have been investigated in three coils with curvature ratios ranging from 11 to 60.7. The ranges of the Reynolds numbers for the gas and the liquid varied from 1500 to 3000 and 620 to 3200, respectively. A model has been proposed that describes the liquid phase RTD as combination of two different residence time distributions applicable for turbulent and laminar liquid flows.

The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors

An experimental study has been aimed at linking the extent of axial mixing in gas and liquid phases in tall bubble column reactors with the respective bubbling regimes (homogeneous, transition, heterogeneous) encountered in such reactors and with corresponding macro-scale flow patterns of both phases. The experimental programme, carried out in a bubble column reactor of 0.14 m in diameter and 4.1 m in height, included determination of gas and liquid phase residence time distribution (RTD) characteristics for different modes of primary gas dispersion and flow visualisation experiments aimed at identification of macro-scale flow patterns in the reactor. The experimental results proved an essential effect of gas dispersion mode (bubbling regime) on the extent of gas and liquid phase mixing in the reactor. The respective dependences of Pe G and Pe L on the superficial gas velocity, obtained with the distributing plate generating sequentially homogeneous, transition and heterogeneous bubbling regimes, indicated very sensitively the transition from the homogeneous to the heterogeneous bubbling mode (the onset of the transition bubbling regime) occurring at u 0G = 0.04 m s −1 with the appearance of local macroeddies in the bed. The comparison of our mixing data with corresponding dependences of gas holdup and k La L on the superficial gas velocity revealed the existence of direct links between the variations of RTD and mass transfer characteristics induced by the changes of gas dispersion (bubbling) mode. The results of the analysis of variances of gas phase RTD curves justified (in agreement with corresponding Pe G data) the plug flow approximation for the gas phase in the homogeneous bubbling regime and indicated the convective mechanism of gas mixing within the whole region of heterogeneous bubbling conditions. Analogically, the data for the liquid phase yielded the convective mechanism of liquid mixing within the whole range of experimental conditions. The mixing data obtained for both phases in the heterogeneous bubbling regime were adequately described by the model of consecutive circulation cells with the backflow between adjacent cells, proposed on the basis of the flow visualisation experiments as a realistic simplified representation of the macro-scale flow structures in heterogeneous bubble beds.

Residence time distribution in a packed bed bioreactor containing porous glass particles: influence of the presence of immobilized cells.

An experimental investigation of the liquid phase residence time distribution (RTD) in a packed bed bioreactor containing porous glass particles is presented. For Re < 1, intraparticle forced convection is negligible and only diffusion, characterized by an effective diffusion coefficient, must be considered to describe the mass transfer process between the extraparticle and the intraparticle fluid phase. For Re > 1, the mass transfer rate becomes dependent on the liquid flow rate, indicating the existence of intraparticle convection. A model including axially dispersed flow for the external fluid phase and an 'apparent' effective diffusivity that combines diffusion and convection, predicts experimental RTD data satisfactorily. Yeast cells immobilized inside the porous glass beads did not affect the mass transfer rate at low biomass loading. At high biomass loading (0.02 g yeast cells g-1 carrier), the mass transfer rate between the extraparticle and intraparticle fluid phase was significantly decreased. Comparison of the RTD data from experiments performed in the presence and absence of cells in the external fluid phase revealed that the mass transfer rate is influenced by the cells immobilized inside the porous particles and not by the cells present in the external fluid phase.

The Hydrodynamic Characteristics of Cocurrent Downflow and Cocurrent Upflow Gas-Liquid-Solid Catalytic Fixed Bed Reactors: the Effect of Pressure

While most catalytic fixed bed gas-liquid reactors of the petrol industry work at quite high pressures, the academic scientific work in this field concerned itself almost exclusively with the domain of approximatively atmospheric pressures. The authors present the results of some years of experimental investigations on the hydrodynamic characteristics of trickle bed reactors and lately of cocurrent upflow reactors. During the last years, results were also obtained under pressures up to 8 MPa. The measurements were made in a small scale cold flow equipment (diameter 23 mm). Different aqueous and organic more or less viscous, eventually coalescence inhibiting liquids, four gases and a number of non porous more or less wettable particles were used. The liquid holdup was determined in all cases by measuring liquid phase residence time distribution by different tracers. The following conclusions may be drawn:(a) In the high interaction regime, it is the inertia of the gas and the liquid phases which is the main cause of the dissipation of mechanical energy. In this regime, results obtained in cocurrent upflow and downflow are approximately equal. (b) Most correlations of literature are unable to predict the effect of pressure on the pressure drop or the liquid holdup. (c) The gas viscosity has no influence on the hydrodynamics. It is therefore possible to simulate for example hydrogen under high pressure conditions by another gas of the same density (at a much lower pressures). A critical evaluation of the correlations and/or models of literature is presented, concerning their ability to represent the different characteristics as a function of pressure.

Liquid residence time distributions in immobilized cell bioreactors

Previous work has demonstrated that high ethanol productivities can be achieved using yeast or bacterial cells adsorbed onto the surface of ion exchange resin in vertical packed bed bioreactors. The present work quantitatively characterizes the overall degree of backmixing in such reactors at two scales of operation: 2.0 and 8.0 L. Stimulus-response experiments, using two solvents (2,3-butanediol and 2-ethoxyethanol) as tracers, were performed to measure the liquid phase residence time distribution (RTD) during continuous ethanol fermentations using the yeast Saccharomyces cerevisiae and the bacterium Zymomonas mobilis at the 2-L scale, and with S. cerevisiae at the 8-L scale. In order to separately determine the effects of liquid flow rate and gas evolution on the degree of mixing, stimulus-response experiments were also performed in the systems without microbial cells present. The evolution of CO(2) was found to dramatically increase the extent of mixing; however, the tanks-in-series model for non-ideal flow represented the systems adequately. The packed beds were equivalent to over 70 tanks-in-series during abiotic operation while during fermentations, with similar liquid flow rates, they ranged in equivalence from 35 to 15 tanks-in-series. This increased knowledge of the overall degree of mixing in packed bed, immobilized cell bioreactors will allow for more accurate kinetic modelling and efficient scale up of the process.

Energy effectiveness and working characteristics of different tower reactors for aerated slurry systems

Decisive hydrodynamic and mass transfer characteristics of different types of tower reactors (rotating disc reactor, single and multistage sieve-tray bubble columns, tower reactor with ejector gas distributor) as well as the energy effectiveness of their performance were compared with the purpose to establish a quantitative basis for the qualified choice of the proper reactor type according to demands of specific reaction processes. Selected design parameters included gas and solid phase holdup, kLaL, liquid phase residence time distribution, and axial distribution of the solid phase, the experiments were carried out in a wide range of solid phase concentration (0-20 wt. %) and particle sizes (2.3-280 μm). The experimental results proved that due to their favourable suspension characteristics and operation stability the rotating disc reactors can be advantageously used for slow reaction processes with low demands on the intensity of interfacial gas-liquid contact which can be carried out at low gas flow rates. On the other hand the multistage bubble column reactors proved to be superior devices for transport–controlled reaction processes regarding both the achievable rate of interfacial mass transfer and the overall energetic efficiency of phase contacting.

Scaling-up of packed columns Part I

Mass transfer experiments in three different columns, of 0.14, 0.29 and 0.44 m diameter respectively, which were packed with 8.4 mm Raschig rings are reported. The packed height amounted to 2.07 m. Acetone was stripped from a water-acetone mixture by means of air under carefully controlled experimental conditions, maintaining a stripping factor equal to unity. The purpose of the experiments was to develop reliable scaling-up rules for packed columns. Liquid phase residence time distribution measurements were performed to supplement the results of the mass transfer experiments. Neither the mass transfer efficiency nor the axial dispersion in the liquid phase was found to depend on the column diameter. The absence of any influence of the column diameter on the variables which were investigated is assumed to be due to the fact that both the gas and the liquid distributors in the various columns were of identical design.

Kinetics of the selective hydrogenation of crotonaldehyde to crotyl alcohol

Selective hydrogenation of phenylacetylene is an important reaction for the removal of a small amount of phenylacetylene from styrene monomer. In this work, the kinetics of phenylacetylene hydrogenation over an egg-shell Pd/Al2O3 catalyst in the presence of excess styrene was investigated by using a laboratory-scale fixed-bed reactor. The liquid-phase residence time distribution (RTD) in the reactor under different operating conditions was measured by tracer pulse method, and the Peclet number was determined based on the experimental data and the RTD model. The kinetic experiments showed that an increase in temperature and pressure or a decrease in the liquid hourly space velocity gave rise to increased conversion of phenylacetylene and decreased concentration of styrene. A rigorous mathematical model combining reaction kinetics, external mass transfer, intraparticle diffusion as well as axial dispersion of the liquid phase was developed to describe the selective hydrogenation of phenylacetylene in the reactor, and the kinetic and adsorption parameters involved in the kinetic model were estimated by minimization of the sum of squares of relative residuals between observed and model-derived concentrations of different components. The kinetic model can describe the phenylacetylene hydrogenation over the Pd/Al2O3 catalyst very well, and the activation energies for the hydrogenation reactions of phenylacetylene to styrene, styrene to ethylbenzene and phenylacetylene to ethylbenzene were 48.6, 52.2 and 57.9 kJ/mol, respectively.