Liquid Phase Residence Time Distribution Research Articles

Interplay of Particle Suspension and Residence Time Distribution in a Taylor–Couette Crystallizer

In small-scale continuous crystallization, particle suspension and residence time distribution are critical factors determining operability and product quality. Here, the Taylor–Couette crystallizer stands out for its high flexibility. Its characteristic vortex structure intensifies local mixing, thus improving the suspension and simultaneously narrowing the residence time distribution, whereby these effects can be adjusted by operating and design parameters. However, the operating window is limited by the prerequisite of sufficient particle suspension. In this study, we investigated the suspension behavior and its impact on the attainable liquid phase residence time distribution and the flow regimes observed. For this purpose, the just-suspended rotation rate was visually determined for different design and operating parameters. A correlation was regressed from experimental data, showing that this rotation rate was mainly affected by the radius ratio of the rotor and stator. In addition, the liquid phase residence time distribution was measured by tracer experiments in regions of sufficient suspension, validating a correlation from the literature. With a combination of both correlations, the design parameters of the apparatus can thus be optimized according to the goal of, for example, a narrow residence time distribution in the suspended state.

Archimedes tube crystallizer: Design and characterization for small-scale continuous crystallization

Various continuous crystallizer concepts have been developed for the small-scale production (V˙≤100mLmin−1) of APIs and fine chemicals. The existing concepts differ in terms of achievable residence times and residence time distribution (RTD) width and can be classified qualitatively in a portfolio on the basis of these characteristics. The classification demonstrates, that it is still challenging to achieve long, flexible residence times in combination with narrow RTD width leading to narrow particle size distributions. Thus, an air-segmented coiled tube crystallizer was developed based on the Archimedes screw principle: The Archimedes tube crystallizer (ATC). In this device, coiled tubing is employed to convey individual fluid batches by rotation. Thereby, the air-segmented flow is generated by a specifically designed inlet tank mounted on the rotational axis. As a first step towards the implementation of the ATC concept in industry, characterization experiments were focused in this work. In a Design of Experiment, liquid phase RTD with tracer sodium chloride/water and solid phase RTD with material system L-alanine/water were investigated. The experimental results proved narrow RTD close to ideal plug flow for both liquid and solid phase over the complete operating window. Hence, the ATC represents a reasonable expansion of the continuous crystallizer portfolio.

Solid-liquid axial dispersion performance of a mesoscale continuous oscillatory flow crystalliser with smooth periodic constrictions using a non-invasive dual backlit imaging technique

A dual backlit imaging technique has been developed for liquid and solid phase residence time distribution (RTD) measurements in a mesoscale (millilitre) continuous oscillatory flow crystalliser with smooth periodic constrictions (herein known as the SPC mesoscale crystalliser). The pixel-based technique enables reliable determination and direct comparison of experimentally determined hydrodynamic parameters for the liquid and solid phase without concern for errors that may be introduced by utilising different measurement techniques for each phase. Additionally, the non-invasive technique offers benefits over traditional intrusive methods as demonstrated herein. Results of solid-liquid RTD experiments reveal that for a set of oscillatory flow conditions, particles do not experience the same degree of axial dispersion and mean residence time as the continuous liquid phase. The SPC mesoscale crystalliser, however, provides solid-liquid plug flow at low net flow rates. Findings in this work emphasise the importance of characterising solid phase axial dispersion for active pharmaceutical ingredient (API) systems during continuous crystallisation development to identify minimum dispersion operating conditions essential for solid-liquid plug flow crystallisation.

Performance, mixing and kinetics study in the flotation column reactor using coal

ABSTRACT A flotation column has been designed and characterized with respect to gas velocity, slurry velocity, frother concentration, gas holdup, bubble surface area flux for the study of the mixing property, liquid phase residence time distribution, flotation recovery and rate constant. The study was performed using coal as solid. A strategy of integrating chemical kinetics into the flotation process to test the floatability behaviour of coal and to estimate floatability parameter of coal has been developed. The results indicate that the coal floatability parameter is 3.05 × 10−4. A relationship between gas holdup and bubble surface area flux was developed.

Revisiting the reaction kinetics of selective hydrogenation of phenylacetylene over an egg-shell catalyst in excess styrene

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 57.9kJ/mol, respectively.

Design and development of energy efficient continuous cooking system

For large scale cooking, it is desirable to implement continuous cooking. In addition to the usual advantages of continuous cooking (size of cooker, uniform cooked quality, possibility of process control), there is a possibility of increasing the thermal efficiency by preheating water against the hot cooked food leaving the cooker. In the present work, solid and liquid phase residence time distribution (RTD) have been measured in 120mm diameter and 1.6m long continuous cooker. Three levels of screw speed (1, 3 and 5rpm), two levels of solid flow rate (12and 24kg/h) and three levels of liquid flow rate (15, 25 and 35 lph) were selected. The solid phase was always found to move in a plug flow manner as the minimum Peclet number was found to be 81.5. This information was found to be useful for deciding the capacity of the continuous cooker. Thus, the time required for batch cooking was found to be equal to the minimum residence time in a continuous cooker at equivalent operating condition of temperature.

Modeling the diffusion-free liquid phase residence time distribution of Taylor flow by the unit cell concept: Progress and limitations

In this paper we study the diffusion-free liquid phase residence time distribution (RTD) of laminar Taylor flow in square mini-channels numerically and analytically. We evaluate the RTD caused by the non-uniform laminar velocity field for co-current upward and downward Taylor flow from detailed numerical simulations obtained with a volume-of-fluid method by diffusion-free Lagrangian tracking of a set of virtual particles. The numerical RTD curves are used to develop a compartment model for the RTD of a Taylor flow unit cell (which consists of one gas bubble and one liquid slug) in the fixed frame of reference. While the new model is conceptually similar to models from literature, it is refined with respect to (i) the delay time (i.e. the residence time of the fastest liquid elements, modeled by a plug flow reactor) and (ii) the difference in slope of the RTD at small and large residence times (modeled by two parallel continuous stirred tank reactors with different mean residence time). It is shown that the refined model fits the numerical unit cell RTD reasonably well for different flow conditions. From the unit cell RTD model, the RTD for two and three unit cells in series is computed by a convolution procedure. The agreement between the numerical and the convolution-based RTD for multiple unit cells is not satisfactory and indicates the inappropriateness of the convolution procedure for computing the diffusion-free RTD of multiple unit cells in Taylor flow. The failure of the convolution procedure is attributed to the dividing streamline, which separates in Taylor flow the liquid phase in two regions, one with recirculating and one with bypass flow. In the absence of diffusion, the diving streamline is never crossed. As a consequence, locations of tracer particles in neighboring unit cells are not independent which invalidates the unit cell convolution approach.

Liquid phase axial mixing in solid–liquid circulating multistage fluidized bed: CFD modeling and RTD measurements

Liquid phase residence time distribution (RTD) studies have been performed in conventional solid–liquid fluidized bed (SLFB) and solid–liquid circulating multistage fluidized bed (SLCMFB). The riser column was made up of 50mm i.d. and 2m long glass pipe while the multistage down comer column (glass) consisted of seven stages of 100mm i.d. and 100mm long sections each having perforated plate as a distributor (having 480 holes of 2mm diameter). RTD experiments for SLFB were carried out in the column having the same diameter as the downcomer of SLCMFB. RTD has been estimated for both the riser column and the multistage column of SLCMFB. Computational fluid dynamic (CFD) simulations of SLFB and riser section of SLCMFB have been performed to predict the RTD. In all the above cases good agreement was found between the CFD predictions and the experimental measurements. Ion exchange resins and glass beads were used as a solid phase and water as a fluidizing medium. The dispersion characteristics of SLFB and SLCMFB have been investigated for resin particles with size range of 0.36–0.72mm and glass beads with size range of 0.1–0.7mm. It was observed that the liquid phase axial dispersion coefficient depends strongly on superficial liquid velocity, particle size and particle density. Based on the experimental data, empirical correlations have been proposed for liquid phase axial dispersion coefficient and have been found to be applicable to all the available data in the published literature.

Residence time distribution and modeling of the liquid phase in an impinging stream reactor

Based on some experimental investigations of liquid phase residence time distribution (RTD) in an impinging stream reactor, a two-dimensional plug-flow dispersion model for predicting the liquid phase RTD in the reactor was proposed. The calculation results of the model can be in good agreement with the experimental RTD under different operating conditions. The axial liquid dispersion coefficient increases monotonously with the increasing liquid flux, but is almost independent of gas flux. As the liquid flux and the gas flux increase, the liquid dispersion coefficient of center-to-wall decreases. The axial liquid dispersion coefficient is much larger than that of center-to-wall, which indicates that the liquid RTD is dominated mainly by axial liquid dispersion in the impinging stream reactor.

Liquid phase residence time distribution for gas–liquid flow in Kenics static mixer

The residence time distribution (RTD) of the liquid phase for co-current gas–liquid upflow in a Kenics static mixer (KSM) with air/water and air/non-Newtonian fluid systems was investigated. The effect of liquid and gas superficial velocities on liquid holdup and Peclet number was studied. Experiments were conducted in three KSMs of diameter 2.54 cm with 16 elements and 5.08 cm diameter with 8 and 16 elements, respectively, of constant L e/ D e = 1.5 for different liquid and gas velocities. A correlation was developed for Peclet number, in terms of generalized liquid Reynolds number, gas Froude number and liquid Galileo number, where as for liquid holdup, a correlation was developed as a function of gas Reynolds number. The axial dispersion model was found to be in good agreement with the experimental data.

Hydrodynamics and liquid phase residence time distribution in mesh microreactor

This paper is focused on the experimental analysis of residence time distribution and phase hold-up in a mesh microreactor. A microreactor, where a finely weaved mesh is sandwiched between two flat plates with specific inlets for gas and liquid, is proposed. The microvolumes formed upon sandwiching the mesh are totally connected and thus it yields a view of several interconnected microvolumes. This system is easy to build and does not need precision micromachining. A high-speed photographic analysis yielded the phase distribution for different mesh types over a wide range of operating gas and liquid flow rates. The RTD was studied by measuring the liquid phase conductivity at the outlet of the reactor. Channeling prevailed for mesh with smaller open area. The ADEM was used for fitting the tracer curves with tailing ends. The RTD and the image analysis for all the mesh types showed hysteresis when the gas flow rate was maintained constant and the liquid flow rate was gradually increased and then decreased.

Liquid-Phase Residence Time Distribution for Two-Phase Flow in Coiled Flow Inverter

The coiled flow inverter (CFI) is an innovative device, which has potential for the intensification of processes currently carried out in conventional mixers. Step response experiments were carried out in a CFI to study liquid-phase residence time distribution (RTD) for gas-liquid flow under the conditions of both negligible and significant molecular diffusion using Newtonian fluids. A total of 16 CFIs of different curvature ratios ranging from 6.7 to 20, dimensionless pitch from 1 to 2.5, and number of bends from 1 to 15 were investigated. The range of Dean numbers for the gas and the liquid phase was varied from 235 to 1180 and 3.16 to 1075, respectively. The reduction in dispersion number is nearly 2.6 times for two-phase flow in CFI with 15 number of bends as compared to a straight helix under identical process conditions. A modified axial dispersion model is proposed to describe the RTD. The efficiency of the device is characterized by a mixing criterion.

RTD IN TRICKLE BED REACTORS: EXPERIMENTAL STUDY

Axial dispersion has been measured in a laboratory-scale model trickle bed reactor at different liquid and gas flow rates under co-current down-flow condition. Air was used as gas phase and water or aqueous polyethylene glycol solution was used as liquid phase, covering the Reynolds number range of 0–164 for gas phase and 4–86 for liquid phase. Liquid phase residence time distribution was measured using a 1 M KCl pulse tracer. The axial dispersion model was used to simulate the observed RTD, and liquid phase PeL and BoL were estimated as model parameters. Based on the results, it was observed that under low interaction flow regime (trickle flow), liquid phase Bodenstein number, BoL, increased gradually with liquid phase Reynolds number, ReL, and was found to be nearly independent of gas phase Reynolds number, ReG. Near the flow transition point, BoL or PeL decreased, and in the high interaction regime it showed an oscillatory response. In the trickle flow regime .

Liquid Flow Behavior in Catalyst-Containing Pockets of Modular Catalytic Structured Packing Katapak SP

An element or section of Katapak-SP, a modular catalytic structured packing material (MCSP), consists of catalyst-filled pockets separated by one or more corrugated sheets of a large specific area, high capacity structured packing. Full active wetting of catalyst particles contained in catalyst-filled pockets is essential for its reactive performance. In this study the liquid-phase residence time distribution in catalyst-filled pockets of Katapak-SP was determined as a function of the superficial liquid load. The liquid load corresponding to full saturation of the pockets appeared to be the best condition for the flow pattern within the pocket. Then in an MCSP, reaction occurs at best flow condition. The liquid in excess is rejected and descends in the separation section of the element. Due to the difference in porosities, the residence times in the pocket and the separation part of the catalytic packing differ significantly. However, the liquid streams from pockets and separation compartments mix and redistribute at the transitions between rather short packing elements, and this ensures a relatively good overall performance, which can be manipulated to some extent by changing the ratio of catalytic to separation sections. The best performance of MCSPs is at liquid loads just above the catalytic load point with corresponding vapor loads around hydraulic loading.

A model for the residence time distribution of bubble-train flow in a square mini-channel based on direct numerical simulation results

This paper presents an original method for evaluating the liquid phase residence time distribution in bubble-train flow using data from direct numerical simulations (DNS). The method is a particle method and relies on the uniform introduction of virtual particles in the volume occupied by the liquid phase within a single flow unit cell. The residence time distribution is obtained by statistical evaluation of the time needed by any particle to travel an axial distance equivalent to the length of the flow unit cell. Residence time curves are evaluated from DNS data of bubble-train flow in a square mini-channel for different lengths of the flow unit cell. The curves obtained are well fitted by an exponential relationship, which has been developed on basis of a two-tanks-in-series compartment model, where the first tank is a plug flow reactor and the second is a continuous stirred tank reactor.

Mixing Time, Homogenization Energy and Residence Time Distribution in a Gas-Induced Contactor

Mixing time, homogenization energy and residence time distribution (RTD) experiments have been carried out in a gas-induced contactor in order to determine mixing characteristics of the vessel. Effects of sensor location, impeller speed, gas induction, and gassing on mixing time have been studied. Experiments on a semi-technical scale vessel have identified optimal mixing time conditions and optimal impeller speeds for most efficient homogenization energy. The effects of liquid flow rate and impeller speed on liquid phase RTD have also been investigated. The contactor mixing behaviour was modelled by a continuous stirred tank reactor (CSTR) with dead zones and bypassing. Good agreement between experimental and model results was observed. Analysis shows that contactor performance is close to that of an ideal CSTR; however, dead zones (as large as 16.7%) at low liquid flow rates and bypassing (as large as 10%) at high liquid rates need to be considered.

Liquid phase residence time distribution for a two-phase countercurrent flow in a packed column with a novel internal

The liquid phase residence time distribution (RTD) for gas–liquid countercurrent flow in a packed column with a novel internal was measured by conductivity measurements and an air–water system. The RTD of a liquid tracer is well represented by the ADM and PDE models. At lower gas flow rates, the Peclet number of the liquid in the packed column with the internal is lower than that without the internal; at higher gas flow rates, it is vice versa, especially with an internal with a higher volume fraction. The distribution of the liquid RTD can be improved by using suitable geometric parameters of the internal to give a larger volume fraction and a lower stage height.

A Strategy for Scaling Up the Fischer–Tropsch Bubble Column Slurry Reactor

For successful scale-up of the bubble column slurry reactor for Fischer–Tropsch synthesis, we need to have proper description of hydrodynamics and transport phenomena (gas and liquid holdups, liquid-phase residence time distribution, gas–liquid mass transfer, heat transfer to cooling tubes) as a function of reactor scale (column diameter and height) and operating conditions (superficial gas velocity, system pressure, slurry concentration, . . .). We discuss and develop a scale-up strategy using computational fluid dynamics, with a minimum amount of experimental input from hydrodynamic studies carried out on a relatively small scale.

Liquid phase mixing in 2-phase liquid–solid inverse fluidized bed

Liquid phase residence time distribution studies are reported in 2-phase inverse fluidized bed for the first time in the literature. Using a pulse tracer technique and deconvolution method of analysis, RTD of the system, residence time, Peclet number and dispersion coefficient are determined. The liquid phase axial dispersion coefficient increases with increase in liquid velocity and Archimedes number and is independent of static bed height. An empirical correlation has been proposed for liquid phase axial dispersion coefficient in 2-phase IFB.

Scale up studies on partitioned bubble column reactors with the aid of CFD simulations

In an earlier publication on partitioned bubble columns [Catal. Today 69 (2001) 165–170], liquid phase residence time distribution was measured in columns of 0.10, 0.15 and 0.38 m diameter at superficial gas velocities, U G, in the range 0.05–0.4 m/s. In the present study, CFD simulations, using two-dimensional Cartesian geometry and the Eulerian framework, are used to provide a theoretical background to our earlier experimental work. Both CFD simulations and experiments show that the (superficial) liquid exchange velocity, U ex, at the partition plate is independent of column diameter; the exchange velocity U ex is however found to be a strong function of the open area of the partition plates. The height of the gas cap forming underneath partition plates decreases significantly with increasing column diameter; this is a desirable result from the point of view of scale up. It is concluded that CFD simulations can be a powerful scale up tool.