Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Radioisotope tracer study in trickle bed reactors

The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air‐water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing.     

Flow Regime Maps for Flow of Liquid Through Downcomer in a Turbulent Bed Contactor

The provision of a downcomer to classical turbulent bed Contactor (TBC) increases the gas treating capacity of the equipment. When the downcomer is provided, it is expected that all the liquid passes through the downcomer only without any liquid flow through the distributor. In the present study, the operational regime for the flow of liquid only through the downcomer is experimentally evaluated for various geometric parameters and particle characteristics. It is observed that the preferred operational regime without dumping and weeping increases with an increase in Archimedes number and downcomer diameter, and decreases with an increase in static bed height and downcomer weir height. Correlations are proposed to satisfactorily predict experimentally observed operational regime.     

Axial and lateral dispersion of fine particles in a binary-solid riser

Axial and lateral mixing of fine particles in a binary-solid riser have been investigated using a phosphor tracer method. The measured bimodal residence time distribution (RTD) demonstrated two types of axial dispersions of the fines: the dispersion of discrete particles and that of clusters. A proposed one-dimensional, bimodal dispersion model describes the bimodal RTDs very well. The axial Peclet number of the fines is not sensitive to the fraction of coarse particles, gas velocity and solids circulation rate. Lateral solids dispersion was determined by measuring the solids RTD at different radii using a point source tracer injection. A two-dimensional dispersion model describes the measured RTDs satisfactorily. Lateral solids mixing decreased as coarse particles were added into the riser. Correlations of the axial and lateral Peclet numbers obtained fit the experimental data well.     

The gas-liquid contacting effects on liquid dispersion in pilot scale upflow hydrotreaters

A model for the estimation of the extent of the non-ideal liquid flow from RTD data at operating conditions in small-scale hydrotreaters has been developed accounting for the gas oil evaporation and gas-liquid mass transfer. Non-ideal flow is described by the axial dispersion model. Gas-liquid phase mass transport and equilibrium are considered by simulating the petroleum gas oil fraction with five pseudocomponents. Liquid holdup and axial dispersion coefficients for an upflow reactor are estimated by fitting the response curves after a step change of the tracer concentration at the inlet of a fixed bed operating at reaction conditions to the model predictions. The results obtained with the developed model are compared with those estimated by a simplified pseudohomogeneous model. It has been found that the model parameters estimated by the developed model are compatible with those determined with a mock up system of toluene and nitrogen at ambient conditions.     

TRANSFERABLE TRACER ANALYSIS FOR MULTIPHASE REACTOR

In order to determine the mixing and transfer parameters of a multiphase reactor, we present here a dynamic method involving the injection of a pulse on the gas inlet concentration, and the detection of the dissolved gas in the liquid at two levels in the column. The two phase dispersion model is used involving four parameters: the liquid holdup (or mean residence time), the liquid and gas Peclet numbers, and the volumeric mass transfer coefficient. They can be identified by deconvolution of the two signals using Fast Fourier Transform. The detection is performed by electrochemistry, the same electrodes being able to detect oxygen, the transferable tracer, but also ferricyanide as a tracer of the liquid phase. The coupling of these two methods gives satisfactory results. Different combinations have been tested; the best results, for both accuracy and optimization convergence, are obtained when Pe₁ is derived from liquid RTD and the other three parameters, t, Pe_g and k₁a are optimized from transferable tracer experiments.     

Material transport in a continuous rotary drum. Effect of discharge plate geometry

Experimental results on the influence of the discharge plate geometry on the dimensionless residence time distribution (RTD) for material transport in a continuous rotary drum are described. The RTD obtained by a stimulus-response technique for the different discharge plates can be described well by the axial dispersed flow model. Based on the characteristic Peclet number of the flow regime, material flow tended more towards the plug flow condition at an intermediate size discharge opening. Calculation of the axial dispersion coefficient in each case revealed that the open-ended drum behaved more like an ideal mixer. The implication of these results on the design of continuous rotary devices is discussed.     

Flow regimes in a turbulent bed contactor

Experiments were conducted in a turbulent bed contactor (TBC) with and without downcomer operating in Type I and Type II modes to obtain the flow regime maps. The variables include flow rate of the gas and liquid phases, size and density of the particles, bed height, free open area of the supporting grid, and diameter and weir height of the downcomer. Static, partially fluidized, completely fluidized, flooding and annular regimes are identified depending on geometric and operating conditions. The effect of these variables on flow maps is discussed and correlations are proposed for delineating the flow regimes.     

小型气-液-固流化床液相的停留时间分布

采用脉冲示踪技术,研究了3 mm床径的小型气-液-固流化床内液相停留时间分布。以KCl为示踪剂,液相为去离子水,气相为空气,固相为平均粒径0.123~0.222 mm的玻璃微珠和氧化铝颗粒,测量流化床出口液相的电导率,得到其停留时间分布曲线。结果表明,增大表观液速和表观气速,分布曲线变窄,平均停留时间缩短,Peclet数增大;固相的存在使液相的平均停留时间增长。表观液速1.96~15.70 mm×s^-1,表观气速1.18~1.96 mm×s^-1的条件下,流动接近层流;平均停留时间的范围为（19.6±0.34）s~（48.0±0.92）s,建立的Pe经验关联式对实验结果有较好的预测,偏差在±25%以内。研究结果对于小型三相流化床的设计放大具有指导意义。     

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.     

Criteria for axial dispersion effects in adiabatic trickle bed hydroprocessing reactors

This paper first outlines an approximate solution to the governing equations for an adiabatic hydroprocessing trickle bed reactor operating in the presence of axial dispersion. The approximate solution agrees very well with the rigorous numerical solution for Peclet numbers greater than approximately three.Using the approximate solution, criteria for significant axial dispersion effect are obtained. These criteria indicate that at high conversions, an adiabatic operation produces a larger axial dispersion effect than the isothermal operation. At low conversions, opposite results are obtained.The derived criteria are used to evaluate the orders of magnitude of Peclet number required to avoid axial dispersion effect in pilot scale adiabatic reactors for (a) residual hydrodesulfurization (b) hydrocracking of gas oils and (c) denitrogenation of shale oils. The calculations indicate that the axial dispersion effect is of less importance in case (c) than in cases (a) and (b).Finally, the role of heat effects on the axial dispersion in a vapor phase fixed bed adiabatic reactor is evaluated.     

Axial dispersion characteristics in three phase fluidized beds

Axial dispersion coefficients in three-phase fluidized beds have been measured in a 0.152 m-ID x 1.8 m high column by the two points measuring technique with the axially dispersed plug flow model. The effects of liquid velocity (0.05–0.13 m/s), gas velocity (0.02–0.16 m/s) and particle size (3-8 mm) on the axial dispersion coefficient at the different axial positions (0.06–0.46 m) in the bed have been determined. The axial dispersion coefficient increases with increasing gas velocity but it decreases with an increase in particle size and exhibits a maximum value with an increase in the axial position from the distributor. The axial dispersion coefficients in terms of the Peclet number have been correlated in terms of the ratio of fluid velocities, the ratio of the panicle size to column diameter, and the dimensionless axial position in the bed based on the isotropic turbulence theory.     

Axial dispersion in two phase co-current flow with fast and instantaneous reactions

Approximate solutions to the governing equations for two phase co-current flow with fast and instantaneous reactions between a gaseous and a liquid species are considered. These solutions are obtained by perturbation technique for large Peclet numbers of the gas and the liquid phases and by Galerkin's method for small and intermediate values of the gas and liquid Peclet numbers. The approximate solutions are compared with a numerical solution obtained under the limiting condition of the gas phase Peclet number becoming infinite. Criteria for axial dispersion in a two phase column are also obtained based on the perturbation solutions.     

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.     

A combined method to facilitate the high dispersion modeling for wastewater treatment bioreactors with closed boundaries

Two tanks in series and axial dispersion models are used frequently for bioreactors flow regime modeling, especially the wastewater treatment reactors. Using the tanks in series model is simple, but the axial dispersion model is dependent on the boundary conditions. In case of high dispersion and closed boundaries, axial dispersion model will be a complicated method. In this paper by combination of tanks in series and axial dispersion models, a new simple model is presented to facilitate the axial dispersion solution for closed boundary condition. For the various studied Peclet numbers, the maximum peak concentration error is calculated by combined method equal to 3% in comparison with the error for open solution that increases progressively and exceeds 14%. Furthermore the combined model peak time errors are zero for all the Peclet numbers while are more than 10% for the open solution.     

Liquid backmixing in oscillatory flow through a periodically constricted meso-tube

This paper deals with the measurement and modelling of axial liquid dispersion in a 4.5 mm internal diameter tube provided with smooth-periodic constrictions (meso-tube) in steady and oscillatory flow conditions. The residence time distribution (RTD) in the meso-tube was monitored for a range of fluid oscillation frequency (f) and amplitude (x₀) at laminar flow. The RTD response was modelled with three hydrodynamic models: (i) tanks-in-series, (ii) tanks-in-series with backflow and (iii) plug flow with axial dispersion. The steady flow through the meso-tube at flow rates up to 21.30 ml/min resulted in broad RTDs, mainly due to the parabolic velocity profile. The use of fluid oscillations allowed a fine-control of the axial liquid dispersion in the meso-tube due to generation of secondary flow in the regions between the constrictions. The axial dispersion coefficient D was reduced by up to 13-fold in comparison with the steady flow situation. Values of x₀ ≤ 1 mm and f = 10 Hz generally resulted in a maximum reduction in axial dispersion through, therefore maximum improvements in RTD. The tanks-in-series model was generally not capable of predicting RTDs in the meso-tube. The potential of this platform for the continuous, sustainable production of added-value products is herein demonstrated.     

Residence time distribution measurements in an external-loop airlift reactor: Study of the hydrodynamics of the liquid circulation induced by the hydrogen bubbles

A detailed study of the residence time distribution (RTD) analysis of liquid phase has been performed in an external-loop airlift reactor of 20 L nominal volume, regarded as a global unit and discriminating its different sections (riser, gas–liquid separator and downcomer) using the tracer response technique. The reactor was used as an electrochemical reactor in order to carry out the electrocoagulation/electroflotation (EC/EF). The gas phase created in the riser is the hydrogen produced by water electrolysis.In order to use this reactor for a continuous EC/EF, hydrodynamic studies were carried out to control the operating conditions and to help modelling the electrocoagulation. Current density, position of the electrodes in the riser and the volumetric liquid flow (inlet flow) are the key parameters for the hydrodynamics.The experimental results revealed that both in the downcomer and the riser–separator zones, the flow model is axial dispersion. Interesting results were obtained:

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  The superficial liquid velocity (ULd) at the downcomer, decreased when the volume inlet flow increased (0<QL<2 L/min).

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  The Peclet number obtained in the downcomer was correlated to the current density and the electrodes position.

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  In the riser–separator zone the Peclet number decreased with the superficial liquid velocity in the riser indicating that the dispersion increased with an increase of turbulence created in the separator by an increase of liquid velocity.

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  The percentage of flow that quits the reactor without reacting increased when the main flow increased and the current intensity decreased. The global RTD can be reconstituted by the signal resulting from the junction and that from riser–separator and downcomer zone by using the convolution technique. The experimental results confirm this reconstitution. The experiments confirm also that the liquid crosses the reactor without achieving loops in the case of the continuous flow.

     

浮选柱中液相轴向返混的研究

An experimental study on the axial dispersion of liquid was carried out in a 0.382-m-ID flotation column packed with different structured packings or free of packings. The correlations of axial Peclet numbers with the liquid and gas superficial Reynolds numbers were developed for various packings. Among the packings tested, it is found that in the column packed with 250Y or 350Y packings the axial dispersion is the lowest. The addition of frother can decrease the axial dispersion. By the simulation analysis of the one-dimension dispersion model of packed flotation column, it is found that small axial dispersion, high collection rate constant and low axial liquid velocity can increase the collection zone recovery.     

Liquid-phase axial mixing study in a mobile bed contactor with low density particles

Axial mixing of the liquid phase in a 0.15 m ID mobile bed contactor has been studied for spherical, cork and irregular-shaped particles of densities 53, 183 and 112 kg m^?3, respectively, using the air-water system. The range of gas and liquid flow rates covered were from 0.5 to 5 m s^?1 and from 0.011 to 0.044 m s^?1, respectively. The experimental breakthrough curves were interpreted by means of the axial dispersion model. Correlations for the Peclet number and axial dispersion coefficient have been proposed. The results indicate that near plug flow of the liquid phase could be achieved with higher static bed heights if the congregation of particles at the wall could be avoided.     

双颗粒提升管中细颗粒的混合行为研究(Ⅰ)──轴向混合行为

利用磷光颗粒示踪技术对双颗粒提升管中细颗粒的轴向混合行为进行了研究测得反映细颗粒轴向扩散的停留时间分布曲线呈双峰分布，这表明细颗粒存在两种类型的扩散形式：弥散颗粒扩散和颗粒团扩散本文提出的一维两组分扩散模型可较好地描述停留时间分布曲线的双峰分布.实验结果表明细颗粒的两种轴向混合行为有较大的差别，而总包Peclet数几乎不随操作条件及床内粗细颗粒比例变化