Low backmixing in multistage agitated contactors used as reactors

The objective of the study was to test the validity of an available backmix correlation in the low backmix regime required by multistage agitated contactors used as reactors. The correlation was found to be inapplicable, and new equation forms were developed to fit the new experimental data. Keys to low backmix rates are a small annular area around the shaft compared to the column cross-sectional area and an extended annular path length.     

Liquid backmixing and circulation in rectangular downcomer

1 INTRODUCTIONDowncomers,employed in tray columns,are widely used nowadays in industrialseparation processes.They function as channels for the liquid phase to fall fluentlyfrom one tray onto its neighbouring places for the held-up bubble to separate fromthe liquid phase.It is well known that liquid flow patterns and mixing characteristicsin downcomers are closely related to the initial liquid distribution on the tray,andhence to tray efficiency.For a long period of time,little attention has been paid to     

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.     

Particle-liquid mass transfer in three phase sparged reactor

Particle-liquid mass transfer in a three phase sparged reactor has been studied over a wide range of particle sizes, for the chum-turbulent regime. The particle-liquid Sherwood number has been correlated in the usual form with the Reynolds and Schmidt numbers. Use of the hindered particle settling velocity in the Reynolds number yields good agreement (±20%) with the present as well as most of the literature data. The proposed correlation also holds for power law non-Newtonian liquids when the effective viscosity is used.     

Continuous phase axial mixing in rotary disk contactors

This paper presents a review of important investigations on axial mixing in the continuous phase in rotary disk contactors (RDC) and analyses the various literature data in terms of the flow number (D_R N/u_e) and a geometry factor G_f. A new model equation has been developed to represent continuous phase axial mixing in the RDC taking into account the factors affecting the recirculation flow within the compartment and the main flow through the compartment in terms of disk-Reynolds number and modified tube-Reynolds number respectively. A satisfactory expression for G_f has been obtained on the basis of the present proposed model and analysis of data of available literature and those of the present work, covering laboratory and industrial size columns of 10 different investigators varying from 3.5 cm to 218 cm dia involving 19 different column geometries. The following forms of generalized correlation for continuous phase axial mixing in RDC have been suggested.      

Liquid holdup measurements in turbulent bed contactors by a tracer technique

An experimental technique to eliminate the effects of sensor dynamics when a tracer is used to measure the hydrodynamics of a turbulent bed contactor is described. The resulting liquid holdup data are compared both with predictions from the pressure drop measurements and with the available correlations in the literature. The results show that the pressure drop method and Handl's (1976) equation under-predict liquid holdup, while Rama et al.'s (1983) and Kito et al.'s (1978) equations over-predict the results.     

Dispersed phase hold-up in mechanically agitated gas–liquid contactors

In the present study, in mechanically agitated gas–liquid contactors (MAC), a generalized correlation for the direct estimation of dispersed gas phase hold-up is proposed. This present proposed new correlation in terms of fundamental operating variables, involving gas velocity, impeller speed in terms of Froude number, vessel geometry and the physical properties, is more accurate than those previously reported and also simpler to use. © 1998 SCI     

Gas hold-up in highly viscous pseudoplastic non-newtonian solutions in three phase sparged reactors

Gas hold-up structure was studied in a three phase sparged reactor of 200 mm diameter. Air and aqueous solutions of carboxymethyl cellulose were used as gas and liquid phases. Spherical glass beads formed the solid phase. The superficial gas velocity, rheological properties, particle size and solid loading were varied over a wide range and their effects on gas hold-up structure were studied. A correlation for the estimation of the gas hold-up as a function of superficial gas velocity, apparent liquid viscosity, particle settling velocity and solid loading was established.     

Critical impeller speed for solid suspension in multi-impeller three phase agitated contactors

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0–15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n_isg) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle—liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.     

A correlation for dispersed phase axial mixing coefficients in rotating disc contactors

A correlation for the estimation of dispersed phase axial mixing coefficients in terms of all the operating and fundamental variables was established. The proposed correlations are found to be a good representation of the present data (260 data points) obtained using four liquid‐liquid systems from two different sources. Copyright © 2005 Society of Chemical Industry     

苯乙炔选择性加氢滴流床反应器内的液相轴向返混特性

苯乙炔选择性加氢是从裂解汽油C₈馏分中回收苯乙烯的关键反应,本文采用滴流床反应器对该反应体系的液相轴向返混行为进行了研究。首先利用脉冲示踪法测得了不同操作条件下的液相停留时间分布密度;然后基于固定床轴向扩散模型,通过有限元正交配置法和Levenberg-Marquardt非线性最小二乘法,计算得到了Péclet数和液相平均停留时间;最后考察了主要操作条件对液相轴向返混的影响。研究结果表明,增大液体流量、气体流量、温度以及压力均可减小液相返混,而增大颗粒粒径会使液相返混加剧。     

Effects of internals on phase holdup and backmixing in a slightlyexpanded-bed reactor with gas-liquid concurrent upflow

Five different internals were designed, and their effects on phase holdup and backmixing were investigated in a gas-liquid concurrent upflow reactor where the spherical alumina packing particles of three diameters (3.0, 4.5 and 6.0 mm) were slightly expanded under the conditions of varied superficial gas velocities (6.77×10^-2-3.61×10^-1 m·s^-1) and superficial liquid velocities (9.47×10^-4-2.17×10^-3 m·s^-1). The experimental results show that the gas holdup increases with the superficial gas velocity and particle size, opposite to the variational trend of liquid holdup. When an internal component is installed amid the upflow reactor, a higher gas holdup, a less liquid holdup and a larger Peclet number characterizing the weaker backmixing are obtained compared to those in the bed without internals under the same operating conditions. Additionally, the minimal backmixing is observed in the reactor equipped with the internals with a novel multi-step design. Finally, empirical correlations were proposed for estimating gas holdup, liquid holdup and Peclet number with the relative deviations within 11%, 12% and 25%, respectively.     

Mass transfer in gas sparged membrane cells

Rates of mass transfer for the transfer of NaCl across a high permeability Cuprophane membrane in a gas-stirred batch dialysis cell were measured by the conductivity method. Gas sparging was found to increase the rate of mass transfer across the membrane by an amount ranging from 18 to 360% depending on the superficial gas velocity and the physical properties of the solution. The overall mass transfer coefficient was related to the superficial velocity by the equation

Investigation of feedpipe backmixing in an agitated vessel

Feedpipe backmixing in an agitated vessel was investigated using a newly developed conductivity technique. By this technique, the onset of feedpipe backmixing could be detected and the penetration depth of the vessel fluid into a feedpipe was determined. For a given feedpipe flowrate. critical agitator speeds to eliminate feedpipe backmixing were determined using Rushton six-bladcd disk turbine impeller (6BD) and high efficiency, axial-flow type 3-bladed impeller (HE-3) of 8.89 and 12.70 cm diameters in 11.2 liter reactor. The ratio of the feedpipe velocity to the critical agitator speed (v _f /v _i ) was determined as a function of feedpipe Reynolds number (N _R,T ). The conductivity technique was successful either in the laminar regime, the transitional regime, or in the turbulent regime in the feedpipe.     

Phenol degradation in rotating biological contactors

The biodegradation of synthetically‐prepared phenol wastewater was studied in a single stage, bench‐scale rotating biological contactor (RBC). The effect of process variables, namely rotational speed (40–175 rpm), input phenol loading (1754–3508 mg phenol m^?2 h^?1), input hydraulic loading (8.77–23.42 dm³ m^?2 h^?1), and temperature of wastewater (20–30 °C) on the amount of phenol removed in the system was investigated. It was observed that an increase in the speed of rotation significantly improved the performance. An increase in the hydraulic loading rates caused a reduction in the phenol removal rate, while an increase in the organic loading rate resulted in an improvement in performance. An increase in temperature caused an increase in the microbial activity and therefore gave better performance. A mathematical model has been developed based on oxygen transfer and kinetics of biodegradation. © 2002 Society of Chemical Industry     

A backmixing model describing micromixing in single-phase continuous-flow systems

A new model is presented which describes the mixing of fluids entering a continuous-flow reactor. The model contains only one parameter characterizing the micromixing rate, in a turbulent field. It pays special attention to the dilution of the entering stream of fluid by the surrounding older fluid. The dilution causes not only a change in concentration, but also a huge expansion of the stream.The model retains the distinction between micromixing and macromixing. The greater part of this study bears on fast reactions carried out in stirred ve     

Mass transfer at gas sparged spherical electrodes

Rates of mass transfer were measured for the cathodic reduction of K₃ Fe(CN)₆ in a large excess of sodium hydroxide at a single sphere cathode stirred by oxygen evolved at a horizontal disc anode placed below. The effect of the oxygen discharge rate and sphere diameter were studied at different ferricyanide concentrations. The rate of mass transfer was found to increase by an amount ranging from 66% to 450% over the natural convection value depending on the operating conditions. The data were correlated using hydrodynamic boundary layer theory and Kolmogoroff's theory of isotropic turbulence. The data were found to fit the equation: Sh = 2 + 0.21 Sc ^0.33 (ed _s ⁴/v ³)^0.268 where e is the rate of energy dissipation/unit mass of the liquid. Previous data on mass transfer at fixed beds of spheres stirred by counterelectrode oxygen bubbles were also correlated.     

Interfacial areas in agitated gas-liquid contactors

Chemical reactions with known chemical and physical properties were used for the determination of integral values of the interfacial areas created with an agitator in a gas-liquid contactor. The reactions between oxygen and a sulphite solution and between carbon dioxide and sodium hydroxide solutions were chosen. For the first system the chemical reaction in the liquid boundary layer is proved to be rate determining under our experimental conditions. The existence of two regions is demonstrated:

• 1. 

  (a) A region without agitation effect. At agitation rates below a minimum agitation rate n0D interfacial areas are not affected by the stirring; they only depend on the gas load vs and the type of gas sparger.

• 2. 

  (b) A region with agitation effect. At agitation rates above n0D the interfacial areas are linearly dependent on the agitation rate and independent of the gas load and type of gas inlet.

The value of n₀D depends on the tank to agitator diameter ratio and probably is independent of the liquid viscosity. Generally valid correlations for the minimum agitation rates and the interfacial area under fully baffled conditions could be derived for a great number of geometrically similar impellers of different sizes in vessels with diameters ranging from 0.14 to 0.90 m. Although vessels with a diameter larger than 1 m were not investigated the method of correlation seems to give good results for scaling-up. The influence of the physical properties of the liquid remains uncertain.     

Gas/liquid mass transfer in sparged agitated slurries

Volume referred mass transfer coefficients k_La were determined for six slurry systems in an agitated vessel of standard configuration. The measurements were carried out under variation of power input, gas flow rate and solids concentration. The k_La data can be fitted well by correlations proposed for agitated gas/liquid tanks. Preference is given to the concept introduced by Zlokarnik (Adv. Biochem. Engng8, 133, 1979) as by means of this the experimental Stanton numbers can be described excellently as function of only one dimensionless group which involves total power input and slurry phase properties.