Performance characteristics of mechanical foam‐breakers fitted to a stirred‐tank reactor

In a stirred‐tank reactor (STR), a comparison of the performance of mechanical foam‐breakers: a six‐blade turbine (F‐B), a six‐blade vaned disk (V‐D), a two‐blade paddle (T‐P), a conical rotor (C‐R), a fluid‐impact dispersion apparatus (FIDA) and a rotating disk mechanical foam‐breaker (MFRD) was carried out using defined foaming media. The foam‐breaking ranges (relative to the gas superficial velocity, U_g) of the T‐P, C‐R and FIDA were inferior to that of the F‐B, V‐D and MFRD. The power consumption, P_kc, for foam‐breaking in the MFRD was the lowest among the F‐B, V‐D and MFRD. Operation of the F‐B and V‐D in the STR caused a considerable amount of liquid droplets from the collapsed foam to be entrained with the exhaust air. © 2002 Society of Chemical Industry     

Mechanical control of foaming in stirred‐tank reactors

The foam‐breaking characteristics of rotating‐disk mechanical foam‐breakers (MFRDs) fitted to stirred‐tank reactors (STRs) containing various foaming liquids were evaluated. The critical disk rotational speed, N_c, required for foam‐breaking and the liquid hold‐up, ?_L, in ascending foam reflected, respectively, the foam‐breaking behaviour of MFRDs and the foaming behaviour of STRs. Empirical equations for the prediction of N_c and ?_L, which can be applied independently of the type, concentration and temperature of foaming liquid, were obtained. The foam‐breaking power, P_kc, of MFRDs was also clarified in relation to the level of ?_L which is related to the difficulty or ease of mechanical foam‐breaking. © 2001 Society of Chemical Industry     

Gas holdup of rotating foam reactors measured by γ‐tomography—effect of solid foam pore size and liquid viscosity

Rotating foam reactors have already shown to give high mass transfer rates compared to stirred tank reactors. For a deeper insight into the hydrodynamics of these reactors, the hydrodynamics of rotating foam reactors were studied using γ‐ray tomography. The two‐phase flow through the foam block stirrer is mainly influenced by the solid foam pore size and the liquid viscosity. For low viscosity, the optimal foam block pore size was identified in the range between 10 and 20 pores per inch (ppi). With smaller pore size, the gas holdup inside the foam block strongly increases due to bubble entrapment. For higher viscosity, pore sizes larger than 10 ppi have to be used to achieve a sufficient liquid flow rate through the foam block to avoid a strong gradient over the reactor height. The effect of the hydrodynamics on the gas–liquid and liquid–solid mass transfer and the reactor performance are discussed. © 2012 American Institute of Chemical Engineers AIChE J, 59: 146–154, 2013     

Foam Rupture by Mechanical and Vibrational Methods

The need to destabilize foams and control their formation arises in many industrial processes. Mechanical foam breakers, mechanical vibration and ultrasonic vibration are examples of physical destructive techniques which do not suffer from the drawbacks associated with the use of chemical antifoams and defoamers. This paper reports on recent studies in this area. Two novel designs of mechanical foam breaker are presented which are superior to traditional foam breakers in controlling unwanted foaming in both bubble columns and stirred vessels. The effectiveness of mechanical and ultrasonic vibrations in destabilizing foams is shown to be governed by vibrational amplitude and frequency, and foam structure. A fundamental understanding of foam destabilization by these physical techniques is crucial to further advancement in the design of efficient and robust foam breakers and the development of enhanced non‐invasive vibrational destructive techniques.     

Mechanical foam breaking – a physical model for impact effects with high speed rotors

During recent years in chemical engineering processes for instance with aerated stirred tanks containing surface-active components foams and their mechanical control have become very important. However, there are still no rules for calculating mechanical foam breakers. As a contribution to foam control the present paper discusses the fundamentals of foam breaking by means of foam densities and gives a physical model for the mechanisms of a mechanical foam breaker when the main breaking effect is based on impact. The mechanisms were proved by measuring pressures in a foam breaker box. Diameters of secondary foam bubbles produced by foam breakers are calculated and discussed. Experimental investigations were made with different surfactants in aqueous solutions. The paper points out their influence on the process of foam breaking.     

The Use of Positron Emission Particle Tracking in the Study of Multiphase Stirred Tank Reactor Hydrodynamics

This paper describes the use of positron emission particle Tracking (PEPT) in the analysis of local particle and fluid velocities in solid‐liquid stirred tank reactors agitated with a Rushton turbine and an upward‐pumping pitched blade turbine. PEPT captures the full three‐dimensional characteristics of hydrodynamics and mixing in stirred vessels, allowing the analysis of the two‐phase flow fields. Furthermore, by comparing the liquid and particle velocities, the spatial and temporal variation of the relative particle‐liquid velocity can be estimated. Such information reveals considerable heterogeneity in the vessel and facilitates the evaluation of impeller design, particularly with the aim of minimizing mass transfer limitations.     

三叶后掠-HEDT组合桨搅拌釜内流场的模拟及实验

对应用于聚乙烯聚合反应中的三叶后掠-HEDT组合桨的搅拌釜内流场进行了模拟研究,分析组合桨的离底距C ₁、桨间距C ₂以及转速N的变化对搅拌釜内流场的影响,利用PIV实验对模拟结果进行了验证;将该组合桨与三叶后掠-六直叶圆盘涡轮组合桨进行了模拟对比研究。结果表明：当桨间距与釜内径的比为0.35时,釜内桨叶间的流体流动效果最好,该条件下能够改善搅拌釜上层流体的速度分布;当离底距与釜内径的比值为0.29时,组合桨下方出现了整体的环流,有利于釜底流体的混合;桨叶转速N=90 r/min时釜内流体速度分布均匀,同时上层HEDT桨叶产生的射流方向趋于水平。两种组合桨的对比研究表明：二者流型相近,但前者搅拌功率能够得到明显降低。研究结果可为三叶后掠-HEDT组合桨在聚乙烯聚合反应釜中的工程应用提供参考。     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

机械搅拌式发酵罐中的消泡技术研究与探讨

分析了好氧性微生物发酵过程中由于搅拌、气体引入和微生物代谢等产生泡沫的原因和泡沫对发酵生产过程的危害。介绍了工业生产中所采用的化学消泡和机械消泡方法、常用的几种化学消泡剂和机械消泡结构形式,以及它们在使用过程中的优点和不足之处,并提出了一种新型机械消泡装置——变径孔式消泡器,这种消泡器是依靠泡沫通过变径孔通道时所产生的压力变化和转动时所产生的剪切力来实现消泡目的的,消泡效果良好。     

The Two‐Phase Flow in an Axially Stirred Vessel Investigated Using Phase‐Doppler Anemometry

The phase‐Doppler technique has been used to characterize the two‐phase flow of liquid and particles in a stirred vessel agitated by a pitched blade turbine. The number of measurement locations used is considerably larger than in previous investigations and the behaviour of four different types of particles is studied. The fluid phase and particle phase flow is studied with particular emphasis on the relative velocities of the two phases. The largest slip velocities in the tank were found just beneath the impeller, and large slip velocities generally coincide with large velocity gradients. Generally, particles lag in relation to the fluid when the fluid flow is directed upwards, and vice versa, but exceptions to this are not unusual.     

Hydromechanics of separation of gas—liquid systems with foam structure

The operation of mechanical foam breakers (filtering, centrifugal, nozzle, disk, paddle, drum, and jet foam breakers) is modeled, and their main characteristics are calculated. This will allow development of a rational and optimal design of equipment for foam breaking in technological processes.Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 38, No. 6, 2004, pp. 604–610.Original Russian Text Copyright © 2004 by Vetoshkin.     

Effect of Ethylene Glycol on the CO2/Water Gas‐Liquid Mass Transfer Process

The effect caused by the presence of ethylene glycol on the gas‐liquid mass transfer velocity of CO₂ in a aqueous phase has been studied. In this study two different gas‐liquid contactors have been used, a bubbling stirred reactor and a flat surface stirred vessel. The first contactor, gas phase, was introduced using a porous bubbling plate. The influence of operational variables, stirring rate, gas flow rate and ethylene glycol concentration were studied. The experiments were carried out at 298.15 K using a semicontinuous regime. The final aim was to obtain empirical equations that allow the calculation of the mass transfer velocity for this system a priori.     

A Multi‐Block Approach to Obtain Angle‐Resolved PIV Measurements of the Mean Flow and Turbulence Fields in a Stirred Vessel

Two‐dimensional Particle Image Velocimetry (PIV) measurements have been used to characterize the complex turbulent flow generated by a T/3 45° pitched‐blade down‐flow turbine, operated at Re ≈ 5 · 10⁴, in a fully turbulent stirred vessel. To maintain high spatial resolution when viewing the whole vessel, a multi‐block approach has been developed, which combines data from different fields of view into a composite flow map. Using 500 measurements of instantaneous u and v velocity fields, angle‐resolved mean velocity maps and turbulence properties, such as the RMS velocities and the turbulence kinetic energy, have been estimated near to the blade, as well as in the bulk of the vessel, at a spatial resolution of between 1 and 2 mm. Vorticity maps have also been calculated to help visualize the trailing vortex structures close to the impeller blades and integral length scales have been estimated from the two‐dimensional spatial auto‐correlation function. It is shown than the common assumption that the integral length scale is about half the blade width is an overestimate close to the impeller and an underestimate far from the impeller.     

Power Consumption in Gas‐Liquid Contactors Agitated by Double‐Stage Rushton Turbines

The dependence of power consumption on impeller spacing in unaerated and aerated gas‐liquid contactors agitated by dual Rushton turbine systems was studied, and the gas flow rate and viscosity effects were measured in relation to these parameters. The experiments were carried out in a 0.19 m i.d. vessel stirred by two Rushton turbines with a diameter d = 0.10 m; with blade length and blade height 0.25 d and 0.2 d, respectively. In tap water the impellers acted independently for spacings greater than 1.65 d, while in glycerol solutions the two impellers already acted independently at an impeller spacing equal to 1.2 d. In aerated systems, a notable increase in the power consumption with increasing impeller spacing could be detected for small gas flow rates and low viscosities, while a decrease in the Newton number with increasing Froude number could be observed at constant impeller spacing. The Newton number was not affected by flow number at high viscosity values.     

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas‐Liquid Contactors

Gas‐liquid contacting in mechanically agitated vessels is widely used in the process industry. Bubble size measurements at different flow regions in the vessel provide useful information for the mechanisms of gas dispersion and gas‐liquid flow. In this work, bubble size distributions and distributions of Sauter mean bubble diameters at four different regions in air‐water and air‐NaCl solution systems agitated by a six‐blade disk turbine are measured by using the photographic method. The effects of gassing rate, impeller speed and electrolyte presence in the system have been examined.     

聚合物稳泡剂耐温性研究进展

在油气井开发过程中,虽然对稳泡剂的稳泡效果认识较早,但很少对其耐温性能进行对比研究,随着油气资源找寻方向由浅层油藏逐渐转变为高温油藏、裂缝型以及低渗油藏,聚合物稳泡剂的耐温性能成为重要研究课题。该文对稳泡剂的3个主要施工领域:泡沫压裂液稳泡剂、泡沫驱油稳泡剂、泡沫洗井液稳泡剂的耐温性进行对比分析,阐述了其在油田应用中的优势与不足,针对国内外聚合物稳泡剂耐温性差、稳泡效果不明显、合成过程复杂等问题,提出改进合成工艺、纳米复配、功能性单体自组装等建议,来达到提高聚合物耐温性与稳定性的目的。     

Optimisation of vertical axis wind turbine: CFD simulations and experimental measurements

A system for the conversion of kinetic energy of wind into thermal energy has been developed which can replace relatively expensive electro‐mechanical equipment. The system consists of a vertical axis wind turbine (VAWT) which is coupled with the shaft of a stirred vessel. In the present work, computational fluid dynamic (CFD) simulations have been performed for the flow generated in a stirred tank with disc turbine (DT). The predicted values of the mean axial, radial and tangential velocities along with the turbulent kinetic energy have been compared with those measured by laser Doppler anemometry (LDA). Good agreement was found between the CFD simulations and experimental results. Such a validated model was employed for the optimisation of drag‐based VAWT. An attempt has been made to increase the efficiency of turbine by optimising the shape and the number of blades. For this purpose, the combination of CFD and experiments has been used. The flows generated in a stirred tank and that generated by a wind turbine were simulated using commercial CFD software Fluent 6.2. A comparison has been made between the different configurations of wind turbines. Results show that a provision in blade twist enhances the efficiency of wind turbine. Also, a wind turbine with two blades has higher efficiency than the turbine with three blades. Based on the detailed CFD simulations, it is proposed that two bladed turbine with 30° twist shows maximum efficiency. © 2011 Canadian Society for Chemical Engineering     

Particle‐resolved PIV experiments of solid‐liquid mixing in a turbulent stirred tank

Particle Image Velocimetry (PIV) experiments on turbulent solid‐liquid stirred tank flow with careful refractive index matching of the two phases have been performed. The spatial resolution of the PIV data is finer than the size of the spherical, uniformly sized solid particles, thereby providing insight in the flow around individual particles. The impeller is a down‐pumping pitch‐blade turbine. The impeller‐based Reynolds number has been fixed to Re = 10⁴. Overall solids volume fractions up to 8% have been investigated. The PIV experiments are impeller‐angle resolved, that is, conditioned on the angular position of the impeller. The two‐phase systems are in partially suspended states with an inhomogeneous distribution of solids: high solids loadings near the bottom and near the outer walls of the tank, much less solids in the bulk of the tank. The liquid velocity fields show very strong phase coupling effects with the particles increasingly attenuating the overall circulation patterns as well as the liquid velocity fluctuation levels when the solids volume fraction is increased. © 2017 American Institute of Chemical Engineers AIChE J, 63: 389–402, 2018