Effect of oscillation amplitude on velocity distributions in an oscillatory baffled column (OBC)

This paper presents a numerical study of the effect of oscillation amplitude in oscillatory baffled column (OBC) using computational fluid dynamics. The numerical work was carried out for single phase liquid flow for an unsteady 3-D model using commercial software, Fluent (2006). This work was concentrated on the effect of oscillation amplitude. Three amplitudes of 5, 10 and 15 mm with constant frequency of 1 Hz are applied. Vortex and cycle average velocities at different points are analyzed. The studies show the maximum velocity for 5 mm, 10 mm and 15 mm in an OBC are 0.11 m/s, 0.25 m/s and 0.40 m/s respectively in the first cycle of oscillation. At a constant frequency, greater oscillation amplitude displaces the liquid to a further distance and builds a larger vortex. Vortex length was 1.5 times bigger when oscillation amplitude changes from 5 mm to 10 mm and 2 times when the amplitude is triple from 5 mm. The detailed validation is presented somewhere else; this research is focused on the effect of oscillation.     

Flow patterns and oil—water dispersion in a 0.38 m diameter oscillatory baffled column

We report our experimental flow visualization observations of flow patterns and experimental oil‐water dispersion measurements in an oscillatory baffled column (OBC) of an internal diameter 380 mm. Both types of experiments were carried out covering an identical range of oscillation frequencies, amplitudes, orifice diameters and baffle spacings. The flow visualization observations show that eddy mixing has been achieved in the pilot OBC and the intensity of which is largely dependent on the operational and geometrical parameters tested, which is similar to that in a smaller scale OBC. The scale‐up correlation was found to be linear. The oil‐water dispersion measurements show that the degree of the dispersion depends significantly on the oscillation frequency and amplitude with an increase in either leading to an increase in dispersion. The effect of the orifice diameter on the oil—water dispersion is also evident, but the effect of the baffle spacing is much weaker. Based on the experimental data we have established a correlation relating the degree of oil—water dispersion to the power input to the system. We have also compared the power requirement to achieve a complete dispersion in the pilot OBC with that in a bench scale OBC of 50 mm diameter and found that the energy dissipation is more economical in the large scale application.     

EFFECTS OF HALL CURRENT AND CHEMICAL REACTION ON OSCILLATORY MIXED CONVECTION-RADIATION OF A MICROPOLAR FLUID IN A ROTATING SYSTEM

This article is concerned with the analysis of the effects of thermal radiation on oscillatory mixed convection flow of a micropolar fluid in a rotating frame of reference in the presence of transverse magnetic field and Hall current. The influence of a first-order homogeneous chemical reaction and heat source effects is also analyzed. The governing partial differential equations with the appropriate boundary conditions are reduced to a set of ordinary differential equations using similarity transformations. The dimensionless governing equations for this investigation are solved analytically after using small perturbation approximation. The effects of various parameters on the velocity, temperature, and concentration fields as well as on skin-friction coefficient, Nusselt number, and Sherwood number with their amplitude and phase are discussed in detail. Numerical results are discussed with the help of graphs and tables. Present results are also compared with previously published work.     

Volumetric power consumption in baffled shake flasks

For the cultivation of microorganisms, baffled shake flasks are employed when increased levels of oxygenation and mixing are required. Their use has been discouraged, however, due to the danger of a wetted sterile plug and the lower reproducibility of the experimental results. Consequently, there are only few studies dealing with this type of shaken bioreactor, and there is practically no characterization of this reactor type from a chemical engineering viewpoint. Therefore, a systematic study to elaborate the basic characteristics of the volumetric power consumption and the unfavorable out-of-phase phenomenon in baffled shake flasks is undertaken. A new type of measuring device was developed to measure the volumetric power consumption in a single shake flask. The volumetric power consumption was found to increase with increasing shaking frequency and with decreasing filling volume. Further, an independency of power consumption on the shaking diameter was observed as long as the fluid motion is in-phase. A comparison of two different baffle geometries demonstrated that deeper baffles cause more resistance to fluid flow. For the commonly employed shaking diameter of 25 mm, the investigated baffled flask types may not be operated in the in-phase state. A larger shaking diameter must therefore be employed. It was found for the first time that for all in-phase conditions, the dimensionless Newton number Ne^′ is independent of the Reynolds number Re. Power consumption in baffled shake flasks may therefore be described by a characteristic Ne^′ only dependent on the filling volume V_L and the flask type. Even though there are quantitative differences, a qualitative similarity between fluid flow in stirred tanks and shake flasks has been demonstrated.     

The effect of axial impeller geometry on the link between power and flow numbers

Stirred vessels are used to facilitate mixing processes across a wide range of industries. Their performance can often be predicted with certain characteristics of the agitator, like the dimensionless power and flow numbers. Since there exists a large number of agitator designs and geometries, it is desirable to be able to predict these characteristics using models rather than rely on previous experimental data. In this study, we use an angular momentum balance combined with computational fluid dynamics to correlate the power, flow, and mixer geometry across a wide range of down-pumping pitched blade turbine geometries. The models developed from the results allow us to predict the power from the flow (or vice versa) for the geometries investigated. We tested two methods for the measurement of the flow rate and found that the choice of measurement method can affect the relationship between the power, flow, and impeller geometry.     

The influence of hydrodynamic and mass transfer entrance effects on the operation of a parallel plate electrolytic cell

Experimental measurements of mass transfer in an electrochemical flow cell of rectangular cross section with different hydrodynamic entrance and electrode lengths have been made. For fully developed flow, average Sherwood numbers under laminar conditions vary with Graetz number to a power 0·30. For turbulent flow, fully developed mass transfer conditions occur about twelve equivalent diameters along the electrode and are best represented by the Chilton-Colburn analogy which predicts Sherwood numbers varying with Reynolds number to a power of 0·8 and Schmidt number to a one-third power. For shorter electrodes Sherwood numbers can be adequately correlated by an expression with Reynolds number to a two-thirds power and dimensionless electrode length to a power of −0·2. For hydrodynamic entrance lengths of not less than eight equivalent diameters, data in the laminar region can be expressed by an emperical boundary layer type of equation which includes terms for the hydrodynamic entrance length and electrode length. In the turbulent regime substantially developed flow occurs after eight entrance lengths and correlations with fully developed flow equations are satisfactory     

Characterisation of flexible baffles in an oscillatory baffled column

In our previous studies the volume‐averaged strain rates, of 2–22 s^?1, were obtained in an oscillatory baffled column (OBC) based on velocity measurements over a half baffled cell for oscillatory Reynolds numbers of 1000–4030. These values are very low compared with those in a traditional stirred tank vessel (at least >100 s^?1) for similar operational conditions and the same power consumption. It was also observed that the volume‐averaged strain rates in the OBC fluctuated with the phase of oscillation over any cycle, with high values coinciding with eddy generation, and low values with eddy cessation. The objective of this study is to show that such fluctuations can be attenuated by employing innovative flexible baffles, whose inside edges move with the fluid oscillation. In this paper experimental measurements of velocity vector maps and strain rate distributions using Digital Particle Image Velocimetry (DPIV) are presented for both conventional and flexible baffles in an OBC. Mixing characterisation, in terms of axial dispersion coefficients, are compared for both baffle designs. The results show that the flexible baffles can reduce the fluctuations and magnitudes of the volume‐averaged strain rates in the OBC without compromising the mixing performance. Low and uniform strain rate distributions in time and space are essential biochemical, biomedical and pharmaceutical applications where shear sensitive cultures are involved. © 2001 Society of Chemical Industry     

Simulation of oscillatory baffled column: CFD and population balance

In the present work, an attempt has been made to combine population balance and a CFD approach for simulating the flow in oscillatory baffled column (OBC). Three-dimensional Euler-Euler two-fluid simulations are carried out for the experimental data of Oliveira and Ni [2001. Gas hold-up and bubble diameter in a gassed oscillatory baffled column. Chemical Engineering Science 56, 6143-6148]. The experimental data include the average hold-up profile and bubble size distribution in the OBC. All the non-drag forces (turbulent dispersion force, lift force) and the drag force are incorporated in the model. The coalescence and breakage effects of the gas bubbles are modeled according to the coalescence by the random collision driven by turbulence and wake entrainment while for bubble breakage by the impact of turbulent eddies. Predicted liquid velocity and averaged gas hold-up are compared with the experimental data. The profile of the mean bubble diameter in the column and its variation with the superficial gas velocity is studied. Bubble size distribution obtained by the model is compared with the experimental data.     

Convective heat transfer for power law fluids in packed and fluidised beds of spheres

The forced convection heat transfer characteristics for an incompressible and steady flow of power law liquids in fixed and extended beds of spherical particles has been studied numerically. The sphere-sphere hydrodynamic interactions have been accounted for by using a simple cell model. Within the framework of such a cell model, the momentum and energy equations have been solved using a finite difference method to obtain the velocity and temperature fields. Extensive numerical estimates of the local and average Nusselt numbers as functions of the physical, rheological and kinematic variables have been presented and discussed for the two commonly employed thermal boundary conditions. In broad terms, the Nusselt number for power law fluids (both shear-thinning and shear-thickening conditions) normalized with respect to the corresponding value for a Newtonian fluid shows weak additional dependence on the power law flow behaviour index. The shear-thinning behaviour is seen to promote heat transfer and as expected the shear-thickening behaviour impedes heat transfer in fixed and fluidised beds. All in all, the present results encompass wide ranges of conditions as follows: Reynolds number: 1-500; Peclet number: 1-500; bed voidage: 0.4-0.8 and the flow behaviour index: 0.5-1.8 thereby covering extremely shear-thinning and shear-thickening types of fluid behaviours. The paper is concluded by presenting detailed comparisons with the limited analytical and/or experimental results available for liquid-solid mass transfer in such systems.     

Predictions of pressure drop for modified power law fluids in conduits of three different cross-sectional shapes

Numerical solutions are presented for fully developed laminar flow for a modified power law fluid (MPL) in conduits of arbitrary cross sections. The solutions are applicable to pseudoplastic fluids over a wide shear rate range from Newtonian behavior at low shear rates, through a transition region, to power law behavior at higher shear rates.The analysis identified a dimensionless shear rate parameter which, for a given set of operating conditions, specifies where in the shear rate range a particular system is operating, i.e. in the Newtonian, transition, or power law regions.The numerical results of the friction factor times Reynolds number for the Newtonian and power law region are compared with previously published results showing agreement within 0.05% in the Newtonian region, and 0.9% and 5.1% in the power law region.     

Design of stable steam injectors for continuous heating

Continuous direct steam injection systems are used in industry to rapidly raise the temperature of process streams either for heating or for sterilization purposes. High heat transfer rates can be achieved using this method, as compared with other methods e.g. shell and tube heat exchangers. Currently, there are no rational procedures available for designing steam injectors for stable operation. Flow visualization studies and pressure measurements have shown that three flow regimes; bubbling, jetting and intermittent steam/water flow exist, in direct steam injection into continuously flowing water. These flow regimes are a function of process conditions and orifice diameter. A semi-quantitative flow regime map for a range of process conditions has been drawn up. Bubbly flows give rise to the highest levels of noise due to bubble oscillations. This type of flow should therefore be avoided. The most stable flow regime, in terms of noise levels generated and hydrodynamic considerations is the jetting regime. Models have been derived for each flow regime; good agreement is found between experimental and theoretical data. A dimensionless number (Bubble/Jet number) has been defined, which can be used to predict transitions between bubbling and jetting flows. This dimensionless number, together with the flow regime map can be used to design stable, relatively quiet steam injection systems.     

A numerical study of the accelerating motion of a dense rigid sphere in non-newtonian power law fluids

The equations of motion of an accelerating sphere falling through non-Newtonian fluids with power law index n in the range 0.2 ≤ n ≤ 1.8 were integrated numerically using the assumption that the drag on the sphere was a function of both power law index and terminal Reynolds number, Re_t For 10^?2 ≤ Re_t ≤ 10³ both dimensionless time and distance travelled by the sphere under transient conditions showed a much stronger dependence on the flow behaviour index, n, for shear-thinning than for shear-thickening fluids. The form of this dependence is investigated here. Furthermore, results in four typical shear-thinning fluids suggested a strong correlation between the distance and time travelled by the sphere under transient conditions and the value of the fluid consistency index. The analysis reported herein is, however, restricted to dense spheres falling in less dense fluids, when additional effects arising from the Basset forces can be neelected.     

Effects of mixing,seeding, material of baffles and final temperature on solution crystallization of l-glutamic acid in an oscillatory baffled crystallizer

In this paper, we report the effects of mixing intensity, seeding, composition of baffle material and final temperature on meta-stable zone width (MSZW) and crystal polymorph in solution crystallization of an industrially important compound, l-glutamic acid, in an oscillatory baffled crystallizer (OBC). The results show that the MSZW decreases with increasing of mixing; meta-stable α crystals are transformed into stable β crystals with enhanced mixing intensity. Seeding meta-stable α crystals in operational conditions that promote β crystals leads to the formation of α crystals allowing co-existence of both forms; while seeding stable β crystals in conditions that favour α form allow β crystals prevailing in all conditions. Smoother surface of baffle material in OBC exhibits larger MSZW and favours meta-stable crystals, while rougher surface has smaller MSZW with stable crystals dominating. The meta-stable crystals gradually change into the stable form when final cooling temperature is closer to its nucleation temperature. The outcomes from this work indicate that by controlling process parameters desirable crystal polymorph can be obtained in the OBC.     

Polymer branching and first normal stress differences in small-amplitude oscillatory shear flow

General rigid bead-rod theory explains polymer viscoelasticity from macromolecular orientation. By means of general rigid bead-rod theory, we relate the normal stress differences of polymeric liquids to the branch position on a backbone branched macromolecule. In this work, we explore the first normal stress differences coefficients of different axisymmetric polymer configurations. When non-dimensionalized with the zero-shear first normal stress difference coefficient, the normal stress differences depend solely on the dimensionless frequency. In this work, in this way, we compare and contrast the normal stress differences of macromolecular chains that are branched. We explore the effects of branch position, length, functionality, spacing, and multiplicity, along a straight chain, in addition to rings and star-shaped macromolecules in small-amplitude oscillatory shear flow.     

Ecoulement de fluides newtoniens et non newtoniens à travers des lits fixes stratifiés

Fixed beds of plate shape particles have been studied in two different “flow-particle” configurations. From pressure drop measurements performed for a Newtonian fluid flow, two characteristic structural parameters have been determined by using a capillary - type model. The differences concerning the hydrodynamic behaviour of the two configurations are discussed and a factor characterizing the bed anisotropy is proposed. A dimensionless equation allowing the pressure drop determination for Newtonian and non-Newtonian purely viscous fluid flow in the two configurations, for a large range of Reynolds numbers, is proposed and tested with experiments.     

非牛顿流体的非等温搅拌流动

采用流线迎风Petrov-Galerkin(SUPG)有限元解能量方程与一般Galerkin有限元解动量方程相结合的方法对Carreau流体的非等温搅拌流场进行了数值模拟。并与牛顿流体等温搅拌流动的相应结果作了比较,表明剪切变稀和温度效应对搅拌流动的流型、温度分布产生很大的影响。冷模试验结果应用于热模工况时会产生较大的误差。     

Gas hold-up and bubble diameters in a gassed oscillatory baffled column

In this paper attempts are made to address how bubble behaviour in a batch oscillatory baffled column (OBC) contributes to the overall measured enhancement in mass transfer. A CCD camera is used to measure the bubble size distribution and the gas hold-up in the OBC. The experimental results of Sauter mean diameter and gas hold-up are correlated as a function of the power dissipation and superficial gas velocity, in order to allow for comparisons with published correlations for the mass transfer coefficient. In general, an increase in the oscillatory velocity causes an increase in the hold-up and a decrease in the Sauter mean diameter. Furthermore, we are able to show that the changes in the gas hold-up contribute more than the mean bubble size to the control of the mass transfer coefficient.     

Modeling of hydrodynamic cavitation reactors: a unified approach

An attempt has been made to present a unified theoretical model for the cavitating flow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase flow as the basis. This model has been used to describe the radial motion of bubble in the cavitating flow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an orifice plate. Simulations of the bubble dynamics in a venturi flow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity fluctuations the radial bubble motion in case of an orifice flow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation.     

An Investigation into Parameters Affecting Crystal Purity of Urea in a Stirred Tank and an Oscillatory Baffled Crystallizer

Confidential industrial trials showed that higher purity crystals were produced in an oscillatory baffled crystallizer (OBC) compared to a stirred tank crystallizer (STC) when operating at similar conditions. This trend is verified in this work using urea as the test compound that is different from that was employed in industrial tests. We have observed that a higher supersaturation level at nucleation coupled with a lower nucleation temperature was measured in the STC at all investigated conditions compared to that in the OBC. This led to a higher nucleation rate in the STC, consequently producing smaller crystals. Crystal size distributions and imaging analyses suggest that these smaller crystals were more likely to form agglomerates in the STC, trapping either mother liquor or impurity or both and leading to the lower purity observed when compared to that in the OBC.     

Hydrodynamics and power consumption of a reciprocating plate gas–liquid column

An experimental program has been initiated to use a reciprocating plate column for aerobic fermentations. This paper reports the results of the initial phase consisting of the hydrodynamic studies and the evaluation of the power consumption as a function of the level of agitation and gas superficial velocity. The variation of the gas holdup is presented and compared to a correlation. It is shown that a simple correlation can predict adequately the gas holdup when water is used. The power consumption, necessary for scale-up and design considerations, was evaluated using the pressure variation at the base of the column, the pressure difference across the column and the force exerted on the central shaft to reciprocate the plate stack. Results for the power consumed by the fluid mixture obtained with the three methods are presented and analyzed. The three methods give approximately the same value of the power consumption.