Liquid-phase backmixing in bubble columns, structured by introduction of partition plates

Installation of perforated sieve plates into a bubble column has the effect of introducing structure into an otherwise chaotic hydrodynamic behaviour. In this study, we focus on the reduction of backmixing of the liquid phase in compartmentalised bubble columns. Liquid-phase residence time distribution (RTD) measurements were carried out in bubble columns with diameters D_T=0.10, 0.15 and 0.38 m with air–water system operating at superficial gas velocities of U_G=0.05–0.4 m/s. Partition sieve plates with open areas of 18.6 and 30.7% were used in the studies. The measured data on RTD were interpreted in terms of an axial dispersion model extended to allow for liquid interchange between compartments. The interchange velocity was found to be strongly dependent on the free area of the plates but practically independent of the column diameter.     

Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (D_ba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onD _ba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate D_bc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (U_p/U_g).     

填料塔内流体轴向返混系数的确定

阐述了以示踪响应技术为基础确定填料塔内流体轴向返混系数的一种方法,首先对实验得到的示踪剂停留时间分布曲线采用等距法进行异常数据的剔除,再采用五点三次平滑公式进行滤波处理。然后用一维轴向扩散模型对曲线进行拟合,根据扩散模型在一定初始条件和边界条件下的解析解,采用时间域最小二乘法确定了轴向返混系数。结果表明,阐述的方法具有较高的精度。示踪停留时间曲线的实验测定值与其拟合值的均方差在0．034～0．096之间。     

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.     

周期性沟槽通道内自激震荡流及换热特性

应用数值模拟方法,分析了以周期性方式布置不同结构通道内产生的周期性自震荡流动现象,并研究了自激震荡流对下壁面物块传热特性的影响。计算采用低雷诺数、二维、非稳态层流模型。采用布置斜板和布置两个不同弯曲方向叶片等三种方式来诱导自激震荡流。分别对不同结构通道内流体的流场、温度场、努塞尔特数和表面摩擦系数变化规律进行了对比分析。计算结果发现：三种结构由稳态过渡到自震荡流时对应的临界雷诺数都很低,分别为620、480、400,并且有较低临界雷诺数的通道结构对应较强烈的震荡,表现为沟槽区域的强烈涡结构运动,及由此带来的更好换热效果。此外,表面摩擦系数因通道涡结构相异而有不同的变化趋势。     

Characterization and optimization of an oscillatory baffled reactor (OBR) for ozone-water mass transfer

Ozone-water mass transfer was investigated using an oscillatory baffled reactor (OBR) operated as a semi-batch and as a co-current up flow continuous reactor. The effects of input ozone concentration, input gas and water flow rates, and oscillation conditions on gas hold up, volumetric mass transfer coefficient and mass transfer efficiency were determined. The same reactor was operated as a baffled column (without oscillation) and as a bubble column to assess the effect of the reactor arrangement on the mass transfer. The results show that the OBR was 5 and 3 times more efficient for ozone-water mass transfer than the baffled and bubble columns, respectively. The enhancement obtained with OBR over the baffled column reactor was found to decrease with gas flow rate due to changes in bubble flow pattern from homogenous to heterogeneous. Under continuous flow conditions, the performance of the baffled reactor and the OBR were found to be twice efficient for ozone-water mass transfer than when operating under semi-batch conditions. The mass transfer effeciency (MTE) was found to increase from 57% using the baffled reactor to 92% with OBR under continuous flow at water and gas superficial velocities of 0.3 and 3.4 cm s⁻¹, respectively.     

The influence of mixing on lysozyme renaturation during refolding in an oscillatory flow and a stirred-tank reactor

Protein refolding is a key unit operation in many processes that produce recombinant biopharmaceuticals using Escherichia coli. Yield in this step generally controls overall process yield, and at industrially relevant protein concentrations is limited by aggregation. While most refolding operations are optimised with respect to chemical environment, the physical processes affecting yield have been neglected. In this study, we demonstrate that refolding yield for the model protein lysozyme is dependent on mixing intensity during dilution refolding. This is shown for two different reactor configurations: a standard stirred-tank reactor and a novel oscillatory flow reactor. We further show that the effect of mixing is dependent on the type of chaotrope employed for denaturation. Yield falls significantly when mixing intensity is decreased following urea denaturation, while the effect of mixing is not apparent when guanidine hydrochloride is employed as the denaturant. In batch tests we further confirm that, for urea, the “path” of dilution affects yield, and hence the observed sensitivity to mixing is not unexpected. We conclude that mixing is a critical parameter that must be optimised in industrial reactors, along with the usual chemical and protein-specific parameters.     

Evaluation on the degrading behavior of melt polyolefin elastomer with dicumyl peroxide in oscillatory shear flow by Fourier transform rheology

The degradation of melt polyolefin elastomer (POE) at the presence of dicumyl peroxides (DCP) was estimated at elevated temperature in oscillatory shear flow. Large amplitude oscillatory shear (LAOS) experiments followed by Fourier transform rheology (FTR) were carried out to detect and evaluate the branching architecture of the products. The third complex harmonic (I₃^∗) and other two parameters, small strain elastic shear modulus (M) and large strain elastic shear modulus (L), which describe the nonlinearity and elasticity of a material obtained from FTR, are mainly used to characterize the topological structure of polymer chains. The results indicate the degradation appeared just after a large amount of the long chain branches (LCB) created rather than as soon as the reaction started when the strain was applied within the linear viscoelastic regime of the original POE at high frequencies. This is different from our previous result that the dominant reaction was coupling in linear shear flow. The threshold strain for degradation decreased with the oscillatory frequency, and the frequency owned a different acting mechanism from the strain amplitude to cause the degradation reaction. Moreover, there is a kind of selectivity of shear rate on the polymer chains for degradation. Low frequency results in short linear scission segments and a long branched chain suffers from degradation more than once. At high frequency, the possibility of degradation at the sites near the branching points of LCB increases.     

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.     

The intensification of gas-liquid flows with a periodic, constricted oscillatory-meso tube

The experimental study of gas dispersion in a vertical periodically, constricted, oscillatory meso-tube (OMT) is herein presented. Water was continuously pumped through the OMT in the laminar flow regime along with an oscillatory flow component superimposed into the net flow in a range of fluid oscillation frequency (f) and centre-to-peak amplitude (x₀) of and 0-3 mm, respectively, in the presence of a very low superficial gas velocity . Bubble images were recorded with a CCD camera and analysed with Visilog^® software. A bimodal distribution of bubble size was in general observed but the bubble size was found strongly dependent on the oscillatory flow mixing conditions imposed into the fluid. A number fraction of micro-bubbles (with an equivalent diameter, D_eq, equal or bellow 0.2 mm) up to 60% was generated with increasing values of x₀ (i.e. 3 mm) and values of f in the range . Furthermore, it is demonstrated that the Sauter mean diameter, D₃₂, and the specific interfacial area, a, can be fined tune by setting both f and x₀ in this studied range. The high number fraction of micro-bubbles was concluded to have a positive impact in enhancing the liquid-side mass transfer coefficient, k_L. Globally, the differences in bubbles sizes were found to play a marginal effect in the global enhancement of the k_La in the meso-tube in comparison with the intensive contact experimented by the bubbles rising in the oscillatory flow. The higher order of magnitude of the k_L values found in this work (up to ) is promising for running numerous industrial gas-liquid flows processes through smaller and better, while aeration of biotransformations can be run more efficiently, as supported by our recent proof-of-concept studies carried out in the platform.     

The axial dispersion performance of an oscillatory flow meso-reactor with relevance to continuous flow operation

A laboratory scale continuous oscillatory flow meso-reactor was developed and residence time distribution (RTD) studies were carried out in order to establish certain process characteristics of the system. In particular, the dispersion coefficient as a function of the primary variables was established. Using optical probes the axial dispersion was investigated by monitoring the response of a pulse dye tracer at different locations within the meso-reactor. Three cases, net flow without oscillation, oscillation without net flow, and oscillation plus net flow were studied over a range of oscillation frequencies, amplitudes, and net flow rates. Both the imperfect and the perfect pulse injection methods were used to determine the axial dispersion coefficient for the system with and without net flow. The axial dispersion coefficient and the dimensionless dispersion number were analysed in the context of different flow conditions. A correlation was established and demonstrated that the axial dispersion within the meso-reactor could be quantified as a function of flow conditions. The results showed that the laboratory continuous flow meso-reactor was able to produce plug flow with modest axial dispersion over a wide range of parameter space, thereby indicating efficient mixing and effective RTD performance.     

Influence of liquid properties on flow regime and backmixing in a special bubble column

An experiment aimed to link the extent of axial mixing in a special configuration bubble column reactor with different liquid properties (water, 10% K₂CO₃ solution, 20% K₂CO₃ solution, paraffine). The experimental results proved that, increase of liquid viscosity will delay the mean residence time and weaken gas axial backmixing. Increased surface tension leads to lower flow regime transition point and higher overall gas holdup. Surface tension is the dominant factor to influence of gas axial backmixing degree. A simple RTD model for homogeneous–heterogeneous regime is developed in the column of 0.1 m diameter and the corresponding correlation of gas axial dispersion coefficients is . The model is verified by experiments with air/water/paraffine system. Good agreement is found. As a byproduct, a non-empirical formula for gas holdup results, _g/(1−_g)⁴ = 0.579 (u_gμ/σ)^0.918 (μ⁴g/ρ_Lσ³)^−0.252. But both correlations cannot be available for K₂CO₃ solution with addition of small quantities of surface tension in pure liquid.     

Numerical study of the flow pattern and heat transfer enhancement in oscillatory baffled reactors with helical coil inserts

Oscillatory baffled reactors (OBRs) are a means of process intensification as they allow processes with long residence time to be converted from batch to continuous processing. Helically baffled OBRs have only been developed at “mesoscale” so far, but at this scale have displayed significant advantages in terms of the increased range of conditions over which plug flow is achieved. Scale-up studies are underway to determine whether this is replicated at larger scales. This paper reports fluid mechanical modeling of a helically baffled oscillatory flow for the first time. Time-dependent flow structures induced in tubular reactors have been analyzed on the basis of periodic, laminar flow numerical simulation. A reversing swirled core flow and its interaction with the unsteady mechanism of vortex shedding downstream of the wires has been described. This has allowed greater understanding of the flow structures, which will underpin optimal design and scale-up. The potential for heat transfer enhancement is discussed, considering the compound effect of oscillatory motion and helical coil inserts. The results show that the heat transfer for the helical baffled tube could be enhanced by a factor of 4 compared to a smooth tube in the tested range of oscillation conditions.     

A simplified method of obtaining the backmixing coefficient of a solvent extraction column from tracer pulse data

A method based on transfer function matching is used to obtain the backmixing coefficient of a solvent extraction column. The method demonstrates the exploitation of the symbolic manipulation capability of the current generation of scientific computing software. It starts with the dynamic model of Dongaonkar et al. and makes use of a set of tracer pulse data of Dongaonkar for a Kühni column. Compared to the existing methods of evaluating backmixing coefficient, this new method is more direct and simple to implement on computers. It has the potential of being applicable to more elaborate dynamic models of plate columns.     

Liquid flow texture analysis in trickle bed reactors using high-resolution gamma ray tomography

Trickle bed reactor performance and safety may suffer from radial and axial liquid maldistribution and thus from non-uniform utilization of the catalyst packing. Therefore, experimental analysis and fluid dynamic simulation of liquid–gas flow in trickle bed reactors is an important topic in chemical engineering. In the present study for the first time a truly high-resolution gamma ray tomography technique was applied to the quantitative analysis of the liquid flow texture in a laboratory cold flow trickle bed reactor of 90 mm diameter. The objective of this study was to present the comparative analysis of the liquid flow dynamics for two different initial liquid distributions and two different types of reactor configurations. Thus, the hydrodynamic behavior of a glass bead packing was compared to a porous Al₂O₃ catalyst particle packing using inlet flow from a commercial spray nozzle (uniform initial liquid distribution) and inlet flow from a central point source (strongly non-uniform initial liquid distribution), respectively. The column was operated in downflow mode at a gas flow rate of 180 L h⁻¹ and at liquid flow rates of 15 and 25 L h⁻¹.     

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     

Computational fluid dynamic modelling of flow patterns in an oscillatory baffled column

Numerical simulations to date within the context of oscillatory flow in a baffled column have been limited to flows in a symmetrical regime, i.e. eddies are generated symmetrical to the central line of the column where the oscillatory Reynolds numbers are below 400. In this paper, 3-D computational fluid dynamic (CFD) simulation of flow patterns of oscillatory flow in a baffled column has, for the first time, been carried out and the results extended to all regimes of oscillatory Reynolds numbers covering from symmetric to asymmetric flows. The flow patterns simulated have also been validated by both direct flow visualisation and by digital particle image velocimetry measurements. The success of such CFD simulations opens doors for many potential studies, from optimisation of geometry for plug flow to suspension of particles, and from droplet breakage and coalescence to mass/heat transfer of particles.     

Transition of the flow in a symmetric channel with periodically expanded grooves

Transition of the flow in a periodically grooved channel is numerically investigated for periodicity indices m=1 up to 6 by assuming the two-dimensional and fully developed flow field, where m is defined as a number of grooves in which the flow repeats periodically. Critical Reynolds numbers for the onset of a self-sustained oscillatory flow from a steady-state flow are evaluated by numerical simulations. It is found that the bifurcations occur at the critical Reynolds numbers as a result of Hopf bifurcation, and a period in the streamwise direction of the oscillatory flow is twice as long as the groove pitch of the channel. In addition, flow visualization with the aluminum dust method is carried out to confirm the results obtained from the numerical simulations. The experimental results are in good agreement with the numerical ones.