Droplet size and velocity profiles in liquid-liquid horizontal flows

The size and vertical distribution of drops were studied experimentally in dispersed liquid-liquid pipeline flows. Under most conditions the pattern was dual continuous where both phases retain their continuity and there is entrainment in the form of drops of one phase into the other. The investigations were carried out in a stainless steel test section with 38 mm ID with water and oil (density and viscosity ) as test fluids. Mixture velocities from 1.5 to and input oil volume fractions from 20% to 80% were used. A dual sensor impedance probe allowed drop chord length and drop velocity measurements at different locations in a pipe cross section.It was found that in dual continuous flows drop concentration and size decreased with increasing distance from the interface. There were only small differences in size between oil drops in the lower water continuous layer of the flow and water drops in the upper oil continuous layer. Mixture velocity did not affect significantly the drop size of either phase since higher velocities that would result in smaller drops were accompanied by increased entrainment of one phase dispersed into the other that favoured larger drops. The Rosin-Rammler function was found to fit satisfactorily the experimental drop size distributions, while literature correlations on entrained and maximum drop sizes in a turbulent field underpredicted the values found experimentally.     

循环流化床液液雾化过程转型的实验研究

建立了循环流化床液液雾化的实验装置,对水在非相溶介质中喷射雾化进行了研究。采用高分辨率数码摄像仪实时采集不同工况条件下的雾化形态,对比分析了雾化过程转型现象;通过图像处理与数值计算相结合的方法,对实验结果进行了分析。结果表明,雾化过程与相间相对雷诺数密切相关,在相对雷诺数为1 632时,由鼓泡型转变为连续射流型,而当相对雷诺数增加到1.14×105时,雾化则由连续射流型转变为紊态射流型;在雷诺数大于1.62×105后,液滴粒径急剧减小。在紊态射流形态中,雾化液滴数多且粒径微小,符合液液循环流化床制取流体冰技术对液滴尺度的要求。     

Effective interfacial area in agitated liquid-liquid continuous reactors

A chemical method has been used to quantify the effective interfacial area in a baffled continuous stirred liquid-liquid reactor. Two and four straight paddle impellers were used in the experimental runs, at 34°C, with hold-up fractions of dispersed organic phase between 0.061 and 0.166 and stirring speed ranging from 360 to . Influence of the residence time on the formation of the interfacial area generated in this system was not registered; however, differences were reported between continuous and batch mode operations.The interfacial area was correlated to hold-up fraction and Weber number by a new empirical model proposed in this work. This model allows to use only one equation to calculate the interfacial area in this continuous stirred reactor in the wide range of operating conditions tested (490<We<9600), which include different flow regimes. This is a relevant contribution as previous studies in this field only contemplate turbulent flow. In the transitional regime the mean drop size diameter decreases abruptly with Weber number, but this pattern changes in the higher range of Weber where the dispersed drops become smaller very smoothly. This pattern does not depend on the agitator used or hold-up fraction. The mean drop size diameter is smaller for the four paddle impeller and increases with hold-up fraction. The model developed may be applicable to dispersions in aromatic nitration reactors, improving its operation and design.     

Transition between stratified and non-stratified horizontal oil-water flows. Part II: Mechanism of drop formation

The conditions and mechanism of drop formation at the interface of oil-water wavy stratified flows that lead to the onset of drop entrainment and the transition to dual continuous flow pattern were investigated both experimentally and theoretically. Experimentally, high-speed video imaging was used to capture the mechanism of drop detachment from waves during oil and water stratified flow in a diameter horizontal acrylic pipe. The visual observations revealed that the faster phase undercuts the other one while the waves present in both phases deform until drops are detached. The wave deformation was attributed to the drag force, that originates from the relative movement between the two phases, exceeding the stabilising surface tension force. Based on this force balance an equation was developed that related the wavelength to the amplitude that can lead to drop detachment. This drop entrainment equation and the wave stability equation suggested in part I of the paper [Al-Wahaibi, T., Angeli, P., 2007. Transition between stratified and non-stratified horizontal oil-water flows. Part I: Stability analysis. Chemical Engineering Science, in press, doi:10.1016/j.ces.2007.01.024 ], defined three regions in a wave amplitude versus length graph, namely the stable waves, the unstable waves and the drop entrainment region. The intersection of the lines produced by these two equations gives the critical minimum wave characteristics for drop formation. These agreed well with experimental data when a new correlation for the drag coefficient on the waves was used, suitable for liquid-liquid flows. Also the characteristics of waves that were experimentally found to form drops fell within the predicted entrainment region.     

Mixing of stabilised drops in suspension polymerisation

In suspension polymerisation it is sometimes necessary to add material to the reactor after the reaction has started. When that happens, the new material and existing drops can remain segregated for significant amounts of time. Hashim and Brooks (Chem. Eng. Sci. 57 (2002) 3703) showed that the viscosity of drops affects both their sizes and their rates of coalescence. In the work reported here, further clarification of the drop mixing process is achieved by using pairs of stabilised dispersions. Solutions of polystyrene (PS) in styrene formed the dispersed phase and the tracer die technique was used to determine the extent of drop mixing. Drop mixing rate increased when the polymer content of all the drops increased from 0 to but further increases in polymer content lead to a reduction in mixing rate. Drop viscosity affected the mixing rate both directly and indirectly, because viscosity affected drop size and that influenced the drop-mixing rate. With increased polymer content, the larger drops made little contribution to the mixing. Experimental results were compared with the prediction of a model developed previously by Alvarez et al. (Chem. Eng. Sci. 49(1) (1994) 99). The model is consistent with the initial coalescence rates that were deduced from the experimental measurements. Drops containing PS mixed more quickly with drops containing PS than with drops containing PS. In those cases, the initial drop size distribution was relatively wide but, gradually, became narrower (the larger drops disappeared). With no polymer in the dispersed phase, the mixing of pre-dispersed drops was slower than the mixing that occurred when a batch of non-dispersed material was added to a stabilised dispersion (i.e. batch mixing). But, with of polymer in the dispersed phase, the mixing rate of two stabilised dispersions was similar to the batch mixing rate (even though the added polymer solution was not initially protected by the stabiliser). Mixing of two stabilised dispersions, with drops containing different amounts of PS, indicated that drop viscosity influences the mixing rate more than the difference in drop sizes.     

Analysis of a new type of high pressure homogeniser. Part B. study of droplet break-up and recoalescence phenomena

Calculation of the flow pattern in a new small homogenising valve design (Stansted, U.K.), able to reach operating pressure as high as , was investigated in the first part of this paper using a Computational Fluid Dynamics method. Numerical simulation results are used in the present paper to better understand the emulsification process in the Stansted high-pressure homogeniser.Deformation of drops is supposed to occur in the intense elongational flow caused by the restriction between the piston and the seat of the valve. Deformation may be mainly followed by drop disruption in the narrow valve gap. Break-up probably also occurred in the highly turbulent region, located just at the exit of the gap, and underlined by the numerical investigation. Cavitation and the rate of recoalescence, first assumed from numerical results, are determined thank to experimental methods. Intensities of both phenomena strongly increase with homogenising pressure. Final droplet size of model oil-in-water emulsions is then the result of equilibrium between droplet break-up and recoalescence, which strongly depends on operating pressure.     

Capillary jet instability under the influence of gravity

The focus of this paper is on the effect of gravity stretching on disturbed capillary jet instability. Break-up and droplet formation under low flows are simulated using finite difference solution of a one-dimensional approximation of disturbed capillary jet instability chosen from the work by Eggers and Dupont (J. Fluid Mech. 155 (1994) 289). Experiments were conducted using water and aqueous glycerol solutions to compare with simulations. We use a gravity parameter, G, which quantifies gravity stretching by relating flow velocity, orifice size and acceleration and is the reciprocal of the Froude number. The optimum disturbance frequency was found to be inversely proportional to G. However, this relationship appears to be complex for the range of G's investigated. At low G, the relationship between and G appears to be linear but takes on a weakly decaying like trend as G increases. As flows are lowered, the satellite-free regime decreases, although experimental observation found that merging of main and satellite drops sometimes offset this effect to result in monodispersed droplet trains post break-up. Viscosity did not significantly affect the relationship between the disturbance frequency and G, although satellite drops could be seen more clearly close to the upper limit for instability at high G's. It is possible to define regimes of satellite formation under low flows by considering local wavenumbers at the point of instability.     

液液雾化特性与粒径分布规律

液液雾化过程是液液循环流化床的关键技术之一,在流化床常态实验装置上,采用快速摄像与图像处理相结合的方法,获得了实验流量范围内液滴形成过程的图像以及形成液滴的粒径信息,运用数学分布函数对液滴的粒径分布进行了研究。研究结果表明,各流量工况下,液滴的粒径分布与Rosin-Rammler分布符合得较好;在实验流量范围内,形成液滴的区域有单液滴形成区域、过渡区域和多液滴形成区域,且液滴中位径的总体趋势是减小的,当水的流量为50 ml·min-1时,形成液滴的粒径主要集中在0.7～1.0 mm之间;在单液滴形成区域和多液滴形成区域,液滴粒径的均匀性先减小后增大,中位径呈减小趋势,但在多液滴形成区域两者的变化幅度较小,在过渡区域,液滴粒径的均匀性与中位径基本不变。研究结果可为液液循环流化床基于设计粒径的要求合理选取运行流量提供可靠依据。     

Tomographic analysis of dry and semi-wet bed fluidization: the effect of small liquid loading and particle size on the bubbling behaviour

The hydrodynamic changes resulting from the addition of very small quantities of a non-volatile liquid into a cold conventional fluidized bed has been investigated, and compared with the effects of increasing the particle size in a dry bed. Three different particle mixtures belonging to Group A/B, Group B and Group B/D were assessed. The changes in regime transition velocities, pressure drop, bubble rise velocity, bubble frequency and bubble flow rate have been quantified by employing Electrical Capacitance Tomography measurements. A new analysis method for measuring the effective interparticle forces (F_ip) and the effective drag force (F_d) in a dry fluidized bed is described, and the results are presented in terms of different force ratios including the single particle weight (W). It is shown that the addition of a few drops of liquid to a dry bed of Group B or B/D introduces similar hydrodynamic changes (except, in terms of bubble frequency) as that of increasing the bed particle size, and these particular changes shift the powders away from Group A/B behaviour. It is also illustrated that for beds of different particle sizes, a typical bubbling behaviour can be achieved at a specific gas velocity, this velocity coincides with the point of equality in hydrodynamic force ratios and E_ip/F_d.     

Modelling of high viscosity oil drop breakage process in intermittent turbulence

A multifractal model of the fine-scale structure of turbulence is applied to describe breakage of viscous drops of immiscible liquid immersed in a fully developed turbulent flow. A population of drops whose diameter falls within the inertial subrange of turbulence is considered here. The population balance equation is used to predict the drop size distributions. Calculations are performed for binary and multiple breakage. Several daughter distribution functions are applied and the results of their application are compared with experimental data. Experimental investigations of drop breakup were carried out in a flat bottom stirred tank having the diameter of and equipped with Rushton type agitator and four baffles. Silicone oils with viscosity of 10, 100, 500 and 1000 m Pa s were dispersed in the aqueous continuous phase. Measurements were performed using high resolution digital camera. Experimental results as well as numerical simulations show that after the initial period of multiple breakage, the strongly asymmetric type of binary breakage dominates.     

Transformation of single drop breakup from binary to ternary and multiple in turbulent jet flows

By releasing liquid drops in turbulent jet flows,we investigated the transformation of single drop breakup from binary to ternary and multiple.Silicone oil and deionized water were the dispersed phase and con-tinuous phase,respectively.The probability of binary,ternary,and multiple breakup of oil drops in jet flows is a function of the jet Reynolds number.To address the underlying mechanisms of this transfor-mation of drop breakup,we performed two-dimensional particle image velocimetry(PIV)experiments of single-phase jet flows.With the combination of drop breakup phenomenon and two-dimensional PIV results in a single-phase flow field,these transformation conditions can be estimated:the capillary number ranges from 0.17 to 0.27,and the Weber number ranges from 55 to 111.     

Crystallization of alpha-lactose monohydrate in a drop-based microfluidic crystallizer

A microfluidic process for producing crystals of controlled size by confining the crystallization within drops is demonstrated. The process consists of a drop producing stage which segments the mother liquor into monodisperse drops followed by crystallization in a temperature controlled tubular crystallizer. The process is implemented to produce lactose crystals with significantly narrower crystal size distribution (CSD) as compared with crystals produced in a stirred bulk crystallizer. By controlling the drop size and initial supersaturation the mean crystal size can be controlled. The distribution of the number of crystals per drop and the CSD are measured as a function of supersaturation at temperatures between 20 and for 150 and drops. In the case where drops contain only one crystal, a very narrow CSD is obtained with a coefficient of variation of crystal size as low as 7%. The crystallization of lactose in a microfluidic tubular crystallizer is modeled by treating nucleation in drops as a Poisson process with a nucleation rate based on classical nucleation theory. Experimental results are in good agreement with predictions from a Poisson process model over the range of temperatures, supersaturations and drop sizes tested.     

Particle growth kinetics of calcium fluoride in a fluidized bed reactor

Crystallization process in a fluidized bed reactor to remove fluoride from industrial wastewaters has been studied as a suitable alternative to the chemical precipitation in order to decrease the sludge formation as well as to recover fluoride as synthetic calcium fluoride.In the modeling, design and control of a fluidized bed reactor for water treatment it is necessary to study the particle growth kinetics. Removal of fluoride by crystallization process in a fluidized bed reactor using granular calcite as seed material has been carried out in a laboratory-scale fluidized bed reactor in order to study the particle growth kinetics for modeling, design, control and operation purposes.The main variables have been studied, including superficial velocity (SV, ), particle size of the seed material (L₀, m) and supersaturation (S). It has been developed a growth model based on the aggregation and molecular growth mechanisms. The kinetic model and parameters given by the equation fits well the experimental data for the studied range of variables.     

The voidage function and effective drag force for fluidized beds

Here, an experimental investigation on the effective drag force in a conventional fluidized bed is presented. Two beds of different particle size distribution belonging to group B and group B/D powders were fluidized in air in a diameter column. The drag force on a particle has been calculated based on the measurement of particle velocity and concentration during pulse gas tests, using twin-plane electrical capacitance tomography. The validity of the voidage function “correction function”, (1−ε_s)ⁿ, for the reliable estimation of the effective drag force has been investigated. The parameter n shows substantial dependence on the relative particle Reynolds number , and the spatial variation of the effective static and hydrodynamic forces. It is also illustrated that, a simple correlation for the effective drag coefficient as function of the particle Reynolds number (Re_p), expressed implicitly in terms of the interstitial gas velocity, can serve in estimating the effective drag force in a real fluidization process. Analysis shows that, the calculated drag force is comparable to the particle weight, which enables a better understanding of the particle dynamics, and the degree of spatial segregation in a multi-sized particle bed mixture. The analogy presented in this paper could be extended to obtain a generalized correlation for the effective drag coefficient in a fluidized bed in terms of Re_p and the particle physical properties.     

Three-dimensional vortex simulation of particle-laden air jet

The effect of particle diameter on the gas-particle two-phase compound round jet is numerically analyzed by the three-dimensional vortex method presented in a prior study. The air jet issues from a round nozzle into the co-flowing air stream, where the Reynolds number based on the air velocity at the nozzle exit is 2×10⁴ and the velocity ratio between the co-flowing stream and the jet at the nozzle exit is 0.27. The flow direction is vertical downward. Spherical glass particles having diameters 60, 80 and are loaded from the nozzle. The mass loading ratio is 0.27. The analysis made clear the air turbulent modulations due to the particles, such as the relaxation of velocity decay, the increment and decrement of momentum diffusion at the developing and developed regions, respectively. It also clarified that the air turbulent modulations become markedly as the particle diameter decreases.     

EXPERIMENTAL AND NUMERICAL STUDY ON GAS FLOW IN THE GRID ZONE OF JETTING-FLUIDIZED BEDS

A grid model describing the gas flow and interchange in the grid zone of jetting fluidized beds is proposed. Based on this model, longitudinal gas concentration profiles in the jet and annulus are calculated. The longitudinal gas concentration distribution is also experimentally investigated in a jetting fluidized bed with an inside diameter of 50 mm at the ambient temperature, and a jetting fluidized bed with an inside diameter of 80 mm at high temperatures. Comparison between the calculated and experimental results has shown that the experimental profiles can be qualitatively predicted by the grid model. The results indicated that the concentration in the grid zone depends on the gas exchange between the jet and the annulus, and the net gas flow from the jet to the annulus. The gas exchange rate is mainly affected by the inlet gas velocity from the nozzle. The present study is thought to be helpful to understand the grid gas behavior in the jetting fluidized bed coal gasifier.     

The influence of moisture on the fluidization behaviour of porous pharmaceutical granule

The influence of moisture content on the fluidization behaviour of placebo pharmaceutical granule has been studied in a 14 cm diameter cylindrical fluidized bed column. The dry granule has a mean diameter of and exhibits a bimodal size distribution with modes of 169 and . Bed pressure drop profiles and tapped density measurements were generated for granule moisture contents between 5 and 30 wt%, which corresponds to typical final and initial moisture contents experienced during drying. At high moisture contents, the wet granule exhibits Geldart C type powder behaviour as channelling and defluidization exist. As the moisture content is reduced, the granule fluidity improves and demonstrates behaviour characteristic of Geldart B powders. The changing fluidization behaviour was quantified using parameters such as the full support velocity, full support bed voidage and Hausner ratio. These parameters were found to increase significantly above granule moisture contents of 10 wt%. The increase in the Hausner ratio suggests that the interparticle force load in the bed increases. This change in interparticle force load is responsible for the increase in the full support velocity and bed voidage.     

Fluidization behavior in a circulating slugging fluidized bed reactor. Part II: Plug characteristics

In the transporting square nosed slugging fluidization regime () a bed of polyethylene powder with a low density () and a large particle size distribution () was operated in two circulating fluidized bed systems (riser diameters 0.044 and 0.105 m). A relation was derived for the plug velocity as a function of the gas velocity, solids flux, riser diameter, particle size range and particle and powder properties. The influence of the plug length on the plug velocity, the raining rate of solids onto and from the plugs and the influence of the particle size range on the plug velocity is accounted for.