Gas absorption at a liquid surface agitated by vortex rings

Underwater vortex rings have been formed at orifices of between 25.4 and 63.5 mm diameter and projected upwards at the surface of the water. The conditions of ring formation at which significant surface deformation occurs have been correlated. The absorption of pure carbon dioxide by the water surface with impinging vortex rings has been measured and related to the following variables: time of fluid impulse through the orifice, orifice diameter, volume of displaced liquid, and depth of submergence. A simplified mass transfer model based on the penetration theory approximately correlates the data.     

Velocity and momentum of vortex rings in relation to formation parameters

The formation of ring vortices under water has been studied at orifices of diameter 2.54 to 6.35 cm and formation times from 0.04 to 0.68 s, with orifice Reynolds number between 3 × 10⁴ and 3 × 10⁵. Ciné photographs of the formation process were taken, and the initial velocity of the vortices was found in general to be one half of the velocity of flow through the orifice. The translational momentum of the vortex rings was measured by a surge-tube method. It was concluded that the formation process is most efficient when the length of the impulsive jet is not more than two orifice diameters.     

Hydrodynamic Characterization of SPLITT Fractionation Cells

Abstract

The efficacy of SPLITT fractionation requires an absence of hydrodynamic mixing between laminae constituting the thin liquid film streaming through a SPLITT cell and it requires structural elements capable of splitting the film evenly along streamplanes. These requirements are examined here by both experimental tests and by a numerical analysis of flow properties near the inlet splitter. The experimental tests, involving dye injection and the injection of pulses of latex particles that may or may not be driven across flow laminae by gravity, show that SPLITT cell performance is close to that of ideal theory at low Reynolds numbers. The computer results verify an absence of mixing under these conditions, but when the Reynolds number and inlet flow asymmetry are both high, vortex motion is found near the inlet splitter edge, suggestive of mixing. The conditions leading to vortex formation are defined. It is shown that tapering the splitter edge suppresses vortex formation.     

Ternary nylon-6/rubber/modified rubber blends: Effect of the mixing procedure on morphology, mechanical and impact properties

Ternary polyamide-based blends have been prepared by adding to nylon-6 (PA6) an ethylene-propylene random copolymer (EPM) and the same EPM functionalized by inserting onto its backbone maleic anhydride groups (EPM-g-SA). Two kinds of processing have been used: (a) one-step mixing in which the three components were simultaneously introduced in the mixer; (b) two-step mixing in which the two rubbers EPM and EPM-g-SA were separately premixed before the final mixing with PA6. Also binary PA6/EPM-g-SA blends have been prepared to compare their properties with those of the ternary one.

Mechanical tensile characterization at room temperature and impact Izod tests at different temperatures as well as a morphological analysis of smoothed samples have been performed on all the blends. It has been shown by a model reaction that both in binary and ternary blends an EPM-g-PA6 graft copolymer is formed, which acts as an interfacial agent between the rubbery dispersed phase and the polyamide matrix. The blends obtained by the one-step mixing showed a gross morphology and a very poor impact resistance, whereas the ones prepared by the two-step mixing exhibited very fine morphologies and excellent impact performances. In addition, as shown at least in the case of one ternary blend, there seems to be good morphological stability of these materials after a second processing. This has been attributed to the influence of the interfacial agent formed during the melt mixing of the two premixed rubbers with PA6.     

Evaluation of a novel self-aerating,oscillating baffle column

We describe the operation of a pilot scale oscillating baffle column using a self-aeration system for oxygenation of water. The top baffle has a high constriction ratio and is sufficiently near to the surface of the water such that gas bubbles are generated. This aeration plate is coupled with a series of equally spaced low constriction orifice baffles, which lead to uniform dispersion of entrained air bubbles throughout the liquid volume. Flow visualisation experiments using video and still photography were used to identify two mechanisms for bubble generation: bubble formation under the water surface by surface vortex suction, and bubble generation from the collapse of a surface fountain and subsequent entrainment of bubbles into the bulk. Mass transfer measurements have shown that under most conditions a uniform oxygen concentration could be obtained throughout the column as a result of efficient mixing, and that the sole limitation to mass transfer performance was determined by the aeration mechanism. Initial comparison on the basis of aeration efficiency with other devices reveals a modest oxygen transfer rate, but with low specific power consumption of order 0.3 kW/m³.     

Efficiency of a vortex contact stage in thermal distillation

The results of studying the process of distillation in a column with 21 vortex contact stages have been reported. The partial condensation of an ascending ethanol–water mixture vapor at the contact stages has been shown to intensify the mixture distillation process by threefold compared to adiabatic distillation. The process parameters that intensify heat and mass transfer in thermal distillation have been established. Some relationships for estimating the efficiency of a vortex contact stage have been derived.     

FORMATION AND FLOW OF GAS BUBBLES IN A PRESSURIZED BUBBLE COLUMN WITH A SINGLE ORIFICE OR NOZZLE GAS DISTRIBUTOR

The characteristics of gas bubbles in a 5 cm diameter bubble column equipped with a single orifice of 1,3 or 5 mm diameter were investigated under system pressure of 0.1-15 MPa. The formation of gas bubbles was strongly affected by the system pressure. Under high pressures a dispersed gas jet was formed at gas velocities where spherical gas bubbles would have been formed at atmospheric pressure. The critical gas velocity between the bubbling regime and the jetting regime was correlated with the liquid phase Weber number and the gas phase Reynolds number based on the gas velocity at the orifice. Bubble size and gas holdup in the main part of the bubble column were also affected by the bubble formation pattern at the distributor     

The formation and growth of bubbles at a submerged orifice

A model has been developed to describe the formation of single bubbles at a submerged orifice. The model is based on a modified Rayleigh equation for bubble growth and describes the effect of gas momentum by assuming that the flow field inside the growing bubble is in the form of a circulating toroidal vortex. The equations describing the bubbling system are solved numerically using an explicit finite-difference technique. The model shows that bubble growth is characterised by an initial outward movement of the base of the bubble along the plate floor followed by an inward movement back towards the orifice which leads to a severing of the bubble from the orifice and termination of the growth cycle.Computed bubble growth rates, formation times and chamber pressure fluctuations are shown to be in good agreement with available experimental data for a wide range of system pressure (0–1.37 MN/m²) and computed bubble shapes are similar to those observed experimentally.     

Mixing strategy effect on dispersion of amine during reagent production for flotation applications

Mixing and dispersion strategies for the inline production of amine flotation reagents is explored for 2.54 cm and 5.08 cm process lines and flow rates comparable to current commercial production systems. Through video analysis of injection into a clear pipe, the dispersion effectiveness was visualized and quantified as a variability intensity, and compared for natural‐stream turbulence, orifice‐plate and structured static mixing elements. The results suggest that a single point of energy dissipation was more effective in dispersing the injected amine, with the orifice plates consistently yielding fully‐dispersed reagent solutions. The results suggest that while a structured mixing device with 6 elements did improve dispersion relative to an empty pipe, more mixing elements or a smaller characteristic length (i.e. 2.54 cm mixer) would be better suited to this specific application.     

Studies of gas–liquid flow through reactors internals using VOF simulations

Two-phase flow through reactor internals have been experimentally and numerically studied. Experiments have been carried out with a setup running under ambient pressure for two configurations. The first configuration consists of a mixing box orifice inlet through which liquid flows as a film sheared by a gas flow. The liquid height at orifice inlet is documented over a wide range of liquid and gas flowrates. The second configuration consists of the two-phase flow through a downcomer of a distributing tray. Two and three dimensional computational fluid dynamic (CFD) simulations using the volume of fluid approach have been used to compute both flows for similar flow conditions as used in the experiments. It is shown that the agreement between experiments and calculations is very good. Based on this good agreement, it is finally discussed how CFD can be used to achieve better design rules for gas liquid reactor internals via simulations carried out for industrial process conditions.     

Natural convective heat transfer from open vertical tubes

Natural convection heat transfer in vertical tubes is described by a one-dimensional dispersion model similar to that used previously for fluid mixing in the presence of an unstable density gradient. Temperature profiles have been measured in air in 2 m high insulated vertical tubes (21.3, 24.5, 27.7 cm internal diameter), closed at the bottom and open at the top, and heated at the bottom with a known heat flux. The effective mixing lengths for eddy heat transport are of the same order as the tube diameters.     

Hydrodynamics and liquid phase axial mixing in orifice pipe reactor

Two phase flow in a horizontal pipe, with orifice plates placed at regular intervals as obstructions, was studied for the effect of phase velocities on flow patterns, fractional phase hold-ups, pressure drop and liquid phase axial dispersion. Radioactive technetium-99m (as an aqueous solution of sodium pertechnatate) was used as tracer. A pulse injection technique with two point measurements was employed. Three different orifice diameters were used (8 mm, 16 mm, and 20 mm) in a pipe diameter of 32 mm. The orifice spacing was 500 mm in all cases. Superficial gas (air) velocity was varied over a range from 0.02 m/s to 1.0 m/s and superficial liquid (water) velocity from 0.03 m/s to 0.85 m/s. Different flow patterns under different flow conditions were identified and a generalised flow map is presented. Variations in hold-ups and pressure drop with flow patterns have been explained. Rational correlations have been developed for fractional phase hold-ups and pressure drop. A preliminary comparison of two phase gas-liquid flow in a horizontal pipe with orifice obstructions (to be called orifice pipe reactor), as a gas-liquid contacting device, is made with a conventional bubble column reactor. Recommendations have been made for future work.     

稀释水掺混用薄板式静态混合器结构设计与优化

稀释水掺混是油田或炼油厂原油电脱盐系统的关键环节,直接影响电脱盐的效率和运行能耗。为进一步简化设备结构、助力系统降本增效,提出将薄板式静态混合器应用于稀释水掺混环节。在对薄板式静态混合器进行初步结构设计的基础上,对油水混合情况进行计算流体动力学(CFD)三维数值模拟,以离析强度的平方根(IOS0.5)、管路压降(Δp)等作为评价指标,考察了注水管长度、注水管?弯曲薄板间距、弯曲薄板导向位置、弯曲薄板厚度四个结构参数对油水两相混合程度及运行能耗的影响。以混合管路内径D为基准参照,借助响应曲面法(RSM)对关键结构参数进行优化,并对最优结构参数组合下的混合性能进行预测。优化所得最优结构参数组合为注水管长度为1/3D、注水管-弯曲薄板间距为4/25D、弯曲薄板导向位置为1/8π、弯曲薄板厚度为1/25D,结构优化后的IOS0.5相比优化前降低了43.06%。定性分析薄板式混合器内的水相速度云图和流线图可知,当注水比为2%时,油水两相在注水管下游3D处即可达到均匀混合状态,可见薄板式静态混合器能够在小注水比下快速实现油水均匀混合。     

Axial mixing and scaleup of reciprocating plate columns

Axial mixing measurements in single phase (water) flow have been taken in open-type reciprocating plate columns of diameters 25.4 and 508 mm. In the case of the smaller column, two-phase axial mixing was measured, both in the dispersed phase (water dispersed in n-heptane) and the continuous phase (with n-heptane dispersed in water). Pulse injection of a tracer solution of ammonium chloride and methanol in water was used. Under single phase conditions, the axial dispersion coefficients were found to go through a minimum as the agitation level was increased from zero. The coefficients were nearly an order of magnitude higher in the 508 mm column than in the 25,4 mm column. In two phase flow in the 25,4 mm column, the continuous phase axial dispersion coefficients also went through a minimum as agitation was increased. The dispersed phase axial dispersion coefficients decreased monotonically as agitation was increased from zero. The results of this work and previous data are used in modelling the scale-up of reciprocating plate columns by means of Pratt's simplified technique. The existing empirical scale-up equation is consistent with an assumption that continuous phase mixing increased with column diameter but dispersed phase mixing remains unchanged.     

Mixing in a Taylor-Couette reactor in the non-wavy flow regime

The mixing in a Taylor-Couette flow cell is quantified with laser induced fluorescence (LIF). Time-resolved, two-dimensional measurements of dye concentration have been obtained in the non-wavy Taylor vortex flow regime (Re=330) and analysed in order to characterise the intervortex and intravortex mixing. The results show clear evidence of intervortex mixing especially near the inner wall region and the inflow boundaries, and demonstrate that Taylor vortex flow cannot be simply assumed as a series of well mixed tanks. Intravortex mixing is slow in relation to the mixing between adjacent vortices and is more rapid in the azimuthal direction than the meridional plane. Increasing Re towards the upper limit of the Taylor vortex flow regime (Re=950) results in enhanced mixing despite the apparent absence of an azimuthal wave. Both the intervortex and intravortex mixing times reduce substantially and the intravortex mixing in the azimuthal and meridional planes occur at similar timescales.     

Circumferential mixing in one-phase and two-phase Taylor vortex flows

This work deals with the experimental study of the circumferential mixing in the Taylor vortex flow between coaxial cylinders. Each pair of vortices is considered as a closed chemical reactor and the residence time distribution in this reactor is determined by an electrochemical method. The tanks-in-series model with recirculation is applied to describe the flow in the vortex reactor; the circumferential mixing is characterized by the number of tanks in series and by the mixing time. Experiments were carried out with one-phase flow and with two-phase liquid—liquid flow. It is shown that stirring due to the liquid droplets increases the circumferential mixing.     

Vortex flow of dilute polymer solutions

It has been observed that very d longchain polymers which are effective in turbulent drag reduction inhibit the formation of a vortex or air core as water drains from a tank. This paper considers the fluid mechanical velocity profile measurements have been performed. There appear to be at least two distinct mechanisms for the vortex inhibition—one involving the viscosity enhancement caused by polymer addition, and the other related to the viscoelastic properties of the polymer solutions. This second mechanism is shown to arise due to the generation of high normal stresses as the air core begins to form. The very close correlation between vortex inhibition and turbulent drag reduction suggests that normal stresses may also play an important role in this latter phenomenon.     

Mixing and micromixing times in the forced vortex region of unbaffled mixing devices

Unbaffied mixing devices are characterized by a very particular hydrodynamics. A well defined depression in the liquid surface is formed, resulting from the combined vortex established in the liquid. The combined vortex is composed of one central Forced Vortex Region (FVR) and one annular free vortex region. Because the FVR behaves like a confined mixing zone, this region inside an unbaffled mixing device appears to be quite interesting to carry out certain chemical reactions. In this paper the influence of operating conditions on the mixing time of two reagent feed streams arriving continuously in the FVR of unbaffled mixing devices is investigated. Process parameters investigated are?: stirrer speed, reagent flow rates and feed pipe position. Mixing time correlations are established, which are useful for the scale‐up of this reactor.     

Effect of retempering with superplasticizer admixtures on slump loss and compressive strength of concrete subjected to prolonged mixing

To have concrete possesses the specified engineering properties, its slump loss has to be compensated somehow at construction site so that concrete could be placed and compacted properly. Several attempts have been tried so far to render concrete workable at construction site including starting with a high initial slump at the stationary plant, or retempering with water and/or with chemical admixtures at construction site. In this investigation, ASTM C 494 Type F superplasticizer was used for retempering concrete to restore its initial slump. Concrete mixes having an initial slump of about 19 cm were prepared and subjected to prolonged mixing with different mixing duration such as 30, 60, 90, 120, and 150 min following an initial mixing of 5 min to ensure homogeneity. At the end of each mixing period, cube specimens of 15 cm were cast from concrete retempered to its initial slump level and tested at the age of 28 days for compressive strength. Results revealed that compared to the concrete retempered with water, those retempered with a superplasticizer admixture have yielded significantly higher strength regardless of the mixing duration.     

Sol-gel alumina coating of wire mesh packing

Wire mesh packings have seen increasing applications to multiphase processes in recent years. Despite the high surface area, open structure and thermal and chemical resistance, wire mesh packings have a complex geometry which hinders some chemical applications, including changes of surface properties through application of a uniformly adhesive coating. In this work, the sol-gel deposition method of alumina coating ceramics was investigated for the first time on stainless steel wire mesh by using Dixon rings as example. The kinetics of deposition during the hydrolysis and polycondensation was followed for a range of initial composition of the coating such as the ratios of Al₂O₃ to water, acid content, polyethyleneimine binder content and the number of deposition cycles. Well-adhered alumina with a thickness up to 20 μm was successfully deposited. The molar ratios of acid to alumina and alumina to water of 0.25 and 0.01, respectively, 48 h of the aging time, 96 h of the mixing time and 2.1 g/L of polyethyleneimine binder formed a free of cracks coating of controlled thickness alumina on the Dixon rings.