粘性体系中微观混合实验研究

体系粘度是影响微观混合质量的重要因素。文章在粘度为0.001—0.077 Pa.s的甘油-水溶液中采用碘化钾-碘酸钾平行竞争反应研究搅拌釜中粘性体系的微观混合状况,分析泛能式桨的混合能力。结果表明:随着体系粘度的增加,微观混合效果变差;搅拌转速增加,微观混合效果变好。粘性体系内微观混合质量受到“涡旋卷吸”与“分子扩散”的共同影响。低粘条件下,二叶平桨混合效果优于泛能式桨;粘度增大,单位体积功增大时泛能式桨微观混合效果优于二叶平桨。     

微通道反应器内黏性流体的微观混合

Micromixing efficiency of viscous media in Y-type micro-channel reactor was studied by using iodide-iodate test reaction as working system. Experiments were carried out in water-glycerin mixtures with 7 different viscosities. The experimental results showed that segregation index of the micro-channel reactor increases with the decrease of volumetric flow rate and the increase of solution viscosity. Based on the incorporation model, the micromixing time tm of the micro-channel reactor was estimated in the range of 10－4-10－3 s at different viscosities, which indicated that the micro-channel reactor possesses a much better micromixing performance compared to the stirred tank (tm 0.02-0.2 s).     

Two-colour laser induced fluorescence for the quantification of micro- and macromixing in stirred vessels

Laser induced fluorescence technique (LIF) enables the measurement of the progress in mixing inside the mixing vessel. This is done by injecting a mixture of an inert and a reacting fluorescent dye into the vessel. The inert dye serves as a tracer for the macromixing. The reacting dye is changing its fluorescent characteristics while undergoing a fast chemical reaction with the vessel content and therefore shows the micromixing indirectly. The concentration fields of the dyes are measured simultaneously in an arbitrary plane using the two-colour LIF-technique. Areas of micromixing are detected by calculating the local degree of deviation from the concentration fields. Low Reynolds number measurements with a Rushton turbine show better macro- and micromixing for a dye injection closer to the stirrer shaft compared to a position closer to the centre of the main vortex.     

Macro- and micromixing in a novel sonochemical reactor using high frequency ultrasound

This paper deals with the influence of ultrasound on macro- and micromixing in a new developed sonochemical reactor. Unprecedented piezoelectric transducer arrangement with a high frequency of 1.7 MHz has been used in this novel reactor. Macromixing quality has been investigated visually and the Dushman reaction (iodide-iodate) coupled with a neutralization reaction have been examined in order to characterize micromixing quality. In addition, the effect of liquid viscosity on the segregation index has been studied. The results show that this new developed reactor can establish reasonable macro- and micromixing inside the reactor. Moreover, the performance of this reactor has been compared with a stirred tank reactor equipped with a Rushton turbine impeller. It is found that with the same input electrical power, the obtained segregation index for stirred tank reactor is approximately 10% more than proposed new ultrasound reactor, which means the sonoreactor works more efficiently.     

Using microparticles to enhance micromixing in a high frequency continuous flow sonoreactor

In this paper, an experimental study was conducted to investigate the effect of high frequency ultrasound wave on micromixing efficiency in the presence of polymeric microparticles in a tubular sonoreactor. The size and volume fraction of polymeric microparticles were varied and the micromixing efficiency was studied by adopting the Dushman reaction coupled with a neutralization reaction. In addition, the effect of flow rate and liquid viscosity on the segregation index was studied. The experimental results showed that the movement and dispersion of the polymeric microparticles by ultrasound wave could improve the micromixing efficiency. The results showed that the size of the polymeric microparticles has great effect on mixing. Moreover, it was found that in the presence of these microparticles, segregation index decreases significantly and their effect reduces with increase in the solution viscosity. The presented results show that using high frequency ultrasound waves in the range of MHz and in the presence of microparticles can promise to reach an efficient micromixing in tubular sonoreactors.     

SECONDARY MIXING OF IMPINGING SHEETS

The increase in the overall micromixing rate of two impinging sheets that results from subsequent impingement of the mixed sheet into a barrier was studied using mixing-sensitive chemical reactions. This “secondary micromixing” of the reactants yielded overall micromixing times that were up to an order of magnitude lower than the micromixing times without subsequent impingement of the mixed sheet (primary micromixing). Employing the same diffusion model for secondary micromixing, used previously to explain primary micromixing, suggests that the liquid viscosity, the mixed sheet velocity, and the characteristic dimension of the barrier are important parameters in determining the secondary micromixing rate. In addition, the distance from the impingement zone, at which the barrier is placed, is a key parameter in determining the amount of enhancement in the overall micromixing rate. The greatest enhancement was achieved by placing the barrier at the end of the mixed sheet where it breaks up into droplets. At this point the energy release resulting from the impingement of the mixed sheet into the barrier yielded calculated striation thicknesses that were typically 3 to 4 times smaller than the calculated striation thicknesses resulting from the energy release due only to the impingement of the single sheets.     

Viscometric behaviour of hydrophobically modified poly(sodium acrylate)

Summary Hydrophobically associating poly(acrylic acid) was obtained by reaction of a small amount of octadecylamine on the carboxyl groups of the polymer. In pure water, this modified polymer exhibits a very high thickening power. By increasing the ionic strength of the aqueous solution a strong viscosity increase can be obtained instead of the decrease classically observed with polyelectrolytes. For instance, in 1% NaCl the modified poly(sodium acrylate) is of several orders of magnitude more viscous than the precursor polymer.     

Mixing effects in the bromination of resorcin

The product distribution from the bromination of resorcin (m-dihydroxybenzene) is influenced by mixing in an experimentally convenient range of concentration and mixing intensity at room temperature. The degree of bromination is insensitive to mixing, a fact which can be explained chemically. Similarly explicable is the high sensitivity of the composition of the isomeric dibromoresorcins; the % 2,4-dibromoresorcin formed has been used to characterise mixing in semi-continuous and continuous stirred tank reactor operation. In the latter case the influences of the concentration and flow rate of the feed, turbine speed and feed positions near the turbine were investigated. Although full reaction kinetics are not available, these reactions are fast and sensitive enough to study micromixing.     

Structure and Property of Cationic Hydroxyethyl Cellulose

Through the macromolecule reaction, hydroxyethyl cellulose (HEC) was modified by quaternary ammonium monomer (GTAC) and the cationic HEC(C-HEC) with optimized viscosity was synthesized successfully. The structure of C-HEC was characterized by FTIR and Kjeldahl nitrogen content measurement. The introduction of cationic group onto the molecules of HEC resulted in the increase of the hydrophilicity of HEC. The dependence of the aqueous solution properties of C-HEC on the polymer concentration, GTAC dosage, additional electrolyte, temperature and shear rate were studied comprehensively.     

Measurement of the local intensities of segregation with the tomographical dual wavelength photometry

The newly developed tomographical dual wavelength photometry enables the measurement of the local intensity of segregation at a multitude of points inside the stirred vessel. This is done by injecting a mixture of an inert and a reacting dye into the vessel. The inert dye serves as a tracer for the macromixing, whereas the vanishing of the reacting dye shows the micromixing. The concentration fields of the dyes are measured simultaneously by transluminating the vessel from three directions with superimposed laser beams of different wavelength. The light absorption by the dyes is measured with CCD-cameras and these projections are used for the tomographic reconstruction of the concentration fields. Low Reynolds number measurements with a Rushton turbine show better macro- and micromixing for a dye injection closer to the stirrer shaft compared to a position closer to the main vortex.     

Spreading of Polydimethylsiloxane Drops on Solid Horizontal Surfaces

Effect of the volume of drops, surface energy and roughness of substrate together with temperature and viscosity on the spreading velocity of polydimethylsiloxane (PDMS) drops on solid horizontal surfaces was studied. Spreading velocity was shown to grow with decreasing drop volume, the effect being more pronounced at high viscosities of polymer. The deviation of shape of the spreading drop from that of a spherical segment is more pronounced the higher the surface energy of substrate, the higher the polymer viscosity and the smaller the drop volume. Spreading on a rough surface is slower than on a smooth one owing to the energy barrier created by surface inhomogeneities: the barrier is to be overcome by the spreading liquid. Based on the experimental results a mechanism of spreading of polymer drops is proposed. Changes in potential energy of a drop and in the free surface energy of the system during spreading were compared, allowing a theoretical evaluation of the influence of gravity on the spreading velocity of drops. A theoretical analysis of spreading kinetics of viscous drops is given. The equation proposed agrees well with the experimental results at 90° > θ > 0°.     

高黏微观混合表征新方法及其在双螺杆挤出机中的应用

在双螺杆反应挤出过程中微观混合程度是重要的操作参数。一些特别设计的经典小分子竞争反应体系常用于表征微观混合,但其用于大分子反应的高黏物系时由于大小分子扩散不同容易引起偏差。今建立了一种由苯乙烯-(3-异丙烯基-α,α′-二甲基苄基-异氰酸酯)共聚物(PSt-co-TMI)与9-(甲胺基-甲基)蒽(MAMA)为快反应、PSt-co-TMI与己内酰胺(ε-CL)为慢反应的大分子竞争反应体系,并应用于TSE-20可开启式自啮合双螺杆挤出机中微观混合状况的表征。考察了不同螺杆位置处产物的分离指数和转化率;研究了螺杆转速、喂料速率等对微观混合质量的影响。结果表明:该大分子竞争体系可很好地用于双螺杆挤出机中高黏物系微观混合表征;微观混合在物料熔融后相当短的流程内即完成,微观混合质量在挤出过程稳定后保持不变;对于推进式元件,在相同的喂料速率下随着螺杆转速的增加微观混合质量下降;填充度是影响推进元件中微观混合质量的重要因素,在特定的实验条件随喂料量增大微观混合质量反而改善的原因在于填充度的提高有利于微观混合。     

Makro- und Mikromischen im Rührkessel

Macro- and micromixing in stirred tanks. Macromixing time, based on turbulence theory, is calculated theoretically as a function of the specific power input, the kinematic viscosity, the Schmidt number, and the degree of segregation. A comparison of the micromixing time with data for macromixing found in literature reveals that the micromixing time governs the reaction for very small vessel diameters. If the vessel diameter is greater than the critical size than the turbulent macromixing time controls the mixing process. The micromixing time θ_mikro is calculated by means of the experimental results of the turbulent velocity distributions in different stirred vessels for several geometrical arrangements of stirred tanks and different positions of feed input. The calculated data for turbulent mixing times are only valid for geometrically optimized stirred vessels. The necessary mixing time may increase or, if a chemical reaction of 2nd order takes place, the desired yield of the product may decrease considerably if the geometrical conditions are not taken into account.     

The influence of viscosity on micromixing in turbulent flows

Viscosity has a negligible effect, if any, on macromixing parameters (e.g. velocity distribution and blending time when turbulent flow is fully developed). It does, however, influence micromixing parameters (e.g. Batchelor concentration microscale (νD²/ε)^1/4 and time constant for decay of fine-scale concentration fluctuations). The product distribution of two rapid, competitive, consecutive reactions (diazo coupling between 1-naphthol and diazotized sulphanilic acid) is sensitive to reagent concentration gradients on the molecular scale. It is shown that if all the independent micromixing parameters are kept constant, except the viscosity, the product distribution changes. The viscosity was varied by dissolving less than 0.5wt.% carboxymethyl cellulose (Hercules, type 7MF) in the aqueous reagent solutions. The viscosity then depends upon many factors (CMC concentration, temperature, shear rate, pH, chemical composition of the solution and mixing sequence during make-up of the solution), and CMC is not an ideal additive. Nothing better seems to be available. At least the spectrophotometric analytical method and the rate constants are unaffected by low CMC concentrations (0.5 wt.%). Three reactors (rotor---stator high intensity mixer, flow in a pipe, stirred tank) were operated in the turbulent flow regime. Increasing the viscosity caused more secondary product to be formed. This effect was described quantitatively by our earlier micromixing model.     

粘性体系微观混合的研究进展

概述了粘性体系微观混和理论模型的发展历程,指出用机理模型描述微观混合是未来研究的方向.同时介绍了国内外研究微观混合所采用的主要化学实验方法,包括常用的化学反应体系及所得到的相关结论,且指出离集指数Xs和微观混合速率α是最常用的微观混合性能表征参数,最后对微观混合研究进行了总结与展望.     

Experimental investigation on micromixing characteristics of coaxial mixers in viscous system

An experimental investigation into the micromixing performance of coaxial mixers in a viscous system is reported, in which the iodide-iodate reaction system is chosen to quantitatively characterize the product distributions. The effects of feeding time, feeding position, impeller speed, inner impeller configuration, and operation mode on the segregation index, Xs, are examined. It is revealed that the feeding near the inner impeller benefits micromixing and should be regarded as the preferred position. The presence of the rotating outer impeller causes the micromixing performance of the coaxial mixer to be significantly better than the single-shaft mixer. While an increase in the outer impeller speed has a limited influence on micromixing, the inner impeller speed is the dominant influencing factor, that is, the Xs decreases obviously when the inner impeller speed is increased. On the other hand, the coaxial mixers with multiple and axial inner impellers have a better micromixing performance at the same specific power consumption, P_V, than that with single and radial inner impellers. Among the configurations consisting of a Rushton impeller (RT), six-straight-blade turbine impeller (SBT), and six-pitched (45°)-blade turbine impeller (PBT), the Xs of the coaxial mixer is always the smallest at the same P_V when the PBT + RT configuration is used as the inner impeller. In addition, it is found that the difference in Xs that results from various operation modes is small in terms of power consumption; however, the co-rotation mode is still recommended for the micromixing of the coaxial mixer due to its excellent performance in general.     

Mixing and fast chemical reaction—III: Model-experiment comparisons

According to the diffusion-reaction model developed in Part II, the product distribution from two competitive, consecutive reactions will be a function of the stoichiometric and volumetric ratios of the reagents, the ratio of the rate constants, a time parameter and a Thiele-like modulus. The time parameter is large for fast reactions, signifying that yield and conversion are independent of it (i.e. reaction then goes to completion, whereby the limiting reagent is used up). The diazo coupling reactions of Part I have sufficiently well-defined properties that the models of Part II and [2] for CSTR and batch reactor respectively can be evaluated.In a first model-experiment comparison the Thiele modulus was changed by varying the viscosity and the turbine speed of a CSTR. Despite significant scatter the agreement was good; the size of the reaction zone was about half the Kolmogoroff turbulence microscale. In a second comparison using two geometrically similar reactors the principal variables were mode of operation (semi-continuous and continuous), turbine speed, feed point (3 locations) and scale (0.0025 and 0.063 m³) . The sizes of the reaction zone ranged from 10 to 40% of the Kolmogoroff scale, the ranking being consistent with known flow patterns. Laminar shear of the reaction zone during reaction might explain this result. Various scale-up rules were evaluated. The unexpected and potentially dangerous phenomenon of back-mixing into the feed pipe, with consequent pre-reaction, was observed and studied empirically.     

Mixing and Crystallization Kinetics in Gas‐liquid Reactive Crystallization

A study of gas‐liquid reactive crystallization for CO₂‐BaCl₂‐H₂O system was performed in a continuous flow crystallizer. The influences of mixing on the crystallization kinetics of barium carbonate crystals were investigated. The mixing parameters are stirrer speed, feed concentration, gas‐flow rate, pH of solution, addition rate of NaOH solution, and mean residence time. Under pH‐stat operation, the crystallization mechanism can be assessed by the addition rate of NaOH solution, which acts as an indicator for the absorption rate of carbon dioxide. Assuming a size‐independent agglomeration mechanism, the nucleation rate, growth rate and agglomeration kernel can be obtained, simultaneously, at steady state, by the method of moments. Evidence shows that feed concentration, feed rate, gas‐flow rate, and stirrer speed have a significant influence on the nucleation rates and mean particle sizes. This shows the effect of micromixing. The crystallization mechanism tends to be reaction limited when the feed concentration of barium chloride solution is higher than 5 mM, while at lower stirrer speeds and feed concentrations, the mechanism tends to be both mixing and reaction controlled. The growth rate depends on the mean supersaturation value and the pH of the solution and the mass‐transfer resistance cannot be completely eliminated in this work. For a monodispersal collision model, in the viscous sub‐range of turbulence, the agglomeration kernel can be expressed as β ∝ d^{3 –1/4}, showing a low efficiency of collision. The result is also demonstrated by the agglomeration kernel expression. Comparison with a liquid‐liquid‐mixing reactive crystallization system is also discussed.     

Micromixing of Solid‐liquid Systems in a Stirred Tank with Double Impellers

The effect of solid particles on micromixing has been studied using the competitive iodide/iodate reaction system in stirred, multi‐impeller, solid‐liquid systems. The influences of particle size, impeller speed, solid holdup, feed position, and energy input have been investigated. The change of the segregation index with the power input was more distinguishable only for the 450–600 μm particles as compared with the large ones, at the same solid holdups. Also, for the small ones, cloud formation was observed at a particle concentration of 12.1 wt %. However, the influence of larger particles of 1–1.25 mm on micromixing was negligible, though both energy input and solid loading were increased. Besides, the optimal feed position was identified, and multiple feeds were also explored.     

Micromixing effects on complex reactions in a CSTR

Micromixing effects on consecutive-competing second order reactions in an ideally macromixed CSTR are investigated by the use of the Two Environment Model of Ng and Rippin for mixed feed, and the model of Spielman and Levenspiel for unmixed feed.General plots are obtained for various values of reaction and mixing parameters. Yield of the desired product is found to show a maximum at intermediate states of micromixing for unmixed feed conditions. For mixed feed condition, micromixing effects are found to be very small.