Continuous polymerization of methyl methacrylate in a taylor‐couette reactor. I. Influence of fluid dynamics on monomer conversion

A Taylor‐Couette reactor offers certain advantages for continuous polymerization over other reactor types. These advantages are its rather narrow residence time distribution (series of vortices) and its good heat transfer characteristics. Hydrodynamics in this type of reactor can be controlled by its geometry (diameter, gap, width, length) and its operational parameters (rotational speed, mean residence time, viscosity). In this article, a model is presented which is suited to answer the question of how hydrodynamics influences the productivity of a continuously operated Taylor‐Couette polymerization reactor. To this end, productivity is quantified by the rate of monomer conversion. The model is specified and experimentally validated for the free radical polymerization of methyl methacrylate with the solvent xylene and 2,2‐azoisobutyronitrile as the initiator. The model considers the following four phenomena: (i) Macromixing between the vortex cells is accounted for by an axial dispersion model. (ii) The dependence of viscosity on monomer conversion along the reactor is described by a viscosity model. (iii) Polymerization kinetics and its dependence on hydrodynamics are correlated from experimental data. (iv) The dependence of segregation index I_s on the local energy dissipation is used to characterize micromixing within the vortices. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

泰勒反应器中流体的停留时间分布研究

采用脉冲示踪法,实验考察了泰勒反应器结构参数、操作参数和物料粘度对其停留时间分布的影响规律,发现泰勒反应器中物料的混合程度随泰勒数(Ta)的增大而增大、轴向雷诺数(Reax)的增大而减弱.进一步采用多级混合模型,将泰勒反应器的当量全混釜数(N)与反应器结构参数、泰勒数和轴向雷诺数相关联,提出了一个N的测算公式.在实验考察的范围(Ta=280～3 230; Reax=0.89～6.78)内,由该式计算的N值与实验值很好地相符,具有广泛的适用性.     

Effects of geometry and flow rate on secondary flow and the mixing process in static mixers—a numerical study

A method based on computational fluid dynamics (CFD) for the characterization of static mixers using the Z factor, helicity and the rate of striation thinning is presented. These measures were found to be well-suited for the characterization of static mixers as they reflect the pressure drop, the formation of secondary flow, i.e. vortices, and their effect on the mixing process. Two commercial static mixers, the Kenics KM and Lightnin Series 45, have been characterized. In the mixers investigated, secondary flow is formed in the flow at the element intersections and due to the curvature of the mixer elements. The intensity of the vortices is higher in the Lightnin than the Kenics mixer due to edges in the middle of the Lightnin mixer elements. The formation of vortices affects the Z factor by an increase in the power requirement, and the rate of striation thinning by an increase in the stretching of the striations. The formation of vortices was observed at a Reynolds number of 10 in both mixers with aspect ratios of 1.5. However, the intensity of the vortices was greater in the Lightnin than the Kenics mixer, which was observed in not only the magnitude of the helicity, but also the Z factor, rate of striation thinning and the distribution of striation thickness.The distribution in striation thickness is shifted towards thin striations as the flow rate is increased from below to above the Reynolds numbers of which vortices were first observed, but some striations still pass the mixer elements almost unaffected, which can be seen in the skewness of the distribution of the striation thickness, which shifts from being negative to positive.     

Using Tomography to Characterize the Mixing of Non‐Newtonian Fluids with a Maxblend Impeller

The flow field inside a cylindrical mixing vessel was visualized by electrical resistance tomography (ERT), a non‐intrusive measurement technique. Six tomography planes, each containing 16 sensing electrodes, measured the mixing time in the agitation of pseudoplastic fluid exhibiting yield stress. The effects of various parameters such as impeller types, impeller speed, fluid rheology, power consumption, Reynolds number, and absence of baffles on the mixing time were investigated. The Maxblend impeller was able to improve the mixing performance of non‐Newtonian fluids in a batch reactor. The mixing quality could be further enhanced by decreasing the xanthan gum concentration and using baffles in the mixing vessel.     

Transitional Mixing of Shear‐Thinning Fluids in Vessels with Multiple Impellers

The mixing efficiency of shear‐thinning fluids was evaluated using carboxymethylcellulose sodium salt (Na‐CMC) aqueous solutions of varying mass concentrations and three types of impellers (Rushton turbine (RT), six‐flat‐blade turbine (FBT), six‐pitched‐down‐blade turbine (PBT)) which were mounted on a common shaft in combinations of three, four, and five impellers. The mixing time proved to be dependent on the number of impellers as well as on the distance between. The Reynolds number has a significant influence on the mixing time for all studied systems. The results of power consumption allowed to choose the impeller system with the best efficiency.     

Rheological Characterization of Slurries for the Preparation of Alumina Foams via Replica Technique

Five commercially available thickeners were used to prepare thickened alumina slurries for foam production using the replica technique. The solid content and the type of thickener significantly affected the rheological properties, whereby an increase in the solid content caused both the viscosity and the thixotropy to increase. For the evaluation of the consistency index and of the level of shear‐thinning behavior, the power law fluid model (Ostwald de Waele) was used. The rheological properties play a key role during the coating process in the replica technique. However, relatively little information is available on the effect of slurry rheology on the properties of reticulated foam ceramics. In this study, the influence of viscosity, thixotropy, and the level of shear‐thinning behavior on the coating behavior is evaluated. The results indicate a strong influence of the viscosity and the level of shear‐thinning behavior on coating behavior.     

Flow dynamics in Taylor–Couette flow reactor with axial distribution of temperature

In this study, the flow dynamics of a Taylor–Couette flow with an axial distribution of temperature was experimentally investigated. The flow can be classified into three patterns based on the balance between the centrifugal force and the buoyancy. If the buoyancy is dominant, global heat convection is observed instead of Taylor vortices (Case I). When the buoyancy is comparable to the centrifugal force, the Taylor vortices and global heat convection appear alternately (Case II). If the centrifugal force is sufficiently high to suppress the buoyancy, stable Taylor vortices are observed (Case III). The characteristics of the mixing/diffusion are investigated by conducting a decolorization experiment on a passive tracer. In Case II, the tracer is rapidly decolorized in the presence of the global heat convection instead of the Taylor vortices. This result implies that the interaction between the centrifugal force and the buoyancy would induce an anomalous transport. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1075–1082, 2018     

Power Requirement for Mixing Shear‐Thinning Fluids with a Planetary Mixer

The optimal control of processes dealing with non‐Newtonian liquids requires the knowledge and control of the power demand of the mixing equipment. In this context, an extension of the Metzner and Otto concept to planetary mixers is proposed to adapt this concept to planetary mixers. The theoretical part of this work defines modified expressions of Reynolds and power numbers. These definitions introduce a characteristic velocity u_ch that is used to define the parameter K_s. A planetary mixer is employed to experimentally ascertain this guideline. Power consumption measurements carried out by mixing shear‐thinning fluids permit to determine the K_s factor. This factor varies only slightly with the flow behavior index and may be regarded as a defined constant for this geometry. Finally, experiments with an additional shear‐thickening fluid confirm the validity of this approach.     

Study on the Flow Field inside the Microfiltration Separator with Rotary Tubular Membrane

Abstract

By using a new type of laser surveying instrument named particle image velocimetry (PIV), the flow field inside a rotary tubular membrane separator with a rotating inner tubular microfiltration membrane and a transparent outer cylinder was measured experimentally, and from which some new observations were resulted. Pairs of stable Taylor vortices with similar dimensions and opposite flowing directions were directly visualized by the measured streamlines and vorticity of flow field in the annular gap of the membrane separator. No matter how the axial Reynolds number, radial Reynolds number and Taylor number changed, the dimensions of the Taylor vortices and the distances between the centers of adjacent Taylor vortices were almost the same, but the shapes of the Taylor vortices at lower Taylor numbers were more regular than those at higher Taylor numbers. The Taylor vortices disappeared because of the turbulence when the Taylor number was too high. The maximum axial velocity near the membrane surface was about 20 times larger than the mean velocity of axial flow inside the annular gap, and the maximum outward radial velocity near the membrane surface was even about 3000 times larger than the average velocity of the radial permeating flow through the tubular microfiltration membrane. The large velocities near the membrane surface, which were due to the Taylor vortices, could prevent solid fine particles from depositing onto the membrane surface and/or entering into the membrane pores and therefore result in reduced concentration polarization and reduced membrane fouling. The results in this study provided some valuable guidelines on the hydrodynamic way to reduce membrane fouling.     

Rheology of 1‐butyl‐3‐methylimidazolium chloride cellulose solutions. II. Solution character and preparation

The focus of this article of a three part series is the effects of preparation and composition on the shear rheology of cellulose in the ionic liquid 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl). Included are the effects of three different degrees of polymerization, (i.e., average molecular weight), manual versus high shear mixing, a range of cellulose concentrations, and the effects of controlled amounts of lignin and a hemicellulose. The rheology implies that a gel phase develops at higher degrees of polymerization, higher concentration, and at lower temperatures. The first article focused primarily on shear rheology of cellulose in [Bmim]Cl with a high shear preparation technique, one degree of polymerization, a narrow range of cellulose concentrations, and temperature. The third article focuses on elongational rheology of cellulose in [Bmim]Cl. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

撞击流反应器流场数值模拟及其混合性能优化

利用数值模拟方法分析多喷嘴对称撞击流反应器内部流场以优化反应器结构。研究不同喷嘴数和进料条件对撞击流反应器内速度场、湍流特性及混合效果的影响。结果表明,不同喷嘴数撞击流反应器内流速分布为双峰型,等流速工况下速度梯度随喷嘴数增加而减小,高剪切力分布范围随喷嘴数增加先增大后减小。通过分析湍流尺度分布发现小尺度涡旋主要集中在撞击区,而大尺度涡旋主要集中在发展区,且四喷嘴撞击流反应器平均剪切应力及涡旋尺寸梯度最大,四喷嘴结构更有利于增强流体湍动强度并强化混合。撞击流反应器内平均湍动能随喷嘴数增加先增大后减小,其中四喷嘴撞击流反应器内平均湍动能最大。当撞击流反应器为四喷嘴结构时,其混合效果最好,完全混合时间最短为22 s。在本研究工况内四喷嘴结构为撞击流反应器混合的最优结构。     

Liquid‐phase axial dispersion of turbulent gas–liquid co‐current flow through screen‐type static mixers

This article discusses the characteristics of turbulent gas–liquid flow through tubular reactors/contactors equipped with screen‐type static mixers from a macromixing perspective. The effect of changing the reactor configuration, and the operating conditions, were investigated by using four different screen geometries of varying mesh numbers. Residence time distribution experiments were conducted in the turbulent regime (4500 < Re < 29,000). Using a deconvolution technique, the RTD function was extracted to quantify the axial/longitudinal liquid‐phase dispersion coefficient. The findings highlight that axial dispersion increases with an increasing flow rate and/or gas‐phase volume fraction. However, regardless of the number and geometry of the mixing elements, reactor configuration, and/or operating conditions, the recorded liquid‐phase axial dispersion coefficients in the presence of screens was lower than that for an empty pipe. Furthermore, the geometry of the screen was found to directly affect the axial dispersion coefficient in the reactor. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1390–1403, 2017     

Shear‐rate‐dependent rheology effects on mass transport and surface reactions in biomicrofluidic devices

Consideration is given to shear‐rate‐dependent rheology effects on mass transport in a heterogeneous microreactor of rectangular cross section, utilizing both numerical and analytical approaches. The carrier liquid obeys the power‐law viscosity model and is actuated primarily by an electrokinetic pumping mechanism. It is discovered that, considering the shear‐thinning biofluids to be Newtonian fluids gives rise to an overestimation of the saturation time. The degree of overestimation is higher in the presence of large Damkohler numbers and electric double layer thicknesses. It is also increased by the application of a favorable pressure gradient, whereas the opposite is true when an opposed pressure gradient is applied. In addition, a channel of square cross section corresponds to the maximum fluid rheology effects. Finally, the numerical results indicate the existence of a concentration wave when using long channels. This is confirmed by analytical solutions, providing a closed form solution for wave propagation speed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1912–1924, 2015     

Mixing of a lignin‐based slurry fuel

Lignin‐based slurry fuels are a potential alternative to fossil fuels in kraft pulp mills. Lignogels — mixtures of lignin, fuel oil, water and surfactant — are non‐Newtonian fluids, with shear‐thinning and thixotropic behaviour. Their mixing was investigated in tanks with volumes of 3 and 30 L. An A310 hydrofoil impeller was used in all experiments. Results were compared with measurements in Newtonian fluids, used to characterize the impeller over a broad range of Reynolds numbers (1–500 000). An aqueous CMC solution was also used for characterization of the impeller and estimation of the Metzner‐Otto constant. Results in the transition region were corrected by introduction of two empirical parameters.     

Effect of Inlet Type on Shear Stress and Mixing in an Annular Photobioreactor Involving a Swirling Decaying Flow

Improvement of mixing can lead to an enhancement of photobioreactors productivity, provided that the adverse effect of shear forces is kept below the fragility‐ threshold of cells. Implementation of a swirling decaying flow induced by a tangential inlet in an annular photobioreactor was investigated. The study focused on a compromise solution between the mixing and the resulting shear stress, and various hydrodynamical characteristics were measured for five types of tangential inlets and three Reynolds number values. Comparison with a pseudo‐axial flow induced by a radial inlet indicates that swirling flows are suitable in the particular application of microalgal cultures.     

Mixing of Shear‐Thinning Fluids with Dual Off‐Centred Impellers

Mixing times for inelastic shear‐thinning fluids in stirred tanks have been experimentally investigated using a combination of two off‐centred impellers operating in both co‐ and counter‐rotating modes. A colour‐discolouration technique based on fast acid‐base reaction was used for the determination of the mixing times as well as to reveal the possible presence of caverns and dead regions. A statistical plan of experiments allowed determining the effects of the impeller position, the rotational speed, the flow behaviour index, the impeller type and their mutual interactions. A stronger influence of the impeller position on mixing times was observed for both rotating modes with fluids exhibiting pronounced shear‐thinning. It was also found that segregated regions could be readily destroyed by dual off‐centred impellers as compared with the single centred impeller configuration. Mixed flow impellers were shown to be less efficient in terms of mixing times than radial flow impellers. Results obtained under the best operating conditions were compared to steady stirring experiments showing the potential and drawbacks of the proposed scenarios.     

Power Requirement when Mixing a Shear‐Thickening Fluid with a Helical Ribbon Impeller Type

The optimal design of close clearance impellers requires the knowledge of the power demand of the mixing equipment. In non‐Newtonian mixing, this can be readily obtained using the Metzner and Otto concept [1]. In this work, this concept and the determination of the K_s value for an atypical helical agitator (PARAVISC system from Ekato firm) have been revised in the case of shear‐thinning fluids and a shear‐thickening fluid. For poor shear‐thinning fluids, it has been shown that for our mixing system the K_s value does not vary strongly with the flow behavior index, and may be regarded as a constant for the mixing purpose design. By contrast, for the shear‐thickening fluid, power consumption measurements indicate that the relationship between the K_s values and the flow behavior index is much more complex due to a partial solidification of the product around the impeller.     

Viscosity properties of homogeneous polyelectrolyte complex solutions from sodium carboxymethyl cellulose and poly(acrylamide‐co‐dimethyldiallylammonium chloride)

The viscosity properties of homogeneous polyelectrolyte complex solutions of sodium carboxymethyl cellulose (CMC) and poly(acrylamide‐co‐dimethyldiallylammonium chloride) have been investigated by means of a rotation viscometer at different complexation ratios, shear rates and temperatures. Compared with aqueous solutions of the component polyelectrolytes, such complex solutions can afford substantially increased viscosities at the complexation ratios examined, together with enhanced shear‐thinning rheology and temperature stability. According to this study, it is possible to improve the viscosity properties of water‐soluble polymers by homogeneous interpolyelectrolyte complexation in aqueous solutions. © 2000 Society of Chemical Industry     

Measuring Mixing Time in the Agitation of Non‐Newtonian Fluids through Electrical Resistance Tomography

Electrical resistance tomography (ERT), which is a non‐invasive and robust measurement technique, was employed to visualize, in three dimensions, the concentration field inside a cylindrical mixing vessel equipped with a radial‐flow Scaba 6SRGT impeller. The ability of ERT to work in opaque fluids makes this technique very attractive from an industrial perspective. An ERT system with a 4‐plane assembly of peripheral sensing rings, each containing 16 electrodes, was used to measure the mixing time in agitation of xanthan gum solution which is a pseudoplastic fluid with yield stress. An image reconstruction algorithm was used to generate images of the tracer distribution within the sensing zone. In this study, the effect of impeller speed, fluid rheology, power consumption, and Reynolds number on the mixing time was investigated.     

Temporary networks in polymer‐modified asphalts

The viscosity functions of several polymer‐modified asphalts (PMAs) were studied at different temperatures in steady‐state rate sweep tests. The materials were obtained by mixing different base asphalts with either styrene‐butadiene‐styrene (SBS), ethylene‐vinylacetate (EVA) or reactive ethylene terpolymers (RET). The first two polymers form a physical network that is swollen by the asphalt, while the latter is functionalized with glycidylmethacrylate (GMA) and can crosslink and/or chemically bond with the molecules of asphaltenes. In the presence of SBS or EVA, at certain temperatures, the viscosity curves exhibit a Newtonian behavior at low shear rates, followed by two distinct shear‐thinning phenomena. In some cases, the first shear‐thinning is preceded by a small shear‐thickening region. Similar phenomena are not present in the viscosity curves of the RET‐modified asphalts and can be related to a temporary nature of the physical polymer network. Polym. Eng. Sci. 44:2185–2193, 2004. © 2004 Society of Plastics Engineers.