A macroscopic agglomeration kernel model for gibbsite precipitation in turbulent and laminar flows

A macroscopic agglomeration kernel model has been developed that is capable of describing gibbsite agglomeration over a broad range of process conditions, including both the laminar and turbulent flow regimes. The agglomeration kernel model was derived using chemical reaction engineering principles and data from an extensive experimental program covering a wide range of temperatures, supersaturations, seed sizes, shear rates and mixing regimes. The experimental precipitation data in the laminar flow regime were generated using a novel Taylor-Couette precipitator. Data in the turbulent regime were generated in a stirred reactor. The developed agglomeration kernel model incorporates terms for the collision, capture, rupture and cementation rates affecting the formation of agglomerates. The model is shown to be able to predict the data from independent experiments. The proposed model also captures the complex non-linear shear rate and size-dependency observed experimentally, e.g. (1) for small particles the agglomeration kernel exhibits a maximum with respect to the shear rate, increasing at low shear rates in the laminar flow, but decreasing in the unstable Taylor-Couette flows and turbulent regimes; (2) for large particles the agglomeration kernel decreases monotonically with increasing shear.     

UV Disinfection of E. coli Between Concentric Cylinders: Effects of the Boundary Layer and a Wavy Wall

UV inactivation of E. coli in a plug flow reactor between concentric cylinders was investigated. The concentration boundary layer thickness was computed for laminar, turbulent and Taylor-Couette flow in terms of the respective mass transfer Sherwood number. It is demonstrated that the concentration boundary layer is thin and that the mass transfer coefficient is large and comparable in size for both turbulent and laminar Taylor-Couette flow in contrast to laminar flow. Computation of the fluence distribution for each flow pattern indicate that turbulent and especially Taylor-Couette flow subject E. coli to an equal flux of photons corresponding to ideal plug flow. However, experiments with turbulent flow that require large axial velocities indicate that very long reactor lengths are necessary to inactivate E. coli. Finally, rotor wavy wall modifications are explored to increase the inactivation of microorganisms in Taylor-Couette flow.     

环流反应器的流动、混合与传递特性

由于环流反应器内存在定向循环,与鼓泡塔反应器相比,其混合性能大幅提升,已广泛应用于许多工业过程,如发酵、反应器结晶等工业过程,近年来成为国内外学者研究的热点。针对环流反应器内操作条件下的流动形态、流体力学（包括相含率分布、循环液速、混合时间以及离集指数等）及传质/传热特性,总结了其最新研究进展,分析了相含率尤其是固含率变化对反应器中关键参数如循环液速和化学反应速率的影响,展望了从机理上研究相互耦合的多相流动、传质/传热和化学反应规律,为进一步推动其工业应用提供参考。     

Analysis of dominant flow structures and their flow dynamics in chemical process equipment using snapshot proper orthogonal decomposition technique

Snapshot proper orthogonal decomposition (POD) technique has been applied to reveal the dominant flow structures, their dynamics and length scales in six widely used industrial equipments (stirred tank, bubble column, Taylor-Couette flow (annual contactor), ultrasonic reactor, jet reactor, and channel flow). The variation in length scale of structures within an equipment, with change in its operating conditions (Reynolds number and power input) or change in its geometric configuration (sparger and impeller designs), has been brought out in this work. The planar data set for POD analysis was obtained from particle image velocimetry (PIV) and large eddy simulation (LES) studies. The dominant spatial topology was analyzed by using the velocity and vorticity POD modes. The modes have revealed the following flow structures: the ascending streaks and bursts in channel flow, the vortex tube and leading edge vortices in jets, the irregular small chaotic vortices in Taylor-Couette flow, the variation in plume oscillation and flow structures in the vortical region of bubble column resulting from changes in sparger design, the high intensity vortices near the source of ultrasound in the ultrasonic reactor and the effect of impeller designs on dominant flow structures and near blade vortices in the stirred tank. The length scales of structures are obtained by applying image processing on the spatial modes. The dynamics of these flow structures in each of the items of equipment is captured by reconstructing the flow field using appropriate spatial and temporal modes that contribute to these structures. Further, a unique attempt has been made to correlate the length scale distribution with the mixing time.     

Flow regime, hydrodynamics, floc size distribution and sludge properties in activated sludge bubble column, air-lift and aerated stirred reactors

This paper presents a comparative study how reactor configuration, sludge loading and air flowrate affect flow regimes, hydrodynamics, floc size distribution and sludge solids-liquid separation properties. Three reactor configurations were studied in bench scale activated sludge bubble column reactor (BCR), air-lift reactor (ALR) and aerated stirred reactor (ASR). The ASR demonstrated the highest capacity of gas holdup and resistance, and homogeneity in flow regimes and shearing forces, resulting in producing large numbers of small and compact flocs. The fluid dynamics in the ALR created regularly directed recirculation forces to enhance the gas holdup and sludge flocculation. The BCR distributed a high turbulent flow regime and non-homogeneity in gas holdup and mixing, and generated large numbers of larger and looser flocs. The sludge size distributions, compressibility and settleability were significantly influenced by the reactor configurations associated with the flow regimes and hydrodynamics.     

Flow regime maps and optimization thereby of hydrodynamic cavitation reactors

Hydrodynamic cavitation reactors are known to intensify diverse physical and chemical processes. In this article, flow regime maps have been proposed that give an overview of the operation of hydrodynamic cavitation reactor for different combinations of design and process parameters. These maps are based on simulations of cavitating flow using mathematical model that couples continuum mixture model with diffusion limited model. Specific flow regimes have been identified depending on the energetics of the collapse of cavitation bubble as sonophysical, sonochemical, and stable oscillatory (no physical or chemical effect). The radial motion of the bubble in the cavitating flow is governed by the mean and turbulent pressure gradients, which in turn, are decided by the design parameters. An analysis of variations in the pressure gradients in the cavitating flow with design parameters has been given. The flow regime maps form a useful tool for identification of most optimum set of design parameters for hydrodynamic cavitation reactor for a physical or chemical process. © 2012 American Institute of Chemical Engineers AIChE J, 58: 3858–3866, 2012     

Pressure drop and its reduction of gas–non-Newtonian liquid flow in downflow trickle bed reactor (DTBR)

The influences of concurrent flow of air–Newtonian and non-Newtonian liquid systems on pressure drop and on its reduction in downflow trickle bed reactor are presented in the present work. The pressure drop at different flow regimes in the trickle bed is enunciated by the dynamic interaction model based on the framework of the momentum balance. From the analysis, it is observed that the non-ideality factor of bubble flow regime is higher than that of pulse and trickle flow regimes which may influence efficiency of the reactor. The present work also concludes that the percentage of pressure reduction increases with increasing the surfactant concentration. However there is a limitation of change of concentration, above which no more reduction can be obtained. The present study may be useful for further understanding and modelling of multiphase reactor with non-Newtonian liquid, which has great industrial applications.     

气液固流化床流动介尺度模型研究进展

气液固流化床是一类重要的多相反应器,在化工及相关过程工业中有着广泛的应用。然而,由于对该类反应器内复杂的多相流动结构的定量描述十分有限,目前其设计和放大仍主要依赖经验,致使放大成功率低,反应结果达不到预期效果。因此,建立和完善气液固流化床内的三相流动机理模型,是实现该类反应器科学设计和放大的关键环节。对气液固流化床内的三相流动机理模型的研究进展进行了分析,着重总结了三相流动介尺度机理模型研究的新进展,并指出了存在的问题和进一步研究的方向,希望为该类反应器的基础研究和工业应用提供参考。     

A combined CFD modeling and experimental study of pyrolytic carbon deposition

Understanding the chemical reaction mechanism and deposition kinetics is of great importance to guide the production of pyrolytic carbon (PC). A practical approach to mimic the commercial chemical vapor deposition (CVD) process and eventually predict the PC deposition rate is highly desired. In this work, a simplified two-step reaction mechanism was proposed for the CVD of PC, with the first step in the gas phase and the second step on the substrate surface. The kinetic parameters were determined by trial and error using a computational fluid dynamics simulation. The velocity, temperature, and concentration profiles in a cold-wall, forced-flow reactor were modeled based on the geometry and experimentally determined boundary conditions. The computed PC deposition rates for substrate temperatures between 700 and 3000 °C were in good accordance with experimental results. Rate limiting steps were observed for both the deposition experiments and simulations. Mass-transport-limited and reaction-limited regimes were identified in wide temperature and flow rate ranges. A higher deposition rate was found in a cold-wall reactor compared with those in an insulated reactor or a hot-wall reactor. Finally, the PC microstructure was characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy, and X-ray diffraction, demonstrating progressive development of graphitization with increasing deposition temperature.     

Comparison of flow-reversal, internal-recirculation and loop reactors

In this work, we compare the performance of flow-reversal, internal-recirculation and loop reactors. In the absence of analytical results we use asymptotic, approximate and simulated solutions and present some experimental results. As criteria for comparison we use the maximal temperature achieved and the robustness of solution.Experiments and simulations of ethylene oxidation in the flow-reversal and internal-recirculation reactor, showed that the technically simpler inner-outer internal-recycle reactor may operate better at low flow rates than that with flow reversal, but the conclusion is reversed at high flow rates. Using approximate solutions, we show the dependence of the maximal temperature on the inner-outer heat-transfer coefficient.Loop reactor can generate rotating pulse solution: we simulate such solutions for two asymptotic cases where the ratio of switching velocity (i.e., unit length/switching time) to pattern velocity is either around unity or very large. We compare them with solutions of 4-8 units reactors. The slow-switching regimes require a delicate control. The fast-switching solution is robust but its peak temperature depends on the kinetic parameters and reactor length, compared with that of the flow-reversal reactor where it depends mainly on bed conductivity.     

In situ MRI of the structure and function of multiphase catalytic reactors

NMR imaging (MRI) was used to study the distribution of the liquid phase in an operating trickle bed reactor using hydrogenation of -methylstyrene or n-octene-1 as representative examples. In a single pellet reactor, the existence of oscillating regimes under unchanged external conditions was shown. The experiments with packed beds have demonstrated the non-uniform distribution of the liquid phase over the bed, the presence of partially liquid-filled or completely dry catalyst particles in the operating reactor, and the existence of liquid phase transport between liquid-filled and dry catalyst particles. Detection of spatially resolved NMR spectra was used to characterize chemical conversion variations within the operating reactor. Preliminary MRI results for an operating monolithic reactor were obtained. It was found that MRI can be used to directly image solid materials using NMR signal detection of nuclei other than ¹H. In particular, imaging of alumina using ²⁷Al NMR signal appears highly promising for the development of novel MRI applications in chemical engineering and catalysis, including spatially resolved NMR thermometry.     

EXPERIMENTAL DETERMINATION AND MODELLING OF LIQUID-SOLID MASS TRANSFER COEFFICIENTS IN A THREE-PHASE REACTOR

Apparent mass transfer coefficients for solid dissolution in a liquid with and without a chemical reaction were experimentally determined in a fixed bed three phases reactor with downward cocurrent gas and liquid flows. The chemical system selected was benzoic acid, sodium hydroxide in aqueous solution, and atmospheric air. Continuous gas, pulse and dispersed bubble regimes were studied and the results were correlated obtaining apparent mass transfer coefficient as a function of liquid and fluid volumetric flow. It was found that gas flow effect on mass transfer coefficient was small over continuous gas and dispersed bubble regimes, but appreciable over pulse regime. Additionally, it was found that the mathematical model that best described the mass transfer process under pulse regime, by using the increment factor due to the instantaneous chemical reaction, is the film theory     

Characterization of fluidization regime in circulating fluidized bed reactor with high solid particle concentration using computational fluid dynamics

The hydrodynamics inside a high solid particle concentration circulating fluidized bed reactor was investigated using computational fluid dynamics simulation. Compared to a low solid particle reactor, all the conventional fluidization regimes were observed. In addition, two unconventional fluidization regimes, circulating-turbulent and dense suspension bypassing regimes, were found with only primary gas injection. The circulating-turbulent fluidization regime showed uniformly dense solid particle distribution in all the system directions, while the dense suspension bypassing fluidization regime exhibited the flow of solid particles at only one side system wall. Then, comprehensive fluidization regime clarification and mapping were evaluated using in-depth system parameters. In the circulating-turbulent fluidization regime, the total granular temperature was low compared to the adjacent fluidization regimes. In the dense suspension bypassing fluidization regime, the highest total granular temperature was obtained. The circulating-turbulent and dense suspension bypassing fluidization regimes are suitable for sorption and transportation applications, respectively.     

串行流化床内气固流动控制

针对化学链燃烧分离CO₂技术特点，在一串行流化床（循环床+喷动床）冷态实验装置上，以CaSO₄载氧体为实验原料（d_p= 0.6 mm），研究串行流化床气固流动特性。基于床内压力分布特征，提出将循环床（空气反应器）沿床高方向划分为鼓泡段和快速流化段2个流型区域，将喷动床（燃料反应器）沿床高方向划分为喷动段、鼓泡段和悬浮段3个流型区域，得出串行流化床内气固流动控制机理。研究并考察了循环床流化风速度、喷动床喷动风速度对串行流化床内反应器间（空气反应器和燃料反应器）气体串混、颗粒循环速率以及床层压降的影响。研究结果表明，流化风是床内颗粒循环的驱动力，流化风速度应控制在 3.77～4.05 m·s^-1；喷动风速度对床内颗粒循环以及系统稳定运行起着关键作用，建议将喷动风速度控制在0.42～0.56 m·s^-1。     

Experimental study of flow regimes in three‐dimensional confined impinging jets reactor

Dynamic behaviors in a three‐dimensional confined impinging jets reactor (CIJR) were experimentally studied by a flow visualization technique at 100 ≤ Re ≤ 2000 and 2 ≤ D/d ≤ 12 (where D is the reactor diameter and d is the nozzle diameter). The effects of inlet Reynolds numbers (Re) and geometry configurations of the CIJR on the flow regimes have been investigated by a particle image velocimetry and a high‐speed camera. Results show that with the increasing Re, a segregated flow regime, a radial deflective oscillation, an axial oscillation and a vortex shedding regime emerge in turns in CIJR. A map of parameter space formed by the inlet Reynolds number (Re) and the normalized reactor diameter (D/d) has been presented. The effects of jet instability and confined boundary of the chamber on the flow regimes and their transition are also investigated and discussed. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3033–3045, 2014     

Identification of flow regime transitions in an annulus sparged internal loop airlift reactor based on higher order statistics and Winger trispectrum

Higher order statistics and Wigner higher order moment spectra were used to extract useful flow regime characteristics from wall pressure fluctuation signals in an annulus sparged internal loop airlift reactor. It is found that the pressure fluctuation in the airlift reactor is a typical nonlinear and non-stationary process, which exhibits different frequency characteristics depending on flow regimes. Analysis methods based on bispectrum and Wigner trispectrum are powerful tools to reveal frequency characteristics of pressure signals. To identify flow regime transitions in the reactor, two new characteristic quantities, namely average bispectrum and generalized average frequency, are defined from bispectrum and Wigner trispectrum of the pressure signal, respectively. Two flow regime transition points corresponding to three flow regimes in the reactor are successfully detected by using these two characteristic quantities.     

Multiwalled carbon nanotube deposition profiles within a CVD reactor: An experimental study

A number of proposed applications of carbon nanotube (CNT) arrays require that uniform deposition of well-aligned CNTs is achieved. The CNT deposition profiles inside a chemical vapor deposition (CVD) reactor are strongly dependant on the reaction temperatures, feed gas flow rates, carrier gas flow rates and reactor geometry. In addition, objects placed in the path of the flow of feed material could affect the deposition patterns. In this paper, an experimental study aimed at achieving better control of the deposition patterns of CNTs is presented. Multiwalled CNTs were grown on a long substrate by the catalytic CVD of a xylene/ferrocene solution. The deposition patterns on the substrate were examined for different furnace temperatures, xylene/ferrocene feed rates and carrier gas flow rates. Small objects representative of electronic devices were placed at different locations on the substrate and their effect on the deposition patterns was explored. The effect of changing the height and the gap distance between these objects was also studied.     

Computational fluid dynamics in the development of catalytic reactors

Computational fluid dynamics is becoming an important tool in the study of chemical engineering processes and apparatuses (in particular, the share of works with the application of this method is nearly 6% of the total number of all chemical engineering works issued by Elsevier Science Publishers in 2010). The possibilities of computational fluid dynamics are demonstrated using examples from three different chemical engineering fields: developing a method for loading a tubular reactor for the steam conversion of natural gas, studying heat transfer in a reactor for the hydrogenation of vegetable oils upon the replacement of a catalyst, and investigating the transitional processes in an automobile neutralizer. The results from computational fluid dynamics are verified by comparing them with experimental data in developing a method for loading a tubular reactor, using the problem of decelerating a catalyst particle with a flow of air as an example. The obtained data are compared with classical measurement data on the aerodynamic drag of a ball and a cylinder and represent the further development of works on the flow around particles of complex shape. In this work, the results from inspecting a reactor for the hydrogenation of oils with allowance for the possible heating and uniform distribution of a flow before its entering the catalyst bed are presented. It is shown that the construction of the reactor does not ensure homogeneity of the reaction flow at the desired level and requires modification of heating elements. The efficiency of computational fluid dynamics for investigating fast processes with a chemical reaction is exemplified by studying the transitional processes in an catalytic automobile neutralizer (the effect of flow dynamics and heat transfer on the thermal regime in a honeycomb catalyst particle is very difficult to study by experimental methods). The application of computational fluid dynamics allows us to reduce considerably the time and cost of developing and optimizing the designs of efficient catalytic fixed-, fluidized-, or moving-bed reactors (particularly multiphase stirred (slurry) reactors), along with mixers, adsorbers, bubblers, and other chemical engineering apparatuses with moving media.     

反应器的热衡算方程精解

植根于热力学分析 ,提出严谨的反应器热量衡算方法 ,对国内外一些论著中的疑点、误点进行了辨析 ,得出了理想流动反应器之一的平推流反应器 (plugflowreactor)热衡算精确、通用的新型微分方程和过程方程 ,试图为精确进行反应器的热量衡算重新探讨理论依据。这些方程可用于管式反应器沿程各截面以及整个反应器的精确热计算。作者论述谨以平推流反应器为代表 ,但其方法不失通用性 ,可望在反应器热量衡算中广泛应用     

Scale-up concept for modular microstructured reactors based on mixing, heat transfer, and reactor safety

Microstructured reactors are characterized by rapid mixing processes and excellent temperature control of chemical reactions. These properties allow the safe operation of hazardous chemistry in intensified processes. Problems occur during scale-up of these processes, where heat transfer becomes the limiting effect. With high flow rates and transitional or even turbulent flow regimes in small channels, rapid mixing and excellent heat transfer can be maintained up to high production rates. For exothermic reactions, limits for parametric sensitivity and safe operation are shown from literature and combined with convective heat transfer for consistent scale-up. Good knowledge of reaction kinetics, thermodynamics and heat transfer is essential to determine runaway regions for exothermic reactions. From these correlations, consistent channel design and continuous-flow reactor setup is shown.