Influence of the meandering channel geometry on the thermo-hydraulic performances of an intensified heat exchanger/reactor

In the global context of process intensification, heat exchanger/reactors are promising apparatuses to implement exothermic chemical syntheses. However, unlike heat exchange processes, the implementation of chemical syntheses requires to control the residence time to complete the chemistry. A way to combine the laminar regime (i.e. enough residence time) with a plug flow and the intensification of both heat and mass transfers is the corrugation of the reaction path.In this work, the experimental set-up is based on plate heat exchanger/reactor technology. 7 milli-channel corrugated geometries varying the corrugation angle, the curvature radius, the developed length, the hydraulic diameter and the aspect ratio have been designed and experimentally characterized (heat transfer, mixing times, pressure drops, RTD). The objectives were to assess their respective performances to derive some correlations depending on the channel design.The results confirmed the benefits of the reaction channel corrugation. Heat and mass transfers have been intensified while maintaining a plug flow behaviour in the usually laminar flow regime. Moreover, whatever the meandering channel's curvature radius, the results highlighted the relevance of considering the Dean number as the scale-up parameter. This dimensionless number, more than the Reynolds number, seems to govern the flow in the wavy channels.     

Generation of Silica Nanoparticles from Tetraethylorthosilicate (TEOS) Vapor in a Diffusion Flame

Silica (SiO2) nanoparticles were synthesized by the gas phase thermal oxidation of tetraethylorthosilicate (TEOS) in a laminar diffusion flame reactor. Characteristics of the formation of silica nanoparticles along the axial distance above the burner outlet were investigated. Effects of maximum flame temperature, TEOS concentration, residence time, and water vapor on the particle size were also investigated. Silica nanoparticles less than 20 nm in average particle diameter were synthesized in all of the experiments. Morphological changes of particles were found along axial distance above the burner outlet; many small aggregates of particles (dp = ～ 7 nm) were found up to 2 cm, isolated smaller particles ( dp = ～ 5 nm) at 4 cm, and aggregates of bigger particles (dp = ～ 10 nm) at 10 cm. Larger particles at higher TEOS concentrations are generated in the flame synthesis. As the maximum flame temperature increased, the average particle size of silica also increased. Smaller particles were produced with decrease of the residence time of TEOS vapor in the flame. The average particle size decreased with the injection of water vapor to the flame.     

Aerosol reactor design: Effect of reactor geometry on powder production and vapor deposition

Production of Knudsen aerosols from gaseous precursors in the absence of coagulation is analyzed in reactors with laminar flow. Tubular, annular and parallel plates configurations are considered. Annular and parallel plate reactor geometries can produce more monodisperse aerosols than tubular reactors at the expense of lower yields. For equivalent conditions, these geometries give more vapor deposition and lower rates of particle formation than tubular reactors, a result of importance in chemical vapor deposition.     

Residence Time Distribution of a Capillary Microreactor Used for Pharmaceutical Synthesis

The main drivers for application of small-scale reactors in the pharmaceutical industry are the possibility of rapid synthesis and screening of novel drugs as well as the readiness of the scale-up. The characterization of fluid flow pattern was performed through step-up and step-down residence time distribution experiments using a tracer at six different flow rates. Four single-parameter models were considered for representing deviations from ideal plug flow and ideal laminar flow in tubes. The model that provided the best results was the axial dispersion model and the Peclet and Reynolds numbers could be well correlated. Obtained Peclet values from 44 to 244 were close to Pe > 100, in which axial dispersion can be neglected and the reactor can be considered as plug flow reactor.     

Characterization of Residence Time Distribution in a Plug Flow Reactor

One of the biggest advantages of plug flow reactors lies in their narrow residence time distribution. The pulse experiment, as a common method on acquiring that distribution, relies on the tracer injection being a perfect pulse. A deviation from a perfect pulse leads to erroneous results if not taken into account. With a numerical analysis of experimental data, this effect is quantified in turbulent and laminar flow regime and the results are compared to an analytical method. Significant deviations occur mostly in the turbulent regime, which has the greatest technical relevance.     

液固下喷自吸环流反应器流体力学特性

采用改进皮托管测定了液固下喷自吸环流反应器中的液体平均速度及液相湍流强度分布,采用PV4A颗粒浓度速度测量仪测定了固含率分布。试验结果表明:随着液体喷射流量的增大,液体平均速度、液相湍流强度及固含率均增大;在导流筒内液相平均速度近似管流分布,在环隙中的速度分布基本均匀;在导流筒和环隙内固含率沿径向分布均匀。该研究对于下喷环流反应器的放大与优化有重要指导意义。     

Gas-phase synthesis of nanoparticles: scale-up and design of flame reactors

The scale-up and design of flame aerosol reactors are investigated for synthesis of silica and titania nanoparticles. In specific, coflow burners of different dimensions are studied at various precursor, fuel, and oxygen flow rates. The influence of the flame enthalpy content on product primary particle size is investigated by changing the fuel from methane to propane or hydrogen. Operation lines relating product particle size with reactant outlet conditions, burner size, and flame enthalpy are developed, showing how the different reactors can produce silica or titania nanoparticles of the same size. A scale-up procedure developed for fumed silica is extended to the synthesis of titania nanoparticles covering production rates of 2-200 g/h. At high fuel-oxidant velocity difference at the burner outlet, the operation of diffusion flame reactors converges to that of premixed ones.     

Design of a Flexible Pilot Plant Reactor for the Steam Cracking Process

A design technique for a pilot plant reactor of single diameter is presented to scale up or down steam cracking coils of different configurations like mono‐tubular, classical, and reversed splits. Using dimensional analysis, two criteria are selected in establishing partial similarity between different scales, the mean residence time, and the axial pressure profile in the reactor, in addition to preserving the flow pattern within the turbulent region. The sensitivity and accuracy of the proposed method is compared to another conceivable alternative that focuses on the lateral gradients. The pilot reactor coil is adapted for any large‐scale reactor by the adjustment of feed flow rate and the effective length exposed to the firebox heat flux. Simulation results for naphtha cracking in a commercial split coil and also the equivalent pilot plant reactors are used for verification and validation of this method.     

Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows

Thermophoretic deposition of aerosol particles (particle diameter ranges from 0.038 to 0.498 μm) was measured in a tube (1.18 m long, 0.43 cm inner diameter, stainless steel tube) using monodisperse NaCl test particles under laminar and turbulent flow conditions. In the previous study by Romay et al., theoretical thermophoretic deposition efficiencies in turbulent flow regime do not agree well with the experimental data. In this study, particle deposition efficiencies due to other deposition mechanisms such as electrostatic deposition for particles in Boltzmann charge equilibrium and laminar and turbulent diffusions were carefully assessed so that the deposition due to thermophoresis alone could be measured accurately. As a result, the semiempirical equation developed by Lin and Tsai in laminar flow regime and the theoretical equation of Romay et al. in turbulent flow regime are found to fit the experimental data of thermophoretic deposition efficiency very well with the differences of less than 1.0% in both flow regimes. It is also found that Talbot's formula for the thermophoretic coefficient is accurate while Waldmann's free molecular formula is only applicable when Kn is greater than about 3.0.     

Computational fluid dynamics simulations of pressure drop and heat transfer in fixed bed reactor with spherical particles

Fixed bed reactors are among the most important equipment in chemical industries as these are used in chemical processes. An accurate insight into the fluid flow in these reactors is necessary for their modeling. The pressure drop and heat transfer coefficient have been studied for the fixed bed reactor with tube to particle diameter ratio (N) of 4.6 and comprising 130 spherical particles using computational fluid dynamics (CFD). The simulations were carried out in a wide range of Reynolds number: 3.85≤Re≤611.79. The RNG k-ɛ turbulence model was used in the turbulent regime. The CFD results were compared with empirical correlations in the literature. The predicted pressure drop values in laminar flow were overestimated compared with the Ergun’s [27] correlation and underestimated in the turbulent regime due to the wall friction and the flow channeling in the bed, respectively. It was observed that the CFD results of the pressure drop are in good agreement with the correlations of Zhavoronkov et al. [28] and Reichelt [29] because the wall effects have been taken into account in these correlations. Values of the predicted dimensionless heat transfer coefficient showed better agreement with the Dixon and Labua’s [32] correlation. This is explained by the fact that this correlation is a function of the particle size and shape in the bed.     

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.     

Modelling and design of thin-film slurry photocatalytic reactors for water purification

Photocatalytic oxidation processes are highly effective clean technologies for the degradation and mineralization of a wide variety of priority pollutants in water and wastewater. However, the application of heterogeneous photocatalysis for wastewater treatment on an industrial scale has been impeded by a lack of mathematical models that can be readily applied to reactor design and scale-up. As a results current photocatalytic reactors in research and development have been designed by empirical or semi-empirical methods only.In this paper, a simple and generic mathematical model for steady-state, continuous flow, thin-film, slurry (TFS) photocatalytic reactors for water purification using solar and UV lamps is presented. The model developed is applicable to TFS flat plate and annular photoreactors of (a) falling film design or (b) double-skin design, operating with three ideal flow conditions: (1) falling film laminar flow, (2) plug flow and (3) slit flow. The model is expressed in dimensionless form and scale-up of TFS photocatalytic reactors can be carried out by dimensional analysis. In addition, the model parameters can be estimated easily from real systems and model solutions can be obtained with little computational effort.Comparison of a number of ideal flow systems shows that both falling film laminar flow and plug flow operation modes give higher performance than the slit flow system. Slit flow operation mode results in lower conversions due to the non-correspondence of fluid-residence time and the transversal radiation field. The effect of optical thickness, on reactor performance and the evolution of radial profiles of a model pollutant with photoreactor length are presented for each of the operation modes. The falling film laminar flow system was found to be more efficient than the plug flow system when the reactor conversion is above 80%. For lower reactor conversion the plug flow system was found to be marginally more efficient than the falling film laminar flow system. A methodology for the optimal geometrical design of a highly efficient configuration of TFS photocatalytic reactors is also presented. The mathematical models presented may be used as a tool for the design, scale-up and optimization of these types of photocatalytic reactors.     

气液混合式撞击流反应器流场特性数值模拟

撞击流反应器具有高效传质和相间强相互作用等优点,在工业上被广泛应用.基于传统撞击流反应器构建了新型二路加速管同轴对置撞击流反应器,进行了以空气为连续相、液态水为离散相的流场混合特性模拟,分析了不同气相流速下气液高速混合流动过程,研究内部流场速度、压力分布及颗粒直径与停滞时间的变化特性.结果表明,流场分布关于撞击面对称,...     

Scale-up of nanoparticle synthesis in diffusion flame reactors

The scaling-up of diffusion flame aerosol reactors is investigated for synthesis of nanoparticles. Three co-flow burners of different dimensions are studied at various precursor, oxidant and fuel flow rates. An operation line relating product particle size with reactant outlet conditions, flow rate and burner size is developed showing how the three reactors can produce silica and titania nanoparticles of the same size and morphology. This operation line shows the limit of fast reactant mixing where the diffusion flame aerosol reactors perform as premixed ones. An operation diagram is obtained for different silica production rates and a scale-up procedure is developed.     

Modelluntersuchungen zur Partikelbeanspruchung in Reaktoren

This paper reports systematic studies on hydrodynamic stress in agitated vessels with baffles, reactors with predominantly laminar flow, SERALE viscosimeters, bubble columns, and gas-driven loop reactors. The stress on particles is determined by way of the destruction of model particle systems, with the kinetics of destruction, the equilibrium particle diameters, and the enzyme activity after a given time serving, respectively, for assessment of the stress. Thereof one obtaines an indication of the methodology necessary for determining the stress and it permits selection of appropriate reactor systems according to the criterion of particle stress. In the case of reactors with turbulent motion without dominant laminar flow or gas/liquid interfaces give analogous results, permitting the assumption that they are also applicable to other particle systems. Specifically for stirred reactors a geometric function is derived from the experimental results which permits prediction of stress caused by various types of impellers. Impellers with large blade areas relative to the dimensions of the tank produce less shearing forces owing to their uniform power input, in contrast to small and especially axial flow impellers. In laminar flow or reactors with gas/liquid interfaces the level of particle stress depends upon the proclivity of the particles to enter the boundary layers. It can therefore be deduced that particles with different surface properties, which lead to differing interactions with boundary layers and interfaces, will also experience different kinds of particle stress on laminar flow or in aeration processes. Thus the scope for application of the results obtained with model particle systems to other particle systems is limited in such reactors.     

Numerical simulation of flow field and residence time of nanoparticles in a 1000-ton industrial multi-jet combustion reactor

In this work, by establishing a three-dimensional physical model of a 1000-ton industrial multi-jet combustion reactor, a hexahedral structured grid was used to discretize the model. Combined with realizable k–ε model, eddy-dissipation-concept, discrete-ordinate radiation model, hydrogen 19-step detailed reaction mechanism, air age user-defined-function, velocity field, temperature field, concentration field and gas arrival time in the reactor were numerically simulated. The Euler–Lagrange metho...     

Fluidization behavior in a circulating slugging fluidized bed reactor. Part I: Residence time and residence time distribution of polyethylene solids

Square nosed slugging fluidization behavior in a circulating fluidized bed riser using a polyethylene powder with a very wide particle size distribution was studied. In square nosed slugging fluidization the extent of mixing of particles of different size depends on the riser diameter, gas velocity, hold up and solids flux in the riser. Depending on the operating conditions the particle residence time distribution of a riser in the slugging fluidization regime can vary from that of a plug flow reactor to that of a well-mixed system.Higher gas velocities cause shorter particle residence times because of a significant decrease in the hold-up of particles in the riser at higher gas velocities. A higher solids flux also shortens the average residence time. Both influences have been quantified for a given polyethylene-air system.Residence time and residence time distribution were determined for different particle size and the influence of gas velocity, solids flux, hold up and riser diameter was studied. When comparing data from segregation and residence time experiments it is clear that segregation data can predict the spread in residence time as a function of overall residence time, particle size and gas velocity. The differential velocity between small and large particles found in the segregation experiments can predict the spread in residence time as found in the residence time distribution experiments with a powder with a broad particle size distribution. Raining of particles through the slugs was studied as a function of plug length, gas velocity and pulse length. It was found that raining is not the determining mechanism for segregation of particles.     

Wärmeübergang in kurzen,wandgängig gerührten Apparaten

Scraped surface heat exchanger are often used in industrial settings for the treatment of systems with solid particle formation or higher viscosities. In reactors with a small length‐diameter ratio, the backmixing has thereby a significant influence on the heat transfer. In the experiments presented here, it can be shown that the heat transfer coefficient strongly depends on the flow pattern within the laminar flow regime, and that, depending on the running conditions, an increased rotor speed may have a negative effect on the heat transfer rate.     

Agitation scale-up effects during VCM suspension polymerization

Experimental data are presented showing the effect of reactor agitation intensity and reactor size on final resin properties during VCM suspension polymerization. Experiments were carried out in three stirred batch polymerization reactors covering a broad range of vessel sizes (bench scale, pilot plant, and commercial production units). Reactors' shapes were geometrically similar. The same charge recipe and operating procedure was also used for all three reactors. The effects of major agitation parameters such as impeller diameter, width, and speed are correlated against resin properties using the Weber number. The same characteristic U-shaped curve is found for all three reactors when mean particle diameter is plotted versus Weber number. However, the curves do not lie on top of one another but are spread apart, the larger reactors having a higher Weber number. Another interesting feature is that the coefficient of variation (particle size standard deviation divided by mean diameter) decreases dramatically as reactor size is increased. Other resin properties also show improvement upon scale-up. In summary, resin properties continue to improve as reactor size is increased over the range studied (bench-to-commercial large reactor scale), but a correct application of the complex scale-up technology must be employed to take advantage of this observation.     

泰勒反应器应用技术进展

作为基于泰勒涡流原理制得的一种新型反应器,泰勒反应器得到了日益广泛的应用,呈现出良好的发展前景。本文讨论了内圆筒转速、轴向流速、半径比及纵横比等操作参数对泰勒反应器性能的影响,综述了泰勒反应器在颗粒制备、光催化降解、生物培养等领域的应用现状,针对气体通入影响、流动特性改进、反应器放大等应用问题提出了相应解决办法,并指出寻求合理的反应器放大方法以及对反应器放大后进行稳定性和可靠性研究是该领域今后研究的重点。