Application of kinetics and computational fluid dynamics in pinene isomerization

A reliable kinetic model to describe the effects of various factors on the reaction rate and selectivity of pinene isomerization is developed. Furthermore, computational fluid dynamics (CFD) is applied to simulate the solid- liquid dispersion in reactor. The catalyst TiM is obtained by improving the composition and structure of hydrated titanium dioxide. The kinetic equation of pinene isomerization is deduced based on reaction mechanism and catalyst deactivation model. The kinetic equation of pinene isomerization reaction is fitted, and the results show that the fitted equation is correlated with the experimental data. The rate and selectivity of pinene isomerization reaction are affected by the amount of catalyst, deactivation of catalyst, structure of catalyst, reaction temperature and water content of catalyst. The solid-liquid distribution of the reactor is calculated by computational fluid dynamics numerical simulation, and the solid-liquid dispersion in commercial scale reactor is more uniform than that in lab-scale reactor.     

An integrated dynamic model for reaction kinetics and catalyst deactivation in fixed bed reactors: skeletal isomerization of 1-pentene over ferrierite

Catalyst deactivation in fixed beds is a critical issue in many industrial processes. A general dynamic model for fixed bed reactors involving catalyst deactivation was presented and simplified. The simplifications were based on the pseudo-steady state hypothesis for the fluid phase. Catalyst deactivation was treated by a linear model with respect to surface intermediates. A semi-analytical solution was obtained for the surface intermediates. The approach was applied to skeletal isomerization of 1-pentene over ferrierite. The comparison of experimental results and model predictions revealed that the model is able to describe the essential features of catalyst deactivation in skeletal isomerization and it can be useful for process scale-up.     

计算流体力学在化学工程中的应用

计算流体力学(CFD)用于求解固定几何形状设备内的流体的动量、热量和质量方程以及相关的其他方程,已成为研究化工领域中流体流动和传质的重要工具。本文概述了CFD的基本原理以及CFD在化学工程领域方面的应用,重点介绍了CFD在搅拌槽、换热器、蒸馏塔、薄膜蒸发器、燃烧等方面的应用。     

计算流体力学用于搅拌器流场研究及结构设计

采用计算流体力学(CFD)模拟方法对现有同轴搅拌器的流场结构进行了模拟,同时进行了混合时间的数值研究,并与实验结果相比较,证实了模拟方法是可行的。针对现有搅拌器存在的问题,提出了3种改进型的搅拌器,并进行CFD模拟研究,发现桨Ⅲ所需混合时间最短,桨Ⅱ和桨Ⅰ次之;桨Ⅲ消耗的功率最大,而桨Ⅰ和桨Ⅱ所需功率最少。     

Prolonging catalyst lifetime in supercritical isomerization of 1-hexene over a platinum/alumina catalyst

A series of experiments were carried out for the isomerization of 1-hexene in the temperature range and pressure range 10-100 bar, representing operating points both above and below the critical point of 1-hexene. At constant temperature of , increasing the pressure from 10 to 100 bar led to a substantial conversion increase up to a maximum of 78%. At each pressure, loss of conversion of 1-hexene was observed over the course of 8 h reaction time, which was attributed to the formation of oligomers and eventually coke upon the catalyst surface. Loss of conversion occurred initially more rapidly at the intermediate pressures of 40 and 70 bar, compared with at 10 and 100 bar, where a sustained but gradual decrease of conversion occurred. With increasing temperature in the range , conversion was highest at the condition , 40 bar, which is closest to the critical point of 1-hexene. Higher concentrations of oligomers, which act as coke precursors, were detected with increasing temperature and pressure of the reaction. The mass fraction of coke deposited upon the catalysts was dependent upon operating conditions and was within the range 0.63-1.36%, whilst the metal dispersion reduced from 26.8% for the fresh catalyst to 2.82-4.61% for the range of coked samples. A detailed examination of the void space structural changes, occurring after coking under both sub-and super-critical conditions, has been made. In particular, the void space structures were characterised in terms of external accessibility using percolation theory. The catalysts which were operated at reaction conditions where rapid initial deactivation occurred displayed a lower apparent connectivity in comparison with catalysts operated at conditions favouring a gradual sustained deactivation. It has also been found that, under certain conditions, the apparent connectivity of the remaining pore network can, unexpectedly, appear to increase following coking. This has been attributed to the initial loss of the most inaccessible pores within the network.     

Ethylbenzene disproportionation and p-xylene selectivity enhancement in xylene isomerization using high crystallinity desilicated H-ZSM-5

Desilication accompanied with minimum loss of crystallinity effect of a high alumina ZSM-5 zeolite on the isomerization reaction of ethylbenzene/xylene mixtures has been considered.Desilication was assessed through XRF,XRD,FTIR,TEM,nitrogen adsorption/desorption,NH_3-TPD,~(29)Si and~(27)Al MAS NMR analytical techniques.Desilication was accompanied with the creation of super acid sites.There exists a limit(Si/Al molar ratio of9.67)for keeping high crystallinity and obtaining improved catalytic performance.Desilication promotes ethylbenzene conversion by disproportionation and trans-alkylation reactions while the same reactions are limited for the xylene isomers.The p-xylene approach to equilibrium improves by more than 7% at 400℃ and a WHSV of 2 h~(-1)for the optimum sample with respect to the parent zeolite.At the same conditions,the optimum sample exhibits the maximum ethylbenzene conversion of 89%,i.e.more than 40%w.r.t.of the parent zeolite.However,the xylene yield decreases only 3%.     

分子蒸馏过程的计算流体力学模拟

采用计算流体力学(CFD)方法,分析了蒸发器上液体负荷、刮膜器旋转速度对蒸发器上液膜厚度变化及液膜中速度矢量的影响。将CFD模拟结果与文献报道的计算值和实验值进行比较,发现具有较好的吻合性,同时可以用模拟计算结果解释一些实验现象。采用脉冲注射示踪剂法测定了蒸发器上流体的平均停留时间分布,与CFD模拟的在刮膜器作用下蒸发器上流体平均停留时间分布数据进行比较,二者的偏差在±8%以内,验证了CFD模拟的准确性。     

SWA/TiO_2微球的制备及催化α-蒎烯异构化性能

采用溶胶-凝胶法和浸渍法,制备了纳米负载型H4SiW12O40（SWA）催化剂,考察了载体种类、SWA浸渍浓度对催化剂性能的影响。结果表明,用TiO2固载、8%SWA浸渍所得催化剂SWA/TiO2的催化性能较佳。利用XRD、TEM和BET比表面测定技术对其结构进行了表征。结果显示,该催化剂颗粒为圆球形,粒径为40～50 nm,具有较好的分散性。载体TiO2的引入明显增大了SWA的比表面积。将纳米SWA/TiO2用于催化α-蒎烯异构化反应,实验结果表明,该催化剂具有较好的催化活性和选择性,异构化反应的主产物是莰烯。在适宜的实验条件下,α-蒎烯的转化率达98%,莰烯的产率达58%。与其它负载型催化剂比较,SWA/TiO2具有用量少、活性高、反应时间短等特点。     

Simulation of fine particle formation by precipitation using computational fluid dynamics

The 4‐environment generalized micromixing (4‐EGM) model is applied to describe turbulent mixing and precipitation of barium sulfate in a tubular reactor. The model is implemented in the commercial computational fluid dynamics (CFD) software Fluent. The CFD code is first used to solve for the hydrodynamic fields (velocity, turbulence kinetic energy, turbulent energy dissipation). The species concentrations and moments of the crystal size distribution (CSD) are then computed using user‐defined transport equations. CFD simulations are performed for the tubular reactor used in an earlier experimental study of barium sulfate precipitation. The 4‐EGM CFD results are shown to compare favourably to CFD results found using the presumed beta PDF model. The latter has previously been shown to yield good agreement with experimental data for the mean crystal size at the outlet of the tubular reactor.     

Comparison of various radiation-cooled dew condensers using computational fluid dynamics

Radiation-cooled dew water condensers can serve as a complementary potable water source. In order to enhance passive dew collection water yield, a Computational Fluid Dynamics (CFD) software, PHOENICS, was used to simulate several innovative condenser structures. The sky radiation is calculated for each of the geometries. Several types of condensers under typical meteorological conditions were investigated using their average radiating surface temperature. The simulations were compared with dew yield measurements from a 1 m² 30°-inclined planar condenser used as a reference. A robust correlation between the condenser cooling ability and the corresponding dew yield was found. The following four shapes were studied: (1) a 7.3 m² funnel shape, whose best performance is for a cone half-angle of 60°. Compared to the reference condenser, the cooling efficiency improved by 40%, (2) 0.16 m² flat planar condenser (another dew standard), giving a 35% lower efficiency than the 30° 1 m² inclined reference condenser, (3) a 30 m² 30°-inclined planar condenser (representing one side of a dew condensing roof), whose yield is the same as the reference collector, and (4) a 255 m² multi-ridge condenser at the ground surface provided results similar to the reference collector at wind speeds below 1.5 m s^− 1 but about 40% higher yields at wind speeds above 1.5 m s^− 1.     

Deactivation kinetics of Escherichia coli cells correlated with intracellular superoxide dismutase activity in photoreaction with titanium dioxide particles

In the photoreaction of Escherichia coli IM303 (superoxide dismutase (SOD)‐deficient mutant) and MM294 (wild‐type strain) with TiO₂ particles, the viability of strain IM303 decreased linearly with photoreaction time, while the time profile of the viability of strain MM294 exhibited a curved form. Using strain MM294 with varied initial SOD activities, the TiO₂ photoreaction tests were conducted at incident light intensities of I₀ = 4,8 and 14 W m^?2, and the time profiles of bacterial viabilities were analyzed on the basis of the series‐event model. The value of n (corresponding to the step number in the series reaction kinetics described by the model) increased with an increase in initial SOD activity (A_SOD,0), giving a mean value of A_SOD,0/n = 7.1 × 10^?9 U cell^?1 under the conditions examined. SOD activities in the cells of strain MM294 with A_SOD,0 = 1.9 × 10^?8 and 4.0 × 10^?8 U cell^?1 decreased with the progress of photoreaction conducted at I₀ = 14 W m^?2. The transition of intracellular SOD activities expressed was in agreement with the observed data by considering the changes in bacterial cell populations with varied SOD activities based on the proposed model. © 2002 Society of Chemical Industry     

Simulations of photodegradation of toluene and formaldehyde in a monolith reactor using computational fluid dynamics

In this study, simulations were conducted on a monolith reactor for the photodegradation of toluene and formaldehyde. The monoliths in the reactor were treated as porous zones and the photocatalytic oxidation occurring on the monolith surfaces was modeled using Langmuir–Hinshelwood kinetics. A discrete ordinates model was used to simulate the light intensity with a novel approach, which involved an adjustable parameter—the absorption coefficient of the channel wall, for modeling the local light intensity across the porous media. The advantage of this approach was that despite its simplicity, it was able to capture and visualize the local light profile across the monolith channels and to integrate it into the reaction kinetics. Although it required a trial‐and‐error to determine the correct value of the channel wall absorption coefficient, the proposed model achieved a reasonable agreement between the simulation results and published experimental data. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Simulating multiphase flow in a two-stage pusher centrifuge using computational fluid dynamics

The design of two-stage pusher centrifuges have developed rapidly, but a good understanding of the theory behind their practice is a long-standing problem. To better understand centrifugal filter processes, the computational fluid dynamics (CFD) software program FLUENT has been used to model the three-dimensional geometry and to simulate multiphase flows based on Euler-Euler, moving mesh, dynamic mesh and porous media models. The simulation tangential velocities were a little smaller than those for rigid-body motion. In the stable flow region, the radial velocities were in good agreement with the theoretical data. Additionally, solid concentration distribution were obtained and also showed good agreement with the experimental data. These results show that this simulation method could be an effective tool to optimize the design of the two-stage pusher centrifuge.     

Analysis of energy dissipation in stirred suspension polymerisation reactors using computational fluid dynamics

This work evaluates the spatial distribution of normalised rates of droplet breakage and droplet coalescence in liquid–liquid dispersions maintained in agitated tanks at operation conditions normally used to perform suspension polymerisation reactions. Particularly, simulations are performed with multiphase computational fluid dynamics (CFD) models to represent the flow field in liquid–liquid styrene suspension polymerisation reactors for the first time. CFD tools are used first to compute the spatial distribution of the turbulent energy dissipation rates (ε) inside the reaction vessel; afterwards, normalised rates of droplet breakage and particle coalescence are computed as functions of ε. Surprisingly, multiphase simulations showed that the rates of energy dissipation can be very high near the free vortex surfaces, which has been completely neglected in previous works. The obtained results indicate the existence of extremely large energy dissipation gradients inside the vessel, so that particle breakage occurs primarily in very small regions that surround the impeller and the free vortex surface, while particle coalescence takes place in the liquid bulk. As a consequence, particle breakage should be regarded as an independent source term or a boundary phenomenon. Based on the obtained results, it can be very difficult to justify the use of isotropic assumptions to formulate particle population balances in similar systems, even when multiple compartment models are used to describe the fluid dynamic behaviour of the agitated vessel. © 2011 Canadian Society for Chemical Engineering     

Design study of printer nozzle spray dryer by computational fluid dynamics modeling

Abstract

Milk powder production is one of the most energy-consuming processes in the dairy industry. To reduce production costs and increase spray dryer efficiency, the EU sponsored an international project named ENTHALPY. One of the results of the project was the development of mathematical models and computational fluid dynamics simulation methodology for the milk spray-drying process. This article introduces the methodology of CFD model development and presents simulation results as the basis for further drying tower design optimization. The CFD simulations were performed for a new design of monodisperse multistream atomizer. As a result of the CFD simulation, sets of parameters such as drying efficiency, protein thermal degradation, collision frequency, and wall deposition were obtained.     

Analysis of shear-induced coagulation in an emulsion polymerisation reactor using computational fluid dynamics

Shear-dependent coagulation is a costly problem for the latex manufacturing industry, due to product degradation and reactor downtime. In this study, a method for calculating the shear-dependent coagulation rate in emulsion polymerisation is developed. The method combines simple models for coagulation (only binary collisions being considered) with the effects of rheology on the flow field, using computational fluid dynamics (CFD) to solve the detailed flow field in the reaction vessel. By using the local shear rates (LSR), the method developed provides a more detailed and system-specific assessment compared with using an average shear rate (ASR) for calculating the coagulation rate. The difference in the predictions between the ASR and the proposed LSR method was investigated. It was found that the ASR and LSR methods predict different coagulation rates, especially for more sophisticated coagulation models where the coagulation rate is not linearly dependent on the shear rate. The LSR method was also used to study the effect of the rheology of the latex, of the impeller speed and of the reactor design on the coagulation rate. It was found that the LSR method is useful for providing both visual and numerical means to identify regions with elevated coagulation rates in the modelled reaction vessel. The treatment provides estimates of the amounts of coagulum formed on the vessel walls and on the impeller.     

Integrated FCC riser—regenerator dynamics studied in a fluid catalytic cracking pilot plant

In this paper a dynamic simulator of the fluid catalytic cracking (FCC) pilot plant, operating in the Chemical Process Engineering Research Institute (CPERI, Thessaloniki, Greece), is presented. The operation of the pilot plant permits the execution of case studies for monitoring of the dynamic responses of the unit, by imposing substantial step changes in a number of the manipulated variables. The comparison between the dynamic behavior of the unit and that predicted by the simulator arise useful conclusions on both the similarities of the pilot plant to commercial units, along with the ability of the simulator to depict the main dynamic characteristics of the integrated system. The simulator predicts the feed conversion, coke yield and heat of catalytic reactions in the FCC riser on the basis of semi-empirical models developed in CPERI and simulates the regenerator according to the two-phase theory of fluidization, with a dilute phase model taking account of postcombustion reactions. The riser and regenerator temperature, the stripper and regenerator pressure drop and the composition of the regenerator flue gas are measured on line and are used for verification of the ability of the simulator to predict the dynamic transients between steady states in both open- and closed-loop unit operation. All the available process variables such as the reaction conversion, the coke yield, the carbon on regenerated catalyst and the catalyst circulation rate are used for the validation of the steady-state performance of the simulator. The comparison between the dynamic responses of the model and those of the pilot plant to step changes in the feed rate and preheat temperature reveals the ability of the simulator to accurately depict the complex pilot process dynamics in both open- and closed-loop operation. The dynamic simulator can serve as the basis for the development of a model-based control structure for the pilot plant, alongside its use as a tool for off-line process optimization studies.     

计算流体力学在氧化沟设计中的应用

利用计算流体力学对氧化沟的倒伞曝气机及推流器建立叶轮机械驱动模型,建立氧化沟污水-污泥多相流模型,利用计算流体力学软件Fluent对倒伞曝气机及推流器驱动的氧化沟进行流场模拟,能够较为准确的预判各种情况下的流场,有效地整合表面曝气机与潜水推流器的统一,能够很好地对氧化沟工程设计,设备配置及布置等起优化作用,并对污水厂运营起指导作用。     

Chemistry and radiation in oxy-fuel combustion: A computational fluid dynamics modeling study

In order to investigate the role of combustion chemistry and radiation heat transfer in oxy-fuel combustion modeling, a computational fluid dynamics (CFD) modeling study has been performed for two different oxy-fuel furnaces. One is a lab-scale 0.8 MW oxy-natural gas flame furnace whose detailed in-flame measurement data are available; the other is a conventional 609 MW utility boiler which is assumed to be operating under oxy-fuel combustion condition with dry flue gas recycle. A new model for gaseous radiative properties is developed, validated, and then implemented in the CFD simulations. The CFD results are compared to those based on the widely used model in literature, as well as the in-flame measurement data. The importance and advantage of the new model for gaseous radiative properties have been well demonstrated. Different combustion mechanisms are also implemented and compared in the CFD simulations, from which significant difference in the predicted flame temperature and species is observed. This difference is consistent with those expected from the equilibrium calculation results. As a conclusion, the appropriate combustion mechanisms applicable to oxy-fuel combustion modeling are identified. Among the key issues in combustion modeling, e.g., mixing, radiation and chemistry, this paper derives useful guidelines on radiation and chemistry implementation for reliable CFD analyses of oxy-fuel combustion, particularly for industrial applications.     

计算流体力学在填料塔中的应用

本文综述了计算流体力学在填料塔中的应用和进展情况,并根据研究方法的不同,将其分为两类,即整体平均的研究方法和单元综合的研究方法。并对各研究方法的优势和不足及其应用前景进行了评述,认为用计算流体力学来研究填料塔内流体的流动状况是今后的发展趋势。