Computer Simulation of the Performance of a Downer‐Regenerator CFB for the Partial Oxidation of n‐Butane to Maleic Anhydride

A reactor model for a downer‐regenerator circulating fluidized‐bed (CFB) during the partial oxidation of n‐butane to maleic anhydride is presented. Upflow reactors (risers) suffer from severe solids back mixing and gas‐solids‐separation, in comparison down flow reactors exhibit a more uniform gas‐solids flow and reduced backmixing, resulting in narrower residence time distributions. Due to the sensitivity of the VPO catalyst to over‐reduction, downer reactors present an interesting alternative to riser reactors. The reactor models for the downer and the regenerator fluidized‐bed are coupled with reduction and oxidation kinetics for the catalyst, respectively. The influence of the solids residence time distributions for the combined system of both reactors on the oxidation state of the catalyst is explored by a novel newly developed oxygen loading distribution. Simulation results suggest the limited solids‐flux in downers restrict the maximum butane concentrations, while the scale‐up is predicted to be uncritical.     

Mixing performance comparison of milliscale continuous high‐shear mixers

Mixing performance of two continuous flow millilitre‐scale reactors (volumes 9.5 mL and 2.5 mL) equipped with rotor‐stator mixers was studied. Cumulative residence time distributions (RTD) were determined experimentally using a step response method. Distributions were measured for both reactors by varying impeller speed and feed flow rate. The mixing effect was determined by measured RTDs. Computational fluid dynamics (CFD) were used to verify that the residence time distribution in the measurement outlet agreed with the outlet flow. The mixing power of both reactors was determined using a calorimetric method. The reactor inlet flow rate was found to affect mixing performance at 1–13 s residence times but the effect of impeller speed could not be noted. Both milliscale reactors are close to an ideal continuous stirred‐tank reactor (CSTR) at the studied impeller speed and flow rate ranges. The specific interfacial area was found to depend on the reactor inlet flow rate at constant impeller speed for the case of copper solvent extraction.

     

MeOH to DME in bubbling fluidized bed: Experimental and modelling

Methanol dehydration over a ZSM‐5 containing catalyst was studied in a fluidized bed reactor. At temperatures ranging from 250 to 325°C, methanol conversion varied from 30% at a contact times of 0.14 s and approached 100% of the equilibrium conversion at a contact time starting from 10 s. Sequential and parallel reactions were negligible at low temperatures while hydrocarbon formation became appreciable at 325°C. Online gas analysis by mass spectrometry provided real‐time measurements at a frequency of 4.4 Hz that allowed for fast determination of steady‐state conditions. Gas phase residence time distribution (RTD) measurements indicated that axial dispersion was essentially negligible at short contact times with a shallow bed of catalyst. With longer residence times, the flow pattern could be approximated by six continuously stirred‐tank reactors (CSTR) in series. Both the simple 1D hydrodynamic model and a detailed multi‐zone fluidized model were used to interpret the experimental data to derive a kinetic expression for the dehydration of methanol to di‐methyl ether (DME). The expression includes the reverse reaction that is most often neglected in the literature. The reaction data were best fit with the kinetics based on the 1D model. The fluidized bed is a viable reactor type for kinetic measurements of highly exothermic reactions where hotspots and radial and axial temperature gradients are problematic in fixed beds.     

内置螺旋挡板流化床颗粒停留时间分布

采用脉冲示踪法在内置螺旋挡板冷态鼓泡流化床上研究了螺旋挡板、加料速率、流化风速、颗粒粒径和床料高度对颗粒在流化床内停留时间分布的影响. 结果表明,颗粒停留时间的无量纲方差从无螺旋挡板时的0.558减小到有螺旋挡板时的0.085,螺旋挡板可有效抑制颗粒返混,增大颗粒运动的平推流趋势;加料速率增大为约2倍时,停留时间减小为约50%,流动更趋向于平推流;床料高度增加,颗粒返混加剧,颗粒平均停留时间及无量纲方差均增大,颗粒运动向全混流靠近;随流化风速增大,颗粒平均停留时间变长;实验范围内,颗粒粒径对颗粒停留时间分布影响不大.     

Axial gas dispersion in a fluidized bed of polyethylene particles

Gas mixing behavior was investigated in a residence time distribution experiment in a bubbling fluidized bed of 0.07 m ID and 0.80 m high. Linear low density polyethylene (LLDPE) particles having a mean diameter of 772 Μm and a particle size range of 200-1,500 Μm were employed as the bed material. The stimulus-response technique with CO₂ as a tracer gas was performed for the RTD study. The effects of gas velocity, aspect ratio (H₀/D) and scale-up on the axial gas dispersion were determined from the unsteady-state dispersion model, and the residence time distributions of gas in the fluidized bed were compared with the ideal reactors. It was found that axial dispersion depends on the gas velocity and aspect ratio of the bed. The dimensionless dispersion coefficient was correlated with Reynolds number and aspect ratio.     

Radioisotope tracer study in trickle bed reactors

The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air‐water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing.     

A hybrid CFD—reaction engineering framework for multiphase reactor modelling: basic concept and application to bubble column reactors

The coupling of turbulent mixing and chemical phenomena lies at the heart of multiphase reaction engineering, but direct CFD approaches are usually confronted with excessive computational demands. In this hybrid approach, the quantification of mixing is accomplished through averaging the flow and concentration profiles resulting from a CFD flow field calculation and a computational (“virtual”) tracer experiment. Based on these results, we construct a mapping of the CFD grid into a generalised compartmental model where the chemistry calculations can be efficiently carried out. In contrast to the empirical models used in the residence time distribution (RTD) approach, the compartmental model in this methodology, owning to its CFD origins, retains the essential features of the equipment geometry and flow field. A procedure for extracting the mixing information from k-ε based CFD codes is outlined, but the main concept of the approach is not restricted to any particular type of turbulence modelling, and will therefore benefit from future developments. A phenomenological model of mass transfer and chemical reaction, based on the penetration theory, is employed to simulate the interfacial phenomena in gas-liquid reactors, and a study of CO₂ absorption into alkali solution is presented to demonstrate the method.     

Entwurf und Betrieb eines Rohrreaktors mit enger Verweilzeitverteilung

In process engineering the residence time is an important design parameter, and a narrow residence time distribution is advantageous to avoid possible by-products in complex chemical reactions. A good radial mixing with low axial dispersion provides a narrow residence time distribution in a tube reactor. The axial dispersion of laminar flow in a straight tube is very high and generates a wide residence time distribution. However, secondary flows improve the radial mixing, which are investigated in this paper for curved tube reactors. Design notes for good radial mixing and geometric designs of tube reactors with baffles are presented.     

管式搅拌反应器中流动特性实验及模型研究

为进一步研究带机械搅拌装置的新型管式反应器内流动特性,采用刺激-响应技术,测定流体在不同操作条件下的停留时间分布(RTD)曲线并与无搅拌停留时间分布曲线作对比,计算了平均停留时间分布的统计特征值。用混合时间表征混合特性,用Peclet操作准数表征轴向扩散特性。结果表明,适当增大转速、流量或降低水位,都有利于反应器内流体的均匀混合。大体上随流量的减少和液位的升高停留时间有延长的趋势,转速变化对停留时间的影响不显著。在搅拌转速不超过400 r/min时,混合时间随着搅拌转速的增大而缩短。在实验范围内,反应器相当于3个串联全混槽反应器。     

Mixing time in bioreactors

The mixing time in bioreactors is discussed. Theoretical correlations for the mixing time in stirred tank and bubble column reactors respectively, are derived. They are based on Kolmogoroff's theory of isotropic turbulence and the relationships between the mixing time and the axial dispersion coefficient in the turbulent flow regime. The predictions of the proposed correlations are in reasonable agreement with several available data and correlations. A wide divergence among the data for the mixing time reported in the literature is also discussed.     

不同推进式桨叶对搅拌反应器内气液两相混合特性的影响

为了研究不同推进式桨叶对搅拌反应器内气液两相混合特性的影响,以某搅拌反应器的推进式桨叶为研究对象,将搅拌聚合物简化为含5%气体的清水介质,基于螺旋桨叶片设计方法和CFD流场仿真技术,采用VOF多相流模型和RNG k-ε 湍流模型,对四种推进式桨叶内部气液两相流动进行数值分析,实现了推进式桨叶参数设计和性能优化。分析设计转速在400 r/min时的径向速度、0~18 s的时间范围内气体体积分数的变化、无量纲气体体积分数以及无量纲轴向速度,来评价四种推进式搅拌反应器搅拌性能的剪切、混合、分散。研究结果表明：变螺旋角(FDC-450-γ)非对称桨叶的流动更均匀、混合速率更快和剪切分散能力能强。通过对四种不同推进式桨叶的比较分析,为后续的研究和工程实践奠定了基础。     

The effect of downstream synthesis gas feeding on Fischer–Tropsch product distributions

The current study examines the effect of downstream synthesis gas feeding on Fischer–Tropsch product distributions in two series plug flow reactors over a precipitated iron catalyst. Synthesis gas at a constant H₂:CO ratio of 2.5 was fed into the entrance of a first reactor, and the resulting products were fed into a second reactor along with fresh synthesis gas of varying H₂:CO ratios (3, 2, 1, and 0.5). The selectivity of C₅₊ hydrocarbons in the downstream feeding cases was over two times lower than the corresponding “top feeding” cases, in which an equivalent amount of synthesis gas was fed through both series reactors without the addition of downstream synthesis gas. These observations were attributed to the effects of bed residence time and H₂:CO ratios as influenced by the simultaneous water–gas shift reaction.     

Paste Residence Time in a Spouted Bed Dryer. III: Effect of Paste Properties and Quality Interactions

The aim of this study was to evaluate the effects of paste properties on residence time during drying in a spouted bed dryer with inert bodies. The effect of paste solids content, surface tension, and viscosity on the residence time distribution and the mean residence times were studied using factorial experimental designs. The inert bodies used were glass and polyethylene beads. The mean residence times varied from 13.6 to 16.3 and 12.2 to 17.7 min for drying on glass and polyethylene beads, respectively. The analysis of variance showed that mean residence times significantly depended on solids content and surface tension for glass beads and also on viscosity for polyethylene beads. The residence time distributions for all conditions studied fitted well to the perfect mixing cell when applying the continuous stirred vessels in series model analysis. The powder density, flowability, and particle size depended on paste properties and inert type.     

Paste Residence Time in a Spouted Bed Dryer. III: Effect of Paste Properties and Quality Interactions

The aim of this study was to evaluate the effects of paste properties on residence time during drying in a spouted bed dryer with inert bodies. The effect of paste solids content, surface tension, and viscosity on the residence time distribution and the mean residence times were studied using factorial experimental designs. The inert bodies used were glass and polyethylene beads. The mean residence times varied from 13.6 to 16.3 and 12.2 to 17.7 min for drying on glass and polyethylene beads, respectively. The analysis of variance showed that mean residence times significantly depended on solids content and surface tension for glass beads and also on viscosity for polyethylene beads. The residence time distributions for all conditions studied fitted well to the perfect mixing cell when applying the continuous stirred vessels in series model analysis. The powder density, flowability, and particle size depended on paste properties and inert type.     

Age distribution and the degree of mixing in continuous flow stirred tank reactors

New development of mean age theory is discussed for quantitative analysis of mixing and age distribution in steady continuous flow stirred tank reactors. A new relationship between the moments of age and the moments of residence time are derived. With this new relationship the variance of residence time distribution can be computed much more efficiently and accurately. The relationships of three existing variances of age are described and a new set of variances and the degree of mixing are defined. The theory is used to characterize mixing performance in a CFSTR with different layouts of an inlet and an outlet. Mean age and higher moments of age in the reactors are obtained from CFD solutions of their steady transport equations. The spatial distribution of mean age reveals details of the spatial non-uniformity in mixing. Variances of age and the degree of mixing discussed by Danckwerts and Zwietering are computed for the first time in the literature for non-ideal stirred tank reactors. It is found that although these measures are useful, certain key features in non-uniform mixing are not reflected accurately. Results show that the new set of variances and the degree of mixing more accurately characterize the non-uniform mixing in the reactors.     

内循环式无梯度反应器的研究

设计了一种内循环式无梯度反应器,可用以进行气固相催化反应动力学的研究。反应器的结构简单。采用电磁搅拌器以克服旋转轴径向密封的困难。催化剂可静置于多孔板上,或放在转篮中与轴一起运动。催化剂的粒度从零点几毫米至工业粒度均可适用。经停留时间分布测定证明反应器内气体的流动状况属于理想混合。径向及轴向的温度差甚小。这种反应器可以在800℃以下的温度操作。 研究了不同的气体体积流量下停留时间分布和搅拌速率间的关系。为了达到完全混合状态,较高的气体流量要求较高的搅拌速率。此外,在不同的搅拌速率下,还测定了反应器出口反应物的浓度,当搅拌速率增加至某一定数值后,组成不再改变。     

Onset of pulsing in gas-liquid trickle bed filtration

When liquid suspensions containing low concentration of fine solids are treated in catalytic packed bed gas-liquid-solid reactors, which are operated in trickle flow or near the transition between trickle and pulse flow, plugging develops and increases the resistance to two-phase flow. Also due to obstruction, such accumulation of fines in the catalyst bed shifts progressively the flow pattern from trickling to pulsing flow. The progressive onset of pulsing flow along the packed bed was estimated using a sequential approach based on combining a “large time-scale” unsteady-state filtration solution of two-phase flow with a “short time-scale” solution of a linear stability analysis of two-phase flow. Space-time evolution and two-phase flow of the deposition of fines in trickle bed reactors under trickle flow regime was described using a one-dimensional two-fluid model based on the volume-average mass and momentum balance equations and volume-average species balance equation for the fines. The model hypothesized that plugging occurred via deep-bed filtration and incorporated physical effects of porosity and effective specific surface area changes due to the capture of fines, inertial effects of phases, and coupling effects between the fines filter rate equation and the interfacial momentum exchange force terms. The transition between trickle flow and pulse flow regimes was described from a stability analysis of the solution of the transient two-fluid model around an equilibrium state of trickle flow under pseudo steady state conditions. The impact of liquid superficial velocity, viscosity and surface tension, gas superficial velocity and density, feed fines concentration, and fines diameter on the transition between trickle and pulse flows in the presence of fines deposition was analyzed.     

Numerical Simulation of Macroscopic Mixing in a Rushton Impeller Stirred Tank

The macroscopic mixing in a stirred tank with different tracer injection locations, impeller speeds and impeller positions is simulated numerically by solving the transport equation of the tracer based on the whole flow field in the baffled tank with a Rushton disk turbine numerically resolved using the improved inner-outer iterative procedure. Predicted mixing time is compared well with the literature correlations. The predicted residence time distribution of the stirred tank is very close to the present experimental results. The effect of the installation of a draft tube on the mixing time and residence time distributions is addressed.     

Paste Residence Time in a Spouted Bed Dryer. IV: Effect of the Inert Particle Size Distribution

The residence time distribution and mean residence time of a 10% sodium bicarbonate solution that is dried in a conventional spouted bed with inert bodies were measured with the stimulus-response method. Methylene blue was used as a chemical tracer, and the effects of the paste feed mode, size distribution of the inert bodies, and mean particle size on the residence times and dried powder properties were investigated. The results showed that the residence time distributions could be best reproduced with the perfect mixing cell model or N = 1 for the continuous stirred tank reactor in a series model. The mean residence times ranged from 6.04 to 12.90 min and were significantly affected by the factors studied. Analysis of variance on the experimental data showed that mean residence times were affected by the mean diameter of the inert bodies at a significance level of 1% and by the size distribution at a level of 5%. Moreover, altering the paste feed from dripping to pneumatic atomization affected mean residence time at a 5% significance level. The dried powder characteristics proved to be adequate for further industrial manipulation, as demonstrated by the low moisture content, narrow range of particle size, and good flow properties. The results of this research are significant in the study of the drying of heat-sensitive materials because it shows that by simultaneously changing the size distribution and average size of the inert bodies, the mean residence times of a paste can be reduced by half, thus decreasing losses due to degradation.