尼龙6聚合管内微分发热量与温度分布

<正>1引言 尼龙6是合成聚酚胺生产中最重要的产品之一[1]，目前工业上多数采用水解聚合。本文涸绕尼龙6聚合反应工程开发的一系列工作都是为这一技术路线国产化提供理论基础。近卫0年来，尼龙6聚合的各种主、副反应精确的动力学数据已有报道[2,3]，目前的重点已转向工业化生产反应器中各种物理过程的模拟，诸如传热、流动、混合、停留时问分布、蒸发脱水以及过程优化控制等领域。     

Dispersion in fully developed flow through regular porous structures: Experiments with wire-mesh sensors

Within this study the wire-mesh sensor is proposed as a suitable device to investigate radial and axial dispersion in tubular reactors. Axial dispersion in turbulent flow through a regular highly porous structure is addressed and the effect of reactor length on the estimated axial dispersion coefficient is discussed. We state that the gradual increase of turbulence intensity in the entrance section of the porous structure is an effect which leads to a length dependence of the dispersion coefficient. Furthermore, we present measurements of radial mixing by the wire-mesh sensor and compare it to reference measurements using laser induced fluorescence. By that we discuss the possible bias from non-homogeneous tracer distribution between the electrodes. Despite the differences of the measurement principles we found a good qualitative agreement between the results.     

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.     

Mixing–segregation phenomena of binary system in a fluidized bed

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D., 1.8 m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.     

On the residence time distribution in reactors with non-uniform velocity profiles: The horizontal stirred bed reactor for polypropylene production

Horizontal stirred bed reactors for polypropylene production have a typical axial powder mixing pattern which is unique for polyolefin gas-phase reactors. The polymer, although adhering to the catalyst particles, has a shorter mean residence time and a broader residence time distribution than the catalyst. Both polymer and catalyst residence time distributions, strongly depend on the temporal catalyst activity profile. The powder mixing pattern in a horizontal stirred bed reactor results from two transport effects: the continuously increasing powder net flow in the downstream direction caused by the particle growth, and the simultaneous stirring flows with equal intensity in the up- and downstream directions. Both together lead to a residence time distribution of the reacting catalyst particles between fully backmixed and plug flow. A modelling approach is proposed which describes the axial flow contributions of stirring and polymerisation separately, and which is more accurate and flexible than simple cells-in-series models used in most papers so far. For large-scale reactors, residence time distributions are predicted from experimental pulse-response curves obtained in a miniaturised mixing model without reaction. Residence time distributions equivalent to “three to five” well-mixed reactors in a series reported in the literature are experimentally accessible only with difficulties under polymerising conditions. With the modelling approach proposed in this paper, the “reacting” RTD can be calculated on the basis of the cold-flow experimental data. We compare the proposed modelling concept with simple cells-in-series models by simulating catalyst yields, particle size distributions and catalyst transitions using two model catalysts with different activity profiles. The selection of the right modelling approach has indeed a significant influence on the simulation results, especially if the throughput or the axial velocity profile is changing and the catalyst is not deactivating.     

Polymerization of styrene in a continuous filled tubular reactor

Free radical solution polymerization of styrene has been studied using a binary mixture of symmetrical bifunctional initiators in a filled tubular reactor packed with static mixers. Owing to intensive radial mixing induced by the static mixers, a near plug flow pattern was obtained in the reactor with some axial dispersion effect. The axial mass dispersion coefficient was determined from the residence time distribution experiment and a dynamic axial dispersion model has been developed and solved to investigate steady state and transient behavior of the filled tubular reactor. With a solvent volume fraction of 0.3, the monomer conversion up to 70% was obtained without fouling problems in the temperature range 90 to 120°C. The experimental filled tubular reactor was operated under various reaction conditions and a reasonably good agreement between the model and the experimental data was obtained without using any adjustable parameters.     

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.     

Superior mixing performance for airlift reactor with a net draft tube

A modified networks-of-zones model is developed to investigate the mixing performance of three tower-type bioreactors, namely airlift, bubble column and net column (a short notation for airlift reactor with a net draft tube) reactors. A key parameter β, that characterizes the interaction intensity between the neighboring uprising and down-coming streams, is identified to play a decisive role in determining the mixing characteristics of the three tower-type reactors. The concentration dynamics and mixing behaviors of the three types of reactor are studied with a maximum non-zero eigenvalue analysis (the slowest mode analysis). The model predictions are validated with experiments of heat mixing. The superior mixing performance of the net column reactor over the airlift and bubble column reactors is clearly revealed with the present model and is experimentally verified, and can be linked to an optimum mass transfer between the neighboring uprising and down-coming streams, provided by the net draft tube. This optimum mass transfer is a direct result of a balanced flow distribution in the axial and radial directions.     

Modeling of a Buss‐Kneader as a polymerization reactor for acrylates. Part I: Model validation

The Buss‐Kneader is generally known as a compounding device. Although a reasonable number of papers have been published on extruders as polymerization reactors, only little is known about the behavior of the Buss‐Kneader when used as a polymerization reactor. Its good mixing properties in the radial and axial directions make it a suitable reactor for exothermal polymerization reactions. This paper describes experiments with the co‐polymerization of n‐butyl acrylate and hydroxyethyl methacrylate in a Buss‐Kneader. For model calculations the Buss‐Kneader was treated as a plug flow reactor with axial dispersion. Experimental results on axial temperature profile, monomer conversion and molecular weight are compared with model calculations. Model parameters are based on independently measured data on the heat transfer coefficient, axial dispersion and polymerization kinetics.     

Solids mixing in a fluidized bed riser

The solids mixing in a riser with a height of 10 m and 0.186 m inner diameter was investigated by using pneumatic phosphor tracer technique. Considering the shielding effect of the bed material on the light emitted from the phosphor tracer particle, a modified method for the phosphor tracer measurement is proposed. And then the curves of particle residence time distribution were obtained. The experimental results show that the particle diffusion mechanism can be explained by the dispersions of dispersed particles and particle clusters in the axial direction, and as well the core-annulus nonuniform distribution of the solids fraction in the radial direction of the riser. Moreover, based on the experimental results, a two-dimensional dispersion model was established to predict the solids axial and radial diffusion. Furthermore, the effects of superficial gas velocity and solids circulating flux on the axial and radial Peclet number of the particles were discussed; two empirical correlation formulas about the axial and the radial Peclet numbers were given; the calculated values agree well with the experimental results.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Effect of impeller type on the mixing in torus reactors

Torus reactors are characterized by a homogeneous fluid circulation without dead zones. Torus reactors were used for applications in biotechnology, food processing, polymerization and liquid waste treatments. The relatively simple extrapolation of performances, due to the absence of dead volume, is one of the main advantages of this reactor, with low shear stresses and an effective radial mixing allowing efficient heat dissipation. This study is based on the mixing in order to analyse the fluid circulation, mainly in turbulent flow regime, and to characterize the torus reactor with the axial dispersion plug flow model. The objective of this study is to characterize the flow and the mixing in the torus reactors in batch and continuous modes. The mixing analysis was made according to the flow parameters and to the geometrical characteristics of the reactor and impeller. The mixing in the torus reactor can be characterized by the Péclet number, Pe_D, defined with torus diameter. A representative model based on plug flow with axial dispersion and partial recirculation was proposed.     

Effective dispersion model for flow-through catalytic membrane reactors combining axial dispersion and pore size distribution

In a flow-through catalytic membrane reactor, catalyst is immobilized in the pores of a membrane, which is convectively passed by the reaction mixture, allowing for high catalytic activity and a potentially narrow residence time distribution (RTD). As the membrane geometry prevents measurement of a meaningful RTD, a residence time distribution model is introduced, which accounts for deviations from ideal plug flow behavior induced by a non-ideal pore size distribution and by axial molecular diffusion. Both effects are combined to an effective dispersion model with a single dimensionless parameter, which is a function of pore geometry, axial velocity and molecular diffusion coefficient. As a result of the developed model, recommendations for optimum reactor operation are given.     

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.     

下喷式环流反应器液相流动特性研究

液相停留时间分布分析用于下喷式环流反应器导流筒顶部区域、底部区域、环隙流体流动特性研究。分别将轴向扩散模型应用到各个区域,实验结果表明轴向扩散模型能较好的预测反应器内流体的停留时间分布。采用最小二乘法拟合实验响应曲线,得到模型方程参数。结果表明各个部分的Pe值均随液体喷射速度的增大而减小。导流筒顶部区域的Pe值变化范围为:25.4~6.6;导流筒底部区域的Pe值变化范围为:45.4~11.6;环隙的Pe值变化范围为:60.0~39.2。结果表明导流筒顶部区域返混最大,环隙区域接近于平推流。反应器混和时间随液体喷射速度的增大而减小,变化范围为:88.3~12.5 s。     

旋转盘式混合器混合过程数值模拟

基于旋转盘式混合器的研制实践,采用Polyflow软件在分析其速度和剪切速率分布的基础上,研究了物料粒子的动态混合过程;并从物料停留时间和最大剪切应力分布角度分析了旋转盘式混合器的分布和分散混合性能。结果表明,旋转盘式混合器可提供强大的剪切、置换和压缩作用,随着动盘旋转体转速增加,分散混合性能增强,不利于轴向分布混合。     

Residence time distribution and dispersion of gas phase in a wet gas scrubbing system

Residence time distribution (RTD) of exhaust gas in a wet scrubbing system was investigated for application to the removal of SO _x , NO _x or dust included in exhaust gas. The mixing of gas phase in the wet scrubbing system was also examined by considering the axial dispersion coefficient of gas phase. Effects of gas amount (velocity), liquid amount (velocity) and solid floating materials on the residence time distribution (RTD) and axial dispersion coefficient of exhaust gas were discussed. The addition of solid floating materials could change the RTD and thus dispersion of exhaust gas in the scrubbing system. The mean residence time and axial dispersion coefficient of exhaust gas were well correlated in terms of operating variables.     

Flow visualization and modelling of a filter-press type electrochemical reactor

A study of flow visualization and residence time distribution is provided in order to model the flow between two electrodes in a commercial filter-press reactor, the ElectroSynCell® from Electrocell AB. Flow visualization indicates that both axial and lateral dispersion phenomena occur and a global plug flow behaviour is observed. The flow distribution is asymmetric due to the design of the inlet system in the active zone. The flow throughout the cell is described by a dispersed plug flow model for which the mean residence time and the Pe´clet number are determined. The reaction area and the inlet system are separately analysed by locating conductimetric probes inside the electrochemical cell. The reaction area is also well described by a dispersed plug flow model, and characterized by high dispersion. The inlet system is, respectively, described by a dispersed plug flow model and by a cascade of continuous stirred tank reactors. The high number of reactors in the cascade denotes a quasi plug flow behaviour. The results are confirmed by two cascades of continuously stirred tank reactors in series. The dispersion coefficients obtained throughout the reaction area of the cell are not constant. This shows that the flow is not well established at the entrance of the reaction zone and depends on the entrance conditions.     

Residence Time Characteristics of the Novel Archimedean Screw Crystallizer/Reactor

This work presents a novel design of a continuous crystallizer, the Archimedean screw crystallizer reactor (ASKR) with integrated Archimedean screw as conveying element. By the screw, the axial motion of the product solution is initiated, and an undesired axial mixing is restricted. This structure results in a narrow residence time distribution and, thus, promotes a defined product quality. In order to quantitatively assess the flow characteristics of the crystallizer and to highlight the influence of variable operating parameters such as rotational speed and volumetric flow, investigations of the residence time behavior with variation of the operating parameters are reported.     

Mixing in reactive extrusion of low-density polyethylene melts: Linear vs. branched

The melt flows of linear low-density polyethylene (LLDPE) and branched low-density polyethylene (LDPE) have been compared in a fully intermeshing co-rotating twin-screw extruder. The polyethylene melts were selected in order to investigate the effects of the melt rheology on the mixing. Their shear vicosity curves are quite similar, but the LDPE has a markedly higher apparent extensional viscosity over a wide range of stretch rates. The stagger of the paddles in the mixing zone of the extruder creates axial pressure-driven axial flow can have significant extensional strain components. Residence time distributions obtained in the melt zones of the extruder with tracer dye reveal that the LDPE has a narrower residence time distribution than the LLDPE over a wide range of operating conditions. The axial dispersion for the LDPE is significantly lower than the axial dispersion for the LLDPE. This is attributed to the greater extensional viscosity of the LDPE. During the reactive extrusion process, solid maleic anhydride and polyethylene were added at the feed port but the peroxide provides better control of the crosslinking reaction. Residence time distributions measured for the chemically more reactive LLDPE melt indicate reduced levels of axial mixing with reaction. The reduction in mixing is due to a crosslinking reaction that occurs in parallel to the grafting reaction. This change in mixing is smaller than the difference in mixing between LDPE and LLDPE.