EFFECTS OF HYDROCARBON LIQUID FEED IN POLYETHYLENE POLYMERIZATION PROCESS ON PARTICLE SURFACE TEMPERATURE

In the fluidized bed gas phase polymerization of polyethylene (PE), the heat generated by the exothermic polymerization process is dissipated into the gas mixture flowing past the polymer particles. The polymer particle temperature is determined by the extent of convective heat transfer and other mechanisms of heat removal. In addition to the heat removal by convective heat transfer, liquid hydrocarbon (HC) is often injected into the reactor to further remove heat by evaporation but without partaking in the reaction. The effects of adding this liquid HC on the particle surface temperature have been investigated numerically by means of a one-dimensional polar model. Results indicate that the primary mechanism for removal of the heat of polymerization from the particles is by means of convective heat transfer to the bulk gas, which amounts to 99.5% removal of total heat of polymerization. The PE particle temperature rises only by 1-2°C above the surrounding bed gas mixture. The addition of liquid HC to the feed, however, has a pronounced effect on controlling the reactor gas temperature as most of this liquid is evaporated to the gaseous phase before it reaches the polymer particles. To state it clearly, heat of polymerization is transferred from the particles to the reactor bulk gas predominantly by convection, and part of this heat is subsequently absorbed by evaporation of the fresh liquid HC in the feed. Comparison with a detailed computational fluid dynamic (CFD) model of polymerization in a generic gas phase reactor has also been conducted. The results confirm that the particle temperature rise above the reactor gas temperature is consistent with the one-dimensional model. However, local gas temperature variations are present in the reactor due to the unsteady gas-solid hydrodynamics. Hence, there are some zones that are a few degrees hotter/colder than the bulk reactor temperature with corresponding increase/decrease in particle temperature in these zones.     

CONVECTIVE HEAT TRANSFER CHARACTERISTICS OF GAS IMPINGEMENT ON SMALL HEATED DEVICE IN LIQUID COOLANT BATH

Heat transfer characteristics of a small heated device have been investigated in a liquid bath with gas jetimpingement as function of gas flow rate,coolant temperature,liquid phsicochemical properties,heat flux,heat source size,ambient pressure and the distance between jet and heated wall.The experimental results show that the agitation of liquid caused by gas jet bubbles increases greatly therate of heat transfer,and the evaporation of coolant near the wall,which was due to the concentration differencebetween gas-liquid interface and bulk gas phase,gives additional enhancement of heat transfer.The rate ofevaporation related to the bubble growth was mathematically formulated.By using the simultaneous heat and mass transfer model,the convective heat transfer coefficient and masstransfer coefficient can be deduced from the experimental results.In addition,the local heat transfer coefficient and the distribution of evaporation heat flux on the smallheated surface are investigated mathematically and experimentally.     

乙烯气相聚合流化床反应器内Geldart B类和Geldart D类颗粒流动特性的三维数值模拟

乙烯气相聚合流化床反应器的设计、操作和优化依赖于对聚合物颗粒粒径大小和分布、气泡运动特性及聚合反应状况的准确描述。采用Eulerian-Eulerian 双流体模型和群体平衡模型耦合方法对某乙烯气相聚合中试规模的工业流化床反应器分别处于常规聚合工艺(属Geldart B 类颗粒)和免造粒工艺(属Geldart D 类颗粒)时床体的气固流动特征以及不同颗粒类型对反应器操作状态和颗粒运动特性的影响进行了三维数值模拟研究。与传统聚乙烯生产工艺相比,免造粒工艺时的Geldart D 类聚合物颗粒更易聚集于气体入口处区域,而且会产生明显的旋涡并出现较大的气泡。研究结果可为免造粒聚乙烯生产工艺的工业推广应用提供参考。     

丁二烯气相聚合过程中聚合物颗粒增长的动态研究

An experimental apparatus composed of microscope, video camera, image-processing, and mini reactor which can be used for real-time measurement of the growth of polymer particle in gas phase polymerization was built up to carry out dynamic study of gas phase polymerization of butadiene by heterogeneous catalyst based on neodymium(Nd). The studies of the shape duplication of polymer particles and catalyst particles and the growth rate of polymer paxticle were made. Results show that the appaxatus and procedure designed can be well utilizedto make dynamic observation and data collection of the growth of polymer particle in gas phase polymerization.A phenomenon of shape duplication of polymer particles and catalyst particles was observed by the real-time measurement. The result also concludes that the activity of individual catalyst particle is different, and the effect of reaction pressure on the growth of polymer particle is significant.     

Modeling of transfer phenomena on heterogeneous Ziegler catalysts: Differences between theory and experiment in olefin polymerization (an introduction)

This article begins by briefly reviewing the more important contributions to the area of modeling heat and mass transfer, and particle growth during the polymerization of olefins on Ziegler–Natta catalysts. It is shown that these models are capable of identifying the critical areas involved in heat and mass transfer, and of modeling polymerizations where the observed activity is less than approximately 5,000 g of polymer per gram of catalyst per hour (g/g/h). However, it is not possible to use these models “as-is” to model more modern catalysts whose activity levels can surpass the 50,000 g/g/h mark because they predict prohibitively large concentration gradients inside the growing particles during slurry polymerizations, and temperature gradients outside the particles during polymerization in the gas phase. An analysis of the mass and heat transfer Peclet numbers (Pe) reveals that certain simplifying assumptions may not always be valid. Pe values in the transition range suggest that convection inside the particles during polymerization in the liquid phase may help to explain why observed mass transfer rates are higher than the predicted rates. In an opposite vein, a Pe analysis shows that conductive heat transfer may play an important role at length scales characteristic of those in the early stages of polymerization. A new mechanism for heat transfer at reduced length scales is proposed. © 1995 John Wiley & Sons, Inc.     

Experimental investigation of vinyl chloride polymerization at high conversion—reactor dynamics

A series of suspension polymerizations of vinyl chloride monomer (VCM) was carried out in a 5-L pilot plant reactor over the temperature range, 40–70°C. The reactor pressure and monomer conversion were monitored simultaneously every 7–8 min. The critical conversion X_f, at which the liquid monomer phase is consumed, was considered to occur when the reactor pressure fell to 98% of the vapor pressure of VCM for suspension at the polymerization temperature. The reactor model predictions of pressure are in excellent agreement with the experimental data over the entire conversion and temperature ranges studied. The mechanism of reactor pressure development for VCM suspension polymerization is discussed herein in some detail. For isothermal batch polymerization, the reactor pressure falls in two stages due to the effect of polymer particle morphology on pressure drop. The first stage is due to the volume increase of the vapor phase as a result of volume shrinkage due to conversion of monomer to polymer. The monomer phase is not yet consumed at this stage, but it is trapped in the interstices between primary particles creating a mass transfer resistance; therefore, the reactor pressure drops slowly. The second stage is due to both the volume increase of the vapor phase and to the monomer in the vapor phase diffusing into the polymer phase because of the subsaturation condition with respect to monomer in the polymer phase. The reactor pressure drops dramatically with an increase in monomer conversion at this stage. The present model can be used to predict reactor dynamics during suspension polymerization under varying temperature and pressure conditions.     

TRANSPORT PHENOMENA ASPECTS OF THE FREE-RADICAL RETROGRADE-PRECIPITATION POLYMERIZATION (FRRPP) PROCESS

We have been studying free-radical polymerization that is accompanied by phase separation above the lower critical solution temperature. In the past, we have experimentally shown evidence of hot regions in the reactive system. We have also shown in the past that eventually the system exerts control over the rate of propagation as well as termination. In this work, we invoke a concept in polymer physics (the coil-to-globule transition) to help explain the mechanism of thermal trapping within the polymerization zones. The diffusivities of polymer chains at different stages in the reaction are calculated using appropriate methods. From the diffusivities, the propagation and termination rate coefficients are calculated using the Achilias-Kiparissides gel effect model. With experimental kinetic data, we then estimate rates of monomer consumption within polymer-rich particles. Using a pseudo-steady-state heat transfer model, we are able to show that interior temperatures of polymer-rich particle domains greater than about 1 mm can reach spinodal temperature values at the early stage of polymerization. Polymer-rich particle sizes are obtained from the same reactor system whereby a small amount of crosslinker is added to preserve particle morphology. This experiment indicates that even under turbulent flow conditions, relatively large particles can exist in the reactor fluid. This agrees with the physical implications of the coil-to-globule transition. However, since these particles were obtained during the period of slow conversion rate, our heat transfer calculations indicate that interior particle temperatures would be almost the same as surface temperatures. This points to an unknown radical-trapping mechanism at this stage of the polymerization process.     

Transport phenomena aspects of the free-radical retrograde-precipitation polymerization (FRRPP) process

We have been studying free-radical polymerization that is accompanied by phase separation above the lower critical solution temperature. In the past, we have experimentally shown evidence of hot regions in the reactive system. We have also shown in the past that eventually the system exerts control over the rate of propagation as well as termination. In this work, we invoke a concept in polymer physics (the coil-to-globule transition) to help explain the mechanism of thermal trapping within the polymerization zones. The diffusivities of polymer chains at different stages in the reaction are calculated using appropriate methods. From the diffusivities, the propagation and termination rate coefficients are calculated using the Achilias-Kiparissides gel effect model. With experimental kinetic data, we then estimate rates of monomer consumption within polymer-rich particles. Using a pseudo-steady-state heat transfer model, we are able to show that interior temperatures of polymer-rich particle domains greater than about 1 mm can reach spinodal temperature values at the early stage of polymerization. Polymer-rich particle sizes are obtained from the same reactor system whereby a small amount of crosslinker is added to preserve particle morphology. This experiment indicates that even under turbulent flow conditions, relatively large particles can exist in the reactor fluid. This agrees with the physical implications of the coil-to-globule transition. However, since these particles were obtained during the period of slow conversion rate, our heat transfer calculations indicate that interior particle temperatures would be almost the same as surface temperatures. This points to an unknown radical-trapping mechanism at this stage of the polymerization process.     

两相流载气蒸发研究和传热数据关联

两相流载气蒸发是在蒸发器的垂直加热管中引入一种惰性气体,使液体的蒸发在气液两相界面上进行的过程.本文研究了载气蒸发的流型区域和传热规律,对载气蒸发的特性进行了机理分析,并以叠加模型关联了实验数据,获得了准数方程式.     

The leidenfrost reactor: a new general purpose direct-cooled reactor and its use for low temperature polymerizations

Summary Inspired by the Leidenfrost phenomenon a direct-cooled general purpose low-temperature reactor has been developed and its applicability for polymerizations was demonstrated. The Leidenfrost reactor is efficiently cooled by introducing relatively inexpensive liquid N₂ (LN₂) directly into the bulk of polymerization charges and the rapid evaporation of the coolant chills the system. The gaseous N₂ generated provides an inert gas blanket that protects moisture and/or air sensitive charges. The installation of expensive cooling jackets or coils is superfluous and stirring is optional. The virtually complete recovery of cooling energy is possible. In the Leidenfrost reactor the LN₂ heat sink is virtually in contact with the heat source so that the path of heat transfer is practically nil. Reactor fouling is eliminated which is of particular interest in butyl rubber manufacture. Constant temperature control is readily accomplished by regulating the input rate of the coolant and undesirable temperature jumps are suppressed. Continuous operation is possible. The operation of the Leidenfrost reactor is illustrated by the polymerization of isobutylene at –60°C.     

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation     

高温流化床乳化相与浸没表面间传热过程的数值模拟

应用建立的高温流化床乳化相与浸没表面间传热的表面-颗粒-乳化团理论模型,针对9种Geldart B类粒子高温流化床乳化相与浸没表面间的动态传热过程进行了数值模拟,发现在距浸没表面1个粒径范围内,乳化相的不均匀性和温度分布的不均匀性非常显著;随着乳化相在表面停留时间的延长,乳化相与浸没表面间的换热系数减小,在接触时间较短时换热系数下降较快. 对于所用Geldart B类粒子和950℃床温,传导换热系数占总换热系数的85%~45%,辐射换热系数占15%~45%,对流换热系数一般占10%左右;在粒径小于0.6 mm时,对流换热对总换热的贡献相对很小.     

Bifurcation Analysis of the Fluidized Bed Polyethylene Reactor with Internal Cooler

A mathematical model has been developed to simulate a gas‐phase ethylene polymerization reactor with internal cooler. The model was analyzed to determine the effects of reactor operating conditions on dynamics and stability. The reactor model employed assumed that both the gas and polymer phase in the reactor are well mixed. Comparing the present model to one with external heat exchanger confirms that, in either form, gas‐phase polyethylene reactors are prone to show unstable steady states, limit cycles and excursions toward unacceptably high temperature steady states. It was also observed that, with internal cooler, minor design changes in the cooler area available for heat transfer and in the inlet temperature of the coolant have a significant effect on the low stable steady state range of catalyst feed rates. With internal cooler, the suitable operating range increased with the increase in the area available for heat transfer. This effect is insignificant in the case of a reactor with external heat exchanger. Manipulating the reactor coolant inlet temperature and/or gas velocity can increase the stability range in the reactor with internal cooler as against one with external heat exchanger.     

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

ABSTRACT

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation     

松木屑颗粒热解过程中的热/质传递规律

为探索在固定床反应器中有机固废颗粒热解过程中的热量、质量传递机理,本研究从颗粒尺度上对有机固废松木屑颗粒热解过程建模分析,模型中考虑了焦油的二次裂解反应及挥发分在颗粒孔隙中的质量、动量传递过程,并采用达西定律模拟了挥发分在颗粒孔隙内的流动现象,对颗粒热解过程的吸热反应以及挥发分逸出时的对流换热对颗粒温度的影响进行考察。基于两步反应动力学模型,探讨了不同颗粒尺寸、热解温度对有机固废松木屑颗粒热解过程的影响。结果表明,热解吸热反应和挥发分的对流换热阻碍了热量向颗粒中心的传递,延长了颗粒达到均温的时间;松木屑颗粒热解时,颗粒内会存在明显的温度梯度,在颗粒表面主要受化学反应动力学限制,在颗粒内部则主要受热量传递过程限制。此外,热解温度越低,粒径越大,颗粒内部的传热阻力越大。松木屑颗粒完全热解所需时间会随着颗粒粒径的增大而增加,但当颗粒粒径在10mm以上时,随着颗粒粒径的增大,颗粒完全热解所需时间的增量要大于10mm以下颗粒。     

Modeling of vaporization and cracking of liquid oil injected in a gas-solid riser

Vaporization and cracking of liquid oil injected in a gas-solid riser (fluid catalytic cracking riser reactor) was computationally studied in this work. Evaporation of a single drop injected in a stream of gas-solid mixture was analyzed first. A model for simulating evaporation of a drop considering heat transfer from the gas phase as well as from the solid particles was developed. The model relates the evaporation rate of droplet with rate of collisions of solid particles, specific heat capacities of solid and liquid, latent heat of vaporization, relative velocity of gas and liquid and temperatures of three phases. The understanding gained from such a model was then extended to simulate evaporation of liquid drops injected in FCC risers. The Eulerian-Lagrangian approach was used to simulate simultaneous evaporation and cracking reactions occurring in FCC riser reactors. A commercial CFD code, FLUENT (of Fluent Inc., USA) was used. Four and ten lump models were used for simulating cracking reactions. Appropriate user defined functions were developed to implement heterogeneous kinetics and heat transfer models in FLUENT. A special algorithm was developed to calculate accumulated coke on catalyst particles. A boiling point range was considered for simulating realistic oil feedstock. The model was first evaluated by comparing predicted results with published industrial data. The simulations were then carried out to understand influence of key design and operating parameters on performance of FCC riser reactors. The parameters studied included; initial oil droplet distribution, catalyst inlet temperature, catalyst to oil ratio and thermal cracking. The approach, model and results presented here would be useful for optimization of FCC operation, cost to benefit analysis of new FCC nozzles and related decision-making.     

直流蒸汽发生器蒸干及蒸干后传热数值分析

准确预测直流蒸汽发生器流动沸腾及蒸干对其设计、安全可靠运行极其重要。通过对B&W公司直流蒸汽发生器进行合理简化,引入两流体三流场数学模型及壁面热通量分区模型,分别进行基于常热通量和耦合传热的蒸汽发生器流动沸腾数值模拟。结果表明：蒸干发生时传热性能急剧下降,常热通量边界下壁温升高的幅度相当大（约300 K·m^-1）,而耦合传热边界下壁温飞升幅度约为25 K·m^-1,与实际情形相一致;两种热边界中预热区会发生过冷沸腾,壁面处传热由液相对流换热、淬火换热和蒸发换热3部分构成,核态沸腾区蒸发换热为主要换热方式,同时伴随着液相对流换热和淬火换热,蒸干发生时淬火换热和蒸发换热全部降到0,在蒸干后传热区域换热方式为气相对流换热。     

AN INTEGRAL MODEL FOR DRYING OF HYGROSCOPIC AND NONHYGROSCOPIC MATERIALS WITH DIELECTRIC HEATING

Heat and mass transport phenomena in drying assisted by microwave or radio-frequency dielectric heating are analyzed. When drying at temperatures near boiling point or with high temperature gradients, the effect of the gas phase pressure gradient on moisture transfer within the solid can be important. The governing heat and mass transfer equations, including consideration of internal heat generation and the effect of the gas phase pressure gradient, are derived and solved in a one-dimensional system using an integral method. The integral model has been used to simulate dielectrically-enhanced convective drying of beds of polymer pellets, glass beads and alumina spheres with flow over the bed surface. Model predictions of drying rates and temperatures agree well with experimental data for these cases.

The model provides a relatively fast and efficient way to simulate drying behavior with dielectric heating, and may be useful in design and optimization of dielectrically-enhanced convective drying processes.     

Polymerization of olefins through heterogeneous catalysis. III. Polymer particle modelling with an analysis of intraparticle heat and mass transfer effects

Propylene and ethylene polymerization in liquid and gas media are described by a multigrain particle model. Intraparticle heat and mass transfer effects are investigated for a range of catalyst activities. For slurry polymerization, intraparticle mass transfer effects may be significant at both the macroparticle and microparticle level; however, for normal gas phase polymerization, microparticle mass transfer effects appear more likely to be important. Intraparticle temperature gradients would appear to be negligible under most normal operating conditions.     

控制气相PE流化床高度对装置运行周期的影响

分析了聚合过程中控制流化床床高对聚合物粒径,催化剂聚合产率和装置运行周期的影响.增加流化床床高,有利于聚合物粉料对反应器上部扩大段的冲刷,有效减少了粉料堵塞循环回路和反应器结块的发生几率,提高了聚合反应器运行周期.