Multiplicity of steady states in fluidized bed reactors—V: The effect of catalyst decay

A simplified two-phase model is used to investigate the behaviour of non-isothermal fluidized bed reactors experiencing catalyst decay. The investigation shows that for highly exothermic reaction it is almost always desirable to operate the reactor at the middle unstable steady state, since it gives higher accumulative yield than both the high and the low temperature steady states. A simple feedback control scheme with time varying set point is suggested to stabilize the middle steady state. The dynamic behaviour and stability of the system is investigated for the open-loop reactor (uncontrolled) and the closed loop reactor (controlled).     

Multiplicity patterns in atmospheric fluidized bed coal combustors

A new mathematical model has been developed for the analysis of the steady state characteristics of Atmospheric Fluidized Bed Coal Combustors. A physical understanding of the different processes occuring within the bed is incorporated in the development of detailed mass and energy balances for the different reactor phases (bubble and emulsion phase, sorbent and coal particles). Care is placed on the modelling of the homogeneous and heterogeneous oxidation reactions, the growth of bubbles within the bed by coalescence and the heat transfer between the solids and the immersed steam generating tubes. Furthermore a population balance for the coal particles which accounts for variable size feed, fines elutriation and solids overflow enables the calculation of two very crucial bed operating variables: the carbon loading and the combustion efficiency. The model also predicts the temperature and the O₂ and CO concentrations within the combustor.The numerical solution of the dimensionless model equations is based on their reduction to a system of three nonlinear algebraic equations. Three steady states are obtained in the industrially important region of design and operational characteristics. They correspond to a high, intermediate and low reactor temperature and combustion efficiency. At the same time the respective values of carbon loading and O₂ concentrations are low, intermediate and very high. The results indicate that there exists a critical value of the coal feed rate, or of the superficial gas velocity, below which the bed is extinguished.     

流化床中甲烷芳构化过程

在石英流化床反应器中研究了无氧条件下甲烷直接催化转化制备芳烃的过程.发现催化剂的诱导期长短、甲烷的总转化率与温度、甲烷分压及甲烷空速相关.在973 K下,液体产品中苯的选择性与萘的选择性随着催化剂的失活呈现不同的变化趋势.所得主要技术指标（甲烷转化率、苯的收率与选择性等）与固定床微型反应器中的结果相近.还研究了催化剂上的积炭对甲烷转化率、催化剂失活的影响,为将来的进一步研究提供了基础.     

Flue gas desulfurization in an internally circulating fluidized bed reactor

An internally circulating fluidized bed reactor (ICFBR) was used as a desulfurization apparatus in this study. The height of the bed was 2.5 m, and the inner diameter was 9 cm. The bed materials were calcium sorbent and silica sand. The effects of the operating parameters of the flue gas desulfurization including relative humidity, particle size of the calcium sorbent, inlet concentration of SO₂, difference between the superficial gas velocities in the draft tube and the annulus, and superficial gas velocity in the draft tube on SO₂ removal efficiency (RE) were investigated. It was found that when the relative humidity (RH) was varied from 40% to 80%, the steady state RE had a largest value of approximately 15% when the relative humidity was 60%. When RH = 50%, 60% and 70%, RE decreased initially and then increased. After that RE decreased again until a steady state was reached. In addition, RE decreased with increasing calcium particle size or inlet SO₂ concentration. A larger difference between the superficial gas velocities in the draft tube and the annulus had a higher RE resulting from increasing reactivity of the calcium sorbent caused by a higher attrition rate. Moreover, a higher attrition rate had a higher total volume of the flue gas treated. Finally, a model to predict the steady state RE in ICFBR was proposed. It assumed that the draft tube section was a bubbling fluidized bed while the annulus section was a moving bed. In addition, the effects of the calcium sorbent conversion, attrition rate and gas-bypassing fractions on RE were also taken into account in this model. It was found that the values of RE predicted by this model agreed with the experimental results.     

Multiplicity of the steady state in fluidized bed reactors—IV. Fluid catalytic cracking (FCC)

A mathematical model for fluid catalytic cracking units is developed. The model takes into account the kinetics of the cracking reactions, as well as the kinetics of coke combustion in the regenerator. A numerical scheme is developed for the solution of the model equations. It is found that multiplicity of the steady states extends over a wide scope of operating variables and parameters. The model investigates the effects of catalyst circulation rate and gas oil flow rate, which have a strong effect on the reactor temperature and hence yield and selectivity. The phenomenon of hysteresis has been investigated. The model can be used for yield optimization and steady state control.     

Photocatalytic oxidation of ethyl alcohol in an annulus fluidized bed reactor

The photocatalytic oxidation of ethyl alcohol vapor in an annulus fluidized bed reactor of 0.06 m I.D. and 1.0 m long was examined. The TiO₂ catalyst employed was prepared by the sol-gel method and was coated on the silica gel powder. The UV lamp was installed at the center of the bed as the light source. The effects of the initial concentration of ethyl alcohol, the power of UV-lamp, the photocatalysts with different preparation methods, and the superficial gas velocity on the reaction rate of ethyl alcohol decomposition were determined. It was found that, at 1.2 U_mf of flow rate, about 80% of ethyl alcohol was decomposed with initial concentration of 10,000 ppmv and the increase of superficial gas velocity reduced the reaction rate significantly.     

Modelling and simulation of an oxychlorination reactor in a fluidized bed

Taking 1,2‐dichloroethane from the oxychlorination reaction is a commercially very important process due to the large application of the 1,2‐dichloroethane in the chemical industry of PVC production. This work presents the modeling and simulation of an oxychlorination reactor with a fluidized bed. The pseudo‐homogeneous model with one‐dimensional flow in steady state was applied based on the theory of fluidized bed in two phases. It allows the sensitivity analysis of the operational and project parameters of the reactor. The ordinary differential equations system that represents the mathematical model of the reactor was solved through the application of the numerical method of Newton–Raphson's. The results obtained have proved that the developed model represents the system suitably, in spite of the one‐dimensional model. The effect of different parameters was investigated through the sensitivity analysis, and the results show that the parameters that have the largest influence on the reactor performances are: fluidized bed height, bubble diameter, residence time, cupric chloride weight in the catalyst, and emulsion phase temperature.     

厌氧流化床处理红霉素废水的研究

研究了中温(33～35℃)条件下,厌氧流化床(AFB)反应器处理红霉素废水的运行特性.结果发现:进水COD为6 900 mg/L、HRT为4.4 h时,COD、BOD的去除率分别达到了76.6%、65.9%.最后利用Garrett-Sawyer关系式求出了难生物降解的有机物浓度,并利用Monod模型对实验结果进行了动力学分析,得出了红霉素废水中温条件下厌氧消化过程的基质降解规律.     

Propenoxide isomerization in a fluidized bed reactor

Propenoxide isomerization, over lithium orthophosphate as a catalyst, was investigated in a fluidized bed reactor. A mathematical model of the process was developed and its kinetic parameters identified. There is a high degree of selectivity for allyl alcohol.     

流化床电化学反应器研究进展

流化床电化学反应器是一种三维颗粒电极反应器,以其比表面积大、传质速率高而备受关注。就流化床电化学反应器在导电机理、数学模型、结构放大以及应用领域的研究现状和进展进行了综述,阐明了该类反应器开发、设计和模型化方面存在的问题,并提出了研究方向。     

A loop three-phase fluidized bed reactor

An airlift three-phase reactor in which solid particles are fluidized in the riser by the recirculating liquid was developed. Three particle types of 1.7–8.6 mm in diam., porous and nonporous, spheres and cylinders, were studied in the water–air system. Little effect of the alumina porous particles (apparent density 1.47 × 10³ kg/m³) on the gas holdup and liquid superficial velocity was found. On the other hand, the heavier, nonporous ceramic particles (density 2.4 × 10³ kg/m³) had a significant effect on both design parameters. The axial distribution of particles was also a function of the particle density and liquid superficial velocity.     

微型流化床内松木屑和煤泥等温混合热解特性

采用微型流化床反应器对松木屑和煤泥的等温混合热解气体释放行为进行实验研究。探讨不同温度和掺混比例对CH₄、CO、CO₂与H₂释放特性的影响,并通过模型配合法求解其动力学参数,研究松木屑和煤泥混合热解过程的相互作用。通过FT-IR检测发现煤泥的主要成分为含有C-O和C==O键的芳香化合物,松木屑则以带-OH键的长链脂肪烃为主。在等温稳定反应阶段,松木屑热解气体生成速率高于煤泥,随着生物质掺入比例的不断提高,混合原料气体生成反应速率亦呈现不同程度的增加。利用模型积分法求解了松木屑、煤泥及其混合物热解气体生成动力学参数,并通过实验值和计算值对比筛选出了最概然机理函数。通过活化能对比发现,混合热解对4种气体组分生成具有不同的影响作用,其中CO实验活化能明显低于计算值,表现为二者协同作用利于CO的生成释放;对H₂而言,在75%混合比例条件下,混合反应导致其生成活化能呈现协同负效应,使得活化能实验值明显高于计算值;相较而言,CH₄在混合热解过程影响相对较弱,并呈现小幅度的协同负效应,而CO₂的生成特性则受混合比例的影响较为明显。     

Syngas combustion characteristics of four oxygen carrier particles for chemical-looping combustion in a batch fluidized bed reactor

Syngas combustion characteristics of oxygen carrier particles have been investigated. Experiments were performed on four oxygen carrier particles in a fluidized bed reactor. All four oxygen carrier particles showed high gas conversion, high CO₂ selectivity, and low CO concentration in the reducer and very low NOx (NO, NO₂, N₂O) emissions in the oxidizer. Moreover, all particles showed good regeneration ability during successive reduction-oxidation cyclic tests up to the 10 ^th cycle. The results indicate that inherent CO₂ separation, NOx-free combustion, and long-term operation without reactivity decay of oxygen carrier particles are possible in a syngas fueled chemical-looping combustion system with NiO/bentonite, NiO/NiAl₂O₄, Co _x O _y /CoAl₂O₄, and OCN-650 particles. However, Co _x O _y /CoAl₂O₄ represented slight decay of oxidation reactivity with the number of cycles increased and the oxidation rate slower than other particles.     

Gas mixing in a turbulent fluidized bed reactor

A series of experiments has been conducted to study mixing and hydrodynamic behaviour of a downward facing sparger in a turbulent fluidized bed reactor. Using pressure measurement techniques, two flow discharge modes were identified around the sparger by injecting a gas tracer into the bed. These are bubbling and jetting conditions. Experimental results show that, under bubbling conditions, bubbles tend to keep their identity, while under jetting conditions a highly turbulent heterogeneous area is formed around the injection point. Due to attrition and erosion of internal heating or cooling surfaces in industrial reactors, the dominant discharge mode is the bubbling pattern. Therefore, in this investigation, the bubbling pattern is studied by measuring the radial and axial dispersion of gas tracer injected to a hot fluidized bed reactor of 20 cm diameter of FCC and sand particles. A three‐phase model is also proposed in order to predict the mixing length. In addition, the effect of sparger configuration on tracer gas mixing was examined for FCC particles.     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.