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.     

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.     

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.     

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。     

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.     

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.     

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.     

三氯氢硅合成流化床反应器工艺改造

针对三氯氢硅生产过程中存在的流化床反应器指形管磨损漏油问题,通过对布风板改造、指形管改造、消除局部涡流、监测漏油等措施,收到了良好的效果。     

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.     

The analysis of an isothermal batch electrochemical reactor with a secondary cathodic reaction

An analysis has been made to examine the effect of a constant applied electrolysing voltage on the performance of an isothermal batch electrochemical reactor in which the rate of the cathodic reaction is limited by mass transfer of a reactant to the electrode surface, a simultaneous secondary reaction occurring at higher cathode potentials.Numerical solutions are presented for a hypothetical process study to show how average production rates for various percentage conversions of reactant are influenced by the magnitude of the electrolysing voltage and the corresponding electrical energy requirement. The most noticeable effect is that the electrical energy requirements for a given production rate are very strongly influenced by both the degree of conversion of reactant and by the extent to which the secondary reaction occurs.     

Gasification of coal and PET in fluidized bed reactor

Blended fuel comprising 23 wt.% polyethyleneterephthalate (PET) and 77 wt.% brown coal was gasified in an atmospheric fluidized bed gasifier of laboratory-scale. The gasification agent was composed of 10 vol.% O₂ in bulk of nitrogen. Thermal and texture analyses were carried out to determine the basic properties of the fuel components. The influence of experimental conditions, such as the fluidized bed and freeboard temperatures on major and minor gas components and tar content, as well as features of the blended fuel gasification in comparison with the single coal gasification, were studied. In the case of coal with PET gasification, only the fluidized bed temperature showed significant influence on CO, CO₂, CH₄ and H₂ content in the producer gas, whereas the effect of the freeboard temperature was insignificant. In single coal gasification both temperatures had considerable and almost the same influence. The content of minor components, such as ethane, ethylene, acetylene and benzene, was found to be more dependent on the freeboard temperature than on the fluidized bed temperature. It was observed that the higher the freeboard temperatures get, the lower is the concentration of the minor components, with the exception of acetylene. The absolute contents of almost all minor and tar components were approximately three times higher in blended fuel gasification than that in single coal gasification. Finally, partition of carbon (char) and selected metals into bottom and cyclone ash in gasification of both fuels is discussed.     

Feedback linearizing control of a fluidized bed reactor

The design of an adaptive nonlinear controller for the control of a fluidized bed reactor is derived by using exact linearization techniques. Reset action and parameter adaptation are used to make more robust the precise compensation of nonlinear terms, which is called for in the linearization technique. A nonlinear antiwindup mechanism is introduced to handle reset windup problem and to provide fast response without large overshoot. Simulation results show that the proposed adaptive controller guarantees good setpoint tracking. The developed estimation algorithm allows accurate estimation of the parameters for which the regressor component is not zero.     

流化床反应器的改进与优化

通过对流化床内件的改进和优化,解决了反应器反应时放出的大量热量对设备整体的影响。