Fixed bed reactor modelling and experimental data for catalytic dehydrogenation in seasonal energy storage applications

Dehydrogenation of methylcyclohexane to toluene is studied by modelling and experimentally in two fixed bed reactor systems in the laboratory and pilot plant scale. Kinetic and transport parameters are estimated and measured. Interstage membranes for hydrogen separation are used to improve efficiency and exceed equilibrium conversion by 1–12%. The complexity of modelling the laboratory scale necessitates a pseudo-homogeneous two-dimensional model to compensate for radial gradients giving satisfactory agreement between simulated and measured data. The pilot plant scale system is simpler to model, a pseudo-homogeneous one-dimensional model describes the system with good accuracy. The highest yield achieved in the pilot plant scale system is 98%.     

Safe design of cooled tubular reactors for exothermic,multiple reactions. Consecutive reactions

The model of the pseudo-homogeneous, one-dimensional, cooled tubular reactor is applied to two consecutive, irreversible first order reactions. A criterion is derived to obtain a desired integral yield. Based on this criterion three requirements are formulated, which enable us to choose the relevant design and operating conditions. If any of the requirements are met, the reactor is also safe with respect to runaway. In an illustration the results are applied to the production of phthalic anhydride via the oxidation of naphthalene. It is shown that the requirements formulated can be used for the design of the reactor and for its immediate adjustment to a change in operating conditions. In view of the special behaviour of consecutive reactions in a tubular reactor a fine tuning of the operating conditions remains necessary after this adjustment.     

列管固定床反应器内CO氧化偶联制草酸二甲酯反应模拟及优化

针对列管式固定床反应器中的单根反应管,采用在接近工业条件下获得的CO氧化偶联制草酸二甲酯动力学方程,建立了一维、二维拟均相模型,并与单管实验结果进行了对比,结果表明一维拟均相反应器模型更能准确描述单管反应器内的CO偶联反应。进一步利用一维拟均相模型模拟计算了操作参数对床层热点温度、反应转化率、产物选择性及床层压降的影响,分析了反应器热点温度对操作参数的敏感性。计算结果表明：冷却介质温度对反应管热点温度、亚硝酸甲酯转化率有较大影响,是需要严格控制的工艺指标;较低的空速容易引起反应器飞温;反应器进口压力、原料气进料温度和反应物组成在计算范围内对反应器热点温度影响相对较小。为了提高偶联反应器的负荷和强化床层内的传热效果,可以将进料空速提高至4000 h^-1,同时,可以通过将反应器进口压力增大至500 kPa来降低压缩机能耗。研究结果可为现有列管式CO氧化偶联反应器的改进和工艺优化提供参考。     

气体流动条件下钴基催化剂固定床的传热

在气体流量4~8 Nm3/h、气体分布器进口温度190~210℃、加热管壁温约240℃的条件下,对气体流动时活性组分呈蛋壳型分布的钴基催化剂固定床的传热进行了实验研究,建立了二维拟均相传热模型,利用正交配置法和Levenberg-Marquardt法对其求解,得到了钴基催化剂床层径向有效导热系数及壁给热系数的关联式,并将传热参数与由气体处于静态时固定床的有效导热系数计算而得的固定床传热参数值进行了比较,在气体入口温度范围内考察了其对固定床传热参数的影响. 结果表明,实验所得传热参数与文献值的最大偏差绝对值均在15%以内.     

Recuperative coupling of exothermic and endothermic reactions

Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improves the thermal efficiency of processes and reduces the size of the reactors. One type of reactor suitable for such a type of coupling is the heat exchanger reactor. In this work, a one-dimensional pseudo-homogeneous plug flow model is used to analyze and compare the performance of co-current and counter-current heat exchanger reactors. A parametric analysis is carried out to address the vital issues, such as the exit conversion of the endothermic reaction, the temperature peak (hot spot) of the exothermic reaction and the reactor volumetric productivity. The measures to reduce the hot spot by different catalyst profiling techniques are also addressed. Some features of the dynamic behavior exhibited by these reactors, which are important from design, operational and control point of view, are presented.     

Autothermal reactors with internal heat exchang—I Numerical and experimental studies in the multiple steady-state region

Detailed numerical and experimental studies in the multiple steady-state region of an autothermal reactor with internal countercurrent heat exchange were conducted. The mildly exothermic water-gas shift reaction was used in the studies. In this distributed-parameter system multiple steady states were found experimentally. Although significant radial and longitudinal temperature gradients exist, one-dimensional (radially-lumped) simulations predict mean behavior quite well. For two-dimensional simulations, Legendre polynomial trial functions give faster convergence than Jacobi polynomials when using orthogonal collocation methods. Inflections in the radial temperature gradients were measured near the hot spot although not predicted by the simulations. Significant temperature jumps at the inner wall upstream of the hot spot were also measured. Production of hydrogen could be conducted at surprisingly mild conditions if the reactor could be controlled in a stable mode essentially identical to the middle unstable steady state.     

耦合传质的羰基化固定床反应器传热模拟分析

固定床反应器中进行强放热反应时, 反应器的热点温度对操作参数变化敏感,容易引起飞温,导致转化率下降,影响催化剂寿命。为强化羰基化固定床反应器内热质传递与化学反应的协同性,建立考虑颗粒内扩散影响的羰基化固定床反应器拟均相一维传热模型,考察操作参数对床层热点温度、反应转化率、床层温升的影响。不仅体现传热传质和反应的协同作用,而且影响关系明晰、求解方便。为保证反应转化率,本实验条件下确定催化剂颗粒直径小于等于1.5 mm。反应器入口温度/冷却剂油温既要满足床层热稳定性需求,又要使反应转化率和床层温升都在合理范围内。模拟结果表明在床层入口温度升高的同时,可通过降低冷却剂油温获得良好的反应转化率和较小的床层温升。在此基础上,考察入口环氧乙烷浓度对反应转化率和床层温升的影响。本研究可为固定床反应器满足转化率要求、床层合理温升而选择催化剂颗粒直径、床层入口温度、冷却剂油温和床层入口浓度等操作参数提供计算依据。     

Tubular reactor design—II A modified one dimensional model

A modified one-dimensional model (MODM) is developed for tubular packed reactors with exothermic reactions. The radial temperature and conversion profiles are represented by equations whose parameters are functions of axial position. The parameters depend upon the centerline temperature and conversion, the mean temperature and conversion, and the conversion at the reactor wall. These five variables are represented in terms of five ordinary differential equations which are solved simultaneously. The heat loss to the surroundings is determined from the temperature gradient and the thermal conductivity at the pipe wall. The heat generation due to reaction is obtained from a cross-sectional average reaction rate.The model is used to simulate the oxidation of sulfur dioxide and the oxidation of naphthalene to phthalicanhydride. Although the MODM requires about half the computer time required for the two-dimensional model, it was nevertheless found to yield results of comparable accuracy for both the systems. The average values of conversion and temperature from the new model are also shown to be more accurate than those obtained with the conventional one-dimensional model (CODM). Furthermore the new model also accurately predicts the heat loss at the wall and provides information on the hot spot temperature and location whereas the CODM does not.     

CO氧化偶联反应器模拟与优化

CO与亚硝酸甲酯(MN)氧化偶联制草酸二甲酯(DMO)是合成气制乙二醇过程的关键步骤，现有工业装置存在效率低的问题。采用包括副反应(生成碳酸二甲酯和甲酸甲酯)动力学的动力学模型和二维两相反应器模型，对CO氧化偶联的移热式固定床反应器进行建模，研究了换热方式及操作条件对反应器性能和安全性的影响。结果表明，以温度的二阶导数作为飞温的判据是灵敏和可靠的。与常规的逆流和恒温移热方式相比，并流移热使反应器形成更为均匀的温度分布，有利于提高反应器产能。增加入口MN含量会升高反应器MN转化率和热点温度；但是，由于CO、NO和反应物MN之间存在竞争吸附，增加入口CO和NO含量会导致MN转化率和热点温度降低，所以增加入口压力导致MN转化率降低。且热点温度对MN和NO的含量更为敏感，应严格控制入口MN和NO的含量。采用遗传算法进行反应器工况寻优，确定了最优的反应条件，可提高单台反应器对应的乙二醇(EG)年产能至12万吨。     

内冷管型甲醇合成塔的拟均相二维模型

本文考虑床层径向流速分布,建立了冷管型甲醇合成塔的拟均相二维模型,得出床层径向温度与浓度分布。结果表明,床层中近冷管处温度较低,活性未充分发挥;而床层中部分区域,温度偏高,可能过热,催化剂易于老化失活,在工业生产中,尤其是催化剂还原过程中,必须对此充分重视。     

固定床反应器进行n级反应的飞温判据

本文以一级反应系统参数敏感性理论为依据，给出ｎ级反应系统以ε为扰动参数的临界热点温度表达式，内插得到该系统的飞温判据。结果表明：反应级数越大，系统抵抗外部环境干扰的能力越强，安全操作区域越大，参数敏感区域越小。     

Modeling of transient heterogeneous two-dimensional catalytic packed bed reactor

A two-dimensional transient catalytic packed bed model, incorporating all transport parameters and resistances, along with boundary conditions based on a catalytic single pellet has been developed. Thermal conduction through the solid phase is included in the model. The overall steady state reactor performances of packed bed reactor using a model proposed in this study are compared with those from different models which are often used for a packed bed reactor. The model presented is very useful in the presence of internal temperature and concentration gradients in the catalyst pellets. The dynamic behavior in feed temperature change is examined during ethane hydrogenolysis. A transient thermal runaway is observed by feed temperature decrease. The sensitivities of the computation to each physical parameter and the effects of some simplifying assumptions in the model are also analyzed. The magnitude and position of hot spot in catalytic packed bed reactor are relatively sensitive to thermal parameters and characteristic parameters of a catalyst pellet.     

A GENERAL RUNAWAY CRITERION FOR FIXED-BED REACTORS

The safe design and operation of fixed bed reactors require the consideration of potential runaways. The steep temperature rise may affect the selectivity, activity and stability of catalysts and in certain cases the reactor safety.

Mathematical models simulate the runaway conditions. Such simulations require extensive calculations to define the range of safe operation. A new runaway criterion was developed to replace the need to solve the differential reactor balances. The criterion is simple, defined by one algebraic equation. It can be applied to any kinetic expression. Extensive testing shows an excellent agreement between the runaway criterion and the two-dimensional model predictions. It also predicts the runaway conditions in agreement with experimental data.     

Safe design of cooled tubular reactors for exothermic,multiple reactions; parallel reactions—I: Development of criteria

Previously reported design criteria for cooled tubular reactors are based on the prevention of reactor temperature run away and were developed for single reactions only. In this paper it is argued that such criteri a should be based on the reactor selectivity, from which eventually a maximum allowable temperature can be derived. To this end and for the pseudo-homogeneous, one dimensional model of a cooled tubular reactor in which two parallel, irreversible first order exothermic reactions are carried out, two criteria are developed for the safe design and operation of the reactor. The criteria enable us to choose tube diameters and operating conditions, which are safe in view of the derived selectivity and of possible runaway as well. The method outlined can be used in the initial design stage and requires kinetic information on both the desired and the undesired reaction.     

Effects of process and design parameters on heat management in fixed bed Fischer-Tropsch synthesis reactor

A two-dimensional pseudo-homogeneous model of wall-cooled fixed bed Fischer-Tropsch synthesis (FTS) reactor with Co/Re/γ-Al₂O₃ catalyst was developed to study the effect of process and design parameters on heat generation and removal characteristics. The influence of liquid-phase formation on heat transport was accounted for by using two-phase correlations. The effect of intraparticle diffusion on heat generation was considered. Detailed numerical simulations were performed to analyze the effect of process and design parameters on the reactor performance in terms of heat management. Results show that thermal behavior of FTS fixed bed reactors is very sensitive and any large disturbances can lead to temperature runaway. Large tube diameters are shown to be particularly unfavorable, with d _t >5 cm resulting in axial and radial gradients greater than 20 K and 13 K, respectively. The importance of detailed reactor modeling when designing and optimizing FTS fixed bed reactors is highlighted.     

Experimental and simulation study of the temperature distribution in a bench-scale fixed bed Fischer–Tropsch reactor

The temperature distribution in a bench-scale fixed bed Fischer–Tropsch reactor using Co-based catalyst was investigated under conditions of 2 MPa and 458 K at various syngas partial pressures and space velocities. The single-tube reactor had a diameter of 0.05 m, which is representative of the diameters used in industrial applications. With a special designed temperature measurement, the detailed temperature distribution in a bench-scale reactor was reported for the first time. The changes of maximum temperature in the bed and hot spot region were discussed at different N₂ flow rate and gas hourly space velocity. A 2D pseudo-homogeneous fixed bed reactor model was developed using ANSYS Fluent. A position-dependent heat-transfer coefficient, which considered more accurate in temperature prediction, was applied. The model was validated against both the reaction results and the measured temperatures. The inferred properties within the reactor were analyzed to give insight as to how to increase the reactor production capacity.     

提升管循环流化床气,固局部滑落速度的分布

本研究采用光纤激光多普勒测速仪（ＬＤＶ），通过两列信号处理系统，同时测定了气体和颗粒局部速度，研究了低密度循环流化床提升管内局部滑落速度的变化规律。实验结果表明，提高颗粒循环速率，在任一径向位置的气固滑落速度均增加；提高气体表观速度，反而使气固之间局部滑落速度减小。由于固体颗粒在近壁区的团聚行为，在近壁区气、固局部滑落速度出现一个极大值。     

Investigation of a fixed-bed pilot plant reactor by dynamic experimentation. Part 2. Simulation of reactor behaviour

Data collected with forced uncorrelated sinusoidal changes of the process variables are used to simulate the behaviour of a non-isothermal non-adiabatic pilot plant fixed-bed reactor. The test reaction applied is the hydrogenation of toluene to methylcyclohexane on an industrial nickel catalyst. A pseudo-homogeneous two-dimensional model as well as a one-dimensional model are applied to describe the reactor behaviour. The model parameters are estimated by defining an objective function based upon residuals of the discretized differential equations instead of upon deviation from the observed variables. This procedure leads to a drastic reduction of computer expenditure. Measured and simulated temperature and concentration profiles are compared on the basis of a number of plots and fitting quality criteria. The model parameters estimated from data collected during dynamic runs are comparable to those obtained from a series of stationary experiments (2₃ experimental design). However, with the dynamic experiments the data necessary for modelling can be collected much more efficiently.     

Parametric sensitivity in co-currently cooled tubular reactors

An intrinsic criterion was derived for parametric sensitivity and runaway in co-currently cooled tubular reactors. It is essentially based on the approach of Van Welsenaere and Froment (1970) developed for the constant wall temperature case and which makes use of fundamental properties of the trajectory in the temperature vs. partial pressure plane. The criterion limits the maximum allowable temperature on an objective basis and permits the calculation of the corresponding critical inlet values for the operating variables, like partial pressure of the reactant, temperature of the reactor fluid, temperature of the cooling medium, coolant flow rate, reactor throughput ... . In addition to the rigorous treatment, a simple extrapolation procedure is presented, which proves to be very useful and accurate, provided the conditions are not too severe.