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。     

Dynamic modeling and simulations of the behavior of a fixed‐bed reactor‐exchanger used for CO2 methanation

A multidimensional heterogeneous and dynamic model of a fixed‐bed heat exchanger reactor used for CO₂ methanation has been developed in this work that is based on mass, energy and momentum balances in the gas phase and mass and energy balances for the catalyst phase. The dynamic behavior of this reactor is simulated for transient variations in inlet gas temperature, cooling temperature, gas inlet flow rate, and outlet pressure. Simulation results showed that wrong‐way behaviors can occur for any abrupt temperature changes. Conversely, temperature ramp changes enable to attenuate and even fade the wrong‐way behavior. Traveling hot spots appear only when the change of an operating condition shifts the reactor from an ignited steady state to a non‐ignited one. Inlet gas flow rate variations reveal overshoots and undershoots of the reactor maximum temperature. © 2017 American Institute of Chemical Engineers AIChE J, 64: 468–480, 2018     

丙烷脱氢Cr_2O_3/Al_2O_3催化剂烧焦再生过程的模型化与模拟(英文)

A heterogeneous model is developed for the regeneration of the Cr2O3/Al2O3 catalyst for the propane dehydrogenation process by considering the internal mass transfer and external mass/heat transfer during the coke combustion. Simulation shows that under practical operating conditions, multi-steady states exist for the catalyst pellets and the catalyst temperature is sensitive to gas temperature. However, at increased mass flow rate or lowered oxygen concentration, multi-steady states will not appear. Under the strong influences of film diffusion, the coke in the packed bed reactor will first be exhausted at the inlet, while if the film diffusion resistance is decreased, the position of first coke exhaustion moves toward the outlet of the reactor.     

U型管式换热器进口截面流场数值模拟

建立了U型管式换热器进口截面的三维稳态流动数学模型,求得了U型管式换热器内部的压力场、速度场和温度场分布;在此基础上对U型管式换热器内部的温度场、压力场和速度场进行了讨论。研究结果表明,一方面,增大U型管式换热器进口热流体速度,可以增加U型管式换热器的换热量,增大出口截面的速度,增大内部压强,提高内部温度;另一方面,运行时间越长,U型管式换热器内部温度越低、出口截面速度越大,总传热率越低,压力损失先减小后趋于稳定;同时,离U型管换热器越近的外导流筒冲刷腐蚀越严重。     

纵翅片结构形式对管换热器性能的影响

提出了一种可改善换热效率的百叶窗式纵翅片换热管的结构模型,对其进行简化,采用Fluent软件对换热管烟气侧流动与传热过程进行数值模拟,对比了两种百叶窗纵翅片与普通纵翅片的换热效率与压降,结果发现：百叶窗式纵翅片传热效果比普通纵翅片高130%以上。模拟了6组不同流体入口速度下传热与压降的变化情况,分析了百叶窗翅片间距与倾斜角度对传热与压降的影响,结果表明：入口速度越大,进出口温差越小,压降越大;翅片间距越大,进出口温差和压降都越小;翅片倾斜角度越大,换热效果相差不大,压降越大。搭建了简易实验平台对模拟结果进行验证。     

Experimental validation of a new heat transfer intensification method for FCC external catalyst coolers

In petroleum refining industry, external catalyst cooler is a key device in FCC units processing heavy residue feedstock. In this study, a new heat transfer intensification method was proposed for FCC external catalyst coolers, which aims to increase their bed-to-wall heat transfer coefficient by enhancing the internal solids mixing and thus the particle renewal on heat tube surface by a double-distributor design. To validate this idea, a large cold model with similar heat tube design and heat transfer mechanism to industrial catalyst coolers was built. Heat transfer coefficient and axial profiles of particle concentrations were measured under different operating conditions. The experimental results proved the feasibility of this heat transfer intensification idea. Higher bed-to-wall heat transfer coefficient, smaller fluidizing gas usage and higher adjustment flexibility are realizable in the new catalyst cooler. It is also learned from this study that uniform gas distribution, limited wall effect, good fluidization state are necessary to achieve good heat transfer performance in FCC external catalyst coolers. An effective height was speculated from the axial tube wall temperature distributions, within which the heat transfer intensification of the new catalyst cooler is effective. This effective height is also found to rise with increasing superficial gas velocity.     

气相法聚乙烯工艺冷凝态操作模式的稳定性和动态行为

气相法聚乙烯工艺冷凝态操作模式由于显著提高了循环气移热能力和反应器时空产率,已成为流化床乙烯聚合工艺的主流操作模式。建立了气相法聚乙烯工艺冷凝态操作模式的数学模型,包括流化床反应器模型,多级换热器模型和反应温度、压力以及循环气组成的控制模型。基于此,采用流程模拟方法,计算了系统在反应器温度采用闭环控制时的稳态解;根据系统对小扰动的动态响应特点,定性判断了反应器温度采用开环控制和闭环控制时聚合反应系统的稳定性;考察了系统对1-己烯分压和催化剂进料速率的阶跃响应特性。结果表明,反应器温度采用闭环控制时,聚合反应系统在所考察操作条件下均是稳定的,而采用开环控制时,解曲线被分叉点分割为稳定区域和不稳定区域。反应器温度对1-己烯分压阶跃变化的动态响应表明聚合反应系统存在长、短周期两类振荡,表明冷凝态操作模式下乙烯聚合反应过程是一个多控制回路耦合的复杂过程。     

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。     

Steady-state multiplicity in a solid oxide fuel cell: Practical considerations

Steady-state multiplicity in a solid oxide fuel cell (SOFC) in three modes of operation, constant ohmic external load, potentiostatic and galvanostatic, is studied using a detailed first-principles lumped model. The SOFC model is derived by accounting for heat and mass transfer as well as electrochemical processes taking place inside the fuel cell. Conditions under which the fuel cell exhibits steady state multiplicity are determined. The effects of operating conditions such as convection heat transfer coefficient and inlet fuel and air temperatures and velocities on the steady state multiplicity regions are studied. Depending on the operating conditions, the cell exhibits one or three steady states. For example, it has three steady states: (a) at low external load resistance values in constant ohmic external load operation and (b) at low cell voltage in potentiostatic operation.     

列管式固定床F-T合成反应器的模拟及优化

针对列管式固定床F-T合成反应器的特点建立了一维拟均相数学模型。通过对模型计算与试验结果进行对比分析,表明该模型可以较好的描述反应过程。并从固定床反应器的操作热稳定性出发,对列管式固定床F-T合成反应器的最大允许管径和最大传热温差进行了计算。结合模型计算结果得出了合适的反应管直径与冷却介质温度范围,可以优化列管式固定床F-T合成反应器的设计。     

Mathematical modeling of a moving bed reactor for post‐combustion CO2 capture

A mathematical model for a moving bed reactor with embedded heat exchanger has been developed for application to solid sorbent‐based capture of carbon dioxide from flue gas emitted by coal‐fired power plants. The reactor model is one‐dimensional, non‐isothermal, and pressure‐driven. The two‐phase (gas and solids) model includes rigorous kinetics and heat and mass transfer between the two phases. Flow characteristics of the gas and solids in the moving bed are obtained by analogy with correlations for fixed and fluidized bed systems. From the steady‐state perspective, this work presents the impact of key design variables that can be used for optimization. From the dynamic perspective, the article shows transient profiles of key outputs that should be taken into account while designing an effective control system. In addition, the article also presents performance of a model predictive controller for the moving bed regenerator under process constraints. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3899–3914, 2016     

Mathematical modeling of turbulent heat and mass transfer with chemical conversions

A transient model of heat and mass transfer with nonlinear sources (sinks) caused by first-and second-order chemical reactions is developed. The model uses a matching condition (equal temperature and local flux values) at the reaction zone-coolant interface. A finite-difference numerical solution to the problem is obtained using the alternating direction method. The model is tested by application to fast polymerization processes. The effect of the coolant velocity, reactor radius, and coolant temperature at the reactor inlet on the polymerization efficiency is studied.     

Modeling and comparison of different syngas cooling types for entrained-flow gasifier

A three-dimensional numerical model has been developed for studying the multiphase flow and heat transfer process in the syngas cooler. The realizable k−ε turbulent model and discrete random walk (DRW) model were adopted to simulate the gas phase and particle phase flow fields, respectively. The discrete ordinate model (DOM) was applied to solve the radiative heat transfer equation, and the gas radiative properties were calculated by weight-sum-of-gray-gases model (WSGGM). The ash particle radiative properties were also considered in the radiative heat transfer calculation. The convection heat-transfer between the gas phase and discrete phase is also considered. The flow field and temperature distribution results are in good agreement with the experimental data. Firstly, the results indicate that the RSC should be the better choice for integrated gasification combined cycle (IGCC) power plant. For cooling-syngas quenching cooler, the outlet region has higher risk of fouling and slagging because the outlet gas and particle temperature are about 940 °C, and exceed the criteria temperature 760 °C (suggested in the literature). Secondly, when the inlet velocity and flow rate of the quenching gas are fixed, the more the quenching gas inlets are, the better the flow field and temperature field are. A recirculation region with the diameter about 1.5–2.0 m is formed in the center of the cooler under the quenching gas profile, and the intensity of the reflux flow increases with the number of quenching gas inlets. The particles are rapidly quenched when the particle flow through the quenching gas profile. Furthermore, the temperature of the quenching gas, and the temperature of the water in the tubes of membrane wall also have important effect on the temperature field in the RSC.     

Energy efficient and controlled flow processing under microwave heating by using a millireactor–heat exchanger

A continuously operated microwave heated millireactor setup has been developed for performing reactions of highly microwave absorbing media in a controlled and energy efficient manner. The setup consists of a tubular reactor integrated with a heat exchanger. A microwave transparent liquid was used as coolant to extract the excess heat from the reaction mixture, thus controlling the temperature of the reaction mixture by avoiding overshoots and subsequent boiling. A reactor‐heat exchanger shell and tube unit with a diameter of the inner tube of 3·10^?3 m and a shell of 7·10^?3 m inner diameter has been manufactured in quartz. The unit size was defined based on simulation with a heat‐transfer model for the microwave cavity part. Microwave heating was incorporated as a volumetric heating source term using the temperature‐dependent dielectric properties of the liquid. Model predictions were validated with measurements for a range of 0.167·10^?6 to 1.67·10^?6 m³/s flow rates of coolant. The outlet temperature of both the reaction mixture and the coolant, were predicted accurately (tolerance of 3 K), and the process window was determined. The model for the reactor part provided the required length of the reactor for a hetero‐geneously catalyzed esterification reaction. The predicted conversions, based on the obtained temperature profile in the reactor packed with the catalyst bed, known residence times and kinetics of the esterification reaction, were found to be in good agreement with the experimental results. Efficient utilization of microwave energy with heat recovery up to 20% of the total absorbed microwave power and heating efficiencies up to 96% were achieved. It has been demonstrated that the microwave heating combined with millireactor flow processing provides controlled and energy efficient operation thus making it a viable option for a fine chemical production scale of 1 kg/day (24 h period). © 2011 American Institute of Chemical Engineers AIChE J, 58: 3144–3155, 2012     

Characterization of Soot Deposition and Particle Nucleation in Exhaust Gas Recirculation Coolers

Cooled exhaust gas recirculation (EGR) is used to control engine out NO_x (oxides of nitrogen) emissions from modern diesel engines by re-circulating a portion of the exhaust gases into the intake manifold of an engine after cooling it through a heat exchanger commonly referred to as an EGR cooler. However, EGR cooler fouling due to presence of soot particles and hydrocarbons (HC) in engine exhaust leads to a decrease in cooler efficiency and increased pressure drop across the cooler. This can adversely affect the combustion process, engine durability, and emissions. In this study, a multicylinder diesel engine was used to produce a range of engine out HC and soot concentrations to investigate soot deposition and particle nucleation in an EGR cooler. A portion of the engine exhaust was passed through an EGR cooler, while particle size and HC concentration measurements were made at the cooler inlet and outlet. Tests were conducted over a range of EGR cooler coolant temperatures and engine out soot and HC concentrations to determine the impact on the nucleation and accumulation modes of the exhaust particle size distributions. A reduction in the accumulation mode particle concentration at the EGR cooler outlet was observed for high soot concentrations indicating soot deposition within the EGR cooler. As the EGR coolant temperature was reduced, the outlet accumulation mode particle concentration was reduced further, indicating increased soot deposition in the cooler due to increased thermophoresis. There were no signs of diffusiophoresis due to HC diffusion within the cooler over the range of conditions used in the study. A significant increase in outlet nucleation mode particle concentration was observed for the low soot concentrations. This mode increased with either increasing HC concentration or decreasing coolant temperature, indicating the saturation ratio (SR) dependence of the nucleation mode formation. However, as the soot concentration was increased, the nucleation mode disappeared because of HC adsorption onto the increased soot surface area.

Copyright 2012 American Association for Aerosol Research     

Dynamic behaviour of a semibatch non-isothermal gas-liquid reactor

A dynamic model to predict transient temperature profiles of a nonisothermal semibatch gas-liquid reactor was developed. The enhancement factor for non-isothermal gas absorption was calculated by extending the model of Bhattacharya et al. (AIChE J (1987) 33 1507) to incorporate the effect of finite gas film resistance. For a particular combination of values of heat of reaction, heat transfer and activation energy parameters, transient temperature profiles of the semibatch reactor were found to be highly parameter sensitive. In the reactor model, gas phase dynamics were taken into account and heat generated in the reactor was assumed to be removed by a coolant flowing through a cooling coil.     

转化过程气体换热器的副线调节性能——温度的影响

对转化过程采用圆缺形折流板和双圆缺形管束排列的气体换热器 ,推导出操作气量负荷率、管壳程副线率以及换热面积富裕系数与换热器管壳程进出口温度变化值之间的定量解析关系式 ,它们仅是换热器管壳程进出口温度设计值的函数。管壳程进出口温度变化共有 58种组合形式。最小换热面积富裕系数为温度变化引起换热量变化及换热器平均温差变化两部分的乘积。     

Operating strategies for Fischer-Tropsch reactors: A model-directed study

A comprehensive parametric study for a Fischer-Tropsch (FT) synthesis process has been conducted to investigate the relation between process parameters and reactor characteristics such as conversion, selectivity, multiplicity, and stability. A flexible model was employed for this purpose, featuring the dependence of Anderson-Shultz-Flory (ASF) factor on composition and temperature. All variable process parameters in industrial FT reactors were subject to variation, including reaction temperature, reactor pressure, feed ratio, inlet mass flux, feed temperature, heat transfer coefficient, catalyst concentration, catalyst activity, etc. While typical trade-off was encountered in most cases, i.e., the change of a parameter in one direction enhances one aspect but deteriorating another, the change of feed conditions gave some promising results. It has been found that decreasing the feed rate (or increasing the residence time) and/or lowering the feed concentration can successfully enhance the conversion up to more than 90% for our specific case, without hurting the product selectivity as well as effectively condense the region of multiple steady states. The benefits and limitations accompanied with the variation of the parameters were discussed in detail and a rational start-up strategy was proposed based on the preceding results. It is shown that the decrease of inlet mass flux (say, 85% decrease of the feed rate or 60% decrease of the feed concentration from the nominal condition chosen here) or the decrease of H₂/CO ratio (specifically, below about 0.25), or their combination can eliminate multiple steady states. The resulting unique relation between temperature and manipulated variable (i.e., coolant flow rate) appears to assure a safe arrival at the target condition at the start-up stage. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

聚丙烯环形反应器模拟与分析——(Ⅰ)定态特性和可操作域

以工业装置为背景,建立了聚丙烯环形反应器数学模型,经模拟计算,研究了聚丙烯环形反应器的工艺特性,工艺参数对反应器操作状态的影响,并对反应器的可操作域作了分析。     

Modelling of fluidized bed reactors—VI(b): The nonisothermal bed with stochastic bubbles

Two stochastic nonisothermal fluidized bed reactor models are developed to investigate the significance of the fluctuating nature of fluidized beds on reactor performance. Fluctuating bubble size distributions within the bed are simulated by stochastic mass and heat transfer coefficients. Results of hybrid computer simulations indicate that randomness can enhance or inhibit reactor performance depending on the operating parameters of the nonisothermal model. Bubble and dense phase concentration statistics are fairly similar to those of corresponding isothermal models because dense phase temperatures are relatively insensitive to transfer coefficient fluctuations due to the high dense phase beat capacity. However, the corresponding stochastic isothermal models predict decreases in conversion with increasing variance in the transfer coefficients for all operating conditions. Results indicate that a deterministic system with two stable steady states may have fewer stable random stationary solutions. The existence of the stationary states is dependent on fluctuation frequency and variance of the transfer coefficients.