OPTIMUM REACTOR DESIGN IN METHANATION PROCESSES WITH NONUNIFORM CATALYSTS

A generic methodology is developed to design a heterogeneous catalytic reactor for methanation processes. For the optimization of a heterogeneous catalytic reactor, nonuniform catalyst pellets such as a layered catalyst are considered with respect to reaction type, reactor performance, and component distribution inside the catalyst. Heterogeneous uniform and nonuniform catalyst models were developed to analyze the effect of mass and heat transfer between both bulk phase and catalyst surface and inside a catalyst pellet. Then, concentration profiles of hydrogen and carbon monoxide in the catalyst pellet and along the reactor axis were obtained by analyzing simulation results. It was shown that the application of different types of nonuniform catalyst pellets at a certain number of separate zones within a reactor could produce higher catalyst performance than a reactor with uniform catalyst. Furthermore, it proved a significant decrease of catalyst deactivation behavior such as coking and sintering. Layered catalysts were optimized to maximize an overall reactor performance over the catalyst lifetime, achieving capital cost reduction characterized by reactor size, catalyst amount, and degree of catalyst deactivation. Last, temperature control throughout the reactor operating periods was strategically planned for a reactor operation with distribution of nonuniform catalyst pellets. This methodology can also be usefully applied to the design of heterogeneous catalytic reactors for other processes such as hydro-treating process and cracking process.     

QUALITATIVE OBSERVATIONS OF HEAT AND MASS TRANSFER EFFECTS ON THE BEHAVIOUR OF A TRICKLE BED REACTOR

This paper examines the effect of simultaneous heat and mass transfer on the hydrogenation of cyclohexene in a trickle bed reactor with particular attention given to the problem of liquid phase evaporation and transition to the gas-phase regime of operation. The reaction rates are obtained as a function of temperature and hydrogen flow rate; the concentration of the substrate in the feed displays considerable hysteresis due to an abrupt increase of the reaction rate arising from temperature gradients within the bed and in the gas film surrounding the catalyst pellet, during the transition from the liquid to the gas-phase regime. The transition is accompanied by the change of apparent kinetics of the model reaction as well as by a change of regime and operation of the pellet. In the liquid phase a pellet originally showing inter-phase and intra-particle diffusion resistances changes into the gas-phase regime with a large resistance due to inter-phase diffusion.     

内冷式刮膜薄膜蒸发器的传热性能研究

在内冷式刮膜薄膜蒸发器中试装置上以粘性流体甘油为物料进行传热性能研究 ,发现刮膜薄膜蒸发时夹套导热油侧传热系数为液膜侧传热系数的 1.5～ 2 .0倍 ,传热热阻主要在液膜侧。实验还发现 ,随进料流量增大 ,进料温度升高 ,刮板转速增大 ,蒸发温度降低 ,操作压力降低 ,液膜侧传热系数会增大。结合因次分析和实验数据 ,关联物料物性、操作条件及设备尺寸等因素得到液膜侧传热系数的准数关联式。     

Estimation of concentration and temperature profiles for methane-steam reforming reaction in a porous catalyst

This work represents the results of the theoretical study of the distribution of concentration and temperature in porous catalyst, carried out over spherical catalyst pellets for the methane-steam reforming reaction over Ni/MgAl₂O₄ catalyst. Kinetic information has been taken from a recent study [J. Xu, F.G. Froment, AIChE J. (35) (1989) 88–96] for the above reaction. The mass and heat transfer distribution coupled with chemical reaction, outside and within the catalyst particles for methane-steam reforming reaction was estimated in this present study. A computer program was written to calculate both concentration profile and temperature profile for the methane-steam reforming reaction.     

MATHEMATICAL MODELING OF HETEROGENEOUS CATALYTIC REACTORS

It is well known that in the practical employment of catalysts in commercial reactors, physical processes may play an important and even dominant role in the overall conversion scheme. Such physical processes may include mass transfer both within and external to the catalyst particles and the analogous effects of heat transport. In addition, for packed beds, dispersion of mass and heat in both the axial and radial directions may be important. The intra-particle effects arise because of the significant size of the catalyst particles which in turn is dictated by the requirement that the pressure drop within the reactor shall not be excessive. In practice, catalyst pellets up to about 1 cm in diameter are often employed.     

MATHEMATICAL MODELING OF HETEROGENEOUS CATALYTIC REACTORS

It is well known that in the practical employment of catalysts in commercial reactors, physical processes may play an important and even dominant role in the overall conversion scheme. Such physical processes may include mass transfer both within and external to the catalyst particles and the analogous effects of heat transport. In addition, for packed beds, dispersion of mass and heat in both the axial and radial directions may be important. The intra-particle effects arise because of the significant size of the catalyst particles which in turn is dictated by the requirement that the pressure drop within the reactor shall not be excessive. In practice, catalyst pellets up to about 1 cm in diameter are often employed.     

异型管曲率对气液膜分布及凝结换热特性的影响

基于双膜理论及边界层理论,建立了圆管、椭圆管、滴形管的管外气、液膜厚度及传热系数的数学模型。以圆管传热系数为依据进行了模型验证,计算结果与实验值的平均偏差约为6%,基本符合工程实际要求。在给定条件下,通过对不同曲率下各管型管外传热系数的计算,得到了气、液膜厚度的分布规律及换热特性。结果表明：在有效换热面积相同时,三种管型的气、液膜厚度,传热系数在一定角度下,随曲率的增大而增大;当曲率相同时,椭圆管传热系数最大,圆管最小。同时,分析了不同管型及其曲率对气、液膜排泄的影响机理和分离机理。为强化换热提供一定理论指导。     

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.     

HEAT TRANSFER IN A THIN LIQUID FILM IN THE PRESENCE OF AN ELECTRIC FIELD

Heat transfer enhancement in an evaporating thin liquid film utilizing a electric field under isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, capillary pressure, and an electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of the electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that applying an electric field can enhance heat transfer in a thin liquid film significantly. In addition, utilizing electric fields on the evaporating film will be a way to expand the extended meniscus region to attain high heat transfer coefficients and high rates of heat flux.     

HEAT TRANSFER IN A THIN LIQUID FILM IN THE PRESENCE OF AN ELECTRIC FIELD

Heat transfer enhancement in an evaporating thin liquid film utilizing a electric field under isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, capillary pressure, and an electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of the electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that applying an electric field can enhance heat transfer in a thin liquid film significantly. In addition, utilizing electric fields on the evaporating film will be a way to expand the extended meniscus region to attain high heat transfer coefficients and high rates of heat flux.     

Methanol synthesis in a three‐phase catalytic bed under nonwetted condition

To overcome the heat removal problem encountered in methanol synthesis at high syngas concentrations in the gas phase, a three‐phase nonwetted catalytic system was established by introducing an inert liquid medium into a fixed‐bed reactor. To form a repellent interface between the liquid and the catalyst, the catalyst was modified into hydrophobic, while the liquid medium was chosen as a room temperature ionic liquid with hydroxyl groups. The liquid‐solid contact angle was measured to be 115°, and only 20% of the catalyst external surface was wetted by the liquid. Under three‐phase condition, the reaction rate was measured to be 60%–70% of gas‐phase reaction, while it was merely 10%–20% for the fully wetted catalyst. From the resistance analysis on the mass transfer and reaction steps, the overall reaction rate is expected to increase further if the surface could be more wet proofed. © 2016 American Institute of Chemical Engineers AIChE J, 63: 226–237, 2017     

双搅拌釜中脱硫催化剂再生的传质反应过程

在双搅拌釜中,考察了ＴＨＳ、ＡＤＡ、ＴＦ三种催化剂的再生氧化传质速度随液相传质数变化的规律,并以效果最好的ＴＨＳ的对象,进一步考察了催化剂浓度、温度、ｐＨ等因素对其再生氧化传质速度的影响。研究了用液相催化氧化法脱硫化氢的催化剂再生的传质反应过程。     

柱状阳离子交换树脂上乙基叔丁基醚的合成

将国产粒状强酸性阳离子交换树脂S - 5 4制成柱状催化剂 ,在其作用下 ,以乙醇和叔丁醇为原料 ,于常压液相条件下合成乙基叔丁基醚 (ETBE) .考察了温度、催化剂用量、反应物初始摩尔比以及水的存在等因素对反应速度的影响 ,建立了一个 7参数动力学模型 ,模型与实验数据吻合较好 .并将柱状和粒状S - 5 4树脂对该反应体系的催化效果进行了比较     

EXPERIMENTAL DETERMINATION AND MODELLING OF LIQUID-SOLID MASS TRANSFER COEFFICIENTS IN A THREE-PHASE REACTOR

Apparent mass transfer coefficients for solid dissolution in a liquid with and without a chemical reaction were experimentally determined in a fixed bed three phases reactor with downward cocurrent gas and liquid flows. The chemical system selected was benzoic acid, sodium hydroxide in aqueous solution, and atmospheric air. Continuous gas, pulse and dispersed bubble regimes were studied and the results were correlated obtaining apparent mass transfer coefficient as a function of liquid and fluid volumetric flow. It was found that gas flow effect on mass transfer coefficient was small over continuous gas and dispersed bubble regimes, but appreciable over pulse regime. Additionally, it was found that the mathematical model that best described the mass transfer process under pulse regime, by using the increment factor due to the instantaneous chemical reaction, is the film theory     

SO_2在钒催化剂上的氧化反应动力学

本文通过SO_2在钒催化剂熔融液相中反应特性的分析,提出了一种新的液相反应机理及液相反应本征动力学模型.再将液相传质与液相本征动力学模型结合得到“微观”动力学模型,避开了液相有效因子的复杂求解.认为不同温度范围必需建立不同的模型.在推导S_(101)型工业催化剂原粒的宏观动力学模型中,采用了简化的孔扩散模型.根据对实验数据的回归,得到两温度段的宏观动力学模型如下:r_1=k_1po(_2)~318pso(_2)~112[(1-3/2β)~112/(1+k_1(pso_3/pso_2)~(1/2)]~112(高温段)r_2=k_2po(_2)~114pso(_2)~112[(1-3/2β)~112/(1+k_1(pso_3/pso_2)~(1/2)]~112 (低温段)     

The measurement and prediction of temperature increase in single catalyst pellets

Temperatures within and near to single pellets of tin/antimony oxide used to catalyse the oxidative dehydrogenation of butene to butadiene have been measured and the results compared with values predicted by a mathematical model. The observed temperature difference was up to 10°C, depending on the reaction conditions, with the major change in temperature occurring across the gas film near to the surface. The mathematical model describes these observations with reasonable accuracy, and discrepancies are discussed in terms of the parameters that are used in the calculation. The derived temperature difference is found to be particularly sensitive to values of bulk concentrations and film heat transfer coefficient, and to be less responsive to values of bulk density and effective diffusivity.     

A lumped parameter approximation to a general model for catalytic reactors

Consideration ig given to the solution of the highly exothermic fixed bed catalytic reactor problem taking into account heat and mass transfer resistances inside the catalyst pellets and across the external fluid film, as well as radial temperature and concentration gradients in the fluid phase. Even on large digital computers, the computation time is excessively long if the sets of differential equations are solved simultaneously. By considering the intra-particle equations in detail, it is shown that they may be reduced to a lumped parameter form which permits rapid evaluation of an effectiveness factor. The functional dependence of this effectiveness factor is determined primarily by the external film resistance. While still retaining the characteristics of the general problem, the lumped parameter approximation may be solved in a substantially shorter time, thus permitting its use in optimization and control studies.     

气液旋流器内液相平均停留时间

旋流设备内伴有传热、传质或反应过程时,介质停留时间是其关键参数。在冷模实验装置上,采用持液量法,对气液旋流器内液相平均停留时间进行了研究。结果表明,液相停留时间随入口含液率增大明显降低,随入口气速增大降低较小。气液界面剪切力远小于液相重力是入口气速对液相停留时间影响较小的主要原因。基于液膜受力平衡,建立了气液旋流器内液相平均停留时间模型。模型预测停留时间与实测值总体吻合良好,在液膜Reynolds数Re_l< 1200范围内,模型预测停留时间偏大,讨论了模型预测偏差与液膜流型的关系。     

降膜蒸发管内液体夹带及传热恶化特性的研究

对垂直管内蒸汽和水逆向流动时的降膜蒸发、蒸汽夹带液体及传热恶化特性进行了详细的试验研究,发现降膜蒸发管工质入口端的壁温首先突然升高和出现波动.通过分析其机理和影响因素,给出了传热恶化的预测关系式.     

木质素悬浮液管式降膜蒸发传热研究

管式降膜蒸发器由于具有传热系数高、传热温差小等优点,已应用于多个工业领域,对管式降膜蒸发器的研究也成为近年来传热技术领域的一项重要课题.文中设计了一套管式降膜蒸发装置,以木质素悬浮液为工质,对影响降膜蒸发传热的因素进行了实验研究,通过冷膜实验测试了所设计的筛板分布器的性能,得出筛板分布器能较好地使物料在换热管内壁上成膜...