EFFECT OF PORE DIFFUSION IN METHANOL SYNTHESIS

Pore diffusion limitations dampen the sensitivity of temperature runaway to variations in the coolant temperature in methanol synthesis reactors. For an isothermal pellet and low product concentration, the relationship between catalyst effectiveness and the Thiele parameter can be represented with a single curve. However, when equilibrium is approached or at high product concentrations, the relationship between catalyst effectiveness and the Thiele parameter becomes a function of catalyst surface temperature. For large temperature gradients in the catalyst, the effectiveness factor can significantly increase and parametric sensitivity in the relationship between catalyst effectiveness and the Thiele parameter may result.     

EFFECTS OF PORE DIFFUSION ON THE SYNTHESIS OF METHANOL

The hierarchical model concept is applied for solving of the test-problem. Three levels of hierarchy are defined. On the first one the reaction rate at an inner point of a pore in the catalyst pellet is examined. On the 2-nd level, taking pore diffusion into account and introducing the so-called distortion factor, the mean reaction rate concerning a pellet is calculated. On the 3-rd level, the rate of reaction at an inner point of the reactor is given, and the parametric study of industrial reactor is carried out.     

THE EFFECT OF PORE DIFFUSION ON THE SHAPE OF THE PH AND TEMPERATURE PROFILES OF IMMOBILIZED ENZYMES

The effect of diffusion on the pH and temperature profiles of immobilized enzymes has been quantitatively assessed using the theory of pore diffusion. The pH profiles of immobilized enzyme systems varied from those of soluble ones to the square root of the relative values of the soluble ones depending on the degree of pore diffusion limitation. Also the decrease in activation energy was predicted quantitatively with considerable accuracy in terms of effectiveness factor. The experimental pH profiles of a few immobilized enzymes were compared with those theoretically predicted.     

UCKRON II TEST PROBLEM THE EFFECTS OF PORE DIFFUSION

The UCKRON I test problem was reworked with the inclusion of diffusional limitations to the kinetics. For this purpose, the rate expressions, i.e. the full kinetic model and the Langmuir-Hinshelwood model, proposed in the previous workshop, were assumed to represent the true kinetics.

A simple mathematical model based on simplified effective diffusivities and the “Dusty-Gas” model were used to take account of pore diffusion resistance within the catalyst particle. These models predicted effectiveness factors based on the reaction at the bulk conditions. The effectiveness factors were then correlated against temperature and the partial pressure of reacting species.

These correlations were then used to predict the temperature profiles within the water cooled plug flow reactor for various values of cooling media temperature.     

二级可逆反应等温催化剂有效因子的简化计算

将等效温度法的应用拓宽至二级可逆反应,推导出球型等温催化剂有效因子的近似解析解,采用一氧化碳变换反应宏观动力学结果加以验证,证明该法简便可靠。     

氧化钙催化活性在焦油裂解过程中的变化规律

在以循环床锅炉循环灰为热载体、部分气化产生的半焦为锅炉燃料的煤的部分气化技术中,循环灰中的氧化钙对降低焦油产率、提高煤气产率是有利的。以焦油中的苯和甲苯为对象,用固定床反应器实验研究了焦油裂解过程中反应对氧化钙催化活性的影响,测定了失活系数,探讨了失活机理。实验结果表明,随着积碳量的增加,氧化钙颗粒的比表面积和空隙率减小,裂解反应速率下降,裂解产物中碳的总产率增加。     

EFFECTS OF SOLUTE ADSORPTION ON HINDERED DIFFUSION UPTAKE RATES IN FINITE BATH EXPERIMENTS

This work investigates the effects of solute adsorption on hindered diffusion behavior in porous catalysts. A mathematical model describing the adsorptive diffusion process is developed. The model, termed the shrinking pore model, incorporates the local reduction in catalyst pore diameter due to the adsorption of solute molecules on the pore walls. The influence of the adsorbed solute layer is found to depend on two additional parameters, reflecting the relative degree of adsorption and molecule/pore size ratio. Hindered diffusion experiments are performed for diffusion controlled adsorptive uptakes of two solute molecules, quinoline and polystyrene, from cyclohexane on a porous catalyst. Comparison of the experimental data and model simulation results shows that for the larger polystyrene solute the shrinking pore model better represents the uptake behavior than the conventional model which assumes constant catalyst properties, e.g. pore diameter, during the uptake process. Experimental measurements were found to be in good agreement with model simulations after accounting for additional hindered diffusional effects due to an adsorbed solute layer on the pore walls. The additional hindrance due to the adsorbed solute was found to be very significant for the uptake of the larger polystyrene solute, whereas it was not significant for the smaller quinoline solute.     

EFFECT OF DIFFUSIONAL RESISTANCE ON THE REGENERATION OF COKE-DEACTIVATED SUPPORTED METAL CATALYST

This paper presents the solution of the model of the simultaneous diffusion and reaction of the uniformly coked catalyst undergoing regeneration with gaseous reaction with the solid coke. The reaction is assumed to be catalytic hence it obeys Langmuir-Hinshelwood kinetics. The resulting -gas and coke mass balance non-linear coupled equations were solved by finite difference technique. Profiles of coke and gas consumption within the pellet are presented and the effect of diffusional resistances are evaluated by the effect of a sufficient variation in Biot number.     

ANALYSIS OF THE PERFORMANCE OF NON-UNIFORMLY ACTIVE CATALYST PELLETS FOR A NON-ISOTHERMAL SERIES REACTION

An analysis of the influence of the non-uniform distribution of the active species in a catalyst pellet on the effectiveness factor and selectivity for the case of a series non-isothermal reaction is carried out. The case in which the desired reaction has a lower activation energy than the undesired reaction is considered. The effect of different reaction parameters on the selectivity for a single pellet is studied, and the optimum activity distribution is determined for high values of Thiele modulus. It is shown that temperature gradients within the pellet have a detrimental effect on local and overall selectivity and that these gradients are a strong function of the distribution of catalytically active material within the pellet.     

吸附密度对蛋白质在离子交换吸附剂中孔扩散系数的影响

通过间歇吸附动力学实验,采用孔扩散模型研究了牛血清白蛋白和γ-球蛋白在阴离子交换剂中的扩散行为,考察了蛋白质初浓度和平均吸附密度对孔扩散系数的影响.结果表明,随蛋白质初浓度和平均吸附密度增大,孔扩散系数均呈指数下降,且蛋白质分子尺寸越大下降趋势越明显.     

INFLUENCE OF CHAR PROPERTIES ON REACTION RATES

The reaction of coal char is affected by the char morphology, which is dependent on the temperature and pressure at which the char is prepared. Char properties, such as surface area (by CO₂ adsorption) and diffusion coefficients of CO₂ in char have been measured for chars prepared at 900-1200°C and at pressure to 16 atm at 900°C. The diffusion coefficient results strongly indicate Knudsen or activated surface diffusion. The surface areas and diffusion coefficients decrease in general at higher preparation temperatures, but have a maximum at 1000°C. There is an apparent relationship between these observations and the reactivity results which demonstrate unusual behavior above 1000°C.     

INFLUENCE OF CHAR PROPERTIES ON REACTION RATES

The reaction of coal char is affected by the char morphology, which is dependent on the temperature and pressure at which the char is prepared. Char properties, such as surface area (by CO₂ adsorption) and diffusion coefficients of CO₂ in char have been measured for chars prepared at 900-1200°C and at pressure to 16 atm at 900°C. The diffusion coefficient results strongly indicate Knudsen or activated surface diffusion. The surface areas and diffusion coefficients decrease in general at higher preparation temperatures, but have a maximum at 1000°C. There is an apparent relationship between these observations and the reactivity results which demonstrate unusual behavior above 1000°C.     

HYDRODEMETALLATION CATALYST DEACTIVATION—EFFECT OF METALS ACCUMULATION ON HYDROGENATION AND HYDROGENOLYSIS ACTIVITIES

The deactivation of hydrodemetallation catalyst was investigated in diffusion-free conditions using a model oil system. The hydrodemetallation activity of a CoMo/γ-alumina catalyst was studied using nickel or vanadyl etioporphyrins as model compounds and the hydrodesulfurization and hydrogenation activity using dibenzothiophene and naphthalene. The experiments were carried out over sulfided catalysts at 320°C and 4.83 MPa total pressure. The catalyst activity for nickel etioporphyrin hydrodemetallation was maintained at least up to 50% metal loading. The hydrogenolysis activity decreased with increasing nickel and vanadium on the catalyst but the hydrogenation activity increased. The changes in activity were attributed to the different activities of the various sulfide phases present in the catalyst system.     

分子吸附对聚乙烯醇在氧化铝粉末压缩体中扩散系数的影响

利用氧化铝粉末压缩球状体和Ｆｉｃｋ第二扩散定律,系统地研究了聚乙烯醇在多孔介质和实验系统ｐＨ值为８．４０±０．２条件下的扩散行为,以及聚乙烯醇在球体外表面吸附对扩散系数的影响．实验结果和相应计算得到的扩散系数表明：发生在氧化铝表面的分子吸附,不仅导致扩散平衡状态时的Ｍｔ（聚乙烯醇在时间ｔ的扩散量）高于理论计算值Ｍ∞,而且使实验初始阶段计算的扩散系数远大于其它过程扩散系数．由分子吸附产生的实验误差和扩散系数计算误差,可通过修正浓度分配系数Ｋ,或者直接从Ｍｔ中去除可能存在的吸附量得到解决．     

PERFORMANCE OF AN ANNULAR CATALYTIC THREE-PHASE REACTOR FOR A SECOND ORDER REACTION

A novel annular catalytic three-phase reactor is theoretically investigated for a second order hydrogenation reaction. The results indicate that the ratio of the gaseous reactant to the liquid reactant within the reaction zone of the annular reactor can be adjusted by changing the reactor dimensions and the concentration of the gaseous reactant in the feed. As a consequence, the annular reactor may have a significant advantage over the trickle-bed reactor with respect to the selectivities that may be achieved in the reactor.     

分布活化能模型用于模拟蒸馏的初步研究

对分布活化能模型向二级反应体系中的应用进行了推广,将最新的Miura分析方法应用于煤焦油馏分的模拟蒸馏曲线分析,发现所得活化能数据同非线性动力学回归分析结果非常接近,初步表明了该法应用于二级反应体系的可行性。     

拟稳态法测定气相扩散系数

根据拟稳态一维扩散原理,建立了气相扩散系数的测定装置。在该装置上,测定了8个体系的气相扩散系数,其中CH_2Cl_2-空气,CH_2Cl_2-H_2,CH_3CH_2Br-空气,CH_3Cl_2Br-H_2,CHCl_3-H_2等5个体系的气相扩散系数实验值,迄今尚未见文献报道。实验结果表明,本测试装置稳定性好,实验误差小,实验装置体积小,测试过程较为简便。     

FUNDAMENTAL MODEL OF TRANSPORT AND REACTION IN A CYLINDRICAL CATALYST PORE

The kinetic theory of gases is used to arrive at integral equations which govern free molecular flow and heterogeneous kinetics in a single, cylindrical, isothermal, uniform catalytic pore. The formulation is in terms of reactant fluxes to the catalytic surface and overall or reactive sticking factors. Focus is on simple m-th order reactions which depend on the surface concentration of a single species. Effectiveness factors for the pore, as well as reactant flux profiles obtained using this fundamental model, are compared to conventional continuum diffusion-reaction model predictions of effectiveness factors and reactant concentration profiles. There are distinct differences between the predictions of the two models for moderate to high sticking factors (0·1 to 1). For very small sticking factors, as commonly found in practical catalytic reaction systems, predictions of both models are essentially the same. The widely used continuum diffusion-reaction method provides good predictions of effectiveness factors and reactant concentration profiles, although it is not rigorous in its treatment of species transport.     

MODELLING THE DEACTIVATION KINETICS OF CYCLOHEXANE DEHYDROGENATION ON PLATINUM/ALUMINA CATALYST

Two reaction deactivation kinetic models of the deactivation of cyclohexane on platinum/alumina catalyst have been proposed based on the coking mechanism of Corella and Asua and the reaction mechanism of Susu el al. The difference in the models was in the number of active sites participating in the coking reaction represented by h with h = 1 in model 1 and h ≠ 1 in model 2. The models were dynamic models and parameter estimates were carried out by a method that did not require decoupling of the main from the coking reaction. The model formulation has made possible the estimation of the level of residual activity by the incorporation of a factor, ƒ, that represents incomplete deactivation. For the deactivation kinetic data studied, a high value of ƒ; was obtained indicating a low residual activity. The models were found to give a good fit with the data. The standard deviation of the models from experimental were 10.21 % for model 1 (h ≠ 1) and 12.63% for model 2 (h ≠ 1), None of the models is superior to the other, though for having a lower standard deviation model I is better. It is therefore appropriate to propose that deactivation by coking in this work occurred preferably by a single site mechanism of the rate controlling step of the coking reaction.     

浸渍对炭-石墨材料孔隙孔径的影响

以进口Ф 288 mm接头为试样,对其进行了二浸三烧及石墨化处理,并逐阶段对不同孔隙孔径的填充情况做了图像分析的微区定量测试,从测试结果中可看出浸渍的效果与炭－石墨材料孔隙孔径的大小有直接关系。