Multicomponent mass diffusion in porous pellets: Effects of flux models on the pellet level and impacts on the reactor level. Application to methanol synthesis

A heterogeneous model has been derived for a fixed packed‐bed reactor producing methanol. Several closures for the intra‐particle mass diffusion fluxes; Maxwell–Stefan, Wilke, dusty gas and Wilke–Bosanquet, have been compared on the level of the catalyst pellet and the impacts of the different particle flux closures on the reactor performance are investigated. A preparatory study of the transport phenomena on the pellet level is recommended prior to any large‐scale reactor simulation to determine what are the rate determining transport mechanisms. Hence, if Knudsen diffusion is apparent on the level of the pellet, a combined bulk and Knudsen diffusion model should naturally be used in the reactor simulations as well, because Knudsen diffusion can influence significantly on the reactor conversion. Minor differences are observed between the diffusion flux models on both pellet and reactor level. Hence, for the reactor operation conditions applied in this study, the Wilke model is a good approximation to the rigorous Maxwell–Stefan model, and similarly, the Wilke–Bosanquet model is an appropriate model to use in replacement for the dusty gas model. Moreover, variable pressure and viscous flow can be neglected in the pellet model, as the effect of these contributions are not visible at neither pellet or reactor level. © 2011 Canadian Society for Chemical Engineering     

模拟单颗粒催化剂上多组份反应扩散过程的通用软件的开发

本文在尘气传递模型的基础上,开发了在微机上实现的模拟单粒催化剂多组份多反应体系反应扩散过程的通用软件。它可用于横拟速率方程为任意形式的,扩散系数随温度、浓度变化的,不同形状颗粒、粒外有或无扩散效应时等温或非等温的反应扩散过程。本软件具有管理功能强、模型精度高、算法先进以及适用性广等特点,并配有彩色图形输出,是辅助催化剂设计和分析反应器性能的有效工具。     

Transesterification of soybean oil to biodiesel over kalsilite catalyst

The transesterification reaction of soybean oil with methanol over kalsilite-based heterogeneous catalysts was investigated. The kalsilite was synthesized from potassium silicate, potassium hydroxide, and aluminum nitrate aqueous solutions by controlling the pH value at 13. After calcination in air at 1200°C, a very porous kalsilite (KAlSiO₄) was obtained with surface pores ranging from 0.2 to 1.0 μm. However, this kalsilite had relatively low catalytic activity for the transesterification reaction. A biodiesel yield of 54.4% and a kinematic viscosity of 7.06 cSt were obtained at a high reaction temperature of 180°C in a batch reactor. The catalytic activity of kalsilite was significantly enhanced by introducing a small amount of lithium nitrate in the impregnation method. A biodiesel yield of 100% and a kinematic viscosity of 3.84 cSt were achieved at a temperature of only 120°C over this lithium modified catalyst (2.3 wt-% Li). The test of this lithium modified catalyst in pellet form in a laboratory-scale fixed-bed reactor showed that it maintained a stable catalytic performance with a biodiesel yield of 100% over the first 90 min.     

Modeling of multicomponent mass diffusion in porous spherical pellets: Application to steam methane reforming and methanol synthesis

The main purpose of this paper is the derivation and evaluation of various diffusion flux models. For this aim, a comprehensive catalyst pellet problem has been simulated for two test cases: the steam methane reforming (SMR) and the methanol synthesis, as these two important chemical processes cover various aspects of a chemical reaction. The pressure, temperature, total concentration, species composition, viscous flow, mass and heat fluxes within the porous spherical pellet are included in the transient pellet model. Mass diffusion fluxes are described according to the rigorous Maxell–Stefan and dusty gas models, and the respectively simpler Wilke and Wilke–Bosanquet models. Simulations are performed with these fluxes defined according to both the molar averaged and mass averaged definitions. For the mass based pellet equations, a consistent set of equations is obtained holding only the mass averaged velocity. On the other hand, the closed set of molar based pellet equations hold both the molar averaged and mass averaged velocities as the fundamental energy balance and the momentum balance (Darcy law) are derived according to the mass averaged velocity definition, whereas the diffusion fluxes are defined relative to the molar averaged velocity. Identical results of the molar and mass based pellet equations were not obtained; however, the deviations are small. It is anticipated that these discrepancies are due to some unspecified numerical inaccuracies. However, efficiency factors have been computed for both processes and the values obtained compare well with the available literature data. Furthermore, efficiency factor sensitivity on parameters like pore diameter, tortuosity, temperature and pressure have been accomplished, and the classical simplifications of the pellet equations have been elucidated: isothermal condition, constant pressure, and neglecting viscous flow. The following conclusions are established for the reactor operating conditions used in the present work. The methanol synthesis: The simulation results of the methanol synthesis indicate that the classical assumptions are very fair for this process. Moreover, both Wilke and Wilke–Bosanquet models are good replacements for the more rigorous Maxwell–Stefan and dusty gas models. However, the simulation results are affected by Knudsen diffusion, thus the diffusion flux is most appropriately described by the Wilke–Bosanquet model. The SMR process: Knudsen diffusion hardly influences the results of the highly intraparticle diffusion limited SMR process. As the Wilke model does not necessarily conserve mass, we recommend the Maxwell–Stefan model because the simpler Wilke closure deviates with several percents. However, it is not elucidated whether these deviations are numerical problems arising from the large gradients of this process, or related to the choice of diffusion model. Isothermal and isobaric conditions can be assumed within the particle, but significant external temperature gradients are observed. Convective fluxes are much less than the diffusive fluxes, hence viscous flow can be neglected.     

The dissolution of cupric hydroxide films from copper surfaces

The dissolution of cupric hydroxide films grown under well defined potentiostatic conditions has been studied in lithium hydroxide as a function of hydroxide ion concentration, electrode rotation speed (at a rde, and formation potential. The dissolution reaction was followed by monitoring the amount of cupric hydroxide on the copper surface electrochemically at any given time. The open-circuit potential was used as an indication of when the dissolution of when the dissolution reaction was complete.

The dissolution reaction was found to be jointly controlled by surface kinetics and diffusion of the dissolution product away from the surface. The effect of incomplete dissociation of LiOH was also considered.     

Physical and electrochemical characteristics of nickel hydroxide as a positive material for rechargeable alkaline batteries

Nickel hydroxide is widely used as an active material in pasted-type nickel electrodes. Physical properties of several nickel hydroxide powders have been examined by laser diffraction, BET, X-ray diffraction, thermal analysis and SEM. Nickel hydroxide powder with a smaller crystalline size shows better charge-discharge cyclic characteristics. The chemical diffusion coefficients of the proton in the nickel hydroxide sample are measured by a current pulse relaxation method. Nickel hydroxide with a smaller crystalline size shows a higher proton diffusion coefficient, giving excellent cycling behaviour.     

高镁锂比盐湖卤水中锂镁沉淀法的分离研究

采用改进的沉淀法进行锂镁分离,解决我国青海高镁锂比盐湖卤水中提锂的工艺难题.配制镁锂浓度比为44的模拟卤水(含110 g/L Mg2+、2.5 g/LLi+),以氢氧化钠为主沉淀剂,以吐温-80、聚丙烯酰胺及晶种为辅助沉淀剂,来改进氢氧化镁沉淀的颗粒大小及其形态,使生成的氢氧化镁易于过滤,锂离子吸附损失减小.取10 mL模拟卤水,以50 mL(2 mol/L)氢氧化钠、0.5 mL吐温-80、0.03 g聚丙烯酰胺、0.03 g晶种构成复合沉淀剂,反应温度为60℃,溶液pH 12～13,则沉淀除镁率达99.9％,锂离子吸附损失率低于2％,过滤速度大大提高.采用SEM、XRD、粒度分布、热分析对氢氧化镁晶体的形貌和结构进行了分析,结果表明辅助沉淀剂的加入能有效地改变氢氧化镁沉淀的颗粒大小及其形态.     

溶析结晶法提纯氢氧化锂的研究

文章研究了不同温度下氢氧化锂-乙醇-水三元体系的平衡溶解度,结果表明,在同一温度下,氢氧化锂在乙醇-水混合溶剂中的溶解度随乙醇/水质量比的增加显著降低,而温度的改变对氢氧化锂的溶解度影响很小。绘制了相应的LiOH-C2H5OH-H2O三元体系相图,利用相图研究了以乙醇为溶析剂、采用溶析结晶法分离提纯氢氧化锂的工艺,析出的产品经检测可达到电池级单水氢氧化锂的标准。     

Effective Diffusivities and Convective Coefficients for CaO‐CaSO4 and CaO‐CaCl2 Pellets

Diffusion and convective flow in the pores of pellets formed by compressing mixtures of calcined limestone and CaSO₄/CaCl₂ powders have been studied experimentally by using the single pellet moment technique. The experiments were conducted in a diffusion cell by flowing nitrogen gas (carrier) through both faces of the pellet. Limestone powder was calcined in an atmosphere of N₂ at 800 °C and mixed with CaSO₄/CaCl₂ for diffusion experiments. Effective diffusivity of helium has been estimated by exposing the upper face of the pellet to a pulse of and matching the response peak on the lower face of the pellet with theoretical expressions. The values of the effective diffusivities increased with temperature, but decreased with increasing CaSO₄/CaCl₂ content in the pellet. The convective flow contribution to the diffusion flux was found to increase with increasing pressure drop across the pellet.     

用MgF2作为镁离子扩散源实现铌酸锂单晶光纤包层

首次利用ＭｇＦ２作为镁离子内摭用源以实现铌酸锂单昌光纤的芯－包层波导结构，此包晶纤比末包晶纤的损耗降低约１０倍。对镁离子内扩散机理进行了分析。对扩散表面进行了Ｘ射线衍射分析和扫描电镜观察，发现当镁离子内扩散超过一定程度时，在镁的扩散层出现ＬｉＮｂ３Ｏ８物相结构，该相结构的出现与铌酸锂体仙锂离子的外扩散及扩散表面的氟离子存在有关，     

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.     

Purification process for an inorganic rechargeable lithium battery and new safety concepts

We have investigated an inorganic lithium battery system in which LiCoO₂ is used as the positive electrode and lithium, intercalated into graphite, serves as negative electrode. The conducting salt is lithium tetrachloroaluminate (LiAlCl₄). The electrolyte is based on SO₂. It has been shown that a layer of lithium hydroxide is present on the surface of the lithium cobalt oxide. This has a negative impact on the stability of the electrode. To improve stability, we have developed a purification process for removing the lithium hydroxide from the surface of the positive electrode. After purification the cells show no significant change in either capacity or internal resistance when cycled. Up to 70% of the theoretical capacity of electrodes which have been purified in this way can be used without any negative effects being observed. To prevent the deposition of metallic lithium leading to a hazardous situation, a new safety concept was developed whereby local short circuits are allowable. Safe functioning of the new concept has been demonstrated with tests on complete cells.     

THERMAL AND MASS DIFFUSIVITY FROM DYNAMIC SINGLE PELLET EXPERIMENTS

The single-pellet moment techniques previously developed for the evaluation of effective diffusion coefficient and effective thermal conductivity of a porous solid are combined and a procedure is proposed for the simultaneous evaluation of these constants from a single set of pulse response experiments. It is shown that both temperature and concentration response curves can be measured from a single pulse of oxygen injected into hydrogen carrier gas flowing past a catalyst pellet. It is shown that the zeroth moment of the concentration response peaks are sufficient for finding the effective diffusivity while both zeroth and first order moments are required for thermal conductivity. Experiments conducted at 110^°C with a boehmite pellet of porosity 0.57 gave a thermal conductivity value of 3.6 x 10^-4 cal/cm ^°K s and an effective diffusivity (for H₂O) of 0.054 cm2/s. The adsorption equilibrium constant of H₂O was determined as ρ_pK = 24.5 from the first moment data. If sorption resistance to diffusion through the pellet is included in the diffusion coefficient, an apparent value of 0.0011 cm2/s would be obtained for D_eff. This result indicates the importance of separation of diffusion and sorption parameters in the analysis.     

A numerical study of pellet model consistency with respect to molar and mass average velocities,pressure gradients and porosity models for methanol synthesis process: Effects of flux models on reactor performance

The objective of this work is to compare mass- and mole based diffusion flux models, convection, fluid velocity and pore structure models for methanol synthesis process. Steady-state models have been derived and solved using least-squares spectral method (LSM) to describe the evolution of species composition, pressure, velocity, total concentration and diffusion fluxes in porous pellets for methanol synthesis. Mass diffusion fluxes are described according to the rigorous Maxwell Stefan model, dusty gas model and the more simple Wilke model. These fluxes are defined with respect to molar- and mass averaged velocities. The different effects of choosing the random- and parallel pore models have been investigated. The effects of Knudsen diffusion have been investigated. The result varies significantly in the dusty gas model. The effectiveness factors have been calculated for the methanol synthesis process for both mass- and mole based pellet models. The values of effectiveness factors for both mass- and mole based pellet models do not vary so much. The effect of Wilke-, Maxwell–Stefan- and dusty gas mass diffusion fluxes on the reactor performance have been studied. Steady-state heterogeneous fixed bed reactor model is derived where the intra-particle mass diffusion fluxes in the voids of the pellet are described by Wilke-, Maxwell–Stefan- and dusty gas models. Furthermore, the total computational efficiency of the heterogeneous fixed bed reactor model is calculated with several closures for the intra-particle mass diffusion fluxes. The model evaluations revealed that:

-

The mass- and mole based pellet models are not completely consistent. However, the small deviation (less than 2%) between mass- and mole based pellet models is due to the model equations are not fully consistent. If one pellet model is to be chosen for the methanol synthesis process, the optimal diffusion flux model is the Maxwell–Stefan model.     

The design of catalyst pellets

Because of the limitations of the effectiveness factor concept to few simple types of complex reactions and independent diffusion coefficients, it is necessary to solve the balance equations individually for each component in the general case. Therefore the characteristic equations for a catalyst pellet design were generalized for any type of complex kinetic model with regard of the multicomponent diffusion relations. In order to supply the necessary parameter values, conventional models for the physical properties of the catalyst are used. The whole modelling served as a basis of a computer program for several purposes: The control of transport limitations, the evaluation of the pseudohomoheneous production rates and the optimization of the physical properties of the catalysts. The importance of an appropriate catalyst pellet design and the application of the computer program is demonstrated by several examples.     

苛化提纯盐湖锂矿回收含氟碳酸锂的研究

主要论述了一种提纯盐湖锂矿和回收含氟碳酸锂的方法。工艺流程：盐湖锂矿（含氟碳酸锂）通过一次水洗涤除去其中所含的大部分可溶性杂质后,按一定配比将其投入到石灰乳料浆中加热到90~95 ℃反应4 h,过滤后得到氢氧化锂溶液,将氢氧化锂溶液在100~120 ℃下进行加压浓缩4 h精制得到钙镁离子和硅含量较低的氢氧化锂溶液;向精制后的氢氧化锂溶液中通入食品级二氧化碳沉锂得到工业级碳酸锂,或继续浓缩制备氢氧化锂;用以上工艺生产得到的工业级碳酸锂通过二次碳化、阳离子交换树脂除去钙镁离子、重结晶可得到硅含量为10×10^-6以内的高纯碳酸锂,或浓缩得到钙含量为5×10^-6、镁含量为2×10^-6以内的单水氢氧化锂。     

Optimization of catalyst pellet structures and operation conditions for CO methanation

A fundamental understanding of the effects of catalyst pellet structures and operation conditions on catalytic performance is crucial for the reactions limited by diffusion mass transfer. In this work, a numerical investigation has been carried out to understand the effect of catalyst pellet shapes (sphere, cylinder, trilobe and tetralobe) on the reaction-diffusion behaviors of CO methanation. The results reveal that the poly-lobe pellets with larger external specific surface area have shorter diffusion path, and thus result in higher effectiveness factors and CO conversion rates in comparison with the spherical and cylindrical pellets. The effects of operating conditions and pore structures on the trilobular catalyst pellet with high performance are further probed. Though lower temperature can contribute to larger effectiveness factors of pellets, it also brings about lower reaction rates, and pressure has little impact on the effectiveness factors of the pellets. The increase in porosity can reduce the pellet internal diffusion limitations effectively and there exists an optimal porosity for the methanation reaction. Finally, the height of the trilobular pellet is optimized under the given geometric volume, and the results demonstrate that the higher the trilobular catalyst, the better the reaction performance within the allowable mechanical strength range.     

High-order approximations for noncyclic and cyclic adsorption in a biporous adsorbent

An accurate and realistic model for transient diffusion and adsorption in a biporous pellet is typically represented by two coupled second-order partial differential equations. The model, however, has been rarely used in practice because of its mathematical complexity and bulky numerical computation, and approximations of the model have been used instead. But the accuracy of the available approximations has been limited and not enough for detailed analysis and simulation of the mass transfer process. Therefore, in this study, we develop for the first time high-order approximations, of up to third order, for noncyclic and cyclic adsorption in a biporous pellet, respectively. The approximations are in the form of a state equation which consists of first-order differential equations ; the number of the equations is the same as the approximation order. The approximations are easy to use and their accuracy dramatically increases with increasing approximation order, so that the second-or the third-order approximations can effectively substitute the complex biporous diffusion model.     

The effect of film resistances on the fouling of catalyst pellets—II transient analysis

A transient analysis of the fouling of porous catalyst particles has been made using two models, in one of which an isothermal pellet is assumed while in the other intraparticle gradients of both temperature and concentration are considered. To account for the relative rate of fouling compared with the main reaction and hence to determine whether transient or pseudo-steady analysis is appropriate a new parameter termed the characteristic parameter for fouling is employed. The effect of this parameter and Lewis number are considered and a comparison is made with pseudo-steady state analyses. Finally a comparison is made of catalyst multiple steady states under fouling conditions using both types of analysis.     

气体、溶液中的非电解质和电解质在多孔颗粒内扩散的对比研究

实验测量了气体、溶液中的非电解质以及电解质三类性质相差较大的体系在相同的多孔颗粒内(孔隙率0.234～0.632)的有效扩散系数,并计算出曲折因子.探讨了由D_(?)=D_oε/τ定义的曲折因子对孔隙结构和扩散组分特性的依赖关系.