On the use of internal mass transfer coefficients in modeling of diffusion and reaction in catalytic monoliths

We utilize the recently developed concept of internal or intraphase mass transfer coefficient to simplify the problem of diffusion and reaction in more than one spatial dimension for a washcoated monolith of arbitrary shape. We determine the dependence of the dimensionless internal mass transfer coefficient (Sh_i) on washcoat and channel geometric shapes, reaction kinetics, catalyst loading and activity profile. It is also reasoned that the concept of intraphase transfer coefficient is more useful and fundamental than the classical effectiveness factor concept. The intraphase transfer coefficient can be combined with the traditional external mass transfer coefficient (Sh_e) to obtain an overall mass transfer coefficient (Sh_app) which is an experimentally measurable quantity depending on various geometric and transport properties as well as kinetics. We present examples demonstrating the use of Sh_app in obtaining accurate macro-scale low-dimensional models of catalytic reactors by solving the full 3-D convection-diffusion-reaction problem for a washcoated monolith and comparing the solution with that of the simplified model using the internal mass transfer coefficient concept.     

Two-resistance mass transfer models for the adsorption of dyestuffs from aqueous solutions using activated carbon

The adsorption of telon blue (acid blue 25) dye and deorlene yellow (basic yellow) dye on to carbon has been studied. A two-resistance mass transfer model has been developed based on film resistance and homogeneous solid phase diffusion. The model may be applied over a wider range of operating conditions than previous models since it has more extensive analytical components. The variables investigated were initial dye concentration and solid: liquid ratio, and the experimental and theoretical results were in good agreement. The adsorption of telon blue on to carbon has been described using an external mass transfer coefficient of 2.0 × 10⁻³ cm s⁻¹ and a homogeneous solid phase diffusion coefficient of 2.0 × 10⁻⁹ cm² s⁻¹. The adsorption of deorlene yellow has been described using an external mass transfer coefficient of 1.0 × 10⁻³ cm s⁻¹ and a homogeneous solid phase diffusion coefficient of 3.0 × 10⁻¹⁰ cm² s⁻¹.     

DAMO过程中甲烷传质模型的建立与验证

以甲烷为电子供体还原亚硝酸盐的反硝化型甲烷厌氧氧化（Denitrification Anaerobic Methane Oxidation,DAMO）是最近环境科学与工程领域的一个重大发现。虽然DAMO过程释放的自由能非常大,但是DAMO微生物生长却极其缓慢,细胞倍增时间长达一个月以上,DAMO富集培养物难以获得。由于甲烷是一种不易溶于水的气体,因此一般认为甲烷是DAMO微生物富集培养的限制因素,对此建立了甲烷在气-液-菌三相间的传质模型,研究了甲烷溶解度和传质阻力对DAMO微生物富集培养的影响。根据模型分析,得到了DAMO比活性与气相甲烷分压和甲烷传质阻力间的关系式,并通过DAMO活性试验验证了该关系式。     

Influence of dissolved hydrocarbons on volumetric oxygen mass transfer coefficient in a novel airlift contactor

Hydrocarbon compounds are sparsely soluble in aqueous systems but, nonetheless, their presence can influence significantly mass transfer behavior in gas-liquid systems. water-p-xylene and water-p-xylene-naphthalene mixtures were employed in order to determine the influence of dissolved hydrocarbons on mass transfer of oxygen from air bubbles to water. The surface renewal-stretch model has been modified for predicting the volumetric mass transfer coefficient, (K_La)_h, in the presence of surface contaminant molecules, including hydrocarbon compounds and surfactants. Theory and experimental oxygen transfer results were found to be in satisfactory agreement with average absolute deviation of 15%. Pendant drop and contact angle measurements by axisymmetric drop shape analysis were carried out to determine the reduction in surface tension of water due to the addition of p-xylene and naphthalene. Molecular orientation caused by instantaneous attraction of the polar moieties of the organic compounds toward the water interface has been found to be the main cause of reduction in surface tension. It was predicated that changes in gas-liquid mass transfer behavior resulted from surface contamination and that the significant parameter was the reduction in surface tension.     

气液传质理论研究进展

文内对气液传质的理论研究进行了评述，并预测了未来的发展方向     

Experimental study of mass transfer limitations in Fe- and Cu-zeolite-based NH3-SCR monolithic catalysts

An experimental study of steady-state selective catalytic reduction (SCR) of NO_x with NH₃ on both Fe-ZSM-5 and Cu-ZSM-5 monolithic catalysts was carried out to investigate the extent of mass transfer limitations in various SCR reactions. Catalysts with different washcoat loadings, washcoat thicknesses and lengths were synthesized for this purpose. SCR system reactions examined included NO oxidation, NH₃ oxidation, standard SCR, fast SCR and NO₂ SCR. Comparisons of conversions obtained on catalysts with the same washcoat volumes but different washcoat thicknesses indicated the presence of washcoat diffusion limitations. NH₃ oxidation, an important side reaction in SCR system, showed the presence of washcoat diffusion limitations starting at 350 °C on Fe-zeolite and 300 °C on Cu-zeolite catalysts. Washcoat diffusion limitations were observed for the standard SCR reaction (NH₃+NO+O₂) on both Fe-zeolite (≥350 °C) and Cu-zeolite (≥250 °C). For the fast (NH₃+NO+NO₂) and NO₂ SCR (NH₃+NO₂) reactions, diffusion limitations were observed throughout the temperature range explored (200–550 °C). The experimental findings are corroborated by theoretical analyses. Even though the experimentally observed differences in conversions clearly indicate the presence of washcoat diffusion limitations, the contribution of external mass transfer was also found to be important under certain conditions. The transition temperatures for shifts in controlling regimes from kinetic to washcoat diffusion to external mass transfer are determined using simplified kinetics. The findings indicate the necessity of inclusion of mass transfer limitations in SCR modeling, catalyst design and optimization.     

一种氨水垂直降膜吸收传质模型

在Harriott理论的基础上,提出一种新的垂直降膜吸收传质模型,将降膜过程中漩涡的作用点由一个几何点扩展到一个区域,将整个降膜流动过程划分为非湍流区和全湍流区.非湍流区内,波峰和波谷域采取了不同的Levich厚度处理方法.在全湍流区,对波动速度进行了修正.理论数据与氨水光管降膜吸收实验数据相比较,偏差在非湍流区不超过10%,在全湍流区不超过15%.扩散系数、Levich厚度及波动周期对传质系数的影响被表示为两个量纲1量之间的关系.     

叉流热源塔传热传质模型的建立及实验验证

热源塔作为新型的热质交换设备,在热源塔热泵机组运行过程中起着重要作用。其在冬季运行时从空气中吸收热量,为热泵机组提供低品位热量。热源塔与冷却塔在传热传质上存在一定异同点,指出了冷却塔与热源塔在传热传质上存在热阻、液体物性、潜热换热量比例、循环水/液体流量、飘液对系统的影响、热量传递方向和换热量大小等方面的差异。根据热源塔与冷却塔差异建立叉流热源塔传热传质数学模型,并采用实验验证模型的准确性。结果表明叉流热源塔潜热百分比低于35%,模型结果与实验结果相比换热性能误差低于10%,该模型能够较为精确地对叉流热源塔换热性能进行模拟。     

液-液微尺度混合体系的传质模型

针对微尺度液-液混合体系,考察了流量对膜分散萃取过程的影响,并根据传质过程方程,计算了各种条件下的传质系数和传质速率;采用现有的传质模型分别计算分散相和连续相的分传质系数,然后根据传质阻力的加合性得到总传质系数;应用理论传质系数计算传质效率,与实验值进行了比较.研究结果表明,在微尺度混合条件下,直接影响传质系数的因素是停留时间和液滴直径,传质系数随着停留时间的减小而增大.膜分散萃取的传质系数可以达到1.2×10^-4m•s^-1,比传统的萃取方式大10～100倍;不能像塔式萃取设备一样,用简单地忽略某一相的传质阻力或用总体平均的简化计算公式来计算微尺度混合的传质性能;考虑滴内滴外传质系数,并考虑时间的影响,利用现有公式分别计算滴内滴外传质系数,并采用阻力加合,可以较为准确地计算微混合条件下的总传质系数,计算值与实验值符合很好.     

Enhancement of oxygen mass transfer rate in the presence of nanosized particles

Absorption of gases into a liquid is essentially important for two- or three-phase reactions, because the diffusion of a sparingly soluble gas, like oxygen, across a gas-liquid interface generally limits the reaction rates. Using a third, dispersed phase, the mass transfer rate could be significantly increased. The question arises how the absorption rate can be described in the presence of very fine, nanometer size particles or droplets. Its mathematical model should take into account the specific properties of the nanoparticles, e.g. the Brownian motion of particles, its effect on the diffusion of the bulk phase molecules, the mass transfer rate into the nanoparticles, its dependency on the particle size, etc. The mass transfer rate of oxygen, in the presence of nanometer size, organic droplets, has been investigated both experimentally, using organic submicron n-hexadecane droplets, and theoretically. The effect of the Brownian motion of the nanoparticles as well as its effect on the diffusivity in the nanofluid has been discussed. Accordingly, the enhanced diffusion coefficient, due to the convective motion of the continuous liquid phase induced by the moving particles, has been predicted and its effect on the mass transfer enhancement has been calculated using both homogeneous and heterogeneous mathematical models. The predicted data were compared to the measured ones.     

液体中气泡上浮与传质过程的耦合模型

针对液体中气泡上浮与传质这一非稳态、强耦合过程,分析气泡的受力情况,考虑到非恒定Basset力的影响,得出了气泡瞬态加速度模型;利用绕球流动传质边界层模型,并引入非平衡传质理论,构建了气泡的瞬态非平衡传质模型;进而依据气泡质量变化率将两模型耦合,以此构建了完整描述这一过程的耦合模型。计算实例表明,Basset力对难溶性气泡的运动过程无明显影响,但对易溶性气泡影响显著;传质条件则对两类气泡都具有重要影响,且该模型中引入非平衡传质理论后,计算值与难溶性气泡的实验结果吻合更好。     

旋转填充床内高黏介质脱挥过程的传质模型(英文)

A mass transfer model for devolatilization process of highly viscous media in rotating packed bed(RPB) was developed based on penetration theory and mass conservation.Before establishing the model,some mass transfer experiments of thin film were conducted in a designed diffusion cell including vacuum and feeding system. In this study,acetone was used as the volatile organic compound(VOC) and syrup as the highly viscous media.The thickness of thin film was changed by using different liquid distributor.It was found that bubbling played an important role in the devolatilization.The correlation of diffusion coefficient of acetone in highly viscous dilute solution was proposed.The relative error between predicted and experimental data was within the range of ± 30% for diffusion coefficient of acetone in syrup.A comparison of experimental data of RPB with model indicated that the relative error was within ± 30% for efficiency of acetone removal.     

颗粒分布对吸附热变换器内传热传质的影响

采用直接接触法提高吸附热变换器内的传热传质速率,回收热水直接生成过热蒸汽。对蒸汽生成过程进行数值建模,耦合质量、能量和动量方程。气液固的三相计算被合理简化成两个由一个移动水-气界面连接的两相区域计算。模拟研究填充床内的沸石颗粒分布如松散-密集型和密集-松散型对蒸汽生成的影响。两种填充床生成蒸汽的总质量相同。密集-松散型生成蒸汽的时间短,但生成速率快。密集-松散型生成蒸汽与入水的时间比值为58.8%,生成的蒸汽均保持在峰值,最高温度达249℃,系统整体温升达139℃,而松散-密集型生成的蒸汽仅有1/3保持在峰值。密集-松散型出口处与水-气液面产生蒸汽的质量比值大,表明该床层的颗粒分布更有利于蒸汽的快速通过。     

电渗析过程传质模型的研究进展

电渗析是一种利用离子交换膜和电势差从溶液及其他不带电组分中分离出离子的物质分离过程,该技术具有适应性强、预处理简单、能耗低、环境污染小等优点,被广泛应用于化工、生物等领域的分离纯化过程。本文主要介绍了用于电渗析分离过程的6种传质模型,总结了各模型的优势及存在的问题,指出限制电渗析技术进一步发展的主要原因是对包含物质传递、浓差极化、流体流动行为、电解质溶液-膜平衡等复杂现象的电渗析过程进行理论和实验研究难度大,而传质模型化为电渗析分离过程的物质传递研究提供了一条有效途径,有助于深入研究电渗析过程中物质的传递机理,准确预测分离性能并导向性优化电渗析结构设计和操作工艺。并且提出未来电渗析传质模型的研究方向是结合经验方程或传质系数进一步优化传质模型,并采用仿真工具模拟传质过程,提高模型的准确性和普适性。     

流化床液固两相传质过程的模拟研究进展

液固两相流化床具有液固相接触效率高、传质和传热性能好、颗粒分布均匀等优点,已被广泛应用于众多工业过程中。然而,流化床中与传质过程耦合的颗粒流化的复杂非线性特征及其湍动特性,使得对传质过程特性的研究十分困难。且仅依靠实验观测和理论预测难以揭示多相流相互作用规律,无法获得全面和详细的速度场和浓度场分布情况。近年来,数值模拟的快速发展为深入探索流化床中液固两相流动行为及其与传质过程耦合问题提供了重要的途径。本文对流化床液固两相流动与传质过程模拟方法进行了综述,并对其未来研究趋势进行了展望。借助于计算传质学理论可以更精确地预测局部浓度的分布情况,进而可以深入分析液固两相流化床中的传质过程规律与传质特性,为液固两相流化床的设计和优化提供理论基础。     

基于格子Boltzmann方法模拟纳米流体强化传质过程

使用Boltzmann方法对纳米流体中的传质过程进行了模拟,给出了修改后的Boltzmann方程和计算纳米流体中扩散系数的方程,通过两种方法,基于有限体积颗粒的LBE方法和基于点源颗粒的LBE方法进行了模拟,并与宣益民的实验进行了对比。最后计算了CO2在纳米流体中的扩散系数,计算结果表明,纳米流体因为其纳米颗粒的微扰动对传质有着很大的强化效果,为CO2吸收提供了一种新思路,并且证明了纳米流体的强化传质主要是靠对流传质。     

Effect of surfactants on liquid-side mass transfer coefficients

In the present paper, the effect of liquid properties (surfactants) on bubble generation phenomenon, interfacial area and liquid-side mass transfer coefficient was investigated. The measurements of surface tension (static and dynamic methods), of critical micelle concentration (CMC) and of characteristic adsorption parameters such as the surface coverage ratio at equilibrium (s_e) were performed to understand the effects of surfactants on the mass transfer efficiency. Tap water and aqueous solutions with surfactants (cationic and anionic) were used as liquid phases and an elastic membrane with a single orifice as gas sparger. The bubbles were generated into a small-scale bubble column. The local liquid-side mass transfer coefficient (k_L) was obtained from the volumetric mass transfer coefficient (k_La) and the interfacial area (a) was deduced from the bubble diameter (D_B), the bubble frequency (f_B) and the terminal bubble rising velocity (U_B). Only the dynamic bubble regime was considered in this work (Re_OR=150-1000 and We=0.002-4).This study has clearly shown that the presence of surfactants affects the bubble generation phenomenon and thus the interfacial area (a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient (k_La) and the liquid-side mass transfer coefficient (k_L). Whatever the operating conditions, the new k_La determination method has provided good accuracy without assuming that the liquid phase is perfectly mixed as in the classical method. The surface coverage ratio (s_e) proves to be crucial for predicting the changes of k_L in aqueous solutions with surfactants.     

A mathematical model of heat and mass transfer in Yankee drying of tissue

Final dewatering in the production of dry creped tissue is performed by Yankee drying, which includes hot pressing followed by simultaneous contact and impingement drying. The present study models Yankee drying and compares simulation results to the data obtained from trials on a pilot tissue machine. It advances models published previously by the representations developed for the transport of heat in the pressing stage and for the heat transfer involved in the dehydration of the cylinder coating spray. The model predicts an average specific drying rate within 4% in the range of the experimental data used.     

A three-zone mass transfer model for a rotating packed bed

A rotating packed bed (RPB) is recognized for its merits in chemical process intensification. In most studies of RPB mass transfer modeling, however, the effects of the end and cavity zones have not been taken into consideration, since it was very difficult to distinguish the end and bulk zones by hydrodynamics and mass transfer process. In this work, the radial thickness of the end zone was obtained by developing a probability method and imaging experiments to separate the end and bulk zones. A three-zone model, including end, bulk, and cavity zones, of the overall gas-side volumetric mass transfer coefficient (K_Ga)t was first established. Experiments of dissolved MEA chemisorption of CO₂ were carried out to validate the proposed three-zone mass transfer model. The results of the MEA-CO₂ absorption experiments showed that the experimentally obtained values of CO₂ absorption efficiency were in agreement within ±20% with the model predictions.     

A combined model of mass transfer coefficients for contaminated drop liquid-liquid systems

Mass transfer rates to and from drops in liquid-liquid extraction equipment are often likely to be reduced by the presence of surface active contaminants. For industrial column design circumstances it is ideally required to account for the extent of contamination in a quantitative manner yet existing design procedures do not allow this to be done for the typical intermediate Reynolds number region of 10 to 100. A method is proposed for correcting continuous phase and drop mass transfer coefficients for the deleterious effects of contamination using only one contamination parameter which needs to be determined experimentally.