Modeling of a spiral-wound module and estimation of model parameters using numerical techniques

A model for a spiral-wound reverse osmosis system using the three-parameter membrane transport model by Spiegler-Kedem is presented. The pressure drops in the permeate channel, feed channel and also the variation of the mass transfer coefficient along the feed channel were taken into account. An analytical solution was not possible due to the large number of nonlinear model equations; therefore, a computer solution utilizing finite differences was employed. The data generated by the simulation of the proposed model clearly indicate that neglecting the variation in the mass transfer coefficient and pressure drop along the flow channels can lead to errors in permeate concentration, though the effect on permeate flow rate may not be significant. The significance of the reflection coefficient in the membrane transport model was also investigated. A method for estimation of the model parameters is also presented; previously reported experimental data were analyzed. Using this parameter-estimation program, a correlation for the mass transfer coefficient in the feed channel is proposed and compared with the correlation available in the literature.     

A multiscale mass transfer model for gas-solid riser flows: Part II—Sub-grid simulation of ozone decomposition

This article is to test the EMMS-based multiscale mass transfer model through computational fluid dynamics (CFD) simulation of ozone decomposition in a circulating fluidized bed (CFB) reactor. Three modeling approaches, namely types A, B and C, are classified according to their drag coefficient closure and mass transfer equations. Simulation results show that the routine approach (type C) with assumption of homogeneous flow and concentration overestimates the ozone conversion rate, introduction of structure-dependent drag force will improve the model prediction (type B), while the best fit to experimental data is obtained by the multiscale mass transfer approach (type A), which takes into account the sub-grid heterogeneity of both flow and concentration. In general, multiscale behavior of mass transfer is more distinct especially for the dense riser flow. The fair agreement between our new model with literature data suggests a fresh paradigm for the CFB related reaction simulation.     

Mass transfer in the core-annular and fast fluidization flow regimes of a CFB

In gas-solid reactors, particularly circulating fluidized beds (CFB) it is becoming increasingly more important to be able to predict the conversion and yield of reactant species given the ever rising cost of the reactants and the ever decreasing acceptable level of effluent contaminants. As such, the development and use of predictive models for the reactors is necessary for most processes today. These models all take into account, in some manner, the interphase mass transfer. The model developer, unless equipped with specific experimentally based empirical correlations for the reactor system under consideration, is required to go to the open literature to obtain correlations for the mass transfer coefficient between the solid and gas phases. This is a difficult task at present, since these literature values differ by up to 7 orders of magnitude. The wide variation in the prediction of mass transfer coefficients in the existing literature is credited to flow regime differences that can be identified in the cited literature upon careful inspection.A new theory is developed herein that takes into account the local hydrodynamics. The resulting model is compared with data generated in the NETL cold flow test facility and with values from the literature. The new theory and the experimental data agree quite well, providing a fundamentally based mass transfer model for predictive reactor simulation codes.     

Numerical evaluation of mass transfer coefficient in stirred tank reactors with non-Newtonian fluid

In fermentation processes, a constant supply of oxygen is fundamental for cell growth. The supply rate is controlled by the volumetric mass transfer coefficient. The literature reports few numerical studies evaluating the volumetric mass transfer coefficient for aerated systems with non-Newtonian fluids in stirred tanks. The aim of this work was to undertake a numerical study of the main hydrodynamic and mass transfer parameters, including average gas hold-up, and power number. Xanthan gum solutions were used to simulated. The simulations were performed with different impeller rotational speeds (600 to 1000 revolution per minute) and specific gas flow rates (0.4 to 1.2 volume of gas per volume of liquid per minute), adopting an Euler-Euler approach and assuming uniform spherical bubbles. The turbulence was simulated with k?ε turbulence model and sst shear stress transport turbulent model. The numerical results were compared with experimental values available in the literature. The results showed good agreement between the numerical and experimental values of gas hold-up, power number, and volumetric mass transfer coefficient. The sst shear stress transport turbulence model provided better results, compared to the standard k?ε model, for simulation of volumetric mass transfer coefficient in a non-Newtonian fluid under the conditions used. Simulations for uniform bubbles with 3 millimeters diameter gave mass transfer coefficient values that were close to the experimental data.     

建材VOC散发过程模拟与传质参数测定新方法

建材中挥发性有机化合物(VOC)的散发是一个复杂传质过程。为准确把握传质特性,首先建立了一套描述干建材散发行为的显性完全解析模型,适用于模拟对人体最不利的无换气情况;代入有关文献中的传质参数预测了环境舱浓度,与文献中对应的实验数据及数值算法预测值吻合良好。然后基于对模型的分析提出一套简便快捷的实验方法,能够利用不同VOC背景值下干建材在密闭舱中散发的平衡浓度或逐时浓度,求取预测散发过程的4个重要的传质参数:可散发浓度C₀、扩散系数D、分配系数K和对流传质系数h_m;实验部分测算了两类密度板中甲醛散发的C₀、D、K、h_m,代入数值算法预测了密闭舱和直流舱的环境舱浓度,与实验数据吻合良好。该套模型和测定方法能够应用于建材散发的模拟研究。     

半弧面斜叶桨的流体力学性能与氧传质特性

氧传质系数是气液搅拌反应器设计的关键参数,研究新型搅拌桨的氧传质性能对气液两相搅拌反应器的强化有着重要的意义。本文实验研究了气体分布器、搅拌转速、气量对氧传质系数、搅拌功耗及气含率的影响,结果表明,氧传质系数随搅拌转速和气量的增加而增加;并建立了氧传质系数与搅拌功耗和表观气速的经验公式,为进一步放大应用提供了基础。采用欧拉-欧拉多相流模型及群体平衡模型对半弧面新型斜叶桨进行了计算流体力学（CFD）数值模拟研究,模拟研究了不同结构、搅拌转速、气量下的流体力学性能和氧传质系数,模拟计算结果与实验值的相对偏差在20%以内;这为研究这一半弧面新型斜叶桨提供了一种可靠的数值模拟方法;优化了半弧面新型斜叶桨的结构,提高了搅拌釜的氧传质效率。     

微通道内MEA/MDEA混合溶液吸收CO2的传质特性

研究了T形微通道内N-甲基二乙醇胺（MDEA）和单乙醇胺（MEA）混合水溶液吸收CO₂的传质过程。考察了弹状流型下气液两相流量、MEA和MDEA浓度对液侧传质系数k_L和体积传质系数k_La的影响。液侧传质系数和体积传质系数均会随着MEA浓度的升高而升高。与MEA相比,MDEA浓度的提高对传质影响较小。传质系数会随着液体流量的增大而增大,但气体流量的变化对其影响较小。体积传质系数随液体流量的增大而增大,但随气体流量的增大先增大,之后趋于稳定。考虑到化学反应对传质的强化作用,引入了Hatta数,提出了一个新的体积传质系数预测式,预测效果良好。     

气升式内环流反应器流场及传质特性数值模拟

采用双流体模型和气液二相流体动力学理论建立了气升式环流反应器流体流动的数学模型,在此模型的基础上利用溶质渗透理论和各向同性湍流理论建立了局部液相体积传质系数数学模型。采用计算流体软件F luent对气升式环流反应器内气液二相流动状况进行模拟,模拟结果较好地解释了气升式环流反应器内的流动行为及传质特性。模拟计算值与文献实验值的吻合说明了模型的可行性。     

Axial Mixing and Mass Transfer Performance of an Annular Pulsed Disc-and-Doughnut Column

The annular pulsed disc-and-doughnut column (APDDC) is an important type of extraction equipment in the PUREX process, which has been used in several commercial reprocessing plants. As there are few mass transfer experimental results reported in the literature, the axial mixing and mass transfer performance of an APDDC was studied for both extraction and stripping processes in the present work. Two parameters in the axial dispersion model (ADM), namely, the axial dispersion coefficient of the continuous phase and the number of mass transfer units, were regressed by correlating ADM with experimental concentration profiles. The influence of flow rate and pulsation intensity on these parameters was also investigated. Models developed for the PDDC were tested for correlation with APDDC experimental data and suitable models and conditions were determined. The height of a mass transfer unit was also calculated, which highlights the impact of axial mixing on mass transfer performance. Moreover, the influence of internal wettability on mass transfer performance was discussed.     

Hydrodynamics and Transport Processes during Centrifugal Short‐Path Distillation

To simulate the centrifugal short‐path distillation process, both two phases and interfacial transport are taken into account simultaneously for the first time. A new computational fluid dynamics model based on the volume‐of‐fluid and species transport methods is built up to analyze the detailed flow and transfer characteristics. A binary system with dibutyl phthalate‐dibutyl sebacate (DBP‐DBS) is used as an example for the investigation with both numerical and experimental methods. The residence time and the effects of operating parameters such as evaporator temperature and feed flow rate are explored comparatively. The simulation result for the liquid‐film thickness shows a satisfactory agreement with literature data. On the basis of the simulation results, we may also obtain detailed characteristics of the heat and mass transfer such as gradients in temperature and concentration and the liquid overall mass transfer coefficient.     

Mass Transfer from Wooden Surfaces and Internal Moisture Non-Equilibrium

ABSTRACT

The transfer of moisture from a wooden surface and the corresponding mass transfer coefficient in relation to the heat transfer coefficient are considered. Experimental data are reviewed that show that the mass transfer coefficient often is one order of magnitude smaller than what could be expected from the analogy between heat and mass transfer. The nature of this deviation and its influence on wood drying models is discussed. It has been suggested by Wadsö.that moisture flow inside the wood exhibits non–Fickian behaviour and that this may be due to a slow desorptiodabsorption by the wood fibre. A model is thus developed where the normal assumption of internal local equilibrium between the vapour and bound phases is replaced by a mass transfer resistance. The model is applied to one–dimensional moisture flow in a body of finite thickness and solved analytically. Numerical values calculated with the model are analysed in order to establish whether “apparent” non–Fickian behaviour of the kind observed could be explained by such a model. It is also analysed whether the model could explain an apparent decreased mass transfer coefficient at the surface.     

中空纤维透析器中纤维管填充密度对传质影响的数值模拟

通过数值模拟研究了纤维管填充密度（27.18%～67.11%）增加时传质系数的变化趋势,同时考察了流体速度对传质系数的影响.模拟结果表明：随着填充密度的增加,传质系数先增加后下降;流体速度不仅影响传质系数的大小（速度越大,传质系数越大）,也影响最佳填充密度（传质系数最大值对应的填充密度）的大小（速度增大,最佳填充密度也增大）.模拟结果可为透析器的优化设计提供参考.     

超重力旋转床中水脱氧过程的模型化研究

超重力旋转床是一种高效的强化传质和混合的新型设备。今提出了超重力旋转床中的水脱氧过程的传质模型,分别采用欧拉方法和拉格朗日方法对超重力旋转床中的气相和液滴的运动行为进行了数值模拟;在此基础上计算了液滴的传质系数,计算结果和实验结果符合较好,平均误差为7．9％。当超重力旋转床中液体存在的主体形式更接近于液滴时,模型计算结果误差减小。进一步讨论分析了液体和气体流量、转速以及填料内径的变化对于超重力旋转床体积传质系数的影响,分析表明旋转填料对液体剧烈地剪切破碎分散作用是强化传质过程的主要原因。     

氨法烟气脱硫SO2吸收传质系数研究

喷淋塔氨法脱硫技术被广泛应用于净化烟气的SO₂,传质系数是喷淋吸收塔重要的设计和运行参数,但目前文献中有关氨法脱硫传质系数的报道很少,还有待进一步研究。在喷淋塔中对氨法脱硫SO₂吸收传质过程进行了实验研究,结合对液滴和塔壁液膜运动的计算,得到不同实验条件下SO₂的吸收传质速率,并建立了氨法脱硫SO₂吸收传质系数表达式。该传质系数包含浆液pH、烟气流速u_g和液气比L/G等主要参数,能够反映不同pH、u_g和L/G条件下SO₂在单位气液接触面积上的传质速率。对比验证结果表明,该传质系数计算得到的SO₂吸收传质速率与实验值之间的相对误差小于±12%,二者能够较好地吻合。建立的传质系数表达式能够为喷淋塔氨法脱硫的优化设计和运行提供理论参考。     

Application of surface-renewal-stretch model for interface mass transfer

A new surface-renewal-stretch (SRS) model was developed to correlate experimental data for the time-average overall mass transfer coefficient, K_L,av, in liquid-liquid and gas-liquid mass transfer systems. The model is based on the equation of continuity, which includes both turbulent and convective mass transfer at the liquid-liquid and gas-liquid interfaces. The model incorporates Dankwerts surface-renewal model with the penetration theory for surface stretch proposed by Angelo et al. [Angelo, J.B., Lightfoot, E.N., Howard, D.W., 1996. Generalization of the penetration theory for surface stretch: application to forming and oscillation drops. A.I.Ch.E. Journal 12 (4) 751-760]. We used our new SRS mass transfer model to correlate successfully the existing interface mass transfer experimental data from published literature. As a result, the experimental mass transfer coefficient data was predicted with a high degree of accuracy.     

Methanation in a fluidized bed reactor with high initial CO partial pressure: Part II— Modeling and sensitivity study

A model of a bench-scale methanation reactor was set-up by modifying the classical two-phase model approach and introducing an additional bulk flow from bubble to dense phase to consider the volume contraction of the methanation reaction. The model uses experimentally determined kinetics and hydrodynamic correlations from literature. It was satisfyingly validated by comparing the calculated gas concentration profiles with the experimental data, especially with respect to initial reaction rates and reactor exit concentrations.A sensitivity study with respect to different bubble size correlations, mass transfer rates and considering or neglecting the bulk flow (influence of volume contraction caused by the methanation reaction) was carried out. It showed that the bubble size correlation by Werther and the resulting gas concentration profiles fit the measured data better than the computed gas concentration profiles using the bubble size correlation by Rowe.Neither a variation of the mass transfer coefficient nor neglecting the bulk flow in the fluidized bed model did yield further improvement of the calculated concentration profiles.     

随机填充中空纤维膜组件中非稳态渗透传质数学模型

Based on the membrane-based absorption experiment of CO2 into water, shell-side flow distribution and mass transfer in a randomly packed hollow fiber module have been analyzed using subchannel model and unsteady penetration mass transfer theory. The cross section of module is subdivided into many small cells which contains only one hollow-fiber. The cross sectional area distribution of these cells is presented by the normal probability density distribution function. It has been obtained that there was a most serious non-ideal flow in shell side at moderate mean packing density, and the large amount of fluid flowed and transferred mass through a small number of large voids. Thus mass transfer process is dominated by the fluid through the larger void area. The mass transfer process in each cell is described by the unsteady penetration theory. The overall mass transfer coefficient equals to the probability addition of the mean mass transfer coefficient in each cell. The comparisons of the values calculated by the model established with the empirical correlations and the experimental data of this work have been done.The predicted overall mass transfer coefficients are in good agreement with experimental data.     

多组分流动沸腾传热研究

多组分混合物流动沸腾传热系数算式中的液相传质系数，是描述传质阻力的关键参数，但是，当前既无合适的计算方法也无经验数据，只能借用物理和化学吸收传质系数的经验值。本文给出一个简化计算方法。用此法对有机水溶液系统进行预测计算，所得传热系数与实测值比较，其平均绝对偏差为１０．７２％。     

微曝氧化沟气液两相传质模型构建及传质影响因素分析

基于ANSYS Fluent软件建立了微曝氧化沟气–液两相流动和溶解氧输运模型,对比不同工况下氧体积传质系数的实验测量值和模拟结果,误差在7%以内。采用验证可靠性的模型模拟研究了气泡直径、曝气量和横向流动速度对微曝氧化沟内氧传质的影响。结果表明,气泡直径由1.5 mm增至3 mm时,氧体积传质系数由15.80 h?1降低至5.83 h?1;曝气量由0.5 m3/h增大至2 m3/h时,氧体积传质系数由4.21 h?1增至14.15 h?1,减小气泡直径和增大曝气量能明显提高氧体积传质系数。微曝氧化沟内气–液相间传质及溶解氧的分布受横向流动影响,开启单台和两台推流泵时,氧体积传质系数分别比无横向推流工况增大27.7%和42.4%,横向流动能有效提高气泡羽流内的气含率,增强氧传质效果。     

Mass transfer coefficient prediction method for CFD modeling of riser reactors

In gas-solid reactors, particularly circulating fluidized beds (CFB) and riser it is becoming increasingly more important to be able to predict the conversion and yield of reactant species given the ever rising cost of the reactants and the ever decreasing acceptable level of effluent contaminants. As such, the development and use of predictive models for the reactors are necessary for most processes today. These models need to take into account the interphase mass transfer. Unless equipped with specific experimentally based empirical correlations for the reactor system under consideration, the modeler is required to go to the open literature to obtain correlations for the mass transfer coefficient between the solid and gas phases. This is a difficult task at present since these literature values differ by up to 7 orders of magnitude as found in the literature dating back to 1949. The wide variation in the prediction of mass transfer coefficients in the existing literature is credited to flow regime differences that can be identified in the cited literature upon careful inspection.Applying a new theory developed by Breault and Guenther [1] that takes into account the local hydrodynamics, a predictive methodology is presented that allows the local mass transfer coefficient to be calculated based upon local properties in a CFD simulation. A sample calculation is presented and compared with the data used in developing the theory as well as correlation for the literature.