Experimental study of particle-to-liquid mass transfer in two-phase fixed and fluidized beds

Primary experimental results for local particle-to-liquid mass transfer in two-phase fixed and fluidized beds and in free flows are reported. A hypothesis on the mechanism of the effect of turbulent pulsations is suggested.

---

, . .

---

     

液固流化床内床层动态特性的CFD模拟

在对液固流化床内流动特性进行理论分析的基础上,采用多相流CFD方法模拟了二维矩形流化床内水和玻璃球体系的流体动力学特性。实验结果表明,流体速度突变后,液固流化床从一个平衡状态向另一个平衡状态过渡。当液体速度减小时,床层表面及分隔界面的变化与理论模型预测一致;由于重力不稳定性的影响,当液体速度增大时,床层表面稳定上升但与时间不呈线性关系,且分隔界面难以分辨。进而膨胀后停留时间的长短对收缩过程的影响也可以忽略。尽管床层收缩时间明显小于实际膨胀时间,但是与预报的理想膨胀时间比较接近。模拟结果与前人的实验研究和理论分析相吻合,表明多相流CFD模拟可以很好地预测液固流化床的床层动态行为。     

Estimation of heat transfer parameters in packed beds from radial temperature profiles

A fast and accurate estimation technique for the heat transfer parameters of the two-dimensional pseudo-homogeneous reactor model is presented. It is shown how previous difficulties in the parameter estimation can be avoided by a more logical choice of these parameters.     

Estimation of heat transfer parameters in packed beds from radial temperature profiles

A fast and accurate estimation technique for the heat transfer parameters of the two-dimensional pseudo-homogeneous reactor model is presented. It is shown how previous difficulties in the parameter estimation can be avoided by a more logical choice of these parameters.     

Mass transfer,adsorption and reaction in slurry reactors

The performance of three-phase isothermal slurry reactors with continuous gas flow and batch liquid may be affected by gas-bubble-to-liquid and liquid-to-catalyst-particle mass transfer, intraparticle diffusion in the pores of the particles, and adsorption and surface reaction. In dynamic operation, e.g. by introducing a pulse of reactant in the gas feed, the rates of adsorption and surface reaction are not necessarily equal. Equations for a first-order reaction are developed for the zeroth and first moments, in both the effluent gas and the batch liquid, in terms of the separate rate constants for adsorption and surface reaction and the other pertinent rate and equilibrium constants. The results show that under some operating conditions it should be possible to evaluate, from experimentally determined moments, the separate values of the adsorption and reaction rate constants. This would eliminate the assumption that one of the steps is rate controlling.Dynamic adsorption and reaction data are apparently unavailable at present to apply in the derived equations. However, for the special case of adsorption in the absence of reaction, for which experimental data are available, the predicted moments agree with the experimental values.     

Simple capillary flow porometer for characterization of capillary columns containing packed and monolithic beds

A simple capillary flow porometer (CFP) was assembled for through-pore structure characterization of monolithic capillary liquid chromatography columns in their original chromatographic forms. Determination of differential pressures and flow rates through dry and wet short capillary segments provided necessary information to determine the mean diameters and size distributions of the through-pores. The mean through-pore diameters of three capillary columns packed with 3, 5, and 7 μm spherical silica particles were determined to be 0.5, 1.0 and 1.4 μm, with distributions ranging from 0.1 to 0.7, 0.3 to 1.1 and 0.4 to 2.6 μm, respectively. Similarly, the mean through-pore diameters and size distributions of silica monoliths fabricated via phase separation by polymerization of tetramethoxysilane (TMOS) in the presence of poly(ethylene glycol) (PEG) verified that a greater number of through-pores with small diameters were prepared in columns with higher PEG content in the prepolymer mixture. The CFP system was also used to study the effects of column inner diameter and length on through-pore properties of polymeric monolithic columns. Typical monoliths based on butyl methacrylate (BMA) and poly(ethylene glycol) diacrylate (PEGDA) in capillary columns with different inner diameters (i.e., 50–250 μm) and lengths (i.e., 1.5–3.0 cm) were characterized. The results indicate that varying the inner diameter and/or the length of the column had little effect on the through-pore properties. Therefore, the through-pores are highly interconnected and their determination by CFP is independent of capillary length.     

Measurement of the fluidized velocity in gas-solid fluidized beds based on AE signal analysis by wavelet packet transform

An energy distribution theory was presented based on regular evolvement of energy fraction of acous-tic signals with fluidization velocity. Wavelet packet analysis was used in processing the acoustic sig-nals originated from particle impact on the wall of a fluidized bed. A new criterion of judging incipient fluidization(Umf) velocity and minimum turbulent velocity(Umt) was proposed according to the energy distribution theory. Experiments were performed with five groups of high density polyethylene(PE) particles and one bimodal PE to acquire incipient fluidization velocity and minimum turbulent velocity by using the criterion. The feasibility of this method in obtaining characteristic fluidization parameters was further verified by comparing it to results from the pressure drop method and the empirical value from industry.     

Rational description of trickle flow through packed beds part II: Radial spreading of the liquid

We investigated the liquid spreading in a packed bed under a trickle-flow regime following two approaches. First, we derived analytical relations aflow to be computed, assuming the liquid spreading to be a homogeneous markovian process, the transition probabilities of which are calculated supposing a l algorithm based on a constrained entropy maximization. We show that the usual radial dispersion coefficient is mainly related to the geometrical aspect mean density flow distribution. This coefficient cannot represent the evolution of local quantities such as liquid flow-rate distributions.     

Low flow rate modifier addition in packed capillary column supercritical fluid chromatography

A new method to accurately deliver small amounts (0.5 to 20 mol%) of modifier into CO₂ was used to study the effects of three different modifiers (methanol, water, and formic acid) in packed capillary column SFC. The method allows the use of different modifiers, with minimal instrument modification. The effects of the different modifiers at different concentrations on retention and peak shape are shown by analyzing a polarity test mixture and a sample of free fatty acids.     

The development of a new LDF mass transfer correlation for adsorption in fixed beds

In this study, a general LDF model has been introduced to predict mass transfer rate through adsorbents with the macropore diffusion as the controlling step. Using this relation eliminates the need for solving the time-consuming diffusion equation to find mass transfer rate through the porous adsorbent. The proposed relation was successfully applied in the general mathematical model for an adsorption fixed bed. This correlation was adjusted to be capable of predicting the mass transfer rate in a wide range of gas adsorption systems reported in the literature. This correlation was used in 21 different adsorbent and adsorbate systems. The results demonstrated an excellent agreement between the correlation results and those obtained using Fickian diffusion equation. By applying the developed LDF model instead of diffusion model, a great deal of CPU time can be saved. The latter characteristic will be very important when this model is employed in commercial software such as Aspen Adsorption or Prosim Dynamic Adsorption Column.     

T形微通道中气泡分散流的传质性能

在T形微通道中,以错流剪切的分散方式实现了微米级分散气泡的制备,并以NaOH水溶液吸收CO2为对象,考察了气．液微分散体系的分散规律和传质性能．通过考察两相流速对气泡分散尺寸的影响,建立了预测气泡形成尺寸的数学模型．根据气泡的初始分散尺寸、流动阶段的体积变化以及传质完成后的尺寸,首次测定和区分了气泡形成阶段和运动阶段的传质量,建立了原位测定气泡分散流传质系数札的方法,并考察了两相流量对札的影响．结果表明,由于微通道中气泡的形成时间很短,形成阶段的传质量在总传质量中所占的比例很低．气泡分散流的传质系数主要受液相流量的影响,气相流量的影响基本可以忽略．基于实验结果,建立了计算传质系数鼠的无因次准数关联,计算结果与实验结果符合良好．     

Thermal particle base methods in fluid flow and heat transfer

Journal of Thermal Analysis and Calorimetry -     

Determination of adsorption isotherms of proteins by H-root method: comparison between open micro-channels and conventional packed columns

This communication compares the accuracy of a micro open parallel plate system (microOPPS) with a conventional packed column for predicting isotherm data by using the H-root method (HRM). HRM is restricted to compounds obeying the Langmuir isotherm model. The performance of the two chromatographic systems was simulated by using comprehensive mathematical models. Operating conditions were varied and their effects on the accuracy of predictions was evaluated. Better accuracy in the isotherm predictions was obtained with the packed column due to its higher efficiency. However, good predictions can be obtained with the microOPPS with the advantage of significantly lower sample consumption.     

Enantioselective separations by packed column subcritical and supercritical fluid chromatography

Enantioselective separations have been one of the most successful applications of supercritical fluid chromatography (SFC). Although analytical scale separations have dominated the literature, the use of SFC for preparative chiral separations is growing. Both analytical and preparative scale SFC separations seek to take advantage of the high efficiency, high throughput, and rapid method development associated with the technique. This review will cover recent developments in the application of SFC to enantioseparations.Contribution of the National Institute of Standards and Technology. Not subject to copyright.     

Determination of the interparticle void volume in packed beds via intraparticle Donnan exclusion

Interparticle void volumes and porosities of packed capillaries have been determined using intraparticle Donnan exclusion of a small, unretained, co-ionic tracer (nitrate ions). The operational domain of this approach has been characterized for bare silica, reversed-phase, and strong cation-exchange materials (with different particle sizes and intraparticle pore sizes) in dependence of the mobile phase ionic strength. Interparticle porosities agree well with those analyzed by inverse size-exclusion chromatography (ISEC). Limitations to the use of Donnan exclusion (electrostatic exclusion) and ISEC (mechanical exclusion) arise as either type of exclusion becomes noticeable also in the cusp regions between the particles, or as the intraparticle pores are so large that complete electrostatic and size-exclusion are difficult to realize. Our data confirm that intraparticle Donnan exclusion presents a most simple, fast, and reliable approach for the analysis of packing densities.     

射流流化床煤气化炉中气固流动和传热、传质的CFD模拟研究

通过双流体模型对射流流化床煤气化炉进行了CFD（Computational Fluid Dynamics,计算流体力学）模拟。模拟着重分析了流化床气化炉气固流动的特性和传质、传热过程。结果表明,流化床中气固两相的传热、传质过程与气体和颗粒的运动特性密切相关。