Prediction of Two‐Phase Frictional Pressure Drop for the Concurrent Gas and Newtonian Liquids Upflow through Packed Beds

Correlations were developed to predict frictional pressure drop for concurrent gas‐liquid upflow through packed beds covering all the three identified flow regimes, i.e. bubble flow, pulse flow and spray flow. The observation that the gas and liquid flow rates have different influences on the two‐phase pressure drop in different flow regimes, was taken into consideration in the development of these correlations. More than 600 experimental pressure drop data from the present study and literature covering a wide range in gas‐liquid systems, flow rates and column packing were used.     

Magnetic Resonance Visualisation of Flow and Pore Structure in Packed Beds with Low Aspect Ratio

Different NMR techniques were combined to obtain the structure and velocity information for a systematic investigation of fixed beds with low aspect ratio (tube diameter to particle diamter, d_t/d_p) in the range 1.4 to 32. The structure of the void space was determined for a variety of packed beds of glass beads or regular and irregular porous pellets by magnetic resonance imaging (MRI). Based on the images the radial distribution of the voids within the bed was obtained. Ordering effects were found even for non‐spherical and polydisperse particles, and a maximum of the fluid density near the tube wall was confirmed for all pellet geometries and sizes. By combining MRI with velocity encoding, velocity profiles and distributions of flow velocity components of a single fluid phase through packed beds have been acquired. The radial velocity distribution follows an oscillatory pattern which largely reflects the ordering of the particles, which can be accessed from the density distribution of the interparticle fluid. Maximum velocities of up to four times the average value were found to occur near the tube wall. This wall effect was observed for all but the smallest particles, where the aspect ratio was d_t/d_p = 32. Moreover, a visualisation of flow pattern in the presence of packed particles was achieved by using a tagging technique, and the stationary flow field could be identified for an experimental time of several hours.     

Simulation of Turbulent Flow in a Packed Bed

Numerous models for simulating the flow and transport in packed beds have been proposed in the literature with few reported applications. In this paper, several turbulence models for porous media are applied to the gas flow through a randomly packed bed and are examined by means of a parametric study against some published experimental data. These models predict widely different turbulent eddy viscosity. The analysis also indicates that deficiencies exist in the formulation of some model equations and selection of a suitable turbulence model is important. With this realization, residence time distribution and velocity distribution are then simulated by considering a radial profile of porosity and turbulence induced dispersion, and the results are in good agreement with the available experimental data.     

Holdup and Pressure Drop of Packed Beds Containing a Modular Catalytic Structured Packing

A comparative experimental study has been carried out to establish the hydraulic behavior of a new modular catalytic packing (Sulzer's Katapak^®‐SP 12) and the hybrid packed bed consisting of a catalytic packing section placed in between two sections comprising elements of a high capacity structured packing (MellapakPlus 752.Y). Air‐water experiments were carried out at ambient conditions using a Perspex column with an internal diameter of 0.45 m. As expected, the liquid holdup and the pressure drop of the combined bed were between those measured for beds consisting purely of the catalytic and structured packings. However, unlike the two reference beds, the combined bed exhibited a clear upper gas load limit due to a pronounced liquid buildup at the transition from the structured packing to the top element of the catalytic packing section. Also it appeared that the Delft model, with appropriate packing geometry modifications is capable of reliably predicting the preloading region holdup and pressure drop of a hybrid packed bed containing Katapak‐SP.     

Experimental and Theoretical Investigation of Concentration and Temperature Profiles in a Narrow Packed Bed Adsorber

Significant radial profiles of concentration and temperature may exist in packed bed adsorbers, leading to a different breakthrough behavior at the wall than in the core of the bed. Respective data have been gained by near‐infrared tomography for zeolite and water in relatively narrow tubes, and are compared with the predictions of a two‐dimensional model. Though the agreement is satisfactory in total, some deviations from the experimental results have been observed. Model reductions can not be recommended due to complex interactions between thermal effects, channelling, and non‐linear equilibrium in the dynamic process.     

复杂性在气固流化床流型识别中的应用

研究了气固流化床从起始流化态、鼓泡态、湍动态直至快速流化的四个典型流型下,压力脉动时间序列的算法复杂性和涨落复杂性随表观气速的变化趋势。实验结果表明,这两种复杂性参数对于流化床流型变化敏感,但在同种流型下,在操作条件的较宽变化范围内保持稳定。基于这一特性,故将复杂性测度与多元统计分析中的距离判别方法相结合,建立了一种只需获得一定长度的压力脉动时间序列,无需知道具体操作条件的流型识别方法。     

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles (, ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered (; ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G_s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.     

Identification of Flow Regimes in Slot‐Rectangular Spouted Beds using Pressure Fluctuations

Pressure fluctuation data recorded in slot‐rectangular spouted beds of 1.44 mm diameter glass beads were analyzed with the aid of statistical and chaotic tools. The column width was maintained at 150 mm, while the thickness and slot width were both varied. It is shown that there are significant shifts in both statistical and chaos measures as the gas flow rate is increased. The results suggest that pressure fluctuations can be used to provide diagnostics of flow regime transitions in spouted beds when viewing is impossible.     

Distinction between Electrostatic and Electrokinetic Effects on the Permeability of Colloidal Packed Beds

Electrostatic and electroviscous effects can significantly decrease the permeability of packed beds which consist of colloidal particles. This results in poor filterability of colloidal suspensions. Electrostatic effects refer to the dependency of the structure of the packed bed on the particles' tendency to agglomerate or disagglomerate. This tendency is influenced by the particle charge, and thus the pH and the ionic strength of the suspension. Electroviscous effects relate to the increased flow resistance of the pores due to a streaming potential being established when the electrochemical double layer of the particles is sheared off. It is difficult to distinguish electrostatic and electroviscous effects because they are interrelated. The comparison of permeability measurements of both TiO₂ and Al₂O₃ with a classical permeability model demonstrates the influence of the two different effects on permeability.     

Flow of a FENE Fluid in Packed Beds or Porous Media

An analysis describing viscoelastic flow of a FENE fluid in packed beds or porous media is presented based on the capillary hybrid model of the flow. A close similarity is revealed between the functional relationship of the reduced elongational viscosity to and that relating the reduced mean elongational viscosity λ_H ? to the Deborah number which is utilized in the flow expressions. The agreement obtained between the predicted and experimentally determined evaluations of the resistance factor, including the effects of variation of polymer concentration, molecular weight and solvent quality was found to be satisfactory. Onset Reynolds numbers for enhanced flow resistance are also predicted successfully.     

Study of Binary‐Solid Liquid Fluidized Beds

A proper characterization of the hydrodynamics of binary‐solid liquid fluidized beds is an important first step in its effective utilization. Of particular importance in this connection is to be able to predict the unique hydrodynamic phenomenon of the layer‐inversion, which is associated with the change of the stratification pattern of the two solid species in the fluidized bed brought about due to a change either in the liquid velocity or the bed composition. Past few years have witnessed the development of several models for the prediction of the layer‐inversion phenomenon. It is, therefore, important to analyze their predictions in the light of the growing wealth of the literature data. Such an exercise is important to examine the underlying assumptions and propose modifications to improve their predictive capability. Predictions of well‐known layer‐inversion models are compared with the experimental data reported in the literature including our own which comprises of solid species of widely different size, and therefore, provide an important test of predictive capability of models.     

Heat transfer characterization of gas-liquid flows in a trickle-bed

Instantaneous local fluid-solid heat transfer coefficient (h_t) in a laboratory scale trickle-bed was measured using a constant-voltage anemometry technique. It was observed that convective heat transfer rate in the liquid-rich pulses was approximately 4 times that in gas-continuous bases for the air-water system. Time-averaged heat transfer rate was found to be positively influenced by both gas and liquid flow rates, with a stronger dependence on the latter. Heat removal efficiency, taking pressure drop penalty into account, suggested an optimum at intermediate liquid flow rate. Based on the measurements, a four-parameter heat transfer model featuring heat transfer coefficients in liquid-rich pulses (h_tp) and gas-continuous bases (h_tb), pulsing frequency and pulse fraction was developed to characterize transient h_t under various flow regimes. This model can be used in any trickle-bed reactor simulation that accounts for the dynamic interactions of catalytic reactions and heat transfer. It was found that while h_tp and h_tb correspond to liquid-solid and gas-solid heat transfer, respectively, and are determined mainly by the fluid properties, pulsing frequency and pulse fraction are the factors characterizing different flow regimes. Pulsing frequency, which can significantly impact reaction, may be tuned by selecting appropriate packing size, since smaller sizes generate higher frequency pulses. For example, a two-fold higher frequency was detected in packing as compared to that with packing. Flow regime evolution along the column axial location was identified visually, while the dispersed bubbling flow retreating to pulsing flow owing to gas bubble coalescence was evidenced by the heat transfer measurements.     

气固流化床压力脉动信号的Hilbert-Huang变换与流型识别

采用Hilbert-Huang变换(HHT),提取出气固流化床压力脉动信号的各阶内禀模态函数(IMFs),进一步证明了压力脉动信号是由复杂的不同波间和波内频率调制成分所组成,具有气固两相运动相互调制的非线性特征。分析各阶内禀模态函数的能量分布及其转换,发现不同频段IMF的能量与流型状态之间有着很好的对应关系,能够从整体上反映流化床的流化状态,从而提出了应用IMF中频段能量进行流化床流型识别的新方法。该方法只需一个压力脉动信号,算法简单、实用,没有需要主观决定的参数,具有较好的工业应用前景。HHT分析比现有的分析方法更能深入地揭示流化床内的非线性流体动力学特征。     

Drift flux concept in two- and three-phase reactors

The drift flux concept has been used to describe and analyse some hydrodynamic parameters such as the flow regimes and the retention phases in three-phase fluidised and fixed bed reactors. The effects of the gas, liquid and solid properties and the characteristics of the apparatus (distributor quality) on the hydrodynamics have been studied. Two hydrodynamic data banks as well as experimental results have been used to study the influence of the coalescence inhibiting behaviour of liquids and to determine flow regime transitions and phase retentions. In heterogeneous regime, the ratio of the drift flux density to the superficial gas velocity tends to a limit, approximately independent on the liquid velocity in both fixed and fluidised beds. Two correlations of the drift flux have been developed by this analysis; the first of which is valid in the three-phase fluidised beds and the other in the three-phase fixed beds.     

Hydrodynamics and Flow Development in a 15.1 m Circulating Fluidized Bed Riser

Hydrodynamics and flow development are studied in a long riser circulating fluidized bed reactor (15.1 m). Optic fiber probes were used for the measurement of local solids distribution. Pressure drops were also measured with pressure transducers along the riser. The flow development in the riser center is almost instant with solids holdup remaining constant and low, and particle velocity remaining high along the riser. The particle flow is firstly developed from center, and then towards the wall. The riser height is an important factor for the design of circulating fluidized bed reactors. Increasing the solid circulating rate significantly slows down the flow development process, while increases in the superficial gas velocity accelerate it.     

Fluid Flow Characteristics of String Reactors Packed with Spherical Particles

A hydrodynamic investigation of three geometries of string pellet reactors filled with spheres was conducted. Two geometries were circular spiral channels, while the third was a straight horizontal square channel. Stimulus‐response experiments provided data for residence time distribution analysis from which Pe numbers and liquid holdup were deduced. Flow regimes and transitions were determined from visual observations through the transparent tube wall. For the whole range of the experimental conditions applied in this work and for all reactors, the ratio of gas to liquid velocities, V_g/V_l, is a controlling parameter for Pe number, holdup, and pressure drop.     

Experimental Investigation of Pressure Drop Hysteresis in a Cocurrent Gas–Liquid Upflow Packed Bed

Extensive experimental work on hysteresis in a cocurrent gas–liquid upflow packed bed was carried out with three kinds of packings and the air–water system. However, only when packed with small glass beads (f1.4 mm) was the bed pressure drop hysteresis observed. Two more liquids with different liquid properties were employed to further examine the influence of parameters on pressure drop hysteresis. The similarity of pressure drop hysteresis in packed beds was concluded in combination of experimental evidence reported in literature.     

Mechanism of Local Fluidization in Converging Packed Beds

The velocity profile on the bed surface of two‐dimensional linear‐converging beds with 15° and 30° wall angles was measured at different superficial velocities using hot‐wire anemometry. The results of the velocity measurements indicate that local fluidization in the corners is caused by the geometry‐induced maldistribution of fluid flow, and it occurs when the velocity in the corners exceeds the minimum fluidization velocity of particles. The results of pressure measurements within the bed show the same trend as the velocity profile, providing a qualitative verification of the velocity profile measurement. It is shown that the variation of measured pressure drop over the bed with velocity does not agree with the Ergun equation at high superficial velocities due to the severe maldistribution of fluid flow.