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.     

Identification of Flow Regimes in a Concurrent Gas Liquid Upflow through Packed Beds

The flow regimes bubble flow, pulse flow, and spray flow were identified by visual observation in a packed column. Three gas‐liquid systems (air/water, air/56 % glycerol, and air/monoethanolamine) and four column packings (Raschig rings, Intalox saddles, and two sizes of spheres) were investigated to cover wide ranges of physical properties of gas liquid systems and characteristics of column packings affecting the flow regime transition. Criteria for the flow regime transition were developed in terms of system and operating variables.     

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.     

Influence of Water and Supercooling on Permeability and Compressibility of Acrylic Acid Crystal Beds

The performance of a solid‐liquid separation process is often limited by size distribution and morphology of the crystals. To predict the filtration behavior, a robust and applicable model based only on crystallization process parameters is necessary. Therefore, a model has been successfully developed for the industrial system of aqueous acrylic acid melt to predict compressibility and permeability of a crystal bed. For this purpose, common equations are transferred successfully to nonspherical particles. Chord length distributions (CLDs) obtained from an inline focused beam reflectance measurement (FBRM) probe and crystal aspect ratios have been used in combination to determine the major input parameters for the model. Integration of a complex mathematical restoration of crystal size distribution (CSD) from CLD data by the use of existing models is avoided. Considering acceptable fault tolerances of the model, a simple approach used as a robust industrial application is employed. The adjustment of all required constants used in the permeability model was based on simple filtration tests.     

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.     

填充床中空隙率分布的研究综述

综述了充床中空隙率径向分布的几种主要测量技术和数学模型,概括了空隙率分布的一般规律,为试验研究和工程设计工作提供重要依据。     

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.     

Packed bed combustion of char particles: experiments and an ash model

This paper presents the results of a series of experiments on the packed bed combustion of coke particles, including temperature and gas concentration profiles in the bed and measurements of particle size, void fraction and sphericity on both fuel and ash. An earlier numerical model is extended to include the effects of ash on the bed void fraction and other properties as well as on heat conduction within the bed and heat and mass transfer between particulate and gas. The model is shown to give good agreement with experimental data. It is demonstrated that the physical properties of ash—particle size, void, and sphericity—can have a considerable influence on other bed processes.     

Filtration of Colloidal Suspensions – MRI Investigation and Numerical Simulation

The principle mechanisms of solid‐liquid separation processes are sedimentation and filtration, both including the formation and compression of a liquid‐saturated bulk. The compressive properties of the bulk determine the operating parameters of solid‐liquid separation devices and the achievable separation results. Information about the solids volume fraction of the bulk is essential for a better understanding of the physical mechanisms and precise modeling. A numerical model for the calculation of the local solids volume fraction during formation and compression of filter cakes and sediments was developed. The calculated results are compared with experimental NMR data.     

The Effect of Particle Size Distribution on Pressure Drop through Packed Beds of Cooked Wood Chips

The pressure drop of process liquors through columns of wood chips determines the operability, efficiency and control of both batch and continuous pulp digesters and the quality of the pulp produced from them. Pressure drop was measured through columns of industrial white spruce chips (produced with a chipping head‐rig) as a function of the chip size distribution and the extent of delignification. Flow resistance depended on the porosity of the chip bed which was affected by the kappa number of the chips (which affected their flexibility) and chip size distribution, the compaction forces applied to the column, and the liquid superficial velocity. The chip beds were compressible and inelastic. Previous work from the literature using the Ergun equation to characterize pressure losses through chip beds is examined and compared with results of this work.     

Effects of Draft Tubes on Particle Velocity Profiles in Spouted Beds

The vertical particle velocity profiles in a full‐column cylindrical conical spouted bed, with or without a draft tube, are measured using a fibre optic probe system. The profiles have different characteristics for a draft tube spouted bed (DTSB) than for a conventional spouted bed (CSB). The spout of a CSB consists of a central flow where particle velocities fit exponential distributions, and a boundary layer where particle velocities are nearly uniform. The spout of a DTSB has no boundary layer and its radial particle velocity profiles are approximately linear. The particle velocities in the spout of a DTSB increase when superficial gas velocity increases, draft tube diameter decreases, or when entrainment height decreases. A kinematic model has been used to simulate the granular flow in the annulus of a CSB and DTSB, and they are compared with the experiments. The particle velocities in the annulus of a DTSB are much lower than that of a CSB. Their radial profiles are also different with a CSB. The dependence of particle velocities in the annulus of a DTSB on superficial gas velocity, draft tube diameter, and entrainment height are also discussed. One concludes that the draft tube diameter and entrainment height are two key factors for the solid circulation rate of a DTSB.     

Effect of Non‐Newtonian Characteristics on Convective Liquid‐Solid Heat Transfer in Packed and Fluidised Beds of Spherical Particles

In this work, the field (continuity, momentum and thermal energy) equations togetherwith a cell model have been solved numerically to elucidate the influence of non‐Newtonian (Power law rheology) liquid characteristics on liquid‐solid heat transfer in packed and fluidised beds of spherical particles. The results presented herein relate to wide ranges of conditions of bed voidage, power‐law index and Peclet number but are limited to low Reynolds number (≤1) flow conditions. Within the range of conditions, the effect of power‐law index is found to be small and this is also consistent with the available experimental results on liquid‐solid mass transfer in these systems.     

减小甲醇合成催化剂床层温差的工艺设计

ICI低压甲醇合成工艺中,合成塔内冷热气体直接混合传热,造成催化剂床层间温差较大,对催化剂及甲醇合成都有不利影响。本文对造成问题的原因进行了分析,提出了改进设计。     

CFD-based analysis of the wall effect on the pressure drop in packed beds with moderate tube/particle diameter ratios in the laminar flow regime

In this work, the influence of the confining walls on pressure drop in packed beds is studied numerically for moderate tube/particle diameter ratios. Two different configuration types are investigated, a regular type and an irregular one. The regular configurations follow the atomic body-centered cubic and face-centered cubic structure in ideal crystals, whereas the modified ballistic deposition method is employed to generate the irregular configurations. To validate the simulation results, four experimental pressure drop correlations are used, namely by Ergun (1952) [1], by Carman (1937) [2], by Zhavoronkov et al. (1949) [3] and by Reichelt (1972) [4]. Simulation results for the regular configurations are in a good agreement with the Carman correlation. For the irregular configurations, which are closer to real packed beds, agreement with the correlations of Zhavoronkov et al. (1949) [3] and Reichelt (1972) [4] is better. This is explained by the fact that the latter two correlations take the wall effect into account. CFD simulations provide useful data on the flow field inside packed beds which can be used for the improvement and further optimization of the design and operation of packed bed reactors.     

Experimental and CFD Simulation Study for the Wetting of a Structured Packing Element with Liquids

The hydrodynamics of liquid flow in packed columns affects the column performance from the point of view of heat and mass transfer. The interfacial and the specific wetted areas are decisive in this case. The complex three‐dimensional liquid flow on a single structured and flat packing element of Rombopak 4M was investigated. It consists of four connected wavy inclined plates in an X‐shape configuration. The geometric characteristics of the packing were related to the fluid mechanics of the liquid distribution. CFD simulation results for different cell sizes and flow rates, obtained using the VOF (volume of fluid) model, are presented as being capable of describing this complex geometry. With the help of the CFD simulation and the experimental results from Rombopak 4M, correlations from the literature describing the interfacial and wetted area and liquid holdup in packed columns were adjusted to describe the hydrodynamic performance of Rombopak 4M.     

Local Structure Effects on Pressure Drop in Slender Fixed Beds of Spheres

For slender fixed beds, the void fraction and flow properties are complex topics. Different factors can influence the local bed structure. For statistical analysis, 2800 fixed beds have been generated and the impact of friction factor and reactor-to-particle diameter ratio on the distribution has been shown. With particle-resolved computational fluid dynamics, all local structure effects are taken into account for the flow simulations. Pressure drop measurements and simulations showed that these effects can lead to areas with low flow resistance, leading to overestimated pressure drop by typical correlations up to 85 %.     

Impacts of Bleaching and Packed Column Steam Refining on Cocoa Butter Properties

Natural and alkalized cocoa butters were bleached and subsequently steam refined in a continuous packed column at temperatures ranging between 160 and 220 °C. None of the processes evaluated gave rise to any detectable formation of trans fatty acids, interesterification or polymerization. For the pressures and steam injection rates used, packed-column steam refining required a minimum temperature of 170 °C to achieve acceptable taste. Bleaching was highly effective in preventing darkening at high steam-refining temperatures, as well as in removing alkaloids, such as theobromine and caffeine, before steam refining. The impacts on the crystallization properties of cocoa butter were studied using DSC and P-NMR. The more significant changes in crystallization kinetics and equilibrium values can be reliably predicted on the basis of FFA removal from the butter.