Rheology,Sedimentation, and Filtration of TiO2 Suspensions

It is shown that with aqueous suspensions of colloidal oxide and hydroxide particles simple rheometric and sedimentation experiments allow the determination of the optimum pH for filtration and washing. This approach is more practical than zeta potential measurements, as explained by a theoretical review and illustrated by experiments with industrial TiO₂ white pigments of varying surface chemistry. Besides, the solids content of the filter cake and its dependence on pH is correctly predicted from the rheology of the suspension, thus giving an idea of the mechanism of filter cake formation. This follows from a comparison to the results of filtration experiments. There, the influences of filter vacuum as well as of pH and solids content of the pigment feed suspension on filtration capacity and washing efficiency have been studied. The experiments also illustrate an inexpensive method of how to find the optimum set of parameters for an economical drum filter operation in the plant.     

Structure Characterization of Filled Polymers by Means of MRI

The structure of foams and filled polymers can be analyzed by means of Magnetic Resonance Imaging (MRI). It is also possible to observe the deformation behavior of the structure of foams and filled polymers in situ. A displacement experiment was performed in a magnet and observed by MRI. The NMR images are analyzed by image processing. Average distances between particles are estimated by the use of the autocorrelation function and the spectrum of the autocorrelation function. The spectrum shows the spatial frequency of the distances. The displacement field was calculated by the cross‐correlation function. Thus, information concerning the particle distances and micromechanical deformation can be obtained by NMR imaging methods by combining autocorrelation and cross‐correlation. Such data are important for the evaluation of the mechanical strength of filled polymers.     

Highly Resolved Determination of Structure and Particle Deposition in Fibrous Filters by MRI

A new method for the quantitative three‐dimensional structure determination of a coarse filter medium as well as the particle deposition inside this medium is described. The filter structure was imaged at a voxel resolution of 59 μm which is close to the mean fiber diameter, therefore allowing individual fibers and their position to be determined. The data processing method for determining the filter structure as well as the quantification of the mass deposited in the filter are explained in detail. For particle deposition investigations the spatial resolution used was 235 μm. Hereby, a linear correlation between deposited mass and signal intensity was observed.     

Numerical Simulation for Particle Penetration Depth Distribution in Deep Bed Filtration

A two‐dimensional model has been developed to simulate particle penetration through porous media. The particle penetration depends on many parameters including the Reynolds number, particle drag coefficient, the ratio of the diameter of injected to filtered particles, fluid velocity, and pore size, etc. The numerical model for separation efficiency in periodic porous media was studied. Previous work has described the effects of injected particle size, Reynolds number and particle drag coefficient. In this study, the porous media flow is modeled (solution of the Navier‐Stokes equations) by using the finite element method, and the analysis is restricted to the case of two‐dimensional periodic porous media. The effects of these factors and particle depth distribution in porous media are investigated. It is noted that the results for the three Reynolds numbers 1, 16.56, and 100, are qualitatively similar, and about 40 % of particles are trapped in the top part of the filter.     

Application of Magnetic Resonance Imaging (MRI) to Determine the Influence of Fluid Dynamics on Desulfurization in Bench Scale Reactors

The influence of fluid dynamics on the hydrodesulfurization (HDS) reactions of a diesel oil in bench‐scale reactors was evaluated. The porosities and liquid saturations of catalyst beds were quantified by using the MRI technique. The gas‐liquid systems used in the experiments were nitrogen diesel and hydrogen diesel. An apparatus was especially constructed, allowing in situ measurements of gas and liquid distributions in packed beds at elevated pressure and temperature up to 20 bar and 200 °C, respectively. The reactor itself had a length of 500 mm and an internal diameter of 19 mm. The packed beds used in this MRI study consisted of: (1) 2 mm diameter nonporous spherical glass beads and (2) 1.3 mm diameter porous Al₂O₃ trilobes having the same size as the original trilobe catalyst used in HDS bench‐scale experiments. The superficial gas and liquid velocities were set within the range of trickle flow, e.g., u_0G = 20–500 mm/s and u_0L = 0.1–6 mm/s. In parallel with the MRI experiments, the hydrodesulfurization of a gas oil was investigated in a bench‐scale plant. Its reactor had the same dimensions of the trickle‐bed column used in the MRI experiments and was filled with original trilobe catalyst. These catalytic experiments were carried out at a wide range of operating conditions (p = 30–80 bar, T = 300–380 °C, LHSV = 1–4 h^–1). The results of both fluid dynamic and catalytic reaction experiments were then combined for developing a simulation model to predict the HDS performance by accounting for fluid dynamic nonidealities.     

Combination of Time‐Dependent Self‐Diffusion Measurements (Dynamic Imaging) with Magnetic Resonance Imaging

The new NMR method, demonstrated in the present work, allows within local small volumes the determination of integral structural parameters like the surface‐to‐pore volume ratio or the tortuosity of a fluid‐filled porous medium. A combination of conventional imaging (MRI) with measurements of observation‐time dependent self‐diffusion (dynamic imaging) was used.     

Measurement and Simulation of Fluid Flow in Agitated Solid/Liquid Suspensions

Numerical and experimental investigations were carried out to improve the knowledge of the flow field in solid/liquid suspensions in agitated vessels and to examine the ability of the commercial CFD, code CFX 4.2, for this application. The numerical results were compared with our own experimental results of time resolved Laser‐Doppler‐Velocimeter (LDV) and power input measurements with particle concentrations of up to 15 percent of the volume. To perform LDV measurements within the solid/liquid flow the refractive index matching method was used. Results of the CFD simulation show the necessity to use high grid resolution, the k‐ϵ turbulence model and an advanced discretization scheme to achieve a grid‐independent solution. Simplifications of the grid geometry were shown to be an acceptable way to minimize the time required for calculation. The comparison of experimental and numerical results shows good agreement for velocities and power input in the single‐ and in all two‐phase flow cases. Good agreement with regard to the kinetic energy k is also apparent, except in the region near the stirrer blades, where local maxima are underestimated by the simulation.     

Modeling of Droplet Separation Kinetics in Granular Bed Filters Based on MRI Measurements

In droplet separation by granular bed filters, the transient loading regime plays an important role because, for gases with low droplet concentrations, steady state will be reached only after a long time. A mathematical model describing this transient loading regime as well as steady state was developed. It is based on differential balances for the dispersed droplets and for the separated liquid. The time‐dependent and spatially resolved liquid loading is calculated and compared with data obtained by magnetic resonance imaging (MRI), a noninvasive measuring technique. The good agreement between simulated and measured loading curves proves the general applicability of the model.     

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.     

糠醇树脂碳化过程的核磁共振研究

引用质子的连续波和核磁共振研究糠醇树脂在６０～８６０℃温度范围的碳化过程。并按热处理时发生的结构变化和化学变化解释该结果。加热到２４０℃时分子量和交联度增大;３２０℃时产品充分交联,形成坚硬的结构;３８０℃以上放出氢气并产生顺磁中心,实验证明温度是生产的关键。     

Characterization of Structure and Transport in Porous Media by Pulsed Field Gradient (PFG) NMR Technique. Part I: Master Curve and Characteristic Inner Length

Systematic experimental studies focusing on the practical application of observation time dependent pulsed‐field‐gradient (PFG) NMR were performed. The objective was to provide engineering scientists with a reliable experimental tool for characterizing the structure and transport in fluid‐filled porous media. Observation time dependent self‐diffusion in glass bead packs as model systems was investigated, where the diffusing species (molecules of the solvent or dissolved particles) served as probes for the confining geometry in the porous medium. First, the basic question whether the obtainable structure information is independent of the actual mobility of the diffusing probe particles was examined experimentally. It could be demonstrated that plotting the normalized time‐dependent diffusion coefficient D(t)/D₀ versus the actual migration length l_D(t) during a given observation time t indeed yields a characteristic “master curve” that is independent of the mobility of the diffusing species, thus reflecting, as desired for a reliable method, solely the effects of the confining geometry of the porous system of interest. It was further shown that from the master curve a new quantity, i.e., a “characteristic inner length” or “correlation length” ξ_D can be derived that corresponds to a path length in the porous medium, after which particles in the pore fluid experience an averaged restricting geometry and diffuse with an effective diffusion coefficient D_eff. It turned out that ξ_D is surprisingly short, that is, in the order of the bead radius. Moreover, it was demonstrated that normalization of this migration length with the bead radius yields a common master curve for all monodisperse bead packs used and thus, it is obviously possible to derive and record master curves for different kinds of packs, beds or other porous media as references that can be used to characterize or certify the kind of the porous matrix of interest.     

Nuclear magnetic resonance imaging of an operating gas–liquid–solid catalytic fixed bed reactor

This work reports our pioneering application of the nuclear magnetic resonance imaging (MRI) technique to the dynamic in situ studies of gas–liquid–solid reactions carried out in a catalytic trickle bed reactor at elevated temperature. The major advance of these studies is that MRI experiments are performed under reactive conditions. We have applied MRI to map the distribution of liquid phase inside a catalyst pellet as well as in a catalyst bed in an operating trickle-bed reactor. In particular, our studies have revealed the existence of the oscillating regimes of the heterogeneous catalytic hydrogenation reaction caused by the oscillations of the catalyst temperature and directly demonstrated the existence of the coupling of mass and heat transport and phase transitions with chemical reaction. The existence of the partially wetted pellets in a catalyst bed which are potentially responsible for the appearance of hot spots in the reactor has been also visualized. The combination of NMR spectroscopy with MRI has been used to visualize the spatial distribution of the reactant-to-product conversion within an operating reactor.     

Magnetic Resonance Imaging in Chemical Engineering: Basics and Practical Aspects

The huge potential of nuclear magnetic resonance imaging is increasingly exploited in chemical engineering. However, optimal use of this rather complex method requires some background knowledge. In this paper, first the basic principles are presented in a form dedicated to readers with an engineering formation. Subsequently, practical aspects such as resolution in space and time are discussed. Emphasis is put on established methods implemented on standard magnetic resonance imaging (MRI) hardware, as mainly used in the research presented in this issue.     

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.     

Predictions of flow instabilities in the shear‐thickening regime with an improved bautista–manero–puig model

The trajectories of chaotic fluctuations in dilute micellar solutions, which typically occur during the creation and destruction of shear‐induced structures in the flow inception of a 0.4 wt % cetyltrimethylammonium tosilate aqueous solution and an aqueous solution of 3 mM cetyyltrimethylammonium bromide‐sodium salicylate, are predicted here with an improved Bautista–Manero–Puig model. To determine the constancy of the model parameters with time, we applied successfully an integro‐differential approach. This methodology revealed that all the model parameters, but the elastic modulus is constant. The nature of the predicted fluctuations was analyzed by employing the exponent of Lyapunov. This analysis demonstrates that the fluctuations are chaotic deterministic in Regimes I and IV, whereas they are random in Region II. We found that the predictions of the modified model follow the experimental data and reveal that these fluctuations are related to the creation of elastic structures at the inception of shear flow. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3735–3745, 2018     

Newtonian and Non‐Newtonian Low Re Number Flow Through Bead Packings

The results of measurements of velocity distributions of Newtonian and non‐Newtonian fluids flowing through porous media are described in this contribution. The porous matrix was modeled by glass beads of different diameters forming a random bead packing confined by a circular tube. These packings were passed through by aqueous solutions of glucose and xanthane gum. Nuclear magnetic resonance (NMR) methods were applied to investigate the flow field in the packing. Spatially resolved and integral displacement distribution measurements were reported.