A Comparison of Hydrodynamics and Structure in Random Spherical and Cylindrical Packed Beds

Catalytic packed bed reactors are widely used in industrial processes. The particle packing structure in the bed introduces complex flow patterns in the system. Therefore, understanding the hydrodynamics of the fluid is key in designing these kinds of reactors since they highly affect the heat and mass distribution in the reactor. Conventional support in these reactors is cylindrical particles, as they can be prepared via extrusion. However, the orientational freedom of the cylindrical particles compared to spheres results in very different flow behavior. Therefore, this study investigates the single-phase flow behavior in packed beds of both spherical and cylindrical particles.This study compares experimental measurements of the flow field using Magnetic Resonance Imaging (MRI) with detailed flow simulations using a ghost cell Immersed Boundary Method (IBM). To enable a quantitative comparison, the configuration of the packed beds is reconstructed from MRI for both the spherical and cylindrical packings. The flow distribution in the beds is compared at several Reynolds numbers. In addition, the range of Reynolds numbers is extended in the IBM simulations. The challenges and opportunities of both the MRI experiments and the fully resolved simulations for investigating flow in chemical reactors are discussed

A comparison of hydrodynamics and structure in a random packed bed with cylindrical particles

Catalytic packed bed reactors are widely used in industrial processes. The particle packing structure in the bed introduces complex flow patterns in the system. Therefore, understanding the bed hydrodynamics is key in designing these reactors since it significantly affects the heat and mass transport in the reactor. In industrial reactors often cylindrical particles are used, as they can be prepared via extrusion. However, the orientational freedom of the cylindrical particles compared to spheres results in distinctly different packing configuration and associated flow behavior. Therefore, this study investigates the single-phase flow behavior in slender packed beds of cylindrical particles. This study compares data on flow measurements obtained from Magnetic Resonance Imaging (MRI) with detailed data obtained from fully resolved simulations using a ghost cell Immersed Boundary Method (IBM). To enable a quantitative comparison, the configuration of the packed bed is reconstructed from MRI. The flow distribution in the beds is compared on a one-to-one basis at several Reynolds numbers. The challenges and opportunities of both the MRI experiments and the fully resolved simulations for investigating flow in chemical reactors are discussed. <br/

Hydrodynamics inside packed beds of spherocylinders; Magnetic Resonance Imaging and Pore Network Modelling approaches

Packed bed reactors are one of the central processing units of chemical and petrochemical industries. A detailed understanding of the hydrodynamics of the flow passing through packed bed reactors is of great importance in improving the design and performance of these reactors. One parameter that affects the hydrodynamics inside packed bed reactors is the packing configuration, i.e., the packing size and shape. The influence of the packing structure on the flow becomes more significant in the case of slender packed beds, where the column to particle diameter ratio is small. In this work, two approaches are employed to investigate the hydrodynamics in packed beds of spherocylinders with different aspect ratios: an experimental approach, Magnetic Resonance Imaging (MRI), and a numerical approach, Pore Network Modelling (PNM). The 3D structure images and the flow fields are obtained using two different sequences of MRI. From the structure images, pore network models are extracted. By implementing the numerical flow analysis on the pore network models, the flow fields are calculated from PNM and compared to the ones from MRI. The comparison shows a good correspondence between PNM and MRI, implying that PNM can describe hydrodynamics well in the slender packed beds of spherocylinders

Numerical and experimental study of the flow distribution inside slender packed beds of spherocylindrical particles

In this paper, Magnetic Resonance Imaging (MRI) and a Pore Network Model (PNM) are used to characterize the flow in packed beds of spherocylindrical particles. PNM is chosen as it is a relatively fast numerical approach, which provides local information on the bed flow pattern. MRI scans of packed bed reactors provide detailed information on the bed structure and the flow. In this study, the packed beds are reconstructed from the MRI images. The impact of the image quality on the PNM’s flow field prediction is assessed. It is shown that improved image quality significantly enhances prediction accuracy. With a sufficient image quality, PNM is able to closely match MRI in predicting the flow fields and capture important characteristics such as wall channeling

Hydrodynamics inside packed beds of spherocylinders; Magnetic Resonance Imaging and Pore Network Modelling approaches

Packed bed reactors are one of the central processing units of chemical and petrochemical industries. A detailed understanding of the hydrodynamics of the flow passing through packed bed reactors is of great importance in improving the design and performance of these reactors. One parameter that affects the hydrodynamics inside packed bed reactors is the packing configuration, i.e., the packing size and shape. The influence of the packing structure on the flow becomes more significant in the case of slender packed beds, where the column to particle diameter ratio is small. In this work, two approaches are employed to investigate the hydrodynamics in packed beds of spherocylinders with different aspect ratios: an experimental approach, Magnetic Resonance Imaging (MRI), and a numerical approach, Pore Network Modelling (PNM). The 3D structure images and the flow fields are obtained using two different sequences of MRI. From the structure images, pore network models are extracted. By implementing the numerical flow analysis on the pore network models, the flow fields are calculated from PNM and compared to the ones from MRI. The comparison shows a good correspondence between PNM and MRI, implying that PNM can describe hydrodynamics well in the slender packed beds of spherocylinders

Micellization of a weakly charged surfactant in aqueous salt solution: self-consistent field theory and experiments

Self-consistent field (SCF) calculations and light scattering experiments were performed to study the pH and salt response of micelles composed of surfactants with a single weak acid group in aqueous salt solution. To this end, the common surfactant Brij 35 was oxidized to yield a polyoxyethylene alkyl ether carboxylic acid with a single terminal weakly charged carboxylic acid group in alkaline media. At low pH values, the micellar hydrodynamic radii (Rh) are independent of the salt concentration. By contrast, at pH values around the acid dissociation constant (pH ≈ pKa ± 1), the micellar radius decreases upon increasing pH until a salt-dependent plateau value is reached. The reduction in micellar size is more pronounced for lower salt concentrations. The SCF computations are in qualitative agreement with the experimental results and further reveal a limiting value for Rh corresponding approximately to the Debye length λD. Self-assembly into micelles is suppressed for low salt concentrations that would yield Rh < λD. Instead, the surfactants remain as unimers in solution. The results are summarized in a state diagram displaying the preferred surfactant configuration in solution as a function of Rh/λD, pH and salt concentration

Micellization of a weakly charged surfactant in aqueous salt solution : Self-consistent field theory and experiments

Self-consistent field (SCF) calculations and light scattering experiments were performed to study the pH and salt response of micelles composed of surfactants with a single weak acid group in aqueous salt solution. To this end, the common surfactant Brij 35 was oxidized to yield a polyoxyethylene alkyl ether carboxylic acid with a single terminal weakly charged carboxylic acid group in alkaline media. At low pH values, the micellar hydrodynamic radii (Rh) are independent of the salt concentration. By contrast, at pH values around the acid dissociation constant (pH ≈ pKa ± 1), the micellar radius decreases upon increasing pH until a salt-dependent plateau value is reached. The reduction in micellar size is more pronounced for lower salt concentrations. The SCF computations are in qualitative agreement with the experimental results and further reveal a limiting value for Rh corresponding approximately to the Debye length λD. Self-assembly into micelles is suppressed for low salt concentrations that would yield Rh < λD. Instead, the surfactants remain as unimers in solution. The results are summarized in a state diagram displaying the preferred surfactant configuration in solution as a function of Rh/λD, pH and salt concentration