Hannsjörg Freund

Numerical simulations of single phase reacting flows in randomly packed fixed-bed reactors and experimental validation

Abstract Randomly packed fixed-bed reactors are widely used in the chemical process industries. Their design is usually based on pseudo-homogeneous model equations with averaged semi-empirical parameters. However, this design concept fails for low tube-to-particle diameter ratios (=aspect ratios) where local phenomena dominate. The complete three-dimensional (3D) structure of the packing has therefore to be considered in order to resolve the local inhomogeneities. New numerical methods and the increase of computational power allow us to simulate in detail single phase reacting flows in such reactors, exclusively based on material properties and the 3D description of the geometry, thus without the use of semi-empirical data. The successive simulation steps (packing generation, fluid flow and species calculation) and their validation with experimental data are described in this paper. In order to synthetically generate realistic random packings of spherical particles, we apply a Monte-Carlo method. The subsequent numerical simulation of the 3D flow field and coupled mass transport of reacting species is done by means of lattice Boltzmann methods. The simulation results reveal that not only the local behaviour but also integral quantities like the pressure drop depend remarkably on the local structural properties of the packings, a feature which is neglected when using correlations with averaged values.

Analysis of the flow field and pressure drop in fixed-bed reactors with the help of lattice Boltzmann simulations

The pressure drop of technical devices is a crucial property for their design and operation. In this paper, we show how the results of lattice Boltzmann simulations can be used in science and engineering to improve the physical understanding of the pressure drop and the flow inhomogeneities in porous media, especially in sphere-packed fixed-bed reactors with low aspect ratios. Commonly used pressure drop correlations are based on simplified assumptions such as the capillary or tortuosity model, which do not reflect all hydrodynamic effects. Consequently, empirical correlations for certain classes of media have been introduced in the past to bridge the gap between the models and the experimental findings. As is shown in this paper by the detailed analysis of the velocity field in the void space of packed beds, the pressure drop is due to more complex hydrodynamics than considered in the above-mentioned models. With the help of lattice Boltzmann simulations, we were able to analyse the different contributions to the total dissipation, namely shear and deformation of the fluid, for different geometries over a wide range of Reynolds numbers. We further show that the actual length of the flow paths changes considerably with the radial and circumferential position.

Selective oxidation of cyclohexanol to cyclohexanone in the ionic liquid 1-octyl-3-methylimidazolium chloride

Ionic liquid (IL) 1-octyl-3-methylimidazolium chloride was found to effectively intensify cyclohexanol oxidation and resulted in 100% conversion of cyclohexanol with 100% selectivity to cyclohexanone using hydrogen peroxide as an oxidant and WO(3) as a catalyst. The effect of the IL as a solvent is discussed with the support of COSMO-RS theory.

Optimal Reactor Design for the Hydroformylation of Long Chain Alkenes in Biphasic Liquid Systems

Abstract In this work, we apply our recently proposed reactor design methodology based on elementary process functions to the hydroformylation of long chain linear alkenes in a biphasic ionic liquid system with Rh/TPPTS catalyst. A potential selectivity increase towards the linear aldehyde of 35% compared to a reference case from literature is identified and an approximation of the best reaction route is proposed.

Influence of Resolution of Rasterized Geometries on Porosity and Specific Surface Area Exemplified for Model Geometries of Porous Media

Rasterized representations of geometrical structures are commonplace in science and engineering. They are used in analysis and design of complex geometrical structures; however, the introduced errors for volume and surface estimation are often not considered in detail. To provide insight and information on these effects, in this study model geometries of porous media (simple cubic, body-centered cubic, face-centered cubic) are used to investigate the influence of resolution (voxels per length) on volume and surface approximation. The numerically obtained results are compared with analytical solutions for porosity and specific surface area. Small deviations from the real volume are found for the rasterized geometry at reasonable resolution. For the estimated surface area, in contrast, when using marching cubes considerable deviations from the analytically calculated surface area are found even at relatively fine resolutions. These findings are especially important for the use of rasterized voxel data as input for engineering correlations to estimate characteristic physical transport properties such as pressure drop or effective heat transport.

pH effects in the acetaldehyde–ammonia reaction

The pH dependency of the reaction of acetaldehyde and ammonia to form the acetaldehyde-ammonia trimer has been studied in detail. The acetaldehyde-ammonia trimer is a molecule of interest in organic synthesis, since it can be used as a substrate in many reactions involving acetaldehyde or ammonia. This trimer is well known in the literature but no references are present so far to describe its formation from ammonia sources other than ammonium hydroxide. The focus of this study is on describing the course of reaction after addition of acetaldehyde to solutions of ammonia and various acids. Products have been analysed by means of 1H-NMR and IR spectroscopy and the complete range of pH values has been covered. Depending on the pH, two reaction regimes can be distinguished. At high pH, only the trimer is formed. In contrast, at low pH, only low quantities of the trimer are produced and the nature of the applied acid has a distinct effect on the reaction outcome. Inorganic acids result in low trimer concentration and high quantity of unreacted ammonia. Polymer formation dominates with simple carboxylic acids. Complex organic acids, such as e.g. maleic or nicotinic acid, lead to comparable quantities of the trimer and acetaldehyde. Based on our results, we propose some adjustments to the traditional reaction scheme developed for acetaldehyde-ammonia trimer formation at high pH.

A new reaction route for the synthesis of 2-methyl-5-ethylpyridine

In this work, a novel synthesis route to produce 2-methyl-5-ethylpyridine (MEP) from the cyclic acetaldehyde ammonia trimer (AAT) is explored. The reaction was studied in a semi-batch reactor in the presence of different promoters to adjust the pH of the reaction solution. Among various ammonium salts tested as promoters, ammonium acetate was identified as the most suitable promoter for the reaction. By using a Design of Experiments (DoE) approach, the temperature and concentration of reactants and the promoter were identified as the most important/decisive parameters influencing the course of the reaction. Additional mechanistic investigations were carried out to assess the effect of these parameters in detail and to clarify the by-product formation via oligomer formation.

Dynamics of Liquid-Liquid Systems Based on Linear Thermodynamics of Irreversible Processes

Abstract The use of non-equilibrium models for integrated processes involving liquid–liquid systems has increased in recent years. These processes often exhibit complex dynamic behavior. These dynamical systems still pose many open questions, e.g. with regard to the sources of multiple steady states (MSS). This article analyzes the effect of mass transfer on the MSS of these systems. A generalized non-equilibrium modeling approach based on linear thermodynamics of irreversible processes (LTIP) is presented, and the dynamics of the system is studied systematically. It is shown that the non-linearity present in even the simplest non-ideal activity model acts as a source for MSS. The parameters that affect the solubility, e.g. temperature, can play a critical role on the existence of MSS in the system. A geometrical visualization of the MSS is also illustrated.

Detailed Simulation of Transport Processes in Reacting Multi-Species Flows Through Complex Geometries by Means of Lattice Boltzmann Methods

New numerical methods such as the lattice Boltzmann approach together with the increasing computational power can provide a detailed insight into the flow and transport processes in complex three dimensional geometries. The present paper demonstrates this approach by investigating inhomogeneities in the 3-D flow field of reacting species in a tubular fixed bed reactor of small tube-toparticle diameter ratio. For such reactors detailed 3-D simulations are of technical and economical interest as conventional modelling methods cannot correctly predict the local flow behaviour. However, these local inhomogeneities can significantly influence the reactor performance. Beside the discussion of the reaction engineering results, also the computational performance of the applied method is described.