Structure-heat transport analysis of periodic open-cell foams to be used as catalyst carriers

Longitudinal Relaxation (T 1) of Methane/Hydrogen Mixtures for Operando Characterization of Gas-Phase Reactions

Catalytic hydrogenation reactions are important in a modern hydrogen-based society. To optimize these gas-phase reactions, a deep understanding of heat, mass, and momentum transfer inside chemical reactors is required. Nuclear magnetic resonance (NMR) measurements can be used to obtain spatially resolved values of temperature, gas composition, and velocity in the usually opaque catalytic macrostructures. For this, the desired values are calculated from measured NMR parameters like signal amplitude, T ₁, or T ₂. However, information on how to calculate target values from these NMR parameters in gases is scarce, especially for mixtures of gases. To enable detailed NMR studies of hydrogenation reactions, we investigated the T ₁ relaxation of methane and hydrogen, which are two gases commonly present in hydrogenation reactions. To achieve industrially relevant conditions, the temperatures are varied from 290 to 600 K and the pressure from 1 bara to 5 bara, using different mixtures of methane and hydrogen. The results show that hydrogen, which is usually considered to be nondetectable in standard MRI sequences, can be measured at high concentrations, starting at a pressure of 3 bara even at temperatures above 400 K. In the investigated parameter range, the absolute T ₁ values of hydrogen show only small dependence on temperature, pressure, and composition, while T ₁ of methane is highly dependent on all three parameters. At a pressure of 5 bara, the measured values of T ₁ for methane agree very well with theoretical predictions, so that they can also be used for temperature calculations. Further, it can be shown that the same measurement technique can be used to accurately calculate gas ratios inside each voxel. In conclusion, this study covers important aspects of spatially resolved operando NMR measurements of gas-phase properties during hydrogenation reactions at industrially relevant conditions to help improve chemical processes in the gas phase.

Low Heat Capacity 3D Hollow Microarchitected Reactors for Thermal and Fluid Applications

Lightweight reactor materials that simultaneously possess low heat capacity and large surface area are desirable for various applications such as catalytic supports, heat exchangers, and biological scaffolds. However, they are challenging to satisfy this criterion originating from their structural property in most porous cellular solids. Microlattices have great potential to resolve this issue in directing transport phenomena because of their hierarchically ordered design and controllable geometrical features such as porosity, specific surface, and tortuosity. In this study, we report hollow ceramic microlattices comprising a 10 μm thick hollow nickel oxide beam in an octet-truss architecture with low heat capacity and high specific surface area. Our microarchitected reactors exhibited a low heat capacity for a rapid thermal response with a small Biot number (Bi << 1) and large intertwined surface area for homogeneous flow mixing and chemical reactions, which made them ideal candidates for various energy applications. The hollow ceramic microlattice was fabricated by digital light three-dimensional (3D) printing, composite electroless plating, polymer removal, and subsequent thermal annealing. The transient thermal response and fluidic properties of the 3D-printed microstructures were experimentally investigated using a small-scale thermal and fluid test system, and analytically interpreted using simplified models. Our findings indicate that hollow microarchitected reactors provide a promising platform for developing multifunctional materials for thermal and fluid applications.

Effect of the Zirconia Particle Size on the Compressive Strength of Reticulated Porous Zirconia-Toughened Alumina

Reticulated porous ceramics have attracted researchers owing to the separation and collection properties of porous materials and the combined high thermal resistance and chemical stability of ceramics. Among various kinds of reticulated porous ceramics, we investigated the feasibility of using reticulated porous Zirconia-toughened Alumina as applications such as dielectric barriers, insulators, and filters with acceptable properties. An acceptable range of the compressive strength for reticulated porous ZTA applications is approximately 1 MPa. However, when the pore density of the reticulated porous ZTA specimen prepared using coarse zirconia was 60, maximum compressive strength of 1.63 MPa was obtained. To enhance the compressive strength of reticulated porous ZTA specimens, rheological control of the ZTA slurry is most important by optimizing the viscosity of the ZTA slurry, and the composition (average particle size, solid loading, organic binder, and thickener) of the ZTA slurry was controlled. The optimized processing conditions to enhance the compressive strength of reticulated porous ZTA specimens were determined. Consequently, we enhanced the compressive strength of the reticulated porous ZTA specimens from 0.37 MPa to 3.11 MPa by optimizing the ZTA slurry when the solid loading content, the pore density, the sintering temperature, the amount of PVA, and the amount of thickener were 66 wt.%, 60 PPI, 1600 °C, 2 wt.%, and 0.15 wt.%, respectively.

Numerical Simulation of Heat and Mass Transfer in an Open-Cell Foam Catalyst on Example of the Acetylene Hydrogenation Reaction

In the present work, based on numerical simulation, a comparative analysis of the flow of a chemically reacting gas flow through a catalyst is performed using the example of selective hydrogenation of acetylene in a wide range of flow temperatures variation. Catalyst models are based on open-cell foam material. A comparison is also made with calculations and experimental data for a granular catalyst. The porosity and cell diameter were chosen as variable parameters for the porous catalyst. The results of numerical studies were obtained in the form of component concentration fields of the gas mixture, vector fields of gas movement, values of conversion, and selectivity of the reaction under study. The parameters of the porous material of the catalyst are determined for the maximum efficiency of the process under study.

Scalable 3D-printed lattices for pressure control in fluid applications

Additive manufacturing affords precise control over geometries with high degrees of complexity and pre-defined structure. Lattices are one class of additive-only structures which have great potential in directing transport phenomena because they are highly ordered, scalable, and modular. However, a comprehensive description of how these structures scale and interact in heterogeneous systems is still undetermined. To advance this aim, we designed cubic and Kelvin lattices at two sub-5 mm length scales and compared published correlations to the experimental pressure gradient in pipes ranging from 12-52 mm diameter. We further investigated all combinations of the four lattices to evaluate segmented combinatorial behavior. The results suggest that a single correlation can describe pressure behavior for different lattice geometries and scales. Furthermore, combining lattice systems in series has a complex effect that is sensitive to part geometry. Together, these developments support the promise for tailored, modular lattice systems at laboratory scales and beyond.

The Effects of a Zirconia Addition on the Compressive Strength of Reticulated Porous Zirconia-Toughened Alumina

Porous ceramics have attracted researchers due to their high chemical and thermal stability. Among various types of porous ceramics, reticulated porous ceramics have both high porosity and good permeability. These properties of porous ceramics are difficult to replace with porous metals and polymers. ZTA is used in a variety of applications, and a wealth of experimental data has already been collected. However, research reports on reticulated porous zirconia-toughened alumina (ZTA) are insufficient. Therefore, we prepared reticulated porous ZTA via the replica template method. In this study, various processing conditions (average particle size, zirconia content, solid loading, dispersant, and thickener) were adjusted to improve the compressive strength of the reticulated porous ZTA. As a result, the optimized processing conditions for improving the compressive strength of reticulated porous ZTA could be determined.

Melting of Paraffin Waxes Embedded in a Porous Matrix Made by Additive Manufacturing

The recent advances in additive manufacturing technology have widened the choice of materials that can be printed, opening new frontiers in the field of heat transfer devices. This paper explores the use of a solid porous matrix in which paraffin waxes, having different melting temperatures (42, 55, and 64 °C), were embedded. The solid matrix is made by additive manufacturing. The parent cell of the porous matrix occupies the volume of a cube with an edge of 5 mm. The entire 3D printed matrix has a square base with an edge of 100 mm, and it has a height of 20 mm. The solid matrix was printed between two plates, each one with a thickness of 10 mm, where thermocouples were inserted, and it was tested in an upright position, laterally heated applying three different heat fluxes (10, 15, and 20 kW m−2). The experimental results are given in terms of the temperature of the heated side, as well as of the phase change material, during the heating process. The temperature reached by the heated side and the time needed to completely melt the paraffin waxes are compared at the different working conditions. Furthermore, the thermal conductivities and diffusivities of the three paraffins and of the parent material of the porous matrix were experimentally evaluated.