Open Cellular Structure Research Articles

Gum Hydrocolloids Reinforced Silver Nanoparticle Sponge for Catalytic Degradation of Water Pollutants

The accumulation of organic contaminants including dyes in aquatic systems is of significant environmental concern, necessitating the development of affordable and sustainable materials for the treatment/elimination of these hazardous pollutants. Here, a green synthesis strategy has been used to develop a self-assembled gum kondagogu-sodium alginate bioconjugate sponge adorned with silver nanoparticles, for the first time. The properties of the nanocomposite sponge were then analyzed using FTIR, TGA, SEM, and MicroCT. The ensued biobased sponge exhibited hierarchical microstructure, open cellular pores, good shape memory, and mechanical properties. It merges the attributes of an open cellular porous structure with metal nanoparticles and are envisaged to be deployed as a sustainable catalytic system for reducing contaminants in the aqueous environment. This nanocomposite sponge showed enhanced catalytic effectiveness (km values up to 37 min−1 g−1 and 44 min−1 g−1 for methylene blue and 4-nitrophenol, respectively), antibacterial properties, reusability, and biodegradability (65% biodegradation in 28 days).

Microwave-responsive SiC foam@zeolite core-shell structured catalyst for catalytic pyrolysis of plastics

Catalytic pyrolysis is a promising chemical recycling technology to supplement mechanical recycling since plastics can be broken down into monomers or converted to the required fuels and chemicals. In this study, a microwave (MW) -responsive SiC foam@zeoltie core-shell structured catalyst was proposed for the catalytic pyrolysis of polyolefins. Under microwave irradiation, the SiC foam core works as both microwave adsorber and catalyst support, thus concentrating the generated heat energy on the ZSM-5 zeolite shell, where the catalytic reaction takes place. SiC foam with an open cellular structure can also improve the global transport of mass and heat during plastics pyrolysis. In this work, the effects of the SiO2/Al2O3 ratio and alkaline treatment of ZSM-5 zeolite coated SiC foam under MW irradiation on the variations in product distribution from low-density polyethylene (LDPE) pyrolysis were investigated at 450 °C. The results indicated that the appropriate acidity and pore structure were crucial to upgrading gas and liquid products. Particularly, the creation of a mesoporous structure in ZSM-5 zeolite via alkaline treatment could improve the diffusion of large molecules and products, thus significantly increasing the selectivity of high-valued light olefins and aromatics while inhibiting the formation of unwanted alkanes, which are expected in the chemical industry. Concretely, the concentration of olefins in gas increased to 51.0 vol% for ZSM-5(50)-0.25AT, and 65.6 vol% for ZSM-5 (50)-0.50AT, compared with 45.2 vol% for the parent ZSM-5(50). The relative concentration of aromatics in liquid decreased from 96.6% for ZSM-5(50) to 75.9% for ZSM-5(50)-0.25AT, and 71.1% for ZSM-5(50)-0.50AT. Given the respective yield of gas and liquid, the total selectivity of C2–C4 olefins and aromatics for mesoporous ZSM-5 zeolites could reach 58.6–64.9% during LDPE pyrolysis, which were higher than that for the parent ZSM-5 zeolite.

Improving the radial heat transport and heat distribution in catalytic gas-solid reactors

Improving the radial heat transport or the heat distribution is a paramount aspect for the sustainable and safe operation of catalytic gas-solid reaction systems. In this perspective, the recent advances in this field are summarized and critically discussed, i.e., particle shape modification of random packed beds, structured reactors including foams, monoliths, and periodic open cellular structures, internals in packed beds, as well as alternative concepts for energy input like induction, Ohmic, and microwave heating. The role of detailed modeling to advance this field will be emphasized and future research needs are presented.

Effect of gas nitriding on 316 L stainless steel lattice manufactured via selective laser melting

Nitriding is well-known as a surface treatment method to improve the wear and fatigue properties. This work demonstrates the capability of nitriding in improving the overall mechanical performance of the open cellular structure, by taking the 316 L stainless steel lattice structure fabricated by additive manufacturing as an example. The microstructure, composition distribution, and compressive properties of the obtained structure were analyzed. It is found that the effective elastic modulus, compressive proof strength, and energy absorption capacity of the lattice were improved after conventional gas nitriding. A nitrided layer with a thickness of about 60 μm was formed on the surface of the nitrided parts, including an outer layer of chromium nitrides and an inner layer containing expanded austenite. The nitrided lattices with thinner struts were strengthened more due to the higher proportion of the nitrided layer. Meanwhile, the lattices were embrittled after nitriding. However, the toughness of the lattice can be compensated by adjusting the lattice structure, such as changing the angle between the strut and the compression direction so that a lattice with good strength and toughness can be obtained.

The role of network topology on the uniaxial tensile behaviour of low density random open cellular structures

The effect of cellular structure on the mechanical properties of cellular solids is included in the prefactors of the relative density term in the Gibson and Ashby scaling laws. The effect of cell shape, cell size distribution, cell shape anisotropy, cell wall curvature and strut cross sectional variation have been known to affect these prefactors. This paper investigates the dependence of these prefactors on the network connectivity in random honeycombs and three-dimensional random open cellular structures. Beginning with intact structures, the network connectivity is reduced by randomly removing struts. A scaled genus density, that remains independent of sample volume, absolute cell size, and the relative density, has been defined and taken as a quantitative measure of the global connectivity. The variation in the overall stiffness and offset yield stress due to loss in connectivity was found to clearly depend on the scaled genus density when the relative density is kept constant, indicating that the geometric prefactors are functions of scaled genus density. The prefactors for intact perturbed cubic voronoi, perturbed BCC voronoi structures, and perturbed cubic body diagonal lattices with widely differing scaled genus density also confirm this strong correlation. We also show that the prefactors can be factored to quantitatively decouple the effects of anisotropy and scaled genus density.

Numerical and experimental investigations of novel nature inspired open lattice cellular structures for enhanced stiffness and specific energy absorption

Lattice structures are meant for light weight structure with a high strength to weight ratio used to carry high loads and high energy absorption capabilities. In the lattice structures, the non-stochastic arrangement of members has the ability to control and customize their mechanical properties. The recent developments in additive manufacturing provides freedom in design and gives flexibility to manufacture tailored complex novel lattice structures. In the present investigation, the traditional 2.5D honeycomb and the tessellated open lattice structures with basic crystalline arrangements of the members are considered to study the load carrying capacity, stiffness and energy absorption capabilities by keeping constant volume for each lattice. The results through the experiments and finite element analysis are compared with a novel open lattices structure to show significant improvement in stiffness and high energy absorption with the volume of the material. However, the novel open lattice structure has displayed prominent post yield plateau stresses and energy absorption with sacrificing stiffness and strength significantly.

Review on Additive Manufacturing of Catalysts and Sorbents and the Potential for Process Intensification

Additive manufacturing of catalyst and sorbent materials promises to unlock large design freedom in the structuring of these materials, and could be used to locally tune porosity, shape and resulting parameters throughout the reactor along both the axial and transverse coordinates. This contrasts catalyst structuring by conventional methods, which yields either very dense randomly packed beds or very open cellular structures. Different 3D-printing processes for catalytic and sorbent materials exist, and the selection of an appropriate process, taking into account compatible materials, porosity and resolution, may indeed enable unbounded options for geometries. In this review, recent efforts in the field of 3D-printing of catalyst and sorbent materials are discussed. It will be argued that these efforts, whilst promising, do not yet exploit the full potential of the technology, since most studies considered small structures that are very similar to structures that can be produced through conventional methods. In addition, these studies are mostly motivated by chemical and material considerations within the printing process, without explicitly striving for process intensification. To enable value-added application of 3D-printing in the chemical process industries, three crucial requirements for increased process intensification potential will be set out: i) the production of mechanically stable structures without binders; ii) the introduction of local variations throughout the structure; and iii) the use of multiple materials within one printed structure.

Numerical and Experimental Investigation of Pressure Drop in Periodic Open Cellular Structures for Intensification of Catalytic Processes

Periodic Open Cellular Structures are envisioned as potential enhanced catalyst substrates for heat and mass transfer-limited processes. To enable their rational design, in this study, we propose a combined numerical and experimental approach to assess the pressure drop in the tetrakaidekahedral and diamond lattices since generalized correlations are not present in the literature. Deviations are observed between the model predictions available in the literature, which are possibly due to the different methods of investigation, that is, numerical on ideal geometries and experimental on reproductions. To reconcile the two approaches and prevent discrepancies, careful attention is paid here to the quality of 3D-printed replicas for experimental investigation to obtain results representative of ideal lattices. Computational Fluid Dynamics simulations and experiments are then employed together to cross-validate the results and then to perform a parametric analysis of the effect of the morphological properties on the pressure drop. The effect of the cell size and the porosity are discussed, enabling the derivation of engineering correlations for the prediction of the pressure drop across the lattices within the range of 1 < Reds < 300. Finally, the performances of lattice materials are compared with those of conventional structured supports by evaluating the trade-off between the fluid–solid mass transfer rate and the pressure drop, which is crucial for several catalytic processes. Results show that the diamond lattice outperforms other cellular materials and can outperform ceramic honeycomb monoliths at low Reynolds numbers.

Recent Advances in the Development of Highly Conductive Structured Supports for the Intensification of Non-adiabatic Gas-Solid Catalytic Processes: The Methane Steam Reforming Case Study

Structured catalysts are strong candidates for the intensification of non-adiabatic gas-solid catalytic processes thanks to their superior heat and mass transfer properties combined with low pressure drops. In the past two decades, different types of substrates have been proposed, including honeycomb monoliths, open-cell foams and, more recently, periodic open cellular structures produced by additive manufacturing methods. Among others, thermally conductive metallic cellular substrates have been extensively tested in heat-transfer limited exo- or endo-thermic processes in tubular reactors, demonstrating significant potential for process intensification. The catalytic activation of these geometries is critical: on one hand, these structures can be washcoated with a thin layer of catalytic active phase, but the resulting catalyst inventory is limited. More recently, an alternative approach has been proposed, which relies on packing the cavities of the metallic matrix with catalyst pellets. In this paper, an up-to-date overview of the aforementioned topics will be provided. After a brief introduction concerning the concept of structured catalysts based on highly conductive supports, specific attention will be devoted to the most recent advances in their manufacturing and in their catalytic activation. Finally, the application to the methane steam reforming process will be presented as a relevant case study of process intensification. The results from a comparison of three different reactor layouts (i.e. conventional packed bed, washcoated copper foams and packed copper foams) will highlight the benefits for the overall reformer performance resulting from the adoption of highly conductive structured internals.

Scalable, process-oriented beam lattices: generation, characterization, and compensation for open cellular structures.

Additively manufactured lattices are emerging as promising candidates for structural, thermal, chemical, and biological applications. However, achieving a satisfactory prototype or final part with this level of complexity requires synthesis of disparate knowledge from the distinctly digital and physical processing stages. This work proposes an integrated framework for processing self-supporting, open lattice structures that do not require supports and facilitate material removal in post-processing steps. We describe a minimal yet comprehensive design strategy for generating uniform lattice structures with conformal open lattice skins for an arbitrary unit cell configuration. Using continuous liquid interface production (CLIP™) on a Carbon M1, printability is evaluated for five unique bending-dominated lattice structures at unit cell length scales from 0.5 - 3.5 mm and strut diameters ranging from 0.11 - 1.05 mm. Using a cubic lattice as a basis, we further examine dimensional fidelity with respect to 2D lattice void dimensions and part position, finding differences between length scales and within parts, due to physical processing artifacts. Finally, we demonstrate a functional grading strategy based on process control methods to compensate for dimensional deviations. Using an iterative approach based on a naïve process model, deviation of the planar strut radius in a cubic lattice was decreased by approximately 85% after two iterations. These insights and strategies can be readily applied to other structures, characterization techniques, and additive manufacturing processes, thereby improving the exchange of information between digital and physical processing and lowering the energy barriers to producing high-quality lattice parts.

Influence of biodegradable polyurethane foam on biocoenosis and sludge activity in reactors simulating low-load wastewater treatments

Hydrophilic and fully aliphatic polyurethane (PUR) foam exhibiting a highly open cellular structure was used as a biodegradable biofilm carrier that would serve simultaneously as a nutrient source. A one-month laboratory testing in a reactor simulating domestic wastewater treatment plants was conducted based on chemical and hydrobiological analyses as well as evaluation of microorganism development and enzymatic activity. It was found that PUR foam can successfully serve as a carrier for microorganisms and simultaneously can be biodegraded when the supply of nutrition is limited.

Development of a generalized thermal resistance model for the calculation of effective thermal conductivities in periodic open cellular structures (POCS)

Periodic open cellular structures (POCS) are a group of porous media with promising attributes for the use in heat transfer enhancement. They consist of a periodic arrangement of unit cells that offer a vast design freedom and therefore high adaptability to the needs of a specific application. In order to use POCS efficiently, a fundamental understanding of the relation between their geometry and thermal transport properties is essential.This contribution focuses on the investigation and modeling of the effective thermal conductivity. It is based on a homogenization approach that averages the heat conduction properties over an entire unit cell, while neglecting the influence of the fluid. To this end, the investigation provides detailed insight and an important component for the subsequent model development on the overall heat transfer capabilities of POCS, including convection and conjugate heat transfer. A numerical simulation setup is developed, which allows the determination of this quantity in isotropic as well as anisotropic POCS with varying unit cell type, porosity, tapering and strut cross-sectional shape. The results indicate the impact of both struts as well as nodes, on the heat transfer and form the foundation for a new generalized thermal resistance network model. In contrast to similar approaches, the impact of multidimensional heat transfer within the nodes is considered enabling an accurate prediction of the effective thermal conductivity of all POCS investigated in this work. Due to its modularity, it is easily transferable to other unknown geometries, thus contributing to the toolbox for the efficient design of POCS.

Additive manufacturing of architected ceramics foam for noise reduction

Porous foams are routinely used for noise control applications; their open cellular structure provides good sound dissipation performance over a broad frequency range while retaining low bulk density. However, conventional acoustic foams cannot satisfy the mechanical and flammability constraints frequently encountered in aerospace applications. While fiberglass blankets are currently used for high temperature noise dissipation applications, their batch-to-batch performance variability and lack of mechanical stiffness remains an engineering hurdle. In this study, we investigate the viability of additively manufacturing ceramic foams for high temperature noise dissipation applications. We demonstrate the use of a low-cost, material extrusion 3D printer to fabricate ceramic foams with controlled porous architectures that can be tuned to provide exceptional sound absorption properties. The printed and sintered ceramic foams are tested using a normal incidence impedance tube and the effect of various print, sintering, and geometrical parameters on the acoustical impedance are studied. Our work shows that low-cost additive techniques present a viable step toward scalable fabrication of ceramic foams with tailored mechanical and acoustical properties.

Polyurethane/polydopamine/graphene auxetic composite foam with high-efficient and tunable electromagnetic interference shielding performance

In this work, two kinds of auxetic composite foams of polyurethane (PU) with polydopamine (PDA) and graphene (GR) (PU/PDA/GR) were prepared by dip-coating and ultrasonic immersing, then followed by triaxial compression-heating process. The electromagnetic interference (EMI) shielding effectiveness (SE) value of the PU/PDA/GR auxetic composite foam with a Poisson's ratio of −2.36 from PU foam with open cellular structure reaches 59.75 dB, which is 8.9 times of the positive Poisson's ratio composite foam with the same GR content. Furthermore, the relationship of the EMI shielding performance of auxetic foam with different forms of reentrant structure was studied for the first time, in which the absolute value of negative Poisson's ratio was used to quantitatively describe the difference of the reentrant structure in the obtained auxetic foam. The larger the absolute value of negative Poisson's ratio is, the better the EMI shielding performance of the composite foam is. The high absorption EMI SE and the existence of reentrant structure are the key factors for the high EMI shielding performance of the auxetic composite foam. In addition, it is found that the EMI SE of the auxetic foam can be adjusted via simple heating treatment, showing that the auxetic composite foam may be used as a temperature-regulated EMI shielding material in the future.

Detailed investigation of liquid distribution and holdup in periodic open cellular structures using computed tomography

Periodic open cellular structures (POCS) show great potential for process intensification in catalytic reactors due to their favorable properties regarding heat transfer, pressure drop and liquid distribution. In multiphase applications, POCS can be employed as multifunctional catalyst carrier with integrated liquid distribution function for enhancing the performance of heterogeneously catalyzed reactions. This applies particularly to trickle bed reactors where a uniform liquid distribution is of great importance. In this contribution, the liquid distribution of three POCS of different unit cell geometry is investigated using a custom-made computed tomography scanner with high spatial resolution. POCS with the Kelvin, diamond and DiaKel unit cell, respectively, were manufactured via fused deposition modeling out of acrylonitrile butadiene styrene. The liquid distribution was examined in dependence of the unit cell as well as for different liquid and gas flow rates. The three POCS show different behavior for the liquid distribution and holdup within the structures. POCS with Kelvin and DiaKel unit cell geometry show a higher liquid holdup as well as a more uniform liquid distribution compared to diamond unit cell POCS. Also, the liquid filling between the individual POCS that are stacked in the column is dependent on the unit cell.

A Fundamental Investigation of Gas/Solid Heat and Mass Transfer in Structured Catalysts Based on Periodic Open Cellular Structures (POCS).

In this work, we investigate the gas–solid heat and mass transfer in catalytically activated periodic open cellular structures, which are considered a promising solution for intensification of catalytic processes limited by external transport, aiming at the derivation of suitable correlations. Computational fluid dynamics is employed to investigate the Tetrakaidekahedral and Diamond lattice structures. The influence of the morphological features and flow conditions on the external transport properties is assessed. The strut diameter is an adequate characteristic length for the formulation of heat and mass transfer correlations; accordingly, a power-law dependence of the Sherwood number to the Reynolds number between 0.33 and 0.67 was found according to the flow regimes in the range 1–128 of the Reynolds number. An additional −1.5-order dependence on the porosity is found. The formulated correlations are in good agreement with the simulation results and allow for the accurate evaluation of the external transfer coefficients for POCS.

Development and evaluation of a new particle tracking solver for hydrodynamic and mass transport characterization of porous media – A case study on periodic open cellular structures

The detailed understanding of local mass transport characteristics in porous catalyst supports is essential for the evaluation of their performance in reaction engineering applications. This applies particularly for the development and assessment of novel concepts such as additively manufactured periodic open cellular structures (POCS). A major drawback of the frequently used Eulerian based mass transport simulation methods is their tendency to overestimate molecular diffusion due to artificial numerical diffusion, especially in the range of high Péclet numbers (boundary-layer dispersion and pure mechanical dispersion regime) that is often encountered in industrial applications.To overcome this problem, we present a new efficient solver implementation for the OpenFOAM® simulation toolbox based on the Lagrangian Random Walk Particle Tracking method to calculate the mass transport via a deterministic convective and a stochastic diffusion term. Besides the numerical calculation of effective diffusion as well as dispersion coefficients, the new solver is also capable of analyzing residence time behavior, flow tortuosity and contact patterns with catalytically active surfaces of catalyst support structures in detail. Following a thorough validation study, we demonstrate the evaluation options provided by the results of this new solver named disTrackFoam on the example of three POCS with distinct unit cell types.

Rich H2 catalytic oxidation as a novel methodology for the evaluation of mass transport properties of 3D printed catalyst supports

In this work, we propose a novel methodology for the evaluation of the external mass transfer properties of 3D printed catalyst supports. This protocol relies on the use of a lab scale SLA 3D printer with a resin characterized by a high heat deflection temperature (HDT) for the manufacturing of samples at a lower price and with higher accuracy than equivalent metallic 3D printed structures. Periodic open cellular structure (POCS) samples with Tetrakaidecahedron unit cells (TKKD) were 3D printed and catalytically activated by depositing a 3% Pd/CeO2 washcoat by spin-coating. The washcoat was then consolidated with a two-step heat treatment composed by in-situ calcination in N2 and reduction in N2/H2 stream. Catalytic tests of rich H2 combustion showed the possibility to reach the external mass transfer control at temperatures below the resin HDT. Sherwood numbers were eventually estimated from the oxygen conversions under full external mass transfer control assuming a PFR behaviour. To validate the methodology, 3D printed replicas of open cell foams were also tested, and the results were successfully compared against a well-established literature correlation. Moreover, a one-to-one comparison was performed between the Sherwood numbers of a resin 3D printed structure, tested with the proposed methodology, and a metallic 3D printed structure, tested with the conventional CO oxidation approach. The two methods lead to superimposed results, thus providing the experimental evidence of the equivalence of the two methodologies for the evaluation of the external mass transport properties of complex catalyst substrates.

Systematic Experimental Investigations and Modeling of the Heat Transfer in Additively Manufactured Periodic Open Cellular Structures with Diamond Unit Cell

Additively manufactured periodic open cellular structures (POCS) represent a new class of structured catalyst carriers due to their favorable characteristics such as high specific surface area, low pressure drop, and remarkable heat transport properties. The periodic, well-defined, and reproducible geometry makes systematic investigations and geometric optimization possible. For a detailed optimization, models for the description of the transport characteristics of POCS are necessary. Besides the geometric parameters of the unit cell, the unit-cell geometry itself has a major effect on the dispersive thermal conductivity. POCS based on the diamond unit cell show a superior dynamic thermal conductivity in comparison to cubic structures and are therefore examined in detail. From the measurement results and the corresponding model parameters, influences of the geometric parameters of the structures are found. This leads to new design correlations for the heat transport in POCS. With this, a model-based optimization of POCS for a specific application, such as strongly exothermic reactions, is possible.

Packed-POCS with skin: A novel concept for the intensification of non-adiabatic catalytic processes demonstrated in the case of the Fischer-Tropsch synthesis

Reaction heat removal/supply is a key issue in the development of intensified compact reactors for non-adiabatic catalytic processes. In this work, using the exothermal Fischer-Tropsch synthesis as a critical case, we show the possibility to boost the heat transfer performances of a packed-POCS (Periodic Open Cellular Structure) reactor by adding an outer metallic thermally connected skin to the conductive cellular internals. To this aim, the performances of a highly active Co/Pt/Al2O3 catalyst, packed in the form of 300 μm particles into the metallic structure 3D-printed in AlSi7Mg0.6, are assessed at industrially relevant operating conditions and compared with those of the same catalyst particles packed into the same POCS printed without the skin. Outstanding performances (CO conversion in excess of 80 %) are reached with flat axial and radial temperature profiles along the catalytic bed, in line with the excellent temperature control enabled by the conductive POCS. The presence of the skin strongly decreases the overall external temperature difference thanks to the enhanced thermal contact between the reactor wall and the ordered cellular structure, which governs the wall heat transfer. Accordingly, thanks to the presence of the skin, the temperature profile inside the reactor can be controlled much more effectively, thus enabling new operating windows that are not commonly accessible.