Pareto-optimal design and assessment of monolithic sponges as catalyst carriers for exothermic reactions
Kiewidt, Lars ; Thöming, Jorg - \ 2019
Chemical Engineering Journal 359 (2019). - ISSN 1385-8947 - p. 496 - 504.
Monolithic catalyst - Multi-objective optimization - Open-cell foam - Pareto-optimal - Solid sponge - Tradeoffs
Monolithic sponges combine low pressure losses and excellent heat transport properties and are consequently considered as promising catalyst carriers for fixed-bed reactors. Insights on how to design porosity and window size of monolithic sponges to resolve conflicting relations between low pressure losses, high thermal conductivities, and high space-time-yields (STY), i.e., a high catalyst inventory, are still unknown, especially at pilot or production scales. This study quantifies the outlined tradeoffs and assesses the potential of monolithic sponges as catalyst carriers compared to conventional packed beds of pellets. A state-of-the-art heterogeneous reactor model was applied in combination with a genetic multi-objective optimization algorithm to predict Pareto-optimal sets of sponge designs (max. STY, min. Δp,ΔTmax⩽ΔTtol). As example, the methanation of CO2 was chosen. The Pareto-optimal set of sponge designs shows that small windows are necessary to obtain high space-time-yields comparable to the ones of conventional packed beds. As a consequence, the expected low pressure loss cannot be achieved. Because of excellent heat transport properties, which are weakly dependent on the throughput, monolithic sponges however allow stable operation under varying gas loads. The results demonstrate that monolithic sponges will probably not replace packed pellet beds of pellets for the steady-state production of chemicals. Instead, they provide a competitive option for small-scale, decentralized production for example within chemical energy storage and CO2 utilization.
Multiscale modeling of monolithic sponges as catalyst carrier for the methanation of carbon dioxide
Kiewidt, Lars ; Thöming, Jorg - \ 2019
Chemical Engineering Science: X 2 (2019). - ISSN 2590-1400
Methanation - Monolithic catalyst - Multiscale modeling - Open-cell foam - Reactor modeling - Solid sponge
Solid sponges provide large surface areas, low pressure drops, and excellent heat transport properties and are thus promising monolithic catalyst carriers. Their potential compared to randomly packed beds at industrial scales, however, is largely unknown. To facilitate scale-up and future simulation studies, we present a 2-d mulstiscale reactor model for catalytic sponges. Therefore, we couple a 2-d pseudo-homogeneous reactor model with a 1-d reaction–diffusion model to explicitly consider heat and mass transfer and diffusional limitations at the catalyst scale. A comparison of simulated temperature profiles with experimental ones during CO2 methanation at the lab scale demonstrates the validity of the developed model. Further, the results show that it is necessary to include heat and mass transport at the catalyst scale to adequately simulate concentration and temperature distributions in solid sponges although the applied catalyst layers are typically thinner than 100 μm. The presented model thus allows to obtain insights into the interplay between heat and mass transport at both, the reactor and the catalyst scale, and provides a solid foundation for scale-up and techno-economic studies to assess the performance of solid sponges as catalyst carrier at industrial scales.
Cobalt@Silica Core-Shell Catalysts for Hydrogenation of CO/CO2 Mixtures to Methane
Ilsemann, Jan ; Straß-Eifert, Angela ; Friedland, Jens ; Kiewidt, Lars ; Thöming, Jorg ; Bäumer, Marcus ; Güttel, Robert - \ 2019
ChemCatChem 11 (2019)19. - ISSN 1867-3880 - p. 4884 - 4893.
CO methanation - Core-shell catalysts - Heterogeneous catalysis - Nanostructures
COx hydrogenation reactions for hydrocarbon synthesis, such as methane, are becoming more and more important in terms of the energy transition. The formation of the byproduct water leads to a hydrothermal environment, which necessitates stable catalyst materials under harsh reaction conditions. Therefore, novel nanostructured core-shell catalysts are part of scientific discussion, since these materials offer an exceptional resistance against thermal sintering. Here we report on a core-shell catalyst - Co@mSiO2 - for the hydrogenation of CO/CO2 mixtures towards methane. CO methanation experiments reveal a rapid temperature-depended deactivation for temperatures above 350 °C caused by coking and possible blocking of the pores. In comparison to a Co/mSiO2 reference catalyst with the same Co particle size a significantly higher methane selectivity was found for CO2 hydrogenation, which we attribute to the confinement effect of the core-shell structure and therefore a higher probability of CO readsorption. Finally, the simultaneous CO/CO2 co-methanation experiments show a high flexibility of the catalyst materials on different gas feed compositions.
Coupled conjugate heat transfer and heat production in open-cell ceramic foams investigated using CFD
Sinn, Christoph ; Pesch, Georg R. ; Thöming, Jorg ; Kiewidt, Lars - \ 2019
International Journal of Heat and Mass Transfer 139 (2019). - ISSN 0017-9310 - p. 600 - 612.
Computational fluid dynamics (CFD) - Conjugate heat transfer - Open-cell foam - Pseudo-homogeneous model - Solid sponge - Volumetric heat source
Combining low pressure drop and remarkable heat transport properties, open-cell foams offer a combination that makes them a highly attractive option as monolithic catalyst support. The coupled thermal behavior of foams and fluids during heat production, e.g., exothermic chemical reactions, is still not thoroughly described despite their potential to optimize temperature control in fixed-bed reactors. Hence, the aim of this study is to get deeper insight into coupled conjugate heat transfer and heat production in open-cell foams used in a tubular reactor with constant wall temperature. Therefore, we conducted - μCT-based CFD simulations of open-cell foams with artificial heat sources that mimic the heat of reaction during an exothermal chemical reaction and allow to study the effect of heat production on heat transfer while requiring lower computational cost than simulating actual chemical reactions. We implemented a range of heat source intensities, that covers typical exothermic reactions, to study their effect on heat flows as well as temperature fields and used CO 2 methanation as a case study. We further quantified the influence of superficial velocity, heat source intensity, and material on the temperature fields inside the foam and found conduction being the dominant heat transport mechanism. We further evaluated the feasible range of even more simplified pseudo-homogeneous models and found high thermal conductivities and low superficial velocities to be appropriate. In conclusion, the presented approach offers the possibility to study thermal effects regarding catalytic supports and give valuable insight in heat transport mechanisms under relevant process conditions in heterogeneous catalysis (heat sources: 5–150 W for a reaction volume of 1.2 × 10 −5 m 3 , superficial velocities 0–0.51 m s −1 , thermal conductivities 5–50 W m −1 K −1 ).
3D characterization of gas phase reactors with regularly and irregularly structured monolithic catalysts by NMR imaging and modeling
Ulpts, Jürgen ; Kiewidt, Lars ; Dreher, Wolfgang ; Thöming, Jorg - \ 2018
Catalysis today 310 (2018). - ISSN 0920-5861 - p. 176 - 186.
Gas phase reaction - In-operando - Magnetic resonance - Reactor simulation - Regularly and irregularly structured monolithic catalysts - Temperature measurements
A heterogeneously catalyzed gas phase reaction process was characterized regarding temperature and concentration profiles by means of three dimensional (3D) 1H magnetic resonance spectroscopic imaging (MRSI), using the exothermal ethylene hydrogenation as an example. Here, temperature mapping was achieved by using specifically designed thermometers filled with ethylene glycol. The impact of heat and mass transfer on the process performance was investigated by using two different monolithic catalysts with completely different heat and mass transfer characteristics: a regularly structured honeycomb monolith and a irregularly structured open-cell foam packing. The influence of these characteristics on the reaction zones within the monolithic catalysts was demonstrated by simulations that were based on 2D reactor models. To evaluate the applicability of temperature and concentration mapping by 1H MRSI for model validation, a predictive two dimensional model of the process was applied. The resulting simulations of temperature profiles and concentration distributions were in very good agreement with the experimental data with deviations below 9%. Conventional mass spectroscopic measurements provided further evidence of the accuracy of 3D MRSI measurements as well as the 2D reactor model.
|Tradeoff analysis of monolithic sponges
Kiewidt, Lars ; Thöming, J. ; Bitter, J.H. - \ 2018
Electrodeless dielectrophoresis: Impact of geometry and material on obstacle polarization
Pesch, Georg R. ; Kiewidt, Lars ; Du, Fei ; Baune, Michael ; Thöming, Jorg - \ 2016
Electrophoresis 37 (2016)2. - ISSN 0173-0835 - p. 291 - 301.
Insulating posts - Insulator-based dielectrophoresis - Multipole expansion - Particle Trapping - Polarization
Insulator-based (electrodeless) dielectrophoresis (iDEP) is a promising particle manipulation technique, based on movement of matter in inhomogeneous fields. The inhomogeneity of the field arises because the excitatory field distorts at obstacles (posts). This effect is caused by accumulation of polarization charges at material interfaces. In this study, we utilize a multipole expansion method to investigate the influence of geometry and material on field distortion of posts with arbitrary cross-sections in homogeneous electric fields applied perpendicular to the longitudinal axis of the post. The post then develops a multipole parallel or anti parallel to the excitatory field. The multipoles intensity is defined by the post's structure and material properties and directly influences the DEP particle trapping potential. We analyzed posts with circular and rhombus-shaped cross-sections with different cross-sectional width-to-height ratios and permittivities for their polarization intensity, multipole position, and their particle trapping behavior. A trade-off between high maximum field gradient and high coverage range of the gradient is presented, which is determined by the sharpness of the post's edges. We contribute to the overall understanding of the post polarization mechanism and expect that the results presented will help optimizing the structure of microchannels with arrays of posts for electrodeless DEP application.
Coatings of active and heat-resistant cobalt-aluminium xerogel catalysts
Schubert, Miriam ; Schubert, Lennart ; Thomé, Andreas ; Kiewidt, Lars ; Rosebrock, Christopher ; Thöming, Jorg ; Roessner, Frank ; Bäumer, Marcus - \ 2016
Journal of Colloid and Interface Science 477 (2016). - ISSN 0021-9797 - p. 64 - 73.
CO methanation - Coatings - Cobalt catalyst - Fischer-Tropsch - Xerogel
The application of catalytically coated metallic foams in catalytic processes has a high potential for exothermic catalytic reactions such as CO2 methanation or Fischer-Tropsch synthesis due to good heat conductivity, improved turbulent flow properties and high catalyst efficiencies. But the preparation of homogenous catalyst coats without pore blocking is challenging with conventional wash coating techniques. Here, we report on a stable and additive free colloidal CoAlOOH suspension (sol) for the preparation of catalytically active Co/Al2O3 xerogel catalysts and coatings. Powders with 18 wt% Co3O4 prepared from this additive free synthesis route show a catalytic activity in Fischer-Tropsch synthesis and CO2 methanation which is similar to a catalyst prepared by incipient wetness impregnation (IWI) after activating the material under flowing hydrogen at 430 °C. Yet, the xerogel catalyst exhibits a much higher thermal stability as compared to the IWI catalyst, as demonstrated in catalytic tests after different heat agings between 430 °C and 580 °C. It was also found that the addition of polyethylene glycol (PEG) to the sol influences the catalytic properties of the formed xerogels negatively. Only non-reducible cobalt spinels were formed from a CoAlOOH sol with 20 wt% PEG. Metallic foams with pores sizes between 450 and 1200 μm were coated with the additive free CoAlOOH sol, which resulted in homogenous xerogel layers. First catalytic tests of the coated metal foams (1200 μm) showed good performance in CO2 methanation.
Solid Sponges as Monolithic Catalyst Supports for CO2Methanation – Experimental Realization and Structure Optimization
Kiewidt, L. ; Thöming, J. - \ 2016
Chemie Ingenieur Technik 88 (2016)9. - ISSN 0009-286X - 1 p.
Solid sponges are applied as catalyst supports in a bench-scale fixed-bed reactor (25 x 100 mm) for CO2 methanation to demonstrate their potential for process intensification. The structure e.g. porosity pore size spatial distribution of the porosity and pore size is optimized to tune the heat transport properties locally and to determine the optimized temperature profiles for process intensification.
NMR-basierte ortsaufgelöste Charakterisierung von reaktiven StrÖmungen in porÖsen Systemen
Ulpts, J. ; Dreher, W. ; Kiewidt, L. ; Thöming, J. - \ 2016
Chemie Ingenieur Technik 88 (2016)9. - ISSN 0009-286X - p. 1280 - 1281.
In situ analysis of gas phase reaction processes within monolithic catalyst supports by applying NMR imaging methods
Ulpts, Jürgen ; Dreher, Wolfgang ; Kiewidt, Lars ; Schubert, Miriam ; Thöming, Jorg - \ 2016
Catalysis today 273 (2016). - ISSN 0920-5861 - p. 91 - 98.
3D magnetic resonance spectroscopic imaging - Catalytic monolith - Gas phase reaction - Non-invasive concentration measurement
Measuring spatially resolved concentration distributions in gas phase reaction systems is an important tool to validate simulation calculations, improve the understanding of transport processes within the catalyst, and identify potentials for improvements of monolithic catalyst supports. The commonly used measurement methods for such opaque systems are invasive and, thus, might be misleading due to alteration of the system. To overcome this issue, a 3D magnetic resonance spectroscopic imaging (MRSI) method was developed and implemented on a 7-Tesla NMR imaging system to map the concentration distributions within opaque monolithic catalysts using the ethylene hydrogenation process as case study. The reaction was catalyzed by a coated sponge packing or a honeycomb monolith within an NMR compatible packed bed reactor. Temperatures at the inlet and the outlet of the catalyst beds were simultaneously determined by analyzing the spectra of inserted ethylene glycol filled glass capsules. Steady state concentration profiles and temperature levels were measured at different reaction conditions. In order to prove the plausibility of the measured spatial distributions of compound concentrations, the experimental results were compared to a 1D model of the reactor based on kinetic data from literature. Furthermore, a comparison with integral concentration measurements using a mass spectrometer demonstrated deviations below 5%. The results show that 3D MRSI is a valuable and reliable tool to non-invasively measure spatially resolved process parameters within optically and/or mechanically inaccessible structured monolithic catalyst supports, even if only standard thermal polarization is exploited and the use of expensive and technically challenging signal enhancement techniques (hyperpolarization) is avoided. We expect that 3D MRSI can pave the way toward deeper insight into the interactions between catalyst, catalyst support, and gas phase.
Multicomponent gas diffusion in nonuniform tubes
Veltzke, Thomas ; Kiewidt, Lars ; Thöming, Jorg - \ 2015
AIChE Journal 61 (2015)4. - ISSN 0001-1541 - p. 1404 - 1412.
Analytical transport model - Classical Maxwell-Stefan equations - Experiments on conical tubes - Gas multicomponent diffusion - Two-bulb diffusion experiment
In many technical processes gas, multicomponent diffusion takes place in confinements that are rarely uniform in direction of their long axis (e.g., catalysts pores). Here, we show that in conical tubes multicomponent diffusion is hindered. This effect increases with ratio of inlet to outlet cone radius Λ, indifferent of the orientation of the tube. Based on the Maxwell-Stefan equations, predictive analytical solution for ideal multicomponent diffusion in slightly tapered ducts is developed. In two-bulb diffusion experiments on a uniform tube, the results of Duncan and Toor (1962) were reproduced. Comparison of model and experiment shows that the solution presented here provides a reliable quantitative prediction of the temporal change of H2, N2, and CO2-concentration for both tube geometries, uniform and slightly conical. In the demonstrated case (Λ=3.16), mass diffusion is 68% delayed. Thus, for gaseous diffusion in "real," typically tapered pores the transport limitation is more serious than considered so far.
Predicting optimal temperature profiles in single-stage fixed-bed reactors for CO2-methanation
Kiewidt, Lars ; Thöming, Jorg - \ 2015
Chemical Engineering Science 132 (2015). - ISSN 0009-2509 - p. 59 - 71.
Chemical energy storage - Methanation - Process intensification - Semenov number - Thermal optimization
The catalytic conversion of carbon dioxide into methane, known as Sabatier process, is a promising option for chemical storage of excess renewable energy and greenhouse gas emission control. Typically externally cooled fixed-bed reactors (FBR) using supported nickel or ruthenium catalyst are applied. The Sabatier process, however, is strongly exothermic and leads to substantial hot spots within the reactor at stoichiometric feed ratios. Although high temperatures increase the reaction rate in general, they thermodynamically limit the achievable methane-yield in the Sabatier process. Here, we present an easy-to-use method based on a Semenov number optimization (SNO) to compute optimal axial temperature profiles in single-stage fixed-bed reactors that account for kinetic and thermodynamic limitations simultaneously, and thus result in maximized yield for a fixed reactor length. In a case study on CO2-methanation, these temperature profiles result in a twofold improvement of the methane-yield compared to isothermal and adiabatic operation, and thus demonstrate the high potential of thermal optimization that lies in the Sabatier process. The SNO-method provides a valuable tool to compute optimal temperature profiles, and allows intuitive insight into the key parameters for thermal process intensification. Further, it can readily be transferred to other processes that suffer from the dilemma between kinetic and thermodynamic limitations. Our findings illustrate the attractiveness of the SNO-method to compute optimal temperature profiles in fixed-bed reactors, and the need for catalyst supports with enhanced and tailorable heat transport properties.