Adoption of 3D printed highly conductive periodic open cellular structures as an effective solution to enhance the heat transfer performances of compact Fischer-Tropsch fixed-bed reactors

Abstract Heat transfer is universally recognized as a key challenge for the intensification of the Fischer-Tropsch (FT) process in compact fixed-bed reactors. For the first time in the scientific literature we demonstrate experimentally that the adoption of a highly conductive periodic open cellular structure (POCS, 3D-printed in AlSi7Mg0.6 by Selective Laser Melting) packed with catalysts pellets is a promising solution to boost heat exchange in fixed-bed FT reactors. This reactor configuration enabled us to assess the performances of a highly active Co/Pt/Al2O3 catalyst packed into the POCS at process conditions relevant to industrial Fischer-Tropsch operation. Unprecedented performances (CO conversion ≈ 80%) could be thus achieved thanks to an outstanding heat management. In fact, almost flat axial and radial temperature profiles were measured along the catalytic bed even under the most severe process conditions (i.e. high CO conversions corresponding to high volumetric heat duties), demonstrating the effective potential of this reactor concept to manage the strong exothermicity of the FT reaction. The heat transfer of the packed-POCS reactor outperformed both packed-bed and packed-foam reactors, granting smaller radial temperature gradients in the catalytic bed, as well as smaller temperature differences at the reactor wall, with larger volumetric power releases. The strengths of the packed-POCS reactor configuration are its regular geometry, which enhances the effective radial thermal conductivity, and the improved contact between the structure and the reactor wall, which governs the limiting wall heat transfer coefficient

Storage Material Effects on the Performance of Ru-Based CO2 Capture and Methanation Dual Functioning Materials

In this study, a systematic investigation on Dual Functioning Materials (DFMs) for the capture and methanation of CO₂ is carried out. The attention is focused on the nature of the CO₂ adsorbent component (storage material, SM) varying between alkaline (Li, Na, K) and alkaline-earth (Mg, Ca, Ba) metal oxides in combination with Ru, both supported on an Al₂O₃ support. Combining gas phase reactivity analysis and FT-IR characterization, the samples are characterized in terms of CO₂ storage capacity. It is found that all the SM-containing samples adsorb significant amounts of CO₂ as carbonate species, with the higher amounts being adsorbed when the more thermally stable species are formed, i.e., when Ca, Ba, or K are employed as SMs. In all cases, the hydrogenation of the adsorbed carbonates to CH₄ occurs at lower temperature, if compared to their thermal desorption. However, in the case of Ca- and Ba-based DFMs, resilient carbonates are present on the material surface. It was found that the SMs able to form the more thermally stable carbonates upon CO₂ adsorption also showed the best performances in capture/methanation cycles at 350 °C, even if some residual carbonates were left on the DFM after the hydrogenation step. In particular, the following order of reactivity has in fact been observed in terms of CH₄ production: Ru–K ≥ Ru–Ba > Ru–Ca > Ru–Na ≫ Ru–Mg ≅ Ru–Li ≅ Ru. The presence of steam and O₂ during the capture step has a detrimental effect on the CO₂ adsorption for all samples and, as a result, on CH₄ production due to the competition of CO₂ and water for the same adsorption sites. Thus, only SMs able to form strongly bound carbonates species upon CO₂ exposure can retain significant CO₂ storage capacity also in the presence of water in the adsorption feed

Ceria-based catalysts for NOx removal in NSR processes: A fundamental study of the catalyst modifications explored by in situ techniques

In this work, a fundamental and systematic study was conducted, leading to a better understanding of the phenomena occurring on the catalyst’s surface during the NOx reduction process in NSR systems. For this purpose, ceria-based catalysts, with Cu in substitution of noble metal, have been synthesized and deeply characterized by means of XRF, XPS, in situ (XRD, Raman spectroscopy and DRIFTS), temperature-programmed reduction under H2 (H2-TPR) and under NO reaction (NO isothermal reaction + NO-TPR). The whole results show the key role of copper to promote the reducibility and the creation of oxygen vacancies, allowing a high NO consumption and fast kinetics of N2O and N2 formation, until the oxygen vacancies consumption takes place. The study of the surface reactions taking place in the formation of adsorbed NOx species and the oxygen vacancies consumption with NO uptake is complex; however, a hydroxyl consumption route is found to be involved. The reduction of NO provided higher levels of N2 at higher temperatures; also, a very high efficiency of the previously created oxygen vacancies was found for this process.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Science and Innovation (PID2019-105542RB-I00 project) and the UE-FEDER funding. Martínez-Munuera also acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603)

Silver-based catalytic materials for the simultaneous removal of soot and NOx

The potential of silver-based catalysts in the simultaneous removal of particulate matter (soot) and NOxis investigated in this work and compared with that of a model Pt-Ba/Al2O3catalyst. The Ag (5 wt%) - Ba (10 wt%)/MO (MO = CeO2, ZrO2, Al2O3) and Ag (5 wt%) - Sr (10 wt%)/CeO2catalysts have been prepared by incipient wetness impregnation and characterized by BET, XRD, HRTEM, XPS and temperature-programmed reduction (TPR) experiments. The behavior of the catalyst in the soot combustion (under loose conditions) and NOxremoval has been separately analyzed by means of temperature programmed oxidation (TPO) and isothermal concentration step change (ICSC) experiments, respectively. The results show that all the catalysts are active in soot combustion with a significant decrease of oxidation onset temperature compared to uncatalyzed soot oxidation. The removal of NOxin the absence and in the presence of soot was investigated under cycling conditions, i.e. alternating lean-rich phases according to the LNT strategy. It has been found that the Ag-based samples are able to simultaneously remove soot and NOx. In particular, comparing the behavior of the prepared catalysts, the Ba-containing systems showed higher NOxstorage capacity than Sr-catalyst; also, the nitrogen selectivity increased even if resulted lower than the traditional LNT Pt-based catalyst. A detrimental effect of soot on the NOxstorage activity has been also observed.Postprint (author's final draft

Design and commissioning of a thermal stability test-rig for mixtures as working fluids for ORC applications

A novel test-rig for studying the thermal stability of mixtures as working fluids for ORC applications was designed and commissioned at the Laboratory of Compressible-fluid dynamics for Renewable Energy Applications (CREA) of Politecnico di Milano, in collaboration with the University of Brescia. The set-up is a standard one, in which a vessel containing the fluid under scrutiny is placed in a vertical oven for ~ 100 hours at a constant temperature T = Tstress. During the test, the pressure P is monitored to detect thermal decomposition of the fluid. After the test, the vessel is placed in a controlled thermal bath, where the pressure is measured at different value of the temperature T, with T < Tstress and T < Tc (Tc critical temperature). The resulting isochoric pressure-temperature dependence is compared to that obtained before the fluid underwent thermal stress. If departure from the initial fluid behavior is observed, significant thermal decomposition occurred and a chemical analysis of the decomposition products is carried out using gas chromatography and mass spectroscopy. The novelty of the set-up is the possibility of taking samples of both liquid and vapor phases of the fluid, a capability that was introduced to study thermal decomposition of mixtures, whose composition depends on the pressure and temperature, as well as to capture the more volatile products of thermal decomposition of pure fluids and mixtures. Preliminary experimental results are reported for the pure siloxane fluid MDM (Octamethyltrisiloxane, C8H24O2Si3)

Intensifying heat transfer in Fischer-Tropsch tubular reactors through the adoption of conductive packed foams

open5openFratalocchi, Laura; Visconti, Carlo Giorgio; Groppi, Gianpiero; Lietti, Luca; Tronconi, EnricoFratalocchi, Laura; Visconti, Carlo Giorgio; Groppi, Gianpiero; Lietti, Luca; Tronconi, Enric

Study of N2O formation over Rh- and Pt-based LNT catalysts

In this paper, mechanistic aspects involved in the formation of N2O over Pt-BaO/Al2O3 and Rh-BaO/Al2O3 model NOx Storage-Reduction (NSR) catalysts are discussed. The reactivity of both gas-phase NO and stored nitrates was investigated by using H2 and NH3 as reductants. It was found that N2O formation involves the presence of gas-phase NO, since no N2O is observed upon the reduction of nitrates stored over both Pt- and Rh-BaO/Al2O3 catalyst samples. In particular, N2O formation involves the coupling of undissociated NO molecules with N-adspecies formed upon NO dissociation onto reduced Platinum-Group-Metal (PGM) sites. Accordingly, N2O formation is observed at low temperatures, when PGM sites start to be reduced, and disappears at high temperatures where PGM sites are fully reduced and complete NO dissociation takes place. Besides, N2O formation is observed at lower temperatures with H2 than with NH3 in view of the higher reactivity of hydrogen in the reduction of the PGM sites and onto Pt-containing catalyst due to the higher reducibility of Pt vs. Rh