Novel mechanistic view of catalytic ozonation of gaseous toluene by dual-site kinetic modelling

The catalytic ozonation of VOCs is a promising approach for degradation of indoor VOCs, such as gaseous toluene. However, the mechanism and relevant kinetic steps involved in this reaction remain unclear. In this study, the catalytic ozonation of toluene over MnO2/graphene was investigated using the empirical power law model and classic Langmuir-Hinshelwood single-site (denoted as L-Hs) mechanism. The apparent activation energy determined using the power law model was 29.3±2.5 kJ mol−1. This finding indicated that the catalytic ozonation of toluene over MnO2/graphene was a heterogeneous reaction, and the Langmuir-Hinshelwood mechanism was applicable. However, the L-Hs mechanism did not fit the experimental data, suggesting that the reaction was non-single-site governed. A novel Langmuir-Hinshelwood dual-site (denoted as L-Hd) mechanism was then proposed to explain the experimental observations of the catalytic ozonation of toluene over MnO2/graphene through a steady-state kinetic study. This mechanism was based on the hypothesis that MnO2 was responsible for ozone decomposition and toluene adsorption on graphene; these two types of adsorption were coupled by an adjacent attack. Furthermore, XPS results confirmed the presence of a strong connection between MnO2 and graphene sites on the surface of MnO2/graphene. This connection allowed the adjacent attack and validated the dual-site mechanism. The L-Hd model was consistent with the predicted reaction rate of toluene removal with a correlation coefficient near unity (r2 = 0.9165). Moreover, the physical criterion was in accordance with both enthalpy and entropy of toluene adsorption constraints. Fulfillment of mathematical and physical criteria indicated the catalytic ozonation of toluene over MnO2/graphene can be well described by the L-Hd mechanism. This study helps understand the catalytic ozonation of toluene over MnO2/graphene in a closely mechanistic view

Dry (CO2) reforming of propane over bimetallic Mo-Ni/Al2O3 catalyst: catalyst synthesis and reactor operation

Hydrocarbon reforming using CO2, a greenhouse gas, as a feedstock has attracted increasing attention due to the benefits of chemical valorisation of natural gas and CO2, which have an adverse impact on the environment. For hydrocarbon dry reforming, where the product stream H2: CO ratio is less than 3, synfuel production is more amenable for downstream methanol and other oxygenated synthesis. Dry reforming of propane has attracted much interest because of the associated lower reforming temperature and lower vapour pressure of propane compared with methane at ambient temperature, which makes it more favourable for fuel cell cars with internal reforming. However, the formation of carbon on Ni catalysts is well known, thus the addition of Mo to Ni would help in mitigating carbon deposition through possible conversion of the Mo oxide to a Mo carbide phase. Several authors reported high activity, stability and carbon resilience of the Mo-Ni catalyst during methane steam and dry reforming. Furthermore, it has been shown that potassium promotion also retards the nucleation of carbon. As a result, the present project investigates both reactor operation and the performance of Mo-Ni/Al2O3 catalyst as well as the effect of K-promotion on propane dry reforming at temperatures of 773 -973 K .The catalysts were prepared using wetness co-impregnation method. Alumina supported bimetallic 5(wt%)Mo-10(wt%)Ni was doped with 2.5 (wt%) K. Various characterization techniques were employed to measure the physicochemical properties of the catalysts. Specifically, N2-physisorption, H2-chemisorption, temperature-programmed calcination (TPC), temperature-programmed reduction (TPR), NH3 and CO2- temperature-programmed desorption (TPD), X-ray diffraction (XRD) and Total organic carbon (TOC) analyses were performed. K-promotion enhanced the BET surface area, pore volume, metal dispersion and metal surface area. XRD analysis of calcined catalysts confirmed the presence of metal oxides. TPD experiments revealed the acid : basic site ratio of 8.3 for Mo-Ni , while K-promotion decreased the value to 7.5 ,suggesting that basicity of the catalyst was improved by K addition.Catalytic reaction studies were carried out in a stainless-steel quartz fixed-bed reactor (ID = 15 mm ID) co-axially placed within a temperature-controlled tubular furnace and loaded with 0.5g of catalyst. Both Mo-Ni and K-containing catalysts were found to be promising for dry reforming of propane due to their high activity and stability under different operating conditions

Prediction of solid formation conditions in mixed refrigerants with iso-pentane and methane at high pressures and cryogenic temperatures

High boiling-point components in mixed refrigerants can improve the performance of natural gas and hydrogen liquefaction facilities. However, such heavy compounds can freeze out from the refrigerant mixture, posing blockage and plant shutdown risks for cryogenic heat exchangers. To improve the predictions of these conditions, freezing and melting temperatures of pure iso-pentane and (methane + iso-pentane) binary systems were measured at temperatures down to 87.5 K and pressures up to 13 MPa. The iso-pentane melting data are compared with predictions of a thermodynamic model embedded in the ThermoFAST software package. Adjusting the model's fusion molar volume change parameter to force agreement with the measurements reduced the deviations of the experimental data from the model by over 90% relative to the default parameter value. The measured melting data for binary mixtures were used to confirm solubility predictions for iso-pentane in mixed refrigerants. Adding 20 mol% iso-pentane to a methane-rich refrigerant increases the available duty for cooling natural gas or hydrogen from (313–123) K by a factor of three. This improvement outweighs the risk of freeze-out in this refrigerant with the melting temperature being 98 K, which is 15 K lower than the minimum temperature needed for LNG production

Natural gas density measurements and the impact of accuracy on process design

The liquefaction of natural gas is an energy intensive process, requiring at least 5% of the energy associated with methane's lower heating value. Key to estimating and optimizing these energy requirements are process simulations which rely upon calculated thermophysical properties of the natural gas. In particular, the prediction of thermophysical properties of natural gas mixtures at pressure-temperature conditions close to the mixture's critical point or cricondenbar is challenging but important as often natural gas processes operate close to these conditions. In this work, we present a comprehensive study of two natural gas related systems: (CH4 + C3H8 + CO2) and (CH4 + C3H8 + C7H16) with n-heptane fractions up to 15 mol%. High accuracy measurements of densities, at temperatures from 200 K to 423 K and pressures up to 35 MPa are presented. The extensive experimental data collected for these mixtures were compared with the GERG-2008 equation of state, as implemented in the NIST software REFPROP. The relative deviations of the measured densities from those calculated using the GERG-2008 model range between (−2 to 4)% for all mixtures, presenting a systematic dependent on mixture density and n-heptane content. Finally, a case study is presented that probes the impact of the accuracy of density on the pinch point in a simulated LNG heat exchanger. An uncertainty in the density of 1% is shown to cause significant 30% reduction in the minimum approach temperature difference, suggesting that accurate thermophysical property calculations are key to reducing over-design of processing plant

Modelling of Liquid Hydrogen Boil-Off

A model has been developed and implemented in the software package BoilFAST that allows for reliable calculations of the self-pressurization and boil-off losses for liquid hydrogen in different tank geometries and thermal insulation systems. The model accounts for the heat transfer from the vapor to the liquid phase, incorporates realistic heat transfer mechanisms, and uses reference equations of state to calculate thermodynamic properties. The model is validated by testing against a variety of scenarios using multiple sets of industrially relevant data for liquid hydrogen (LH2), including self-pressurization and densification data obtained from an LH2 storage tank at NASA’s Kennedy Space Centre. The model exhibits excellent agreement with experimental and industrial data across a range of simulated conditions, including zero boil-off in microgravity environments, self-pressurization of a stored mass of LH2, and boil-off from a previously pressurized tank as it is being relieved of vapor

Phase equilibrium studies of high-pressure natural gas mixtures with toluene for LNG applications

© 2020 Elsevier B.V. To prevent possible freeze out in the main cryogenic heat exchanger (MCHE) used in liquefied natural gas (LNG) plants, new and accurate phase equilibrium data are required to improve the predictive reliability of existing models, in particular cubic equations of state (EOS). In this work, the vapor-liquid equilibrium (VLE) of a ternary methane + propane + toluene (methylbenzene) mixture was studied over a wide range of conditions with toluene as the minor component in both the liquid and vapor phases. Measurements were conducted along different isochoric paths at temperatures between (213 and 298 K) and pressures up to 8.3 MPa, to obtain data at conditions relevant to the operation of LNG scrub columns. The measured VLE data were compared to results calculated with the HYSYS Peng Robinson (PR) equation of state (EOS) that is used widely in LNG industry. The amount of toluene in the vapor phase was found to be under-predicted by the HYSYS PR EOS by an average of around 77% at lower temperatures, with the error increasing as temperature and toluene concentration decreased. The current work demonstrates that the HYSYS PR EOS as well as other cubic EOS substantially under-predict the possible toluene content of saturated vapours that could be present in the overhead of the LNG scrub column. Using the ThermoFAST model recently developed and optimised for the calculation of solid-liquid equilibrium conditions in LNG production, this work further demonstrates that the 77% increase in the toluene content of a saturated vapor entering the MCHE, corresponds to a 7 K increase in the solid formation temperature, which could significantly increase the likelihood of a blockage in the MCHE and thus possible shutdown of the LNG plant. The experimental and modelling work presented here underscores the importance of improving predictions of the allowable threshold concentration of heavy components in fluids entering cryogenic heat exchangers in LNG plants

Hydrogen liquefaction: a review of the fundamental physics, engineering practice and future opportunities

Hydrogen is emerging as one of the most promising energy carriers for a decarbonised global energy system. Transportation and storage of hydrogen are critical to its large-scale adoption and to these ends liquid hydrogen is being widely considered. The liquefaction and storage processes must, however, be both safe and efficient for liquid hydrogen to be viable as an energy carrier. Identifying the most promising liquefaction processes and associated transport and storage technologies is therefore crucial; these need to be considered in terms of a range of interconnected parameters ranging from energy consumption and appropriate materials usage to considerations of unique liquid-hydrogen physics (in the form of ortho–para hydrogen conversion) and boil-off gas handling. This study presents the current state of liquid hydrogen technology across the entire value chain whilst detailing both the relevant underpinning science (e.g. the quantum behaviour of hydrogen at cryogenic temperatures) and current liquefaction process routes including relevant unit operation design and efficiency. Cognisant of the challenges associated with a projected hydrogen liquefaction plant capacity scale-up from the current 32 tonnes per day to greater than 100 tonnes per day to meet projected hydrogen demand, this study also reflects on the next-generation of liquid-hydrogen technologies and the scientific research and development priorities needed to enable them