ACS Engineering Au最新文献

In an effort to estimate the feasibility of heat recuperation from an internal combustion engine (ICE) by steam reforming (SR) or by decomposition of the fuel, we study here the required size of a reformer heat exchanger in order to power a 3.7 kW engine. To that end, we experimentally test the heat transfer in a structured commercial reactor with ∼0.39 m² of heat transfer area in an ∼1 L unit. We then simulate the required length for evaporation and reforming of several fuels, using published kinetics with a highly active catalyst, under a fixed exhaust temperature of 973 K, and study the effect of pressure and steam-to-fuel ratio. Both co- and counter-current schemes are considered. Methanol decomposition is probably the best solution from the energy point of view. However, it is known to lead to deactivation. Methanol SR (with S/M = 1) requires about 2 L of reformer-HE and seems to be a reasonable solution, yielding a chemical energy gain of ∼16%, a value close to the asymptotic thermodynamic value. Moreover, the presence of CO₂ in the reformate is known to mitigate to NO_x emissions down to zero-impact levels. Ethanol SR (with S/E = 1 or 3) yields poor results since CH₄ is an intermediate, which requires high temperatures for reforming; operating ESR requires exhaust temperatures of ∼1250 °K or higher. While such high temperatures may be attained and may yield an energetic gain of more than 20%, it will require modification of the process. Methylal SR (S/MA = 1) yields good results as well.

The by-products generated from the whiskey distillation process consist of organic liquids with a high chemical oxygen demand (COD) and residues with a high solid content. Low-carbon strategies that repurpose and valorize such by-products are now imperative to reduce the carbon footprint of the food and beverage industries. The operation of a two-phase anaerobic digester to produce volatile fatty acids (VFAs) and biogas may enable distilleries to transition toward a low-carbon bioeconomy. An example of such a system is a leach bed reactor connected to an expanded granular sludge bed (LBR-EGSB) which was designed, commissioned, and conceptually validated in this paper. Several design improvements progress the LBR-EGSB beyond previous reactor designs. An external gas–liquid–solid separator in the EGSB was used to capture any residual gases produced by the effluent and may reduce the amount of methane slippage and biomass washout. The implementation of a siphon-actuated leachate cup is a low-cost alternative that is less prone to actuation malfunction as compared to electrically actuated solenoid valves in previous reactor designs. Furthermore, replacing fresh water with distillery’s liquid by-products as leachate promotes a circular repurpose and reuse philosophy. The system proved to be effective in generating VFAs (10.3 g VFAs L^–1_Leachate), in EGSB COD removal (96%), and in producing methane-rich biogas (75%_vol), which is higher than the values achieved by traditional anaerobic digestion systems. The LBR-EGSB could ultimately provide more by-product valorization and decarbonization opportunities than traditional anaerobic digestion systems for a whiskey distillery.

Froth flotation is the most versatile process in mineral beneficiation, extensively used to concentrate a wide range of minerals. This process comprises mixtures of more or less liberated minerals, water, air, and various chemical reagents, involving a series of intermingled multiphase physical and chemical phenomena in the aqueous environment. Today’s main challenge facing the froth flotation process is to gain atomic-level insights into the properties of its inherent phenomena governing the process performance. While it is often challenging to determine these phenomena via trial-and-error experimentations, molecular modeling approaches not only elicit a deeper understanding of froth flotation but can also assist experimental studies in saving time and budget. Thanks to the rapid development of computer science and advances in high-performance computing (HPC) infrastructures, theoretical/computational chemistry has now matured enough to successfully and gainfully apply to tackle the challenges of complex systems. In mineral processing, however, advanced applications of computational chemistry are increasingly gaining ground and demonstrating merit in addressing these challenges. Accordingly, this contribution aims to encourage mineral scientists, especially those interested in rational reagent design, to become familiarized with the necessary concepts of molecular modeling and to apply similar strategies when studying and tailoring properties at the molecular level. This review also strives to deliver the state-of-the-art integration and application of molecular modeling in froth flotation studies to assist either active researchers in this field to disclose new directions for future research or newcomers to the field to initiate innovative works.

We report the investigation of a chalcopyrite leaching process that implements millimeter-sized glass beads that are stirred in the leach reactor to combine particle grinding, mechanical activation, and surface removal of reaction products. The paper focuses on demonstrating the impact of the so-called attrition-leaching phenomenon on the leaching rate of a chalcopyrite concentrate and provides a first understanding of the underlying mechanisms. For this purpose, we have compared the copper leaching yield for different configurations under controlled chemical conditions (1 kg of glass beads and 84 g of chalcopyrite concentrate in 2.5 L of H₂SO₄-H₂O solution, pH = 1.3, E_h = 700 mV vs SHE, and T = 42 °C). On top of elemental analysis of the leach solution with time, we provide a full characterization of the solid residue based on X-ray diffraction, elemental analysis, and sulfur speciation. We demonstrate that glass beads led to a remarkable enhancement of the leaching rate in conditions where particles were already passivated by simple leaching and even when large amounts of solid products (elemental sulfur and jarosite) were present. An in-depth evaluation of particle size distribution showed that particle breakage occurred during a rather short time (a few hours) at the beginning of the runs, transforming the initial particles with d_4/3 = 30 μm to finer particles with d_4/3 = 15 μm. Then, particle breakage almost stopped, while an attrition phenomenon was evidenced, inducing the formation of very fine particles (<1 μm) and aggregates concomitantly with copper leaching.

A novel chemical looping (CL) process was demonstrated to produce acetaldehyde (AA) via oxidative dehydrogenation (ODH) of ethanol. Here, the ODH of ethanol takes place in the absence of a gaseous oxygen stream; instead, oxygen is supplied from a metal oxide, an active support for an ODH catalyst. The support material reduces as the reaction takes place and needs to be regenerated in air in a separate step, resulting in a CL process. Here, strontium ferrite perovskite (SrFeO_3−δ) was used as the active support, with both silver and copper as the ODH catalysts. The performance of Ag/SrFeO_3−δ and Cu/SrFeO_3−δ was investigated in a packed bed reactor, operated at temperatures from 200 to 270 ^°C and a gas hourly space velocity of 9600 h^–1. The CL capability to produce AA was then compared to the performance of bare SrFeO_3−δ (no catalysts) and materials comprising a catalyst on an inert support, Cu or Ag on Al₂O₃. The Ag/Al₂O₃ catalyst was completely inactive in the absence of air, confirming that oxygen supplied from the support is required to oxidize ethanol to AA and water, while Cu/Al₂O₃ gradually got covered in coke, indicating cracking of ethanol. The bare SrFeO_3−δ achieved a similar selectivity to AA as Ag/SrFeO_3−δ but at a greatly reduced activity. For the best performing catalyst, Ag/SrFeO_3−δ, the obtained selectivity to AA reached 92–98% at yields of up to 70%, comparable to the incumbent Veba-Chemie process for ethanol ODH, but at around 250 ^°C lower temperature. The CL-ODH setup was operated at high effective production times (i.e., the time spent producing AA to the time spent regenerating SrFeO_3−δ). In the investigated configuration with 2 g of the CLC catalyst and 200 mL/min feed flowrate ∼5.8 vol % ethanol, only three reactors would be required for the pseudo-continuous production of AA via CL-ODH.

Natural gas is expected to be the major energy source in the near future, and storing it in the form of gas hydrate is a safe, clean, and economical approach. However, required thermodynamic conditions and slow kinetics are the key challenges that need to address for process viability. This study involves an experimental investigation of methane and cyclooctane sH hydrate formation for possible applications in gas storage using thermodynamics, kinetics, morphology, and Raman analysis. The hydrate formation is carried out at such thermodynamic conditions where only sH hydrate would form. The four-phase (L_w-L_HC-H-V) sH hydrate equilibrium is studied for the methane and cyclooctane system via dissociation along the phase boundary method which is a robust method as it delivers a greater number of equilibrium data points in a single experimental run compared to other available methods. The sH hydrate formation helps in lowering the equilibrium conditions compared with sI hydrate formation. The slow sH hydrate formation kinetics can be improved by using low tryptophan concentrations. In this work, 0.1 wt % is the optimum tryptophan concentration as the gas uptake, and the hydrate formation rate is found to be the highest compared to 0.01, 0.05, and 1 wt % tryptophan concentrations. Here, we also visually investigate the sH hydrate formation and observed that the hydrate formation occurs below the interface for the system with no tryptophan; however, hydrate formation occurrence above the interface increases with an increase in the tryptophan concentration. The increase in the hydrate formation could be dedicated to the increased gas uptake due to the increasingly porous nature of hydrate formation. The Raman analysis confirmed the presence of methane and cyclooctane in sH hydrate cages. The higher intensity of the peaks using tryptophan additionally confirms the higher hydrate formation compared to the system with no tryptophan.

Solid flow in a Geldart’s group B circulating fluidized bed (CFB) riser is complex, and it exhibits backflow and recirculation in the riser. A single radioactive tracer particle is used to measure the overall and sectional residence time distribution in a CFB riser at a gas velocity of 7.6–9.2 m/s and a solid flux of 100–200 kg/m²s. At the same time, radioactive particle tracking (RPT) data are used to measure the trajectories of the tracer particle and its length distribution at the bottom and middle sections of the riser. Both residence time distribution (RTD) and trajectory length distribution data obtained from RPT and RTD experiments are processed and compared. Results show that the bottom section has higher back mixing than the middle section. The results also show that back mixing in both the sections reduces with an increase in the gas inlet velocity and reduces marginally with an increase in the solid flux. Results confirm that RPT and RTD data are highly correlated and can be used with the same accuracy to quantify the macromixing behavior of any process vessel/reactor.

Atmospheric carbon poses an existential threat to civilization via global climate change. Hundreds of gigatonnes of carbon dioxide must be removed from earth’s atmosphere in the next three decades, necessitating a low-cost, energy-efficient process to extract low concentrations of carbon dioxide for conversion to a stable material permanently stored for thousands of years. In this work, the challenge of removing gigatonnes of CO₂ is described via the scale of effort and the thermodynamics of collecting and reducing this diffuse chemical, the accumulation of which imparts a substantial entropy penalty on any atmospheric carbon capture process. The methods of CO₂ reduction combined with upstream direct air capture (DAC) including absorption, membrane separation, and adsorption are compared with biomass torrefaction and permanent burial (BTB). A Monte Carlo model assesses the mass, energy, and economics of the full process of biomass torrefaction from biomass collection and transport to stable carbon burial to determine that 95% of scenarios could remove carbon for less than $200 per CO₂-tonne-equivalent. Torrefied carbon is further discussed for its long-term stability and availability at the scale required to substantially mitigate the threat of climate change.

A unconventional stirred tank of geometry typically adopted for the production of biogas is experimentally investigated with pseudo-plastic model fluids. The apparent viscosities of the fluids, based on the Metzner–Otto method, are in the range of 39–264 mPa·s, resulting in a range of rotational Reynolds number equal to 17–648. The power consumption of the three top-entering agitators is measured by a strain gauge technique, and the power number curve is obtained in the full range of flow regimes, going from laminar to fully turbulent conditions. The flow field measured by particle image velocimetry allows us to observe the fluid circulation patterns and their variations in different operative conditions. The measurements reveal relatively low axial and radial velocities, especially toward the bottom of the tank, that may hinder solid feedstock suspension and subsequent biogas production. Significant changes in the flow patterns are observed with small variations in the impeller speed and the mixture viscosity. The homogenization dynamics of a tracer obtained by planar laser-induced fluorescence leads us to estimate the dimensionless mixing time, a trend similar to that observed for conventional stirred vessel geometries. The detailed fluid dynamics information collected by a combination of different techniques can contribute to optimize the energy requirement and to avoid failure of the biogas production due to poor fluid mixing.