1,720,977 research outputs found
Methodologies for the design of solar receiver/reactors for thermochemical hydrogen production
Full text linkThermochemical hydrogen production is of great interest due to the potential for significantly reducing the dependence on fossil fuels as energy carriers. In a solar plant, the solar receiver is the unit in which solar energy is absorbed by a fluid and/or solid particles and converted into thermal energy. When the solar energy is used to drive a reaction, the receiver is also a reactor. The wide variety of thermochemical processes, and therefore of operating conditions, along with the technical requirements of coupling the receiver with the concentrating system have led to the development of numerous reactor configurations. The scope of this work is to identify general guidelines for the design of solar reactors/receivers. To do so, an overview is initially presented of solar receiver/reactor designs proposed in the literature for different applications. The main challenges of modeling these systems are then outlined. Finally, selected examples are discussed in greater detail to highlight the methodology through which the design of solar reactors can be optimized. It is found that the parameters most commonly employed to describe the performance of such a reactor are (i) energy conversion efficiency, (ii) energy losses associated with process irreversibilities, and (iii) thermo-mechanical stresses. The general choice of reactor design depends mainly on the type of reaction. The optimization procedure can then be carried out by acting on (i) the receiver shape and dimensions, (ii) the mode of reactant feed, and (iii) the particle morphology, in the case of solid reactants
Wall heat transfer coefficient and effective radial conductivity of ceramic foam catalyst supports
No full textIn this work, different tubular solid-foam packed-beds were experimentally investigated under air flow in order to determine the effective conductivity and wall-to-bed heat transfer coefficient. Several foam types with different material, namely silicon carbide and alumina, and cell density were tested. Foam samples were experimentally studied using different air flow rates and wall temperatures ranging between 200 and 500 °C. Axial and radial steady-state temperatures in the bed were experimentally measured and analysed with a two-dimensional, one-equation model. A Chilton–Colburn-type correlation based on the reactor diameter as characteristic dimension was proposed for the wall-to-bed heat transfer coefficient, which proved good to describe the behaviour of all tested foams. Effective radial thermal conductivity and pressure drop in the foams were also characterized
Toward minimal complexity models of membrane reactors for hydrogen production
Full text linkMembrane reactors are inherently two-dimensional systems that require complex models for an accurate description of the different transport phenomena involved. However, when their performance is limited by mass transport within the reactor rather than by the selective product permeation across the membrane, the 2D model may be significantly simplified. Here we extend results previously found for methane steam reforming membrane reactors to show that such simplified two-dimensional model admits either a straightforward analytical solution for the cross-section averaged concentration profile, or can be reduced to a 1D model with an enhanced Sherwood number, depending on the stoichiometry of the reaction considered. Interestingly, the stoichiometry does not affect the expression of the enhanced Sherwood number, indicating that a versatile tool has been developed for the determination of membrane reactor performance at an extremely low computational cost and good degree of accuracy
A discussion of possible approaches to the integration of thermochemical storage systems in concentrating solar power plants
Full text linkOne of the most interesting perspectives for the development of concentrated solar power (CSP) is the storage of solar energy on a seasonal basis, intending to exploit the summer solar radiation in excess and use it in the winter months, thus stabilizing the yearly production and increasing the capacity factor of the plant. By using materials subject to reversible chemical reactions, and thus storing the thermal energy in the form of chemical energy, thermochemical storage systems can potentially serve to this purpose. The present work focuses on the identification of possible integration solutions between CSP plants and thermochemical systems for long-term energy storage, particularly for high-temperature systems such as central receiver plants. The analysis is restricted to storage systems potentially compatible with temperatures ranging from 700 to 1000 ◦C and using gases as heat transfer fluids. On the basis of the solar plant specifications, suitable reactive systems are identified and the process interfaces for the integration of solar plant/storage system/power block are discussed. The main operating conditions of the storage unit are defined for each considered case through process simulation
A hybrid numerical approach for predicting mixing length and mixing time in microfluidic junctions from moderate to arbitrarily large values of the Péclet number
No full textWe investigate numerically the homogenization process of a diffusive species in a mixing channel of arbitrary length downstream a microfluidic cross-junction. The channel length, λα, necessary to achieve a prescribed level of mixedness, α, is targeted as primary quantity of interest, and its dependence on the Reynolds number, Re, on the flow ratio between the impinging currents, R, and on the Schmidt number of the solute, Sc, is analyzed. The accurate numerical solution of the mass transport equations up to values Pe=ReSc≃106 of the Péclet number is here made possible by a hybrid numerical approach. This approach combines a recently proposed Monte Carlo method, enforced near the impinging zone, with a pseudo-transient 2D formulation of mass transport in the mixing channel, where purely axial (Poiseuille) flow settles in. At values of the flow ratio significantly different from unity (e.g. those used in flash nanoprecipitation) a non-trivial dependence of λα on Re is found at fixed Sc and R. This result is interpreted based on the spectral (eigenvalue/eigenfunction) structure of the 2D generalized Sturm-Liouville pure diffusion problem defined onto the cross-section of the mixing channel. Hinging on this interpretation, we show that for Sc⩾103 the dependence of λα on Sc at fixed Re and R can be singled out and theoretically predicted. By this property, universal ready-to-use curves yielding the mixing length for each assigned geometry can be constructed, which could be used to correlate the mixing time with other phenomena (chemical reactions, phase changes) occurring alongside the mass transport
Estimate of the height of molten metal reactors for methane cracking
Full text linkMethane Cracking represents one of the most promising routes to CO2-free hydrogen production.The methane decomposition reaction is typically carried out in fixed or fluidized catalytic beds, where the metal catalyst is supported on porous ceramic particles. By proper choice of the metal catalyst, the catalytic reaction environment allows to obtain sizeable reaction rates at operating temperatures as low as 700°C. Besides, in solid catalytic beds, the catalyst is swiftly deactivated due to the massive (i.e. stoichiometric) deposition of the solid carbon product. One way to bypass carbon deposition is to use a molten metal bath (which may or may not contain catalytic metal components) as a reaction environment, where methane bubbles are introduced at the bottom of the bath and are progressively converted as they rise through the liquid metal. The key point of this process is that, owing to a large density difference between the solid carbon phase and the molten metal, the solid product of the reaction floats on top of the liquid metal and can be thus mechanically skimmed. In this article, we develop an analytical approach to the estimate of the bath height, which constitutes one of the most critical design parameters of the process. Specifically, based on the observation that in practical applications the reacting bubble is in the kinetics-controlled regime, we obtain the conversion vs time solution for a bubble of given initial size. On the assumption of ideal gaseous mixture behaviour, the knowledge of the conversion curves allows to estimate the bubble diameter as a function of time during the rise of the bubble through the molten metal. This piece of information is then post-processed to obtain the bubble motion as a function of time. The elimination of the time parameter between the two solutions allows to construct a conversion-height map for different diameters of the bubbles
Two-dimensional modeling and experimental investigation of an inverse molten carbonate fuel cell
No full textMolten carbonate electrolyzers (MCECs) represent an innovative method for the conversion of electrical energy into chemical energy, coupling the advantages of both low- and high-temperature electrolysis processes. In the present work, a planar MCEC was experimentally tested under different temperature, gas flow rate and composition conditions. A 2D model was developed for the first time and validated against experimental data. The model was found to accurately describe the behavior of the cell, both in terms of the relationship between applied voltage and flowing current and of product gas composition. The model was then used to predict thermal effects in case of adiabatic operation of the cell, showing that, in the absence of temperature control, the cell temperature could increase significantly and that the presence of thermochemical reactions alongside the electrochemical processes could significantly affect the behavior of the cell
Biogas upgrading through CO2 removal by chemical absorption in an amine organic solution. Physical and technical assessment, simulation and experimental validation
No full textAn experimental and modelling study of CO2 removal from a simulated biogas feed by chemical absorption in an organic solution of 2-amine-2-methyl-1-propanol in an ethylene glycol and n-propanol solvent is presented. Absorption was carried out under different temperature, feed flow rate, and feed recirculation conditions. Regeneration was carried at different temperatures. Cyclability tests showed that the absorption capacity remained stable starting from the fourth cycle. In the conditions analyzed, higher temperatures and liquid recirculation favor absorption. With all other conditions constant, the CO2 absorption efficiency increased from 72% to 87% when the temperature increased from 23 to 45°C. At 33°C, liquid recirculation enhanced the absorption efficiency from to 93%–97%. A model was developed and validated against experimental results. Absorption and desorption rates are proportional to the carbon dioxide and AMP concentrations and to the alkyl carbonate concentration, respectively. The two rate constants were fitted from the experimental data: their values at 30°C are 0.033 s−1 (kmol/m3)−1 and 1.5 × 10−6 s−1, respectively. The model indicates that the beneficial effect of temperature and liquid recirculation is due to the increased mass transfer coefficient of CO2 from the gas to the liquid solution, which increased from 4 × 10−3 s−1 at 20°C to 1.3 × 10−2 s−1 at 70°C and by a factor of 8.8 as a consequence of feed recirculation. An increase in the biogas flow rate reduced the absorption efficiency by decreasing the contact time. Higher temperatures also increased the rate of CO2 desorption
Power to liquid through waste as a carbon source. A technical and economic assessment
No full textThe combination of a power to methanol process with the use of waste for carbon supply turns out to be a cleaver solution for sustainable chemical production and waste management as well as long-term energy storage. In the current work, an innovative scheme which integrates waste and energy conversion to produce methanol is addressed. Within the proposed configuration syngas is produced from waste gasification and enriched with hydrogen produced through water electrolysis, in order to accomplish methanol synthesis requirements. A techno-economic and environmental analysis of the hybrid scheme is proposed together with its comparison with waste to methanol and direct power to methanol technologies. Direct waste conversion into methanol is today the most attractive solution. In a near future, however, considering the increase of the renewable share of power and reduction of power cost, the hybrid scheme may become a quite attractive solution
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