Toward minimal complexity models of membrane reactors for hydrogen production

Membrane 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

Analysing the performance of MCECs over a wide range of operating temperatures

Hydrogen production through water electrolysis has gained significant attention in the past years as a means of tackling the problem of the imbalance between the intermittent rate of electricity production from renewable sources and the continuous electricity demand from end users. Recently, much of the effort has been shifted toward the electrolysis of steam rather than water, for example in solid oxide cells, which operate at temperatures around 800°C. In this manner, part of the energy required for the conversion to hydrogen is provided as heat rather than electricity. At the same time, the high temperature levels require the use of highly resistant materials, which increase the overall cost of the process. An interesting alternative is represented by molten carbonate electrolysis cells (MCECs), operating at temperatures well below 700°C. In the present work, a molten carbonate cell was operated in a lower temperature range (490-550°C) by changing the composition of the electrolyte mixture. The data obtained, along with experimental results at higher temperature (570-650°C) available in the literature, was analyzed using a 0D model accounting for Ohmic and activation overpotentials to determine the correlation between current and potential. It was found that, while the dependence of Ohmic losses on temperatures is discontinuous when cell operation is switched from the lower to the higher temperature range, activation losses vary with continuity. This result provides important insight on the performance of MCECs that can serve as a basis for future studies

Estimate of the height of molten metal reactors for methane cracking

Methane 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

A discussion of possible approaches to the integration of thermochemical storage systems in concentrating solar power plants

One 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

High temperature stability of Terphenyl based thermal oils

Thermal oils are nowadays widely used as heat transfer fluids or cooling media in industrial and energy production plants. Currently, very few data are available about their thermal stability in function of the operating temperatures, which is a crucial parameter to estimate oil structural changes and their possible effects the maximum fluids lifetime. The present work is concerned with ageing tests on a commercially used thermal oil at temperatures higher than the nominal working ones, including a full post-test characterization. At this aim, a dedicated experimental set-up was designed and constructed to study the degradation kinetics, and to qualitatively and quantitatively analyze the released gases. As a result, the kinetic parameters were estimated, along with the related changes in the oil thermos-physical properties

Modeling and design of membrane reactors

Membrane reactors are attracting increasing interest because of the opportunity they represent in increasing the efficiency of small-scale systems. Their use in gas phase reactions has been proposed for a variety of applications, where they may act either as selective extractors or as distributors. In particular, membrane reactors are generally employed for the selective permeation of hydrogen. For instance, they have been proposed for hydrogen production through reactions, such as steam reforming of hydrocarbons, water gas shift, propane and ethane dehydrogenation, and ammonia decomposition. Such applications require the use of Pd-based membranes, through which hydrogen permeates selectively, enhancing conversion and allowing the production of pure hydrogen. Perovskite-based membranes, which present a high selectivity towards oxygen permeation, are instead used as distributors for reactions, such as the partial oxidation of methane or ammonia, autothermal reforming, and oxidative dehy-drogenation of alkanes. In this case, the use of the membranes allows the achievement of uniform species concentrations along reactors, leading to a higher product selectivity; however, they may also be used as extractors to enhance conversion. Processes that take advantage of oxygen extraction include the coupling of oxygen-consuming reactions with water splitting, thermal decomposition of CO2, and NOx decomposition. In other applications, the reaction is localized on the membrane, which acts as the catalyst and separator at the same time. The modeling of membrane reactors is essential to exploit all the benefits that can be derived from their optimal design, but it represents an ongoing challenge because of the complexity of describing systems in which the transport of mass, momentum, and energy are strongly coupled. With reference to mass transport, the effects of convection, dispersion, reaction, and permeation should, in principle, be simultaneously accounted for. Gas composition may affect membrane permeance and the coupling of the rates of permeation and reaction can result in multiple steady states. The reaction and permeation may cause a change in density that affects momentum transport. Furthermore, temperature gradients may be formed as a consequence of the heat of reaction, energy transport associated with the permeation, and the potential presence of a heating system. The purpose of this Special Issue is to publish research papers on advances in membrane reactor modeling and design, as well as review papers. Potential topics include the modeling of: Membrane reactors for enhanced conversion/product selectivity Membrane reactors for controlled feed distribution Membrane reactors for coupled reactor systems Catalytic membrane reactor

Methodologies for the design of solar receiver/reactors for thermochemical hydrogen production

Thermochemical 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

Removal of xenobiotics in a two phase sequencing batch reactor: kinetics and modelling

The objectives of the paper are to verify the potentialities of a sequential two phase partitioning bioreactor in degrading xenobiotics and to evaluate the kinetic parameters for modelling the system. The target compound investigated was the 4-nitrophenol. Preliminary tests were carried out to define the solvent most appropriate for the compound. Among the three investigated solvents 1-undecanol, 2-undecanon and oleyl alcohol, the 2-undecanon was chosen because of the higher partition coefficient of 30 and the negligible formation of emulsions. Moreover, the tested solvent showed satisfactory "biocompatibility" characteristics for the biomass with minor effects on the intrinsic kinetics. Parallel batch kinetic tests were then performed with the conventional one phase and the two phase systems. In the two phase system the biomass is exposed for all the time to 4NP concentrations that are significantly lower if compared to the conventional system and, for the highest concentration (450 mg/l) in the two phase system a reduction of the reaction time is observed depending on the biomass concentration. Kinetic parameters were also evaluated in both cases by fitting of the experimental data with a modified form of the Haldane equation