1,721,002 research outputs found
Transport-permeation regimes in an annular membrane separator for hydrogen purification
No full textA transport model is proposed, accounting for full coupling between mass and momentum transport, for a H2/CO2 mixture entering a packed-bed annular volume in the presence of a hydrogen permeable membrane whose permeation process is governed by Sieverts' law. As a consequence of mass/momentum coupling, radial convective fluxes typically arise, which, in turn, impact upon the fine structure of radial concentration profiles of the species. As overall indicators of equipment performance, we target the total hydrogen permeate flowrate and recovery, and investigate their dependence on the operating pressure while keeping inlet gas velocity constant. We show that the permeate dependence on pressure displays a sharp cross-over between a transport-limited linear regime and a square-root membrane-limited regime, ultimately ensuing from Sieverts' law. Based on the closed-form solutions of simplified equations describing the two limiting regimes, a complete analytical prediction of the permeate on the operating pressure is developed. An important fallout of this approach is the identification of a critical pressure which yields optimal operating conditions, allowing the largest hydrogen production compatible with sustained values of the recovery at fixed gas velocity. The closed-form prediction of the permeate flowrate is tested in a range of parameters that mimic a wide variety of operating conditions, from millimeter-scale laboratory prototypes up to parameter values pertaining to full scale industrial equipment. Comparison of model predictions vs. experimentally determined values of the recovery available in the literature proves the fitness of the analytical prediction to industrially relevant process conditions. The same approach developed here can be used with minor modifications in all of the cases where the permeation flux through the membrane can be modeled by a power-law expression characterized by an exponent strictly lower than unity. © 2016 Elsevier B.V
How does radial convection influence the performance of membrane module for gas separation processes?
No full textA two-dimensional axial-symmetric isothermal model, based on full coupling between mass and momentum transport, has been developed to describe the separation of a binary gaseous mixture in a packed bed membrane module. Steady-state conditions have been studied. The gaseous mixture to be separated enters an annular gap between two co-axial cylinders. The inner wall of the outer cylinder is impermeable to both components, whereas a membrane, with infinite selectivity towards one of the components, is supported onto the outer wall of the inner cylinder. A radial flux of the permeating components is therefore present. The main focus was on the determination of the influence of radial convection on the performance of the separator, which has been analysed in terms of three dimensionless groups. Different transport regimes could be identified, corresponding to different values of the dimensionless groups. The impact of radial convection has been assessed by comparing model predictions with those of a fully uncoupled one-dimensional model. A discrepancy up to 20% of the recovery has been observed in industrially relevant ranges of the parameters
Catalyst development for steam reforming of methane and model biogas at low temperature
No full textLow temperature steam reforming (400-550°C) for the production of hydrogen offers significant advantages compared to the conventional process. The milder operating conditions lead to lower operation costs and cost of construction materials. Additionally, no CO shift reactor is required due to favorable temperature for the WGS reaction. In this work, we report the catalytic performance of Ni and Rh catalysts supported on La2O3-ZrO2 and La2O3-CeO2-ZrO2 for their application in a multifuel membrane reformer operating at low temperature. The performance of the catalysts is assessed in different operating conditions in methane steam reforming (GHSV, temperature, H2O/CH4ratio) as well as in reforming of model biogas. Stability tests were conducted up to 90h on stream (1bar and 7bar) and the tendency toward carbon formation was investigated. All catalysts were active in the reforming reactions at 400-550°C and the catalysts supported on La2O3-CeO2-ZrO2 showed superiority in activity and stability probably due to the presence of ceria in the support which contributes to the reforming rate and the resistance to carbonaceous deposits. Ni(10)CeZrLa exhibited remarkably stable performance with minimum amount of carbon formed after 90h (ca. 0.05wt%). TPO and TPH analysis of the carbonaceous deposits showed that the dominating type of carbon is highly reactive and can be easily removed by oxidation or hydrogenation at 500°C. This fact makes the catalyst even more promising for the proposed low temperature process, since the catalyst can be hydrogenated by using part of the H2 production stream without further heating of the reactor. © 2015 Elsevier B.V.
Multi-objective optimization of a hydrogen production through the HyS process powered by solar energy in different scenarios
No full textThermochemical or hybrid cycles powered by concentrated solar energy are a very promising way to produce an effective clean hydrogen through the water splitting, in terms of greenhouse gas (GHG) emissions and power production sustainability. SOL2HY2 is an European project focused on this goal. It deepens the so-called HyS process in a closed or partially open version using a proper SO2 depolarized electrolyser, and moreover, it investigates key materials and process solutions, along the entire production chain. However, the identification of the best solution to obtain a suitable hydrogen in terms of cost, efficiency, availability of energy and material, sharing of renewable energy source, continuity of operation in different locations and plant sizes, poses many challenges in terms of flexibility and complexity of the system. In fact, it involves various chemical equipment, different solar and thermal storage technologies, and variable operative conditions with different reaction temperatures and mixture concentrations. Hence it arises the importance to have a tool for the investigation of this system. In this paper, data analysis and multi-objective techniques are used to study and optimize the process under consideration. Several mathematical methods have been exploited to make the best use of the available data, such as Design of Experiments techniques, meta-modeling strategies and genetic algorithms. All these methods have been implemented in the open source environments Scilab and R. © 2018 Hydrogen Energy Publications LL
Experimental and theoretical study of VOC removal by adsorption onto activated carbon in a fixed bed column.
No full textIn this work, an experimental and theoretical study on the adsorption of vapors of volatile organic compounds (VOCs) in a fixed bed column is presented and developed; the analyzed system is composed by three kinds of VOCs, that is dimethoxy-ethane (DME), isopropyl alcohol (IPA) and methyl-tertbutyl-ether (MTBE) and a commercial activated carbon as adsorbent material.
Nowadays VOC are widely used as dissolving and cleaning agents in many industrial processes such as printing, manufacturing of magnetic tapes, electronic chips, pharmaceutical and cosmetic synthesis and are therefore frequently present in the gaseous industrial effluents.
Unfortunately VOC are a class of hazardous pollutants, due to their harmful effects of human and animal health and the environments. It is well documented that VOC inhalation and longterm exposure can cause serious health effects; moreover they are included in the “ozone precursor substances”, that means substances which contribute to the formation of ground-level ozone. Therefore, control of VOC emission is mandatory for the industries, according to the environmental regulations issued world-wide.
Different techniques are available to destroy VOC by different types of thermal, catalytic or biological oxidation or to remove VOC from gaseous effluents by absorption, adsorption or membrane separation. Among the last type of technologies, adsorption is one of the most extensively used, because of the high selectivity and capacity of the adsorbent material, the ease of operation, the low capital and operating costs, and the compactness of equipment; due to its high separation efficiency even at low concentrations and at low operating costs, adsorption is also employed after one or more previous treatment processes, which operate a more gross separation to reduce pollutant’s concentrations to low levels. Furthermore, adsorption in fixed bed columns is suitable to treat effluents of variable composition and can tackle the problem of accidental increases of pollutant concentration without compromising the effluent quality, but at the expense of a reduction of the column’s useful life.
Several adsorbent materials can be used to adsorb VOCs: activated carbon, zeolite, silica gel, MOF, carbon molecular sieve. In particular, activated carbon is a good adsorbent with high adsorption capacities towards polar and non-polar compounds; it can be produced by a great variety of raw materials, such as hazelnut shell, rubber-seed shell, palm kernel shell, coconut shell, rice husk, wood; this large availability of waste material makes the activated carbon relatively cheap and available. However, its removal efficiency can be negatively affected by gas relative humidity, competitive adsorption of different contaminants and slow mass transfer rate. In particular, in the pharmaceutical industry in which the production is predominantly in batch mode, the gaseous streams contain several VOCs and competitive adsorption usually occurs. Furthermore, the gaseous composition can vary periodically in relation to the planning of production.
The effluent is treated in a fixed bed column packed with activated carbon before its release into the atmosphere; it is clear that it is difficult to study the column behavior, and hence, to predict its life time in such operating conditions. Indeed, as mentioned before, when a column is fed with a multicomponent stream, the competitive adsorption for the adsorbent sites usually occurs; moreover, because of the discontinuity nature of the feeding stream, the column is
subjected to adsorption and desorption phases which take place along the bed until its break occurs.
Therefore, the development of a fixed-bed mathematical model able to describe the complex system behavior is crucial if you want to predict the performance of such separation process; to this aim an adequate understanding of both equilibrium and kinetics of adsorption and desorption is essential. In order to obtain this information, the experimental response must be matched with the theoretical response, calculated from the dynamic model of the system. On
the other hand, from the perspective of the fixed-bed column design, the model can represent a useful tool to predict the dynamic behavior of the column in a wide range of operating conditions, without recourse to extensive experimentation.
Single VOC removal by adsorption process is widely analyzed in the literature, where a variety of VOCs as well as kinds of models to describe their adsorption in several adsorbent materials can be found; far fewer studies focusing on the adsorption of multicomponent mixtures in dynamic conditions are instead available and even fewer studies on the desorption phase have been carried out.
In this research work, adsorption and desorption of the three aforementioned organic solvents, widely used in pharmaceutical industries, on a commercial activated carbon are experimentally and theoretically studied in a fixed bed column
Integration of photovoltaic and concentrated solar thermal technologies for H2 production by the hybrid sulfur cycle
No full textIt is widely agreed that hydrogen used as energy carrier and/or storage media may significantly contribute in the reduction of emissions, especially if produced by renewable energy sources. The Hybrid Sulfur (HyS) cycle is considered as one of the most promising processes to produce hydrogen through the water-splitting process. The FP7 project SOL2HY2 (Solar to Hydrogen Hybrid Cycles) investigates innovative material and process solutions for the use of solar heat and power in the HyS process. A significant part of the SOL2HY2 project is devoted to the analysis and optimization of the integration of the solar and chemical (hydrogen production) plants. In this context, this work investigates the possibility to integrate different solar technologies, namely photovoltaic, solar central receiver and solar troughs, to optimize their use in the HyS cycle for a green hydrogen production, both in the open and closed process configurations. The analysis carried out accounts for different combinations of geographical location and plant sizing criteria. The use of a sulfur burner, which can serve both as thermal backup and SO2 source for the open cycle, is also considered. © 2017 Author(s)
Il campione aziendale e le sue dinamiche - Indicatori dell'annata agraria 2006-07
No full textIl lavoro collacabile nella Collana INFORMAZIONE AGRARIA è caratterizzato dalla volontà di proseguire le attività di monitoraggio e valutazione dei risultati economici, delle aziende agricole dell’Umbria nel periodo in oggetto. Proprio per queste motivazioni, il lavoro è un vero e proprio compendio statistico piuttosto che un studio interpretativo delle dinamiche del settore agricolo. Nella prima parte l’obiettivo è quello di fornire gli indicatori economici di sintesi aziendali e colturali
New solid phase of KNO3 - NaNO3 salt mixtures studied by neutron scattering and differential scanning calorimetry analysis
No full textWith the aim of supporting the thermodynamic modeling of phase equilibria of molten salt mixtures, this article investigates the solid phases present in NaNO3 and KNO3 binary mixtures. To that end, an innovative approach based on the combined use of high-temperature Neutron Scattering Analysis and Differential Scanning Calorimetry was proposed. A solid phase not reported before was detected close to the solidus curve in the central part of the phase diagram, and its diffraction pattern was assigned to a mixed nitrate phase, i.e. a solid solution of the two salts. © 2018 Author(s)
Conceptual study of the coupling of a biorefinery process for hydrothermal liquefaction of microalgae with a concentrating solar power plant
No full textA conceptual analysis of the coupling of a concentrating solar power plant with a chemical process for hydrothermal liquefaction (HTL) of microalgae to biocrude was performed. The two plants were considered coupled by molten salt recirculation that granted energetic supply to the chemical process. Preliminary estimations have been done considering a solar field constituted by 3 linear parabolic solar collectors rows, each 200 m long, using a ternary molten salts mixture as heat transfer fluid, and a chemical plant sized to process 10 kT/y of microalgae. Under adopted conditions, we have estimated a minimum selling prize of the biocrude that is similar to that achieved in non-solar HTL processes. © 2017 Author(s)
Uncertainty quantification in a hydrogen production system based on the solar hybrid sulfur process
No full textWater splitting through the Hybrid Sulfur (HyS) process powered by solar energy is a promising pathway to the production of green hydrogen. The main challenges to the development of this process are related to the intrinsic variability of the solar resource, which, besides requiring the deployment of innovative process solutions, introduces significant elements of uncertainty in the plant design. In this paper, the Polynomial Chaos Expansion (PCE) method is applied for the uncertainty quantification (UQ) in this kind of systems. In particular, a forward analysis dealing with the evaluation of the output probability distributions is performed. This is carried out in terms of the input probability distributions, and the analysis is focused on how uncertainty is propagated from the input to the output. Moreover, a comparison between the PCE method and the standard Monte Carlo analysis (using the Latin Hypercube Sampling method) is performed. The obtained results show the advantage of the PCE approach in terms of convergence rate and the number of function evaluations. Finally, a sensitivity analysis through Sobol’ indices has been carried out, which highlighted the influence of each variation in the input on the output
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