Produzione di idrogeno in reattori a membrana con catalizzatori di nuova generazione

Responsabili di progetto

Giuseppe Barbieri, Vitali Bakhtadze

Accordo

GEORGIA - SRNSF - Shota Rustaveli National Science Foundation

Bando

CNR/SRNSF 2014-2015

Dipartimento

Scienze chimiche e tecnologie dei materiali

Area tematica

Scienze chimiche e tecnologie dei materiali

Stato del progetto

Nuovo

Proposta di ricerca

In the last few decades a great deal of attention has been paid to the use of hydrogen as an energy carrier to be employed for clean energy production by means of new technologies, such as polymer electrolyte membrane fuel cells.
Unfortunately there are obstacles to the use of this newer technology, high-purity hydrogen is required for a typical polymer-electrolyte membrane fuel-cell with the CO concentration usually under 20 ppm, in addition there are difficulties in storage and transport of the said gas.
Hydrogen production can be carried out in several ways, but currently a large hydrogen supply is obtained cost-effectively and in the most part from natural gas by means of methane steam reforming reaction carried out over Ni-catalyst at 700-800°C with large carbon formation over the catalyst surface and inside the pores.
However, in recent years the process of making synthesis-gas by methane conversion also from CO2 has attracted special attention. Investigations, performed in the last few years in the Laboratory of Catalysis of the Institute of Inorganic Chemistry and Electrochemistry, on methane conversion over 4Co-Mn catalyst showed a selectivity in hydrogen and CO between 95-98% when volume rates of methane of 1000-7000 hour-1, feed molar ratio of CH4:CO2 =1.0-1.3 and temperatures of 800-850°C were operated. No carbon deposition on the catalyst surface was observed after 50 hours of operation.
The main method of carbonic acid conversion of methane is dissociative adsorption of CH4 at acid centers of the catalyst surface by the formation of surface carbon and its further interaction with CO2. Along with this, a high acidity of the surface is favorable to catalyst coking. The stabilization of the active component in nanorange is one of the methods to avoid carbon accumulation on the catalyst surface, which may be attained by the use of a secondary carrier with a highly developed surface and by predetermined functional characteristics.
In spite of the availability of the process of methane conversion by carbonic acid, up till now, it is of no industrial use. The easy formation of carbon on the surface of nickel catalysts and a reduction of their activity is one of the basic reasons for this. On the other hand, even though the conversion of methane by vapour is a mature and well-established technology for industrial production in the hydrogen market on a worldwide scale, also by the use of the well-known Ni catalyst, the entire process is essentially complex. Also, this method, based on large-scale plants, is not advised for small-scale productions of ultra-pure hydrogen because of the outsized amount of equipment and uneconomical downscaling. Therefore, the miniaturization of equipment, together with the identification of suitable catalysts, can be considered a fundamental strategy towards the real application of these technologies for hydrogen production. Small-size reactors technology offers a great advantage over conventional macro-scale reactors since it is flexible for a wide range of applications and can make the scale-up of a process straightforward.
When, in addition to the smaller size of the reactor, a selective and continuous removal of hydrogen is operated in the same apparatus, a higher pure hydrogen recovery as well as chemical conversion can be reached. This allows the thermodynamic limit of traditional reactors to be exceeded and operations in less severe conditions for an equilibrium limited reaction. The membrane reactor is an attractive technology able to combine two different unit operations in a single device (i.e. reaction and separation) by integrating a hydrogen perm-selective Pd-based membrane in a conventional reformer. The membrane allows the selective removal of most of the hydrogen produced from the reaction volume, with consequent improvement of methane conversion together with the recovery of a pure hydrogen stream not requiring any further separation. As mentioned previously, currently, conventional nickel catalysts are usually used in methane steam reforming since they are very active, efficient and allow operating at higher space velocities despite problems of coking. Therefore a more compact reactor can be designed, in combination with efficient mass and heat exchange, properly distributing the active component always required to save precious metals. This last aspect can regard indistinctly both exothermic and endothermic reactions. A similar approach can be followed for the conversion of methane by carbonic acid. In this latter case, a great deal of attention has to be focused on the selection of the catalysts that is determined by the following factors:
o Stability of cobalt containing catalysts even at the formation of surface carbon in large quantities;
o Ability of the carriers of manganese and alumo-calcium oxides to reduce the formation of elementary carbon;
o Heat resistance of manganese low oxides (MnO, Mn3O4) at high temperatures (1500°C) and their probable ability to inhibit the sintering of metallic cobalt crystals.

This proposal deals with the development of a hydrogen production unit based on a membrane reactor where the proper integration of membrane technology with the most appropriate catalyst for the chosen reaction will allow the attainment of high conversions and hydrogen recovery at temperatures lower than the industrial cycle, while reducing catalyst coking.
ITM-CNR will study the methane conversion as a function of operating parameters such as, for instance, temperature (400-550°C), pressure (up to 10 bar), permeate pressure, feed molar ratio (as low as possible (closer to the stoichiometric one)), catalyst weight and membrane surface, etc. The Institute of Inorganic Chemistry and Electrochemistry will prepare/develop catalysts and study all the aspects also related to catalysts use in membrane reactors.
An important aspect to take into account is the analysis of phenomena such as coke formation during the reaction. The membrane presence influencing the ratio of reactants/products and allowing operation at lower temperature will condition the coke formation and this aspect, quite different from that in conventional reactors, will be jointly and widely investigated.

Obiettivi della ricerca

The main objective is the development of new hydrogen production, a membrane reformer, operating with developed catalysts able to reduce coking over the surface and inside the pores of the catalysts.


This will be pursued by investigating the following aspects:
o Research of the phase genesis of the secondary carriers of the catalysts according to the condition of synthesis; the catalysts, containing manganese and cobalt oxides, will be synthesized for methane conversion by carbonic acid; oxide-granulated and block shaped secondary substrate metal catalysts with nano-composite plating will be synthesized for air-purifying and some chemical-engineering processes. The results of the investigation may probably be used for obtaining secondary substrate metal anodes, promoted by oxides and noble metals, which are active electrode-catalysts in the technology of obtaining thermal hydrogen.
o Determination of the thermal strength and stability of the catalysts in the oxidation process of methane; the operating resource for a catalyst will be determined: efficiency, resistance against coalification, and heat stability. Various physical-chemical methods will be used in the investigations: thermal, x-ray phase, electron microscopy, porometry and others.
o Integration of membrane and catalyst: design of the membrane reactor.
o Identification of the proper operating conditions maximizing the membrane reactor performance and reducing the undesired effect such as coke formation.

Ultimo aggiornamento: 02/05/2024