Analysis of technologies for hydrogen purification with application to steam reforming process

Hydrogen is one of the most abundant elements on the Earth, but it is not available in free and molecular form since it is combined with other chemical elements. Therefore, it is considered an energy vector. It was discovered in 1766 by Cavendish, but its name was coined by Lavoisier in 1783. At the beginning of 1800s, hydrogen was produced by electrolysis and it was combined with methane as "rich town gas" for lightning and cooking. Then, at the end of 19th century, it was substituted by oil and natural gas resources. In 1911 it was used as feedstock for ammonia production; in 1949 the first catalytic reformer was commercialized and, in 1961, hydrogen found application as fuel for rocket spacecraft. The word "hydrogen economy" was used for the first time after oil crisis of 1970s. It was referred to an economy where hydrogen had a key role in decarbonising several energy sectors. However, in the last three years the GHG emissions have grown of about 10% reached in 2019 the new record of 59.1 GtCO2e. In order to reduce global warming, the European Commission has launched in 2019 the European Green Deal with the scope of achieving the neutrality in the GHG emissions in 2050. In this scenario, several countries have been working on national hydrogen strategy focusing on green hydrogen. Nowadays hydrogen, indeed, can be divided into three types: grey, blue and green hydrogen. The first one includes hydrogen produced from fossil fuel such as steam methane reforming; the blue hydrogen, instead, is grey hydrogen with application of CCS. Finally, the green hydrogen is produced from electrolysis, in which electricity is provided by renewable resources. However, the cost of green hydrogen is still three times greater than that produced with CCS and the current hydrogen demand is covered for half from steam reforming of natural gas. This research project, carried out within the framework of the project "Pure hydrogen from natural gas reforming up to total conversion obtained by integrating chemical reaction and membrane separation", financially supported by MIUR (FISR DM 17/12/2002)-Italy, has the scope to find a way to increase the efficiency of steam reforming by means of technologies for hydrogen purification saving CO2 emissions. The removal, indeed, of one product in this case hydrogen, allow to overcome the thermodynamic limit of the conventional process. The experimental tests, performed in the Chieti pilot plant, have been analysed in detail finding a mass transfer correlation able to describe the permeation of hydrogen through the palladium membranes. Three different geometries have been considered taking into account both concentration polarization and membrane permeance. The results have shown that for Re <20,000 the concentration polarization is the limiting step, while for 20,000 <25,000 the two resistances can be compared and for Re >25,000 the membrane resistance is the controlling phenomena. Finally, a commercial steam reformer has been compared with two emerging technologies: the Membrane Reactor (MR) and the Reformer and Membrane Module (RMM). The rate of production and permeation of hydrogen have been estimated by means of the aforementioned mass transfer correlation and the kinetic model of Xu and Froment. The two velocities are equal only at the end of the reactor with a mean rate of hydrogen production ten times greater than that of permeation. Furthermore, the analysis of key parameters such as Gas Hourly Space Velocity (GHSV), Steam to Carbon (S/C) and Hydrogen Recovery Factor (HRF) has shown thar the RMM configuration can increase the methane conversion of about 30% than conventional steam reforming. A RMM composed by two reactors and membranes as the Chieti pilot plant, indeed, ensure a methane conversion of 92% with a HRF of 94%.