High Pressure Hydrogen Dehydration Using Ionic Liquids

Abstract

Renewable energy projects, such as wind and solar (PV) parks, are rapidly being built to replace fossil fuel based energy sources. The major drawback of electricity production from renewable sources is that they are intermittent and therefore lead to amismatch between the supply and the demand of energy. Energy storage in the form of hydrogen production is a potential candidate to overcome this problem. However, due to its lowvolumetric energy density, hydrogen must be compressed to use it for storage or transportation purposes. HYET BV has developed an Electrochemical Hydrogen Compressor (EHC) that is capable of compressing hydrogen up to P = 1000 bar, and have the potential of bringing compression costs down to 3 kWh/kg. As the compressed hydrogen is saturated with water, it must be dehydrated before it is used for refuelling Fuel Cell Electric Vehicles (FCEV), as the ISO 14687-2:2012 standard has limited the water concentration to 5 ¹mol water per mol gas mixture. In this work, a landscaping studywas performed to search for potential drying methods, includingmembrane separation, adsorption, absorption and cooling. To select the best method for hydrogen dehydration, it is essential to have a good understanding of the thermodynamics at the given pressure conditions, beginningwith the saturated water content in compressed hydrogen. To the best of our knowledge, the only experimental data describing the water content in the H2-H2O mixture for pressures exceeding 300 bar are from 1927 and are limited to T = 323.15K [J. Am. Chem. Soc., 1927, 49, pp 65-78]. In this thesis, it was concluded that conventional Equations of State (EoS) failed to predict the equilibrium coexistence compositions of the liquid and gas phase. Therefore, molecular simulations were used, which adequately predicted the water content in compressed hydrogen. The second part of this thesis was to select the best drying method for hydrogen dehydration. Previous work showed that absorption using Ionic Liquids (IL) could be an interesting alternative to traditional dryingmethods. Therefore, absorption experiments were performed on the ILs 1-Ethyl-3-methylimidazolium acetate [EMIM][Acetate], 1-Butyl-3-methylimidazoliumchloride [BMIM][Chloride], and 1-Butyl-3-methylimidazolium octyl sulfate [BMIM][Octyl Sulfate] to test the feasibility of ILs for hydrogen dehydration. The ILs were selected on stability, safety, and on the activity coefficient of water in the given IL. The experiments showed that ILs can be used as an absorbent to dry hydrogen gas. A part from the initial selection criteria, the experiments showed that surface tension is an important criterion for future IL selection, as it is strongly related to foam production. Finally, the very hygroscopic ILs tested turned out to be capable of absorbing large amounts of water, but a lot of energy was required to regenerate the ILs, making them less attractive as a potential absorbent.