(662c) A Thermodynamic Model of an Adsorbed Natural Gas Tank Containing a Metal-Organic Framework Adsorbent

Containing a Metal-Organic Framework Adsorbent

Hongda Zhang and Randall Q. Snurr

Department of Chemical & Biological Engineering

Northwestern University, Evanston, IL 60208  USA

Adsorbed natural gas (ANG) has many advantages, including higher safety and lower cost, compared with traditional compressed natural gas storage for vehicular applications. Metal-organic frameworks (MOFs) are a promising class of adsorbents for ANG. However, in addition to methane, commercial natural gas always contains small amount of impurities including ethane and propane. These higher hydrocarbons are more easily adsorbed by the adsorbents due to their stronger interactions with the adsorbent framework atoms. In order to study the effect of these impurities on the performance of an ANG tank, we combined GCMC molecular simulations and macroscopic thermodynamics to develop a model for an ANG tank. Central to our approach are the use of Ideal Adsorbed Solution Theory (IAST) to predict mixture isotherms from single-component isotherms and the Peng-Robinson equation of state to calculate fugacity coefficients at high pressure. With this model, the performance, especially the deliverable energy, of the natural gas storage system with different MOFs can be tested over many operation (adsorption/desorption) cycles. Furthermore, screening of a small MOF database containing 120 structures has been carried out. Based on the screening result, good MOF candidates have been identified, and some interesting trends have been observed. Some MOFs that perform well with pure methane are not among the top materials when considering natural gas that includes ethane and propane. The observed trends can be used to develop guidelines for us to search for and design better MOF materials.