(571e) Accurate High Pressure CH4 Equilibrium Isotherms on NaY Using Adva and Xemis Apparatuses

In a recent publication, an international interlaboratory study led by the US National Institute of Standards and Technology (NIST) has been presented with the measurements of high-pressure CH₄ adsorption isotherms on a NaY reference sample at 25 °C up to 7.5 MPa [1]. The effort involved 28 laboratories across the world and generated a reference CH₄ isotherm for NaY with a 95% uncertainty interval.

In this work, we have extended the study by comparing CH₄ isotherms on the same NIST reference sample (RM 8850) at different temperatures carried out using two different techniques (volumetric and gravimetric) across two different laboratories who participated in the original NIST round-robin effort. Measurements were carried out using a built-in-house Adsorption Differential Volumetric Apparatus (ADVA) and a commercial Hiden Isochema XEMIS microbalance.

Relying on the measurement of the differential pressure between two symmetric branches (sample and reference), ADVA has the key advantage, over conventional volumetric systems, of ensuring the same accuracy in the entire pressure range available. This is particularly beneficial when adsorption isotherms up to high pressure need to be measured. Being completely designed and built in house, a specific focus was put in minimising the volumes, allowing accurate equilibrium measurements using a small amount of adsorbent.

The XEMIS is a unique gravimetric system characterised by a symmetric geometry, which ensures high accuracy measurements at high pressures in a wide range of temperatures (77 to 773 K) thanks to the ultra-low total buoyancy offered by the system. The microbalance provides a weight measurement resolution of 0.2 μg, with a long term stability of ± 5 μg.

Thanks to their individual designs, both systems are capable of generating highly accurate isotherms using very small amounts of sample (< 100 mg for both systems). This is a key feature when working with novel adsorbents which are often synthesised in small quantities.

In this work, as part of the comparison, we discuss the challenges and best practices to obtain reliable results using high pressure volumetric and gravimetric systems when dealing with small amounts of sample. Reliable high pressure isotherms are particularly important to validate molecular simulations and increase accuracy in the prediction of adsorption process modelling, therefore rigorous interpretation of the data is as important as the accuracy of the data acquired. For this reason, the CH₄ equilibrium isotherms on NaY at different temperatures are finally converted into absolute isotherms and interpreted using a thermodynamically consistent model.

[1] Nguyen, H.G.T., Sims, C.M., Toman, B. et al. A reference high-pressure CH4 adsorption isotherm for zeolite Y: results of an interlaboratory study. Adsorption 26, 1253–1266 (2020). DOI: 10.1007/s10450-020-00253-0