title: Adsorption differential volumetric apparatus (ADVA) for high pressure equilibrium and kinetic measurements
Creators: name: Rea, Riccardo
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: subject: adsorption
subject: high pressure
subject: kinetics
subject: equilibrium
subject: modelling

description: Adsorption based technologies are largely used in industrial applications for separating mixtures into single components. The past two decades have seen an exponential growth in the development of promising materials for high pressure applications, such as H2 recovery and purification, biogas upgrade and gas storage, such as CO2 from capture, and CH4 and H2 for energy purposes. From the point of view of process intensification and energy optimisation, there is a great need of highly accurate and reliable high pressure data. This thesis aims to characterise and validate the use of a novel adsorption differential volumetric apparatus (ADVA) for the accurate measurements of high pressures adsorption equilibrium and kinetics. Two systems have been optimised as part of this work: ADVA-270 and ADVA-60. The methodology for the equilibrium has been developed almost entirely in ADVA-270, meant for equilibrium measurements. It accommodates transducers for high pressures, up to 270 bar. With the optimised experimental protocols for the accurate volumes calibration and equilibrium experiments, the system proved to be very accurate even when testing sample masses lower than 100 mg. High pressure isotherms on NaY for different gases were measured at three temperatures. To assess the reliability and accuracy of the ADVA-270 results, the isotherms were compared with those coming from independent measurements at the Hiden Isochema Laboratories using similar sample mass. The isotherms were modelled using a thermodynamically consistent Langmuir model. To tackle the necessity to use absolute isotherms for high pressure applications, a novel experimental method is presented for the direct measurements of absolute isotherms in a volumetric system. The method, experimentally straightforward to implement, does not require additional ex-situ and time consuming tests, it is not destructive and just relies on a low pressure gas expansion on the fully saturated sample. To validate the protocol, a series of absolute isotherms were measured using ADVA-270 on commercial 13X. The method, tested on a hydrophilic material, provides, for the first time, an alternative to conventional procedures to calculate the absolute adsorbed amount. For kinetic analyses the new ADVA-60 was designed and built following all the stages from prototyping to the final operational configuration. The similarities between the two ADVAs made it possible to transfer the experimental protocols optimised for volumes calibration and adsorption equilibrium tests. ADVA-60 is optimised to measure fast kinetics at pressures up to 60 bar, features a fast data acquisition system (up to 300 Hz) and slightly smaller volumes than ADVA-270 for increased precision. The new ADVA-60 was used to investigate the mass transport inside a commercial carbon molecular sieve, used for high pressure air separation and biogas upgrade. O2/N2 and CO2/CH4 separation capability was investigated by measuring the adsorption kinetics of the individual gases up to 5 bar. The kinetic data were modelled using the isothermal and non-isothermal kinetic models to assess the transport mechanism and concentration dependence of the time constant. The mass transport exhibits a super-Darken concentration dependence, partially agreeing with other authors findings. Final remarks on advantages and limitations of the technique are provided together with potential future developments of the instrument and the methodology.