Separation of noble gas mixtures of (Xe, Ar and Kr) using gas hydrate technology.

Abstract

The separation and purification of xenon from xenon + argon + krypton mixtures is an important research area owing to the increasing industrial demand for the gas. While cryogenic distillation, membrane technology and adsorption using metal-organic frameworks (MOF's) are used as separation methods, these methods are energy-intensive and sometimes financially non-viable. A comprehensive theoretical investigation of these methods is presented in this work. Hydrate-based separation technology has been reported to provide a possible solution. This study aimed to assess the performance of gas hydrate technology in separating and purifying noble gas mixtures. Hydrate technology is an interesting application, it has and is being investigated for the application in hydrocarbon gas capture and storage, carbon dioxide capture and storage, food concentration and refrigeration amongst other application. The motivation of this work was to find a cost-effective separation technology for separating and purifying xenon from the xenon + argon + krypton mixtures. Previous studies investigated the application of gas hydrate technology in separating xenon from binary noble gas mixtures, including an investigation of the effect of hydrate promoters and other factors in the capture of xenon in the hydrate phase. This study builds upon previous work focusing on the ternary mixtures of (Xe + Ar + Kr). Experimental measurements of gas hydrate phase equilibria for gas mixtures of (argon + krypton +xenon), along with compositional analysis of the hydrate and vapour phases using gaschromatography were performed. The isochoric pressure search method was used for measurements, with the use of a 52 ml stainless steel equilibrium cell. Different gas mixtures with various compositions ranging from 19 to 70 mol% xenon were investigated. To check the reliability of both the experimental equipment and procedures used in this study, dissociation data for the simple carbon dioxide + water system were measured. The newly measured data were compared with those in literature and were found to be in agreement with an acceptable uncertainty range. The instruments that were used were calibrated and the calibrations were verified. A thermodynamic model based on van der Waals and Platteeuw (vdW–P) solid solution theory was used to predict the hydrate equilibrium conditions for the Xe + Ar + Kr + water systems. An average absolute deviation (AAD%) of 1.4% between the experimental and predicted hydrate dissociation conditions was obtained. The consistency between modelled results and the novel measured experimental data demonstrated the validity of the proposed method. Concurrent to measuring thermodynamic equilibrium data, equilibrium compositional data for the systems studied were measured. The results indicate that the concentration of xenon has the highest increase in the first and second hydrate stages, reducing the concentration effect as the number of stages increases. For a mixture with 40.7 mol % argon, 33.6% krypton and 25.7% xenon, a concentration increase from 25.7% to 80.4% of xenon was achieved using two hydrate formation and dissociation stages. These findings were used to evaluate energy loads for the hydrate-based separation method. The results obtained were compared to the results obtained from an Aspen® simulation of the conventional cryogenic distillation process to determine energy loads for the conventional cryogenic distillation process. Results of the comparison revealed that the hydrate-based separation method presents a 20% energy cost advantage over cryogenic distillation.