Hydrate-based gas separation for working fluid mixtures: Application to composition-adjustable organic Rankine cycle

Abstract

Composition-adjustable organic Rankine cycle (ORC) is deemed to improve the overall energy efficiency compared to the conventional ORC, bringing more energy saving and emission reduction. Inspired by hydrate-based gas separation, the hydrate-based composition regulation of working fluid was proposed and investigated experimentally using R22/R125 mixtures as feed gas. To improve the hydrate formation kinetics, stirring was employed and boosted the initial hydrate formation rate by about 200% compared with the unstirred cases. Cases with more R22 fraction in the feed gas showed faster initial hydrate formation rate. Partition of components between gas and hydrate phases was observed in a kinetic separation manner, i.e., R22 led the hydrate formation at the early stage while R125 was enclathrated more at the later stage. With proper formation time (45 – 135 min), the R125 fraction in the gas phase could increase by 9.5 – 18.7 mol% from various feed gas compositions (30 – 70 mol%). In the hydrate phase R22 was typically enriched with the best separation factor varied between 2.71 and 10.27. Compared to hydrate formation at a constant temperature (275.5 K), the temperature ramping down mode (from 278.5 K to 275.5 K) slowed down the hydrate formation kinetics but amplified the separation performance, with the best R22 separation factor 2.84 times higher than that of the constant-temperature case achieved at the same time. Visual images revealed a “wall-climbing” hydrate growth pattern for R125/R22 mixed hydrate formation particularly in unstirred conditions. These results support the proposal of a hydrate-based composition-adjustable ORC and could lead to further development of this technology with practical insights.