Design of an SF6 recuperation system

Numerous families of particle detectors at CERN utilize gas mixtures containing fluorinated gases, which have significant environmental impacts due to their high Global Warming Potential (GWP). To comply with European emissions regulations, it is essential to either reduce the consumption of these gases or significantly limit their emissions through abatement. Specifically, in Resistive Plate Chambers (RPCs), a mixture of R134a (95.2%), iC4H10 (4.5%), and SF6 (0.3%) is used. Isobutane, being an organic gas, acts as a quencher, absorbing photons produced from recombination processes and limiting the formation of secondary avalanches. SF6 is an electronegative gas that captures free electrons, reducing free charges in the gas volume and suppressing the onset of streamers. This thesis focuses on the simulation, design, and development of a recovery system for SF6, the fluorinated gas with the highest GWP (23,600), from a mixture of R134a, iC4H10, and N2 originating from the exhaust of the R134a recovery system. The goal is to achieve a recovered gas purity between 90% and 99% SF6, with a molar recovery rate of at least 60%. Cryogenic distillation has been selected as the primary separation method due to its efficiency in liquefying modest gas flows. An overview of CERN and the CMS experiment is provided, including the operation of RPCs and the role of individual gases within these detectors. The detailed examination of the RPC gas system includes the mixer module, humidifier, pump module, and gas purifier, to comprehensively understand the gas path and purification processes involved. Insights into the R134a recovery system are crucial for the subsequent design of the SF6 recovery system, which will operate in series. The thesis also includes testing a prototype for SF6 separation, supported by simulations to validate experimental conditions. This prototype, already present in CMS, consists of a single-stage buffer and it will be used for testing the cryogenic distillation of SF6 from other components. The aim of the test is to evaluate the solubility of R134a in SF6 and to see if the separation occurs or not. Theoretical studies using binary diagrams for SF6 and R134a, utilizing the McCabe-Thiele approach, estimate the theoretical stages required for separation. The introduction of isobutane as a third component and the analysis of the ternary diagram of these components, derived using Aspen Plus, are also covered. The core of the thesis is the simulation and design of a new SF6 recovery system. This system comprises two distillation columns arranged in series. The output from the top of the second distillation column is an SF6/N2 mixture with a 70/30 composition. This mixture is then processed through a membrane, where it is expected to be concentrated to a composition of 95% SF6 and 5% N2. An economic analysis evaluates the feasibility of implementing recovery systems to reclaim gases compared to abatement systems. The thesis concludes with a discussion of the results, offering real-world perspectives on the strengths and weaknesses of the recovery system.