R134a separation and recuperation from the gaseous mixture used in the Resistive Plate Chamber detectors at the LHC Experiments

Resistive Plate Chambers (RPC), working at the Large Hadron Collider (LHC) at CERN, are used with a mixture containing C2H2F4 (R134a) and SF6. For the Compact Muon Solenoid the working mixture has the following composition: R134a 95.2%, iC4H10 4.5% e SF6 0.3%. The CMS Experiment is only one of the four big Experiments displaced long the LHC with ALICE, ATLAS and LHCb. Muons represent a very clean probe for the detection and characterisation of the collision occurring in LHC Experiments. The specific composition of the mixtures allows to have the best performance from the detectors themselves. Although these gases are necessary for the optimization of the detectors’ performances, they present some related problematics. There are two main problems for the RPC mixture: 1. R134a are SF6 Green-House Gases (GHG) with high Global Warming Potential (GWP) values, 1430 and 22800 respectively. 2. R134a and iC4H10 form a minimum boiling azeotrope with composition 65/35 which is impossible to separate through simple distillation. Some gases with high GWP values have been regulated in recent year from the EU. In order to reduce emissions from these gases, the EP-DT Gas Team at CERN consider different approaches. One of the possible ways is the one discussed in this thesis which is the separation of R134a from the RPC mixture. To reach this goal, a system for the separation of R134a from other components of the RPC mixture was built. Final purpose of the process is the recuperation and storage of the R134a to be re-used in the detection system. At the beginning it seemed hard to reach high efficiency in term of recuperated gas since a better understanding of the phenomena occurring in the system was necessary. Because of its difficult treatise, the “azeotrope issue” was investigated through the possible way to separate the gaseous azeotrope itself. By the moment that the injected mixture is 95/5 in R134a/iC4H10, a distant point from the azeotrope composition point (65/35), it was possible to achieve the R134a separation via simple distillation. The outcome vapour from buffer was enriched with the azeotropic mixture, while the outcome liquid (almost) pure R134a. In this thesis modifications to parameters and to the system itself were followed by test aimed to the verification of the separation results through gas-chromatographic and mass spectrometry analyses. Starting with the injected mixture with composition stated above (and with different inject flow tested), it was possible to achieve after several tests – aimed to improve the efficiency and data consistency of the recuperation process – pure R134a with no SF6 and iC4H10 in the recuperated gas with only few ppm of air. Moreover, the efficiency of the performed tests were stabilized between 80 and 90% in term of recuperated R134a