Development of a CF4 recuperation plant for the Cathode Strip Chambers detector at the CERN Compact Muon Solenoid experiment

Cathode Strip Chambers (CSC) are used in the end-cap region of the Muon spectrometer for the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). The nominal gas mixture contains 40% Ar, 50% CO₂ and 10% CF₄. The main function of CF₄ is to prevent polymerisation processes that would result on deposits on the wires as proved by long-term ageing tests performed during the past few years. CF₄ is an expensive gas; the current gas operational cost in the CMS CSC is about 100 kCHF/year, in a closed-loop gas system that is operated with a 90% re-circulation rate. Unfortunately, it is not possible to increase the re-circulation fraction because N₂ from air constantly diffuses into the detector. However, the possibility of recuperating the CF₄ would offer several advantages, both in terms of cost and flexibility of operation. A CF₄ recuperation plant has been proposed. The present layout is based on four stages: (1) CO₂ bulk separation in a polymeric membrane, (2) residual CO₂ removal in a 4 Å molecular sieve, followed by (3) CF₄ adsorption a in 13X molecular sieve and (4) CF₄ compression and storage. A prototype with all the functionalities (automatic actions, on-site regeneration, and gas analysis with an infrared device) has been built and it is currently being tested. Preliminary results are presented in this paper. During the tests, several candidate membranes have been characterized and the optimizations of both CO2 and CF4 adsorption stages have been carried out. The overall CF₄ recuperation efficiency is about 65%. N₂ contamination in the recuperated gas is substantially reduced.