Regeneration of the purifier is initiated by the operator, and is carried out automatically by the system controller. The operator will become aware that regeneration is necessary by the output of the onboard analytical equipment. A combination of process analyzers — oxygen, moisture, and TCD (thermal conductivity) — samples from the bottom of the purifying media bed. Sampling from this position provides indication prior to purifier breakthrough. Other sampling points will be strategically selected for testing system purity and troubleshooting (tube bundles, before and after compressors, etc.); however, the default sample position is in the purifier bed.
Once regeneration is initiated, the liquid nitrogen supply will be stopped, and the remaining liquid boiled off by the introduction of warm (vaporized) nitrogen. A mechanical vacuum pump on the downstream side of the system will pull a vacuum on the purifier pulling out the air contaminants as well as the condensable contaminants. The time for a typical regeneration is 10-12 hours, and is limited by how quickly the purifier can be warmed up to room temperature. Upon completion of the regeneration cycle, liquid nitrogen is re-introduced to the system.
Once complete, the system controller will valve back in the purifier, and normal operation can resume. During regeneration, the system controller displays ‘Regeneration in Process’ and will not allow the system to recover helium. Other parts of the system may still be used during this time.
The efficacy of the system to produce highly pure helium to the correct specification has been proven in three steps. The operation was initially tested at the Airgas final assembly plant near Cleveland, OH, and as first installed at VACCO. As the plant runs, purity is monitored at four locations at the plant. Finally, periodic samples are taken and sent to an outside lab for verification of helium purity.
This article was contributed by Bret Shelton of VACCO Industries and Brad Wallace of Airgas Specialty Gases. For more information, visit https://info.hotims.com/61068-115.