Testing of a practical low pressure permeator

Pd membrane permeators have long been used in tritium processing to separate gas mixtures into a pure-hydrogen permeate fraction, and a hydrogen-depleted retentate fraction. However, the retentate fraction has previously always required further purification (generally by oxidation followed by adsorption) to reach tritium concentrations low enough for environmental release. This report describes the development and testing of a practical-scale, low-pressure permeator to produce a hydrogen-depleted retentate fraction that can be stacked directly to the environment without further processing. This performance is attained by linking the permeator to a high vacuum system and by constructing the permeator with high vacuum conductance. The permeator incorporated three 1/8" Pd/Ag tubes in a 6" shell. The pumping train consisted of turbomolecular pump, followed by a diaphragm pump. For this phase of testing only the H₂ performance was measured with tritium operation planned for later. This device has been tested with total feed flowrates up to 20 SLPM and various feed hydrogen concentrations. Permeate pressures were in the range 10^-5 of 10(-2) torr range. Temperatures used were 350 to 450 °C. Presented are the design features of the permeator, and the results of testing this prototype device. Retentate hydrogen concentrations in the low ppm range were routinely achieved