For a practical air detritiation system, the effect of the residual tritiated water left on the silica was demonstrated. The residual tritiated water significantly reduced the detritiation factor of the air detritiation dryer. To maintain the detritiation factor more than 350, the space velocity lower than 140 h-1 should be selected for the design and the operation of an air detritiation dryer packed with silica gel. The HTO/H2O separation factor evaluated using a dryer with residual tritiated water was 1.17 on the average. Silica gel is superior to commercial molecular sieves in dehydration ratio at room temperature. Dehydration from silica gel was dependent on the accumulated amount of purge gas, independent on flow rate. Dehydration from silica gel showed a slight pressure dependency. Consequently, silica gel is a promising desiccant agent for air detritiation dryer.