invention may advantageously be used either when the gas is to be reacted in a single reactor vessel or by two vessels connected in series in either case the gas may first be flowed through a chiller which will precipitate some of the heavier hydrocarbon gases but also water vapor and a heater as in conventional practice using a pervious bed constituted of 34 sulfatreat 34 as set forth in a system having a single reactor tower when the h 2 s level monitored by sampling at or in advance of the reactor outlet shows a sudden rise water is injected into the inflowing gas the amount injected may be relatively small the drop in the h 2 s level which follows signals that the bed has been revived thereafter such small amounts of water are injected either continuously or at intervals to avoid recurrence of increase in h 2 s this is continued until the known capacity of the oxide is substantially say at least three fourths utilized when a single reactor vessel is utilized the vessel may advantageously be equipped with at least one sampling port either at mid depth of the vessel or somewhere between mid depth and the reactor outlet while experience at some installations has shown that unusually dry gas may dry reactor beds within as little as seven days at other installations where the gas is fairly moist and not heavily polluted with h 2 s the same may flow for a year or more without drying the bed.