1.    In an Outside-In Purifier, hydrogen moves toward a palladium-alloy wall, where it is removed by diffusion. The vector for the direction of gas flow is essentially perpendicular to the wall.

 2.    Hydrogen carries all impurities toward the wall, increasing the concentration of the impurities as the wall is approached.

 3.    Oxygen moves, with other impurities, towards the wall.  The innermost oxygen burns catalytically with hydrogen to form steam, which is trapped at the wall as an impurity.

4.    The steam layer prevents further catalytic combustion and oxygen continues to build up on the far side of the steam layer in the same manner as the non-reactive (inert) impurities.

 5.    Oxygen can build up without limit until it reaches the Lower Explosive Limit, where it can flash spontaneously or until the inert shield layer of steam, nitrogen, etc. ruptures (because of eddy currents, geometric irregularities or the like) and the oxygen rich layer impinges on the catalytically active palladium surface and burns.  The potential temperature change for the reaction is not a function of the initial oxygen concentration under these circumstances and can be much higher than calculated from the feed gas composition.

      With the outside-in geometry, it is possible to generate destructively high surface temperatures at the permeation barrier with relatively small amounts of oxygen in the feed gas. 

       This problem does not exist in RSI PURIFIERS' Inside-Out Purifiers in which the permeation tubing is well swept and the major flow is parallel to the permeation surface rather than perpendicular to it.  Oxygen is carried immediately to the wall and burns catalytically at low concentration and therefore with a small temperature change.  Combustion products and inert impurities are pushed ahead of the incoming gas stream and are concentrated in a predictable manner as hydrogen is lost through the permeation tube wall.

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