In the commercial startup of processes utilizing high-pressure hydrogen, e.g. hydrocracking, it is common practice to carry out a preliminary pressure test of the catalyst-loaded reactor in order to insure against possible leakage of combustible gases during subsequent processing. Commonly, for safety reasons an inert gas is employed for this pressure testing, nitrogen being preferred for economy reasons. For most operations nitrogen performs very satisfactorily, but for reasons which are still largely conjectural, at elevated temperatures (above 200.degree. F) nitrogen has been found to have a substantial adverse effect upon certain catalysts comprising a Group VIII noble metal supported on crystalline aluminosilicate zeolites, wherein the zeolite support has been substantially converted to a hydrogen and/or dehydroxylated form. In some cases, it has been found that after undergoing nitrogen pressure testing, the activity of such catalysts for hydrocracking and/or hydrogenation is drastically reduced, to levels of only a small fraction of the initial fresh activity. Surprisingly, seemingly analogous catalysts based on metal-stabilized crystalline zeolites, such as magnesium-stabilized zeolites, do not appear to be affected adversely by high-temperature nitrogen. Another puzzling aspect of the invention is that the damage appears to occur only when the catalyst is in an oxidized state; in the reduced state, little or no damage occurs.
In view of the foregoing, it would appear that the above noted damage could be avoided by either of two possible expedients. Firstly, since the nitrogen damage becomes significant only at temperatures above about 200.degree. F, pressure testing could be carried out at temperatures sufficiently low to avoid significant damage. However, this alternative would be hazardous in many cases due to the "hydrogen-embrittlement" which many catalytic reactors composed of ferrous alloys may have previously undergone through extended use at high hydrogen pressures. When such embrittlement occurs, the "transition temperature" of the reactor walls may substantially incrase, to levels in the range of about 200.degree.-300.degree. F. The transition temperature is the temperature below which cracks in the reactor walls will propagate (at a given operating pressure) in an instantaneous, catastrophic manner, and above which cracks will be arrested by the inherent toughness of the metal. It is hence considered a prudent safety measure to conduct pressure testing (at pressures in excess of about 700 psig) only at temperatures above about 200.degree. F, and preferably above about 300.degree. F.
In view of the foregoing hazards, and in view of my discovery that high temperature nitrogen has a much less adverse effect upon the catalyst when in a reduced state, the present invention is directed to the feature of prereducing the catalyst with a noncombustible mixture of nitrogen and hydrogen prior to the high temperature pressure testing. Catalysts of the present description are ordinarily loaded into reactors in a calcined, oxidized state, and later activated by reduction and dehydration with high-pressure, high-temperature hydrogen following the pressure testing. I have now found that it is perfectly feasible to reduce the catalyst with dilute hydrogen at relatively low pressures and temperatures prior to pressure testing, thereby avoiding to a major extent the above noted nitrogen damage during pressure testing, and then complete the activation-dehydration as an integral part of the normal process startup procedure. The prereduction is carried out using a mixture of nitrogen and hydrogen in which the hydrogen concentration is below that required to give a combustible mixture, i.e., below about 6 volume percent. This procedure permits prereduction to be carried out safely without danger of fires or explosions in the event of leakage from the reactor. If desired, the same dilute nitrogen-hydrogen mixtures may be utilized for the subsequent pressure testing.