Nickel sulfide catalysts regeneration

The first step in regenerating the coated, inactive nickel sulfide catalysts is to remove the reactants and products of the catalytic reaction aloi with the extraneous material coating the catalyst. In most cases, extraction of the catalytic mass with solvents, or purging with steam is adequate for this clean-up. The more resistant film of coke or tar is then removed by oxidation with air under carefully controlled conditions to avoid over-heating the catalyst. During this oxidation, the nickel sulfide is usually oxidized to NiO. This NiO is then reduced and sulfided in the same manner as in the initial preparation of the catalyst. 

Sulfur material balances on the system showed that the sulfur contained in the coal charge became primarily distributed between the ash and methanation catalyst while some was lost to the system. The buildup of sulfide sulfur on the nickel catalyst amounted to 25-45% of that initially contained in the coal. This acquisition of sulfur by the methanation catalyst indicates that periodic catalyst regeneration would probably be necessary to maintain it in a sufficiently active form. In spite of the coal s predominant form of sulfur being organic, the sulfur remaining in the ash was predominantly in the sulfate form. 

The catalysts/adsorbents are frequently nickel based [255], containing high amounts of nickel similar to hydrogenation catalysts. The operating temperature is in a range between 100 and 300 °C. The catalysts form nickel sulfides and their regeneration is possible by treatment in air. An alternative are the nickel/zinc oxide catalysts, which initially form nickel sulfide and subsequently zinc sulfide, while the nickel is regenerated [109]. 

In fact in the case of nickel transformation to nickel sulfide (reaction 5.10), passing pure gas over the poisoned anode can regenerate the electrochemically inert metal sulfide to the original, active catalyst ... 

The usual catalysts are based on cobalt, nickel, molybdenum and tungsten sulfides, generally combined and deposited on alumina. The most widely used formula is a composite sulfide of molybdenum and colbalt oo alumina. Run length and catalyst life are longer than those of tbe catalytic systems employed in brst step hydrogenation, Le. 6 to 12 months and 3 to 5 years, and the regeneration method is identical... 

Organo-metallic compounds contained in the feed will be decomposed and the metals will be retained on the catalyst, thus decreasing its activity. Since metals are normally not removable by oxidative regeneration, once metals have poisoned a catalyst, its activity cannot be restored. Therefore, metals content of the feedstock is a critical variable that must be carefully controlled. The particular metals which usually exist in vacuum gas oil type feeds are naturally occurring nickel, vanadium and arsenic as well as some metals which are introduced by upstream processing or contamination such as lead, sodium, silicon and phosphorous. Iron naphthenates are soluble in oil and will be a poison to the catalyst. Iron sulfide as corrosion product is normally not considered a poison to the catalyst and is usually omitted when referring to total metals. 

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