Nickel tungstate

Effect of Catalyst The catalysts used in hydrotreating are molybdena on alumina, cobalt molybdate on alumina, nickel molybdate on alumina or nickel tungstate. Which catalyst is used depends on the particular application. Cobalt molybdate catalyst is generally used when sulfur removal is the primary interest. The nickel catalysts find application in the treating of cracked stocks for olefin or aromatic saturation. One preferred application for molybdena catalyst is sweetening, (removal of mercaptans). The molybdena on alumina catalyst is also preferred for reducing the carbon residue of heating oils. 

In treating cracked stocks such as steam cracked naphtha or visbreaker naphtha, which are highly olefinic in nature, nickel molybdate or nickel tungstate catalysts are generally employed. These catalysts have much higher activity for olefin samration reactions than does cobalt molybdate. 

It should also be noted that this process does not alter the smoke point. Smoke point is a function of aromatics content and mild Hydrofining does not hydrogenate aromatics. To accomplish this, treating over a more active catalyst such as nickel tungstate at pressures of at least 800 psig is required. 

Hydrotreating catalysts are composed of cobalt or nickel molybdate or nickel tungstate on an alumina or zeolite support. The materials are sulfided with hydrogen sulfide (H2S) before use, but the final catalysts may retain some oxide and be of complex composition. 

NiW04 NICKEL TUNGSTATE 1230 Pb3(As04)2 LEAD ARSENATE 1275... 

As an example, we can represent the synthesis of nickel tungstate as follows ... 

Catalyst (sulfided) Early units nickel molybdate/alumina high nitt ogen conversion little cracking. Later units nickel tungstate/alumina High nitt ogen conversion up to 40% cracking. 

A typical catalyst can operate for a number of years, and any slow loss of activity can be compensated for by a gradual increase in process operating temperature until the reactor temperature limit is reached. Regeneration is then possible, either in the reactor or externally, by burning accumulated coke in dilute air. This restores the activity of most nickel/molybdate or nickel/tungstate catalysts, which must then be resulfided before further use. 

---