Nickel catalysts, coking

Nickel catalysts are also used for steam methane reforming. Moreover, nickel catalysts containing potassium to inhibit coke formation from feedstocks such as LPG and naphtha have received wide appHcation. 

The presence of contaminant metals on the equiUbrium catalyst can significantly increase the catalyst coking tendency, which in turn results in an increase in regenerator temperature if all other factors remain unchanged. As one example, if the metals on an FCCU equiUbrium catalyst increased from an equivalent-nickel value of 2000 wt ppm to 3500 wt ppm, the catalyst coke factor would increase 30—50%. If all controllable parameters remained constant, the regenerator temperature would be expected to increase 35—50°C and conversion would drop. 

Baker, R.T.K, Chemistry and physics of carbon, Marcel Dekker, New York, 14, 1978, Trimm, D.L., The formation and removal of coke from nickel catalyst, Catal. Rev. Sci. Eng., 1977, 16, 155 189. 

Rostrup-Nielsen, J. R. 1974. Coking on nickel catalysts for steam reforming of hydrocarbons./. Catal. 33 184-201. 

Higher Hydrocarbons. - A number of papers describing the steam reforming of higher hydrocarbons are particularly concerned with the subject of carbon deposition on the catalysts. The subject of carbon deposition on nickel catalysts is considered to be somewhat outside the subject of this review, especially as the subject is covered by two excellent recent discussions of papers on carbon deposition and coking during steam reforming, methanation, and other reactions.202 203... 

Sulphur is the most severe poison for steam reforming catalysts. A detailed study of sulphur contamination is provided in [7], On the other hand, sulphur may have a positive effect too, because it may depresse coke formation on nickel catalysts [16],... 

Earlier studies in nickel catalysts resistance to coking in steam reforming showed that the carbon deposition rate depends not only on such direct factors as nickel dispersion [10] or the support composition [11], but also on indirect factors, connected with the preparation and pretreatment conditions of the systems, the latter influence the coking rate by causing changes of the direct factors [12]. 

The length of the induction period is affected by all the same factors which affect the coking rate of nickel catalysts in the steam reforming of hydrocarbons. A high dispersion of nickel, an increase in MgO content in the support or a small additive of a promoter all cause a prolongation of the induction time. 

Supported and coprecipitated nickel catalyst represent an interesting alternative, from the economical point of view, to other selective hydrogenation catalysts such as Pt or Pd, of higher performance but also with a higher price. In the present study, a coprecipitated NiO/NiAl 04 catalyst has been selected to cany out the selective hydrogenation of acetylene to ethylene as a test reaction. One important characteristic of this process is the large amount of coke which may be generated [l]. 

As time progresses, the catalyst in the HDS reactor decays because of metal (vanadium and nickel) and coke depositions. The deposition of these metals occurs nonuniformly along the length of the reactor (more deposits occur near the reactor inlet than at the reactor outlet). In normal plant operations, the catalyst activity decline is counterbalanced by a rise in feed temperature, a reduction in the amount of quench fluids fed to the reactor or both, so as to achieve the same quality product. The process is terminated upon the attainment of a maximum allowable temperature (MAT) anywhere in the reactor. The catalyst bed is then regenerated. The time required to achieve the MAT is often called the reactor cycle life. 

Effects of Potassium on the Catalytic Behavior of Coked Nickel Catalysts in Hydrogenation and Hydrogenolysis Reactions... 

Coking resistant Nickel Catalysts for Hydrocarbon Steam Reforming Song Ruojun, Zhang Liangqu and Guo Shendu... 

EFFECTS OF POTASSIUM ON THE CATALYTIC BEHAVIOR OF COKED NICKEL CATALYSTS IN HYDROGENATION AND HYDROGENOLYSIS REACTIONS... 

In summary, although a good correlation between nickel surface area and activity for hydrogenation of benzene has often been found on clean nickel, this seems not to be the case on coked nickel catalysts. Indeed, the growth of well-defined forms of carbon is expected to change the nature of the active sites, potentially giving rise to a new catalyst. 

COKING-RESISTANT NICKEL CATALYSTS FOR HYDROCARBON STEAM REFORMING... 

Nickel catalysts find wide industrial application in the hydrogenation of unsaturated organics, steam reforming and meihanation. However, deactivation of these catalysts through coking is a serious problem. 

---