Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

24 Nickel Based Catalysts
Compensation of sulfur poisoning effect
One of the most important parameters that affect sulfur poisoning is the space velocity. So with the
proper selection during the process the effect could be eliminated. Alternatively, a desulfurization
process of the feed prior to the reaction over the catalyst is recommended. A commonly used procedure
is the conversion of H2S into metal sulfides by the addition of ZnO which is the universal
sulfur adsorption material in modern desulfurization units for cylindrical pellets (Aasberg-
Petersen,2011).
Desulfurization: ZnO +H2S → ZnS+ H2O (29)
Regeneration: ZnS + 3/2 O2 → ZnO + SO2 (30)
This conversion occurs in the temperature range of 315-530°C which produces SO2 during regeneration
through oxidation at 590-680°C (Mondal,2011).
It has also been reported by many researches that the use of guard bed of dolomite before the gas
enters the catalyst bead could really decrease the concentration of sulfur in the gas feed (Sutton,2001).
3.1.2 Carbon Formation
There is not a single chemical mechanism regarding carbon formation. The coke deposition depends
on the nature and the surface of the catalyst, Figure 13, (Lassi,2003), the operating conditions
which affect the reactions that occur and the composition of the feed. Especially, with the
increase of pressure the carbon formation is enhanced (Hepola,1997b).
Figure 13: Carbon formation (Lassi,2003)
The carbon can either react with water or form products such as H2, CO2 or CO. or pass through
series of steps leading to carbon deposition (Zhang,2007). Coke formation is the main deactivation
reason of catalysts (Pfeifer,2008). Three main carbon types exist: pyrolytic, encapsulating and
whisker carbon (Sehested,2006).
Pyrolytic: When the catalyst’s activity is decreased, higher hydrocarbons don’t decompose and so
they are exposed to higher temperatures and form this type of carbon (Sehested,2006). It can be
avoided through proper design of the catalyst (Aasberg-Petersen,2011).
Encapsulating carbon (gum): This type of carbon is formed during the reforming of heavy hydrocarbon
feeds with a high content of aromatic compounds. It consists of a thin CHx film or of a few
layers of graphite in between the nickel particles (Sehested,2006). The formation of this type is
enhanced by low temperatures and the high final boiling point of the hydrocarbon mixture. Therefore
after pressurized tests it was reported that a dense layer of encapsulating carbon was formed
(Hepola,1997a).
Whisker: It is considered as the most destructive form of carbon. It is formed because of the reaction
of hydrocarbons or CO of the nickel particle on the one side of the catalyst and the nucleation
of graphite carbon as a carbon whisker on the other side.
A correlation between carbon formation and small concentrations of H2S has been reported indicating
that small amounts of sulfur enhanced the carbon formation compared to higher amounts, dur-