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

26 Nickel Based Catalysts
argon flow that contained 45 vol% vapors but none of these procedures had as good results as the
first one. What was interesting was the fact that the activity of the regenerated catalysts decreased
more rapidly with the gasification time due to sintering (Yamaguchi,1986). Regeneration is considered
as the removal of carbon deposits could also be achieved by putting the catalyst into H2O or
CO2 stream (Wang,1998).
This procedure was followed also by Arauzo et al. (Arauzo,1997). They put the catalysts on stream
of H2O or CO2 or mixtures of both and achieved complete gasification of secondary coke and soot
that was deposited on the catalyst surface. This method was tested into different states regarding
the catalytic reforming.
In the first test the regeneration was made once the run of the experiment was finished, so a slow
combustion in situ at 500 °C was realized. The catalyst regained its original color and it appeared
completely clean of coke. In the second test the regeneration was made during the run of the experiment
by using different gasifying mediums like small amount of oxygen (0.25 L/min of air) and
low sand/catalyst ratio was chosen in order to emphasize on the effect of coke gasification. The
catalyst regained similar activity to the fresh catalyst but there was a small loss of activity by attrition.
3.3 Reduction of the Ni-based catalysts
It has been reported that the reduction procedure affects significantly the poisoning of the catalyst
due to sulfur (Hepola,1997a). Therefore it has a clear effect on the catalyst activity and life (Sutton,2001),
(Pfeifer,2008). According to tests made by M.P. Aznar et al. it was concluded that when
catalysts are previously reduced the conversion of tars by steam reforming is higher (Aznar,1993).
Temperature control during these procedures is important in order to eliminate an exothermally
generated over-temperature condition that could possibly damage the catalyst (Elliott,1993).
The usual procedures are:
a) Reduction can be performed by using only the synthetic gas mixture, which is flown on the
catalyst. Then a slow reduction procedure is performed with a (50%-50%) N2/H2 mixture
over 12-16 hours at the heating rate of 50 °C/h to determine whether the synthetic gas had
any effect on the sulfur poisoning of the catalyst (Hepola,1997a), (Hepola,1997b).
b) Zhang et al. (Zhang,2007) reduced the catalysts at 700°C, with a flowing mixture of (50%-
50%) N2/H2 for 2.5 hours at a flow rate 80ml/min. For each test there were filled 0.5 ml of
20 and 30 mesh particle size catalysts into the reactor.
c) The sample catalyst is heated up under a constant flow of He (20ml/min) and then reduced
at a constant flow of for hydrogen for 1 hour at 850 °C (10 ml/min).When the demanding
reaction temperature is reached the catalyst is charged with steam and toluene (Pfeifer,2008).
d) Activation in situ. In the catalytic bed with a flow of hydrogen at 450°C for 2 h followed by 1
hour at 750 °C (Aznar,1993).
e) The catalyst is placed inside the reactor to be purged with nitrogen and pressurized with
hydrogen to about 1 MPa. Then it is heated slowly to the reduction temperature (300-
400°C). Finally it is subjected at 700°C in H2 stream for at least 3 hours, usually overnight
(Elliott,1993).
f) The simplest process would be to put the catalyst in hydrogen stream for at least 4 hours
at 700°C (Yamaguchi,1986).
g) By heating the catalyst and keeping it at 900°C for 1h in (50%-50%) N2/H2 mixture
(Simell,1997).
h) Under the reactants gas mixture for 30 minutes at 750 °C (Świerczyński,2007).