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