3.6. Time on stream study
The progress of hydrogen and sulfur yield with time on stream
was analyzed for the case of 3J1 catalyst for an extended 45 h run, as
shown in Fig. 10a. This sulfur yield was estimated based on the effluent
hydrogen sulfide in the dry gas stream. Sulfur yield increased
over the first 2 h, after which it decreased steadily, indicating sorption
of sulfur at the start up. The driving force for adsorption was
highest at the start when the surface of catalyst had no sulfur.
After the catalyst became saturated with sulfur, it was anticipated
that the yield of sulfur would stay at maximum. Instead, this value
decreased in parallel with that of the hydrogen yield. This observation
can be explained by the detection of unreacted organic sulfur
in the liquid effluent. The gas chromatograph peak corresponding
to thiophene (present in equilibrium in vapor phase) was seen at
a retention time of 10 min during a typical gas chromatographic
run. Although the GC was not calibrated for thiophene, it was seen
that the intensity of this peak increased with the progress of reaction.
Fig. 10b shows data for both Ni/b and 3J1 catalysts. In case
of Ni/b catalyst, the peak of unreacted thiophene was seen right
after the first few minutes of the start of the reaction, whereas
with 3J1 catalyst, the unreacted thiophene was seen only after 37 h
on stream. The observed deactivation in this case can be reasonably
attributed to reduced conversion of hydrocarbon which, in
turn, may be directly correlated to reduced thiophene conversion to
hydrogen sulfide. Similar results were obtained by Mayne et al. [35]
who concluded that reaction conditions under which thiophene
exited unconverted from the reactor led to continued deactivation
of the catalysts during their study of isooctane reforming on
Ni-based catalysts.