The
kinetics and diffusion effects of methanol steam reforming
reaction over commercial Cu/ZnO/Al2O3 catalyst
was studied for steam to methanol (S/M) ratios of 0 to 1 and pressures
below 6 bar. Our objective is the development of a novel high-pressure
propulsion technology based on the concepts of thermochemical recuperation
(TCR) and onboard hydrogen production. A simple kinetic model assuming
methanol decomposition followed by water–gas-shift was used
to estimate the rate constants (k
MD, k
WGS). The apparent activation energy of k
MD was estimated as 45–55 kJ/mol for
large pellets and S/M = 1.0, 0.5, and 0.0; k
MD for S/M = 0 (and the conversions obtained) were smaller
than those of S/M = 1, probably due to CO inhibition. At temperatures
above ∼500 K, the WGS is at equilibrium. Strong pore-diffusion
limitations are evident at 1 bar for the 3 mm catalyst, evident experimentally
as well as by analysis; the apparent k
MD is almost diffusion free for particles of 0.7 mm in diameter. This
resistance increases, of course, with P. The selectivity
of CO (dry basis) increases with W/F
meth (weight of
the catalyst divided by flow rate of methanol in units of kgcat s
mmol−1) and increases with decreasing particle size.
As the pressure increases, the ratio of CO and CO2 increases
moderately up to 4 bar; at 6 bar the change is drastic. Similar observations
were made with S/M = 0.5 and 1.0. Deactivation rates and coke formation
were also studied and were found to be marginal under atmospheric
pressures over a period of 10 h and became evident only at S/M = 0
and 275 °C; at 6 bar the decline was evident already at S/M =
0.5. The source of deactivation was attributed to coking, a conclusion
based on TPO of spent catalyst. In the case of S/M = 1 and 0.5, and
at 6 bar, there was a shift in the composition from CO2 to CO with time over a period of 10 h at 275 °C. However, the
carbon deposition in all cases was estimated to be about the same.
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