Experimental studies of intrinsic kinetics and diffusion during methane steam reforming

Hou, K

Abstract

The intrinsic kinetics and diffusion behaviour of methane steam reforming have been
investigated in this work. Measurements of effective diffusivities of the gases present in
methane steam reforming have been carried out by using the steady-state technique over
wide ranges of temperature and pressure in a modified Wicke-Kallenbach (W-K) type
diffusion apparatus. The effects of diffusion limitation on the reactions were examined at
atmospheric pressure in a pellet reactor; and the intrinsic kinetics of methane steam
reforming have been studied on a commercial nickel/alumina catalyst (ICI 57-4) in an
integral reactor. A simulation study has been carried out to determine the effects of
hydrogen removal on the performance of a membrane reactor and the catalyst activity for
methane steam reforming.
For the measurements of effective diffusivities, the temperature and pressure dependencies
of effective diffusivities of gases measured have been obtained, and the tortuosities of
pellets used for the gases measured have been estimated by using the parallel path pore
model. At ambient pressure, the temperature exponent values range from 1.0 to 1.25. This
indicates that the diffusion occurs in the transition region. At pressures up to 1MPa, the
diffusion lies mainly in the bulk diffusion region. The pressure exponent was generally less
than 1.0 with values lying between 0.4 and 0.85 except for gas-water vapour pairs where it
is close to 1.0. The tortuosities estimated for the pellets varied from 1.84 to 2.51 for
different gases at ambient pressure, but decreased with increase in pressure.
Using the pellet reactor that couples the diffusion and reaction for methane steam reforming,
experimental results, which were obtained over catalyst pellets without holes, show that the
diffusion rate of methane into the catalyst pellets almost totally controlled the reaction rate. Under such diffusion limitation, other conditions did not show any apparent effects on the
reaction. For catalyst pellets that contained four holes, the diffusion limitation on reaction
was considerably reduced. However, catalyst activity did not play an important role in
affecting the reaction. It was found that the effects of diffusion limitation on the catalyst
pellet with a higher activity are greater than on catalyst pellet with a lower activity.
For the study of the intrinsic kinetics of methane steam reforming, the effects of
temperature, pressure, and ratio of steam/methane on reactions have been investigated
experimentally under condition of no diffusion limitation. The effects of total pressure on
initial reaction rates indicated that the rate controlling steps of steam reforming are surface
reactions between adsorbed species. The experimental results confirmed that both CO and
CO2 are primary products of steam reforming. Six detailed reaction mechanisms were
considered by combining different adsorption behaviour of methane and steam on the nickel
catalyst. For methane steam reforming, accompanied by water gas shift on the catalyst used,
intrinsic rate equations were derived by using the Langmuri-Hinshelwood-Hougen-Watson
(LH-HW) approach and Freundlich's non-ideal adsorption concept. Applying the method of
parameter estimation and model discrimination, the new model was determined and the
parameters in this model were determined as statistically significant and thermodynamically
consistent.
The influence of hydrogen removed on the catalyst deactivated by hydrogen sulphide
poisoning and carbon formation has been simulated for methane steam reforming in a
tubular catalytic reactor with a hydrogen permeable wall. The effects of the main variables
on H2S tolerance and the tendency to carbon formation on the catalyst have been
investigated. The simulation demonstrated that the hydrogen removed by the membrane may cause more extensive catalyst deactivation with the H2S tolerance decreasing and the
tendency to carbon formation increasing as the proportion of hydrogen removal increased.
The simulation also showed that the benefit of using a membrane reactor may not be
achieved for feedstocks with a high H 2S level when a high proportion of hydrogen is
removed. A higher applied pressure and a more efficient desulphurisation technique need to
be employed to compensate for the influence of significant hydrogen removal on the
catalyst activity for methane steam reforming in a membrane reactor operated at low
temperatures.

Citation

Hou, K. Experimental studies of intrinsic kinetics and diffusion during methane steam reforming. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Sep 23, 2011
• Publicly Available Date: Sep 23, 2011
• Award Date: Jan 1, 1998