Modelling Studies To Proper Size A Hydrogen Generator For Fuel Cells

G. Maggio, V. Recupero, R. Di Leonardo, M. Lagana Istituto CNR-TAE, via S.Lucia sopra Contesse 39, 98126 S. Lucia, Messina, Italy.

Introduction

Based upon an extensive survey of literature [1-3] a mathematical model has been developed to study the temperature profile along the catalytic bed of a reactor for the methane partial oxidation. The model allowed a preliminary design of a 5 Nm3 syngas/h prototype to be integrated with second generation fuel cells as hydrogen generator (in the framework of the EC-JOU2 contract). This design was based on some target features, including the choice of a GHSV (gas hour space velocity) equal to 80000 h"1, a catalyst particle size of 1/8", a molar air/methane ratio of 2.7 (i.e. OJCU4=O.S3), a linear velocity in the catalytic bed of about 2 m/sec, and an inert/catalyst ratio 3:1. Starting from this data, the work has been concerned with the identification of the controlling regime (kinetic or diffusional), and then with the estimation of the gas composition and temperature profiles along the reactor. A comparison between experimental and model results has also been accomplished.

Modelling approach

Identification of the controlling regime is paramount for calculating the basic dimensions of the reactor and for determining the degree of partition of the inert diluent solid alongside the whole length of the catalytic bed to assure quasi-isothermal conditions, in order to avoid hot spots. As it is generally agreed [1-4] that the catalytic selective partial oxidation of methane (CSPOM) reaction is very fast, i.e. the reaction is said to be at equilibrium even at very high space velocity (in the order of 104 -105 h"1), the overwhelming influence of mass and heat transfer effects is likely expected. Our results, derived from well known equations [5-6] and based on data reported in Tab.I, confirmed this statement and the targeted GHSV upper limit of 80000 h"1 should be seen as a compromise between productivity and detrimental effects by „ , local overheating.

5 Nm3 reactor data- To obtain a careful estimation of the longitudinal temperature and composition profiles, various partial methane oxidation reaction models, derived from literature [7-8], have been considered and analyzed. However, the model results seem to confirm [7] that the reaction pathway involves initial conversion of some CH4 to C02 and steam, followed by a sequence of steam reforming and water gas shift reactions, to give equilibrium product yields.

Having assumed and proved that the reaction'is controlled by external mass transport, it was further _ supposed that the composition of the reaction mixture at the catalyst surface is in thermodynamic equilibrium with the mixture in the bulk flow.

Temperature

800°C=1073K

Pressure

2 ata

Inlet total gas flow rate

6.85 m3/h= 305.61 mol/h

Inlet gas composition

CH4=27.0%, 02=14.6%, N2=58.4%

Reactor internal diameter

5.0 cm

Catalytic bed length

17.3 cm

Mean particle diameter

1/8 inch

Bed void fraction

0 0

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