Steam reforming of methane, ethane, propane, butane, and natural gas over a rhodium-based catalyst

doi:10.1016/j.cattod.2009.01.008

Catalysis Today

Volume 142, Issues 1–2, 15 April 2009, Pages 42-51

https://doi.org/10.1016/j.cattod.2009.01.008 Get rights and content

Abstract

Steam reforming of methane, ethane, propane, butane, and a sulfur-free natural gas is studied over a rhodium-based monolithic honeycomb catalyst. The product distribution is analyzed as function of temperature (250–900 °C) and steam-to-carbon ratio (2.2–4) for two honeycomb channel densities (600 and 900 cpsi) and an uncoated monolith by gas chromatography and mass spectroscopy. The reactive flow in the single monolith channel is modeled by a two-dimensional flow field description coupled with detailed reaction mechanisms modeling surface and gas-phase kinetics. Ethane, propane, and butane are converted at much lower temperature than methane, also in natural gas mixtures. An impact of the presence of the higher hydrocarbons on methane conversion in steam reforming of natural gas is found. Steam reforming in the pure gas phase occurs only above 600 °C and the product spectrum differs from that of catalytic conversion.

Introduction

Steam reforming of hydrocarbons is an important chemical processes [1], [2], [3] providing synthesis gas (H₂ and CO), which can subsequently be converted to numerous valuable basic chemicals. The most prominent catalysts for the reforming of natural gas are nickel, which is the conventional catalyst in industry, and rhodium. According to literature, rhodium has been extensively studied as catalyst for steam reforming (SR) of methane [4], [5], [6], [7], [8], [9], [10], [11] and propane [12], [13], [14], [15], [16], but no study has been found for SR of ethane and only one for SR of butane [17].

Modeling of catalytic SR of natural gas has mainly been based on global kinetic expressions [18], [19], [20] and thermodynamic calculations [21]. Even though, a variety of detailed multi-step surface reaction mechanisms have been published over the last years, for instance for modeling partial and total oxidation of hydrocarbons over Pt [22], [23], [24], [25], [26], Ni [27], [28], [29], [30], [31], and Rh [29], [30], [31], [32], [33], there is no mechanism available which covers SR of natural gas, which in fact consist of more than methane.

This article presents an experimental and modeling study on steam reforming of methane, ethane, propane, butane, and natural gas over a rhodium-based catalyst. The final objective of this study is the development of a detailed mechanism including conversion of the higher alkanes present in natural gas and potential gas-phase reactions.

Subsequently experiments using ethane, propane, and butane were performed. In natural gases alkanes higher than butane are only found as traces with concentrations less than 0.04% [34] and were not investigated as their influence is negligible.

Section snippets

Experimental set-up

In this study steam reforming of the different alkanes is investigated at temperatures ranging from 300 to 1000 °C. The experiments were carried out in a tubular flow reactor schematically depicted in Fig. 1. Distilled water for steam production was kept in a pressure reservoir at 3 bar. The flux of liquid water to the vaporizer was electronically controlled by a Bronkhorst Hi-Tec LiquiFlow. The gas compounds argon, methane, ethane, propane, butane, and natural gas were electronically controlled

Modeling and simulation approach

In modeling the reactor, the following simplifications have been made:

(1)
Since the measured temperature difference between the front and back of the catalyst were always below 10 °C, isothermal conditions can be assumed. The foam monolith in front of the catalyst ensures a uniformly distributed inlet flow at the catalyst front face. Hence, all channels behave essentially alike and only one channel of the monolith needs to be analyzed.
(2)
The coating led to almost round channel cross-sections.

Surface reactions

The development of the surface reaction mechanism for steam reforming of natural gas proceeded over several steps. The new mechanism is based on mechanisms previously developed in our group for autothermal reforming (also known as oxidative steam reforming) [43] and partial oxidation [33] of methane over Rh. The autothermal reforming mechanism extends the latter one by introducing four additional reactions of surface-adsorbed HCO. HCO was suggested by Yan et al. [44] to play a crucial role in

Results

The steam-to-carbon ratio (S/C), which is the total number of water molecules divided by the total number of carbon atoms in the feed gas, serves as parameter to describe the feed composition. For propane, for example, S/C 3 means 9 molecules of water per molecule propane. For natural gas, an average carbon content per molecule was calculated based on the gas composition. The S/C calculation also accounts for the amount of CO₂ contained in the natural gas.

Conversion, selectivity and yield are

Discussion

Conversion (Fig. 2, Fig. 4, Fig. 5, Fig. 7, Fig. 8, Fig. 9) and product selectivity (Fig. 3, Fig. 6, Fig. 13) in steam reforming of alkanes depend on S/C. The conversion of all alkanes slightly increases with increasing S/C for a given temperature. The decreasing increments of conversion with more steam addition (Fig. 2) is caused by the thermodynamics of the system (Fig. 12). CO selectivity strongly increases with rising temperature but falls with rising S/C (Fig. 3, Fig. 13) according to the

Conclusion

Based on extensive experimental studies of SR of the major single alkane components of natural gas and SR of a sulfur-free natural gas mixture, a detailed reaction mechanism for the catalytic conversion over a Rh-based catalyst was developed and evaluated by comparison of experimentally derived and numerically predicted conversion and selectivity. The mechanism was implemented into a two-dimensional flow field description in a single monolith channel and also coupled with an elementary step

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Steam reforming of methane, ethane, propane, butane, and natural gas over a rhodium-based catalyst

Abstract

Introduction

Section snippets

Experimental set-up

Modeling and simulation approach

Surface reactions

Results

Discussion

Conclusion

Advances in Catalysis

Catalysis Today

Applied Catalysis A: General

Applied Catalysis A: General

Catalysis Today

Catalysis Today

Catalysis Today

Applied Catalysis A: General

Studies in Surface Science and Catalysis

Catalysis Today

International Journal of Thermal Sciences

Fuel Processing Technology

Applied Catalysis A: General

Catalysis Today

Applied Catalysis A: General

Journal of Power Sources

Theochemistry

Surface Science

Chemical Engineering Science

Catalysis Today

Surface Science

Catalysis Today

Surface Science Reports

Advances in Catalysis

Journal of Catalysis