Thermodynamic analysis of adsorption enhanced reforming of ethanol

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Abstract

Adsorption enhanced reforming (AER) is a novel hydrogen generation method. To study hydrogen yield and graphite formation in the AER process, a recently developed thermodynamic code for reaction–adsorption systems is extended to account for the formation of condensed species. Parametric analysis of the steam reforming of ethanol is performed to study the effect of adsorption and other parameters (temperature, pressure, steam/carbon ratio and O2/ethanol ratio in the feed) on the yield of hydrogen and the formation of graphite. It is shown that with CO2 adsorption, the hydrogen yield is enhanced while the graphite formation is suppressed. Based on the thermodynamic analysis, AER is a potential low-temperature method for hydrogen generation.

Introduction

As reducing the demand on fossil resources has been a public concern, hydrogen, being a potential carrier of clean energy, is now an important topic that may lead into a new era of energy research. Among various industrial techniques, steam reforming of hydrocarbons accounts for about 50% of the hydrogen produced over the world. In the United States, 95% of the hydrogen production is made through steam reforming of methane [1]. In this sense, steam reforming serves as a foundation in America's transition to a hydrogen economy.

The past decade witnessed a significant development in steam reforming. On the feedstock side, the reforming of biomass-derived hydrocarbons (e.g., bio-ethanol) is widely acknowledged as a potentially viable, renewable and carbon-neutral (or even carbon-negative in conjunction with sequestration) process [2], [3], [4]. On the technology side, the steam reforming combined with CO2 removal using adsorption [5], [6], [7], [8], [9], [10] or membrane [11] is shown to significantly enhance the yield and purity of hydrogen. When CO2 is removed from the reforming process, the thermodynamic limitation of the water gas shift reaction (CO + H2O = CO2 + H2) is circumvented and therefore, the chemical equilibrium shifts to the right, resulting in an enhancement in the extent of the forward reaction. Removing CO2 from the reactive system enables a relatively lower reforming temperature, thus alleviating the requirement for both the catalyst and the reactor. Moreover, the higher purity in the syngas leaving the reformer makes it easier for downstream hydrogen separation, which is typically achieved through a pressure swing adsorption (or PSA) unit. Extensive experimental and computational studies indicate that reforming with simultaneous CO2 removal is a very promising technique for enhanced hydrogen production [11], [12], [13], [14]. Ongoing research is conducted by various companies such as Chevron, Shell and Intelligent Energy to commercialize these novel reforming technologies [8], [15], [16].

Despite their wide utilization for hydrogen generation, the reforming processes (including the aforementioned enhanced reforming processes) might be accompanied by unfavorable formation of graphite under certain conditions, which could lead to catalyst deactivation and reformer malfunction due to its accumulative nature [12], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Studies have shown that the formation of graphite can be suppressed by using high steam/carbon (or S/C) ratios in the feedstock [17], [20], [24], [28], adding oxygen to the feed [20], doping with a trace amount of sulphur, tin, chromium, molybdenum, potassium, and gold on the catalyst [18], [23], [24], [25], [29], adjusting reforming temperatures [30], enhancing support (e.g., alumina, silica, magnesia, zirconia, and ceria [25], [31]), and adding sodium [32] etc. However, due to the inherent complexity of various coupled physicochemical phenomena involved in the reforming process, no general fundamental understanding is currently available towards the graphite formation and catalyst deactivation [12], [21], [26]. Bridging such a gap could germinate solutions to a longer catalyst lifetime, which will lead to significant economical benefits in the reforming process. Thermodynamic modeling has been a powerful tool in the analysis of hydrogen generation and fuel cells [33], [34], [35], [36], [37] and may provide insight into graphite-free operating windows for the AER process. However, to the best knowledge of the author, a generic thermodynamic model that accounts for gaseous reaction, adsorption, and formation of condensed species is not yet available. Motivated by this, a comprehensive thermodynamic model is developed in this work to calculate the equilibrium compositions in both the gaseous and the condensed phases of general reaction–adsorption systems. The thermodynamic analysis is carried out for steam reforming of ethanol under various operating conditions and it shows that adsorption enhanced reforming (AER) is advantageous over conventional reforming processes not only in terms of hydrogen yield but also in graphite suppression. Such an analysis might be used to guide the design and operation of an AER process.

Section snippets

Equilibrium of reaction–adsorption systems with formation of condensed species

The aim of this work is to develop a comprehensive thermodynamic model to analyze reaction–adsorption systems coupled by formation of condensed species. In the context of AER of hydrocarbons, the adsorbed species of interest is CO2, and the condensed species is graphite [5], [10], [13], [14], [38], [39]. Multi-component adsorption and multi-component condensation might also be included in the current model to handle other condensed species and the adsorption of gaseous species besides CO2.

The

Thermodynamic analysis of the effect of CO2 adsorption on steam reforming of ethanol

A reactive system consisting of steam and ethanol is studied in this section. The steam reforming of biomass-generated ethanol would be a potentially viable and renewable process for hydrogen production [2], [3], [4]. Experimental studies have shown that for a system of hydrocarbon (e.g., methane, methanol or ethanol) and steam, the yield and purity of hydrogen can be significantly improved if CO2 is adsorbed [5], [6], [7], [8], [9], [10]. Calculations in this section will provide a

Concluding remarks

Thermodynamic analyses have been carried out for AER of ethanol. It is shown that adsorption of CO2 suppresses the formation of carbon in the reforming process. When CO2 adsorption is combined with high S/C ratios, increased pressures and partial oxidization, the formation of carbon can be further reduced. Because adsorption also enhances hydrogen yield and purity, AER would be an efficient way of making hydrogen at a reduced temperature as compared to conventional steam reforming.

It is worth

Acknowledgements

This work is supported by the American Chemical Society Petroleum Research Fund (PRF# 50095-UR5) and the Provost's Teacher-Scholar program at Cal Poly Pomona. Industrial feedback from Dr. Durai Swamy at Intelligent Energy, Long Beach, CA is gratefully acknowledged. The author would also like to thank the anonymous reviewers for their valuable comments and suggestions.

References (43)

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