Elsevier

Fluid Phase Equilibria

Volume 261, Issues 1–2, 1 December 2007, Pages 375-381
Fluid Phase Equilibria

Hydrogen solubility of mixed naphthenes and aromatics for a new hydrogen storage medium in fuel cell system

https://doi.org/10.1016/j.fluid.2007.07.062Get rights and content

Abstract

Hydrogen solubility was measured for four equimolar binary solvent mixtures, benzene + toluene, cyclohexane + methylcyclohexane, benzene + methylcyclohexane and cyclohexane + toluene, and an equimolar quaternary, benzene + cyclohexane + methylcyclohexane + toluene, in the pressure range from 1.077 to 4.580 MPa at 303.15 K. The hydrogen solubility increased linearly with the pressure for all systems. The binary interaction parameters, in a mixing rule of Peng–Robinson (PR) equation of state, were determined for benzene + toluene and cyclohexane + methylcyclohexane from the vapor–liquid equilibrium data of literature, and that for benzene + methylcyclohexane and cyclohexane + toluene from the hydrogen solubility data in the binary solvent mixture. Using the optimized binary interaction parameters, PR equation well predicted the experimental data for the quaternary.

Introduction

Hydrogen has been paid much attention as a clean energy to establish a fuel cell system. However, it is difficult for hydrogen to be stored with high contents because of the low critical temperature, 33.2 K [1]. To overcome the difficulty, some methods have been proposed [2], [3]. For example, NaAlH4 shows high hydrogen storage ability due to its hydrogen production by a thermal decomposition. The ability is as much as that of hydrogen cylinder loaded up to 70 MPa. Some naphthenes have been also expected to show hydrogen storage ability by use of following catalytic dehydration reaction [2], [3], [4], [5]:

The theoretical mass content of hydrogen was 7.19 mass% for cyclohexane and 6.16 mass% for methylcyclohexane, respectively. Fig. 1 shows a fuel cell system utilizing the dehydration reaction. In the system, naphthene is sent from a fuel tank to a heat exchanger. In a membrane reactor, the vaporized naphthene is catalytically converted to aromatic and hydrogen. The most part of hydrogen selectively permeates through the membrane. The mixture, composed of aromatic, unreacted naphthene and residual hydrogen, is sent to the heat exchanger, and then cooled to separate vapor and liquid phase. After passing thorough an adsorption column, the vapor was added to the permeated hydrogen. Though cyclohexane has a high hydrogen storage ability, the melting point of benzene and cyclohexane is around 278 K [1]. Comparing with those substances, that of methylcyclohexane and toluene is 146.55 and 178.16 K [1], respectively. The mixture, cyclohexane + methylcyclohexane, will be used as a new hydrogen storage medium in the lower temperature range. Therefore, the experimental data of hydrogen solubility or phase behavior are essential for designing the heat exchanger to separate and recover hydrogen effectively. In our previous studies [4], [5], hydrogen solubility in two pure naphthenes, cyclohexane and methylcyclohexane, and two pure aromatics, benzene and toluene, and the two equimolar binary solvent mixtures, benzene + cyclohexane and methylcyclohexane + toluene, were measured by use of a synthetic type apparatus at 303.15 K. In this study, hydrogen solubility was successively measured in four equimolar binary solvent mixtures, benzene + toluene, benzene + methylcyclohexane, cyclohexane + methylcyclohexane and cyclohexane + toluene, and an equimolar quaternary, benzene + cyclohexane + methylcyclohexane + toluene, at 303.15 K.

The binary interaction parameters, in a mixing rule of Peng–Robinson (PR) equation of state [6], were determined for benzene + toluene and cyclohexane + methylcyclohexane from the vapor–liquid equilibrium (VLE) data of literature, and those for benzene + methylcyclohexane and cyclohexane + toluene from the hydrogen solubility data for binary solvent mixture. Using the optimized binary interaction parameters, PR equation well predicted the experimental data for the quaternary.

Section snippets

Materials

Hydrogen was purchased from Takachiho Trading Co., Ltd., Tokyo, with the stated purity of 99.99+%. Other chemicals were special grade reagents supplied form Wako Pure Chemical Industries, Ltd., Osaka, Japan. The stated purity of benzene, cyclohexane and toluene was 99.5+%, and that of methylcyclohexane was 98.0+%. All chemicals were used without further purification.

Hydrogen solubility measurements

Fig. 2 shows a schematic diagram of experimental apparatus. The apparatus was based on a synthetic method. The advantages of the

Results and discussion

Table 1 is the summary of the experimental data, and Fig. 3, Fig. 4 show the hydrogen solubility in benzene + toluene and cychohexane + methylcyclohexane at 303.15 K. In the figures, the solubilities in pure solvent [4] were also shown. As shown in the figures, the hydrogen solubility increased linearly with the pressure for all systems. The hydrogen solubility in C7 aromatics or naphthenes was larger than that in C6 aromatics or naphthenes. For the mixture, the hydrogen solubility was smaller than

Conclusion

Reliable hydrogen solubility data were obtained for two binaries, benzene + methylcyclohexane and cychohexane + toluene, and for a quaternary, benzene + cychohexane + methylcyclohexane + toluene, at 303.15 K. The hydrogen solubility increased linearly with the pressure following the Henry's law for all systems. The hydrogen solubility can be predicted with PR equation and the mixing rule based on van der Waals one fluid model. Then, binary parameters for binary solvent mixture can be optimized not only by

Acknowledgements

The authors thank Dr. N. Taoka of Osaka New Material Center, Japan for his assistance, and wishes to acknowledge the Ministry of Economy, Trade and Industry, Japan, and New Energy and Industrial Technology Organization (NEDO) for financial supports of this research (No. 06000531-0).

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