Thermal Conductivity of Hydrocarbon Liquid Organic Hydrogen Carrier Systems: Measurement and Prediction

In this study, the thermal conductivity of diphenylmethane, benzyltoluene, and dibenzyltoluene and their fully hydrogenated counterparts relevant in the context of the liquid organic hydrogen carrier (LOHC) technology was investigated. For the determination of the thermal conductivity between 298.15 and 323.15 K at atmospheric pressure with an expanded uncertainty of 5% (k = 2), a steady-state guarded parallel-plate instrument was used. To discuss the thermal conductivity over a broader temperature range, measurements were also performed for the density of the pure LOHCs and binary mixtures of diphenylmethane and dicyclohexylmethane between 293.15 and 363.15 K by vibrating tube densimetry as well as for the thermal diffusivity and the Fick diffusion coefficient of binary mixtures of diphenylmethane or dicyclohexylmethane with dissolved hydrogen between 323 and 573 K and pressures between 2.7 and 6.2 MPa by dynamic light scattering (DLS). In general, the measured thermal conductivities of the compounds under investigation decrease with increasing temperature and molar mass and with the degree of hydrogenation. For mixtures of diphenylmethane and dicyclohexylmethane, the concentration dependency of the thermal conductivity shows a distinct nonideal behavior. Based on the measured thermal conductivities of the present pure substances and reference data from the literature for selected cyclic hydrocarbons, a prediction method for the thermal conductivity of such systems could be developed, which requires the molar mass and density as sole input parameters. This correlation represents the thermal conductivity of the selected dehydrogenated and hydrogenated substances and of the studied binary mixture between 298 K and their normal boiling temperature, typically within 5%. By comparison with the thermal conductivity data for diphenylmethane and dicyclohexylmethane deduced from the measurement results for thermal diffusivity and density and from literature data for the specific heat, it was found that the prediction method also allows for a reliable estimation of the thermal conductivity of LOHCs up to a temperature of 573 K, which is the relevant temperature range for hydrogen release from the hydrogenated LOHC compounds.