Liquid organic hydrogen carrier (LOHC) technology enables hydrogen storage at ambient conditions as hydrogen is chemically bound to a carrier molecule. The LOHC system benzyltoluene (H0-BT)/ perhydro benzyltoluene (H12-BT) has the best combination of low vapor pressure and viscosity of all pure hydrocarbon LOHC systems [1]. The endothermal dehydrogenation of H12-BT is typically conducted in fixed bed reactors, feeding H12-BT as a liquid. The dehydrogenation unit is a multiphasic system with changing hydrodynamics as gaseous hydrogen is released resulting in evaporation of the LOHC. Under technically relevant conditions the LOHC evaporates completely in the reactor resulting in a reduced heat transfer in the catalyst bed and a decrease in productivity [2]. Furthermore, catalyst deactivation is more pronounced in pure gas phase dehydrogenation as coke precursors can no longer be washed off the catalyst surface. So, multiphasic conditions are highly beneficial for the performance of a dehydrogenation unit.
Evaporation of H12-BT can be prevented by lowering temperature or increasing pressure. However, both parameters would negatively affect the catalyst activity of endothermal dehydrogenation. Therefore, it is not desirable to prevent complete evaporation by lowering temperature or increasing pressure. The evaporation of H12-BT also depends on hydrogen to LOHC ratio. A low hydrogen to LOHC ratio prevents the evaporation of H12-BT, but results in a low utilization of the LOHC, which negatively impacts the economics of LOHC technology. Therefore, we evaluated a loop concept for the dehydrogenation of H12-BT. In this concept H12-BT is diluted with a recycled H0-BT stream, reducing the hydrogen to LOHC ratio while still maintaining high LOHC utilization. We will show that the reduced catalytic activity due to the decreasing concentration of H12-BT can be compensated by improved heat transfer and longer residence time due to reduced evaporation resulting in a performance boost.
References
[1] https://doi.org/10.1039/D1SE01767E
[2] https://doi.org/10.1016/j.ijhydene.2024.02.096
