Isopropanol–acetone–hydrogen chemical heat pump (IAH-CHP) is a system having the ability to improve energy-grade and to achieve energy storage simultaneously. Acetone hydrogenation is an important part of IAH-CHP and directly affects the performance of IAH-CHP. In this work, the fully three-dimensional simulations on acetone hydrogenation in randomly packed-bed reactors with small tube-to-particle diameter ratio D/dp were carried out by Computational Fluid Dynamics (CFD). The study focused on the heat and mass transfer characteristics at inter- and intra-particles of the bed. The results indicate that the resistance of mass transfer at intra-particles is maximum in the entire transport process. Particleto-fluid heat transfer is the main resistance of heat transfer in the bed. In addition, the synergy between transport of components and reaction rate at intra-particles was analyzed for various structural parameters, such as porosity εp, pore diameter do, particle diameter dp and tube-to-particle diameter ratio D/dp, in order to improve the utilization efficiency of catalyst. This study provides a detailed understanding on transport characteristics both at intra-particles and in fluid zone in randomly packed-bed reactors with small D/dp, providing a guideline for catalyst and reactor design for this type of chemical heat pump.
Conclusions
3D simulations on both transport and reaction at catalyst particles and transport in fluid zone in randomly packed beds with small D/dp were carried out by CFD in this study. At first, a three-dimension randomly packing structure of packed-bed reactors with D/dp = 1.5 and D/dp = 3.0 are built using DEM. Then, catalyst particles were assumed as solid in order to assure that only diffusion exists at microporous particles. The reaction and transport at catalyst particles were achieved by using user defined scalars and user defined function, whereas transport of components in fluid zone did so by using the species transport equation inherent in CFD software. And the transport of components was coupled at the interface of catalyst particles and fluid zone by user defined function. The study focused on the heat and mass transfer characteristics at inter- and intra-particles of the bed and the synergy between transport of components and reaction rate at intra-particles. The major conclusions can be drawn as follows:
(1) The concentration gradient at the inner of catalyst particles is greater than that in fluid zone near particles. Mass transfer at intra-particles is main resistance of mass transfer in the bed. The temperature gradient at catalyst particles is less than that in fluid zone near particles. Particle-to-fluid heat transfer is main resistance of heat transfer in the bed;
(2) Acetone productivity increases with the increase of porosity, whereas selectivity of isopropanol first increases and then decreases with it, leading to a maximum. Both acetone productivity and selectivity of isopropanol decrease with the increase of pore size. Acetone productivity increases with the decrease of particle diameter, but selectivity of isopropanol decreases with it. Acetone productivity increases with the increase of tube-to-particle diameter ratio, while selectivity of isopropanol decreases with it.
The results have been published on Applied Thermal Engineering 94 (2016) 238–248.
