Multi-scale Study On Transfer And Reaction Performance Of Exothermic Reactor Of Isopropanol-acetone-hydrogen Chemical Heat Pump

The energy efficiency in China is much lower than developed countries and many other countries, so there is a lot to do to make the improvement. Waste heat recovery, especially the low-temperature waste heat recovery, is increasingly becoming an important way to enhance energy efficiency and to solve the energy crisis. The low quality of low-temperature waste heat makes it difficult to be used directly, so most is discharged into the air, and hence a large amount of energy is wasted. The best way to take advantage of low-temperature heat is to upgrade its temperature, so the upgraded heat can be used directly. If this measure is realized, not only the enery efficiency can be improved hugely, but also the energy utilization scope can be expand enormously, such as low-temperature natural energy, including solar energy and geothermal energy.Chemical heat pump (CHP) is the best choice to upgrade low-temperature heat for the reason of high efficiency, no pollution, low energy consumption, large extent of temperature promotion, and so on. Among many CHPs, Isopropanol-Acetone-Hydrogen (IAH) system is one of the most prospective CHPs. IAH-CHP takes advantage of a pair of reversible chemical reactions, the endothermic isopropanol dehydrogenation reacting at about80℃and the exothermic acetone hydrogenation reacting at about200℃, to upgrade the low quality heat and make it possible to be used directly in industry.This dissertation studies the high-temperature exothermic reaction of acetone hydrogenation in IAH-CHP on multi-scales of molecular scale, porous catalyst scale, reactor scale and system scale by adopting both experiment and numerical simulation on the background of deep utilization of low-temperature boiler exhaust gas.Firstly, on the molecular scale, the amorphous alloy Raney Ni is used as catalyst for the kinetic experiment of high-temperature acetone hydrogenation, through which the three reaction mechanisms and corresponding products are confirmed, and the Langmuir-Hinshelwood kinetic equation of every reaction are promoted based on the experiment data and reasonable assumptions. The influences of the experimental conditions including space velocity, operating pressure, reaction temperature,and hydrogen flow rate are conducted and analyzed.Secondly, on the porous catalyst scale, in order to study the synergistic effect of chemical reaction and heat and mass transfer in porous media, a microscale simulation on porous catalyst particles is carried out. The simulation focuses on the influence of species diffusion on species distribution, temperature field, reaction rate, acetone conversion and isopropanol selectivity. The simulation results show that the micropore diameter is the determinant of diffusion coefficient, and there is a very thin transient zone around the catalyst particle, and both the species diffusion coefficient and the reaction rate gradients in this zone are very sharp. The reactions are taking place inside the catalyst particles, so the temperature is a little higher than the external flow. When the catalyst particle diameter increases, the acetone conversion and the isopropanol selectivity and yield increase. When the micropore diameter increases, the the acetone conversion and isopropanol selectivity decrease, and the isopropanol yield increases firstly and then decreases. According to the simulation results, the recommended values of catalyst particle and micropore are suggested in this dissertation.Then on the reactor scale the transfer and reaction performance of reactor is simulated to study the influence of catalyst particle diameter, catalyst thermal conductivity, reactor diameter and space velocity on the gas-solid temperature difference and the reaction by using the non-heat-balance porous media model. According to the simulation results, the gas-solid temperature difference is too little to affect the reaction. A larger catalyst thermal conductivity can enhance the heat transfer capability of the reactor remarkably and reduce its temperature obviously, and a larger reactor diameter will increase the reactor temperature. A larger space velocity will decrease both the acetone conversion and isopropanol selectivity, but can increase the isopropanol yield, so a larger space velocity is beneficial to the system efficiency. Based on the simulation, the best reaction temperature and hydrogen-acetone mole ratio are optimized via theoretical design calculation by analyzing their influences on reaction energy quality, reaction heat amount, acetone conversion and pressure drop. In order to solve the problems of large pressure loss and low heat transfer capability in randomly packed bed reactor, this thesis proposes a structured packed bed reactor and makes3D simulation on the reactor, and the simulation results indicate that this structured packed method can reduce the pressure loss effectively and has a better heat transfer capability.At last on the system scale a new high efficient IAH-CHP with multi-in-series exothermic reactors is proposed and studied. Contrasting with traditional IAH-CHP with one single exothermic reactor, this new system gets huge promotion in reaction heat released, and the system performance and exergy efficiency are also improved. At the same amount of heat released, the new system has a sharp decrease in material flow rate and catalyst packing amount, as well as the heat load of reboiler, compressor and heater, so the system performance and exergy efficiency have a certain improvement. Another advantage of the IAH-CHP with multi-in-series exothermic reactors is the heat released from different reactors can be used in different ways to reduce exergy loss produced by energy mixing.