Molten salt chemical looping for reactive separation of HBr in a halogen-based natural gas conversion process

Abstract

Hydrogen bromide (HBr) oxidation to molecular bromine (Br2) is demonstrated in a chemical looping process using a molten bromide salt. The two-step process is operated at 500°C and first contacts oxygen with molten KBr-LiBr-NiBr2 to form Br2 gas and a suspension of nickel oxide (NiO) particles in one reactor. The oxide suspension is then contacted with HBr to regenerate the bromide salt and produce steam. Sixty-eight metal oxides/bromides were considered. The cyclic interconversion between oxide and bromide, by alternating exposure to HBr and oxygen, at a single temperature was only possible with nickel. In contrast to solid-based chemical looping systems, the liquid bromide salt (NiBr2 dissolved in KBr-LiBr eutectic) was found to be cycleable without attrition or deactivation. Further, when mixtures of olefins and hydrogen bromide were reacted with the oxide suspension, selective oxidation of HBr was observed without hydrocarbon oxidation. High selectivity for HBr oxidation is due to the solubility of HBr in the molten salt, which allows contact with NiO, whereas, the insoluble hydrocarbons do not contact the reactive oxide. A process model that makes use of reactive separation of HBr from hydrocarbons and process intensification using molten salt-based chemical looping is presented as a potentially lower cost alternative to a process model using conventional separations in bromine-based methane conversion. The total heat exchanged in a corrosive environment in the molten salt based process is 205MW, and the heat exchanged in a corrosive environment in the conventional process is 581MW.