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 (Br 2 ) 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-NiBr 2 to form Br 2 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 solidbased chemical looping systems, the liquid bromide salt (NiBr 2 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 205 MW, and the heat exchanged in a corrosive environment in the conventional process is 581 MW.

A Review of Methane Activation Reactions by Halogenation: Catalysis, Mechanism, Kinetics, Modeling, and Reactors

“…Fluidized bed reactors have also been considered with advantages of easy temperature control and coked catalyst regeneration [90]. In a similar way as methanol is coupled to gasoline components on H-ZSM-5 or to olefins on SAPO-34, methyl halides can also be used as precursors for higher hydrocarbons [18,90]. Methyl halides can be obtained from methane and elemental halogen in one step by either free radical homogeneous or heterogeneous catalytic reactions [30,91].…”

“…Hydrogen halide regeneration into halogen is necessary for economic, environmental, and safety reasons. It was studied by Upham et al [18] using chemical looping of a molten salt. They considered 68 different metal bromides/oxide systems and chose the KBr-LiBr-NiBr 2 eutectic system.…”

“…NiO is a suspension of solid particles in a LiBr-KBr system while NiBr 2 is soluble in LiBr-KBr [18]. This feature is especially attractive, since mechanical attrition of particles cause problems with the increase of the surface area or with filtering.…”

“…As NiO was consumed, the solid phase disappeared and was reformed in the regeneration step (Equation (25)). Upham et al [18] also considered using a molten phase on a SiO 2 solid carrier in a packed-bed and found that this option was not the best because of the lower overall activity in HBr capture. Considerable cracking of propylene, the model compound for higher HCs, and associated coking were also observed when they used SiO 2 supported NiO/NiBr 2 .…”

“…Considerable cracking of propylene, the model compound for higher HCs, and associated coking were also observed when they used SiO 2 supported NiO/NiBr 2 . The probable cause for less coking and cracking of HCs in the molten phase, as compared to a SiO 2 supported system, was suggested to be the considerably lower solubility of HCs in the molten salt, as opposed to the solubility of HBr [18]. Hydrogen bromide capture and regeneration of bromine took place at approximately the same temperature ranges as methane activation with bromination and downstream coupling (385-530 • C), making the proposed Br 2 regeneration very suitable for integration into the entire gas to liquid process [18].…”