GRT has developed a process for the selective partial oxidation of methane to produce liquid fuels and high value commodity chemicals. The process does not utilize a synthesis gas intermediate and is tolerant to common methane contaminants including the high concentrations of carbon dioxide present in biomethane.
Three steps are involved. The first step, “activation”, uses bromine to activate a C-H bond in methane producing methylbromide and HBr.
CH4 +Br2 ---> CH3Br + HBr (1)
In the second step, “coupling”, the methyl bromide produced in activation is reacted over proprietary zeolite-type catalysts to form higher molecular weight hydrocarbons and HBr. Through careful design of the catalysts and choice of reaction conditions, GRT has shown that the products of the coupling reaction can be controlled to produce gasoline range molecules, aromatics for jet fuels, and a range of valuable chemicals. For “gasoline”, an idealized equation for the overall reaction can be written as:
8 CH4 + 8 Br2 ---> 8 CH3Br + 8 HBr ---> “C8H16” + 16 HBr (2)
Here “C8H16” is used to represent the mixture of actual hydrocarbon products making up gasoline. The third step, regenerates molecular bromine by oxidation of the HBr that is produced as a byproduct in both the activation and coupling reactions (reactions 1 and 2). This step is crucial since the process cannot tolerate loss of bromine. Perhaps the most cost effective small scale process option that GRT has developed for regeneration involves the use of supported metal oxide solid reactants that neutralize HBr and transform the metal oxide into a metal bromide. Bromine is regenerate by contacting the metal bromide with air, transforming the metal bromide back to an oxide.
“C8H16” + 16HBr + 8MO ---> “C8H16” + 8MBr2 + 8H2O (3)
8MBr2 + 4O2 ---> 8MO + 8Br2 (4)
GRT's initial solid reactant process configuration, referred to as the switched bed process, utilizes three or more fixed bed reactors containing the solid reactant operating in parallel so that while one bed is in HBr capture mode, another bed is in purge mode, and a third bed is in regeneration mode. By switching the role of the beds, quasi continuous operation is possible.
This talk will focus on the development of solid reactants that enable the switched bed solid reactant process to operate in an efficient and cost effective manner and on reactor design modeling to use data from small scale laboratory reactors to make design demonstration and commercial scale reactors.
The requirements on the solid reactant materials are quite daunting. 1. The solid reactant must have a large capacity for the capture and regeneration of Br2. The size of the solid reactant fixed beds required is of course inversely proportional to capacity. Practical considerations require a capacity of >2 mmol/g.
2. The capacity must be stable for thousands of capture/regeneration cycles. The large capacity requirement implies the material must be highly dispersed, i.e. present as nano scale particles. Sintering and deactivation with cycling is a serious problem.
3. The rates of capture of HBr and regeneration of Br2 must be high enough to give a sharp reaction front in the fixed bed reactor so that breakthrough of HBr and O2 can be avoided. High rates are also beneficial in that they enable faster cycle times and thus smaller amounts of solid reactant.
4. The material must have minimal or no degradation of the hydrocarbon products
5. The material must be packaged on a support with strength and mechanical integrity over cycles that are suitable for commercial operation.
We have developed synthesis strategies and methods for solid reactant materials that address these requirements. In addition we have developed a testing protocol for these materials to guide the synthesis and optimization of better materials. We initially use testing at atmospheric pressure to get data rapidly on a large number of materials. For the best materials, we then conduct laboratory scale tests at pressures up to 15 atm. to give data suitable for scale-up using kinetic data and reactor design models.