(370i) Chemical Looping for the Oxidative Cracking of Shale Condensates

Authors: 
Neal, L., North Carolina State University
Haribal, V. P., North Carolina State University
Li, F., North Carolina State University
Production of olefins by thermal cracking of light paraffins and naphtha is an energy intensive process (20-40 GJth/tonne ethylene). The burning of fuel required to meet this energy demand for olefin production makes cracking one of the single largest industrial contributors to anthropomorphic CO2 emissions. In the case of naphtha cracking, 1.8-2 kg of CO2 can be emitted per kg ethylene. To address the environmental challenges for olefin production, we have previously introduced several redox systems for the Chemical Looping Oxidative Dehydrogenation (CL-ODH) of ethane to ethylene. CL-ODH can greatly reduce the primary energy demand and CO2 emissions for ethylene production, and it is well positioned to take advantage of ethane produced by the North American shale gas boom, cracking of heavier components is still an important concern. Not only is naphtha cracking the primary source of olefins in large parts of the world, ethane cracking does not produce the same amounts of industrially important C3+ olefins and di-olefin. Additionally, while shale-gas has increased the supply of ethane dramatically, shale-oil and gas have also flooded the market with condensates/ultra-light crude fractions that are unsuitable for use as transportation fuel. To take advantage of this market shift while simultaneously decreasing the energy demand and environmental impact of naphtha cracking, we propose a two-step Redox (or chemical looping) Oxidative Cracking (ROC) scheme where hydrogen, produced from the cracking of light liquid paraffins, is selectively oxidized by lattice oxygen from a redox catalyst. Regeneration of the lattice oxygen in a subsequent step heats the redox catalyst, providing the thermal energy needed for the cracking reaction. This process leads to minimal parasitic energy loss compared to cryogenic air separation and significantly reduced CO2/NOx emissions, while promoting olefin formation through the removal of hydrogen. In the current study, a promising mixed-metal-oxide based oxygen carriers are demonstrated for ROC. The general reaction pathways are elucidated. Simulations are also presented that indicate that the ROC process can reduce the overall energy demand and CO2 emissions by half, while yielding increased C3+ products vs. ethane cracking or CL-ODH. Simulations also show that energy savings in the system significantly exceed the loss of hydrogen byproducts.
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