(723d) Natural Gas to Liquid Transportation Fuels, Olefins, and Aromatics (GTL+C2_C4+C6_C8)

The United States faces major challenges towards the production of hydrocarbon fuels and high value chemicals. These major challenges comprise of high crude oil prices, volatility of the global oil market, and excessive greenhouse gas (GHG) emissions. A recent review has shown that these challenges can be addressed using single or hybrid feedstocks such as coal, biomass, and natural gas towards the production of liquid transportation fuels [1].  However, high value petrochemicals, such as C2 to C4 olefins and C6 to C8 aromatics, are an important part of refineries since they can substantially increase the profitability of these plants. Therefore, hybrid energy systems that produce liquid fuels from alternative feedstocks should also consider the production of C2 to C4 olefins and C6 to C8 aromatics. The primary C2 to C4 olefins are ethylene, propylene, and butadiene, whereas the key C6 to C8 aromatics include benzene, toluene, and the xylene isomers. Moreover, recent progress in the shale gas industry provides cheap and abundant natural gas feedstock that can be utilized towards fuels and chemicals production. Recent work has already shown that the production of liquid fuels from natural gas (GTL) can be competitive with typical petroleum processes [2]. Therefore, this work considers the production of liquid fuels concurrently with C2 to C4 olefins and C6 to C8 aromatics, starting from a natural gas feedstock.

A techno-economic and environmental assessment of a novel refinery that converts natural gas to liquid fuels, C2 to C4 olefins, and C6 to C8 aromatics (GTL+C2_C4+C6_C8) will be performed using an optimization-based framework. The process synthesis superstructure will contain multiple natural gas conversion pathways. Synthesis gas (syngas) produced from the reformers will be directed to either the Fischer-Tropsch refining section or to the methanol production section. The Fischer-Tropsch hydrocarbons will be upgraded to fuel-grade quality products in either a ZSM-5 reactor or through a series of treatment units. The methanol will either be converted into gasoline or olefins. The olefins may be directed to the olefin purification section or further processed to produce gasoline and distillate. Aromatics will either be separated from naphtha produced from the refinery or through conversion of liquefied petroleum gases (LPG). Any CO2 produced in the plant can be vented, sequestered, or recycled back in the refinery.

The optimization-based framework consists of process design, process synthesis, and global optimization strategies to determine the optimum plant topology under different scenarios [2-13]. Simultaneous heat, power, and water integration is present in the model to keep the utilities costs at minimum levels. This framework ensures the selection of the process that produces the liquid fuels, C2 to C4 chemicals, and C6 to C8 aromatics in the most preferable way (lowest cost or highest profit). The superstructure proposes different methods of production of syngas, hydrocarbons, and chemicals; therefore the tradeoffs between each method for the proposed refinery will be investigated. Different case studies are presented to explore the effect of the plant capacity and olefins/aromatics production levels. The key foundations, topological decisions, economical and environmental aspects are discussed.

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