(165h) Natural Gas to Liquids, Olefins, and Aromatics: A Systematic Approach for the Optimal Production Trade-Offs

Natural gas to liquid transportation fuels processes¹ have been investigated for several decades, but were considered economically unfavorable because of high natural gas costs. Due to recently discovered sources of shale gas, coupled with improvements in the technology used to extract this resource, the abundance of natural gas within the United States soared in recent years. This drastic increase in the supply of natural gas has caused prices to plummet, from $7.99/MMBTU in 2008 to $2.99/MMBTU in 2015.² Additionally, recent studies investigating the coproduction of liquids and valuable chemicals, such as aromatics and olefins, have shown how these coproducts can significantly increase the profitability of refineries.^3,4However, the profitability of refineries that produce a wide spectrum of products depends intricately on the ratios of these products. Therefore, we investigate the economic effects of natural gas to liquids, olefins, and aromatics refineries to determine and assess the technological and environmental tradeoffs with multiple GTL+C2C4+C6C8 alternatives using an important production parameter.

An optimization-based comprehensive process synthesis framework for the conversion of natural gas to liquid transportation fuels, olefins, and aromatics is proposed.^3,4 Several direct and indirect natural gas conversion technologies are compared to determine the optimal processing pathway.¹ Numerous hydrocarbon production alternatives are incorporated, including Fischer-Tropsch refining and methanol synthesis. Multiple commercial and novel technologies for the production of the olefins and aromatics are included. The process synthesis superstructure includes an olefins purification section and aromatics complex for the production and upgrading of these high-value chemicals. The main olefins investigated include ethylene, propylene, butenes and butadiene. The main aromatics produced include benzene, toluene, and the xylenes. A large-scale nonconvex mixed-integer nonlinear optimization (MINLP) model is formulated to encompass each of these alternatives.⁵ The MINLP model is solved using a powerful deterministic global optimization branch-and-bound framework and the optimal solution obtained is mathematically guaranteed to be within a few percent of the best possible value. This process synthesis framework provides an adequate baseline for comparing competing technologies.^6-9 The key production parameter is a function of the important products output from the refinery, namely, gasoline, diesel, kerosene, aromatics, and olefins.

Several case studies are presented to investigate the effect of plant capacity and production ratios on the overall profit of the refinery. The major topological decisions as a function of the production parameter will be discussed and the economic and environmental tradeoffs of the refineries will be presented.

1. Baliban, R. C.; Elia, J. A.; Floudas, C. A. Novel natural gas to liquids (GTL) technologies: Process synthesis and global optimization strategies. AIChE Journal 2013, 59, 505–531.

2. Energy Information Administration, Available at: http://www.eia.gov/dnav/ng/hist/rngwhhdm.htm, 2015

3. Onel, O.; Niziolek, A. M.; Elia, J. A.; Baliban, R. C.; Floudas, C. A. Biomass and natural gas to liquid transportation fuels and olefins (BGTL+C2_C4): Process Synthesis and Global Optimization. Industrial & Engineering Chemistry Research 2014, 54, 359-385.

4. Niziolek, A. M.; Onel, O.; Elia, J. A.; Baliban R. C.; Floudas, C. A. Coproduction of Liquid Transportation Fuels and C6_C8 Aromatics from Biomass and Natural Gas. AIChE Journal 2015, 61, 831-856.

5. Baliban, R. C.; Elia, J. A.; Misener, R.; Floudas, C. A. Global Optimization of a MINLP Process Synthesis Model for Thermochemical Based Conversion of Hybrid Coal, Biomass, and Natural Gas to Liquid Fuels. Computers and Chemical Engineering 2012, 42, 64-86.

6. Niziolek, A. M.; Onel, O.; Hasan, F. M.; Floudas, C. A. Municipal solid waste to liquid transportation fuels – Part II: Process synthesis and global optimization strategies. Computers and Chemical Engineering 2014, 74, 184-203.

7. Niziolek, A. M.; Onel, O.; Elia, J. A.; Baliban, R. C.; Xiao, X.; Floudas, C. A. Coal and Biomass to Liquid Transportation Fuels: Process Synthesis and Global Optimization Strategies. Industrial Engineering & Chemistry Research 2014, 53, 17002-17025.

8. Baliban, R. C.; Elia, J. A.; Floudas, C. A. Novel natural gas to liquids (GTL) technologies: Process synthesis and global optimization strategies. AIChE Journal 2013, 59, 505–531.

9. Baliban, R. C.; Elia, J. A.; Floudas, C. A. Biomass and Natural Gas to Liquid Transportation Fuels: Process Synthesis, Global Optimization, and Topology Analysis. Industrial & Engineering Chem Research 2013, 52, 3381–3406.