(211f) Natural Gas to Liquid Transportation Fuels Utilizing Chemical Looping Technologies for Syngas Generation: Process Synthesis and Global Optimization

Authors: 
Tso, W. W., Texas A&M University
Niziolek, A. M., Princeton University
Onel, O., Princeton University
Floudas, C. A., Texas A&M University
Even with advances in electric vehicles and improved vehicle fuel economy, the demand for liquid fuels in the transportation sector is projected to remain steady through 2040 [1]. Liquid fuels produced from domestic feedstocks would partially offset this demand and could ultimately reduce dependence on foreign crude imports, enhancing national energy independence. A potential feedstock is natural gas due to its high hydrogen to carbon ratio and methane-rich composition, increasing the overall carbon conversion to liquid fuels instead of CO2. Natural gas to liquid fuels (GTL) processes may also be economically promising due to an abundance of inexpensive natural gas in the United States (2015 spot price: $2.62/MMBtu [2]).

Previous multi-scale engineering [3] work by Baliban et al. [4] demonstrated that optimal GTL processes can be economically competitive with petroleum refineries. The primary natural gas conversion technologies investigated were syngas generation via autothermal reforming or stream reforming, partial oxidation to methanol, and direct oxidative coupling to olefins, before further upgrading to liquid fuels via methanol synthesis or Fischer-Tropsch processes. Natural gas conversion is low through partial oxidation and oxidative coupling routes, and while it is greater through traditional reforming, high pressure operation limits the overall conversion.

However, recent developments in chemical looping [5, 6] as an alternative for syngas generation from natural gas have significantly expanded the potential GTL capabilities. Chemical looping can be operated at low pressure and offers close to complete conversion of natural gas in a single pass. A high concentration of syngas can be produced without using pure oxygen, eliminating the need for additional air separation and syngas conditioning units. This could greatly reduce the capital and operational costs associated with generating syngas in GTL processes and improve the overall process efficiency.

In this work, two chemical looping technologies are incorporated into a GTL process superstructure[4, 7, 8, 9] as alternatives to reforming for syngas generation. All process technologies are rigorously modeled and together with other important process components, such as hydrogen and oxygen production, wastewater treatment, and light gas handling, form a large-scale nonconvex mixed-integer nonlinear model (MINLP). A deterministic global optimization branch-and-bound algorithm is used to solve the MINLP [10]. Simultaneous heat and power integration is also performed to minimize utility cost [11]. This process synthesis framework provides an illuminating means to systematically compare chemical looping technologies against other competing technologies in a GTL process.

Several case studies are examined to highlight the prospective benefits of chemical looping technologies over other conversion routes. The effect of plant capacity and production ratios on the overall profit of the GTL process is analyzed. Major topological decisions on process technologies will be discussed. Economic and environmental trade-offs will also be presented.

[1] U.S. Energy Information Administration. Annual Energy Outlook 2015 with projections to 2040. Available at: https://www.eia.gov/forecasts/aeo/pdf/0383%282015%29.pdf. Accessed May 2016.

[2] U.S. Energy Information Administration. â??Henry Hub Natural Gas Spot Priceâ?. Available at: http://www.eia.gov/dnav/ng/hist/rngwhhdm.htm. Accessed May 2016.

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[4] Baliban, R. C.; Elia, J. A.; Floudas, C. A. Novel Natural Gas to Liquids Processes: Process Synthesis and Global Optimization Strategies. AIChE Journal 2013, 59 (2), 505-531.

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[6] de Diego, L. F.; Ortiz, M.; García-Labiano, F.; Adánez, J.; Abad, A.; Gayán, P. Hydrogen production by chemical-looping reforming in a circulating fluidized bed reactor using Ni-based oxygen carriers. Journal of Power Sources 2009, 192 (1), 27â??34.

[7] 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 Chemistry Research 2013, 52 (9), 3381-3406.

[8] 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 2015, 54 (1), 359-385.

[9] 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 (3), 831-856.

[10] 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 & Chemical Engineering 2012, 42, 64-86.

[11] Baliban, R. C.; Elia, J. A.; Floudas, C. A. Simultaneous process synthesis, heat, power, and water integration of thermochemical hybrid biomass, coal, and natural gas facilities. Computers & Chemical Engineering 2012, 37, 297-327.