(156c) Small-Scale Gtl Processes for Utilizing Stranded Gas: Model Identification, Process Synthesis, and Global Optimization

Authors: 
Onel, O., Princeton University
Niziolek, A. M., Princeton University
Floudas, C. A., Princeton University
Butcher, H., Texas A&M University
Wilhite, B., Texas A&M University

Advances in the shale gas industry have generated abundant and inexpensive sources of natural gas in the United States, [1] with almost 500,000 active natural wells operating across the nation [2]. There are also many sources of “stranded gas” that are uneconomic to produce due to their low energy content, impurities, small volume of production, or lack of infrastructure. Therefore, this stranded gas is typically vented or flared.  Gas-to-Liquids (GTL) processes have been considered as an alternative to monetize the stranded gas [3,4] and mitigate the negative environmental impacts. A recent review outlined that many GTL technologies are commercially and technologically developed [5]. A superstructure based global optimization framework is introduced for a GTL refinery that considers multiple conversion pathways and upgrading technologies [6]. The objective is to find the best processes and topological alternatives across different scales, products, and feedstock compositions [6].

As a novel natural gas conversion alternative, an annular microchannel reactor (AMR) is designed for the catalytic steam reforming of methane for small-scale applications [7]. Through the use of a rigorous computational fluid dynamics (CFD) model, the AMR is simulated and validated against experimental data [8]. Although the technology is promising for small scales and CFD simulations provide an accurate representation, the model [8] is computationally too expensive to consider in a superstructure-based approach [6]. Therefore, a grey-box model identification approach is developed to build a surrogate model that will accurately represent AMR behavior. A non-linear non-convex parameter estimation model is built and optimized for this motive to find a mathematical model that predicts the syngas effluent within less than 1% of the CFD simulation results.

The surrogate model is then implemented to a GTL process superstructure [6] for small-scale GTL applications. A set of case studies along with sensitivity analysis to observe the performance of the AMR at these scales is studied.  The results suggest that the break-even oil prices can be as much as $20/bbl lower for technologies that utilize AMR compared to traditional reforming technologies.

References:

[1]: Energy Information Administration, Annual Energy Outlook, 2014

[2]: Energy Information Administration, Natural Gas Monthly, April 2015

[3]: Lichun, D.; Shun’an, W.; Shiyu, T.; Hongjing Z.; GTL or LNG: Which is the best way to monetize “stranded natural gas?, 2008, Petroleum Science, 5(4), 388-394.

[4]: Wood, D. A.; Nwaoha, C.; Towler, B. F; Gas-to-Liquids (GTL): A review of an industry offering several routes for monetizing natural gas, 2012, Journal of Natural Gas Science and Engineering, 9, 196-208.

[5]: Floudas, C. A.; Elia, J. A.; Baliban, R. C.; Hybrid and single feedstock energy processes for liquid transportation fuels: A critical review, 2012, Computers and Chemical Engineering, 41, 24-51

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

[7]: Wilhite, B. A.; Breziner, L.; Mettes, J; Bossard, P.; Radial Microchannel Reactors (RMRs) for Efficient and Compact Steam Reforming of Methane: Experimental Demonstration and Design Simulations, 2013, Energy & Fuels, 27, 4403-4410.

[8]: Butcher, H.; Quenzel, C. J. E.; Breziner, L.; Mettes, J.; Wilhite, B. A.; Bossard, P.; Design of an annular microchannel reactor (AMR) for hydrogen and/or syngas production via methane steam reforming, 2014, International Journal of Hydrogen Energy, 39, 18046-18057.