(563b) Optimal Production of Light Olefins from "Wet" Shale Gas: An Integrated NGL Cracking and Dry Gas Reforming Approach

High levels of natural gas production that stemmed from the shale gas industry lowered this feedstock price significantly [1]. Therefore, production of “wet” gas that contains natural gas liquids (NGLs), such as ethane, propane, butane, and natural gasoline, is favorable to extract for commercial purposes [1]. In some wet shale plays, the combined ethane and propane composition can exceed 30% [2]. These recent trends motivate industrial effort to build several new ethane crackers with a combined ethylene production capacity of 12.5 million tonnes/year in the United States [3]. 

Light olefins including ethylene, propylene, and butene isomers are valuable petrochemical intermediates and make up two-thirds of the petrochemicals market [4]. These olefins can be produced from alternative feedstocks such as biomass and natural gas via thermochemical conversion and reforming [4]. However, extraction of NGLs prior to methane conversion is an opportunity for refineries to maximize carbon conversion and avoid high reforming costs. A demethanizer column is utilized [5,6] to recover more than 83% of ethane and more than 99% of higher hydrocarbons in the NGL stream. The NGLs can be cracked or further separated for C₃ and C₄ dehydrogenation processes. A rigorous mathematical model is developed for the hydrocarbon cracker using the well-established cracking kinetics in the literature [7,8]. Dry gas is converted via reforming or direct conversion processes introduced into a process superstructure [4,9]. The light olefins are produced and purified to product quality with an extensive superstructure that considers multiple production, conversion, and purification technologies.

The process superstructure that is developed is solved using a novel branch and bound global optimization framework. The objective is to maximize the profit of light olefin production from the integrated NGL extraction and dry gas conversion plant. A techno-economic analysis will be presented with major topological decisions across several case studies. Different natural gas compositions will be shown to present the topological trade-offs for various NGL compositions.

References:

[1]: Kopalek, M.; High value of liquids drives U.S. producers to target wet natural gas resources, 2014, Energy Information Administration

[2]: Hill, R. J.; Jarvie, D. M.; Zumberge, J.; Henry, M.; Pollastro, R. M.; Oil and gas geochemistry and petroleum systems of the Fort Worth Basin, 2007, AAPG Bulletin, 91(4), 445-473.

[3]: Chang, J.; New projects may raise US ethylene capacity by 52%, PE by 47%, 2014, ICIS Petrochemicals.

[4]: 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, 2015, Industrial and Engineering Chemistry Research, 54, 359-385.

[5]: Luyben, W. L.; NGL Demethanizer Control, 2013, Industrial and Engineering Chemistry Research, 52, 11626-11638.

[6]: Luyben, W. L.; Effect of Natural Gas Composition on the Design of Natural Gas Liquid Demethanizers, 2013, Industrial and Engineering Chemistry Research, 52, 6513-6516.

[7]: Sundaram, K. M.; Froment, G. F.; Modeling of thermal cracking kinetics – I: Thermal cracking of ethane, propane and their mixtures, 1977, Chemical Engineering Science, 32(6), 601-608.

[8]: Sundaram, K. M.; Froment, G. F.; Modeling of thermal cracking kinetics – II: Cracking of iso-butane, of n-butane, and of mixtures ethane-propane-n-butane, 1977, Chemical Engineering Science, 32(6), 609-617.

[9]: 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.