(374f) Shale Gas to Light Olefins: Global Optimization of an Integrated NGL Recovery, Steam Cracking, and Methane Conversion Superstructure

Authors: 
Onel, O., Princeton University
Niziolek, A. M., Princeton University
Floudas, C. A., Texas A&M University
Due to the decreased prices of natural gas in recent years [1], production of â??wetâ? gas that contains natural gas liquids (NGLs), such as ethane, propane, butane, and natural gasoline, is favorable to extract for commercial purposes [2]. In some wet shale plays, the combined ethane and propane composition can exceed 30% [3]. These recent trends motivate industrial efforts to build several new ethane crackers with a combined ethylene production capacity of 12.5 million tonnes/year in the United States [4]. Although the NGLs can be effectively used as petrochemical feedstocks toward light olefins, the optimal processing technology (catalytic and/or thermal), feed composition (mixed feed or a pure feed), and operating conditions are unknown.

Previous work has shown that light olefins including ethylene, propylene, and butene isomers can be produced from natural gas via reforming in a profitable manner out of a superstructure of process alternatives [5]. However, extraction of NGLs prior to methane conversion is an untapped opportunity for these refineries that can simultaneously maximize carbon conversion and avoid high reforming costs. This NGL recovery section is introduced to the existing superstructure of novel/competing process alternatives. A demethanizer column is utilized [6,7] to recover more than 83% of ethane and more than 99% of higher hydrocarbons in the NGL stream. The NGLs can be further separated and sent to steam cracking or catalytic dehydrogenation alternatives. Several steam cracking reactors incorporate a pure or mixture of hydrocarbon feed to produce olefins. The operating conditions and reactor topologies of the steam cracking alternatives are dynamically optimized using the well-established cracking kinetics in the literature [8,9]. Catalytic dehydrogenation process alternatives incorporate commercially available catalysts for propane and/or isobutane dehydrogenation. The methane rich gas from the demethanizer is converted via reforming or direct conversion processes introduced into the process superstructure [5].

The mathematical model of the overall process superstructure 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, cracking, catalytic dehydrogenation, and methane conversion refinery. A technoeconomic 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 in the NGL recovery section. The net present values can be significantly improved (>10%) through the use of the integrated superstructure.

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