(574c) Comprehensive Process Modeling for Converting Natural Gas to Lower Olefin Via Reforming and Fischer-Tropsch Process

Synthesis gas or syngas, mixture of hydrogen (H₂) and carbon monoxide (CO), generated from the upgrading of traditional chemical feedstocks such as natural gas, coal and biomass, is utilized industrially for the production of value-added chemicals which include chemical intermediates (olefins, alcohols), fuels and synthetic wax. Syngas conversion to products is commonly practiced via the Fischer-Tropsch synthesis (FTS) process, where metal catalysts (e.g., solid iron- and cobalt-based catalysts) are contacted with syngas at elevated temperatures and pressures to produce hydrocarbons (e.g., olefins and paraffins) and/or oxygenated organic products. However, these processes also coproduce C1 byproducts which include carbon dioxide (CO₂) and methane (CH₄). Furthermore, catalysts/processes for Fischer-Tropsch may pose disadvantages due to low conversion/activity resulting in high concentrations of unconverted syngas in the product mixture. In this presentation, we discuss scenarios to maximize the yield of olefins from the syngas conversion reaction and analyze consequent carbon efficiency and energy consumption.

A process model is developed in process flowsheeting software for lower olefin production based on natural gas as a feedstock for syngas production via methane reforming. Steam methane reforming and dry reforming are considered for this study. The heat balance for reformer is carried out using a fuel-fired furnace. Syngas is fed to an olefin synthesis reactor; excess H₂, if any, is separated and can be used as a fuel. The product stream from the syngas conversion/olefin synthesis reactor, composed of unreacted CO and H₂, CH₄, CO₂, and heavier hydrocarbon products is subjected to a series of separation steps including H₂ membrane separation, CO₂ separation units, and a sequence of distillation column including de-methanizer, de-ethanizer, C2-splitter, de-propanizer, C3 splitter, etc. to produce final olefin products. The unconverted reactants can be recycled to appropriate units in the system. A heat and mass balance is performed and several scenarios are evaluated based on catalyst performance and recycle options.

The results show that conversion in olefin synthesis reactor has a profound effect on recovery section. The low boiling-point components (CO + H₂) results in an aggressive cryogenic process and/or high pressure systems to recover lower olefins (C2–C4) from the product mixture where temperature as low as -200 ^oC have to be implemented for separation of olefins. Yield vectors are generated based on a series of catalyst/s and results of these sensitivity analysis suggest that separation can be achieved at higher temperature. Further, recycle options to the reformer were evaluated. Although the recycle results in higher overall yield of lower olefins, this also results in higher load on the furnace and results in lower energy efficiency and thus, similar overall carbon efficiency.

An integrated process model including reformer, reformer furnace, olefin synthesis reactor, recovery units is developed to evaluate carbon efficiency and energy consumption for natural gas to lower olefin production via syngas conversion. This work demonstrates process modeling capabilities that enables comparison between catalysts that have contrasting yields and selectivity, and provides direction for future development efforts.