(240a) Emission and Cost-Aware Optimization of Electrified Reactors for Ethylene Production Via Steam Cracking

Steam cracking of hydrocarbons like ethane and naphtha to produce ethylene, a crucial platform chemical with a global production of over 200 MT/year, is estimated to result in nearly 270 million metric tons (MT) of CO₂ emissions annually. These emissions arise from the combustion of fuel gas to supply the heat input for the reaction carried out at around 1100 K. With the declining cost of variable renewable electricity (VRE) supply, there is a growing interest in electrification based decarbonization of steam cracking processes, through use of novel electrified reactor concepts [1,2,3]. In particular, a few studies have quantified the potential of so-called parallel wire electric resistance heated (ERH) reactors [1,2], involving internal heating via resistively heated wires placed along the fluid flow direction, to increase ethylene yields vs. conventional reactors while also reducing reactor sizes. These studies also highlight the potential for such reactors to operate dynamically in response to volatility in the electricity supply. Here, we systematically evaluate the design and operational performance of such ERH reactors for ethane steam cracking, addressing two relatively understudied aspects: (a) the impact of multiple design degrees of freedom on operational performance and (b) the reactor's ability to operate dynamically in response to electricity supply volatility and its implication for optimal reactor design and eventually process design.

We first discuss the impact of reactor design parameters on reactor performance through a 1-D simulation model of the ERH reactor. The model accounts for mass-energy balance coupling, reaction kinetics associated with multiple radical reactions involved, as well as an estimate of coke formation. The framework enables the quantification of the impact of key reactor parameters, including steam-to-hydrocarbon weight ratio, reactor geometry (length), and reaction heating power intensity (kW/m of reactor length), which is influenced by the applied voltage and wire spacing. For example, we identify various combinations of heat power intensity and space time (residence time) that can achieve the same ethylene yield, with increasing heating power intensity (increasing operating costs) requiring lower space times (lower capital costs) to achieve the same yield and vice versa. Additionally, we observed that a higher heating power density, which allows for a smaller reactor design (lower space time), also reduces pressure drop and enables lower pressure operation—beneficial for mitigating coke formation. This illustrates opportunities for reactor design optimization considering economic and environmental metrics of interest.

To explore this further, we developed an optimization model to evaluate the cost-optimal design and operation of the electrified reactor under temporally variable electricity supply, while considering constraints related to reactor operational performance as a function of design parameters as well as production constraints and operating temperature limits (metallurgical limits of the reactor). Preliminary model evaluations highlight that dynamically adjusting reactor operation in response to grid electricity supply fluctuations can reduce costs compared to steady-state operation under plausible reactor capital costs and electricity supply scenarios for the U.S. electric grid. The implementation of operational flexibility leads to a decrease in the reactor's capacity utilization, resulting from a strategic allocation of production to periods of low electricity prices. The presentation will conclude by discussing how these reactor-level insights can be incorporated in the optimization of the overall process, where one needs to also consider operational flexibility constraints and sizing decisions of downstream unit operations (e.g., ethylene purification).

References:

[1] Agrawal, R.; Chen, Z.; Oladipupo, P. (2023). Electrically heated dehydrogenation process.S. Patent No. 11,578,019.

[2] Balakotaiah, V.; Ratnakar, R. R. Modular Reactors with Electrical Resistance Heating for Hydrocarbon Cracking and Other Endothermic Reactions. AIChE J. 2022, 68(2), e17542.

[3] Stevenson, S. A.; Ward, A. M.; Huckman, M. E.; Hongbing, J. I. A. N.; Oprins, A. J. M.; Dijkmans, T.; Chen, L.; Schroer, J. W.; Broekhuis, R. (2024). Furnace including heating zones with electrically powered heating elements and related methods. S. Patent Application No. 18/682,762.