Exploring Propane Conversion Via Unconventinal Piston Reactor – a Modeling Study to Guide Process Development and Economics

The commodity chemical industry is a major consumer of energy and is responsible for producing around 25% of global greenhouse gas emissions [1, 2]. Hence, decarbonization of the chemical industry is an imperative step towards achieving a sustainable and carbon-neutral energy system. With the growing prominence and cost-effectiveness of renewable energy sources, the paradigm shift toward electrification via utilizing novel reactor systems that exploit renewable electricity to drive chemical processes appears to be the most viable strategy for reducing the carbon footprint of chemicals[3]. The ability to convert electrical energy into chemical energy offers a means to store electricity and overcome the intermittency of renewable sources such as solar and wind.

The piston reactor is an innovative concept that converts electrical energy into mechanical work and, subsequently, into chemical energy storage molecules[4, 5]. It operates much like a conventional combustion engine, cyclically compressing and expanding a feed gas within very short timeframes, typically in the millisecond range. This compression generates high temperatures, reaching up to 1500 K, and pressures of several hundred bars, which can initiate reactions to produce common chemicals. The rapid gas expansion leads to quick cooling, helping the reactor maintain a desired non-equilibrium state and preventing secondary reactions of metastable species that could yield undesired by-products[6]. In addition to its capabilities, piston reactors are known for their simplicity, compactness, safety in operation, and ability to promptly adapt to variations in feed conditions[7].

The piston reactor can produce a range of chemical products through different reaction pathways. Therefore, it is crucial to identify promising products and their associated reactions from a multitude of emerging possibilities through early-stage evaluation. This assessment is vital for the early screening of promising directions, helping avoid extensive and time-consuming experimental work. The experimental landscape for the piston reactor is vast, involving various operational conditions, making it challenging to pinpoint favorable operating regions and the best pathways. Additionally, evaluating the feasibility of a reaction pathway based solely on experimental metrics like conversion, yield, or selectivity is inadequate. A more comprehensive analysis, considering economic and environmental factors like separation, raw material, product costs, and emissions, is essential. However, a significant challenge in early-stage assessment is the lack of complete data, particularly related to mass and energy balances necessary for a proper process evaluation. Therefore, there is an urgent need for a tool that can quickly and informatively assess and chart the operational space of the piston reactor for specific chemistry, facilitating key decision-making during the laboratory phase.

The current study presents an approach for a quick preliminary assessment of the piston reactor technology to investigate the opportunities and constraints of this reactor concept. For this purpose, a zero-dimensional single-zone timed-dependent thermodynamic model of a single-cylinder piston reactor is implemented. The kinetics are predicted using a detailed gas-phase mechanistic model. Propane is selected as a representative case study because it yields a diverse array of industrially relevant products including but not limited to hydrogen, ethylene, and propylene. The model is used to analyze the solution space which is then evaluated based on economic metrics to identify preferred operating regions. The exploration methodology follows a systematic, model-driven innovation approach to tackle the challenge of assessing opportunities arising from the intermediate mixtures produced during the reactor. Additionally, this work not only aims to develop a novel equipment concept but also to propose a comprehensive packaged solution capable of converting raw materials into value-added products.

In Figure 1, a case study is presented where the piston reactor operates with a propane/argon mixture to explore the potential for endothermic propane pyrolysis. The introduction of argon as a diluent in the feed mixture effectively reduces the heat capacity of the incoming gas, leading to higher temperatures during the compression phase, thereby enhancing conversion. This process generates a mixture consisting of hydrogen and various hydrocarbons, including methane, ethane, and ethylene within the analyzed solution space. Figure 1b provides a preliminary assessment of the added value of these products, considering their price per unit of amount of material fed (propane in this case). This evaluation can be extended to other cases involving different feed combinations or operating conditions, allowing for a comparative analysis of their added value. Additional economic indicators, such as separation costs and feed costs, can be incorporated into the evaluation to identify economically viable regions for further exploration.