process design, techno-economic and life-cycle assessments
Process Design, Techno-Economic and Life-Cycle Assessments
Replacing the steam cracking process with oxidative dehydrogenation for ethylene production offers potential energy and environmental benefits. To evaluate these possibilities, we conducted a study combining conceptual process design, techno-economic analysis, and life-cycle assessments of the oxidative dehydrogenation of ethane (ODHE) for producing ethylene at an industrial scale. For comparison, we also simulated and optimized the conventional steam cracking process of ethane.
The techno-economic analysis results for ODHE, utilizing a boron-containing zeolite chabazite (B-CHA) catalyst, show that it is economically competitive, with a production cost of $790 per ton of ethylene, compared to steam cracking's $832 per ton. However, our cradle-to-gate life-cycle assessment indicates that the ODHE process emits more greenhouse gases, with emissions of 2.42 kg CO2 equivalent per kg of ethylene, compared to steam cracking’s 1.34 kg CO2 equivalent per kg. This discrepancy arises from the significant refrigerant input needed for recovering ethylene from byproducts like CO, CH4, and unreacted oxygen and ethane.
Further scenario analysis reveals that improvements in C2H6 conversion per pass, selectivity to ethylene, and the ratio of ethane to oxygen in the ODHE process could help make it both economically and environmentally viable as a replacement for steam cracking.
1. Introduction
Ethylene plays a central role in the chemical industry and is a fundamental building block in petrochemicals, with a global production capacity reaching 214 million metric tons. Recent expansions in ethylene production in the U.S. due to the shale gas boom are influencing the chemical industry's energy consumption and greenhouse gas emissions. Ethylene is crucial for producing various plastics, particularly polyethylene, which is one of the most frequently used plastics worldwide. Furthermore, ethylene is a precursor for many chemicals, including ethanol and acetaldehyde.
Various technologies exist for producing ethylene, including steam cracking (also referred to as thermal cracking or pyrolysis), catalytic cracking, and dehydrogenation. Currently, steam cracking of hydrocarbons is the most widely used method, but it is energy-intensive, requiring elevated reaction temperatures exceeding 800 °C. Consequently, this energy demand impacts the production costs of ethylene. Ethylene production also ranks as the second-largest source of greenhouse gas emissions within the chemical industry, contributing 1.2 tons of CO2 equivalent emissions for every ton of ethylene produced. Globally, there is a pressing need for technological advancements and process optimizations to enhance the efficiency and sustainability of ethylene manufacturing.
As conventional ethylene production techniques have been optimized over the past eight decades, with thermal efficiencies reaching 95%, replacing these methods poses significant challenges. Alternative pathways for ethylene production are being explored, focusing on sustainable feedstocks, clean energy technologies, and advanced catalysts. Recent findings indicate that electrified steam cracking could offer carbon-neutral ethylene production. However, the oxidative dehydrogenation of ethane (ODHE) continues to be attractive due to its lower energy requirements and inherent coke removal properties enabled by the oxygen in the feedstock.
2. Methodology
2.1 Process Design and Simulation
Process flowsheets for ethylene production via ethane steam cracking and ODHE were developed based on published reports and experimental findings from our lab. Kinetic-driven process simulations were performed using Aspen Plus v12. The RPlug reactor unit was utilized for modeling both ethane steam cracking and ODHE. Due to limited experimental data, side reactions in the ODHE process were modeled using the RStoic reactor block, relying on experimental conversions. Kinetic parameters for the ethane steam cracking reaction were obtained from the literature, while those for the ODHE process came from our lab's experimental data.
2.2 Techno-Economic Analysis
A techno-economic model was established, incorporating total capital investment and manufacturing costs for both ethane steam cracking and ODHE technologies, based on a discount rate of 10%, a tax rate of 30%, a straight-line depreciation method (over 7 years), and a yearly operational period of 8,000 hours. A 20-year discounted cash flow rate-of-return analysis estimated the minimum selling price (MSP) of ethylene that results in a net present value (NPV) of zero. The Aspen Energy Analyzer V12 was employed for optimal heat network design, and the Aspen Process Economic Analyzer V12 was used to support the techno-economic analysis.
2.3 Life-Cycle Assessment
The Life-Cycle Assessment (LCA) was conducted to compare ethane steam cracking and ODHE, adopting a cradle-to-gate system boundary that includes raw material extraction, utility generation, and production stages. The unit-specific inventory was detailed, with characterization data sourced from Ecoinvent 3.9, subsequently characterized for lifecycle impact assessment using the ReCiPe method.
3. Results and Discussion
3.1 Ethane Steam Cracking Process
The steam cracking process consists of three main stages: (1) steam cracking, where ethane and steam are introduced into the cracking reactor, resulting in ethylene production; (2) water removal, using triethylene glycol for absorption; and (3) ethylene recovery, where the product stream is compressed and cooled before distillation. Key operating parameters are detailed in accompanying tables.
3.2 ODHE Process
The ODHE process involves four stages: (1) oxidative dehydrogenation, where mixed ethane and oxygen react at moderate temperatures; (2) dehydration and decarbonization of resulting gases; (3) purification using absorption columns; and (4) product recovery through distillation.
3.3 Techno-Economic Analysis
The ODHE process demonstrates a minimal selling price of $790 per ton of ethylene, indicating its economic competitiveness relative to steam cracking. Although steam cracking has slightly lower manufacturing costs and capital investments, the lower operating temperature of ODHE results in reduced energy demand and earlier investment recovery.
3.4 Life-Cycle Assessment
Heat integration significantly mitigates carbon emissions for both processes, simultaneously bridging the environmental impact gap between ODHE and steam cracking.
4. Conclusions
This study assesses the feasibility of replacing the steam cracking process with ODHE for industrial-scale ethylene production. The findings show that while the ODHE process is economically competitive, it produces higher greenhouse gas emissions. Improving catalyst performance and optimizing reaction conditions, in conjunction with alternative separation technologies, can enhance the process's environmental performance.
Conflicts of Interest
There are no conflicts to declare.
Acknowledgements
The authors acknowledge financial support from various grants.
References
For further details and supplementary information, please refer to the electronic supplementary information available.