(658a) Design of CO2 Conversion to Dimethyl Carbonate By the Process-to-Planet Multiscale Modeling Framework

Total U.S. greenhouse gas (GHG) emissions have increased by 3.5 percent from 1990 to 2015 and CO₂ emissions from the fossil fuel combustion account for approximately 77 percent of the GHG emissions [1]. To close a chemical carbon cycle, technological strategies, such as carbon capture and conversion, have been developed [2]. CO₂ conversion technologies not only utilize CO₂ captured from industrial emissions, but also produce valuable products [3]. But, those solutions might cause unintended harm to the environment because processes for the carbon capture and conversion cause additional CO₂ emissions. The Process-to-Planet (P2P) multiscale modeling framework has been developed to design engineering process models by accounting for their upstream life cycle up to the entire economy scale [4,5]. Solving design problems with the P2P modeling framework prevents the environmental impacts from shifting outside the systems boundary.

In this work, the P2P modeling framework is applied in designing a dimethyl carbonate (DMC) synthesis process. DMC is an environmental-friendlier substitute reagent for toxic substances such as phosgene and dimethyl sulfate [6]. Since conventional DMC synthesis routes use or produce toxic substances, alternative routes that are environmentally friendly have been developed. Kongpanna et al. screened various DMC synthesis routes and identified potentially sustainable routes that utilize CO₂ [3]. In this study, the DMC synthesis route from urea and methanol is selected for the application of the P2P modeling framework. Urea is prepared from ammonia and CO₂ that is captured from industrial activities. Life cycle inventory (LCI) data are collected from the U.S. LCI database [7,8] to develop the process-based life cycle (value chain scale) model. The economy scale model is developed using the U.S. industrial input-output data [9] and U.S. GHG emissions data [1]. Based on the flow diagram from the reference article [3], the engineering process (equipment scale) model is developed and its design variables are identified. To connect multiscale models, cutoff flows across different scales are identified. The P2P model for the DMC synthesis process is constructed using the General Algebraic Modeling System (GAMS) to optimize design variables. Optimal designs are identified in consideration of a trade-off between maximizing net present value and minimizing environmental interventions.

The Life Cycle Assessment (LCA) approach has been developed to account for multiple spatial scales. The hybrid LCA approach combines value chain scale and economy scale models [10]. The P2P modeling framework integrates the hybrid LCA approach with the engineering process model. In this work, the difference between each approach in total environmental interventions and optimal values of design variables is discussed. Also, the insights gained by applying the P2P modeling framework to designing the CO₂ conversion process are discussed.

Although CO₂ conversion processes utilize CO₂ to produce desired products, in most cases, their net CO₂ emissions are still positive. Also, the demand for products from CO₂ conversion process is limited. Therefore, technological solutions only account for a small portion of total GHG emissions that need to be reduced. To close the carbon cycle toward sustainability, other strategies, such as ecological and agricultural solutions, need to be combined with technological strategies. In such integrated systems, since food, energy, and water (FEW) flows interact across multiple scales, the FEW flows need to be quantified and considered as optimization objectives to account for the nexus of FEW systems. For example, an integrated model with the P2P modeling framework and an agro-ecological model, such as the Soil and Water Assessment Tool (SWAT) model, could be developed to assess multiple flows at the nexus of FEW systems toward closing the carbon cycle. The integrated model prevents the shifting of environmental impacts not only across spatial scales, but also across multiple flows.

References

[1] USEPA. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2015. U.S. Environmental Protection Agency, Washington, D.C., 2017.

[2] George A. Olah, G. K. Surya Prakash, and Alain Goeppert. Anthropogenic chemical carbon cycle for a sustainable future. Journal of the American Chemical Society. 133 (33): 12881-12898, 2011.

[3] Pichayapan Kongpanna, Varong Pavarajarn, Rafiqul Gani, Suttichai Assabumrungrat. Techno-economic evaluation of different CO₂-based processes for dimethyl carbonate production. Chemical Engineering Research and Design. 93: 496–510, 2015.

[4] Rebecca J. Hanes and Bhavik R. Bakshi. Process to planet: A multiscale modeling framework toward sustainable engineering. AIChE Journal. 61 (10): 3332-3352, 2015.

[5] Rebecca J. Hanes and Bhavik R. Bakshi. Sustainable process design by the process to planet framework. AIChE Journal 61 (10): 3320-3331, 2015.

[6] Pietro Tundo and Maurizio Selva. The chemistry of dimethyl carbonate. Accounts of Chemical Research. 35 (9): 706–716, 2002.

[7] U.S. Life Cycle Inventory Database. National Renewable Energy Laboratory, 2012.

[8] Rolf Frischknecht et al. The ecoinvent database: Overview and methodological framework. International Journal of Life Cycle Assessment, 10(1):3–9, 2005.

[9] U.S. Bureau of Economic Analysis. Interactive data: Industry data input-output. http://www.bea.gov/iTable/index_industry_io.cfm, accessed November 1, 2016.

[10] Clark W. Bullard. Net energy analysis: Handbook for combining process and input-output analysis. Resources and Energy. 1 (3): 267-313, 1978.