(399b) Economic and Environmental Analyses of Optimal Integrated Processes to Produce Chloromethanes Production from Wastes.

Securing the drinking water supply is one of the most daunting challenges in the upcoming decades.¹ Population growth combined with the depletion of freshwater resources due to climate change is intensifying water scarcity. The percentage of the population without an affordable supply of water is estimated to rise from 41% nowadays to 52% in 2050.² To face this challenge, desalination technologies such as evaporation, pervaporation, electrolysis or reverse osmosis (RO) become necessary to ensure the supply and minimize the population affected. Among them, RO has been elected as the most popular one, around 85% of the plants in the world,³ due to its better economic performance. However, RO plants generate environmental issues such as excessive energy use,⁴ and the acidification and increase of salinity in the marine environment of the discharge.⁵

This work provides an economic and environmental (LCA) comparison of two processes (direct chlorination of methane and hydrochlorination of methanol that include two alternative configurations) to produce chloromethanes from waste brine and CO₂. The processes consist of the electrolysis of the brine to produce the chlorine and hydrogen. On the one hand, methane is produced from CO₂ hydrogenation and next, chlorine is used to obtain the chloromethanes. On the other hand methanol is produced also form Co2 hydrogenation. Next HCl is produced using a Deacon reactor or a burner. The HCl is used to synthesize the chloromethanes. Both processes are modeled using a mathematical optimization approach. The mathematical optimization framework is particularly interesting in the direct chlorination process where a chain reaction takes place and the residence time in the reactor defines the product distribution obtained for the chloromethanes. All the units are modelled using first principles, mass and energy balances, thermodynamic and phase equilibria and experimental data. In particular, the direct chlorination is modelled using an orthogonal collocation approach to capture the chain reaction that takes place since the residence time in the reactor defines the product distribution obtained for the chloromethanes.

Among the three alternatives, the hydrochlorination processes turned out to be more profitable and show lower environmental impacts than the direct chlorination process. The highest profitability is obtained with a hydrochlorination process based on a Deacon reactor for the production of HCl when Cl₂ is sold as by-product. However, if Cl₂ cannot be sold as a by-product, the use of a burner to produce the HCl is suggested as the technology of choice. The conversion in the reactor to chloromethanes in the direct chlorination process is low, being the process less profitable (a minimum selling price of $5/kg_{methyl-chloryde}) than with hydrochlorination of methanol (~$1.8/kg_{methyl-chloryde}). The hydrochlorination process presents the lowest environmental impacts, generating 3.6 kg_CO2/kg_CH3Cl produced versus 13.3 kg_CO2/kg_{methyl-chloride} for direct chlorination of methane. The emissions of the hydrochlorination process are mainly due to the electricity consumption in the electrolyzer and can be reduced to avoidance of ~-1.5 kg_CO2/kg_CH3Cl if renewable energy is used as supply.

References

(1) United Nations. The Sustainable Development Goals Report 2022. Clean Water and Sanitation. . Available in: https://www.un.org/sustainabledevelopment/water-and-sanitation/ Last access: 1st February 2023 2022.

(2) United Nations. The United Nations World Water Development Report 2018. Available in: www.unwater.org/publications/world-water-development-report-2018/ Last access 1st February 2023 2018.

(3) Peter H. Gleick. The World’s Water. Volume 7; 2011.

(4) Shahabi, M. P.; McHugh, A.; Anda, M.; Ho, G. Environmental Life Cycle Assessment of Seawater Reverse Osmosis Desalination Plant Powered by Renewable Energy. Renew Energy 2014, 67, 53–58. https://doi.org/10.1016/J.RENENE.2013.11.050.

(5) Zhou, J.; Chang, V. W. C.; Fane, A. G. Environmental Life Cycle Assessment of Reverse Osmosis Desalination: The Influence of Different Life Cycle Impact Assessment Methods on the Characterization Results. Desalination 2011, 283, 227–236. https://doi.org/10.1016/J.DESAL.2011.04.066.