Techno-economic evaluation of different CO2-based processes for dimethyl carbonate production
Introduction
Carbon dioxide accumulation in the atmosphere is a major cause of concern with respect to the increasing global temperature of the earth and severe climate changes. The CO2 is primarily released from long-term storage via combustion of fossil-fuel. It has been estimated that the worldwide energy-related CO2 emissions are increasing at a rate of about 2.1% per year (Xu et al., 2010). It is therefore necessary to decrease the emission of CO2 to the atmosphere on a global scale. The CO2 emissions from the petrochemical sector, for example, oil refineries, LNG sweetening, ammonia, ethane and other petrochemical process and ethylene oxide to atmosphere are estimated around 1460 MtCO2/year, while, CO2 utilization in chemical process such as urea, methanol, dimethyl ether, tert-butyl methyl ether (TBME) and organic carbonate is estimated around 178 MtCO2/year (Aresta et al., 2013). Although, no single solution will be sufficient in reducing this large net CO2 emission, a potential strategy could be to more utilize CO2 as a chemical feedstock for conversion to more valuable chemicals (Centi and Perathoner, 2009). However, the utilization of CO2 for the production of fine chemicals is severely limited by the reaction equilibrium in most cases and they have been widely reported (Omae, 2012). The high stability of carbon dioxide leads to a very low driving force, which has to be compensated if higher value chemical products are to be produced what is necessary is to first create a full reaction tree of higher value chemicals that can be produced directly or indirectly with CO2 as a reactant. This requires each synthesis route to be investigated for thermodynamic feasibility and availability of catalysts, when necessary. Having the reaction tree, different synthesis routes can be investigated to find the best set of value-added products by CO2 utilization and thereby reduction of net CO2 emission as a first step, the synthesis routes for a selected set of higher value products could be investigated based on known reaction data.
This work focuses on the evaluation of the production of dimethyl carbonate (DMC) by several reaction routes. DMC is an important carbonylating and methylating reagent used in various fields such as medicine, pesticides, composite materials, flavoring agent and electronic chemicals (Omae, 2012, Pacheco and Marshall, 1997). Although processes for the production of DMC are well-established, for example, BAYER (Kricsfalussy et al., 1996), UBE (Matsuzaki and Nakamura, 1997) and ENIChem (Tundo and Selva, 2002), the synthesis of DMC utilizing CO2 is an option worth investigating since it offers direct benefits to the environment while creating valuable products from the emitted and undesired CO2. In this paper, CO2 based processes for production of DMC are selected for evaluation and compared according to a set of performance criteria that includes yield, energy consumption and CO2 emission. For a consistent comparison, the various criteria are evaluated for the same product specification (that is, a fixed purity) and per unit mass of the desired product.
Section snippets
DMC production process alternatives
The production of DMC is classified here in terms of two main types, namely conventional processes and CO2-based processes. Among the conventional processes, the productions of DMC from phosgene, through partial carbonylation of methanol (BAYER process) and from methyl nitrile (UBE process) are well-known. The processes utilizing CO2 include direct synthesis with methanol and integrated processes involving intermediate compounds such as urea, propylene carbonate and ethylene carbonate, which
Screening of process routes
The objective of this analysis is to preselect three of the most promising process alternatives as candidates for further analysis based on the thermodynamic feasibility of their synthesis routes together with environmental, safety and health concerns.
Performance evaluation
Because of the concerns on issues such as the depletion of natural resources, environmental–safety–health impacts, as well as sustainability of the chemical process, it is not enough to simply find the optimal chemical process converting given raw materials to specified products. It is necessary to also make the process sustainable. In this work, the well-known sustainability metrics (Azapagic, 2002, Carvalho et al., 2008) together with life-cycle assessment factors and some green chemistry
Conclusion
Different processes for DMC production based on CO2 utilization have been investigated. The processes include the direct route of reacting CO2 with methanol and indirect routes of converting CO2 with ammonia, ethylene oxide and propylene oxide to urea, ethylene carbonate and propylene carbonate, respectively, and then further reacting them with methanol to DMC. Although the values of Gibbs free energy indicate advantage for the conventional processes (phosgene route, carbonylation of CO route
Acknowledgments
The financial support from The Thailand Research Fund is gratefully acknowledged. In addition, the first and the last authors would like to acknowledge the Ph.D. scholarship from Dussadeepipat Scholarship, Chulalongkorn University and the stay as a visiting researcher at the CAPEC-PROCESS Center at DTU Chemical Engineering.
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