Techno-economic evaluation of different CO2-based processes for dimethyl carbonate production

https://doi.org/10.1016/j.cherd.2014.07.013Get rights and content

Highlights

  • Techno-economic evaluation of CO2-based dimethyl carbonate production processes.

  • Reactions include CO2, urea, ethylene carbonate and propylene carbonate routes.

  • Energy consumption, net CO2 emission, atom efficiency and LCA were determined.

  • The performances were compared with those of the conventional BAYER process.

  • The ethylene carbonate route is the most promising process for DMC production.

Abstract

In this work, several chemical processes for production of dimethyl carbonate (DMC) based on CO2 utilization are evaluated. Four CO2-based processes for production of DMC are considered: (1) direct synthesis from CO2 and methanol; (2) synthesis from urea; (3) synthesis from propylene carbonate; and (4) synthesis from ethylene carbonate. The processes avoid the use of toxic chemicals such as phosgene, CO and NO that are required in conventional DMC production processes. From preliminary thermodynamic analysis, the yields of DMC are found to have the following order (higher to lower): ethylene carbonate route > urea route > propylene carbonate route > direct synthesis from CO2. Therefore, only the urea and ethylene carbonate routes are further investigated by comparing their performances with the commercial BAYER process on the basis of kg of DMC produced at a specific purity. The ethylene carbonate route is found to give the best performance in terms of energy consumption (11.4% improvement), net CO2 emission (13.4% improvement), in global warming potential (58.6% improvement) and in human toxicity-carcinogenic (99.9% improvement) compared to the BAYER process. Also, the ethylene carbonate option produces ethylene glycol as a valuable by-product. Based on the above and other performance criteria, the ethylene carbonate route is found to be a highly promising green process for DMC 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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