(203p) Innovative Production of Dimethyl Ether From Synthesis Gas

Dimethyl ether (DME) is a sustainable substitute for diesel fuel. Its application involves both the chemical and automotive industries. In recent years the global market for DME has increased – especially in emerging countries like China. The trend indicates increasing future demands [1]. In this project, natural gas (e.g. from biomass) and carbon dioxide (e.g. from power plants) are utilized as raw materials in a dry reforming process to produce syngas. Syngas production is followed by direct DME synthesis, in which conventional methanol synthesis and DME synthesis are integrated into a one-step process over a bifunctional catalyst [2], resulting in a simplified overall process design. The objective of this project is to evaluate and analyze process design, especially with respect to sustainability and environmental impact.

The production of dimethyl ether consists of three main parts. The first part is syngas production from methane and CO₂ by dry reforming process. In this process, equimolecular methane and CO₂ is converted to syngas, the main components of which are H₂ and CO in a molar ratio of approximately 1:1. This reaction occurs over a Ni/La₂O₃ catalyst at 800 °C and 1 bar with a methane conversion degree of 97 % [3]. In the second part, DME is produced from syngas. Equimolecular H₂and CO is converted to DME over a bifunctional catalyst operating at 260 °C and 50 bar. Downstream separation process is employed in the third part to separate pure DME as product and recycle unreacted material to the two reactor systems. A purge containing unreacted syngas and inerts is included to avoid accumulation in the process.

The resulting process design is similar to direct DME synthesis schemes reported by JFE and KOGAS processes [2, 4]. However, the proposed process design utilizes dry reforming for syngas synthesis – a reforming process not yet applied by the industry. The use of dry reforming provides syngas of an ideal composition for direct DME synthesis, while lowering the amounts of natural gas consumed and carbon dioxide emitted during production. The mass ratio of product to raw materials is approximately 1.8 while the energy consumption is approximately 4.6 MJ/kg product, without considering heat integration. The estimated cost of production, considering 10 years of operation of the base case design, is approximately $0.32/kg product. Thus, a potential exists for more sustainable DME production in terms of environmental impact due to the consumption of carbon dioxide during the reforming process.

The process design is done by utilizing a systematic approach by performing a 12-task design procedure [5]. The feasibility of the process is verified and basic information about raw materials, product and process is collected in tasks 1-3. Subsequently, mass- and energy balance calculations are performed in tasks 4-6, where design decisions are made for units needed and the operating conditions of them. Hereafter, more rigorous models are applied for the unit operations in task 7. The sizing and costing of each unit is performed to enable economic evaluation of the entire process in tasks 8-9. Finally, cost and sustainability optimization of the design are performed in tasks 10-12. Optimization includes heat integration, environmental and sustainability analysis. The sustainability analysis identified bottlenecks in the base case design and addressing them led to the innovative aspects of the process design.

The presentation will highlight the innovative aspects of the process design and quantify them in terms of improvements in sustainability metrics and environmental impacts. The results also show that the more sustainable alternative is also the more economical.

References:
[1] T.H. Fleisch, A. Basu, R.A. Sills, Introduction and advancement of a new clean global fuel: The status of DME developments in China and beyond, Journal of Natural Gas Science and Engineering, 9, 2012, 94-107
[2] T. Ogawa, N. Inoue, T. Shikada, Y. Ohno, Direct Dimethyl Ether Synthesis, Journal of Natural Gas Chemistry, 12, 2003, 219-227
[3] X. E. Verykios, Catalytic dry reforming of natural gas for the production of chemicals and hydrogen, International Journal of Hydrogen Energy, 28, 2003, 1045-1063
[4] SH. Lee et al., Scale up study of DME direct synthesis technology, Technical Report, Korea Gas Corporation, 2001. Available: http://www.igu.org/html/wgc2009/papers/docs/wgcFinal00745.pdf
[5] Rafiqul Gani, Systematic Process Design in 12 Hierarchical Steps, Course Lecture Notes, DTU, Lyngby, 2012