Synthesis of Dimethyl Ether from Natural Gas: CO2 Utilization Process

Dimethyl ether (DME) is an important chemical product which can be used as fuel, solvent, refrigerant and aerosol. DME’s most promising application is the substitution of liquefied petroleum gas as environmental friendly solution to the liquid fuels problem. A plant for DME production from natural gas is designed in this work by applying the 12-tasks systematic process design method [1]. The goal of this work is to design a sustainable and economically feasible process by utilizing global warming gas CO₂for producing a highly valuable product, DME. The conceptual process design is first performed in the MSc-level course “Process Design: Principles and Methods” and then extended as sustainable process design project at the Department of Chemical and Biochemical Engineering in Technical University of Denmark.

The two main steps required to produce DME from natural gas are: syngas has to be synthesized first, followed by a direct reaction combing production and dehydration of methanol to make DME. The production of syngas in the first step is carried out through tri-reforming since carbon production could be avoided and thus the catalyst is protected from poisoning. In the following step, the H₂/CO ratio is adjusted to avoid water-gas shift reaction, which makes CO₂as side-product instead of water.

The whole project is divided into twelve different tasks [1], in which different design decisions are made in a hierarchical manner. First, all the information about products and selected synthesis process are collected and an initial processing path synthesized in tasks 1-3. Then, the preliminary design decisions for the main design variables are made and verified through simple mass balance based simulations in task 4. Then, both energy and mass balance are verified through a process simulator (PROII) using simple as well as rigorous simulation models in tasks 5-7. At this point, a preliminary synthesis-design of the process is obtained. The next tasks (8-9) involved the sizing and costing calculations to establish a base case design. In the final tasks (10-12), the base case design is analyzed and targeted improvements are made through optimization, heat integration, and environmental impact analysis [2] to obtain a more sustainable and improved process design. A net carbon dioxide emission for this process is also calculated. Based on the design, twenty thousand metric tons of DME, with a 99.7% purity, are produced per year.

In this work PROII is used for simulating the process, while ECON is applied for economic evaluation. Also many other tools are used: ICAS [3] for additional property prediction, SustainPro [4] for sustainability analysis and process “hot-spot” (bottleneck) identification and LCSoft [5] for lifecycle assessment factor and environmental indicators, including the ECO efficiency indicators.

References

[1] D. K. Babi, "Teaching Sustainable Process Design using 12 systematic Computer Aided Tasks". Computer Aided Chemical Engineering, 2015, vol. 37, pp. 173-178.

[2] L. T. Biegler, I. E. Grossmann, and A. W. Westerberg, Systematic methods of chemical process design. 1997.

[3] R. Gani, G. Hytoft, C. Jaksland, and A. K. Jensen, “An integrated computer aided system for integrated design of chemical processes”. Comput. Chem. Eng. Jul. 1997, vol. 21, no. 10, pp. 1135–1146.

[4] A. Carvalho, H. A. Matos, and R. Gani, “SustainPro—A tool for systematic process analysis, generation and evaluation of sustainable design alternatives”. Comput. Chem. Eng.Mar. 2013, vol. 50, pp. 8–27.

[5] S. Kalakul, P. Malakul, K. Siemanond, and R. Gani, “Integration of life cycle assessment software with tools for economic and sustainability analyses and process simulation for sustainable process design”. J. Clean. Prod.May 2014, vol. 71, pp. 98–109.