Process Simulation and Environmental Aspects of Dimethyl Ether Production from Digestate-Derived Syngas

Abstract

The production of dimethyl ether from renewables or waste is a promising strategy to push towards a sustainable energy transition of alternative eco-friendly diesel fuel. In this work, we simulate the synthesis of dimethyl ether from a syngas (a mixture of CO, CO₂ and H₂) produced from gasification of digestate. In particular, a thermodynamic analysis was performed to individuate the best process conditions and syngas conditioning processes to maximize yield to dimethyl etehr (DME). Process simulation was carried out by ChemCAD software, and it was particularly focused on the effect of process conditions of both water gas shift and CO₂ absorption by Selexol^® on the syngas composition, with a direct influence on DME productivity. The final best flowsheet and the best process conditions were evaluated in terms of CO₂ equivalent emissions. Results show direct DME synthesis global yield was higher without the WGS section and with a carbon capture equal to 85%. The final environmental impact was found equal to -113 kgCO₂/GJ, demonstrating that DME synthesis from digestate may be considered as a suitable strategy for carbon dioxide recycling.

Keywords: carbon footprint; digestate; dimethyl ether; gasification; process simulation.

Figure 1

Procedure scheme to obtain the optimal bio-DME environmental assessment.

Figure 2

Block flow diagram of the production of DME by gasification of digestate and direct synthesis.

Figure 3

Thermodynamic analysis scheme.

Figure 4

Theoretical equilibrium DME yield as a function of reaction temperature and pressure, for the raw syngas with the following molar composition: H₂/CO = 0.87, CO₂/CO = 0.94. Lines are only a guide for the reader.

Figure 5

Theoretical equilibrium DME yield as a function of the initial CO₂/CO molar ratio, reaction temperature and pressure, for syngas with H₂/CO = 0.87. Lines are only a guide for the reader.

Figure 6

Theoretical equilibrium DME yield as a function of the initial CO₂/CO molar ratio, reaction temperature and pressure, for syngas with H₂/CO = 2. Lines are only a guide for the reader.

Figure 7

Theoretical equilibrium DME yield derivative with respect to CO₂/CO molar ratio as a function of the initial CO₂/CO molar ratio, reaction temperature and pressure, for syngas with H₂/CO = 0.87. Lines are only a guide for the reader.

Figure 8

Theoretical equilibrium DME yield derivative with respect to CO₂/CO molar ratio as a function of the initial CO₂/CO molar ratio, reaction temperature and pressure, for syngas with H₂/CO = 2. Lines are only a guide for the reader.

Figure 9

Theoretical equilibrium DME yield (global or single-pass) with respect to the CO₂ capture, fixing the WGS conversion and the purge ratio to 0% and 10%, respectively.

Figure 10

Theoretical equilibrium DME yield (global or single pass) with respect to the WGS conversion, fixing CO₂ capture and the purge ratio to 85% and 10%, respectively.

Figure 11

Theoretical equilibrium DME yield (global or single pass) with respect to the purge ratio, fixing WGS conversion and the CO₂ capture to 0% and to 85%, respectively.

Figure 12

Flowsheet diagram for the optimal process result.

Figure 13

Equivalent CO₂ emissions of the optimal flowsheet.

Figure 14

Environmental impact comparison between bio-DME from digestate with CCS, bio-DME from digestate without CCS, DME from biomass indirect liquefaction [56] and diesel.

Figure 15

Effect of initial digestate moisture content on carbon dioxide emission of the proposed process plant for DME synthesis.