(310e) Process System Engineering Perspective on Production of Higher Alcohols from Carbon Dioxide

Using captured CO₂ as a chemical feedstock has been championed as a strategy to close the anthropogenic carbon cycle and reduce the net carbon emissions. A plethora of products of CO₂ utilization have been proposed and investigated in the literature, including Fischer–Tropsch fuels, aromatics, polymers, carbonates, carboxylic acids, ethers, methanol, and higher alcohols. Nevertheless, not only the synthesis process but the scale of production should also be considered. Compounds with small demands are not ideal candidates as CO₂ utilization products, since their contributions do not match the magnitude of the current global emissions. Energy, construction and polymer industries are attractive since they have the potential to utilize a large amount of CO₂. In this context, we examine the prospects of using CO₂ to produce chemical energy vectors in the form of higher alcohols. Several studies in the literature have investigated the use of propanol (C3OH), butanol (C4OH), pentanol (C5OH) and hexanol (C6OH) as fuels or fuel additives, some of which exhibited comparable engine performance to gasoline and reduced emissions.

Higher alcohol synthesis from CO₂ and green H₂ can occur via syngas or a direct conversion. At the present, thermocatalytic conversion of CO₂ to higher alcohols has not demonstrated sufficient yield and selectivity for industrial implementation. While several other research groups are directing their efforts towards catalyst development, it is equally important to examine higher alcohol synthesis from a process system engineering perspective. Here, we assumed perfect catalysts in terms of conversion and selectivity for higher alcohol synthesis from CO₂, designed a process and performed techno-economical and environmental analysis for each of the C3-6 alcohols.

The results show that the C5OH and C6OH plants have the potential to be economically viable at the assumed CO₂ and H₂ costs of 80 $/t and 2500 $/t, respectively. On the other hand, C3OH and C4OH plants cannot break even at the assumed raw material costs, but can do so if the H₂ cost drops to 1700 $/t. Common to all four alcohols, the raw material costs constitute about 83% of the cost of manufacturing, emphasizing the need of cheaper green H₂. The utilities costs account for about 12% of the cost of manufacturing in each plant. Since C3OH forms a homogeneous azeotrope with water, their separation requires an extractive distillation with entrainer such as ethylene glycol. C4OH forms a heterogeneous azeotrope with water and requires two reboiled absorbers and a phase-separation decanter for purification. As a result, the purification steps in these two plants are more energy intensive. The synthesis of these two alcohols will benefit from innovative reactor design such as membrane reactors, which simultaneously allow partial water removal and enhance reaction conversion. Due to the large amount of wastewater generated in the process, the wastewater treatment cost accounts for about 5% of the cost of manufacturing. On-site wastewater treatment system will be beneficial in high-capacity plants.

The CO₂ emissions of each plant were computed by adding direct emissions (i.e. combustion of fossil fuels for energy) and indirect emissions (associated with raw materials and electricity). The emissions in C3OH, C4OH, C5OH and C6OH plants are about 64%, 63%, 55% and 49% of the amount of CO₂ consumed by each plant, respectively. The higher percentages in C3OH and C4OH plants were due to higher energy demands for their separation processes.

The results also reveal that the hydrogen wastage (fraction of H₂ feed forming by-product water) of the four C3-6 alcohol synthesis processes was about 62% each. The “wasted” H₂ was used for the reduction of the oxygen moiety in CO₂, and therefore, its loss was unavoidable. Given the costly production of green H₂, this hydrogen wastage is an impediment to the economic performance of higher alcohol synthesis.

Finally, while C3OH and C4OH plants are not economically viable as standalones, we argue that they can be integrated with other plants, which have higher profits but more CO₂ emissions. This synergistic integration allows a win-win exchange of economic and environmental benefits. An example of such is demonstrated in this study.

Overall, this study performed a techno-economical and environmental assessment of four higher alcohol synthesis processes with an assumption of perfect catalysts. This allowed us to underscore the technological gaps to overcome, and set the direction for future research parallel to catalyst development.