Abstract
The use of captured CO2 as a raw material is a quite old concept that has however received more and more attention recently. Indeed, carbon capture units are increasingly being developed as well as new technologies for the storage, the utilisation and the transformation of this captured CO2. This is driven by the increasing necessity to move towards more sustainable production processes and to mitigate greenhouse gases emissions.
The storage of CO2 in earth’s layers being a cost only technology, the alternative consisting in the production of novel chemical products or key substitutes to fossil-based chemicals seems attractive. In this perspective, two processes for dimethyl carbonate (DMC) production from captured CO2 are discussed. The selected pathways both differ from usual dimethyl carbonate units in the selected raw materials and in the choice of energy used. Both processes rely on the direct synthesis of DMC from methanol and carbon dioxide. Each implies the utilisation of a dehydration agent to consume the water produced by the direct reaction and leading to a yield increase. Both alternatives are designed for an annual production of 20,000 tonnes of DMC and their respective mass and energy balances are simulated in Aspen Plus. Subsequently, a techno-economic analysis is performed to asses the viability of each process. From those analyses, it turns out that the direct synthesis of DMC using 2-cyanopyridine (2-CP) as dehydration agent leads to a revenue of 37 Me/year whereas the use of ethylene oxide (EO) as dehydration agent leads to 47 Me/year revenue. The difference comes from the ethylene glycol produced and sold. However, if the net present value (NPV) is regarded, the production of dimethyl carbonate using 2-cyanopyridine seems to be the most interesting production process.
In order to end up with a process as sustainable as possible, a sustainable way for methanol production is studied. This one implies the production of hydrogen from water electrolysis which is discussed as well. The results show that the electrolysis cell consumes around 40 kWh/kg of H2 produced. This consumption is a hint showing the important energy requirements to produce methanol in a sustainable way. However, if renewable energy resources are considered, this process would be an attractive alternative compared to the classical methanol consumption in terms of CO2 emissions. Nevertheless, the production cost of this methanol coming from renewable resources should not be neglected as it is shown that it is around 1,799 e/tonne while the actual purchase cost is 423 e/tonne. A whole energetic integration with the DMC production unit could be considered to optimise the overall energy consumption.