Economic, exergic, and environmental assessment of energy-saving separation process for carbon dioxide direct hydrogenation to methanol

The rapid increase of carbon dioxide content in the atmosphere has led to environmental problems such as the greenhouse effect, carbon dioxide hydrogenation to methanol will help to alleviate the burden on the environment. Energy-saving processes were proposed for separating methanol products from crude methanol produced through the direct hydrogenation of carbon dioxide. A flash system and nitrogen stripper were applied to remove carbon dioxide and hydrogen in the carbon dioxide hydrogenation products because of the high solubility of carbon dioxide in methanol. Conventional distillation (CDiC), differential pressure thermal coupling (DPDiC), and double effect distillation (DEDiC) were established to obtain high-purity methanol from crude methanol with high water content to achieve energy conservation. Total annual cost (TAC), carbon dioxide emissions, and exergy efficiency were conducted to evaluate the proposed distillation processes. The results show that the three-stage flash evaporation is more energy efficient than the two-stage flash evaporation, with an exergy loss of 16.7% and 24.0%, respectively. The DEDiC process has the highest exergy efficiency of 25.6%, while the DPDiC process demonstrates better economic and environmental performance. Thus, the DPDiC process provides an energy-saving option for crude methanol refining.