Internal carbon loop strategy for methanol production from natural gas: Multi-objective optimization and process evaluation

Abstract

In this study an internal carbon loop strategy for producing methanol from natural gas (NG) by introducing CO2 to enhance methanol productivity and minimize greenhouse gas (GHG) emissions. The overall process consists of a steam methane reforming (SMR) process (hydrogen production), methanol synthesis process (carbon utilization), crude methanol separation process, chemical absorption process (carbon capture), and CO2 compression and storage processes (carbon storage). Methanol was produced using hydrogen, carbon monoxide, and carbon dioxide as the products and by-products of the steam methane reforming process. In particular, the pressure of the compressed CO2 for storage was utilized to enhance profitability. A strategy for injecting the pressurized CO2 into the reforming reactor, methanol reactor, or splitting it between the two reactors was examined. The process variables used were the steam/natural gas ratio (1.5, 2.0, 2.5), CO2/natural gas ratio (0–0.5), natural gas flow rate (3000 kmol/h), reforming reaction pressure (5 barg), and methanol reaction pressure (80 barg). Methanol production was 2800–3000 tonMeOH/day under according to operating conditions. Carbon pricing was considered to assess the feasibility of installing and operating a carbon capture and storage (CCS) unit. The effect of the process economic and environment variables was investigated on total energy duty, levelized cost of methanol (LCOM) from a techno-economic analysis (TEA) perspective, and global warming potential (GWP) from a life cycle assessment (LCA) standpoint using local sensitivity analysis (LSA). Subsequently, multi-objective optimization (MOO) for levelized cost of methanol and global warming potential was performed to find optimal operating variables and establish various relevant scenarios. Scenario analysis reveals that implementing an internal carbon loop strategy through relatively simple retrofits can increase levelized cost of methanol by 12.8% while reducing global warming potential by 41.5%.