The sustainable design of producing low-carbon methanol requires promoting technological advancement, adopting alternative feedstocks, implementing carbon circularity, and finding reliable and affordable renewable energy sources. Natural gas is one of the most abundant sources; however, it emits carbon dioxide (CO2) when it is used as the primary feedstock for chemical products. This work aims to develop a new conceptual design to produce blue and green methanol. The proposed system includes the conventional methanol process of partial oxidation, while CO2 emitted from the separation unit was recycled and used to produce green methanol by combining with hydrogen from an electrolyzer. The blue methanol process, green methanol process, and electrolyzer were simulated by using Aspen Plus software. Heat integration was conducted to minimize the heating and cooling utilities required by the methanol plant. Furthermore, a heat pump was introduced to mitigate waste heat and enhance the energy efficiency. To escalate the sustainability aspect by minimizing freshwater use, the reverse osmosis (RO) was incorporated to recycle and use the flowback and produced water of shale gas production. The energy system was designed to ensure providing a steady-state thermal and electric energy and overcome the fluctuations in solar energy and weather conditions, including solar parabolic trough collectors (PTC), a thermal energy storage, a steam generator, and a back-pressure turbine. The technical-economic-environmental performance of the system under the uncertainty of solar energy intensity and natural gas prices was evaluated by considering two scenarios, with and without the heat pump. Accordingly, the historical data of the solar direct normal irradiance (DNI) and natural gas prices were statistically analyzed to study the behavior of the data distribution. Afterward, a Monte Carlo analysis was performed to generate 100,000 random samples for each data set. The multi-scenario stochastic mixed-integer nonlinear programming (MINLP) model was formulated using LINGO Software to maximize the annual net profit after tax. The optimal case shows that the blue/green methanol production cost is lower when using a heat pump. The environmental assessment shows a significant reduction in the CO2 emission and freshwater use for the overall system boundary.
Keywords: Green Methanol, Blue Methanol, Heat Integration, Simulation, Stochastic Optimization
