Detailed assessment of dynamic startup, shutdown, and flexibility of the adiabatic, gas-and water-cooled methanol fixed bed reactor: Comparison for power to methanol application

Integration of intermittent renewable energy with methanol synthesis requires that the reactor and separation system be flexible and tolerant to frequent stop–start operation. In this paper, the dynamic load flexibility, start-up, and shutdown characteristics of the water-cooled, gas-cooled, and adiabatic fixed bed reactor are investigated under hot and cold start modes. Parameters such as the flow rates, compositions, reactor temperature evolution with start-up and shutdown, start-up and shutdown time, and the heat transfer characteristics of the reactor are assessed. Different standby and heat management strategies are discussed. It is shown that the load flexibility range of 20–110 % is attainable in the adiabatic and water-cooled reactor systems applied for mildly exothermic methanol synthesis. The gas-cooled reactor system can tolerate a load range of 40–110 %. The gas-cooled reactor has a shorter start-up (21 min) and shutdown time (6 min) followed by the water-cooled and thereafter adiabatic reactor when nitrogen is used for hot standby management. Using H₂ during hot standby shortens the start-up time of the reactors up to 15, 30, and 36 min for the gas-cooled, water-cooled, and adiabatic reactors, respectively. While to reduce cost and alleviate safety concerns during long-duration cold standby, the use of N₂ is recommended. Cold start-up can take several hours and depends on thermal transients of the catalysts and reactors. Reactors can tolerate fast ramp rates up to 2.22 %/min for load change range (up and down to 20–110 %), up to 2.7–16.67 %/min for start-up, and 0.067–16.67 %/h for shutdown. Fast ramp rates reduce the CO₂ emissions during the start-up. Reactors can operate safely with no load/ramp-induced wrong-way behavior and violation of path constraints.