Subsequently, lifecycle GHG emission factors (SI, Table ) and economic factors (SI, Table ) are considered within the mass and energy balances calculated in the overall flowsheet simulation, thus resulting in the objective values. The optimizer varies the values of decision variables and improves the objectives iteratively. After the surrogate-based optimization is completed, we use the obtained values for the decision variables to perform rigorous simulations for individual sub-systems, as to validate the optimal solution.