/Mathematical optimization
Once a process can be written down, designing it becomes a question of choosing. Optimization makes the choice exact: variables, constraints and an objective, and a solver that searches a space far too large to try by hand. I like how much engineering judgment turns out to fit in that form.
The discrete choices are what make it hard. Whether to build a unit at all is a yes-or-no variable, so a design problem becomes a mixed-integer program, and the solver has to reason about a lattice of integer points instead of a smooth region.
A superstructure takes that to the whole flowsheet. It holds every candidate technology and every link between them, and the solver keeps only the ones worth building.
Real objectives also pull against each other. Cost and emissions rarely fall together, so the useful answer is the whole curve of trade-offs, and a decision maker picks a point on it.