I have to optimize a large CO2 piping distribution network. CO2 is coming from a header and getting distributed into sub headers which supply CO2 to desired delivery points. There are no loops. I can optimize header pressure, header diameter, subheader diameters, delivery piping diameters to reduce overall costs. Diameters can only be from the standard table, meaning discrete values. I can also choose to have pressure reducing valves installed any where. How can i approach this problem? Which method is more appropriate?