An $85\%$ removal efficiency is to be achieved for a reactant which is known to decay by a $1^{st}$ order reaction. For a given flow rate $Q$ and a $1^{st}$ order reaction rate $k,$ compare the system size for a single plug flow reactor and one, two, three and four equal$-$sized CSTR$\text{'}$s in series. Assume steady$-$state conditions. Put your answers in terms of other variables. Is there any advantage in running plug flow reactors in series? Why or why not?