The plug flow model assumes there is no boundary layer adjacent to the inner wall of the pipe.
Essentially no back mixing is assumed with "plugs" of fluid passing through the reactor.
This results in differential equations that need to be integrated to find the reactor conversion and outlet temperatures.
Other simplifications used are perfect radial mixing and a homogeneous bed structure.
An advantage of the plug flow model is that no part of the solution of the problem can be perpetuated "upstream".
This allows one to calculate the exact solution to the differential equation knowing only the initial conditions.
In laminar pipe flow, the velocity profile is parabolic.
Plug flow will be achieved if the sublayer thickness is much less than the pipe diameter (
is the Darcy friction factor (from the above equation or the Moody Chart),
In this regime the pressure drop is a result of inertia-dominated turbulent shear stress rather than viscosity-dominated laminar shear stress.