Plug Flow Reactor (Tubular Reactor) - Notes

3. 8.1 Plug Flow Reactor (Tubular reactor) Batch reactor Reactants are initially charged into a container, well mixed and allowed to react for a certain period. The resultant mixture is then discharged. This is an unsteady-state operation where composition changes with time, however, at any inst...

3. 8.1 Plug Flow Reactor (Tubular reactor) Batch reactor Reactants are initially charged into a container, well mixed and allowed to react for a certain period. The resultant mixture is then discharged. This is an unsteady-state operation where composition changes with time, however, at any instant the composition throughout the reactor is uniform (because the reactor mixture is perfectly mixed). 8.2 CSTR It is a well-mixed stirred tank operated continuously and is normally run at steady-state (the effluent reactant and product concentrations are constant over time). Because of the perfect mixing the reactants entering the reactor assumed to be instantaneously distributed over the total reactor volume. As a result, there are no spatial variations in concentration, temperature or reaction rate. Thus, the concentration and temperature in the exit stream is the same. 8.3 Plug Flow Reactor (PFR) This consists of a pipe (tube), in which the reactants are continually consumed as they flow along the length of the reactor. The concentration varies continuously in the axial direction through the reactor, as a result of which the reaction rate, which is function of concentration, also varies axially. This is normally operated at steadystate. The necessary and sufficient condition for plug flow is for the residence time in the reactor to be the same for all fluid elements. Actually, the flow is highly turbulent and the flow field may be modelled by that of a plug flow. 8.4 Resident time - In a batch reactor each and every fluid element remains exactly the same reaction time in the reactor volume. - In a CSTR, the separate fluid elements have a wide range in residence times. In systems where mixing is highly non-ideal, the residence time distribution (RTD) is needed to characterise the mixing that occurs in the reactor. Real reactors never fully follow the ideal flow patterns. Deviations can be caused by channelling of fluid, recycling of fluid and creation of stagnant regions. The RTD of a reactor is a 27