Isothermal Plug Flow Reactors (PFRs): Summary
The answers to the ConcepTests are given below and will open in a separate window.
Key points from this module:
- Material balances on individual components are most useful for steady-state PFR design: 0 = In – Out + Generation by reaction.
- All molecules spend the same time in a PFR (unlike a CSTR or laminar flow reactor).
- A plug flow reactor has no radial gradients.
- The mass balances for a PFR are best solved numerically in general, with one mass balance for each component in the system. For real systems where multiple reactions occur and the reactor is non-isothermal, the ordinary differential equations must be solved numerically.
- Using mass flow rates as the dependent variables makes the equations easier to solve with multiple reactions and mole changes (gas-phase reactions).
- For gas-phase reactions, the volumetric flow rates can vary significantly between the inlet and outlet of the reactor because of a mole change due to reaction (e.g., A → 2B), temperature change, or pressure change. If more gas-phase molecules are in products than reactants, the volumetric flow rate increases moving through the reactor. If fewer gas-phase molecules are in products than reactants, then the volumetric flow rate decreases moving down the reactor.
- The mass balance equations are identical for a non-isothermal PFR except the rate constant (k) is a function of temperature so it changes with distance down the reactor so mass balances must be solved simultaneously with an energy balance.
- Plug flow reactors are used for continuous, large-scale production. Plug flow reactors often contain a solid catalyst.
- Potential concerns with using PFRs are poor mixing and hot spots.
From studying this module, you should now be able to:
- Determine the size of an isothermal PFR for a single reaction, given the rate constant, order of reaction, inlet reactant concentration, and inlet flow rate.
- Determine the conversion in an isothermal PFR for a single reaction, given the rate constant, order of reaction, inlet reactant concentration, and inlet flow rate.
- Use the ideal gas law to account for changes in the volumetric flow rate in the plug flow reactor due to changes in the number of gas-phase moles in a reaction.
- Explain how the volumetric flow rate changes with distance down an isothermal PFR for a gas phase reaction.
- Predict how reactant flow rates change with the feed volumetric flow rate increases to an isothermal PFR.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder