Adiabatic PFR with feed-product heat exchange - Web Labs at www.ReactorLab.net - by Richard K. Herz
The system inlet flow enters the "cold side" of a heat exchanger, then flows to the inlet of an adiabatic, packed-bed, plug-flow reactor. The reaction is A → products over catalyst pellets. The reaction is first-order in reactant. For simplicity, the reaction equilibrium composition is specified to be almost all product under all conditions, i.e., an essentially irreversible reaction. The concentration of reactant A is represented as Ca. Constant fluid density is assumed. The variable Kf300 is the value of the forward reaction rate coefficient at 300 K.
When the reaction enthalpy (heat of reaction) is negatively valued, the reaction is exothermic and heats the fluid, which tends to increase the reaction and energy release rates. The fluid leaving the reactor enters the "hot side" of the heat exchanger. For an exothermic reaction, this heats the inlet flow. This type of system is related to an "autothermal reactor" in which the heat exchanger is integrated with the reactor.
This system has feedback of energy. As the lab starts running from the default conditions, notice the temperature change flow to the right in the cold side of the heat exchanger (bottom color panel), then from left to right in the reactor (top panel), then from right to left in the hot side of the exchanger, just above the cold side. This heats the cold side again and the process is repeated again in a feedback loop.
This system can exbibit hysteresis. Click the Run button and let the system reach steady state at the default conditions with an inlet T of 340 K. Then decrease the System inlet T to 330 K and wait for steady state. Note the relatively high conversion of reactant to product.
Next, decrease the inlet T to 320 K and wait for steady state. Finally, increase the inlet T back to the previous 330 K and wait for steady state. Note the relatively low conversion of reactant compared to the earlier result at 330K. Thus, there are multiple steady states at 330 K.
Many industrial ammonia synthesis reactor (ammonia converter) designs contain a reactor-heat exchanger structure like this.
At any instant, there are not exact matches between the heat exchanger cold outlet temperature and the reactor inlet temperature, nor between the reactor outlet and exchanger hot inlet because of transit times in the connecting pipes.
Currently in this simulation, the only heat capacities which affect the dynamic response are that of the fluid and catalyst. The heat capacities of the reactor and exchanger bodies may be added later. The residence times of the reactor and heat exchanger are set equal.
Interesting dynamics can be observed. For example, let the system reach steady state at the default input values. Then double the flow rate from 0.005 to 0.01.
Parameters remaining constant: