Ask Greg McMillan

Greg's Response:

For gas reactants and a gas product, a pressure loop controls the material balance and provides the time available for reaction at a given production rate by manipulating the discharge product flow to balance the total feed flow. The residence time for these fast reactions is small (e.g., a few seconds) but still must be kept above a low limit. A fluidized catalyst bed is used to promote reaction rate. If a temperature loop manipulates the leader gas reactant flow, the production rate is automatically maximized by the temperature and pressure controllers for a given cooling rate established by boiler feedwater (BFW) flow and the number of coils in service. Direct manipulation of feed rate by the temperature control is possible in gas reactors because the additional time lag for composition response is negligible due to the small residence time and the inverse response at the control points is negligible due to the fast reaction, high heat release, and catalyst heat capacity.

A gas reactor with a fluidized catalyst bed may develop hot spots from localized high reactant concentrations due to a non-uniform flow distribution and no back mixing. Numerous separate cooling coils are used so operations personnel can switch coolant coils in or out of service to deal with hot spots and changes in production rate. However, the switching causes a disturbance to the temperature controller as fast as the BFW on-off valves can move. Numerous thermowells each with multiple sensors traverse the reactor. The average temperature is computed for each traverse with the highest average selected as the control temperature. A feedforward signal can provide preemptive correction for the disruption of coil switching by means of a gain and velocity limit set to match the BFW on-off valve installed characteristic slope and stroking time.

The plug flow of reactants through the reactor provides a tight residence time distribution. 

A dynamic simulation that includes all the phases, process time constants, mixing delays, measurement and valve 5Rs, and all thermal time constants is critical for detailing and tuning the best control strategy. 

For much more knowledge, see the ISA book Advances in Reactor Measurement and Control (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).

Top Ten Mistakes made in Fluidized Bed Gas Reactor Temperature Control

1. Use of orifices instead of venturi tubes to provide better reactant flow measurement 5Rs.
2. Use of tight shutoff valves instead of throttling valves to provide better feed valve 5Rs.
3. Poor distribution of feed streams.
4. Poor distribution of catalyst.
5. Coating or plugging in catalyst bed.
6. Hot spots in catalyst bed triggering side reactions.
7. Slow temperature measurement response due to protective tubes or thermowells.
8. Use of thermocouples instead of RTDs to eliminate drift and improve sensor resolution.
9. Not rejecting outlier temperature measurements in computing average bed temperature.
10. Not including enough sensors in cross sections for computing average bed temperature.