Exothermic CSTR temperature control with cooling jacket: select the recommended fail-safe valve action and controller action pairing for stable, safe regulation using cooling water.

Introduction / Context:
For an exothermic continuous stirred-tank reactor (CSTR), cooling reliability is critical to prevent thermal runaway. The control loop usually manipulates cooling water flow via a control valve on the jacket or coil. Choosing the correct valve fail action and controller action ensures both safety and proper loop direction.

Given Data / Assumptions:

• Process: exothermic CSTR with water-jacket cooling.
• Controlled variable: reactor temperature (high risk if it increases).
• Manipulated variable: cooling water flow through a valve.
• Air failure should favor increased cooling (fail-safe).

Concept / Approach:
An air-to-close valve (fail-open) will open upon loss of air, increasing cooling water flow and enhancing safety. Loop direction: when temperature rises above set point, the controller must open the cooling valve. For an air-to-close valve, increasing controller output pressure would close the valve; therefore the controller must be indirect (reverse) acting so that a rise in measured temperature decreases controller output, opening the valve.

Step-by-Step Solution:

Verification / Alternative check:
Typical P&IDs for exothermic services show fail-open cooling valves with reverse acting temperature controllers to achieve the correct sign.

Why Other Options Are Wrong:

Common Pitfalls:
Overlooking fail-safe hardware intent; always design for safe state on instrument failure.

Final Answer:
Air-to-close valve with the controller indirect (reverse) acting