Question

Temperature control of an exothermic chemical reaction taking place in a CSTR is done with the help of cooling water flowing in a jacket around the reactor. The types of valve and controller action to be recommended are

• A. Air to open valve with the controller direct acting
• B. Air to close valve with the controller indirect acting
• C. Air to open valve with the controller indirect acting
• D. Air to close valve with the controller direct acting

Hint:

Fail-safe logic for exothermic reactors: Always maximize cooling utilities upon power failure. Maximizing flow requires a Fail-Open valve, which is physically built as an Air-to-Close valve.

Answer 1

The Correct Option is B

Concept: Designing safe control loops requires selecting a fail-safe valve mode alongside a matching controller configuration to ensure stability.
• Air-to-Open (Fail-Closed FC): The valve requires air pressure to open; it snaps shut if instrument air pressure fails.
• Air-to-Close (Fail-Open FO): The valve requires air pressure to close; it opens fully if instrument air pressure fails.
• Direct-Acting Controller: Output increases as the measured PV increases (Output \(\uparrow\) when PV \(\uparrow\)).
• Indirect-Acting (Reverse-Acting) Controller: Output decreases as the measured PV increases (Output \(\downarrow\) when PV \(\uparrow\)).

Step 1: Selecting the fail-safe valve mechanism.
The system runs an exothermic reaction. If the temperature rises uncontrollably, it can cause a thermal runaway explosion. If the facility loses instrument air power, we must ensure maximum cooling water flow continues to cool the vessel. Therefore, the cooling water control valve must default to wide open when unpowered. This requirement dictates an Air-to-Close (Fail-Open) valve configuration.

Step 2: Deciding controller action for loop stability.
Let's trace the physical feedback mechanism to ensure negative feedback control:
• Suppose the reactor temperature (PV) increases.
• To counteract this increase, we need more cooling water, which means the Air-to-Close valve must open wider.
• To open an Air-to-Close valve further, the pneumatic controller signal output to the valve must decrease (venting pressure allows the valve spring to push it open).
• Because the controller output must decrease when the process temperature increases, the controller must be configured as indirect-acting (reverse-acting).