Ask Greg McMillan

Greg's Response:

A vessel is defined here as storage, recycle, blend, or surge volume where there is no heat of reaction and no agitator. Fortunately, most vessel volumes are so large that the process time constant is extremely large and, consequently, the rate of change of temperature is exceptionally slow. This is fortunate because the only mixing typically provided in storage and feed tanks is provided by incoming flows (e.g., feed and recirculation) and convective currents. The addition of eductors and the location of dip tubes or spargers can greatly reduce the loop dead time magnitude. Ideally, thermowells should be installed in the elbows of recirculation lines. If they must be installed in the vessel, they should be in areas where the fluid velocity is largest, to reduce the measurement time constant, but farthest away from steam spargers, to avoid erratic nonrepresentative temperature readings. Storage and feed tank temperature loops often have a process time constant so large that a high controller gain can be used despite a high process dead time from poor mixing.

For throttling of coolant flow to coils or jackets of small vessels with a continuous discharge flow, the same concerns described for heat exchangers about a limit cycle from excessive dead time and process gain at low flow exist. Therefore, gain scheduling or signal characterization are useful techniques. Also, a feedforward signal can be computed, based on a simple energy balance as was done for the heat exchanger. A better solution is to keep a high constant coil or jacket recirculation flow and use a coil or jacket inlet secondary temperature control manipulate a makeup coolant flow to inlet with pressure control manipulation of return flow from coil or jacket outlet. When heating is required, steam flow to a steam injector in a high flow coil or jacket recirculation line manipulated by inlet temperature control provides the best dynamics. For vessel zero discharge flow, set point profiles and proportional-plus-derivative (PD) controllers (like those used on batch reactors) can help prevent overshoot.

Simulations that include operating conditions and equipment design with Digital Twin can help find and confirm the best control strategies.

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