In a self-control system for maintaining a constant temperature, how is temperature lag addressed?
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In DCS systems, during the reaction process heated by heat transfer oil, automatic control valves adjust the supply amount of hot oil based on the temperature inside the reactor. During the constant-temperature maintenance phase, due to the significant lag in temperature response, the temperature often exceeds the specified range. Have any of you sea friends encountered such a problem? How was it solved? Let’s discuss it. .
1. First, consider the selection of the primary sensor: use mica sheet thermistors for temperatures ranging from -100 to 600 degrees, while thermocouples should be used for higher temperatures. 2. Thermal grease or thermal oil can be applied to the top of the temperature sensing sleeve to facilitate heat transfer. 3. Ensure that the temperature-sensing elements of the thermistors and thermocouples are in contact with the bottom of the sleeve, thereby accelerating heat transmission
The key is the time it takes for the overall temperature of the reactor to reach equilibrium, not whether the thermometer itself is delayed in reading. If the thermometer indeed has a significant delay, it is best to use an armored version without a protective cover. Regarding the lag issue, the only thing I can do is lower the temperature control point.
There are two reasons for the delay: process and instrumentation. The issue with the instrument has been explained by a few people upstairs ； There are also process-related issues; there is a certain delay in the transfer of the heat from the heating oil to inside the tank. Suggestion: Increase the differential effect.
It’s a real tough problem – a typical second-order lag system (adding heat transfer oil in between actually increases the order). Adding a derivative is useful, but it can cause larger fluctuations if not done properly. It’s better to have a weaker adjustment effect, and adjust manually when it’s not stable. For small disturbances, it is better to use feed flow rates and/or inlet temperature as feedback signals; this way, once stabilization is achieved, the system can remain stable in the face of general disturbances as well. Or adopt advanced control schemes.
It is recommended to use advanced temperature control instruments as well as precise thermocouples. We use a small DCS temperature control system from Euroland, which provides very stable temperature control with variations within 0.1. The thermocouple wires are custom-made by us.
It depends on the size of the reactor; in some cases, the temperature response is too slow, and even a strong differential effect doesn’t help
In terms of instruments: use heat resistors with high thermal conductivity; include the inlet water temperature and outlet water temperature in the PID control, utilizing differentiation； In terms of process control: Regularly check the heat conduction conditions inside the tank to maintain a high thermal conductivity and stabilize the heat release during the polymerization reaction.
1. Enhanced differential action. 2. Investigate the cause from a process perspective to determine whether the flow rate of the heat transfer oil is unstable; if so, increasing the heat transfer oil flow rate can be used as a temperature cascade control mechanism for the secondary loop