The steam supply flow rate to the reboiler fluctuates greatly
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Our company has a distillation unit that uses saturated steam at 1.0 MPa as the heat source. The designed flow rate is 6 T/H; the steam inlet is controlled by a regulating valve. A drain trap is installed after the reboiler, and the drained water is sent to a flash tank at 0.4 MPa where it flashes to produce 0.4 MPa steam, which is then fed into the pipeline system. The current situation is that there are significant fluctuations in steam pressure, which in turn causes large fluctuations in the temperature at the bottom of the tower. The steam traps have been checked, and the reboiler has also had its air removed, but the cause remains unidentified. Currently, the system is stable only when steam is supplied directly from the bottom (as per the original design). Does anyone know why there are such large fluctuations in the steam supply?
Is the reboiler feed unstable?
To some extent, this phenomenon occurs during flashing
Which feed do you mean by the reboiler feed?
Vaporized? Is there any solution?
Add a flash tank stage and give it a try
Firstly, it’s not clear how the control for the reboiler section is configured; it would be best to provide a PID diagram. I speculate that the steam feed control valve is controlled by the reboiler outlet temperature; based on this, the causes of steam fluctuations are generally due to the control valve and the steam trap. Please rule out the problem as follows: 1. Control feedback lag, resulting in fluctuations in steam flow ； 2. Is the selection of the control valve correct? 3. Is there significant air entrapment when the drain valve discharges fluid? Recommended modification plan 1: Add steam flow control (secondary control) to perform cascade control of the steam feed valve with the reboiler outlet temperature (primary control). Advantages: Minimal modification required. Option 2: To reduce the impact of the steam trap, it is recommended to move the steam feed control from the inlet to the outlet of the reboiler condensate. Steam feed should be controlled by regulating the condenser liquid level (secondary control) and the temperature at the reboiler outlet (primary control). Advantages: Reduces steam feed pressure drop and improves thermal efficiency. Both solutions employ cascade control for regulation. The above are merely my personal opinions; I welcome corrections from experts!
Even with the steam control valve fully opened, the flow rate still fluctuates greatly; I don’t understand why
I recommend an article titled “Analysis and Optimization of Typical Temperature Control Schemes for Distillation Column Reboilers,” published in the 6th issue of Chemical and Pharmaceutical Engineering in 2015. I came across it by chance, and it should be useful to you.
It is now controlled by direct steam at the bottom (as in the original design). Was the original design to have steam entering from the bottom? Can I take a look at the picture?