What are the possible reason for column pressure (1-2 KSC pressure) fluctuation. if the column pressure increases, will the separation increase, and vice versa?
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23/03/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
More tips: I have been assuming that the fluctuation is over a period of minutes, not as air or water temperatures change with the weather, and I will continue to assume that.
The design of the pressure control system on the column could be at fault. For example, if it has a flooded condenser, and there are non-condensibles, that alone can cause erratic pressure swings. If it has a gas blanketed accummulator, the control valve sizes may not be correct. There are two good designs for hot vapor bypasses for pressure control. However, if the two designs are mixed up, it will not work well. This was a common error in the 1980s and 1990s. In one of the correct designs, the vapor control valve size is as much as six times as large as for the other correct design, to give you an idea of the consequences of mixing up the designs.
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23/03/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Let's take the 2nd question first: Increasing pressure makes the separation worse directionally. That's because k values of adjacent components converge as pressure increases.
First question: Many things can cause a particular control to fluctuate. One control in a 50 control column sequence, for example, can cause virtually all of the controls to fluctuate. This has been found to be true in countless troubleshooting experiences. In your overall system, approach the controls one at a time. Set it to manual and observe the entire system. If the problem persists, place that same control on bypass (outside) and observe. If the problem persists, return that control to automatic and proceed to the next control and repeat the procedure. If a control is causing the problem, you will eventually find it. One time, the very first control we checked was the one causing the entire unit to fluctuate. That control valve needed a valve positioner and had none. They thought I was a genius, but it was pure luck.
Other things can cause pressure fluctuations, such as fluctuating feed rate, but, in that case, a well-tuned pressure controller and properly sized control valve may be able to prevent pressure fluctuation. Sometimes the residence time beyond the baffle in a kettle reboiler is too short, causing control fluctuation in the entire process. There can be other equipment issues, as well. Start by checking the instrument controls as suggested.
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23/03/2010
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keith bowers, B and B Consulting, kebowers47@gmail.com
Flooding (surging) in the column will cause pressure surges. Increasing column will decrease separation.
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