Re: Controlling Flooding Correlations

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Posted by Tod Peterson on April 05, 2000 at 00:15:01:

In Reply to: Controlling Flooding Correlations posted by Sri Sampath on February 15, 2000 at 21:04:50:

I think this is a very good question. I have been requested many times to control a "calculated" flooding limit because of delay or difficulty in getting working DP measurements. I have always refused because of the many difficulties and assumptions that would go into making it work. However, I have been forced to install a "crude" flooding limit constraints when a flooding problem was discovered too late to do anything about.

I think the biggest problem with a calculated flooding limit is, "calculating the flooding limit". There are just too many things that are difficult or impossible to measure, which can have a significant effect on the flooding behavior. Usually flooding is most heavily influenced by the volume of vapor travelling up the column. Reboiler heat input is usually measurable. Gas volume is also influenced by column pressure, so also should be pressure compensated to be accurate.

The biggest problems are the unmeasurables. When ambient temperature is cool, internal condensation reduces the gas traffic and gives more margin for flooding or increases fractionation. This can be a significant effect which is very difficult or impossible to measure. Other things which can have a significant effect on the flooding limit is feed composition and tray fouling. Taking these factors into consideration will probably make it necessary to add a significant safety margin to any flooding calculation. If the economics are there for maximum feedrate and/or fractionation, a good DP indicator cannot be beat.

One time I was forced to add a flooding constraint to a column with no DP measurement. The client insisted the column was a long way from flooding so we went without an indication. Later when the column did flood (at higher rates than run previously run before, just before commissioning) we had to do something. I used a simple calculation of Reflux + overhead product as a VERY simplified measure of gas flow up the column. Since they were over-refluxing the column anyway, this simple calculation was enough to do the trick.

I know there are better calculations than reflux + product to predict the onset of flooding. If you have any experience using such a correlation in a controller, please share with us what kind of equation and results you were able to obtain. I would be very interested in the results.

Tod

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: I think this is a very good question. I have been requested many times to control a "calculated" flooding limit because of delay or difficulty in getting working DP measurements. I have always refused because of the many difficulties and assumptions that would go into making it work. However, I have been forced to install a "crude" flooding limit constraints when a flooding problem was discovered too late to do anything about.
: I think the biggest problem with a calculated flooding limit is, "calculating the flooding limit". There are just too many things that are difficult or impossible to measure, which can have a significant effect on the flooding behavior. Usually flooding is most heavily influenced by the volume of vapor travelling up the column. Reboiler heat input is usually measurable. Gas volume is also influenced by column pressure, so also should be pressure compensated to be accurate.
: The biggest problems are the unmeasurables. When ambient temperature is cool, internal condensation reduces the gas traffic and gives more margin for flooding or increases fractionation. This can be a significant effect which is very difficult or impossible to measure. Other things which can have a significant effect on the flooding limit is feed composition and tray fouling. Taking these factors into consideration will probably make it necessary to add a significant safety margin to any flooding calculation. If the economics are there for maximum feedrate and/or fractionation, a good DP indicator cannot be beat.
: One time I was forced to add a flooding constraint to a column with no DP measurement. The client insisted the column was a long way from flooding so we went without an indication. Later when the column did flood (at higher rates than run previously run before, just before commissioning) we had to do something. I used a simple calculation of Reflux + overhead product as a VERY simplified measure of gas flow up the column. Since they were over-refluxing the column anyway, this simple calculation was enough to do the trick.
: I know there are better calculations than reflux + product to predict the onset of flooding. If you have any experience using such a correlation in a controller, please share with us what kind of equation and results you were able to obtain. I would be very interested in the results.
: Tod

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