3 replies to this topic

[Nina1119]

Hi,

We have a packed bed(random packing) system with the liquid being aqueous MDEA solution with the foaming factor of 0.75. I am trying to calculate the pressure drop using this factor. As the company software we use does not have any feature to use this to estimate the pressure drop across the bed, I divide the liquid and gas flow rates with the foaming factor resulting in increase of the flow rates and use these flow rates to calculate the pressure drop across bed. Is this method accurate? Can anybody help me with a better way of calculating pressure drop using the foaming factor?

Thank you.

[Zauberberg]

For practical purposes (in terms of evaluating tower diameter) this is a reasonable approach. The presence of foam (scaled to 0.75 system factor) means that the density of bulk liquid (foam) inside the downcomers and trays/packing would be 75% of pure liquid density. This translates into higher residence time - for trays, and higher holdup time - for packed beds. In other words, 75% approach to flood for a system with 0.75 system factor is equivalent to 100% flood for a non-foaming system.

If you look at the generalized pressure drop correlation, you will see that the foaming factor does not figure explicitly in the equation, but the phase density does play. Considering the same fact presented above, i.e. that we should consider "aerated" liquid phase inside the tower (with lower bulk density than the pure liquid), this would result in higher X and Y values in the correlation and move us towards the right side on the column DP/Capacity graph (see uploaded spreadsheet).

For gas sweetening, you are not actually interested in pressure drop - you want to be below the flooding limit, and that is why you need accurate hydraulic calculations. Having said that, I would strongly recommend that you verify your calculation results with the vendor (Koch-Glitsch, Sulzer, or whoever is the supplier of tower internals), as over-simplifying the calculation approach could lead to quite expensive errors.

Attached Files

•  Packed Tower Sizing.xls   350.5KB   173 downloads

[Art Montemayor]

Nina1119:

Kindly read the attached document.

I have deleted the other multiple posting you made of this same topic in the Refinery Forum. I will delete all multiple postings that you make, so please read the Forum Terms & Rules that you agreed to when you registered as a member.

Thank you.

Attached Files

•  Forums Registration Terms & Rules.doc   45.5KB   35 downloads

[katmar]

Nina, your proposed calculation is correct in terms of the usual definition of the foaming derating factor and is probably the best available non-proprietary method. The definition of this foaming factor is given on page 17 of the document at http://www.sammichae...ftp/class14.pdf . Although this document refers to tray towers the definition is the same.

I would use this calculation with some caution because it assumes that the derating factor remains constant for all pressure drops. The derating factor is probably experimentally determined close to the flooding point and is likely to be most accurate there. Fortunately, that is probably the condition in which you are most interested so you can use the experimentally determined foam factor with some confidence. You should still follow Zauberberg's advice and ask your packing supplier for their experience.

Although I agree with Zauberberg's description of how the foaming decreases the liquid density, I do not agree that a foam factor of 0.75 is the same thing as having a liquid density of 75% of the pure liquid density. The foaming factor is much more empirical than that. Decreasing the liquid density to 75% of the pure liquid value in a packed column calculation will usually have a much smaller effect than applying a foam factor of 0.75 in terms of the above mentioned definition. Even decreasing the gas density to 75% of the actual value will have a smaller effect than multiplying the velocity by 1/0.75. This is because, even though decreasing the density to 75% results in the same velocity as dividing the velocity by 0.75, for a given velocity the pressure drop increases with gas density.