2 replies to this topic

[Nick4]

What is the physical meaning of the packing factor (Fp) for an absorption column?  E.g. what does it mean if comparing one value to another?

 

I am wondering because we did an experiment where we measured the pressure drop for different gas flow rates and liquid flow rates, and then we calculated the Fp.  I found that the Fp would decrease as the liquid flow rate was increased from 1 to 2 to 3, to 4 L/min.  I'm just trying to explain why this trend was observed. 

[PingPong]

Packing factor F_p is a characteristic of the packing, not of the type of column or liquid.

 

Originally (long time ago) the packing factor was defined as the packing's surface area per unit volume, divided by the cube of the packing void fraction: F_p = a/ε³ and so its dimension was/is  ft²/ft³  or  1/ft  or  ft^-1

Nowadays however packing factors are sometimes reported in  m^-1  so always check dimension before you use them.

 

Nowadays the values of F_p often do not correspond anymore with a/ε³ but are determined by the vendor so as to match a Generalised Pressure Drop Correlation (GPDC). See books and articles of Ralph Strigle and Henry Kister, or old brochures of Norton (which was taken over by KochGlitsch). This is due to more efficient packing shapes that can have a higher a without creating too much pressure drop. This is especially true for structured packings.

 

From the above it follows that the higher the value of F_p in a series of sizes of the same packing family, the more efficient the packing size is (lower HETP or HTU), but also the more pressure drop it will give and the sooner it will flood. Both effects are due to the fact that high F_p means high contact area between gas and liquid due to high a of the packing.

 

Small packing sizes have a higher F_p than bigger packing sizes of the same family, so for example: IMTP40 has a lower HETP (more efficient) than IMTP50 but requires a bigger column diameter (due to less hydraulic capacity) to limit pressure drop and avoid flooding.

Edited by PingPong, 08 March 2014 - 11:43 AM.

[breizh]

Nick4,

 

This document attached may help you too.

 

Breizh