8 replies to this topic

[Kim Bryan]

Hi everyone, I am tasked to an initial design of packed column for a grassroots project. My lead engineer told me to size the packing based on KG Tower version 5.2 and I have no complains on the software itself. Most of my problems arise on the checkpoints of the design. My current checkpoints are

 

1. Flooding [Capacity, Const. L/V % in KG Tower].

This should be around 70% to 80%. Everybody knows this from their college education

 

2. Vapor Momentum [Fs m/s*(kg/m3)^0.5 in KG Tower]

Our current criteria is 1.5 to 3.0 m/s*(kg/m3)^0.5. I'm not really sure about this. But if I am to relate it with piping momentum which is usually greater than 1000 Pa (or 32 m/s*(kg/m3)^0.5 in the same unit), this is very low. I guess the Reynolds Number for columns is quite low.

 

3. Liquid Load [m3/hr/m2 in KG Tower]

The criteria here is that the liquid load be close to 50m3/hr/m2 (Liquid Flow rate/cross section of column). I really don't get this. I've also searched the internet for this criteria but found vague answers. The closest I've got is that this criteria determines whether liquid distributors are needed.

 

Thank you for any help.

[abhi_agrawa]

Kim,

 

My understanding is:

 

A. Based on Flooding Capacity the column diameter for the tower is determined.

 

B. Now that the tower diameter is fixed, you can calculate the resulting irrigation rate (m3/m2/h). Based on the irrigation rate and the type of packing selected, the liquid distributor will be designed. I believe that you would always need a liquid distributor.

 

Hope this helps,

Abhishek

[Kim Bryan]

Thank you abhi_agrawa!

 

But what I'm confused about is the criteria itself. Why is my Lead Engineer wants me to balance the other 2 criteria (Flooding and Vapor Momentum) with a 50m3/h/m2 Liquid Load. Is the value 50m3/h/m2 Liquid Load correct?

 

I guess low Liquid Load will result to channeling (where most liquids are brought to one side of the column because of there is so much vapor relative to the liquid going downwards).

[PingPong]

Depending on its type and size or specific area, a packing requires a minimum wetting rate, but normally that requirement is a factor 10  lower than your number.

 

You should ask your lead engineer why he wants that high liquid load of 50 m³/h/m².

[Kim Bryan]

Thank you, PingPong!

 

So it's only around 5 m3/h/m2? Is this value from plant operation experience?

 

The rationale behind 50m3/h/m2, as he said, is for the better distribution of liquid in the packing. Too low of a liquid load will result to channeling since it would just be "blown" off by the incoming vapors.

[PingPong]

So it's only around 5 m3/h/m2? Is this value from plant operation experience?

Packing vendors quote minimum wetting rates (minimum liquid loads) in their brochures. The highest number I have ever seen is 5 m³/m²h (2 GPM/ft²) for polymer random packing. For metal random (dumped) packings it is lower. See for example page 11 in: http://www.koch-glit...ry/KGMRP-02.pdf

 

For metal structured packing it is even much lower, only 0.2 m³/m².h: http://www.sulzer.co...ed_Packings.pdf

 

See also minimum loads for liquid distributors in: http://www.koch-glit...rary/KGMTIG.pdf

 

Of course that does not mean that you should aim at these minimum liquid loads, but it illustrates that there is no problem operating well below the 50 m³/m²h that your lead engineer seems to prefer.

 

However: apart from a minimum liquid rate, you should also pay attention to the Flow Parameter X = (L/G)^.√(ρ_G/ρ_L) in your column. That should preferably not be lower than 0.01 to avoid risk of liquid being blown up (entrained). Note however that the Flow Parameter can not be influenced by the column diameter. To Increase it would require changes to the material balance of your design.

Edited by PingPong, 16 September 2015 - 05:51 AM.

[alchemist]

Hi,

 

I am still a bit confused on the specific liquid loading.  I checked the cited references and found several references to very high values for the loadings

 

For example, page 9 of the reference:  http://www.koch-glit...rary/KGMTIG.pdf

 

gives specifications for Model 116 INTALOX® Deck Distributor  (Model 117 Redistributor)

 

     Diameters 36 - 240 in. [150 - 6000 mm]

    Liquid rates between 4 - 80 gpm/ft2 [10 - 200 m3/m2 h]

 

Page 20 of the reference:  http://www.sulzer.co...ed_Packings.pdf

has a plot for selection of the optimum distributor plate as a function of liquid loading and column diameter.  The plot has loadings as high as 300 m3/m2.h. 

 

Also, does the pressure have a bearing on the liquid loading.  The post-combustion CO2 capture columns operate just slightly above atmospheric pressure whereas the pre-combustion CO2 capture columns could be as high as 500 psia. Also, the pre-combustion columns are rarely above 10-12 ft in diameter.

 

 

Anybody have any further insights into this ?

 

-AC

 

 

So it's only around 5 m3/h/m2? Is this value from plant operation experience?

Packing vendors quote minimum wetting rates (minimum liquid loads) in their brochures. The highest number I have ever seen is 5 m³/m²h (2 GPM/ft²) for polymer random packing. For metal random (dumped) packings it is lower. See for example page 11 in: http://www.koch-glit...ry/KGMRP-02.pdf

 

For metal structured packing it is even much lower, only 0.2 m³/m².h: http://www.sulzer.co...ed_Packings.pdf

 

See also minimum loads for liquid distributors in: http://www.koch-glit...rary/KGMTIG.pdf

 

Of course that does not mean that you should aim at these minimum liquid loads, but it illustrates that there is no problem operating well below the 50 m³/m²h that your lead engineer seems to prefer.

 

However: apart from a minimum liquid rate, you should also pay attention to the Flow Parameter X = (L/G)^.√(ρ_G/ρ_L) in your column. That should preferably not be lower than 0.01 to avoid risk of liquid being blown up (entrained). Note however that the Flow Parameter can not be influenced by the column diameter. To Increase it would require changes to the material balance of your design.

 

[PingPong]

The above discussion in 2015 was about the minimum liquid loading required, to assure sufficient packing wetting and avoid bed channeling.

 

You are free to operate at much higher liquid loadings, as a result of your specific design or application.

[alchemist]

Thanks for the clarification.  I somehow missed that point. 

 

-AC