Please help! HTU of random packing saddle #1

[tch422]

Hi Guys,

I need your help on absorption HTU. Please refer to attached. It's related to base system HTU from KGa curve. How do I get HTU using formula HTU= G/ Kga?

Below is the test conditions:
Bed height 3.05m
Liquid concentration: 4% NaOH
Conversion to carbonate:25%
Inlet CO2 concentration: 1%
Gas rate: 900 lb/hr ft3 atm, divide by air density 29 lb/lbmol= 31 lbmol/hr ft3 m
Pressure: 1 am

Assumptions:
1) This is chemical absorption so consider gas phrase only.
2) Assume at liquid rate 8 gpm/ft2 using Saddle #1. The Kga value is ~ 3 lbmol/hr ft3 atm
3) Using HTU= G/Kga= 31/3= 10.3 ft!
4) This HTU value is very high and not reasonable.

Thanks for attention and please help !

 

[georgeverghese]

This value for Kga of approx 50 in SI units is almost exactly the same as that published in table 14-2 in Perry Chem Engg Handbook 7th edition ( abstracted from Norton) for the exact same service for 25-38mm Intalox saddles, so there are no errors in the units here in this graph.

For dilute concentrations of the solute in the gas ( which is the case here), I dont see any error in your calculation.

 

[tch422]

Thanks for checking. Please see normal range of HTU taken from Chemical Process Equipment. As you can see, the range falls within 3.6 ft -0.8 ft. Same test conditions. So, there is something wrong with my calculation. Could you please help again?

 

[Latexman]

The following conversion is wrong, or the units on RHS is wrong:

Gas rate: 900 lb/hr ft3 atm, divide by air density 29 lb/lbmol= 31 lbmol/hr ft3 m

1 atm = 10.3326 m H20 column.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[tch422]

Pardon me for the mistake in pressure unit. Thank you. No unit conversion is needed as they have the same unit

Gas rate: 900 lb/hr ft3 atm, divide by air density 29 lb/lbmol= 31 lbmol/hr ft3 atm
Pressure: 1 atm

2) Assume at liquid rate 8 gpm/ft2 using Saddle #1. The Kga value is ~ 3 lbmol/hr ft3 atm
3) Using HTU= G/Kga= 31/3= 10.3 ft!

 

[georgeverghese]

For what absorption service is this Hog taken from? Same as CO2 - dilute NaoH or some other service? CO2 - dilute caustic is a relatively weak chemical reaction so Hog will be much higher than for other services where there is a stronger chemical reaction.
Stronger chemical reaction than this case leads to higher Kga which translates to lower Hog or HTU.
Also these reported values are for overall Kga, so no need to add a correction factor for liquid phase resistance ie Hg or HTU = Gm/Kga.

 

[georgeverghese]

If you look at table 14-3 in the same chapter in Perry 7th edn, Kga values range from <10 to in excess of 300 in SI units (kgmoles/hr.m3)for the same type of packing. Kga values are NOT a function of the packing type alone, these values are strongly influenced by solute and solvent pair affinity, gas and liquid flows, solute and solvent concentration and system P/T.

 

[georgeverghese]

These graphs you have posted for "typical HTU" are for the ammonia water system, which I found from Perry 6th edition in the chapter 18 on packed tower hydraulics design. This is a much more reactive system that this 1%CO2 in 4%wt caustic solution, as evidenced by the high Kga value of 340kgmoles/hr.m3.atm as published in Perry ( so roughly 7x higher Kga than for the CO2 - NaOH system). So the same packing in this NH3 - H20 system would maybe give you a HTU of 10/7 = 1.5ft which is what you are mistakenly expecting to get.

So these graphs for Hog you have posted later are the wrong comparison basis, since it is not for the CO2-NaOH system at the same operating conditions as that for the first graph posted.

There is some abnormality in the Norton Kga numbers (or so it seems to me too) for this 900lb/hr ft2 gas rate for 25mm Intalox ceramic packing in this CO2 - NaOH system. The Kga value published is almost the same as that for 500lb/hr/ft2 gas rate. Extrapolating from the 500lb/hr/ft2 data, the Kga at 900lb/hr/ft2 ought to be approx ((900/500)^0.7) * 56 = 84kgmoles/hrm3 at 1 atm. The basis for this approximate numerical extrapolation is found in Perry. But the published value is only 59kgmoles/hr.m3.atm. The authors in Perry have commented on this anomaly and have not offered an explanation why this is the case. You may query Norton why this is the case.