Calculation of NTU for Scrubber

[DonyWane]

I am performing some design calculations for a HCl / KOH scrubber. I have calculated the height of a transfer unit assuming that the mass transfer characteristics behave like a HCl / H2O scrubber (but with H+ concentration always at zero because of the excess KOH I know that I have).

Next (and what I at first thought would be easier), I need to calculate my NTU for the column (a radomly packed bed, with Intalox Metal Tower Packing, 1.5"). I can show that my equilbrium mole fraction in the vapor phase is essentially zero, reducing the common NTU equation to:

NTU = ln(ybottom/ytop)

I have several operating scenarios, one of which is a 50/50 N2 and HCl offgas. For this system, I calculated that I needed essentially 6 or 7 transfer units. However, for my 100% HCl offgas case, I cannot figure out how to calculate how many transfer units I have. Is there an inherent problem from a mass transfer standpoint as to why you must dilute to some value (say 80% or 50% HCl in N2 or Air), or can you scrub a 100% HCl offgas? If so, how do you calculate how many transfer units you will need in your column?

Thanks!

 

[sshep]

DonyWane,
I think that you will have trouble defining an operating line for a graphical (or equivalent) analysis of a 100% HCL case regardless how you worked around the equilibrium assumptions in light of the acid-base reaction. Obviously L/V at the top of your tower is zero for the 100% HCL case so the classical methods used for designing absorbers will not apply. You might search this forum for previous threads on caustic scrubbing for more info because that topic has been posted several times.

In any event, HCL offgas concentrations this high should usually either be condensed, or absorbed in a FFA to generate a product which has value rather than nuetralized and wasted.

best wishes, sshep

 

[StoneCold]

Donywane
What metal are the Intalox Packing made of that they can stand up to HCL and KOH? You might be much better off with a plastic packing.

Regards
StoneCold

 

[DonyWane]

StoneCold,

Thanks for your suggestion. Our process involves very high temperatures - and although we use a series of eductors/quenches to cool the stream - we have a requirement to protect the packing at elevated temperatures that are beyond the acceptable range for plastic packings.

I have since answered my own question. We have some instrument purges that give us about 1 SCFM of nitrogen flow through the packed tower. As a result, I was able to get away from the 100% HCl assumption.

Thanks for your replies!

 

[siretb]

One always has some inerts ....
Even with high temperatures, with a quench and some safety design rules (like emergency water that would spray in the column should the quench fail) it is still possible to use plastic. If you do not want plastic, an empty tower with spray nozzles should work, too.

I would question the use of KOH: The KOH is quite expensive, and you should be able to do most of the job with plain water (no reagent). If you need to go to real low HCl emissions, then consider a second stage. Note that a quench could be the first stage and that it will collect a lot of HCl, as long as it bleeds, if properly designed. The quantity of reagent (potass) will be much less and it could quickly pay off. Of course, you may need to, eventually, neutralize the bleed, but this would be done later, for instance at the waste water tretment, and you would use a cheaper reagent, like lime.
To sum up, I would do: a full quench , with a significant bleed, catching moost of the HCl
Followed by a scrubber. Bleeed of scrubber goes to quench.