18 replies to this topic

[vicki15]

Hi!!
Ive got to design a absorption column and to do part of my design i need to know the solubilty of so2 and nox in caustic soda and i cant find it anywhere! i was just wondering if anyone could help! thank you!

[djack77494]

I don't think you're likely to find that data. Sulfur and nitrogen oxides are acid gases that will react with an alkaline solution like caustic. As long as you maintain an alkaline pH, I think it would be reasonable to think of the absorption as being irreversible. The solution essentially will absorb all the acid gases it can get.
Doug

[joerd]

I don't think you're likely to find that data. Sulfur and nitrogen oxides are acid gases that will react with an alkaline solution like caustic. As long as you maintain an alkaline pH, I think it would be reasonable to think of the absorption as being irreversible. The solution essentially will absorb all the acid gases it can get.
Doug

if equilibrium is reached. The mass transfer and kinetics can limit the amount that is absorbed. So there is no clear cut answer to the question - like you said.

[djack77494]

Joerd,
I'm a little confused by your talk of equilibrium. When you reach the point where you cannot absorb additional acid gases, you have exhausted your caustic and you no longer have an alkaline solution. I suppose that ideally you might strive to reach that point for the sake of maximum caustic utilization, but pH is a tricky thing and can swing rapidly fron high to low pH as you approach "equilibrium". Therefore, practically I don't think you'd ever get too close to a pH of 7. If you accept that, then you are always operating with an alkaline sorbent solution and you never reach solution/gas "equilibrium". You can reach steady state by operating the process continuously and setting a "blowdown or drawoff" flowrate to produce the desired solution pH. The desired pH would definitely be in the alkaline range.
Doug

[vicki15]

hi!!thank you! does that mean that nox and sox are completly soluable in naoh?if so how do i do my equilibrium line?also as a reaction accurs and heat is given out that effects my design do i have to add in a cooling coil or something?? thank you!

[djack77494]

In answer to your question, yes, I do think NOx and SOx are completely soluble in caustic solution. You have a mass transfer problem and mass transfer limitations will dictate how much of the acid gas is absorbed into the solution. Once in the aqueous phase, I think it safe to assume that there is an instantaneous, irreversible reaction that eliminates NOx and SOx as "dissolved gases".
Doug

[siretb]

SOx will be soluble in caustic, and you will not get much problems if you keep the pH above 6.5
Concerning NOX, usually you have both NO and NO2 ; the partition depends on the upstream process. In incinération you have about 10% NO2 and 90% NO
NO2 is soluble in caustic, NO is not. To scrub NO you need an oxidizing agent, like sodium chlorite, hydrogen peroxide, ....
You will find a lot of litterature concerning NOx wet scrubbing.
Some people also use ozone of phosphorus to oxidize (in the gas phase) the NOx all the way to nitric acid.

If you plan to use caustic, keep in mind that at a high ph (>8.5) you'll start collecting the CO2, if any is present.

[djack77494]

Siretb makes two very good points. One is that a high values of pH, you will absorb CO2 as well as the other acid gases. Since you call this an absorption column and never mentioned CO2, I assumed that it was not present. Let us know if CO2 is a concern. Siretb's second point was to differentiate between the two major components of what we refer to as "NOx". One readily dissolves in water to produce an acidic solution; the other is much more reluctant to do so. I failed to address this point. Throughout this dialog, I have always considered the solution to be at a high (though undefined) pH, containing more caustic (NaOH) than anything else. If you introduce large concentrations of other constituents, the conclusions I have inferred may change.
Doug

[vicki15]

hi!!
no there is no CO2 in my absorption column. Only NOx and SOx. The NOx and SOx are quite dilute and make up about 17% of the incoming gas the rest is steam. Does this mean i can still consider it to be total absorbtion in my column?my tutor said that for my equilibrium line i should look at the solubilty of nox and sox in water as they react with the water first and then with caustic soda and the reaction with the water is the rate determining step-is this correct? Thank you for you help.

[siretb]

17% is not dilute at all. It's very concentrated.
If you are using caustic you cannot calculate the absorber based on the SO2 (or NO2) solubility, because the reaction SO2 + NaOH --> NaHSO3 or Na2SO3 (depending on pH) is a very fast reaction, that will speedup the transfer. (refer to Perry's handbook for the basics, see enhancement factor)
You may have two limiting factors
the gas side mass transfer coefficient (at low concentrations)
the liquid side mass transfer coefficient (at high SO2 concentrations, for example)
so in a such case you need to worry about both sides, liquid and gas.

Since NO is barely soluble and will not readily react with NaOH, you may use solubility data to calculate your equilibrium line and select your operating line.
For SO2, at a pH high enough (say > 7) there is almost (almost) no SO2 as such in the liquid phase. so the equilibrium line is almost flat, and the only controlling parameters will be the kg.a and kl.a . The main criteria for selecting L/G would then be how concentrated you would allow the bleed to be (obviously less than saturation)
for NO2, the solubility is limited, but there will be some enhancement because of reaction with NO2

Are you a student?
If your need more help, please supply the actual flowrate, full composition incl. concentrations of NO / NO2 / SO2 , and temperature / pressure , along with specifications

I think that you'll need more than caustic if you have much NO (nitrogen monoxide)

[vicki15]

hi

my flow rate going into my absorption column is 3652.707 kg/hr and 88% of it is steam, 1.5% is SOx and 10% is NOx. is this a very concentrated stream? also do you know how much nox and sox i am legally allowed to give off in my plant? im still a little bit confused about what i have to do for my equilibrium line. thank you so much for your help!

[siretb]

We need NO and NO2 SE-PA-RA-TE-LY
, not lumped together as NOx
T and P were not supplied, also

[vicki15]

the temperature is 30C and the pressure is atmospheric! do you think having a plated column would be better than a packed column. i think that a it will be acidic a packed column would be best but if heat is given off in the raction than a plated column would be best. so im not quite sure what would be best to use. Thank you.

[siretb]

Going acidic may be an option for some NOx. It's not easy to select the proper operating conditions. A plate column, then may be preferable to a packed column if you want to go acidic (because of kinetics). Otherwise a packed colmun would be a better choice.
In your case, I do not have enough information to advise what would be best.
Now, I will not discuss this matter any further if you don'nt give the full composition, including NOx speciation.

[siretb]

If you do not find data for the same system, you may look in the litterature (or vendor) data for another system and use dimensionless numbers like the Sherwood. You keep the same sherwood and back calculate the Kg.
Reinhardt Billet 's Packed tower ISBN 3-527-28616-0 has lots of useful correlations for Kl.a Kg.a and holdup in packed towers

[katchum]

Hello

I too am designing a scrubber for HCl, SO2 with NaOH scrubbing. Now my question is: what is the pH needed to say that the reactions in the scrubber are irreversible, meaning NTU = ln(y1/y2)?

And, should I measure that pH at the flow out side of the liquid of the bottom of the scrubber?

Also, is it common for scrubbers that your G to L ratio is constant?


Thanks

Albert

[j_claire]

hi! i've read your topic and my problem is related to it. i am also tasked to design a scrubber to treat NOx emission. the NOx that we produced is not from burning process but from the silver leaching process. we use 68% nitric acid for the process and it is heated up until leaching is finished. is it correct to assume that only NO2, water vapor and O2 will be present in the flue gas? what's the best treatment to reduce our NO2 concentration from about 40000 mg/m3hr to 500 mg/m3? the company would want a 2 stage scrubber that would also have the capacity to treat its own scrubbing waste. is it to complicated or what?