6 replies to this topic

[greatparesh]

Hi friends,

In my Urea Granulation plat, the exhaust gases pass from granulation section to th stack and finally into the environment. Now, gases coming out from the stack are havibng very high concentration of ammonia (NH3) exceeding the environmental norms.

So, now i want to reduce this ammonia from exhaust gas. What are the technologies available? Scrubbing of this exhaust gas with sulfuric acid (H2SO4) to get ammonium sulfate and ammonia free exhaust gas is one technology. Can anybody elaborate on this technology and any other technologies for removal of ammonia from exhaust gases?

P.S. Water cannot be used here. If you have books/papers for references, please let me know. The properties of gas at the stack are

Temp=45 deg C
Pressure=0.0006 kg/cm2g
NH3 Conc.=80 mg/Nm3 gas
Allowable NH3 conc=50 mg/Nm3 gas
Total Gas Flow Rate = 6e5 Nm3/hr

Waiting for your replys.

Regards
Paresh Jain'
jain.paresh@gmail.com

[djack77494]

Paresh,
If possible, the best way to reduce the ammonia concentration of the exhaust gases is to minimize their production or release from the upstream processing. Ammonia and ammonium salts can be difficult to capture at low concentrations, and are often environmentally objectionable. Ammonia will frequently combine with acid gases and generate an aerosel that consists of extremely fine particles. The resultant salts form a characteristic "blue haze" at the stack. Once the solids are generated in the process, they are extremely difficult to capture.
Doug

[greatparesh]

Hi Doug,

You are right. At these low concentration it becomes really difficult to reduce ammonia conc. However, i have no control on upstream process side and only option avaiolable is reduce ammonia in exhaust gas by acid scrubbing. If you make a note of concentrations of ammonia, in wt%, Inlet=0.015 wt% and Outlet=0.005 wt %. With such a low conc change of ammonia, MB suggests, very very low flowrate of liquid (H2SO4, water, (NH4)2SO4 compared to gas flowrate.

How do u handle such conditions? i mean it is giving rise to very high Diameter column !! please guide.

Regards
Paresh Jain

[siretb]

You have a very large flowrate, and certainly the first option is to limit as much as possible the incoming level of ammonia.
The pressure is almost atmospheric.
For such a high flowrate, scrubbing with water, or acidic water should do, considering that you need only less than 50 mg. Will be expensive.
Now for the design, you'll hace to trade of between higher pressure drop and a column of a lesser diameter, or a larger column.

Please explain what you mean by "water cannot be used here". Do you rule aou any scrubbing involving water?

[djack77494]

My guess would be that a Scrubber with structured packing would be best for this job. The pressure drop can be quite small and you would need a minimum irrigation (circulation) rate. I would opt for using an acidic solution, perhaps an inhibited H2SO4 solution to effect the removal of ammonia. At moderate to low(ish) values of pH, this will be effective and you have the bonus of being able to control emissions (within limits) by pH adjustments. The key parameter will be efficient contact/mass transfer.

There is some good news. Because of the very low pressure, low liquid circulation rates, and minimum height of packing that will be required, the cost of the Scrubber will be low compared to what it would be if one of these factors was not true. There's also some bad news. Because of the low pressure and high mass flowrate, your gas volumetric flowrate is high, requiring a large (i.e. expensive) Scrubber. The choice of sorbent and metallurgy will have a major impact on your capital costs. I don't think you can do better than this approach, however.

HTH,
Doug

[greatparesh]

Dear friends,

Thanks for your replys.

Doug, i think circulation rate required will be higher. Bcos, you have very high gas flow rate and to avoid channeling with NOT VERY LARGE DIAMETER column, you will need a high circulation rate.

i think acid scrubbing will be a very good option because if i have an acid which can instantaneously react with ammonia then i can easily get to that low conc of 30 ppm in outlet. However, there are many techniques in the market for ammonia removal these days. Some of them are
1. Scrubbing with Water
2. Acid Scrubbing (Sulfuric Acid, Hydrochloric Acid, Nitric Acid etc.)
3. Ion Exchange Resins
4. Adsorption by Zeolites....

Can any one has done comparative study (pros and cons) of these technologies? Or has any details of these techniques? Especially for my case where ammonia in the feed itself is very less i.e 150 ppm (0.015 wt%) and outlet desired is 30 ppm, which technique will be best suitable ?

Is ammonia removal by acid the best option or should i go for water scrubbing or the latest techniques like adsorption by zeolites and ion exchange ?

Awaiting your responses.

Thanks
Paresh Jain

[djack77494]

Paresh,
The only other additional comment I am able to offer at this time is to beware of water washing. Water has virtually no buffering capability, depending on its purity. Therefore, a small amount of absorbed ammonia will quickly drive the pH upwards. This greatly diminishes its ability to absorb additional ammonia.