Production of Aqua Ammonia 1

[athark]

I am working on a study to produce Aqua ammonia within our facility. Currently we produce anhydrous ammonia, and most it is shipped outside as agricultural grade. Now we have a need for two grades (19% and 25%) of aqua ammonia.

In our existing facility, currently we recover ammonia gas from NH3 KO drum overhead, and is eventually cooled down to liquid (anhydrous) ammonia. I have one option of using liquid anhydrous ammonia, passing it through vaporizer and then mixing ammonia gas with water in a mixer/tank.

The other option will be taking some of the ammonia gas overhead of KO drum and contacting it with water in a mixer/tank to get required concentration of ammonia solutions. Currently ammonia gas is not analyzed.

Some of my quetions are.

Can we mix directly liquid anhydrous ammonia and water ?

What are the most common methods of producing aqua ammonia ?


I will appreciate, if anyone can have an input.
Thanks

 

[djack77494]

I have seen gaseous ammonia being absorbed into an aqueous circulating solution as a method of making aqua ammonia. I'm not sure it would be wise the mix liquid anhydrous ammonia directly with water, as the reaction might be overly vigorous.

 

[BenThayer]

the only way i have seen aqua ammonia produced for commercial use was mixing anhydrous ammonia with high quality water.

the system i was responsible for used boiler feed water quality water (de-ionized or the hardness salts will precipitate out) and commercial grade anhydrous ammonia and had well water coolers.

the system had a back-pressure control to minimize flashing and to have essentially all of the cooling take place before exiting the exchanger system. the controls were essentially a "set and forget" on the water feed through a meter and then a ratio control on the ammonia with another flow meter.

the system was designed in the mid fifties and was still operational to the late 90's when i changed jobs.

the storage tanks will need some type of CBV and you might want them to vent to a scrubber (we did and then used that water for make-up to the system). we produced 29.6% NH3 in water and this has a higher vapor pressure than the strengths you noted.

ammonia/water aqua generators or converters were available commercially but i have lost track of who made them.

we were a complete ammonia/urea/nitric acid/UAN production facility and aqua was a small part of what we had on site even though we shipped 4-6 trucks per day.

good luck (and if the ratio is off, the vigorous reaction will tell you to make adjustments if the smell of the storage tank doesn't)

the absorbing the vent makes sense depending on what you already have for a recovery system off your ammonia plant.

but then you cannot produce ammonia without the plant on line. consider that against your customer requirements.

we were able to produce our aqua essentially without the ammonia unit on line (even to the extent that our site could unload railcars to our site if sales in the area warranted it).

 

[athark]

Thanks Benthayer and djack for your valuable input.

Ben, For the facility you mentioned, I will assume that liquid anhydrous ammonia was vaporized before mixing with de-ionized water for producing aqua ammonia.

We already have storage tanks for aqua ammonia, since currently aqua ammonia is bought over the fence for internal consumption in our facility. I am trying to sieze opportunity of using our own anhydrous ammonia for aqua ammonia production.

Our Aqua ammonia requirement each month is roughly:

28,000 gallons (delivered in 4500 gallons batches) for 19.5% strength

62,000 gallons (delivered in 6000 gallons batches) for 24.5% strength.


The way I imagine, we will be producing aqua ammonia in batches. So for scoping study, i will anticipate having following equipment

* Vaporizer (to convert liquid ammonia to gas before contacting with water)

* Contacting/mixing vessel (For mixing water and ammonia gas.

* Storage Tanks (Do I need one or two storage tank ? Since we will be producing two strengths of aqua ammonia in batches, and then transfer it to existing aqua ammonia tanks in our facility).

* Water pump
*Liquid anhydrous ammonia pump
*19.5% aqua ammonia pump
*24.5% aqua ammonia pump
*Analyzers

Flow scheme:
a) Liquid anhydrous ammonia is pumped to vaprizer.

b) Ammonia Gas from Outlet of vaporizer is injected to Contacting vessel, and the deionized water is introduced to mixing vessel.

c) From mixing vessel,Aqua ammonia is transferred to tank meant for 19.5% strength (here 19.5% is achieved by water feed, or having some analyzer).
Similarly from mixing vessel, 24.5% tank strength is also filled.

Any input/ suggestions?? Keeping in mind my monthly requirements, what size mixing vessel should I use ??

Thanks,
athark

 

[kenvlach]

I haven't worked with ammonia (other than very small lab quantities), but perhaps you don't need the vaporizer [or if you use one, use the product for heating].

Use the enthalpy of solution (30.5 kJ/mol) to cancel out the heat of vaporization of liquid ammonia (+23.35 kJ/mol at BP of –33.4 °C) plus heating of the ammonia (for gas, 35.06 J/mol/°K). Adjust your input water temperature to keep the product from getting too warm.

 

[HEC]

Fischer Porter produced vapour mixing equipment. They were bought out by ABB they may be worth contacting for information.

Mark Hutton

 

[BenThayer]

we did not use ammonia vapor. we used liquid at about 34°F from an intermediate product stream from the ammonia unit.

as an alternate we had some ammonia bullets that were at ambient temperature that we could feed to the system.

the key is maintaining the pressure on the system to prevent the ammonia from flashing when it reacts with the water. your strength should be easier to make because there will be more water and the adiabatic temperature gain would be less.

for a first pass, take the heat of reaction of 0.2# of nh3 with 0.8 # of water with both starting at 60°F and see what the temperature rise would be and then you will need that much cooling to bring back to 60°F. these are not exact numbers but just meant to illustrate. whatever temperature you reach, you will have that much vapor pressure. you will need to keep at least that much pressure on the system while you cool it.

is most of your ammonia produced and stored at -28°F in atmospheric tanks?

we sold essentially all of the aqua we produced. we used just a little bit to mix with the -28°F product going to storage to meet the 0.2% minimum water content for the commercial grade.

using ammonia vapor will increase your cooling requirements quite a bit.

 

[gr2vessels]

Just a small addition to Ben's post. Sulzer has several static mixer design arrangements for the production of aqua ammonia. They can provide the mechanical design for it, if required. They can supply the mixing only elements, to fit / retrofit an existing piping system. The static mixer is at almost ridiculously small size for a large production volume. Also, they can provide the process design to your specification, it's all too easy.
Cheers,
gr2vessels

 

[BenThayer]

i never saw the thing disassembled so i never saw the ammonia sparger or mixer details.

however, i was always quite fond of the sulzer gas mixers. we had one for the air/nh3 mixer for the acid unit.

 

[athark]

Thanks,for all the great input.

Ben, to your earlier question.

We currently store liquid ammonia in bullets at 66 degF and 103 psig.