8 replies to this topic

[djack77494]

during injection of water in liquid ammonia line is there any chance of water freezing,

I would say yes, there's a pretty good chance you'll freeze the water line if it is not properly designed.

is this proposal is correct interm of safety wise,

The reaction between anhydrous ammonia and water is quite strong and may generate "knocking" or water hammer effects. You may want to search for "aqua ammonia" to see if you can find out anything on the details of how to safely do this.

addition of water will increase the temperature of ammonia due to sensible heat and heat of absorption
what will impact on refrigeration load?

That effect should be quite small given the concentrations you are considering.

I fail to see why you would be doing what you propose, however. Normally, you'd want to keep anhydrous ammonia bone dry. The dryer the better. Why do you want to humidify it?

[latexman]

djack,

We get liquid anhydrous ammonia at the unit I'm at via a CS pipeline that is several miles long. I've been told by the expert here that ammonia with a little water in it is considerably less corrosive to CS than 100% anhydrous ammonia. In fact, he said the folks that supply our ammonia add the water to anhydrous like talha describes, except their ammonia is at about 450 psig and ambient temperature, so they do not have the freezing issue. Our water spec is < 1%. Apparantly all this is done to make the pipeline less expensive. It is quite old (not sure how old) and is in excellent shape, so they must be right. Water is somewhat detrimental to the catalyst in one of our reactors, so I know this decision was not taken lightly years ago by my predecessors. We just replaced a 50 year old pressure vessel we use for surge capacity in 2007. It was in great shape too, it just did not comply with OSHA's relatively new 1910.111. It's MAWP was too low and it's metallurgy did not meet the brittle fracture specifications in the event of a large leak and auto-refrigeration to -33 F. There were some other issues that could have been fixed without buying a new pressure vessel. Those were geared to preventing illegal drug makers from stealing ammonia to make methamphetamines and for process safety (excess flow valves). Anyway, thats why folks add water to ammonia.

[Zauberberg]

Just a wild thought: can you inject water in the form of dilluted ammonia solution? That could prevent any kind of freezing and it will not affect the quality of process stream.

[djack77494]

Just a wild thought: can you inject water in the form of dilluted ammonia solution?

That sounds like an excellent idea, Zauberberg. A product known as "Aqua Ammonia" is widely available as a commodity product. As I recall, it is a bit more than 30 weight % ammonia in water. Adding aqua ammonia I believe would be much safer than adding pure water. tala, I don't have any designs for making aqua ammonia from anhydrous, but such schemes exist. The methods they employ ahould be applicable to your situation of getting a bit of moisture into your anhydrous ammonia. Probably other technologies/know-how, such as dilution of concentrated sulfuric acid would also be applicable.

[Qalander (Chem)]

i also worked on this point in detail as the concentration of ammonia increase in water, the freezing point of water will automatically decreased. I know this is feasible and safe. Roughly 10% concentration in water would be around -13 deg C
Let see why water freezes in above case because of heat transfer
heat transfer will occur in two way i.e. conduction and convection
If we talk about the convection then our understanding is heat transfer due to motion of fluid here in our case also both fluid will mixes
what i mean to say that direct contact of both will give the aquous so we get the freezing depression when water is direct contact with liquid ammonia

now only chance of water freezing is due to conduction heat transfer.
I am also worry about this conduction will this occur or not?

Dear talha,Another wild thought coming your way.

I am not so sure but are there any possibilities whatsoever of Hydrates formation (probably in the presence of some foreign materials) and may be it's not water freezing?

Explore the wild thought althpugh it is just hypothetical only.