liquid Anhydrous Ammonia (15barg) expand to gas (1 bar) 2

[megeng]

Hi, we are introducing anhydrous ammonia at 15 bar gauge into a tank at 1 bara and 85 degrees Celcius. We meed to keep the temparatue at 85 and on expansion the anhydrous goes to -37 degrees. What is the safest method of introduction?

 

[dcasto]

Then you'll need to heat the NH3 before the valve to 100 C. The NH3 will then be 85 C at 1 barg and 1 barg only.

Or just do nothing, as the pressure in the tank increases, the temperature will increase to.

It all depends on what you want to do.

 

[BenThayer]

the title of the post says that this is a liquid. you cannot heat liquid ammonia to 85°C without a significant increase in pressure. forgive me as i regress to non SI units.

100°F is a nominal 200 psig. your 15 barg must be about 100°F so i would not heat the liquid until you really review the properties of ammonia at 85°C (about 185°F!) the vapor pressure of ammonia at 185°F (your 85°C) is 669 psi.

heating the liquid will only change what percent of the liquid will flash to a vapor. it will not change that most of it will still enter as a liquid.

you need to better define what you are doing:

what is in the tank when you add the ammonia?

is there a liquid in the tank or vapor?

is there ammonia vapors or is there just an inert such as nitrogen? this will impact the potential temperature of the liquid in the tank. if there is only nitrogen, then the tank will actually have a partial pressure of 0 psia ammonia and could cool below your proposed -37

what is the metallurgy of the tank?

can it withstand rapid localized cooling if the liquid is going to fall to the bottom of an empty tank?

 

[megeng]

The tank is 316ss and is carrying a nickel sulphate slurry at 85C. once the ammonia is bubbled through there should be no anhydrous vapours going to vent.

I've been looking at trying an ammonia vaporiser but the ones available are for small scale use (for farmers) and the project doesn't want large amounts of cash spent(if one needs to be specially built).

We are keeping the temperature of the tank using heating coils and low pressure steam, therefore I can't allow the temperature of the anhydrous to drop the temperature inside the tank by too much.

It is also quite a problem to find a valve that can take both a liquid and a gas at high pressure even though we using a 2 inch line to convey the anhydrous ammonia

 

[Latexman]

I believe decasto’s advice is thermodynamically sound. If the liquid ammonia is vaporized at 15 barg, the resultant temperature is 41^o C. Then, the vapor can be superheated to 100^o C (I got 103^o C) such that if it was throttled (adiabatically expanded) from 15 barg and 103^o C to 1 bara it would cool down to 85^o C.

Good luck,
Latexman

 

[megeng]

Thanx latexman

Do you know of where I can get some information about anhydrous vaporization. I only seem to be finding small units used on farms, there are none that can deal with our flow of 5500 kg/h.

 

[mxmaciek]

Small google digging can be beneficial, i.e. this link:

http://www.algas-sdi.com/ammonia-products.html

Even if max range ends with 4400kgs/hr, you can ask them for individual approach or consider use of two vaporisers working paralelly.

 

[megeng]

Thank you,

I gound that site before and have their info...I do however live in South Africa and this company is a US Company and I haven't been able to find any agents this side of the world.

 

[mxmaciek]

Well, seems that you're too late, because they were at The 2007 World LP Gas Forum, October 24-26 in Cape Town, South Africa.

I can suppose that some contact / materials / representatives shall be available still, try to find it locally.

 

[BenThayer]

latexman, you might be correct but since decasto's post had said to heat the ammonia "before the valve," i was concerned he meant the liquid.

i would not worry about looking specifically for an ammonia vaporizer. you really just need a "vaporizer" in general.

i have seen ammonia vaporized in the tubes with steam on the shell side (not ideal and i would not recommend) and also using waste heat from cooling water. the cooling water was in the tube side and ammonia in the shell of a kettle style reboiler.

have to be careful to maintain minimum pressure on the ammonia side if no water flowing or subject to freeze-ups.

the design of the vaporizer is going to be just as dependent on the rest of your deliver system and the sophistication of your controls.

since you need to only maintain enough pressure on the system to deliver into a 1 bara vessel, i would be most surprised that you cannot utilize some type of waste heat or even ambient sidearm heaters to keep pressure in the tank.

what are your normal ambient temperatures there?

 

[megeng]

Our ambient temperatures are 30 degrees C. We calculated that to vaporize would require a large amount of energy. we have 32 barg steam available which we were told to use sparingly and 3 barg steam.

 

[mxmaciek]

megeng,
if you have steam available, I can supose that you have some power plant in nearby. How about use of "waste" heat, i.e. hot cooling water after the condenser?

 

[Latexman]

Our anhydrous ammonia tank has a coil in the bottom of it which uses 1 barg steam to control pressure in the head space of the tank at 10 barg. Liquid ammonia is pressure fed (no pump) to a vaporizer which uses cooling water to vaporize the ammonia. Vapor ammonia then flows to a superheater which uses 2 barg steam to superheat the vapor.

32barg steam would be overkill!

Good luck,
Latexman

 

[dcasto]

for small scale vaporizers look at LNG vaporizers used in peak shaving sites. They are basically swimming pool heaters with some added safety systems. LPG vaporizers will work also.