Heating ammonia above it's critical temperature

[abutt116]

Hi Guys,

If ammonia feed to reactor at urea plant is heated above it's critical temperature, what are the negative and positive effects on the process and plant life.

Thanks.

 

[TD2K]

Ammonia's critical temperature is only about 270F which is not a particularly high temperature. The main significance is if you are above the critical pressure is that no matter how high the pressure goes, you do not create a liquid but rather the material remains as a supercritical gas.

What exactly is your concern? The critical temperature is a thermodynamic issue, it's not a safety point. Oxygen and nitrogen at room temperature are well above their critical temperatures also.

 

[TD2K]

Let me add that ammonia either above or below its critical temperature is a toxic gas. See it's MSDS for more information

http://www.chem.tamu.edu/class/majors/msdsfiles/msdsammonia.htm

 

[abutt116]

Thanks for reply.
I am worried about that above critical temperature ammonia will be gas regardless of the pressure. But as a principle our ammonia feed to urea reactor should be in liquid state. Secondly at present we are heating our ammonia feed above critical temperature but unable to have design pressure (3200 psi)in reactor because we are facing safety popping (which has set pressure of 3400psi) at about 3100psi. I was thinking that giving ammonia in gas form might be the cause for safety popping.

 

[TD2K]

If you have a safety popping at 3100 psig instead of 3400 psig there are several possibilities:

- pressure flucuations.
- incorrect PSV set pressure when it arrived from the PSV shop (mistakes happen)
- incorrect temperature correction for set pressure at operating temperature versus temperature when the PSV is bench tested (if it's a conventional PSV).
- physical problem with your PSV. For example, if it's a pilot operated PSV and the pilot is plugged so the system pressure isn't being passed to the PSV dome, the PSV can start to relieve below set point since the system pressure is used to hold the PSV closed until the set pressure is reached and the pilot releases the pressure on top of the dome (allowing the PSV to open).

For your process, what is the design feed temperature at the location you are worried about? What are the operating limits from your operating manual, PFDs, operating limits, etc. What I'm getting at is how did you conclude that the operating temperature of the system could be the problem.

 

[abutt116]

Thanks for very detailed guidance.

Now there are two things. (1)Is there any effect of giving gas ammonia instead of liquid ammonia on the safety popping at very high pressure? (2)Design ammonia temperature is 250F and we are heating ammonia upto 285F.Is this contributing to safety popping? Bench test has been conducted many times regarding every other issue.What i found that when heating ammonia feed to reactor above critical temperature, there is hammering in the ammonia heater and when we are below critical temperature, there is no hammering in the ammonia heater.

Thanks.