Deciding on the high-temperature trip and alarm levels for AN (Ammonium Nitrate) plant service

By Ron Peddie

This paper looks at the instrumentation of an Ammonium Nitrate plant and explains the variety of temperature trips and alarms.
For instance, 200ºC is used as an alarm on a pipe reactor.
Or maybe even as low as 105ºC on a pump
The register of temperature will show several temperatures normally in groups.
Each trip or alarm temperature setting is the result of a risk assessment.

Maximum temperature for AN stability

The most dangerous event to avoid is a runaway reaction/thermal decomposition.
Decomposition means that some AN molecules have become unstable, generating sufficient heat to decompose more AN rapidly. [1]. Decomposition is the AN molecule breaking into NOx, ammonia, water, and nitrogen. This may start as an endothermic process producing NOx. Eventually, the decomposition will become exothermic, and this rapidly escalates. This is a thermal runaway.
AN initially undergoes a reversible endothermic decomposition to ammonia and nitric acid. At more elevated temperatures, exothermic reaction paths dominate, and a runaway/ thermal explosion will result.
In this situation, you can have an explosion.
The temperature at which this process becomes unstable is determined experimentally – so you will see different figures quoted.
The temperature number produced depends on the nature of the experimental test—the lowest onset temperatures occur at near adiabatic conditions. Confinement also affects results, as the primary decomposition reaction is partially suppressed.
Often, you will see temperatures between 190ºC and 230ºC
It should be noted that all experimental verification will produce a higher maximum safe temperature than the actual safe temperature. This means the experimental figures may not be on the safe side, and some additional safety margin in temperature level is needed.

This is the case with DSC/DTA studies – a near adiabatic test (ARC or Dewar) will give a slightly conservative result, assuming no contamination in the sample or vessel.
This is because the scale of a larger vessel cannot release heat like a small experimental study.
Adiabatic tests are preferred.
Also, you cannot measure the inside of a vessel, and there may be a lag in response.
The experimental difficulties have been closely examined, leading to better experimental methods, such as ARC (Accelerating Rate Calorimeter Testing,) which produces maximum temperature figures closer to the actuality.
This can be a complex area and was the subject of a paper at ANNA in 2015 [2]
Even with the ARC reading, a safety margin band of 20ºC IE is needed. The actual safe temperature may be 20ºC lower than the take-off temperature detected. This accounts for various other factors, like contamination or loss of heating control. Or the location of the temperature probe in local static media
In practice, the temperature alarm should be set by comparing the maximum temperature required by the process plus a margin.

For neutralisers

The maximum acid strength and the pressure determine the temperature for a neutraliser. So, it is normal to see a variation in the trips and alarms. Generally, the risk assessment team will decide on an abnormal temperature about 10 ºC above the expected maximum for an alarm and perhaps 15 ºC above the expected maximum for an intervention such as a deluge of water.
It should be noted that neutralisers are thermally stable when in full operation. This is because they release huge amounts of heat, such as process steam. A small amount of contamination cannot overpower this heat loss.
A pipe reactor is a special case as the AN is in the form of a mist in the pipe and flashes immediately afterwards. The geometry of a mist does not allow a thermal runaway; thus, pipe reactors can have a higher trip and alarm point on the pipe itself.

Figure 1Geometry inside a pipe reactor showing protection against thermal runaway

The danger period for neutralisers is during start-up or shutdown. The tragic accident at Port Neal [1] is an example of an accident with a neutraliser in a shutdown. Due to high-temperature steam, this accident also involved overheating.

Figure 2 Situation at Port Neal in 1993, where a thermal runaway occurred

For process tanks

The expected temperatures on process tanks follow on from the neutraliser or evaporator. Again, the risk assessment team would set these at about 10ºC above the expected minimum temperature. These temperatures will vary according to the maximum expected temperature in a high concentration prilling unit, making a dense fertiliser grade as high as 190 ºC or slightly more.
In an explosives grade plant, they will be around 160 ºC, reflecting the use of lower strength solution.
In other tanks used in recycling recovery, the temperature levels would be lower still, and a risk assessment would decide on a temperature trip and alarm that would alert the operator without becoming a nuisance by going off too frequently.
The philosophy is to keep the alarm temperature as low as possible to indicate an undesirable deviation in process operation as quickly as possible.

For evaporators

The falling film evaporators used in ammonium nitrate have specific design features for thermal safety.
The control of the vacuum maintains lower temperatures in a vacuum evaporator generally used in explosives-grade production.
However, vacuum evaporators cannot reach high concentrations and also produce liquid effluent. In these cases, an air-swept evaporator is used. The main safety feature of an air-swept evaporator is that evaporation takes place sub-cooled; in other words, the AN does not boil in an air-swept evaporator. With this safety feature, the air-swept evaporator can be fed with higher-temperature steam than other parts of the plant to process to a high concentration. This steam is likely too hot to be used elsewhere in the plant.

Figure 3 Geometry inside a falling film evaporator, which allows the use of higher temperature steam than the rest of the plant

For pumps

Pumps have a different problem than other parts of the plant. If the pump is deadheaded, it can start overheating with the heat supplied by the motor and cause an explosion, which is normally a reasonably small thermal incident. To protect against this, a high-temperature trip is normally used. This will be set at about 20ºC above the normal operating temperature. For a high-concentration pump, this would be about 170ºC if the pump is running a 91% melt at 150 Deg C.
Another method used for safety is a submerged pump. Submerged pumps can be designed with an internal kickback, meaning deadheading is impossible. These pumps have also been fitted with high-temperature trips, but these can be physically challenging to install

For steam systems

Many major accidents in AN production are caused by the steam heating temperature being too high. Therefore, the temperature is generally about 20 degrees above the temperature to prevent the Ammonium Nitrate solution from fudging.
For a high-density prill plant, this maximum temperature might be about 190ºC. For a low-density plant, it might be about 160 ºC
Another powerful protection for steam systems is limiting the steam pressure. A pressure relief valve can reliably do this. Again, the settings will be different depending on the application.
For a high-density plant, the steam may be at about 10 Barg with a saturation temperature of 180ºc and an RV set at 12 Barg.
For a low-density plant, the steam may be at about 6 Barg with a saturation temperature of 160ºc and an RV set at 8 Barg.
The risk team should always be aware of the solution's boiling point compared with the steam temperature. If the steam is below the boiling point, this mitigates against the concentration increasing, perhaps to dangerous levels.

Other thermal protection

The plant should have no dead spots where decomposition can proceed, possibly without being noticed. So, when shut down, the plant must be drained clear, or the risk assessment must be clear there is sufficient protection.
Contamination
Contamination can lower the safe maximum operating temperature of AN. Areas of the plant where contamination can build up should be avoided. This will mean maintaining reasonable velocity levels in pipes and avoiding areas of contamination. Because of the danger of corrosion products, the neutraliser device should be swept with steam before starting up.

Conclusion

Temperature monitoring alarming and interlocks on an AN plant provide a solid layer of safety protection. Each temperature setting should be decided by risk analysis and logic.

References

[1] United States- Environmental Protection Agency, "Chemical Accident Investigation Terra Industries Nitrogen Fertilizer Facility," US EPA, Kansas City, 1993.
[2] R. Turcotte, M. Braithwaite and R. Peddie, "A review of Testing Methods and interpretation of their results for the Thermal stability of Ammonium Nitrate," in ANNA proceedings, Eindhoven, 2015.