Inline pH Measurement in Hot Undiluted Ammonium Nitrate

How a Pressurizable pH Sensor Works at 130 °C

The pH of stored liquid ammonium nitrate must be constantly controlled to prevent decomposition. Storage temperature is too high for traditional pH sensors. An advanced, pressurizable pH sensor provides continuous and accurate measurements in the harsh conditions.

Ammonium nitrate (AN) is a globally important inorganic salt. It can be used as a pure fertilizer or, more commonly, is blended with other valuable fertilizer products. It is also a very strong oxidizer and as such has many applications as an explosive or explosive component. Over 20 million tons of AN are produced globally per year. AN can be manufactured as a solid crystal; however, heightened security measures worldwide have forced many facilities to store it in solution in liquid form, depending on the need and downstream use.

Continuous pH control during storage is required to avoid AN decomposition. But, liquid AN is an extremely aggressive medium for pH sensors, so measurement in diluted AN, requiring a slipstream, and dilution and evaporation equipment, is the costly alternative.

How Ammonium Nitrate Liquor Is Produced and Kept Stable

The majority of AN is produced through a neutralization reaction of ammonia and nitric acid at high temperature and pressure. This is accomplished through various industrial processes ranging from the UCB process, SBA process, Chemico process, Stamicarbon process, and the NSM / Norsk Hydro process. Each process varies the reactor type and specifics with respect to temperature and pressure. Regardless of the process used the end product is AN in aqueous solution, which can be dried and granulated for further use. Commonly, the AN is stored hot in highly concentrated solution of 80 % or more. This is known as ammonium nitrate liquid or liquor.

As can be seen in figure 1, high storage temperatures are required (to achieve a high-concentration, low-volume product) as lower temperatures will cause crystallization of solid AN within the storage vessel. A prevalent storage risk is decomposition, where nitrogen species (namely ammonia) are lost. To prevent this, a strict pH range of the AN is required with storage values being neutral or alkaline, as acidic conditions enhance decomposition.

Commonly, ammonia loss will decrease the pH, thus ammonia gas can be added through a circulation system when the pH drops below acceptable levels. Circulation systems should always be installed to ensure a homogeneous solution (~ 5 – 10 % tank volume per hour). Too high a pH level can have environmental issues with an ammonia odor being present in the vicinity.

Crystallization point of ammonium nitrate liquor vs concentration

Why Traditional pH Sensors Fail at 130 °C

Problems arise when measuring pH at AN storage temperatures, typically 130 °C, which are normally too high for in-line pH sensors. At such temperatures, pH sensors experience rapid evaporation of internal electrolyte. Sensor lifetimes of less than a week are very common.

Cooling down the concentrated solution for measurement is not an option, as crystallization will occur, as mentioned above. Therefore, it is common practice to dilute the AN with hot condensate to a 10 wt % solution in a slip stream in which pH is measured after cooling the flow to around 25 °C – 60 °C. With a change in temperature and concentration, the pH also changes and compensation and set points must be observed. The pH of a neutral 10 wt % AN solution will be in the region of 4.5 at 25 °C.

Before it is returned to storage, the diluted AN used for measurement must be re-concentrated again through evaporating the dilution water. The required energy and infrastructure for sampling and concentrating make this a costly but necessary operation if traditional pH sensors are being used.

Measuring pH in the circulation loop of an ammonium nitrate storage tank

Direct pH Measurement at Storage Temperature

Complex and costly slipstream and concentration systems are not required if an InPro 2000i pH sensor from METTLER TOLEDO is used (figure 2). This is because the InPro 2000i can measure directly at the hot storage temperature.

Its electrolyte chamber is pressurizable, and having the electrolyte under pressure elevates its boiling point substantially; therefore, loss of electrolyte through evaporation is greatly reduced. Pressurizing the sensor to 5 bar allows continuous and accurate operation at 130 °C process temperature. The pressure can be preset using an InTrac 776e housing to higher values, further reducing the electrolyte evaporation that is a problem with all other pH sensors. The pressurized liquid also avoids reference system contamination due to the steady outflow of electrolyte.

In addition, the housing allows for sensor retraction and rinsing with hot condensate during retraction to ensure that AN does not crystallize within the housing. An M400 2-wire transmitter completes the measurement loop

ISM Diagnostics for Sensor Health Monitoring

InPro 2000i features METTLER TOLEDO Intelligent Sensor Management technology. ISM increases measurement uptime, simplifies sensor handling, and lowers operating costs. These benefits are possible due to the specially developed microchip embedded in ISM sensors.

For the InPro 2000i this circuitry allows pH calculation within the sensor itself. The pH value is then transferred to the connected transmitter via a highly stable digital signal. The microprocessor also contains advanced diagnostic algorithms that provide continuous data on sensor “health”, so operators know when sensor cleaning, calibration or replacement will be required.

Learn more about the InPro2000i

Download the Full Application Note

Now that you have learned how continuous inline pH measurement helps improve safety, maintain product quality, and simplify operations in hot ammonium nitrate processes, download the full Application Note for a more detailed explanation of the application, key measurement challenges, and the benefits of direct inline monitoring without sample conditioning.

Discover how accurate, real-time pH measurements can support safer storage, reduce maintenance efforts, and improve process reliability.