Neutralisation Process Control in an Ammonium Nitrate Reactor using a Redoxan Controller

Control of pH is of extreme importance in the neutralisation reaction process of nitric acid and ammonia to produce ammonium nitrate. Many accidents, including explosions with fatalities, have been reported related to problems in the pH control of ammonia nitrate reactors. The state of the art for this control is a standard pH probe measuring the pH of a sample, usually at 10% of concentration and at a temperature lower than 100^oC. The sample needs to be diluted and refrigerated to avoid crystallisation and damage in the pH electrode. Based on the sampling system, the pH measure is delayed, it is quite unreliable and attention-demanding; typical failures of the system are crystallisation of the sample, uncontrolled dilution water, uncontrolled cooling and delays of the response.

In Fertiberia-Sagunto the pH control of the reaction is of greater importance due to the designed working conditions at high temperature (180^oC) and at low pH (close to 1). The original pH control (standard system plus tritrometer) was not safe enough and caused many problems in the pH control system of the reactor. Fertiberia has recently installed a new control system concept based on sensors that measure free nitric acid or ammonia content. They are installed directly in the exit pipe of ammonium nitrate solution from reaction, at high temperature. This system gives an instant response of the excess content of the reactants, without delays and directly translatable to pH. Several months of operation have proved the reliability of this control system and the consistency of the results.