In the synthesis section of an urea plant we find the reactor where the carbon dioxide and the ammonia react to urea. This urea reaction is an equilibrium reaction.To achieve the optimum yield of urea, it is necessary that in the stripping urea process the reactants are present at an optimum specific ratio. This will lead to a stable operation of the urea synthesis section, minimum consumption of energy and maximum yield of urea. This ratio, which needs to be controlled within a tight range, is called the N/C molar ratio.
Only a limited number of possibilities to analyse the N/C ratio are available. Some plants still operate the plant by taking manual samples to do laboratory analysis, but safety risks and inaccuracies are inherently connected by the high pressure of the process. Most operators therefore like to work with on-line analysis nowadays, moreover as it speeds up the start up of the urea plant, as in this phase the flow measurements of carbon dioxide and ammonia are many times unreliable. The analyser is even essential if you want to come to Advanced Process Control. In this case the analyser will generate the required input signal for an automation system like a distributed control system to control the flows of ammonia and carbon dioxide. The only proven technology for the on-line measurement of the N/C ratio is by means of a density measurement of the urea reactor effluent stream.The density of the urea reactor effluent stream is known to be linear with the N/C ratio under certain temperature and pressure conditions; these are achieved in the N/C metering system.

2009 Spie Brochure NC meter.pdf