Analysis of NH₃ and CO₂ concentration in a wet gas stream from Solvay crude soda production.

MARCIN MASEK – GAS SAMPLE SP. Z O.O.

The method for producing soda ash was developed in the early 1860s when two Belgian brothers, Ernest and Alfred Solvay, devised an improved synthetic method for soda manufacturing.

The production of soda (sodium carbonate) utilizes limestone (calcium carbonate) and rock salt (sodium chloride). In practice, auxiliary materials are also necessary, such as coke and ammonia, which acts as a catalyst.           niezbędne jest zastosowanie materiałów pomocniczych (koksu, amoniaku jako katalizatora).

The Solvay process  relies on  multiple operations, including the thermal decomposition of limestone, the absorption of ammonia by brine, and the carbonization of the resulting solution by saturating it with CO2. It also involves the separation of crystallizing sodium bicarbonate (NaHCO3) and its subsequent calcination, as well as the recovery of ammonia from the liquid after bicarbonate crystallization using milk of lime.

The optimization of the process should allow for increased productivity and a simultaneous reduction in the consumption of auxiliary raw materials, thereby generating additional revenue. To achieve this, a precise and continuous measurement of both CO2 and NH3 concentration in the process gas is essential. Such a measurement presents a pioneering engineering challenge due to the high and variable concentration of ammonia in the gas (>10%), coupled with the presence of water at the saturation point and dissolved ammonium, carbonate, and chloride ions, which tend to crystallize at temperatures below 55°C.

A significant problem is the presence of salts in the collected sample, whose crystals block the free flow of gas from the process pipeline to the analyzer. These salts must be removed from the sample within a properly designed and constructed sample collection, transport, and preparation system. Simultaneously, the de-salting process must not alter the composition of the gas being measured before it is supplied to the NH3/CO2 analyzer.

An additional challenge is the correct selection of the measurement method and the design of the analytical system. The widely used absorption spectroscopy requires "sample extraction," which means drawing the sample from the process installation and transporting it to the analyzer without disturbing its composition or losing the components being measured.

Another difficulty to overcome is that NDIR analyzers (which use absorption in the near-infrared spectrum) are typically used for measuring ammonia in dry gases. Conversely, measuring ammonia in wet gases (saturated with H2O) is, according to accepted practice, done using laser spectroscopy. It is worth noting here that this method has only been verified for ammonia concentrations ranging from single ppm to 1% in wet gases. Consequently, higher concentrations of ammonia in wet gases have not yet been successfully measured, requiring the application of innovative solutions and R&D work to, for example, select an IR spectrum band where the NH3 measurement will not be interfered with by other components present in the gas.

An individually configured analyzer guarantees reliable CO2 measurement thanks to a specially designed sample preparation system, which features: