Abhinav Gaur

Henry’s Law, which states that the solubility of a gas in a liquid is proportional to its partial pressure above the liquid (C = kH × P), applies strictly to physical absorption, meaning when the gas does not chemically react with the solvent. However, in the case of CO₂ reacting with NH₃, as occurs in urea production, we are dealing with chemical absorption, in which ammonium carbamate (NH₂COONH₄) is formed. This is an exothermic and reversible reaction:

2NH3(l)+CO2(g)⇌NH2COONH4(aq)\mathrm{2NH_3 (l) + CO_2 (g) ⇌ NH_2COONH_4 (aq)}2NH3​(l)+CO2​(g)⇌NH2​COONH4​(aq)

In this context, Henry’s Law cannot be directly applied to CO₂ solubility, since the gas does not remain in its molecular form—it reacts and forms a new chemical species. However, in certain sections of the process, especially in the urea stripper, Henry’s Law can be indirectly applied to calculate the release of unreacted CO₂ and NH₃ from the solution as they return to the gas phase.

In the Saipem stripper, the purpose is to remove excess ammonia and carbon dioxide from the urea solution via thermal decomposition under reduced pressure and elevated temperature. During this step, ammonium carbamate decomposes back into NH₃ and CO₂, and this reversible reaction allows the gases to be released. At this stage, equilibrium between the liquid and gas phases is established, and Henry’s Law helps estimate the partial pressures of CO₂ and NH₃ in the gas phase, but only after they have been regenerated from the carbamate.

Therefore, Henry’s Law is used in modeling the release and recovery of gaseous components in the stripping section, but it does not describe the initial chemical absorption of CO₂ in liquid ammonia, which falls outside the scope of the law. In that sense, it is used in combination with equilibrium constants of chemical reactions and mass balances, forming part of an integrated thermodynamic model of the urea process.

While Henry’s Law is not applicable to the chemical absorption of CO₂ into NH₃ (formation of ammonium carbamate), it plays a valuable role in the stripping phase of the Saipem process, helping to quantify and model the release of CO₂ and NH₃ gases as they transition from liquid to vapor phase after the decomposition of carbamate.

Dragan Ugrinov Dear Sir,

Thanks for your quick and informative response but my confusion is regarding implementation of Henry's Law during decomposition of ammonium carbamate. Actually, I want to know that whether decomposition of ammonium carbamate in stripper is based on Henry's Law and excess ammonia in stripper helps in carbamate decomposition?

I am attaching a file where something like this is described which is creating all this confusion. Or it may be that i am unable to understand clearly what writer want to say. Kindly help me in understanding this fact.

Abhinav Gaur

• Yes, in the Saipem urea process, the stripper uses conditions that promote ammonium carbamate decomposition and gas removal.
• No, the decomposition itself is not directly based on Henry’s Law — it is a chemical process.
• However, Henry’s Law is useful for modeling partial pressures and gas-liquid equilibrium after the decomposition, especially for the stripping of CO₂ and NH₃.
• Excess ammonia helps by lowering the partial pressure of CO₂ and improving stripping efficiency.

Dragan Ugrinov

Can you explain point 3 and 4 in detail?

Because I want to know deeply about;

• How Henry’s Law is useful for modeling partial pressures and gas-liquid equilibrium after the decomposition, especially for the stripping of CO₂ and NH₃?
• How Excess ammonia helps by lowering the partial pressure of CO₂ and improving stripping efficiency?