Return to aqueous electrolyte page

The CO₂ – NH₃ – H₂O system

The phase behavior and the thermal properties of this system is important because of its potential use for removing carbon dioxide from flue gas in post combustion carbon capture processes. One such process is the chilled ammonia process (Eli Gal, Ultra cleaning combustion gas including the removal of CO2, World Intellectual Property, Patent WO 2006022885 (2006)). In the chilled ammonia process, a slurry consisting of a liquid in equilibrium with solid ammonium bicarbonate (NH₄HCO₃) is produced in an absorber. The slurry releases CO₂ at a relatively high pressure after being heated in a desorber. After being cooled again, the liquid is being led back to the absorber.

Speciation

The CO₂ – NH₃ – H₂O system is apparently a ternary system. But due to hydrolysis, a number of ionic species are formed in the system. This phenomenon is also called speciation, the distribution of an element among defined chemical species in a system. In the ternary CO₂ – NH₃ – H₂O system, the following species are formed in various amounts: H⁺, OH^-, NH₄⁺, NH₂COO^-, HCO₃^-, CO₃^2-.

The formation of the carbamate ion NH₂COO^- is central for understanding the properties of the CO₂ – NH₃ – H₂O system. The carbamate ion is produced by the reaction:

The CO₂ – NH₃ – H₂O system was modeled with the Extended UNIQUAC model. (Thomsen K and Rasmussen P, “Modeling of Vapor-liquid-solid equilibrium in gas-aqueous electrolyte systems”, Chemical Engineering Science, 54(1999)1787-1802).

Figure 1 shows the experimentally measured and the calculated speciation in a 6.226 molal NH₃ solution at 40°C. The amounts of NH₃, NH₄⁺, and NH₂COO^- are shown in the figure to the left, the amounts of CO₂, CO₃^2-, and HCO₃^-  in the same solution are shown to the right. The experimental data come from Lichtfers PhD Dissertation, University of Kaiserslautern, Germany, 2000. The calculated values come from the Extended UNIQUAC model. There is a good agreement between the experimental data and the corresponding calculated values. It is seen that the amount of carbamate apparently goes through a maximum at approximately 3 mol CO₂ per kg water. This maximum occurs at a pH about 9 and at a loading of 0.5 mol CO₂ per mol NH₃.

It is worth noticing that the Extended UNIQUAC model parameters for the ternary CO₂ – NH₃ – H₂O system were published in 1999. In figure 1, model calculations are compared with experimental data published in 2000. The model parameters were based solely on vapor-liquid equilibrium data, solid-liquid equilibrium data and thermal property data. The calculation in figure 1 therefore represents prediction rather than correlation of speciation in the system.

Vapor – liquid equilibrium

Precise vapor-liquid equilibrium calculations are important for designing processes such as carbon capture processes using ammonia. The Extended UNIQUAC model was fitted to experimental data in the temperature range from 0 to 110°C and is able to reproduce data with high accuracy in this temperature range and at pressures up to 100 bar. At temperatures above 110°C the accuracy is decreasing.

Sample calculations performed with the Extended UNIQUAC model are shown in figures 2 and 3. The calculated data are compared with experimental values in order to document the quality of the fit. Partial pressures of NH₃ are shown in the figure to the left, partial pressures of CO₂ are shown in the figure to the right. The calculations were performed for solutions containing a fixed amount of ammonia while different amounts of carbon dioxide were added. Each ammonia concentration is represented by a branch in the diagram.

It is seen that the partial pressure of ammonia is decreasing by addition of CO₂ while the CO₂ partial pressure is increasing. At a loading of approximately 1 mol of CO₂ per mole NH₃, the partial pressure of CO₂ is seen to increase rapidly. This is because ammonia and carbon dioxide react one to one. When the available ammonia is neutralized by carbon dioxide, the vapor pressure of carbon dioxide goes up because the solubility of CO₂ in solutions with low pH is not very high.

Solid – liquid equilibrium

In the chilled ammonia process, solid NH₄HCO₃ is formed in the absorber. It is therefore important to be able to calculate solid-liquid equilibrium in the system accurately. The available experimental data for solid-liquid equilibrium in the system are scarce and they are inconsistent. The researchers who reported on solid liquid equilibrium in the system do not agree on what solid phases are formed in the system. It was found that the experimental data reported by Jänecke, (Jänecke E, Zeitschrift für Electrochemie, 35(1929)716-728) seemed to be most reliable and only these were used for determining parameters in the model. 

The phase diagram in figure 4 shows the composition and temperature ranges the different solid phases precipitate in. The phase diagram does not show the concentrations of CO₂ and NH₃, only the ratio between the amounts of these two species. The 40°C isotherm is shown in figure 4. At 40°C, four different solids can precipitate. The isotherm in figure 4 shows all concentrations at saturation. Some disagreement between the calculated and the experimental data for ammonium carbamate (NH₂COONH₄) solubility is seen. 

According to the experimental data of Jänecke, the following solids can precipitate in this system in the temperature and pressure range considered here:

·         NH₄HCO₃, Ammonium bicarbonate

·         (NH₄)₂CO₃·2NH₄HCO₃, Sesquicarbonate

·         (NH₄)₂CO₃·H₂O, Ammonium carbonate monohydrate

·         NH₂COONH₄, Ammonium carbamate

These solids will precipitate from a chilled ammonia solution of sufficient ammonia concentration when carbon dioxide is added (the solution is loaded with carbon dioxide). Due to constraints caused by Gibbs Phase rule, the two partial pressures remain constant while two solid phases are present simultaneously. In a three component system with four phases, there is only one degree of freedom (see invariant points). If the composition of the gas phase or of the liquid phase is changed, the pressure will change. If the total pressure and the temperature are kept constant, the composition of the gas or of the liquid therefore can not change. The overall reaction taking place as CO₂ is added to the system is:

This is shown in figure 6 for a temperature of 10°C, a total pressure of 1 bar and a 27 wt% ammonia in water solution. In the figure to the left, the partial pressures of carbon dioxide and ammonia as function of CO₂ loading are shown. At low loadings, ammonium carbonate monohydrate is precipitating. The partial pressure of ammonium is decreasing as carbon dioxide is added, and that of carbon dioxide is slightly increasing. As the loading increases, pH of the solution decreases and at a loading slightly above 0.5, ammonium bicarbonate becomes superaturated and starts precipitating. The number of solid phases is therefore two and the total number of phases is four. The system has only one degree of freedom. When carbon dioxide is added, the amount of solid ammonium carbonate is increasing and the amount of solid ammonium carbonate monohydrate is decreasing. At a loading of 0.78, the last ammonium carbonate monohydrate has disappeared. The system now has only one solid phase, one liquid phase and a gas phase. Two degrees of freedom are available and the vapor pressure increases upon further loading with carbon dioxide.

The graph to the right in Figure 6 shows the amounts of solids precipitating from a solution consisting of approximately one kilo water. The precipitated amount of ammonium carbonate monohydrate reaches a maximum at a loading of 0.51. At a loading of 0.78 mol CO₂/mol NH₃, the last ammonium carbonate monohydrate has disappeared.

The kinetics of this solid-solid transition has not been described in the literature and it is not known if the CO₂ absorption or the solid-solid transition is the rate limiting step for the absorption process.

Figure 6: The figure to the left shows the partial pressures of carbon dioxide and ammonia during loading of a 27wt% ammonia solution with carbon dioxide. The figure to the right shows the corresponding amounts of the two solids present in this loading range. These amounts are based on a solution with approximately 1 kg water.

The graphs in figure 6 were calculated for 10°C and for a 27wt% ammonia in water solution. The same scenario is valid in a wide temperature range. When the loaded slurry is heated to 50°C all ammonium bicarbonate seems to dissolve in the solution. The graphs were calculated in Microsoft Excel. Vapor-liquid-solid equlibrium calculations are performed by calling a DLL file through the Visual Basic interface. The DLL file contains the Extended UNIQUAC model and the algorithm for the vapour-liquid-solid equilibrium calculation (Flash calculation).