2nd Post Combustion Capture Conference (PCCC2)
Ternary Phase Diagrams and Experimental Investigation of the
Particle Formation Kinetics for the CO2-NH3-H2O System
Daniel Sutter, Matteo Gazzani, Marco Mazzotti*
Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, 8092 Zurich, Switzerland
Abstract
Information on the kinetics of particle formation in the Chilled Ammonia Process may help to improve the control of the current
realization of the process which avoids solids. Additionally, the implementation of a dedicated solid formation step in the process
has the potential to increase the efficiency of the capture process by reducing the energy demand of the regeneration section.
Ternary phase diagrams for the thermodynamically complex CO2-NH3-H2O system were constructed and their use in process
development and in the experimental investigation of crystallization kinetics is presented based on examples. The design of such
experiments is discussed. Experimental results confirm the applicability of standard spectroscopic tools and Focused Beam
Reflectance Measurements for the investigation of crystallization kinetics.
Keywords: Chilled ammonia; solid formation; nucleation; growth rate; crystallization; phase diagram; ternary system; post combustion capture
1. Introduction
The Chilled Ammonia Process (CAP) is a promising candidate among the post-combustion CO2 capture
technologies, particularly because of the advantages concerning cost, and global availability of its absorbent and
because of the absence of toxic degradation product emissions. Alongside relatively slow absorption kinetics, solid
formation in the process that potentially leads to clogging of process equipment is a known complication. There is
potential to improve efficiency and reliability of the current CAP that intends to avoid solid formation by
investigating the kinetics of particle formation in the system. This knowledge might enable a more precise control of
the current process, and might allow integrating solid formation into the process in a dedicated process step.
Ternary phase diagrams for the complex CO2-NH3-H2O system were constructed. The presentation will highlight
the use of such diagrams in understanding the thermodynamics of the system, in the design of an experimental
campaign to study the kinetics of particle formation, and in process development. The experimental investigation of
nucleation and growth rates will be introduced and experimental results be presented.
2. Ternary phase diagram for the CO2-NH3-H2O system
An existing thermodynamic model that applies the extended UNIQUAC model and the SRK equation of state [1]
was used to construct isothermal ternary phase diagrams for the CO2-NH3-H2O system. Figure 1 shows the resulting
* Corresponding author. Tel.: +41 44 632 2456; fax: +41 44 632 1141.
E-mail address: [email protected]
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ternary phase diagram for 10°C and standard atmospheric pressure as an example. The straight line connecting the
carbamate and the carbonate solids in Figure 1 points to the H2O corner. The fact that this graphical information
corresponds directly to the decomposition of ammonium carbonate into ammonium carbamate and water (see Eq. 1),
is only one example of how the diagram supports the understanding of the thermodynamic relations.
(NH4 )2 CO3  H2O
NH2COONH4  2H2O
(1)
Figure 1: Isothermal ternary phase diagram for the CO2-NH3-H2O system in weight fractions for 10°C and atmospheric pressure. The phase
regions of particular interest are labeled with S, L, and V for solid, liquid, and vapor phases, respectively. There is an approximately triangular 2phase (S+L) region for each solid, bound by two straight tielines and the solubility isotherm. The solubility isotherm is the nonlinear boundary
between the S+L region of each solid and the single-phase zone (L) close to the H2O corner. Only phase regions that can accurately be disclosed
with the available thermodynamic model are plotted. We expect, for example, a small vapor-only region close to the NH3 corner.
3. Experimental investigation of particle formation
The kinetics of particle nucleation and growth are crucial prerequisites for rate-based modeling and design of
chilled ammonia absorption processes including solid precipitation. The quantitative experimental exploration of the
CO2-NH3-H2O system is complicated by the relatively high and interdependent vapor pressures of CO2 and NH3.
The establishment of the vapor-liquid equilibrium (VLE) leads to changes in the aqueous concentration of both
species and accordingly to variations in the speciation equilibria. Temperature and pressure variations, as well as the
consumption of aqueous ions by solid formation, result in continuous changes in the VLE. Therefore, the design of
the experimental setup aims at minimizing the volumetric ratio of vapor and liquid phase.
The setup consists of a gas-tight titanium crystallization reactor with external temperature control. The pressure
can be confined with a backpressure regulator, which is set to standard atmospheric pressure in order to resemble the
absorber conditions. The design of reactor and lid allow high liquid levels, effectively minimizing the volumetric
ratio of vapor phase and liquid phase to about 5%. At the conditions of the absorber with an aqueous NH3 loading of
below 40 wt.-%, the system’s total vapor pressures is below 1 bar, as the absence of a vapor phase in Figure 1
shows. Assuming ideal gas properties for the vapor phase, the density of water for the liquid phase, and typical
concentrations of the aqueous CO2 and NH3 species that lead to solid formation, the number of solute molecules in
the vapor phase is approximately a factor of 104 lower than the number of solute molecules in the liquid phase.
Thus, neglecting the VLE effects on the liquid phase concentrations seems justifiable.
The growth rate of e.g. ammonium bicarbonate crystals can be explored with seeded batch desupersaturation
experiments. The seed particles with known particle size distribution (PSD) grow, while the concentration of the
solutes decreases to saturation. Attenuated Total Reflectance – Fourier Transform Infrared (ATR-FTIR) or Raman
Author name / Energy Procedia 00 (2013) 000–000
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spectroscopy allow measuring the solute concentration in-situ. A fit of the desupersaturation profile to a growth
model yields the kinetic parameters for particle growth. Subsequent cooling crystallization experiments reveal the
kinetic parameters for nucleation. Continuous cooling of a saturated solution leads to nucleation, when the
concentration exceeds the metastable limit. The appearance of particles can be detected by Focused Beam
Reflectance Measurements (FBRM). A model that incorporates growth and nucleation can then be fitted using the
obtained particle size distribution, the number of particles, and the desupersaturation profile.
ATR-FTIR and Raman spectroscopy are able to track the concentrations of carbonate, bicarbonate, and
carbamate ions [2,3]. Figure 2 shows the ATR-FTIR and FBRM signals resulting from one of our cooling
crystallization experiments. Raman spectroscopy additionally offers the potential to detect the solids in suspension,
and can thus be used to determine which solids are formed. The spectroscopic tools need to be calibrated. Reference
solutions will be prepared in the gas-proof reactor by dissolving defined amounts of solid ammonium bicarbonate in
demineralized water and ammonia solutions. Complete dissolution will be ascertained by FBRM. A thermodynamic
model will be fitted to the spectroscopic measurements using nonlinear regression as described by [2].
Figure 2: IR absorbance at 1360 cm-1, FBRM counts per second, and reactor temperature for a cooling crystallization experiment. The local
maximum in temperature is due to the heat of crystallization, when particles are formed. The FBRM starts to count particles at the same time,
when the temperature starts increasing. The IR absorbance indicates the consumption of solute by solid formation. At 1360 cm-1, the bicarbonate
species leads to a peak due to –CO2 symmetric stretching [2].
4. Summary
In summary, the presentation will introduce ternary phase diagrams for the CO2-NH3-H2O system and
demonstrate their use as a tool to achieve a more rigorous understanding of the complex thermodynamic relations in
the system with a focus on solid formation. The design of an experimental setup and campaign for the investigation
of the kinetics of particle formation in this system, which exhibits high vapor pressures of two solutes, will be
presented as well as up-to-date experimental results.
References
[1] Darde, V, van Well, WJM, Stenby, EH, Thomsen, K. Modeling of carbon dioxide absorption by aqueous ammonia solutions using the
extended UNIQUAC model, Ind Eng Chem Res 2010;49:12663-74
[2] Richner, G and Puxty, G. Assessing the chemical speciation during CO2 absorption by aqueous amines using in-situ FTIR, Ind Eng Chem
Res 2012;51:14317-24
[3] Wen N and Brooker, MH. Ammonium Carbonate, Ammonium Bicarbonate, and Ammonium Carbamate Equilibria: A Raman Study, J Phys
Chem 1995;99:359-68