Modeling of Carbon Dioxide Absorption by Aqueous Ammonia Solutions Using the Extended UNIQUAC Model

A comparative study of thermodynamic models to describe the VLE of the ternary electrolytic mixture H2O–NH3–CO2

This study aims to ascertain the influence of the activity coefficient model and equation of state for predicting the vapor–liquid equilibrium (VLE) of the multi-electrolytic system H2O–NH3–CO2. The non-idealities of the liquid phase are described by the eUNIQUAC and eNRTL models. The vapor phase is modeled with the Nakamura equation, which is compared with the ideal gas assumption. The models are validated with experimental data from literature on total pressure and ammonia partial pressure. Results show that the models UNIQUAC and NRTL without dissociation can only reproduce the experimental conditions in the absence of CO2. When the electrolytic term is considered, the eUNIQUAC model is able to reproduce the experimental data with greater accuracy than the eNRTL. The equation of state which describes the vapor phase plays no major role in the accuracy of the VLE prediction in the operational conditions evaluated here. Indeed, the accuracy relies on the activity coefficient, therefore the ideal gas equation can be considered if the non-idealities of the liquid phase are described by a well-tuned model. These findings could be useful for equipment design, flowsheet simulations and large-scale simultaneous optimization problems.

Enhancing the anaerobic digestion process through carbon dioxide enrichment: initial insights into mechanisms of utilization

Carbon dioxide (CO₂) enrichment of anaerobic digesters (ADs) without hydrogen addition has been demonstrated to provide a potential solution to manage CO₂ streams generated in the water and organic waste sectors, with concomitant increases in methane (CH₄) production. This study investigates the CO₂ utilization mechanisms, by considering chemical and biological pathways in food waste and sewage sludge ADs. Methanosaetaceae was observed to be the dominant methanogen in sewage sludge ADs (Abundance of 83.8-98.8%) but scarce in food waste units (3.5-5.8%). Methanosarcinaceae was dominant in food waste (14.3-32.4%), likely due to a higher tolerance to the free ammonia nitrogen concentration recorded (885 mg L^-1). R_{Methanosaetaceae} (ratio of Methanosaetaceae fluorescence signal between test and control) of 1.45 and 1.79 were observed for sludge ADs enriched once and periodically with CO₂, respectively (p-value  < .05), suggesting a higher Methanosaetaceae activity associated with CO₂ enrichment. Reduction of CO₂ by homoacetogenesis followed by acetoclastic methanogenesis was proposed as a CO₂ utilization mechanism, which requires validation by radiolabelling or carbon isotope analysis.

Estimating speciation of aqueous ammonia solutions of ammonium bicarbonate: application of least squares methods to infrared spectra

The knowledge of the speciation and of the supersaturation of aqueous solutions of CO₂ and NH₃ is pivotal for the design and optimization of unit operations, e.g. absorption or crystallization, in the framework of ammonia-based CO₂ capture systems.

A low-energy chilled ammonia process exploiting controlled solid formation for post-combustion CO2capture

A new ammonia-based process for CO₂capture from flue gas has been developed, which utilizes the formation of solid ammonium bicarbonate to increase the CO₂concentration in the regeneration section of the process. Precipitation, separation, and dissolution of the solid phase are realized in a dedicated process section, while the packed absorption and desorption columns remain free of solids. Additionally, the CO₂wash section applies solid formation to enable a reduction of the wash water consumption. A rigorous performance assessment employing the SPECCA index (Specific Primary Energy Consumption for CO₂Avoided) has been implemented to allow for a comparison of the overall energy penalty between the new process and a standard ammonia-based capture process without solid formation. A thorough understanding of the relevant solid–solid–liquid–vapor phase equilibria and an accurate modeling of them have enabled the synthesis of the process, and have inspired the development of the optimization algorithm used to screen a wide range of operating conditions in equilibrium-based process simulations. Under the assumptions on which the analysis is based, the new process with controlled solid formation achieved a SPECCA of 2.43 MJ kg_CO2⁻¹, corresponding to a reduction of 17% compared to the process without solid formation (with a SPECCA of 2.93 MJ kg_CO2⁻¹). Ways forward to confirm this significant improvement, and to increase the accuracy of the optimization are also discussed.

Design and Optimization of the Preparation of Calcium Carbonate from Calcium Sulfate and Ammonium Bicarbonate

Simulation on single factor effect was used for the design and optimization of the preparation of calcium carbonate from calcium sulfate (DH) and ammonium bicarbonate. This study shows that simulation on single factor effect is effective because the experimental results are close to predicted results. Furthermore, response surface method based on a central composite design was used to determine the range of parameters to achieve a highly efficient conversion of DH. The results indicate that the significant parameters that affected the conversion of DH were ratio of carbon to sulfur, temperature, concentration of ammonium bicarbonate, and stirring speed. The strength order of factors is as follows: ratio of carbon to sulfur > concentration of ammonium bicarbonate > stirring speed > temperature. A quadratic polynomial equation was established using multiple regression analysis. The optimum parameters were determined as follows: 2.10 for ratio of carbon to sulfur, 320.35 K for temperature, 337.31 rpm for the stirring speed, and 1.75 mol · L−1 for bicarbonate concentration. The corresponding conversion rate of the experimental result was 99.7%, which was highly consistent with the predicted value of 99.9%. Based on model and the optimum parameters, products of vaterite, with ammonium sulfate crystal of grade A, can be obtained. Equipotential lines of conversional rate and desired process conditions were provided as well.