Document Type : Research Paper
1 M.S. Student, Department of Chemical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
2 Assistant Professor, Department of Chemical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
Hot potassium carbonate (PC) solution in comparison with amine solution had a decreased energy of regeneration and a high chemical solubility of . To present vapor and liquid equation (VLE) of this system and predict solubility, the ion specific non-electrolyte Wilson-NRF local composition model (isNWN) was used in this study; the framework of this model was molecular. Therefore, it was suitable for both electrolyte and non-electrolyte solutions. The present research employed the NWN model and the Pitzer-Debye-Hückel theory in order to assess the contribution of the excess Gibbs energy of electrolyte solutions in a short and long range. The data of solubility in water and the system of aqueous were correlated in the model considering a temperature range of and a pressure range of and . The average absolute error of ( ) and ( ) systems were and respectively. The results and comparisons with other models proved that the experimental data were exactly correlated in the model.
The global climate is changing due to the continuous increase in the temperature of the earth, which is of great concern for the environment. The major reason for the global warming is the increasing production of more than the nature can accept. is increasingly released into the atmosphere through using fossil fuels and industrial processes (Višković et al., 2014). There are different methods available for the elimination of from flue gases of power plants and gas streams. CO2 capture by using chemical absorption is a mature technology currently used (Zhenqi et al., 2012; Meihong et al., 2011; Zhao et al., 2012). Utilization of amine solution is the most popular method for chemical absorption (Afkhamipour et al., 2013; Austgen et al., 1991; Dang et al., 2003; Sidi-Boumedine et al., 2004) . However, amines have some disadvantages; for instance, these solutions have shown excessive energy requirement for regeneration, easy degradation, and strong corrosion to equipment. Appropriate absorbers must have a fast reaction rate and a lower heat of regeneration. Hot potassium carbonate solution has a decreased energy of regeneration (Kohl et al., 1997) and a high chemical solubility of (Todinca et al., 2007). An equilibrium thermodynamic model is required to anticipate the total pressure of the solution as well as the partial pressure of . Similar systems can be thermodynamically modeled through many available models to achieve acceptable simulation and optimization for a chemisorption process (Afkhamipour et al., 2013). These models can be categorized into three groups: 1) a state equation through phi-phi method as reported by Gubbins and Buttom (Button et al., 1999) and Huttenhuis et al. (Huttenhuis et al., 2008); 2) semi-empirical models such as the Kent-Eisenberg model (Kent et al., 1976) 3) excess Gibbs energy or gamma-phi method such as the works of Austgen et al. (Austgen et al., 1991) and Li and Mather (Li et al., 1998; Qian et al., 1995). This research has investigated the free energy of surplus Gibbs to demonstrate the systems VLE ( ) and ( ) through the thermodynamic modeling. In these systems, the local composition statement was used regarding the short-range contribution, and one of the Debye-Hückel (DH) equations was employed for long range electrostatic interactions (Zhao et al., 2000; Messnaoui et al., 2008). Haghtalab and Mazloumi (Haghtalab et al., 2009) applied Wilson-NRF model to electrolyte solutions, which were considered as an ion-pair. The model had a salt specific parameter as well as two adjustable parameters per each salt. However, Mazloumi (Mazloumi et al., 2015) had used this model using ion specific parameters for strong aqueous binary and ternary electrolytes. The results were highly correlated with the experimental data. The Wilson-NRF model was utilized as the thermodynamic model for non-electrolytes in the current work as presented in the study of Mazloumi (Mazloumi et al., 2015).
2. Thermodynamic framework
The solubility and the other solute species quantity may be predicted by the thermodynamic modeling (Anderko et al., 2002). A few equations must be considered to model a system like potassium carbonate solution thermodynamically, and a thermodynamic approach should be utilized for the VLE calculations.
2.1. Standard state
The equilibrium of the system was defined through chemical potential. In this condition, each component of all the phases should have equal chemical potential values. These values of solvent (water) in an aqueous system can be defined by a mole fraction-symmetrical equation (Afkhamipour et al., 2013):
In which, is the standard chemical potential of pure water. also represents symmetrical water activation coefficient, and is the water mole fraction; stands for absolute temperature, and is the universal gas constant. The chemical potential values of the solute in the system is calculated by the following equation:
2.2. Chemical equilibrium
The chemical reactions of the liquid phase must be taken into consideration for speciation calculations. The ionic complex species are produced when reacts with PC in an aqueous solution. The subsequent reactions are given by:
The stoichiometric technique was utilized in this study to create the calculation. This technique was correctly investigated by several researchers (Al-Rashed et al., 2012; Cullinane et al., 2002).
Equation 6 calculates the chemical equilibrium constant,