1. Introduction
CO2 is considered as one of the gases that causes the greenhouse effect and changes in the weather conditions as one of the most important pollutants in the atmosphere. Considering that the amount of CO2 production increases day by day, it is important to use an effective and economical way to absorb this gas. Chemical absorption is one of the most commonly used methods for CO2 absorption. One way that improves the absorption of CO2, it is to mix an amine such as triethanolamine (TEA) with a solvent such as piperazine whose solubility is more limited in water and can act as an accelerator to increase the Co2mass transfer rate for amine systems. This absorbent solution has the potential and the ability to take advantage of both solvents. The process of chemical absorption by alkanolamine solutions is used to sweeten gases containing combination of H2S and CO2. Amines are obtained from the combination of alcohol and ammonia. In TEA and N-methyl-ethanolamine, ammonia molecules are directly attached to the nitrogen atom instead of hydrogen atoms, and are called third-type amines.
In 2004, Yeon et al. studied an absorption reaction in a large laboratory experiment of carbon dioxide absorption proces alkanolamine, piperazine and TEA and kinetics. Absorption process was carried out in range of 303-382 K and in a mixture of piperazine with 0.26 and 0.64 molar plus 0.38, 1.13 and 2.26 molar TEA. Also, the absorption rate was obtained as a function of the partial pressure of carbon dioxide. According to the results obtained in their research, the initial concentration of adsorbent and the partial pressure of carbon dioxide gas have increased the rate of absorption. Tounsi et al. (2005) measured the new equilibrium data for CO2 in the diethanolamine (DEA)+methanol (MeOH)+H2O system under temperature conditions of 323.15 and 15.32 K, and pressure up to 10 MPa. They used kinetic and mass balance methods to generate their solubility data. They used kinetic and mass balance methods to generate their solubility data. To fit the experimental data, they used the Kenneth-Eisenberg model to have a good match. In 2008, Archane et al. measured the CO2 solubility data in a mixture of DEA+MeOH+H2O with a constant concentration of DEA of 30% by weight and a change in the concentration of methanol from 0 to 30% by weight at a temperature of 158.29 K. They saw low changes in the balance of data by changing the concentration of methanol. Also, from the equation of electrolyte state was used to model and calculate the equilibrium properties of the system and to report new parameters. In 2008, You et al. studied the effect of adding 2-amino-2-methyl-1-propanol (AMP) on CO2absorption by aqueous ammonia solution at 298 K. Experimental data are not presented in the table and they graphically capture the adsorbent efficiency in removing CO2 from an gas containing 15% vol. CO2 and 85% vol N2. They showed that the addition of AMP would reduce the evaporation of ammonia and, to a certain extent, also increase the CO2 absorption capacity. Yang et al. (2010) measured the solubility of CO2 in an aqueous mixture of AMP and piperazine (as activator) at 313-335 K and up to 139.9 kPa. The concentration of AMP in the mixture of solvents was 26.8% and 17.9%, while the concentration of piperazine was 12.9% and 4.3% by weight, respectively. They observed that at constant temperature and constant concentration of total amine, the solubility of CO2 increased with increasing relative pressure. Also, at constant temperature and constant AMP concentration, adding piperazine increased the solubility of CO2. Schüler et al. (2012) reported the data on the solubility of CO2 in a mixture of water and methanol at 298 K and 333 K at atmospheric pressure. They showed that the solubility of CO2 increases with the increase in methanol amount in the mixture, but at higher temperatures it decreases solubility. In 2013, Kariznovi et al. measured and predicted the density, viscosity, and solubility of CO2 for various alcohols such as methanol, ethanol, 1-propanol at 303.2 K and 323.2 K and pressure up to 6 MPa. They also observed that increasing the dissolution of CO2 in alcohols has led to an increase in density and a decrease in the viscosity of saturated fluid. Empirical solubility data and saturated fluid density data were predicted by two EOSs of PR and SRK. Their modeling results showed that both state equations in this range of pressure and temperature are in good agreement with the experimental data, but the saturation fluid density calculated by the SRK equation of state was much better than the PR equation of state. In 2013, Kim et al. examined the absorption of carbon dioxide in laboratory by using different alkanolamines and compared the results of each. They were able to measure the solubility of carbon dioxide in alkanolamines at temperatures of 313, 333, and 353 K. The results of this study showed that the amount of heat of absorption is directly related to the heat required for absorbent recovery, and also depends on the type of amine and the amount of carbon dioxide. In 2014, Murshid et al. also measured the solubility of CO2 in aqueous solution of AMP+ piperazine at two different temperatures of 303.15, 333.15 and various concentrations. Their measurements were carried out at pressure range of 5-60 bar and found that adding piperazine to the AMP aqueous solution increased the loading rate at high pressures. It also evaluated the effect of pressure on the solubility, while solubility decreases with increasing temperature. In 2014 Haghtalab et al. using a high pressure chamber, the equilibrium data of CO2 solubility in aqueous solution DIPA+AMP+PZ at K 313.15, 328.15 and 343.15 K and measured the pressure range 1-40 bar. They also obtained empirical data on the solubility of CO2 in dual aqueous solution of diisopropanolamine (DIPA)+AMP and DIPA+ piperazine in the above conditions. They subjected the results to partial CO2pressure versus acid gas loading (mol CO2/mol Amine). They observed that the combination of AMP- piperazine as an activator increases CO2 loading, which means that the absorption of CO2 in a DIPA aqueous solution increases. They also provided another work for the MDEA+AMP+PZ system in 2015 in the same conditions, and reported that piperazine in aqueous solutions of alkanolamines led to a slight variation in CO2 pressure versus CO2 loading, and with increasing pressure, the absorption of CO2 equilibrium increases and with decreasing temperature, the absorption of CO2 in the mixture of alkanolamines decreases. Shokohi et al. (2015) measured the solubility of CO2 in a mixture of aqueous solution of AMP+MDEA with different weight percentages, and temperatures of 313.13, 333.15, and 353.15 K up to a pressure of 2.5 MPa. They reported that the solubility of CO2 decreases with increasing temperature, while increasing with increasing pressure. Addition of AMP can also improve the loading capacity of the MDEA. In 2015, Al-Salem conducted a study on the removal of carbon dioxide from the exhaust gas from Kuwait refineries using a mixture of several amines. This research was conducted to identify and provide an appropriate method for reducing the pollution of the Kuwaiti oil industry. In 2016, Arentte investigated the absorption of carbon dioxide economically by activated charcoal with the aim of providing a model for comparing the use of new and renewable energies and carbon dioxide absorption methods. In 2016, Chu et al. examined absorption of carbon dioxide with monoethanol amine. In this study, they simulated numerical modeling of a full absorbed tower to remove carbon dioxide from monoethanolamine gas and compared their results with laboratory values ​​and other simulation results. Then, this absorption column was used for the industrial scale used and the effect of operational pressure and tower height was studied on the carbon dioxide mass transfer and energy consumption. The results showed that optimal operating pressure is atmospheric pressure and the optimal height of the absorbed columns in this work is about 8 meters. For the minimum energy consumption, the surface area per unit volume and the porosity of fill materials should be as large as possible.
In this research, a general model is proposed to calculate the carbon dioxide solubility in amine solutions, which will be used the CPA equation of state to predict the properties of the system. Finally, the results and outputs of the simulated model, which include the amount of CO2 solubility in this system and in different conditions, will be compared with and verified by the laboratory data in the articles.