Among the greenhouse gases, carbon dioxide (CO₂) is the most influential gas on global warning. There are various techniques used for the removal of CO₂, including reactive absorption, membrane separation, biological fixation, and hydrate base separation (Gaspar et al., 2011; Norouzbahari et al., 2016; Pashaei et al., 2017). Reactive absorption seems to be the most practical and effective technique. Reactive absorption is an important unit of several chemical and petrochemical industries. Chemical reactions in the absorption of undesirable gases bring many advantages in terms of operating conditions such as increasing mass transfer while decreasing the operating total pressure (Schneider et al., 2003; Khan et al., 2011; Pashaei et al., 2016).

Recently several important industrial reactive absorption processes have been successfully investigated both theoretically and experimentally by a number of researchers. A wide variety of chemical solvents have been employed, where aqueous solutions of alkanolamines have found comparatively widespread applications (Pacheco and Rochelle, 1998). Recently, mixed solutions including blend amines and hydroxide solutions are used in the absorption of undesired gases (Ghaemi et al., 2011).

Usually reactive absorption processes occur in the aqueous electrolyte solutions (Ahmadi et al, 2008). In these solutions, the components are in ionic and molecular form, whereas in the gas phase the components are only in molecular form (Ghaemi et al., 2009). The ionic species affect the amount of acidity and basicity of the liquid phase, which has an impact on the absorption rate of the gases, especially on acidic and basic gases like CO₂ and NH₃. In the previous researches, the effects of all species in the liquid phase on the absorption of gas components have not been considered (Ghaemi et al., 2011b; Noeres et al., 2003).

An experimental investigation of reactive absorptions can lead to better understanding of the process behavior and designing more efficient operating conditions (Ghaemi et al., 2011c). Therefore, in this work, a procedure is introduced to study the reactive absorption of CO₂ in an aqueous electrolyte solution of NH₃, H₂O, and NaOH applying the principles of electrolyte solutions (Beutier and Renon, 1978). In addition, all ionic and molecular species concentration profiles along the column height have been obtained. Furthermore, the influence of ionic and molecular species on the absorption rate of CO₂ has experimentally been studied.

2. Modeling of electrolyte systems

According to works of Edwards et al., thermodynamic modeling of electrolyte solutions is based on three principles (Edwards et al., 1978; Etemad et al., 2015):

1- Overall mass balance of electrolytes in liquid phase:

2- Electroneutrality:

Electrolyte solutions are electrically neutral even though they may contain millions of charged ions (cations and anions). This means that the molar concentration of positive charges in an electrolyte solution always equals the molar concentration of negative charges. Equation 2 mathematically represents this principle:

3- Chemical equilibria:

Equilibria of chemical reactions taking place in the solution (dissociation of water and electrolytes and reactions between electrolytes and/or products of their dissociation) with the deviations from the ideal solution properties taken into account (Kontogeorgis and Folas, 2009):

where, K is the association equilibrium constant of reaction r, and  is the stoichiometric coefficient of i in reaction r; and  respectively represent the molality and activity coefficients of i.

Deviations of aqueous electrolyte solution properties from the ideal solution properties are expressed by the means of the excess Gibbs free energy G^E. The classical way of calculating the activity coefficients of real species dissolved in the solution is based on the semi-empirical Pitzer equation for G^E (Pitzer and Mayorga, 1973; Pitzer, 1973):

where, f₁(I)is the Debye–Huckel–Pitzer function describing the effect of long-range ion–ion electrostatic interactions depending on the ionic strength of the solution.  are the parameters of binary short-range intermolecular interactions. f₂(I) is a function describing the effect of ionic strength, on binary short-range intermolecular interactions.  is the parameter of ternary short-range intermolecular interactions. Differentiation of Equation 4 yields the activity coefficient of the dissolved species i (Kamps et al., 2000; Norouzbahari et al., 2015):

The activity of water follows the Gibbs–Duhem equation:

3. Reactive absorption of CO₂ in an NaOH/NH₃/H₂O system

When CO₂ is absorbed in an NaOH/NH₃/H₂O solution, the following chemical reaction equilibria in the liquid phase occur: the protonation of ammonia (R1), the formation of bicarbonate (R2), the dissociation of bicarbonate to carbonate (R3), the formation of carbamate from ammonia and bicarbonate (R4), the autoprotolysis of water (R5), and the dissolution of NaOH (Brettschneider et al., 2004; Ghaemi et al., 2011).

The model of the present system consists of the flowing equations:

1-       Mass balances of the electrolytes:

2-      Charge balance in the solution:

3-      Equilibria of chemical reactions:

In the liquid phase, there are ten molecular and ionic species, including NH₃, CO₂, H₂O, Na⁺, NH₄⁺, HCO₃⁻, CO₃²⁻, H⁺, OH⁻, and NH₂COO⁻. The activity coefficients of these species were obtained using Equations 5 and 6. The temperature dependence of dissociation equilibrium constants are determined using the correlations reported elsewhere (Ghaemi et al., 2011c).

4. Absorption pilot plant

The schematic of the applied absorption pilot plant is shown in Figure 1. The column is made of a glass cylinder with a diameter of 105 mm containing three packing sections. A tray is mounted between these sections to redistribute the liquid flow. On each tray, two temperature sensors are installed to measure gas and liquid temperatures. Liquid and gas samples can also be taken using two valves, which are installed on the trays. The packing beads are ceramic Rasching rings with a diameter of 0.5 inches. Gas and liquid samples are taken at four points of the column as shown in Figure 1. Temperatures are measured at four positions along the column to give temperature profiles for gas and liquid phases.

Figure 1

a) An image of the absorption pilot plant used in this work, and b) a schematic of the pilot plant.

5. Experiments

The experimental data were obtained in steady state conditions at atmospheric pressure. At the beginning of the experiments, only nitrogen gas was fed to the column, and then liquid and the other gases feed flow rates and temperatures were adjusted to the desired values by four flowmeters. After reaching a steady-state condition, which was indicated by a steady-state temperature profile and fixed concentration of the analyzer, the gas and liquid samples were taken. Duplicate liquid and gas samples were taken with a time interval of about 10 minutes. Gas samples were sent to an online gas analyzer and liquid samples were analyzed offline.

The pilot plant experiments have been carried out in different conditions in terms of inlet liquid phase concentrations. Liquid flow rate was 80 lit/hr. for all of the experiments. The inlet and outlet conditions of the absorption column are given in Table 1.

For testing the repeatability of the conducted experimental runs, one experimental run was repeated for three times, and a standard deviation equal to 0.0015 was obtained for CO₂ mole fraction at top of the column (gas outlet), which indicates satisfactory repeatability.

Table 1

Operating conditions of the absorption column in two different experiments.

6. Measurements

The liquid samples were analyzed offline with an ion chromatography (IC-762 type from Metrohm Company) based on the conductivity detection method. As mentioned above, the results are averages of the two samples taken in the same condition but at different times. Here, the average deviation for the determination of CO₂ and NH₃ was given in the range of 4–6%. The online gas analyzer is a MCS100 from SICKMAIHAK Company, which is an extremely compact multi-component infrared photometer for extractive continuous monitoring of flue gases. In this process, H₂O, NH₃, and CO₂ concentrations were measured in the gas phase. The response time of MCS100 analyzer is less than 2 seconds with average measurement deviation of less than 2% for NH₃ and CO₂.

7. Results and discussion

The experiments were carried out in a steady state condition. Table 2 shows temperatures and component concentrations of both liquid and gas phases. The solubility of carbon dioxide in aqueous solutions is very different. The experiments were conducted to study the impacts of liquid feed concentration and liquid and gas temperature on CO₂ absorption. The results of Table 2 reveal that the absorption of CO₂ increases as a result of a decrease in the liquid feed temperature. The experiments showed that increasing NaOH concentration in the inlet liquid, increases CO₂ absorption during the column height. It is clear that at a high concentration of NaOH in the liquid phase, the concentration of hydroxide ion is high, so reaction rate between CO₂ and hydroxide will increase. Therefore, as Table 2 shows the absorption rate is increased by increasing NaOH concentration.

Table 2

Temperatures and component concentrations of pilot plant experiment.

Figure 2

Gas phase mole fractions along the column height.

Figure 2 shows component mole fractions in gas phase along the column height for Experiment 1. CO₂ is gradually transferred to the liquid phase throughout the column, while water and ammonia are transferred from liquid to gas phase. As a result, nitrogen partial pressure in the gas phase is decreased. Water vapor concentration curve in Figure 2 shows that there is no water in the gas feed although when the gas phase enters the column, its water content starts to rise due to the mass transfer from the liquid phase to the gas phase. Since the sum of all mole fractions equals 1, nitrogen concentration is descending at the bottom of the column and ascending at other sections as a result of the variations in the concentrations of the other components in the gas phase.

Figure 3

Ionic and molecular concentration of carbon dioxide species along the column in Experiment 1.

In the liquid phase, there are molecular and ionic species, and the compositions are obtained using the equations of electrolyte systems. Figures 3 and 4 show molecular and ionic carbon dioxide species concentrations. According to these figures, when liquid move from top to bottom of the column, molecular CO₂ and HCO₃^- concentrations increase duo to an increase in CO₂ absorption from gas phase. On the other hand, CO3^-2 and NH₂COO concentrations decrease due to chemical reactions. From z=0.2 to z=0, CO₃^-2 and NH₂COO concentrations increase because of the decomposition of CO₂ and HCO₃^- species. In the electrolyte systems, the concentration of CO₂, because of chemical reactions, is low, so in this system, as show in Figure 4, the concentration of CO₂ is low.

Figure 4

Ionic and molecular concentration of carbon dioxide species along the column in Experiment 1.

Figure 5

Temperature gradients of liquid and gas phases along the column height in Experiment 2.

The absorption of CO₂ is exothermic, causing high variation in temperature of both phases in the first section of the column (0-0.2 m). In this section, reaction heat increases liquid phase temperature, and because liquid and gas phases are in close contact, the gas phase temperature is also increased. The temperature gradients for both phases are shown in Figure 6. Between the column height of 0.2 and 0.4 m, the temperature of liquid phase has a maximum, and in this point the absorption rate is high.

Figure 6

The acidity and basicity of solution along the column height in Experiment 2.

Figure 6 indicates the acidity and basicity of the liquid phase along the column height. Mixed ammonia and sodium hydroxide solution is a basic solution, and the concentration of OH^- is higher than that of H⁺. Therefore, the inlet solution is basic, and, due to the absorption of acidic gas (CO₂), the basicity of the solution decreases, which in turn leads to a lower rate of CO₂ absorption.

8. Conclusions

In this work, the reactive absorption of carbon dioxide in an aqueous solution of NH₃, H₂O, and NaOH was experimentally investigated. The electrolyte principles of aqueous solutions were used to calculate the concentrations of the ionic and molecular components in the liquid phase. The experiments were conducted to obtain the data employing a pilot plant packed bed absorption column. The results revealed that carbon dioxide absorption occurred gradually throughout the column. The data also indicated that carbon dioxide absorption rate strongly depended on the concentrations of free ammonia and hydroxide ion. Finally, the results showed that CO₂ absorption rate was increased by increasing sodium hydroxide concentration.

Nomenclature