In energy industry as petrochemicals and electricity generation, absorption technology is looked as the main one for CO₂ capture from flue gases. In this work, absorption solubility of CO₂ in aqueous solutions of four branched polyamines were determined at 313.15 K and 393.15 K using the constant-volume method combined with gas chromatography analysis. The relationship between molecular structure of these polyamines and absorption performance is discussed. It is found that the capture performance is affected by the number of amino groups, the carbon number between the amino groups and the chain length of polyamines. The corresponding values of CO₂ absorption heat were also estimated using the Gibbs-Helmholtz equation and were analyzed from the view of molecular conformation.

Keywords: CO₂ capture, absorption, branched polyamine, solubility

Global climate warming associated with increased emissions of CO₂ from anthropogenic sources have become one of the most critical worldwide issues of the current age. A multitude of technological advances^1–3 have been developed to reduce CO₂ emissions from combustion exhaust gases from petrochemicals and electricity generation. Nowadays amine-based aqueous solutions are the most commonly used absorbents for the absorption process.⁴ In this case, finding new solvents that offer highly cyclic absorption capacity is important. Compared with monoamine solvents, polyamines solvents ⁵ are expected to have lower volatility along with higher CO₂ loading capacity and mass transfer rate. For example, diethylenetriamine (DETA) entails a higher mass transfer rate, ⁶ higher cyclic capacity,^7–9 and significant lower heat of absorption ¹⁰ than monoethanolamine (MEA). Tetraethylenepentamine (TEPA) shows outstanding potential for CO₂ absorption, and it can maintain a high absorption rate as well as cyclic absorption capacity.¹¹ One mole TEPA removes three times more CO₂ per cycle than 1.0 mole MEA. Cyclic absorption capacity is the solubility difference between absorption and desorption. These investigations reveal that polyamines make a good prospect for chemical absorption. Singh et al showed that an increase in chain length between the amine and different functional groups, result in a decrease of absorption rate whereas, the absorption capacity was increased in most absorbents.^12,13 Machida et al.¹⁴ observed that alkyl chain length between two amines has an important role in CO₂ solubility.¹⁴ Although some fragment solubility data for polyamines have been reported in previous literature, the data for the polyamines in different structures are insufficient.

In this work, the vapor-liquid phase equilibria of CO₂ in aqueous solutions of four branched polyamines were measured at 313.15 K (absorption process) and 393.15 K (desorption process). Based on the solubility data, the corresponding absorption heats () were calculated to analyze the energy consumptions for CO₂ capture with these solutions.

Materials

CO₂ and N₂ with a volume fraction of 0.99999 were supplied by Millennium City Gas. The solvents used are listed in Table 1. The solution was prepared using ultrapure water, which was taken from the Center 120 FV-S ultrapure water machine. The resistivity of ultrapure water is 18.2 MΩ·cm at 298.15 K. All components were used without further purification.

Apparatus and procedure

The constant-volume method, combined with gas chromatography analysis was used to measure the differing levels of solubility. A detailed description of the vapor–liquid equilibrium apparatus can be obtained from previous literature.^15,16 It is comprised of two stainless steel tanks for buffer and reaction. Both tanks are equipped with temperature transducers (PT-100, Kunlunhaian Co.) and pressure transducers (JYB-KO-HAG, Kunlunhaian Co.). The accuracy of the temperature and pressure transducers is ±0.1 K and ±0.5%, respectively. In accordance with the Peng-Robinson (PR) equation, the precise amount of CO₂ in the gas phase was determined using its volume, pressure, and temperature.¹⁷ The principle of this method is to accomplish a known volume of gas with the known-volume polyamine solutions. The amount $n_{{\text{CO}}_{\text{2}}}^{\text{all}}$  of CO2 gas introduced into the reaction tank was determined by the change in the buffer tank, before and after injection. After equilibrium was achieved at a constant temperature, the amount of CO2 gas absorbed in the solutions $n_{{\text{CO}}_{\text{2}}}^{\text{l}}$  was equal to $n_{{\text{CO}}_{\text{2}}}^{\text{all}}$  subtracted by the amount of CO2 in the vapor phase $n_{{\text{CO}}_{\text{2}}}^{\text{g}}$ . The CO2 solubility , ${\alpha }_{{\text{co}}_{\text{2}}}$ , in the liquid phase was defined as $n_{{\text{CO}}_{\text{2}}}^{\text{l}}$  divided by the mole number or mass of amine. The experimental error of CO2 loading is ±8%.

At low partial pressure (<10 kPa), the pressure transducer error may have a greater impact on results. Therefore, the CO₂ partial pressure was obtained using gas chromatography (Agilent 7890). The gas phase is primarily composed of CO₂, with minor components of N₂, H₂O, and amine. The partial pressure of amine is small enough to be considered negligible. The partial pressure of water was calculated according to Raoult’s law. Prior to the introduction of CO₂ into the reaction tank, the partial pressure of N₂ was calculated by subtracting H₂O partial pressure from total pressure. Correspondent to the increase in CO₂ amount, the solution’s volume change remained negligible and the N₂ partial pressure was assumed constant. The apparatus was verified by determining the solution of CO₂ in 30 mass% MEA solution at T=313.15K and 393.15K. The results are compared with reported data.^18–20 Our results are agree with reported data.

CO₂ solubility levels in aqueous solutions of four branched polyamines at 313.15 K and 393.15 K were measured using the constant-volume method combined with gas chromatography analysis, and the results are shown in (Figures 1–3). The concentration of amines were set to be 2.5M, which correspond to about 25% - 40% in weight percentage, which is the usual range used in CO₂ absorption processes. The measured corresponding solubility levels are expressed in two forms, both involving the mole number of CO₂; in one form, it is dissolved in per mole of amine, and in the second, per kilogram of amine. The relationship between solubility and molecular structure Table 2 can be also seen in Figures 1–3.

1. Relationship between the solubility and molecular structure.

1. Effects of different types of amino groups: DMPDA and TMTMDA all have two amino groups, though these groups are different in type. Figure 1 demonstrates that as the number of tertiary amino groups increases, cyclic absorption capacity also increases in the high CO₂ partial pressure region. The reaction mechanisms for tertiary and primary amines differ with each other. Theoretically, one mole of CO₂ reacts with one mole of tertiary amines but with two moles of primary amines. In other words, tertiary amines have a higher CO₂ loading capacity than primary amines if CO₂ partial pressure is high enough. So at low temperatures, the absorption capacity can be enhanced by increasing the number of tertiary amino group. However, as shown by lines at 393.15K in Figure 1A, Figure 1b, the absorption capacity is reduced with the increasing number of tertiary amino groups at high temperatures, which means that bicarbonate-the product of tertiary amine is unstable and easily decomposes at high temperatures. DAMDPA and DMDPTA have three amino groups. Regarding DAMDPA and DMDPTA, the secondary and primary amines, respectively, of dipropylenetriamine are substituted by a tertiary amino group. So DAMDPA has two primary amino group and one tertiary amino group, while DMDPTA has one primary, one secondary and one tertiary amino group. The difference between them is a primary amino group compared with a secondary amino group. As shown in Figure 2. The solubility of CO₂ for DAMDPA is slightly higher than that for DMDPTA both at 313.15K and 393.15K in the lower pressure range. That means a primary amino group can absorb a little bit more CO₂ than a secondary amino group in the lower pressure range. Compare the results in Figure 2 with those in Figure 1, one can see that the difference of a tertiary amino group with a primary amino group is larger than the difference of a secondary amino group with a primary amino group.


Figure 1 Solubility of CO₂ in 2.5 mol·L^-1DMPDA, and TMTMDA solutions.

●: DMPDA 313.15 K; ○: DMPDA 393.15 K; ▲TMTMDA 313.15 K; △TMTMDA 393.15 K.



Figure 2 Solubility of CO₂ in 2.5 mol·L^-1 DAMDPA, and DMDPTA solutions.

● DAMDPA 313.15 K; ○ DAMDPA 393.15 K; ▲ DMDPTA 313.15 K; △ DMDPTA 393.15 K.

2. Effects of the number of amino groups: The solubility levels per mole of amine and per kilogram of amine for DMDPTA and DMPDA are presented in Figure 3A, Figure 3b, respectively. A similar conclusion can be obtained regarding the two subfigures: DMDPTA has higher absorption capacity than DMPDA in unit of mole of CO₂/mol of amine, whereas the capacity is relatively lower in unit of mole of CO₂/kg of amine.


Figure 3 Solubility of CO₂ in 2.5 mol·L^-1 DMDPTA and DMPDA solutions.

■ DMDPTA 313.15 K; □ DMDPTA 393.15K; ● DMPDA 313.15K; ○ DMPDA 393.15K.

3. CO₂ absorption heat: As one of the important properties, the absorption heat in the aqueous solution of amine can be estimated using the Gibbs-Helmholtz equation.²¹ According to the equation, the absorption heat with different amine aqueous solutions can be calculated from the corresponding solubility data. Results are presented in Figure 4. There is not data for TMTMDA because of no data at the same loading at two temperatures. Values of the absorption heat for DAMDPA, DMPDA and DMDPTA are between -75 and -65 kJ/mol. The comparison of DAMDPA and DMDPTA, offers the conclusion that the absorption heat decreases with the secondary amino group compared that with the primary amino group. When the loading goes larger, the absorption heat decreases first for DMPDA because of DMPDA has only two amino group, while DAMDPA and DMDPTA have three amino group.

Figure 4 Heats of absorption of CO₂ with aqueous solutions of amines.

▲: DAMDPA; △: DMPDA; ▼: DMDPTA.

CO₂ solubility in aqueous solutions of four branched polyamines are measured and compared with each other. The cyclic absorption capacity increases with an increase in the relative amount of tertiary amino groups in the amines, while primary and secondary amines have similar absorption capacity. For polyamine containing three or more amino groups, the situation is more complex due to impact of the steric hindrance. The high theoretical absorption capacity of tertiary amines, steric effects, and the instability of bicarbonates should be simultaneously accounted for the total results. As the number of amino groups increases, the ability to absorb CO₂ declines. The value of absorption heat is crucial for the energy consumption to desorb CO₂, and it decreases as alkyl groups on nitrogen atom increases.

This work was supported by National Science and Technology Support Program of China (No. 2015BAC04B01) and the National Natural Science Foundation of China 21276010 and 51373019).

The authors declare no conflict of interest.

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