Solubilities of CO 2 in 1-Allyloxy-3-( 4-Nonylphenoxy )-2-Propanol

1-allyloxy-3-(4-nonylphenoxy)-2-propanol polyoxyethylene ethers (ANAPEs), a new type of absorbent, are polymeric surfactants with different adduct numbers. In this work, ANAPEs, including SN-10 with adduct number of 10 and SN-15 with adduct number of 15, were prepared for CO2 absorption using the isochoric saturation method. Densities of the ANAPEs at atmospheric pressure were measured by a 5.567 ± 0.004 cm3 pycnometer, which decreased with increased temperature. Solubility data of CO2 in ANAPEs were measured within the pressure range of 0 – 600.0 kPa and temperature range of 303.15 – 323.15 K at 10 K intervals and could be calculated on the basis of experimental data of p, xCO2 and bCO2. The solubility of CO2 in absorbents increased linearly with increasing pressure and decreased with increasing temperature at all the pressures. The solubility of CO2 in SN-15 is the highest at all temperatures, but almost the same with SN-10 at 303.15 K over pressures (p < 350kPa), which indicates physical dissolution process. Henry’s constants were determined from solubility data. With increasing temperature, Henry’s constants increased. Thermodynamics of CO2 absorption were calculated including enthalpy, entropy, and Gibbs energy. The absolute value of ΔsolH based on Hx of SN-15 is largest at 303.15 K and indicates stronger SN-15/CO2 interactions, consistent with solubility of CO2 based on Hx. The negative enthalpy demonstrated exothermic process, which means the dissolution of CO2 in ANAPEs is favourable enthalpically. The ΔsolG shows positive value.


Introduction
As one of the most abundant greenhouse gases, CO 2 has achieved great attention in academic and industrial areas.A number of separation technologies have been applied to capture CO 2 , and liquid absorption is the most widely used method. 1,27][18][19][20][21] Compared with ILs, the surfactants are cheap, low-toxic, and structurally tunable. 22In our previous work, we have investigated the solubility of CO 2 in a series of fatty amine polyethylene ethers (FAPEs) with various oxyethylene (EO) chain numbers at T = (303.15,313.15 and 323.15)K, and p = (100 to 550) kPa.The results demonstrated high solubility of CO 2 in FAPEs with high EO content.Currently, the surfactants systems used in CO 2 absorption are vacant and it is significant to develop and diversify CO 2 absorbents with different structures. 23allyloxy-3-(4-nonylphenoxy)-2-propanol polyoxyethylene ethers (ANAPEs) are polymeric surfactants with different adduct numbers.This work introduces a new type of absorbent and provides the thermodynamics in polymeric surfactants with different adduct numbers.The solubility of CO 2 in SN-10 and SN-15 were studied in a pressure range of 0 - 600 k Pa, and temperature range of 303.15 - 323.15 K. Henry constants were determined from the solubility data.Thermodynamics of CO 2 absorption were calculated, including enthalpy, entropy, and Gibbs energy.

Apparatus
The stainless steel apparatus was an upgraded version, based on our previous glass equipment, 24 as shown in Fig. 1.The apparatus included a CO 2 gas cylinder ( 1), two water baths (2, 5), a CO 2 gas reservoir (GR, 3) with magnetic stirrer, a CO 2 gas equilibrium cell (EC, 4) with magnetic stirrer, and two pressure transmitters (6, 7).The volumes of EC and GR were determined using the previous method 25 with the results of 141.61 cm 3 and 370.99 cm 3 , respectively.The temperatures of water baths were accurately controlled with a precision of ± 0.05 K.The pressures were monitored by pressure transmitter (Fujian Wide Plus Precision Instruments Co., Ltd, Wide Plus-8, 0 to 600.0 kPa, with an accuracy of 0.1 % full scale).

Methods
The CO 2 solubility was measured using the isochoric saturation method. 25The weights of the ANAPEs were measured using an electronic analytical balance (Mettler-Toledo AL204) with an uncertainty of 2 • 10 −4 g.Temperatures were controlled by two water baths.After vacuum drying for 24 h at 350 K, approximately 15 - 40 g ANAPEs were loaded into EC and degassed under vacuum at 343.15 K while stirring for 1 h.The entire system was controlled at a specified oven temperature using water baths, and evacuated to pressures p 1 for 1 h after cooling.The pressure then reached the scheduled value p 2 by feeding the CO 2 from gas cylinder into GR.By opening the valve between EC and GR, the CO 2 was brought into EC and absorbed by ANAPEs with magnetic stirring, which can facilitate CO 2 absorption.When the pressure of the EC reached equilibrium, which was normally after 4 h, the equilibrium pressures of CO 2 in EC and GR were recorded as p 3 and p 4 , respectively.The amount of absorbed CO 2 could then be calculated from the difference between the initial pressure of the GR and the final pressures of EC and GR.With the same procedure, the repeated experiments were fulfilled at the same equilibrium temperature.
3 Results and discussion

Density
The densities of the ANAPEs at atmospheric pressure were measured by a 5.567 ± 0.004 cm 3 pycnometer, which was calibrated in advance with double distilled water at 303.15 K.The results demonstrated that the densities of ANAPEs decreased when increasing the temperature, as shown in Table 1.CO 2 solubility in ANAPEs could be calculated based on the experimental data of p, x CO2 and b CO2 .Due to the low pressure of ANAPEs, its effect on CO 2 solubility could be neglected, and the gas phase was assigned to pure CO 2 .Therefore, the amount of absorbed CO 2 can be calculated by where n CO2 is amount of absorbed CO 2 in ANAPEs, n 0 is the initial amount of CO 2 in GR, n 1 and n 2 are equilibrium amounts of CO 2 in GR and EC, respectively.The values of n 0 , n 1 and n 2 were obtained from Soave-Redlich-Kwong (SRK) equation on the basis of experimental PVT data.The volume of liquid in EC was derived directly from the mass and density of ANAPEs at different temperatures, and the volume expansion of liquid in EC could be neglected.
The molar fraction (x CO2 ) and molality (b CO2 ) of CO 2 were obtained by the following equations: where n ANAPE is the amount of ANAPE and m ANAPE is the mass of ANAPE.
Fig. 2 shows the dependence of the CO 2 solubility on temperatures and pressures.It is evident that the solubility of CO 2 in SN-10 is positive linear with CO 2 pressure, and elevating temperature decreased the absorption of CO 2 .Fig. 3 shows that the dependence of CO 2 solubility on pressure in SN-15 was similar to that in SN-10, but SN-15 had a higher capacity of CO 2 than SN-10.All these phenomena demonstrated that the absorption process of CO 2 in ANAPEs might be merely physical. 26

Henry constants
Henry's constant, which is presented in terms of H x or H b , is the key parameter for designing the gas absorption process. 27In this work, the gas phase was assumed to comprise pure CO 2 and its pressure was relatively low, which means the fugacity of gas was approximately equal to equilibrium pressure of CO 2 . 28,29Therefore, Henry's constants of CO 2 in ANAPEs can be obtained from the experimental data of CO 2 solubility.As shown in Table 3, both H x and H b increased with the temperature, which means that the CO 2 solubility decreased with increasing temperature, as shown in Fig. 2. Additionally, Henry's constant H x of CO 2 in SN-15 is lower than that in SN-10 under the same temperature, which means SN-15 possesses a higher CO 2 capacity than SN-10.

Thermodynamics
Temperature had a significant effect on Henry's constant, and thus an empirical equation was used to describe this relationship as where B i is the optimized coefficient and can be obtained using a linear regression of multiple-variables calculation.
The values of B 0 - B 2 are listed in Table 4.The thermodynamic properties of the absorption process of CO 2 in ANAPEs can then be calculated from the following equations: ( where Δ sol G, Δ sol H, Δ sol S are the standard Gibbs energy, enthalpy and entropy changes under the pressure of 0.1 MPa, respectively.The changes in thermodynamic properties at 303.15 K and 0.1 MPa are listed in Table 5.
Δ sol H is related with the interaction between gas liquid, its negative value that the absorption of 2 is exothermic.According to the molecular points, Δ sol S represents the degree of order between solute and solvent. 30From Table 5, the absolute value of Δ sol H of SN-15 is larger than that of SN-10, which means that there is a stronger interaction between CO 2 and SN-15.Additionally, a higher negative value of SN-15 indicates its interaction with CO 2 .These account for the behaviour as shown in Fig. 3.

Table 1 -
Solubility data of CO 2 in ANAPEsTable2shows the solubility of CO 2 in ANAPEs within the pressure range of 0 - 600.0 kPa at temperature range 303.15 - 323.15 K.In Table2, p, x CO2 and b CO2 stand for CO 2 equilibrium pressure above the liquid absorbent, amount fraction of CO 2 in liquid phase, and molality of CO 2 in liquid phase, respectively.

Table 2 -
Solubility data of CO 2 in ANAPEs at different temperatures kg−1

Table 4 -
Values of coefficients B 0 , B 1 , and B 2 for the equation

Table 5 -
Calculated Gibbs energy, enthalpy and entropy of the solutions at 0.1 MPa and 303.15K Pa and a temperature range of 303.15 - 323.15 K using the isochoric saturation method.When increasing temperature, the solubility of CO 2 in absorbents decreased and Henry's constants increased.The absolute value of Δ sol H based on H x of SN-15 is the highest at 303.15 K and indicates stronger SN-15/CO 2 interactions, which is consistent with the solubility of CO 2 based on H x .The negative enthalpy demonstrated an exothermic process, which means the dissolution of CO 2 in ANAPEs is favourable enthalpically.The Δ sol G shows positive value.

Table 3 -
Experimentally determined Henry's constants (H x , based on amount fraction; H b , based on molarity) of CO 2 in solutions at various temperatures Solutions H x ⁄ Mpa H b ⁄ Mpa kg mol −1 -standard Gibbs energy of solution, kJ mol −1 Δ sol H -enthalpy of solution, kJ mol −1 Δ sol S -entropy of solution, J mol −1 K −1 H b -Henry's constants based on molarity, Mpa kg mol −1