CHAPTER 1: CO 2 CAPTURE
7. Conclusions and recommendations
7.1 Conclusions
CHAPTER SEVEN: CONCLUSIONS AND RECOMMENDATIONS
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–! Refractive index values of the solutions decrease upon increasing the temperature at any constant concentration but increase with the increase in the concentration of alkanolamines at any constant temperature.
In fact, by measuring the thermodynamic properties of a solution, the deviation of the solution from the ideality can be obtained. In addition, the molecular interactions as well as structural arrangement can be interpreted. To achieve these goals, excess molar volume, partial molar volume, partial molar volume at infinite dilution, and apparent molar volume were evaluated from the density experimental data. Also, the deviation in viscosity and refractive index from ideality were calculated in this study.
The values of VE for all the solutions were negative and absolute values of VE decrease with increase in temperature. The temperature dependent negative values of VE reveal that the volume shrinkage takes place when alkanolamine and ethanol are mixed. The value and sign of VE depend on which of the chemical, physical and geometrical factors in the mixture are predominant over the others. In this study, the absolute values of VE for the systems follow the order: MEA+ETOH>
MDEA+ETOH >DEA+ETOH based on the following interpretations:
–! The self- associated bonds of MEA molecules are weaker than that in DEA and MDEA, therefore, at certain temperature, the number of available MEA species to form a cross complex with ethanol is larger than DEA and MDEA. Consequently, the volume contraction in MEA+ETOH mixture is larger than that of MDEA+ETOH and DEA+ETOH.
–! The self-associated bonds for MDEA molecules are weaker than DEA due to the strict hindrance effect of the methyl group attached to the N atom of MDEA, therefore, the number of available MDEA species at the certain temperature is higher than DEA and hence, volume contraction is larger for MDEA+ETOH mixture than DEA+ETOH mixture.
–! Based on the geometrical effect, the smaller molecule of ethanol can better pack in the voids presents in large molecules of MDEA and DEA compared to MEA which leads to increase in the volume contraction as well as the absolute value of VE, hence, the volume
contraction in DEA+ETOH mixture is larger than that of MDEA+ETOH and MEA+ETOH.
–! The overall VE value of MEA+ETOH mixture is the most negative among all amines. This trend indicates that the chemical effect seems to be dominant over other factors in the presently investigated mixtures.
–! The value of VE becomes less negative with increase in temperature. When temperature increases the self- and cross-associated bonds of the components decrease and the value of VE become less negative.
Partial molar volumes at infinite dilution were also calculated from two different methods. Results indicate that the values of the partial molar volume at infinite dilution for presently studied mixtures calculated from equations 6-8 and 6-9 are in a good agreement with the values obtained from equations 6-10 and 6-11. All values of the partial molar volumes at infinite dilution are less than corresponding molar volumes of pure components due to the packing effect and formation of hydrogen bond between solvent and solute.
After measurement of the experimental data for densities, viscosities and refractive indices of the investigated binary mixtures at different temperatures, some correlations were used to correlate the experimental data.
Redlich-Kister type of equation was used to calculate the value of excess molar volume for the studied mixture. The results show that this equation can be precisely used to correlate the excess molar volume with the average absolute deviation less than 0.008 for all mixtures at different investigated temperatures.
Three different models including modified Graber equations (6 and 8 parameters) and Jouyban- Acree Model were applied to correlate the densities of the mixtures. The %AARD values obtained from the Modified Graber methods are very close for all three mixtures. Hence, it is expected that the concentration of the second component (x2), which is ethanol in this study, does not impact the density properties of the mixtures noticeably. Also, the %AARD of Jouyban−Acree Model are
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larger than Graber methods for MDEA+ETOH and DEA+ETOH, however, this amount is the lowest for MEA+ETOH mixture.
The viscosity deviations of the binary mixtures of MEA+ETOH, MDEA+ETOH and DEA+ETOH were measured in the temperature range of 293.15 to 333.15 K and correlated with the Redlich- Kister type of equation. The value of ∆^ for all the solutions are negative and increase with increase of temperature for all mixtures. The negative value of viscosity deviation is attributed to the weak interactions between unlike molecules. Also, the difference in the molecular size of the unlike components affects the sign and value of viscosity deviation. Hence, in this study, the smallest value of viscosity deviation corresponds to DEA+ETOH and the highest value is related to MEA+ETOH binary mixture.
The viscosities of the studied binary mixtures were also correlated using different models including McAllister Model, Jouyban-Acree Model, Herraez Model, and Redlich-Kister equation. Results show that % AARD of the presently used models for all the systems are among 0.1 to 2.1 mPa.s which indicate that all the models can estimate the experimental viscosity data satisfactorily.
Among these models the best results for present mixtures is attributed to the Redlich-Kister model with the lowest %AARD of 0.12 mPa.s for MEA+ETOH, 0.21 mPa.s for MDEA+ETOH and 1.24 mPa.s for DEA+ETOH solution. It should be mentioned that in comparison of different models, number of adjustable parameters should also be considered. Of course, correlations with high number of adjustable parameters can yield better results.
The experimental data for refractive indices of the studied mixtures were correlated using different models including Lorentz-Lorenz, Gladstone-Dale, Heller, Weiner, Arago-Biot, Eykman and Newton. All these correlations estimate the experimental data well within the acceptable %AARD ranged from 0.16 % to maximum 0.44 % for the MEA+ETOH mixture, 0.15 % to maximum 0.36
% for the MDEA+ETOH and, 0.44 % to maximum 0.91 % for the MEA+ETOH mixture. Further, the lowest deviation from experimental data was determined from the Weiner relation.
The modified Graber equations (6 and 8 parameters) and Jouyban-Acree model were also applied to correlate the experimental refractive indices of the studied mixtures. All the models for three
binary mixtures estimate the experimental refractive index values perfectly. The maximum deviation from the measured refractive index data is attributed to the Jouyban-Acree Model with
%AARD of 0.11 for MEA+ETOH, 0.24 for MDEA+ETOH and 0.27 for DEA+ETOH solution.
The lowest deviation from experimental data is attributed to the modified Graber equation (8 parameters) with %AARD of 0.04 for MEA+ETOH, 0.07 for MDEA+ETOH and 0.08 for DEA+ETOH solution.