The diffusivity coefficient of the amine in the aqueous amine solution

The assumption of pseudo-first order was evaluated by considering the two conditions given in Eqs. (7)(8). The Hatta number was calculated from Eq. (10) and is given in Appendix B.

First, as the Hatta number was at all time higher than 3, the condition given in Eq. (8) was fulfilled. Then, for the condition given in Eq. (9), the infinite enhancement factor, E, was set equal to 5 times the value of the Hatta number and the lowest acceptable diffusivity coefficient of the amine in the aqueous amine solution, Damine, solution, was calculated from Eq. (11). The Damine, solution for the solvents studied in this work is not known so this exercise also studies how sensitive the pseudo-first order assumption is for the choice of Damine, solution. Often, in the literature, Damine, solution is assumed similar to the Damine, solution measured for another amine. The lowest acceptable Damine, solution is shown in Fig. 4 and is compared to the diffusivity coefficient of 4 M MDEA in the MDEA solution, DMDEA, solution, experimentally determined by Snijder et al. (1993) in the temperature range 25–75 °C. The comparison shows that the lowest acceptable Damine, solution ranges from 4 to 6 times lower than DMDEA, solution. This indicates that the pseudo-first order assumption made in this work is valid as long as the Damine, solution for the blended amines studied in this work is not more than 4 times smaller than that of MDEA. A lower value can be considered unlikely since the experimentally determined Damine, solution for other amines like MEA and diethanolamine (DEA), with similar concentration/viscosities, varies less than two times the value of the DMDEA, solution (Snijder et al., 1993). Overall, it can be concluded that the kinetic experiments in this work were carried out under pseudo-first order conditions.

Fig. 4. The diffusivity coefficient of the amine in the aqueous amine solution, Damine,solution, as a function of the solution viscosity, μ. (Δ) Snijder et al. (1993); coloured lines, lowest acceptable Damine,solution as calculated from Eq. (11).

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