For the present study, it was assumed that the gas phase was completely backmixed when the speed of agitation was more than the critical speed of agitation. Some exberiments were, however, carried out where the effect of resi- dence tilme distribution of the gas on the cal- culation of a is important. Lean carbon dioxide (carrier gas-air) was absorbed in aqueous solu- tions of M.E.A., D.E.A. and D.I.P.A. dissolved in D.E.G.-water mixtures. The physical proper- ties of the solutions were the same. The values of a for the three alkanolamines are reported in Table 2. It can be observed that when a is based on log mean partial pressure of the solute gas, that is, plug flow model, the value of a decreases as the reaction rate constant of the amine increases (khl.E.A. > k,.,.,. > &.,?A.). However, when the gas phase is assumed to be completely backmixed and the driving force is based on the outlet partial pressure of the solute gas, then the values of a for the various systems are found to be practically the same. To have a further check that the gas phase is completely backmixed, the value of a was obtained by the air oxidation of sodium dithionite in D.E.G.-water mixture, having practically the same physical properties as that of the alkanolamines. For the oxidation of dithionite, as the change in partial pressure of oxygen is insignificant, the residence time distri- bution will have practically no effect on the value Table 2. Residence time distribution of gas Mode of operation: continuous. Mode of gas dispersion: sparged contacting. T = 20 cm, D = 8 cm, B/T = 0.34 and H/T = 1. Gas flow rate, V, = 2.50 cmlsec. System: CO, (carrier gas-air) - Aqueous solutions of alkanolamines a (cm-‘) based on Solute gas CO, CO, CO, No. Absorbent M.E.A.-D.E.G.- water D.E.A.-D.E.G.- water D.I.P.A.-D.E.G.- water M.E.A.-D.E.G.- water D.E.A.-D.E.C.- water D.I.P.A.-D.E.G.- water Na&O,-D.E.G.- NaOH-water 0.139 1.58 1.156 0.023 0.064 5.10 750 1.37 3.82 1.87 1.526 0.139 1.29 0.875 0.139 1.58 1.140 0.139 0448 0.076 0.0177 0.036 0.086 5.02 750 0.105 4.87 750 0.059 5.10 950 2.00 3.58 2.65 3.64 1.50 5.00 2.38 5.08 1.87 1.486 0.139 0.076 5.02 950 1.29 0.820 0.139 0.119 0.0862 0.210 0.067 0.177 0.099 4.87 950 3.36 4.85 0.199 4.58 750 3.47t 39Of tFor the sake of comparison. 467
v. D. MEHTA and M. M. SHARMA of a. Run No. 7 in Table 2 for the oxidation of dithionite indicates the value of a to be 3.90 cm, which is about the same as that for the absorp- tion of lean carbon dioxide in alkanolamine solutions, based on backmixed model for the gas phase. This clearly indicates that the gas phase is almost perfectly backmixed when the speed of agitation is more than the minimum speed of agitation, no (found from Eq. (1)). Table 2 also indicates that at the same volu- metric flow rate of the gas and the same partial pressure of the solute gas, although the outlet partial pressure of the solute gas changes by a factor of three, the value of a based on the completely backmixed model, remains prac- tically the same. This observation contradicts the reported findings of Linek and Mayrho- ferova , reported elsewhere.