For the present study it was assumed that the gas

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 [3], reported elsewhere.