6
92
J . Chem. Eng. Data 1997, 42, 692-696
Tem p er a tu r e Dep en d en cy of th e Equ ilibr iu m Con sta n t for th e
F or m a tion of Ca r ba m a te F r om Dieth a n ola m in e
Moh a m ed Kh eir ed d in e Ar ou a , Abd elba k i Ben Am or , a n d Moh d Za k i Ha ji-Su la im a n *
Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
The equilibrium constant for the formation of diethanolamine carbamate was determined experimentally
-
3
4
at (303, 313, 323, and 331) K for ionic strengths up to 1.8 mol dm , the inert electrolyte being NaClO .
A linear relationship was found to hold between log K and I0 . The thermodynamical constant has been
.5
3
determined and expressed by the equation log K
1
) -5.12 + 1.781 × 10 K/T.
In tr od u ction
Aqueous alkanolamine solutions are frequently used to
remove acidic components such as H S and CO from
process gas streams. Among the industrially important
alkanolamines which have been used for this purpose are
monoethanolamine (MEA), diethanolamine (DEA), and
methyldiethanolamine (MDEA) (Astarita et al., 1983).
When primary or secondary amines such as DEA or MEA
disturbed during the analysis. This method requires the
separation of the precipitate from the filtrate, which is a
rather tedious technique and gives poor reproducibility.
Recently Haji Sulaiman et al. (1986) developed another
technique based on titration with NaOH to determine
2
2
species concentration in CO
2
+ alkanoamine + water
systems. This technique is simple to perform and gives
reproducible results. The method is used in this work to
study the effects of temperature and ionic strength on the
equilibrium constant for the formation of carbamate from
DEA.
react with CO
a result, the maximum CO
is 0.5 mol of CO /mol of amine. However, at high CO
partial pressures carbamates may hydrolyze to generate
free amines which can further react with additional CO
2
, stable carabamates are usually formed. As
2
loading that can be attained
2
2
Exp er im en ta l Setu p a n d P r oced u r e
2
to give loadings higher than 0.5. The carbamate-forming
reaction is thus an important step in the reaction mecha-
nism especially for those involving primary and secondary
amines. The instability of these carbamate ions that are
formed in an aqueous system of tertiary amines has been
argued to be responsible for the high sorption capacity of
such solutions. For the same reason, sterically hindered
amine which has been designed to induce such instability
The chemicals used in the investigation were obtained
from Merck (98% diethanolamine and 99% sodium per-
chlorate monohydrate) and May & Baker (99% sodium
bicarbonate). All these compounds were of p.a. quality and
were used as received. Standard aqueous sodium hydrox-
-
3
ide (1.0 mol‚dm ) solution was provided by Reagecon.
Equilibrium experiments were carried out by adding a
3
2
(Sartory
predetermined amount of powder NaHCO
0.2 mol‚dm
3
to 100 cm of
showed excellent performance for removing CO
and Savage, 1983). Despite the importance of this car-
bamate-forming reaction for the absorption of CO in
-
3
DEA solution. Experimental runs were
performed at different ratios (0.5, 1.0, and 1.5 mol NaHCO
2
3
alkanolamine solutions, experimental data on the equilib-
rium constant for its formation is still scarce in the
literature. It is a common approach among investigators
to consider the equilibrium constant as an additional
parameter to be fitted along with the interaction param-
eters to the VLE data (Kent and Eisenberg, 1976; Desh-
mukh and Mather, 1981; Austgen et al., 1989; Haji-
Sulaiman and Aroua, 1986). This procedure is likely to
introduce significant errors in predicting the equilibrium
concentrations of bicarbonate and carbamate ions espe-
per mol DEA) of bicarbonate to total amine. In order to
vary the ionic strength of the solution, various amounts of
-
3
inert salt NaClO (0.0, 0.5, 1.0, and 1.5 mol‚dm ) were
4
added. The system was left to equilibrate at (303, 313, 323,
and 331) ( 0.5 K for about 24 h and finally titrated with
-
3
1.0 mol‚dm
NaOH solution using a PC controlled
Metrohm 716DMS Titrino autotitrator which utilized the
DET (Dynamic Equivalence-point Titration) technique for
the determination of the end point. In this technique the
volume increments are adopted to the slope of the titration
curve, thus providing accurate results in the shortest
possible time (less than 5 min). Due to the short analysis
time, the equilibrium of the system is not expected to be
disturbed (Haji-Sulaiman et al., 1986). The end points are
determined automatically from the first derivative of the
titration curve where the largest change in the solution
pH with the addition of an incremental volume of NaOH
is detected. All determinations were carried out in tripli-
cate, and the results were reproducible within 4% of NaOH
volume as shown by the data in Table 2.
cially at low CO
2
loading (Haji-Sulaiman et al., 1986).
The lack of experimental data on the equilibrium con-
stant of carbamate formation in the literature is mainly
attributed to the difficulty in measuring the carbamate ion
concentration accurately. Chan and Danckwert (1981)
have proposed an experimental technique to determine the
concentrations of carbamate ion and thus the equilibrium
constant for its formation. In this method, barium chloride
and an excess of sodium hydroxide are added to an aliquot
sample of the solution which precipitates carbonate and
bicarbonate but leaves carbamate in the solution. The
clear solution is further titrated by HCl for the final
determination of the carbamate ions. As claimed by the
investigators, the equilibrium of the system was not
Th eor y
When an aqueous solution of DEA is reacted with
bicarbonate, the carbamate is formed according to the
following reaction (reaction i)
*
Author to whom correspondence should be addressed.
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