J . Chem. Eng. Data 1999, 44, 887-891
887
Equ ilibr iu m Con sta n t for Ca r ba m a te F or m a tion fr om
Mon oeth a n ola m in e a n d Its Rela tion sh ip w ith Tem p er a tu r e
Moh a m m ed Kh eir ed d in e Ar ou a , Abd elba k i Ben a m 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 carbamate from monoethanolamine was evaluated at various
temperatures of 298, 308, 318, and 328 K and ionic strengths up to 1.7 M. From the plot of log10 K versus
I0.5, the variation of the thermodynamical constant with temperature follows the relationship log10 K1 )
-0.934 + (0.671 × 103)K/T.
In tr od u ction
and subsequently the equilibrium constants. From the plot
of apparent equilibrium constant expressed in terms of
species concentration versus the square root of the ionic
strength, the equilibrium constants at infinite dilution at
various temperatures were estimated. In a related work,
Haji-Sulaiman et al. (1998) used these values of equilib-
rium constant for carbamate formation to analyze their
equilibrium data for CO2 in DEA and DEA + MDEA
mixtures as well as other data reported in the literature
using the modified Kent-Eisenberg model. Within reason-
able accuracy, they obtained excellent agreement between
experimental and predicted CO2 loading.
Removal of acid gases such as CO2 and H2S using
aqueous solutions of alkanolamines is an industrially
important process. The presence of the alkanolamine
enhances the removal of the acid gas components through
reactions in the liquid phase. The reactions of H2S with
an aqueous alkanolamine proceed according to parallel
acid-base reactions. However, CO2 reacts directly with a
primary or secondary alkanolamine to form carbamate,
which is one of the major reaction products. This step in
the reaction mechanism has been argued to be responsible
for the high rate of CO2 absorption by primary and
secondary alkanolamines as compared to those for tertiary
alkanolamines. This reaction limits the ultimate stoichio-
metric loading to 0.5 mol of CO2/mol of amine. At high CO2
partial pressures, hydrolysis of carbamate can occur to
yield free amines which would further react with additional
CO2 to give loadings higher than 0.5.
Despite the importance of the carbamate-forming reac-
tions, reliable values of the equilibrium constant for its
formation are not readily available in the literature. The
data on the apparent equilibrium constants for diethanol-
amine (DEA) and monoethanolamine (MEA) reported by
J ensen et al. (1954) and Chan and Danckwerts (1981) were
obtained over a very limited range of operating conditions
and could not be used in VLE models where thermody-
namic constants are generally required. Due to the scarcity
of this important data, it is a common approach among
investigators to consider the equilibrium constants as
adjustable parameters to be fitted together with other
interaction parameters to alkanolamine + CO2 VLE data.
The values of equilibrium constants generated in this
manner were found to give erroneous results when used
to determine the species concentrations such as that of
carbamate in the system (Haji-Sulaiman and Aroua,1996).
Recently, the authors (Aroua et al., 1997) applied the
technique developed by Haji Sulaiman et al. (1996) to study
the temperature dependence of the equilibrium constant
for the formation of carbamate from DEA. In this tech-
nique, equilibrium solutions of DEA + NaHCO3 with
different ionic strengths were titrated with NaOH. This
information, together with equations describing the equi-
librium of the system and mass and charge balances, was
used to evaluate the concentrations of the different species
The results to be discussed in this paper are an extension
of the earlier work which estimates the equilibrium con-
stant of carbamate formation from MEA. A relationship
on its variations with temperature is also proposed.
Exp er im en ta l Setu p a n d P r oced u r e
The chemicals used in the investigation were obtained
from Merck (98% monoethanolamine and 99% sodium
perchlorate monohydrate), May & Baker (99% sodium
bicarbonate), and Reagecon (standard 1.0 M aqueous
sodium hydroxide solution). All these chemicals were of p.a.
quality and were used as received.
A similar experimental procedure to that employed in
the previous work was used here (Aroua et al., 1997).
Equilibrium experiments were performed by adding a
predetermined amount of NaHCO3 to exactly 100 mL of
0.100 ( 0.005 M MEA solution. The concentration of the
MEA solution was checked by titration with a standard
0.1 M HCl solution. Experimental runs were conducted at
different ratios (0.5, 1.0, and 1.5 mol of NaHCO3 per mol
of MEA) of bicarbonate to total amine. The ionic strength
of the solution was varied by adding various amounts of
inert salt NaClO4 (0.0, 0.5, 1.0, and 1.5 M), and the system
was left to equilibrate at (298, 308, 318, and 328) ( 0.5 K
for about 24 h. A Labb-Line Orbit air bath shaker was used
to control the temperature of the reacting mixtures. Finally,
the equilibrated solution was titrated with 1.0 M NaOH
solution using a PC-controlled Metrohm 716 DMS Titrino
autotitrator which utilized the DET (Dynamic Equivalence-
point Titration) technique for the determination of the end
point from the first derivative of the titration curve. The
analysis requires less than 5 min to perform, and within
this short analysis period, the equilibrium of the system
was not expected to be disturbed. All determinations were
carried out in triplicate.
* To whom correspondence should be sent. Telephone: 3-7595292.
Fax: 3-7595319. E-mail: mzaki@fk.um.edu.my.
10.1021/je980290n CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/30/1999