To understand and optimize CO2 absorption in binary
amine systems, we experimentally and theoretically investigated CO2 absorption using typical amines and blended amines in a polypropylene
hollow-fiber membrane contactor. The amines studied were monoethanolamine
(MEA), diethanolamine (DEA), and N-methyldiethanolamine
(MDEA), and their aqueous blends of MEA/MDEA, DEA/2-amino-2-methyl-1-propanol
(AMP), and MDEA/piperazine (PZ). The predicted results, including
overall mass transfer coefficients and CO2 removal ratio,
agreed very well with those determined experimentally. For single
amines, the optimal concentration was around 30 wt % for MEA and 20
wt % for DEA. MDEA concentration had little effect on the overall
mass transfer coefficient. We optimized the formulation of blended
amines using theoretical analysis. The optimal compositions in MEA/MDEA,
DEA/AMP, and MDEA/PZ systems were respectively 30 wt % MEA, with MDEA
in proportions from 0.1 to 0.3; 15 wt % DEA, with AMP in proportions
from 0.5 to 0.8; and 20 wt % MDEA, with PZ in a proportion of 0.3.
To further understand the CO2 membrane absorption process,
we also analyzed individual mass transfer resistances as a function
of additive concentration in blended amines and the effects of liquid
velocity on the overall mass transfer coefficient. This shows that
CO2 absorption is controlled by the liquid side for DEA/AMP
blends and by combined liquid–gas phases for MEA/MDEA blends.
For MDEA/PZ blends, control of CO2 absorption is characterized
by a gradual transition from liquid side controlled to liquid–gas
combined controlled as the concentration of PZ increases.
Trace carbon dioxide (CO2) removal from an enclosed
space was studied experimentally and theoretically in a hollow-fiber
membrane contactor using monoethanolamine (MEA) solution. Changes
in trace CO2 removal performance with liquid flow rate,
gas flow rate, absorption temperature, and CO2 loading
were investigated individually in an apparatus under well-defined
and controlled experimental conditions. We developed a two-dimensional
(2-D) mathematical model to predict and further analyze the experimental
results. The modeling results agreed well with the experimental work.
Liquid flow rate positively influences trace CO2 removal,
but is not recommended for operation at a very high level. Increasing
the gas flow rate improves CO2 absorption flux at the cost
of reducing CO2 removal efficiency; the optimal gas flow
rate for the trade-off between CO2 removal efficiency and
absorption flux is presented. The optimal absorption temperatures
change with liquid CO2 loading for trace CO2 removal. CO2 removal efficiency is a decreasing function
of CO2 loading; the recommended CO2 loading
of MEA solution is below 0.35 mol CO2/mol MEA. To predict
the influence of membrane wetting on trace CO2 removal,
we also propose a model that incorporates membrane wetting. The minimum
breakthrough pressure are 0.38 and 0.0581 MPa for fresh membrane and
old membrane, respectively. Membrane wetting significantly deteriorates
trace CO2 membrane absorption performance, with CO2 removal efficiency decreasing suddenly once the membrane
is wetted.
Amino acid salts
have greater potential for CO2 capture
at high temperatures than typical amine-based absorbents because of
their low volatility, high absorption rate, and high oxidative stability.
The protonation constant (pK
a) of an amino
acid salt is crucial for CO2 capture, as it decreases with
increasing absorption temperature. However, published pK
a values of amino acid salts have usually been determined
at ambient temperatures. In this study, the pK
a values of 11 amino acid salts were determined in the temperature
range of 298–353 K using a potentiometric titration method.
The standard-state molar enthalpies (ΔH
m
0) and entropies
(ΔS
m
0) of the protonation reactions were also determined
by the van’t Hoff equation. It was found that sarcosine can
maintain a higher pK
a than the other amino
acids studied at high temperatures. We also found that the CO2 solubilities and overall mass-transfer coefficients of 5 m′ sarcosinate (moles of sarcosine per kilogram of
solution) at 333–353 K are higher than those of 30% MEA at
313–353 K. These results show that some possible benefits can
be produced from the use of sarcosine as a fast solvent for CO2 absorption at high temperatures. However, the pronotation
reaction of sarcosine is the least exothermic among those of all amino
acids studied. This could lead to a high regeneration energy consumption
in the sarcosinate-based CO2 capture process.
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