University of Groningen
Carbon dioxide removal processes by alkanolamines in aqueous organic solvents
Hamborg, Espen Steinseth
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Publication date:
2011
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Hamborg, E. S. (2011). Carbon dioxide removal processes by alkanolamines in aqueous organic solvents
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Abstract
Acid gas removal by absorption has been used to separate carbon dioxide (CO2 ) and other
acid gases such as hydrogen sulfide (H2 S), sulfur dioxide (SO2 ), carbonyl sulfide (COS), carbon
disulfide (CS2 ), and mercaptans from natural gas, hydrogen, and other gas streams since the
1930s. [1, 2] The basic process covering this application, in which an acid gas is absorbed from
a gas stream into an aqueous solution of an (alkanol)amine, was patented as early as 1930 by
Bottoms [3]. The basic principles of this process are still very much like the ones used for acid
gas removal today; the untreated gas stream enters the absorber at the bottom of the column
where it is contacted with the solvent at ambient temperatures. The solvent flows from the
top countercurrently down the column where it gradually takes up more acid gas by a chemical
reaction, until it leaves the bottom of the column as a rich absorbent. The treated gas leaves
the top of the absorber to be used for other purposes or released to the atmosphere. The rich
solvent is heated in a heat exchanger before it is directed to the top of the desorber column
where the (alkanol)amine is regenerated with steam at elevated temperatures, in the range of
100 to 120◦ C. The acid gas is chemically released from the (alkanol)amine and flows up through
the desorber column together with evaporized water. The evaporized water is condensed from
the stripping gas in the overhead condenser, thus providing pure acid gas(es) which can be
used for other purposes or geologic sequestration. The regenerated and lean alkanol(amine) is
directed back to the top of the absorber via a heat exchanger and a cooler to reach ambient
temperatures.
The aforementioned process has been used for the removal of acid gases from medium to
high pressure gas streams since the 1930s primarily to reach a desired (market) gas composition
of the treated stream. The treated gas stream would usually be used in a process somewhere
else or in the case of natural gas used for energy production in any form. The removed acid
gas is used for other (commercial) purposes, vented, or stored as solid waste. An example of
commercial use of a removed acid gas is the use of removed CO2 from natural gas for enhanced
oil recovery. CO2 is then utilized as pressure support in order to maintain the pressure in the oil
reservoir for prolonged crude oil extraction. [1] With increased awareness of the consequences of
CO2 emissions to the atmosphere, the focus of removing CO2 from low pressure gas streams has
gained increased attention the last years. [4] The concept of removing CO2 from low pressure
gas streams, i.e. flue gases, was for the first time evaluated in the early 1990s. [5,6] In particular,
this applies to the removal of CO2 from the flue gases of fossil-fueled power plants.
Triethanolamine (TEA) was the first alkanolamine which became commercially available
and was used in early acid gas treating plants. Because of its low capacity, reactivity, and
poor stability, TEA has been largely displaced by monoethanolamine (MEA), diethanolamine
(DEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and piperazine (PZ) due
to commercial interests. Industrial processes have successfully been developed in the past where
the absorbent is usually based on (mixtures of) the aforementioned (alkanol)amines. [1] In the
vii
current work, efforts have been made in order to develop and understand the fundamentals
of; (1) a CO2 removal process based on alkanolamines in aqueous organic solvents, and (2)
gas-liquid desorption processes. These efforts are summarized below.
The dissociation constants of protonated 2-amino-2-ethyl-1,3-propanediol (AEPD),
2-amino-2-methyl-1-propanol (AMP), diethylmonoethanolamine (DEMEA), DIPA, dimethylmonoethanolamine (DMMEA), MDEA, MEA, 1-amino-2-propanol (MIPA), methylmonoethanolamine (MMEA), TEA, the first and the second dissociation constants of PZ
and hydroxyethylpiperazine (HEPZ), the second dissociation constant of β-alanine, taurine,
sarcosine, 6-aminohexanoic acid, DL-methionine, glycine, L-phenylalanine, L-proline, and
the third dissociation constants of L-glutamic acid and L-aspartic acid were determined by
electromotive force measurements from 293 to 353 K. The dissociation constants of protonated
triethylamine (TREA) were determined with the same technique from 293 to 333 K. In addition,
the dissociation constants of protonated MEA and MDEA were determined in methanol-water,
ethanol-water, and t-butanol-water solvents also with the same technique. The alcohol mole
fractions were ranging from 0.2 to 0.95 and the temperatures from 283 to 323 K, 283 to 333
K, and at 298.15 K, respective to the different solvents. The experimental results have been
reported with the standard state thermodynamic properties for the different compounds and
solvent compositions and compared to available literature values. The dissociation constants
and the thermodynamic properties of the (alkanol)amines and amino acids presented provide
information about the use of these compounds and solutions as possible absorbents for acid gas
removal.
Liquid phase mass transfer coefficients were measured in a controlled environment during
gas absorption into a liquid and gas desorption from a liquid in a batch operated stirred
tank reactor over a wide range of operating conditions. At identical operating conditions,
the mass transfer coefficients for absorption and desorption appeared to be the same within
the reported experimental uncertainty. The desorption mass transfer coefficients depend, in
the same manner as the absorption mass transfer coefficients, on the physico-chemical and
the dynamic properties of the system, and were thus related by the Sherwood, Reynolds, and
Schmidt numbers. Desorption mass transfer processes can be further described by the wellknown film theory, the penetration theory, the surface renewal theory, etc. in the same manner
as absorption mass transfer processes.
The chemical enhancement factors were measured in a controlled environment for absorption
and desorption mass transfer processes in aqueous 2.0 M MDEA solutions at temperatures of
298.15, 313.15, and 333.15 K and the loading of CO2 ranging from 0 to 0.8 in a batch operated
stirred tank reactor. At identical operating conditions, the chemical enhancements factor for
absorption and desorption also appeared to be the same within the reported experimental
uncertainty.
The forward and reverse kinetic rate parameters were determined for CO2 absorption and
desorption mass transfer processes in aqueous 2.0 M MDEA solutions at temperatures of 298.15,
313.15, and 333.15 K and the loading of CO2 ranging from 0 to 0.8. The derived kinetic rate
parameters were based on the results of experimental work in a controlled environment in a
batch operated stirred tank reactor. Within applied experimental conditions it was shown that;
(1) the forward and reverse kinetic rate parameters derived by an analytical relation based on the
Higbie penetration theory were within 25 % of those numerically derived by a system of partial
differential equations based on the Higbie penetration theory. The analytical relations were
based on reversible reactions of finite rate in solutions of different CO2 loading and diffusivities,
and (2) the reaction order of the forward reaction in solutions of different CO2 loading was close
to unity, and in agreement with the proposed reaction mechanism. Arrhenius type of equations
already developed for correlation of forward kinetic rate parameters were further modified in
order to sufficiently correlate reverse kinetic rate parameters. These types of equations thus
formed a tool for the correlation and prediction of reverse kinetic rate parameters for engineering
purposes. The experimentally determined forward and reverse kinetic rate parameters were
accordingly found to be related by an overall temperature dependent chemical equilibrium
constant.
Based on the experimental results described above, process concepts of using alkanolamines
in aqueous organic solvents were evaluated by process simulations using the Procede Process
Simulator. MDEA, methanol, and ethanol were chosen as the respective alkanolamine and
organic compounds, and some additional experimental values of the CO2 vapor liquid equilibria
in 3 kmol·m−3 MDEA were determined in methanol-water and ethanol-water solvents. The
available experimental results were implemented into the Procede Process Simulator. The
simulator was used to simulate a CO2 removal plant with 90 % CO2 removal from the flue
gas of a power plant based on available specifications of an 827 MWe pulverized coal fired
power plant. A solvent of 3 kmol·m−3 MDEA in aqueous methanol solution was considered for
conceptual purposes. The results indicatively showed a maximum decrease in the reboiler duty
of the desorber of about 7.5 % at methanol fractions of about 0.06 compared to alkanolamines
dissolved in purely aqueous solutions and a reboiler temperature decrease with increasing
methanol fractions. Further experimental results are, however, necessary in order to more
precisely simulate CO2 removal processes by alkanolamines in aqueous organic solvents.