Note: Descriptions are shown in the official language in which they were submitted.
13407~~
TERTIARY ALKANOLAMINE ABSORBENT CONTAINING
AN ETHYLENEAMINE PROMOTER AND ITS METHOD OF USE
CKGROUND OF THE INVENTION
For many years, carbon dioxide has been removed from
gaseous mixtures with various absorbent liquids.
Alkali metal. salts such as carbonates, phosphates,
borates, and phenates of sodium and potassium are one
category of absorbent liquid. The carbon dioxide
absorption rates of such salts is, however, rather low,
and, therefore, it has been necessary to add promoting
agents to thesN salts. British Patent No. 798,856 to
S.P.A. Vetrocoke discloses that an inorganic or organic
compound of trivalent arsenic is useful in activating
such salts. :In Astarita et al.'s "Promotion of C02
Mass Transfer In Carbonate Solutions", Chemical
Enc~ineerina Science, Vol. 36, pp. 581-88 (1981.), it is
mentioned that arsenious acid, ethanolamines, and amino
acids promote the absorption of carbon dioxide by
carbonate-bicarbonate salts. U.S. Patent No. 4,094,957
to Sartori et al., U.S. Patent No. 4,112,052 to Sartori
et al., U.S. Patent. No. 4,217,237 to Sartori et al.,
U.S. Patent No. 4,405,577 to Sartori et al., U.S. Patent
No. 4,405,578 to Sartori et al., U.S. Patent No.
4,405,579 to Sartori et al., and Sartori et al.'s
"Sterically Hindered Amines for C02 Removal from
Gases", ~ndustr~ial Engineering Chemical Fundamentals,
Vol. 22, pp. 239-49 (1983) ("Sartori article") all
disclose activating a basic salt for removing carbon
dioxide from gaseous mixtures with sterically hindered
1340~~8
-2-
amines or amino acids (i.e. a primary amine in which
the amino group is attached to a tertiary carbon atom
or a secondary amine in which the amino group is
attached to a secondary or tertiary carbon atom).
Alkanolamine:~ in aqueous solution are another class
of absorbent liquid for removal of carbon dioxide from
gaseous mixturEa. Alkanolamines are classified as pri-
mary, secondary,, or tertiary depending on the number of
non-hydrogen substituents bonded to the nitrogen atom of
the amino group. Monoethanolamine (HOCH2CH2NH2) is an
example of a well-known primary alkanolamine. Con-
ventionally used secondary alkanolamines include
diethanolamine ((HOCH2CH2)2NH) and diisopropanol
amine ((CH3CHOHCH3)2NH). Triethanolamine
((HOCH2CH2)3N) and methyldiethanolamine
((HOCH2CH2)2NCH3) are examples of tertiary
alkanolamines which have been used to absorb carbon
dioxide from industrial gas mixtures. These
alkanolamines are not only useful in absorbing carbon
dioxide, but they have also been employed to absorb
hydrogen sulfide or carbonyl sulfide from gas mixtures
which may or ma~~ not contain carbon dioxide.
After absorption of carbon dioxide and/or hydrogen
sulfide and/or carbonyl sulfide in an alkanolamine
solution, the :solution is regenerated to remove absorbed
gases. The regenerated alkanolamine solution can then
be recycled for further absorption. Absorption and
regeneration are usually carried out in different
separatory columns containing packing or bubble plates
for efficient operation. Regeneration is generally
achieved in 2 stages. First, the absorbent solution's
pressure is reduced so that absorbed carbon dioxide is
vaporized from the solution in one or more flash
regenerating columns. Next, the flashed absorbent is
I3~0~6~
-3-
stripped with steam in a stripping regenerating column
to remove residual absorbed carbon dioxide. With
primary and secondary alkanolamines, the nitrogen reacts
rapidly and directly with carbon dioxide to bring the
carbon dioxide into solution according to the following
reaction sequence:
2RNH2 ~~ C02 ,~ RNHCOO- + RNH3
where R is an alkanol group. To obtain concentrations
of carbon dioxide in solution which are greater than 0.5
mole of carbon dioxide per mole of alkanolamine, a
portion of the' carbamate reaction product (RNHC00-)
must be hydro7.yzed to bicarbonate (HC03-) according
to the following reaction:
RNHCOO~ + H20 ~ RNH2 + HC03-
There is a characteristic equilibrium between the
carbamate (RNHCOO-) and bicarbonate (HC03-) ions
for each alkanolamine which determines the vapor-liquid
equilibrium or solution phase concentration of carbon
dioxide for any gi en gas phase pressure of carbon
dioxide. The alkanol substituent groups R which are
attached to the nitrogen atom of any alkanolamine affect
the basicity of the alkanolamine and its reactivity
toward and vapor-liquid equilibrium with carbon dioxide.
In formin~~ a carbamate, primary and secondary
alkanolamines ~sndergo a fast direct reaction with carbon
dioxide which makes the rate of carbon dioxide absorp-
tion rapid. However, considerable heat is required to
break the bond between the alkanolamine and carbon
dioxide in the carbamate and regenerate the absorbent.
In addition, primary and secondary alkanolamines have a
limited capacity to absorb carbon dioxide due to the
formation of atable carbamates. The Sartori article
1340~~8
-4-
teaches that loading of such alkanolamines is improved
by incorporating sterically hindered amines. Meanwhile,
British Patent No. ?98,856 activates primary alkanol-
amines, like ethanolamine, with arsenious oxide.
Unlike primary and secondary alkanolamines, tertiary
alkanolamines cannot react directly with carbon dioxide,
because their amine reaction site is fully substituted
with substituent groups. Instead, carbon dioxide is
absorbed into solution by the following slow reaction
with water to form bicarbonate:
R3N + C02 + 1H20 ~ HC03- + R3NH+
Because tertiary alkanolamines do not bond with carbon
dioxide, they can be economically regenerated often by
simply reducing pressure in the system (i.e. flash
regenerating). little or no thermal regeneration is
required. Although the absence of a direct reaction
with carbon dioxide makes regeneration of tertiary
alkanolamines more economical, large solvent circulation
rates and high liquid to gas ratios (i.e. high liquid
loadings) in tlae absorber are required due to the slow
absorption of .carbon dioxide. Consequently, systems
utilizing tertiary alkanolamines require absorption
columns of increased height and diameter compared to
systems employing either primary or secondary
alkanolamines.
In order t.o increase the rate of carbon dioxide
absorption by aqueous tertiary alkanolamine solutions,
promoters have been added. In U.S. Patent No. 4,336,233
to Appl et al ("Appl patent"), a piperazine promoter is
incorporated in an aqueous methyldiethanolamine
solution. The process disclosed by the Appl patent
"employs aqueous solutions of a bottom product obtained
as a by-product from the synthesis of ethylenediamine
134Q7~~
_5-
from monoethanolamine and ammonia. this material also
contains 0.3 percent by weight, based on piperazine of
the following by-products: NH3, ethylenediamine, MEA,
and further nitrogen-containing products." The by-
I
products are merely said to "not interfere with the
process according to" the Appl patent.
Promoted met.hyldiethanolamine solutions have both an
increased rate of carbon dioxide absorption and an
increased capacity for carbon dioxide compared to
unpromoted methyldiethanolamine. Improved rate of
absorption is particularly evident at low levels of
carbon dioxide loading and diminishes as such loading
increases. The full benefit of the promoter is found in
processes which employ thermal regeneration in addition
to flash regeneration of the absorbent to maintain the
low loading levels necessary to produce a gas product
with low levels of carbon dioxide. Flash regeneration
alone is sufficient for bulk removal of carbon dioxide
from high pressure gases (i.e. carbon dioxide partial
pressure greater than 50 psia) where low carbon dioxide
specifications in the product gas are not needed.
DESCRIPTION OF THE INVENTION
The absorption of carbon dioxide from gas mixtures
with aqueous absorbent solutions of tertiary alkanol-
amines is improved by incorporating at least one
alkyleneamine promoter in the solution. The alkylene-
amine is incorporated in an amount sufficient to enhance
substantially 'the carbon dioxide absorption rate by at
least 10%, usually by 25-200 % and/or the capacity of
the tertiary alkanolamine in water by as much as 70%.
The tertiary alkanolamines utilized can be any of a
1340'~~~
-6-
variety of compounds suitable for absorbing carbon
dioxide from ~gas mixtures. Examples of these absorbent
alkanolamines include: methyldiethanolamine, triethanol-
amine, dimethylethanolamine, diethylethanolamine,
methyldiisopropanolamine, and mixtures thereof. Methyl-
diethanolamine and triethanolamine are the preferred
tertiary alkanolamines with methyldiethanolamine being
most preferred.
The promoter utilized by the present invention is
generally at least one alkyleneamine defined by the
formula:
H2N(CxH2xNH)nH
wherein x is 1 to 4, while n is 1 to 12. More
preferably, the present promotion additive is at least
one ethyleneamine or propyleneamine selected from a
homologous series of polyamines, all of which contain
two primary amino groups with other amino groups, if
any, being secondary. Ethyleneamines which are useful
in the present invention are defined by the formula:
H2N(C2H4NH)nH
where n is 1 to 12. Specific examples of such ethylene-
amines include ethylenediamine, diethylenetriamine,
tetraethylenepentamine, and mixtures thereof. Of these
promoters, ethylenediamine is most preferred.
The alkyleneami.ne-promoted tertiary alkanolamine
absorbents may additionally contain conventional
corrosion inhibitors to prevent corrosion caused by
carbon dioxide absorbed in the tertiary alkanolamine
absorbent solution. When corrosion in the process is
controlled, higher alkanolamine strengths can be
utilized to reduce absorbent circulation rates,
equipment size, and operating costs.
I34~~~8
_7_
The alkyleneamine-promoted tertiary alkanolamine
absorbents of the present invention can be prepared as
an aqueous solution which is ready to be used in the
field. In ready-to-use form, the aqueous absorbent
solution may contain 35-65 wt.% water, preferably 45-55
wt.% water, anc9 most preferably 50 wt.% water. The
alkyleneamine promoter in the promotion additive is
present in an amount sufficient to enhance the rate at
which cool (to 100-130'F) aqueous tertiary alkanolamines
absorb carbon dioxide from warm (150-200'F) gas mixtures
in a gas-liquid contact apparatus. Such enhancement in
the rate of carbon dioxide absorption is achieved when
the alkyleneamine- promoted tertiary alkanolamine
achieves at least 10%, usually 25-200%, faster carbon
dioxide removal than the same absorbent without the
alkyleneamine. This generally occurs when the
alkyleneamine is 0.5-10 wt.% of the aqueous absorbent
solution, preferably 2- .6 wt.%, and most preferably 3-4
wt.%. An effective amount of the corrosion inhibitor,
if any, for purposes of the present invention is about
0.05-0.1 wt.% of the aqueous absorbent solution. The
tertiary alkanolamine comprises the remainder of the
absorbent solution.
Alternatively, the alkyleneamine-promoted tertiary
alkanolamine can be prepared as a concentrate, shipped
to the facility where it is to be utilized, and then
diluted with water prior to use. In its concentrated,
non-aqueous fo:-m, the absorbent contains 1.0-20 wt.% of
alkyleneamine promoter, preferably 4-12 wt.%, and most
preferably 5 t:o 8 wt.%. The tertiary alkanolamine con-
stitutes the remainder of the concentrated absorbent.
The mole ratio of alkyleneamine to alkanolamine is 0.01
to 0.5, preferably 0.05 to 0.27, most preferably 0.07 to
0.17.
-g-
In use, the aqueous ethyleneamine-promoted alkanol-
amine solution (i.e. lean absorbent) at temperatures of
120°F to 200°F is contacted with an industrial gas
containing carbon dioxide at partial pressures of 25
sia to 250 psia or higher in an absorption zone and
P
both a product gas substantially free of carbon dioxide
(i.e. 0.01 volume % to 0.1 volume % for trim gas removal
and 3-5 volume % for bulk gas removal) and an aqueous
alkyleneamine-promoted alkanolamine solution laden with
absorbed carbon dioxide and optionally absorbed hydrogen
sulfide or carbonyl sulfide (i.e. rich absorbent) are
withdrawn from the absorption zone. In addition, the
aqueous alkyleneamine-promoted alkanolamine can be used
to treat gases with hydrogen sulfide and/or carbonyl
sulfide containing little or no carbon dioxide. The
absorption zone is preferably a gas-liquid contact
column containing bubble plates or packing to improve
absorption.
To conserve absorbent, the rich absorbent is
regenerated in at least one regenerating column and then
recycled to the absorption column. Such regeneration is
achieved by first removing absorbed carbon dioxide from
the rich absorbent by reducing the absorbent's pressure
in at least one flash regenerating column (i.e. flash
regenerating) t:o pressures as low as 7 psia and as high
20 psia and i=hen stripping the flashed absorbent with
steam in a strapping regenerator column operating at 235
to 265°F to remove absorbed carbon dioxide to a residual
loading of approximately 0.005 to 0.05, preferably 0.01
moles of carbon dioxide per mole of alkanolamine. The
flash regenerating column and the stripping regenerator
column both contain either packing or bubble plates for
more efficient regeneration. The carbon dioxide-
containing gas. produced in each regenerating column can
,.._.
i34o7ss
_g-
either be removed from the absorption/regeneration
installation separately or the gas generated by the
stripping regenerator column can be passed into the
flash regenerating column and removed with the gas
generated therein. Alternatively, regeneration can in
some cases be ca~~ried out with just a flash regenerating
step due to the ease with which tertiary alkanolamines
are regenerated.
A wide variety of industrial gases (e. g. fuel gases,
gasification product gases, refinery gases, synthesis
gases, natural gas, and enhanced oil recover associated
gases) can be treated with alkyleneamine promoted
tertiary alkanolamine absorbent solutions utilizing the
above-described absorption/regeneration scheme. These
gases can include compounds in the range of amounts
shown in Table 1.
TABLE
Compound mount (Volume percent)
C02 10-40%
CO 20-40%
0-80%
C 0.1-90%
N2H2n + n(n=2-6)
0-20%
H2S 0-0.1%
COS
Synthesis gas containing the compounds in the amounts
shown below in Table 2 is particularly well suited to
treatment with absorbent solutions in accordance with
present invention.
TABLE 2_
Co~~a mou t (Volume percent)
C02 15-35%
CO 20-40%
H2 20-40%
0.1-5%
N2 0-20%
134078
-10-
2 0-3%
COS 0-0.1%
Typically, contacting such gas with absorbents in
accordance with the present invention reduces the carbon
dioxide content: in the gas by between 75 and 100 vol.%
to yield gases of less than 3 vol.% carbon dioxide. The
alkyleneamine-promoted tertiary alkanolamine also re-
duces the hydrogen sulfide content in the gas by between
99.5 and 100 vol.% to less than 0.0004 vol.% and reduces
the carbonyl sulfide content in the gas by 50-90 vol.%
to less than 0.001 vol.%.
ERIE;F DESCRIPTION OF THE DRAWINGS
Figure 1 is. a process flow diagram for an absozption
and regeneration process utilizing a tertiary alkanol-
amine absorbent containing an alkyleneamine promoter
according to the present invention.
Figure 2 is a graph showing vapor-liquid equilibrium
data and rate of absarption data for absorbent solutions
with 50 wt% inethyldiethanolamine alone or with 3
wt% piperazine, ethylenediamine (i.e. EDA), or die-
thylenetriamine (i.e. DETA).
AILED DESCRIPTION OF THE DRAWING
In a preferred embodiment, as shown in the process
flow diagram of Figure 1, carbon dioxide is removed in
two absorption columns connected fn series. Synthesis
gas 8 is first directed into bulk C02 absorber 2 where
it is contacted countercurrently with lean bulk
absorbent ~. ~r bulk absorber product gas ~ from which
65 to 85% of the carbon dioxide in the synthesis gas
X340768
-11-
has been removed is withdrawn from bulk C02 absorber 2
as an overhead product, while rich bulk absorbent p is
withdrawn as a bottom product. The bulk absorbent
product gas ~ is then conveyed into a lower portion of
trim C02 absorber 4 and countercurrently contacted
with lean trim, absorbent F which removes substantially
all the remaining carbon dioxide. A substantially
C02- free trial absorber product gas ~ is removed from
trim C02 absorber 4 as an overhead product, while rich
trim absorbent Ci is withdrawn as a bottom product.
Regeneration of the rich absorbent streams is also
carried out in two columns. Rich bulk absorbent _D
undergoes a pressure drop through turbine 6 and then
passes into flash regeneration column 8 having a top
section 8A and a bottom section 8B. Gases g are
conveyed by vacuum compressor 9 from bottom section 8B
to top section 8A, _while liquid O flows from top section
8A to bottom aection 8B by gravity. In flash regenera-
tion column 8 which operates at pressure of 7 to 20
psia, some of the absorbed C02 flashes and is removed
from flash regeneration column 8 as an overhead product
Product ~ then passes through condenser 20 which is
cooled (to 100-130°F) with water to permit non-
condensibles to be separated and removed from reflux
drum 10 as a gaseous product T, while condensibles ~C are
withdrawn as a liquid product. Flash regenerated
absorbent ~i i.s withdrawn from flash regeneration column
8 as a bottom product which can either be combined with
rich trim absorbent ~ and directed by turbine 6 to bulk
C02 absorber 2 as lean bulk absorbent $ or be conveyed
by pump 12 t;o stripping regeneration column 14. In
stripping regeneration column 14, residual carbon
dioxide is rE:moved as stripping regeneration overhead
product ~I which is then recycled to flash regeneration
13~O~1G~
-12-
column 8 from which carbon dioxide is removed from the
system. Such recycling enhances the flashing of carbon
dioxide in flash regeneration column 8. Stripping
regeneration bottoms product ~1 is either conveyed
through steam reboiler 16 and returned to stripping
regeneration column 14 or combined with make-up water ~
and condensate ~C and recycled by pump 18 to trim Co2
absorber 4 as lean trim absorbent
To reduce process energy requirements and improve
absorption effic;iency, a portion of stripping regenera-
tion bottoms product ~1 is cooled from about 250°F to
170°F by incoming flash regenerated absorbent ~i in heat
exhanger 22. That portion of bottoms product ~1 which
becomes lean trim absorbent ~ is further cooled (to
100-130°F) with water in cooler 24. As a result of its
heat exchange contact with stripping regeneration
bottoms product ~1, flash regenerated absorbent H_ is
preheated (to 150-235°F) prior to entering stripping
regeneration column 14.
~XAMPL~ 1
In commercial use, 142,333 standard cubic feet per
minute of synthesis gas g produced by the partial
oxidation of natural gas with steam and air could be
treated in accordance with the process flow diagram of
Figure 1 for ultimate use of the purified synthesis gas
in a 1200 ton pe:r day ammonia product facility.
Synthesis gas ~ containing 41.9 mole % hydrogen,
41.0 mole % nitrogen, 15.7 mole % carbon dioxide, 0.7
mole % carbon monoxide, 0.5 mole % argon, 0.2 mole %
methane, and 0,.1 mole % helium enters the bottom of bulk
C02 absorber 2 which is 11 feet in diameter and
contains two 2!3 foot packed beds of structured stainless
steel packing at 170°F and 400.0 psia. In bulk C02
134078
-13-
absorber 2, synthesis gas ~ countercurrently contacts
lean bulk absorbent $ which is an aqueous solution
containing 47 weight percent methyldiethanolamine and 3
weight percent ethylenediamine. Lean bulk absorbent B
is fed to the top of bulk C02 absorber 2 at a rate of
6609 gallons per minute at a temperature of 155'F, and
at a loading of 0.27 moles of carbon dioxide per mole
of methyldiethanolamine. Carbon dioxide is removed from
synthesis gas ~~ at a rate of 2746 moles per hour so that
bulk absorbent product gas ~ which has a temperature of
155°F and 399 psia contains 3.95 mole % carbon dioxide.
Bulk absorbent product gas ~ then enters the base of
the 36 tray trim C02 absorber 4 and is counter-
currently contacted with lean trim absorbent ~ which is
also an aqueous solution of 47 weight % methyldiethanol-
amine and 3 weight percent ethylenediamine which has
been regenerat~sd to a lean C02 solution loading of
0.01 moles o:E carbon dioxide per mole of. methyl-
diethanolamine. Lean trim absorbent ~ is fed into the
top of trim C02 absorber 4 at a temperature of 105°F
and at a flo~arate of 1368 gallons per minute. Carbon
dioxide is removed from bulk absorber product gas C at a
rate of 769.3 moles per hour so that the carbon dioxide
content of the trim absorber product gas ~ is reduced to
1000 volumetric ppm or 0.1 mole %.
Rich bulk absorbent p which is discharged from the
bottom of the bulk C02 absorber 2 at a temperature of
181°F, at a flowrate of 6935 gallons per minute, and
with a molar carbon dioxide content of 0.48 is fed into
top section 8A of column 8. Top section 8A is 11 feet
in diameter, 39 feet. high, and filled with a 20 foot bed
of stainless steel packing. This section operates at
about 20.2 psia so that C02 is flashed at a rate of
1971.2 moles per hour which reduces the carbon dioxide
1340"ls~
-14-
loading in the absorbent to 0.33 moles of C02 per mole
of methyldiethanolamine. This flashing reduces the
absorbent temperature to 166'F. Flashed absorbent Q is
then conveyed t:o bottom section 8B which is 42 feet
high. In this section, the absorbent is vacuum flashed
at 9.7 psia to remove another 750.7 moles of C02 per
hour so that the flash regenerated absorbent ~I has a
loading of 0.2'7 moles of C02 per mole of methyl-
diethanolamine~and a temperature of 155°F.
A 81.6 percent portion of flash regenerated
absorbent ~i is directly recycled to the top of bulk
C02 absorber 2, while the remainder is heated to 233'F
by stripping regeneration bottom product ~1 in heat
exchanger 22. From heat exchanger 22, preheated, flash
regenerated absorbent ~i is fed into the top of stripping
regeneration column 14 which is 11 feet in diameter and
contains 17 sieve trays.
In stripping regeneration column 14, 793.5 moles of
carbon dioxides per hour are produced as stripping
regeneration overhead product ~I, so that the stripping
regeneration bottom product ~1 has a carbon dioxide
loading of 0.01 moles per mole of methyldiethanolamine.
Bottom product ~t is discharged at a rate 1388 gallons
per minute and at a temperature of 253°F. Heat exchange
with flash regenerated absorbent ~i reduces the tempera-
ture of stripping regeneration bottom product ~1 to
168°F, while cooler 24 reduces the temperature of what
is now lean trim absorbent ~ to 105°F.
Reboiler 16 heats bottom product Z1 at a rate of 55.9
MM BTUs per hour which is equivalent to 15.9 M BTUs per
mole of carbon dioxide.
EXAMPLE 2
A series of experiments were conducted in a
,.~ 134flr1~8
-15-
specially desigr,~ed racking autoclave to determine the
rate of carbon dioxide absorption and the vapor-liquid
equilibrium for :several absorbent solutions.
The autoclave is a stainless steel cylinder which is
I
surrounded by a steam jacket around which are electric
heaters. Carbon dioxide is charged to an accumulator
cell from a carbon dioxide cylinder. The carbon dioxide
is then transferred from the accumulator cell to the
autoclave through a port in the autoclave. A port is
also provided :in the autoclave through which absorbent
solution can b~. charged. The autoclave is rocked by an
electric motor.
The head is provided with a port through which a
thermocouple is inserted for measurement of the auto-
clave temperature. The accumulator cell's pressure drop
is used to calculate the gram-moles of carbon dioxide
charged to the autoclave. Rate of absorption data is
obtained by measuring the autoclave pressure vs. time.
The following aqueous absorbent solutions were
prepared:
No. Agueous. Absorbent
1 50 wt.~. methyldiethanolamine alone
50 wt.~~ methyldiethanolamine plus 3.0 wt.$
pipera~; ine
3 50 wt.~s methyldiethanolamine plus 3.0 wt.~
diethy7.enetriamine
50 wt.3~ methyldiethanolamine plus 3.0 wt.$
ethylenediamine
Each of the aqueous absorbent solutions was
separately evaluated in the rocking autoclave by
initially charging one of the absorbent solutions to the
autoclave and heating the solution to a 70°C run
1340~~~
-16-
temperature. The autoclave was then pressurized with
carbon dioxide from the accumulator cell, with the
pressure change: of the accumulator cell being recorded.
When autoclave pressurization is complete, a starting
pressure is rE:corded, and a stop watch and the rocking
motor are started simultaneously. The rate of carbon
dioxide absorption is measured by recording the
autoclave pres:~ure drop with time. Equilibrium in the
autoclave is <sssumed when the autoclave pressure ceases
to change. A:Eter the equilibrium autoclave pressure is
recorded, the autoclave containing the carbon dioxide-
laden absorbent solution is again pressurized with
additional carlbon dioxide and the procedure is repeated
until the final experimental solution loading or equili-
brium vapor pressure of carbon dioxide is achieved.
This procedure is repeated several times for a given
solution until a set of carbon dioxide absorption rate
data and vapor-liquid equilibrium data is obtained.
Each of the 4 absorbent solutions are subjected to this
testing technique.
For each charge of carbon dioxide to the autoclave,
an equilibrium carban dioxide vapor pressure is measured
subsequent to the change in autoclave pressure with
time. Equilibrium solution loading is then calculated
as the total moles of carbon dioxide charged to the
autoclave from the accumulator (calculated from
accumulator pressure drop using a non-ideal equation of
state) minus the moles of carbon dioxide remaining in
the autoclave vapor space (calculated from autoclave
equilibrium vapor pressure again using a non-ideal
equation of state). The resultant vapor liquid
equilibrium data is plotted as carbon dioxide vapor
pressure versu:~ solution loading.
The rate of absorption in moles of carbon dioxide
134076
-17-
absorbed per liter of solution in the autoclave per -
minute are calculated from autoclave pressure change
with time again using a non-ideal equation of state.
Gram-moles of carbon dioxide are converted to cubic
feet, and the rate of absorption is divided by an
average partial pressure driving force in atmospheres to
obtain the final rate of absorption in terms of cubic
feet of carbon dioxide per hour-liter-atmospheres. The
rate of absorption data can then be plotted versus
equilibrium solution loading.
From the carbon dioxide partial pressure, the
absorbent solution's carbon dioxide loading, and the
solution absorption rate values for each carbon dioxide
pressurization, a pair of curves can be prepared for
each of the absorbents as shown in Figure 2. One set of
curves are the carbon dioxide partial pressure vs.
carbon dioxide loading curves having a generally
increasing slope. Another set of curve are the rate of
absorption vs. carbon dioxide loading curves having a
generally decreasing slope.
The curves representing the equilibrium between
carbon dioxide in the vapor phase (i.e. carbon dioxide
partial pressure) and carbon dioxide loading in the
solution (i.e. moles carbon dioxide per mole of amine)
can be used to determine the absorbent solution's
circulation rage, while the rate of carbon dioxide
absorbed by the solution vs. the carbon dioxide loading
curves are used to determine the mass transfer rates
from which ab:~orbent column staging requirements can be
ascertained.
~nhancement in the rate of absorption and solution
capacity is most pronounced under trim absorber
conditions where solution loadings are defined as lean
and semi-rich. The difference between the commercially-
1340~1G~
-18-
utilized lean solution loading of 0.01 nolss C02/mole
amine and a semi-rich solution loading in ~guilibrium
with a C02 partial pressure of 20 psia, as would exit
the trim absorber and enter the bulk absorber, is then
calculated using the curves from Figure 2 for each of
the four absorbent solutions. J~s a result, the four
solutions were found to have the following loading
differentials, absorption rates, and required circulation
rates set forth below in Table 3.
p
1 0.26 112.2 4.0 2.7
2 0.39 64.9 19 3.8
3 0.42 63.3 18 3.7
4 0.45 49.9 25 4.2
A= Aqueous Absorbent No.
B=Loading Differential--i.e. semi-rich minus lean
(moles carbon dioxide per mole of amine).
C=Circulation Rate (gallons of solvent per pound-
mole C02).
D~Lean Solution hbsorption Rate (cubic feet per
hour-atmosphere).
E=Semi-Rich Solution J~rbsorption Rate (cubic feet
per hour atmosphere).
From Table 3, it is apparent that the methyldiethanol-
amine promotes! with ethylenediamine and diethylene-
triamine unexpectedly have a higher carbon dioxide
capacity compared to unpromoted methyldiethanolamine and
~34o~r~g
-19-
to methyldiethanolamine promoted with piperazine. As a
result, methy:ldiethanolamine solutions promoted with
ethylenediamine and diethylenetriamine are capable of
absorbing more carbon dioxide during trim absorption.
In addition, '.Cable 3 shows that methyldiethanolamine
promoted with ~thylenediamine has lean and semi-rich
carbon dioxide absorption rates unexpectedly higher than
those of unpromoted methyldiethanolamine alone and
methyldiethanolamine promoted with piperazine. The lean
and rich carbon dioxide absorption rates of methyl-
diethanol amine promoted with diethylenetriamine are
unexpectedly better than those of methyldiethanolamine
alone and about the same as those of methyldiethanol-
amine promoted with piperazine.
Although the invention has been described in detail
for the purpose of illustration, it is understood that
such detail i:a solely for that purpose and variations
can be made therein by those skilled in the art without
departing from the spirit and scope of the invention
which is defined by the following claims.
