Field of invention:
The present invention relates to an absorbent liquid having improved absorption capac.ity for acid
gases from gas stream comprising of tertiary amine, primary amine, activator and water. The
present invention also relates to a process of removal of acid gases from the gas stream using the
absorbent liquid based on activated methyldiethanolamine.
Background:
Natural gas is the major source of fuel used in industries. The natural gas comprises of
hydrocarbons mainly methane and other impurities like C02, H2S. The concentration of
impurities varies from source to source. C02 concentration in natural gas varies from 5% to 80%
by volume & H2S concentration varies from 100 ppm to 40000 ppm by volume. Presence of C02
in natural gas reduces the calorific value of the gas and increases the storage and transportation
costs. However, H2S is a toxic gas, which on burning in furnaces & in other applications results
in formation of SO2, which causes environmental pollution.
There are several methods used to remove these two acid components from gaseous stream.
Aqueous solutions of commonly available alkanol amines are generally used to scrub the acid
gas in absorption column. Different types of amines are used to remove C02 & H2S from the
acid gases. Selection of the amines depends on the CO2 and H2S concentration in the gas stream,
operating pressure and temperature of the gas, application of the treated gas and the capital &
operating costs of gas purification.
GB 1,058,304 describes a process for removing acid gases from gaseous streams by contacting
the gaseous streams with an aqueous solution comprising sulpholane and a secondary
alkanolamine or morpholine or derivatives of morpholine, e.g., 2,6-dimethylmorpholine, 2,6-
diethylmorpholine, 2,3,5,6-tetraethylmorpholine,2 -methylmorpholine, 2-ethylmorpholine and 2-
methyl-6-ethylmorpholine. The patent specification does not teach the concept of improved
working capacity as defined hereinafter or acid loading capacity. More importantly, the patent
specification does not teach the use 'of an absorbent liquid comprising methyldiethanolamine
(MDEA) in combination with a primary amine and a sulpholane as provided by the instant
invention.
US 6337059B1 discloses a composition for removal of acid gases such as C02, H2S, COS from
gaseous streams by using aqueous solution comprising more than 1.1 mole piperazine per liter of
aqueous solution and about 1.5-6 mole MDEA per liter of aqueous solution. The composition
further comprises a physical solvent selected from the group. consisting of sulpholane or
methoxytriglycol.
US 7758673B2 discloses a process for removal of C02 from a gas stream, using an absorption
column having 5-80 contacting trays with an aqueous washing solution, containing 20-45. parts
by weight water, 20-35 parts by weight sulpholane and 40-55 parts by weight of an amine,
selected from monoethanolamine (MEA), diethanolamine (DEA), triethanolmine (TEA),
diisopropanolamine (DIPA) and MDEA. Further, the aqueous solution also contains piperazine. -
The amount of CO2 in the feed gas stream is 1-45 mol%.
US 6290754B1 discloses a regenerative process for deacidification of gas containing C02 and
liquid hydrocarbons in an absorption zone, using absorption liquid, comprising of MDEA and an
activator. The activator is a compound of a particular formula, disclosed in the specification.
US 5861051A discloses a process for the absorption of C02 and H~Su's ing an aqueous solution
comprising about 25% to 50% by weight MDEA and 3% to 15% by weight 2-(2-aminoethoxy)-
ethanol. The ratio of MDEA and 2-(2-aminoethoxy)-ethanol in the aqueous solution is
maintained around 2: 1. .
US 6165432~4 discloses a composition and process for removal of acid gases using an aqueous
solution comprising a mixture of a tertiary and primary alkanolamine. The primary alkanolamine
is 2-amino-1-butanol in concentration ranging between 1-30% by weight, while the tertiary
alkanolamine is MDEA and is present in concentration between 20-60% by weight.
It is evident from prior art, various mine based solvents are used for chemical absorption of
C02, and this includes MEA, DEA, diglycolamine, DIPA, TEA, etc. and for physical absorption,
absorbents include N-methylpyrrolidone, polyethylene glycol dimethyl ether, methanol,'
propylene carbonate, etc. The above listed absorbents are incapable of giving a high acid gas
loading, which is desired for the efficient removal of these gases.
None of the prior art documents discloses the acid gas loading aspect of the absorption liquid,
which is described as the moles of C02 absorbed per mole of amine used and is one of the
efficiency determining factors in the absorption of acid gases. It is this property of the absorbent
liquid that determines the usability of a particular solvent (though there are other factors such as
the operating temperature, volatility, composition of different amines, etc.) in absorbing acid
gases, such as COz and H2S. Furthermore, many of the cited and non-cited documents did not
even disclose the C02 partial pressure in the feed and/or product stream, the operating
temperature and pressure.
Therefore, in order to overcome the shortcomings in the above mentioned prior arts, the present
invention discloses a selective solvent composition comprising MDEA, hexamethylenediamine
(HMDA) and sulpholane in a particular concentration range, so as to give an enhanced acid gas
loading and removal efficiency. Further, the present invention also discloses a process for the
removal of acid gases (especially C02 and H2S) by absorption of the said gases in an absorption
column, operating at an optimum'temperature and pressure.
It is evident that there is still a great need and interest in the gas purification industry for
alkanolamine compositions which are aqueous blends of a primary mine and tertiary
alkanolamine which will be effective in the removal of acid gases from gas streams and will have
low degradation, corrosivity and better efficiency compared to absorption blends known in the
art.
It has now been discovered that an aqueous mixture comprising a tertiary alkanolarnine and
optionally a primary amine and activator is not only effective in removing acid gases from gas
stream but it also has unexpectedly high acid gas loading, removal efficiency, low degradation,
corrosivity.
Summary of the Invention:
The present invention discloses a unique solvent composition of amines and an activator for acid
gases absorption with acid gas loading ranging between 0.75 to 1.15 moles of C02 absorbed per
mole of amine. The composition of the present invention consists of chemical absorbents
(primary and tertiary amines) for absorption of C02 at very low partial pressures as well as
physical absorbent (sulpholane) for high partial pressures.
Detailed descri~tiono f the invention
The present invention relates to an absorbent liquid having improved absorption capacity for acid
gases from gas stream comprising of:
35 to 50 w/w% of tertiary amine,
0 to 15 W/W% of primary amine,
0 to 15 W/W% of activator and
water;
wherein the acid gas is loaded in the range of 0.75 to 1.15 mole of acid gas per mole of amine.
One embodiment of the present invention, wherein the tertiary amine is selected from
methyldiethanolamine (MDEA), triethanolamine, trimethylamine and triphenylamine.
In another embodiment of the present invention, wherein the tertiary mine is MDEA.
In still another embodiment of the present invention, wherein the primary amine is selected from
methoxyglycol, 2-amino- 1 -butanol, hexamethylenediamine (HMDA), methylamine and
ethanolamine.
Yet another,embodiment of the present invention, wherein the concentration of HMDA is in the
range of 5-1 5 w/w%.
A further embodiment of the present invention, wherein the activator is selected from piperazine
and sulpholane.
Yet another embodiment of the present invention, wherein the activator is sulpholane.
In another embodiment. of the present invention, wherein the concentration of Sulpholane is in
the range of 5- 15 w/w%.
In still another embodiment of the present invention, wherein the process for the removal of acid
gases from the gas stream; comprises:
contacting the.absorbent liquid with the gas stream, wherein the absorbent liquid comprises of 35
to 50 w/WO/o of tertiary amine, 0 to 15 w/& of primary amine, 0 to 15 W/W% of activator and
water at a temperature of 20' to 45' C and at a pressure range of 2-110 kg/cm2g,
wherein the acid gases are absorbed by the liquid till the equilibrium is attained.
- Yet another embodiment of the present invention, wherein the gas stream is refinery off gas, bio
gas, natural gas or synthesis gas.
In still another embodiment of the present invention, wherein the acid gas is COz and/or H2S.
Yet another embodiment of the present invention, wherein the amount of C02 absorbed is 0.5 -
50% by volume and amount of H2S absorbed is between 0 - 4% by volume and maximum partial
pressure of 10.4 kg/cm2.
In still another embodiment of the present invention, wherein the base chemical is MDEA.
Yet another embodiment of the present invention, wherein the amount of HMDA is in the range
of 0 to 0.4 moles per mole of MDEA.
In still another embodiment of the present invention, wherein the amount of HMDA is in the
range of 0.25 - 0.4 mole per mole of MDEA. '
Yet another embodiment of the present invention, wherein the amount of sulpholane is 0 to 0.3
mole per mole of MDEA.
In still another embodiment of the present invention, wherein the amount of C02 absorbed is
0.75 to 1.15 mole per mole of amine.
Yet another embodiment of the present invention, wherein regeneration of liquid is carried out at
a pressure ranges between 2 to 3 kg/cm2a and low pressure steam is used for regeneration of
liquid.
In still another embodiment of the present invention, wherein the concentration of H2S in the
treated gaseous stream is reduced to 4 to 100. ppm by volume.
The solvent of the present invention is a mixture of chemical as well as physical absorbents.
Primary and tertiary amines are used as chemical absorbent;, which can absorb C02 at very low
partial pressures. The activator acts as a physical absorbent, capable of absorbing C02 at high
partial pressures.
The primary amine, used in the present invention is hexamethylenediamine (HMDA) in
concentration ranging between 0-15% by weight while the tertiary amine, Nmethyldiethanolamine
(MDEA), is in the range of 35-40% by weight.. The physical absorbent,
sulpholane, is in the range of 0-1 5% by weight. The concentration of the absorbent liquid to be
used is decided based on the feed concentration of C02 and H2S. This implies that the
concentration of the absorbents can be altered depending on the separation desired. For instance,
if a gas stream containing high amount of C02 is to be used,as a feed, the concentration of
sulpholane, which is desirable for H2S separation, is reduced and the concentrations of other
absorbents are increased, in order to enhance the separation of C02. Similarly, the concentration
of the absorbents can be varied to get higher separation of H2S along with C02.
Addition of HMDA to the aqueous solution containing MDEA, results in an improvement of the
mass transfer. The acid gas loading of only MDEA solution comprising of 50% by weight
MDEA is increased by 40% due to the addition of 10% HMDA, keeping the total concentration
of alkanol amine to 50% of the aqueous solution of absorbent. Further improvement of the acid
gas loading is done by adding sulpholane 'to the aqueous solution of MDEA and HMDA.
Improvement is achieved by 10% due to addition of sulpholane by 15 wt%.
The present invention also aims to provide a process to remove acid gases like C02 and M2S
from gas streams at low operating & capital cost. The gas sweetening process uses various
solvents to remove the acid gases but the economy of the process depends on acid gas loading of
the solvent used for removal of acid gases, the thermal .& chemical stability of the solvent,
operating pressure of the absorption column as well as the concentration of acid gases in the feed
gas & treated gas leaving from the absorption column.
The present invention, therefore, further relates to a process for removing acid gases from a
gaseous stream which contains COz and H2S, whose total partial pressure goes beyond 10.4
kg/cm2.~hega s comes in contact with an aqueous absorbent comprising of at least one tertiary
amine and one primary amine & sulpholane at a concentration of at least 10% of total solution.
1 C02/H2S is absorbed in the solvent and the partial pressure of the C02/H2S in the feed gas
decreases, until equilibrium is achieved.
The acid gas loading of the absorbent increases with increase in C02/H2S partial pressure. This
reduces the circulation rate of aqueous absorbent solution to the absorber and regenerator. High
acid gas loading also reduces the lean amine flow to the absorption column & the size of the
absorption column.
The most widely used equipment for carrying out the mass transfer operation related to the
absorption of the said gases is an absorber. The absorber is generally a tall and vertical tower, the
diameter of which is decided by the gas flow velocity inside the column. In the present invention,
the solvents are chosen such that their chemical and physical properties are good enough to
withstand such high pressure and temperature prevailing in the absorber and stripper column
respectively. Absorption, which is a low temperature and high pressure process, is carried out in
the present invention, to absorb the acid gases like C02 and H2S in the aqueous absorption liquid.
The temperature and pressure conditions are maintained at an optimum level, in order to enhance
1
I the absorption of C02/H2S in the solution. Once the acid gases are absorbed in the solution, the
treated gas leaves the absorption column and the rich mine is processed in the regenerator
column, for the regeneration of solvent.
The regeneration of aqueous solution of absorbent after absorption of C02/H2S from gaseous
stream is carried out at low pressure in the regenerator which uses trays for mass transfer.
Depending on the acid gas pressure required in the downstream Sulphur Recovery Unit, the
regenerator is operated. The regenerator bottom temperature is maintained at 120-130° C by
using a reboiler where low pressure steam is used as heating media.
Experimental set-up:
Laboratory study was conducted to generate vapor liquid equilibrium data for different
solvent combinations. The schematic of the experimental setup is shown in Figure 1. The
experimentation was carried out using gas containing C02 only because handling of H2S is a
problem in laboratory. H2S itself is a toxic compound and causes health problem. The
experimental setup for VLE measurement consists of a 732 ml stainless steel cylindrical
high pressure equilibrium cell (7). The cell was connected to gas cylinder (1) containing
mixture of C02 & N2 (40% COz) through a regulator (2) and an isolation valve (4). A
pressure gauge (5) was attached for the measurement of pressure inside the equilibrium cell.
A vent (6) and a feed inlet (3) were provided at the top of the equilibrium cell.
It should be noted here that the equilibrium cell (7), as the name suggests, is a cylindrical
vessel in which absorption occurs. The cell is the representation of an ideal tray or plate in
the absorber column. Therefore, the gases are not removed, until the equilibrium is attained.
Figure 1: Schematic of the experimental setup
In order to carry out the experimentation, mother solution consisting of 40 wt% MDEA, 10 wt%
HMDA, 15 wt% sulpholane and water was prepared. 50 ml of solution was fed in the
equilibrium cell (7) through the feed inlet (3) for each experiment. The quantity of the solution
fed to the cell can be altered, depending upon the volume of the equilibrium cell. The
equilibrium cell was pressurized to desired pressure by injecting the gas through regulator (2)
and an isolation valve (4). The composition of the gas was analyzed by using a gas
chromatography. The valve was closed tightly for isolation of equilibrium cell from any feed gas
entry. The initial pressure of the equilibrium cell (7) indicated by pressure gauge (5) was
-recorded at zero time.
It was observed that as the gas entered the cell, i.e. at time t=O, there was no change recorded in
the pressure gauge. But as the time passed, the equilibrium cell pressure gradually fell and the
data was recorded. The fall in pressure suggests the absorption of C02 in the solution. It was
observed that after some time equilibrium was attained, which was monitored by the steady state
achieved by the pressure gauge. Once the steady state attained, the cell was recharged with the
feed gas, in order to saturate the solution with maximum possible COz in it. The pressure was
maintained to the initial value, for each fresh recharge, and the downfall of pressure was
recorded with time.
The experiment was terminated when there was no further drop in pressure, as observed from the
pressure gauge. The experiment was then repeated at different values of pressure and
concentration of the aqueous solution and feed gas stream.
Results:
Experiments were conducted at pressures ranging from 12 kg/cm2g to 25 kg/crnzg and 25 OC.
Tables 1 & 2 show the pressure variation and acid gas loading with time. Table 3 shows
experimental data with different concentration of HMDA and sulpholane at 25 kg/cm2g and 25
OC. Table 4 shows the comparative results obtained with time for MDEA only and
MDEA+HMDA solution at 25 kg/cm2g and 25 OC. Each experiment attained saturated condition
with respect to C02 absorption in the aqueous solution of the arnine.
Table 1:
Operating Pressure : 12 kg/cmZg (partial pressure of C02=S.2 kg/cm2)
Solvent : MDEA 40% (wt) + HMDA 10% (wt) + 15%(wt)
Sulpholane
Operating Pressure : 17 kg/crn2g (partial pressure of C02=7.2 kg/cm2)
Solvent : MDEA 40% (wt) + HMDA 10% (wt) + 15%(wt)
Sulpholane
Time
(Min)
0
1
3
5
7
9
11
13
17
19
25
3 1
35
Time
(Min)
0
1
3 .
5
7
9
11
13
17
2 1
33
Pressure at
equilibrium cell
(kg/cm2g)
12.5
10.8
9.9
9
8.4
8
7.8
7.5
7
6.8
6.5
6.1
6
Loading ( Mole of C02 per Mole
of Amine)
0
0.217595341
0.332792875
0.447990408
0.524788764
0.575987668
0.601587119
0.639986297
0.703984927
0.729584379
0.767983557
0.819182461
0.831982186
Pressure at equilibrium
cell (kg/cm2g)
18
16
15.3
14.1
13.3
13
12.5
12.2
12
11.2
11
Loading ( Mole of C02 per Mole
of Amine)
0
0.25599452
0.3455926
0.49918931
0.60158712
0.6399863
0.70398493
0.7423841
0.76798356
0.87038136
0.89598082
Table 2:
Operating Pressure : 21 kg/cmZg (partial pressure of C02=8.8 kg/cm2)
Solvent : MDEA 40% (wt) + HMDA 10% (wt) + lS%(wl)
Sulpholane
-
Operating Pressure : 25 kg/cmZg (partial pressure of C02=10.4 kg/cm2)
Solvent : MDEA 40% (wt) + HMDA 10% (wt) + IS%(wt)
Sulpholane
Time
(Min)
0
1
3
5
8
10
12
14
16
20
22
24
30
34
Time
(Min)
0
1
3
5
7
9
11
13
15
17
19
21
23
27
33
53
Pressure at
equilibrium cell
(kg/cm2g)
21
19
18
17.5
17
16.7
16.5
16.2
16
15.8
15.6
15.5
15.2
15
Loading ( Mole of CO2 per Mole
of Amine)
0
0.357144835
0.535717252
0.625003461
0.714289669
0.767861395
0.803575878
0.857147603
0.8928620867
0.92857657
0.964291054
0.982148295
1.035720021
1.071434504
Pressure at equilibrium
cell (kg/cm2g)
26
24
22
21.5
21
20.7
20
19.8
19.5
19.2
19
18.8
18.5
18.1
18
17
-
Loading ( Mole of C02 per Mole
of Amine)
0
0.255994519
0.511989038
-
0.575987668
0.639986297
0.678385475
0.767983557
0.793583009
0.831982186
0.870381364
0.895980816
0.921580268
0.959979446
1.01117835
1.023978076
1.151975335
Table 3:
The results (refer Tables 1, 2 & 3) clearly show that at a constant operating pressure, the C02
loading increases with time. This may be attributed to the fact that high operating pressure of the
column tends to increase the driving force for mass transfer. Hence, more and more C02 is
absorbed in the solution, resulting in the drop in the partial pressure of C02 and ultimately a drop
in the operating pressure and an increase in the COz loading.
Solvent : MDEA 40% (wt) + HMDA 7% (wt) + Sulpholane 7% (wt)
Similarly, at different operating pressures and at fixed value of pressure in the equilibrium cell,
there is an increase in the C02 loading. This may be attributed to the fact that absorption, being a
high pressure and low temperature process, increases with the increase in the operating pressure.
This means, higher the operating pressure, higher will be the absorption, but limited to the fact
that high operating pressure vessels are difficult to design and incur more operating cost than a
substantially low operating pressure vessel.
29 18 0.995063
Table 4:
Table 4 further gives a comparative data to show the comparison between aqueous solutions
containing only MDEA and MDEA+HMDA. It can be observed from the above table, that the
loading capacity of aqueous solution containing only MDEA (-0.96) is lesser than the one
containing MDEA+HMDA (=0.98), which is further less than the aqueous solution containing
MDEA+HMDA+Sulpholane (-1.15). Hence, it can be concluded that the composition of the
present invention is capable of imparting a much higher absorption capacity (or loading capacity)
~ .-.
for acid gases.
Operating Pressure : 25 kg/cm2g (partial pressure of C02=10.4
kg/cm2)
Solvent : MDEA 50% (wt)
Time
(Min)
0
I
3
5
7
9
13
17
19
29
Operating Pressure : 25 kg/cmZg (partial of C02=10.4 kg/cm2)
.- - .-
Solvent : MDEA 40% (wt) + HMDA 10% (wt)
Time
(Min)
0
I
3
5
7
9
I I
23
3 7
39
Pressure at
equilibrium cell
(kg/cm2g)
25
23
22
2 1
20
19.6
19
18.5
18.2
18
Pressure at
equilibrium cell
(kg/cm2g)
' 25
22.9
21.5
2 1
19.5
19.1
18.5
18 .
17.5
17.3
Loading ( Mole of C02 per Mole
of Amine)
0
0.2'15733
0.413599
0.551465
0.689332
0.744478
0.827 198
0.896 13 1
0.937491
0.965064
Loading ( Mole of C02 per Mole
of Amine)
0
0.26787
0.446449
0.5 10228
0.701563
0.752586
0.82912
0.892898
0.956677
0.9821 88

Claims
We claim:
1. An absorbent liquid having improved absorption capacity for acid gases from gas stream
comprising of:
35 to 50 w/w% of tertiary amine,
0 to 15 W/W% of primary amine,
0 to 15 W/W% of activator and
water;
wherein the acid gas is loaded in the range of 0.75 to 1.15 mole of acid gas per mole of
amine.
2. The liquid as claimed in claim 1, wherein the tertiary mine is selected from
methyldiethanolamine (MDEA), triethanolamine, trimethylamine and triphenylamine.
3. The liquid as claimed in claim 2, wherein tertiary amine is MDEA.
4. The liquid as claimed in claim 1, wherein the primary amine is selected from
methoxyglycol, 2-amino-1-butanol, hexamethylenediamine (HMDA), methylamine and
ethanolamine.
5. The liquid as claimed in claim 4, wherein the primary amine is HMDA and used in the
concentration in the range of 5-1 5 w/w%.
6. The liquid as claimed in claim 1, wherein the activator is selected from piperazine and
sulpholane.
7. The liquid as claimed in claim 6, wherein the activator is sulpholane and used in the
concentration in the range of 5-1 5 w/w%.
8. The process for the removal of acid gases liom the gas stream, said process comprising:
contacting the absorbent liquid as claimed in.claim 1, with the gas stream, wherein the
absorbent liquid comprises of 35 to 50 w/w?! of tertiary amine, 0 to 15 w/w% of primary
amine, 0 to 15 w/WO/o of activator and water at a temperature of 20" to 45' C and at a
pressure range of 2- 1 10 kg/cm2g,
wherein the acid gases are adsorbed by the liquid till the equilibrium is attained.
9. The process of claim 8, wherein gas stream is refinery off gas, bio gas, natural gas or
synthesis gas.
10. The process as claimed in claim 8, wherein acid gas is C02 andlor H2S
11. The process as claimed in claim 8, wherein the amount of C02 absorbed is 0.5 - 50% by
volume and amount of H2S absorbed is between 0 - 4% by volume and maximum partial
pressure of 10.4 kg/cm2.
12. The process as claimed in 8, wherein the base chemical is MDEA.
13.The process as claimed in 8, wherein the amount of HMDA is in the range of 0 to 0.4
moles per mole of MDEA.
14. The process as claimed in claim 8, wherein the amount of HMDA is in the range of 0.25
- 0.4 mole per mole of MDEA.
15. The process as claimed in claim 8, wherein the amount of sulpholane is 0 to 0.30 mole
per mole of MDEA.
16. The process as claimed in claim 8, wherein the amount of COz absorbed is 0.75 to 1.15
mole per mole of amine.
17. The process as claimed in claim 1 to 16, wherein regeneration of liquid is carried out at a
pressure ranges between 2 to 3 kg/cm2a and low pressure steam is used for regeneration
of liquid.
18. The process as claimed in claim 8, wherein the concentration of H2S in the treated
gaseous stream is reduced to 4 to 100 ppm by volume.