Note: Descriptions are shown in the official language in which they were submitted.
~37Z3
The present invention relates to a process for the
removal of acidic gases from a gaseous mixture containing
the same.
Processes for the removal of acidic gases, such as H2S
and C02, from gaseous mixtures containing the same are well
known in the art. In general, these processes involve
scrubbing the gaseous mixture with a liquid absorbent in
an absorption zone whereby the acidic gases are removed
from the gaseous mixture and a loaded absorbent stream
(generally called the fat absorbent) is obtained which is
passed to a regeneration zone where the absorbent is heated
and/or stripped with solvent vapour, e.g., steam, resulting
in the release of the acidic gases. The regenerated absorbent
(generally called the lean absorbent) is returned into
contact with the feed gas mixture in the absorption zone
while the evolved acidic gases are passed to a cooler/
condenser in which the solvent vapours are condensed and
separated from the acidic gases.
For a variety of industrial applicationS,it is further
necessary or desirable to reduce the sulphur content of a
~ gaseous mixture containing significant quantities of COS
- and/or mercaptans in addition to H2S and C02 to low levels
prior to further processing and/or utilization of the
gaseous mixture. For example, sour gas available from
certain natural gas reservoirs is known to contain up to
about 0.1% by volume (1000 ppm) COS and some 0.01% by
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11137Z3
volume (100 ppm) mercaptans in addition to substantial
quantities of H2S and C02. Since the COS and mercaptans
present in the sour gas make-up part of the total sulphur,
substantial removal of same, in addition to the H2S
present, is necessary to meet many of the specifications
for conventional end uses of such gas, e.g., residential
heating and industrial uses. Further, in conventional
partial combustion processes utilizing sour liquid hydro-
carbon oils or sulphur-containing coals as the primary
fuel source, a crude synthesis gas product is obtained
which typically contains 100 to 1500 ppm COS in addition
to the H2S and C02 partial combustion by-products. In
many cases this crude synthesis gas product is subject to
further processing, e.g., contact with sulphur-sensitive
~ C0-shift catalysts in hydrogen manufacture, or funnelled
to industrial and consumer end-uses, e.g., as energy
source in gas turbine generation of electricity or as a
town gas for private consumption, which makes it desirable
or even essential that the total sulphur content of the
combustion gas be reduced to very low levels.
Although the commercially available gas-treating
processes are generally satisfactory with respect to the
degree of removal of undesirable gas contaminants, there
is still room for further improvement of such processes.
A major cost factor for these regenerative processes
constitutes the stripping steam requirement for regener-
` ~137Z3
ating the fat absorbent which can be very high for many solvents.
The said steam requirement is greatly dependent on the amount
of absorbent circulating, so that reduction of this absorbent
circulation would necessarily bring about a reduction in the
steam consumption. The amount of absorbent to be used in a
particular case is dictated by the partial pressures of the
acidic gas components in the gaseous mixture to be treated and
the desired degree of gas purification. A reduction in the
absorbent circulation would also contribute to further savings
from a capital cost point of view in that smaller absorption
and regeneration columns are required and in that less absor-
bent solvent for filling the columns is necessary.
This invention seeks to provide a process for the -
removal of acidic gases as identified to a very high degree
while using a reduced absorbent circulation.
Accordingly, the present invention relates to a pro-
cess for the removal of àcidic gases from a gaseous mixture
containing the same by contacting the said gaseous mixture
countercurrently in a contacting zone at a temperature in the
range of 15 to 135C and at a pressure of from 1 to 100 kg/cm2
abs., with an aqueous absorbent comprising an amine with a
PKb Of 3-14 at 25C in an amount of 10 to 70~ by weight and
sulfolane and/or its derivatives in an amount of 25 to 70~ by
weight, which aqueous absorbent is introduced at the top of the
contacting zone, and a fat aqueous absorbent is finally with-
drawn from the contacting zone at the bottom thereof for
regeneration by means of heating and/or stripping, in which
process at an intermediate point at the lower part of the said
contacting zone the said aqueous absorbent is withdrawn from
said contacting zone as a semi-fat absorbent and is reintroduced
into the said lower part of said contacting zone after having
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1~137Z3
been cooled externally, for further contacting with the said
aqueous mixture.
The process of the present invention is particularly
suitable for treating gaseous mixtures comprising acidic
gases at relatively low partial pressures. This may be the
case when the mixture to be treated comprises little acidic
gas components or when it contains the said gas components
at relatively low pressures~ The sum of the partial pressures
of hydrogen sulphide and carbon dioxide should preferably be
lower than 3.0 bar, and more preferably lower than 2.0 bar
at the temperature applied during absorption. The greatest
benefit will be derived from the process of the invention
when applying it to gaseous mixtures having a hydrogen sul-
phide partial pressure in the range of from 0.025 to 0.6 bar
at a lean absorbent temperature of 30 to 65C. In applying
the process of the invention for the removal of carbon dioxide
from gaseous mixtures comprising substantially no hydrogen
sulphide (less than 0.1% vol. H2S) the carbon dioxide partial
pressures may vary in the range of from 0.1 to 2.0 bar at a
lean absorbent temperature as indicated.
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~113723
The process of the invention is also very suitable for
application in those areas of the world where deep cooling
to temperatures below 50C is not well possible because
the ambient temperature in these areas during at least
part of the year does not drop below the figure indicated.
Even in these instances intermediate cooling to temper-
atures of about 65C may considerably add to reducing the
absorbent circulation.
The contacting zone used preferably comprises of from
10 to 50 contacting layers, such as valve trays, bubble
cap trays, baffles and the like. The aqueous absorbent
is suitably withdrawn at an intermediate point above the
second contacting layer but below the fifteenth contacting
layer of the absorption zone if 20 layers or more are
applied or below the tenth contacting layer if less than
20 layers are applied.
The aqueous absorbent used in the present process
comprises a "chemical" solvent part, i.e., the weak basic
component, and a "physical" solvent part, i.e., sulfolane
and/or its derivatives.
The chemical solvent part comprises certain components
or mixtures of these components of the class of amines.
; For the process according to the invention these amines
should have a weakly basic character. (The basic strength
of a compound is conveniently expressed in terms of the
negative logarithm of the basic dissociation constant,.
Thus, a strong base has a low pKb, while a weak base has
1113723
a value approaching the upper limit PKb = 14). Bases suit-
able for the process according to the invention have a
PKb at 25C in the range of 3 to 14. Alkanolamines are
particularly suitable, especially those amines having
1 to 4, and preferably 2 to 3, carbon atoms per alkanol
radical. Typical species are, inter alia, monoethanol-
amine, diethanolamine, methyldiethanolamine, di-iso-
propanolamine, triethanolamine and mixtures thereof.
Especially the dialkanolamines can be used with advantage.
The physical solvent part comprises certain com-
ponents or mixtures thereof, selected from the group of
cyclotetramethylene sulphones. The derivatives from the
basic sulphone cyclotetramethylene sulphone (thiophene
tetrahydro-1,1-dioxide) which is also known as sulfolane,
should preferably have not more than 4, more preferably
not more than 2 alkyl substituents in the tetramethylene
sulphone ring. Sulfolane itself is the preferred species
of this class of compounds.
It has been found advantageous to use in the process
an aqueous absorbent which preferably contains the weak
basic component within the range of 20-55%wt. An absorbent
containing 30-45%wt of the weak basic component has been
found very suitable. The sulfolane or its derivatives are
preferably present in the absorbent in an amount of 30 to
55%wt. The balance of the mixed solvent consists essentially
of water, generally in the range of 5-35%wt.
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~1~3723
The descending aqueous absorbent can be withdrawn
from the absorption column by means of a total draw-off
tray. Such a tray divides the column in two sections
with respect to the flow of absorbent, but has a passage
for the rising gas mixture to allow it to pass from the
lower section into the upper section. The absorbent col-
lected on the draw-off tray is withdrawn from the ab-
sorption column and externally cooled by indirect heat
exchange to a temperature in the range of from 20 to
70C. More preferably, the withdrawn aqueous absorbent
which is semi-fat, is cooled to a temperature which is
~; the same as the temperature of the aqueous absorbent
introduced at the top of the absorption column. There-
after the cooled, semi-fat absorbent is reintroduced
into the column in the lower section, preferably directly
under the total draw-off tray. It will be evident by the
position of the total draw-off tray that the lower section
of the column is shorter than the upper section of the
column, which latter section encompasses more absorption
trays.
It is also possible to use two absorption columns of
different column length and arranged such that the gaseous
mixture contacts first in the shorter one of the two
columns a cooled semi-fat absorbent introd~ced at the top
of this shorter column and then countercurrently contacts
a lean absorbent in the taller one of the two columns.
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~1137;:3
The fat absorbent leaving the shorter column is regener-
ated, cooled and introduced at the top ~ the taller column
as the lean absorbent.
The principal attainment of the process of this in-
vention is the high degree of removal of acidic gas com-
ponents from gaseous mixtures at very low partial pressures
of these acidic gas components and at reduced absorbent
flows. Because a low temperature has particularly a favour-
able effect on the absorption of hydrogen sulphide and/or
carbon dioxide, intermediate cooling of the absorbent before
its introduction into the lower section results in more
favourable absorption equilibria for hydrogen sulphide
and/or carbon dioxide in the bottom of the absorption
column. When a gaseous mixture enters the lower section
of the absorption column countercurrent to a descending
stream of cold liquid absorbent, a high degree of ab-
sorption is obtained even at the relatively low partial
pressures of the acidic gases. This high degree ~ ab-
sorption is maintained as the total amount of acidic
gas components progressively decreases as the gas mixt-
ure proceeds upward in the column, because in proceeding
upwards the gaseous mixture is contacted with an absorbent,
the temperature whereof is decreasing.
Gaseous mixtures which can advantageously be treated
by the process according to the invention, include natural
gas, refinery gas or synthesis gas obtained by the partial
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1113723
oxidation of heavy oil fractions or coal. The process is
particularly attractive for the treatment of gases con-
taining high amounts of acidic gases, such as hydrogen
sulphide and carbon dioxide at relatively low partial
pressures. The gaseous mixtures to be treated in ac-
cordance with the process of the present invention may
also contain carbonyl sulphide in amounts of from 0 to
1500 ppmv and mercaptans in amounts of from 0 to 500 ppmv.
The process may be adapted to treat such feedstocks which
also contain higher molecular weight hydrocarbons.
An important step in the process of the invention
comprises intimately contacting the gaseous mixture and
the aqueous absorbent at pressures in the order of 1 to
100 kg/cm2 abs. A preferred pressure range is from 5 to
70 kg/cm2 abs. A highly preferred aspect of the operation
of the absorption column is to conduct absorption under
countercurrent contacting at temperatures in the range of
about 15C to about 135C, more preferably of about 30C
to 80C and wherein the temperature of the bottom of the
absorption tower is about 5C to 30C higher than the
temperature in the top part of the absorption zone.
; Normally, intimate contacting is effected in a vertical
column, the sweet dry gas leaving the column near or at
the top while the fat absorbent solution leaves at or
near the bottom of this column.
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11137Z3
The fat absorbent contains dissolved acidic gases,
such as hydrogen sulphide and carbon dioxide, carbonyl
sulphide and/or mercaptans, together with possible con-
taminating proportions of hydrocarbons, originally present
in the gaseous mixture. Under the preferred absorption
conditions, the proportion of hydrocarbons contaminant to
hydrogen sulphide is maintained at a low level.
The fat absorbent is conducted from the bottom portion
of the absorption column under high pressure to a regener-
ation zone wherein the pressure is reduced to 0.0-4 kg/cm2
gauge for the purpose of removing the acidic gases and
other absorbed gaseous contaminants. In this zone, referred
to as a gas stripper or stripper column, the solution is
heated to a temperature sufficient to volatilize the
acidic gases and other absorbed gaseous contaminants,
and water therefrom, which leave the stripping column at
the top. The fat absorbent may also be sent firstly to a
flashing zone wherein a major part of the absorbed acidic
gases and other absorbed gaseous contaminants is removed
from the absorbent by reducing the pressure to 1.5-8 kg/cm2
abs. Thereafter the absorbent is regenerated as described.
One of the chief advantages of the use of the aqueous
absorbent of the present invention is experienced during -
the stripping operation.
It has been found that in stripping the fat absorbent
; to remove the acidic gases absorbed the steam requirement
,,
--11--
7Z;~
per unit volume of absorbent is more or less constan~.
Thus, by reducing the solvent circulation less steam has
to be used. It is possible to employ stripping temperatures
between about 100C and 190C at pressures between about
0 and 4 kg/cm2 gauge. The use of these low regeneration
temperatures has a distinct advantage in that any thermal
degradation of the organic solvent fractions of the ab-
sorbent is greatly reduced.
Fig. 1 shows a schematic flow diagram of an embodiment
of the process of the invention, in which a gaseous feed
stream is countercurrently contacted with a descending ab-
sorbent flow. Apart from the heat exchangers and pumps
shown any further ancillary equipment has been omitted.
Fig. 2 shows a different embodiment of the process of
the invention in which two separate absorption columns are
used instead of a single column divided into two separate
sections.
In Fig. 1 a gaseous mixture comprising acidic gases is
introduced into the lower section of an absorption column 2
via a line 1. The absorption column 2 is provided with
trays of which the lowest one is indicated by a and the
top one by b. The said column is divided in two sections
by means of a total draw-off tray 4 which is provided
somewhere between the second and tenth tray. The gaseous
mixture is introduced below the first tray (tray a) and
the aqueous absorbent is introduced above the top tray
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(tray b) via a line 9. The gaseous mixture rises in the
lower section and passes via the total draw-off tray 4
into the upper section of the column. It is counter-
currently contacted by the descending aqueous absorbent.
As result of the released heat of absorption during the
absorption of acidic gases the aqueous absorbent is
gradually increasing in temperature. The warmed-up ab-
sorbent is withdrawn from the column via a line 5 and
externally cooled in a heat exchanger 6 and re-introduced
into the lower section of the absorption column 2 by
means of a pump 7 and a line 8. The gaseous mixture in
the lower section is contacted with a cooled and semi-fat
aqueous absorbent. As a result of the absorption of the
acidic gases the temperature of the absorbent rises again.
The treated gaseous mixture which is now substantially
free of acidic gases and/or carbonyl sulphide and/or mer-
captans, if present, leaves the column via a line 11. The
hot, fat absorbent leaves the column 2 at the bottom
thereof via a line 12 and is heat-exchanged with hot,
regenerated absorbent in a heat exchanger 10. Thereafter,
the fat aqueous absorbent is introduced into a stripper
column 13 for regeneration.
The stripper column operates at a reduced pressure
with respect to the pressure applied in the absorption
column. It is heated by means of a reboiler 18. Hot
aqueous absorbent is withdrawn at the bottom of the
11137Z~
stripper column 13 via a line 14 and further heated in the
reboiler 18. It is re-introduced into the stripper column
via a line 19. The acidic gases stripped from the ab-
- sorbent leave the stripper column via a line 20. The
lean regenerated absorbent leaves the stripper column 13
via a line 15 and is pumped back to the absorption column 2
by means of a pump 16. The hot lean absorbent is cooled
by heat exchange in the heat exchanger 10 as set out above
and further cooled in a cooler 17.
The acidic gases obtained in the process of the in-
vention can be used for the preparation of elemental sulphur.
This may be done by the well-known modified Claus process.
Depending on the H2S/C02 ratio in the acidic gases they may
be fed directly - preferably after cooling and condensation
of the water vapour - into the thermal stage of the sulphur
recovery unit or they should be subjected to a further en-
richment step prior to their introduction into the said
thermal stage.
In Fig. 2 the same reference numerals have been used
for identical parts. Instead of a single column 2 two
separate columns 2a and 2b are used. The gaseous mixture
leaves column 2a - comparable with the lower section of
the column 2 in Fig. 1 - via a line 40 at the top and
enters column 2b at the bottom thereof. The semi-fat ab-
sorbent leaves column 2b at its bottom and is introduced
after cooling at the top of column 2a.
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11137Z3
The invention will now be illustrated with the fol-
lowing Examples.
EXAMPLE I
A natural gas feed stream comprising 3 vol.% H2S and
12 vol.% C02 was fed at a temperature of 40C and a pressure
of 12 bar abs. and a flow rate of 100,000 Nm3/h to an
absorber/stripper unit as shown in Fig. 1. The aqueous
absorbent usedc~prised 45% of di-isopropanolamine, 40%
of sulfolane and 15% by weight of water. It was introduced
at the top of the absorber column at a temperature of 41C.
The column was provided with 40 trays, a total draw-off
tray being installed between the 3rd and 4th tray. The
temperature of the semi-fat absorbent withdrawn from the
column was 66C. The absorbent was externally cooled with
water and reintroduced at a temperature of 41C directly
under the draw-off tray. The temperature of the fat ab-
sorbent leaving the column was 57C and passed at indirect
heat exchange with the hot, lean absorbent to the regener-
- ation column. It was introduced at a temperature of 91C
into the regeneration column operating at a pressure of
2.0 kg/cm2 abs. The regenerated lean absorbent left the
regeneration column at a temperature of 137C. After heat
exchange with the fat absorbent and cooling with water
it was introduced at a temperature of 41C at the top of
the absorber column.
7~3
The treated gas mixture leaving at the top of the ab-
sorber column comprised less than 4 ppm H2S and 500 ppm C02.
The amount of absorbent flow was 360 m3/h and the
amount of steam required for the regeneration 43 tons/h.
Processing of the above feed in a standard gas treat-
ing unit not having external cooling facilities as shown
in the Figure and having 35 trays, would have required
500 m3/h of absorbent and 57 tons/h of steam to obtain
the same degree of purification of the gaseous mixture
treated.
EXAMPLE II
In the same manner as described in Example I two
further experiments were carried out with feed gases of
different compositions and at different flow rates. The
reaction conditions and the results obtained have been
tabulated in the Table. In this Table also the conditions
and results of the experiment of Example I have been in-
serted as Experiment No. 1.
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1~137Z3
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