Note: Descriptions are shown in the official language in which they were submitted.
~,2~5;~
PROCESS FOR THE REMOVAL OF C02
AND~ IF PRESENT, H2S FROM A GAS MIXTURE
The invention relates to a process for removal
f C2 and, if present, H2S from a gas mixture.
In many cases it is necessary to remove C02 and~
if present, H2S and other sulphur-containing impurities
such as COS from gas mixtures. The removal of H2S
and/or other sulphur-containing impurities from gas
mixtures may be necessary in order to render these
gas mixtures suitable for catalytic conversions using
sulphur--sensitive catalysts, or to reduce environmental
pollution if the said gas mixtures or combustion gases
obtained therefrom are discharged to the atmosphere.
Examples of C02-containing gas mixtures from
which H2S and/or other sulphur-containin compounds
generally have to be removed are gases obtained by
partial combustion, or complete or partial gasi-
fication, of oil and coal, refinery gases, town gas,
natural gas, coke-oven gas, water gas, propane and
propene.
Removal of C02 from gas mixtures, either as such
from a gas mixture which contains no or virtually no
, ~
76
H2S (e.g. natural gases), or in admixture with H2S in
case the latter compound is present in the gas mixture,
is often necessary to bring the gas mixtures on a
desired calorific value and/or to avoid corrosion in
transport lines and/or to avoid freezing in cryogenic
equipment and/or to avoid transport of C02 which is
o~ no value in the gas mixture ultimately to be used for
- a certain purpose.
In many cases the C02 and, if present the H2Sg
will be removed from the said gas mixtures using
liquid solvents, which will often be basic. At least
part of the C02 present in the gas mixtures will be
absorbed in the liquid solvent together with at least
part of the H2S, if present. The H2S and C02 (which
in this speci~ication are also indicated as acid
gases) will be removed from the said gas mixtures
at the pressure of the gas mixture concerned, i.e.
in many cases at elevated pressure.
The loaded solvent obtained after absorption of
C02 and, i~ present, H2S from the gas mixture is to
be regenerated partly or totally, during which re-
generation H2S, if present, and C02 are set free.
In case H2S is present in an appreciable content
in the gas obtained after regeneration of the loaded
solvent, this gas cannot be discharged to the
atmosphere before at least most of the H2S has been
~Z~5Z'~¢
removed therefrom. The H2S is very suitably removed
from this gas by converting it to elemental sulphur3
which is separated off. The conversion of H2S into
elemental sulphur is generally carried out in the art
by means of a Claus process in which some o~ the H2S
; is oxidiæed to S02, and sulphur and water are formed
by reaction of H2S with S02, with or without the
assistance of a suitable catalyst. In order to be able
to carry out a Claus process, the molar percentage of
H2S in a mixture with C02 must be at least about 15.
If this percentage is between about 15 and about ~0,
the Claus process can be carried out by separating one-
third part of the gas, combusting the H2S therein to
S02~ and subsequently mixing the resultant S02-con-
taining gas with the balance of the H2S-containing
gas, afker which the Claus reaction can be further
carried out at elevated temperature and preferably
in the presence of a catalyst. In case the gas con-
tains about 40% mol H2S or more, the Claus process
can be carried out by combusting the gas with a
quantity of air which is sufficient to conYert one-
third part of the H2S into S02, and subsequently re-
acting the H2S and S02 to form sulphur and water.
In many instances the gas set free during re-
generakion of the loaded sol~ent is unsuitable for use
in a Claus process, the H2S content thereof beirng too
l~S~16
low and further processes for increasing the H2S
content are to be carried out with ~uch a gas.
In case the gas set free during regeneration
should have an H2S content which is sufficiently high
for use in a Claus process it may nevertheless be of
importance to increase the H2S content thereof, because
in the latter case the total amount of gas to be used
in the Claus process is lower, and accordingly the
installations can be of smaller size.
The increase in H2S content of the gas set free
during regeneration of the loaded solvent can, of
course, be achieved by preferential absorption of the
H2S ~rom that gas in a suitable solvent, and regener-
ation of that solvent after loading. However, such a
second absorption process is unattractive in view of
the extra installations to be built and the extra
amount o~ energy needed for regeneration of that
loaded solvent.
The invention provides a process for the re-
moval of C02, and if present,H2S from a gas mixture3in which process the energy needed for the regener-
ation of the solvent loaded with C02 and, if present~
H2S is very low, and in which process gases with
high H2S contents suitable to be used in-a Claus
process are obtained with a single absorption step
from gas mixtures containing H2S.
-- 5 --
Accordingly, the invention provides a process for
removal of CO2, and, if present, ~2S from a gas mixture, which
process is characterized in that:
a) the gas mixture is contacted at an elevated pressure of
at least 5 bar countercurrently with a solvent which comprises a
tertiary amine and a physical absorbent;
b) the loaded solvent obtained is flashed at least once by
pressure release to a pressure which is above the total partial
pressure of the CO2 and H2S present in the loaded solvent at the
prevailing temperature;
c) the loaded solvent obtained in step b) is flashed at least
once by pressure release to a pressure which is below the total
partial pressure of the CO2 and H2S present in the loaded solvent
at the prevailing temperature,
and semi-lean solvent obtained in step c) is, optionally after all
or part thereof has been totally regenerated, used as solvent in
step a).
The solvent comprises a tertiary amine, a physical
absorbent and preferably water.
Acid gases are able to react with tertiary amines. Very
suitable tertiary amines are aliphatic, in particular those which
contain at least one hydroxyalkyl group per molecule. Examples
are triethanolamine, tripropanolamin~, triisopropanolamine,
ethyldiethanolamine, dimethylethanolamine, diethylethanolamine.
276
Preference is given to ~ethyldiethanolamine.
A physical absorbent is a compound in which acid
gases are soluble, but without undergoing a reaction
therewith. Very suitable physical absorbents are
sulfolane and substituted sulfolanes, alcohols with
1-5 carbon atoms per molecule (e.g. methanol), tetra-
ethylene glycol dimethyl etherg N-methylpyrrolidone,
alkylated carboxylic acid amides (e.g. dimethylform-
amide). Preference is given to sulfolane. The word
"sulfolane" denotes the compound "tetrahydrothiophene
1,1-dioxide".
The contents of tertiary amine and physical ab-
sorbent (and, i~ present, water) in the solvent may
vary between wide limits. Very suitably ~he solvent
contains in the range of from lG to 60%w of tertiary
amine, prefe~ably methyldieth~nolamine, in the range
o~ from 15 to 55%w of physical absorbent, preferably
sulfolane, and in the range o~ from 5 to 35%w of water.
It is essential that the solvent used in the
process according to the invention comprises a tertiary
amine and a physical absorbent. In modified processes
solely differing from the process according to the
invention in that the solvent contains a secondary
and/or a primary amine instead of a tertiary amine or
contains a tertiary amine but nc physical absorbent,
less C02 is set free in the flashing in step c)~ and
accordingly, in case H2S was pre.sent in the original
gas mixture, the .semi-.lean solvent ob.tained in the
partial regeneratïon in step c) contains H2S and C0~
in a lower molar ratio than obtained with the process
according to the invention. Moreover, total regener-
ation (which in general is carried out by stripplng
with steam) of the said semi-lean solvent requires in
the case of the above modified processes more steam and
yields a mixture of C~2 and H2S in a less favourable
molar ratio for a Claus process than in the case of the
process according to the invention.
A modified process differin~ from that according
to the invention in that the solvent comprises one or
more physical absorbents only, amines being absent,
requires in many cases more solvent and more absorption
trays in an absorption column used in step a) to
:~ achieve the same amount of acid gases absorbed than
~: ~ the process according to the invention. Moreover, more
non-acid gases are absorbed in solvents which com-
prise one or more physical absorbents only than in the
solvents used in the process according to the in-
vention, which non-acid gases are set free during the
flashing in step b). In case a solvent is used which
comprises one or more physical absorbents only, this
amount of non-acid ga.ses is so greak that it is not
attractive to use it as fuel (as can be done with the
276
non-acid gases set free in step b~ in the process
accordlng to t.he invention, and accordingly th.ese non-
acid gases need repressurizing (with the aid of
capital intensive compressors) prior to recycling to
step a).
The contacting o~ the gas mixture with the
solvent in step a) is carried out at elevated pressure,
which is considered to be a pressure of at least 5,
in particular of at least 10 bar. Pressures in the
range of from 20 to 100 bar are very sultable.
The contacting of the gas mixture with the
solvent is very suitably carried out in a contacting
one~ e.g. an absorption column which comprises in the
range of from 15 to 80 contacting layers, such as
valve trays~ bubble cap trays, ba~fles and the like.
It has surprisingly .been found that by using the
solvent in the process according to the invention the
H2S can substantially be removed from the gas mixture
used as feed whi.le regulating the amount o~ C02 which
~: 20 is le~t in the purified gas. This regulation can be
achieved by regulating t.he solvent circulation, i.e.
the ratio of solvent fed to the extracting zone and
the amount of gas mixture fed thereto. In case no or
hardly no H2S: i5 pre.sent in the gas mixkure, .the
amount of C02 removed therefrom can also .be
regulated by the solvent circulation. If desired,
~)5;~
the C02 can be removed to a very great extent. The
solvent circulation can still be further reduced, if
desired, by removing loaded solvent at an intermeLdiate
point from the contacting zone at the lower part there-
of, externally cooling the removed loaded so]vent, andreintroducing it to the lower part of the contacting
zone for further contacting of the gas mixture to be
purified, e.g. as describe~ in British patent speci-
fication 1~589 3 231.
The temperature during the contacting of the gas
mixture and the solvent in step a) may vary between
wide limits. Temperatures in the range of from 15 to
110C are very suitable, temperatures in the range of
from 20 to 80C are preferred.
In step a) all or the greater part of COS, if
present, is removed from the gas mixture.
~; The loaded solvent obtalned from step a) contains
CO~g H2S (if any) and, in general, amounts of dis-
;~ solved non acid components from the gas mixture to be
purified~ e.g. hydrocarbons and/or hydrogen and/or
carbon monoxide. These non-acid gases are to be re-
moved at least partially from the loaded solvent by
flashing in step b) to a pressure which is higher than
the total partial pressure of the acid gases present
in the loaded solvent. In this way only very small
amounts of acid gases are released from the solvent
~5%7~
together wi.th the non-acid ~ases, e.g. hydroca~bons
and/or hydrogen and/or carbon monoxi.de. If .desired,
the gas mixture obtained from the flashing in step b)
may be recirculated to step a), but in order to avoid
recompressin~ this gas mixture is preferably used for
any other purpose, e.g. as fuel gas (if desired after
removal of all or part of the ~2S present, e.g. by
contacting the said gas mixture with a small amount
of lean solvent). Non-acid gases have to be removed
: 10 from the loaded solvent before this solvent is flashed
to a pressure which is lower than the total partial
pressure of the acid gases, because otherwise the
hydrocarbons and/or hydrogen and/or carbon monoxide
would be set free together with an appreciable amount
of acid gases. As in many cases these acid gases or
: combustion gases obtained therefrom are to be discharged
to the atmosphere, the hydrocarbons and/or hydrogen
and/or carbon mo~oxide would be discharged or burnt
simultaneously, which would be a waste of these valuable
compounds.
~:~ Although the loaded solvent may be flashed in step
b) several times, each time at a lower pressure, in
most cases the greater part of dissolved non-acid com-
ponents will be removed in one flash step~ and for
that reason it is pre.ferred to flash the loaded solvent
once in step b).
7'6
The loaded solYent obtained in step b~ - which
besides acid gases contains only small amounts of other
dissolved compounds - is flashed in step c~ to a pres-
sure below the total partial pressure of the acid gases
in the said loaded solvent at the prevailing temper-
ature. It has been found that in the process according
to the invention (in which a solvent comprising a
tertiary amine and a physical absorbent is used) the
amount of C02 set free is much higher than in modified
processes differing solely from the process according
to the invention in that the solvent contains a
secondary and/or primary amine instead of a tertiary
, amine or contains a tertiary amine, but no physical
absorbent. In case the loaded solvent also contains
H2S the gas set free after the pressure release in
step c) has a much higher molar ratio of C02 to H2S
than the molar ratio of these gases originally present
in the loaded solvent. It is of advantage to heat the
loaded solvent, e.g. to a temperature in the range of
~rom 45 to 110C before or during flashing in step c),
because in that case the molar ratio of C02 to H2S in
the gases set free after the pressure release is still
further increased. The above modified process in which
a solvent is used which comprises a secondary or
primary amine and a physical absorbent or a solvent
which comprises a tertiary amine and no physical
12
absorb.ent, gives a much lo~er molar ratio o~ C0.2 to
H2S in t:he gas:.s.et ~ree a~ter the pres.sure re.lease.
Because the gas: set free in step c) has a higher
molar ratio of C02 to H2S than that o~ t~e original
loaded solvent, the molar ratio of H2S to C02 present
in the solvent remaining after the pressure release
in step c) is higher than originally.
Because at each pressurerelease in step c) the
molar ratio of ~2S to C02 in the remaining solvent
is lncreased, it may be of advantage to ~lash the
loaded solvent in step c) at least twiceS each time
to a lower pressure or at a higher temperature in
case the original gas mixture contains H2S. In general,
the pressure of the loaded solvent after the pressure
release in step c) will be about atmospheric.
In step c) a large amount of C02 is set free, and
accordingly the loaded solvellt is in fact regenerated
to an appreciable extent, yielding semi-lean solvent.
In case the original gas mixture was substantially
free from H S the semi~lean solvent obtained in step c)
contains C02 as the only acid gas, and it is prefer- -
entially used as such at least partlally as solvent
in step a); in many cases the amount of C02 present
in the semi-lean solvent obtained in ~tep c) will be
so low that it is preferred to use this semi-lean
solvent as the only solvent in skep a). If desired,
13
part or all o~ khe semi-lean solYent obtained in
step c) can be totally regenerated (e.g. b~ stripping
with steam) and used as solvent in step a~. In case
totally regenerated solvent and semi-lean solvent are
bokh used as solvents in step a) the former is prefer-
entially introduced into the contacting zone at a point
further removed from the inlek of khe gas mixture than
the semi-lean solvent.
: ~:
In case the original gas mixture did not only
contain C02 but also H2S the acid gases set free during
the flashing in step c) in a number of cases will con-
tain so small amounts of H2S that they can be dis~
; charged to the atmosphere after incineration. If
desired or needed the H2S present in the gases set
free during the flashing in step c) may be removed
there~rom by contacting these gases with a solvent
under conditions which favour preferential removal of
H2S over C02. ~or such removal, e.g. a mixture com-
prising an amine and optionally a physical absorbent,
may suitably be used, in particular the solvent to be
used according to the present invention. In order to
achieve a high selectivity for H2S removal the con-
tacting is very suitably carried out in a column with
less than 20 contact ;brays and at high gas velocities,
e.g. as described in British patent speci~ication
1,362,384.
~2C~t~7~;
In cas:e H2~ ~as present in the original gas mixture
the semi~lean solvent obtained in step c~ contains H2S
and C02 in a high molar ratio. In view ofthe high
content of H2S this semi-lean solvent is not suitable
to be used as solvent in step a) and the acid gases
present therein have to be removed therefrom. The acid
gases are removed from the said semi-lean solvent by
total regeneration to yield lean solvent. The regener-
ation is very suitably carried out by heating in a
regeneratlon column (e.g., to a temperature in the
range of from 80 to 160C), which heating is preferably
carried out by stripping with steam.
The gas obtained during this regeneratlon has
such an H~S content that it can be suitably used in a
;~ 15 Claus process for the preparation of sulphur.
~.
The lean solvent obtained after regeneration can
very suitablybe~eused in step a), and also for con-
tacting the flashed gas from step b) or c) if desired.
It will be clear that in order to keep the
amount of energy needed in the process as low as
~ .
possible, it is of advantage to carry out heat exchange
of process streams where appropriate.
EXAMPLE 1
10,000 kmol~h of a gas mixture (composition 80%v
methane) 5%v ethane, 3%v propane, 1%v butane, 1%v
H2S and 10%v C02) are introduced into the bottom of
J~6
an absorption column containing 30 valve tra~s, at
a temperature o~ 40C and a pressure of 50 bar. This
gas mixture is countercurrentl~ contacted with 300 m3/h
of a lean solvent consisting of methyldiethanolamine
(50%w), sulfolane (25%w) and water (25%w). Purified
gas leaves the top of the absorption column in an
amount of 95~1.4 kmol/h; this gas contains 644 kmol/h
C2 and less than 4 volume parts per million (ppm) of
H2S -
The loaded solvent (300 m3/h) is removed from the
bottom of the absorption column; it contains 99.7
kmol/h H2S and 356 kmol/h C02. This loaded solvent is
flashed to a pressure of 15 bar at a temperature of
69.2C. The gas flashed off ~45 kmol/h) contains 1.4
kmol/h H2S, 10.7 kmol/h C02, the balance consists of
hydrocarbons. The loaded solvent obtained after this
first flash contains 98.6 kmol/h H2S and 345 kmol/h
C02. It is heated by heat exchange with lean solvent,
and flashed to a pressure of 1.3 bar at a temperature
of 70C. The gas set free during this second flash
(293.6 kmol/h) consists of 35.6 kmol/h H2S and
258 kmol/h C02. It is contacted countercurrently with
148 m3/h lean solvenk in a second absorption column
with 13 valve trays at a temperature of 40C and a
pressure of 1.1 bar, yielding 212 kmol/h of a gas
which consists of C02 containing 300 ppm H2S.
" ~2~5276
16
The loaded solvent .ohta.ined in t:he last-ment.ioned
: absorption column is regenerated together with the
semi-lean solvent obtained after the second flash.
Th.is semi-lean solvent contains 63 kmol/h H2S and
87 kmol/h C02 The regeneration is carried out by
stripping with steam, yielding a gas which consists
of 98.6 kmol/h H2S and 133 kmol/h C02, which gas is
very suitable to be used in a Claus process. The lean
: solvent obtained after regeneration (448 m3/h) is
: lO partly (148 m3/h) recycled to the second absorption
; column and partly (300 m3/h) ~after heat exchange
with the loaded solvent from the first flash) used
as lean solvent in the absorption column.
EXAMPLE 2
lOaO00 kmol/h of a gas mixture (composition
90.65%v methane and 9.35%v C02) are introduced into
the bottom o~ an absorption column containing 20
valve trays at a temperature of 35C and a pressure
of 91 bar. This gas mixture is countercurrently con-
tacted with 844 m3/h of a solvent which consists o~
methyldiethanolamine (50%w), sulfolane (25%w) and
water (25%w). This solvent is semi-lean solvent; it
contains 1374 kmol/h C02 and no methane. The gas
leaving the top of the absorption column (9069
kmol/h) consists. ~or 98%v of methane, the balance
being C02.
2~'~
The loaded solvent ~844 m3/h) is removed from
the bottom of the absorption column; it contains
2131 kmol/h C02 and 174 kmol/h methane and has
temperature of 53C. This loaded solvent is flashed
to a pressure of 24 bar and a temperature of 51C.
The gas flashed off (204 kmol/h) ~onsists of ~59
kmol/h methane and ll5 kmol/h C02. The loaded solvent
obtained after khis first flash (8ll4 m3/h) contains
2086 kmol/h C02 and 15 kmol/h methane. It is heated
; lO and flashed to a pressure of 1.3 bar and a temper-
ature of 40C. The gas set free during this second
flash (727 kmol/h) consists of 15 kmol/h methane and
712 kmol/h C02. The semi-lean solvent obtained in
khe second flash (844 m3/h) contains 1374 kmol/h C02
and no methane, and is introcluced at the top of the
absorption column as solvent for the gas mixture to
be purified as described abover
Comparative Experiment
For comparison the same process as described in
Example 2 is carried out with a solvent consisting of
diisopropanolamine (5~%w), sulfolane (25%w) and water
(25%w) (not according to the invention). For the re-
moval o~ the same amount of C02 from the feed gas~ and
the use of non-regenerated semi-lean solvent in the
absorption step, a solvent circulation about five
times as high as that needed in the process according
76i
to the invention described in Example 2 is needed.
Moreover, about five times as much methane is absorbed
per hour in the absorption column, and released in
the first flashj the amount of methane set free in
this first flash is so high that for economical
reasons this gas has to be recompressed and re-
cycled, which makes the installation of expensive
compressors necessary.
