Note: Descriptions are shown in the official language in which they were submitted.
~9~3
The invention relates to a method for increasing the H2S/C02 ratio
in a gas mixture (~ereinafter referred to as the starting gas) in order to
prepare a gas mixture having an H2S~C02 ratlo suitable for use in a Claus
process G~ereinafter referred ~o as the desired gas). -
In man~ cases it is necessary to remove H2S from gas mixtures, for
example in order to render these gas mixtures suitable for catalytic conver-
sions using sulp~ur-sensitive catalysts, or in order to reduce environmental
pollution if before or after combustion the said gas mixtures are discharged
to the atmosphere.
Examples of gas mixtures from Nhich H2S generally has to be removed
are gases obtained by complete or partial combustion or gasification of oil
and coal, refinery gases, town gas, natural gas, coke-oven gas, water gas,
propane and propylene.
In many cases the H2S will be removed from the said gas mixtures
using liquid absorbents, which will often have a basic character. In a large `
number of cases the gas mixtures to be purified will also contain C02, at
least some of which will be absorbed in the liquid absorbent together with
the H2S. The H2S and C02 will be remo~ed from the said gas mixtures at the ;
pressure of *he gas mixture concerned, i.e., in many cases at elevated pres- ;
sure. By heating the loaded liquid absorbent, for example with steam, a gas
mixture is obtained consis~ing substantially of H2S and C02 in the ratio in
which these substances were present in the loaded liquid absorbent.
This H2S and C02-containing gas mixture cannot be discharged to
the atmosphere before at least most of the H2S has been removed therefrom.
The H2S is very suitably removed from this gas by converting it into Plemental
sulphur, 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 of the H2S is oxi-
dized to SO2, and sulphur and water are formed by reaction of H2S with S02,
~ith or withou~ the assistance of a suitable catalyst.
- 2
3995~3
For the operatian of a Claus process with a gas mixture comprising
su~s~antIall~ H2S and C02, the method to be followed depends on the ratio of
H2S and C02 in this gas mixture, or in other words, on the molar percentages
of H2S based on the total amount of H2S and CO2 present. Three different H2S
concentration ranges can be distinguished for the operation of a Claus pro-
cess:
1) If more than approximately 40 mol. % of the gas mixture
consists of H2S ~and the rest is virtually entirely C02~,
the first stage of the Claus reaction can be carried out
~y combusting the gas mixture with a quantity of air
~hich is sufficient to convert one-third part of the H2S
into S02. In one or more subsequent reactors~ sulphur
and water are then formed, preferably in the presence of
a suitable catalyst.
2) If the gas mixture comprising substantîally H2S and
C02, consists of between 1520 mol. % and approximately
40 mol. % of H2S, the above-mentioned method encounters
difficulties and the Claus process has to be carried out
b~ separating one-third part of the gas and combusting
2a t~e H2S therein to S02, and subsequently mixing the
resultant S02-containing gas with the rest of the H2S-
containing gas, after which the Claus reaction can be
further carried out at elevated temperature and preferably
in the presence of a catalyst.
31 If less than approximately 15-20% by volume of the gas
mixture consists of H2S (and the rest virtually entirely
of C02), a Claus process cannot be carried out in the
above-mentioned manner. The combustion in the gas mix-
ture has to be additionally s-upported. This may be
effected, for example, by recycling the sulphur formed
~ ~i
``` ~L~3995~3
in the Claus process to the H2S and C02-containing gas,
or by oxidation of the latter gas wlth pure oxygen
instead of air, or 6y injecting a fuel ~for example a
hydrocarbon) into the gas before it is combusted in the
thermal stage of the C]aus process. These processes are
generally expensive and not very attractive technically.
Another possibility to raise the H2S/C02 ratio in the mixture con-
sists in selectively absorblng H2S from the gas mixture in an appropriate
aasorbent, and subsequently liberating the H2S (mixed with the absorbed C02)
~rom the loaded absorbent in a regeneration stage. In case the H2S concentra-
tion in the gas mixture is low, an excessive amount of C02 is still absorbed
in the absor6ent during the selective absorption, when using this method. For
that reason it sometimes happens that no gas mixture having the desired com-
position is o6tained after liberation of the absorbed gas from the absorbent
loaded in this way~
There is therefore a need for a process whereby the quantity of
H2S in a gas mixture, in particular in a gas mixture otherwise consisting sub- -
stantially of C02, can be increased according to desire. It will be endeavour-
ed to control the H2S content in such a way that either a Claus process can
2a be carried out according to the system described first (addition of air to the
entire mixture) or according to the system described second (separation of
one-third part of th0 gas mixture and complete combustion of the H2S present
therein, followed by com6ination with the rest of the gas mixture).
Because there are in general no great differences between the costs
of investment and operation of the embodiment-s of the Claus process mentioned
in 1~ and 2), a process whereby the quantity of H2S in a gas mixture can be
increased according to desire will be of importance mainly to increase H2S
concentrations of below 15 - 20 % to 20 % or more. At the same time it will
be endeavoured not to increase the H2S concentration much higher than is
3~ needed in order to be a61e to carry out the method of the Claus process de-
~99513
scribed in 2), because higher concentrations do not offer any important ad-
vantages in the Claus process, whereas they do require more absorption and
regeneration capacity and energy.
~ le invention provides a method ~or the prepara~ion of a gas mix-
ture having a desired H2S/C02 ratio, independent o the originally present
H2S~C02 ratio in the starting gas, as a result of which an optimum use of
Claus plants and absorbent/regeneration equipment can be obtained.
According to the invention there is provided a method for increas-
ing the H2S/C02 ratio in a gas mixture ~hereinater referred to as the start-
lQ ing gas) in order to prepare a gas mixture having an H2S/C02 ratio suitable
for u~e în a Claus process (hereinafter referred to as the desired gas),
~hich process is characterized in that at least certain components of the
starting gas are absorbed in an aqueous solution of an amine ~further to be
called an amine liquid absorbent) in an absorption column, in that the absorb-
ed gas is liberated from the resultant loaded amine liquid absorbent in a re-
generation column, in that a portion of the gas becoming available from the
regeneration column is treated with amine liquid absorbent in an absorption
column under such conditions that H2S is absorbed selectiv01y relative to C02
in the amine liquid absorbent and the loaded amine liquid absorbent obtained
2n in this manner is introduced into the said regeneration column, and in that
the magnitude of the said portion of the gas becoming available from the re-
generation column is so selected that this gas is the desired gas.
The method according to the invention may be carried out in a num-
ber of different ways, depending on ~he composition of the starting gas, andon the H2S/C02 ratio in the desired gas.
In addition to H2S and C02, ~he starting gas will generally contain
other c~mponents, and in a first absorption step the starting gas will be
treatsd with an amine liquid absorbent in which H2S and C02 are absorbed,
w~ile the other gases of the starting gas are not or only slightly absorbed.
3~ Such a treatment yields an amine liquid absorbent loaded with H2S and C02,
~ .
1~99~i~3
the ratio of these gases ~n t~e am~ne l~qu~d a~sorbent remalning the same or
substantially the same as that in the starting gas.
In a number of cases it will be possible to prepare the desired gas
according to the invention by introducing the above-mentioned H2S and C02-
loaded amine liquid absorbent into a regeneration column and to treat a pro-
portion of the gas liberated from the regeneration column, which gas comprises
substantially H2S and CO~, in an absorption column under such conditions that
H2S is absorbed in the amine liquid absorbent selectively relative to C02.
Such an absorption column is hereinafter also referred to as the selective
absorption column. T~e H2S-loaded amine liquid absorbent becoming available
from the selective absorption column is subsequently supplied to the regenera-
tion column simultaneously with the afore-mentioned loaded amine liquid ab-
sorbent (which will generally be loaded almost exclusively with H2S and C02).
Both streams can be introduced into the regeneration column separately~ but
it is preferred to mix the streams before supplying them to the regeneration
column. In the regeneration column the C02 and H2S are completely or sub-
stantiall~ completely removed from the amine liquid absorbent by heating,
very suitably~ heating with steam.
The gas mixture liberated from the regeneration column must have
2~ the desired composition, and this composition will be dependent on the quanti-
t~ of H2S whlch is supplied to the regeneration column in the form of H2S-
loaded amine liquid absorbent originating from the selective absorption
column. The latter quantity of H2S is dependent on the quantity of H2S sup-
plied to the selective absorption column, because that H2S is absorbed sub-
stantiall~ completely in the amine liquid ab~orbent in the said absorption
column.
Because not more than a portion at most of the C02 is absorbed in
the amine liquid absorbent in the selective absorption column, the H2S/C02
ratio i~n the loaded amine liquid absorbent in the selective absorption column
~11 be far ~igher than in the a~ore-mentioned loaded amine liquid absorbent
995~1l3
to ~e supplied to the regeneration column; as a result the H2S/C02 ratio in
the regeneration column and therefore in the gas mixture becoming available
therefrom is higher than ln the afore-mentioned loaded amine liquid absorbent
to be supplied to the regeneration column. The H25/C02 ratio in the gas mix-
ture li~erated from the regeneratlon column ~s consequently determined by the
quantity of H2S supplied from this gas mixture to the selective absorption
column; that quantity of H2S i5 determined by the portion of the gas mixture
liberated from the regeneration column which is supplied to that absorption
column.
By controlling the magnitude of the portion of the gas mlxture
liberated from the regeneration column and supplied to the selective absorp-
tion column, it is therefore possible according to the invention to adjust as
desired the H2S~C02 ratio in the gas mixture liberated from the regeneration
column; this ratio is naturally limited in that the quantity of C02 present
in the afore-mentioned loaded amine liquid absorbent ~generally loaded ex-
clusively with H2S and C02) to be supplied to the regeneration column will
invaria~ly be present in the gas liberated from the regeneration column.
In a number of cases it will not be possible to obtain any gas hav-
ing a desired composition by means of this method9 because the ratio of the
2Q quantities of H2S and C02 in the starting gas is too low. Also, in certain
cases it is possible to obtain a gas having the desired composition, but to
that end ~o large a proportion of the desired gas becoming available from the
reg~neration column has to be passed to the selective absorption column that
the method becomes technically and economically unattractive.
In such cases it is possible to carry out the method according to
the invention in another manner. It is then very suitable for the H2S and
C2 a~sorbed in the amine liquid absorbent from gases containing other com-
ponen*s as well, to be liberated from the loaded amine liquid absorbent in a
` reg~neration column. Subsequently, the H2S is selectlvely absorbed in amine
liquid absorbent in an absorption column from the resultant gas (which in
-- 7 --
~C~9~ L3
this embodiment IS considered as startlng gas), and the resultant H2S-loaded
amine liqu~d a~sQr~ent ~s supplied ~o a regeneratlon column.
A port~on of the desired gas becoming avallable from the said re-
generation column ~s treated with amlne liquid absorbent ln a selective ab-
sorption column, and the H2S-loaded amine llquid a~osorbent obtained from the
said selective afisorption column ls also supplied to the regeneration column.
It is possible to use two selective absorption columns, namely one
for the treatment of the starting gas and one for a portion of the desired
gas. If desired, the loaded amine liquid absorbent obtained by selective
absorption o the starting gas ma~ be introduced into the selective absorp-
tion column ~or the desired gas, where it can serve as absorbent for the said
desired gas. In the selective absorption column for the desired gas complete-
ly regenerated amine liquid absorbent may optionally be introduced in addi-
tion.
It will be clear from the above that by the term "amine liquid ab-
sorbent" is meant not only an aqueous solution of an amine containing substan-
tially no gas (i.e., completely regenerated), but also such a solution partly
loaded wit~ gas, provided that this load is such that H2S can still be ab-
sorbed from the gas to be treated.
2Q ~t is generally preferred to use a single selective absorption
column in which both the above-mentioned gases, i.e., starting gas and de-
sired gas, are treated with amine liquid absorbent. These gas streams may be
introduced into the selective absorption column either together or separately.
In the latter case it is advantageous to introduce the desired gas at a point
below the point of introduction of the starting gas.
It may occur that fluctuations in the H2S/C02 ratio occur with
time in the starting gas containing H2S and C02 in a ratio lower than that
desired. In order to keep the H2S/C02 ratio constant or substantially con-
stant in the gas mixture to be obtained according to the method of the inven-
tion, the quantity of gas mixture to be supplied to the selective absorption
3~3
column will have to ~e varied (and if desired also the quantity of amine li-
quid absorbent to ~e supplied to the said column). It may even occur that
the H2S~C02 ratio ln the starting gas is the same or higher than the desired
ratio, so that no urther H2S enrichment of the gas mixture o6tained from the
regeneration column ig necessar~. ~n that case a method according to the in-
vention is not necessary~ and the introductlon of the desired gas into the
selective absorption column can be terminated; if the desired gas is the only
gas being supplied to the selective absorption column, the said column can be
shut down completely.
As an illustration o.~ a process in which the quantity of the gas
becoming available and the H2StC02 ratio~therein ma~ fluctuate, mention may
be made of the gasification o coal. First, most of the H2S and C02 will be
removed b~ non-selective absorption from a gas obtained in this process, and
subsequently H2S and C02 will be liberated Erom the thus loaded amine liquid
absorbent. H2S is removed by selective absorption in a selective absorption
column from the resultant gas mixture consisting substantially of H2S and
C02, and by regeneration of the resultant H2S-loaded liquid amine absorbent
a gas is obtained which under normal process conditions is suitable for a
Claus process. If in the case of reduction of the supply of gas obtained by
2Q coal gasification and/or decrease of the H2S content thereof no gas is ob-
tained ~hich is suitable for the Claus process, it is possible according to
the method of the invention to introduce into the said selective absorption
column a portion of the gas becoming available from the regeneration column.
The amine liquid absorbent used may be an aqueous solution of
primary, secondary~ and tertiary amines or mixtures thereof. As an example
mention may be made of ethanol amine; preference is given to polyalkanol
-~ amines, in particular di-isopropanol amine and methyl diethanol amine.
If desired~ the aqueous solution of an amine may also contain non-
~aslc compounds capable of physically absorbing ~2S and C02. Examples of
;
,;'"~1
5~3
this type o~ compounds are N-methyl pyrrolidone methanol and cyclotetramethy]-
ene sulphones, in particular sulfolane.
The structure of and the conditions in the absorption column are so
controlled that H2S ~s selectively absorbed rom the gas mixture, which other-
~ise consists largely of C02. The process is very sui~ably operated counter-
currently in a tray column, the gas mixture to be absorbed being introduced
at the ~ottom o~ the column. The absorption column pre~erably contains fewer
than 20 trays, and the gases are passed through the column a~ a consid0rable
rate, preferably at a rate of at least 1 m/sec., based on what is known as
the "active" or "aerated~' surface of the contact tray. If desired, a packed
~ed absorption column may also be used, having a packed height of preferably ''
less than 10 m.
Preferably, the gas mixture to be absorbed is contacted in each
selective absorption column with the amine liquid a'bsorbent at approximately
atmospheric pressure, although higher and lower pressures are by no means ex-
cluded. The temperature of the amine liquid absorbent in the selective absorp- ; -
tion column may vary within wide limits; temperatures below 55C, in particu-
lar from 15 to 50C, are very suitable, although higher temperatures are by
no means excluded.
The resultant gas mixture containing H2S and C02 in the desired ra-
tio (i.e., for use in a Claus process containing either 20 mol. % or slightly
more o H2S or 40 mol % or slightly more of H2S, as explained above) is very
suita~ly subjected to a Claus reaction, in w~ich sulphur is formed. The off-
gases from the Claus process, which still contain some H2S, can be discharged
to the atmosphere after combustion, or their content o$ sulphur compounds can
be further reduced to a considerable extent by reduction and selective absorp-
tion in a liquid absorbent of the gas obtained after reduction, for example
as described in Canadian patent ~47,473.
The gas becoming available from the selectiveiabsorption column,
3a which gas comprises substantially CO2 but Will generally still contain a minor
- 10 -
9~3
quantity o~ H2S, can also be dischar~ed to the at~sphere after combustion.
Com~ust~on and/or discharge thereo~ may o~ course ~e e~ected together with
other sulphur-containing gases, such as the above-mentioned Claus off-gases.
In case two absorption columns are present, the gas ~ecoming available from
the selective absorption column may also (if desired partlally~ he recycled
to the first absorption column.
The invention ~ill now be elucidated with re~erence to Figures 1 and
2, each of ~hich represents an embodiment o~ the invention.
In Figure 1 a C02-containing gas which must be freed of H2S is sup-
lQ plied via a line 1 to an absorption column 2. The purified gas is discharged
via a line 3. The ~12S and CO2-loaded amine liquid absorbent is discharged
via a line 4 and passed to a regenera~ion column 5. The gas becoming avail-
a~ole from the regeneration column 5 via a line 6 ~the desired gas) is divided
into two portions. One portion is discharged via a line 7, for example to a
Claus plant 8~ the discharge gas of which is discharged to the atmosphere via
a line 9, an incinera~or 10 and a line 11. The second portion of the gas
becoming available from the regeneration column 5 is supplied via a line 12
to a selective absorption column 13 according to the invention. The gas li-
berated ~rom the said absorption column is either supplied via a line 14 and
2~ the line 9 to the incinerator 10, or it is recycled via a line 19 to absorp-
tion column 2. The H2S-loaded am me liquid absorbent obtained from the ab-
sorption column 13 is supplied to the line ~ via a line 15. At the point
where the line 6 branches into t~e lines 7 and 12 there is a mechanism by
means of which the distribution of the gas stream from the line 6 between the
lines 7 and 12 can be controlled.
The amine liquid absorbent becoming availa~le from the regeneration
column via a line 16, which a~sorbent is free or substantially free of CO2
and H2~, ~s supplied via lines 17 and 18 to the absorptiGn columns 2 and 13,
respectively.
3~ ln Figure 2, a gas comprising substantially HzS and CO2 is supplied
~;
.
~95~5~L3
via a line lal to a selective absorption column 102. The gas issuing from
the said a~sorption column is discharged via a line 103, and, because it still
contains only very little H2S, ~t may, if desired, Be discharged to the at-
mosphere after combustion. T~e H2S and C02-loaded amine liquid absorbent is
passed via a llne 104 to a regeneration column 105. The gas becoming avail-
abl0 ~rom the said column via a line 106 ~the desired gas) is divided into
two portions. One portion is discharged via a line 107, for example to a
Claus plant Cnot shown). The second portion of the gas becoming available
from the regeneration column 105 is supplied via a line 112 to the selective
absorption column 102. T~e amine liquid absorbent becoming available from
t~e regeneration column 105, whlch absorBent is free or substantially free of
H25 and CO2, is passed via a line 117 to the aBsorption column 102. At the
point where the line 106 branches into the lines 107 and 112 there is a
mechanism by means of which the distribution of the gas stream between the
lines 107 and 112 can be controlled.
EXAMPLE r
The reference numerals refer to Figure 1
A gas stream of 80,000 Nm3/h, comprising substantially H2 and CG
and having an H2S content of 0.3 mol. % and a CO2 content of 7.0 mol. %, sup-
2~ plied via the line 1 to the absorption column 2, which contains 15 contact
trays. Via line 17 a 30 % by weight solution of methyl diethanol amine in
t~ater is supplied at a rate of 140 m /h, and the gas mixture is treated there-
~th at a temperature of ~5C and a pressure of 20 bar abs. lhe purified gas
escaping via the line 3 contains 3.3 % by volume of CO2 and 10 ppm by volume
of ~2S. The resultant H2S and CO2-loaded absorbent, in which the H2S/C02
molar ratio is 0.078, is supplied via the line 4 to the regeneration column 5.
TRis regenerat~on column, whic~ contains 14 trays, is heated with steam to a
Bottom temperature of 110C, the bottom pressure being 1.4 bar abs. Via the
line 6 a gas stream of 4150 Nm3/h leaves the regeneration column, of which gas
stream 2950 Nm3/h is supplied via the line 12 to the absorption column 13,
9~3
whi~ch conta~ns 9 contact trays~. A 30 % by weight solution of methyl diethanol
amlne in water is supplied ~45 m3/h) to this column via the line 18, and the
gas~mixture IS` treated therewith at a temperature of ~5C and a pressure of
1.1 Bar a~s. The gas velocity is 1.5 m/sec. In the loaded absorbent dis-
charged via the line 15, the H2S/CO2 molar ratio is 3.17, and the said loaded
absorbent is mixed with the absorbent present in t~e line ~, whlch absorbent
contains H2S and C02 in a ratio lower than the desired rat~o. Via the line
14 a gas containing 90 mol. % of CO2, 10 mol. % of H20 and 1500 ppm by volume
of H2S is discharged from the absorption column 13.
lQ ~s a result of a method according to the lnvention, the H2S/C02
ratio, wh~ch was 0.078 in the absorbent in the line 4, has been increased to
0.28 in the gas mixture becoming available from the regeneration column via
the line 6. The said gas mixture contains 20 mol. % of H2S, 70 mol. % of CO2
and 10 mol. % of water. It is supplied to a Claus plant 8 via a line 7.
`~ The absorbent freed cf C02 and H2S and becoming available from the
regeneration column 5 via the line 16 is supplied to the absorption columns
2 and 13 via the lines 17 and 18, respectively.
EXAMPLE II
The reference numerals refer to ~igure 2.
2~ A gas stream of 10,000 Nm3/h~ comprising 90 % by volume of C02, 7 %
by volume of H2O, 2 % by volume of hydrocarbons and 1 % by volume of H2S, is
introduced between the eighth and ninth tray from the bottom of the absorption
column 102 ~hich contains 18 trays). The pressure in the column is 1.05 bar
abs. and the temperature ~0C. The gas becoming available from this column
via the line 103 ~9,790 Nm /h) still contains 100 ppm by volume of H2S and
further gl % by volume of C02, 7 % by volume of H20 and 2 % hy volume of hydro-
carbons. The loaded absorben~ in column 102, comprising a 24 % by weight solu-
tion of methyl diethanol amine in water, is introduced via the line 104 into
the 12-tray regeneration column 105. The regeneration column is heated with
3a s~team to a bottom temperature of 110C, the pressure being 1.4 bar abs. From
,~, .
~95~l~
this column vi~a the line lR6 ~he desired gas ~ecomes available at a rate o:E
2260 Nm /h and having a composition of 47 % by volume of H2S, 47 % by volume
f C2 and 6 % ~ volume of H2O. Of this gas~, 210 Nm3/h is discharged via
t~e line 107 to a Claus plant, and 2050 Nm3/h is introduced below the first
tray from the 6Ottom of a6sorption column 102 via the line 112. The gas velo-
city in this column is 1~5 m/sec~ The a~sor~ent regen0rated in the regenera-
tion column 105 is supplied via the line 117 to the a6sorption column 102 at
a rate o~ llO m3/h.
, ~ . .
