Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
108Ss9~
The invention relates to a process for the removal
of acid gases from a gas or liquid mixture by contacting
the said mixture with a solution containing water and an
amine in which at least two alkanol groups which are bound
to nitrogen are present, followed by the removal of the
acid gases from the resulting acid gas-loaded solution
by heating.
For many years aqueous solutions of di- and trialkanol
amines have been used for the removal of acid gases, by
which in this patent application are meant hydrogen sulphide
and carbon dioxide and also compounds which are relatively
easily convertible into ;hese materials such as carbon
oxysulphide, from liquids, e.g. hydrocarbon mixtures,
and in particular from gases, such as natural gas and
refinery gases. These amines are very suitable to reduce
to lower values the acid gas contents of the gases or
liquids to be purified, also when the pressure of the
gas to be purified is not high, e.g. atmospheric. The
reduction of the content of acid gases, in particular
H2S, to a minimum is becoming increasingly important
owing to the environmental regulations imposed by the
authorities. In this cGnnection mention may also be
made, for example, of the removal of sulphur compounds
from off-gases from sulphur-removal plants (in particular
Claus plants) by converting all sulphur compounds in
these off-gases into H2S, followed by intensive H2S-removal
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from the resulting gas by means of the said amines
until a gas having a very Low H2S content is obtained.
As already stated, the reduction to very low
values of the acid gas content of the gases or liquids
to be purified is quite possible. The regeneration
of the amine solutions loaded with acid gases must
be carried out thoroughly to the effect that the
remaining content of ac d gases in the regenerated
amine solution becomes so low that this solution
can be re-used to reduce to the desired low values
the acid-gas content of the gases to be purified.
This thorough regeneration requires much heat; steam
is generally used as the source of this heat. It
is considered a drawback that a large quantity of
steam is necessary because the generation of steam
is expensive.
It has now been found that the quantity Or steam
required for the desired regeneration of the amine
solution can be limited considerably if certain
compounds are added to the amine solutions before the
extraction of the acid gases from the mixtures to be
purified. This not only makes the quantity of steam
needed in order to obtain a very low residual quantity
of acid gas in the regenerated amine solution smaller
than in the conventional process, but also allows those
parts ~ the plant where steam is produced and where
the regeneration takesplace to be smaller-dimensioned,
thus resulting in substantial capital savings.
10~35~90
In accordance with the present invention there is
provided a process for the removal of acid gases from a gas
or liquid mixture by contacting the said mixture with a
solution containing water and an amine in which at least
two alkanol groups which are bound to nitrogen are present,
followed by the removal of the acid gases from the resulting
acid gas-loaded solution by heating, characterized in that
before said solution is contacted with the said mixture,
sulphuric acid, an acid salt of sulphuric acid, oxalic
acid, an acid salt of oxalic acid or a salt, of which the
cation is removed from the ~olution before or during the
heating step, is added to the solution in an amount such
that during the removal of the acid gases by heating, 0.1
to 15 moleiO of the amine in the solution is present in the
acid form.
In general the solutions containing water and an
amine, in which at least two alkanol groups which are bound
to nitrogen are present (which amine is referred to below
as alkanol amine), will compri~e no substantial quantities
of other compounds. It is al~o poqsible, however, that a
con~iderable quantity of a physical solvent for acid gase~
i8 present, for example sulfolane, ~-methyl-pyrrolidone,
propylene carbonate or methanol.
Suitable alkanol amines are secondary amines, such
as diethanol amine and dipropanol amine, and in particular
diisopropanol amine and tertiary amines, such as tri-
ethanol amine and in particular methyl di-ethanol amine.
The quantities of the alkanol amine which are pre-
sent in the aqueous solution may vary between wide limits.
These quantities are very suitably 10-70~o by weight in parti-
cular 15-3~/o by weight, based on the total solution.
D _ 4 _
1~855~0
The compound to be added serves to ensure that
0.1-15 mole % of the amine in the solution is in the acid
form, i.e. the protonated form, during the removal of the
acid gases from the loaded solution.
As the compound to be added use is very suitably
made of an acid having a PK which under the conditions in
which the acid gases are removed from the solution is lower
than the PK Of the amine present which contains at least
two alkanol amine groups which are bound to nitrogen, with
the result that the amine is immediately converted into the
acid form. Sulphuric acid and oxalic acid are particularly
preferred.
By acids are, also meant acid salts, e,g. NaHSO4,
thus the acid salts of sulphuric acid and oxa~ic acid may
also be employed in accordance with the invention.
It is also possible to use acids the PK of which
is higher than the PK of the amine; it will then be
necessary to use an excess of equivalents of acid in order
to convert a desired quantity of the amine into the acid
form,
It is of course also possible to add the acid
together with an optionally equivalent quantity of an
am~ne, in which two alkanol groups which are bound to
nitrogen are p~esent, to the said solution. This amine
is preferably the amine which is present in the solution
as its main component.
~,
108~590
It is further possible that the compound to be
added does not immediately result in the protonation
of the amine, but that the said compound must be subjected
to a conversion after addition to the said solution. The compound
to be added is, for example, very suitably a salt of an
acid of which salt the cation is removed from the solution
before or during the heating step to remove the acid gases.
The æaid cation may, for example, be removed as a volatile
base by evaporation. As examples of such cations may be mentioned
cations of volatile amines (e.g. methyl amine, ethyl amine)
and in particular ammonium. Ammonium sulphate is very
suitably used as the compound to be added.
It is also possible to use as the compound to be added
a salt of an acid the cation of which is removed by precipitation
(possibly followed by filtration or centrifuging) before the
heating step to remove the acid gases. Examples are salts
of metals which have poorly soluble hydroxides, e.g.
aluminium, iron.
In this application, by "removal of a cation from the
solution" is also mean~ the binding of the cation in the
solution in such a way that the amine is converted into
the acid form by means of the anion which is added as a
salt together with the said cation. As an example of such
a removal may be mentioned complexing of a cation, if desired
only under the condîtions in which the regeneration of the
acid gas-loaded solution is carried out.
The quantity of the said compound to be added to the
above solution may vary. It will be best to determine
1085590
empirically in each separate case what quantity of a
certain compound results in an optimum saving in the
amount of heat required for the regeneration of the
! B acid gas-loaded solution. In general the quantity of
compound to be added will ~ 1-5 mole %, based on alkanol
amine.
Removal of the acid gases from the gas mixtures to
be purified and regeneration of the loaded alkanol amine
solution may be carried out in conventional equipment for
these operations, e.g. in absorption and regeneration
columns p~ovided with trays. In a number of cases a
somewhat larger number of trays will have to be used
in the absorption column and/or regeneration column in
the process according to the invention than in the
conventional process (i.e. using an alkanol amine without
the addition of the said compound), but the costs involved
are amply compensated by savings in those parts of the plant
where steam is produced and where the regeneration of the
acid gas-loaded solution is carried out, and in the quantity
of steam required.
In some cases the gases resulting from the process
according to the invention have such a low H2S-content
that they can be vented to the atmosphere without incineration.
This may in particular be the case for gases resulting
from removal of H2S according to the invention from gases
obtained by conversion into H2S of all sulphur compounds
present in off-gases of sulphur-removal plants, in particular
Claus plants.
1~85590
-- 8 --
EXAMPLE I
a hydrocarbon gas mixture containing 7.2% by volume
of H2S wa;. treated countercurrently with an aqueous
solution containing 27% by weight of di-isopropanol
amine at a pressure of 7 bar abs. and a temperature of
40C in an absorption column havin~ 15 valve trays.
The gas throu~hput rate was 230 Nm3/h, the liquid throughput
rate 0.65 m3/h. The purified gas mixture contained H2S
in a quantity of 10 parts by volume per million (ppm).
The aqueous solution loaded with H2S was regenerated
in a regeneration column having 16 valve trays by means
of low-pressure steam of which 220 kg/m3 of the aqueous
solution was required to remove therefrom such a quantity
of H2S that the solution could be reused for the purification
f the gas mixture to an H2S content of 10 ppm.
When a quantity of 0.015 mole of (NH4)2S04 or the
same quantity of oxalic acid per mole of di-isopropanol
amine had been added to the aqueous solution in advance,
the quantity of steam required for the regeneration
was found to be only 100 kg/m3 of the aqueous solution.
EXAMPLE II
The off-gas from a sulphur removal plant (Claus
process) had the following composition:
H2S 0.4 %vol
S2 0.2 "
COS O . 1 "
S (as S1) 0.1 "
C2 2.6 "
N2 64.6 "
H20 32.0 "
1085590
g
This off-gas was heated and passed together with a
gas containing H2 and C0 over a sulphided cobalt/
molybdenum on aluminil catalyst at 360~C. The resulting
gas (in which substantially all of the S02, COS and S
originally present had been converted to H2S) was cooled
to 40C in a cooling tower, in which the gas was contacted
with a circulating water stream. The gas from the top of
this column contained 1.1% by volume of H2S and 3.6%
by volume of C02. This stream was treated with 0.24 m3/h
of an aqueous solution containing 25% by weight of
di-isopropanol amine at 1.05 bar abs. and 40C in a
column provided with 11 valve trays at a gas throughput
rate of 95 Nm3/h. Upon regeneration of the loaded aqueous
solution with 280 kg of steam per hour in a regeneration
column provided with 16 valve trays, a purified gas
with 225 ppm of H2S was obtained. When a quantity of
0.02 mole of (NH4)2S04 per mGle of di~isopropanol amine was
added to the aqueous solution, a purified gas was found
to be obtained which contained only 25 ppm of H2S at the
same throughput rates and at the same quantity of steam
used.
EXAMPLE III
A quantity of 250 Nm3/h of a gas mixture obtained in
the gasification of oil, which gas mixture contained 0.5%
by volume of H2S and -4.5% by volume of C02, was treated
with 0.32 m3~h of an aqueous solution containing 22%
by weight of methyl-di-ethanol amine at a pressure of
1085590
-- 10 --
20 bar abs. and a temperature of 40C in a column with
20 valve trays, bringing the H2S content in the purified
gas to 4 ppm. In order to obtain this amount of H2S a
quantity of,50 kgth of ~team was necessary for the
regeneration of the loaded solution in a column with 16
valve trays. When a quantity of 0.023 mole of H2SO4 per
mole of methyl-di-et~anol amine was added in advance to
the aqueous solution, a quantity of only 28 kg of steam~hour
was found to be necessary to obtain a purified gas with
4 ppm of H2S under otherwise identical conditions.
EXAMPLE IV
A fresh aqueous solution of 209 k.moles di-isopropanol
amine per m3 was loaded with H2S to a concentration of
0.14 mol H2S/mol di-isopropanol amine by bubbling pure
H2S through the solution at ambient temperature. The
loaded solution was heated to 90C and introduced at a
rate of 4.05 kg/hr at the top of a regeneration column
eqU~ed with 50 trays. The H2S was stripped from the solution
with the aid o~ saturated steam which was introduced at a
temperature of 103C at the bottom of the column at
a rate of 0.52 kg/hr. The stripped solution was removed
from the bottom of the column (comparative Experiment A).
Two experiments (B and C) were performed with the
~ same aqueous solution of di-isopropanolamine to which
5 0.02 k.moles H2SO4 per k.mole di-isopropanol amine had
been added before loading with H2S. In the first experiment
the oonditions used were the ~ame as those applied in
. .
~0855~0
experiment A, in the second experiment the conditions
differed in that the rate of saturated steam used
was 0.~1 kg/hr.
In the table the H2S contents of the stripped
solution and the rate of steam used are recorded. As
can be seen less steam is needed to obtain nearly the
same H2S content in case H2S04 is present in the solution
of di-isopropanol amine (experiment C) or with the same
amount of steam a much lower content of H2S in the stripped
solution is obtained (experiment B).
Table
.Experiment H S0 added to kg steam mol H2S/mol di-isopropanol
a~in4e so.lution per hr. amine in stripped solution
_
A No 0.52 0.0062
B Yes 0.52 0.0012
C Yes 0,31 0.0074
;
