Note: Descriptions are shown in the official language in which they were submitted.
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Removal of C02 and/or H?S from gases
The present invention relates to a process for
removing C02 and/or H2S from gases by means of an aqueous
absorption liquid.
It has been disclosed, for example by A.L. Kohl
and F.C. Riesenfeld in Gas Purification, 3rd edition,
1979, that aqueous solutions of monoethanolamine or di-
ethanolamine or m;xtures of cyclotetramethylene sulfone
and an aqueous solut;on of d;isopro~anolamine can be used
as solvents for remov;ng C02 and/or H2S from gases. In
these processes, the solvent laden w;th C0z and, where
relevant, H2S has to be regenerated in a stripp;ng column
by feeding in steam, and this ;nvolves the consumpt;on
of a cons;derable amount of energy. In the removal of
C02 and, ;f necessary, H2S from natural gases containing
higher hydrocarbons by means of a mixture of cyclotetra-
methylene sulfone and an aqu00us solution of diiso-
propanolamine, there is the additional disadvantage that
the higher hydrocarbons have a relatively h;gh solubility
;n th;s solvent, so that the acid;c gas taken off at the
top of the stripping column has a relatively high content
of hydrocarbons, which, where the acidic gas contains
H2S, can Lead to difficulties in a downstream Claus
plant. Moreover, primary or secondary alkanolamines,
such as monoethanolamine or diethanolamine, can as a rule
only be used as aqueous solutions having a relatively low
concentrat;on of these alkanolam;nes, s;nce the use of
h;gher concentrat;ons may cause severe corrosion damage
to parts of the plant.
There was therefore a need for a process for
remov;ng C02 and/or H2S from gases, in which the dis-
advantages of the conventional processes can be avoided.
It ;s an obj~ect of the present invention to pro-
v;de a process for remov;ng C02 and/or H2S from gases,
in which both capital costs and energy costs can be re-
duced compared w;th the conventional processes~
We have found that this and other objects and
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advantages are achieved, in accordance with the invention,
by a~process for removing C2 and/or H2S from a gas con-
taining C02 and/or H2S by means of an aqueous alkanolamine-
containing absorption liquid, wherein, in an absorption
stage, the gas containing C02 and/or H2S is treated at from
40 to 100C with an aqueous absorption liquid which con-
tains from 20 to 70 % by weight of methyl-diethanolamine,
the treated gas is taken off at the top of the absorption
stage, the aqueous absorption liquid laden with C02 and/or
H2S is removed at the bottom of this stage and is regener-
ated by being let down in two or more flash stages, the
final ~lash stage being operated under reduced pressure
and the latter being generated by means of a mechanical
apparatus for producing reduced pressure and a steam-jet
ejector, which are connected ;n series, and the regener-
ated absorpt;on liquid is recycled to the absorption
stage.
In an advantageous embodiment of the process, the
water losses resulting from removal of water in the gas
streams taken off at the top of the absorption stage and
from the flash stages are compensated by feeding in, at
the bottom of the penultimate flash stage, an amount of
steam corresponding to the water loss.
In the novel process, the solvent laden with C02
and/or H2S is regenerated solely by flashing, without the
use of a stripping column, so that a substantial reduction
in both capital costs and energy costs is achieved.
Furthermore, the novel process permits the use of a rela-
ti~vely high concentration of methyldiethanolamine in the
3~0 absorpt;on liqu1d, without resulting in corrosion damage
in the gas washer. Another advantage of the process is
that the water balance~of the gas washer can be controlled
in~a slmple manner.~ Using this procedure, it is possible
to regulate not only the water balance of the gas washer
~` 35 but also its heat balance, so that any heat exchanger pre-
s;ent in the gas washer~for regulating the heat b~alance can
be smaller or, if appropriate, can be omitted. The
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procedure according to the invention gives a regenerated
absorption l;quid which has a low C02 and/or HzS content,
so that it is possible to circulate smaller amounts of
absorption Liquid; this results in a corresponding saving
of energy~
In another preferred embodiment of the process, the
steam-jet ejector is downstream of the mechanical apparatus
for generating reduced pressure. It may be advantageous if
the gas taken off at the top of the final flash stage is fed
in~ together with the steam used to operate the steam-jet
ejector, at the bottom of the penultimate flash stage.
Examples of gases which can be treated using the
novel process are coal gasification gases, synthesis gases,
coke oven gases and, preferably, natural gases. The process
is advantageously used for removing C0z and/or H25 from
natural gases which contain higher hydrocarbons in addition
to methane. These higher ~ydrocarbons are in general C2-
C30-hydrocarbons, in part;c~lar C2-C1z-hydrocarbons, which
as a rule are aliphat;c, eg. ethane, propane, isobutane, n-
butane, isopentane, n-pentane, the hexanes, heptanes, oc-
tanes, nonanes ancl decanes and the higher homologs. The
higher hydrocarbons can contain~ in addition to the ali-
phatic hydrocarbons, aromatic hydrocarbons such as ben-
zene. In general, the natural gases contain from 0.1 to
40, preferably from 0.5 to 30, in particular from 1 to 20,
mol X of the higher hydrocarbons.
The gases contain in general from 1 to 90, prefer-
ably from 2 to 90, in particular from 5 to 60, mol % of
CQz. They can also contain H2S as a further acidic gas,
or can contain HzS alone, for example in an amount from
a few mol ppm,~for example 1 mol ppm, to 50 mol %, pre-
- ferably from 10 mol ppm to 40 mol %O
The solvent used for the novel process is an
aqueous absorption liquid which contains from 20 to 70,
~; ~ 35 ~preferably from 30 to 65, in particular from 40 to 60, %
by weight of methyldiethanolamine. An aqueous methyl-
d1ethaAolamine solution is advantage~usly employed, for
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example an aqueous solution of technical-grade methyl-
diethanolamirie. In an advantageous embodiment of the pro-
cess, an aqueous methyldiethanolamine solution ~hich addi-
tionally contains from 0.05 to 1, in particular from 0.1
S to 0.8, in particular from 0.1 to 0.6, mole/liter of a
primary amine or alkanolamine, such as monoethanolamine,
or preferably a secondary amine or alkanolamine, advantage-
ously methylmonoethanolamine, very particularly advantage-
ously piperazine, ;s used.
The aqueous absorption liquid containing ~rom 20
to 70% by weight of methyldiethanolamine may additionally
contain a physical solvent. SuitabLe physical solvents
are, for example, N-methylpyrrolidone, tetramethylene sul-
fone, methanol and oligoethylene glycol dialkyl ethers,
such as oligoethylene glycol methyl isopropyl ether
~SEPASOLV MPE)~or oligoethylene glycol d;methyl ether
~SELEXOL)~ The physical solvent is present in the absorp-
tion liquid in general in an amount of from 1 to 60, pre-
ferably from 10 to 50, in particular from 20 to 40, % by
weight.
The novel process is carried out as follows: the
gas conta;ning C02 and/or H2S is first treated, in an
absorption stage, with the methyldiethanolamine-containing
absorption liquid. During this procedure, the temperature
of the absorption l;quid in the absorption stage is main-
tained at from 40 to 1û0C, preferably from 50 to 90C,
in particular from 60 to 80C. In general, the pressure
used in the absorption stage is from 10 to 110 bar. The
absorption stage ;s advantageously an absorption column,
in general a packed column or a column equipped ~;th trays.
Advantageously~ the gas to be treated is fed in at the
bottom and the absorption liquid is fed in at the top of
~the absorption column, the acidic gases C02 and/or HzS
~being washed out by a countercurrent procedure. While any
H2S present is advant~ageously washed out to a substantial
extent, in general so that the treated gas has an H2S
~ content of not more than lZO, preferably not more than 10,
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in particular not more than 3, mol ppm, it may be advan-
tageous to wash out the C02 ~rom the gas so that the treated
gas contains not more than about 0.5 - 6, preferably from
0.5 to 5, in particular from 1 to 4, mol X of C0~. The
treated gas is advantageously taken off at the top of the
absorption stage, expediently at a point above the feed of
the absorption liquid. The absorption liquid laden with the
acidic~gases C02 and/or H2S is advantageously taken off at
the bottom of the absorption s~age.
The laden absorption liquid is then regenerated in
2 or more, advantageously from 2 to 5, preferably 2 or 3,
flash stages, the final flash stage being operated under
reduced pressure, and the latter being generated by means
of a mechanical apparatus for producing reduced pressure
and a steam-jet ejector, which are connected in series,
and any water losses resulting from removal of water in the
gas streams taken off at the top of the absorption stage and
of the flash stages may be compensated by simultaneously
feeding in, at the bottom of the penultimate flash stage,
an amount of steam corresponding to the water loss. Pre-
ferably, a pressure of from about 0.3 to about 1, prefer-
ably from 0.4 to about 1, in particular from 0.5 to about
0.9, bar is maintained in the final flash stage. Examples
of suitable mechanical apparatus for producing reduced
pressure are vacuum pumps and, preferably, compressors,
eg. screw compressors or centrifugal compressors. The
steam-jet ejector is preferably downstream of the mechani-
cal apparatus for producing reduced pressure. In general,
the flash stages are operated at from 35 to 100C, prefer-
ably fram 45 to 90C, in particular from 55 to 85C.
To compens~ate for water losses which ar;se in theprocess as a result of water being removed in the gas
streams taken off at the top of the absorption stage and
from the flash stages, an amount of steam corresponding
35~ to the water loss is ~advantageously fed in at the bottom
of the penultimate flash stage. As a rule, the water
~; present in the gas streams taken off is substantially
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removed in the form of steam. Low-pressure, medium-
pressure or high-pressure steam, eg. steam under 1~5-100
bar, can be fed to the bottom of the penultimate flash
stage. Preferably, low-pressure steam, eg. steam under
1.5-10~ advantageously 1.5-S, bar, is used, since this
steam is in general cheaply available.
The gas taken off at the top of the final flash
stage can be released to the atmosphere or, if it still
contains HzS, can be worked up by oxidizing the H2S, for
example in a Claus unit. In an advantageous embodiment
of the process, the steam-jet ejector is do~nstream of
the mechanical apparatus for producing reduced pressure,
and, expediently, the gas taken off at the top of the
final flash stage ;s fed ;n, together with the steam
used to operate the steam-jet ejector, at the bottom of
the penultimate flash stage.
Where the steam used to operate the steam-jet
ejector ;s fed in at the bottom of the penultimate flash
stage, the ejector is advantageously operated using an
amount of steam corresponding to that required to compen-
sate the water losses in the process. However, it ;s also
possible to operate the steam-jet ejector with an amount
of steam smaller than that required to compensate the
water losses, and in add;tion to feed in the lacking
amount of steam at the bottom of the penultimate flash
stage. The steam-jet ejector can be operated using
medium-pressure or high-pressure steam. Medium-pressure
steam, eg. under 5-Z0, preferably 5-10, bar, is preferably
; used.
The penultima~e flash stage is advantageously oper-
ated under a pressure of about 1-30, preferably about 1-25,
in particular about 1-Z0, bar.
Flashing is advantageously carried out using flash
chambers which can, for example, also be in the form of
columns. These flash chambers need not contain special
baffles,~ although columns equipped with baffles, eg.
packed columns, may also be used.
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The acidic gases CO2 and/or H2S are advantageously
taken off at the top of the final flash stage or (where
the gas taken off at the top of this stage is fed in, to-
gether with the steam for operating the steam-iet ejector,
at the bottom of the penultimate flash stage) at the top or
the penuLtimate flash stage. The regenerated absorption
liquid taken off at the bottom of the final flash stage is
recycled to the absorption stage.
The Example which follows ilLustrates the invention
in more detail, the course of the process being shown
diagrammatically in the figure.
In the-figure, a gas which contains COz and/or
HzS, for example a natural gas containing higher hydro-
carbons, eg. aliphatic C2-C10-hydrocarbons, is passed under
superatmospheric pressure, via line 1, into the bottom of
absorption column 2~ At the same time, 20-70% strength
by weight aqueous methyldiethanolamine solution is passed,
as an absorption liquld, v;a line 3 to the top of the
absorpt;on column. The absorption liquid, which is fed
countercurrent to the gas, becomes laden with the ac;dic
gases C2 and/or H2S, and the laden absorption liquid
is taken off at the bottom of the absorption column via
line 4. The washed gas is taken off at the top of the
absorption column v;a l;ne 11. The stream 4 of laden
absorption liquid is then let down into a flash chamber 5,
for example via a valve or, preferably, an expansion tur-
bine. In this stage, a flash gas is liberated from the
absorpt;on l;quid and ;s taken off at the top of flash
chamber 5 v;a l;ne 6.~ At the bottom of flash chamber 5,
the absorpt;on l;qu;d which has been p~rtially let down
is taken off v;a line 7 and~ if necessary, heated in heat
exc~hanger 8, for example by from 0 to 15C, and the absorp-
t;on l;quid, wh;ch may or may not have been heated, is let
down into a second flash chamber 9 in which reduced pres-
sure down to, for example, 0.3 bar is maintained by meansof the mechanical apparatus 14 for producing reduced
pressure, for example a compressor, and the steam-jet
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ejector 13. The steam-jet ejector 13 ;s advantageously
supplied~ via line 12, with the amount of steam required
to compensate the water losses of the system. The gas
taken off at the top of flash chamber 9 is fed, together
with the steam used for operating the steam-jet ejector
13, via line 1û to the bottom of the f;rst flash chamber 5~
The Example which follows illustrates the invention.
EXAMPLE
The gas washer used comprises an absorption column
and, downstream from this, three flash chambers in succes-
s;on. In the absorpti~n column, 3.15 kmol/h of a C02-
containing natural gas are washed, under 75 bar, with a
50% strength by weiyht aqueous methyldiethanolamine solu-
tion as the absorption liquid.
The gas to be treated has the following composition:
C2 10.0 mol%
CH4 75.0 mol%
higher hydrocarbons
(C2 C12-hydrocarbons) 15.0 mol~
The temperature of the absorption liquid in the
feed to the absorption column is 70C. The C02 content
in the washed gas iS less than 2 mol %. The laden wash
liquid leaving the absorption column is let down to 30 bar
in a first flash chamber. In this procedure, 0.011 mol/h
of a hydrocarbon-rich intermediate flash gas having a C02
concentration of less than 4 mol ~ is liberated from the
solution and taken off at the top of the first flash
chamber. The partially let down absorption liquid is then
heated in a heat exchanger, after which it is let down to
1.5 bar in a second flash chamber. In this procedure,
0.26 kmollh of a C02-rich flash gas having a C0z concen-
tration of more than 99 mol % is liberated, and removed
from the top of the second flash chamber.
The absorption liquid taken off at the bottom of
the second flash chamber is passed through a heat exchanger
and ~hen finally let down in a third fLash chamber in
which a pressure of 0.7 bar is m~intained by means of a
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steam-jet ejector and a compressor. The gas taken off at
the top of the third flash chamber is passed, together with
the steam used for operating the steam-jet ejector, into the
bottom of the second flash chamber. The absorption liquid
S taken off at the bottom of the third flash chamber is
recycled to the top of the absorption column with the aid
of a circuLatory pump.
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