Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
o . æ ~ ~1 J ~69/~70
REMOVAL OF C02 AND/OR H2S F`ROM CRACKED GASES
It is known to remove C02 and/or H2S from cracked gases by
regenerative scrubbing using aqueous solutions of alkali metal
salts of aminoacids as ~he scrubbing liquorO However, the conven~
tional process has the disadvantage that the absorption capacity
of the regenerated scrubbing liquor decreases in the cour~e of
time, so that all or part o~ the scrubbing liquor must be replaced
at certain lntervals.
We have found an advant~geous process for removi.ng C02 and/or
H2S from a cracked gas by brin~ln~ the said gas, at ~rom 20 to 60C
10 and under atmospheric pressure or superatmospheric pressure, into
oontact, in an absorption zone, with an aqueous solution o~ the
alkali metal salts of aminoacids as the scrubbing liquor, freeing
the solution leaving the absorption zone from C02 and/or
H2S by heating in a desorption zone if necessary after first lett~
ing down the pressure, and then recycling the so-regenerated scrubb-
ing liquor to the absorption zone, wherein the scrubbing liquor
used is an aqueous solution of an alkali metal salt of a N-dialk~l-
~-aminomonocarboxylic acid.
The li~e of the scrubbing liquor is much longer in the proce~s
20 according to the lnvention than ln the conventional process.
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The process ~ccording to the lnvent.ion ls used with particular
advantage for removing C02 and/or H2S from a cracked gas~ containing
ethylene, which is obtained by thermal cracking of vaporiz~ble hydro-
carbons in the presence of steam in an indirectly heated tubular
r~actor at gas exit temperatures of above 760C.
It was surprising that using the process o~ the invention the
li~e of the scrubbing liquor can be extended wlthout f'urther addi~
tlves, since aerman Published Application 2,211,640 dl~closes that
when using alkali metal salts o~ lower ~waminoAcldsg whlch ma~ be
monosubstltuted at the amino group, as a s~:rubblng l~quor which can
be regenerated, it is necess~ry to add certain compounds containlng
amino groups, such as hydrazlne or phenylh~drazine, to the scrubb~
ing liquor, to extend the life of the regenerable scrubbing liquor.
The regen~rable scrubbi~g liquor used for the process of the
invention is an aqueous solutlon of an alkali metal salt of a N-
dialkyl-~-aminomonocarboxyllc acid carrying two alkyl groups on the
nitrogen atom. Pr~ferably9 the potassium salts of the N-dialkyl~
aminomonocarboxylic acid are used. Preferred alkyl groups are those
of 1 to 6 carbon atoms and especially of 1 to ~ carbon atoms,
Examples o~ suitable alkyl groups are ethyl~ n propylJ lsopropylg
n~bu~yl, isobutylJ pentyl, hexyl, cyclohexyl and especially methylO
ExampIes of sultable N-dialkyl-~-aminomonocarboxylic acids are N-
dimethyl-~-aminopropionic acid, N diethyl-~-aminopropionic acid, - -
disthylglycine and dimethylglyclnsO The use o~ an aqueous solution
of the potassium salt of dimethylgylcine ls particularly advantageous,
The density of the regen~ble scrubbing liquor~ at ?OOC, is
advantageously ad~usted to from ~05 to 1025l especially from 1015
to 1.200 The amou~t of scrubbing l~quor to be used depends~ egO,
on the amount of H2$ and C02 which ls to be removed9 on the concen~ :
tration o~ th~se compounds in ths gas mixture,c~ ~e d~rbd~ l p~ty
a~:~2'p~ ar W~ !p~: andwa~k~g t~ and on the residen¢e time
o~ the gas mixture. The most advantageous conditions can easily
be determined ~rom case to caseO
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O~ZO 31,369/370
~ he process is carried out under atmospherlc pressure or super~
atmospheric pressure, preferably ln the pressure range of from 5 to
50 atmospheres absolute and especially from 10 to 30 atmospheres
absoluteO It may be carried out in one or several stagesO
Temperatures o~ from 20 to 60C, preferably from ~5 to 50C,
are used i~ the absorption zone. The temperatures in the desorption
zone are in general ~rom 95 to 110C
It may be advantageous to add an al}cali metal hydroxlde and/or
an alkali metal salt o~ an acid having a pK o~ at least 5 to the
10 aqueous solution o~ the alkali metal salt of the N~dialkyl~amino-
monooarboxyllc acld, Preferably, the alkali metal salt~ of acids
which have a pK o~ at least 5 and are vola~ile under the temperature
oonditions of the desorption ~one are added. The alkali metal
hydroxide and/or alkall metal salt used are in general sodlum com~
pounds and especially potassium compounds. Examples Or su~table addi~
tives are sodium hydroxide~ sodium ~rbonate, sodium bicarbonate and
especially potassium hydroxide9 potassium carbonate and potassium
b$carbonate. The additive(s) may be introduced as such or ln the
form of a solution, eg, an aqueous solution. The amount of additive~s)
2Q may be varied depending on the nature o~ the cracked gas, In general,
from 0.001 to 1~ preferably from 0.01 to 005, m~-equivalent of the
additive(s) is introduced per m~ (S.T.P.) o~ the cracked gas which
is to be scrubbedO The additive(s) may be introduced oontinuouæly or
batchwise.
Preferably, the process according to ths invention ls used for
the treatment of ethylene-containing cracked gases which are obtal~ed
by thermal cracking of vaporizable hydrocarbons ln the presence o~ -
steam i~ an indirectly heated tubular reactor at gas e~lt temperatur~
above 760C. Preferably~ the gas exit temperatures are from 800 to
30 870 C and especially ~rom 810 to 860Co The weight ratiG o~ steam
to vaporizable hydrocarbon is in general O ~1: 2 0 0 9 The residence time
of the hydrocarbon in the tubular reactor i~ adv~ntageously from Ool
to 2 seconds, pre~erably ~rom 0.1 to 1 second and especially from
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~ ~ 7 8 ~ oOZ. ~ 6~/~70
0.1 to 0.5 ~econd~ Suitable vaporizable hydrocarbons are crude oil
and especially naphtha, gasoline, diesel oil or gas oilO
It may be advantageous~ in order to avoid polymer formation
and polymer deposition, ~o bring the aqueous sollltion of the alkali
metal salt of the Ndlalkyl-~aminomonocarboxylic acld into contact
with a hydrocarbon mixture which contains at least 40% of benzene
and lts homologsJ before and/or a~ter the desorption zone and, after
this treatment, to separate the hydrooarbon mixture agaln ~rom the
aqueous solution. Examples o~ auitable homologs Or benzene are tolu~
10 ene, the xylenesJ ethylbenzene and methylethylbenzene~ A hydrocarbon
mixture which contains ~rom 50 to 90 percent by weight of aromatics
o~ 6 to 9 carbon atoms ln the molecule is used pre~erentiallyO Small
amountsJ eg. up to a total o~ about 5 percent by weight, of poly~
merizable compounds, eg. styrene, c~clopentadiene or methylstyrene9
are not troublesome. Furthermore, the hydrooarbon mixture may con-
tain up to 60 percent by we~ghtJ preferably up to 50 per~ent by
weight, o~ non-aromatic hydrocarbons of 5 to about 12 carbon atoms
in the molecule.
The treatment of the aqueous solution o~ the alkali metal salts
o~ the ~-aminocarboxylic acld with the hydrocarbon mixture may be
carried out directly arter the absorption o~ C02 and/or ~ S or after
the desorption o~ the C02 and/or H2S. It is ad~antageous to mi~ the
aqueous solution of the alkali metal salt o~ the N-dialh~l~amino
monooarboxyli¢ a¢id, which is to be purifiedJ thoroughl~ with the
hydro¢arbon mlxture. The treatment with the hydrocarbon may be
carried out under atmospheric pressure or superatmospheric pressure
eg. at up to 30 atmospheres absolute~ The amount o~ hydrocarbon
mixture to be used dep~nds3 eg., on the treatment temperature, on
the amount of polymer to be removed and on the extent to which the
hydrocarbon mixture has been mixed with the solutlon which is to be
puri~ied.
EXAMPLE 1
A mlxture o~ naphtha o~ boiling range ~rom 40 to 180C and
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~8~'~5 ~o ~ 69/~70
steam~ in a weight ratio of naphtha to ~team o~ 1~0.5, i3 cracked
in a tubular reactor~ with a gas exit temperat-ure o~ 820C and a
residence time of 0.4 second. The cracked gas ls cooled to about
40 C in several stages by conventional methods and is then compressed
to about ~0 bars in a turbo-compressorO To remove the H2S and C02
constituents, the cracked gas is withdrawn9 after ~he fourth stage
of the compressor~ under a pressure o~ 17 bars a~d at ~C, and red
at a rate o~ 60Jooo m3 (S.T,P.)/hour to the lower part o~ a tray
column. The cracked gas consists essentially of ethylene~ methaneg
hydro~en and propylene, the remainder being composed o~ hydrocarbons
o~ 4 to 7 carbon atoms, The H2S oontent i~ 250 ppm and the C02
content 50 ppm. At the top o~ the scrubber column, 7 m3/hour o~ an
aqueous solution of the potassium salt of dimethylgl~olne, of den~ity
1.17, are introduced at 40C, The laden liquor is withdrawn at the
bottom of the scrubber columnJ and mixed thoroughly with a stream
of 2.5 m3/hour of a hydrocarbon mixture, containing about 60% of
benzene and its homologs, toluene and xylene, and the resultlng mix-
ture is fed to a separator. A~ter separation of the two phases, the
scrubbing liquor is introduced at the top of the regenerating column
and is freed from the absorbed C02 and H2S by heatin~ to about 110C~
The liquor regenerated in this way is returned to th~ absorptlon
co}umn after having undergone heat e~change. The residual contents
in the scrubbed crackad gas are found to be 10 ppm of C02 and 4 ppm
of H2S~ Even a~ter an on-stream period of two months, virtually
no decrea~e in the activity o~ the scrubbing liquor was detectableO
; A comparative experiment is carried out a~ descr~bed in the
preceding paragraph except that instead of the potasslum salt o~
dimethylglycine the potassium salt of N-methyl~ aminopropionic acld
ls used. After an on-stream period of two months, the activlty of
the scrubblng liquor had greatly decreased~ so that 130 ppm o~ H2S
and 30 ppm of C02 were found in the scrubbed gas~
EXAMPLE 2
The procedure described in the first paragraph o~ Example 1 is
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~ 5 0,Z. ~1,369/~70
followed, A~ter an on~stream period of 4 monthsJ the resldual
contents in the scrubbed gas had risen to 14 ppm of C02 and 17 ppm
f ~2S~
If the procedure described in the preceding paragraph is
followed but each day 8 kg o~ potassium hydroxide ln the form o~ a
50~ strength aqueous solution are added to the sorubbing liquor,
corresponding to 0.1 mg-equivalent o~ potassium hydroxide per m~
(S.T.P.) of scrubbed gas, no decrease in the activity o~ the scrubb--
ing liquor is dete¢table after an on-stream period o~ 4 monthsO
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