Note: Descriptions are shown in the official language in which they were submitted.
897 o.z. 31,905
REMOVAL OF C2 AND/OR H2S FROM GASES
The present invention relates to a process for removin~ H2S
and/or C02 from gases which contain these constituents, especially
as impurities, by washing the gases with a solvent whlch contains
the methyl isopropyl ethers of polyethylene glycols.
The use of organi¢ solvents or aqueous solutions of organic
solvents to remove undeslred acid constituents, eg. H2S and C02,
from natural gases and synthesls gases, has been disclosed. A re-
view article in H~drocarbon Processing, April 1975, pages 84 - 105,
may be mentioned as representative of the extensive prior art,
The solvents for the selective removal of H2S in the presence
f C2 comprise two groups. Firstly, there are chemical solvents,
r, eg. a~ueous solutions of methyldiethanolamine and solutions of salts
of C~-aminocarboxylic acids, eg. glycine or alanine (Alkazid ~ ),
the selectivity of whlch is due to the fact that they dlssolve H2S
; many times more rapidly than they dissolve C02. Secondly, there are
physical solvents, eg. N-methylp~rrolidone (Purisol ~ ) and the di-
methyl ethers of polyethylene glycols (Selexol ~ ), whlch thermo-
dynamically dissolve more H2S-th~n C02.
~; In additlon to the solubility of a gas ln a solvent, from which
the minimum amounts of circulating solvent are calculated, the rate
o~ solution of the gas in the solvent is of great importance, since
it determines the size of the absorber.
It is an ob~ect o~ the present invention to provide a solvent
which not only exhibits a high rate of solution of H2S but in which
H2S is also adequately soluble.
-` 1(~91897
The use of dimethyl ethers of polyethylene glycols or
their mixtures to remove CO2 and/or H2S from gases is disclosed
in ~.S. Patents 2,649,166, 3,362,133 and 3,533,732. Germain Laid-
Open Application DOS 2,263,980 discloses alkylpolyethylene gly-
col tert.-butyl ethers as solvents for acid gases.
It is true that as a rule the above solvents exhibit
adequate absorption of H2S andtor CO2 and also satisfactory
viscosity characteristics; according to the experiments describ-
ed in German Laid-Open Application DOS 2,263,980 the unsymmetric-
al ethers have somewhat higher absorption capacities than the
dimethyl ethers described in the above U.S. Patents. However,
the rate of absorption of H2S, both with dimethyl ethers and
with methyl tert.-butyl ethérs of polyethylene glycols, is not
satisfactory in every case.
We have found, surpr~singly, that the above object
is achieved by the use of methyl isopropyl ethers of polyethylene
; glycols. These have a higher rate of solution of H2S than the
conventional dimethyl ethers and alkyl tert.-butyl ethers; it
; is therefore possible to choose a relatively smaller absorber.
Accordingly, the invention relates to a process for
removing CO2 and/or H2S from gases which contain these consti-
tuents, by washing the gases in an absorption zone under pres-
sure with a solvent comprising one or more alkyl ethers of
polyethylene glycols of from 2 to 8 - /CH2-CH2-O/- units, to
the temperature of the solvent at the top of the absorption
zone not exceeding 50C, with subsequent regeneration of the
solvent, methyl isopropyl ethers of ethylene glycols being used
as the solvent.
Gases which may be purified in this way are coke oven
gases, coal gasification gases, synthesis gases and, preferably,
natural gases, from which H2S is to be removed selectively.
According to the invention, the methyl isopropyl
` 1(~9189 7
ethers of polyethylene glycols of the following formula,
which contain from 2 to 8 ethylene glycol groups (i.e.
n = from 2 to 8) can be used as solvents:
H
H3C-O-(CH2~O)n-~-cH3
CH3
The use of ethers with from 3 to 7 ethylene glycol groups is
preferred; from the point of view of the rate of solution of
H2S, the compound with 3 ethylene glycol groups, ie. the methyl
isopropyl ether of triethylene glycol, has proved'best, whereas
compounds with 6 to 8 ethylene glycol groups are more suitable
for removing CO2. However, in practice mixtures, obtained by
synthesizing these compounds in the presence of stro'ngly acid
; cation exchange resins, are as a rule employed (cf. Berman
printed Application 2,544,569). I mixtures of monomethyl
ethers with 3 to 5 ethylene glycol,units are reacted with
propylene in accordance with German printed Application
2,544,569 and the low-boiling constituents are removed, the
residual mixtuxe of monomethyl ethers and methyl isopropyl
ethers may be employed as the solvent.
As a rule 7 the solvents are employed in a virtually
, anhydrous form. If steam stripping is carried out in the
desorption column, the water content of the solvent should not
exceed 8% by weight, based on the solven~.
, From the point of view of the ability to dissolve CO2
and H2S, the!methyl isopropyl ethers of polyethylene glycols
behave like physical solvents, ie. Henry's law applies as a
good approximation, and thermodynamically more H2S than CO2
is dissol~ed,
The process according to the invention can be carried
out at normal or superatmospheric pressure, advantageously at
H2S partial pressures greater than 0.05 bar and especially
-2a-
1C~91897
greater than O.S bar. When removing C02 from gases not containing
H2S, the C02 partial pressure should advantageously be greater
than 4 bars and especially greater than lO bars. The washing
process may be carried out in one stage or two stages. The
choice of washing process as a rule depends on the partial
pressures of the gases to be washed out and on the final purity
required, or on the permissible heat consumption or stripper
gas consumption.
The process according to the invention may be carried
out either with packed columns or with columns fitted with
exchange trays. The temperature of the solvent at the top of
the a ~ ber should not exceed 50C, since, the higher the
temperatures, the lower is the gas loading of the solvent.
The absorption is as a rule carried out at from 20 to 40C.
The top temperature of the absorber iS fixed in accordance
with the conventional criteria and as a rule depends on the
desired degree of purity and on the temperature of the cool-
ing water.
The rich solvent can be flashed in one or more stages,
eg. using a flash turbine, before it is substantially regenera-
ted in a packed desorption column or a desorption column
equipped with trays, using stripping gas or steam which can be
injected directly or can be generated by adding from 2 to 8%
by weight, especially from 3 to 5% by weight, of water to the
solvent and employing indirect heat exchange. The solvent can
also be stripped with an inert gas.
If, after flashing, the stripping is carried out in
a column, it is advantageous to choose a pressure of from 1.1
to 1.5 bars in th ~ain flashing stage.
The solvent running into the desorption column can be
heated by means of the solvent discharged, in a countercurrent
heat exchanger. The temperature at the bottom of the absorber
--3--
1091897
as a rule is from 110 to 140C, especially 115 to 130C. Thesolvent is conveyed by means of a pump to the top ~f the absorber
via a cooler which can be used to set up the desired top tem-
perature of the absorber.
If the wash is carried out in two stages, only a part
of the ~olvent, coming from the desorption column, is fed to
the top of the absorber, while the remainder is fed, at a some-
what higher temperature, to another point of the absorber as it
comes from the main flashing stage.
Figures 1 and 2 show two preferred process flow charts
for carrying out a one-stage wash and a two-stage wash (rough
wash and fine wash~, respectively.
The one-stage wash as shown in Fig. 1 is particu-
larly suitable or gase~ with low partial pressures o the com-
ponents to be washed out.
A rough wash using a flashing circuit may be carried
out as follows (cf. Figure 1):-
. ~he gas to be washed is supplied through line 11 tothe absorption column 1 through which it flows from bottom to
top countercurrent to the solvent which is charged at the top
od the column. The washed ~treated) gas leaves the absorption
column 1 at the top via line 12. The solvent loaded with sour
gas leaves column 1 at the bottom and is flashed through a
- flash turbine 4 into a flash column 2. It is then supplied
: via heat exchanger 7 to the desorption column 3. The degassed
solvent leaves the desorption column at the bottom and is forced
by pump 5 via solvent cooler 9 into the top of the absorption
column. The flash gas from the flash stage leaves column 2
at the top through line 13. The off-gas from desorption
column 3 leaves at the top and is then cooled in offgas cooler
10. The heat balance of the wash is matained by heat exchanger
8 at the bottom of column 3
.
--4--
1Q91897
In the Figures,thenumbers denote the following:
1. Absorption column
2, Flash column
3, Desorption column
4, Flash turbine
5. Solvsnt pump
6. Condensate pump
7. Solvent / solvent heat exchanger
8. Reboiler
g. Solvent cooler
10. Off-gas cooler
11. Crude gas
12. Treated gas
13. Flashing gas ~inert ga~ ~ component washed out)
14, Off-gas ~component washed out)
/
:, /
//
' /
/_
.
1C~91897 o.z, ~1,905
Figure 2 shows a pre~erred flow diagram for two-stage washing
(rough and fine washing) with one flashing stage and one desorption
stage (stripper)O The absorption column 1 comprises two sections 21
(rough wash) and 22 (rine wash). The solvent loaded with sour gas is
flashed, as in Fig~ 1, in turbine 4 and column 2. The solvent leav-
in~ flash column 2 at the bottom ls divided lnto two streams. One
portlon Or the stream goes to rough wash column 21 after passlng
through pump 25, while another portion of the flashed solvent passes
through heat exchanger 7 to the top of desorption column ~. Reboiler
8 converts some of the solvent into vapor with which the solvent ~n
column 3 is stripped from sour gas. The solvent stream thus regene-
rated is pumped by pump 5 through heat exchangers 7 and 9 for cool-
ing, and then fed to fine wash column 22. The off-gas leaving at
the top of desorption column 3 is cooled in off-gas cooler 10.
In this Flgure, the numbers denote the following:
21. Rough wa~h column
22. Fine wash column
25, Solvent pump 2.
In addition to their ability to dlssolve H2S and C02, the methyl
~o lsopropyl ethers of polyethylene glycols are able to absorb water.
Hence, the solvents to be used accordlng to the lnventlon can also
be used for conditlonlng natural gases. In that case, the water con-
talned in the natural gas would be removed at the top or the stripper
(compare position 3 in Figures 1 and 2). If the solvent of the in-
vention is used for this purpose, the procedure followed would be as
; described in German Laid-Open Application DOS 2,4~7,576, whlch pro-
poses a process for conditioning natural gases by means of solvents
other than those now proposedO
The present invention is illustrated by Examples 1 and 2 whlch
follow. Comparative Example 1 compares the rate of absorption of H2S
by methyl isopropyl ethers of polyethylene glycols wlth the rate of
absorption by the ethers mentioned in U.S. Patent ~9~62,1~ (e) and
German Laid-Open Application DOS 2,26~9980 (f)9 and Comparative Ex-
--6--
1091897
-- o.Z. ~1,905
ample ~ the stabllity of the methyl tertO-butyl ethers of ~OS
2,26~,98C wlth the methyl isopropyl ethers of the invention.
EXAMPLE 1 ~ Selective H2S removal
2C)0 m~(S.TOP.)/h of a dry synthesls gas at 16 bars and 50C
are supplied to a packed column of 00~ m diameter packed to a height
of 7.5 m. The composition Or the gas is as follows (in % by vol.):
C~ 400
CO 4608
002
N2 0.2
Ar -4
H2 48.0
H S oO4
COS 24 vol. ppm
The gas is washed countercurrently with 1.6 m3/h Or a solvent
comprlsing 90~ w/w of asymmetrical methyl isopropyl ethers of poly-
ethylene glyaols [26 w~.% trl, 36 tetra, 2~ penta, 11 hexa and
4 hepta~ , 6~ of slmllarly composed monomethyl ethers and 4% of wa-
ter, the fe¢d temperature being relatively unfavorable at 50C.
The treated gas leaving the top of the absorber contalns 209% v/v
;~ 20 C02, 8 vol. ppm Or COS and o.8 vol.% H2S. The wash liquld loaded
with sour gas has a temperature of 51C at the bottom of the ab-
sorber. It is regenerated by flashing to 1025 bars and strlpping
with steam in a desorption column (bottoms temperature 1~0C), al-
:
- lowed to cool to 50 and returned to the top or the absorber.
i~i
` EXAMPLE 2 - Joint removal of H2S and C02
The method of Example 1 is followed, but 7 m~ (S.T.P.~ wash
liquid is used per hourO At the top of the absorber the treated gas
contains 1200 vol. ppm C02, ~ 1 vol. ppm H2S and ~-8 vol~ ppm COS.
COMPARATIVE EXAMPLE 1
,- ,
Table 1 which follows shows the transfer coefficients Kg for
the solvents of the lnvention and for varlous solvents of the prior
~0 art. The Kg values were determined in a ~et stream chamber, the Kg
--7--
1091897
.., O.Z. 31J905
~ e Or the met~lyl isoprlopyl ether of triethylene glycol being
taken arbltraril~ as 1.
TABLE 1
relative mass transfer
~a~ Metl-yl isopropyl ether of tr~ethylene glycol
(b~ Methyl isopropyl ether of tetraethylene glycol o.86
~c) Methyl isopropyl ether of pentaethylene glycol 0.79
(d~ Undistilled m~xture of a, b and c 0.72
~e) Mixture of dimethyl ethers of polyethylene 0 57
glycols with low-boiling constituents
10 (f) Methyl tert.-butyl ether of triethylene glycol 0.79
(g) Methyl tert~-butyl ether of tetraethylene glycol o.58
COMPARhTIVE EXAMPLE 2
(a) Table 2 which follows shows the results of' comparative
experiments on the decomposition of~ the methyl tert.-butyl ether
Or tetrae~hylene glycol (A~ and of the corresponding methyl iso-
p~opyl ether ~) with sulfurlc acid. In each case, 30 g of the ether
(A) or (B) were heated with 2 drops of concentrated sulfuric acld
at 140C (A) or 270C (B only) for 1 hourO In the case of compound
(B), a further 2 drops of concentrated sulfuric acid were after-
ward added at the higher temperature and the material was heated
0 for a further 2 hours at 270C. In each case, the isobutene or iso-
propylene eliminated was determined.
TABLE 2
Proportion decomposed A B
in ~
Amount of concentrated H2S04
2 2 4
140C, arter 1 hour 100~ O
-- _.
270 C
after 1 hour - O
after 3 hours _ 2~
Table 2 shows that the solvents to be used according to the
invention are substantially more stable in an acid medium than
--8--
1(~918~7
OOZ. 31,905
the solvents of the prlor art~ as may be seen rrom the low degree
of decomposltion~
~b) In a rurther experimentJ the rate of decomposition of the
: ethers (A) and (B) was determlnedO For this purpose, 100 g portions
of the ethers were heated wlth 5% of the acld lon exchanger used
rOr the manufacture of the ether (B) (a sulfonated crosslinked poly-
styrene resln ln the H+ form) at 70C, and the rate of elimlnation
of olefins was measuredO If the rate constant of the decomposition
reactlon for (B) is taken as = 1, a value o~ 562 is ~ound ror the
compounds (A) of the prlor art~
~
.,
''
'
