Note: Descriptions are shown in the official language in which they were submitted.
5~i~;3
PROCESS FOR TH~ PRODUCTION OF ELEMENTAL SULPHUR
The invention relates to a process for the production of
elemental sulphur from hydrogen sulphide-containing gases ac-
cording to a Claus-type process having at least a thermal zone
in which a hydrogen sulphide-containing gas is combusted with
a first free oxygen-containing gas, followed by one or more
catalytic zones in which hydrogen sulphide and sulphur dioxide
react together to form elemental sulphur and water.
H2S-containing gases may originate from, for example,
thermally or catal~tically processing crude mineral oils or
products derived from such oils, or from processing coke-oven
gas. Examples of such catalytic processes are desulphurization
processes carried out in a petroleum refinery. Natural gas may
also contain H2S. H2S may be removed from H2S-containing gases
by means of absorption in a regenerable absorbent. Regeneration
of the absorbent yields a gas having a higher H2S content than
the H2S-containing starting gas and usually also containing C02.
H2S may also be present in ammonia(NH3)-containing waste
gases. The above thermal and catalytic processes and processing
of coke-oven gas may also give rise to the formation of MH3-
containing waste gases. Such processes yield liquid or gaseousproduct streams containing ammonia, which must be removed there-
from. Ammonia may be removed from such streams by washing with
water at, for example, elevated pressure and reduced -temperature.
Washing is mostly carried out with an abundant quantity of water,
so that dilute, ammonia-containing solutions are formed. Steam
stripping of ammonia-containing solutions yields water suitable
for discharge into open surface water and an ammonia- and water
vapour-containing waste gas. Such waste gases may also contain H2S.
~ :~&5~6~
In a Claus~-type process the following reaction takes place
with regard to sulphur formation:
6H2S ~ 32 ' 6S + 6H2o (1)
which can be considered as occurring in two steps:
2X2S + 32 = 2S02 + 2H20 (2)
4H2S + 2S02 ~---- 6S + 4X20 (3)
In Claus-type processes an H2S-containing gas is partially
or completely combusted with a free oxygen-containing gas in a
furnace, also re~erred to herein as "thermal zone", followed
by one or more catalytic zones in which reaction (3) is carried
out. The gases ~ormed in the thermal zone are suitably cooled in
a waste heat boiler prior to their introduction into the first
catalytic zone. Thus, heat is recovered by producing steam; it
is desirable to recover as much steam as possible. The amount
of the s-team generated in the waste heat boiler will, i.a.~ depend
on the temperature of the gases formed in the thermal zone. This
temperature, in turn, will be determined by the temperature and
composition (H2S content and content of hydrocarbons and ammonia,
if any) of the H2S-containing feed gases and of the temperature
and composi-tion of the oxygen-containing gas supplied to the
thermal zone.
If the H2S content of the H2S-containing gas is low, for
example less than 20% by volume, and the balance consists of
non-combustible gases, for example carbon dioxide (C02), then
the requisite temperature for the combustion of the H2S cannot
be attained in the thermal zone. In this case the thermal zone
is difficult to operate, the combustion not being sufficiently
stable and ammonia - which is o~ten present in H2S-containing
gases - is only pa~tly burned. Moreover, the amount of steam
produced in the waste heat boiler is low. These difficulties
particularly hold when the free oxygen-containing gas has a low
temperature, for example below 0C and, particularly, below
S76~
-20 C, as may occur in areas with a very cold season. These
difficulties may be avoided by applying special measures,
such as combusting the H2S-containing gas with oxygen-enriched
air or pure oxygen, but these special measures are expensive.
In one version of the Claus-type process, referred to as
"straight-through operation", sufficient ~ree oxygen-containing
gas is used to convert the ~2S to elemental sulphur in accordance
with reaction (1). This implies that the ~uantity of ~ree oxygen-
containing gas is such that only one third of all of the H2S can
be oxidized to S02 in accordance with reaction (2), the oxygen
contained in the free oxygen-containing gas being consumed ac-
cording to reaction (2). As long as the H2S-containing gas has a
high H2S content (more than, for example, 40% by volume) and a
low content of non-combustible gases such as C02, they can be
readily combusted with production of elemental sulphur in the
thermal zone according to reaction (3). Reaction (3) is also
referred to as "Claus reaction". Thus, the effluen-t gas from
the thermal ~one finally contains H2S, S02, sulphur vapour~
nitrogen, water vapour, C02, C0, C0S, CS2 and h~drogen. C02 and
C0 are present, because they are formed by combustion of hydro-
carbons which are present in most of the H2S-containing feed
gases and part of the C02 is present, because most of the H2S-
containing feed gases also contain C02.
The effluent gas from the thermal zone usually has a temper-
ature in the range of from 600C to 1650C and is suitably firstcooled in a waste heat boiler and then in a condenser to a temper-
ature which is sufficiently low to condense lmost all of -the
sulphur vapour present. The latter temperature is suitably in the
range of from 170C to 200C. Alternatively, the effluent gas
from the thermal zone is cooled in one step to a temperature in
the range of from 170 C to 200C with production of steam~
After cooling of the effluent gas from the thermal zone to
a temperature in the range of from 170C to 200C the gases from
which almost all of the sulphur has been removed are re-he~ted
5P7~
to a temperature in the range of ~rom 230C to 280C and intro-
duced înto a first catalytic zone (a ca-talytic zone is herein~
after also referred to as "Claus stage")~ where H2S reacts with
S2 to produce more elemental sulphur in accordance with
reaction (3~. This production of elemental sulphur is increased
by the prior condensation of elemental sulphur. It is common use
to cool the effluent of the first Claus stage to a temperature
in the range of from 160 C to 170 C in order to condense elemental
sulphur which is withdrawn. Then, the gas stream freed from
elemental sulphur is heated to the reaction temperature required
for the next Claus stage, usually to a temperature in the range
of from 200C to 280C. The effluent of the second Claus stage
is cooled to a temperature in the range of from 160C to 170C
in order to condense elemental sulphur which is withdrawn. The
second Claus stage may be followed by a third or fourth Claus
stage in order to increase the sulphur production. Sulphur
production is further increased by a t~ fftep indirect cooling
of the effluent of the last Claus stage with wa-ter, in the first
and second step producing steam of a pressure of, for example,
3.5 and 2 bar, respectively.
Catalysts suitable to catalyze reaction (3) are well known.
Preferably~ activated alumina or bauxite is useda but any other
catalyst promoting this reaction may be used as well.
In another version of the Claus type process, referred to
as "spli-t-flow operation"g -the H2S-containing gas is split up
into a first and a second portion, representing about one third
and about two thirds, respectively, of the total H2S-containing
gas. The first portior. is combusted with a free oxygen-containing
gas in an amount stoichiometric with respect to reaction (2),
all or almost all of the H2S being oxidized to S02. In this
case a negligible amount of elemental sulphur, if any, is
formed. Thus~ the effluent gas from the thermal zone essentially
contains S02, nitrogen, water vapour, C02, C0 and hydrogen.
35~7~i~
In split-flow operation, the effluent gas from the thermal
zone is suitably cooled in a waste heat boiler and then combined
with the said second portion of the H2S-containing gas. The gas
mixture thus obtained is introduced at a temperature usually in
the range of ~rom 230C to 280C into the firs-t Claus s-tage~
The operation is then continued as described above for the
straight-through operation. Split-flow operation is particularly
suitable when gases having a relative~y low H2S content, for
example less than 40% by volume, have to be combusted.
If acceptable, the gases which have le~t the cooler after the
final Claus stage in straight-through or split-flow operation
may be incinerated in the presence of fuel gas and the combustion
gases thus obtained released into the atmosphere. ~his procedure,
however, has as a disadvantage that the heat present in these
combustion gases is lost.
It is an object of the present invention to recover heat
from the combustion gases mentioned above.
It is a second object of the present invention to provide a
process which allows a high recovery of steam from the heat
present in the gases formed in the thermal zone of a Claus-
type process.
It is a third object of the present invention to provide a
process which enables gases having a low H2S content to be com-
busted without being forced to apply oxygen-enriched air or
pure oxygen.
A further object of the present invention is to provide a
process which can be carried out with the aid of free oxygen-
containing gases having a low temperature.
A fourth object of the present invention is to provide a
process which allows a higher ammonia conversion to nitrogen and
water in the thermal zone, thus providing an environmental clean
process.
Accordingly, the invention provides a process for the
production of elemental sulphur from hydrogen sulphide-containing
5'7~3
gases according to a Claus-type process having at least a thermal
æone in which a hydrogen sulphide-containing gas is combusted
with a first free oxygen-containing gas, fo]lowed by one or more
cataly-tic zones in which hydrogen sulphide and sulphur dioxide
react together to form elemental sulphur and water, in which
process at least a portion of the first free oxygen-containing
gas and/or of the hydrogen sulphide-containing gases is pre-
heated by means of indirect heat exchange with off-gases ob-
tained by incineration of combustible sulphur compounds-con-
taining waste gases with a second free o~ygen-containing gas.
The gases obtained by incineration of combustible sulphur
compounds conta;ning waste gases with a free oxygen-containing
gas are belo~ also referred to as "incinerator off~gases". The
H2S-containing gases may be introduced into the the~mal zone as
one stream or as two or more different streams. Any of these
streams may contain ~H3.
The pre-heating of at least a port;on of the H2S-containing
gases and/or of the first free oxygen-containing gas by means of
indirect heat exchange with incinerator off-gases is suitably
carried out in such a manner so as to maximize heat recovery
from the incinerator off-gases for the production of high pres-
sure steam in the waste heat boiler.
Any portion of the first free oxygen-containing gas and
of the H2S-containing gases which is not preheated by means of
indirect heat exchange with incinerator off-gases may, if
desired, be preheated by any other suitable heat source. This
heat source may be available inside or outside the Claus-type
process. Preferably,all of the first free oxygen-containing
gas and all of the H2S-containing gas are pre-heated by means
of indirect heat exchange with incinerator off-gases. In the
case of split-flow operation, the said pre-heating may take
place before spl;tting up the H2S-containing gas into a first
and a second portion or each of the two portions may be
separately pre-heated after splitting up.
5~76~
The first free oxygen-containing gas and the X2S-containing
gases may be pre-heated by means of indirect heat exchange with
incinerator off-gases starting at the temperature at which they
are available, the former gas at a temperature below, for
example,0 C and the latter gas at a temperature in the range of
from, for example, 10C to 50C. The use of such temperatures
may cause cold spots in the material of the heat exchanger being
in contact with incinerator off-gases. As incinerator off-gases
contain water vapour and sulphur trioxide (S03) these cold spots
may give rise to corrosion of this material. This corrosion is
avoided when, according to a preferred embodiment of the in-
vention, a gaseous effluent is withdrawn from the catalytic zone
or, if more than one catalytic zone is applied, from a catalytic
zone which is final with respect to sulphur formation (the
~Ifinal catalytic zone") and the withdrawn effluent is cooled by
means of indirect heat exchange with waterS to condense elemental
sulphur and generate steam, which steam is used for indirectly
warming up the first free oxygen-containing gas and/or the
hydrogen sulphide-containing gases~ prior to the pre-heating
by means of indirect heating with incinerator off-gases.
Water to be used for indirect cooling of the gases formed in
the thermal zone with simultaneous generation of steam may be
pre-heated in any desired manner and suitably with the aid of
incinerator off-gases. ~hese incinerator off-gases and those to
be used for pre-heating at least a portion of the H2S-containing
gases and/or o~ the first free o~ygen-containing gas may
originate from the same or from different incinerators. Ac-
cording to a preferred embodiment of the invention incinerator
off-gases are first used for pre-heating the water and then
for pre-heating at least a portion of the first free oxygen-
containing gas and/or of the hydrogen sulphide-containing
gases. A]ternatively, the incinerator off-gases are first used
for pre-heating at least a portion of the first free oxygen-
containing gas and/or of the hydrogen sulphide-containing gases
~5 and then for pre-heating the water.
5~7~
The first free oxygen-containing gas, to be supplied to the
thermal zone, is preferably air. Oxygen-enriched air or pure
oxygen may be used if the H2S-containing gas contains so little
H2S that sustaining the combustion becomes necessary.
The second free oxygen-containing gas, required for the
incineration of the combustible sulphur compounds-containing
waste gases, may be heated before it is used for the inciner-
ation of the combustible sulphur compounds-containing waste
gases. This heating may be carried out in any suitable manner,
for example by means of indirect heat exchange with steam; this
heating is very suitably carried out by means of indirect heat
exchange with off-gases obtained by incineration of combustible
sulphur compounds-containing waste gases. According to a pre-
ferred embodiment of the present invention a stream of free
oxygen-containing gas, heated by means o~ indirect heat ex-
change with off-gases obt~ined by incineration of combustible
sulphur compounds-containing waste gases, is split up into the
fir8t stream which is supplied to the thermal zone, and the
second stream which provides free oxygen required for the
incineration of the combustible sulphur compounds-containing
waste gases.
If desired, a portion of the incinerator off-gases may be
mixed with the second free oxygen-containing gas, required for
the incineration; the mixture thus obtained can be incinerated.
The process according to the invention is particularly
suitable for split-flow operation, because the application there-
of in combination with high recovery of heat from incinerator
of~gases allows a higher temperature in the thermal zone and
therefore a more favourable combustion of gases having a low
~2S content, a higher cbnversion of NH3, if present, to nitrogen
and water, and a higher production of steam. In split-flow
operation, all of the H2S introduced into the thermal zone is
combusted to S02 and a hydrogen sulphide-containing gas is
supplied to the first catalytic zone; the H2S-containing gas
to be supplied to the thermal zone and the H2S-containing gas
to be supplied to the first catalytic zone may originate from
different ~ources and have different compositions, but suit-
ably originate ~rom the same source and have the same com-
position. According to a preferred embodiment of the presentinvention, an H2S-containing gas i5 split up into a first and
a second portion, all of -the H2S present in the first portion
being combusted to S02, and the combustion gases thus formed
being cooled7 after which the cooled combustion gases are mixed
with the second portion, the mixture thus obtained being intro-
duced into the ~irst catalytic zone. The said first and second
portion are suitably about one third and two thirds, respectivelyg
o~ the starting H2S-containing gas.
The steam generated in the waste heat boiler by cooling the
gases formed in the thermal zone may be used for any suitable
purpose. Preferably, H2S- and S02-containing gases which have
been cooled to condense elemental sulphur, are re-heated, prior
to introduction into a catalytic zone, by means of indirect
heat exchange with steam generated by indirect cooling of the
gases formed in the thermal zone.
Incinerating combustible sulphur compounds-containing waste
~ases involves conversion of the combustible sulphur compounds
to S02. This incineration may be thermal. However, thermal in-
cineration requires heating of the combustible sulphur compounds-
containing waste gases to a temperature in the range of ~romabout 500C to about 600C, which is costly because of the
necessary heat ;nput by, for example, combustion o~ fuel.
Furthermore~ concomitant presence of water and formation of
relatively large amounts of sulphur trioxide (S03) is a problem
for waste heat recovery with regard to the selection of
materials and the reliability of the equipment used. There~ore,
-the incineration is preferably carried out in the presence of
an incineration catalyst, this allows incineratio~ temperatures
much lower -than 500C. In this case less fuel, i~ any, need to
5~
be burned, thus reducing the amount of S03 formed thereby and
alleviating the said problem.
Accord;ng to a preferred embodiment of the present in-
vention the incineration is carried out at a temperature in
the range of from 150 C to 450 C and in the presence of an
incineration catalyst comprising as the catalytically active
components bismuth in an amount in the range of from o.6 to
10% by weight and copper in an &mount in the range of from 0.5
to 5% by weight, calculated on the total weight of the catalyst.
This method provides subs-tantial conversion of the ~2S, COS~
CS2 and elemental sulphur, and concomitantly provides low S03
formation. The method is economical in that the necessary heat
input is low, as evidenced by relatively low incineration temper-
atures. Moreover, this incineration catalyst shows an activity
which remains constant over a long period.
~ hile the temperatures at which the catalytic incineration
is carried out are not critical, it is an advantage that lower
or more moderate temperatures may be employed. Temperatures of
150C to 450C are quite satisfactory, while temperatures of
250C -to 420C are preferred.
The amou~t of free oxygen supplied to the incinerator is
important, in that a stoichiometric excess, preferably a large
excess, of free oxygen is desired in order to react all the
H2S and any COS and CS2 present. In general, at least twice,
and normally up to five times the stoichiometric amount of free
oxygen required for the reaction may be supplied. Preferably,
an excess of about 20 to about 280% of the stoichiometric
amount of free oxygen, based upon all total combustibles, will
be supplied. Amounts as high as 100 or e~en 200 times the
stoichiometric amount of free oxygen may be supplied, if
desired.
Methods of preparation and the most suitable composition
of this incineration catalyat, suitable sizes of catalyst part-
icles, methods for imparting a surface area to the catalyst
1~57Ç~
1 1
and the conditions at which the incineration may be carried out9
such as the pressure and the gas hourly space velocity are found
in 3ritish patent specification ~o. 1,558,656.
The free oxygen in the second free oxygen-containing gas may
be supplied as air, oxygen-enriched air or pure oxygen or from
other gaseous streams containing significant quantities of free
oxygen and other components which do not interfere significantly
with the incineration.
Any suitable combustible sulphur compounds-containing waste
gas may be used in the process according to the present in-
ventionj H~S-containing waste gases, for example Claus tail
gases, are ver~ suitable. By "Claus tail gases" are mea~t the
remaining gases as obtained after condensation of elemental
sulphur from the gases leaving the final Claus stage. As Claus
tail gases are available in large quantities, ~he off-gases
obtained by inciner~tion of Claus tail gases are also available
in large quantities. From this point of view and as these off-
gases have a relatively high temperature, these gases are very
~0 ~uitable for indirect heat exchange with at least a portion of
the H2S-containing gases and/or the first free oxygen-containing
gas according to the invention.
Claus tail gases still contain sulphur compounds and
elemental sulphur. A typical Claus tail gas may have the fol-
lowir.g composition:
H2S 0.1 - 2% vol. S02 5 ~ 1% vol.
COS 0.01 - 0.2% vol. CS2 0.01 - 0.2% vol.
Selement 0 01 - 0.2% vol H2 - 5% vol.
CO o - 3% vol. C02 2 - 50% vol.
H20 25 - 40% vol. N2 balance
In order to reduce a;r pollution, various procedures have
been developed to remove sulphur compounds and elemental sulphur
from Claus tail gases, and even recover, if possible, H2S or
.~' ' .
,,~''.~
~5~76i~3
12
reaction proaucts of H2S contained therein. Such procedures
considerably reduce the amounts of S02, S03 and water in the
incinerator off-gases. In thîs light the combustible sulphur
compounds-containing waste gases are preferably off-gases of a
process for the remo~al of sulphur compounds and elemental
sulphur from the tail gases obtained by coolîng the gases leaving
the final catalytic zone and separating elemental sulphur from
the cooled tail gases. Such a process for the removal of sulphur
compounds and elemental sulphur from Claus tail gases may comprise
the steps of reducing the sulphur compounds and elemental sulphur
under suitable conditions in the presence of a catalyst, absorbing
the H2S thus formed, followed by desorption of the H2S and, if
desired, introduction o~ the desorbed H2S into the Claus plant.
Thus, the H2S-containing waste gases to be incinerated contain
quite minor amounts of H2S.
According to a preferred embodiment of the present invention,
the process for the removal of sulphur compounds and elemental
sulphur from Claus tail gases comprises passing the tail gases
at a temperature above 175C, together with a free hydrogen-
and/or carbon monoxide-containing gas, over a sulphided Group VI
and/or Group VIII metal catalyst supported on an inorganicoxidic
carrier and passing the reduced tail gases thus obtained through
a liquid and regenerable absorbent for hydrogen sulphide.
Group VI and Group VIII refer to the Periodic Table of the
Elements as shown on the inside cover of "Handbook of Chemistry
and Physics", 59th edition (197O-1979).
After hav;ng passed the final catalytic zone and the relevant
condenser for the recovery of elemental sulphur, the Claus tail
gases normally have a temperature in the range of from 125C to
170C. The said reduction of the Claus tail gases is preferably
carried out at a temperature in the range of from 175C to 450C
and more pre~erably of from 260 C to 400C.
Before being contacted with the sulphided Group VI and/or
Group VIII metal catalyst, the temperature of the Claus tail
7~
gases has to be increase~ to a value above 170 C. This may be
effected in any desired manner, for example by means of in-line
burning, i.e. of thermal combustion of a fuel in the Claus tail
gases. In-line burning, however, has the disadvantages of re-
quiring burners, blowers for the supply of air, brick walllinings and fuel to be burned. Moreover, larger gas volumes have
to be passed over the metal catalyst and soot might be formed~
causing fouling of the metal catalyst. Furthermore~ S03 may be
formed when burning gases containing sulphur compounds and
special precautions have to be taken.
The invention, however, allows obviating these disadvantages
as a result of the high production of h;gh pressure steam, which,
in turn, results from the high heat input in the the~mal zone.
So, in accordance with a preferred embodiment of the present in-
vention, the Claus tail gases are indirectly heated by means ofhigh pressure steam~ generated by cooling the gases formed in the
thermal zone. As the inven-tion allows the production of a large
amount of high pressure steam, the Claus tail gases can thus be
heated to a relatively high temperature; sometimes a small further
increase of temperature is desired before they are contacted with
the Group VI and/or Group VIII metal ca-talyst. This small further
increase may be obtained in any suitable manner, but~ according
to a further aspect of the invention, is preferably carried out
by means of indirect heat exchange with the reduced Claus tail
gases which have been passed over the sulphided Group VI and/or
Group VIII metal catalyst. This indirect heat exchange causes
only a small ~ecrease of the tempera-ture of the latter gases and
may sometimes be omitted.
The said reduction in the presence of the Group VI and/or
Group VIII metal catalyst is preferably carried out in the
presence of at least the stoichiometric quantity of H2 and/or
C0 required for complete conversion of S02 and elemental sulphur
into H2S. Generally, 1.3-2.0 times the required stoichiometric
quantity is use a.
14
Methods of preparation and the most suitable composition
of the sulph;ded catalyst and of the carrier and the conditions
at which the reduction may be carried out, such as the pressure,
the gas hourly space velocity and the composition of the free
hydrogen and/or carbon monoxide-containing gas are found in
British patent specification No. 1,356,28~
Before being contacted with the liquid and regenerable
absorbent for H2S, the reduced Claus tail gases are first cooled,
in accordance with common practice, to a temperature below the
dew point of water. It is preferably cooled to a temperature in
the range of from 6 to 60C. This cooling may be carried out in
two stages, for example by indirect heat exchange with water,
thus producing low pressure steam and then by direct heat exchange
with water. In the latter step most of the water vapour present
in the reduced Claus tail gases is condensed.
The off-gases withdrawn ~rom the liquid and regenerable
absorbent must be heated before incineration, in the case of
catalytic incineration to a temperature of at least 150 C. This
heating may be effected in any desired manner, for example by
means of in-line burning. In-line burning, however, has the
~ame disadvantagesas mentioned above for in-line burning in
Claus tail gases, but to a lesser extent since less sulphur and
less water vapour are present.
The invention, however, allows obviating these disadvantages.
The reduced Claus tail gases, after indirect heat exchange with the
Claus tail gases to be reduced, if this indirect heat exchange is
carried out at all, still have a relatively high temperature;
the heat thereof may be used for any suitable purpose. According
to a feature of the present inven-tion, the off-gases which have
left the liquid and regenerable absorbent are heated, prior to
their inc;neration, by means of indirect heat exchange with the
reduced tail gases passed over the sulphided Group VI and/or
Group VIII metal catalyst, preferably after the above-mentioned
heat exchange of the latter gases with the Claus tail gases.
1~
576~
After cooling, -the reduced Claus tail gases are passed
through a liquid and regenerable absorbent for H2$. Preferably,
this absorbent is an aqueous solution of an amine or a sub-
stituted amine. Absorbents o~ this type are well known in the
art, such as for example an alkali metal salt o~ dialkyl-
substituted amino-acids, for example potassium dimethylamino-
acetate, and alkanolamines. More preferably, a polyalkanolamine,
such as diethanolamine, triethanolamine, diisopropanolamine,
methyldiethanolamine or ethanoldiethylamine is used. Very good
results are obtained when using diisopr~panolamine or methyl-
diethanolamine as absorbent. The alkanolamines are preferably
used in aqueous solutions in a molar concentration of 0.5 to 5
and preferably 1 to 4, relative to the said alkanolamines.
After absorption, the H2S enriched absorbent is regenerated by
heating and/or stripping, which produces an H2S-enriched gas
mixture and a regenerated absorbent which may be re-used.
Reduced Claus tail gases having a high C02 content are prefer-
ably contacted with the H2S absorbent under such conditions
that the H2S is selectively absorbed, thus minimizing the re-
cycle o~ C02 to the Claus plant. These conditions include alow temperature and high gas velocities, the said contacting
taking place in an absorption column having at most 20 or, and
preferably, less than 20 contacting trays. More preferably, the
absorption column has 4 to 15 contacting trays. The gas velocity
to be used is at least 1.5 m/sec, and more preferably 2 to 4 m/sec.
These gas velocities are based on the "active" or aerated tray
surface. A low absorbent temperature enhances the selectivity
of the H2S/C02 separation. The temperature is preferably lower
than 40 C; most æatiæfactory results are obtained at temperatures
in the ran~e o~ from 5 C to 30 C. The reduced Claus tail gases
are contacted with the aqueous aIkanolamine solution at atmospheric
or substantially atmospheric pressuren Contacting is pre~erably
effected countercurrently.
5>~
The unabsorbed part o~ the reduced Claus tail ga~es mainly
cons.ists o~ nitrogen and C02, has a very small content o~
hydrogen and contains traces of H2S, COS and CS~, ~or example
100-1000 parts per m;llion (vol.). Xence, it is acceptable to
release the of~-gases,obtained by incineration o~ this unabsorbed
part,through a stack..
The H2S-enriched gas mixture liberated in the regeneration
is pre~erably recycled to the Claus-type process.
The invention will now be illustrated with the aid of the
~ollowing Examples and the Figure.
The Figuxe shows a schematic representation o~ a Claus plant
using split-flow operation, a process ~or the removal of sulphur
compounds and elemental sulphur, and an incinerator for the
combustion of the off-gases of the latter process.
EXAMPLE 1
Air (2055 kg/h, temperature -30C) is sucked in via a line 1,
a pre-heater 2 and a line 3 by a blower 4. In the pre-heater 2
the air is warmed up by means o~ steam generated in a second
sulphur condenser 5, being the ~inal sulphur condenser. The air
discharged by the blower 4 is conducted via a line 6 (temper-
ature 110C) to a pre-heater 7 in which it is pre-heated to a
temperature of 300C. Alternatively, the pre-heater 2 may be
situated between the blower 4 and the pre-heater 7. The air leaves
the pre-heater 7 via a line 8 and is divided into a stream
(1795 kg/h, the ~irst oxygen-containing gas mentioned herein-
be~ore), conducted via a line 9 to a thermal zone 10 and a stream
(260 kg/h7 the second oxygen-containing gas mentioned herein-
before) conducted via a line 11 to an incinerator 12.
An H2S-containing gas (4305.8 kg/h, temperat~re 24C,
C02 content 70.6%v, E2S content 22.9%v, H20 content 4.6%v,
hydrocarbon content 1.9%v) is supplied via a line 13, a pre-
heater 14, a line 15, a pre-heater 16 and a line 17. In the
pre-heater 14 the H2S-containing gas is pre-heated to a temper-
ature o~ 110 C by means of steam generated in the final sulphur
~ ~ ~ 5
17
condenser 5 and in the pre-heater 16 by means of incinerator off-
gases to a temperature of 260C. The incinerator off-gases
supplïed via the line 18 to and those withdrawn from -the pre-
heater 16 via a line 19 have a temperature of 344C and 140C,
respectively. The ~2S-containing gas conduc-ted via the line 17
is split up into a first stream (one third or 1435.3 kgth) con-
ducted via a line 20 to the thermal zone 10 and a second stream
(two thirds or 2870.5 kg/h) by-passing the thermal zone 10 and
conducted via a line 21. Furthermore, an ammonia-containing gas
(19.2 kg/h, temperature 90C, H2S content 22.2%v, ~H3 content
22.2%v, H20 content 55.6%v) is supplied to the ther~al zone 10
via a line 22. In this example, the NH3-containing gas is not pre-
heated. However, it may be preheated by means of indirect heat
exchange with incinerator off-gases. All of the H2S thus introduced
into the thermal zone 10 is combusted to S02 and the ~H3 is con-
verted to N2 and ~2
The effluent from the thermal zone 10 is conducted via a
waste heat boiler 23 and a line 24 to a first Claus stage 26.
The H2S-containing gas supplied via the line 21 is also con-
ducted via the line 2~ to the first Claus stage 26 where anadditional amount of sulphur is formed. Water (-temperature 148C)
is supplied via a line 27, a heat exchanger 28 and a line 29 to
the waste heat boiler 23. In the heat exchanger 28 the water is
heated to a temperature of 166 C b~ incinerator off-gases supplied
via a line 30 (temperature 380 C) and discharged via the line 18
(tempera-ture 344 C). A par-t of the incinerator off-gases (temper-
ature 344 C~ from the line 18 is conducted to the pre-heater 7
via a line 31, withdrawn from this pre-heater via a line 32
(temperature 140 C~ and discharged into the atmosphere. The steam
generated în the waste heat boiler 23 ~temperature 260 C) is
discharged via a line 33 and part thereof (2385 kg/h~ via a line 34
to a destination outside the process.
The gases w;thdrawn from the first Claus stage 26 are con-
ducted via a line 35 to a first sulphur condenser 36 in which they
5~7~3
are cooled b~ means of indirect heat exchange ~lith water to a
temperature of 170C. Sulphur is withdrawn from the ~irst sulphur
condenser 36 via a line 44. The gases leaving the first sulphur
condenser 36 are conducted via a line 37 to a re-heater 38 in
which they are re-heated to a temperature of 210C. The re-heated
gases leaving the re-heater 38 are conducted via a line 39 to a
second Claus stage 40, where an additional amount of sulphur is
formed. In the Claus stages 26 and 40 the gases are contacted
with aluminium oxide. The gases leaving the second Claus stage 40
are conducted via a line 41 to the second sulphur condenser 5 in
which they are cooled by means of indirect heat exchange with water
to a temperature of 127C. Sulphur is withdrawn ~rom the seco~d
sulphur condenser 5 via a line 480 The gases ~rom the second sulphur
condenser 5 are conducted via a line 42 to a re~heater 43 in
which they are re-heated to a temperature o~ 250C. Steam (temper-
ature 148C) is produced in the first sulphur condenser 36. If
desired~ this steam may be used for pre-heating the air and/or
the H2S-containing gas to be conducted to the thermal zone 10r
Re-heating in the re-heater 38 takes place by means of steam sup-
plied via the line 33, a line 45 and a line 46 and re-heating in
the re-heater 43 by means of steam supplied via the line 45.
Steam (temperature 118C) is produced in the second sulphur con-
denser 5; a portion of this steam is used for pre-heating air in
the pre-heater 2, another portion for pre-heating the H2S-con-
taining gas in the pre-heater 14 and the balance is conducted
to a destination outs;de the process ~the three lines required
are not shown).
~ he Claus tail gases leaving the re-heater 43 are conducted
via a line 49g a heat exchanger 50 in which they are heated to
a temperature of 280C and a line 51 to a reactor 52a to which
hydrogen is supplied via a line 53 and t~e line ,1. The reactor
52 is charged with a sulphided Co/Mo/A1203 catalyst (3.2 parts
by weight of Co, 13.4 parts by weight of Mo, 100 parts by weight
of Al203). In the present case upflow is applied in the reactor 52
alternatively, downflow may be applied.
&5~6~
19
The reduced Claus tail gases (temperature 310C) leaving
the reactor 52 are conducted via a line 54~ the heat exchanger
50 in which they are cooled to a temperature of 280 C, a line 55,
a heat exchanger 56 in which they are cooled to a temperature of
5 120 C and a line 57 to a quench column 58 in w.hich they are
quenched by dîrect contact with water supplied via a line 59;
spent water is discharged via a line 60. The quenched gases
(temperature 40C) are withdrawn from the quench column 58 via
a line 61 and introduced into an absorption column 62. An
aqueous solution of dii.sopropanolamine is introduced at the top
of the absorp-tion column 62 via a line 63. The off-gases from the
absorption column 62 (temperature 40C) are conducted via a line
64, the heat exchanger 56 in which they are heated to a temper-
ature of 226C and a line 65 to the incinerator 12. ~uel gas
15 (15 kg/h) is supplied to the incinerator 12 via a line 66. This
fuel gas is preheated to a temperature of 37C by means of indirect
heat exchange with air from the line 11 (this pre-heating is not
shown). A very small amount of fuel gas has been burned separately
to increase the temperature of the off-gases in the line 65 to a
temperature of 380C. The incinerator 12 is charged with an
incineration catalyst cons;sting of 1% Cu and 3% Bi on Al203,
calculated as % weight on total catalyst.
~ he enriched absorben-t leaves the absorption column 62 via
a line 67, is heated in a heat exchanger 68 and conducted via
a line 69 to a desorpt;on column 70. If desired, par-t of this
enriched absorbent may be used as fresh solvent ~or contacting an
H2S-rich stream and the resulting highly loaded absorbent be re-
generated in the desorption column 70, as described in ~ritish
patent specification 1~461gO70~ An aqueous solution o~ diiso-
propanolamine enrichea with H2S and C02 is supplied from asource outside the process via a line 71 and the line 67 and is
simultaneou~ly regenerated in the desorption column 70. The
absorbent in the column 70 is regenerated at ele~ated. temper-
ature by heating wlth ste~m (not shown). The regenerated absorbent
~ ~-&~
is withdrawn from the desorption column 70 via a line 72 and
conducted via the heat exchanger 68, a line 73j a cooler 7~
and the line 63 to the absorption column 62. Part of the regener-
ated absorbent may be conducte~ for example, from the line 63, to
a destination outside the process and, after bei.ng loaded with
E2S and C02, returned via the line 71.
The E2S-containing gas i5 discharged from the desorption
column 70 via the line 13 and is cooled in a cooler (not shown)
in order to condense water vapour; the condensed water is returned
to the desorption column 70.
EXAMPLE 2
An experiment is carried out which differs from that described
in Example 1 in these respects that in the line up of the process
flow scheme as shown in the Figure the pre-heaters 2, 7, 14 and 16
and the heat exchanger 28 are not present. Instead, a steam heater
;s used for indirect heating of the gases in line 25 to the same
temperature as in Exanple 1. In this case 1515 kg/h of steam is
conducted via the line 3~ to a destination outside the process,
this amount is onl~ 63~ of the corresponding amount produced in
Example 1.
