Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
2002-08-12
2004-10-05
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C423S244100, C423S564000, C423S567100, C423S574100, C423S576800
Reexamination Certificate
active
06800261
ABSTRACT:
The invention relates to a process for the removal of sulphur compounds from gases by catalytic reduction of sulphur dioxide, present in gas mixtures.
The necessity of purifying gases, which are further treated in chemical processes, or supplied to buyers, or discharged to the atmosphere, from sulphur compounds, in particular hydrogen sulphide, is generally known. Accordingly, there exist a number of processes, which are directed towards the removal of hydrogen sulphide from gas.
The best known and most suitable process for removing sulphur from gas by recovering sulphur from hydrogen sulphide is the Claus process. In this process hydrogen sulphide is converted by oxidation to a considerable extent into elemental sulphur; the sulphur thus obtained is separated from the gas by condensation. The residual gas stream (the Claus tail gas) still contains some H
2
S and SO
2
.
The method of recovering sulphur from sulphur containing gases by the Claus process is based on the following overall reactions:
2H
2
S+3O
2
→2H
2
O+2SO
2
(1)
4H
2
S+2SO
2
⇄4H
2
O+6
S
n
(2)
Reactions (1) and (2) result in the main reaction:
2H
2
S+O
2
⇄2H
2
O+2
S
n
(3)
A conventional Claus converter—suitable for processing gases having an H
2
S content of between 50 and 100%—comprises a burner with a combustion chamber and a condenser, the thermal stage, followed by a number of reactor stages—generally two or three. These reactor stages constitute the so-called catalytic stages and consist each of a reactor filled with catalyst and a sulphur condenser.
In the thermal stage, the incoming gas stream, which is rich in H
2
S, is combusted with an amount of air, at a temperature of approximately 1200° C. This amount of air is adjusted so that one third of the H
2
S is fully combusted to form SO
2
in accordance with the following reaction:
2H
2
S+3O
2
→2H
2
O+2SO
2
(1)
After this partial oxidation of H
2
S the non-oxidised part of the H
2
S (i.e. basically two-thirds of the amount offered) and the SO
2
formed react further to a considerable portion, in accordance with the Claus reaction
4H
2
S+2SO
2
⇄4H
2
O+3S
2
(2
a
)
The gases coming from the combustion chamber are cooled to about 160° C. in a sulphur condenser, in which the sulphur formed is condensed, which subsequently flows into a sulphur pit through a siphon.
Thus, in the thermal stage, approximately 60% of the H
2
S is converted into elemental sulphur.
The non-condensed gases, in which the molar ratio of H
2
S:SO
2
is unchanged and still 2:1, are subsequently heated to about 250° C. and passed through a first catalytic reactor in which the equilibrium
4H
2
S+2SO
2
⇄4H
2
O+6
S
n
(2)
is established.
The gases coming from this catalytic reactor are subsequently cooled again in a sulphur condenser, in which the liquid sulphur formed is recovered and the remaining gases, after being re-heated, are passed to a second catalytic reactor.
In the Claus process, H
2
S is not quantitatively converted to elemental sulphur, mainly due to the fact that the Claus reaction is an equilibrium reaction and therefore the conversion of H
2
S and SO
2
to elemental sulphur is not complete:
2H
2
S+SO
2
⇄2H
2
O+3
S
n
(2
b
)
A residual amount of H
2
S and SO
2
remains. Now, generally it is not allowed to discharge residual gas containing H
2
S to the atmosphere, and so the gas is oxidised, with the hydrogen sulphide and other sulphur compounds as well as the elemental sulphur present in the gaseous phase being oxidised to sulphur dioxide. With the environmental requirements becoming stricter, this will not be allowed anymore because the sulphur dioxide emission involved is too high. It is therefore necessary to further treat the residual gas of the Claus installation, the so-called tail gas, in a so-called tail gas treater.
Tail gas processes are known to those skilled in the art. The most well-known tail gas processes are the SCOT process, the BSR Selectox process, the Claus sub-dewpoint processes such as Sulfreen, CBA and MCRC, and the Superclaus™ process.
The SCOT process is an effective process for the treatment of tail gas (See GB-A-1, 356,289). In this process the tail gas, together with hydrogen, is passed over a cobalt oxide/molybdenum oxide catalyst on an Al
2
O
3
carrier, all sulphur components present thus being catalytically reduced to H
2
S. The total amount of H
2
S is then separated from the Claus tail gas in a conventional manner by absorption in a suitable liquid. The H
2
S is recovered from the liquid absorbent and recycled to the Claus thermal stage. One drawback of the SCOT process is that it requires an expensive and complicated installation. Another drawback is the high energy consumption involved in removing the hydrogen sulphide from the absorbent again.
In the SCOT process a hydrogenation catalyst is used which is based on a carrier material, usually &ggr;-Al
2
O
3
with a high specific catalytic surface area of typically more than 300 m
2
/g, the carrier material being provided with active compounds such as molybdenum, cobalt and/or nickel for the hydrogenation function. In the SCOT hydrogenation reactor all sulphur components are converted to H
2
S according to
SO
2
+3H
2
→H
2
S+2H
2
O (4)
1/
n
S
n
(vapour)+H
2
→H
2
S (5)
COS+H
2
O→H
2
S+CO
2
(6)
CS
2
+2H
2
O→2H
2
S+CO
2
(7)
In this process it is essential that all sulphur species are converted to H
2
S down to the ppmv level over the hydrogenation catalyst, in order to prevent corrosion and plugging with solid sulphur in downstream equipment. For instance, partial catalytic hydrogenation of SO
2
to sulphur vapour or a mixture of sulphur vapour and H
2
S is not allowed for the SCOT process. In order to achieve complete hydrogenation to hydrogen sulphide and complete hydrolysis of COS and CS
2
, high catalyst bed temperatures in the range of 280-330° C. as well as low space velocities are required. A process of this kind, using complete conversion of the sulphur species to hydrogen sulphide is described in GB-A 1,480,228.
An alternative way to remove hydrogen sulphide from tail gas is partial oxidation to elemental sulphur, as in the so-called BSR Selectox process, described in U.S. Pat. No. 4,311,683. According to this process the Claus tail gas is hydrogenated, water is removed and the H
2
S containing gas, mixed with oxygen, is passed over a catalyst containing vanadium oxides and vanadium sulphides on a non-alkaline, porous, refractory oxidic carrier.
An important drawback of both the SCOT process and the BSR Selectox process is that in both cases the tail gas, after hydrogenation of the sulphur components to H
2
S, must first be cooled in order to remove the water for the greater part. Water greatly interferes with the absorption or the oxidation of H
2
S. Due to the high investments involved, the costs of tail gas treatments according to these known methods are high.
In the SUPERCLAUS™ process the Claus tail gas is subjected to further treatment, whereby H
2
S present in the tail gas is selectively oxidised to elemental sulphur in a dry bed oxidation stage.
In U.S. Pat. No. 4,988,494, one of the basic patents for the SUPERCLAUS™ process, it is described that the H
2
S concentration in the gas leaving the last catalytic Claus stage is increased to a value ranging between 0.8 and 3% by volume by reducing the quantity of combustion or oxidation air passed to the Claus thermal stage.
The increase of the H
2
S concentration in the Claus tail gas, will result in a decreased SO
2
concentration in said tail gas, however, not to very low levels. For an H
2
S concentration of 0.8% by volume, the SO
2
concentration will be typically 0.03-0.15% by volume, and this will still result in a sulphur recovery efficiency loss of typically 0.09-0.45%.
In EP-A 669,854 a process for sulphur recovery is described wh
Borsboom Johannes
van Nisselrooij Petrus Franciscus Maria Theresia
Gastec N.V.
Langel Wayne A.
Weingarten Schurgin, Gagnebin & Lebovici LLP
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