Process for the recovery of sulphur from a hydrogen...

Details Process for the recovery of sulphur from a hydrogen... Process for the recovery of sulphur from a hydrogen...

2001-05-01

2003-11-25

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Sulfur or compound thereof

Elemental sulfur

C423S574100, C423S576800, C423S564000

Reexamination Certificate

active

06652827

ABSTRACT:

BACKGROUND OF THE INVENTION
In a number of processes, such as the refining of crude oil, the purification of natural gas and the production of synthesis gas from, for example, fossil fuels, sulphur containing gas, in particular H
2
S containing gas, is released. On account of its high toxicity and its smell, the emission of H
2
S is not permissible.
The best known and most suitable process for removing sulphur from gas by recovering sulphur from hydrogen sulphide is the so-called 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 so-called Claus residual gas) still contains some H
2
S and SO
2
.
The method of recovering sulphur from sulphur containing gases by the so-called Claus process is based on the following overall reactions:
2 H
2
S+3 O
2
→2 H
2
O+2 SO
2
  (1)
4 H
2
S+2 SO
2
⇄4 H
2
O,+6
S
n
  (2)
Reactions (1) and (2) result in the main reaction:
2 H
2
S+O
2
⇄2 H
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, the so-called thermal stage, followed by a number of reactors generally two or three—filled with a catalyst. These last stages constitute the so-called catalytic stages.
In the combustion chamber, 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. The 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:
 2 H
2
S+3 O
2
→2 H
2
O+2 SO
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 as to a considerable portion, in accordance with the Claus reaction
4 H
2
S+2 SO
2
⇄4 H
2
O+3 S
2
  (2)
Thus, in the thermal stage, approximately 60% of the H
2
S is converted into elemental sulphur.
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.
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
4 H
2
S+2 SO
2
⇄4 H
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.
When the gaseous feedstock contains H
2
S concentrations of between about 15 and 50%, the above described “straight through” process is not used, but instead a variant thereof, the so-called “split-flow” process. In the latter process one-third of the total amount of feedstock is passed to the thermal stage and combusted completely to SO
2
therein. Two-thirds of the feedstock is passed directly to the first catalytic reactor, by-passing the thermal stage. When the feedstock contains H
2
S concentrations of between 0 and 15 the Claus process can no longer be used. The process then used is, for example, the so-called Recycle SELECTOX process, in which the feedstock is passed with an adjusted amount of air into an oxidation reactor, the so-called oxidation reactor, the so-called oxidation stage. The reactor contains a catalyst which promotes the oxidation of H
2
S to SO
2
, and the amount of oxidation air is adjusted so that an H
2
S:SO
2
ratio of 2:1 is established, whereafter the Claus reaction proceeds. The gas from the oxidation reactor is cooled in a sulphur condenser, in which the sulphur formed is condensed and discharged.
To dissipate the reaction heat generated in the oxidation reactor, a portion of the gas stream coming from the sulphur condenser is recirculated to the oxidation reactor.
It is clear that in the Recycle SELECTOX process, the oxidation stage, which is catalytic and does not lead to high temperatures, is equivalent to the thermal stage in the Claus process. In the following, both the thermal Claus stage and the oxidation stage of the Recycle SELECTOX process are referred to as oxidation stages.
The sulphur recovery percentage in a conventional Claus converter is 92-97%, depending on the number of catalytic stages
By known processes, the H
2
S present in the residual gas from the Claus reaction is converted, by combustion or some other form of oxidation, into SO
2
whereafter this SO
2
is emitted to the atmosphere. This has been permissible for low concentrations or small amounts of emitted SO
2
for a long time. Although SO
2
is much less harmful and dangerous than H
2
S this substance is also so harmful that its emission is also limited by ever stricter environmental legislation.
As has been observed, in the Claus process as described above, in view of the equilibrium reaction which occurs, the H
2
S:SO
2
ratio plays an important role. In order to obtain an optimum conversion to sulphur, this ratio should be 2:1. Generally speaking, this ratio is controlled by means of a so-called H
2
S/SO
2
residual gas analyser. This analyser measures the H
2
S and SO
2
concentrations in the residual gas. A controller then maintains the ratio of 2:1 constant on the basis of the equation
 [H
2
S]−2[SO
2
]=0
by varying the amount of combustion air, depending on the fluctuations in the gas composition and the resulting deviation in the above equation. Such a control of the process, however, is highly sensitive to these fluctuations.
Furthermore, the sulphur recovery efficiency (calculated on the amount of H
2
S supplied) is no higher than 97%, and so the gas flowing from the last catalytic stage the residual gas—still contains substantial amounts of H
2
S and SO
2
, determined by the Claus equilibrium, and this in a molar ratio of 2:1.
The amount of H
2
S present in the residual gas can be separated by absorption in a liquid.
The presence of SO
2
in the residual gas, however, is a disturbing factor during the further processing thereof and must therefore be removed prior to such further processing. This removal and hence the after-treatment of the gas is complicated.
The great disadvantage of the presence of SO
2
is that this gas reacts with conventional liquid absorbents to form undesirable products. To prevent undesirable reactions of the SO
2
, therefore, the SO
2
is generally catalytically reduced with hydrogen to form H
2
S over an Al
2
O
3
supported cobalt-molybdenum catalyst in accordance with the so-called SCOT process. The total amount of H
2
S is subsequently separated by liquid absorption in the usual manner.
In the SCOT process the sulphur components, other than H
2
S, such as SO
2
(sulphur dioxide) and sulphur vapour (S
6
and S
8
) are fully hydrogenated to H
2
S according to the following reactions:
SO
2
+3 H
2
→H
2
S+2 H
2
O  (4)
S
6
+6 H
2
→6 H
2
S  (5)
S
8
+8 H
2
→8 H
2
S  (6)
Other components, such as CO, COS and CS
2
, are hydrolysed according to:
 COS+H
2
O→H
2
S+CO
2
  (7)
CS
2
+2 H
2
O→2 H
2
S+CO
2
  (8)
CO+H
2
O→H
2
+CO
2
  (9)
Above conversions to H
2
S are performed with a cobalt-molybdenum catalyst on alumina at a temperature of about 280-330° C. For the SCOT process it is required that sulphur vapour is hydrogenated to H
2
S, and also that SO
2
is completely converted to H
2
S down to ppm level, to prevent plugging/corrosion in the down-stream water quench column. This type of hydrogenation can be defined as high temperature hydrogenation.
In accordance with another me

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