DESULPHURISATION CATALYST (NICKEL- MOLYBDENUM CATALYST)
The natural gas feed stock supplied to NFCL
contains no H2S, but it is anticipated that future supplies may contain sulphur
compounds which have to be removed in order not to poison the reforming catalysts
and the LT shift catalyst. Natural Gas from battery limit is heated to 385
deg.C in the Feed stock preheater F-203, and is passed through the Hdrogenator.
A bed of Nickel-Molybdenum catalyst is provided to catalyse the hydrogenation of
organic sulphur compounds to hydrogen sulphide. There are two types of organic sulphur
compounds that may be present in the feed stock. One is called 'Normal Sulphur' containing
H2S, COS, CS2 and Mercaptans and the other is called 'Less Reactive Sulphur',
containing Thiophenes, Thioethers etc.
In case of normal sulphur except Mercaptan Hydrogen recycle gas is not
consumed where as for less reactive sulphur, recycle hydrogen is consumed as
per the following hydrogenation reactions:
RSH
+ H2 RH + H2S
(Mercaptans)
R1SR + 2H2
RH + R1H + H2S
(Thioethers)
R1SSR + 3H2 RH + R1H + 2H2S
(Thiophenes)
If sulphur
is present, natural gas is mixed with recycle gas from synthesis gas compressor
first stage discharge with flow of recycle gas around 1306 NM3/hr., in order to
avoid Carbon deposition on the catalyst due to catalytic cracking of higher
hydrocarbons if any. After preheating to
385 deg.C,the gas mixture passes to Hydrogenator Reactor R-201 and reacts to
produce H2S. The above reactions are
exothermic but insignificant (which depends on the type of Sulphur that determines the number of moles
of hydrogen taken up). H2S produced in
R-201 and that already present in Natural Gas is then removed in H2S Absorbers
R-202 A/B, thereby the gas will be free of H2S.
Each absorber contains one bed of Zno
catalyst to absorb the sulphur. The
absorbers are operating in series with the second vessel acting as guard. When the Zno in the first vessel is getting
exhausted, a break through of H2S from the first vessel may be observed. The operation will then continue with the second
vessel in service, while the first vessel is being reloaded with fresh
catalyst. The sulphur content at the
exit of R-202B shall be less than 0.1 ppm on dry volume basis at
all times which is tolerant to reforming catalyst.
The sulphur removal reaction in Zno bed
takes place as follows:
Zno +
H2S
ZnS + H2O
Zno + COS ZnS + CO2
Zno reaction with 'S' depend on :
1. Type
of sulphur compounds.
2.
Temperature: Increase in
temperature will generally increase the ability of Zno to remove sulphur.
3.
Capacity : As Zno reacts with sulphur it gets saturated
with sulphur and looses its activity. Normal life of Zno catalyst depends on the
H2S and sulphur concentration in the natural gas.
Reactor
|
Catalyst
Type
|
Size
mm
|
Volume
M3
|
Bulk Density
|
Bed ht.
mm
|
composition
|
Life time (exp)
|
R-201
|
TK-251
|
5X2.5
|
8.44
|
480
|
2220
|
NiO:2-3%
MoO:10%
|
>5
|
R-202A
|
HTZ-3
|
4
|
10.64
|
1300
|
2800
|
ZnO:99%
|
|
R-202B
|
HTZ-3
|
4
|
10.64
|
1300
|
2800
|
ZnO:99%
|
Guard
|
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