21
Figure B-1-2: Influents/effluents scheme for naphtha hydrotreating unit in normal operations
1.5. Intermittent operations
1.5.1. Catalyst sulfiding
The metals of catalysts used in this process are in the oxide form.
They must undergo a
treatment to recover the active sulfide form. If sulfiding is not complete, it could lead to metal
sintering resulting in poor activity of the catalyst and heavy coke deposits.
This operation is
achieved by injection of the sulfiding agent (dimethyl disulfide (DMDS)) in a circulation of
hydrogen and raw feed. The required amount of DMDS is determined from the decomposition
of DMDS into H
2
S.
Figure B-1-3: Influents/effluents scheme for naphtha hydrotreating unit during catalyst
sulfiding
1.5.2. Catalyst regeneration
When catalysts activity becomes too low, they must be regenerated. This regeneration can be
in-situ or ex-situ.
If the regeneration is in-situ, the procedure includes:
Naphtha
Hydrotreating Unit
Catalysts sulfiding
Inert naphtha
H
2
DMDS
H
2
O
Water
saturated with H
2
S
Butane
Fugitive emissions
FG / FO
Combustion gas
Naphtha
Hydrotreating Unit
Naphthas
H
2
H
2
O
Hydrotreated naphtas
Sour
water
Off-gas
Fugitive emissions
Gas relieves
FG / FO
Combustion gas
22
- Commissioning of ammonia injection lines and caustic soda injection lines to the circuit.
- A first coke combustion step with 0.5% vol. oxygen in the reactor inlet gas.
- A second coke combustion step with 1% vol. oxygen in the reactor inlet gas.
- A finishing phase with 1% vol. oxygen.
- The shut-off of ammonia injection and caustic scrubbing.
- Cooling down of the reactor temperature using the recycle gas circulation
to prepare the unit
for the new start-up.
Chemical reactions occurring during catalysts regeneration are:
- Coke combustion to produce carbon dioxide and water.
- Oxidation of the metallic sulfides on the catalyst to produce sulfur oxides.
- Neutralization reactions.
SO
3
+ 2 NH
3
+ H
2
O → (NH
4
)SO
4
CO
2
+ 2 NaOH → Na
2
CO
3
+ H
2
O
SO
2
+ 2 NaOH → Na
2
SO
3
+ H
2
O
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