3 replies to this topic

[Zauberberg]

Why is it necessary to empty complete Alkylation Unit when unit is in circulation/shutdown regime more than 72 hours? Why is it necessary to perform neutralization and empty the complete unit instead of having time-unlimited isobutane circulation when following conditions are provided:

1. Olefin feed is stopped
2. H2SO4 addition is stopped and acid is evacuated from the reactor to acid settler. Practically, there is no acid anywhere inside the equipment - of course, some amounts of H2SO4 remain inside reactor vessel, but there are no components that can make significant acid dillution over time.
3. Alkylate/refrigerant caustic and water wash are functioning as long as it is necessary to provide adequate pH value of effluent streams.

Why do we must neutralize and empty complete equipment after 72 hours of shutdown time, instead of having time-unlimited circulation/shutdown with equipment being filled with hydrocarbons, and put the unit online again when all start-up conditions are provided?
The unit is licensed by Texaco, with Stratco reactor design and effluent refrigeration system.

Regards,

[vinay]

Why is it necessary to empty complete Alkylation Unit when unit is in circulation/shutdown regime more than 72 hours? Why is it necessary to perform neutralization and empty the complete unit instead of having time-unlimited isobutane circulation when following conditions are provided:

1. Olefin feed is stopped
2. H2SO4 addition is stopped and acid is evacuated from the reactor to acid settler. Practically, there is no acid anywhere inside the equipment - of course, some amounts of H2SO4 remain inside reactor vessel, but there are no components that can make significant acid dillution over time.
3. Alkylate/refrigerant caustic and water wash are functioning as long as it is necessary to provide adequate pH value of effluent streams.

Why do we must neutralize and empty complete equipment after 72 hours of shutdown time, instead of having time-unlimited circulation/shutdown with equipment being filled with hydrocarbons, and put the unit online again when all start-up conditions are provided?
The unit is licensed by Texaco, with Stratco reactor design and effluent refrigeration system.

Regards,

Dear Zauberberg,

Have you figured out the reason for this procedure ? We are currently putting up an alkylation plant based on the same technology & this information looks interesting to me

Regards
Vinay

[Zauberberg]

Hello Vinay,

I think I managed to find what is hidden behind this demand. Under stagnant conditions, unreacted olefins, conjunct polymers and other acid soluble hydrocarbons continue to react with the acid. The question is how much time do we have until unit is again put on-line, or complete neutralization has to be executed.

• Acid will degrade in the presence of hydrocarbons to form water and SO2, which dilutes the acid strength and increases its corrosivity. These reactions are temperature dependent.
• Possibility of line and instrument tap pluggage, as solids/acid tars (sludge-like material) are formed
• Risk of freezing (acid will freeze at temperatures <0C)

Procedure for short/unplanned shutdowns:

(1) When olefin feed is pulled out of the Alky unit, it is recommended that the reactors are flushed with I-C4-rich
Isobutane recycle or refrigerant recycle for at least 20 minutes (the longer the better) before the reaction
section shut down. The essence of this step is to ensure that unreacted olefins and reaction intermediates are
converted to Alkylate. This will help to reduce the undesirable reactions that can occur when the reaction
system is S/D, with acid left under stagnant conditions.
(2) The transfer acid lines (between the settlers, and from the final settler to the acid blow down drum) can be
flushed with hydrocarbons, to remove acid and reduce chances of line pluggage.
(3) Good practice is to deinventory the contactor acid to the settler during unit shutdown process. This is done by blocking in the acid recycle valves (from the settler back to the contactor) and using the impeller to move acid from the contactor to the settler. This makes for easier S/U and reduces chances of damage to the impeller shaft, if a motor start is attempted with the contactor containing normal volume of acid (motor over-amp, impeller shaft imbalance due to density difference between hydrocarbon and acid phase).
(4) When acid is left in the unit during a shut down, it is important that the acid strength be verified, and
adjusted with fresh acid as required, before re-introducing olefin feed. Relying on the last known acid strength
will not suffice, due to the undesirable reactions that can slowly deplete the acid strength especially under
stagnant conditions.

Based on similar experiences in other refineries, 1 to 2 weeks is a reasonable time to leave acid in the settlers without any problems. The exact timing will of course vary on a case-by-case basis (ambient temperatures etc.) Two weeks is an absolute maximum observed.

[vinay]

Thank you dear

Regards,
Vinay