8 replies to this topic

[kpa]

Dear All,

Currently I am working on sour water stripper unit of one of the refinery. This is extended Basic design stage of the project.

Licensor defined the sour water tank as normal conical (Fixed) Roof Tank, and the client is insisting for a floating roof tank. I am not in position to convince the client.


Project Data:

-Tank Pressure / Temp: 25 mm H2O / 51 C
-Tank dimensions: 15200mmID x 14700mm Height.
-Total Flow rate: 125t/h
-Tank material: KCS
-Sour Water Source: NHT/DHT/VDU/HCU (Upstream process units)
-Oil Skimmer package inside tank.

In our design we have Sour Water flash drum (OP= 1.0kg/cm2g) upstream of the tank, where almost all the Hydrocarbons will be removed from the sour water.

For design purpose we have considered 5wt% of hydrocarbons in sour water.

My queries:
1. What factors of sour water should I consider for the tank type selection?
2. What would be the good choice for tank type for sour water service?
3. I am worried for the effects of floating roof tank upon high quantity of hydrocarbon liquid carried than normal operation. Will there be any effect on sealing system?

I am look forward for your expert opinions.

Thank you in advance.

[Zauberberg]

The floating roof design will pose some serious concerns due to evaporation/flashing of H2S/RSH/NH3/CN/etc. from wetted tank wall; I cannot see any particular reason for not choosing fixed roof type. Usually, feed drums in SWS units are horizontal, 3-phase separators. Is your SWS tank feed buffer which feeds the stripper, or you have another surge drum downstream of SWS tank? That can make a difference.

For the sour water parameters, include capacity (inflow), minimum and maximum arrival/storage temperatures, contaminant loads, hydrocarbon content. Basically, there are no mysteries in SWS unit design - it's the one of the most simple process configurations.

[Qalander (Chem)]

The floating roof design will pose some serious concerns due to evaporation/flashing of H2S/RSH/NH3/CN/etc. from wetted tank wall; I cannot see any particular reason for not choosing fixed roof type. Usually, feed drums in SWS units are horizontal, 3-phase separators. Is your SWS tank feed buffer which feeds the stripper, or you have another surge drum downstream of SWS tank? That can make a difference.

For the sour water parameters, include capacity (inflow), minimum and maximum arrival/storage temperatures, contaminant loads, hydrocarbon content. Basically, there are no mysteries in SWS unit design - it's the one of the most simple process configurations.

Dear kpa Hello, Adding to what indicated above by Zauberberg; 1)The ambient conditions do impact non-insulated storages, 2)There should be blanketing system for safe handling/disposal of any serious pollutants tackle.3) Floating Screens help control volatile components evaporative losses.
If proper composition given to the designer/Vendor Sealing system and Materials withstand usually to these somewhat low pressures (Near-Ambient)
Hope this helps
Qalander
Qalander

[kpa]

Thank you Zuberberg and Qalander for your valuable comments on the issue.

Yes in our system there is horizontal flash drum, 3-phase separators upstream of storage tank. There is no surge drum downstream of tank. The vent from feed surge drum is floating with stripper overhead line.

Additionally we have provided Nitrogen blanketing to tank and vent line connected to atmosphere through canister.

My one more concern is the operating pressure of the flash drum and tank as pointed out by Qalander.

Flash drum operating pressure is 1.0 kg/cm2 g and tank operating pressure is 25mm H2O.

Does this pressure difference affect more evaporation or flashing?

[Qalander (Chem)]

Thank you Zuberberg and Qalander for your valuable comments on the issue.

Yes in our system there is horizontal flash drum, 3-phase separators upstream of storage tank. There is no surge drum downstream of tank. The vent from feed surge drum is floating with stripper overhead line.

Additionally we have provided Nitrogen blanketing to tank and vent line connected to atmosphere through canister.

My one more concern is the operating pressure of the flash drum and tank as pointed out by Qalander.

Flash drum operating pressure is 1.0 kg/cm2 g and tank operating pressure is 25mm H2O.

Does this pressure difference affect more evaporation or flashing?

Dear Probably You have either missed out the exact type of tank i.e fixed roof with floating screen or floating roof type,that needs clarification and may impact the flashing or otherwise of the stored mateials.
Generally speaking if any tank has capability to withstand pressure ;then higher pressure above liquid surface usually decreases flashing.
Hope it's helpful
Regards
Qalander

[kpa]

Thank you Dear Mr.Qalander for your reply.

Now after disscussions finally we have reached to the conclusion that we will use fixed roof tank with internal aluminium roof type tank.


Thank you once again.

KPA

[Qalander (Chem)]

Thank you Dear Mr.Qalander for your reply.

Now after disscussions finally we have reached to the conclusion that we will use fixed roof tank with internal aluminium roof type tank.


Thank you once again.

KPA

Dear KPA
I wish best of luck in future endeavors. Gives consolation if we prove useful to fellow colleagues.
Regards
Qalander

[Dacs]

While the topic has been concluded, I'd still like to share my experience with a project similar with what the TS had.

I was assigned to a Sour Water System area which employs a storage tank for sour water.

In the initial stage of the project, the design was to send the vent from the tank (which is a fixed CRT) to blower in which will send the gases to a caustic scrubber.

But along the way, there was a concern for handling of fresh and spent caustic and it was preferable to remove the caustic scrubber entirely.

So we revisited the storage tank concept and we came out with the following:
1. Replace the fixed CRT tank with a floating head one
2. We added a layer of HC (diesel) on top of the liquid in the tank to prevent vaporization
3. In case of vaporization (due to seal leak), we added a carbon filter on the vent line
4. We added a flash drum upstream of the tank (which was absent from the previous design, since the tank was designed for intermittent operation)

In this case, we were able to remove the caustic scrubber system.

[mbeychok]

kpa:

I agree that a cone roof tank is adequate. But it is an incontrovertible fact that the tank vent gases will contain H2S ... and that must be dealt with. Assuming that your sour water stripper is in a petroleum refinery, then I recommend that the vent be routed through a small amine absorber with lean amine solution provided from the refinery's main amine scrubbing unit and the rich amine (from the vent absorber) being returned to the refinery's main amine scrubbing unit.

You mentioned that the tank will be blanketed with nitrogen. That will not prevent the H2S gas from venting from the tank every time the level rises. You also mentioned venting through a cannister. What will the cannister contain that will absorb H2S every time the tank level rises? And what plans do you have for replacing the cannister contents when the material it contains is "spent:"?