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DCU Flare

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This topic contains 7 replies, has 3 voices, and was last updated by  Mike Kimbrell 3 months, 2 weeks ago.

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  • #31358

    Hedewandro Lucredi
    Participant

    We usually send the Off gas from our DCU LPG caustic treatment to SRU incinerator. We usually have some plugging in the off gas pipe like a cement and it is hard to clean this pipe. This off gas has some oxygen, dissulfide gas and some caustic (NaOH) from carry over. Sometimes, during the pipe is been cleaning we send this gas to main flare. I would like to know what is it the off gas destination from DCU LPG caustic treatment? Sometimes send to the flare also ? Is it any problem ?

  • #31360

    Mike Kimbrell
    Participant

    Coker LPG caustic treatment is normally for removal of mercaptans and COS through a UOP Merox unit or a Merichem Thiolex unit. The spent air is typically routed to a fired heater to be combusted. In your case you use the incinerator of the SRU, which is fine.

    I have seen this stream routed to a flare as the spent air stream was not near any fired heaters. My preference is to not route this stream to a flare, but find a heater or incinerator close to the caustic treating unit and route the spent air to that destination. I do not like to put any air stream into the flare header, which is my main objection to this routing. Additionally, whatever is plugging the spent air line will also deposit in the flare header and restrict the flow of hydrocarbons out the flare.

    Do you have any calcium or magnesium (hardness) in the water used to dilute the caustic to proper densities? Hardness can cause emulsions in the caustic and calcium salts can be very difficult to remove. Entrained caustic with calcium in it could be the source of the deposits. Have you analyzed them?

  • #31361

    Hedewandro Lucredi
    Participant

    The water used to dilute the caustic (NaOH) is industrial water, so there is high hardness. We analysed the deposits. 15%-20% is caustic, +/- 10 % is sulphur and lot o iron (corrosion deposits)

  • #31370

    Mike Kimbrell
    Participant

    Part of the reason I prefer not to put the spent air stream into the flare header is oxygen, H2S and liquid water will form elemental sulfur at ambient temperatures. Flare headers nearly always have H2S, so the likelihood of forming solid elemental sulfur is quite high. Is the sulfur in the form of elemental sulfur? That would indicate H2S breakthrough from the caustic treating system, which should not happen unless the caustic gets completely spent. This seems unlikely as the mercaptan removal would suffer before the H2S broke through.

    The spent air stream should not be as corrosive as you indicate. What is the operating temperature of the caustic treating system? What is the oxygen content of the spent air? Excessive oxygen can aggravate corrosion.

    My preference is to still route the spent air stream to a fired heater or incinerator rather than to the flare.

  • #31373

    Hedewandro Lucredi
    Participant

    The sulfur formed is elemental. The temperature of the treating system is +/- 45 C. The oxygen has a excess because the treatment required that

  • #31374

    Evan Hyde
    Keymaster

    You might consider routing the off gas from the treatment unit back to the DCU heater. I have worked with a few plants that have a burner specially designs for this stream. It needs to be handled carefully from a design and interlock perspective. But it can be a shorter run than to the the SRU and reduce the risk of plugging. But I agree with the earlier comments from Mike that the water and loading are the primary causes of issues.

  • #31383

    Hedewandro Lucredi
    Participant

    The problem to send to DCU heater is Emissions increase a lot . In the past we sent to the heater, besides emissions sometimes there are some liquid carryover. This gas was sent to a specific burner and sometimes plugged this burner. As you write there are some risks

  • #31400

    Mike Kimbrell
    Participant

    The formation of elemental sulfur requires H2S, oxygen and liquid water. For elemental sulfur to form in the spent air stream there would have to be H2S that was not contacted by NaOH in the prewash caustic or in the extractor. Maybe the caustic is becoming too far spent before being recharged and that is when the H2S is breaking through.

    A thorough review of the LPG caustic treater operation may reduce the problems you are experiencing. If not, then consider installing a larger knock out pot on the spent air line to prevent the liquid carry over you mentioned from getting into the incinerator.

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