Q & A > Question Details
We are working in a hydrocracking unit and since we start up, we haven't had a good copper strip corrosion data in the light nafta, we have 4a and the best we have achieved consistently is 2b. We lowered the pressure in the main stripper from 125 to 115 psig, increased overhead temperature from 282F to 292F and increased the stripping steam from 8000 lb/h (design) to 9600 lb/h. Also in the debutanizer we have drop the pressure from 160 to 140 ans still we don't have good results. What can we do in order to reach the nafta copper strip corrosion in 1A?


Thank you for your answers, I checked the steam and I saw it is 175 psi and 390F, so we are going to heat up more the steam and we are going to try increasing more the flow, but what could happen if I increase it too much, maybe the control valve 100% open and still not get the copper strip corrosion ok?
15/03/2018 A: Emmanuel Aunde, Refining Company, lemma012@yahoo.com
I will recommend doing a water balance on your stripper to ensure the steam is not condensing in the stripper as this will spell even more problems for you. From the operating parameters, its most likely that your steam is condensing in the stripper.
Increase in steam is limited by the overhead water dew point. If you continue to increase the rate of steam injection, the steam will start condensing in the overhead line once it's partial pressure reaches dew point. So do your dew point calculation to know the limit.
13/07/2016 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
You may want to check salt content & pH of water in HCU fractionator overhead drum. If there is sour water being recycled back to fractionator from overhead drum over and over it can gradually build up. Emulsion of sour water & naphtha can be quite stable with IFT of 6- 8 dynes/cm range - especially if corrosion inhibitor is added which may result in sour water carryover with naphtha. One solution may be high efficiency Liquid Liquid coalescer to break the emulsion & remove any sour water from naphtha stream to less than 15 ppmw.
12/07/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
With due respect to process design, we just went ahead and increased the stripping steam rates till we achieved the product spec (of course steam should be dry as Mr NS Murthy suggested) before ramping up the plant capacity from start up level to normal operating level. This has established the true stripping steam requirements of the stripper. PFD values prove to be directional at times.
09/07/2016 A: Mainak Sarkar, Indianoil Corporation Ltd, sarkarm@indianoil.in
The most common causes for failure of copper strip corrosion test is due to presence of H2S but it may also occur due to presence of mercaptan (RSH). The mercaptan generally form at the outlet of the hydrocracking reactor due recombination of H2S and transient olefin. This mercaptan form in this phenomenon is called recombinant mercaptan. To avoid this problem most licensor use a layer of hydrotreating catalyst at the bottom of hydrocracking reactor.
To confirm the presence of mercaptan in the LN stream, you can go for Sulphur speciation (sulphur compound class) of the LN stream. This is a GC method to identify the compounds of Sulphur present in LN types of stream. Standard instruments of Agilent or Perkin Elmer is available.
If the problem is not due to mercaptan, then stripper performance need to be checked
08/07/2016 A: NS Murthy, Suez, murthy.ns.ext@suez.com
1. Check wetness of steam being used. Ensure that there is at least 50deg C super heat in steam being used in stripper.
2. Carry out pr and temp profile in stripper column whether any under performance or channeling taking place.
3. Look for any water ingress from upsteam system into stripper tower and try to avoid such.
08/07/2016 A: Wang Tzu Hong, Formosa Petrochemical Corporation, 424606@rffpcc.com.tw
We had experience the same issue in VGO unit before, when VGO unit was S/U, product diesel was always fail on copper strip corrosion (4a~2b) and it took few days to get "1a~1b". Through root cause anaylsis, we found sub ppm level H2S (0.3~0.5ppm) in product diesel that cause copper strip corrosion fail, H2S silp from De-C4 stabilizer to downstream column during S/U process, the countermeasure is strip steam of De-C4 shall be on-line as early as possible during S/U process, make sure H2S will not slip to downstream.