Temperature Inversion 2

[Petro0707]

Hi All...

Recently we are facing problem of Tempearature inversion between naphtha draw off & crude fractionator column top temperature.

In normal condition – Naphtha draw off temperature remains around 130 °C & Crude column top temperature remains around 111 °C ( Delta between this two temp. ~ 19 C )

But abnormally after the occurrence of this event as shown in graph, naphtha draw temperature is remaining around 118 C & column top temperature is remaining around 114 C ( Delta ~ 5-6 C) along with sudden increase in Naphtha section Pressure Drop (PDI ) also around 1000-1100 mmwater column ( as shown in the graph)

Pl. see attached graph image by clicking, which indicates trends as specified below:

1.Green colour – Crude column overhead temperature
2.Red colour – Naphtha section pressure drop
3.Pink colour – Naphtha draw temperature

In the trend analysis, you can see sudden dip in Naphtha draw temperature on 16th September.

Naphtha PA return temp. has fallen down by 5 C i.e. from 100 C to 95 C . There is no any significant impact on Naphtha distillation & End point.

I have gone through the thread124-178998.

We have got total 3 trays in Naphtha section which is specifically designed HI-FI trays of MOC Monnel.I simulated the Model but it shows that if efficiency of Trays got reduced in naphtha section then in that case top as well as draw both temp. should fall down but here top temp. is higher and draw temp. has fallen down so, is it related to Tray damage or Pump around reflux distribution nozzle damage..Still not sure.There is reduction of Naphtha PA Duty but there is no any major changes in other PA duties of HK, Diesel, HAGO. Please note that we don’t use cold reflux for the column since very long time.

Request for your help please on this…( Regret for long description)

 

[Petro0707]

Hi 25362 & Milutin,

We use generally phenolic & nonphenolic water as desalter wash water which comes through surge drum , so, with this I am attaching desublimation temperature calculation of NH4Cl for cross validation purpose. The final dissociation constant is in atm.,so, the same after converting into psi, and after have a look in the graph, it seems it falls under deposition region. Here we have tried to calculate NH4+ entry through wash water to CDU column & we assume 20 % of NH4+ gets carried through wash water with crudemix to crude column. Yellow highlighted area indicates that much NH4+ presence in that particular wash water.

Pl. give your suggestion/comment on this.

http://files.engineering.com/getfile.aspx?folder=036fa47b-f701-4ce9-af5c-732099d58fff&file=Calc.xls

 

[25362]

[•] Is there a basis to assume that 20% of wash water NH[sub]3[/sub] migrated to crude ? Otherwise the estimation seems to be right.

[•] By using external (cold) reflux the mol fraction of water drops. Besides, the azeotropes of water with light hydrocarbons should chase away any liquid water present.

[•] It would be worth to read the following US patents:
USP 4430196, USP 4599217, USP 4806229, USP 4855035, USP 5211480, USP 5302253, USP 5714664.

 

[Milutin]

Hi Petro,

I am not sure to what you refer in "Yellow highlighted area indicates...", which yellow area?

You should avoid assumptions like "20 % of NH4+ gets carried through wash water with crudemix to crude column". Because that is base for your calculation.
Right way is Cl, NH3 balance in overhead water.

Regards

 

[Petro0707]

Dear Milutin,

Yes, you are right.In that calculation worksheet yellow highlighted cell (raw) indicates NH3 ppm in phenolic water & non phenolic water which is used as desalter wash water.Through overhead NH4+ & Cl- way, still some of the HCl can react inside the column with NH3 & whatever we get in overhead receiver boot water is the unreacted Chloride ions coming from the column.Still both way we need to assume certain thing.Any proven desublime temperature example calculation through overhead NH4+ & Cl- will be highly appreciated & also it will be very helpful to us.

Thanks


Dear 25362,

Yes, I went through patents which you mentioned, it is really imaginative, especially, I learnt that we need to monitor SO4- in boot water , any organic / unextractable chloride in o/l of desalter too .Whether H2S can form NH4HS ? , of course as per original one of the reference threads of 0707 indicates that yes, H2S takes part in the formation of ammonium disulfide,whether we need to monitor NH4HS desublimation temperature along with NH4Cl too ?

 

[25362]

Apart from the link I provided on Oct. 25, I was given to understand that as long as temperatures are above 70[sup]o[/sup]C there is no deposition of NH[sub]4[/sub]SH.

 

[Petro0707]

Hi 25362 ,Milutin,0707,

How are you ?

We have carried out further analysis of top tray material deposits as well as downcomer weir deposits and we have found & confirmed that there was no any chloride as well as no any sulfide was detected and majority we have found is Ni,Cu,Fe & sulfates.Yes,If NH4Cl depositions would have taken place,some ppm chloride should have reported but it was other way around.Over and above,our Naphtha pumparound sample is having more than 480 min. of oxygen stability number and hardly we could detect dissolved oxygen in wash water also,now,since Ni,Cu (from Monel 400 trays & top shell clading material) was reported in naphtha p/a exchangers,naphtha pump )& in these deposits too, the question is how sulfates can be reported in o/h,what can be the source as 25362 has mentioned it seems hardly any chance for NH4HS depositions because o/h temp. is almost 110 C.Other possibility is any oxygen/air ingress in the column o/h which can accelarate Monel 400 corrosion or H2S reaction mechanisom..for that we need proper ionic model..because..it is still very difficult to find root cause of the problem..

Thanks & Regards,

 

[Petro0707]

Dear 25362,Miultin,0707,

Thanks everybody ...It seems we need to go for Ionic modelling to improve our reliability of OH Corrosion.

 

[Petro0707]

Dear All,

It is really interested case where detail X-ray / scanning electroscopic magnification view of top tray-top and bottom pieces / downcomer pieces talk about cavitation erosion and no any chlorides are detected on the trays but sulfates/sulfides are detected and more predominately Nickel and copper sulfates….and Oxygen is also being reported…we were rigorously thinking about NH4Cl but it could be other way around it seems….Apart from that it is really difficult to distinguish sulfates and sulfides but it is surely sulfates/sulfides,so,what will be your opinions on these particular two pieces sulfates and oxygen…how these two guys creates problem and more important is the source of these guys…how to pinpoint exactly..?

 

[Milutin]

Hi Petro,

I am not sure if you wrote about your case or about some case from literature.
As 25362 quoted in post from 19 Oct 07 sulfuric acid can cause severe corrosion in column top trays and overhead. Source can be oxygen as we earlier discussed.

To check sulfates presence we analyze overhead boot water on sulfates, usually below detection in our case.

Regards,

Milutin

 

[Petro0707]

Dear Milutin,

How are you doing…?

Yes, I wrote for our case and we don’t regularly monitor sulphates in OH boot water but we could do few sulphate analysis in OH boot water in the past and we found sulfates in boot water in the range of 2 – 10 ppm and sulphates in desalter wash water in the range of 2 to 10 ppm and in desalter brine water in the range of 6 to 30 ppm. You mentioned below detection level in your case….does it mean in ppb level? Yes...It seems Monel is not appropriate metallurgy in presence of this much amount of sulphates…It is again a question where no any crude assay data shows sulphates and we do process blend of 8 – 10 crudes….may be due to downstream additives in well ..Sulphates can ingress and end up in CDU OH section or elsewhere….our desalter wash water this much sulphates… (mentioned as above) does it acceptable…?

Thanks & Regards,

 

[25362]

Apart from air-oxidation of organosulphur compounds, one may use Sherlock Holmes thinking tips to identify the origin of sulfates in crude, as follows:

[•] S ranks third after C and H among the constituents of crude oil, and far ahead of O and N.
[•] It is not a major constituent of biogenic molecules, where it ranks well behind O and N.
[•] It must have been picked up en route through geological strata, or while standing in reservoirs.
[•] From isotope measurements experts reached the conclusion that it may derive from sulfates in connate water after the oil has accumulated.
[•] The connate oilfield waters contain frequently more than 10[sup]4[/sup] ppm of dissolved inorganic salts - principally chlorides and sulfates of sodium, potassium, calcium and magnesium.
[•] Thus the presence of sulfates in crudes shouldn't be considered a strange event at all.

 

[25362]

http://www.freepatentsonline.com/4855035.html

under the heading Description of the preferred embodiments gives interesting information on the possible origin of sulfates and bisulfates.

 

[Petro0707]

Oh..Yes,Dear 25362,
Thanks for your reply.

I do agree...
It seems in our case dissolved oxygen in desalter wash water which we could not say very confidently 5-6 ppm but it seems in that range (we could do one or two analysis ) as i mentioned earlier that this desalter wash water is phenolic and nonphenolic water from sour water stripper unit and we don't monitor dissolved oxygen on regular basis and that too storage vessel is having Fuel gas as blanketing (oxygen can dissolve through this to desalter wash water)& your attached reference US patent talks about SO2,SO4 speicies can be form in the presence of Oxygen in wash water through reaction with sulfur compounds in crude oil but how much dissolved oxygen is allowed in desalter wash water is it 2 , 3, 4 ppm ?? or how much ppm of oxygen can form these nasty sulphate species..?? What is the allowable sulphate level in overhead boot water ?? ,The mentioned US Patent also talks about corrosion due to sulphates/so2,so3 on CS and not for MONEL-400 while ours is Monel.

Thanks..