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)

 

[Milutin]

Hi Petro,

How to open picture in proper size?

Regards,

Milutin

 

[Petro0707]

Oh…Hello….Milutin,

Yes,you are right.. somehow it was distorted…pl. try below link for enlarged view of the graph.. This will be more clearer

 

[Milutin]

Hi Petro,


Did anything happens on 16th September, like increase/decrease of throughput, upset, switch to different crude, etc...?

I didn't understand which are your top PA draw/return trays?

Regards,

 

[0707]

Naphtha section pressure increase is an indication that maybe the 3 trays in Naphtha section are plugged with ammonia chloride deposits.

Uneven distribution of Pump around reflux distribution due to nozzle plugging or other mechanical damage is also to consider.

Your HIFI trays are with sieve holes, float valves or fixed valves?

Luis Marques

 

[Milutin]

Hi 0707,

I am not sure in tray plugging with ammonia chloride deposits, because, as Petro explained, they don't use cold reflux (from overhead accumulator) so there is no possibility of recycling back ammonia salts or amine salts back to the column.

Salt deposits increase dP on trays and cause tray flooding, so overhead distillation end point should increase, or gap between overhead and side naphtha draw should become smaller.

Regards,

Milutin

 

[Petro0707]

you can see this attached image that PDI tapping is between 3rd tray & overhead vapour line , this section is having HI-FI trays of sieve trays and even after water wash of around 36 hours, Pressure drop is still 1050 mmwc no any change in pressure drop , so , it seems that Ammonium chloride is not a probable cause.

 

[Milutin]

Hi Petro,

Temperature difference for top P/A decreased, so you lost about 20% top P/A duty for same P/A flow. So as you write other P/A didn't change, I suppose your top P/A flow increased to compensate lower dT. If this is true, increased top P/A flow can contribute to increase of dP in top column section.

How many passes have trays in top section? Vapor/liquid distribution can be affected by distributor or trays damage. If you didn’t have some upsets I don’t see reason for sudden tray/distributor damage, but don’t exclude this scenario.
Reason why asked for throughput change, crude switch etc in my previous post is because it could happen if throughput is lowered and quantity of vapors flowing upward is lowered weeping may develop so it can cause liquid to by-pass trays active area and lowering 4th tray temperature. My next question is what is quantity of overhead naphtha before and after “temperature inversion”?
Sorry for now I have more questions then answers, just to have picture what happens.

P.S.Up till now it is not temperature inversion, becuse 4th tray temperature is higher then top temperature.

Regards,

 

[Petro0707]

Hello…Milutin,

I am attaching herewith more detail profile of Naphtha section flow as well as temperature along with PA duties, this trend is described as below ( Starting from top to bottom with indication of two point values i.e. before the occurrence of event and after ,for your reference pl.) :

1st trend – Delta P across top Naphtha section (Between 3rd tray and overhead vapour line)
2nd trend – Crude column overhead temperature °C
3rd trend – Naphtha draw temperature
4th trend- Naphtha FBP
5th trend – Naphtha PA Duty MMKcal/hr
6th trend – HK PA Duty ,MMKcal/hr
7th trend – Diesel PA Duty,MMKcal/hr
8th trend – HAGO PA Duty,MMKCal/hr
9th trend- Naphtha PA return temperature °C
10th ,11th & 12th trend- Naphtha circulation flow across exchanger 1,2 & recirculation flow respectively.

We have got 4 pass trays.There was no any increment in PA Flow to compensate dT because PA flow infact it decreased (see second graph) compare to earlier .Kindly note that there was neither any sudden throughput disturbance nor any major crudemix changes.Yes you are right it is not temperature inversion but Delta T between Naphtha draw and overhead temperature got reduced.

Regret for so many complicated graphical trend.

 

[0707]

your top aircoolers are working well?

 

[Petro0707]

Dear 0707,

We have got total 24 bundles and we usually monitor Delta P across Fin fan coolers and even though we inject wash water continuously but if this delta P becomes higher we do offline water wash by isolating 4 bundles one after another and we monitor wash water quality too on continuous basis.

 

[Milutin]

Hi Petro,

Thank you for graphs. When wrote overhead naphtha I referred to overhead naphta product.
You could make litle experiment, tray to increase other pumparounds (kero, diesel and HGO) if top pumparound is on auto mode with top temperature it will decrease flow.
In this way you will decrease vapor/liquid traffic in top column section. You should watch naphta pumparound draw tray temperature and dP in pumparound tray section. After this you can make changes in other direction - increasing top pumparound.

Regards,

 

[Petro0707]

Dear All concerned,

Finally we ended up with the shut down and found 1st Monel tray severely corroded, 2nd tray and 3rd trays are corroded in decreasing order , with subsequent down below intake condition, Lot of depositions and under deposit corrosions found, lot of blockage/plugging of Hi –Fi trays sieve holes .

We were observing corrosion and sever salt deposition / fouling in Naptha pumparound circuit i.e. in S-01 A/B & S -04 A/B ( Naptha / Crude Pumparound Heat exchangers- as shown in the figure) with duty of -42 mmkca/hr these exchangers MOC is CS ,- Tube side naptha and shell side is having crude oil , we were observing lot of salt depositions in shell side i.e. outside of tubes , eventhough as indicated in figure, for 2 months we have injected ester based corroision inhibitior also in naptha pumparound pump . In fact we observed Ni & Cu metal traces in corrosion deposition in exchanger ( may be coming from monel metal in CDU Top section)

One of the incident that was noticed during the sudden delta P increment in naptha section, that was we have taken new bundle in line in one of above stated exchangers because these was routine practice to do repairing/cleaning in naptha / crude p/a exchangers and for that we have kept standby exchangers too. As you can see in the figure that apart from Pumparound heat exchangers , we have got hot bypass to mitigate final return temperature to the column.

As I mentioned earlier we stopped cold reflux since last two years.

You can see chemical injection points also in the figure, , before two years we had similar problem of top tray severe corrosion and that time , deposit analysis found almost Ash is 80 %, Loss of ignition : 20.47 % & others are chloride,Ni – 48.96 wt %,Fe-11.53 , Cu=15.6 ,Na,Si-traces

We are still at a loss : as we already maintain CDU overhead top temperature well above i.e. almost 115 C against water dew point temp. 95 C, but reflux return temp. to CDU column is almost remains between 96 to 105 C. whether is it having significance ???

Desalters operates with 97 – 99 % efficiency – two stage, also injecting caustic in d/s of desalters , sufficient wash water that is 5 – 7 wt % on crudemix.

Still at a wonder as all these corrosion issues are nightmare now ?

The parameters of CDU OH boot water analysis pH also remains between 5.5 to 6.5, wash water quality pH is little bit high as that is 7 – 7.5 as we use nonphenolic water .

Request all of you to throw some light.

http://files.engineering.com/getfil...a03c99bf0&file=Detail+Naphtha+section+dwg.gif

 

[25362]

Just a wild thought: Monel 400 is affected (corroded) by a combination of aeration (or oxidizers) and acids. Could the wash water be a source of air entering the system ?

 

[Milutin]

Hi Petro
According to drawing you add NH3 as nutraliser to pumparound return. Adding ammonia or some other neutraliser to pumporound can cause severe corrosion to top trays, because ammonia salt deposition.

Deposits in P/A heat exchanger are partially entrained from column by P/A flow. Also in this P/A flow there is always water present, what can bring free water to top tray, and this water can exist locally on top tray.

I test this on our crude unit taking first droplets from P/A heat exchanger outlet pipe drain. Because that drain is the lowest point free water is accumulated.

Overhead wash water quality is not concern because you don't use cold reflux, what is source of your desalter wash water?

Regards,

Milutin

 

[Petro0707]

Desalter wash water is stripped water from Sour water stripper unit that we use both phenolic and non-phenolic water: On and average Quality is :

pH : 7 – 9
NH3 : 20 -50 ppm
H2S : ~ 20 ppm
Fe - < 1 ppm
Cynaide : 0.1 ppm

Yes we have got Monel 400 & we don’t monitor oxygen in the wash water going to desalter.

 

[Milutin]

There is no oxigen present if wash water is stripped.
Do you realy use NH3 in pumparound return? If yes that is possible reason for tray corrosion.

Regards,

Milutin

 

[Petro0707]

Actually in the shown figure---Red arrow point little got shifted, we inject gaseous NH3 in CDU overhead line and not in the P/A return line…Regret for that…

 

[25362]

Petro0707, a preferred chloride content after succesful desalting is [&le;]1 ptb; what is the level in your refinery ?

 

[Petro0707]

Hi Milutin & 25362..!!

We usually don’t monitor chlorides in desalter outlet but outlet PTB salts from the desalters always remains below 1 PTB . Furthermore I am attaching one file for your detail perusal / analysis for CDU Column overhead temp. , final reflux return temp. (after p/a exchanger and hot bypass both ) , naphtha draw temp. , naphtha section pressure drop of last 9 months for both train 1 & 2, as we got two trains .Invariably you will find that Train 1 naphtha section pressure drop was very consistent & within limit & suddenly it was increased from 500 mmwc to 1000 mmwc with in couple of days. This chart is Individual value & moving range (difference between two successive individual value ) chart. ( One dot means – 24 hr average of that particular date , and X axis represents no. of data points starting from jan 07 to the incident date in the same order, UCL & LCL is calculated by statistical software (we need not worry about ) but we can see red points means assignable cause and others are chance causes.

One thing we are having doubt is: during those days i.e. sudden raise of pressure drop of naphtha section, if somebody has tried to lined up cold reflux which is dead leg or any bypass line which is almost 50 m-dead line for quite few months (we can say it is dead line – may contain salts, water-we don’t know) ,then with in 15 days this kind of corrosion on trays can occur or not due to this phenomenon ?

Wild idea - One another thing that I would like to mention that we usually do cleaning of Naptha p/a exchangers through fire water- containing high salts , of course after that passivation and air purging also we do before taking in line – can this be the cause ?

Can FeS – Iron sulfide which can form in Naptha / Crude p/a exchangers and which can flowed down to CDU top section trays and subsequently leaded to compounded galvanic corrosion , something like that can happen ?

http://files.engineering.com/getfil...a_draw_temp.,overhead_temp._train_1_vs._2.ppt