11 replies to this topic

[Chandra S]

Hi,

 

Would like to get some input to a limitation observed at a condensate distillation column.

 

This column capacity is about 74,000bpd which has a side cut (kerosene) while at top is heavy naphtha minus which the yield is about 80-82vol% and bottom product is diesel.

 

Observed lately, the column capacity limits at 100% load with increase of its bottom pressure drop which then affects the kerosene freezing point (the kero end point increases significantly).

 

Total tray in this column is 32 with 2-pass trays; measurement available for bottom section (total of 16 bottom trays) and top sections trays. The last shutdown of this column for cleaning was in May 2011.

 

So now we are running at 92% with no problem however incresing load further gives higher dp which not seen before. Column scaning done last month but findings on internal damage/missing.

 

Anyone experienced column fouling to limit column capacity? Damage at downcomer could lead to this kind of symptoms.

 

Appreciate your inputs, thank you.

 

 

[Zauberberg]

Increase of kerosene end point indicates poor fractionation between Kerosene and Diesel (bottoms), and almost certainly flooding initiated somewhere below the Kerosene draw-off tray. Higher pressure drop across the column is almost certain indicator of flooding.

 

If column scans didn't show any visible signs of damage, it could be easily happen that the trays are fouled.

 

Are there any other indications? Bottom liquid level fluctuations, sudden increase of Naphtha accumulator level, column temperature profiles etc.?

[Arsal]

Hi Chandra s;

Higher pressure drop is the indication of flooding.If your Scan did not show any damage then check your column Draw off temperature and levels and should calibrate the instruments once to ensure the parameters are correct.

Further Fouling problem usually not occurs in the Main distillation columns ,Normally fouling occurs in the column such as Debutanizer,stabilizers which limits the product yield and defolued method is to flush the column online with calculated amount of water with respect to ammonium chloride Deposition and products specification trends.

If You Explain further like Fluctuations in parameters,Tray type,Lab results.then its more easy to diagnose the real cause............

[Chandra S]

Thanks Zauberberg and Rao Arsalan for the response.

 

The problem is obvious at higher load compared to lower load. No significant sign but at higher load, the temperature profile does change especially increase in kero draw temp.

 

All instrumentations were checked, calibrated and those with variation was replaced accordingly.

 

Any possibility you may bet on downcomer blockage by fouling or internal damage that limiting the capacity at certain higher load?

 

It is 2 pass tray, total 32 trays; tray#1 to tray#9 are Sulzer fixed valve trays (trays numbers are bottom up). Feed at tray #9 and kero draw at tray #16. The rest of trays are of Glitch. The bottom 9 trays was replaced in 2005 during debottlenecking of this unit. Typical freeezing of this unit's kerosene is about -60 to -65 degC, so the upset occur, freezing would be around -18 to -20 degC.

 

[Zauberberg]

Increase of Kerosene draw-off temperature is another sign of flooding - it occurs because hotter liquid is rising from the trays below. You certainly do have a problem there.

 

This rising of liquid may be happening either because the downcomers below the Kerosene draw-off tray are partially choked/blocked, or because vapor velocities across these trays are too high, causing excessive pressure drop which prevents the liquid phase from cascading down the trays. This results in liquid level building up in the downcomers, up to the point where liquid starts overflowing onto the trays above. High pressure drop of vapors across the trays may happen because of blocked tray valves, or maybe because of excessive vapor generation. Is there a reboiler on this column, or a fired heater upstream of the column?

 

A sketch showing column details (feed, draw-off trays, reboiler - if exists, configuration, temperatures) would be very helpful. Especially if you can make two of them - one with normal operating parameters and one with parameters when flooding is observed.

[Arsal]

It is very difficult to say any internal damage in columns without thoroughly understand the scenario some times operating problem is a cause.

Tell me some other parameters at max throughput as well and believe if you explain these parameters and also PFD it quite helpful to resolve problem.

 

1) IBP/FBP/FLASH

2) Kero draw off temperature?

3 Pump around Drawoff / return temperature?

4) Fired Heater Transfer line temperature?

5)Bottom Temperature?

 

Will this problem occur only at max throughput?

Is Unit normal at low throughput?

HOW much yield loss if make on spec kerosene on maximum throughput?

How Much the differential temperature variation across Kerosene stripper?and Stripping steam rate Trend?

Product yield is very important if any internal damage.............

[Chandra S]

Agreed that the column is having a problem. The last column was operated at 100% load was in November 2014 then the load was reduced and when load increased back to 100% in December that when the problem started. No significant other issues was seen in that period.

 

Yes, there is fired heater reboiler for this column. I have tested with high boil up ratio but no significant impact in column temperature profile hence my inclination towards downcomer limitation.

 

When operating at lower load, there is no overlap between kero-diesel and of course kero-heavy naphth minus (column overhead).

 

I attach the condition of normal and during high dp condition.  Condensate Column.pptx   515.87KB   44 downloads  Condensate Column.pptx   515.87KB   44 downloads

[Zauberberg]

It would be interesting to see what happens if you try to pull out more Kerosene during high dP operation. This would temporarily reduce liquid load on the trays below Kerosene tray, and it may show whether the downcomer choking is the problem.

[Arsal]

what is IBP/FBP/FLASH point during upset conditions?

Have you reduce transfer line/ bottom temperature of your column to check the freezing point/FBP?it might helpful?

[Chandra S]

Well yes, during that condition, kero flow will increase more than normal yield temporarily even the column load is reduced which will reduce the flooding.

 

Typical average kero IBP/FPB is 155-158/210-215 with freeze of around -65 to -70 degC and flash about 41-42 degC, so when such upset takes place, IBP/FBP is around 160/288 with freeze point of around -18 to - 23 degC and flash about 38-39 degC.

[Zauberberg]

Do you ever remember this column working properly at the same design rate, with the same (or very similar) type of feed? If yes, then the problem is recent and you definitely have something wrong with the tower.

 

How does the system react if you reduce the reboiler temperature, but still trying to be on spec with Kerosene? You will be getting more Diesel, and a bit lighter Diesel than usual.

 

It is worthwhile passing the tower through KG-Tower or Sulcol software. I am really curious what the outcome would be. Perhaps it can give you further insight.

[Arsal]

It is very clear that your tower does not have a fouling problem, here i want to  discuss you a problem of a tower have fouled.

Debutanizer at platformer unit was fouled and sepration efficiency of the tray was decreased and we did not get on spec LPG and HAVING HIGH WEATHRING...fouling is Occured due to Ammonium chloride depostion on tray beacuse we have high HCL in recycle gas and ammonia as well in the system.So NH4CL was deposit and block tray deck.And we resolve this p[problem by online flushing with water at low temperature (NH4Cl is hydroscopic in nature)

 

But I think condensate is not having such type of high impurities.so there is no chance of foulling.AM i right?or you have high impurities in crude feed?

 

It might be that some portion of tower internals (like Tray packing,Down-comer,Tray deck,Ballast unit) may damage.which disturb separation efficiency at high throughput.

 

When find the Actual problem please share with us.

 

Thank you.