Column Temperature Inversion

[EmmanuelTop]

What are possible explanations for temperature inversion in the top pumparound section of crude oil fractionator? Tower top temperature (vapor phase) is 145C, top pumparound return temperature is 90C, and 3rd tray (pumparound draw-off/accumulator tray) temperature is 135C in liquid phase. Normally, this temperature should be 15-20C hotter than the tower top temperature.

Heating the pumparound return stream by bypassing heat exchangers raises the temperature of 3rd tray almost in linear fashion, while column heat balance remains unchanged (which is expected). This results in somewhat changed temperature profile - 3rd tray becomes 4-5C hotter than the tower top, but still well below normal values.
Trays are completely new, replaced during the 2006 turnaround.

I need to say, for those who may suspect too low pumparound return temperature, that crude oil fractionator operated very well at pumparound return temperatures below 80C. At these times, 3rd tray temperature was 15-20C hotter than the tower top.
Everything points out at mixing of cold (external) and hot (internal) reflux. In other words, it looks like part of cold pumparound stream is bypassing hot vapors passing through openings at trays no. 1 and 2. But, it seems to be impossible, since trays are completely new and properly installed.

Do you have any suggestions about this phenomenon?

 

[25362]

I'm given to understand that in large-diameter towers, if the pumparound return distributor is faulty, the liquid/vapor contact can be largely disrupted.

 

[Milutin]

Dear Mr. Zauberberg,

In my opinion it is most likely that you don't have good contact between cold pumparound liquid and hot upward flowing vapors.

My suggestion, before anybody suggest gamma scanning, try to measure column outside wall temperature. For trays in P/A zone, locate downcomer and measure temperature on that position and on same level around column. Cut hole trough insulation before measurement.
This measurement can give you some ideas what is going inside column.

I am not sure for distributor issue for trayed columns, because if trays are ok, no better liquid distributors then trays.

Regards,

Milutin

 

[25362]

Milutin: I referred only to tray #1 and assumed a large diameter.

 

[0707]

145ºC
o--------------------- Reflux «
Tray 1----------------------------


Tray 2-----------------------------
135ºC

___Naphtha accumulator__ Naphtha draw off________________»
90ºC
o---------------------Naphtha pump around«
------------------------------------


If it is so, my opinion is that every thing is ok with your pump around, your problem is maybe with the cooling of your overhead gas. I think you should check the cooling system of your overhead product. Your overhead air coolers or condensers are maybe fouled.

Good luck

Luis

 

[CJKruger]

EmmanuelTop,

Firstly, most likely the vapor and liquid are not making good contact. Probable reasons are:
(a) a missing tray manway,
(b) missing/incorrect liquid return distributor (for multipass trays),
(c) wrong downcomer clearances and weir heights.

Secondly, if weird stuff happen, check that you are significantly above the water dewpoint at the top of the tower.

 

[Milutin]

If I understand tower temp. profile is:

Top 145degC

Tray #1 ------- <-----90degC P/A return

Tray #2 -------

Tray #3 -^^^^^- ------>135degC P/A draw (normally 165degC)

Regards,

 

[Milutin]

EmmanuelTop do you have lab analysis of top P/A, distillation and density, before this temp inversion and now?

Regards,

 

[reena1957]

Dear Emmanueltop,
Please tell us the following:
1. Is there a top temperature controller acting on the external reflux?
2. Has the Top CR flow now more or less - OR- Is the external reflux flow More or Less for the same crude and same product draws and same throughput?
3. Have you done a heat balance around the top two trays?
4.From this have you calculated the approximate UA for the two trays assuming the two trays to be a cross flow exchanger? Has this value come down? If it is so, then there is problem with the trays. New tray installation without proper supervision and quality check on gaskets, always leads to problems.
With best wishes

 

[EmmanuelTop]

Hello Reena,

1. Yes, there is a column top temperature controller connected with pumparound flow control valve (cascade mode).
2. Top CR flow is the sam as it was before, for the same crude/product draws/throughput.
3. Heat balance is calculated by rigorous column simulation and compared with actual plant data. Much less heat is removed from the column top two trays, compared to regular state. Also, as I mentioned before, there is a significant loss in top pumparound duty - because of lower LMTD in pumparound heat exchangers.

I have checked top pumparound naphtha for density and D86 distillation. Density is 0.7592, and D86 end point is 181C (while overhead stabilized naphtha has density of 0.7382 and D86 end point 175C).
The strange thing is that we had to shut down the unit after the first start up after 2006 turnaround, because of insufficient liquid phase level at diesel P/A accumulator tray. On that occasion, we also checked trays No.1 and 2, and also top P/A accumulator tray No.3 - and they all were in perfect condition - at least they looked good and properly assembled. But, in my opinion, there just cannot be some another reason for temperature inversion besides improper trays installation. Even if reflux distributor is broken, trays should act as a liquid phase distributors.

 

[25362]

Are temperature measuring and transmitting instruments in good working order ?

 

[reena1957]

Dear Emmanueltop,
Apart from checking what 25362 has suggested, I think your requirement of Top CR duty has come down due to probably higher CR duties below the top CR ( as the duty is automatically controlled). I would suggest you reduce the heat duties of the CR below the top CR and attain the same duty of Top CR as before shut down, and then check if the top temperaure is still under control. If the vapours reaching the 3rd tray are not hot enough, the liquid leaving the tray also will not be hot.
Best wishes

 

[EmmanuelTop]

25362:

The temperature measurement is correct. We have cross-checked 3rd tray temperature by measuring top pumparound heat exchangers inlet temperature, and it is the same as on the 3rd tray (pumparound draw-off tray).

Reena:

I do not know if this is possible to lower diesel P/A duty and still maintain column heat balance. Since this is only a two-pumparound fractionator system, lowering diesel CR duty will increase top CR heat removal demand, which is already near its maximum (90% of CR pump maximum flow).

We operated before with 346C crude heater outlet temperature and ~70% of maximum diesel CR flow. For the last two years, crude heater outlet temperature is 355C (design value) and diesel CR flow is 98% of maximum, while the top CR is, as I wrote, around 90% of max flow. I am awaiting for DCS trends 3 years ago, to see if raising the heater outlet temperature and raising diesel CR duty adversely affected column top section temperature profile. I do not think parameters these should have such big influence on top CR, since diesel CR duty increase VS feed enthalpy increase are almost the same.

Thank you all for your ideas.

 

[reena1957]

Dear Emmanueltop,
For the same flow of Top CR, the duty will increase if the draw temperature goes up and the return temperature remains same.
Best wishes.

 

[EmmanuelTop]

Dear Reena,

Decreasing Diesel CR flow from 95% to 70% of maximum duty does not influence the temperature profile of column top section. The only thing that is changing is Top CR heat removal demand, it grows beyond maximum duty that is available in present CR heat exchangers loop - which forced us to use more auxiliary reflux flow from the overhead receiver.

It looks like malfunctioning reflux distributors/trays are the only possible explanation for this phenomenon.

 

[0707]

EmmanuelTop

what is the temperature of your reflux distribution line on top tray?