6 replies to this topic

[Root]

 Over Flash flow.xlsx   132.56KB   93 downloadsHi,
We are processing condensate to produce, Naphtha, Distillate and residue, Feed density is 0.754kg/m^3 and IBP 40^0c and FBP is 350^0C.
Column has 30 bubble cap trays, feed is on tray 25 and over flash is from tray 24.
BPA pump flow is going on two locations, one on tray 21 and another is on tray 19 via feed preheat exchanger.we are keeping 25 m^3/hr flow on tray 21, more than the design flow.(50%).
Even though we are not getting over flash flow and gap between distillate is also very less. please go through the attached file for more information.
Can any one suggest some solution to this problem.
Thanks

Edited by Toor, 22 April 2010 - 11:05 PM.

[Zauberberg]

Toor,

You need to post a sketch for this kind of topics - it would make the picture 100% clear.
Secondly, we are missing some of the key design/operating parameters such are:

- Column Feed temperature
- Operating pressure of the tower
- Design specs (ASTM gaps/overlaps) of Naphtha, Diesel (distillate) and Residue
- Actual ASTM gaps/overlaps of products
- Tower temperature profile (design vs. actual)
- Design vs. Actual feed
- Design vs. Actual yields

The topic appears to be quite interesting and, if you can put a small effort from your side in presenting the data in the right way, I'm sure we can be of some help.

Edited by Zauberberg, 21 April 2010 - 11:33 AM.

[Zauberberg]

Toor, thanks for reloading the post with plant data. I am looking at the information you provided and here are my initial comments:

- Feed FBP is 310C and the Distillate FBP is 313C. It looks like you are taking all the feed (the heavier part of it) out with the Distillate draw.
- Lower Residue yield indicates the same.
- Obviously the overlap between Naphtha and Distillate is too high (-24C) vs. design gap of +28C. That's more than 50C difference in absolute numbers, indicating poor separation.
- What definitely causes this lower separation is poor side-stripper performance. Notice that Distillate IBP is 95C vs. 140C design case. Also, the Flash point (dramatically low!) indicates the same.

I don't know what is the design feed quality for this unit but here are several items you may wish to check:

1. Feed FBP (design case), or the whole distillation curve. Is the design feed heavier?
2. For investigating whether the lack of overflash is due to different feed or damaged trays, gradually reduce distillate draw off rate (without changing any other parameter) and see if there is an increase of Overflash.
3. Troubleshoot the Distillate stripper - something wrong is happening there, it looks like there is no stripping at all. You can confirm that by measuring the temperature difference between stripper feed and stripper bottoms. The bottoms product should be few degC colder than the feed.

I will try to get some more conclusions. Meanwhile, if you can supply is with actual temperature profile of the tower, that would be great. And please let us know about the outcomes when checking these 3 points from above.

Thanks,

[Root]

Toor, thanks for reloading the post with plant data. I am looking at the information you provided and here are my initial comments:

- Feed FBP is 310C and the Distillate FBP is 313C. It looks like you are taking all the feed (the heavier part of it) out with the Distillate draw.
- Lower Residue yield indicates the same.
- Obviously the overlap between Naphtha and Distillate is too high (-24C) vs. design gap of +28C. That's more than 50C difference in absolute numbers, indicating poor separation.
- What definitely causes this lower separation is poor side-stripper performance. Notice that Distillate IBP is 95C vs. 140C design case. Also, the Flash point (dramatically low!) indicates the same.

I don't know what is the design feed quality for this unit but here are several items you may wish to check:

1. Feed FBP (design case), or the whole distillation curve. Is the design feed heavier?
2. For investigating whether the lack of overflash is due to different feed or damaged trays, gradually reduce distillate draw off rate (without changing any other parameter) and see if there is an increase of Overflash.
3. Troubleshoot the Distillate stripper - something wrong is happening there, it looks like there is no stripping at all. You can confirm that by measuring the temperature difference between stripper feed and stripper bottoms. The bottoms product should be few degC colder than the feed.

I will try to get some more conclusions. Meanwhile, if you can supply is with actual temperature profile of the tower, that would be great. And please let us know about the outcomes when checking these 3 points from above.

Thanks,

Hi Zaburger,
I have gone through your input and here is my reponse.
1-Feed is same as design case.
2-Pump back is 20% as compare to 12% design flow therefore, over flash is there !
3-Side cutstripper is performing well, inlet temp is 244^0C and Btm temp 232 and top temp is 235^0C
The only concern is naphtha is demping in distillate and distillate is going with residue.
Please be keep it in mind, there is no stripping steam in fractionator and side cut stripper. Steam is not given to corrosion problem in rectifying and over head section.
Hope you have now more clear picture to find out some other solution.
Thanks

[Zauberberg]

Toor,

It's not surprising that you have Naphtha dumped into Distillate, and Distillate dumped into Residue - because there is no stripping steam (as you say). The confusing part is why the side stripper bottom temperature is significantly lower than the stripper feed, though (if there is no stripping). In any case, based on the ASTM D86 data, I assure you that the side stripper definitely does NOT perform well.

The same applies for Distillate loss into the Residue. This could be overcome by increasing the heater outlet temperature but only if there is sufficient pumparound capacity for additional heat removal requirements in the system.

Regarding your comment No.2 - if I understood you well, by increasing pumpback you are getting overflash that is close to design value?

If I were at your place, I would try to start the stripping steam both in the Main Tower and the Distillate Stripper. Remember, the steam strips off lighter components from the Residue which turns back into the Overflash if this additional quantity of Distillate fraction is not drawn off from the tower through Distillate loop.

[Root]

Toor,

It's not surprising that you have Naphtha dumped into Distillate, and Distillate dumped into Residue - because there is no stripping steam (as you say). The confusing part is why the side stripper bottom temperature is significantly lower than the stripper feed, though (if there is no stripping). In any case, based on the ASTM D86 data, I assure you that the side stripper definitely does NOT perform well.

The same applies for Distillate loss into the Residue. This could be overcome by increasing the heater outlet temperature but only if there is sufficient pumparound capacity for additional heat removal requirements in the system.

Regarding your comment No.2 - if I understood you well, by increasing pumpback you are getting overflash that is close to design value?

If I were at your place, I would try to start the stripping steam both in the Main Tower and the Distillate Stripper. Remember, the steam strips off lighter components from the Residue which turns back into the Overflash if this additional quantity of Distillate fraction is not drawn off from the tower through Distillate loop.

Zauberberg,
Thank you for your valuable suggestions.
Toor

[Zauberberg]

Toor,

Please take extreme caution prior to introducing stripping steam into the column(s). The main steam isolation valve (closer to the tower) should be closed and the drain valve fully open, so as to have all the liquid accumulated inside the line completely drained before the steam is injected into the tower. The line is also fairly cold, so please take sufficient time for warming up the line and draining water from the steam supply system. Injecting wet steam or water into the tower would almost for sure dislodge the trays due to rapid vaporization after contacting hot hydrocarbons and creating a pressure surge.