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We are having three stage ejector system in one of our vacuum column without pre condenser. Offlate this vacuum column overhead pressure has gone up from 35mmHG to 45mmHG. Following were the major observations
a. The condensate ( water + HC) outlet temperature from all the three condensers have gone up by around 10°C
b. Hot well slop make has doubled.
c. No significant change was observed in column overhead temperature
d. No changes in product parameters or other process parameters.
e. Cooling water flow and temperatures were checked and no changes were observed.
f. The wash oil ie., Vacuum diesel oil flow and delta T are same.
At the outset, it looks like hydrocarbon carryover from Vac. Diesel bed. But then, how to findout what has gone wrong. Can forum throw light on troubleshooting options we have.
With best wishes
Sudheer Pai
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Vacuum Column Troubleshooting
Started by sudheer, May 29 2008 06:05 AM
4 replies to this topic
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#2
Guest_srojack_*
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Posted 29 May 2008 - 06:47 PM
Hello Sudheer,
From the info you have provided it would seem that there is nothing wrong mechanically or process wise with the vacuum column. Is it possible that the quality of the charge to the column has changed? Maybe these questions should be first answered:-
1. Has the type of charge stock to the unit changed (is there a "new crude" in the refinery)?
2. Are there any changes to the operating conditions on the unit(s) upstream of the vacuum column (any operating limits being reached on the Topping unit(s) causing atmospheric distillates to be left in the reduced crude)?
3. Assuming there are charge/product heat exchangers, is it possible that leaks exist that would cause migration of charge material into the column via reflux circuits or the recycling of light product into the charge stream causing an artificial loading of the vacuum column overhead?
4. On the vacuum unit is more reflux duty now required to achieve the same overhead temperature?
5. Has the charge heater been recently operating at higher outlet temperatures?
6. Has there been an increase in vacuum off gas make from the column?
7. Has the boiling range of the charge to the unit been reduced (is the unit charge now heavier than before)?
From my experience on vacuum units the changes you have described were common with changes in charge stocks to the unit. Anything that widens the boiling range of the unit's charge would normally show up as additional hot well slop oil. It is possible that your overhead system is adequately sized to handle the additional load without negatively affecting the vacuum gas oil yields (this was my experience on my unit).
With respect to questions 5,6 and 7 you will be looking at the possibility of additional cracking taking place with the unit's charge. Higher heater outlet temperature may promote cracking which will increase vacuum overhead loading. At the same heater outlet temperature "crackability" increases when the charge is heavier (less distillates/more bottoms in charge) with the same results as above.
From the info you have provided it would seem that there is nothing wrong mechanically or process wise with the vacuum column. Is it possible that the quality of the charge to the column has changed? Maybe these questions should be first answered:-
1. Has the type of charge stock to the unit changed (is there a "new crude" in the refinery)?
2. Are there any changes to the operating conditions on the unit(s) upstream of the vacuum column (any operating limits being reached on the Topping unit(s) causing atmospheric distillates to be left in the reduced crude)?
3. Assuming there are charge/product heat exchangers, is it possible that leaks exist that would cause migration of charge material into the column via reflux circuits or the recycling of light product into the charge stream causing an artificial loading of the vacuum column overhead?
4. On the vacuum unit is more reflux duty now required to achieve the same overhead temperature?
5. Has the charge heater been recently operating at higher outlet temperatures?
6. Has there been an increase in vacuum off gas make from the column?
7. Has the boiling range of the charge to the unit been reduced (is the unit charge now heavier than before)?
From my experience on vacuum units the changes you have described were common with changes in charge stocks to the unit. Anything that widens the boiling range of the unit's charge would normally show up as additional hot well slop oil. It is possible that your overhead system is adequately sized to handle the additional load without negatively affecting the vacuum gas oil yields (this was my experience on my unit).
With respect to questions 5,6 and 7 you will be looking at the possibility of additional cracking taking place with the unit's charge. Higher heater outlet temperature may promote cracking which will increase vacuum overhead loading. At the same heater outlet temperature "crackability" increases when the charge is heavier (less distillates/more bottoms in charge) with the same results as above.
#3
sudheer
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Posted 29 May 2008 - 10:52 PM
Thank you very much Mr.Srojack for valuable comments. I can make picture further clear with these inputs. ( pointwise reply to your inputs)
1. We are facing this problem since one month. This started 15 days after a interim shutdown. We run various crudes ranging from light to heavier ones to findout no change in the make. It has remained always high. Even no change is observed in condenser outlet temperature.
2. The stripping steam to crude column was increased by around 15% to drive out lighter ones. The Diesel make in crude column was varied also. But no change was observed.
3. RCO comes directly from crude column bottoms and I do not think exchanger leak in charge product heat exchanger will affect vac. column overhead slop.
4. As we operate with similar crudes, with same Cr flow, supply return temperatures, the duty remains same. ( In fact we are operating with max duty all the time)
5. The charge heater operating temperature was varied and no significant change was observed.
6. To measure vac. off gas make, we do not have meter. But the analysis indicated Nitrogen+ Hydrogen content as 30% ( Liebermann book gave typical analysis as 30%).
7. Already answered this.
Further to these inputs following are my observations
a. The slop oil color looks lighter.
b. The slop oil IBP was 55°C and FBP 280°C. is lighter compared to earlier analysis.
Hope I can get some more inputs from you and forum
Thanks
1. We are facing this problem since one month. This started 15 days after a interim shutdown. We run various crudes ranging from light to heavier ones to findout no change in the make. It has remained always high. Even no change is observed in condenser outlet temperature.
2. The stripping steam to crude column was increased by around 15% to drive out lighter ones. The Diesel make in crude column was varied also. But no change was observed.
3. RCO comes directly from crude column bottoms and I do not think exchanger leak in charge product heat exchanger will affect vac. column overhead slop.
4. As we operate with similar crudes, with same Cr flow, supply return temperatures, the duty remains same. ( In fact we are operating with max duty all the time)
5. The charge heater operating temperature was varied and no significant change was observed.
6. To measure vac. off gas make, we do not have meter. But the analysis indicated Nitrogen+ Hydrogen content as 30% ( Liebermann book gave typical analysis as 30%).
7. Already answered this.
Further to these inputs following are my observations
a. The slop oil color looks lighter.
b. The slop oil IBP was 55°C and FBP 280°C. is lighter compared to earlier analysis.
Hope I can get some more inputs from you and forum
Thanks
#4
djack77494
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Posted 30 May 2008 - 04:21 PM
QUOTE (sudheer @ May 29 2008, 07:52 PM) <{POST_SNAPBACK}>
Hope I can get some more inputs from you and forum
I'm no expert on this, but while we're waiting for one to arrive, I think it could be wise to look at your ATB (Atmospheric Tower Bottoms) since it feeds the Vacuum Column. I'm thinking that something could be pushing more lighter material into the ATB stream. Beyond that, I would look into the column's overall material balance and compare against historical data. One of the things I'd be on the lookout for here is the possibility of a heat exchanger leak and some outside material entering one of the Vacuum Column's streams. If you still have no answers, then I'd begin to suspect some sort of mechanical problems.
#5
Zauberberg
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Posted 31 May 2008 - 08:12 AM
Hello Sudheer,
Apart from excellent replies and recommendations you received from Srojack and Doug, here is what I would like to try, if I were in your position.
Operating vacuum condensers at higher temperature, without any other changes of tower parameters, indicates increased load on the overhead system. This can be due to many possible reasons - and what you should do next is to narrow the search for culprit(s). I'd suggest you to cut down VDU feed rate, or heater temperature - only temporarily - and look at system behaviour.
If problem is in leaking exchangers, it is most likely that cutting feed/temperature will not be beneficial. If excessive amount of light ends is present in VDU feed, than you will easily restore overhead condensing capacity. You can also suffer from condensate back-up in vacuum condensers: surface area available for steam/HC condensation is then reduced, increasing the temperature of condensation and, hence the condensing pressure as well. Check is there any temperature gradient around your vacuum condensers - especially between vapor side outlet and bottom condensate drainage nozzle.
Whatever you do, always do one thing at a time. If you are changing several parameters at once, then you will not be able to determine what is the culprit of increased ejector load.
Best of luck,
Apart from excellent replies and recommendations you received from Srojack and Doug, here is what I would like to try, if I were in your position.
Operating vacuum condensers at higher temperature, without any other changes of tower parameters, indicates increased load on the overhead system. This can be due to many possible reasons - and what you should do next is to narrow the search for culprit(s). I'd suggest you to cut down VDU feed rate, or heater temperature - only temporarily - and look at system behaviour.
If problem is in leaking exchangers, it is most likely that cutting feed/temperature will not be beneficial. If excessive amount of light ends is present in VDU feed, than you will easily restore overhead condensing capacity. You can also suffer from condensate back-up in vacuum condensers: surface area available for steam/HC condensation is then reduced, increasing the temperature of condensation and, hence the condensing pressure as well. Check is there any temperature gradient around your vacuum condensers - especially between vapor side outlet and bottom condensate drainage nozzle.
Whatever you do, always do one thing at a time. If you are changing several parameters at once, then you will not be able to determine what is the culprit of increased ejector load.
Best of luck,
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