3 replies to this topic

[Mike1122]

Hello All,

I'm new to the community and i appreciate any help and guidance you can provide with regards to my question.

I'm currently the process engineer for a refinery crude unit (only a couple years refining experience) and i've noticed that our naphtha (CDU overhead liquid) 95% distillation has been increasing for the same vapor temperature setpoint. To investigate further, i monitored the naphtha-jet cut point along with the increase in 95% distillation.

What the data showed was that over time (maybe about 5 months), the 95% distillation and naphtha-jet cut point continued to increase; however, the difference between 95% distillation and cut point (my way of monitoring the heavy tail) did not change much. My understanding was that 95% minus cut point was always a good way to monitor fractionation performance.

Given that we're operating to a consistent overhead vapor temperature, do you think the increasing 95% and cutpoint is a sign of reduced tray efficiency (corroded top trays, plugged trays, etc), despite not seeing much of a change in delta between 95% and cut point? I would think that any cut point shifts would be correlated to a higher overhead temp... Just looking to see if anyone with more experience than myself can touch on this topic.

Thanks!

[S.AHMAD]

1. Are you referring to TBP cut point?
2. The degree of seperation between adjacent cut is indicated by the gap which is the temperature difference between ASTM 5% of heavier cut (jet-kero) and the ASTM 95% of the lighter cut (naphtha).
3. Negative gap is called overlap.
4. For naphtha-kero cut it should be the gap.
5. Monitoring the gap and overlap is a better approach for tray efficiency.

[Himanshu Sharma]

I assume that you are processing same feed stock for the period you have marked the observations.

As explained by Mr. S.Ahmad , please monitor the difference between" ASTM 5% of heavier cut (jet-kero) and the ASTM 95% of the lighter cut (naphtha)" ,if this gap is constant(direction ally speaking as you mentioned between ASTM 95% and naphtha-jet cut point) then your trays are working fine however a reduction in gap is often observed with the tray fouling/cycle length.

can you explain "the 95% distillation and naphtha-jet cut point continued to increase;". isn't that cut point is a variable you can adjust.

[Zubair Exclaim]

Also consider following

1-During changing weather overall refiney performance changes slightly. some times in summer you get higher cut point on top of the column and in winter you will be getting lower TBP cut point values at the top of the column

2-Crude assay also has some variations, check previous crude assays when you were getting lower TBP and match with current.

3-Also if you are suspicious of tray eficiency try the check the pressure drop accross column. accross tray section wherever possible