7 replies to this topic

[sheiko]

Hello,

A quick question please:

I have a crude distillation column which has a fixed feed temperature and a fixed top pressure, but which stripping steam and reflux have decreased.

However, at the same time, I have noticed that deltaT (bottom temp minus top temp) has increased, which would mean that fractionnation has improved.

Could please help me clarify?

Edited by sheiko, 18 September 2014 - 10:26 PM.

[PingPong]

To check whether fractionation changed you need to look at the change in ASTM 5%-95% gaps or overlaps between the products, and change in product properties, especially all flash points, kero freezing point and smoke point, gasoil pour point and viscosity.

[sheiko]

Thanks.
For sure fractionnation has changed.
What I find strange is that overal deltaT indicates improvement while top reflux and stripping steam has decreased...However thinking about it more, this improvement could be an indication of flooding in the column (deltaP has slightly decreased in the same time).

Edited by sheiko, 19 September 2014 - 04:03 PM.

[PingPong]

I don't see why in a crude fractionator an overall dT increase, due to decrease of stripping steam, would indicate fractionation improvement. It's more complicated than that. If you operate a crude column you should have a simulation model of it, so that you can easily see impact of changing operating variables.

 

Flooding should not occur if stripping steam is decreased. Moreover flooding would result in increased column dP.

[sheiko]

Flooding should not occur if stripping steam is decreased. Moreover flooding would result in increased column dP.

Thanks.
Sorry I have not been precise enough (I'm french => bad english).
I meant, decreasing the steam have resulted in a slight decrease of deltaP and an increase of deltaT => improvement.

I don't see why in a crude fractionator an overall dT increase, due to decrease of stripping steam, would indicate fractionation improvement. It's more complicated than that.

Well I thought overall deltaT was a good indication of fractionnation efficiency of distillation columns in general.

Edited by sheiko, 19 September 2014 - 04:05 PM.

[PingPong]

A "normal" distillation column has a reboiler, so that the bottoms temperature is higher than the feed temperature, and corresponds with the boiling point of bottoms product at the bottoms temperature.

 

In a crude oil fractionator that is not the case, due to the stripping steam. The bottoms temperature is lower than the feed temperature, due to loss of heat of vaporization as result of steam stripping of gasoil components. Less steam stripping would result in higher bottoms temperature as less gasoil would be vaporized.

Less stripping steam in the column overhead, while still producing the same quantity of overhead naphtha product (so same TBP cutpoint) would result in higher overhead temperature, because hydrocarbon partial pressure would be higher. If however the reflux is on temperature control of column tray temperature, and the setpoint of that TC is not changed when stripping steam is reduced, the TBP cutpoint of the overhead naphtha would change, and it becomes a different story.

[sheiko]

Thanks a lot pingpong

[Vinayak Chavan]

Also please note that top temperature has to be maintained above the dewpoint of water to avoid the corrosion due to condesation of water in the top section of the column.