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Question on refinery distillation column behaviour
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George11
science forum beginner


Joined: 13 Dec 2005
Posts: 24

PostPosted: Tue Jul 04, 2006 11:04 am    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

Have you considered some of these other possibilities:
* Instrument drift or failure
* Upset in feed stream concentration
* Other process interactions

You would expect adjacent tray temperatures to move in the same
direction as Tray 11. If tray 11 is not moving similar to, for
example, trays 10 & 12, then this could point to instrument failure.

An upset in feed concentration could also affect the Tray 11
temperature. Does the column data show any evidence of this?

Process Interaction Mapping would also identify any strong correlations
to your Tray 11 temperature loop. See this link for more info:
http://www.expertune.com/PTDetailed.html#Cross



The Romanov wrote:
Quote:
I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?
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John Shaw
science forum beginner


Joined: 25 Mar 2005
Posts: 29

PostPosted: Fri Jun 16, 2006 10:07 pm    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

My guess is that two things happened, perhaps by coincidence, at the
same time.

When the reflux is cooled below its condensation temperature (at the
current pressure), there will be further condensation of vapor at the
point where the reflux enters the column. This condensation increases
the amount of reflux in the column. The vapor condensation due to cool
reflux is known as "internal reflux".

An increase in reflux (due to internal reflux) should decrease the
temperature of the upper trays.

Perhaps something happed at the same time, such as an increase of
heavier material in the feed that would increase the temperature. The
decrease from the cooler reflux must have been less than the increase
resulting from some other effect.

Sometimes (not often, but sometimes) when the operator or control
system makes a change that should have one effect some unrelated thing
happens that has the opposite effect.

Once, when operating a column I increased the reflux flow. What I
didn't know (at the time) was that an upstream change increased the
heavy stuff in the feed, causing the purity of the top (by analyzer) to
fall in spite of my reflux increase. I was confused until I found out
(from analyzer data upstream) of the coincidence.

John


The Romanov wrote:
Quote:
I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?
Back to top
Rob Brendel
science forum beginner


Joined: 04 Feb 2005
Posts: 3

PostPosted: Fri Jun 16, 2006 9:35 pm    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

Can you describe the control system for the column overhead? The
actions of the controllers may have contributed to the phenomenon.

I presume the column pressure is controlled with a valve on the light
gas stream taken from the reflux drum. (In my mind I'm picturing a
column with no bypass around the condenser, by the way, since that
wouldn't make sense here. But please confirm that.) I'm guessing the
column pressure dropped due to the increased cooling, and the offgas
rate was pinched back by the pressure controller. How fast did the
pressure controller respond? If too slowly and the column pressure
dropped sharply, there could have been an increase in the vaporization
in all column sections. That vaporization could have brought heavy
components up the tower, increasing the tray 11 temperature. Looking at
responses of other temperatures in the column (such as the total
overhead vapor) may help confirm or refute this.

How are the drum liquid level and tray temperature controlled? One
probably adjusts the reflux flow, and the other the LPG draw off. But
which way were they lined up when this happened, and how fast did they
react? My guess is that the effect of decreased reflux temperature (and
increased flow, whichever control arrangement you had) on the tray 11
temperature would be quite slow relative to a sudden increase in
vaporization carrying up heavy components.

This might be all wrong, though, if the pressure control was tight and
the liquid flow changes were rapid. In that case post more info about
the dynamic responses of the flows and pressures, and maybe someone
will figure it out.



The Romanov wrote:
Quote:
I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?
Back to top
bruce varley
science forum beginner


Joined: 31 May 2005
Posts: 38

PostPosted: Wed Jun 14, 2006 1:29 pm    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

TheRomanov <datukbendahara69@yahoo.com> wrote in message
news:1150218484.492807.296370@y43g2000cwc.googlegroups.com...
It's was a real one and the situation that happened was a result of
panel operator starting another fin fan cooler. What I don't understand
is the dynamic response of the tray11 temperature. It suppose to drop
as reflux is cooler but the reverse happened. Let me know if you need
more information.

belaid1969@hotmail.com wrote:
Quote:
The Romanov a écrit :

I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?

Hi,

Is your column a real one or design case ? Are you making a dynamical
simulation or a static one?

you can not directly play on the reflux tempreature, it is fixed by the
pressure and the composition of the distillate. reflux temprature is
not a degree of freedom. However to discuss about this case, it is
important to answer my first question.

The first response you're likely to get if the cooling rise is fast is an
increase in vapour rate. This will gnerate a transient increase in V/L ratio
throughout the tower which could cause the temperatures to go high. The new
steady state equilibrium, with associated temperatures, will only emerge
once the liquid rate increases, which takes more time.
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belaid1969@hotmail.com
science forum beginner


Joined: 16 May 2006
Posts: 7

PostPosted: Wed Jun 14, 2006 6:43 am    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

TheRomanov a écrit :

Quote:
It's was a real one and the situation that happened was a result of
panel operator starting another fin fan cooler. What I don't understand
is the dynamic response of the tray11 temperature. It suppose to drop
as reflux is cooler but the reverse happened. Let me know if you need
more information.

belaid1969@hotmail.com wrote:
The Romanov a écrit :

I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?

Hi,

Is your column a real one or design case ? Are you making a dynamical
simulation or a static one?

you can not directly play on the reflux tempreature, it is fixed by the
pressure and the composition of the distillate. reflux temprature is
not a degree of freedom. However to discuss about this case, it is
important to answer my first question.

In fact starting the new fin fan, created a perturbation to the system.
so you are in dynamic regime.
I'm have not really high expertise in distillation dynamics, but it is
well known that the process can show wath we call an ''inverse
response'', this is due to the non linearity of the system and the
coupling of the control loops. But normaly after this transitional
period the temperature profile should go on the right way. so the
question did you notice just a transitional temperature behavior or it
stay a this new level after a period of time (steady state regime).


A high temperature of the tray means that the composition of the tray
changed at constant presure. More heavy component are on the tray 11.

another point is the pressure. If the pressure raised this will also
increase the temperature profile at constant composition. did you check
if the pressure was constant.

However we can make a simulation and this will help to understand the
problem.
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TheRomanov
science forum beginner


Joined: 22 May 2005
Posts: 40

PostPosted: Tue Jun 13, 2006 5:08 pm    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

It's was a real one and the situation that happened was a result of
panel operator starting another fin fan cooler. What I don't understand
is the dynamic response of the tray11 temperature. It suppose to drop
as reflux is cooler but the reverse happened. Let me know if you need
more information.

belaid1969@hotmail.com wrote:
Quote:
The Romanov a écrit :

I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?

Hi,

Is your column a real one or design case ? Are you making a dynamical
simulation or a static one?

you can not directly play on the reflux tempreature, it is fixed by the
pressure and the composition of the distillate. reflux temprature is
not a degree of freedom. However to discuss about this case, it is
important to answer my first question.
Back to top
belaid1969@hotmail.com
science forum beginner


Joined: 16 May 2006
Posts: 7

PostPosted: Tue Jun 13, 2006 8:20 am    Post subject: Re: Question on refinery distillation column behaviour Reply with quote

The Romanov a écrit :

Quote:
I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?

Hi,

Is your column a real one or design case ? Are you making a dynamical
simulation or a static one?

you can not directly play on the reflux tempreature, it is fixed by the
pressure and the composition of the distillate. reflux temprature is
not a degree of freedom. However to discuss about this case, it is
important to answer my first question.
Back to top
TheRomanov
science forum beginner


Joined: 22 May 2005
Posts: 40

PostPosted: Mon Jun 12, 2006 7:26 pm    Post subject: Question on refinery distillation column behaviour Reply with quote

I have a debutanizer column with feed consist of C1 hydrocarbon up to
180degC FBP Heavy Naphtha cut. The column is meant to split light
hydrocarbon which ends up at the top and naphtha (light and heavy
naphtha) down at the bottom. Light hydrocarbon at the top is basically
C1 up to LPG components. LPG is drawn as reflux drum distillate while
C1 and C2 is released as offgas from the reflux drum.

The column overhead cooling is provided by 2 fin fan cooler and a water
cooled trim cooler before condensed vapor enters into the reflux drum.
Some of the liquid from the reflux drum is routed back to the column
top tray as reflux beside being drawn as the distillate.

Control of the top LPG quality is by way of controlling tray 11
temperature.

Theoretically when reflux temperature is low, the tray 11 temperature
would drop. However, an instance happened when a sharp drop in fin fan
and the water cooled exchanger outlet temperature raised the tray 11
temperature drastically. At the same time, the reflux drum level also
drops.

Can someone explain how this could happened?
Back to top
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