7 replies to this topic

[Ben2009]

Hi, I have a question about distillation reboiler, i was asked to tell what will happen if we increase a steam pressure of reboiler from 10 psig to 12 psig, and run the experiment at total reflux? using the temperature data i have seen that temp. of the middle trays going down as steam pressure go up, reflux temp and condenser temp goes up, Why only the temp of the middle trays are going up tray 2-8( out of 12 total trays) any clue why that happen?

Please i need your help.

Thank you

[Zauberberg]

Steam pressure inside the reboiler will not change as long as you keep the tower pressure constant. The best way to look into steam reboilers is that they are actually steam condensers. Steam condensation temperature is driven by the cold process fluid temperature, and furthermore the steam condensation temperature determines the steam condensation (reboiler) pressure.

Steam control valve acts as a "pressure modulating" valve - adjusting condensing pressure based on cold/process fluid boiling temperature requirements.

[Ben2009]

Steam pressure inside the reboiler will not change as long as you keep the tower pressure constant. The best way to look into steam reboilers is that they are actually steam condensers. Steam condensation temperature is driven by the cold process fluid temperature, and furthermore the steam condensation temperature determines the steam condensation (reboiler) pressure.

Steam control valve acts as a "pressure modulating" valve - adjusting condensing pressure based on cold/process fluid boiling temperature requirements.

i was asked to change the steam pressure from 10psig to 12 psig by adjusting the steam control valve, and need to determine its effect, i found out that the temperature of the bottom trays are going up and the top trays stays the same plus the overall efficiency going up. remeber every thing is at totall reflux.

also the reflux temp goes up.

can you tell me why i see this changes?

[Zauberberg]

These changes are logical - that's what happens when you operate steam-driven distillation tower.
By opening the steam control valve:

1. The pressure downstream of the valve increases
2. Condensation temperature increases
3. Heat input increases (UAdT)
4. More vapors flow up the tower
5. The condenser load increases and the overhead receiver runs hotter (hotter reflux)
6. Since it is a total reflux system, now more liquid is cascaded down the tower
7. Trays work with higher vapor/liquid loads and therefore reach higher efficiencies, if flooding limit is not exceeded.

[katmar]

It's fairly easy to come up with qualitative answers that would explain what you have observed, but there is no guarantee that these conditions would pertain in every instance where you raise the reboiler pressure. I'm sure anyone could come up with counter arguments to each of my points below, so take it as a possible answer and not as a definitive answer.

If you raise the pressure in the reboiler by opening the steam valve you increase the condensing temperature of the steam and provide more driving force for getting heat into the column. This means you have a higher boil up and higher traffic in the column. This higher traffic will increase the pressure drop across the trays (or packing) and therefore you should see the base pressure rise. This increase in pressure raises the boiling point of the liquid in the base and it is quite normal to see the base temperature rise in response to increased steam flow to the reboiler (and vice versa).

This higher boil up will result in more reflux in the column (you said you are on total reflux). This will result in a sharper separation in the column and it is possible that there is a zone that now has more of the top (low boiling) product than it had before. In a way you can think of the high boiler being "pushed" down the column. This could lower the temperature of that zone.

The top pressure should not change materially because it is only affected by the pressure drop through the condenser and this is usually low (or the top pressure is controlled at a particular value). So the top temperature will only be affected by the top composition. If the top purity does not change then the top temperature will also not change.

Ideally your reflux should be at the condensing temperature. If your reflux temperature has increased this can only mean that previously the reflux was being subcooled. The increased boil up results in less sub-cooling capacity in the condenser and the reflux temperature rises (but never above the condensing temperature). The situation at the top of the column is a bit ambiguous because in your first post you said the condenser temperature rises, but in your second post you said the top temperatures remain the same. This is contradictory, unless by "condenser temperature" you are referring to the water side. The water side would get hotter because it has to remove more heat.

So there you have some explanations to justify your observations, but remember that there are many other things that could affect the situation as well.

[katmar]

Apologies to Zauberberg! I was disturbed half way through posting my reply, so it took me a long time and I had not seen your last reply before I posted. It looks like I have simply plagiarised your ideas - but that was not so. At least we are in close agreement on what is happening in the column.

[Zauberberg]

Harvey,

Actually your post was much more 'engineering' and complete than my answer. Thanks for elaborating on the subject.

[Ben2009]

Harvey,

Actually your post was much more 'engineering' and complete than my answer. Thanks for elaborating on the subject.

Thank you all, it was very helpfull