8 replies to this topic

[mb23]

My hydrotreater Naphtha stabilizer column top pressure is designed for operating at 10.6kgf/cm2g and bottom reboiler out temperature at 185degC.

What happens if I operate it at lower pressure than design at 8.5kgf/cm2g and less reboiling outlet temperature

How does it affects the top LPG product dew point ?

Will there be any issue w.r.t quality of top and bottom products distillation and quality?

Edited by mb23, 21 January 2023 - 12:20 PM.

[SilverShaded]

Can you condense the vapours at lower pressure? if not more off gases.  Generally speaking lower pressure improve separation and requires less reboiler duty, but you may end up venting off more Gases/LPG.  A simulation would help for your particular feed and operating conditions.

[shvet1]

less pressure => more vapor pressure of bottom product => more heat is required to keep bottom product in spec

[breizh]

Hi,

You may be interested in the documents attached. It will not answer your specific question but will give you a good overview of the process.

Breizh 

[Venkat @89]

less pressure => more vapor pressure of bottom product => more heat is required to keep bottom product in spec

When the operating pressure is low but at same reboiling doesn't the heaviers get carried to top product and Bottom product gets much more heavier right?

To ensure bottoms doesn't carry into top product, does reduction in reboiling is sufficient, while operating at lower pressure than design operating value??

Edited by Venkat @89, 23 January 2023 - 02:22 AM.

[shvet1]

 

less pressure => more vapor pressure of bottom product => more heat is required to keep bottom product in spec

When the operating pressure is low but at same reboiling doesn't the heaviers get carried to top product and Bottom product gets much more heavier right?

To ensure bottoms doesn't carry into top product, does reduction in reboiling is sufficient, while operating at lower pressure than design operating value??

 

I do not understand what "reboiling" stands for. It seems you are lacking of understanding of fractionation basics as question you have asked concerns fundamentals of distillation and gas absorption. You should read relevant chapters in handbooks:

- Perry's Chemical Engineers' Handbook

- Walas' Chemical Process Equipment

- Ludwig's Applied Process Design for Chemical and Petrochemical Plants

- Kister's Distillation Operation

and references in those.

 

Anyway your question is subject to modelling of a particular fractionator and is not able to be answered by a short simple post.

Edited by shvet1, 23 January 2023 - 09:46 AM.

[Venkat @89]

less pressure => more vapor pressure of bottom product => more heat is required to keep bottom product in spec

When the operating pressure is low but at same reboiling doesn't the heaviers get carried to top product and Bottom product gets much more heavier right?

To ensure bottoms doesn't carry into top product, does reduction in reboiling is sufficient, while operating at lower pressure than design operating value??

I do not understand what "reboiling" stands for. It seems you are lacking of understanding of fractionation basics as question you have asked concerns fundamentals of distillation and gas absorption. You should read relevant chapters in handbooks:
- Perry's Chemical Engineers' Handbook
- Walas' Chemical Process Equipment
- Ludwig's Applied Process Design for Chemical and Petrochemical Plants
- Kister's Distillation Operation
and references in those.

Anyway your question is subject to modelling of a particular fractionator and is not able to be answered by a short simple post.

For sure, I shall go through them, thanks ????

Edited by Venkat @89, 24 January 2023 - 03:03 AM.

[SilverShaded]

 

less pressure => more vapor pressure of bottom product => more heat is required to keep bottom product in spec

When the operating pressure is low but at same reboiling doesn't the heaviers get carried to top product and Bottom product gets much more heavier right?

To ensure bottoms doesn't carry into top product, does reduction in reboiling is sufficient, while operating at lower pressure than design operating value??

 

Generally speaking, at lower pressure you need less reflux to get the same degree of separation as the relative volatility of most components (but not all!) increases at lower pressure.  There are two competing effects, heat of vaporisation is higher at lower pressures would would tend to require more reboil duty, but the increased relative volatlity makes separation easier which reduces reboil duty requirement.

Generally speaking, less reboil duty is required at lower pressure and this is a very well known effect and often uses as an energy saving measure.

Edited by SilverShaded, 24 January 2023 - 03:49 AM.

[mb23]

Thank you all for your input.