8 replies to this topic

[Priolo]

Good morning,

 

I'm involved in verification of an engineering subcontractor design of a column separation system where part of distillate is exported as vapour (0.1% wt vapour only): a pressure control is provided reflux drum vapor outlet line.

 

For column pressure control, they are proposing an hot bypass control schema with air cooler as condenser:

I'm quite surprised as, normally, for this schema the liquid at air condenser outlet is subcooled. However in subcontractor designer outlet is partially vaporized. 

 

Does anyone has already seen applied such a scheme? 

 

Thank you

 

A.

Edited by alekso, 06 December 2020 - 09:30 AM.

[Bobby Strain]

A flow diagram and information about the components might help us answer. And a sketch showing elevations of the condenser and reflux drum.

 

Bobby

[breizh]

Hi,

Consider the resource attached to support your work . Good to submit a sketch as requested by Bobby . 

Good luck

Breizh 

[fallah]

 

I'm involved in verification of an engineering subcontractor design of a column separation system where part of distillate is exported as vapour (0.1% wt vapour only): a pressure control is provided reflux drum vapor outlet line.

 

For column pressure control, they are proposing an hot bypass control schema with air cooler as condenser:

I'm quite surprised as, normally, for this schema the liquid at air condenser outlet is subcooled. However in subcontractor designer outlet is partially vaporized. 

 

Does anyone has already seen applied such a scheme? 

 

 

Hi,

 

The air cooler would have been designed such that the air cooler outlet to be in saturated liquid or two phase with most part of liquid; hence no subcooled liquid is expected at the air cooler outlet.

 

Meanwhile the column pressure is normally controlled by a PV along the incoming line to the air cooler (condenser), and the hot by pass around the air cooler which is normally equipped with a PDV would control the required pressure difference between the column and reflux drum. 

[Priolo]

Hi, 

 

Thank you all for your feedback:

The control system is what have been described by Fallah.

The fact that liquid is not subcooled, will not create sudden vaporization in reflux drum when hot bypass is open disturbing so the control?

I know that normally subcooling is desired to avoid such situation : only liquid at L/V interface will be saturated allowing a more smoothly control.

I'm curious to know however your experiences.

 

Thanks.

[Pilesar]

Since this is for pressure control, won't your scheme have some flow in the bypass at all times while operating? Design the control elements and tune the control loops for good control response.

[fallah]

 

Thank you all for your feedback:

The control system is what have been described by Fallah.

The fact that liquid is not subcooled, will not create sudden vaporization in reflux drum when hot bypass is open disturbing so the control?

I know that normally subcooling is desired to avoid such situation : only liquid at L/V interface will be saturated allowing a more smoothly control.

I'm curious to know however your experiences.

 

 

Hi,

 

The hot by pass around the condenser which is equipped with a PDV is normally closed and once it would be opened will release just a lot of vapor to adjust the specified differential pressure; hence there would be no disturbing on the control... 

 

Any other issue...?

[ChEf]

Hi,
to avoid mistakes in the design of distillation systems or to solve design issues, the publications and books of Henry Kister are a more than valid reference. The case you are describing is analyzed and explained there, since it is common control scheme.
"Distillation Operation" and "Distillation Troubleshooting" edited by Mcgraw Hill, are definetly two books that an engineer involved in plant design should consider to have in his/her library.
Regards

[breizh]

Hi ,

To add to my previous reply , let you consider the resource attached (see last pages)

Hope this is helping you .

Breizh 

Edited by breizh, 24 December 2020 - 12:58 AM.