7 replies to this topic

[p_stark95]

Many industrial columns (distillation, absorption etc.) need to be operated at pressures which are not atmospheric. In a vacuum distillation column this maintained with an ejector system which creates the vacuum. How is the pressure created and maintained in a column that requires a positive gauge pressure? Does the incoming fluid create this pressure?

[fallah]

Hi,

 

In brief:

 

The pressure in a distillation column is normally created by a reboiler and is maintained by heat removal from the overhead product through a back pressure controller normally located at upstream of overhead condenser...

[breizh]

Hi ,

Consider the resource attached .

 

Breizh

[Art Montemayor]

You are missing the very important basics of Chemical Engineering by not knowing (or taking into account) the fact that it is the VAPOR PRESSURE of the various components being distilled inside the column that produces the operating pressure of the operation.  Additionally, as Fallah has pointed out, the heat energy added at the reboiler raises the temperature - and, consequently, the vapor pressure.

 

In a distillation process you are dealing with liquids at their boiling points - which means that these liquids are saturated and, as such, generate a saturated vapor pressure while boiling.  Engineering involves the practical application of the physical sciences.  The distillation process is an engineering application of Physical Chemistry.

[p_stark95]

Thank you for the answers. That does help me a lot with my concepts. In the case of urea synthesis in an Autoclave, it says the pressure is maintained at 180 atmosphere. How is it done when no reboiler (as much as I know, or in the flow sheet given in the book shows) is employed? The same goes for the case of a reactor for the production of ammonia, where pressures upto 1000 atm has to be maintained. Also does the feed pressure have any role to play in the overall pressure of any column?

Edited by p_stark95, 24 March 2016 - 09:50 AM.

[fallah]

Hi,

 

It's not necessary in all cases a reboiler to be pressure source, a thermal (electrical) element can do the job in an autoclave as well ; also a compressor or pump can be the pressure source...

 

In a distillation column the operating pressure should not exceed the feed pressure...

Edited by fallah, 24 March 2016 - 12:38 PM.

[Art Montemayor]

As Fallah indicates, the feed pressure has nothing to do with setting the distillation column (or a reactor, or autoclave) pressure.  It is the vapor pressure of the internal components that you are seeing in any pressure gauge that is on the vessel.  IT HAS NOTHING TO DO WITH THE FEED'S PRESSURE.

 

As Fallah states, all you should be concerned about with regards to the feed to the column is that its pressure is high enough to OVERCOME the vapor pressure that already exists inside the column.  Otherwise, it simply won't enter the column ..... the fluids in the column (that exist at a higher pressure) will backflow into the feed line and upstream!

 

The same conditions exist in a reactor or autoclave.  You may not be introducing direct reboiler energy, but you are facilitating either a reaction or another source of energy that raises the vapor pressure(s) or generates a gas product and subsequently raises the internal pressure (which is usually controlled by a back-pressure control valve on the vessel's overhead vapor stream).   The basic philosophy is that you must furnish a source of energy (either as a chemical reaction or externally applied with heat exchange in a reboiler - or electric element, as Fallah states) and be able to regulate and control that introduced energy.  There are some processes where a pressurized inert gas blanket is kept over a liquid reaction, but these are very rare - and understandably difficult to control safely.

 

I think your problem is conceptual and due to a lacking of knowing or understanding the physical processes taking place.  This is either a lack of good teaching or a lack of thorough and detailed study into the matter, the basic science applied, or exposure to the actual processes in an engineering lab or in an actual industrial process.

[p_stark95]

Thank you for the answers. I appreciate the help I received. I am currently an undergraduate student, so yes, I'm still building up my concepts. Thank you again!

Edited by p_stark95, 24 March 2016 - 09:28 PM.