6 replies to this topic

[kemathut]

I have some quastion to ask anyone who specialise in heat exchanger concept ^___^

1. As I have a vertical thermosyphon reboiler (type shell and tube). It act like a vaporizer to boil bottom liquid in distillation column to a vapor phase and return back to the column. Low pressure steam is used as utility to heat it up. The question is that what is the effect if i increase the column liquid level which will effect to the liquid level in reboiler too. The steam required in boiling it up will be lower or higher? In my opinion, steam required should be decrease due to increasing of heat transfer surface between steam and liquid phase or my idea is wrong?

I will appreciate if you will discuss in this topic

Thank you very much 555

[NGL Licensor]

Hi,

The steam consumption represents the reboiler duty. This duty is out of the overall heat balance of the column.

I assume the tower pressure is constant.

Normally the duty is not function of the liquid level as it mainly depends on the separation specifics and hence the steam rate.

However, if you calculate the static pressure difference as a result of level change in the reboiler, you can determine the impact of this change on the latent heat of the bottom product. In other words if you specify DP across the reboiler (as a result of the level change) you will get a corresponding delta in reboiler duty.

The higher the pressure in the reboile, the more LP steam you need to effect the same separation

A.King

[SSWBoy]

There is no control (unless you have a throttling valve on the thermosyphon liquid inlet; typically this is not a cost-effective design, and hence rarely seen) of the liquid level being fed to a thermosyphon. Simply 'it is what it is'.

Static head gain on the inlet to the reboiler will balance the pressure drops across the rest of the circuit. If, for instance, you reduced the line size on the outlet of the exchanger, i.e higher dP, the liquid level would go up and vice versa.

Just as a thought, can we be clear on your terminology? The outlet of a thermosyphon is 2-phase. not simply vapour. Also, you refer to it as a vapouriser; are you sure you're not talking about a kettle reboiler?

[kemathut]

Dear all

Thank you very much for all of your kindness and useful knowlegdes.

Thank you very much again and again

Kemathut

[Technocrat]

Dear Kemathut,

The outlet from the thermosyphon reboiler is not pure vapor but a mixed phase. The liquid level inside the column is maintained below the mixed phase return nozzle to the column. There is no level maintained in the rebolier but the reboiler is full of liquid/mixed phase. The liquid moves through the reboiler due to density difference of pure liquid at the rebolier inlet and mixed phase at the reboiler outlet.

If you increase the liquid level inside the column you can pass more liquid through the rebolier due to extra liquid head driving force available at the reboiler inlet. But in any case the liquid level inside the column shall not exceed above the mixed phase return nozzle to the column otherwise the column will face heavy entrainment and lead to premature flooding of the column.

Regards.

[Zauberberg]

Please go through the attached articles:

http://www.tkk.fi/Un...he2002_102c.pdf
http://www.distillat...question001.htm
http://kolmetz.com/p...portunities.pdf
http://www.allbusine.../1000874-1.html