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#1 apis

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Posted 16 February 2008 - 04:50 AM

Hi everybody,
I have a little bit confusing here.What do you all think about the reboiler and condenser pressure in distillation column?Is it supposed to be same with the pressure in the column or not?In my opinion,it supposed to be different because to let the liquid and vapour flow(in my logic).Please guide me if i'm wrong.But, from my reading, i still can't found how to calculate the reboiler and condenser pressure.Did you know any source that tell us how to determine and calculate the pressure of condenser and reboiler?Please share it with me.I'll find it.It's very important to me.About the product that produce in my distillation column,how can i increase my product composition(bottom product).What i already know now is reboiler duty will influence the bottom product composition(Please guide me if i'm wrong).What else the things that you think will influence bottom product composition

#2 Zauberberg

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Posted 16 February 2008 - 09:51 AM

For pressure difference between reboiler and condenser you can assume theoretical 0.005-0.01 bar pressure drop per tray, in most of applications. Of course, it is logical for reboiler pressure to be higher, you agree with that?
I don't know what is your application, but in general you should know what is the operating pressure of the tower based on available heat sources and temperature levels of condensing utilities (air or water).

Composition of bottom product can be changed by: changing tower pressure, reboiler heat flow, reflux flow, etc.

Questions are too general, so we cannot be of much help to you.
Regards,

#3 Art Montemayor

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Posted 16 February 2008 - 01:55 PM


Apis:

Zauberberg is correct. Your queries are too general and not specific enough. This does not enable us to target a specific problem or question and respond accordingly.

However, inspite of your communications problem, I detect some clues of what is giving you problems and this seems to be what commonly affects other students as well. The basic problem is a mis-understanding (or lack of understanding) of the vapor pressure of liquids and the application of RAOULT'S LAW. I presume you are intimately familiar with both terms.

The pressure existing at the shell-side of the overheads condenser (assuming the condensation is on the shell-side) is the vapor pressure of the liquid fluid being condensed there. This sets that pressure. And as Zauberberg has noted, the reboiler pressure has to be the pressure that is needed to yield the driving force (pressure drop) necessary to achieve the overheads pressue. That would be equal to the overheads pressure PLUS all the the pressure drop realized from the Tower's sump (where the liquid being boiled is) to the Overheads condenser. This same reboiler pressure has to be equal to the vapor pressure of the liquid in the Tower's sump. It is that simple and logical. (of course, I am assuming there is an absence of inerts or non-condensables in the tower)

If you think back on your educational preparation for university, you will recognize that all this engineering theory actually started back in your first high school Chemistry or Physics class where you first studied Dalton's Law of Partial Pressures.

Latent Heat of Vaporization - Equilibrium - Dalton's Law - Raoult's Law - Latent Heat of Condensation: these are the various, basic, logical steps that describe a distillation process. It can all quickly get much more complex than that, but basically that is what you have to understand to proceed further into a design of a distillation column.


#4 apis

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Posted 17 February 2008 - 03:39 AM

Hi Zauberberg and Art Montemayor,

Thank you very much for your precious briefing and information.Actually, i have a vacuum distillation problem here.My distillation operate at 12m bar(0.012 bar).So,what i understand from your information,the pressure drop on each tray is 0.005 to 0.01bar.Let say if i take the pressure drop of each tray is 0.005bar and i assume i have 5 stages in my distillation column(not include the reboiler stage) and my feed(at 0.012 bar) at 3rd stages.Hence, my condenser pressure will become 0.002bar(My operating column pressure-1st stage pressure drop-2nd stage pressure drop=0.012-0.005-0.005).And my reboiler pressure become 0.027bar(My operating column pressure+4th stage pressure drop+5th stage pressure drop+reboiler stage pressure drop=0.012+0.005+0.005+0.005).Is it true like that?What i had understand from your information is like this.Please tell me if i'm wrong?Please guide me.Thanks in advance.I really2 need your help.Please help me.

#5 Zauberberg

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Posted 17 February 2008 - 09:43 AM

Hello Apis,

This 12mbar pressure, where it is measured - at which location in the tower?
Ordinarily, pressure transmitter is placed in the vapor space of 1st tray or column overhead line, but it's not like that in 100% of cases. I've seen applications with pressure measurement and control in overhead receiver (bad practice), or specific tray in the column, or even vapor space above the liquid level in tower sump. This is the first question.

Secondly, in some cases you can assume that feed pressure is equal to the feed tray pressure plus nozzle entrance losses. But, in some other cases when 100% liquid is feeding the tower, this may not be true. So again, you have to specify more details for this particular service.

I would like to give you more detailed answers, but I find your queries too much general. Is it vacuum distillation of heavy crude residue in question, solvent recovery unit, chemical plant, or what? And what is your problem here? Troubleshooting the unit, building a process model, comparison with design conditions etc.?

Regards,

#6 djack77494

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Posted 18 February 2008 - 06:38 PM

General comment - Be careful. Pressure drop in vacuum columns is critical. The dP per theoretical stage should be quite low. Practically, often structured packing and grid packing is used in lieu of trays for these applications in order to minimize pressure drop.

#7 apis

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Posted 19 February 2008 - 03:00 AM

Hi Zauberberg and djack,

Thank you very much cause of your explaination.Ok,the things that i want to do right now is designing vacuum distillation column using HYSYS simulator.The information that i have are the temperature of the feed,pressure and their flowrate.The pressure of the feed that i have is 12mbar and the temperature around 250 degree celcius.The raw material that i have is fatty acids.So,when i start to design my vacuum distillation in HYSYS,we need to key in the pressure of reboiler and condenser.Hence, i don't know what value to put it and how to calculate that reboiler and condenser pressure.Please help me.I also had refer to the book,but they don't show the calculation of reboiler and condenser pressure.Thanks in advance.

#8 Padmakar Katre

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Posted 19 February 2008 - 07:23 AM

QUOTE
So,when i start to design my vacuum distillation in HYSYS,we need to key in the pressure of reboiler and condenser.Hence, i don't know what value to put it and how to calculate that reboiler and condenser pressure.Please help me.I also had refer to the book,but they don't show the calculation of reboiler and condenser pressure.Thanks in advance.

Dear the column pressure is always decided on the basis
1. Available utility conditions (Hot Fluid and its conditions for the Reboiler and the Cool fluid and its conditions for the condenser in general words..........bubble point and the dew point temerature at the selected pressure)
2. Effect of the temerature on the material like degradation in case of thermal sensitive materials in this case we can increase the column pressure because the increased temperature will degrade your material. Now we can lower the value of the pressure as we have to check the dew point temerature at that pressure and it should be alwys more than the cold fluid temperature otherwise you will not get the condenstaion.
3. So far I have seen utilities govern this phenomenon.

If you are using the Hysys then you can check for the condenser load and reboiler load at different pressures and the column bottom(mainly the temperature of your boil-up) and the reflux or the condenser outlet temerature and check availability of the required utilities at that temperature. Apart from this you check the extent of seperation i.e. what is required and what it is at the given condition i.e. pressure.

This is purely a case study which we could perform if we could give the column bottom and top pressure as the independent variables and the other as dependent but pressures as ind varibale we cannot specify.

#9 Art Montemayor

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Posted 19 February 2008 - 07:29 AM


Apis:

While you may be a veteran member of the Forum, you don't read the responses given you. If you had, you would have seen that I not only told you how the pressures in a distillation column are established, but how to calculate them.

Using a simulator program like Hysys without knowing the basic principles of Dalton's and Raoult's Laws is like driving a car without a license. You are going to wind up in a wreck sooner than later.


#10 Ali66

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Posted 26 February 2008 - 02:15 PM

Hi Apis:
Just to confirm some aspects and add a comment:

1) Feed pressure is generally of no measurable effect on column pressure. Feed tray pressure can be very different (much lower) than feed line pressure.

2) In many cases of vacuum distillation (I've seen it in batch distillation and may be true also for continuous distillation) the overhead condensate is subcooled, i.e. its vapor pressure is lower than the overhead pressure. Otherise, you'd be withdrawing a lot of vapors into the vacuum pump and limiting the achievable vacuum. The deep vacuum in many industrial cases is limited by the ingress of air into the system (through leaky joints) and the capacity of the vacuum pump. In this case the overhead pressure is more of a function of the vacuum pump versus leak-in of air.

3) Reboiler pressure always higher than column top pressure. However, the difference for only few stages can be very small if properly selected structured packing or good random packing is used. The "0.1 psi per tray" general rule of thumb makes the use of trays unfovourable in vacuum applications. Avoiding trays, I wouldn't be surprised to see some columns showing the 0.1 psi difference between column top and reboiler as opposed to just per single separation stage.

Ali




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