9 replies to this topic

[daraj]

I am a bit confused here, please help with the basic understanding of the concept. I know that in a vacuum generation/condensing system(such as vacuum drying or pervaporation etc.) where you are trying to maintain a vacuum, a vacuum condenser is placed upstream of  the vacuum pump, in order to reduce load on vacuum pump. The idea here is that due to condensation of the vaporsm in the condenser upstream a partial vacuum is created which helps reduce the load in the vacuum pump. The vacuum pump is then mostly used to pump down the system and remove non-condensibles.

 

But my question is this. Generally, low pressures are for flashing a liquid into vapor state, thats how flash vessels work. By reducing pressure you enocurage liquid to go into vapor phase. But here, in a vacuum condenser we seem to be doing the opposite, condensing a vapor into liquid at low pressures(because vacuum pump is pulling vacuum through the condenser). Wont this require a large surface area and very low temp. cooling fluids? How os this advantageous? The load that you reduce in the vacuum pump would be more than negated by the large surface area requirements of the vacuum condenser, isnt it? Or am i missing something? dew point is low at low pressures, whereas for easy condensation you need a higher dew point.

 

so how does this work? please help

[GimliXJ]

The most common purpose for a vacuum system is to reduce the temperature required to vaporize a liquid.  This may be necessary if the available utilities are not adequate to provide vaporization at atmospheric or pressurized systems.  A vacuum system is also necessary if the chemicals are unstable at higher temperatures, such as thermal cracking or they become reactive at higher temperatures making an undesired by-product.

 

One use for a vacuum condenser and vacuum pump are when the liquid is the desired product and removing the lighter chemicals and non-condensables are the primary objective.  This is the case for vacuum stripper systems.  As for the size of the condenser, cooling mediums and vacuum pump sizing, that is all done based on the needs of the process.

 

A higher pressure, and therefore dew point, does make condensing the stream easier but it may not be desired due to the process chemicals as described above or it may not be economically viable.  You will have to investigate this further.  Will it cost less to install a larger compressor (vacuum pump) to handle the full vapor load and then condense the stream or to remove the condensables first then use a smaller compressor?

[jcbenten]

Look at it this way: the vacuum is pulled on the non-condensibles, vapor of your liquid fills the space, then the chilled condenser drops the vapor.

 

Example with water: Lower the boiling point with vacuum to 170F, boil the water, now your condenser can be cooled with 40F chilled water and it is not too expensive (depending on volumes).  Lower BP liquids (i.e., solvents) will require colder chilling units.  Very Low Temp chillers can drive costs up.

[daraj]

Gim and jcbenten thanks. In my application, say a membrane separation unit, I already have an organc mixture of vapor(which are not unstable at 100C). I need to maintain vacuum on one side of the membrane side in order to facilitate selective diffusion. So my purpose is to constantly remove the vapor as soon as it diffuses through the membrane and maintain a vacuum there. I have seen setups for this on the net and most of them suggest a condenser UPSTREAM of the vacuum pump and they say it reduces the load on the vacuum pump. While this may be true in my case as well, Iam pulling a very high vacuum all the way from membraneunit  through the condenser(in the order of 10-50 mbar) and I thought it would be difficult to condense anything at such low pressures. Either you will need highly chilled medium and/or large surface area both of which would work against inexpensive installations. I am not sure how much of  a tradeoff is there in terms of reducing load on the vacuum pump.

 

In fact it is not only here, but generally speaking, condensers are always placed upstream of pump in all vacuum setups I have seen and I question the tradeoffs behind such a move.

 

Gim, like you suggeted, maybe the tradeoff needs to be worked out

[thorium90]

I believe you are looking at vacuum setups for a different system.

 

If you have enough condensables, it would be advantageous to have a condenser before the vacuum pump to reduce the load as this removes alot of stuff that are easily condensed.

 

If you have mostly non-condensables, there would be no point to have a condenser to condense almost nothing unless you make it very cold to get some stuff to condense.

 

Take for example, you have a mixture of steam and nitrogen at 50/50. Putting this mixture through a condenser, you can remove ~50% of the stuff and the remaining duty can be taken by the vacuum pump.

 

Take the next example, you have a mixture of steam and nitrogen at 1/99. Due to the large percentage of nitrogen, you would have to make it pretty cold to remove ~50% of the stuff.

 

You have read many setups on the net right? Look at the compositions of the stream and you will see that due to the larger fraction of condensables in their streams, it is indeed advantageous to have a condenser before their vacuum pump.

Edited by thorium90, 25 June 2013 - 11:38 PM.

[daraj]

THorium, thanks. Yes I might have some condensibles like water and some highly volatile organic stuff. If the volatile organic vapor is more than 50-60% then I might not use a condenser and use a condenser downstream  of vacuum pump, because you cannot vent this thing to atmosphere. You have to condense it and recycle it in the process, the only question was where to position this condenser and thats where all your suggestions have helped.

[thorium90]

Nevertheless, even if the condensible fraction is small, even 10 or 20%, it is still advantageous to have a condenser before the vacuum pump. If there was a reduction in condensibles, the condenser will just condense less stuff since the cooling water will still be at the same temperature. If the fraction increases, you save electricity.
ie: you dont change the temperature of the cooling medium but rather the amount of condensed stuff changes during operation.

[Art Montemayor]

Open, read, and study the attached workbook (especially the Vent Condenser sheet) that I have uploaded in our Forums:

 

 

 Producing and Maintaining a Vacuum.zip   56.31KB   1397 downloads

[shan]

When vapor stream is condensed in the vacuum conditions, it is a tradeoff of larger condenser surface area with other benefits.
For example, it will generate more power, which is worth more money than additional condenser surface area cost in the turbine life, for a condensing turbine steam to exhaust to vacuum conditions.

[daraj]

Thanks, Thorium, Art and Shan