16 replies to this topic

[Sanveer]

I have been tasked with the deisgn of a multicomponent condenser. The stream to be condensed contains benzene ,toluene and the non condensable gases (hydrogen and methane). This corresponds to group 2's assignment as per the problem statement I have attached.  1307280316_ENCH3EC.pdf1.rtf   10.56MB   66 downloads I have decided to use a TEMA AES exchanger. The cooling fluid is water which I have decided to pass through the tubes. I can easily estimate the tube side heat transfer coefficient using the Sieder and Tate correlation. The difficulty for me arises in estimating the shell side heat transfer coefficient as there is desuperheating, condensation and subcooling occuring along with the presence of non condensable gases. I need some guidance as to how to undertake this calculations. Computer simulations arent allowed and calculations have to be performed by hand. In literature I have reviewed the Bell and Ghaly method is to tedious for hand calculation.

Edited by Sanveer, 28 July 2013 - 05:38 AM.

[Art Montemayor]

Sanveer:

 

Why do you select an AES type of exchanger?  Unless you have bad, fouling cooling water you don't neeed an "S" floating head.  A floating head is expensive, inflates the size of the shell, introduces potential cooling medium leaks into the shell, and is to be avoided if possible.  Floating heads were developed for specific purposes and unless you have dirty cooling water, I don't see the justification for the type selected.

 

Your content of non-condensables in the process side is the key ingredient in this application that can cause you all kind of grief and problems during operation and you should focus your exchanger design on that priority feature.  You must be able to separate the non-condensable gases as they are being differentially separated from your condensed portion - and be able to vent them efficiently.  Here, I have to assume that this is a TOTAL condenser and not a PARTIAL one.  Please be kind enough to confirm this basic data you haven't supplied.  Also, do you want to produce saturated condensate or super-cooled one?  Again, confirm this.

[Sanveer]

The lecturer has specified we must use a floating head with a split ring backing device hence the "S" head was selected. The condenser is a total condenser that is producing subcooled condensate.

[Art Montemayor]

If you have discussed the failings of a typical AES type of exchanger with your instructor and he/she keeps insisting that you use it as a condenser - whether total or partial - then you must proceed with the instructions.   And how do YOU propose to expel the non-condensables in such a device???

 

Into the valley of death rode the five hundred .........

[meetyourmaker]

Pick up a copy of DQ kern's "process heat transfer". In it you will find all you need to know about multicomponent condensation. I believe an "S" type layout can be used even if a non condensable stream is involved, although I would use it ONLY if the cooling water is really dirty otherwise it simply adds cost to the exchanger. otherwise I would go with the much cheaper AEM. what is the fouling factor youre using for water?

Edited by meetyourmaker, 29 July 2013 - 08:50 PM.

[Sanveer]

The fouling coefficient for the water is 5000 W/m^2.K. A nozzle to vent the non condensables will be

placed as shown below:

 

[Art Montemayor]

You show up-and-down segmental baffles.  That exactly the type not to use.  If you must use this type of exchanger, then use at least side-to-side to allow your non-condensables to flow all the way to the end where they will be vented.  and if you use the side-to-side, you will have to use either dummy tubes in the center section or plug it with a barrier box or plate.  Otherwise, you will suffer inefficient by-passing of the bundle through the open area in the center of a 2-pass tubeside bundle.

 

You still have not replied as to whether it is a total or partial condenser.  We can't  read brains yet; so you have to tell us.  Otherwise, how are we supposed to know what you are up to and answer your query correctly?

[breizh]

Consider reading this document ( extract of Boiler, Evaporators & Condensers by Sadic Kakac)

 

Hope this helps

 

Breizh

Edited by breizh, 30 July 2013 - 11:44 PM.

[Sanveer]

_{@ Art Montemayor}

 

_{I have specified in an earlier post that I am designing a total condenser. I have changed my baffle type to side-to-side.}

[meetyourmaker]

I agree with Mr Montemayor, you must have side to side flow of condensate in any condensor so use only vertical cut baffles. Could you state the fouling factor for the condensing stream? I stil feel a much cheaper AEM will do the job here if the condensing stream is not too fouling.

[Sanveer]

 The fouling factor for the condensing stream is 1000 W/m^2.K. Although it is low and an 

AEM type exchanger would suit the scenario I am restricted to use an AES exchanger.

 

 

[meetyourmaker]

get a copy of kern's book, there is a chapter on condensation of mixed vapors, go through it work out the examples yourselves. Post your calculations here so we can further advise. One more thing, in your orignal post you say there is non condensable gas present so how can this be a total condensor?

[Art Montemayor]

Meetyourmaker / Sanveer:

 

I have various arguments against the use of the AES, but Sanveer must go with what he has been assigned.  He has a large 2-phase flow that will cause havoc in his ability to separate out the condensables through the length of the exchanger:

 

H₂ = 52.38 mol%

CH₄ = 35.5 mol%

Benzene + Toluene = 12.12 mol%

 

For all practical purposes, he has a gas stream saturated with condensables and, in my opinion, a 2-phase separation problem - not a condensing problem!  That is why it is so important to select an adequate heat transfer device that will facilitate the ultimate separation of both liquid and gaseous phases BEFORE each exit the exchanger.   His shell side velocities should be controlled as low as he can tolerate with respect to the convection heat transfer coefficient and he could be removing his produced condensate differentially down the length of the exchanger by using draining nozzles at each baffle section.  I have done this many times when confronted with this type of 2-phase flow in heat transfer.

[Sanveer]

Art Montemayor:

 

Thanks for your guidance. I am new to working with two phase flows. Could you suggest a correlation I could use to determine the convective heat transfer coefficient for the shell side?

Edited by Sanveer, 03 August 2013 - 02:08 AM.

[Sanveer]

Art Montemayor:

 

There is a separation unit following the heat exchanger. So I thinks its fine for me not to include drains

on the shell. My instructor has modified the question a bit and the inlet stream is now at 256 degrees Celsius

and 24 bar. This means that the only component changing phase is benzene as toluene is a liquid at the given

conditions. My instructor thinks that the condensation of benzene is insignificant when calculating the heat duty.

He even said take everything to be a vapour. This makes no sense

[srfish]

I am puzzled by the statement: "only component changing phase is benzene as toluene is a liquid at the given conditions". The toluene is a vapor at the inlet conditions and only about 50% condensed at the outlet conditions.

[breizh]

An other good reference on heat transfer is : Heat transfer in process engineering  by Eduardo Cao   (Mac Graw hill )

 

Breizh