7 replies to this topic

[theleftcoast]

Hello,

I am designing a Butane/Light Naphtha (C4 and C5-C7) splitter column as part of my senior project. One of the requirements was to design the column to be operable at 50% capacity. I am interested on how the following tower components will work at low rates.

-Sieve Trays
-Total Condenser (either fin-fan or HX with cooling water on one side)
-Thermosyphon Reboiler

I have not had a problem finding methods to size the equipment at full rate but I haven't found so much info on their performance away from what they were originally sized for. I did a quick search for sieve tray turndown capacity and have found that some trays can be operable at up to 70% turndown (http://www.uop.com/o...still Trays.pdf) so I feel confident they would be OK for this application. For the condenser, I wouldn't imagine any trouble operating at reduced rates but I don't have a resource that says that. For the thermosyphon reboiler, my searches showed that turndown would be more of an issue but again, I don't have a resource. Would it be better to choose a different reboiler configuration for this application? We don't have to extremely detailed equipment design (which I understand can be kind of difficult for thermosyphons) but have to do basic sizing and have a sound basis for our equipment choice. Thanks in advance for any help!

Edit: Grammar

Edited by theleftcoast, 19 April 2010 - 08:15 PM.

[Zauberberg]

For 50% turndown any column internals will work, unless if you chose some really endemic items such are high-capacity trays for revamp projects. Sieve or valve trays and majority of packing will work @ 50% turndown.

There are no doubts for condenser as well, and as for the reboiler I would personally go for kettle if the service is fairly clean since they have the highest turndown capabilities. Thermosyphon will work at 50% capacity as well.

Cheers,

[katmar]

The potential problem with sieve trays and turndown is that eventually there is not enough pressure drop across the tray to prevent the liquid weeping through the perforations. If the tray is designed with a reasonable pressure drop at full rates it should still be OK at 50%, but if you design the tray for a very low pressure drop at full rates (typical in a vacuum column) then it will soon start to weep as you turn it down. I would say 50% turndown is common for sieve trays.

The potential problem in the condenser at low rates will be excessive sub-cooling. This would increase your specific steam consumption, but not be much of a problem other than that. You can prevent this by cutting back on the cooling water flow rate, but this might result in the CW return temperature being rather high, which can accelerate fouling and damage tower packings if excessive. Again, 50% turndown is common in condensers.

A thermosyphon reboiler has to be grossly oversized to cause problems. In this case you could get rapid boiling and vapor blanketing of the tubes, which prevents further boiling. Once the vapor collapses you get another burst of boiling. This is very rare and at 50% I would regard a thermosyphon as safe. In any case, if you do get this "burping" effect it is easily cured by decreasing the steam pressure.

[theleftcoast]

Thanks for the replies. This was just the kind of discussion I was looking for.

[Zauberberg]

Look for some interesting information in the following articles:

http://kolmetz.com/p...portunities.pdf
http://kolmetz.com/p...tion-Design.pdf
http://kolmetz.com/p...iagramPaper.pdf
http://kolmetz.com/pdf/odcdrev65.pdf

[Padmakar Katre]

The potential problem in the condenser at low rates will be excessive sub-cooling. This would increase your specific steam consumption, but not be much of a problem other than that. You can prevent this by cutting back on the cooling water flow rate, but this might result in the CW return temperature being rather high, which can accelerate fouling and damage tower packings if excessive. Again, 50% turndown is common in condensers.

Dear Katmer,
Here I have a confusion what you recommended that the process outlet temperature can be controlled by manipulating the cooling water flow agreed but why there is rise in the cooling water return temperature as primarily I see that the cooling water flow will get reduced (will bring the fouling issues in the cooler) if we keep the cooling water temperature range constant(CW out Temp minus CW inlet Temp) and that's the only way to get rid off the sub-cooling and subsequent increase in heating medium in reboiler.I operated overhead condensers in this fashion only i.e. TIC on process outlet as master and cooling water flow FIC as cascade.
I hope you comment on my confusion.

Edited by Padmakar S Katre, 28 June 2010 - 01:23 AM.

[katmar]

Padmakar, that is a good point and highlights the fact that my comment was based on some simplifications and assumptions that I did not state. I believe that my comment was correct. Let me explain the assumptions and simplifications that I made.

The behaviour of the condenser follows the standard relationship of
Heat transferred = Heat transf coeff x area x temperature difference
usually stated as Q = U.A.Δt

At low rates the heat load, Q, is decreased. The area remains the same and I am assuming as a first approximation the heat transfer coefficient, U, also remains the same - in fact it will decrease slightly with flow rate. So if Q decreases while U.A remains "constant" then Δt must decrease. If the condensed liquid leaves warmer (i.e. prevent the subcooling - this is what we want) then it will actually increase Δt. So for Δt to decrease the cooling water exit temperature has to increase (this is what I warned about). The only way to get this increase is to decrease the cooling water flow rate - as was done automatically by your cascaded FIC. The actual performance will depend on the details of the exchanger and the set up, but as a simplified conclusion the cooling water exit temperature will increase.

In normal plant operation (as you have experienced and described above) this is no problem at all. What I was warning about was theleftcoast's question about extreme turndown. As I stated before, a 50% turndown is common in condensers and it is only when you go beyond this that you may experience problems with fouling in long term operation.

Edited by katmar, 28 June 2010 - 03:47 AM.

[Padmakar Katre]

Dear Katmar,
Thanks for your explanation.