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Total Condenser's Inlet Temp For A Vacuum Distillation Column?

student vacuum distillation vacuum distillation dew point bubble point

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#1 bmwjay

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Posted 11 February 2021 - 08:52 AM

Hi all, 

 

So I am designing a vacuum distillation column's total condenser. I carried out a bubble point calculation for a total condenser to determine the top product temp coming out of the condenser. I was wondering how should one go about finding the inlet temperature for the vapor entering the total condenser (required to find the heat load and proceed with the design further)? My thought process was to carry out a dew point calc as you would for a partial condenser and use the temp obtained from that as your inlet temp for the total condenser and work out the temperature difference that way to determine heat load. Apologies if this is a silly question I did not have the best semester during my unit op classes because of COVID. 

 

Its my first time posting on this forum but I believe you all often require additional details surrounding the problem so here are the details surrounding the column:

 

  1. Condenser temp ~90°C 
  2. Reboiler temp ~155°C
  3. Components and corresponding non-key distributed revised material balance is attached as a table to this post (Click Me).
  • Note the temps are not exactly accurate because i used a similar component to substitute the TPG's Tc, Pc and Acentric factor data whilst doing the shortcut design (FUGK method). HYSYS V9's rigorous and shortcut column have shown 100°C  and 160°C as the condenser and reboiler temps respectively (using NRTL).  Again, apologies if this is a silly question. First time designing a condenser and reboiler for a distillation column (and first time designing a distillation column in general).

 

Regards,

Jay


Edited by bmwjay, 11 February 2021 - 01:04 PM.


#2 Pilesar

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Posted 11 February 2021 - 12:27 PM

The stream to the condenser was at its dewpoint when it left the top of the distillation column. Figure out the pressure there (pressure drop in vacuum systems can be relatively significant even though minor in absolute terms) and calculate the temperature. There will be some  temperature loss to ambient and you will be condensing some in the overhead vapor line at that temperature. A shortcut is just to take the stream from dew point to bubble point to find the approximate duty for equipment design. The performance margin for the equipment should cover simplifying assumptions and there will be a check step to confirm after equipment design is completed. I did not see an attached table, but 90 C seems very hot for a vacuum condenser. I suggest you verify your temperature calculations. Also consider whether subcooling will take place in the condenser.



#3 bmwjay

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Posted 11 February 2021 - 01:16 PM

Apologies, i don't know why the material balance did not upload. I have created a Imgur picture link you can view through the browser and attached it as url link. Alternatively you can also click here https://imgur.com/rrn9KaC. What exactly is the issue with the high condenser temp in your view? I raised this question with my professor (former Chevron O&G engineer) because i found these temperatures odd too and he said the temperature seem normal for the system (Water, Propylene Glycol, Dipropylene Glycol and Tripropylene Glycol). I have also run this simulation on HYSYS and  managed to get similar numbers (100C and 160C). Memeber on another forum suggested fin cooler maybe more suitable but he did not clarify why exactly. Thanks for the reply. I will look into the system again.

The stream to the condenser was at its dewpoint when it left the top of the distillation column. Figure out the pressure there (pressure drop in vacuum systems can be relatively significant even though minor in absolute terms) and calculate the temperature. There will be some  temperature loss to ambient and you will be condensing some in the overhead vapor line at that temperature. A shortcut is just to take the stream from dew point to bubble point to find the approximate duty for equipment design. The performance margin for the equipment should cover simplifying assumptions and there will be a check step to confirm after equipment design is completed. I did not see an attached table, but 90 C seems very hot for a vacuum condenser. I suggest you verify your temperature calculations. Also consider whether subcooling will take place in the condenser.



#4 Pilesar

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Posted 11 February 2021 - 04:34 PM

The temperatures are okay for the system. I don't know what cooling medium you plan. One of the features of water is that dissolved minerals become less soluble as the water gets hotter. When the water contacts metal skin temperatures greater than about 140 F, the water side of the exchanger tends to have increased fouling tendency. That's about the temperature where air coolers can become attractive. This is not a 'rule' but a consideration. For small exchangers like this, it may not matter much to optimize. With 90C condensing, you may want to consider a process stream for cooling. Maybe the column feed stream is cold enough to condense the overhead? Or there is another cold process stream nearby in the plant that needs heating anyway? Or preheat pure boiler feed water at the same time you are condensing glycol? Glycol itself is used as a heat transfer medium and there may be a dependable glycol utility for just such services. If you decide to use cooling water, it is likely that the process stream leaving the exchanger will be subcooled. Subcooling is generally not a problem, but be aware of it.






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