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Simple Batch Distillation. Bottom And Top Temperature


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#1 bmicky

bmicky

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Posted 18 September 2019 - 01:58 AM

Hello,

I try to model the separation of n-pentane from the syntetic seal oil/n-pentane miscible mixture by a simple batch distillation process (reboiler + riser+condenser) at the atmosheric pressure. The n-pentane boiling temperature is 38 deg.C. and oil boiling temperature is about 270 deg.C. According to the oil/n-pentane VLE data (which were created based on the Raoult's law),for example at the reboiler's temperature of 60 deg.C, the equilibrium n-pentane (Y) concentration in vapor is 0.9998 and oil is 0.0002.

Is it correct to assume that at the simple batch distillation process, the vapor (top) dew point temperature, assuming when n-pentane (Y) 0.9998 is similar to the botomm bubble temperature of 60 deg.C ?

On the other hand,once the vapor phase enriched with low boiling n-pentane, I would expect to see the vapor temperature closely to 38 deg.C. Is there a temperatute difference between a reboiler and condenser for a simple batch distllation process ?

 

 

Thanks, Mike



#2 sgkim

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Posted 23 October 2019 - 12:58 AM

Hi bmicky, 

 

Q1:<..Is it correct to assume that at the simple batch distillation process, the vapor (top) dew point temperature, assuming when n-pentane (Y) 0.9998 is similar to the bottom bubble temperature of 60 deg.C ?..>

 

Dew point (DP) refers to the temperature of vapor at which the first drop of liquid starts to condensate, and the bubble point(BP) the liquid mixture temperature at which the first liquid drop vaporize to vapor phase.   Once the first vapor mixture is formed at reboiler bottom temperature(BP), the concentration in the liquid phase may drop.  So the BP shall rise continuously as n-pentane vaporises.  So there will always be concentration gradient between the reboiler top and condenser inlet.    

 

Even if the vapor concentration is maintained under constant value, the temperature gradient will also be made passing through the condenser.   So assuming bottom BP be the same as top DP will generate some error in the simulation.     

 

Under certain cases,  however,  when total reflux are maintained consistently either DP or BP will remain unchanged but still show a slight difference between the value.

 

Q2:<..Is there a temperatute difference between a reboiler and condenser for a simple batch distllation process ? >

Surely, and the difference will increase as low boiler gradually escape from the reboiler.  Approaching to the end point, the vapor of the light component will expose to the somewhat superheated condition. 

 

Stefano Kim 






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