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Distillation Tower Top And Bottom Temperature And Pressure


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#1 student55

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Posted 27 March 2016 - 07:20 PM

How do i calculate distillation tower top pressure?

How do i calculate distillation tower top and bottom temperature ?

Is the top temperature calculate by overhead product dew point and bottom temperature by feed bubble point? Is that right?

and if that right how can i determine dew point without knowing tower top pressure ? 

 

Thanks in advance...



#2 cea

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Posted 28 March 2016 - 12:29 AM

Good question.

 

Column profile is set by two parameters, one is bubble point and second one is available utility conditions (such as, available steam pressure & hence steam temperature in case steam being the heating fluid, that governs column bottom temperature). In case, available utility cannot meet required bubble point condition, you may have to operate column under vacuum, so that bubble point will be less by 5ºC than available utility temperature.

 

Once your column bottom temperature & column bottom pressure gets fixed, subsequently column top temperature is nothing but dew point at given pressure.

 

If you are designing completely new plant with no utility available, you need to optimize the conditions. It is always costly affair to operate column under vacuum. As far as possible, you shall run column near atmospheric pressure. At the same time, raising steam temperature & hence pressure calls for all allied hardware of higher rating. Considering both the aspect in mind, you need to fix column parameters.

 

Hope, this answers your query.



#3 student55

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Posted 28 March 2016 - 06:26 PM

Dear Sir,

Thanks for your reply. 

In case of fixing tower bottom temperature, may i have to calculate bubble point of feed mixture or bubble point of bottom product 

 

and for getting bubble point i have to know pressure, How can i get the pressure for atmospheric pressure column i.e. naphtha splitter,debutanizer etc. if my heating fluid max temperature is 260ºC.



#4 cea

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Posted 29 March 2016 - 06:02 AM

Any column operates on vapor liquid equilibrium. The column bottom temperature shall be bubble point for VLE composition of at bottom most tray. In absence of VLE curve, you will never be able to find out composition of liquid over bottom tray & hence bubble point.

 

Further, VLE curve is to do with composition of more volatile component(s) in vapor & liquid & nothing to do with temperature.

 

You need to get VLE curve first, from where you will get composition of reboiler stage. Once you get composition, you can calculate (by hand calculation / by simulation) the bubble point by keeping pressure above 1 kg/cm2a (to accommodate pressure drop in column, in pipeline & in condenser). Check if this temperature is more than utility condition & alter column pressure accordingly.

 

Hope, this answers your query.



#5 breizh

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Posted 29 March 2016 - 06:16 AM

Hi ,

 

https://escholarship...96n0xv#page-489

 

Consider this resource to support your study .

 

Breizh



#6 breizh

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Posted 31 March 2016 - 09:46 PM

Hi ,

 

Attached another great resource about distillation control .

Hope this helps

 

Breizh

Attached Files



#7 fallah

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Posted 01 April 2016 - 01:30 AM

student55,

 

Distillation tower pressure is mostly fixed firstly at the top section due to higher limitation in the capability of available cooling utility than that of heating utility. Then the top pressure would be dictated by the temperature of cooling utility is to be used in overhead condenser. Considering allowable temperature approach, dew point of the overhead stream can be fixed at cooling utility temperature plus temperature approach. Corresponding pressure of this dew point specifies the distillation column top pressure.

 

After the top pressure is specified, the bottom pressure will be dictated by pressure drop along the column which depends on the relevant selected technology and the load of vapor and liquid inside the column. Corresponding bubble point of bottom pressure will specify the bottom temperature... 



#8 student55

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Posted 01 April 2016 - 06:57 AM

Dear sir, 

Thank you very much for your reply.But unfortunately i have a little doubt.Hope you'll help for this-

 

For specifies top pressure, 

dew point is fixed by cooling utility plus temperature approach.Corresponding pressure of this dew point calculation will be based on regarding what ??:  overhead product composition or feed composition

 

For fixing tower bottom temperature,

may i have to calculate bubble point of feed mixture composition or bubble point of bottom product composition  at tower bottom pressure.

 

Best regards,

student55


Edited by student55, 01 April 2016 - 06:59 AM.


#9 fallah

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Posted 01 April 2016 - 07:39 AM


 

student55,

 

1) Specifying top pressure: Based on overhead product composition

 

2) Fixing bottom temperature: Bubble point of bottom product compostion at bottom pressure



#10 student55

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Posted 02 April 2016 - 01:14 AM

Thank you sir



#11 breizh

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Posted 02 April 2016 - 04:25 AM

studen55,

Certainly you got your answer but you should have been able to answer this question if you have been peforming heat balances around the column .

Once again , consider the resources provided to learn about distillation.

My thoughts .

Good luck.

Breizh


Edited by breizh, 02 April 2016 - 07:48 AM.


#12 fallah

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Posted 02 April 2016 - 08:17 AM

student55,

 

To study about distillation design you are recommended to read "Distillation Design" by: Henry Kister...



#13 S.AHMAD

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Posted 03 April 2016 - 09:51 AM

Let me try to give some brief example.

  1. In distillation design we normally specify the overhead and bottom products.  For example, if we want to separate methanol and water, we may specify overhead product to contain maximum water of .01% (example only) and the bottom to contain 5 ppm methanol.
  2. The pressure of the tower is set based on cooling medium available.  Let's say we can cool down the overhead to 36 oC.  Then the top pressure shall be the vapor pressure of the overhead product corresponds to bubble point at 36 oC.  The bottom pressure is the top column pressure plus total tray or packing pressure drop.  Let say you have 20 trays and the pressure drop per tray is 0.1 psi, then the total pressure is 20 x 0.1 = 0.2 psi, then the bottom pressure is the top pressure + 0.2 psi. pressure.  The bottom temperature shall be the bubble point of the bottom product at that calculated pressure.that is equal to the vapor pressure of the bottom product (e.g. water contains 5 ppm methanol).

 

Hope the above helps to clarify your questions.

 

The above example is for 100% condensation of the overhead product.  However, if the overhead is partially condense then the overhead pressure corresponds to vapor pressure equal the dew point of the off-gas.

 

s.ahmad



#14 Hamed1981

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Posted 18 March 2017 - 04:59 AM

nice discussion



#15 rdcrags

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Posted 25 March 2017 - 10:04 PM

I am glad someone got it right after the false starts. The cooling medium temperature usually sets the pressure.






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