11 replies to this topic

[halkeshhulk]

How to calculate the latent heat required for liquid mixtures,when there is a temperature range for boiling(Bubble point to dew point) unlike pure components?

How to approach and evaluate heat required for boiling the liquid mixtures?

[latexman]

Do you have a simulator? If so, use a flash block with vapor fraction 0, 0.25, 0.5, 0.75, and 1.

[Pilesar]

I have not found a latent heat value for multi-component streams that useful when compared to a heating curve. The 'standard' method used by some commercial simulation programs for reporting latent heat of a multi-component stream is to simulate a dewpoint flash at the stream temperature (allowing pressure to change) and then a bubble point flash at the stream temperature (again allowing the pressure to change.) The difference in the two stream enthalpies divided by the stream mass rate is the latent heat reported for the stream.

 

For relief valve sizing, an alternate formula is used which considers mainly the lighter portion of the stream.

 

For the heat required to boil a liquid mixture, the practical answer is to simulate the stream at initial and final conditions to find the difference in enthalpy. Most simulators will calculate the duty for you using a flash block or heat exchange unit. The simulator should also be capable of generating a heating curve for the stream since the duty is likely not linear with temperature.

[halkeshhulk]

Sir,

I have don't have simulator.



SUM(xi*hvi)

where xi is mole fraction of component i

and hvi is latent heat of component i

As temperature during boiling temperature range (Bubble to dew point) for the mixture is not constant.

Does using above mentioned average equation works well?

Edited by halkeshhulk, 07 October 2023 - 09:09 PM.

[halkeshhulk]

Makes sense sir,But where could I find literature or articles on this?

I have not found a latent heat value for multi-component streams that useful when compared to a heating curve. The 'standard' method used by some commercial simulation programs for reporting latent heat of a multi-component stream is to simulate a dewpoint flash at the stream temperature (allowing pressure to change) and then a bubble point flash at the stream temperature (again allowing the pressure to change.) The difference in the two stream enthalpies divided by the stream mass rate is the latent heat reported for the stream.

For relief valve sizing, an alternate formula is used which considers mainly the lighter portion of the stream.

For the heat required to boil a liquid mixture, the practical answer is to simulate the stream at initial and final conditions to find the difference in enthalpy. Most simulators will calculate the duty for you using a flash block or heat exchange unit. The simulator should also be capable of generating a heating curve for the stream since the duty is likely not linear with temperature.

[latexman]

How many components are in the mixture?

[breizh]

Hi,

You may want to use these estimations.

I will apply the mixing rule or define a pseudo component.

This is just an estimation. 

Like others said, better to use a simulator whenever possible.

Good luck

Breizh

[halkeshhulk]

How many components are in the mixture?

Sir,

Trying to do manual enthalpy calculations for binary component system distillate and bottoms to evaluate condenser and reboiler heat duty, instead of taking enthalpy values directly from text.

[PaoloPemi]

I assume you know, at least, vapor pressure of each binary in the mixture,

a basic approach could be Clausius -Clapeyron equation

 

dPv        dH

------  =  -------

dT         T dV

 

for details see Properties of Gases and Liquids (there are free versions available) or similar books..

[latexman]

Sir,

Trying to do manual enthalpy calculations for binary component system distillate and bottoms to evaluate condenser and reboiler heat duty, instead of taking enthalpy values directly from text.

 

It sounds like the enthalpy-concentration method (Ponchon-Savarit method) to analyze binary fractionating columns would be of great benefit to you.  It should be in your University reference text on distillation.  It was in mine (Unit Operations of Chemical Engineering by McCabe and Smith, 3rd. Ed.)  If not, Google it.  There's Youtube video's and university web sites that have course materials on it.

[breizh]

Hi,

To echo latexman's reply, consider this link (book) to support your work.

Separation Processes, Second Edition (escholarship.org)

Breizh

[halkeshhulk]

Thanks sir.

Hi,
To echo latexman's reply, consider this link (book) to support your work.
Separation Processes, Second Edition (escholarship.org)
Breizh