10 replies to this topic

[Parker]

Hi Everyone,

I am a chemical engineering student and I am working on a project. I am designing a vacuum condenser to condense the vapors of n-Hexane at 0.22 bar and 70 degree Celsius. I am quite confused about the effect of vacuum on the latent heat of hydrocarbons. How does it affect? Does it lower the latent heat or does it remains same?

 

At 14.7 psia, the latent heat of hexane is 144 Btu/lb. How will it change by reducing the pressure to 0.22 bar and at 70 degreecelcius?

Please Help. This step is vital in the designing of a condenser that is actually a heat exchanger

[latexman]

Look up the Watson relation between DHv and T.

[breizh]

http://deepblue.lib.....pdf?sequence=1

 

Consider reading this paper you may find answer to your query.

 

N hexane properties :

 

http://www.cheric.or...rop.php?cmpid=6

 

 

Breizh

Edited by breizh, 13 May 2014 - 10:39 PM.

[xavio]

Parker,

 

Mr. Breizh, as always, has provided a good (and very rare) paper for you.

 

Latent heat of vaporization decreases with increasing pressure, starting from triple point up to critical point.

At vacuum, your hexane will have higher latent heat value than it has at atm.

Google for Clausius-Clayperon equation!

 

By the way, how can you condense hexane at 70C when pressure is 0.22bar only?

Check its vapor pressure curve!

 

Good luck.

 

xavio

[Art Montemayor]

OK Parker, I got your message about you falling behind in your project's schedule.  That is still no excuse for making multiple postings of your one simple topic in our Forums.  Laziness is also no excuse.  You could have gotten a great jump on your project and not fallen behind if you kept up with our Forums or used our SEARCH ENGINE.  You would have discovered that you have all the thermophysical properties for Hexane (and many other compounds) at your fingertips by simply going to the NIST website and downloading your information.

 

I did this for n-Hexane and in less than 7 minutes had the custom-made, workbook I am attaching that contains all the thermo data for Hexane in a range of 0,0 barA to 2.0 barA.  I presume you meant 0.22 bar ABSOLUTE but were too lazy to mark it with an A.

 

I hope you realize now how easy it is to get all the detailed information you need if you organize yourself and consult and ask in a specific and detailed manner.  I expect you to know how to extract the exact latent heat of evaporation for hexane at 0,22 barA using the detailed data I have furnished.  If not, be honest and ask us.

 

 

Attached Files

•  Thermophysical Properties of n-Hexane.xlsx   58.69KB   37 downloads

[PingPong]

Parker,

 

The heat to be removed in the condensor is the enthalpy difference between the vapor at T₁ and the condensed liquid at T₂

 

Only if T₂ is equal to T₁ then the enthalpy difference is the latent heat of the condensed component(s).

 

However, as xavio already pointed out, you cannot condense n-hexane at 0.22 barA and 70 ^oC, so if the condensing pressure is 0.22 barA, then the condensing temperature will be 27 ^oC.

 

It seems to me that you therefor need the enthalpy difference between vapor at T₁ = 70 ^oC and condensed liquid at T₂ = 27 ^oC to calculate the duty of your condensor.

[Parker]

Referring to the above post:

 

Sir, I want to condense vapors of n-hexane from a vacuum flash distillation column, in which the vacuum is maintained through a vacuum condenser. The vapors are coming at 70 degree celcius and a pressure of 0.22 barA is maintained in the column and vacuum condenser.

 

I have to condense these vapors, obviously, at a lower temperature via cold water.

[Parker]

Thanks Xavio,

 

Surely, the condensing temperature will be lower and water will be used as a cooling fluid. 

[Art Montemayor]

 

Parker:

 

You haven’t even looked at the data I submitted.  Your answer is in the data.  You either don’t see it because you haven’t taken the time to look at it, or you don’t understand what the data is telling you.  Let’s analyze what you propose to do:

 

“I want to condense vapors of n-hexane from a vacuum flash distillation column, in which the vacuum is maintained through a vacuum condenser.”

Good.  You seem to realize that you need to maintain a vacuum on the distillation column and that partial vacuum you want to create is 0.22 barA.  That means the vapors are coming out at the top at 0.22 barA -- and those vapors are saturated.  Look at the data and see that the corresponding temperature is 27 ^oC, just as PingPong has stated - and their enthalpy is 265.89 kJ/kg.

 

“The vapors are coming at 70 degree celcius and a pressure of 0.22 barA is maintained …”

You are wrong!  The data proves that as shown above.

 

“I have to condense these vapors, obviously, at a lower temperature via cold water.”

Wrong again.  As stated in the data, the vapors are SATURATED.  When you remove the latent heat of evaporation from the vapors, they will condense into saturated LIQUID.  You can take that saturated liquid (at 27 ^oC) and cool down more if you want to and you have a colder heat sink.  If you do, you form subcooled liquid at 0.22 barA.  If you want to know the amount of latent heat you have to remove in your condenser, subtract the enthalpy of the saturated liquid from the enthalpy of the saturated vapor.  You have both of these values in the submitted data.  What is the problem???

 

The above is what both PingPong and Xavio are trying to show you, but you don’t seem to understand the principles of a distillation column.  I hope this helps you understand what you are lacking in knowledge and allows you to concentrate on what you must study and fully comprehend as a chemical engineering student.  This could be better explained in detail if you had supplied a sketch or drawing of the operation as a Process Flow Diagram.

 

[Parker]

Dear Sir, 

The problem is that: 

Separation is taking place between hexane and an other fluid (boiling point ~350 degree celcius). The effective separation temperature of both mixtures is ~135degree celcius at 1 atm. I have lowered the boiling point of mixture by applying vauum pressure. Now, the mixture effectively separates at 70 degree celcius and 0.22 barA.

 

So, I don't have hexane vapors at saturated state (first thing). They are at 70 degree celcius (superheated state) and I want to condense them.

This is my problem that what will be the enthalpy at 70degree celcius and 0.22barA?

Please guide

[xavio]

Parker,

 

First, I don't know if you have figured out your latent heat or not; it is your original inquiry.

From Mr. Art's data, the value is 364.165 kJ/kg.

I suggest you to verify it.

 

Second, I don't understand the meaning of "effective separation temperature".

I also don't know how you come out with 70 degree celcius and 0.22 barA.

I have to assume that you are sufficiently knowledgable about the separation process you're dealing with.

 

Third, enthalpy of superheated gas is calculated as (H sat. vapor @Tsat) + cp * (T - Tsat).

I assume you know how to use Mr. Art's data.

 

Good luck!

 

xavio