Latent heat of sublimation 2

[ReuvenD10]

hello everybody,

I try to calculate the latent heat of the sublimation process. I google for formula or table/graph and I didn't find.
somebody can help with this?

( I need the latent heat to determine the sublimation time in 30,000 feet altitude, aircraft field )

Thanks.

 

[GrouchyStressedTensePoor]

Hi,

What fluid is being used and what is the pressure?

GSTP

Graduate Mechanical Design Engineer
UK

 

[IRstuff]

Isn't it just the sum of joule heating + heat of fusion + heat of vaporization?

TTFN (ta ta for now)
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[TiCl4]

IRstuff is correct.

 

[Latexman]

Is this for dry ice (CO[sub]2[/sub]) inside the plane?

Good Luck,
Latexman

 

[goutam_freelance]

Hope the following will be useful.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[georgeverghese]

What chemical and what pressure at elev 30 000ft?

 

[Latexman]

My SWAG was dry ice for vaccine transport.

Good Luck,
Latexman

 

[goutam_freelance]

Isn't it just the sum of joule heating + heat of fusion + heat of vaporization?

The 'Joule heating' is not part of heat of sublimation. Of course this Joule heating may be required to bring the solid to saturated solid condition (solid about to sublimate).

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[IRstuff]

And, your example included the Joule heat as well. If the melting and boiling points are not coincident, then you need to heat the liquid up to the boiling point, so the sublimate such a material, that total sum must be supplied to get from solid to vapor. Joule heat can be eliminated only if you are dealing with a material at its triple point.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[georgeverghese]

Solid CO2 sublimes to gas at 30e3ft (at about 35kpa abs) at a temp of about -90degC. Dont have data for heat of sublimation at 35kpa / -90degC, but at 100kpa/-78degC, it is 6000cal/gram mole (from Perry 7th ed, pg 2-151, table 2-190).

 

[goutam_freelance]

If the melting and boiling points are not coincident, then you need to heat the liquid up to the boiling point,

To get the concept right we are heating a solid here, not liquid. Also sublimation happens at constant temperature, similar to boiling and fusion, for a pure substance.

Joule heat can be eliminated only if you are dealing with a material at its triple point.

Sublimation can happen at temperature lower than triple point. Refer PVT surface showing triple point.

Horizontal dashed line is the line of sublimation.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[TiCl4]

My thoughts:

I'm pretty sure you can figure out the sublimation temp from the data Georgeverghese provided.

Hsub1 = Hvap1+Hmelt1 (0 sensible/joule heating for special case of sublimation)

Hsub2 = Hsub1 + dH

In this case, the enthalpy change, dH, from the known conditions Georgeverghese provided(-78 C, 100 kPa) to the new conditions (-90 C, 35kPa) are calculated from the individual enthalpy changes of each phase. In this case,
Hsub2 = Hsub1 + dHsolid + dHvapor

[ul]
[li]dH solid: Change of 12 C for the solid - m*Cp*dT[/li]
[li]dH vapor: Change of 12 C (m*Cp*dT, assuming Cp of the vapor doesn't change much over 12C) for the vapor followed by an isothermal expansion at -90C from 100 kPA to 35 kPa.[/li]
[/ul]

Enthalpy of sublimation is a state function, meaning it shouldn't matter the path you take to get from the given data to the actual conditions. Does anyone gainsay that thought process?

 

[georgeverghese]

Correct there TiCl4.

So the Hs at -90degC = (dH to raise 44grams of solid CO2 from -90degC to -78degC) + 6030 - ( dH to cool 44grams of CO2 gas from -78degC to -90degC)

with both bracketed dH terms expressed as positive values.

From the internet, Cp for solid CO2 in this temp range is 0.284cals/gm/degC, while that for gas is 0.22cals/gm/degC, so there is a net gain of 0.06cals/gm/degC for this overall 12degC transition path plus 6000

So I get Hs at -90degC to be 6030 + (0.06x12x44) = 6062 cals/gm mole.

 

[TiCl4]

Georgeverghese, I don’t see the isothermal expansion in your calcs, but I’m thinking it’s fairly small. Looks like the original value you provided could be used with confidence with an ~5% error or so.

 

[georgeverghese]

Yes, I've deliberately ignored that assuming it is too small to bother with [i.e. d(Cp)/dP in the pressure range 35-100kPa for gas is small]