Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Pressure effect on decomp temp?
Refinery
National Hazard
****




Posts: 370
Registered: 17-2-2014
Member Is Offline

Mood: Still

[*] posted on 16-6-2020 at 12:24
Pressure effect on decomp temp?


How does vacuuming benefits pyrolysis process since it could make the components boil before decomposition?

And how does pressure effect on melting point? Does it correlate same way as boiling point?
View user's profile View All Posts By User
mackolol
National Hazard
****




Posts: 368
Registered: 26-10-2017
Location: Poland
Member Is Offline

Mood: Psychedelic

[*] posted on 17-6-2020 at 02:37


When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms increases.
And due to vibration speed some molecules are fragile enough and just crack, sometimes even before their boiling point.
But when you lower the pressure, the space between atoms increases too, the boiling point lowers, so it's required less energy in form of heat to boil it and they don't crack.

And as for melting point, I have found some interesting topic in quora:
https://www.quora.com/What-is-the-effect-of-pressure-on-melt...
View user's profile View All Posts By User
ThoughtsIControl
Harmless
*




Posts: 48
Registered: 13-10-2019
Location: Proxima Centauri
Member Is Offline

Mood: Yin over yang

[*] posted on 17-6-2020 at 07:28


Pressure shares a direct relationship with temperature. Increasing the pressure using a vacuum has the same effects as using more energy in the form of heat. By lowering the pressure, the temperature at which your solution will boil also lowers. This is also true with the melting point, although most elements have a melting range of a few degrees. If you lower the pressure, you lower activation energy required to cause a phase change.

To add onto the conversation I ask you,

What would happen to the boiling point and the melting point if I were to keep all variables constant except the volume? Would the volume increase or decrease along with the temperature and pressure?

If you're confused, simply use this equation as a reference.. PV = nRT




"I do not see myself as outside, why enter?"
View user's profile View All Posts By User
chornedsnorkack
National Hazard
****




Posts: 377
Registered: 16-2-2012
Member Is Offline

Mood: No Mood

[*] posted on 20-6-2020 at 09:34


Quote: Originally posted by Refinery  
How does vacuuming benefits pyrolysis process since it could make the components boil before decomposition?

Yes - it changes the directions.
Quote: Originally posted by Refinery  

And how does pressure effect on melting point? Does it correlate same way as boiling point?

No.
The volume effect of pure substance melting is small. Therefore the effect of pressure on pure substance melting point is small compared to boiling point effect. And for several substances, volume decreases on melting and pure substance melting point falls with pressure.
Also, if you have pyrolysis, you can have apparent melting which is actually decomposition into solution of starting substance and lower boiling decomposition products. In which case, lowering pressure could prevent the apparent melting by having the lower boiling decomposition products evaporate.
View user's profile View All Posts By User
unionised
International Hazard
*****




Posts: 4293
Registered: 1-11-2003
Location: UK
Member Is Offline

Mood: No Mood

[*] posted on 20-6-2020 at 10:58


There are rare circumstances where the decomposition temperature is a nice simple function of the pressure.
The decomposition of CaCO3 to CaO and CO2 is one of the best known.
And the reason it behaves in a predictable way, is that the system is reversible. CO2 will react with CaO to produce CaCO3.

In general, organics decompose when heated in a way that is not reversible.

So it's a bit imprecise to talk about a decomposition temperature.
If I heat sugar for a while, it turns to caramel.
There are two issues there.
Caramel is a mixture of materials.
And I can make a similar product by heating the sugar to a slightly higher temperature for a shorter time or vice versa.

Since the decompositions don't normally have a well defined temperature, it's hard to define what the effect of pressure would be.

In the case of limestone, the additional pressure of CO2 can force the reaction backwards, but that's not going to happen with caramel.
The major product of decomposition is water (as steam), but adding water to caramel doesn't give you sugar. So raising the pressure wouldn't reverse the reaction.
Similarly, reducing the pressure won't decrease the decomposition temperature.
That happens when there's enough thermal energy to start breaking bonds in the material. And that's not pressure dependent.
View user's profile View All Posts By User

  Go To Top