# Boiling water in a tank?

1. Jan 8, 2009

### katchum

If I heat up a tank with water while it is completely closed, will the water then form bubbles while it heats up above 100 °C?

I know the pressure will increase so I think the bubbles won't form, but I'm not sure.

But for the pressure to increase we need gas, so I would say that the gas would come from the liquid, which means bubbles have to form to get this gas.

2. Jan 8, 2009

### vanesch

Staff Emeritus
In principle, a liquid in a sealed container can never boil in an equilibrium situation (uniform temperature, and slow increase of temperature so that the vapor pressure is in equilibrium with the liquid). You get boiling when the pressure in the liquid is smaller than the vapor pressure for that temperature.

If the temperature is uniform, then the total pressure will be equal to the vapor pressure PLUS the partial pressure of the initial air included in the vessel, and will hence always be larger than the vapor pressure, so no boiling.

However, if:
- the liquid is heated locally, locally the vapor pressure can be larger than the overall pressure, so it can start boiling *there*.
- if the heat is applied quickly, so that there has not yet been an equilibrium between the vapor pressure and the new temperature, the pressure might be lower than the vapor pressure for a while (until enough vapor is produced to reach equilibrium).

3. Jan 8, 2009

### katchum

Okay that's clear. In a tank we can almost certainly say that it won't be uniform. We do not want bubbles to occur which could lead to foaming of our liquid so we're going to increase pressure in the tank above vapor pressure with some inert gas before heating then.

4. Jan 8, 2009

### Staff: Mentor

One thing:
No, liquids expand when heated too. And it doesn't take much heat to make extremely high pressures in a rigid container, so be careful.

5. Jan 8, 2009

### katchum

Well, the experiment would be to heat up some water + methyl cellulose to 120 °C in a closed container + nitrogen blanket. The pressure shouldn't get higher than 2 barg. So I would put a 2 barg nitrogen blanket on it and then heat up from 90 °C to 120 °C. The liquid doesn't expand that much would it? Depends on the amount of gas volume of course. The container is only 60 % full.

6. Jan 8, 2009

### QuantumPion

Nucleate boiling can occur below the boiling point for the fluid on the surfaces of the container (or other nucleation sites, such as soild particles in the fluid). See here for a little bit of info: http://en.wikipedia.org/wiki/Nucleate_boiling

7. Jan 8, 2009

### stewartcs

Take a look here for a visual:

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html#c4

CS

8. Jan 8, 2009

### katchum

Thanks for the sites, I assume this "nucleate boiling" will occur in our tank. I just wanted to make sure because a colleague said there was no boiling to be seen in some tests they did. Maybe they didn't look carefully enough. Better safe than sorry.

I suspect that laboratoria use small containers where mixing is almost perfect, while big tanks will be completely different.

Last edited: Jan 8, 2009
9. Jan 8, 2009

### vanesch

Staff Emeritus
Just to rectify a possible misunderstanding: in order to make vapor, there is no necessity to boil. The vapor can escape from the surface without boiling. Boiling means that vapor is produced INSIDE the liquid (because the pressure in the liquid is lower than the vapor pressure, so that a bubble in the liquid *grows* because at the surface of the bubble, the vapor pressure (= the bubble pressure = the liquid pressure) is smaller than the equilibrium vapor pressure, so more and more vapor escapes from the surface, into the bubble, which grows.
If the pressure in the liquid is larger than the equilibrium pressure, then if a small bubble would form, the vapor pressure (which is the only gas in the bubble) would be larger than the equilibrium vapor pressure, and hence vapor would convert back into liquid in the small bubble. So the bubble would collapse and disappear. It could not grow.

10. Jan 8, 2009

### Waveform

Just a quick thought experiment here using standard measurements.

If one had a closed sealed vessel containing 10 lbs of water at 60°F.

Raising the temperature from 60°F to 212°F will require 1520 btu’s considering it will require 1 btu per lb of water per degree F.

At 212°F it will require a larger quantity of thermal energy (evaporation enthalpy) to in essence, start the ‘boiling process’.
At standard atmospheric pressure (14.696psi) it will require 970.3 btu's.

The water will boil,releasing gas thus raising the pressure. As the pressure rises so does the temperature of the water, however the enthalpy will drop.

Just as a reference, as the pressure increases;
At 50 psi the water temp will be 281°F, the evaporation enthalpy will be 924 btu’s
At 500 psi the water temp will be 467°F, the evaporation enthalpy will now be 755 btu’s
At 2000 psi the water temp will be 636°F the evaporation enthalpy will be 463 btu’s

The boiling process will continue to a pressure of 3,206.2 psi and a water temp of 705.4°F. However at this point the evaporation enthalpy will have reached zero. The boiling would cease.

At that point the density of both the fluid (namely water) and the gas will exist in equilibrium.

I would suspect in a large vessel the ‘boiling ‘ would be quite evident. However as the pressure increase to critical, the actual ‘boiling’ would decrease.

If the vessel were small the boiling would be less evident as the pressure gradient went up.

11. Jan 8, 2009

### Staff: Mentor

You may want to keep the top of the tank reasonably warm too. I remember an old "Mr. Wizard" where they got water to boil in a sealed glass jar using an ice cube. He rubbed the ice cube on the top of the jar so some of the vapor condensed on the cold jar which reduced the vapor pressure and caused the liquid water to boil.

12. Dec 21, 2010

### sparkle123

You can boil in a closed container (high temperatures, ex: pressure cooker). You can also boil in a vacuum (low temperatures)

13. Dec 22, 2010

### Jobrag

If you are going to do this you might want to read up the local codes on the design of pressure vessals.