Using an Inert Ideal Gas as a Propellant
An ideal gas is one that obeys the ideal gas laws. On this page we will be seeing how nitrogen obeys the the Pressure Law and Boyle's Law.
The gas laws only work for A FIXED MASS OF GAS. In these aerosol designs the gas never leaves the can, so the gas laws work.
Inert gases are chemically unreactive. Nitrogen is an inert gas that makes up almost 80% of air.
Liquified gas propellants like butane and propane create a constant vapour pressure so that the pressure inside the can never drops until all the liquid has gone. As propellant vapour escapes from the can, more liquid propellant evaporates to replace it. It is this evaporation that creates the vapour pressure.
One solution for ideal gases is a technology called bag on valve and the use of compressed gases like nitrogen as propellants.
Bag on valve keeps the propellant separated from the product. The propellant does not leave the can; it just squeezes the bag like squeezing a tube of toothpaste. As there is no vapour pressure, the gas pressure will drop as the bag empties. If the starting pressure is high enough, there will still be enough pressure to completely empty the bag.
Assembly stages for a Bag on Valve can
1. Rolled up bag on valve assembly drops into the empty can.
2. An ideal gas (in this case nitrogen) is injected into the space around the rolled up bag at about 2 bar pressure.
3. The pressure is checked. If it is too low the can is thrown out before wasting product by filling it.
4. The product is forced into the bag through the valve The holding tapes break and the bag fills up. This reduces the volume of the trapped nitrogen and so the pressure goes up to about 5 bar. This is an example of Boyle's Law in action.
5. Actuator and cap fitted. Ready to use.
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For viscous products such as shaving gel it would be impossible to fill the can through the valve.
An inner bag is filled with gel before the valve is fitted. This leaves the problem of how to fill the propellant at high pressure.
The only way is through the bottom of the can. A special base design has a rubber bung in the centre. Propellant is injected through a fine needle. The bung then self seals.
Bag in can design for shaving gel. Image: Crown Holdings
Using an ideal gas propellant means the initial filling pressure has to be much higher. The bag is already full when the propellant is injected through the bung. As the product is used up the pressure will drop. To get all the product out the starting pressure has to be around 12 bar.
Even gloopier products!
Some very viscous products including gels, grease and glue are
filled directly into the can. A piston in the can is used to push the
product out. The propellant is filled through a bung in the bottom of
Boyle's Law applies again as this is a fixed mass of gas that does not leave the can.
As the product is used up, the piston goes up. The propellant volume increases so the pressure drops.
The filling pressure has to be high to make sure the piston makes it to the top of the can.
The propellants in these cans is usually nitrogen or compressed air, depending on the product.
The piston design makes sure that the high pressure gas does not leak around the side.
The piston is designed to be gas tight in the can.
1. What is an inert gas?
2. Why is nitrogen used when air would also work?
3. At stage 2 the nitrogen pressure is 2 bar. It fills most of the can. What will happen to the pressure of the nitrogen when the bag is filled with the product at stage 4?
4. The can has a diameter of 4.5 cm and is 10cm high. If the nitrogen pressure is 2 bar at stage 2 and the space left for the nitrogen is only 40cm3 once the bag is filled, what will be the pressure when the bag is full?
5. What will be the pressure in the can when all the product has gone?
6. What two factors affect the pressure inside a bag on valve aerosol?
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