Pure Gases/Liquids – When ordering a pure gas with a low vapor pressure (i.e.: Carbon Dioxide) selecting the right package is paramount. As Carbon Dioxide is added to a cylinder it condenses into a liquid when the pressure inside the cylinder reaches 830 psig. You’ve created a “two phase” mixture of pure carbon dioxide, 830 psig gas at the top of the cylinder and liquid carbon dioxide at the bottom.
If the need is for gaseous Carbon Dioxide a single port valve that withdraws from the top of the cylinder is appropriate. As gas is withdrawn the liquid will evaporate to replenish the headspace of the cylinder until all the liquid is depleted.
For this reason, pressure gauges are not very useful in monitoring the contents of liquid cylinders because the pressure will register 830 psig until all the liquid is gone and then the cylinder will empty of the final gas content very quickly. Cylinder weight is much more useful in monitoring liquid cylinder contents.
If the need is for liquid Carbon Dioxide only a single port valve with a dip tube that extends to the bottom of the cylinder is the appropriate valve. This arrangement utilizes the headspace pressure of the gaseous Carbon Dioxide (830 psig) to “push” the liquid Carbon Dioxide up the tube and out the valve in the liquid phase. This arrangement is only useful for withdrawing liquids.
If the need is for both liquid and gaseous Carbon Dioxide a dual port valve with dip tube is required. This valve arrangement has two independent outlets, one that withdraws gaseous product from the headspace and one that withdraws liquid product via the dip tube.
One product, three valve options, knowing the application is critical to selecting the correct liquid package even for something as simple as a pure “gas” like Carbon Dioxide or Propane.
Mixtures Gas/Liquid – Hydrocarbons, meaning with more carbon content per molecule (i.e.: Butane, Pentane, Hexane, etc), as they get “heavier”, have a lower vapor pressure meaning they start to favor being in the liquid phase rather than the gas phase. This presents a challenge to the user when they want a blend in the gas phase that prefers to be in the liquid phase. This discussion becomes much longer than space permits but we can break the options for these types of blends into three categories;
Low Pressure, Gas Phase – Mixtures with low vapor pressures can be filled to just below the condensation pressure of the mixture in a relatively large, low pressure cylinder (i.e.: TWSCO 100 pound cylinder). The advantage being the product is in the gas phase and ready to use by most analyzers. The disadvantage being the quantity of product received can be relatively small and quickly consumed.
Liquid Cylinder with a Pressure Pad – Mixtures can be placed in a cylinder in the liquid phase with a dual port valve with a dip tube and an inert (i.e.: Helium) pressure pad on the headspace. Typically the pressure pad would be about 200 psig. The pressure pad forces the liquid up the dip tube where it is withdrawn as a liquid and typically converted to a gas (via heat/lower pressure) for use. The advantage of this system is a much larger quantity of product is contained within the cylinder. The disadvantage of this system is you must keep a pressure pad on the cylinder in order to withdraw product, plus the product is dispensed as a liquid.
Piston Cylinders – Mixtures that contain a variety of components at grossly different vapor pressures (i.e.: Carbon Monoxide, Hexane) are best delivered via the piston cylinder. This is a mechanical devise that compresses the mixture (via piston) at very high pressures to deliver a single phase liquid, even when the laws of physics would indicate the given mixture really doesn’t want to be a single phase.
When specifying and ordering a hydrocarbon mixture it is critical to work with a knowledgeable supplier who can help ensure selection of the proper package to ensure success with your application.
