A client inquired why a CO2 vessel can be full by weight, but no product will come out. They mentioned that they'd been able to feed NF Nitrogen into the vessel and then get CO2 out.
"I think the principle issue here is one of pressure. CO2 is one of a few substances that go straight from the gas to solid phase and vice versa at atmospheric pressure. The only way to keep it in liquid form is to maintain adequate pressure as shown on this chart http://www.school-for-champions.com/science/dry_ice.htm.
We’ve found it necessary to use pressure to convert the sold back to a liquid once CO2 turns to dry ice (solid) due to a drop in pressure below the level needed to stay as a liquid. This can be done a number of ways and the addition of N2 as you have done is one. The key in your approach is to be certain that flow is tightly controlled and there are reliefs on all hoses and other trap points to prevent rupturing lines or tanks. Complete removal of all CO2 from the vessel can be pretty time consuming and take several days to accomplish. Once all CO2 is removed from the vessel, it is suggested that you use liquid nitrogen to test all functions of the vessel and perform a thorough leak check.
If you have a leak in a pressure building line or regulator, for instance, you get exactly the opposite result from the one desired when you open the PB isolation valve. The same can occur due to a leak in a valve stem or packing. When the valve is opened and a resultant leak occurs pressure will drop and then dry ice will form."
We’ve found it necessary to use pressure to convert the sold back to a liquid once CO2 turns to dry ice (solid) due to a drop in pressure below the level needed to stay as a liquid. This can be done a number of ways and the addition of N2 as you have done is one. The key in your approach is to be certain that flow is tightly controlled and there are reliefs on all hoses and other trap points to prevent rupturing lines or tanks. Complete removal of all CO2 from the vessel can be pretty time consuming and take several days to accomplish. Once all CO2 is removed from the vessel, it is suggested that you use liquid nitrogen to test all functions of the vessel and perform a thorough leak check.
If you have a leak in a pressure building line or regulator, for instance, you get exactly the opposite result from the one desired when you open the PB isolation valve. The same can occur due to a leak in a valve stem or packing. When the valve is opened and a resultant leak occurs pressure will drop and then dry ice will form."
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