Until a few years ago, refrigerated, urethane insulated CO2 Storage Units were used for virtually all carbon dioxide applications. In recent years, vacuum insulated CO2 storage units are being used.
Most urethane insulated storage units manufactured up until the early 1980s had either mastic or fiberglass outer jackets. Many of these units are still in use throughout the US and Canada. In the early 1980s an aluminum outer jacket was introduced with great success in the industry. Today, these aluminum-jacketed urethane units have had an excellent history of durability and service.
Statements have been made that urethane units become saturated with moisture over a period of time and must be rebuilt. The mastic-jacketed units are much easier to penetrate and if not repaired, the insulation will become saturated, and eventually the storage unit will have to be rebuilt. The aluminum-jacketed units have virtually eliminated the problem of moisture entering the insulation unless the outer skin is punctured and repairs are not made. This has eliminated the need to reinsulate these units and most are still in "like new" condition.
Urethane insulated CO2 rail cars with steel outer jackets have established a life span of approximately 50 years without re-insulating for urethane insulated CO2 units with adequate outer jacket protection.
In general terms, the capital cost of a urethane insulated CO2 storage unit will be less than a vacuum-jacketed storage unit. This is the capital cost for the CO2 storage unit only. The smart consumer will evaluate the total operating cost and life expectancy of the CO2 storage unit including vaporizer and refrigeration system, if required, to determine if a urethane insulated or vacuum insulated storage unit is best for their needs. To develop a good cost analysis, the following information is needed:
Capital cost of CO2 storage unit 
Estimate of equipment maintenance schedule and cost
Estimate of equipment maintenance schedule and cost Capital cost of application equipment Depreciation schedule on applications equipment
Depreciation schedule on applications equipment Depreciation schedule on storage unit Installation cost, including foundation required
Installation cost, including foundation required The insulation factor for a vacuum jacketed versus a urethane insulated storage unit is seven to ten times greater. Generally urethane insulated units have 4" of insulation. This can be increased to 6" as an option. TOMCO Equipment Company and FIBA Technologies, Inc. have run specific tests on various thicknesses of urethane and vacuum insulation using perlite or wrapped insulation. Again, these tests concluded that vacuum insulation was seven to ten times greater that 4" urethane insulation.
Carbon Dioxide units must maintain pressure between 250 and 300 psig for most CO2 applications. When product is withdrawn from the unit, heat is removed and must be replaced to maintain proper working pressure. Due to the increased insulation value on vacuum insulated units there is very little heat entry. Therefore, when CO2 is withdrawn from the unit, BTU's are removed lowering the temperature and pressure. A vaporizer is required to maintain the proper operating pressure. If the pressure inside the unit is allowed to drop below 60.4 psig the CO2 will convert to dry ice inside the unit. The unit will then have to be removed from service for a prolonged period of time to allow pressure to return to normal operating conditions. In this type of application, the heat entry into urethane insulated units can be very helpful.
TOMCO produces vaporizers and refrigeration systems for both urethane and vacuum insulated CO2 units. It is miss information to say that either urethane or vacuum insulated carbon dioxide units will never require a vaporizer or refrigeration system. Most importantly, the application and frequency of usage and amount of product withdrawn will determine the best type of insulation and accessories required.
Several factors should be taken into consideration, when determining the type of units. (1) Where space is limited, vertical CO2 units are more readily accepted. (2) Where there are no limitations on space, horizontal CO2 storage units are a more economical choice. (3) Foundations for vertical storage units are much more costly that for horizontal storage units. (4) Standard vertical CO2 storage units are generally only available in sizes up to 50 tons (45 metric tons) capacity. (5) Standard horizontal units are available up to 120 tons (metric tons) capacity.
(6) Urethane insulated CO2 storage units are manufactured with a manway and can be built at little extra cost with an evaporator coil installed inside the vessel. This allows complete flexibility to add refrigeration if it is ever required. There has been misinformation stating that all urethane CO2 storage units require a refrigeration system. (7) For applications where vapor or a large amount of liquid CO2 is withdrawn on a regular basis, no refrigeration system is required. (8) TOMCO also offers an external refrigeration system for those CO2 storage units without an evaporator coil inside the unit.
(9) Both urethane and vacuum insulated CO2 storage units have a carbon steel inner vessel. These pressure vessels are manufactured in compliance with the ASME code, Section VIII, Division I. (10) The maintenance on urethane and vacuum insulated CO2 storage units differs. If a vacuum insulated unit loses its vacuum, the leak must be located and repaired and a new vacuum must be pulled on the unit. The downtime will vary depending on the extent of damage and the time required to locate the leak, make the repairs and pull a new vacuum on the unit. In most cases, on site repairs can be made. If the exterior of an aluminum jacketed urethane CO2 storage unit is damaged, the repair can be made locally with no downtime. If the outer jacket is not repaired, the insulation will eventually become saturated and the unit will have to be reinsulated.
(11) The maintenance on a vaporizer used on a urethane or vacuum insulated CO2 storage unit is minimal.
(12) Refrigeration systems are predominately used on urethane insulated CO2 storage units. If maintenance is required, certified service repairmen are available throughout the United States.
There is a place in today's market for both urethane and vacuum insulated CO2 storage units. The customer should evaluate their particular needs and determine what type CO2 storage unit is best for their application.
