Nitrogen VS. Carbon Dioxide

Cryogenic Systems Equipment has extensive experience with both cryogens; we understand the physics and economics of these products and how they effect both cryogenic freezer design and performance, and the freezing process as it impacts your product.

There are misconceptions about these cryogens: their production, supply, economics, physical properties, etc. This section of our web site is intended be informative, raise some concerns, and offer reassurance. If you are considering a gas supplier, there may be some tips in here useful for both sides.

If a decision has been made to utilize a cryogenic freezer, the next decision is to choose which cryogen to use, liquid nitrogen or carbon dioxide. There are many misconceptions about these products, and in this section of our web site, we hope to clear some of these up.

There are major similarities:

• Both of these gases are available in plentiful supply throughout most of the country
• Both are delivered and stored as a liquid
• Both are piped to the freezer…

And there are major differences:

• Manufacturing
• Physical Properties
• Freezer Design Considerations
• Others

Manufacture and Supply

Nitrogen
Approximately 80% of the earth’s atmosphere is made up of nitrogen. Nitrogen is separated from air at a separation facility, where air is refrigerated to the point that the major components (nitrogen, oxygen and argon) are liquefied. Air separation is achieved due to the fact that these components liquefy at different temperatures.

Carbon Dioxide
Most of the carbon dioxide (CO₂) sold to the food processing industry is recovered as a by-product of other chemical processes. The primary sources of CO₂ in the United States are ammonia plants, refineries and ethanol processors, industries which generate a large amount of waste CO₂ as a result of manufacturing operations. This waste gas, which would otherwise be vented to the atmosphere, is recovered, cleaned and purified, compressed and cooled into a liquid. Another significant source of CO₂ are natural ground deposits recovered from wells.

Supply Issues
Nitrogen and carbon dioxide are always going to be available in ample supply because there are so many essential industries that use these products as critical raw materials. However, you should be aware that there are times when regional market shortages occur.

Nitrogen shortages can occur when utility companies curtail energy supply to air separation plants (and many other energy intensive industries). The nitrogen supplier must either reduce their production, or be forced to buy power from another source (if available).

CO₂ shortages occur when by-product source plants (the ammonia plants, refineries or ethanol processors) cut back production or operations are suspended. This could happen as a result of mechanical difficulties or poor market conditions.

Whatever the reasons, shortages are generally beyond the control of the industrial gas manufacturer. However, the major LN₂ and CO₂ producers are well aware of their vulnerability to short term production interruptions, and have extensive contingency plans.

Most industrial gas companies (both nitrogen and CO₂) operate from a network of overlapping plants. If one plant is impacted, there is usually product available from another. But the industrial gas supplier will incur significant costs in getting through these short term production interruptions.

As part of your gas supplier selection process, and in order to understand how these issues could impact your total freezing cost, we urge you to discuss the following with your potential or existing gas supplier:

• Their supply and distribution capabilities
• Contingency plans to supply your operations in the event of shortage
• How this will impact your cost

Cryogen Delivery and Storage

Both gases are delivered to you as a refrigerated liquid, and shipped in tractor/trailers. A full load of either cryogen is approximately 20 tons. This is a factor that should be considered when you are selecting a storage vessel. If a 20 ton delivery can be made into your storage vessel, then your gas supplier is able to deliver product to you at their lowest possible distribution cost, which is a savings that could be beneficial to both parties.

Storage vessels for liquid nitrogen are double-walled pressure vessels, normally vertical, with insulation in and a vacuum drawn on the annular space between the vessels. These are called “vacuum-jacketed” storage vessels, and they are essential to prevent excessive heating of the nitrogen liquid. Nitrogen is usually stored (for freezing applications) at less than 40 psi.

Carbon dioxide vessels are also available in a vacuum-jacketed configuration, but also as single-walled, insulated pressure vessels (normally horizontal) that are mechanically refrigerated to prevent warming of the liquid CO₂. Either configuration is excellent, however the vacuum-jacketed system requires less maintenance. Liquid CO₂ is normally stored at 300 psi.

Liquid Nitrogen (LN₂) vessels are sized in gallons, CO₂ vessels in tons. This is a rough comparison of standard vessel sizes:

Unless you gas supplier recommends otherwise, we would suggest that a 30 ton CO₂ vessel, or 9,000 gallon LN₂ vessel be the smallest sizes you consider. These vessels are usually available for lease from your gas supplier, and there are a number of excellent equipment specialists that also have lease and service packages. Storage vessels are usually available for purchase from these same sources.

Another difference between the two cryogens is the method in which they are customarily billed. The products you are buying are LIQUID Carbon Dioxide (LCO₂) or LIQUID Nitrogen (LN₂).

LCO₂ Liquid carbon dioxide is delivered and billed in pounds. LN₂ is often delivered and billed based on its vapor equivalent in units of 100 standard cubic feet (ccf). When comparing the cost of these cryogens, it is helpful to be able to compare them in terms of cost per pound.

Note: what you are really comparing is the cost of BTUs, and depending on the type freezer, the product you are freezing and the heat transfer efficiency of your system, the BTU value of these cryogens can vary. If you would like some assistance with your analysis, please give us a call.

To convert from the cost per ccf of LN₂ to cost per pound of LN₂, use this formula:

Price/ccf N₂ X .138 = Price/lb CO₂

To convert from cost per lb of CO₂ to cost per ccf LN₂, use this formula:

Physical Properties of Nitrogen and Carbon Dioxide

Purchasing a truckload of liquid nitrogen or liquid CO₂ is essentially the same as buying a truckload of BTUs. Your goal is to buy those BTUs at the lowest reasonable cost, and use them as efficiently as possible in your freezing equipment. It is important to know how those BTUs are available to you from LN₂ or LCO₂.

Liquid Nitrogen Refrigeration Looking at the chart above, you will see three BTU values for nitrogen:

What this means to the food processor is that when liquid nitrogen is put in contact with the product to be frozen, it will deliver up to 86 BTUs of refrigeration immediately as it turns from a liquid into a gas. But there are still valuable BTUs in the remaining nitrogen gas. CSE designs nitrogen freezers that take advantage of this fact, so that we are able to extract the maximum amount of refrigeration from both the liquid and vapor nitrogen.

Liquid Carbon Dioxide Refrigeration: Carbon dioxide can exist as a liquid only at high pressure (such as your storage vessel or pipeline to the freezer). When liquid CO₂ is injected into your freezer, there is an instantaneous pressure drop of the liquid to atmospheric pressure. This massive pressure reduction causes the liquid CO₂ to phase into a mixture of solid (dry ice) and gas, at a ratio of approximately 45% solid to 55% gas. The gas portion of this mixture has very little refrigeration value, and little attempt is made in freezer design to strip the BTUs from this vapor, but the solid CO₂, commonly known as dry ice snow, has the useable BTUs.

NOTE: When you apply heat to carbon dioxide dry ice, it does NOT melt. It sublimates, turning directly from a solid to a gas!

From the chart above, we can see that the Latent Heat of Sublimation for solid CO₂ is 246 BTUs per pound. Basically, you are buying a pound of liquid CO₂ to create .45 pounds of dry ice snow! This dry ice snow is driven into the surface of the food product, where it begins to sublimate and give up its refrigeration. CO₂ freezers are designed to take maximum advantage of the BTUs available from dry ice snow.

Both LCO₂ and LN₂ are both excellent cryogens. There are many times when the decision to choose one cryogen over another is purely an economic decision, but at other times there is a distinct processing advantage based on the physical properties of the cryogen, the nature of the product you are freezing and the type and size freezer you need.

CRYOGENIC SYSTEMS EQUIPMENT is not an industrial gas producer, but we have extensive knowledge of both cryogens. If you want assistance analyzing which would be the best cryogen for you, please give us a call. We would be glad to share our knowledge.