How to dose liquid nitrogen
Liquid nitrogen benefits lightweight packaging
In today’s competitive food and beverage market, many companies are looking to reduce costs. One way to do this is to reduce the cost of packaging used in their production facilities. Even small savings on container costs can result in large overall savings because of the quantity of units processed. To accomplish this per-unit savings, containers are made more light weight by removing material.
However, with less material, the containers also are less structurally stable, which can lead to collapse when stacked, or improper labeling and packaging. In carbonated soft drinks (CSDs), the presence of carbon dioxide in the product provides internal pressure, making the package rigid and stable. Liquid nitrogen can be used in non-carbonated beverages such as bottled water, juice and teas to achieve the same effect but without adding the “bubbly” feel of carbonation, which might not be desired.
Gaseous nitrogen has been used to expel oxygen and increase shelf life of products. Liquid nitrogen can serve this same purpose while reducing nitrogen consumption by 80 percent compared with traditional gas tunnels.
Whether pressurizing or inerting food or beverage containers, handling liquid nitrogen on a production line poses challenges. Liquid nitrogen has a boiling temperature of -320 degrees Fahrenheit (-196 degrees Celsius), and it will rapidly boil away when exposed to room temperatures. Therefore, insulated equipment must be used to ensure efficiency and safety. This equipment includes an injection device capable of metering small doses of liquid nitrogen into food or beverage containers, as well as storage vessels or tanks and piping to transport the liquid nitrogen to the injection location.
Generally, storage vessels come in two forms: large bulk tanks and small portable tanks called dewars.
Both designs feature a double-wall construction with the inner and outer walls separated by a vacuum space. This vacuum jacket allows the tank’s outside surface to remain at ambient temperature, while maintaining cryogenic temperature inside. The nitrogen can be held in liquid form for quite a while, but even with this vacuum barrier, the insulation isn’t perfect and tank losses can range between 0.5 percent and 2 percent a day. Large bulk tanks typically are installed outside and require longer piping systems to transfer the liquid to the use point. Dewar tanks are portable and can be situated closer to the use point, requiring a shorter length of hose.
For either type of storage vessel, insulated piping should be used to limit losses and improve efficiency. There are multiple types of insulated piping, but they generally are categorized as vacuum jacketed and non-vacuum jacketed.
Vacuum-jacketed piping is a similar concept to that found in bulk tanks or dewars. An internal pipe is surrounded by a vacuum annulus that provides the insulation between the cryogenic temperature in the pipe and the ambient temperature outside it. This vacuum space greatly reduces the heat losses, giving the pipe its efficiency. Vacuum-jacketed piping is more efficient than non-jacketed piping and offers completely frost-free operation. The vacuum jacket on this type of piping is generated by attaching the pipe to a vacuum pump. In a dynamic-vacuum system, a vacuum pump is continuously pumping while the vacuum quality is consistently improving. With the need for a dedicated vacuum pump running at all times, the operating costs are slightly higher with this style. The vacuum on a sealed-vacuum system typically is evacuated at the factory and then is sealed off. However, over time, this vacuum will gradually degrade resulting in increased heat losses and decreased performance.
Either type of vacuum-jacketed piping can come in rigid or flexible sections. Rigid piping needs to be accurately dimensioned to ensure a proper fit in the field. Flexible piping is fabricated in sections making it easier to install as it’s more adaptable in routing around obstructions. Vacuum Barrier Corp. (VBC) custom manufactures both sealed and dynamic-vacuum versions in a variety of materials and sizes. Non-vacuum jacketed lines often are insulated with foam and are not as efficient with heat losses as high as 20 times that of vacuum-jacketed piping. As the foam degrades over time, it loses its insulating qualities. These piping systems also have larger outer dimensions, making it difficult to route through tight spaces.
Nitrogen dosing equipment is the main component of a liquid nitrogen system. It’s often what production facilities are most interested in as it directly affects their ability to meet pressurization or inerting goals. Typically, these are called dosers, and they must operate frost-free and efficiently during dosing or idle times.
The reliability of a doser on a production line is very important as losses are calculated in minutes of downtime. As with any cryogenic device, internal exposure to moisture must be limited at all times as it’s a doser’s biggest enemy. Care must be taken during nozzle changes and maintenance to prevent contamination by moist air. In certain industries, a requirement might be in place stating that the liquid nitrogen be delivered aseptically, and, therefore, the unit must be capable of being sterilized.
Aside from operating frost-free, a doser also must meet the goals of the production facility. Any bottling or canning operation will be looking for consistent pressurization or inerting of their containers. This requires the doser to consistently output an accurate dose of liquid nitrogen, whether dosing discretely or steady streaming. Too small of a dose can lead to unstable containers and the possibility of collapse. For inerting processes this could lead to food spoilage. If dosed with too much nitrogen, containers are at risk of bulging or bursting, which could cause jamming and down time.
The challenge for the dosing equipment is to reliably and accurately control the liquid nitrogen dose for each container at speeds as fast as 2,000 bottles a minute. In order for the production goals of pressurization to be met, a doser relies on consistent fill heights from the filler. Even a small change in fill height can lead to under or over-pressurization.
The doser does have the ability to adjust to changes in line speed of the filler. As the line ramps up or down, timing automatically is adjusted to ensure each dose enters the container. Likewise, dose compensation adjusts the amount of liquid nitrogen dispensed as the line speed changes. For example, as a line slows down there is more time between filling and capping, which means more time for the nitrogen to boil off. Therefore, a larger dose is dispensed to maintain consistent pressures.
Other factors on the production line also must be taken into account to ensure proper pressurization. Travel time from the doser to the seamer or capper should be minimized to prevent excess boiling or loss of nitrogen. Shaking or bouncing of containers on the conveyors can force nitrogen and product out of the package before closure. Reliable sealing closures also are needed to maintain the pressure within the container after dosing.
VBC offers a range of dosers to accommodate a variety of line speeds and budgets.
It is important to address worker and machine safety when dealing with liquid nitrogen. When boiling from a liquid to a gas, nitrogen expands roughly 700 times. Safety-relief valves are installed on tanks, piping and dosers to prevent over-pressurization and potential equipment ruptures. Where there are shut-off valves in a system, there is potential for nitrogen to be trapped. A safety-relief valve must be placed between any two such valves. On bulk tank-fed systems, the lowest-rated relief device typically is placed outdoors. If a safety-relief valve does relieve, it is safer if it happens outdoors rather than inside where workers are present.
Cost is another key consideration of production facilities, and it’s important to look at the full picture when measuring costs of a liquid nitrogen system. Up front purchase price, installation and operating costs must be jointly considered. When evaluating tank options, large bulk tanks initially cost more, but nitrogen is less costly in bulk. The need to continually change out dewars during a production run can also add hidden cost.
There are more options for cost reduction when it comes to piping. Inexpensive foam-insulated piping can reduce the initial price, but the operating costs associated with using more nitrogen can add up during the life of a system. Vacuum-jacketed piping is more expensive up front, but it’s more efficient and will reduce nitrogen consumption and, therefore, operating costs. As mentioned earlier, reliable operation is a key component for liquid nitrogen dosing systems, and downtime caused by an inexpensive, foam-insulated dosing system can offset any perceived gains from a low initial purchase price.
A production facility considering liquid nitrogen dosing should take into account upfront and operating costs, the reliability of the system from tank through piping to doser, and the safety of their workers. BI