The Right Dose
Modern liquid nitrogen injectors conserve nitrogen, plant space and maintenance. Liquid nitrogen (LN2) injection is used throughout the food and beverage industry for inerting and pressurizing packages and containers. For example, bottlers of noncarbonated soft drinks, juices, water, wine and beer often use liquid nitrogen injection equipment to insert small, measured drops of liquid nitrogen into their containers to displace the oxygen-laden air. Soft food packages also are pressurized by liquid nitrogen injection to protect the package contents from crushing. In both cases, the dosed liquid rapidly evaporates into tasteless, colorless, odorless, inert gaseous nitrogen.
The liquid nitrogen typically is supplied from a storage dewar or bulk storage tank located inside or outside the facility. Because liquid nitrogen is a cryogenic liquid (-320°F [-196°C]), it usually is delivered to the process through a vacuum-jacketed pipe to minimize premature vaporization of the liquid. A liquid nitrogen phase separator often is used on the incoming line to make sure that the right pressure and only very cold liquid are delivered to the injection point just above the container.
When the liquid nitrogen is introduced into the container, the liquid vaporizes due to a temperature change. The liquid nitrogen injection drives out other unwanted gases (including oxygen) and covers the container contents with nitrogen gas. Because the vapor requires more volume than the liquid it replaces, the pressure within the container is increased. Pressure-influencing factors include the liquid level in the container, the temperature of the container contents and the dose droplet size.
Some liquid nitrogen injectors operate on continuous flow at all times. Others operate on discrete dosing when the container production line speed is low and then can be manually converted to continuous dosing at higher speeds. The benefit of discrete dosing is that the valve only opens and closes as each container passes below it, which conserves the amount of liquid nitrogen used in the process.
The type of sensing equipment used in the injector determines how efficiently the equipment will operate. Some injectors use only a basic sensor to turn the injector liquid stream on and off when containers are present. More sophisticated units use electronic controls to adjust discrete injector dosing to account for line-speed variations.
Various orifice configurations also are available to control the type of injection pattern required for specific products. The best units allow simple and quick orifice change-outs only using hand tools.
Production Line Considerations
Key factors to consider when putting together a liquid nitrogen injection system include the type of sensor, sensor location and available space on the production line.
The type of sensor selected primarily depends on the product being processed. For example, optical sensors typically can be used with clear bottles containing water or other transparent liquids; a process with metallic containers might operate better using proximity-switch technology (figure 3). Such systems can be either manually or electronically controlled. One advantage to electronic-controlled units is that multiple “recipes” or settings can be stored in the unit so that the equipment can adjust automatically to different containers or products without having to be recalibrated.
The container sensor location is another factor to consider. Some injectors require the sensor to be located at a precise location near the injector head. Other more-advanced systems allow the sensor to be located anywhere near the head, and the software can be told the location after the sensor is mounted. This feature accelerates setup and installation.
Typically, space is at a premium on packaging and bottling lines. Some of the most advanced nitrogen injectors are designed to minimize space requirements by using a remote overhead liquid reservoir connected to the injector by vacuum-insulated flexible plumbing. The remote reservoir acts as a phase separator, delivering vapor-free liquid at a consistently high density. The injector head is a small, simple device that can be mounted above the production line.
Also, because many production lines operate 24/7, maintenance is inevitable. Product serviceability therefore should be examined when considering a design. In some injectors, the valves and actuator modules can be replaced easily within minutes, thus minimizing downtime.
In conclusion, the state-of-the-art of liquid nitrogen injectors has advanced since the inception of the product more than 20 years ago. Technical advances continue to be made as all injector manufacturers strive to further improve the LN2 dosing process.