Great balls of ice

It's always important to stay cool. But it sometimes isn't easy. Freezing liquids into solids can create materials handling problems, as Messer has found with its freezing processes, Cryopel and Cryobreak.

Cryopel was originally designed for a customer who wanted to make a pelletised ice-cream product, with spherical pellets of a uniform 5mm diameter. The pellets are formed by dripping the liquid ice-cream into a liquid nitrogen bath. The system can handle a maximum of 150kg/hr; above this, the pellets tend to freeze into a lump.

The Cryobreak process, which can handle twice the throughput of Cryopel, works by injecting liquid product into a fast-flowing stream of liquid nitrogen which rips the product stream into pieces and freezes them instantly. Granules produced by this process have an uneven size and shape.

Both processes use only the latent heat of evaporation of nitrogen to freeze the particles. The 'cold content' of the nitrogen gas - the energy needed to chill it down from ambient temperatures to its freezing point - is unused.

To avoid these problems, Messer has designed a new process with a faster production rate which uses the energy of the cold gas. The Cryogen Rapid Pelletiser produces consistently spherical particles of frozen food - ice creams, fruit juices and sauces, for example - at throughputs of 250kg/hr per process module.

Each freezing module - contained inside a ventilated box - has its own open liquid nitrogen reservoir. A low-temperature pump conveys the liquid up to a pelletising channel, which is designed to maintain a laminar flow of liquid along its length. A series of nozzles drip the liquid product into the flowing nitrogen, which freezes the surface of the drops as they flow along the channel. At the end of the pelletising bath, both pellets and nitrogen drop onto a fine-mesh conveyor. The liquid falls through into another channel, which returns it to the reservoir. The pellets continue along the conveyor to the exit of the box. The cold nitrogen vapour from the liquid channels and the reservoir reduces the temperature in the box, and the pellets continue to freeze as they pass along the conveyor. By the time they fall into their receiver vessels, they are frozen solid.

Because the cold energy is used, the process uses 30 per cent less nitrogen than the previous methods, Messer says. Moreover, the high production rates allow it to be used for a far wider range of applications. The machinery could be used to produce granules of starter cultures for yogurt or cheese production, for example - the fast freezing and rapid throughput ensures that the cells in the culture are not damaged.

Outside the food industry, the equipment could freeze peroxides used as oxidising agents in the chemical industry, which need to be maintained at a low temperature to retain their activity. Pelletised peroxides could be introduced to a reactor in their frozen state, and would distribute themselves evenly as they melt, ensuring that the reaction starts simultaneously at all points within the vessel.