How to

3 February 2017

What is Dry Ice?

Dry ice, upon first hearing the term, can seem like a bit of a contradiction. As ice is, as we all know, comprised of frozen water, how could it possibly be defined as dry? Well, therein lies the common misconception; dry ice, unlike its conventional counterpart, isn’t actually water at all.

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Dry ice is in fact frozen carbon-dioxide, and the surprisingly versatile substance actually has a wide variety of uses. Often created as a by-product of other industrial processes, entire industries have sprung up centred around capturing and recycling the gas to make dry ice for purposes ranging from the obvious (refrigeration, medical applications) to the obscure (cleaning, vehicle repair, branding) to the straight-up breathtaking (theatre/special effects).

Although a large amount of carbon-dioxide is, as previously mentioned, created as a by-product of other industries, the subsequent manufacturing of the dry ice itself does require a little bit of work. The process typically starts with liquid CO2, held in storage vessels pressurised to 300psi. This pressurisation is crucial to the process as liquid carbon-dioxide simply doesn’t exist under normal atmospheric pressure; in fact, pressures of at least 5.1 standard atmospheres (520kPa) are required for the substance to form as a liquid at all.

The liquid CO2 is then fired through an expansion valve, entering an empty chamber in which normal atmospheric pressure causes it to rapidly turn to gas. The sudden transition from one state to another results in a substantial drop in temperature. Approximately 46% of the gas will then freeze into dry ice ‘snow’, while the remaining gas is either released into the atmosphere or captured for reuse. The ‘snow’ is then compressed into either blocks or pellets, depending on customer requirements.

The resulting dry ice sits at a temperature of -78.5°C (-109.3°F), and gives more than twice the cooling energy per pound of weight and three times the cooling energy per volume as compared to regular water ice, making it extremely useful for industries dealing in frozen goods.

Now, you may remember me stating earlier that, under standard atmospheric pressure, carbon-dioxide has no liquid form – the very fact that gives it its name. This makes it particularly useful for many industries where the moisture from the melting of conventional ice may have a negative impact on their goods or operations. As dry ice changes directly from solid to gas in a process known as sublimation, this is not an issue. According to Dry Ice Info, dry ice will sublimate at a rate of approximately 5-10 pounds per 24 hour period; as such, they advise purchasing dry ice as close to the time needed as possible.

When handling dry ice it is important to protect your hands with a protective cloth or gloves; brief contact is unlikely to cause any harm, but prolonged contact between your skin and the dry ice can result in injuries similar to a burn. It is also important to make sure when storing dry ice that the room has some form of ventilation. If not, the resulting build-up of carbon-dioxide in the air can be damaging to your health. If you develop shortness of breath, a sudden headache or notice a blue tint to your fingernails or lips, this is a sign that you are breathing in too much CO2 and need to vacate the room.

Sam Bonson

Sam is an aspiring novelist with a passion for fantasy and crime thrillers. He is currently working as a content writer, journalist & editor in an attempt to expand his horizons.