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Chemviron Carbon Greening of Aerosol Propellants

Challenge

Develop a system enabling the use of a benign gas as a propellant.

Problem

Since the demise of chlorofluorocarbons (CFCs), the majority of aerosol propellants have been based on light hydrocarbon components such as propane, butane and iso-butane. Although combinations of these materials have suitable vapour pressure characteristics, they are also classified as Volatile Organic Compounds (VOCs) which contribute both to low-level (tropospheric) ozone and global warming (greenhouse gas) effects. Hydrocarbon propellants come from oil feedstocks and are highly flammable, with significant safety and health issues. Residual hydrocarbons pose a significant problem for recycling of the canisters.

These gases also have very high global warming potentials and their use is limited to non-flammable or non-toxic propellants. Compressed gas propellants, such as carbon dioxide and nitrogen on the other hand are low-cost, low-toxicity, non-flammable, inert materials ideally suited to the environment. However, when deployed in aerosol canisters the pressure in the container falls rapidly as the contents are used and causes undesirable changes in the rate of discharge and spray characteristics, resulting in low customer appreciation.

Solution

Gas storage and aerosol dispensing systems have been developed based upon adsorbed permanent gases. The capability of activated carbon to condense or immobilise gases was exploited, resulting in increased gas storage and delivery capacity compared to usual storage containers. Adsorption onto a material’s porous structure densifies the gas to the point that it achieves a quasi-liquid state. Consequently a carbon-filled container can physically deliver more gas than a non-carbon filled container despite the volume lost to the carbon skeleton.

For the aerosol can and related devices (such as the foghorn), carbon dioxide is usually the preferred gas when used in conjunction with a selected activated carbon.

The activated carbon used for aerosol applications is manufactured from sustainable resources such as coconut shells, the carbonisation and subsequent activation of which produces more energy than the process itself uses. This allows a process of manufacture that doesn’t require external energy input and uses all by-products.

Benefits

There are many benefits of employing this combination of materials in this way: C/CO₂ is non-flammable C/CO₂ is non-toxic and both the carbon and CO₂ are food-quality compliant; there is little scope for substance abuse C/CO₂ is environmentally benign (Although CO¬2 is a greenhouse gas, its deployment in an aerosol canister has, at worst, a neutral effect in terms of global warming because the gas is recovered as a by-product from a number of industrial processes that would otherwise be released to air. For the European market alone, sequestration of the gas in aerosol containers, together with replacement of the hydrocarbon gases, is equivalent to removing 5 million tonnes/year of carbon dioxide from the atmosphere (akin to 1,700,000 mid-sized cars). CO₂ has no propensity to deplete stratospheric ozone.) The working pressure of a given system can be designed based upon the selection of suitable carbon and carbon dioxide quantities

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