Liquid carbon dioxide is carbon dioxide gas that has been highly compressed and cooled to transform it into a liquid state. This form of CO2 does not occur naturally under typical atmospheric conditions; it requires specific pressure and temperature ranges to exist. For instance, it can only remain liquid when the pressure is above 5.1 atmospheres and the temperature is between approximately -56.6°C and 31.1°C, which is its critical point. This unique phase of carbon dioxide finds widespread application across many industries.
Sources of Carbon Dioxide
The raw carbon dioxide gas used for liquefaction originates primarily as a byproduct from numerous industrial processes. Its capture and conversion into liquid form efficiently utilizes existing emissions. One significant source is ethanol fermentation, where CO2 is naturally released during alcohol production.
Similarly, the manufacturing of ammonia, a key component in fertilizers, also generates substantial amounts of CO2 as a byproduct. Natural gas processing plants and certain power generation facilities also contribute to the available CO2 supply. These industrial operations produce concentrated streams of CO2, which are then captured for purification and liquefaction. Some natural wells deep within the earth also contain high-purity CO2, which can be directly sourced after purification.
The Liquefaction Process
Converting gaseous carbon dioxide into its liquid form involves a precise manipulation of pressure and temperature. The process relies on increasing pressure and lowering temperature to force gas molecules closer together, leading to a phase change. This typically begins with the initial compression of the raw CO2 gas. High-performance compressors increase the gas pressure, preparing it for subsequent cooling stages.
Following initial compression, the gas is cooled significantly, often using refrigeration systems or heat exchangers. This cooling reduces the kinetic energy of the CO2 molecules, making them more susceptible to liquefaction under pressure. The pre-cooled, compressed gas then undergoes further compression to achieve the necessary conditions for it to transition into a liquid. Specialized equipment like condensers play a role in this phase transformation by facilitating heat removal.
Purification and Storage
After the carbon dioxide has been liquefied, it requires purification to remove impurities that could compromise its quality or suitability for specific applications. Common contaminants include water, sulfur compounds like hydrogen sulfide, volatile organic compounds, and non-condensable gases such as methane, oxygen, and nitrogen. Purification methods typically involve activated carbon filters to absorb organic pollutants and chemical contaminants. Drying systems, often utilizing molecular sieves, are employed to remove moisture.
Cryogenic distillation is another advanced purification technique where the liquefied CO2 is further cooled and then carefully distilled to separate it from other gases with different boiling points, achieving high purity levels. Once purified, liquid CO2 must be stored under specific conditions to maintain its liquid state. This involves specialized insulated and pressurized containers, commonly known as cryogenic tanks. These tanks are typically double-walled and vacuum-jacketed, designed to minimize heat transfer and maintain the low temperatures and high pressures required. For transport, the liquid CO2 is then transferred from these storage tanks into tanker trucks or other vessels.