Many other examples can be found in the literature about the use of such
supercritical fluid [8].
8.3.4 Solvent and Processing Fluid in the Chemical Industry
SC-CO 2 has been used as solvent in chemical processes. The advantage is that it is
possible to modulate the polarity of the solvent by playing on pressure, and the
solvent can eventually be easily recovered by expansion, collected, recompressed,
and recycled. Such operation demands energy and this is a drawback. In some
specific cases, SC-CO 2 can also be used as solvent-reagent. Interestingly, enzymes
can be used in SC-CO 2 [9]. Moreover, SC-CO 2 can be even mixed up to a given
ratio with solvents such as methanol or ethanol that increases the polarity of the
fluid and helps to extract more polar molecules or to build up solvent media of
variable polarity [9].
SC-CO 2 has been used also as solvent in polymerization reactions or in modifying polymers.
Liquid and solid carbon dioxide are also used for direct injection into chemical
reaction systems to control temperature. As an inert gas, carbon dioxide is used in
the chemical industry to purge and fill reaction vessels, storage tanks, and other
equipment to prevent the formation of explosive gas mixtures and to protect easily
oxidable chemicals from contact with air.
CO 2 is largely used as an alternative to chlorofluorocarbons (CFCs) and
hydrocarbons-based blowing agents, which are being phased out due to their
allegedly detrimental effect on the Earth’s ozone layer, in the 100 kt/y foam
industry. Carbon dioxide gas under pressure is introduced into rubber and plastic
mixes, and foams the material upon pressure release.
The foam blowing industry is divided into two main markets: thermoplastics and
thermosets. Thermoplastics, which are used in residential housing as sheeting and
roofing, as well as in commercial roofing applications, are the smaller of the two.
Although polystyrene is the biggest section of this market, there are sizable segments in polypropylene and polyvinyl chloride as well as several smaller segments.
Thermosets are a much larger market, taking about four times as much business
as thermoplastics. This industry closely follows the plastics market. Polyurethane
agents go into residential sheeting, refrigerator doors, insulating foam for pipes,
spray foam for electrical outlets, and sandwich panels, and also the thin layer of
foam which is placed between layers of metal in walk-in refrigerators, storage
trucks, and other larger refrigerating devices. Flashing (i.e., thin protrusions of
material at mould joints) can be quickly removed by tumbling these articles with
liquid carbon dioxide or crushed dry ice. By injecting liquid CO 2 into the moulded
product immediately after the blowing step, the cooling time can be reduced in an
important way. Cycle times of existing machines can, therefore, be reduced and the
production capacity is increased. Due to increase in performance of today’s blowing
techniques, the use of CO 2 has reduced in popularity, but still shows significant
advantages when used with plastic moulds having an important wall thickness.
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8 Use of CO 2 as Technical Fluid (Technological Uses of CO 2 )
supercritical fluid [8].
8.3.4 Solvent and Processing Fluid in the Chemical Industry
SC-CO 2 has been used as solvent in chemical processes. The advantage is that it is
possible to modulate the polarity of the solvent by playing on pressure, and the
solvent can eventually be easily recovered by expansion, collected, recompressed,
and recycled. Such operation demands energy and this is a drawback. In some
specific cases, SC-CO 2 can also be used as solvent-reagent. Interestingly, enzymes
can be used in SC-CO 2 [9]. Moreover, SC-CO 2 can be even mixed up to a given
ratio with solvents such as methanol or ethanol that increases the polarity of the
fluid and helps to extract more polar molecules or to build up solvent media of
variable polarity [9].
SC-CO 2 has been used also as solvent in polymerization reactions or in modifying polymers.
Liquid and solid carbon dioxide are also used for direct injection into chemical
reaction systems to control temperature. As an inert gas, carbon dioxide is used in
the chemical industry to purge and fill reaction vessels, storage tanks, and other
equipment to prevent the formation of explosive gas mixtures and to protect easily
oxidable chemicals from contact with air.
CO 2 is largely used as an alternative to chlorofluorocarbons (CFCs) and
hydrocarbons-based blowing agents, which are being phased out due to their
allegedly detrimental effect on the Earth’s ozone layer, in the 100 kt/y foam
industry. Carbon dioxide gas under pressure is introduced into rubber and plastic
mixes, and foams the material upon pressure release.
The foam blowing industry is divided into two main markets: thermoplastics and
thermosets. Thermoplastics, which are used in residential housing as sheeting and
roofing, as well as in commercial roofing applications, are the smaller of the two.
Although polystyrene is the biggest section of this market, there are sizable segments in polypropylene and polyvinyl chloride as well as several smaller segments.
Thermosets are a much larger market, taking about four times as much business
as thermoplastics. This industry closely follows the plastics market. Polyurethane
agents go into residential sheeting, refrigerator doors, insulating foam for pipes,
spray foam for electrical outlets, and sandwich panels, and also the thin layer of
foam which is placed between layers of metal in walk-in refrigerators, storage
trucks, and other larger refrigerating devices. Flashing (i.e., thin protrusions of
material at mould joints) can be quickly removed by tumbling these articles with
liquid carbon dioxide or crushed dry ice. By injecting liquid CO 2 into the moulded
product immediately after the blowing step, the cooling time can be reduced in an
important way. Cycle times of existing machines can, therefore, be reduced and the
production capacity is increased. Due to increase in performance of today’s blowing
techniques, the use of CO 2 has reduced in popularity, but still shows significant
advantages when used with plastic moulds having an important wall thickness.
130
8 Use of CO 2 as Technical Fluid (Technological Uses of CO 2 )