The conventional dyeing of textiles requires that an excess of dye is dissolved or in some way "taken-up" in an aqueous or solvent solution. The dye mix is then pumped into a vat containing holding the textile. Typically there is agitation or the dye is recirculated several times through the cloth. At the end of the cycle, the dye mix is pumped to the waste treatment facility. Dyes are notoriously difficult to treat. The process is decidedly unfriendly to the environment.
The use of supercritical CO2 in textile dyeing is an environmentally friendly alternative. Instead of using an aqueous or solvent solution to "take-up" the dye, supercritical CO2 is used. The process proceeds in the same manner as the conventional method, but instead of sending the spent mix to the waste treatment facility, the supercritical CO2 /dye mix is depressurized. The CO2 changes to a gas and all the spent dye falls out and can be reused. In production systems the CO2 is recycled for providing for a completely closed systems and an entirely environmentally friendly approach to textile dyeing.
The operation as shown above follows in this manner:
Supercritical CO2 flows into the dye make-up vessel where it dissolves the dye.
It continues to the dye vessel which contains the textile cocoon.
Dyeing can continue in a static bath or a specially designed pump recirculates the mix several times through the cocoon.
At the end of the cycle, the mix flows to the dye collection vessel where depressurization occurs causing the spend dye to drop out.
The CO2 can then be recycled and the "spent" dye reused.
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Carbon dioxide is in its supercritical fluid state when both the temperature and pressure equal or exceed the critical point of 31°C and 73 atm (see diagram). In its supercritical state, CO2 has both gas-like and liquid-like qualities, and it is this dual characteristic of supercritical fluids that provides the ideal conditions for extracting compounds with a high degree of recovery in a short period of time.
By controlling or regulating pressure and temperature, the density, or solvent strength, of supercritical fluids can be altered to simulate organic solvents ranging from chloroform to methylene chloride to hexane. This dissolving power can be applied to purify, extract, fractionate, infuse, and recrystallize a wide array of materials. Because CO2 is non-polar, a polar organic co-solvent (or modifier) can be added to the supercritical fluid for processing polar compounds. By controlling the level of pressure/ temperature/ modifier, supercritical CO2 can dissolve a broad range of compounds, both polar and non-polar.