30-11-2012, 04:37 PM
DYEING WITH SUPERCRITICAL CARBON DIOXIDEA NOVEL APPROACH
DYEING WITH SUPERCRITICAL.pdf (Size: 519.25 KB / Downloads: 122)
Abstract
The textile industry is believed to be one of the biggest consumers of water. On
average an estimated 100 kg of water is needed to process 1 kg of textile material.
Water is used as a solvent in many pretreatment and finishing processes, such as
washing, scouring, bleaching and dyeing. Although there have been efforts to
reduce the water input such as altering conventional equipment, recycling water
and reusing wastewater—water usage is still high in the textile industry. Nonaqueous
systems of dyeing can reduce or completely eliminate the amount of water
used. Reducing water use provides environmental benefits as well as cost savings.
Among the most promising of the non-aqueous systems is the use of supercritical
carbon dioxide (CO2).
This paper will include what is supercritical carbon dioxide, Dyeing of different
fibres i.e. synthetic and natural fibres using Supercritical carbon dioxide as a
medium, Dyes used & mechanisms involved in their dyeing, Process flow,
equipment and their advantages over conventional dyeing method of dyeing.
Introduction
In conventional textile dyeing large quantities of wastewater are produced. This
environmental and economical burden is avoided when supercritical carbon
dioxide is used as dyeing medium instead of water. Separating residual dye from
the CO2 and recycling of CO2 are easy. Energy is saved because textiles do not
need to be dried after the dyeing process. An additional advantage of scCO2 is the
high diffusivity and low viscosity that allow the dye to diffuse faster towards and
into the textile fibers. This results in a faster dyeing process.
Textiles can be classified into non polar, synthetic polymers (e.g. polyester) and
polar, natural Textiles. The second category can be divided into polymers built
from amino acids (e.g. silk and Wool) or cellulose (e.g. cotton).In polyester
dyeing, scCO2 penetrates and swells the fibers, thereby making them accessible for
dye molecules. Upon depressurization, the dye molecules are trapped inside the shrinking polyester fibers. Polyester dyeing in scCO2 has been studied by several
researchers.
Supercritical fluid
When a solid is heated, the thermal motion of the molecules increases, the solid
melts and a liquid and a vapor phase are formed. In figure 1.1 these three states of
matter are graphically presented. When a vapor below its critical temperature is
compressed, it condenses when the vapor-liquid equilibrium line is crossed. Above
the critical temperature however, the thermal energy of the vapor molecules is so
high, that condensation is no longer possible, no matter how much the pressure is
increased. The vapor-liquid equilibrium line ends at the critical temperature. When
a fluid is above its critical temperature and the corresponding critical pressure, it
cannot be regarded as a vapor or a liquid and it is referred to as a supercritical
fluid.
The most widely used supercritical fluid is carbon dioxide, because it combines a
relatively mild critical point with non-flammability, non-toxicity and a low price.
Because of its green and safe character, it is the best supercritical solvent for textile
dyeing.
EQUIPMENT
A supercritical dyeing process operated as is shown in figure 1.2. During the
dyeing, the CO2is circulated through a heat exchanger, a vessel where the dye is
dissolved and through a vessel where the dye is delivered to the textile. When the
desired coloration is attained, dye I still left in the CO2, which is removed by
passing the CO2 through a pressure reducing valve into a separator vessel. In the
separator, the CO2 is gasified, so that the dye precipitates and the clean CO2 can be
recycled by pumping it back to the dyeing vessel. Pilot plants such as drawn in
figure 1.2 have been constructed but, now a commercial-size machine has been
built by DYCOO Company in 2010. But have very high cost and needs time for
its commercialization.