4. Dyeing under hypercritical conditions.pptx.pdf

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.

1. DYEING UNDER HYPERCRITICAL
CONDITIONS
Burhan Uddin Banna
Lecturer, Wet Processing

2. INTRODUCTION:
▪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 waste water usage is still high in
the textile industry.
▪Non-aqueous 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).

3. ▪Concern over volatile organic solvent emissions and the generation of aqueous waste
streams has prompted a number of chemists and chemical engineers to seek new,
cleaner methods for polymer synthesis and polymer processing.
▪The use of super critical carbon dioxide (scCO2) has attracted particular attention in
both of these areas for the following reasons:

1.CO2 is non-toxic, non-flammable, chemically inert, and inexpensive.
2.Supercritical conditions are easily obtained: Tc (CO2) = 31.1C; Pc,(CO2) = 73.8 bar.
3.The solvent may be removed by simple de pressurization.
4.The density of the solvent can be tuned by varying the pressure.
5.Many polymers become highly swollen and plasticized in the presence of CO2.

4. SUPERCRITICAL FLUID
▪Any substance is characterized by a critical point which is obtained at specific conditions of
pressure and temperature.
▪When a compound is subjected to a pressure and a temperature higher than its critical
point, the fluid is said to be “supercritical”.
▪A supercritical fluid (SCF) is a material that can be either liquid or gas, used in a state
above the critical temperature and critical pressure where gases and liquids can coexist.

5. FIG: SUPERCRITICAL FLUID
Fig: Supercritical Fluid.

6. ▪Triple point. The temperature and pressure at which a substance can exist in equilibrium
in the liquid, solid, and gaseous states. The triple point of pure water is at 0.01°C (273.16K,
32.01°F) and 4.58 mm (611.2Pa) of mercury and is used to calibrate thermometers.
▪The three way junction is there because that is where the three phases (solid, liquid and
vapour) of a substance can coexist in thermodynamic equilibrium with one another.
▪At all other temperatures only two (or one) phases can co-exist together in equilibrium.
▪The triple point represents the combination of pressure and temperature that facilitates all
phases of matter at equilibrium.
▪The critical point terminates the liquid/gas phase line and relates to the critical pressure, the
pressure above which a supercritical fluid forms.

7. Fig: Supercritical Fluid.

8. SUPERCRITICAL FLUID
▪A supercritical fluid may be characterized best by referring to a phase diagram as shown for in
the Fig.
▪A liquid can be converted to a supercritical fluid by increasing its temperature (and
consequently its vapor pressure) and simultaneously increasing pressure.
▪A closed system thus reaches critical values where no boundary between the liquid and gaseous
state can be distinguished, i.e., the supercritical state.

9. WHAT IS SUPERCRITICAL
FLUID DYEING?
▪Here Carbon Dioxide is used as a supercritical fluid as carbon dioxide is readily available,
cheap and is non-toxic and non-flammable.
▪Above the temperature of 31.06 Degree Celsius and pressure of 72.8 atmosphere exhibit
physical properties, which are intermediate between properties of liquid and gas.
▪These are supercritical conditions and can be achieved by using commercially available
equipment.

10. SUPERCRITICAL FLUID DYEING
▪The main advantage of this process is that solvent can be easily turned into gas by simply
releasing the pressure leaving no solvent residues and requires no evaporation or separation.
▪The low viscosity of supercritical fluids and the rather high diffusion properties of the
dissolved molecules are especially promising aspects for dyeing processes.
▪A supercritical dyeing fluid should easily dissolve solid dyestuffs and should penetrate even
the smallest pores without the need of vigorous convection procedures.

12. WHY SUPERCRITICAL DYEING IS BETTER THAN
CONVENTIONAL DYEING TECHNIQUE?
1.Waste water is not generated as water is not used while dyeing
2.No damage to the fiber
3.No need of dispersing or levelling agents
4.Short time as compared to conventional dyeing
5.Low dyes and chemical consumption
6.Carbon Dioxide can be recycled easily hence no air pollution
7.Drying is not required
8.Low energy consumption
9.No effluents
10.Environment friendly process

13. WHAT IS THE DIS-ADVANTAGES OR LIMITATIONS OF
SUPERCRITICAL FLUID DYEING TECHNIQUE?
1.Initial investment is high
2.Processing of long length fabric (continuous process) is not possible
3.At present Supercritical fluid dyeing is only suitable for synthetic fibers

14. DYEING OF PET BY SUPERCRITICAL CO
2
:
▪Materials needed: Polyethylene Terephthalate (PET), Azo Dye, industrial grade Carbon dioxide
with diptube.
▪Apparatus: An apparatus for dyeing in supercritical carbon dioxide is consists of a temperature
controller, a vessel heater which surrounds the vessel, a stainless steel dyeing vessel of 50ml
capacity (with a quick release cap), a manometer, a Varex HPLC (High performance liquid
chromatography) carbon dioxide pump and a cooler for cooling the head of the carbon dioxide
pump.
▪The apparatus was pressure-tested for use up to 350 bars or 5,076.321 PSI(Pounds per square inch)
and 100 degree Celsius.
▪A side arm connects the top and the bottom of the cell outside the heater to allow the
supercritical carbon dioxide to circulate by thermal convection.

15. PROCESS FLOW SHEET

16. DYEING OF PET BY SUPERCRITICAL CO
2
:
Dyeing Procedure:
▪The process of dyeing by supercritical fluid begins with placing of PET packages inside the vessel
in a dry state.
▪CO
2
is allowed to enter the dye vessel and the operational pressure and temperature(conventional
dyeing temp of fibre) achieved.
▪The dye is dissolved in circulating CO
2
in the chamber.
▪The Dyeing takes around half an hour under optimal conditions.
▪The concentration of the dye compound in the supercritical CO
2
determines the shade.
▪This shade can be manipulated by density of supercritical CO2.
▪Small quantities of modifier can increase the solubility of the dye.
▪The dyeing cycle ends with the depressurization of the system and collection of excess dyestuff in
the Recovery Vessel.

18. Thank You!