Unconventional techniques for energy conservation in ... techniques for energy conservation in textile wet processing Dr. S. R. Shah and Dr. J. N. Shah Department of Textile Chemistry, Faculty of Technology ...

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  • Unconventional techniques for energy conservation in textile wetprocessing

    Dr. S. R. Shah and Dr. J. N. ShahDepartment of Textile Chemistry, Faculty of Technology & Engineering. The M. S. University of Baroda, Vadodara 390

    001, Gujarat, INDIA.E-mail: srshah24263@yahoo.co.in

    Keywords : Unconventional technique, ultrasound, plasma, supercrtical fluid, electrochemical, textile wet processing

    1. Introduction

    Power and utility plays a vital role and their cost contributes significantly on total cost of finished textile product. Beingbasic and the oldest industry, it grows with the population rate, so is the need of power. Among all the industries, textilesector consumes about 5 - 8% of the total energy mainly in the form of electrical and thermal. Out of this about 40-45%energy consumed in manufacturing of yarn and fabric and 35-60% energy utilized in wet processing. Textile wetprocessing involved pretreatment, dyeing, printing and finishing, on grey fiber to impart aesthetic values andmarketability. All the four conventional energy sources namely, coal, electricity, oil and gas are utilized in the wetprocessing of textile. Table 1 shows the pattern of steam consumption in a typical composite textile sector. Wetprocessing of textile consumes very small proportion of electrical energy mainly for running of machineries. Fuel interms of coal or oil is used extensively, mainly to generate steam or heat. International energy crisis and escalating costof fuels have diverted all the researchers and industrialists to think for the ways to conserve energy [1-3]. Variousapproaches have been developed and practiced to conserve energy in wet processing namely,

    Developments of machines with low material to liquor ratio. Efficient heat recovery and processing. Developments in specialty chemicals and dyes to reduce processing time or cycles. Optimize wet pickup on fabric to reduce drying energy. Adoption of e-control to minimize unnecessary leakages. Development of techniques to reduce process cycle.

    Table 1 Steam consumption in a typical textile composite mill [1]

    Department Steam consumption (%)Humidification 10Sizing 15Boiler house 05Wet processing 60Leakage 10

    Innovation of unconventional techniques have opened a new era of energy conservation in textile wet processing[4,5]. The important benefits to textile industry of the said technologies are:

    The apparent increase in diffusion rate of chemicals Energy saving s as process operate at lower temperature Increased efficiency of process leads to less effluent Preserved drapability, luster and finish of fiber Overall cost reduction of process

    2. Application of ultrasound [6-9]

    Normal audible sound frequency range for human is about 16 18 Hz (1 Hz = 1 Cycle for second). Ultrasound (US) isa cyclic sound pressure wave with a frequency greater than the upper limit of human hearing. US is thus not separatedfrom "normal" (audible) sound based on differences in physical properties, only the fact that humans cannot hear it. USmainly divided into two groups, namely, power ultrasound (20 KHz 2 MHz) and diagnostic ultrasound (5MHz 10 MHz). The various range of sound shown in figure 1. Power US induces cavitation in liquors, mainly used in textilewet processing while diagnostic US does not induce any cavitation and used mainly in medical imaging.

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  • Figure 1: US range diagram

    US radiation requires medium with electric properties for propagation and with this respect, it differs from light andother form of electromagnetic waves, which travel freely in vacuum. During the propagation of ultrasonic waves, theparticles in the electric medium oscillate and transfer energy through the medium in the direction of propagation. Theycan be propagated either longitudinally or transverse. In gas and liquid medium only longitudinal waves are transferredwhile in solid medium both longitudinal and transverse waves can be transmitted. The longitudinal vibration in liquidproduces the phenomenon of cavitations. This cavitations, is the form of microscopically small bubbles, which duringprocess, expand and collapse violently and generating shock wave. The cavitation formation depends on many factors,namely, frequency and intensity of waves, temperature and vapor pressure of liquid. The important phenomenon intextile wet processing, with respect to ultrasonic waves is micro steaming i.e. large amount of vibration energyaccommodated in relatively small volumes with little heating. The combine effects of cavitations and micro steaminglead to inter molecular tearing and surface rubbing. This reaction behaves as catalyst for the actual process i.e. increasesthe rate of processing.

    The use of US in textile wet processing was commenced in 1941 by Sokolov and Tumanski (Japan). Application ofUS in textile wet processing can be divided into two categories.

    2.1 Application in auxiliary bath preparation

    This application mainly related to the preparation of auxiliary bath such as preparation of sizing bath, emulsion solutionpreparation, dye dispersion preparation and thickener preparation for printing. In conventional process the starch andthickener paste/solution are mixed with water and heated at gelatinizing temperature and held for long period at the saidtemperature. With the help of US, the operation can be conducted more rapidly, even at lower temperature. Further, thequality of solution prepared by US usage is superior in terms of homogeneity. Similarly, US techniques are used toprepare emulsion solution used for lubricants. Ramaszeder prepared water-oil emulsion using US and found stabilitymore than 212 hours. Whereas those prepared by conventional method is only 12 hours. US techniques are widely usedfor dye dispersion preparation either in dyestuff industries or at shop floor in dyeing department.

    2.2 Application in chemical processing of textiles

    Application of US in this category involves, heterogeneous systems (textile subtract and liquor) and used in variousprocesses such as desizing, scouring, bleaching, dyeing and finishing.In the pretreatment and washing operations, the main object is to remove natural and added impurities from the fibersurface while in dyeing and finishing is to transfer or diffuse dyes or chemicals into the fiber structure.

    Dyeing of textile materials with US has been subject to many studies likes, effects of low and high frequency soundwaves, quality of dispersion, solubility of dyes, dye uptake by subtract etc. The frequency suitable for inducing UScavitation is in the range of 20 to 50 KHz. The mechanism of US in dyeing can be explained as follows: When ultrawaves are induced in liquor, the phenomenon cavitation starts and liberates entrapped gases from liquid or porousmaterials (textiles) as a result three dimension reactions occurs, namely, dispersion (breaking up of miscells and highmolecular weight aggregates into uniform dispersion ), degassing (expulsion of dissolved or entrapped gases or airmolecules from fiber capillaries) and diffusion (accelerating the rate of dye diffusion inside the fiber by piercing theinsulating layer covering the fiber and accelerating the interaction between dye and fiber). Dispersion and degassingeffects are catalyzed by mechanical action of cavitations while diffusion is catalyzed by both mechanical action ofcavitations and heating of fiber surface.

    US application in textile finishing can be explored for various types of fabric lamination. For this purpose Kuster,Germany has developed US Laminating Calendars. The important significance of US in textile processing can besummarized as:

    Improved quality products obtained mainly in paste preparation process. Better dye penetration, improved uniformity and fastness properties of dyeing. Energy saving as the process is possible at low temperature compared to conventional process. Consistent and uniform bonding in the finishing process.

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  • 3. Application of plasma [10-14]

    Plasma technology was first developed by M. Faraday in 1880s and had been proposed by I. Langmuir in 1926. Plasmais an ionized gas with equal density of positive and negative charges which exist over an extremely wide range oftemperature and pressure. Plasma consists of free electrons, ions, radicals UV-radiation and other particles dependingupon the gas used. In order to maintain a steady state, it is necessary to apply an electric field to the gas plasma, whichis generated in a chamber at low pressure. Plasma is the 4th state of matter and a gas becomes plasma when the kineticenergy of the gas particles rises to equal the ionisation energy of the gas. When this level is reached, collisions of thegas particles cause a rapid cascading ionisation, resulting in plasma(figure 2b). When the neutral molecules of a gasare energized, e.g. by exposing to high electric field, to a point when some electrons become free and the gas turns intoa mixture of electrons, ionised atoms and molecules, photons and residual neutral species. In this state it behaves as achemically very active environment and there is a high likelihood of surface interaction with organic substrates. It isalso possible to genetate plasma at room temperature. Plasma is generated when an electrical current is applied across adielectric gas or fluid (an electrically non-conducting material) (figure 2a) or in air as shown in (figure 2c).

    (a) (b) (c)

    Figure 2: (a) Generation of plasma, (b) Cascade process of ionization. Electrons are e, neutral atoms o, and cations +, (c)Artificial plasma produced in air.

    The conventional wet treatment used in textile mainly concern with energy, cost and environmental issues.Application of plasma at low temperature in textile processing can prove to be the best alternative for these issues.Unlike conventional wet processes, which penetrate deeply into fibers, plasma only reacts with the fabric surface(nanometer range) that will not affect the internal structure of the fiber. Further, plasma technology modify thechemical structure as well as the surface properties of textile materials, deposit chemical materials (plasmapolymerization) to add up functionality, or remove substances (plasma etching) from the textile materials for betterapplicability. In textile processing, this technology can be explored in various areas like pretreatment, dyeing andfinishing through different methodology vis--vis glow, corona and dielectric barrier discharge methods to addfunctionality and modification of surface properties of textile materials. Plasma technology is a surface-sensitivemethod that allows selective modification in the nm-range. Different reactive species in the plasma chamber (a mixtureof electrons, ionised atoms and molecules, photons and residual neutral species) interact with the substrate surface.Depending on the parameters used, different treatments like cleaning, modification or coating occur.

    3.1 Application of plasma in textile processing

    Plasma techniques can be used from pretreatment to finishing and to produce innovative functional textiles. In the firstpretreatment process i.e. desizing, plasma can be adopted to remove polyvinyl alcohol (PVA) based size from the textilefabrics. Plasma technology involves use of either O2/He plasma or Air/He instead of toxic chemicals and hot waters inconventional process. The said innovative techniques break the chains of polyvinyl alcohol making them smaller andmore soluble.

    Plasma treatment can impart anti-felting, improved dyeability and wetting properties in wool fiber. It reduces thecurling effect by etching off the exocuticle that contains the disulfide linkages which increase cross linking andcontribute towards shrinkage. This procedure also enhances wettability by etching off the hydrophobic epicuticle andintroducing surface polar groups.

    Coloration of textiles can be markedly improved by plasma treatments and effect can be observed on both syntheticand natural fibers. The effect of plasma treatment over conventional process are mainly in wettability and capillarityimprovement, enhancement of surface area, reduction of external crystallinity and creation of reactive sites on the fiberstructure. Plasma treatment on cotton in presence of air or argon gas increases its water absorbency which in turnincrease both the rate of dyeing and the dye uptake.The dye exhaustion rate of plasma treated wool has been shown toincrease by nearly 50% (figure 3).

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  • Figure 3: Dyeability of plasma treated wool fiber

    In the synthetic fibers, plasma cause etching of the fiber and introduction of polar groups leading to dyeabilityimprovement. This has been evaluated through in situ polymerization of polyester, polyamide and polypropylenefabrics. Plasma-induced surface modification of micro denier polyester imparted cationic dyeability on polyester fiber.This technique can lead to a continuous flow system, low energy consumption, and more environmentally friendlyconsumption. Polyamide (nylon6) fabrics have been treated with tetrafluoromethane low temperature plasma, dyed withacid and dispersed dyes and results in higher dye exhaustion.

    In the finishing process plasma application leads to produce various functional effects (table 2). The surface coatingresulted on plasma treatment is shown through scanning electron microscope of cotton fiber (figure -4).

    Table 2 Various application of plasma in textile finishing

    Application Material TreatmentHydrophilic finish PP, PET, PE Oxygen plasmaHydrophobic finish Cotton, P-C blend AiSiloxane plasmaAntistatic finish Rayon, PET Plasma consisting of dimethylsilaneReduced felting Wool Oxygen plasmaCrease resistance Wool, cotton Nitrogen plasmaImproved capillarity Wool, cotton Oxygen plasmaUV protection Cotton/PET HMDSO plasmaFlame retardancy PAN, Cotton, Rayon Plasma containing phosphorus

    Figure 4: SEM showing polymer layers on cotton fiber through plasma

    The important significance of plasma technology in textile wet processing can be summarized as: It is applicable to most of textile materials for surface treatment. Optimization of surface properties of textile materials without any alternation of the inherent proper ties of the

    textile materials. It is dry textile treatment processing without any expenses on effluent treatment. It is a green process without generation of chemicals, solvents or harmful substances. The consumption of chemicals is very low due to the physical process. It is applied for different kinds of textile treatment to generate more novel products to satisfy customer's need

    and requirement. It is simple process which could be easily automated and perfect parameter control.

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  • 4 Application of radio frequency [15-17]

    The temperature of any material can be increased either by direct or indirect heating. The conventional process, whereheat is generated remotely and then transferred to the material by conduction, convection and radiation, are all indirectheating. Direct method of heating are those where the heat is generated within the material itself, and radio frequency(RF) is one of the example. This method of heating is referred to as dielectric heating. The frequency of RF fieldsgenerated by devices of dielectric heating, are centered on 13.56 MHz and 27.12 MHz. RF drying has evolved as animportant step in search for the perfect drying method. The RF generator creates an alternating electric field betweentwo electrodes (figure 5a). The material to be dried is conveyed between the electrodes, where the alternating energycauses polar molecules in the water to continuously re-orient themselves to face opposite poles (figure 5b). Thefriction of this movement causes the water molecule in the material to rapidly heat throughout the materials entiremass, thus drying the material. RF drying generates thermal energy from within the product, there is no external source.This results in selective and endogenous drying, where evaporation rate is directly proposal to the amount ofelectromagnetic energy supplied. The use of RF energy as a means of drying is highly efficient as only the watermolecules excited during the process and energy is not dissipated throughout the mass of the product thereby avoidingunnecessary heating. Furthermore, heating is self regulating. The important advantages of RF dryer can be summarizedas:

    Reduction in energy consumption and environment friendly. Common unit to dry range of products. No dye migration, contamination and radiated heat loss. Ease of operation and uniformity of quality. Improved product quality and workplace environment. Immediate heating i.e. no start up and shut down time required.

    (a) (b)

    Figure 5: (a) Principle of RF generation, (b) RF dryer.

    5. Application of infrared in textile wet processing [18-20]

    Infrared (IR) is radiant energy, more accurately, it is electromagnetic radiation. IR is energy emitted by any object,which has a temperature above absolute zero (i.e. 0K or -2730C) (figure 6a). IR is produced as continuous band ofwavelength in the range of 0.8 microns to 1 mm. As temperature is increased, the intensity of IR radiation increasesconsiderably. For effective heating of the products with IR it is important that the temperature of the IR emitter issignificantly higher than that of the product, so that there is not energy flow to the product. IR for process of heatingdivided into three bands, namely, short (< 2 microns), medium (between 2 and 4 microns) and long (> 4 microns).

    In case of textile material drying process, a combined system, namely, IR and convection or air provides the removalof evaporated water (figure 6b).

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  • (a) (b)

    Figure 6: (a) Principle of IR generation, (b) IR dryer.

    The important advantages of IR techniques in comparison to conventional curing and drying are: Distributes even heat throughout the material Provide well controlled and low intensity heat Intensity of heat can be varied quickly as per the products Lower energy cost and less space required

    6. Application of supercritical fluid [21-23]

    Supercritical fluid (SCF) is defined as subtract above its critical pressure and its critical parameters could change. Thestudy of SCF began in the 19th century, but it was only in the 20th century when the advantages of SCF for chemicalprocesses began to realized. SCF has unique properties that may enhance many types of chemical process operations.An additional advantage of using SCF is that it solve effluent problem to large extent. Super critical carbon dioxide isby far the most widely used SCF, is relatively cheap, non-toxic, and non flammable and has zero ozone depletionpotential (figure 7a). The important properties of super critical carbon dioxide are reported in table-3.

    Table 3 Significant properties of super critical carbon dioxide

    Diffusivity Gas > SCF > LiquidViscosity Gas < SCF < LiquidSurface tension Gas = SCF < LiquidDensity Gas < SCF < or = Liquid

    In the textile wet processing, SCF is mainly tried for the dyeing of hydrophobic fibers which in conventional processrequired very high temperature (130-140oC). Application of the said techniques for wool, cotton and polyamidefibers, creating problems because of the polar nature of dyestuffs used for them. Supercritical dyeing (SCD) involvedonly three components, namely, dyestuff powder, subtract and SCF (mainly carbon dioxide). In the actual processdyestuff powder dissolved in SCF, transferred to fiber surface and finally diffused in the fiber structure (figure 7b).Energy comparison was made for dyeing of polyester fiber with disperse dye using conventional and SCF CO2 wasreported in table-4.

    Table 4 Comparison of energy requirements (KJ)

    Process Conventional SCF CO2Pretreatment 4,555 4,555Dyeing 45,250 30,625Post treatment 3,800 0Total 53,605 34,180Energy saving 49 %

    Actual results vary by country and by dyeing equipments

    The important advantages of SCF CO2 can be summarized as Eliminates water and effluent problems. No need of drying after dyeing process Excellent dyeing results in terms of evenness and fastness properties SCF can be recycled and used in subsequent process. Reduction in energy consumption (about 40-50 %) and air pollution

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  • (a) (b)

    Figure: 7 (a) SCF CO2, (b) CO2 dyeing of PET

    Despite outstanding successes in this area and great potential, there is a lack of understanding of the importance ofdyestuffs solubility in supercritical carbon dioxide to dyeing processes. In addition, there has been little study on thekinetics of dyeing process or supercritical carbon dioxide flow characteristics in porous (textile) materials.

    7. Application of electrochemical technique [24-28]

    Traditionally, electrochemical (EC) techniques have been used for the synthesis of compound or metal recoverytreatment. But more recently, a wide range of other applications have been proposed. Some of them are developed tosolve several technical problems of textile industry. One of the interesting use of EC technique is in the bleaching ofcotton fibers and the bleaching of finished denim fabrics. In order to achieve the visual effect in jeans, the generation ofin situ of hypochlorite by EC reaction has been proposed instead of its addition.

    Their application in sulphur and vat color dyeing processes is also interesting. In this case, dyes are reduced bymeans of EC reaction (instead of sodium dithionite). In this way process becomes cleaner as no addition of chemicalreagents.

    Although EC methods play an important role in different textile processes listed above, their wider range ofapplications are related to color removal in waste water treatment, in particular, in the degradation of non biodegradabledyes. In general, EC methods are cleaner than physicochemical and membranes technologies because the use theelectron as unique reagent and they do not produce solid sludge.

    EC techniques also used to produce smart textiles by obtaining functionalized fabrics. These textiles, with specificproperties can be obtained using EC in the synthesis of conductive polymers, especially conductive fibers. In thiscategory they used, to obtain a stable conducting materials as product, grafting or inserting organic compounds tomodify based fiber and super critical treatment of textile through complex reaction between a metal and chemicalstructure of fiber.

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    Materials and processes for energy: communicating current research and technological developments (A. Mndez-Vilas, Ed.)____________________________________________________________________________________________________

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