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«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»

-- [ Page 55 ] --

Reference literature [308, Spain, 2002], [180, Spain, 2001].

4.6.19 Equipment optimisation in batch dyeing Description Textile equipment manufacturers are becoming more and more aware of the need to conserve water, chemicals and energy. These are key objectives that new machine technologies help to achieve. Moreover, optimisation of equipment benefits not only environmental aspects, but also process economics.

Liquor ratio is one of the parameters that influence the environmental performance of the batch dyeing processes and, recently, a distinct trend has developed among equipment manufacturers toward reducing bath ratios. Terms like “low” and “ultra-low” liquor ratio are commonly used by equipment manufacturers to define the characteristics of batch dyeing machines (see Section 2.7.8.2 for the definition of these terms). Moreover, an outstanding feature of modern machines is that they can be operated at approximately constant liquor ratio whilst being loaded at a level as low as 60 % of their nominal capacity (or even 30 % of their nominal capacity with yarn dyeing machines). Even small size lots can therefore be dyed at optimal/ nominal liquor ratio.

This is particularly important especially for commission companies, which need high production flexibility.

Low bath-ratio dyeing machines conserve chemicals as well as water and energy and also achieve higher fixation efficiency. However, as already explained in other parts of this document (see Section 4.1.4), the total water consumption is determined not only by the liquor ratio of the dyeing step, but also by the rinse and wash processes.

The correlation between bath ratio and total water use is not always exact and factors other than liquor ratio should be taken into account when assessing the environmental performance of a batch dyeing machine.

One important factor is the maximum cut-off between different batches and, in particular, the maximum separation between the exhausted dye bath and the rinsing water.

In some modern batch dyeing machines, instead of rinsing by overflow or by draining the bath and refilling the vessel with rinsing water, the textile material is rinsed in a continuous mode in a separate stream, thus avoiding cooling or dilution of the exhausted bath. In this way, the hot exhausted dye liquor and the rinsing waters are kept as separate streams, which allows them to be re-used or at least be treated separately and thermal energy recovered.

Moreover, various techniques can be applied in order to increase rinsing efficiency. When applicable, mechanical liquor extraction is a possible method for reducing the non-bound water retained by the fabric, which is otherwise carried over by the substrate to the next step.

Expression, suction and blowing air through the fabric are also all available techniques. Vacuum technology is the most efficient, but it is not applicable to all types of fabric and it consumes more energy than expression.

Textiles Industry 343 Chapter 4 Another factor affecting specific water and energy consumption in dyeing processes is the duration of the cycle. Short cycle times mean not only higher productivity, but also a reduction in electrical and thermal energy consumptions. Various techniques can be applied to reduce downtimes in the working cycles. These typically include pumped drain and fill options, charge tanks (which are used for the preparation of the liquor in parallel with other process operations), combined cooling and rinsing systems, etc. In the latter the cooling water is passed through the machine heat exchanger during the cooling step and is then fed directly into the jet as hot fresh rinse water. The quantity of rinsing water can be controlled, depending on the end temperature, the desired rate of cooling and, in some machines, also on the desired quality of rinsing.

Further reduction of the cycle times can be achieved by improving the textile/liquor contact to achieve homogenisation of the bath more rapidly (i.e. by shortening the transition time) after a change in operating conditions (e.g. alkali/dyestuff injection, temperature increase/decrease).

Additional common features of modern batch dyeing machines include:

· automated systems for chemicals/dyes dispensing and dyeing cycle control: this allows improvement of the efficiency and reproducibility of the process. In addition, overuse of chemicals, handling losses and equipment clean-up requirements are reduced · automatic controllers to facilitate liquor level and temperature measurement and control: where machines are equipped only with manual water control valves there is a potential for overfilling and unnecessary wastage of water during filling and rinsing operations. Spillage may also result from excessive boiling during the final stages of dyeing. Modern machines are fitted with process control equipment capable of accurately controlling the level of the liquor and the steam supply · indirect heating and cooling systems: indirect heating and cooling is now common practice in modern batch dyeing equipment to overcome dilution and spillage of water · hoods and doors: vapour losses can be significantly reduced by full closure of the machines.

Main achieved environmental benefits The technological optimisation of dyeing machines described above leads to improved environmental performance in terms of both consumption of resources (water, energy and chemicals) and water pollution. These aspects are summarised in the following table.





–  –  –

Notes:

(1) In particular, the full separation of exhausted hot bath from rinsing water is especially advantageous when dyeing under pH-controlled conditions. In this case, the hot exhausted bath can be directly re-used for making up the next batch because dyeing is started at high temperature.

Table 4.27: Environmental benefit associated with optimisation of batch dyeing machines Operational data Variable according to the type of equipment considered (more information can be found for the examples presented in Sections 4.

6.20 to 4.6.21.3).

Cross-media effects None believed likely.

Applicability Most of the principles described in this section are applicable to all types of batch dyeing equipment. Indeed, the extent to which the liquor ratio can be reduced depends on the type of substrate to be processed. Nevertheless, today equipment manufacturers can offer for each

–  –  –

substrate machines with increasingly reduced liquor ratios compared to conventional machines, whilst maintaining the same quality standard of the final product.

Examples of batch dyeing machines where the principles described are applied are reported in Sections 4.6.20 to 4.6.21.3.

Economics Variable according to the type of equipment considered (more information can be found for the examples presented in the crossed-referenced sections mentioned above).

Driving force for implementation High productivity and reproducibility still remain the main driving forces, followed by savings in water, chemicals and energy consumption.

Reference literature [176, VITO, 2001], [171, GuT, 2001], [177, Comm., 2001], [179, UBA, 2001], [116, MCS, 2001], [120, Brazzoli, 2001], [127, Loris Bellini, 2001].

4.6.20 Equipment optimisation applied to winch beck dyeing machines Description Winch beck dyeing machines are described in detail in Section 10.3.1.1. In many sectors of the textile industry they have been superseded by other types of equipment (e.g. jet), but winch dyeing still remains an important technology when dyeing in piece voluminous textiles such as carpets, upholstery, terry-towels and tubular fabrics. In these types of textiles the softness is determined by the bulk given during dyeing and the winch machine has the advantage of imposing low tension on the fabric while maintaing great mechanical action.

A number of technological improvements have been introduced in this dyeing technique,

principally in [171, GuT, 2001]:

· heating: the liquor in the early winches was usually heated by direct steam injection through a perforated pipe. This system provided both rapid heating and vigorous agitation in the beck, but the dilution effect has to be taken into account. Indirect heating/cooling is now more commonly used to overcome dilution and spilling water · liquor/goods contact: in modern winches both the liquor and the fabric are circulated to improve liquor/goods exchange. The carpet is cycled through the dye liquor, which is in turn continually pumped through a drum filter to remove the lint. This circulation system ensures a more rapid homogenisation of the bath and an even distribution of the colour throughout the carpet · liquor ratio: recent winch becks operate at liquor ratios that are significantly reduced compared to conventional machines. Moreover, an outstanding feature is that small batches can be dyed with approximately the same liquor ratio as for maximum load · rinsing: modern winches are designed to remove the carpet without dropping the bath and without cooling or diluting it with rinsing water. Thanks to the “hot-drawing-out system”, the carpet is automatically taken out of the beck and passed over a vacuum extractor which removes the non-bound water. The recovered liquor is diverted back to the dye bath. The carpet is then sprayed and passed over a second suction section where the rinsing water is collected.

346 Textiles Industry

Chapter 4

In addition to the above, modern winches are fitted with hoods to help maintain temperature and minimise losses. They are also equipped with automated dosing and process control systems for full control of temperature profile and chemicals injection during the dyeing process.

Figure 4.23: Supraflor Carpet Winch [171, GuT, 2001] Main achieved environmental benefits The features described above bring about substantial savings in terms of water, chemicals and energy consumption.

Reductions of 40 to 50 % in fresh water for the total dyeing process (up to 94 % savings in rinsing water) and 30 % in electricity consumption are claimed by the machines’ manufacturers [171, GuT, 2001].

The advanced concept applied in this type of winch beck is that the rinsing step is no longer carried out in batch, but rather in continuous mode in a separate section of the equipment without contact between the substrate and the bath. In this way there is no mixing between the rinsing water stream and the hot exhausted bath, which makes it possible to re-use both streams and to recover the thermal energy.

Operational data

Liquor ratios used in carpet winches are typically 1:30 or higher. Recent open-width winch becks operate at liquor ratios of 1:15 - 1:20, depending on the substrate type, loading and fabric construction. A new type of winch beck for open-width dyeing of carpet operates at liquor ratios ranging from 1:9.5 to 1:19 and is used successfully in several installations.

In textile dyeing, liquor ratios for conventional winch becks are typically in the region of 1:15 - 1:25. Recent textile winch becks have a nominal liquor ratio as low as 1:5 - 1:8 (depending on the type of substrate) [171, GuT, 2001].

Cross-media effects None believed likely.

Applicability New generation winch becks can replace old machines in all applications where the use of winch becks is still necessary (e.g. bulky textile such as carpets, upholstery, terry-towels).

–  –  –

The fundamentals of the new equipment cannot be retrofitted onto existing dyeing machines, they need to be replaced. However, some types of older machines can be upgraded with the special “hot-drawing-out system”.

Economics No data were made available.

Driving force for implementation High productivity and reproducibility still remain the main driving forces, followed by savings in water, chemicals and energy consumption.

Reference plants Many are in operation in textile finishing industries world-wide.

Examples are the Supraflor and the Novacarp Carpet Winch.

Reference literature [171, GuT, 2001] 4.6.21 Equipment optimisation applied to jet dyeing machines New concepts have been introduced in jet machines, which can significantly improve not only the productivity but also the environmental performance of the dyeing process for fabrics that are treated in rope form. Some examples of available techniques are presented in the following sections.

4.6.21.1 Airflow jet dyeing machines Description Jet dyeing is now a mature technology. There is, however, a fundamental innovation, which is the use of air, either in addition to or instead of water, as the motive force in driving the fabric rope (air-jet).

In the latest development, the fabric is moved by means of moisturised air, or a mixture of steam and air, in the total absence of liquid. The dyestuffs, chemicals and auxiliaries are injected into the gas stream. Liquor ratios of 1:2 may be reached for woven PES fabric, while 1:4.5 is the lowest limit achievable for woven cotton fabric with such air-jet machines.

The following figure shows that the bath level is always below the level of the processed textile.

The fabric no longer stays in contact with the liquor (the bath is below the basket holding the fabric in circulation). This means that the bath conditions can be changed without changing the process phase of the substrate (for example, cooling the bath while the substrate is still at high temperature for prolonged fixation, adding chemicals, interchanging the bath with another one).

–  –  –

Figure 4.24: Illustration of an airflow dyeing machine with indication of air circulation and injection of the bath [280, Germany, 2002] The bath-less dyeing operation is one of the main features of this machine, the other one is the separated circuit for liquor circulation without contacting the textile.

By contrast with conventional machines, during the whole rinsing process (see Figure 4.25), the bottom valve is open and the rinsing water, initially sprayed onto the fabric when it enters the jet, is immediately drained without additional contact with the fabric. Thus, rinsing is no longer a batch operation, but instead offers all the potential advantages of continuous processing (time saving, possibility of discharging the bath after high-temperature dyeing at 130 °C for optimum heat recovery, maximum cut-off between hot bath liquors and rinsing water, etc.).

Figure 4.25: Illustration of the rinsing step in an airflow dyeing machine with indication of the open valve to achieve continuous rinsing [280, Germany, 2002] Main achieved environmental benefits The extremely low liquor ratio and the continuous rinsing system results in a virtually non-stop

process with:

–  –  –

· less energy needed thanks to quicker heating/cooling and optimum heat recovery from hot exhausted dye liquors · reduced consumption of those chemicals (e.g. salt) for which dosage is based on the amount of dye bath (about 40 % [179, UBA, 2001]) · less water (up to 50 % water savings are achieved compared to conventional machines operating at L.R. of 1:8 – 1:12) [179, UBA, 2001].

In addition, the release of the exhausted dye bath at process temperature and without dilution by rinsing water opens the way for a very efficient and cost-effective degradation of the remaining dyes and other difficult-to-treat auxiliaries, using the advanced radical treatment process “ETP & ETF” (Enhanced Thermal Fenton reaction & Enhanced Photo Fenton reaction).

Operational data Table 4.28 shows specific input data ranges for cotton dyeing with reactive dyestuffs in a conventional jet operating at L.R. of 1:8 – 1:12 and in the airflow machine described above. The data are derived from measurements taken at production sites.

It is worth adding that the model of airflow described in this section is also designed to maintain its low liquor ratio even with the machine well underloaded.

–  –  –

Table 4.28: Comparison of specific input data for cotton dyeing with reactive dyestuffs in a conventional jet (L.

R. 1:8 – 1:12) and in the airflow machine described above Cross-media effects None believed likely.



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