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| Issue date:14/10/2009 |
| ATA Journal for Asia on Textile & Apparel - Oct 2009 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Stephan Kehry |
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| With mounting expectation of protecting our planet, textile machinery manufacturers, such as Benninger, have emphasised on resource management in the area of wet finishing |
There is no doubt that the increased environmental awareness all around the world has its roots in commercial considerations: Polluted rivers keep tourists away, dried- out fields hardly attract investors, over-salted lakes do not help to accelerate development aid. Whether the motivating factors are commercial by nature or not, textile finishing has a special responsibility as a “water consuming industry”. It is now time to meet this responsibility. The available technology is useful and makes good commercial sense.
Growing water shortages
Only around 10% of total water consumption is accounted for by direct human needs in the world. The lion’s share of 70% is used in agriculture, and the remaining 20% is used in industry. Regional variations give an indication of the degree of industrialisation of the different national economies. For example, China uses 80% of its water to irrigate fields, whereas in Europe half of the water is pumped into factories. All this is in stark contrast to the geographical distribution of the water resource. While China is home to around 20% of the world’s population, it only has access to around 5% of world’s freshwater supplies. Conflicts in this area are already (pre-)programmed.
A large amount of our drinking water is already used during the production of food. For example, it takes 5,000 litres of water to produce just one kilogram of meat for human consumption. But also vegetarians have an impact: The production of both one kilogram of bread or one litre of orange juice entails the use of 1,000 litres of water. In the light of this, daily water consumption of an average household in Switzerland seems to be rather modest at approximately 162 litres per head (including showers, toilet flushing, washing machine, cooking, drinking and washing).
Water and energy always have been the two main components in the production of textile fabrics.
The majority of it goes into the production of the natural fibres (e.g. cotton, wool or linen), the plants need to be watered throughout their entire lifecycle, and the sheep need to be fed and watered. In the case of cotton, this can sum up to as much as 20,000 litres of water per kilogram (in Sudan as much as 29,000L).
Against the background of worsening shortages of global resources, these are alarming figures, but they can be dramatically reduced through innovative irrigation techniques (plantation cultivation / “drip irrigation”). With these new techniques, the production of one kilogram of cotton still requires between 7,000 and 9,000 litres of water, but the downward trend at least gives some cause for hope.
Water is also crucial in the Middle East. The intensive water consumption of the cotton growing industry has strained relations between Turkey, Iraq, Israel and Syria. By contrast, sophisticated irrigation of cotton fields is up to now uncommon in West African countries.
The consequences of irrigation lead the world to salinisation, erosion of the soils, the depletion of water reserves and contamination of groundwater. In view of the expected shortages of the increasingly valuable resource of water, it seems inevitable that the amount of water used to grow cotton will have to be reduced rather in the short term than long term, especially as more and more fields will have to be irrigated as a consequence of climate change.
Besides irrigation of cotton fields, wet finishing is the undisputed number two in the list of resource waste. Starting with sizing and desizing, washing, bleaching, mercerising, then dyeing and printing, perhaps even coating, all along in these processes there is always a washing cycle involved, which may require as much as 20 litre of water per each cycle per kilogram of material. As a result, water consumption may add to as much as 200 litre of water during the wet finishing processes for just one kilogram of grey cotton. By the time a standard men’s shirt is tailored and displayed in a shop, more than 2,000 litres of water have gone into its production and processing (basis: 100% CO, 125 g/m2).
Minimising resources
The scenario described above clearly shows that measures are needed. Benninger has developed a new concept to address the problem on two levels:
MinimisationRecycling
The minimisation of resources consumed is based on the optimisation of processes and machinery. Improved processes and modified recipes lead to improved quality and reproducibility and thus a lower consumption (of resources). The change from exhaust dyeing to a continuous process illustrates a good example.
Based on the optimised consumption, water, energy and other resources then can be recycled. The result is a form of textile production that is both commercially and environmentally sustainable.
Enveloped steam valves minimise energy loss |
Even the longest journey starts with a first step. In the case of resource management, a first step may also be a small one. An effective way to save energy in textile finishing is to enclose steam valves in a housing (pictured). The costs are minimal to allow a short payback period of less than eight months.
A step further would be to enclose an entire cylinder drier in a housing. The insulation not only reduces dissipation, but at the same time the drying efficiency is increased. As a result, the same performance can be achieved with less heating cylinders and thus reduces the total investment outlays for the cylinder dryer. The steam savings add up to more than 15%, therefore the total costs can be recovered in less than two years.
15% cost savings via counter-flow optimisation
More cost savings can be achieved along the liquor path. The magic word here is “counter-flow”: The grey fabric runs through the washing compartments from the entry to the exit, the clean water is passed through the plant from the rear to the front. This means that the cleanest fabric comes into contact with the cleanest washing liquor. By rigorously applying this counter-flow principle, it is possible to save both water and energy.
An example is cold pad batch (CPB) dye washing. During dyeing with the CPB method, a padder with nip-controlled rollers (so-called “swimming”) is used to apply dyestuff to the fabric in a defined manner. After a dwelling time that varies depending on the dyestuff, the excess dyestuff needs to be washed out. A distinction is made between the following processes:
1. Rinsing out dyestuff from the surface
2. Soaping (here, the dyes are moved from the core of the fibres to the surface)
3. Neutralization
4. Washing out of the salts produced during neutralization
This process normally requires 20 litres of water and 1.6kg of steam per kilogram of fabric.
In a first step the counter-flow principle can be rigorously applied to the individual processes. The water used to wash out the salts in the rear compartment is directed around the soaping compartments and is then used again when the surface dyestuffs are rinsed out. As the level of soil is low in the rear part of the washing range, this liquor can be used effectively to wash in the front. In addition, less heating of the water is required for the soaping process, which saves energy in the form of steam.
Less heating of the water is required for the soaping process, which saves energy in the form of steam |
The consumption of resources in this process amounts to 9 litres of water and 0.95 kg of steam for one kilo of fabric. Less energy is required in the downstream drying process, as the temperature is already 40°C higher than in conventional processes.
Following savings can be achieved on the basis of average costs:
Water savings 55%Steam savings 41%Overall cost savings 15%
It is clearly possible to improve the internal cost without new considerable investment. The key to these improvements is process know-how and modern open width plant concepts.
Innovative concepts in handling knitted goods
The choice of the correct plant concept also plays an important role.
In the past, knitwear finishing was largely performed in rope form. The fabric tension and curling of the edges made a continuous handling of these goods impossible and therefore an efficient handling impossible. New drive concepts and adapted fabric guidance systems permit companies for new investments to revert to an open width fabric guidance system based on the Trikoflex principle from Benninger.
The quality benefits of open width treatment are felt in texture and consistency of the surface of the fabric. While jet treatment is a mechanical process, continuous treatment in the Trikoflex washing modules is a gentler method. No creasing or abrasive surface damage (pilling) is expected. Additionally, the individual processes are separated from each other, and with the modern plant controls a clear overview is maintained at all times. Apart from an improved user guidance, this results in an excellent reproducibility, improved quality and reduced subsequent costs.
Major savings of continuous treatment, however, are found in the variable costs.
While the exhaustion dyeing process requires between 70-90 litres on average to dye and wash one kilogram of knitwear, a modern Trikoflex plant uses 18 litres for the same process, which corresponds to a saving of nearly 75%. It also saves in terms of energy consumption. With the facility, continuous treatment uses 4500 kJ per kilo of dyed knit goods compared to the previous 19,100 kJ.
Further savings are possible with water recovery and recirculation.
Water flow in textile finishing plants
A wastewater treatment and a recovery system for selected types of wastewater from textile processes consists of two components: ultrafiltration and reverse osmosis.
The difference between the two systems is largely in the pore size of the filter systems used. While ultrafiltration retains particles of less than 10 to 100 nm (this corresponds to 0.00001 to 0.0001mm), reverse osmosis filters out particles less than 1 nm in size. In comparison, cigarette smoke particles are in the order of magnitude of around 1000 nm, and a human hair is 100,000 nm thick.
Wastewater from the washing process is directed to the ultrafiltration stage. This cleaning stage is made up of tiny, self-cleaning ceramic tubes, which are highly resistant to chemicals and have a service life of around 10 years. High temperatures are no problem for this module. This process leads to a reduction of the chemical oxygen demand (COD) — which acts as an indicator of organic substances in the bath — of 63% during desizing, 80% during bleaching and still 48% during reactive dyeing.
Wastewater treatment system suggested by Benninger
Wastewater samples show effects of the treatment system (Pictured from left: the original wastewater in sample 1 is cleaned in subsequent samples on the right) | |
After the ultrafiltration stage, the wastewater passes through the reverse osmosis stage. Here, the remaining dyes and dissolved salts are separated from the remaining liquor. This effect is the result of a spirally rolled-up polymer, which operates at a pressure of 25 bar. The service life here is three years, and the component is cleaned according to the “Cleaning in Place” method (CIP).
In this method, the surfaces of the system in contact with products are cleaned without major disassembly work. The effect of reverse osmosis can be seen in the difference between sample 2 and sample 3 in water samples shown above. Sample 4 shows normal tap water as a direct comparison.
In order to protect the reverse osmosis process, the hot water from the washing baths is cooled in a heat exchanger after the ultrafiltration stage. After the reverse osmosis, the water can then be heated using this energy. As a result, 70% of the employed energy can be reused. Depending on the level of soil, up to 85% of the water can be recovered.
Restrictions only apply to substances that would cause the filters to block up. These include silicates if the pH value is less than 7, printing paste, silicone oil, fluorocarbons and a few other particular specialities, which can vary from manufacturer to manufacturer and need to be verified for each individual chemical.
Aiming at zero discharge
Given the current pricing pressure in the textile industry, regional and national environmental regulations can often make all the difference between commercial success and failure. Meeting waste disposal requirements brings potential financial benefits.
By combining ultrafiltration and reverse osmosis, plants can operate without any wastewater altogether, a concept known as “Zero Discharge”.
The highly concentrated contaminants accumulated from both filtration stages are collected and transferred to a solidification process. The resulting solids usually have an organic basis and often have an excellent calorific value. This is an opportunity to improve the image of textile finishing and, at the same time, reduce the potential conflict between a company’s own production and the requirements of the community.
How does this filtration impact on cost calculations?
In the example of a washing process as mentioned earlier on, the amount of freshwater is reduced to just 1.5 litres per treated kilogram of fabric. This corresponds to a saving of nearly 92%. More impressive is the potential savings with the investment in a modern open width installation for knitted goods in combination with ultrafiltration and reverse osmosis. Instead of the 70-90 litres consumption in a discontinued process, a modern and more efficient Trikoflex plant uses 4 litres only, achieving a saving of 94%.
Source: Benninger AG
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Copyright © Adsale Publishing Limited. Credit goes to Adsale Industry Portal when used.
We reserve the right to take legal action against any party who reprint any part of this article without acknowledgement. For enquiries, please contact the Editorial Department. |
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