Reducing the liquor ratio in exhaust dyeing of knitwear

 

Technical Description

 

In most dyeing methods, water is an essential medium for dyeing. The liquor ratio in an exhaust dyebath is the ratio of the weight of the dry material being dyed to the water weight of the dyebath. For example, a liquor ratio of 1:10 means that 1 kg fibre is dyed in 10 L water.

The liquor ratio can differ greatly between machine types and their mechanisms. Using low-liquor ratio equipment, the weight of water used to dye a given weight of goods can be efficiently reduced. To have an efficient dyeing operation, dyer has the responsibility of controlling the system variables where liquor ratio is a significant factor.

Textile material to liquor ratio has the highest impact on water consumption in exhaust dyeing. If the liquor ratio is reduced from 1:8 to 1:4, it results in a 50% water and a ~30% energy saving. Low liquor ratio exhaust dyeing machines reduce water consumption in dyeing of textile materials and reduce the usage of salt and alkali as these are added in grams per litre of the total liquor. Ensuring low liquor ratio in knitwear dyeing process with modern machines has a daily saving potential of 1900-2000 m3 water for big units.

Some advantages of using low-liquor ratio dyeing equipment are:

  • reduction in water consumption in dyeing
  • major energy savings 
  • reduction in air pollution due to lower steam consumption
  • improved dye fixation 

To reap the benefits, there has been a drive to develop and use machines that use less water and Conveyer Drive High-Pressure Rapid Dyeing Machine (Figure 1) is a fruit of this venture. This type of machine is applicable to knitwear processing industries where machines are used for discontinuous, i.e. batch, scouring, bleaching, and dyeing.

The operation principle of the dyeing is circulation both of fabric in a conventional dyeing machine and the dye liquid which is pressurized by a pump and it passes through a heat exchanger into a nozzle which generates a jetting or overflowing liquid flow. This flow then returns to a fabric accumulation tank of a dyeing machine. Fabric is driven by a fabric roller into the nozzle so that the fabric is driven by the force induced by the jetting or overflowing liquid flow from the nozzle to circulate.

Figure 1: Diagram of Conveyer Drive High-Pressure Rapid Dyeing machine and Conventional Soak Dyeing machine. Diagram source: https://acmemach.com

On the other hand, Conveyer Drive High-Pressure Rapid Dyeing Machine (Figure 1, on the left and Figure 2) uses a conveyer drive system to move the fabric forward, so water is only needed for pipe circulation of fabric and its dye pick up, whereas in conventional soak dyeing machine, fabric swims in water (Figure 1, on the right and Figure 2). As a result, the Conveyer Drive High-Pressure Rapid Dyeing machine can work with lower liquor ratio. The conveyer drive ensures constant speed of the fabric flow which improves dyeing evenness with less water and low tension on the fabric.

Figure 2: The reason for low liquor ratio in conveyer drive machine compared to the conventional ones. Image courtesy: acmemach.com

Instead of bath rinsing, this machine has shower rinsing system which ensures lower use of water in rinsing processes.

Moreover, the dyeing is controlled by cycle times which ensures different dyeing time for fabrics of different lengths and high pressure ensures good penetration of dyes into the fabrics, ultimately helping in conserving water, chemicals and energy.

Achieved Environmental Benefits

  • Water conservation
  • Chemical saving
  • Reduction in electrical energy
  • Reduction in greenhouse gases

Operational Data

Liquor ratio depends on the water pick up rate of fabrics. So, the nature of fabric also plays a role in determining the liquor ratio.

Dyeing process is controlled either by cycle times or by clock time and total dyeing process time depends on the fabric length or speed of fabrics circulation. Shorter fabric length or faster fabric speed will reduce the total process time.

In the process, fabrics absorb little water, so the fabric has low tension and becomes free of edge curling. Therefore, edges are not needed to be stitched for spandex and tricot fabrics before dyeing.

The implementation of this measure can significantly reduce water and steam consumption in knit dyeing. The water in the range of 655,500-690,000 m3/year can be reduced. Its value could be in the range of USD 100,000 – 200,000 annually. The potential of savings can vary from industry to industry.

Cross Media Effects 

None identified.

Technical Considerations relevant to Applicability

This measure is applicable to all those knitwear dyeing industries where exhaust dyeing machines are used.

This requires investment of machines in the process and technical human resource.

The implementation cost will be USD 1,900,000 for 38 machines (as an example) and the annual O&M cost will be USD 781,698.

Economics

Capital cost = USD 6,000,000 (20 machines)

Annual saving = USD 1,728,400

Annual O&M cost = USD 900,000

Simple payback period = 7.24 years 

Additional benefit will be the decrease in treatment cost of wastewater treatment plant. 

Driving Force for Implementation

  • Financial benefits
  • Resource conservation
  • Sustainability of textile business

Reference Industry

  • Color City Limited, DBL Group, Kashimpur, Bangladesh.
  • Comfit Knit composite Ltd, Gorai, Mirzapur, Tangail, Bangladesh.

Reference


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