**Techno-economic estimation of dyeing PA6 fabric**

The cost of energy is continuously increasing and is becoming a significant share of total cost of processing textiles. Lowering the dyeing temperature brings down the energy requirements which can be done by employing some dyebath additives or by giving some pretreatment to the material to be dyed, during which improvement of the dyeing behaviour of the fibre can be achieved (6). Pooling of techno-economic knowledge across research and application would improve the productivity when faced by lower cost competition (7). The intention is to achieve lower machinery cost, less chemicals and energy consumption, easier wash-off, better reproducibility and rapid as well as easier technical changes for higher value-added products. Attempts are undertaken to perform a comparative techno-economic investigation of two dyeing methods of polyamide fabrics; a conventional dyeing method normally as applied in industry versus a proposed modified one which depends on the usefulness of chemical pretreatment of polyamide fabric with acetaldehyde or a redox system at ambient conditions. The economic aspects of the dyeing processes are pursued, where the variables are brought together, to determine the production costs. The pretreatment of polyamide 6 with either reagent reveals some improvements in dyeing characteristics. This is reflected on the possibility of saving considerable amount of energy, shortening the time of dyeing, increasing the production rate and so lowering the total cost of the dyeing process as well as reducing the environmental impacts (8). Conventional exhaust dyeing of PA-6 fabric was performed at nearly the boil (9). Pretreated PA-6 wet fabrics with either acetaldhyde or glyoxal/H2O2 were exhaust dyed at different temperatures (60o, 70 and 80oC). The pH value of the dyeing bath was adjusted at 4.5 by adding acetic acid (about 1 ml/ litre) and using a liq. ratio of 1:10 (1, 10). The process is wet on wet. Pretreated PA-6 fabrics with acetaldehyde had exhausted the same amount attained by conventional dyeing method (0.92g dye/100g fibre) after about 45 min at 80oC (10). Pretreated PA-6 fabric with the redox system exhausts almost completely the dye from the bath at different temperatures (60o, 70o and 80oC) after about 40, 20 and 10 min respectively (8).

#### **Cost of chemicals and water in conventional and modified dyeing method**

Table 2.5 illustrates comparatively the cost of chemicals, dyes and water consumed in the investigated dyeing processes. PA-6 fabric pretreated with glyoxal/H2O2 attained the lowest cost and on the other hand acetaldehyde pretreated PA-6 fabric is relatively the highest. Specifically, this held true in the element of water consumption.


Table 2.5. The costs of chemicals and water consumed (LE/kg fabric)

#### **Steam consumption**

The amount of steam required to offer necessary heat to the wet processes can be estimated from the following equation (11):

 $\mathbf{Q}$ -W. Cp. $\mathbf{At}$ 

Where Q is the heat required, Cp is the specific heat of matter to be warmed up, W is the weight of the material to be warmed up; and Δt is the temperature difference involved, An initial temperature of 20oC is assumed for water. The heat loss by radiation and by other means can be calculated by assuming that the total heat is twice the heat required for heating up (12). Table 2.6 shows the cost of heat energy per kg of fabric. It is clear that the glyoxal / H2O2 pretreated fabric attained the lowest value.


Assuming that 106 Kcal energy = 60 LE

Table 2.6. Cost of energy consumption for the different dyeing process in (LE)
