An alternative to the discharge of exhausted batch processing baths is to save the bath, analyze it for remaining chemical/dye quantities, and then reconstitute the bath for further use. In this manner, a single processing bath can be reused many times before being discharged. This will significantly reduce the volume of and pollutant concentration in your plants effluent. For example, at one facility wastewater flow was reduced by 34% while COD was reduced by 33% due to a dyebath reconstitution program.
This is a very simple technique to implement and can be applied to both dyebaths and bleach baths. With dyebaths, the potential for significant auxiliary chemical savings makes dyebath reconstitution an economically attractive alternative to conventional batch dyeing. Recent research and applications have shown that dyebath reconstitution is applicable to a large amount of the batch dyeing being performed today, and not only to the repeat shade dyeing applications where it has been practiced in the past. With bleach baths, particularly with synthetic fibers, many cycles of reuse can result in significant savings.
The reconstitution process using dyebaths as an example, can be thought of as consisting of four parts:
Two basic alternatives are available for saving the exhausted dyebath. The dyebath can be pumped to a holding tank, while the product is rinsed in the same machine in which it was dyed. Then as soon as the rinsed product is removed, the dyebath can be returned to the dye machine for the next dyeing cycle.
Alternatively, the dyed product can be pulled from the exhausted dyebath and moved to another machine for rinsing. This alternative eliminates the need for holding tanks and pumps, but requires additional product handling and a spare machine to use for rinsing.
In either scheme the dyebath should be cooled with a non-contact cooling water system. Using contact cooling water will dilute the dyebath, thus increasing the quantities of chemicals required for reconstitution. Alternatively bump-and-run dyeing techniques can be used.
Most auxiliary chemicals do not exhaust to an appreciable degree during the dyeing process. Thus, the make-up quantity is about 10 percent, due to carryoff on the product. Certain auxiliaries, however, will exhaust or be partially depleted during dyeing. Generally, it is sufficient to estimate the degree of exhaustion and the quantity needed to replenish the bath.
The unexhausted dyestuffs, however, must be analyzed to determine the exact quantities remaining in the dyebath. This is to ensure that the proper shade is achieved in the next dyeing cycle. The analysis is performed with a spectrophotometer. Procedures for performing this analysis are presented in Reference 2, along with a computer program which will simplify calculating the required dyestuff additions.
Reconstituting the bath consists of adding back the quantities of water, auxiliary chemicals, and dyestuffs needed for the next dyeing cycle. Water is added to replace any lost through evaporation or in the product. Auxiliary chemicals are also added in proportion to the amount of water added. Any auxiliary chemicals that exhaust during dyeing are added to make-up for such exhaustion losses. In general, these quantities can be estimated and do not require exact analysis. Dyestuff add quantities are determined by subtracting the quantities present in the exhausted dyebath (as determined above) from the recipe quantities for the next shade to be dyed.
The major difference in reusing a dyebath is that the starting temperature is generally higher than that of fresh water. This starting temperature should be tested and evaluated so that spotting and levelness problems do not occur in the reuse dyeings. In general, the loss of heat during storage, and the cooling due to make-up water are sufficient to drop the dyebath temperature to a safe level. This technique has been used on a variety of fibers and dyestuffs.
Examples of full scale applications include:
| Product Woven Fabric Woven Fabric Woven Fabric Ladies' Hosery Carpet Yarn |
Fiber Nomex Cotton Cotton Nylon Nylon Acrylic |
Dyestuff Basic Direct Vat/Sulfur Dispers Acid Basic |
Dye Machines Jet Reel Jig Paddle/Drum Beck Package |
Textile dyeing is a complicated process and consequently there are many factors that may affect the implementation of dyebath reconstitution. Procedures for evaluating the potential for starting a dyebath reconstitution program at your facility as well as procedures for establishing such a program are detailed in Reference 2.
Only a limited amount of equipment is needed to implement a dyebath reconstitution system. This includes: storage tanks, necessary pumps, piping and valves and laboratory equipment. Capital costs for a reconstitution system can be estimated based on the unit costs presented below:
| Spectrophotometer Miscellellaneous Labware Desk Computer Holding Tank (2,000 gal.) Pump, Controls (200 gpm) Piping, Valves, Screen (per tank) |
$3,000 $1,000 $5,000 $6,000 $4,000 $6,000 |
Larger tanks, pumps and piping will be required for large dyebecks, such as in the carpet finishing sector of the industry.
Operating costs for reconstitution systems include the power for dyebath pumping, pump maintenance costs, and miscellaneous labware costs. No additional manpower requirements are included for dyebath analysis, since this takes only a few minutes per cycle. A conservative estimate of annual operating costs is $2,000 per beck.
The estimated cost of implementing a dyebath reconstitution program at a typical dyehouse having five 1,500 gallon becks would be:
|
Capital Costs Spectrophotometer Miscellaneous Labware Desk Computer Holding Tank (2,000 gal.) Pump, Controls (200 gpm) Piping, Valves, Screen (per tank) Total Capital Cost Yearly Operating Costs |
$3,000 $1,000 $5,000 $6,000 $4,000 $6,000 $89,000 $10,000 |
The actual costs will be very site specific and will depend on such factors as the volume of dye liquor to be handled and the physical layout of the production lines.
The economics of dyebath reconstitution will vary depending on a number of factors including the value of the auxiliary chemicals used, the type of dyestuffs, and liquor ratios used in dyeing. Based on the example given above the annual savings for the model facility would be:
| Water Use (30,000 gpd) Sewer Use (30,000 gpd) Energy (22.5 mm Btu/day) Auxiliary Chemicals Total Savings Net Yearly Savings |
$ 3,750 $ 3,750 $ 22,500 $ 75,000 $ 105,000 $ 95,000 |
This would result in a pay back period of only 11 months. After this initial period the dyebath reconstitution system would save $95,000 per year in operating costs.
Use of this technique at a number of plants has resulted in significant savings. Reduction in auxiliary chemical, dye and water/sewer costs have been reduced by as much as 60%, 19%, and 43%, respectively. As can be seen, dye-bath reconstitution is a very simple and economical method which can be used to reduce the cost of raw materials, water and wastewater treatment.
1. Bergenthal, J. F. and A. J. Tawa. Investigation of Textile Dyebath Reconstitution and Reuse: Volume I - Technical report. EPA-600/2-84-lOOa. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina. 1984.
2. Bergenthal, J. F. and A. J. Tawa. Investigation of Textile Dyebath Reconstitution and Reuse: Volume II - Operations Manual. EPA-600/2-84-lOOb U.S. Environmental Protection Agency, Research Triangle Park, North Carolina. 1984.
