|Cleaner Production Measures in Printing and Finishing Operations||USA||1992||Full scale|
MANUFACTURE OF TEXTILES # 40
Cranston Print Works (CPW) provides preparation, printing, and finishing of cotton and polyester/cotton blended fabrics. The Webster, Massachusetts United States, division of CPW prepares, prints and finishes cotton and blended fabrics, which are used to make clothing and other goods. The facility employs approximately 420 people in its three-shift-a-day, six-day-a-week operations. Cranston considers acting as a green company to be an integral part of the company commitment to continuous quality improvement, and it has been recognized by the Massachusetts Audubon Society, the Worcester Business Journal and the American Textile Manufacturers Institute for its environmental achievements.
To achieve the brilliant colors that its customers demand, Cranston Print Works uses azoic, or rapidogen, dyes, which require acid treatment to react the dye molecules so the desired color is achieved. The azoic dye consists of two components. The first component is an aromatic diazo molecule which bonds to the fabric. The second component is a diazonium salt, which only bonds with the diazo molecule to create the desired, brilliant shade in an acid environment. This exposure of the dyed textile to an acidic vapor is known as "acid aging"
At Cranston, a multi-disciplinary team applying the Deming Quality Process chose to adopt control charting for processes throughout the plant. Control charting is a means of monitoring the state of a process as a function of process inputs by illustrating normal process variability and highlighting extreme disturbances. Analysis of the control chart then allows for improvement of the process such that variability is reduced.
Understanding the extent to which a manufacturing process is "under control" is the first step towards improving that process. All manufacturing processes which involve the use of toxic chemicals can employ control charting as a means to assess the efficiency of toxics use. The information gathered on the control charts can highlight areas needing improvements and thus reduce toxics use through process improvements. Acid aging of textiles is unique to azoic dyes, however, recovery of acid vapors may be possible in a variety of processes if the properties of the acid are suitable. Manufacturing operations which need to lower process pH may be able to eliminate the need for acid by substituting carbon dioxide to reduce alkalinity. Using control charting, the management at Cranston thus identified and implemented a set of CP measures involving modification of the acid aging process to minimise environmental impacts and generate economic benefits for the company.
Cleaner Production Principle:
Process modification; Housekeeping
Cleaner Production Application:
Toxic use reduction in printing and finishing operations
Pre-treatment of fabrics entails repeated exposures to alkaline solutions to scour and whiten the fabric and to prepare the fabric surface for quality printing. Due to these processes, the waste stream at Cranston is highly alkaline and has required large quantities of sulfuric acid for neutralization.
The acetic acid used to bond the two components of the azoic dyes is delivered as a vapor to the printed textile through the use of an acid bath. The bath, a 5% acetic acid solution, is kept at 215 F in a vessel called an ager through which the fabric is laced on rollers. In a reservoir, or wet well, at the bottom of the ager, the acid solution is boiled producing the vapor to which the fabric is exposed. Approximately 4% of the acid is either consumed in the dye chemical reactions or remains as residue on the textile to be washed off in a subsequent operation. The 96% balance of the acid vapor in the ager escapes the vessel and is captured in an exhaust system.
Prior to implementation of the in-process acid recovery system, the entire amount of this acidic exhaust was sent to a scrubber and then to drain as wastewater. Cranston installed the new acid ager with a condenser in-line between the ager and the scrubber. This condenser recovers 90% of the acid stream, and the recovered solution is piped directly back into the agers wet well where fresh solution is added to make up for losses. Due to the reduced acid content in the exhaust, water flow to the scrubber can be recirculated, significantly reducing discharge.
Control charting is used to ensure optimum process efficiency, defined here as achievement of highest product quality with minimum use of acetic acid. The chemical reaction that occurs during the acid aging process is optimal in a 5% acid environment. An environment which exceeds 5% acid introduces excess acid into the process, while an environment of less than 5% acid can result in poor print quality. Historically, the process for the acid agers involved the manual addition of acetic acid to the wet well, resulting in very high variability in acid concentration. In order to ensure a high quality product, the amount of acid added to the system was much greater than the chemical reaction required. The control charts for this process highlighted the extreme variability of the range in acid content and the unnecessarily high mean value of acid concentration.
Once the lack of control which this process exhibited was brought to the attention of management, steps were taken to improve the process. An automated acid feed was added to reduce the variability in acid content by approximately 79%. Tighter control of the process allows the mean concentration of the acid solution to be held closer to the optimal 5% without jeopardizing product quality due to insufficient acid content.
The substitution of liquid carbon dioxide (CO2) for sulfuric acid represents another significant process improvement by Cranston Print Works resulting in substantial reductions in toxic chemical usage. The application of CO2 in wastewater treatment effectively eliminated Cranstons facility-wide usage of sulfuric acid. Although using CO2 for the treatment of alkaline wastewater is a technology which is not unique to Cranston, the delivery system which was implemented here is unique.
The trend in annual acetic acid and sulfuric acid usage (lbs.) prior to and after the implementation of the CP measures is shown below:
The wastewater stream at Cranston Print Works has a pH of over 11. This alkalinity must be reduced to a pH of approximately 8 before discharge into the local publicly owned treatment works (POTW). Traditionally this neutralization was accomplished through the use of sulfuric acid, but due to environmental concerns, worker health and safety, and the TUR planning process, Cranston chose to use liquid CO2 to neutralize their wastewater. In 1992, two methods of CO2 delivery were available. They were in-line delivery, injecting gaseous CO2 into wastewater pipes en route to holding tanks, and diffusion delivery, bubbling CO2 up through the wastewater in a holding tank. Neither method was suitable because the length of pipe required for in-line delivery and the holding tank depth required for diffusion delivery were prohibitive at Cranstons facilities.
Cranston Print Works decided to take advantage of the turbulence created by jet aeration headers in the wastewater holding tank which combined 15,000 gal / min of recirculated wastewater with 1,000 ft3 / min of air to reduce the BOD. Forty nozzles on each header pipe act as aspirators that mix the air and wastewater as they leave the wastewater header. Neutralization is accomplished by injecting liquid carbon dioxide directly into the wastewater header. The turbulence created by the jet aeration system accelerates the chemical conversion of carbon dioxide to carbonic acid. The gaseous CO2 when mixed with the liquid, converts to carbonic acid and lowers the alkalinity of the waste stream. The CO2 vendor conducted bench-scale tests with Cranston wastewater to ensure the viability of this new process prior to implementation of the production scale unit. These tests determined that liquid CO2 would be able to meet the neutralization demands.
Substitution of Sulfuric acid by Carbon Dioxide for pH adjustment
Cranston processes 65 million yards of fabric each year. In order to improve color yield in the printing process, raw cotton and other fabrics are bleached, then mercerized in a 22% caustic soda solution. These steps generate highly alkaline wastewater (11.4 pH average) which must be neutralized in the companys treatment facility before it can be discharged. Cranston formerly used sulfuric acid for neutralization. Sulfuric acid accounted for 80% of the toxics usage reported by Cranston under the Massachusetts Toxics Use Reduction Act (TURA).
Alkaline wastewater was neutralized in two 4,3090 gallon sumps. The acid was mixed into the wastewater in the first sump for rough pH control, while the second sump was used for fine-tuning of pH levels. Fully neutralized wastewater was pumped into a 600,000 gallon holding basin for organic equalization and seven day discharge to Websters publicly owned treatment works (POTW).
Spurred by a desire to meet the goals of the Massachusetts Toxics Use Reduction Act, the managers of Cranston Print Works altered their wastewater pH adjustment process to eliminate the use of sulfuric acid.
Cranston became interested in eliminating its use of sulfuric acid because it wished to end its use of substances listed as toxic under TURA. The company also hoped to enhance the safety of employees both at Cranston and at the POTW. Easier pH control was a final, though less important, goal. Cost was not a driving factor.
The company installed a jet aeration system with injected liquid carbon dioxide to replace the two 4,300-gallon sumps in which wastewater neutralization had previously taken place.
Cranston Print Works decided to find an
alternative to sulfuric acid in 1991. Liquid carbon dioxide was chosen as the replacement,
since it performs the same job as sulfuric acid and it is not a toxic material. Liquid
carbon dioxide systems also typically offer greater safety, more precise pH control and
lower operating costs than sulfuric acid neutralization systems.
Once the decision was reached, the companys management teamed with plant operators, three consultants, a liquid CO2 vendor and an aeration equipment vendor to lay plans for the change. The team spent approximately 150 hours devising the innovative system that eventually replaced the old acid neutralization sumps.
Cranston officials worked with the liquid carbon dioxide supplier to run laboratory tests which traced the alkalinity levels of specific waste streams. These tests permitted the development of titration curves, which were used in turn to determine the CO2 requirements of the new neutralization system. These tests had the added benefit of improving the companys technical understanding of day-to-day operations.
The two existing 600,000-gallon basins, now operating in series, were equipped with jet aeration systems designed to feed the liquid carbon dioxide into the wastewater. All plant wastewater is now pumped into the two basins, where liquid CO2 is injected into the aeration liquid header to provide complete mixing with the wastewater. As the CO2 mixes with the water it makes carbonic acid, which is weaker and much easier to monitor and control than sulfuric acid. The carbonic acid dissociates into bicarbonates, carbonates and hydrogen ions, and the hydrogen ions reduce the pH. Using this process, it has become easier for Cranston employees to keep wastewater effluents within the required pH range of 6.5 to 9.0.
Implementation of the complete pretreatment system upgrade took approximately 450 man-hours over an eight-month period. Employees whose work requires knowledge of the waste treatment system, including all maintenance workers and waste treatment operators, were given appropriate training by the CO2 vendor and by Cranstons waste treatment supervisor.
Environmental and Economic Benefits:
Toxic use reduction in printing and finishing operations
Process improvements resulting from control charting have reduced annual acetic acid usage in the acid agers by over 128,000 pounds. The automated feed system has dramatically reduced worker exposure to acetic acid.
Implementation of the acid steam in-line recovery unit with the new acid ager has reduced Cranstons annual acetic acid usage by more than 302,000 pounds.
Water discharge from the scrubber unit has decreased from 56 gal./min. to 15 gal./min. as a result of recirculation.
The substitution of carbon dioxide for sulfuric acid in the treatment of alkaline wastewater has completely eliminated the annual use of 2.66 million lb. of sulfuric acid.
Due to TUR planning and good engineering, Cranston has reduced its use of TUR chemicals by over 3 million pounds per year since 1992.
Costs and Benefits:
Due to implementation of the acid recovery system, Cranston annually saves approximately $78,520 in acetic acid procurement costs and $200,000 in wastewater treatment costs.
Control charting used in the acid agers has resulted in the reduction of acetic acid procurement costs by over $33,280 annually.
The capital expenditure for the purchase and installation of the acid recycling system was $235,000.
The capital expenditure for the Tytronics unit and the automated feed was approximately $18,000.
Substituting carbon dioxide for sulfuric acid in wastewater treatment has reduced procurement costs annually be at least $70,000 and compliance costs by over $3,000.
The capital expenditure for the purchase and installation of the CO2 wastewater system was approximately $93,000.
|Annual Savings||Capital Costs|
|Process changes in Acid Agers||$311,800||$260,000|
|CO2 Substitution||$ 73,000||$ 93,000|
Substitution of Sulfuric acid by Carbon Dioxide for pH adjustment
This $115,000 project eliminated the annual use of 2.66 million pounds of sulfuric acid. Although financial profit was not the main goal of the project, the company saved about $80,000 per year in chemical purchase and maintenance costs.
Installation of the new pH adjustment system cost $115,000. This figure includes the purchase of a used liquid CO2 holding tank. Cranston estimates that it saves approximately $60,000 per year in chemical purchase costs, and another $20,000 in system maintenance. This latter figure includes the annual cost of maintaining the sulfuric acid piping system, valves and pumps, together with an additional $5,000 for an annual recoating of neutralization sumps. These figures indicate that the payback period on the new system is slightly less than 1.5 years.
The new system is generally safer and easier to operate than the old one, since employees no longer require special training in acid handling, spill monitoring and emergency spill response.
Since the new system was installed, Cranston has made further changes to equipment and manufacturing controls that have drastically reduced the amount of caustic wastewater generated by its bleaching and mercerizing processes.
Cranstons replacement of sulfuric acid with liquid carbon dioxide has eliminated the annual purchase of more than 2.66 million pounds of sulfuric acid. The change has also freed the company from a series of onerous requirements associated with the safe use of sulfuric acid. These requirements include special protective equipment to handle the acid, training on acid handling and emergency spill response, and special monitoring of deliveries to detect spills and leaks.
This case study was submitted by the Toxic Use Reduction Institute to UNEP IE. It was edited for the ICPIC diskette in June 1997. It has not undergone a formal technical review.
Subsequently the case study has undergone a technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.