2010-PVA Size Recovery/Reuse via Vacuum Flash Evaporation: An Alternative to Ultrafiltration for Closed-loop Processing
PVA Size Recovery/Reuse via Vacuum Flash Evaporation: An Alternative to Ultrafiltration for Closed-loop Processing
Kishor K. Gupta, Georgia Institute of Technology
Textiles is one of the oldest industries in human history and unfortunately is also one of the largest industrial polluters and consumers of energy. In a textile process known as slashing, warp yarn is coated with a polymeric size which improves its weaving properties. Polyvinyl alcohol (PVA) and starch are the most widely used sizes in slashing, often as 70:30 or 60:40 blends. After weaving, size is removed from the fabrics in the preparation process of desizing.
Desize effluent is a high Biological Oxygen Demand (BOD)/Chemical Oxygen Demand (COD) contributor to a plant’s effluent stream.
Starch itself contributes 50% of the BOD in composite textile effluent (100,000-600,000 mg/l), while no known bacteria attacks and degrades synthetic PVA, resulting in its high contribution to POTW COD loads . The treatment of such large volumes of desize effluent to meet stream discharge permit requirements is costly.
When all-PVA/wax sizes are used, significant incentives exist to recover the material from the desize wash water stream, concentrate it and reuse it. Unfortunately, no effective and efficient means to do this has ever been implemented in the industry. Ultrafiltration (UF), operating on the principle of reverse osmosis (RO), has been used for this purpose in the U.S. on a very limited basis, mainly where forced by regulatory authorities. However, this technique is more than 35 years old, and although improved over time, it still has not found widespread industrial acceptance due to high capital
investment, high operating costs, dependence on spiral-wound polymeric film membranes and poor effectiveness.
Starch sizing is not appropriate for significant recovery and repeated reuse cycles
because the natural material is subjected to chemical/biological breakdown in time by starchase bacterial enzymes, i.e., it “sours.”
A new technology that would eliminate the major disadvantages of the current UF PVA recovery process is Vacuum Flash Evaporation (VFE). The VFE process has been implemented in many industries to concentrate “dirty” waste streams, but has never been demonstrated and implemented for PVA size recovery in the textile industry. In this technology, the excess water in the dilute PVA/wax desize stream is vacuum flash evaporated under controlled pressure.
The clean steam produced from the VFE unit is condensed via heat exchangers and utilized as hot make-up water in any plant wet processes (dyeing, preparation, washing, etc.). In addition, all of the water contained in the desize stream is recovered and recycled back into the plant, giving a true closed-loop character to the overall desizing-size recovery/concentration-reuse sequence of processes and aiding overall economics.
No effluent is discharged from the VFE process.