Polyvinyl alcohol size recovery and reuse via vacuum flash evaporation.

摘要

Polyvinyl alcohol (PVA) desize effluent is a major Chemical Oxygen Demand (COD) contributor to a textile plant's Primary Oxygenation Treatment of Water (POTW) operation, and being biologically inert, it presents a threat to the environment. Unfortunately, no effective and efficient means to treat PVA desize effluent has been implemented in the textile industry. Ultrafiltration (UF) reverse osmosis technology for the recovery and recycling of PVA size is more than 35 years old, but it is not used widely because of its many disadvantages. The situation necessitates a new technology for the recovery and recycling of PVA size which can reduce energy and water consumption in an economical and environmentally-friendly manner. A new technology that would eliminate the disadvantages of the current ultrafiltration process in the recovery of PVA from desize effluent is vacuum flash evaporation (VFE). The VFE process for recovery and concentration has been used in a variety of other industries, but has never been demonstrated for size recovery in the textile industry. The reported research is concerned with the VFE process and its economics, and the properties/utilization of the recovered PVA emanating from the process. This research was divided into four parts. In the first and second parts, simulated, recycled PVA size formulations at different impurities concentration were prepared with or without wax, films were made and yarns were slashed to determine their changes in properties as a function of cotton chemical impurities concentration from 0% to 100%. In the third part and building on the bench scale results, the slashing process was optimized and the concept was demonstrated using the University of Georgia (UGA) WebtexRTM Pilot Slasher. In this demonstration, both ground and pile yarns of a common residential towel construction were slashed, with one conventional cycle followed by five size recovery/recycle slashings. The last part involved the economical feasibility of the VFE technology in recovery and recycling of PVA size. With VFE recovery, recycled size was found to contain extracted cotton chemical impurities which decreased the apparent molecular weight of the recycled PVA size, as they (fatty acids, alcohols, waxes, etc.) were of low molecular weight, thus decreasing the breaking strength of the recycled size film. The initial studies were conducted without the addition of wax to the size. In conventional slashing with wax added to the PVA, film strength also decreased. With the occluded cotton impurities present in the recycled PVA film, no wax was required to be added to the recovered size, as the impurities performed the same film physical property modifications as the wax. An increase in elongation at break of recycled PVA film can render weaving processes more efficient. Mostly cotton is used in the towel industry, but 100% PVA size without substantial added wax (8%) cannot be used to slash the pile yarns of the construction because PVA slashed yarn is stiff in nature, making it difficult to form the loops. However, the recovered PVA size from the developed VFE process showed a decrease in bending length, facilitating the use of PVA size in the towel industry. As recovered PVA was added along with virgin PVA in the size box, the cotton yarn chemical impurity levels continued to increase and reached a steady state after ∼five slashing cycles in a size recovery/reuse sequence. At constant solids concentration, viscosity decreased with the number of size recycles, which aided size yarn penetration at the application temperature (70°C). A small increase and then decrease in slashed yarn load at break occurred with impurities concentration increase in the film; however, slashed yarn load-at-break was always more than 20% that of un-slashed yarn. Slashed yarn elongation at break initially increased then decreased with recycling, but it was always more than or close to that of un-slashed yarn. Slashed yarn hairiness remained nearly constant with recycling. Slashed yarn abrasion cycles to first yarn break initially increased, but remained nearly constant on further recycling. Desized yarn color was also constant with recycling. The overall research results quantified an improvement in recycled size slashed yarn properties over those of greige yarn for towel weaving. Size must be removed from fabric after weaving in a desizing operation (part of the preparation wet process) to facilitate subsequent dyeing and finishing. The recycled size film was easy to wet, which aided the desizing of slashed yarns. Based on the pilot scale trial results, comparative economics were developed for model towel manufacturing plant implementation of the VFE concentration technology for all-PVA size towel weaving, implementation of the analogous UF concentration technology, and the current PVA/starch/wax blend size process with no material/water recovery. Incorporation of the VFE technology for PVA size recovery and recycling resulted in ∼