Research And Design Of Treatment Process Of Press Lye From Viscose Production

This work aims to recycle the sodium hydroxide aqueous solution(so-called press-lye)discharged from viscose production process.The press-lye dissolves the hemicelluloses in the raw pulp.When hemicelluloses in the press lye accumulate to a limited content,the lye must be discharged.Otherwise the quality of end viscose products will be degraded.Although membrane separation techniques can be used to recover part of the alkali,after filtration,the concentrates enriched with hemicelluloses still cannot be recycled and reused.Precipitants were used to precipitate hemicelluloses from press-lye and then were separated from alkali liquor to achieve a full recycling and reusing of materials.According to literature investigation,ethanol,methanol,acetone,isopropanol,tert-butanol,and tetrahydrofuran were chosen as precipitants.The effect of contact time between precipitant and press-lye,precipitant dosage,temperature,mixing mode on removal of hemicelluloses were investigated.The results showed that precipitant dosage affected hemicelluloses precipitation rate to the greatest extent with ethanol and press-lye at a volume ratio of 3:1 steadily precipitated 75% of the original hemicelluloses.Moreover,mixed precipitants research indicated that adding a small dosage of isopropanol or tert-butanol to ethanol can significantly improve hemicelluloses precipitation rate,which can significantly reduce the dosage of precipitants and largely decrease energy consumption.Ethanol was selected as precipitant,and its dosage was three times volume of press-lye.With annual processing capacity of 60 thousand tons press-lye(containing sodium hydroxide 15%(wt))as the target,our focus was on the research and design of distillation separation on the treated lye.In order to minimize energy consumption,single column distillation process,fractional heat pump distillation process,countercurrent distillation process,advection double-effect distillation process and advection three-effect distillation process were compared.It was found that advection three-effect distillation process was the best choice for lower cost consumption.Advection three-effect distillation process has achieved the annual processing capacity of 60 thousand tons of press-lye with the total equipment cost is about 7.5 million RMB and the total energy consumption of treating each ton of press-lye is about 850 RMB.