Study On Ramie Degumming With High-boiling Alcohols And The Solvent Reuse

Ramie is a natural perennial herb,which commonly exists as a single fiber with high strength and high modulus.Ramie fibers have broad applications in the processing of textile fabrics,reinforced composites,and industrial packaging ascribed to the advantages of coolness,antimildew and antibacterial properties.Cellulose is the main constituent of raw ramie,while the non-cellulosic components,such as hemicellulose,lignin,pectin,etc.account for about 20 to 30 wt%of the fiber content,which are collectively defined as gums.Prior to spinning,it is necessary to remove the heavily coated gummy contents from the cellulosic part of ramie fibers to retain the cellulose fiber of higher purity,which is called‘degumming'.Currently,the ramie degumming process commonly used in industry is the traditional alkaline method,that is,high concentration Na OH solution is used for high-temperature boiling of raw ramie.However,the method has many defects such as complicated process and serious environmental pollution.In order to reduce environmental pollution,researchers have proposed many new degumming methods,such as oxidation degumming,biological?enzymes,bacteria?degumming,but high equipment cost,unstable degumming,and great cellulose damage result in the slow promotion process in industry.Hereof,this research extended our knowledge of exploring an environmentally friendly and high-efficiency process for ramie fiber degumming by showing a novel route that worked with high-boiling alcohol solvents.The degumming efficiency and fiber quality were improved by adding catalyst,cellulose protectant and lignin remover.At the same time,feasibility of high-boiling alcohol repeated degumming process and solvent recovery in degummed black liquor was discussed to reduce reagent cost and improve resource utilization.The specific research works of the subject are as follows:?1?The low-carbon high-boiling alcohol solvents,1,2-propanediol,glycerol,ethylene glycol and 1,4-butanediol,were used to degum ramie in one step under pressure,and the investigation regarding different alcohols capable of degumming effect was compared and analyzed.The results demonstrated that among the four sorts of alcohol degumming,fibers treated with 1,4-butanediol were easily brittle due to the serious damage to the cellulose.The yield of fiber with the other three alcohol treatments was high?73-76%?,and the tenacity,linear density and residual gum content?RGC?all fulfilled the requirement of the spinning process.Comprehensive analysis showed that ethylene glycol with relatively low-boiling point and glycerol with relatively low toxicity performed the best degumming effect.The tenacity of fiber with ethylene glycol treatment was the highest?8.27 c N/dtex?,and glycerol treated fibers displayed the lowest RGC?3.3%?.The degumming mechanism of high-boiling alcohols was analyzed based on the experimental results.The increase of thermal energy,kinetic energy and molecular activation energy would promote diffusion and penetration of high-boiling alcohols,the released protons?H⁺?of which would attack the chemical bonds between the colloid molecules under high temperature conditions.The acetic acid,a catalyst originating from the breakage of O-acetyl groups in hemicellulose,was released accompanied by the generation of furfural,which was produced by the dehydration of pentose monosaccharides deriving from the cleavage of the?-glycoside bond.The ether bond between lignin molecules was also prone to fracture and could avoid its aggregation under acidic conditions.The atmospheric conditions were adopted in subsequent studies to reduce energy consumption caused by pressurization in the four sorts of processes.Separately,ethylene glycol and glycerol were selected as solvents,and the improvement of degumming effect was achieved by the addition of additives.?2?The improvement of ramie degumming using ethylene glycol?EG?was performed via adding different amounts of acetic acid?EGAc?or raising the degumming temperature?EG-T?under normal pressure and oil bath conditions.Acetic acid has selective characteristics for the separation of biomass,and can catalyze the cleavage of chemical bonds between gum molecules,while temperature has also great influence on the reaction rate of degumming.The results showed that the tenacity and linear density of fiber treated by EG process?130?,6 h?had reached the first and third national standards,respectively,while the adverse effects would be occurred on the subsequent spinning by the high RGC?12.26%?.In the EGAc system,fibers with a ratio of ethylene glycol/acetic acid?60/40??EGAc-60/40?achieved better physical performances,in which the RGC and linear density of fiber decreased by 21.53%and 8.72%,respectively.In the EG-T system,the best properties of fiber treated with the condition of 200?,80 min?EG-T80?were obtained.Compared with fibers by the treatment of 200?,60 min,the RGC and linear density of fiber by EG-T80 treatment decreased by 26.18%and 8.74%,respectively,and the tenacity increased by22.97%.Comparative analysis was conducted on the fibers treated by the two optimal improved processes?EGAc-60/40,EG-T80?.The results showed that the RGC,linear density,and yield of fiber with EG-T80 were lower than that of EGAc-60/40,indicating that the RGC and fiber quality had considerable improvement via increasing temperature.The DP value of cellulose and tenacity of fiber treated by EGAc-60/40 were significantly lower than that of EG-T80,owing to the catalytic degradation of part of cellulose by acetic acid.In summary,increasing the degumming temperature is more conducive to improving the degumming efficiency and effect.?3?Anthraquinone?AQ?possesses multiple effects in accelerating delignification,scavenging oxidized free radicals,and inhibiting the peeling reaction of carbohydrates in biomass separation.Hence,AQ was added in glycol solvent as cellulose protectant and lignin remover to further improve the removal rate of lignin and reduce the oxidative degradation of cellulose ascribed to degumming under high temperature.The results of degumming effect of ramie in ethylene glycol/anthraquinone?EG-AQ?solvent indicated that DP value of cellulose increased with the increase of AQ concentration.The crystallinity,tensile properties of fiber increased first and then decreased as the AQ concentration increased from 0 and 0.5 g/L,while the linear density and RGC of fiber had the opposite trend.With the most appropriate concentration of AQ?0.3 g/L?,crystallinity and tenacity of fiber increased by 20.53%,9.55%,respectively,and the lignin content,RGC,linear density of fiber decreased by 21.43%,17.91%,7.24%,respectively,while the removal rate of hemicellulose decreased by 3.01%,the content of cellulose increased by 0.74%,ascribed to the protective effect of AQ,hence resulting in stable thermal performance and an increase of nearly 1.46%in fiber yield.Moreover,the RGC of fiber reduced by about 0.5%?0.9%during the same degumming time,and the degumming time could be shortened by more than 25 min when the same RGC of fiber was obtained.The degumming mechanism of AQ was analyzed based on the above conclusions.Anthrahydroquinone?AHQ?ions was generated via the ionization of AQ in ethylene glycol solvent,and the induction of quinone oxygen electronegativity to?-aryl ether bond in the lignin structure caused it to break away from the cellulose fiber.In the meantime,cellulose peeling reaction could be prematurely terminated by the oxidation of the terminal aldehyde groups of the cellulose by AQ.In the whole degumming process,the oxidation and reduction reactions were carried out alternately by the mutual transformation of AHQ and AQ,which not only brought about the removal of lignin,but also diminished the cellulose damage.In summary,AQ,the potential regent in improving degumming efficiency and fiber performances,has tremendous potential for the application of organic solvent degumming of ramie.?4?The lowest RGC of fiber treated with glycerol was achieved under pressure in the previous study,and further exploration of glycerol degumming process under normal pressure in an oil bath was performed to reduce energy consumption.In the meantime,ferric chloride?Fe Cl₃?catalyst was added in glycerol solvent to improved degumming effect?referred to as GL-Fe Cl₃ system?.GL-Fe Cl₃ system possessed the superiority of high catalytic efficiency,and could selectively oxidize the primary C6 hydroxyl group on cellulose to carboxyl group,reduce the amount of cellulose degradation and improve the yield and moisture sorption of fiber.The results showed that higher Fe Cl₃content,longer reaction time and higher temperature could facilitate the removal of gums,while also exacerbate the depolymerization of cellulose.Based on the experimental results of degummed fiber physical properties,the appropriate range of Fe Cl₃ content and degumming time were 0?1.0%,30?150 min,respectively,and degumming temperature was 200?.The response surface methodology was used in this study to investigate the interaction effect between Fe Cl₃content and degumming time on the RGC of fiber in GL-Fe Cl₃,and the optimal GL-Fe Cl₃conditions were listed as follows:Fe Cl₃?%?=1.0%,reaction time?min?=121 min,and the reaction temperature was 200?.After experimental verification,the mean RGC of fiber was 3.88%,which was close to the predicted value?3.96%?,indicating that the model established was more ideal.Compared with the traditional alkaline?Tra-alkali?process,ramie fiber treated in GL-Fe Cl₃ showed higher acid group contents and moisture sorption,especially the higher yield was beneficial to increase fiber production and save costs.Further analysis of the degumming mechanism of Fe Cl₃showed that Fe Cl₃ was given priority to catalyzing the degradation of sugars in hemicellulose to produce furfural,but it could also lead to cellulose degradation.Owing to the glycerol-like cellulose structure,it could be oxidized to ketone or aldehyde by Fe Cl₃,or the isolated pair of electron from glycerol combined with the active site of Fe Cl₃ to form a coordination compound,causing an effective reduction of cellulose degradation by Fe Cl₃.As can be seen that the GL-Fe Cl₃ system has immense potential for the isolation of high-performance ramie single fiber.?5?Given the cost of degumming with high-boiling alcohol solvent,the subject explored the reuse of degumming black liquor?the solution after degumming?.Taking ethylene glycol with lower boiling point as an example,the single-cylinder repeated degumming of ramie with ethylene glycol was adopted,and the organic solvent in black liquor was recovered by vacuum distillation.The results revealed that the wax and pectin in ramie with five times of repeated degumming were all removed,the cellulose content was all above 95.5%,and the yield,linear density,tenacity of fiber were in the range of 71.3?73.5%,4.9?6.0 dtex,5.5?8.7 c N/dtex,respectively,which fulfilled the requirement of Chinese standard,especially the best physical properties of fiber with the first,second and third repeated degumming.It could be seen that the ethylene glycol solvent could be reused at least 5 times.Since some black liquor could be removed with the fiber taking out after each degumming,the remaining black liquor with small amount was not enough to the subsequent degumming after the fifth repeated degumming.At this time,black liquor could be distilled under vacuum,and the results showed that the solvent in black liquor could be almost completely recovered.A relatively complete structure of lignin was contained in the black liquor solid,and the contents of lignin and hemicellulose gradually accumulated with the increase of repeated degumming.Shortly,the single-cylinder repeated degumming of ethylene glycol for ramie and solvent recovery have the prominent feasibility of reducing reagent cost drastically,environmental pollution,and realization of resource recycling.