| Dissolved pulp is a high purity chemical pulp with more than 90% α-cellulose content,which can replace cotton and linen and other natural high cellulose fibers used to produce viscose fiber,acetate,cellulose nitrate,cellulose ether and other high value-added products.It is widely used in various fields of national economy such as textile,tobacco,national defense,electronic product manufacturing and so on.This study explored a process route that was used for the manufacture of high-purity dissolving pulp based on the concept of biomass refining that maximizes the use of the three structural components of plant fiber raw materials,such as cellulose,lignin,and hemicellulose.The paper adopted the more mature sulfuric acid pretreatment,optimized the pretreatment conditions by response surface,and prepared high purity dissolving pulp by sulfate method;used four common organic acids to pretreat eucalyptus,and systematically compared the effects of these four organic acid pretreatments on the performance of dissolving pulp;used lactic acid/choline chloride(DES)low eutectic solvent to prepare high purity dissolving pulp.The pretreatment conditions such as temperature,time and sulfuric acid dosage used for dissolving pulp were optimized using response surface analysis(RSM)with the recovery of reducing sugars and solid xylan removal rate in eucalyptus pretreatment solution as the assessment index.Within the range of conditions explored experimentally,the pretreatment conditions influenced the pretreatment xylan removal rate in the following order of magnitude:acid content > temperature > time.Using optimized pretreatment conditions(T=150℃,t=2h,w=1%),combined with kraft cooking and ECF bleaching,a high-purity dissolving pulp was made: α-cellulose content was 98.67%,viscosity was 379.6m L/g,whiteness was 88.81% ISO,the Fock reactivity was 99.23%,and the degree of polymerization was 515.Dissolving pulp is characterized by high α-cellulose content and reactivity.The material mass balance calculation of the dissolving pulp manufacturing process showd that if the reducing sugars and lignin in the pretreatment solution and the lignin in the cooking black liquor are fully recovered,the theoretical comprehensive utilization rate of biomass reaches 60.03%.The performance of four commonly used organic acids such as acetic acid,citric acid,maleic acid and oxalic acid pretreated to make high purity dissolving pulp was systematically compared.As with sulfuric acid pretreatment,four organic acids pretreatment,the hemicellulose of eucalyptus wood chips could be effectively removed.And the components such as oligosaccharides in the organic acid pretreatment solution could be better recycled,among which acetic acid pretreatment showed good pulp making performance,probably because during acetic acid pretreatment,lignin could dissolve in acetic acid and form a lignin film on the wood chip surface,which slowed down cellulose degradation and loss during pretreatment,while other organic acids caused severe cellulose degradation.Compared with the other three organic acid pretreatment methods,the acetic acid pretreatment method produced the highest yield of dissolving pulp(31.66%)and the highest α-cellulose(98.28%).The comprehensive utilization of biomass for the preparation of dissolving pulp by acetic acid pretreatment was evaluated.Through material mass balance calculation,43.8% of polysaccharides(12.14% reducing sugar,31.66% dissolving pulp cellulose)and 22.24% of lignin(0.29% lignin acetate,21.95% lignin sulfate)can be recovered in the process of acetic acid pretreatment for dissolving pulp,and the theoretical comprehensive utilization rate of biomass can reach 66.04%.The results of the study showed that among the four organic acid pretreatment options,acetic acid pretreatment for dissolving pulp is the most promising method to achieve greater utilization of biomass resources in eucalyptus wood.A new method of preparing high purity dissolving pulp using deep eutectic solvent(DES)was investigated.It was found that eucalyptus wood treated with DES cycle could effectively remove a large amount of lignin and xylan from wood chips;the lignin removed also had good recovery,and DES remained reactive after being used for wood chips treatment several times,and the pretreated eucalyptus could be used for the preparation of dissolving pulp.The recovered lignin was characterized by FT-IR and gel permeation chromatographic(GPC),and the results showed that the characteristic peaks of lignin structure were still present,and the average molecular weight of lignin obtained by DES treatment was larger than that of sulfated lignin.Compared with kraft paper made by hydrothermal pretreatment sulfate(PHK),dissolving pulp made by low eutectic solvent pretreatment sulfate(DES-BKP)has the advantages of high yield(37%),high α-cellulose content(94%),high whiteness(88.98%)and good Fock reactivity(98.9%).However,the morphological analysis of the appearance of the fibers showed that the surface of both dissolving pulp fibers prepared after DES pretreatment was "peeled" with many thin fragments and the content of fine fibers(Fw)in the pulp was high.The average fiber length and fine fiber content analyzed by fiber quality analyzer(FQA)were:(i)DES-BKP dissolving pulp with Lw of 0.623 mm and Fw of 8.46%;(ii)PHK dissolving pulp with Lw of 0.663 mm and Fw of 3.95%.Material mass balance calculations for the preparation of DES-BKP from raw materials through pretreatment,cooking and bleaching processes showed that the solid cellulose recovery was 37%,the by-product recovery in hydrolysis solution was 9.36% and DES lignin recovery was 23.38%,and the theoretical overall utilization rate of biomass was 69.74%.The results showed that DES pretreatment for dissolving pulp preparation technology is feasible with high dissolving pulp yield,cellulose purity and comprehensive biomass utilization rate.The dissolving pulp prepared by hydrothermal pretreatment,dilute sulfuric acid pretreatment,response surface optimized sulfuric acid pretreatment,acetic acid pretreatment and lactic acid/choline chloride pretreatment were compared from the dissolving pulp performance and economic performance,and if the problems of acetic acid recovery and DES reuse can be solved effectively,the preparation of dissolving pulp by acetic acid pretreatment and DES pretreatment will have a broad prospect. |
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