Study On Bamboo Presteaming Kraft Pulping Technology And Mechanism

At present, lack of timber resources has become one of important obstacles forthe fast development of Chinese paper industry. On the other hand, textile industryalso meets the shortage of cotton fiber due to declining of cotton acreage and therising of its price. Therefore, production of dissolving pulp using bamboo can notonly reduces the competition for resources with the paper industry, but also realize thehigh-value untilization of this resource. Thus, in this thesis, good dissolving pulpingwill be produced from green bamboo based on a combination of pre-steampretreatment, kraft pulping and element chlorine free (ECF) bleaching. Moreover, thepentosan removal with different steam pressure and reaction time was also studied inthis thesis, in which its removal kinetic was subsequently developed. Finally, toobtain high-grade dissolving bamboo pulp and control the process, the optimization ofkraft pulping and ECF bleaching were also done.Firstly, the process of pre-steam pretreatment, pentosan removal and its kineticwere studied in this thesis. It was found that the petosan removal was continuouslyincreased while the steam pressure was set as1.0Mpa but the reation time waschanged. When reaction time was kept at30min, the petosan removal was alsoincreased and then basically kept no change once the steam pressure reached1.2Mpa.This is mainly because the petosan removal limit is not achieved if only the reationtime is changed but the steam pressure was kept at1.0Mpa. However, although thereation time was kept at30min, the limit of petosan removal could be reached if thesteam pressure could be changed in the range of0.6to1.4Mpa. Finally, this transitionpoint of steam pressure could be found at1.2Mpa.Based on the study of pre-steam pretreatment and petosan removal kinetic, it wasfound that both steam pressure and reaction time could resulted in the removal ofpetosan from green bamboo. The value of Weisz modulus(MW) is far lower than0.15,indicating that the mass transfer is not existed during the process of green bamboosteam pre-treatment. The petosan removal during steam pre-treatment of greenbamboo can be regarded as first-order reaction after data fitting of kinetic model, which is similar to that of hot-water extraction. Its activation energy andpre-exponential factor are48.06kJ/mol and4330min-1, respectively. A new steamEtreatment factor(?) was proposed using the calculated activationenergy and pre-exponential once the severity factor was not suitable to this process.Based on the data fitting, it was also found that the reaction temperature is same to thetemperature of steam once the steam reaching to the surface of bamboo chips. This isdue to the good permeability of steam. Besides, this steam treatment factor can beused to the pentosan removal during the process of hot-water extraction if the heatingtemperature is calculated using numerical algorithms. To compare the process ofsteam pre-treatment and hot-water on removing pentosan, the steam treatment factorwas regarded as a unified parameter. It was found that the pentosan removal by steamtreatment was significant higher than that of hot-water extraction with the same steamtreatment factor. At the same time, compared to hot-water extraction, steampretreatment also resulted in lower cellulose degradation. These results indicate thatsteam pretreatment has better selectivity than hot-water extraction does, which canbenefit a higher α-cellulose, but lower pentosan and lignin content in dissolving pulp.Carbohydrate analyzing results showed that the sugar in steam pretreatment liquorwas mainly composed by oligosaccharides but not monosaccharides. Total sugarcontent in pretreatment liquor was also consistent with the change in pretreated solids.SEM and FTIR results also improved that the steam pretreatment can destroy thegreen bamboo cell wall and promote the release of pentosan.Moreover, the process of kraft pulping of steam pretreated solids was also donein this thesis. It was found that, under same kraft pulping condition, the higher steamtreatment factor used, the more pentosan removed. On the other hand, with samesteam treatment, more lignin and pentosan could be removed if higher active alkaliused. But both of them had no changed once the active alkali charge was beyond21%.Therefore, the alkali dosage used in kraft pulping should not be more than21%if nomore cellulose degradation was expected. The resulted unbleached pulp at thiscondition has the properties as following: the pulp yield and kappa bumber were 39.36%and5.79, the content of α-cellulose, pentosan and ash were93.23,2.20and0.42%, and the visicosity was26.21mPa.s, respectively.To further remove the lignin and hemicellulose in kraft pulp and obtain thedissolving pulp with high α-cellulose, the ECF bleaching was also conducted. Twobleaching sequences, DED and DEQP, were used, respectively. After optimization, theeffective chlorine used in bleaching stages of D1and D2were1.90and0.40%, whichcould result one pulp with the brightness, kappa bumber and visicosity of88.2%ISO,1.19and9.91mPa.s, the petosan, α-cellulose and ash contents of2.43,97.89and0.15,respectively. Similarly, the optimized chlorine dioxide used in the bleaching sequenceof DEQP was1.9%. With this condition, the pulp with the brightness, kappa bumberand visicosity of87.8%ISO,0.73and9.00mPa.s, the petosan, α-cellulose and ashcontents of3.49,98.11and0.14%was obtained. Finally, both bleached pulps resultedfrom two bleaching sequences can meet the requirement of standard of(FZ/T51002-2006) in textile industry.