Effect Of Autohydrolysis Pretreatment On The Impregnation Of Chemical Liquor In Poplar Wood Chips

At present,some studies have reported the application of autohydrolysis pretreatment,which focus on biomass materials pretreatment before pulping or enzymatic hydrolysis.Autohydrolysis will lead to the changes in the carbohydrates and structure of the biomass materials,and then affect the impregnation of chemical liquor.Impregnation is usually used as the first stage of pulp preparation in traditional pulping,and will affect the follow-up pulping performance in terms of mass transfer and pulping uniformity.However,there is limited work about the effect of autohydrolysis both on the physical and impregnation properties of biomass materials,which are vital significant for pulping and efficient utilization of biomass materials.In this paper,the poplar sapwood chips were autohydrolyzed in hot water with different severity factors,and some properties of autohydrolyzed and unautohydrolyzed chips were examined,including the volume porosity,wettability,wood absorbency,shrinkage and fiber saturation point(FSP)of poplar sapwood chips.Then the NaOH solution was used as the impregnation liquor in the impregnation of the chips.The concentrations of NaOH inside and outside of the chips and dissolved substances were also determined to explore the effect of antohydrolysis on the impregnation and reaction of the NaOH solution with the chips.Finally,the reaction-permeation-diffusion model was built,and then analyzed the permeation,diffusion and reaction of the NaOH using the established model.The results showed that the volume porosity,shrinkage and FSP of the autohydrolyzed chips increased with raising the severity factor,while the wood basic density and surface contact angle decreased.Meanwhile,the FSP increased from 28.62%to 40.21%,as the severity factor reached 3.54.The structure of the autohydrolyzed chips became looser and looser with increasing the severity factor,and the capillary system were also improved.Autohydrolysis promoted the decline in NaOH concentration in the free liquor(i.e.,the impregnation liquor outside the chips,FL).However,the NaOH concentration had a little increase in the entrapped liquor(i.e.,the impregnation liquor inside the chips,EL),and then decreased gradually as the impregnation exceeded 60 min.Moreover,both dissolved substances and lignin concentration rose with increasing the severity factor.It implied that autohydrolysis pretreatment improved the wettability,swelling and absorbency properties of the chips,and facilitated chemical diffusion and permeation.Nevertheless,autohydrolysis also accelerated the delignification and the reaction between NaOH and autohydrolyzed chips during the NaOH liquor impregnation.Meanwhile,autohydrolysis pretreatment significantly improved the degree of penetration in the radial direction of the chips,but indistinctively in other directions.The properties of permeation,diffusion and reaction for NaOH solution were analyzed by using the established reaction-permeation-diffusion model.It can be intuitively illustrated that the reaction rate of the NaOH solution in the first 60 min was the fastest,and that autohydrolysis enhanced the reaction.The diffusion was the main way to transport the NaOH solution into the chips.Autohydrolysis improved both permeation and diffusion of NaOH solution,but the permeation was improved more obviously than the diffusion.