Effect Of Autohydrolysis Pretreatment On The Dissolution Amount Of Acetic Acid Prior To Poplar Chemi-thermomechanical Pulping

Pretreatment of the lignocellulose before CTMP (Chemi-thermomechanical Pulp) can make full use of biomass resources and produce value-added products, However, organic acids (e.g., formic acid, acetic acid) released from the raw matericals in the autohydrolysis pretreatment would decrease pH value of the autohydrolysis liquor, and the acidity of autohydrolysis liquor was increased. The contents of acetic acid have a great influence on the changes of carbohydrates and fiber cell structure of the raw materials. Therefore, the mathematical model of the amount of acetic acid versus pretreatment conditions was established for analyzing regulation mechanism of pre-extraction of hemicelluloses.In this paper, poplar wood chips were used as the raw material. Based on the Box-Behnken design, the effects of temperature, time and ratio of liquid to solid on the autohydrolysis treatment were explored. We selected the yield of the hydrolyzed chips, pH value and amounts of organic acids as the target variables. Furthermore, the relationship between the dissolution amount of acetic acid and pH values of the autohydrolysis liquor was studied, and the amounts of glucose and xylose were also analyzed. This study applied response surface methodology to optimize the operating parameters of the poplar autohydrolysis pretreatment method. Some modern analysis instruments such as FT-IR,XRD, 1H NMR and SEM were employed to characterize physical and chemical changes of the poplar chips and hydrolyzate under different pretreatment conditions.The results show that the factors affecting the degree of the autohydrolysis pretreatment were temperature, time and liquid/solid ratio. The yield of the hydrolyzed chips decreased from 98.99% to 84.81% as the autohydrolysis condition aggravated continually, Meanwhile, the dissolution amount of formic and acetic acids increased, and the pH value changed from 5.68 to 3.33. With the increase in autohydrolysis temperature and the extension of autohydrolysis time, the content of acetic acid was much higher than that of formic acid in the autohydrolysis liquor, and the dissolution amount of acetic acid was the main factor of changing the system's acidity value.Based on the analysis of response surface methodology, a quadratic polynomial model assessing the effects of the three independent variables on the target variables was developed. The results show that the optimal autohydrolysis condition of the three factorswas temperature of 163 ?, for 39 min and liquid/solid ratio of 4:1. Under this condition, the reliability of the model was verified. In this case, the optimized response values could have a 90.19% hydrolyzed chips yield, 3.49 pH, and 0.841% dissolution amount of acetic acid.In addition, as the conditions of autohydrolysis were enhanced, more xylose and glucose could be brought into the solution (the amount of xylose increased most), which could be found through the detection of carbohydrates in the autohydrolysis liquor. The amount of oligosaccharides in the autohydrolysis liquor could be determined after the hydrolysis of 4% sulfuric acid. The results showed that the content of oligosaccharides was much higher than that of monosaccharides, which shows that oligosaccharide was the main form of the dissolved carbohydrates. With the enhancement of the autohydrolysis pretreatment conditions, the components of the raw material were extracted gradually, and the content of each component showed a downward trend.The SEM images showed that more and more pseudo-lignin on surface of poplar wood chips with the pretreatment conditions enhanced. The crystallinity index of the sample was measured with X-ray diffraction (XRD), and the results showed that there was no significant change in the configuration of the raw material after autohydrolysis, and the intensity of diffraction changed to a certain extent. This suggested that, with the increase of the processing conditions, free acetyl content increased, and destroyed the connection of hydrogen bond between the cellulose molecules, thus made the changes in the hemicelluloses with a relatively loose structure and the amorphous region of cellulose.