Characteristics And Obstacle Mechanism Of Recycled Fiber Hydrolysis In Supercritical Water

With increasingly scarce and exhausting fossil fuel and releasing greenhouse gases during the combustion process, seeking the alternative, renewable and eco-environmental sources of energy and chemical has been promoting the research related to biomass converted into fuel and useful chemical. Among biomass resources, recycled plant fiber is one of the most abundant types that have been paid attention to the compatible advantage. Achieve the high efficiency conversion rate from cellulose to fermentable monosaccharide is the key process point need to be made effort to. The experiment was designed to evaluate the hydrolysis characteristics of recycled fiber from waste paper in supercritical water in this paper, then to study the effect of the hydrolysis temperature and recycle times of the fiber on the recycled plant fiber hydrolysis in supercritical water. The kinetics was also investigated for further understanding the characteristic of fiber hydrolysis. Similarity the effect of supermolecular structure on the fiber hydrolysis was discussed to determine the fiber hydrolysis obstacle mechanism in supercritical water.It is showed from the result that the non-cellulose components in original fiber were much lower than that in waste fiber with the increasing of the crystallinity and closed pore in cell wall of the fiber. The longer hydrolysis time was needed for waster fiber to achieve the highest hydrolysis percentage, and the glucose concentration in the liquid part was 45.61mg/L after the fiber hydrolyzing. The concentration of cellubiose, glucose, 5-HMF was increased with the increasing of the reaction time, but was much lower than that obtained from original fiber hydrolysis. The high xylose concentration was obtained from hemicelluloses hydrolysis within 10s. It's hard for the glucose accumulation because the 5-HMFconcentration was increased from the conversion of the glucose with the increasing of the hydrolysis temperature.The effect of hydrolysis temperature on the cellulose and hemicelluloses hydrolysis indicated that the high conversion percentage can be obtained with the increasing of the reaction temperature. But the glucose yield was not able to increase correspondingly. The optimized hydrolysis condition was 380℃with the reaction time of 40s based on the highest glucose yield. The recycled fiber with different recycled times was hydrolyzed under the optimized hydrolysis condition and the result showed that the recycled fiber hydrolysis conversion was decreased with increasing of recycled time. And the highest glucose yield was 5.95% when the reation time was 40s. The xylose yield from hemicelluloses conversion was obtained in 20s. But with the increasing of the reaction time, the effect of recycled time on the yield of 5-HMF, conversion percentage of recycled fiber has begun to drop down. The main reason for this phenomenon was that the recycled fiber hydrolysis was started from the hemicelluloses and the amorphous region of cellulose. And the crystallinity of cellulose was increased. In contrast, it's harder for cellulose to hydrolyse in crystal region and long reaction time makes difference of the recycled hydrolysis small in those aspects.It is concluded from the fiber hydrolysis processes and kinetics model analysis in supercritical water that the fiber was hydrolyzed into the oligosaccharides in the beginning and then converted into monosaccharide mainly, along with Furfural, Furan, Ketone and Oranic acid byproduct. The 5-HMF was the main products converted from oligosaccharides. The kinetic parameter analysis elucidated that the conversion rate of cellulose and hemicelluloses was increased with the increasing of hydrolysis temperature, and that the fiber hydrolysis activation energy was 109.42 E/kJ·mol-1 which was lower than that of glucose with 148.46E/kJ·mol-1. But the activation energy for glucose converting to 5-HMF was 139.49 E/kJ·mol-1 which was not benefited for glucose accumulation.The crystallinity of the fiber was increased from 77.34% to 87.78% when the fiber was treated with the temperature from 60℃to 120℃, and the average fibril aggregate size was also increased from 5.6nm to 7.55nm. But the hydrolysis percentage was decreased from 72.5% to 38.4%. The main reason was that the combined water in the fibril was removed when the fiber was treated with the different temperature. It can help to release the free (-OH) in the surface of the fibril and the free (-OH) was recombined into the intermolecular hydrogen bond O(2)H…O(6) and O(3)H…O(5) which make the amorphous much ordered. The increased crystallinity of fiber was attributed to this. In the hydrolysis process, the intermolecular hydrogen bond O(3)H…O(5) was easier to be attacked and make fibril less ordered and the hydrogen bond was decreased correspondingly.The pressure shows the different effect on the supermolecular and hydrolysis potential of fiber. The crystallinity of the fiber change a little when the fiber was pressed with different time. But the average fibril aggregate size was also increased from 2.48 nm to 3.04nm and average fibril aggregate area was increased from 6.25nm2 to7.46nm2. The hydrolysis percentage was decreased from 75% to 48% when the fiber was pressed within 40s. The intensity of intramolecular hydrogen bond O(6)-H…O(3') increased and then decreased when the pressed time was increased. The intermolecular hydrogen bond O(2)H…O(6) and O(3)H…O(5) showed the opposite vibration. This was because that the intermolecular O(6)-H…O(3') hydrogen bond was broken and much more intermolecular O(3)H…O(5) was formed when the fiber was pressed. And the some small crystal region combined together. In the hydrolysis process, the crystal region and some intermolecular hydrogen bond was broken. And the surface area and surface energy was increased which make the intermolecular hydrogen bond O(3)H…O(5) easier to be formed. So the fiber hydrolysis residue has much intermolecular hydrogen bond O(3)H…O(5) when the fiber was pressed with 20min and 40min.