| During the traditional process of Kraft pulping, approximately 75% of hemicelluloses in woody biomass are lost into the black liquor. The lost hemicellulosic fraction along with lignin is usually used as boiler feed. In fact, the calorific value of hemicelluloses is low, only 13.5 MJ/kg, as half of the calorific value of lignin. That is to say, the hemicelluloses are not properly utilized. Undoubtedly, extracting hemicelluloses out of woody biomass before pulping process is a novel and meaningful technology, which help to cut down the alkali dosage required in the pulping process. Furthermore, the pre-extracted hemicelluloses can be converted to many kinds of value added products such as bioethanol by saccharification and fermentation and enhances its appeal.In this study, concentrated hot-water extract of sugar maple hardwood chips are used as experimental materials and dilute sulfuric acid is adopted as catalyst, acid hydrolysis of concentrated hot-water extract are performed in two different reactors of an oil bath setup and a Parr high pressure reactor. Sequently, 1H NMR spectroscopy and 2D HSQC spectroscopy from 600 MHz Nuclear Magnetic Resonance technology are employed to analyze the concentration of each component in the resulting hydrolysates. The influences of reaction temperature, acid dosage and reaction time on the concentration of each component during acid hydrolysis process are discussed in detail. Accordingly, the optimal technological conditions of dilute acid hydrolysis of concentrated hot-water extract are proposed.Based on the two prevalent kinetic models of acid hydrolysis of lignocellulosic materials, Saeman Model and Biphasic Hydrolysis Model, some improvements are made and a new kinetic model of acid hydrolysis of hemicelluloses is advocated. By performing the new model, a series of kinetic parameters of acid hydrolysis are figured out such as rate constants, apparent activation energies and pre-exponential factors.Some conclusions are drawn according to the experiments as following:1) Dilute sulfuric acid hydrolysis of concentrated hot-water extract can attain the expected goal of the conversion of pre-extracted hemicelluloses to monomeric sugars. Xylose is the most abundant monomeric sugar in the hydrolysate and accounts for more than 80% on a weight basis. There are several other monomeric sugars in the hydrolysate, including arabinose, glucose, mannose, galactose and rhamnose. To an extent, higher temperature, higher acid concentration and longer residence time all favor the increment of the concentrations of monomeric sugars except that of arabinose. Once the concentrations of monomeric sugars obtain the maximum in the hydrolysate, they decrease over prolonged reaction time. Arabinose is confirmed as a readily degradable monomer, the concentration of arabinose is observed decreasing over reaction time in most of the experiments. Acetic acid is the most abundant nonsugar byproduct in the hydrolysate. The acetyl groups cleave from the main chain of xylan very rapidly when the sulfuric acid dosage is more than 1% so that the concentration of acetic acid attains a platform value during the first two-minute reaction or the heating up period, hereafter the concentration of acetic acid keeps nearly unchanged over reaction time. The other nonsugar byproducts include furfural, hydroxymethylfurfural and formic acid, etc. the concentrations of furfural and hydroxymethylfurfural increase with the increments in reaction temperature, acid dosage and residence time, which confirms that more severity in experimental conditions, more pentoses and hexoses degrade. Formic acid generates only when the acid dosage is more than 1%. Once the formation of formic acid starts, the concentration of formic acid goes up very slowly over reaction time. However, the amount of formic acid in the hydrolysate is comparatively small.2) The optimal conditions of acid hydrolysis of concentrated hot-water extract are proposed. As to acid hydrolysis of concentrated hot-water extract No.1 at atmospheric pressure, the recommended technological condition is like that reaction temperature between 95°C and 105°C, the weight ratio of sulfuric acid to the extract equal to 6.2%, reaction time between twenty and thirty minutes. As to acid hydrolysis of concentrated hot-water extract No. 2 when a Parr high pressure vessel is used as reactor, the best technological condition is reaction temperature at 120°C, the weight ratio of sulfuric acid to the extract equal to 1.5%, residence time equal to twenty-five minutes.3) A series of kinetic parameters are figured out by performing the proposed kinetic model of acid hydrolysis of hemicelluloses, including rate constants, apparent activation energies and pre-exponential factors. Under the conditions of reaction temperature between 85°C and 105°C, the mass ratio of sulfuric acid to the extract between 1.5% and 6.2%, the rate constants of hemicellulose hydrolysis reaction in the concentrated hot-water extract No.1 vary from 1.28×10-3 min-1 to 3.29×10-2 min-1, and the rate constants of sugar degradation reaction are in the range of 2.18×10-6 min-1 to 1.74×10-3 min-1. The apparent activation energies and pre-exponential factors of hemicellulose hydrolysis reaction and of sugar degradation reaction are 117.88 KJ/mol,133.24 KJ/mol and 6.58×1014 min-1, 9.12×1014min-1, respectively. The rate constant of furfural (including hydroxy-methylfurfural) degradation reaction keeps being zero. i.e., no furfural or hydroxymethylfurfural degrades under the circumstances. Under the conditions of reaction temperature between 120°C and 150°C, the mass ratio of sulfuric acid to the extract between 0 and 1.0%, the rate constants of hemicellulose hydrolysis reaction in the concentrated hot-water extract No.2 vary from 1.11×10-3 min-1 to 0.236 min-1, and the rate constants of sugar degradation reaction are in the range of 7.03×10-5 min-1 to 1.56×10-3 min-1, and the rate constants of furfural (including hydroxymethylfurfural) degradation reaction are between 5.35×10-5 min-1 and 8.77×10-4 min-1. The apparent activation energies and pre-exponential factors of hemicellulose hydrolysis reaction and of sugar degradation reaction and of furfural (including hydroxymethylfurfural) degradation reaction are 103.11 KJ/mol, 142.61 KJ/mol, 128.77 KJ/mol and 1.18×1012 min-1, 6.01×1014 min-1, 6.61×1012 min-1, respectively.
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