【作者】 胡若飞；

【导师】 林鹿； Shijie Liu；

【作者基本信息】 华南理工大学 ， 制浆造纸工程， 2010， 博士

【摘要】 在传统的硫酸盐法制浆造纸工艺中,约75%的半纤维素流失到黑液废水中。流失的半纤维素只能与木质素一起作为锅炉填料来使用,但半纤维素的燃烧热值并不高,只有13.5 MJ/Kg,约为木质素的二分之一,因而半纤维素其实并未得到优化利用。在制浆之前把半纤维素从木材中抽提出来的做法无疑是一项非常有意义的举措,——此举降低了随后蒸煮过程中碱的用量,抽提出来的半纤维素经糖化发酵可用来生产燃料乙醇等多种高附加值化学品。论文研究工作以热水抽提硬木糖枫的半纤维素浓缩抽提液为原料,以稀硫酸为催化剂,以600兆赫兹核磁共振氢谱（1H NMR）和二维波谱（2D HSQC）为主要分析手段,对浓缩抽提液分别于常压和带压下的最佳水解工艺进行了摸索探讨,重点分析了水解产物中的各组份的含量受水解工艺条件如反应温度、催化剂酸用量、反应时间等因素的影响。同时,在盛行的木质纤维素类生物质水解反应的动力学模型——Saeman模型和两相水解模型的基础进行拓展与改进,提出新的半纤维素稀酸水解反应的动力学模型,运用该模型对浓缩抽提液的稀酸水解反应进行拟合,解析出半纤维素水解反应、单糖降解反应和糠醛与羟甲基糠醛降解反应的速率常数、表观活化能及指前因子等一系列动力学参数。结论如下:1)使用稀硫酸催化水解糖枫热水抽提浓缩液能实现半纤维素木聚糖水解成单糖的目的。木糖是水解液中含量最多的单糖组份,占单糖总量的80%以上,其它单糖包括阿拉伯糖、葡萄糖、甘露糖、半乳糖和鼠李糖。除阿拉伯糖外,其余的五种单糖在水解液中的浓度于一定时期内随着反应温度的升高、催化剂酸用量的增加和反应时间的延长呈上涨趋势;当它们的浓度达到最大值后,继续延长反应时间会导致浓度的下降。阿拉伯糖被证实是一种易降解的单糖,多数实验中只能观察到其浓度随反应时间的延长而下降的趋势。醋酸是水解液中含量最多的非糖类副产物,实验证实当催化剂酸浓度达到一定值（≥1%）时,乙酰基会很快地从半纤维素主链上断裂开来,醋酸浓度往往在反应前几分钟（小于两分钟）或升温过程中就达到一个平台值,此后其浓度基本不发生变化。其它的副产物还包括糠醛、羟甲基糠醛和甲酸等,糠醛和羟甲基糠醛的浓度都是随着反应温度的升高、酸浓度的增加和反应时间的延长而增大的,糠醛和羟甲基糠醛在水解液中的存在证实了五碳糖和六碳糖在反应过程中发生了降解,且反映了反应条件越严厉时五碳糖与六碳糖降解的越多。甲酸是在催化剂酸浓度达到一定值（1%）后才开始产生的,一旦产生后其在水解液中的浓度随反应时间的延长而缓慢增长,但其总量相当有限。2)水解反应的最佳工艺条件。对于浓缩抽提液1#于常压下的水解,推荐水解反应的最佳工艺条件为:反应温度95¹05°C、硫酸与浓缩抽提液的质量比6.2%、反应时间20～30分钟。对于采用高压反应釜做反应器的浓缩抽提液2#的水解,推荐的最佳水解工艺条件为:反应温度120°C、硫酸与浓缩抽提液的质量比1.5%、反应时间25分钟。3)由建立的半纤维素水解反应的动力学模型解析出的系列动力学参数,包括各种反应条件下的各反应的速率常数、表观活化能和指前因子。在反应温度85～105°C、酸浓度1.5～6.2%的反应条件下,浓缩抽提液1#中的半纤维素水解反应的速率常数在1.28×10^-3～3.29×10^-2 min^-1范围内,反应表观活化能为117.88 KJ/mol,指前因子为6.58×1014 min^-1;单糖降解反应的速率常数在2.18×10^-6～1.74×10^-3 min^-1范围内,反应表观活化能为133.24 KJ/mol,指前因子为9.12×1014 min^-1;糠醛与羟甲基糠醛降解反应的速率常数保持为零,表明在此种条件下糠醛与羟甲基糠醛没有发生降解。在反应温度120～150°C、酸浓度0～1.0%的反应条件下,浓缩抽提液2#中的半纤维素水解反应的速率常数在1.11×10^-3～0.236 min^-1范围内,反应表观活化能为103.11 KJ/mol,指前因子为1.18×1012 min^-1;单糖降解反应的速率常数在7.03×10^-5～1.56×10^-3 min^-1范围内,反应表观活化能为142.61 KJ/mol,指前因子为6.01×10¹⁴ min^-1;糠醛与羟甲基糠醛降解反应的速率常数在5.35×10^-5～8.77×10^-4 min^-1范围内,反应表观活化能为128.77 KJ/mol,指前因子为6.61×10¹² min^-1。更多还原

【Abstract】 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.更多还原