【作者】 毛燎原；

【导师】 李爱民；

【作者基本信息】 大连理工大学 ， 环境工程， 2013， 博士

【摘要】 经过60余年的发展,我国已成为全世界最大的糠醛生产和出口国。然而,高污染、高能耗、低产率等问题长期制约着糠醛产业的健康发展。长期生产实践表明,在我国,每生产一吨糠醛排放COD>18000mg/L的废水约20吨、消耗洁净蒸汽超过20吨,同时糠醛渣燃烧产生大量二氧化硫,而实际糠醛产率仅为理论值的30-50%。近年来对于糠醛废水的治理虽已取得较大进展,闭循环式的废水蒸发回用工艺在绝大多数糠醛厂被普遍采用,然而由于制约废水处理效率的污垢问题长期无法解决,从而使得废水实际处理率远远低于设计水平。随着国家对环保要求的日益严格、同业竞争的日益加剧及各类生产成本的不断上升,糠醛生产企业面临严峻的生存困境。本文首先对植物纤维水解方面的研究及糠醛生产现状、研究进展进行了总结。然后结合当前生产实践对糠醛生产的整体物流与能流进行了分析与讨论,重点讨论了糠醛产率、醛汽中糠醛含量(或糠醛生成速率)、锅炉热效率以及蒸发器热效率对当前糠醛生产体系物质与能量平衡的影响,从而确立了本文的研究路线。由于含乙酸的废水回用生产是当前糠醛生产中的必经之路,同时也是本文研究的基本前提,因而乙酸“酸汽”的使用贯穿全文。在此基础上,本文系统地分析了废水中乙酸的来源及产量,对乙酸“酸汽”催化玉米芯水解制取糠醛的可行性进行了另析,重点研究了糠醛在含乙酸蒸汽中自身降解损失动力学,为全文研究中催化剂乙酸“(?)气”的合理使用奠定基础。然后运用动力学方程计算出生产中糠醛在气相中降解损失率(?)占实际生产中糠醛理论产率的4.5%,约占糠醛总损失的10%。进一步表明糠醛生产(?)的主要损失发生在液相反应体系,因此加快糠醛从液相中的分离是提升糠醛产率的关(?)针对单一乙酸酸汽催化玉米芯水解制取糠醛时所需温度较高、反应速率较低、能耗大、废水产量高的问题,进一步研究了路易斯酸催化剂FeCl3、AICl3在糠醛(?)取中的应用。首先从纯戊糖催化脱水动力学以及糠醛在路易斯酸催化条件下降解的动力学研究出发,并对戊糖脱水生成糠醛最高产率进行了模型化分析,通过比较确立了获(?)糠醛最高产率的三因素参数模型,进一步明确了戊糖脱水制取糠醛反应体系中的反应(?)程。通过与H2SO4(?)(?)较两种不同路易斯酸FeCl3、AICl3的催化选择性及动力学参数,结果表明,后者能使戊糖脱水速率常数增大10倍以上,同时糠醛产率也达到70%以上(Al(?)3催化)。在此基础上,进一步研究了采用FeCl3和“酸汽”协同催化玉米芯水解制取糠(?)的催化效果。通过对水解残渣微观结构的观察,发现采用FeCl3催化玉米芯水解时,(?)料颗粒的破碎、微观纤维结构的破坏明显加剧,这将有效增大物料内部及物料之间的(?)道,从而致使体系内传质、扩散效率的提升。基于此,进一步的与研究表明,由于原料被破碎后导致的颗粒物料孔隙率降低、压降增大而促使床层扰动而获得了传质的强化和表面更新速度的加快。此外,通过对AICl3对催化戊糖及玉米芯的单因素影响试验结果的考察,发现AlCl3在催化戊糖及玉米芯制取糠醛方面具有比FeCl3更高的选择性,更有利于提升糠醛产率,这为制取糠醛选择催化剂方面提供了新的方向。为了进一步提高糠醛实际产率,从强化蒸馏分离与强化蒸汽气提两个反面进行了糠醛分离的研究。结果表明：不同价态阳离子对糠醛-水二元体系组分分离的作用效果与离子强度正相关,离子强度越高,“盐析”效果越显著,反之越低；一价阳离子对糠醛-水二元体系组分相对挥发度的作用效果可由Furter方程拟合,二价及三价阳离子对糠醛-水二元体系组分相对挥发度的作用效果较复杂,可由修正型Furter方程拟合。采用浓缩海水获得了与模拟海水极为接近的汽提效果,海水作为盐析剂时与纯NaCl(等浓度)作为盐析剂时获得了相似的盐析速率和糠醛回收率。但由于海盐(或模拟海水)中存在Ca2+及Mg2+,因此总体盐析效果高于等量纯NaCl。为了解决制约废水处理效率的蒸发器结垢问题,首先通过对取自糠醛厂的蒸发器污垢样品的特性及形成过程进行了分析和探讨,根据检测和分析结果对其形成机理做出了推断。进一步的模拟结垢试验表明,采用碱中和可消除糠醛废水蒸发过程换热管的结垢,但由于碱中和需碱量极大,这对于糠醛生产企业而言无法承受。进而本研究提出了采用絮凝法去除溶解木质素、悬浮碎屑以及部分单糖的废水预处理方法。结果表明可绝大程度上抑制污垢的生成,该方法成本低,且不影响废水中的乙酸。从长远角度来看,采用絮凝剂除去溶解木质素及悬浮碎屑一方面保持了废水中乙酸,另一方面成本较低,具有长远的推广价值。最后是前文所有研究的应用、优化与总结,并且提出了糠醛清洁生产的最优工艺。首先进行了以戊糖为原料、以NaCl作为盐析剂、以FeCl3为催化剂制取糠醛的研究,并获得相关反应动力学参数,结果表明盐析剂的存在能大幅提升糠醛产率；然后进行了采用氯盐为盐析剂、以FeCl3作为催化剂、以玉米芯为原料由一步法反应制取糠醛的研究,并获得了各因素对糠醛产率的影响范围。为了进一步获得糠醛生产最佳工艺参数,以富含氯盐的浓缩后海水作为盐析剂,采用响应面分析法,确立了海水浓缩倍数、FeCl3浓度、反应温度、乙酸浓度的最优化组合,对玉米芯一步法水解制取糠醛的整体工艺进行了优化,得到了影响糠醛产率的最佳组合条件,并获得了最高产率的模型；验证试验表明模型的拟合程度很好,试验误差较小,模型成立,可以用此模型对浓缩海水盐析-FeCl3-乙酸催化玉米芯生产糠醛的最佳工艺进行分析,同时也表明该模型能够代替真实试验的结果进行表征。最后,提出了糠醛清洁生产最佳工艺,采用该工艺可实现糠醛生产过程污染物近零排放,同时降低原料生产成本约38%,使糠醛产率提高至80%以上。更多还原

【Abstract】 In the past60years, furfural industry in China has developed into the largest all over the world. However, development of furfural production in China has long been blocked by problems of high pollutants and high energy consumption. It was indicated that there about20t wastewater (COD>18000mg/L) generated and more than25t water steam was consumed to obtained1t furfural. In the recent years, the reuse of furfural wastewater by converting it into steam has been applied by most of the furfural plants in China. However, due to the fouling problem, the actual wastewater treatment efficiency could never reach the design index. Today, furfural plants have fallen into a hobble due to increasingly acute regulation and competition.General situation of furfural production and the research on hydrolysis of lignocellulose and furfural production were summarized firstly. Then, mass and energy balance in furfural production was analyzed and, factors such as furfural yield, furfural concentration in product vapor (or formation rate)、thermal efficiency of boiler and heat exchanger which affect the balance of the system was discussed in detail. It was indicated that any unilateral increasing could never satisfied the need of the running of the furfural production system and systemative method must be applied to promote the under-developed furfural production situation.Since the reuse of wastewater that was rich in acetic acid is an essential part of furfural production and a prerequisite of this research, the origin and output of acetic acid was analyzed at first. Then, furfural degradation kinetics in "acid steam" media was studied so as to confirm the feasibility of wastewater reuse. Then, a calculation that based on this furfural degradation kinetics indicated that there about4.5%of furfural potential yield (about10%of total furfural loss) decomposed in "acid steam" phase. Obviously, it could be inferred that most of the furfural loss was caused by its composed (or polymerized) in the liquid phase. Therefore, promotion in separating furfural from the liquid phase was the crucial step to reduce furfural loss.Aimed at lowering the reaction temperature and increasing furfural formation rate, Lewis acid FeCl3AICl3were applied in catalyzing the dehydration of xylose into furfural. Kinetics of furfural degradation and kinetics of xylose dehydration in Lewis acid media were studied. Additionally, calculated maximum furfural yields were obtained from a three factor model and the dehydration reaction mechanism of xylose was obtained consequently. Furfural selectivity and kinetics parameters indicated that the furfural yield exceeded70%when it was catalyzed by FeCl3or AlCl3and, the dehydration reaction rate of furfural was more than10times higher than that catalyzed by H2SO4. Consequently, FeCl3and "acid steam" co-catalyzed furfural production from corncob was mainly studied in chapter4. A SEM analyse indicated that the microscopic cellulose structure of hydrolyze residue was broken much more severe as FeCl3added and used as catalyst. Then, the micro channels in the particle open and the mass transfer efficiency, the diffuse efficiency in the liquid reaction increased. It was deduced that the mass transfer and the surface renewal could be promoted by reducing the porosity of particle materials layer and increasing pressure drop once the particle material was broken to the ground. In addition, s series contrast experiments indicated that AICl3was more effective for promoting furfural yield.Aimed at promoting the furfural separation from liquid phase and reducing its loss, both of enhancing methods on distillation separation and steam stripping were studied. Experiment results indicated that the enhancing effect of cation on the separation of furfural was depending on its ionic strength, high ionic strength causing high separation efficiency. Additionally, impact effects of one-valence cations on furfural separation from liquid phase could be fitted by Furter equation, but the effect of bivalent and trivalent cations only could be fitted by an amendatory Furter equation due to the complex affect mechanism. In order to save salt resource and production cost, concentrated seawater which rich in metal ions was used as salting out agents. Similar separation efficiencies were obtained when concentrated seawater and simulate seawater was used as salting out agent respectively. But it was also indicate that the furfural separation efficiency in the former was a litter higher due to the effect of Ca2+and Mg2+that contained in concentrated seawater.Fouling problems which severely hampered the evaporation and reuse of wastewater was studied. Firstly, the formation mechanism of fouling in heat exchange tubes was inferred and simulation experiments on the formation and eliminating f fouling for the evaporation of wastewater were conducted. The results indicated that there was almost no fouling formed when the wastewater was neutralized to alkalescence by adding NaOH or Na2CO3. Then, the formation mechanism of fouling could be inferred as following:Acetic acid catalyzed polymerization of dissolving lignin and residual furfural in the wastewater at high temperature. The polymeric substances thus forming and remaining in the heat exchange tubes. Consequently, fouling formed as polymeric substances was hydrothermal carbonized which catalyzed by acetic acid. Although fouling could be eliminated by adding sodium carbonate, few furfural manufactures would apply this method due to the high cost. Therefore, a flocculation method which aimed at removing large particles of dissolving lignin and floatable lignocellulose materials was applied. Simulation experiments results indicated that fouling also could be restrained as a certain cationic polyacrylamide flocculants was used. Therefore, pretreatment of furfural wastewater by flocculation was promising not only because of its simple and low cost, but also almost all of the acetic acid was remained.The last Chapter was the application, optimization, summary and perspective of all the previous fundamental research. There were three steps of research in this chapter. Firstly, xylose degradation kinetics in FeCl3catalyzed and NaCl salting out media was studied. The results indicated that furfural yield could only be promoted dramatically by salting out effect of NaCl. Secondly, furfural production from corncob catalyzed by FeCl3in chloride salting out media was studied, and the concentration range of catalyst and salting out agent, range of reaction temperature were obtained. Thirdly, in order to obtain maximum furfural yield, RSM method was used in optimizing all of the reaction parameters. The element of reaction temperature, FeCl3concentration, acetic acid concentration and concentrated ratio of seawater (or concentration of chlorine salts) optimizes combination was obtained. Furthermore, a maximum furfural yield model was obtained. Finally, an optimal furfural clean production process was establishede as almost all the pollutants could be avoided. Furthermore, the results indicated that the material cost could be reduced38%, and more than80%of the potential furfural yield could be obtained by this process.更多还原