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甘蔗糖蜜是甘蔗制糖行业的主要副产物，含有丰富的蔗糖、还原糖、色素和无机盐，目前主要作为底物生产酒精和酵母，但会产生大量富含色素和无机盐的废水。在以甘蔗榨汁为原料的膜法绿色制糖工艺中试成功后，本课题组提出了以糖蜜为原料的膜法绿色制糖工艺---利用膜分离技术从糖蜜中分离得到白砂糖、色素和还原糖。糖蜜的澄清和脱色处理是整个工艺的重点，决定了白砂糖质量和产量。因此，本文主要聚焦在糖蜜的膜澄清和脱色阶段，研究了膜污染机理、分离机制和膜污染控制，为新工艺的开发和优化提供理论支持。首先，评价了5种商品化聚醚砜超滤膜的糖蜜澄清效果，发现UH050超滤膜最适合糖蜜的澄清操作，不仅可以将悬浮物、果胶等物质完全去除，而且蔗糖截留率最低、膜污染最小。为了更好地控制澄清阶段的膜污染，通过分别关联透过通量与平均跨膜压力、平均膜污染速率和单位能耗的方法测定了不同稀释倍数下的临界通量，建立了稀释-浓缩模式下的临界通量测定新方法。根据临界通量值和不可逆污染的大小，推断出糖蜜的最佳稀释倍数是8倍，临界通量值为26-32 L/(m2·h)，在临界通量下运行可以有效地控制膜污染。最后，通过聚多巴胺涂敷和后续接枝聚合物的膜改性方法，改变聚醚砜超滤膜的亲水性和荷电性，探究膜污染的机理。研究发现，在临界通量下运行，膜面修饰对膜污染的影响较小，但当运行通量大于临界通量时，膜面的亲水性和荷电性对膜污染的影响显著。氧化海藻酸钠改性会使膜荷负电性增多，与糖蜜中的主要污染物（荷负电）之间产生更强的静电排斥作用，从而US100膜的临界通量增大了14.3%，其总污染阻力因操作通量的不同而降低了29.1%-64.9%。 然后，评价了11种商品化超滤膜和纳滤膜的糖蜜脱色效果，发现UP005超滤膜和NF5纳滤膜适合对糖蜜进行脱色操作，色素截留率都超过85%。但是二者截留机理却不相同，UP005膜是因为膜污染产生缩孔效应而实现色素的高截留，而NF5膜是依靠其自身的致密分离层。系统研究了糖蜜各组分对脱色膜截留和污染行为的影响，发现还原糖对蔗糖截留的影响可以忽略，无机盐会争夺膜表面的水化层，导致膜孔溶胀，从而降低对蔗糖的截留。色素等污染物则会抵消无机盐诱发的膜孔溶胀效应，从而提高对蔗糖的截留。最后，通过分析操作参数对膜污染和蔗糖截留的影响，发现高温和低通量会降低蔗糖的截留，高温和高通量会加剧膜污染，搅拌速度对蔗糖截留的影响较小(60 ℃)，但会减轻膜污染。UP005聚醚砜膜的耐高温和耐化学清洗稳定性较强，但是其膜污染严重影响其长期运行稳定性，因此需要对UP005膜进行缩孔和亲水化改性，在保证色素截留率的同时降低膜污染的发生。通过静态吸附和过滤操作研究了UP005的膜污染组成，发现疏水吸附是其发生严重膜污染的主要原因。利用多巴胺辅助膜改性技术对超滤膜进行膜面和孔内修饰，发现这些膜改性技术尽管可以降低静态吸附时的不可逆污染，但却只能部分缓解过滤操作时的孔内污染，这主要是由于改性小分子本体存在疏水位点、亲水大分子难以进入深层膜孔修饰孔壁、超亲水矿化层不稳定等原因造成。因此对糖蜜脱色膜的改性，不仅要增加膜的亲水性和荷负电性，更重要的是对膜孔进行缩孔处理和提高改性层的稳定性，避免孔内吸附污染的产生，使其可以依靠修饰后的膜孔实现色素和蔗糖的有效分离。;Cane molasses, a main by-product in plantation white sugar production, contains a mass of sucrose, reducing sugar, pigments and inorganic salts. And it is mainly utilised as substrate for the production of ethanol and yeast, which generates a large amount of high-strength wastewater with inorganic salts and pigments. After the pilot-scale production white sugar from sugarcane juice by membrane technology was succeed, we make attempt to recover sucrose, pigments and reducing sugar from cane molasses by membrane technology. Thereinto, clarification and decoloration of molasses are the key issues for this new technology, determining the quality and output of white sugar. Therefore, this thesis focuses on the membrane fouling mechanism and its effect on sucrose retention as well as fouling control strategies in the clarification and decoloration of molasses, which can provide theoretical support for the development and optimization of this new technology. Firstly, five commercially-available polyethersulfone (PES) ultrafiltration membranes were selected for the clarification of molasses. It was found that UH050 membrane was the most suitable for the purpose, because it could remove suspended solids and pectin completely, and its sucrose retention and membrane fouling were the lowest. In order to control membrane fouling, the threshold flux for differently diluted molasses was evaluated by associating permeate flux with average transmembrane pressure, average membrane fouling rate or specific energy consumption respectively in dilution-concentration mode, where the amount of raw molasses treated by the membrane should be the same. According to the variation of threshold flux and irreversible fouling, it was deduced that the optimal dilution factor of molasses for clarification by UH050 was 8 times and the threshold flux value was around 26-32 L/(m2·h), below which the membrane fouling could be minimized. Finally, the hydrophilicity and charge pattern of the PES membrane were manipulated via a mussel-inspired coating and subsequent polymer grafting in order to explore membrane fouling mechanism. It was found that the membrane fouling was low when operated at threshold flux, and that the membrane surface hydrophilicity and charge pattern only had significant effect on membrane fouling when the operating flux was greater than threshold flux. Besides, the sodium alginate (ADA) grafting could obviously increase the negative charge on the membrane surface, which generated stronger electrostatic repulsion between foulants and membrane, thus resulting that the threshold flux increased by 14.3% and the total membrane fouling decreased by 29.1-64.9%.Next, eleven commercially-available ultrafiltration and nanofiltration membranes were selected for the decoloration of molasses. It was found that UP005 ultrafiltration and NF5 nanofiltration membranes were suitable to achieve the high selectivity of sucrose and pigments, whose colour retention were both over 85%. However, their separation mechanisms are different. For UP005 membrane, the high pigment retention was mainly caused by the pore narrowing effect due to membrane fouling. While for NF5 membrane, its own dense separation layer resulted in the high pigment retention. Through systematically investigating the effects of different components in molasses on sucrose retention and membrane fouling behaviours, it was found that reducing sugar had little effect on the sucrose retention; while inorganic salts resulted in pore swelling, lowering the sucrose retention; but the pore swelling effect induced by inorganic salts was eliminated by the pigments fouling, regaining the high sucrose retention. Finally, through analysing the effect of operating parameters on the membrane fouling and sucrose retention, it was found that high temperature and low permeate flux could reduce sugar retention, and high temperature and high permeate flux could aggravate membrane fouling, and the stirring speed had little effect on sugar retention but could reduce membrane fouling at 60 ℃.UP005 membrane has strong resistance to high temperature and chemical cleaning, but the serious membrane fouling would influence its long-term and stable operation. Therefore, we attempted to decrease the membrane fouling of UP005 by membrane modification technology, that is, the pore narrowing and hydrophilicity enhancement were realized with nearly negligible effect on pigments retention. Through investigating the membrane fouling resistance under static adsorption and dynamic filtration modes, it was proved that hydrophobic adsorption was mainly responsible for the serious membrane fouling. Then, the membrane surface and pores were modified by mussel-inspired membrane modification methods. It was found that the irreversible fouling under static adsorption was greatly mitigated, while the irreversible fouling under dynamic filtration was only reduced marginally, which was caused by the exposed hydrophobic sites of the small molecules (benzene ring), low accessibility of large molecules (ADA) into the membrane pores and the instability of the super-hydrophilic mineralized layer (TMOS). Therefore, the modification of UP005 membrane should not only increase its hydrophilicity and negative charges of membrane surface and pores, but also narrow the membrane pores and increase the modification layer’s stability. Only by this way, the serious hydrophobic adsorption in the membrane could be avoided, and the separation of pigments and sucrose was able to be achieved by its own dense modified separation layer.
|杨强剑. 膜分离技术在糖蜜澄清和脱色中的应用基础研究[D]. 中国科学院大学,2019.|
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