【作者】 张海阳；

【导师】 匡亚莉；

【作者基本信息】 中国矿业大学 ， 矿物加工工程， 2013， 博士

【摘要】 能源微藻作为一种可再生的生物质能源，具有生长周期短、油脂含量高、低污染、不占用耕地等优点，被认为是未来最有潜力替代化石燃油的生物质能源。但是，目前能源微藻的生产加工成本过高，尤其是采收环节，阻碍其产业化进程。由于微藻细胞个体小、浓度低，且稳定悬浮于培养液中，给采收带来很大的挑战。气浮法作为一种高效的固液分离方法，用于能源微藻的采收具有较大的发展潜力。但是，传统的气浮法利用空压机和填料式溶气罐进行高压溶气，噪音大、能耗高、设备维护复杂；采用机械搅拌式或结构复杂的反应器进行絮凝，停留反应时间较长，且需要额外的能耗，增加运行成本；气泡与絮体只是通过简单的逆流接触碰撞的方式，粘附效率低、牢固性差。基于以上问题，本文在传统气浮法和矿物浮选的基础上，研发了基于射流流场的能源微藻混凝共聚气浮采收系统。系统主要由溢流鼓泡式溶气系统、射流释气混合器、螺旋混凝共聚反应器、嵌套式气浮柱4部分构成。通过射流释气混合器与混凝共聚反应器相结合，让气泡直接参与微藻混凝的过程，在絮体中成核，使更多的气泡被包裹在絮体中，形成“絮体-气泡”共聚复合体，实现了集释气、混药、混凝、粘附为一体的微藻混凝共聚气浮采收。在此基础上对采收条件和作用机理进行了试验研究和理论分析，最终建立了能源微藻混凝共聚气浮采收的技术理论体系。论文首先对溢流鼓泡式溶气系统和射流释气混合器的溶释气效能进行试验研究，考察溶气压力、气液比、流量比、溢流水高度、表面活性剂对释气量和气泡粒径分布的影响。结果表明，增加溶气压力可提高释气量，减小气泡粒径分布，但溶气效率降低；随着气液比的增加，释气量变化较小，气泡平均直径增加；流量比对射流流场影响较大，过大或过小的流量比都会减小释气量，增加气泡的平均直径；随着溢流高度增加，溶气停留时间增加，释气量相应增加，同时气泡平均直径逐渐减小；表面活性剂可降低水的表面张力，抑制气泡兼并，形成的气泡更加微细，同时，由于形成微气泡与溶气水共存的混合水流，增加释气量。通过溶气和释气条件进行优化，该溶气释气系统实现低压高效溶气释气，在压力0.2MPa，溶气效率为96.4%，气泡直径在50-60μm分布。为了研究射流流场的释气机理，对射流释气混合器的气泡析出进行热力学分析，结果表明，析出微气泡的大小与溶气压力和释气压降有关，提高溶气压力和增加释气压降，使释放出的微气泡更小，分布更均匀；同时，结合伯努利方程、物料守衡、射流泵理论方程对射流释气混合器进行分段式能量分析，推导出各段的能量转化效率公式，并简化相应参数，计算出流量80L/h，流量比0.1时的能量转化效率，结果表明，释气能量耗散主要在喉管入口和喉管段完成；最后通过Fluent流场模拟软件对射流释气流场进行数值模拟，考察流量比对压力降、能量耗散速率、紊流强度的影响，结果表明，流量比在0.1-0.15间，基于射流流场环境的释气效率较好。通过正交试验考察了凝聚剂、絮凝剂、pH、混药时间、混凝时间对微藻混凝气浮效率和絮体结构的影响。结果表明，除混药搅拌时间表现不显著外，其它几个因素具有一定的显著性。在正交组合条件下，凝聚剂和絮凝剂用量分别为20，15mg/L、pH为6、混药时间30s、混凝时间10min时，混凝气浮效果最好，气浮采收率高达95.31%。在此基础上，对射流释气混合器的射流流场的混药性能进行优化试验研究，结果分析表明，流量比在0.1-0.2范围，射流流场对药剂的混合效果最佳。设计制作了螺旋混凝共聚反应器，使气泡直接参与微藻细胞的混凝过程，气泡在絮体中成核，随絮体一起并大，形成“絮体-气泡”共聚复合体。并与射流释气混合器相结合，实现集释气、混药、混凝、粘附为一体的微藻混凝共聚。由絮体图像显微系统观察可知：该反应器较传统的逆流接触絮凝，形成的絮体包裹气泡数更多，粘附更牢固。通过试验对反应器的结构参数和操作条件进行优化研究，初步建立了流场参数和停留时间对混凝共聚效果的影响关系，并引入狄恩常数和杰曼诺常数对螺旋混凝共聚反应器的流场进行较为准确的描述，为反应器的结构的进一步优化建立了理论基础。由碰撞粘附机理和效率动力学模型可知，气泡与絮体的碰撞粘附效率与二者的大小有关。在此基础上，通过试验考察混凝共聚反应器中絮体大小和气泡大小对气浮效率和带气絮体上浮速率的影响，从而间接反应二者的碰撞粘附效率。结果表明，在保证絮体结构较为紧密的条件下，增加絮体粒径和减小气泡大小可提高二者的粘附效率，从而提高混凝共聚效率。同时，由试验研究发现，适量的表面活性剂有利于增加细胞表面的憎水基团，提高絮体与气泡的粘附紧密性和粘附数量，但过量时会阻碍絮体与气泡的粘附。设计制作了嵌套式气浮柱，并进行了分段动力学模型分析，在此基础上，通过试验研究，考察气浮柱结构和操作参数对气浮分离效率的影响。结果分析表明，外内筒直径比Rd对分离静态环境影响较大，过大时，紊流强度较大，影响絮体结构和静态分离环境。过小时，增加过渡区长度和浊度，影响底流出水澄清度；管流加速段高度应高于过渡区长度，否则底流出水浊度较大；分离区高度过低时，管流加速段出口对分离区静态分离环境扰动较大，影响气浮效率。过高时，由于壁面摩擦和壁面粘附等影响絮体上浮；在保证采收率变化不大的情况下，可通过增加底流和溢流的流量比提高浮出物的浓缩倍率。在整个微藻混凝共聚气浮采收系统的条件和结构优化的基础上，对实验室小规模的气浮采收系统进行经济效率分析。结果表明，入料流量200L/h时，能耗较低，2.72kW h/kg（干重），浓缩倍率为24.9。以此为基础，计算了实验和工业规模的采收成本，分别为1.94，1.45元/kg（干重），低于常规微藻采收方法，并在能源微藻采收成本要求范围，因此证明该系统适用于能源微藻的采收，具有一定的推广价值。更多还原

【Abstract】 As a renewable biomass energy, energy microalgae has the advantages of short growthperiod, high oil content, low pollution and no land occupation, has been considered the mostpromising future to replace the fossil fuel of biomass energy. However, the current energymicroalgae production and processing costs are too high, especially the recovery process,hindering its industrialization process. As the microalgae cells are small, low concentration, andstable suspended in culture medium, which brought great challenges to the harvest. Flotation asan efficient solid-liquid separation method, has great development potential for harvestingenergy microalgae. However, the dissolved air system of traditional flotation(TF) consituted a aircompressor and filler-type dissolved air tank has shortcoming of noise, high energy consumption,maintaining complex. The mechanical stirring or a complex structure of reactor are used in TFfor flocculation. Reaction time is long and additional energy consumption is needed, whichincrease operating costs. The adhesion efficiency between bubble and floc by countercurrentcontact in TF is low, and the structure of floc is loose.On account of the above problems, this study based on the traditional flotation and mineralflotation was carried out, and a microalgae coagulation copolymerization flotation sytem basedon jet flow field consists of verflow bubbling system, jet release mix reactor(JRMR), spiralcoagulation copolymerization reactor(CCR), and nested flotation column, is established. In thissystem bubbles nucleated in flocs are added to the process of microalgae coagulation.Floc-bubble copolymers adhered more bubbles are produced in this system, which achieverelease air, mixing, coagulation, adhesion as a whole microalgae coagulation-copolymerizationflotation. On this basis, a series of optimization expirements and theoretical analysis were carriedout, and technical theory of microalgae coagulation copolymerization flotation was built.Firstly, a series of experiments on dissolved and released air efficiency of overflowbubbling system and JRMR were carried out to study the effects of dissolved air pressure,gas-liquid ratio, flow ratio, overflow height, surfactant on released air volume and bubble sizedistribution. The results show that with the increasing of dissolved air pressure, the released airvolume increased and the average diameter of bubbles reduced, but the efficiency of dissolvedair decreased. When the gas-liquid ratio increases, the average diameter of bubbles increased, butthe released air volume changed little. Effect of flow rate on the jet flow field is large, too largeor small flow ratio will reduce the release of gas, and increase the average diameter of bubbles.With the overflow height increased, the residence time of dissolved air increases, so the averagediameter of bubbles increased, and the average bubble diameter decreases. Surfactant canreduces the surface tension of water, inhibition of bubble coalescence, form more fine bubbles, at the same time, due to the formation of micro bubbles and dissolved air water mixed flow,increase the released air volume. The low dissolved air pressure and high released air efficiencycan been achieved by this system under the optimal conditions. At a pressure of0.2MPa, thedissolved air efficiency is96.4%and the bubbles diameter range is50-60μm.In order to study the mechanism of released air in jet flow field, thermodynamic analysiswas performed on the JRMR. The results show that bubble size is related to dissolved airpressure, which the higher dissolved air pressure and released air pressure difference, the smallerbubble. At the same time, by using the bernoulli equation, the material balance and thetheoretical equation of jet pump to analysis of segmented energy on JRMR, the energyconversion efficiency formulas are deduced, and simplify the corresponding parameters,calculate the flow rate80L/h, the flow ratio of0.1energy conversion efficiency. It shows that,the air released energy dissipation complete mainly at the throat and throat entrance; Finally, theflow field of the JRMR is simulated by Fluent, and the influence of flow ratio on pressure drop,the energy dissipation rate, turbulent intensity were studied. The results show that, the flow ratiobetween0.1-0.15, the efficiency of gas release based on the jet flow environment is better.Coagulant, flocculant, pH, mixing time, coagulation time effect of coagulation flotationefficiency and floc structure of microalgae was investigated by orthogonal test. The results showthat, in addition to mixing and stirring time performance is not significant, several other factorshave significant certain. In the conditions of orthogonal combination, when the coagulant andflocculant dosage were20,15mg/L, pH6, mixing time30s, coagulation time10min, coagulationflotation efficiency is best, the recovery rate as high as95.31%. On this basis, to optimize theexperimental study about the mixing performance of jet flow field of JRMR, results show that,when the flow ratio in the range of0.1-0.2, jet flow field for coagulant and flocculant mixing isbest.CCR was dsigned and manufactured for coagulation copolymerization of microalgae.In thisreactor, the bubble is directly involved in the coagulation process of microalgae cells, bubblenucleation in flocs, together with the floc become big, to form the "floc-bubble" copolymer. Andwith JRMR combined, realize the integration of released air, mixing, coagulation, adhere formicroalgae coagulation copolymerization. Aerated flocs formed in CCR adhere more bubblesand more strong than them in the traditional countercurrent contact flocculation. Theoptimization experiments of structural parameters and operating conditions of CCR were carriedout, and the effect of flow parameters and residence time on microalgae coagulationcopolymerization was estabished. The Dean number and Germano number were introduced todescribe the coagulation flow field of CCR, which established theoretical basis for structuraloptimization of CCR. According to mechanism and dynamics of adhesion, the adhesion efficiency of bubbles andfloc relative to the size of them. On this basis, a series of experiments were carried out to studythe effect of floc size and bubble size on flotation efficiency and floc floating rate, therebyindirectly reaction the adhesion efficiency. The results show that, when the floc more compactly,the adhesion efficiency of flocs and bubble will been improved by increasing the floc size orreducing the bubble size. Meanwhile, the experimental results indicate that proper quantitysurfactant are better to increase the hydrophobic groups of cells, and the structrue of aeratedflocs adhered many more bubbles is more compactly.The nested flotation column was designed, and its dynamics models were establishedpiecewise. On this basis, the effect of structrue and operating parameters of flotation column onseparation efficiency was studied. Results show that, the outer and inner tube diameter ratiohas a great influence on the static separation environment, when the is too large, turbulentintensity is large, affect the floc structure and static separation environment. When the is toosmall, increase the length of transition zone and turbidity, affect the clarity of bottom water. Pipeflow acceleration section height should be higher than the length of transition zone, otherwise theturbidity of the bottom outflow water will be increased. When the height of separation zone islow, its flow field will been disturbed. When the height of separation zone is high, the separationefficiency may been reduced by wall effect. The concentration factor(CF) can been improved byincreased the flow ratio of underflow and overflow.Finally, the economic efficiency of Microalgae coagulation-copolymerization flotationsystem in lab scale was analysised. Results shows that when the feeding flow is200L/h, thelower energy will been consumed,2.72kW h/kg(dry weight), in the meantime CF of24.9will beobtained. On this basis, the recovery cost of experimental and industrial scale were calculated,respectively is1.94,1.45yuan/kg (dry weight), lower than that of conventional microalgaerecovery method, and meet the energy of microalgae recovery cost requirements, so that thesystem is suitable for harvesting energy microalgae, and has a certain popularization value.更多还原