【作者】 李友凤；

【导师】 叶红齐；

【作者基本信息】 中南大学 ， 化学工艺， 2012， 博士

【摘要】 由于混合直接影响到沉淀法制备功能粉体的性能,为了提高混合器内反应物料的混合效果,本研究根据撞击流微混合器内流体的流动形式及两股流体的撞击方式,提出了直流对撞(M1)、锥流对撞(M2)、直流旋撞(M3)和旋流旋撞(M4)四种不同流体碰撞方式的微混合器。通过实验研究和理论模拟计算,对四种微混合器的液液及气液两相的混合过程进行了探讨。在此基础上,考察了四种微混合器在沉淀法制备硫酸钡(BaSO4)、氢氧化铝(Al(OH)3)、铈掺杂钇铝石榴石(YAG:Ce3+)黄色荧光粉中的应用。1、微混合器液液混合性能：利用饱和KC1溶液为示踪剂,实验研究了物料在微混合器内的平均停留时间分布,结果显示：微混合器内流体几乎接近全混流；撞击区进口管径对停留时间及其分布影响很小；M4型混合器的出峰时间延迟,峰宽变窄,说明旋流旋撞改善了微混合器内物料的返混程度。利用碘化物-碘酸盐平衡竞争反应,实验研究了表征微混合器的液相微观混合特性的离集指数(Xs),并考察了流体的流速、流动形式及撞击方式对Xs的影响。结果表明,实验条件下,微混合器的Xs的大小在3.5×10-3-6.0x10-3之间；对于四种结构微混合器的Xs,发现存在如下规律：XSM4<XSM3<XSM2<XSM11;Xs随着流体流速增大、两股流体流速比减小、撞击区进口管径减小而逐渐减小；Fluent软件模拟了微混合器内液相速度场、速度矢量分布场、浓度场、湍动能k及其耗散速率ε分布场等特性,这些特性分布场很好地反映了微混合器流体的混合情况；根据Xs实验数据和模拟结果ε的平均值分别对微混合器的混合时间tm进行估算,得出其微观混合时间都在1ms数量级。2、微混合器的气液混合特性：用NaOH-CO2-N2和H2O-CO2-N2化学吸收法测量了这四种微混合器在气液两相逆流接触条件下平均相界比表面积(a)及液相吸收传质系数(kL)；研究了微混合器结构形状、管道直径(Φ)、液体流量(qv,L)和气体流量(qv,g)等参数对a和kL的影响。发现a为1.2×105～1.0×106m2.m-3,经数据处理得a与各操作参数的关系有aM1<aM2<aM3<aM4,a∝Φ-2.2～-2.02,a∝qv,L0.55～0.62,a∝qv,g0.38～0.50。微混合器的传质系数kL为0.02～0.12cm.s-’,较常规气液接触设备的高了1-2个数量级,从而大大强化了其气液传质过程。当液体流量qv,L从5L·h-1增加到25L-h-1时,M4的传质系数kL是M1的2.69～3.23倍；当撞击区进口管径d从2mm减小到1mm时,在不同液体流量下M1的传质系数kL提高了1.08-1.36倍；气体流量对kL的影响不大,总体呈上升趋势；单涡模型计算结果表明撞击流微混合器具有促进气液传质效果,能较好地反映kL实验值。Fluent软件中的流体体积(VOF)模型模拟结果显示,实验条件下,撞击流微混合器内的气液两相间存在有肉串流、塞状流、气柱流和半环流等流型,这些流型的出现可以增大气液接触比表面积,从而增强微混合器内气液传质效果。3、液液混合制备BaSO4的研究：以BaCl2和Na2SO4为原料,采用四种撞击流微混合器制备BaSO4的研究表明,M4型混合器与其它混合器相比,获得的BaSO4颗粒的粒径更小,粒度分布更窄；流体流量增大及混合器进口管径减小有利于获得较小粒径颗粒；而初始反应物浓度增加也会导致产品粒径下降,但当浓度超过0.50mol·L-1后,产品粒度基本处于稳定。在优化的条件下,可获得平均粒径约100nm,比表面积为29.25m2.g-1的近椭球形超细硫酸钡粉体。4、气液两相混合制备氢氧化铝：以NaAlO2-CO2溶液为反应体系,利用撞击流微混合器制备拟薄水铝石的研究表明：与直撞M1相比,旋撞M3所得产品粒度小,分布均匀；四种微混合器中旋流旋撞M4更具有优势；随着液体和气体流量的增加,所获得的拟薄水铝石颗粒呈现平均粒径减小,粒度分布变窄的趋势；随着NaAlO2溶液初始浓度的增加,会导致产品粒度增大及其分布变宽的趋势；通过对操作条件进行控制,可获得细长纤维状拟薄水铝石粉末,该粉末粒径仅1Onm左右,长径比约为20,孔容为2.54cm3.g-1,比表面积高达390.6m2.g-1。5、多组分共沉淀制备YAG:Ce3+黄色荧光粉的研究：以碳酸氢氨为沉淀剂,采用撞击流共沉淀法制备的YAG:Ce3+前驱体于1000℃煅烧2h后,完全转变为YAG相,获得分散性良好的球形YAG:Ce3+黄色荧光粉：其晶相转变温度比固相法降低了约500℃；随着初始原料浓度的增大,荧光粉的发光强度逐渐减小；当掺铈浓度为2.6%Ce时,荧光粉发光强度最大；荧光粉的发光强度随着加料速度逐渐增加,当加料速速增大到20mL·min-1以上时发光强度基本保持恒定。控制相同的工艺条件,M3制得荧光粉的发光强度比M1的增大了18.6%；M4比Ml的升高23%。结果表明,旋流和旋撞能进一步强化微混合器内反应物料间的混合效果,有利于制得粒度小,且粒度分布均匀的YAG:Ce3+荧光粉,其发光性能良好。更多还原

【Abstract】 Precipitation is a very important method which is widely used in chemical industry in the production of ultrafine powders. Usually, micromixing affects the performance of the powders in the precipitation. Four types of micromixers, straight flow impinging streams micromixer (Ml), conical flow impinging streams micromixer (M2), straight streams vortex micromixer (M3), and twice vortex streams micromixer (M4) were designed on the base of impinging streams mixer according to flow forms and impinging manners of fluids. Then the mixing characteristics of the four types of micromixers were studied by experimental research and theoretical simulation method. The dissertation had a detailed research that the synthesis of barium sulfate (BaSO4), aluminum hydroxide (A1(OH)3) and cerium-doped yttrium aluminum garnet (YAG:Ce3+) yellow phosphor powders by precipitation methods in the four types of micromixers.1. The mixing performance of liquid-liquid in the micromixers. The residence time distribution experiments were performed in the four micromixers and saturated KC1solution was used as a tracer. The results showed fluid in the micromixers was close to full Francis. The inlets diameter of micromixers had little effects on mean residence time and its distribution. The mean residence time delayed and peak width became narrower in the micromixer M4because vortex improved macromixing process of the fluids. The iodide-iodate fast parallel competing reaction was used to quantitatively determine the segregation index Xs in the four different micromixers. The micromixing performance was characterized with segregation index. The smaller the Xs is, the better the micromixing effects are. The effects of volumetric flow, volumetric flow ratio and micromixer configuration on the micromixing performance Xs were studied in details. The experimental results showed the value of Xs was about3.5×10-3～6.0×10-3and Xs had the following rule, XsM4<XsM3<XsM2<XsMi among the four type micromixers. Xs decreased with increasing flow rate, increased with increasing flow ratio, and increased with increasing inlet diameter of nozzles. Macro properties of velocity field, concentration field, turbulent kinetic energy k and turbulent kinetic dissipation rate ε distribution fields were simulated by Fluent software. Distributions of macro flow field results were consistent with the residence time distribution results from experiments. The micromixing time was both1millisecond grade according to Xs experimental data and s simulated values.2. The mixing performance of gas-liquid in the micromixers. The averaged gas-liquid interfacial specific surface area (a) and liquid-mass transfer coefficient kL were experimentally measured by chemical absorption method, in which NaOH-CO2-N2and H2O-CO2-N2reaction systems were used. The experimental results showed a value was about1.2×105～1.0×106m2·m-3, the relation of the averaged gas-liquid interfacial specific surface area a and operating parameters was, The a value of M1, M2, M3and M4gradually increased at the same technological conditions, a had the proportional relations with d-2.22～2.02, qv,L0.55-o.62and qv,g0.38～0.50.The KL value was about0.02-0.12cm·s-1and it increased at least one or two orders of magnitude higher liquid side volumetric mass transfer coefficients compared with the conventional gas-liquid contactors. Vortex resulted in extra contact areas which can guarantee high mass transfer rates in the subsequent micromixer. The kL value decreased with the increase of liquid and gas flow rate, as well as the decrease of the inlet diameter. When the liquid flow rate increased from5L·h-1to25L·h-1, kL increased about2.69-3.23times. When the inner diameter decreased from2mm to1mm, kL increased about1.08～1.36times. According to the vortex model, the theoretical mass transfer coefficient kL was consistent with the experimental results.3. BaSO4was prepared by liquid-liquid mixing in the micromixers. The experiments were performed with two reactants, BaCl2solution and Na2SO4solution to prepare BaSO4in the four micromixers. The results indicated the particles prepared by impinging streams micromixer M4had smaller mean size and narrower size distribution than the other three micromixers because Vortex could intensify micromixing in the precipitation process. The mean size decreased with the increase of liquid flow rate, initial concentration, as well as the decrease of volumetric ratio of reactants and the inlets diameter size. Barium sulfate superfine ellipsoidal particles were prepared with the size of about100nm and the specific surface area of the particles29.25m·g-1at the best technological conditions.4. Al(OH)3was synthesized by gas-liquid mixing in the micromixers. Fibrous pseudoboehmite particles with average diameter about10nm,200nm in length, surface area of390.6m·g-1and large pore volume of2.54cm3·g-1, were prepared by the carbonation method from sodium aluminates solution and carbon dioxide at the optimum conditions. Vortex resulted in smaller particle size and narrower distribution than straightly impinging streams. The mean size and dimensionless variance decreased with the increase of liquid flow rate, gas flow rate, as well as the decreasing of initial concentration and the inlets diameter size.5. Micromixers were further optimized for the preparation of YAG:Ce3+yellow phosphor, used ammonium bicarbonate as precipitant. The results indicated that YAG:Ce3+phosphor particles with a ellipsoidal shape, a mean particle size of about100nm and a narrow particle size distribution could be successfully prepared in them. The YAG powders without any impurity phases could be obtained at1000℃for2h and its crystalline phase transition temperature was about500℃lower than solid-phase sintered method. The luminescent intensity gradually decreased with the increase of the initial concentration of raw materials. The luminescent intensity first increased then decreased with the increase of Ce%doped concentration, and the best Ce%was2.6%. The luminescent intensity gradually increased the increasing flow rate and which changed little when the flow rate exceeded20mL·min-1. The emission intensity of YAG:Ce3+phosphor prepared by shaped impinging streams micromixer M3had about18.6%higher than that by M1and that from M423%higher than that from M1. The results show that the impinging streams micromixers can intensify micromixing effects among reaction materials. So the YAG:Ce3+phosphor with small particle size and a uniform distribution were prepared, and which has excellent luminescence properties.更多还原