【作者】 李辉；

【导师】 路春美；

【作者基本信息】 山东大学 ， 动力工程及工程热物理， 2016， 博士

【摘要】 伴随石油短缺和环境污染带来的巨大压力,生物柴油作为绿色、环保的可再生燃料已在世界范围内引起广泛关注,但我国在生物柴油的基础研究与产业化进程方面,与生物柴油先进技术国家还存在较大差距,亟待进一步强化发展。现有工业生产一般采用KOH等均相碱催化动物脂肪或植物油脂与甲醇的酯交换反应生产生物柴油。均相碱催化活性高、性能强,但不能重复使用,且反应净化过程会产生大量废水。以CaO为代表的固体碱具有易与产物分离、可重复使用等优点,是近年来酯交换催化剂研究的热点。然而普通CaO微观结构欠发达、表面活性位少,其催化酯交换反应所需时间长、催化剂用量大。造纸白泥是造纸业的固体副产物,目前未有较好的处理方式,其室外堆积、自然沉降不仅占用空间、污染环境,而且浪费钙基资源。造纸白泥以钙基物质为主,具备作为酯交换催化剂的潜质,造纸白泥同时含有Na、K等杂质,决定了其催化酯交换的作用机理、反应性能与常规CaO不同,但目前还未有这一方面的研究报道。为此,本文提出利用工业钙基废弃物——造纸白泥(lime mud, LM)催化酯交换生产生物柴油的研究思路。首先,采用不同方式处理造纸白泥得到固体碱,并从催化剂量、醇油摩尔比等酯交换因素出发,考查其催化活性。并通过负载活性物改性进一步优化其催化性能。同时探究造纸白泥固体碱抗H2O和CO2的性能,借助表征手段揭示Na、K对造纸白泥固体碱微观孔隙结构和催化活性的影响。建立固-液-液非均相催化酯交换动力学模型,计算造纸白泥固体碱催化酯交换的动力学参数。最后利用热重红外联用实验系统,探究生物柴油和原料油的热解特性。揭示不同预处理方式对造纸白泥固体碱催化酯交换活性的作用机制。采用直接煅烧和水合两种方式处理造纸白泥,并借助XRD、Hammett指示剂、N2吸附脱附等表征手段揭示处理方式对其催化活性的影响。同时,在酯交换反应系统中增加低频超声波,以提高造纸白泥固体碱催化酯交换体系的传质性能。造纸白泥主要物相成分为CaCO3,经800℃煅烧(LM800)后转化为CaO,并且碱性强度为9.8<H<12.2、孔径分布集中在2-20nm。水合处理造纸白泥(LM-H600)能够丰富其微观孔隙结构,相对于LM800,LM-H600的比表面积从5.17m2/g增加到7.28m2/g,且孔径分布拓宽至60-110nm。在催化剂量为6wt.%(占原料油重)、醇油摩尔比为12、反应温度为64℃以及反应时间为90min工况下,LM800和LM-H600催化花生油与甲醇酯交换转化率分别为69.28%和92.61%,表明水合处理能强化造纸白泥固体碱的性能,在较低的活化温度条件下,能获得较高的催化效率。在LM800催化花生油与甲醇酯交换反应系统中增加25kHz、450W的超声波,能够使酯交换平衡时间由120min缩短至60min、催化剂量从6wt.%降低至4wt.%、醇油摩尔比由12减少到9。超声波的空化作用加速醇油两相乳化状态的形成,减小传质阻力。同时空化作用产生的冲击波和微射流对醇油相界面和固-液相界面有冲击和剥离作用,能够强化反应体系的传质和乳化,加快酯交换反应的进行。通过浸渍法负载KF改性造纸白泥(LM-H600),并结合XRD、Hammett指示剂、N2吸附脱附等表征手段,同时从酯交换参数的优化出发,研究了活性位负载改性造纸白泥催化酯交换的反应特性与作用机理。相对于LM800和LM-H600, KF/LM-600比表面积和比孔容积有所降低,仅为1.02m2/g、0.0046cm3/g,并伴随颗粒烧结、团聚现象。但KF/LM-600具有高效的催化活性位KCaF3和K20,同时其碱性强度增大为12.2<H<15.0。在催化剂量为5wt.%、醇油摩尔比为12、反应温度为64°C工况下,反应120min KF/LM-600可以实现99.09%的酯交换转化率,说明碱性强度是决定催化活性的首要原因。通过固体混合法(SM)、浸渍法(IM)、共沉淀法(CP)和改进共沉淀法(ICP),研究SrO改性对CaO催化活性的影响。相较于S M、IM、CP制备的Sr/Ca固体碱,Sr/Ca-ICP比表面积最大(3.20m2/g),其活性位流失导致的均相贡献仅为3.16%。在催化剂量为5wt.%、醇油摩尔比为9、反应温度为65℃、反应时间为30min工况下,Sr/Ca-ICP催化棕榈油和甲醇进行酯交换反应,重复使用6次后依然能得到92%以上的酯交换转化率。ICP采用(NH4)2CO3为共沉淀剂,残留在前驱体中的NH4+和CO32-在高温活化过程中分解,不影响Sr/Ca-ICP催化酯交换的稳定性。同时,NH4+和C032-分解产生的C02和NH3有利于Sr/Ca-ICP形成丰富的微观孔结构。对造纸白泥做煅烧、水合、干燥处理(LM700-H),考查造纸白泥固体碱抗H20和C02性能。在催化剂量为8wt.%、醇油摩尔比为15、反应温度为64°C、反应时间为120min工况下,LM700-H催化花生油与甲醇酯交换的转化率为88.53%,而分析纯CaO经同样方法处理后(CaO-H)丧失催化活性。造纸白泥经分级水洗脱除Na、K碱金属后(LM-W3-H700),其比表面积和比孔容积分别由2.15m2/g和0.011cm3/g增大至17.93m2/g和0.12cm3/g,且总碱量从0.61mmol/g增大到5.20mmol/g。LM-W3-H700催化棕榈油与甲醇进行酯交换反应,反应120min达到平衡,并能得到96.79%酯交换转化率,而未经水洗的造纸白泥(LM-H700)在相同反应工况下,仅能实现77.12%的转化率。说明造纸白泥中Na、K碱金属会抑制催化剂孔隙结构的形成,导致比表面积和比孔容积降低,减少催化酯交换的活性位,降低催化活性。通过建立固-液-液非均相催化酯交换的动力学模型,计算LM800和LM-H600催化花生油和甲醇酯交换的反应活化能分别为27.41kJ/mol、44.80kJ/mol,均小于NaOH的反应活化能(56.87kJ/mol)。利用热重红外联用实验系统(TG-FTIR)考查原料油(PO)和生物柴油(POB)的热解特性。通过非预置模型的Friedman微分法以及KAS和FWO积分法计算POB和PO的热解反应活化能分别为43.22-56.22kJ/mol和86.24-157.73kJ/mol。 Avrami理论计算POB和PO热解反应级数分别为1.55-1.82、0.89-1.23。POB和PO热解析出的气体分别有烷烃类、烯烃类、醛类、醚类、CO2。其中,酮类气体和H20只出现在PO的热解气体中。采用Friedman微分法和FWO积分法计算得到棕榈油甲酯(PAB)热解的平均活化能大于餐饮废弃油甲酯(WAB)。通过GC和FTIR证明WAB中的不饱和脂肪酸含量高于PAB,不饱和键受热易分解,导致WAB热稳定性较差,容易发生热解反应,但WAB和PAB热解析出的气体产物类型相同。更多还原

【Abstract】 With increasing the deteriorated environmental problem and the limited fossil fuels, biodiesel has been a spotlight for its unique advantages including being renewable, biodegradable, non-toxic and low emissions. However, there is a big gap of basic research and industrialization for biodiesel between our country and advanced countries, thus it is urgent to develop the biodiesel industry. Biodiesel is normally produced through transesterification under the catalytic effect of homogeneous base catalyst (NaOH or KOH). Although it could achieve high transesterification efficiency, the homogeneous base catalyst cannot be recovered and must be neutralized and separated from the biodiesel phase, with the consequent generation of a plenty of wastewater. As a promising representative of heterogeneous base catalyst, calcium oxide (CaO) could offer simple isolation from the production mixtures and be recycled, requiring no cleansing with water which has been obtained more and more attention. But the application of CaO as transesterification catalyst for biodiesel production is restricted due to the less porous microstructure and poor catalytic sites of normal CaO. Lime mud (LM) is the by-product from paper making industry which is always piled outside leading to space occupation and environment pollution. LM is composed of calcium based material, therefore, it possesses the capability to catalyze transesterification. Meanwhile, the existences of impurities like Na, K make the catalytic activity for LM different from that of CaO and no research has been reported till now. Consequently, this paper employed LM as base catalyst for transesterification. To begin with, the LM base catalysts were derived from the pretreatment and the catalytic activities were evaluated through the transesterification parameters. The catalytic activity for LM and CaO was further promoted by doping with active sites. Also, the stability of LM base catalyst in resisting H2O and CO2 were examined. Effects of Na, K on catalytic activity and micro-pore structure for LM base catalyst were revealed via catalyst characterization. The kinetic parameters were calculated through kinetic model for transesterification catalyzed by solid base. Finally, thermal degradation characteristics of biodiesels and feedstock were evaluated through TG-FTIR.The effects of pretreatment for LM on its catalytic activity were revealed. LM was pretreated by two methods, namely, calcination and hydration. Then, the catalysts were characterized by XRD, Hammett indicator, N2 absorption and desorption and so on. Meanwhile, to strengthen the mass transfer between oil, methanol and catalyst, the low frequency ultrasonic was adopted. The results revealed that LM was composed of CaCO3, after being calcined at 800℃ (denoted as LM800), the CaCO3 was transferred into CaO with basic strength of 9.8<H_<12.2 and the pore diameter was in the range of 2-20nm. LM800 was further pretreated with hydration, which was therefore defined as LM-H600 of which the specific area as well as pore diameter was enlarged. Specifically, the specific area was increased from 5.17 m2/g to 7.28 m2/g and the pore diameter range was enlarged to 60-110nm. With catalyst amount of 6wt.%, molar ratio of methanol to oil of 12, reaction temperature of 64℃, LM800 and LM-H600 could achieve 69.28% and 92.61% transesterification conversion in 90 min, respectively, indicating the macroporous particle of LM-H600 could promote the transesterification rate. The conversion was magnified when the transesterification was added with ultrasonic (25 kHz,450 W), where the reaction time was shorten from 120 min to 60 min, catalyst amount was decreased from 6 wt.% to 4 wt.%, and molar ratio of methanol to oil was reduced from 12 to 9. With the effect of ultrasonic, acoustic cavitation could emulsify oil and methanol, the mass transfer resistance between oil and methanol was significantly reduced. Meanwhile, the impulse wave and microjet derived from acoustic cavitation would crush the interface between methanol-oil and solid-liquid accelerating the phase interface updates. Consequently, the mass transfer and emulsification was greatly intensified and thus the transesterification rate was increased as well.To further improve LM catalytic activity, the catalyst of LM modified with KF was prepared through wet impregnation method. The catalyst was characterized by XRD, Hammett indicator, N2 absorption and desorption and so on to investigate mechanism of the modification with KF. Compared with LM800 and LM-H600, surface area and pore volume of the KF/LM-600 was decreased to 1.02m2/g and 0.0046cm3/g along with crystals sintering and cluster agglomeration. Yet, the new and high efficient phase of KCaF3 and K2O were formed when LM modified with KF and the basic strength was increased to 12.2<H<15.0. With catalyst amount of 5 wt.%, molar ratio of methanol to oil of 12, reaction temperature of 64℃, KF/LM-600 could achieve 99.09% transesterification conversion within 120 min indicating the higher basic strength was principally responsible for the higher catalytic activity.To improve the CaO catalytic activity, CaO was modified with SrO through the solid mixing method (SM), wet impregnation method (WI), co-precipitation method (CP) and improved co-precipitation method (ICP), respectively. Compared with SM, WI, and CP, Sr/Ca-ICP possessed the maximum specific area of 3.20 m2/g and homogeneous contribution of the leached active sites was merely 3.16%. Under the mild condition of catalyst amount 5 wt.%, molar ratio of methanol to oil 9, reaction temperature 65℃ for 30 min, Sr/Ca-ICP could achieve transesterification conversion over 92% after 6 reused times. (NH4)2CO3 was employed as precipitator for ICP, the residue of NH4+ and CO32- would be decomposed in the calcination process, hence, no effects would be caused by NH4+ and CO32-. Meanwhile, the formation of CO2 and NH3 also could promote the abundant micro-pore structure for Sr/Ca-ICP.To evaluate the stability of LM700 as transesterification catalyst in resisting H2O and CO2, deionized water was directly added into LM700 and then was dried in the air, LM700-H was therefore obtained. The transesterification conversion of 88.53% catalyzed by LM700-H could be obtained with catalyst amount 8 wt.%, molar ration of methanol to oil 15, reaction temperature 64℃, reaction time 120 min. Yet, the analytical grade CaO treated by the same procedure exhibited no catalytic activity.Specific area and pore volume were increased from 2.15m2/g and 0.011cm3/g for LM-H700 to 17.93m2/g and 0.12cm3/g for LM-W3-H700 that Na, K in LM was removed. The conversion of 96.79% could be obtained while the transesterification of palm oil and methanol catalyzed by LM-W3-H700. Yet, the conversion achieved by LM-H700 was 77.12% with same reaction parameters revealing Na, K would hinder the catalytic sites distributed evenly and destruct the microstructure structure, leading to a reduction of catalytic activity. LM800 and LM-H600 were employed to catalyze the transesterification of peanut oil and methanol. The activation energies of transesterification catalyzed by LM800 and LM-H600 were separately 27.41 kJ/mol and 44.80 kJ/mol, which were less than NaOH of 56.87 kJ/mol.Thermal degradation of biodiesel (POB) and its feedstock oil (PO) was investigated through TG-FTIR. The activation energy of thermal degradation for PO and POB was calculated through Friedman method, KAS method and FWO method, which were 43.22-56.22kJ/mol and 86.24-157.73 kJ/mol. Reaction order derived from Avrami theory was 1.55-1.82 for POB and 0.89-1.23 for PO. The common functional groups of the evolved products detected by FTIR were alkanes, alkenes, aldehydes, ethers, CO2 for both POB and PO. Particularly, ketones group and H2O were only observed in the evolved products of PO but not detected in that of POB. The average activation energies calculated by both Friedman and FWO of thermal degradation for PAB were higher than those of WAB. According the results of GC and FTIR, the content of unsaturated fatty acid for WAB was higher than those of PAB and the unsaturated bond was easily broken down. Yet, types of evolved products were same for both WAB and PAB..更多还原