Characteristics And Mechanism Investigation On Transesterification Catalyzed By Lime Mud Solid Base For Biodiesel Production

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..