Biodiesel Produced by Two Step Hydroprocessing of Waste Cooking Oil 1. Hydrotreating of waste cooking oil and straight run gasoil mixture

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1 Biodiesel Produced by Two Step Hydroprocessing of Waste Cooking Oil 1. Hydrotreating of waste cooking oil and straight run gasoil mixture RALUCA ELENA DRAGOMIR, PAUL ROSCA *, TRAIAN JUGANARU, EMILIA ELENA OPRESCU Petroleum-Gas University of Ploiesti, Faculty of Petroleum Refining and Petrochemistry, 39 Bucharest Blvd., , Ploiesti, Romania This paper presents the research aimed to use waste cooking oil (WCO) for biofuel production. The experiments consist of hydrotreating mixtures straight run gas oil (SRGO) with WCO 5%, 7,5%, 10% at 360 C and 380 C temperatures, 60 bar pressure, liquid hourly space velocity (LHSV) of 1h -1 in the presence of catalyst Co-Mo/ γal 2 O 3. The research has focused on the influence of temperature and the WCO/SRGO ratio on the yields and the physicochemical properties of the biofuel obtained. The research findings highlight the good characteristics of biofuel that fit with the requirements of EN 590, except the pour point and sulfur content. Keywords: waste cooking oil, biofuel, hydrotreating, pour point, cetane index Energy is a strategic factor in worldwide politics, a vital component and a cost factor for economic development and progress of society as a whole, generating a series of major concern worldwide. World energy consumption doubled between 1970 and 2001, and the current policies regarding production, transport and consumption of energy demand will increase by 53% compared to 2007 [1]. In terms of limiting primary energy resources: oil, coal and gas, to achieve sustainability in this domain, alternative sources are required in order to replace a part of fossil energy. Reserves of oil and gas are not renewable and it is estimates that will end in 41 and 63 years respectively [2]. Moreover, fossil energy sources are primarily responsible for increasing of CO 2 emissions, other greenhouse gas emissions and global warming. Another reason for finding alternative energy sources is determined by the instability of oil prices, which it is a severe constrains for countries with limited resources. Some alternatives as energies: wind, solar, hydro, and nuclear and biofuels are considered but all are still in research and development stages. In the present biofuels have attracted great attention in different countries due to regenerability, biodegradability and low emissions of pollutants. For the EU it is estimated a percentage of biofuels in gasoline and diesel of 10% for 2020 without affecting the food needs of the population [3]. Currently extensively used are the first generation biofuels obtained by hydrolysis and fermentation of biomass as a substitute for gasoline and that biodiesel (fatty acid methyl esters / FAME) obtained by transesterification of vegetable oils for diesel fuel type. Competition between utilization biomass as energy or food source, led to research for the second generation biofuels production, that relies on unconventional technologies, mostly on non-food raw materials and especially on waste feedstock, among which an important place is occupied by WCO. A decisive argument in the utilization of WCO is the price. This is the 2-3 times lower than vegetable oil, which reduces the cost of biodiesel [4, 5]. Worldwide are available significant quantities of WCO. Part of WCO is used for soap preparation and as feed additive for the fodder making, but significant amounts are illegally dumped in rivers and soil, leading to serious problems in the environmental pollution. Since 2002 EU forbidden the use of waste oil in the manufacture of fodder. The amount of WCO varies from country to country. In 2013, in EU and USA, have been generated about tons and 1,5 million tons respectively of WCO from which about 90% was converted into biodiesel [6]. The frying process which is realized at C temperatures in the presence of water and oxygen followed by hydrolytic, oxidation, and cracking reactions which cause major changes in the characteristics of the vegetal oil resulting in viscosity and acidity increases as well as associated with an unpleasant odor and a darker coloration [7]. Animal fats which remain in the waste oil increase the content of free fatty acids that require pretreatment to obtain a high conversion into biodiesel [8, 9]. The second generation biodiesel can be obtained from WCO using traditional transesterification technologies in alkaline catalysis [10, 11], acid catalysis [12, 13], enzymatic catalysis [14] and heterogeneous catalysis [15-17]. Another way of converting waste oil into biodiesel is hydrotreating or hydrocracking of waste oil mixed with fossil diesel using commonly technologies from refineries. Hydroprocessing has been the subject of numerous investigations [18-23] for vegetable oils and waste edible oils [24] and is applied on an industrial scale [25-27]. This paper presents the researches related to hydrotreating WCO mixed with SRGO for biofuel production. The effect of waste oil adding ratio, temperature and LHSV was tracked related to characteristics and yield of biodiesel obtained. For all experiments industrial hydrotreating catalyst type Co-Mo/ γal 2 O 3 was used. Experimental part Feedstock and catalysts Mixtures of 5%, 7,5% and 10% vol. of WCO collected from fast food restaurant and SRGO were used. WCO was collected from a local restaurant after using in intensive frying activities. Before hydrotreating was decanted and * prosca@upg-ploiesti.ro Tel: REV. CHIM. (Bucharest) 66 No

2 Table 1 PROPERTIES OF SRGO AND WCO filtered to remove materials particles remaining after cooking. Main characteristics of WCO and SRGO are presented in table 1 and diesel distillation curve of SRGO in table 2. Industrial catalyst type Co/Mo was activated by sulfurization with light naphtha fraction containing 1000 ppm sulfur in the presence of hydrogen atmosphere at 280 C at a pressure of 15 bar and LHSV of 2h -1. Activation is considered finished after H 2 S formation in the reaction gases revealed by the appearance of the yellow colour of 5% aqueous solution of cadmium acetate used as indicator. Micropilot plant Hydrotreating experiments were carried out on a micropilot plant whose scheme is shown in figure 1. A fixed bed reactor with 60 cm 3 capacity was used. Hydrotreating experiments were performed at 360 C and 380 C with 60 bar pressure, LHSV of 1h -1 and H 2 flow rate of 1 L/min. The liquid phase is dried with CaCl 2 in order to remove traces of water which results in deoxygenation reactions of fatty acids from WCO and weighed then to establish the yield in hydrofined product. Biodiesel derived from hydrotreating is characterized to determine the density (EN ISO 12185), distillation curve (ASTM D86), pour point (SR 13552), flash point (SR 5489), viscosity (ISO 3104) and sulfur content (EN ISO ). Also cetane index method ASTM D 4737 using distillation curve and hydrotreated product density was calculated. Results and discussions This paper conducted hydrotreating experiments of SRGO with 5, 7,5 and 10% vol. WCO mixtures at 360 C and 380 C with LHSV of 1 h -1 and a pressure of 60 bar. WCO adding ratio and reaction parameters influence has been tracked with regarding to yields and quality of hydrotreating 278 Fig. 1 Micro-pilot Plant Table 2 THE ASTM DISTILLATION CURVE FOR STRAIGHT RUN GAS product. Yields in products were determined through material balance on each experiment. The dried hydrotreated product was stabilized by TBP distillation by light fraction removal C. Hydrotreated product fraction with initial boiling point greater than 185 C is assimilated as biofuel. The gas phase is the product which distilled until 40 C. After hydrotreating reactions small amounts of water resulted was eliminated by adsorption in CaCl 2 and mentioned in material balance as quantity loses. WCO characteristics changed compared to virgin oil table 1 as a result of cracking, oxidation and hydrolysis reactions occurring at the cooking temperature in presence of water and air. Triglyceride hydrolysis led to free fatty acids content increase and acidity index increase from 0.6 for virgin oil to 1.24 mg KOH/g. As a result of cracking reactions, glycerides and saturated fatty acids were broken into smaller molecules type alkanes, alkenes and small fatty acids. Increasing of un-saturation degree is evidenced by growing iodine value from 118 to 138.4g/100g compared with virgin oil. In operating conditions triglycerides condensation reactions occur which form dimers acids which also contribute to the increase in viscosity of waste oil from 34.1 to 58.9mm 2 /s. Animal fats which contaminates vegetable oil contain saturated fatty acids with long carbon chains which conduct to increasing of: density, freezing point, viscosity and surface tension of WCO. Increasing of surface tension is associated with increasing foaming tendency and a decrease of phase separation during processing. By hydrotreating of WCO and SRGO mixture the content of paraffins C15, C17, C19, etc. increases due to the oil triglyceride transformation by decarboxylation reactions and C16, C18, C20, etc. by deoxygenation [18, 19, 27]. Also by decarbonylating reactions, product with less than 14 carbon atoms in the molecule can appear. Experimental results are presented in tables 3 and 4. The yields of hydrotreated mixtures of WCO and SRGO are shown in table 3 and figure 2. For all adding ratio of WCO to SRGO, apart of the reaction temperature, by hydrotreating the biodiesel is obtained with a high yield of over 87% wt. Adding ratio of WCO especially influences on the side products obtained (H 2 O, CO 2 ), which increase easily with its growth. So by increasing ratio of 5 to 10% water yield increased with 0.46 at 360 C and 0,6% at 380 C due to fatty acids deoxygenation reactions and the polyglycerides from the WCO.The temperature is the decisive parameter which controls products yields obtained in hydrotreating. Thus, by increasing the reaction temperature from 360 to 380 o C light products (gas and petrol) yield increases by approximate % wt. after the hydrocracking side reactions occurring in hydrotreating process. REV. CHIM. (Bucharest) 66 No

3 Table 3 THE YIELDS FROM HYDROTREATING SRGO AND WCO MIXTURES Fig. 2. Influence of temperature and WCO content on products yields obtained by hydrotreating of WCO and SRGO mixtures. Table 4 CHARACTERISTICS OF THE BIOFUEL The physico-chemical properties of biofuel obtained by hydrotreating of WCO and SRGO mixtures are presented in table 4. The data in the table highlights the influence of hydrotreating temperature and WCO ratio in mixtures. Density of hydrotreated product assimilated as biofuel, with values between and g/cm 3, significantly decrease after hydrotreating, compared to WCO density (0.902g/cm 3 ) and just a bit lower than the density of SRGO ( g/cm 3 ). Decrease of density occurs due to hydrogenation reactions which remove oxygen containing groups from the WCO structure and partially of the aromatic hydrocarbons from SRGO. Also secondary hydrocracking reactions led to formation of hydrocarbons with lower molecular weight compared with components present in the waste oil or SRGO used as feedstock. At the same adding ratio of WCO the reaction temperature determine density decreasing due to deoxygenation reactions and the saturation of aromatics from SRGO. Due to the high density of the WCO for the same reaction temperature, the density of biofuel obtained increases slightly with WCO ratio. In all cases, the density of biodiesel obtained by hydrotreating is within the limits of standard EN 590 for diesel fuel quality (density kg /m 3 ). The viscosity of biodiesel obtained from hydrotreating of WCO and SRGO mixtures, decreases comparing with viscosity of the two feedstock. The reduction is notable in relation to WCO viscosity from 58.9 mm 2 /s to less than 4.5 mm 2 /s, which is the upper limit of acceptable viscosity as per EN 590standard. Viscosity reduction is explained by the modification of the WCO s molecular structures and from SRGO by reactions of deoxygenation, decarboxylation, cracking and saturation that causes the conversion of triglycerides into smaller molecules and removing of carboxyl groups. Driven by increased reaction rate, temperature increasing causes a slight decrease of viscosity. At the same reaction temperature, with increasing ration filler, the viscosity of biofuel obtained from hydrotreating increases (fig. 3). Fig. 3 Influence of temperature and WCO content on the biofuel viscosity obtained by hydrotreating of WCO and SRGO mixtures REV. CHIM. (Bucharest) 66 No

4 Fig. 4 Influence of temperature and WCO content on the cetane index of biofuel obtained from hydrotreating mixtures of WCO and SRGO. Flash point of the stabilized hydrofined mixture decreases with C than the WCO and insignificant in relation to fossil gasoil. By hydrogenation of triglycerides with higher flash point, lower flash point paraffinic hydrocarbons are obtained. Also hydrocracking and hydrogenolysis side reactions which accompany the main hydrogenation reaction, generates light hydrocarbons from both WCO and SRGO. In accordance with those mentioned, at a stable reaction temperature, increasing of WRCO adding ratio lead to a slight increase of the hydrofined mixture flash point and increasing hydrotreating temperature will slightly decrease flash point table 4. By stabilization of hydrofinated product, the light hydrocarbons formed by side reaction are removed, therefore biodiesel flash point does not decrease significantly after mixtures hydrotreating being within the limits of standard EN 590. Pour point of biofuel resulted from hydrotreated mixtures is higher than SRGO pour point due to increasing of normal paraffins content which results from polyglicerides hydrogenation of WCO. From the same cause biofuel freezing point increase with WCO adding ratio increasing. By increasing the reaction temperature, due to intensifying of hydrocracking side reactions, isoparaffins are formed which cause a slight decrease of pour point. Pour point can be corrected by sequential coupling of hydrotreating with an hydroisomerization or mild hydrocracking processes from which most n-paraffins with carbon atoms resulted from WCO hydrogenation as well as from SRGO to be converted into isoparaffins with a pour point of about C lower than linear paraffins with the same number of carbon atoms in the molecule. One of the most important characteristics of diesel fuel is cetane index calculated according to ASTM-D4737. The data in table 4 represent the values of cetane index greater than the minimum allowed by EN 560 (CI> 50) for all products of WCO and SRGO hydrotreated mixture, justified by the high content of paraffinic hydrocarbons which are formed by triglyceride conversion. Cetane index increases with WCO adding ratio and reaction temperature figure 4. At higher adding ratios and reaction temperatures the paraffinic character of biofuel increases also the cetane index by default. Another characteristic of diesel fuel is lubricity which is defined by the size of the wear scar diameter (WSD) determined by HFRR method. SRGO have natural lubrication strengthened by sulfur and nitrogen heterocompounds from its composition. The ester group of the WCO gives it a good lubricity as well [27]. After hydrofining the hydrogenated product lubricity decrease with WSD increasing μm comparing with SRGO, especially by 280 Fig. 5 Influence of temperature and WCO content on the lubricity of biofuel obtained from WCO and SRGO mixtures hydrotreating advanced elimination of sulfur compounds from 7215 ppm to about ppm in biodiesel. In all cases the WSD is less than 460μm as required by the EN 590 standard. Influence of mixture ratio and hydrotreating temperature on biodiesel lubricity which resulted from WCO and SRGO is shown in table 4 and figure 5. The data presented in figure 5 show a decrease in scar diameter (WSD) with increasing of WCO content in the hydrotreated mixture due to triglycerides transformation from WCO in normal paraffin with long chain carbon atoms, which have a very good lubricity. High paraffins content from biofuel compensate partially the lubricity loss caused by the removal of sulfur compounds. With increasing of hydrotreating temperature, the lubricity decrease as a result of intensification hydrocracking reaction which generates paraffins with small number carbon atoms in the molecule and hydrogenolysis reactions which determine lowering the sulfur content from the hydrotreated product table 4. By hydrotreating the sulfur content decrease significantly compare with fossil diesel, from 7215 ppm to ppm, without meeting the 10 ppm sulfur limit as required by EN590 standard. The sulfur content in the biofuel is lower for mixture with greater WCO content and those hydrotreated at higher temperatures. WCO is practically free of sulfur and at higher temperatures the hydrogenolysis reaction of the sulfur compounds is intensified. To reduce sulfur content as per standard limits it is recommended increasing hydrotreating severity by pressure and reaction time increasing or the sequential coupling of hydrotreating with a hydroisomerization or hydrocracking process. Conclusions Diminishing of fossil fuel resources, price instability and severe constraints requested by environmental protection are strong reasons which impose finding other sources of fuel cheaper and environmentally friendly. The most promising alternatives include biodiesel made from virgin oil or WCO. Using WCO is both a cost reduction method for biodiesel manufacturing and an efficient way to solve pollution problems caused by storage. The advantage of feedstock low cost is partially attenuated by technological difficulties encountered in the classical transesterification process with methanol, due to negative changes of vegetable oil properties in the frying process. The research study develop hydrotreating of WCO and SRGO mixtures at 60 bar and 360/380 o C, as an option for obtaining biofuel. As a result of the following reactions: hydrogenation, decarbonylation, deoxygenation and cracking, the WCO triglycerides turns mostly in C 14 -C 20 paraffin with linear structure. The heterocomponents and aromatics hydrocarbons from SRGO are eliminated and saturated. The results of these transformations presented in the research study revealed good quality characteristics of REV. CHIM. (Bucharest) 66 No

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