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Transesterification of rubber seed oil to biodiesel over a calcined waste rubber seed shell catalyst: Modeling and optimization of process variables Samuel Erhigare Onoji, Sunny Esayegbemu Iyuke, Anselm Iuebego Igbafe, and Michael O. Daramola Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017
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Energy & Fuels
Transesterification of rubber seed oil to biodiesel over a calcined waste rubber seed shell catalyst: Modeling and optimization of process variables Samuel Erhigare Onoji, *, †, ‡ Sunny E. Iyuke, †, ‡ Anselm I. Igbafe, § and Michael O. Daramola† †
School of Chemical & Metallurgical Engineering, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein 2050,
Private Bag 3, Johannesburg, South Africa ‡
Petroleum and Natural Gas Processing Department, Petroleum Training Institute, PMB 20, Effurun, Nigeria
§
School of Chemical & Petroleum Engineering, Afe Babalola University, PMB 5454, Ado-Ekiti, Nigeria
*Corresponding e-mail:
[email protected] ABSTRACT In the present study, waste rubber seed shell (RSS) obtained from our previous study was investigated as a plausible solid base catalyst for the transesterification of esterified rubber seed oil (RSO) to biodiesel. TGA, XRF, XRD, GC-MS, SEM, and N2 adsorption/desorption analysis (BET) were used to characterize the catalyst. Central composite design (CCD) was employed to design the experiments conducted to study the influence of the process variables (reaction time, methanol/oil ratio, and catalyst loading) on biodiesel yield. Response surface methodology (RSM) technique, was used to optimize the process, and the quadratic model developed was statistically significant with F-value of 12.38 and p-value ( 95% at optimized conditions. In sub-Saharan Africa, Nigeria has an estimated 18 million hectare (ha) of wastelands in the South-South geopolitical zone suitable for the cultivation of rubber tree (Hevea brasiliensis), a nonedible oil bearing plant for latex production and seed oil for biodiesel [1]. Studies by several researchers have shown that a seed of rubber tree contains 35–60% oil that portrays it as a better competitor to other non-edible oils for biodiesel production [1, 12-14]. Currently, an insignificant fraction of rubber seeds are utilized for rubber plant breeding process, and many of the seeds are left to rot away. The rubber seed shells (RSS) generated from seeds during oil extraction, pose a 3 ACS Paragon Plus Environment
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waste disposal problem to rubber seed oil (RSO) millers. In this study, calcined RSS was investigated as a plausible source of direct heterogeneous base catalyst for the transesterification reactions of esterified RSO to rubber seed oil methyl ester (RSOME) popularly called biodiesel. This will no doubt reduce biodiesel production cost. To the best of our knowledge, such investigations on RSS as a source of heterogeneous catalyst for biodiesel production are not available in the literature. Thermo-gravimetric analysis (TGA) was employed to determine a suitable calcination temperature of RSS for biodiesel synthesis. Fatty acid methyl ester (FAME) content was determined by gas chromatography-mass spectrometry (GC-MS). Structural properties of the raw and calcined RSS were examined by X-ray diffraction (XRD). Surface morphology of the catalyst was examined by using Scanning electron microscopy (SEM). X-ray fluorescence (XRF) and N2 physisorption (at 77.3 K) were used to evaluate the elemental composition, and the textural property (specific surface area, pore volume, and pore-size) of the raw and calcined RSS, respectively. Reaction time, methanol/oil ratio, and catalyst loading for the transesterification process were optimized via response surface methodology (RSM) based on central composite design (CCD). The biodiesel produced was characterized to determine its fuel properties and applicability. 2. EXPERIMENTAL SECTION 2.1. Materials and chemicals. Rubber seed (Hevea brasiliensis) oil utilized in the study was previously extracted by the authors with n-hexane from seeds handpicked from the plantations of Rubber Research Institute of Nigeria (RRIN), Iyanomo, Benin City [14]. The rubber seed shells freed from the kernel were stored in polyethylene bags for analyses. All chemical reagents used were of analytical grades manufactured by BDH Chemicals Ltd., Poole England, and GFS Chemicals, Inc., 867 McKinley Ave., Columbus, OH 43223.
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2.2. Methods. 2.2.1. Catalyst preparation and characterization. The rubber seed shell (RSS) was washed 3-4 times with hot distilled water to get rid of dust and solid impurities. It was then dried in a laboratory oven at 110 oC for about 5 h, and stored in a desiccator. The cooled dried RSS was ground to powder with a grinder, and sieved through a 60-mesh size. The powdered raw RSS was analyzed by thermo-gravimetric analyzer (TGA) in the range of 30-950 oC at 10 oC/min, under nitrogen environment to determine a suitable calcination temperature of the catalyst. The RSS powder was calcined from 40-800 oC at heating rate of 10 oC/min in an electric muffle furnace (Carbolite, Parson Lane, Hope Valley S33 6RB, England, Model: RWF 12/5) for about 3 h to remove organic materials in it and stored in screwed bottles for analyses. Raw and calcined RSS were dissolved in deionized water to determine the basic property (pH). Energy dispersive X-ray spectroscopy (EDX 3600B-XRF Skyray Instrument) analysis was employed to determine the elemental compositions of the raw and the calcined RSS. Samples were pulverized using Knife Mill GRINDOMIX (model: GM20), and pelletized with Mini-pellet press kit Asia (model: GS01152). The equipment was calibrated using pure silver standard sample. The diffraction patterns of raw and calcined RSS samples were analyzed with a diffractometer (XRD EMPYREAN mini-material analyzer, manufactured by PANalytical B.V., Holland) to observe their amorphous and crystalline structures. Sample was pulverized to homogeneous size and loaded into XRD sample holder. The diffractometer equipped with PIXcel-3D detector employed Cu-Kα1 radiation source (λ =1.54059 Å) at 45 kV and 40 mA. Scanning was recorded in a continuous mode over 2-Theta range from 4.0131o to 79.9849o with a step size 0.026o at 13.77 second per step. The size and morphology of catalyst was examined with scanning electron microscope (SEM-Phenom ProxMVE 016477830, manufactured by Phenom World, Eindhoven, Netherlands). The RSS sample was dispersed on a stub using a sticky carbon tape before coating with palladium gold. Vacuum was created before carrying out the analysis by inserting the stub into the equipment. 5 ACS Paragon Plus Environment
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Surface area and porosity, pore volume, and pore-size distribution are important parameters in heterogeneous catalysis and catalyst design [15]. Prior to analysis, about 335.3 mg of raw sample was weighed and outgassed at 250 oC for 11 h under a stream of nitrogen gas to remove moisture and impurities. Similar analysis was carried out using 382.7 mg calcined RSS catalyst at same conditions. These analyses were to evaluate the textural properties of the raw and calcined RSS in order to determine the influence of calcination temperature. The accessibility of active sites is determined by the total surface area, and thus related to catalytic activity. Gold App surface area and porosity analyzer (Model: V-sorb 2800p) was used to estimate the surface textural properties of the raw and calcined RSS from isotherms generated by Brunauer-Emmett-Teller (BET) model (eq. 1) [16] using nitrogen adsorption/desorption at 77.3 K.
(P / Po ) = (C − 1) . P + 1 V [(1 − P / Po )] Vm C Po Vm C
(1)
where P is the partial pressure, Po is the saturation pressure, Vm is the maximum amount of nitrogen adsorbed/unit mass of catalyst at high pressure conditions required to form a complete monolayer over the entire surface of the catalyst, and C is the BET constant related to isosteric heat of adsorption. Barrett-Joyner-Halenda (BJH) model calculated the pore-size distributions of the raw and calcined RSS. The total pore volume is defined as the volume of liquid N2 corresponding to the amount adsorbed at a relative pressure of P/Po = 0.997, after which N2 desorption commences. 2.2.2. Acid-catalyzed esterification of rubber seed oil. The rubber seed oil used in the study had an initial acid value of 18.02±0.141 mg KOH/g oil, and FFA level of 9.01±0.07% obtained from authors' previous study [14]. Reduction of FFA to 99%) and stirred for 5 min. The mixture was added to the oil maintained at 60 oC, and the reaction continued for 1 h at a stirring rate of 600 rpm. The esterified oil was transferred into a separating funnel and allowed to stand for 2 h. The pretreated oil was separated from the water formed, and excess methanol was evaporated in a rotary evaporator prior to acid value determination by standard titration procedure described by ASTM. Similar experiments was carried out using 3, 4.5, and 6 % vol/vol H2SO4 and their acid values determined were recorded in Table S1. The 6% vol/vol H2SO4 obtained for
