Effects of process parameters variations and optimization of biodiesel production from orange seed oil using raw and thermal clay as catalyst

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Introduction
Due to the high demand of fossil fuels for energy and transportation, leading to the depletion of the ozone layer and the negative health hazards, research into alternative sources of energy which would not only substitute the conventional energy resource, but also keep the environment free from pollution were explored. At the center of this alternative energy resource was biodiesel, which is made from vegetable oils, animal fats or recycled greases using transesterification, esterification, batch process and pyrolysis. It can be used as a fuel for vehicles in its pure form, but it is usually used as a petro-diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from dieselpowered vehicles (Victor, 2017). Over dependence on fossil fuels for energy production is a major treat to the environment due to environmental pollution which has championed the search for the need of an alternative source of energy which will be less harmful to the environment. In 2016, Port Harcourt had an environmental treat in the form of black dust (soot) which is formed as a result of incomplete combustion of fossil fuels as a result of industrial actions, which affected the health of people in that locality. Researchers carried out various research and discovered that fuel can be gotten from plants (biofuel), mixed with diesel blends to become a biodiesel. A lot of feed stocks were considered.
Vegetable oil is an essential feedstock for the production of biodiesel. Banerjee (2014), investigated on Bio-diesel production from orange peels (Citrus Senesis) as feedstock, it was obtained from the results that the optimal biodiesel yield was 93% under the conditions of 80-83 °C, 1:3 oil to ethanol mole ratio, 70 minutes reaction time and sodium hydroxide as catalyst. Bull and Obunwo, (2014); Mishra et al. (2015), studied the effect of homogeneous catalysts on biodiesel production from crude neem oil feed stock and cost analysis on the production of biodiesel. The maximum average yield of the biodiesel was 72 ± 2% from a reaction time of 75 minutes, a reaction temperature of 50 °C, and methanol to oil molar ratio of 4:5:1 and 1% KOH as alkaline catalysts. Biodiesel production from refined cotton seed oil (Onukwili et al. 2016),biodiesel production from shea-butter using response surface methodology, in which an optimal biodiesel production was achieved at an optimal yield of 92.16% under the conditions of 40 °C for reaction temperature, an agitation speed of 800 rpm and molar to oil ratio of 7:1, Ajala et al. (2016), studied the use of beef, sheep tallow, poultry oil gotten from animals and waste cooking oil as feed stocks for biodiesel production. Montefri and Obbard (2010); Zhang et al. (2003) stated that it will be of great importance if the feedstock will include soaptocks, acid oils, used cooking oils, animal fats, non-food vegetable oils and microorganisms such as algae. Zullaikah et al. (2005), produced biodiesel from rice bran oil, (Agbede 2012), extracted oil from three fruit seeds (mango, tangerine, African star) for their worthiness for production of biofuel, and (Savariraj et al. 2011), worked on reducing viscosity and increasing calorific value using mango seed oil. Ibifubara et al. (2014), discovered that WCO as a feed stock reduced biodiesel production cost by about 60 -70 percent because the feedstock cost constitutes 70-95 percent of the cost of biodiesel production. It is thus needful according for them to promote the global use of biodiesel, low-cost non-edible oils and waste vegetable oils for utilization as feed stocks for producing biodiesels. Yousef et al. (2013) investigated the physico-chemical properties of two types of shahrodi grape seed oils (Lal and Khalili) which was extracted using Soxhlet method and petroleum ether as solvent. Variables such as fatty acid, peroxide value, soapy number, acidity, were placed on considerations. The experiments revealed that, Linoleic acid, fatty acid content was almost equal to 65.39% of all fatty acids which gives oxidation reaction resistance. It was concluded that, Lal variety was better than Khalili for oil content and for low peroxide value. Heydarzaeh et al. (2018), described an alternative energy for the replacement of fossil fuels which has been developed. Biodiesel synthesis as a renewable energy was derived in a continuous packed column reactor. Free fatty acids (FFA) were esterified with ethanol in a heterogeneous catalytic reaction. The catalytic reactor had great potential as FFA introduced to the top of column, flow down ward, reached to catalyst surface and interacted with ethanol on the active site. The ester product was instantaneously formed. In this catalytic reaction, effects of mass ratio of the free fatty acids to ethanol along with reaction temperature in the range of 150-250 °C were considered, as reaction temperature increased, esterification reaction was enhanced. From the data obtained it was concluded that optimal conditions of molar ratio 3:5 and temperature of 250 °C helped in conversion of 90% free fatty acids into ethyl esters. According to (Sirajudin et al. 2013), palm oil is a potential alternative energy source, which will possibly replace the non-renewable fossil fuels, such as gasoline, kerosene and diesel oil. During usage, biofuel produces low pollutants than fossil fuel. The research was conducted through a catalyst synthesis and the catalytic cracking process. The catalytic cracking process was accomplished in a fixed bed micro reactor with temperatures ranging from 350 -500 °C and Nitrogen gas flow rates ranging from 100 -160 ml/min for a period of 120 min. It was discovered that at a temperature of 450 °C and a flow rate of 100ml/min nitrogen produced the highest yield of gasoline fraction of 28.87%, 16.70% kerosene and 1.20% diesel. It was proven that synthesized HZSM-5 catalysts met the standards of a catalyst in producing biofuel by the catalytic cracking of vegetable oils. Important properties of the biodiesel like density, flash point, calorific value and viscosity have also been estimated (Banerjee, 2014). Orange seed were obtained from orange found in the family Rutaceae, these seeds have the ability to yield oil and have been underutilized for any industrial or commercial purpose. The research work deals with the extraction of bio-oil from orange seeds using Soxhlet extractor thereby converting it into biodiesel by transesterification method and the characterization of the bio-oil and biodiesel to obtain their physiochemical properties. Optimization was also carried out using response surface methodology of central composite design (CCD).

Materials and Methods
Orange fruits was purchased from vendors in Marian market, Calabar Municipality in bags. The orange seeds obtained from the parent fruits was air dried, sorted to remove impurities and was grinded using an industrial blender. 100g of grinded seeds was weighed into a semi-permable cotton material and placed into the timble of a 500ml Sohxlet extractor while 400ml n-hexane was measured into a 500ml flat bottom round flask. The Sohxlet with the extraction timble containing the sample in a semi-permable membrane was connected with the condenser which was fitted to the flat bottom round flask containing n-hexane. The oil was heated to 105 °C to remove the moisture content before starting the reaction. Methanol and NaOH were added to oil and 300 rpm stirring speed was used. The method used in the production of the biodiesel was trans-esterification process and the variables for the reaction were methanol to oil ratio (4:1, 6:1, 8:1, 10:1, 12:1), catalyst concentration (1, 2, 3, 4, and 5%), reaction time (30, 60, 90, 120 and 150 minutes), reaction temperature (35, 45, 55, 65 and 75 °C), and reaction speed (150, 200, 250, 300, 350 and 400 rpm). The biodiesel was washed with hot water and dispensed into a 250 ml beaker. It was heated at 105 °C to remove water molecules from the biodiesel. The biodiesel was allowed to cool and stored in calibrated specimen bottles. The glass reactor was set on a heating mantle with electromagnetic field which will enable the agitation effect of the stirring nob. Initially, the reactor was preheated to eliminate residual moisture. A reflux condenser with cold water circulating at the outer jacket was fitted to mid neck of the reactor. Mercury in glass thermometer held in plastic bung was fitted to the right neck of the reactor. The left neck of the reactor was left open initially for input of reactants (oil mixture, methanol and modified catalyst) after which it was closed. Calculated amount of 'methanol' and modified 'catalyst' was added in the amounts established for each experiment and pre-stirred for 10 minutes for proper dissolution after which, 50ml of oil mixture (orange seed oil) was added and the stirring system switched on also at the established 'speed', taking this moment as time zero of the reaction. Each reaction was allowed to last for the required 'time' at specific 'temperature'. After the methanolysis reaction finished, the trans-esterification product was allowed to stand for twelve (12) hours in a separating funnel for glycerol separation. The crude glycerol was removed through the funnel tap leaving the methyl esther, (biodiesel) behind. The biodiesel was washed with hot water and dispensed into a 250 ml beaker. It was heated at 100 °C to remove water molecules from the biodiesel. The biodiesel was allowed to cool and stored in metric specimen bottles.     The coefficient estimate represents the expected change in response per unit change in factor value when all remaining factors are held constant. The intercept in an orthogonal design is the overall average response of all the runs. The coefficients are adjustments around that average based on the factor settings. When the factors are orthogonal the VIFs are 1; VIFs greater than 1 indicate multi-colinearity, the higher the VIF the more severe the correlation of factors. As a rough rule, VIFs less than 10 are tolerable. The equation in terms of coded factors can be used to make predictions about the response for given levels of each factor. The coded equation is useful for identifying the relative impact of the factors by comparing the factor coefficients.  The combined effect of catalyst concentration and temperature on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 1. This showed an increase in the biodiesel yield when the temperature was 65 °C. The combined effect of catalyst concentration and reaction time on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 2. This showed an increase in the biodiesel yield when the reaction time was 120 minutes. The combined effect of catalyst concentration and agitation speed on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 3. This showed an increase in the biodiesel yield with increase in agitation speed. The combined effect of methanol/oil ratio and reaction time on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 4. This showed an increase in the biodiesel yield at a reaction time of 90 minutes and showed a decrease when the time was further increased. The combined effect of methanol/oil ratio and agitation speed on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 5. This showed an increase of 67.5 % in the biodiesel yield at an agitation speed of 250 rpm and a further increase in agitation speed caused a decrease in the biodiesel yield. The combined effect of temperature and reaction time on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 6. It showed an increase in the biodiesel yield when the temperature was 75 °C and reaction time of 150 minutes. The combined effect of temperature and agitation speed on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 7. At a low temperature of 35 °C and agitation speed of 150 rpm, the biodiesel yield was 53.50%, but when the agitation speed was increased to75 rpm at a temperature of 350 0 C, the biodiesel yield increased to 70.08%. The combined effect of reaction time and agitation speed on the biodiesel produced from the orange seed oil at methanol/oil mole ratio 12:1 (w/w) and catalyst loading 4% wt is shown in figure 8. This showed an increase in the biodiesel yield of about 68% when the agitation speed was 250 rpm and reaction time of 120 minutes and showed a decrease when the reaction time and the agitation speed was further increased.

Conclusion
With the recent decline in the oil industry and a search for a cleaner environment and a safer energy, it can be concluded that orange seed oil can be applied for biodiesel production. Also heterogeneous catalyst as such from clay composing of kaolinite, can be used to speed up the reaction. The raw clay catalyst produced varied effect on the biodiesel yield. From the matrix design, set of optimal condition for biodiesel production is; catalyst concentration 3wt %, methanol to sample ratio in mole 12:1, Temperature 65 °C, reaction time 150 minutes and agitation speed 300 rpm. Analysis using central composite design suggests that all the process parameters had significant effect on biodiesel yield. There is also the reaffirmation of cleaner energy as CO2 and CO emitted by biodiesel during fuel combustion test was less than the quantity emitted by petrol diesel.

Disclosure of conflict of interest
No conflict of interest to be disclosed.