Development of Process Intensive in-Situ Biodiesel Production Technique Using Non-Edible Oil Bearing Feedstock; Process Optimization and Performance Studies

Abstract

Biodiesel is produced through the transesterification reaction of extracted oil that is conventionally carried out using mechanical stirring, making it a two-step process (oil extraction and transesterification reaction). The present study demonstrates an innovative single-step energy-efficient, environmentally friendly biodiesel production process, which eliminates the oil extraction stage (an energy-intensive step) using a novel coordinated ultrasound-microwave reactor. Both normal and gamma-irradiated castor seeds are used for the in-situ biodiesel production, and the respective optimum reaction conditions obtained using response surface methodology. The effect of gamma rays dosages on castor seeds and optimum conditions of transesterification reaction parameters for biodiesel production along with kinetic and energy analyses are investigated in the present study. The preliminary cost analysis of the present innovative method is briefly reported. Moreover, the present work also includes engine performance studies and analyses of combustion behavior of the binary (castor biodiesel + diesel) and ternary blends (castor biodiesel + n-butanol + diesel) using a single zone combustion model and modified multiple Wiebe functions. The optimum process parameters for normal and gamma-irradiated seeds were determined to maximize the biodiesel yield and compared. The maximum biodiesel yield of 93.5 ± 0.76 % is achieved from normal castor seeds for optimum reaction conditions of 350:1 methanol to oil molar ratio, 1.71 % wt. catalyst amount, 44 °C reaction temperature, and 30 min of reaction time using response surface methodology (RSM) coupled with central composite design (CCD). There was no improvement observed in biodiesel yield beyond the optimum conditions due to the higher viscosity, density, and unsaturation level of the castor oil obtained from normal castor seeds. Gamma irradiation was found to affect the fatty acid profile of the oil obtained from the castor seeds. The proportion of ricinoleic fatty acid is decreased in the fatty acid profile of gamma-irradiated castor seeds compared to normal castor seeds. Squalene formation was observed, and its proportion increased up to 2.07% in castor seeds irradiated with a dosage value of 9 kGy. The viscosity of castor oil obtained through gamma-irradiated castor seeds was decreased to 195 cSt (for 9 kGy) compared to 210 cSt of castor oil obtained through normal castor seeds. The optimum reaction conditions for gamma-irradiated castor seeds are obtained using RSM coupled with Box-Behnken Design (BBD), which includes: gamma radiation dosage 9 kGy, methanol to oil molar ratio of 288:1 (6.5 ml of methanol per gram of castor seeds; v/w), catalyst amount of 1.33 % wt., 52.5°C reaction temperature and 30 min of reaction time. Under these optimized reaction conditions, the biodiesel yield obtained from gamma-irradiated castor seeds is 96.04 ± 0.53 %, which is 2.5% higher than the biodiesel yield obtained from non-irradiated castor seeds. The comparison of optimum process parameters show that methanol to oil molar ratio is reduced by 17.71 %, while catalyst loading is reduced by 22.22 %, making the process environmentally and economically viable. It is found that the reaction rate of the transesterification has been improved by 30-40%, and activation energy has been decreased by 50% (14 kJ/mol) in the case of gamma-irradiated seeds when compared to the transesterification reaction of non-irradiated castor seeds (28.27 kJ/mol). The gamma irradiation causes the formation of squalene (0.40 to 2.07%) which is not observed in the oil compositions of non-irradiated castor seeds. Physico-chemical properties of biodiesel were compared with biodiesel standards (ASTM D6751 and EN 14214) and found satisfactory for both cases. The castor oil methyl ester (COME) obtained at optimum reaction conditions was used to prepare ternary blends of n-butanol, COME, and diesel in various proportions. The ternary blends of n-butanol and produced COME with petro-diesel are used to investigate its effect on combustion, engine performance, and emission characteristics of fourstroke single-cylinder diesel engine to obtain optimum blending proportion and simultaneously reduce UHC, CO, and NOx without affecting engine performance. A quick and simple approach involving a single zone combustion model coupled with double and triple Wiebe functions has been successfully applied to analyze the combustion behavior of binary and ternary fuel blends respectively. Neat diesel was used as a baseline fuel to compare combustion, performance, and emission characteristics with binary and ternary fuel blends (B30, B20Bu10, B10Bu20, and B15Bu15). The apparent heat release rate (AHRR) calculated from the model is in good agreement (RMSE ≤ 1.33) with experimental values. Peak AHRR values (J/°CA) for fuel blends B30, B20Bu10, B15Bu15, B10Bu20, and neat diesel were found to be 26.1, 29.4, 32.7, 37.1, and 36.1, respectively. The ignition delay period was not affected for the B30 blend; however, it was observed to be increased by 2.89 to 3.35°CA for n-butanol blended fuels. Results of the B30 fuel blend showed a reduction in BTE by 1-3% with an increase in greenhouse gas emissions. The results of the ternary fuel blend (B15Bu15) showed improvement in engine performance characteristics, whereas NOx emission was reduced in the range of 20-60% compared to B30. A detrimental effect on engine performance, emission characteristics, and combustion behavior was observed for B10Bu20. Therefore, B15Bu15 was observed as an optimum fuel blend. A preliminary cost analysis of the in-situ biodiesel production using a hybrid reactor is carried out. It suggests that castor biodiesel production can be proved to be a profitable and sustainable business and has the potential of expansion in the countries focusing on biodiesel blending targets in the near future to mitigate the effect of air pollution due to emissions from automobile vehicles. Moreover, the gamma irradiation as a pretreatment process for seeds feedstock does not require any additional retrofitting facility for the existing biodiesel and oil extraction plants, making the present novel approach recommendable for industrial application. Thus, current study offers new insights to propose a pretreatment process for biodiesel production at industrial scale in near future.