Elsevier

Renewable Energy

Volume 142, November 2019, Pages 207-214
Renewable Energy

Pilot-scale production of biodiesel from waste cooking oil using kettle limescale as a heterogeneous catalyst

https://doi.org/10.1016/j.renene.2019.04.100Get rights and content

Highlights

  • In a pilot-scale microreactor waste cooking oil was converted to biodiesel.

  • Kettle limescale was used as a heterogeneous catalyst in transesterification reaction.

  • In the optimal conditions maximum biodiesel purity was obtained 93.41%.

  • Box-Behnken experimental design was used to determine the optimal conditions.

Abstract

This study aimed to evaluate and optimize a pilot-scale microreactor to convert waste cooking oil (WCO) into biodiesel using kettle limescale. Box-Behnken design was used to determine the optimum conditions for producing biodiesel. Effects of main variables including reaction temperature, catalyst concentration, and alcohol/oil volume ratio were evaluated at a constant residence time of 10 min. Based on the results of analysis of variance, the quadratic regression model had the best coefficient of determination (R2=0.9930) and adjusted coefficient of determination (RAdj.2= 0.9804). After the optimization of temperature, catalyst concentration, and methanol/oil volume ratio, the residence time was optimized to achieve the maximum purity of the produced biodiesel. At a reaction temperature of 61.7 °C, catalysts concentration (oil based) of 8.87 wt %, methanol to oil volume ratio of 1.7:3, and a residence time of 15 min, we observed the optimal conditions for obtaining a maximum biodiesel purity of 93.41%.

Introduction

Since a long time ago, petroleum has been the main source of energy. However, due to the limitations in the use of fossil fuels and because of environmental concerns, many studies have been conducted to investigate and achieve alternative sources of energy [1]. The renewable energy is the most promising alternative source of energy because it is available everywhere and contains lower amounts of nitrogen and sulfur, making it more environmentally friendly than fossil fuels [2,3].

Biodiesel is a mixture of alkyl esters that can be obtained through the transesterification of various renewable feed stocks such as vegetable oils or animal fat [4,5]. In this process, triglycerides reacts with an alcohol and produces esters and glycerol. Among the renewable sources of energy, waste cooking oils and animal fat are the best sources of waste materials with a high potential for the production of an alternative fuel to be used instead of diesel fuels.

The transesterification reaction can be implemented in various reactors using acid, alkaline, or an enzymatic catalyst. Normally, homogeneous catalysts such as NaOH or KOH are used for the production of biodiesel under normal condition at a reaction temperature between 50 and 65 °C [6,7].

The use of homogeneous catalysts for the production of biodiesel results in a better yield in a short time; however, in order to consider environmental concerns, achieve a better level of catalyst removal and reusability, and ensure a less corrosive character, the use of heterogeneous catalysts for biodiesel production has recently increased [[8], [9], [10]].

Since various parameters are effective in the transesterification process, many researchers have investigated the optimization and modelling of the process using the conventional and response surface methodology (RSM) [[11], [12], [13]].

Mohadesi et al. [14] optimized the process of biodiesel production in a continuous microchannel with a heterogeneous catalyst (CaO/MgO). They obtained a purity of 99.31% after 10 min at a temperature of 70 °C using a catalyst concentration of 7.875 wt %, methanol to oil volume ratio of 1.75:3, and n-hexane to oil volume ratio of 0.575:1. Biodiesel can be produced via batch or continuous processes, however many researchers have studied the effects of different parameters on the FAMEs yield or have optimized the reaction conditions in a batch process [[15], [16], [17]].

On the other hand, many studies have investigated the effects of different parameters on continuous production of biodiesel and conducted economic analyses with the aim of scaling up the yield of fatty acid methyl esters (FAMEs) in different continuous reactors such as fixed bed reactors, packed bed reactors, etc. [[18], [19], [20]].

Santana et al. [21] experimentally and numerically investigated the production of biodiesel from sunflower oil in a micro-device using ethanol and sodium hydroxide catalyst. As reported, the highest percentages of biodiesel was 95.8% that was obtained using an ethanol/oil molar ratio of 5 and a catalyst concentration of 0.85% at a temperature of 50 °C.

Many studies have investigated biodiesel production from waste vegetable oils and animal fat on a laboratory scale, however in recent years a small number of studies have focused on the production of biodiesel on larger scales. The researchers have statistically optimized operation conditions to obtain a high conversion efficiency [[22], [23], [24], [25]].

Alptekin et al. [26] used corn oil, chicken fat, and fleshing oil as the feed stocks together with potassium hydroxide and methanol for the transesterification reaction in a pilot plant. The results of their study indicated that the fuel properties of methyl esters in the pilot plant experiments were very close to those observed in a laboratory scale experiment.

In another study, Carlini et al. [27] used sodium hydroxide as an alkaline catalyst to conduct a pilot-scale transesterification process; they also used waste cooking oil with an acid value of 2.12 mg KOH/g oil. Moreover, they compared various types and concentrations of catalysts including NaOH and H2SO4 and investigated the effects of different concentrations of alcohol as well.

A review of the literature shows that little research has investigated the optimization of continuous process of biodiesel production from waste cooking oil, as compared with fresh vegetable oils, using heterogeneous catalyst in the transesterification reaction. Hence, the present research evaluated biodiesel synthesis in a pilot-scale microreactor using waste cooking oil, methanol, and kettle limescale. In order to optimize the continuous production of biodiesel, this study also investigated the influence of main parameters, including temperature, catalyst concentration, methanol to oil molar ratio, and residence time on the percentage of FAME.

Section snippets

Materials and microreactor details

The waste cooking oil was obtained from restaurants in the city of Kermanshah. The fatty acid composition and physical properties of the collected waste cooking oil were tested and the results were similar to those reported in Aghel et al.'s study [28]. The acidity number was calculated 0.04 mg KOH/g oil (according ASTM D664). Moreover, the density and kinematic viscosity of the oil were measured through D941 and D445 methods, respectively. The saponification index of the oil was determined

Catalyst characterization

The XRD patterns of the kettle limescale and calcined kettle limescale at 900 °C for 2 h are shown in Fig. 2. As clear in this figure (Fig. 2), kettle limescale shows the peaks related to CaCO3 and MgCO3. At calcination temperatures of 900 °C, the major peaks were only related to CaO and MgO, which indicates full conversion of calcium carbonate to calcium oxide at the calcination temperature at 900 °C. This trend was observed for the catalyst (DM water unit sedimentation) used in Moradi et al.

Conclusion

The studied pilot-scale microreactor was successfully utilized to produce biodiesel from waste cooking oil using kettle limescale as the catalyst. The highest biodiesel purity i.e. 93.41% was obtained at a temperature of 61.7 °C, with a catalyst concentration of 8.87 wt %, methanol/oil volume ratio of 1.7:3, and residence time of 15 min. High values of the determination coefficient (R2) proved that RSM model was more accurate in estimating the optimum process variables. The experimental results

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