Subbaiyan Naveen ,Kannappan Panchamoorthy Gopinath, *,Rajagopal Malolan ,Ramesh Sai Jayaraman ,Krishnan Aakriti ,Jayaseelan Arun

1 Department of Chemical Engineering,SSN College of Engineering,Kalavakkam 603110,India

2 Centre for Waste Management,International Research Centre,Sathyabama Institute of Science and Technology,Jeppiaar Nagar (OMR),Chennai 600119,India

Keywords:Biodiesel Biofuel Solar energy Optimization Pongamia oil Solar reactor

ABSTRACT Over exploitation of non-renewable energy reserves will lead to increase in price of petroleum fuels.Therefore there is a need for suitable and sustainable substitutes (renewable resource) for conventional fuels.In this study,an efficient and environmental friendly method for production of bio-diesel from Pongamia(Karanja)oil has been developed using a solar reactor.During the experimental study,the maximum temperature attained by the pongamia oil during the transesterification process was 64.1 °C.The transesterification reaction was studied by varying different parameters such as reactant flow rate(5-20 L·h-1),stirring speed(150-450 r·min-1),catalyst mass loading(0.5%-2%)and methanol to oil ratio(3:1 to 15:1).The maximum biodiesel yield was 83.11%at a flow rate of 5 L·h-1,stirring speed of 350 r·min-1,a methanol to oil ratio of 15:1,catalyst mass loading of 1%and reaction time of 270 min.The physical and chemical properties of biodiesel was analyzed as per American Society for Testing Materials(ASTM)standards and it had density of 938 kg·m-3,viscosity (28.7×10-6 m2·s-1),acid value (9.45 mg KOH·(g oil)-1)and flash point(215°C).The energy efficiency of solar heating process was determined by comparing the net energy ratio of direct heating process and solar heating process.For solar heating the net energy ratio(NER) was found to be 31.85 against 5.73 for direct heating.Similarly,net energy efficiency index was calculated for 10 kg production scale and was found to be increasing when scaled up which means that the solar heating process is more effective even in scaled up production.

1.Introduction

Climate change and global warming are regarded as the most pressing environmental problems across the world.As fossil fuels are a major cause for these environmental problems,the use of alternative sources of energy that are renewable,clean and help reduce carbon dioxide and other greenhouse gas emissions should be prioritized[1].Biomass acts as the base material for most alternative fuels.Typically,the conversion of biomass to energy occurs through processes such as combustion.However,concerns over energy efficiency and pollution have resulted in the gradual sidelining of this technology in order to prioritize sustainable development.Biomass-derived liquid fuels such as bio-diesel,bio-oil,bioethanol and bio butanal are explored by researchers around the globe [2].Bio-fuels are produced through fermentation (bioethanol),trans esterification (bio-diesel),gasification (syngas),pyrolysis (bio-oil) [2].Bio-diesel is defined as the mono-alkyl esters of long-chain fatty acids that have been derived using the trans-esterification of renewable feedstocks,such as biomass,vegetable oil or animal fats,and alcohol with or without a catalyst.The properties of bio-diesel are very similar to those of conventional petrol-derived diesel and hence bio-diesel has been proposed as a cleaner alternative to liquid fossil fuels [3].

The cost of raw material accounts for about 75%of the total cost of biodiesel production and hence it is important to choose an appropriate feedstock in order to ensure a low biodiesel production cost [1].Bio-diesel has been successfully produced from edible plant feedstock (edible oils) but this pathway is generally not preferable since the feedstock for biomass is usually derived from food crops and cultivation of these crops requires large areas of land,water and fertilizers for growth [4].As a consequence,biodiesel derived from edible oils tend to have high production costs.Along with this it is a concern that bio-diesel production may compete with food supply in the long run.For a developing country like India,these concerns are especially pressing.Hence,non-edible plant oil sources and waste products of edible oil industry are the primary options as the feedstock in order to produce biodiesel meeting the international standards [5].

Karanja (Pongamia pinnata)is a tree species that is naturally found in temperate and humid regions with moderate rainfall such as India.It has the capacity to grow on most types of soil and shows optimal growth between temperatures of 20°C and 35°C[6].Pongamia oil is non-edible in nature and hence is relatively inexpensive when compared to edible oils.The presence of toxic flavonoids makes the oil non-edible but the oil has been shown to possess excellent medicinal properties[7].Pongamia oil has predominantly found use in soap and detergent making and in the tanning industry.Recently,Pongamia (karanja) oil has been explored as a possible bio-diesel feedstock and has yielded promising results [8,9].

The production of bio-diesel occurs through trans-esterification reaction.Trans-esterification (also called alcoholysis) is the reaction of a triglyceride such as a fat or an oil with an alcohol to form esters and glycerol.A catalyst is generally used to improve the reaction rate and yield.Since the reaction is reversible,excess alcohol is used to shift the equilibrium to the products side [10].The reaction results in the formation of fatty acid methyl esters(FAME).FAME composition is the important measure that determines the quality of bio-diesel.Among the alcohols that can be used in the trans-esterification process are methanol,ethanol,propanol,butanol and amyl alcohol.Methanol and ethanol are used most frequently.Methanol is polar in nature and it has a very short chain structure,hence it easily reacts with triglycerides and sodium hydroxide [11].Typically,different methods such as microwave irradiation [12] and direct heating [13] are used to provide the energy required for the trans-esterification reaction.These methods are expensive and result in an elevated cost of the product bio-diesel.However solar heating has time limitations since sun is available only during day times.A possible alternative source of heating is solar energy.Solar energy is particularly abundant in India and the country has nearly 300 clear sunny days and receives 4-7 kW·h·m-2per day of solar radiations [14].Hence using solar energy is a financially,environmentally and technologically feasible method of producing bio-diesel.

While conversion of Pongamia oil to bio-diesel has been explored in the past,most of the studies are based on an artificial source of heating.To the best knowledge of the authors,no study has so far explored the possibility of using a solar reactor for the trans-esterification reaction.This fact is the primary novelty of this study.This study focuses on a solar reactor for the efficient conversion of Pongamia oil into bio-diesel utilizing methanol and sodium hydroxide as an alkaline catalyst.The impact of parameters such as temperature (60.8-64.1 °C),reaction time (60-270 min),stirring speed (150-450 r·min-1)and flow rate (5-20 L·h-1)amidst others has been discussed in detail.

2.Materials and Methods

2.1.Materials

Pongamia oil was purchased from a local marketplace in Chennai,India.Pongamia oil is non-edible in nature and cheaper than edible vegetable oils.Pongamia oil possess a molecular weight of 892.7 g·mol-1[15].Sodium hydroxide(NaOH)was used as catalyst and methanol used as solvent.Chemicals used were 99%purity and of AR grade.The purchased oil was heated to 80°C,cooled and then filtered to remove existing impurities.Methanol was chosen due to its noted suitability to Pongamia oil as shown in literature.The biodiesel produced using methanol was found to the lowest value of viscosity when compared to other oils due to least steric hindrance.The yield produced using methanol was also considerably higher than other alcohols [16].The reactants and oil were stored at a temperature of 30 °C.

2.2.Solar reactor and experimental procedure

The experimental setup consisted of a copper tube(outer diameter:0.00635 m,wall thickness:24 standard wire gange (swg),total length:5 m) wound in a spiral shape in order to maximize heat transfer area.The copper tube was coated with a layer of black paint (0.1 mm thickness) in order to improve its heat absorption capacity.The setup was covered in a glass box(1000 mm×1000 mm×110 mm)in order to further improve heat transfer and to trap heat within the setup,resulting in increased temperatures.The main heat transfer mechanisms involved in this setup is conduction and convection.The glass cover is provided to ensure that there are no heat losses due to wind.The flow of reactants into the reactor was controlled using a peristaltic pump.Further mixing was effected by a stirrer in a beaker in order to improve the conversion efficiency.The experimental setup has been illustrated in Fig.1.

Biodiesel synthesis from Pongamia oil was carried out using the solar reactor followed by mixing in a beaker using a magnetic stirrer in order to enhance the bio-diesel yield.Around 100 ml of Pongamia oil (93.4 g) was heated for an hour using the solar heater.The quantity of methanol and NaOH utilized varied based on the catalyst loading and methanol to oil ratio.The catalyst was premixed with methanol.Parameters such as catalyst mass load(0.5%,0.75%,1%,1.5% and 2%),methanol to oil ratio (3:1,6:1,9:1,12:1 and 15:1),reaction time (0-270 min),reactant flow rate (5 L·h-1,8 L·h-1,14 L·h-1,17 L·h-1and 20 L·h-1) and magnetic stirrer speed (150 r·min-1,200 r·min-1,250 r·min-1,300 r·min-1,350 r·min-1,400 r·min-1and 450 r·min-1)were varied simultaneously.Every 30 minutes 10 ml of sample was collected from the beaker and the oil conversion percentage was calculated.The samples taken from the solar reactor were analysed by gas chromatography mass spectrometry (GC-MS) to check for fatty acid methyl ester (FAME) content and subsequently conversion.

2.3.Analysis of raw materials and products

Acid value of the filtered Pongamia oil and the produced biodiesel was determined using titration method.The density and viscosity were measured using a hydrometer and viscometer as shown in literature.Flash and fire points were determined by using a differential scanning calorimeter DSC-60.The fatty acid profile of the biodiesel produced was determined by GC-MS technique.GCMS-QP2010 type spectrometer was used at an injection temperature of 260 °C.The column specifications are length of 30 m and diameter of 0.53 mm with thickness of 1 μm.Column oven temperature was fixed at 60 °C with helium acting as the carrier gas.A split ratio of 10:1 was set.The range of mass to charge (m/z) ratio was set between 50 and 1000.

2.4.Energy efficiency calculations

To determine the energy efficiency of the solar reactor,net energy ratio(NER)and net energy efficiency index(NEEI)were calculated using equation (1) and (2) respectively,

Fig.1. Experimental setup of the solar reactor.

Where,Eoutis the calorific value of the total biodiesel produced in MJ,Einis the total input energy utilized by the process for the production of biodiesel,NERSolaris net energy ratio of solar heated transesterification process and NERDirectis net energy ratio of direct heated transesterfication process.

To compare the solar reactor with traditional biodiesel reactor,experiments were conducted using a magnetic stirrer with hot plate.A beaker of volume 1.5 L was used as a reactor.Temperature of the reaction was maintained at 64 °C.Catalyst loading was maintained as 1% and stirring speed was fixed to 350 r·min-1.About 85% conversion was achieved after 90 min.

For comparison of effect of scaling up in NER and NEEI,simulative calculations were done for two different processes based on biodiesel production capacity(1 kg and 10 kg).For 10 kg of biodiesel production,a bigger reactor was used.A mechanical stirrer was used for thorough mixing of huge volume of reactants.1500 W mantle heater was used for the heating.

3.Results and Discussion

3.1.Characterization of raw material

Raw Pongamia oil was initially analysed for its properties.It can be seen from Table 1 that raw Pongamia oil has a varying viscosity and density due to the geographical conditions(temperature,location,etc).Generally,all type of oil exhibits varying physio-chemical properties at varying temperature conditions.In our study,pongomia oil was extracted at 37 °C (average temperature in India),but in other studies the temperature conditions varied based on their geographic environmental conditions.Though the variations are seen,it acts as a viable candidate for biodiesel production (see Table 2).

3.2.Reactor studies

The conversion of Pongamia oil to biodiesel using the solar reactor was carried out in different months across March to October of 2019.Solar-powered transesterification experiments were carried out under following conditions:reactant flow rate (5-20 L·h-1),stirring speed (150-450 r·min-1),catalyst mass loading (0.5%-2%)and methanol to oil ratio(3:1 to 15:1)respectively.The month of May produced the highest outlet temperature of 64.1 °C and consequently the highest conversion of 83.11%.This can be attributed to May typically being hotter than other months of the year and being the peak of Indian summer.Readings for the month of May have been demonstrated in this study.The results produced in different months ranging from March to September were measured and have been shown in Fig.2.Despite the relatively cooler temperatures in the month of October,conversion above 70% was still noted.

Table 1 Comparison of collected Pongamia oil with literature survey

Table 2 Properties of biodiesel obtained from Pongamia oil

Fig.2. Comparison of maximum outlet temperature and conversion obtained across different months.

3.3.Effect of reactant flow rate

The flow rate was varied from 5 to 208 L·h,14-1 L·h-1,17 L·h-1and 20 L·h-1) and corresponding conversion values were noted.Fig.3 depicts the variation of conversion accompanying an increase in reactant flow rate.From the results,it was seen that 5 L·h-1the best conversion results among the different flow rates tested and a maximum conversion value of 83.11%at a reaction time of 270 min.Reactant flow rate has a peculiar effect on the extent of trans-esterification reaction.Higher reactant flow rates result in superior mixing but adversely affect the retention time[22] of the reactants in the solar reactor.Similarly,low flow rates increase the retention time but result in inadequate mixing.Increased retention time results in increased heat transfer and increased reaction time resulting in enhanced conversion.Tangyet al.corroborated the inferences with their study reporting poor conversions at low flow rates [23].In this study,low flow rates were found to be the more suitable option due to recirculation of substrate(pongamia oil)in the reactor,which ensures proper mixing of reactants at a later stage.The retention time within the solar reactor was found to be a stronger influence on conversion when compared to mixing.

Fig.3. Effect of flow rate on conversion at 350 r·min-1 stirring speed,270 min reaction time,15:1 methanol/oil ratio and 1% (mass) catalyst loading.

3.4.Effect of stirring speed

The effect of different stirring speeds(150-150 r·min-1)on conversion percentage was depicted in Fig.4.From the results,it was seen that the maximum conversion was found to reach a constant value beyond 350 r·min-1.Recently,in a study it was reported that the maximum soybean biodiesel yield was obtained at temperature of 70 °C,catalyst of 1.5%,time of 30 min,oil to ethanol ratio of 1:10 and stirrer speed of 350 r·min-1[24].However,the maximum conversion value of 83.11%was not obtained at earlier times at stirrer speed of 350 r·min-1.At higher stirrer speed(350 r·min-1,400 r·min-1and 450 r·min-1),the maximum conversion value was obtained after 270 min of reaction.This may be due to the time taken by the reactants in the beaker to reach the optimum temperature to initiate transesterification reactions.A study by Rahmanet al.reported an optimum flow rate of 400 r·min-1to maximize biodiesel production,comparable to the 350 r·min-1required by the solar reactor to achieve desired heat energy for effective conversion [25].The improvement in conversion upon the increase of stirring speed can be attributed to the improved mass and heat transfer caused by mixing before a constant value is reached.

3.5.Effect of reaction time

Fig.4. Effect of stirring speed on conversion at 5 L·h-1 flow rate,270 min,15:1 methanol/oil ratio and 1% (mass) catalyst loading.

Fig.5. Effect of reaction time on biodiesel production at 350 r·min-1 stirring speed,5 L·h-1 flow rate,15:1 methanol/oil ratio and 1% (mass) catalyst loading.

The extent of reaction had a clear link to the duration of the experiment.Three different trials were carried out to study the influence of reaction time on temperature.The trials were conducted under the same conditions (stirring speed of 350 r·min-1,flow rate of 5 L·h-1,methanol/oil ratio of 15:1 and catalyst mass loading of 1%)in order to optimize reproducibility.The effect of different reaction times on conversion,with the other parameters fixed has been explained in Fig.5.From these trials,it was seen that the time period of 270 min was required to produce conversion efficiency of 83.11%,before that it had taken 150 min to achieve 70%conversion.It is crucial to take into account economic considerations when deciding the duration of the reaction,with increased conversion accompanied by increased energy expenses when reaction is allowed to occur for longer durations.The reaction time obtained for the solar reactor was competitive with other models to convert Pongamia oil to biodiesel.A reaction time of 2 h was required to achieve 95%conversion by conventional heating as reported by Anjanaet al.[17].Verma and Sharma [26] reported a reaction time of 90 min for 91%conversion of Pongamia oil to biodiesel using KOH as a catalyst at a temperature of 66.8°C.The comparatively longer reaction time needed by the solar reactor can be explained by non-uniform heating as opposed to conventional methods.However,this is offset by the mixing process which ensures uniform distribution of temperature at a later stage and the significant costs and energy savings offered by this method.

3.6.Effect of methanol to oil ratio

Fig.6. Effect of methanol to oil ratio on biodiesel conversion at 350 r·min-1,5 L·h-1 flow rate,270 min reaction time and 1% (mass) catalyst loading.

The methanol to oil (M/O) ratio was varied from 3:1 to 15:1(3:1,6:1,9:1,12:1 and 15:1 mole ratio) in order to understand the link between the M/O ratio and conversion.The conversion increased with an increase in the M/O ratio.15:1 M/O ratio resulted in a maximum conversion value of 83.11%.The stoichiometry of the reaction demands 3 moles of methanol for one mole of the oil for the reaction to be carried out.In conventional processes,the methanol to oil ratio was increased beyond 3:1 to increase the conversion efficiency.Fig.6 depicts the variation of conversion with M/O ratio across a 270 min reaction time.Where it can be seen that the conversion of 3:1 M/O ratio is sub-optimal and inefficient.Higher proportion of methanol in transesterification reaction enhance the reaction rate and consequently end up in higher conversion [27,28].Increasing the M/O ratio beyond 15:1 did not result in an increase in maximum conversion even though the conversions at earlier stages of the reaction were markedly higher.Hence,15:1 M/O ratio can be viewed as the saturation ratio for this study.

3.7.Effect of catalyst loading

The catalyst mass loading was varied from 0.5% to 2% and the resulting conversion was checked.Clearly,the conversion increases as the catalyst mass loading is increased from 0.5% to 1%but there is a sharp decrease noted after a critical catalyst mass concentration of 1%.A catalyst mass loading of 1%results in a maximum bio-diesel conversion value of 83.11%.Optimal usage of catalyst is essential as excess catalyst concentration leads to saponification.This manifests in a decreased bio-diesel yield due to the formation of soap by-product[29,30].Fig.7 is a plot of conversion against different catalyst concentrations,from which it can be seen that excess catalyst loading is more detrimental to transesterification process than deficient catalyst loading.

3.8.Properties and composition of biodiesel product

The properties of the biodiesel produced from pongamia oil via transesterification process was summarized in Table 3.The obtained values were also found to conform to the norms determined by the Bureau of Indian Standards(BIS)and American Society for Testing Materials(ASTM).The major factor that determines the fuel property was density.Any changes in fuel density leads to non-uniform fuel supply into the combustion chamber of engine.The resistance of fuel flow in the engine was effected by the viscosity of the bio-diesel.Fuels with high viscosity does not get easily atomized during cold weather conditions.This causes delay in combustion and end up in higher amount of carbon deposition.Flash point is an important measure which provides the data on minimum temperature needed for the vapours to give spark.Flash point of fuel is where the fuel starts burning continuously.

Fig.7. Effect of catalyst loading on biodiesel conversion at 350 r·min-1 stirring speed,5 L·h-1 flow rate,270 min reaction time and 15:1 methanol/oil ratio.

Table 3 List of major FAME compounds found in bio-diesel obtained from pongomia oil

The fatty acid profile of the obtained biodiesel was determined by GC-MS analysis (Fig.8).Different peaks were recognized by comparison to existing peaks in standard GC-MS library and shown in Table 3.Analysis revealed the presence of methyl esters of hexanoic acid (C6),heptanoic acid (C7),octanoic acid (C8),nonanoic acid (C9),4-nonenoic acid (C9),decanoic acid (C10),10-undecenoic acid (C11),dodecanoic acid (C12),hexadecanoic(palmitic) acid (C16),heptadecanoic acid (C17),octadecanoic(stearic) acid (C18),oleic acid (C18),linoleic acid (C18),eicosanoic acid(C20),docosanoic acid(C22)and hexacosanoic acid(C26).These results are in accordance with existing literature in which the presence of stearic acid,linoleic acid,palmitic acid,oleic acid and eicosanoic acid were predominantly observed [31].

3.9.Energy efficiency calculations

Net energy ratio(NER)was calculated for both solar heating and direct heating trans-esterification processes.Energy input values were calculated for both processes and was found to be 2.7 MJ for direct heating and 0.486 MJ for solar heating for 1 kg biodiesel production scale.If the production is scaled up to 10 kg,the energy input values have increased to 8.75 MJ and 0.81 MJ for direct and solar heating processes,respectively.Energy output value was calculated based on calorific value of biodiesel produced,which was 15.53 MJ·kg-1.Based on these input and output values,NER values obtained are 5.73 and 31.85 for direct and solar heated trans-esterification processes respectively.These values are only based on the energy inputs given for trans-esterification process.Only after distillation,washing and drying,the entire energy requirements can be calculated.To compare the energy efficiency of the solar reactor,the trans-esterification process alone which was carried out in the reactor was taken for NER calculations.Net energy efficiency index (NEEI) was calculated as a ratio of NER values of solar and direct heating processes.It was found to be 5.55.

To analyze the effect of scaling up in NER and NEEI values,calculations were done with the energy input values of 10 kg biodiesel production process.Only trans-esterification process was considered.The NER values obtained were 17.69 and 191.11 for direct and solar heating processes.NEEI value has increased to 10.8.This shows that scaling up in to major quantities will increase the energy efficiency of solar heating process.

Considering the further steps of distillation,washing and drying,the energy efficiency values eventually will be reduced further.Assuming,solar distillation and solar drying,the NER values of solar reaction process will not change significantly,whereas,direct heating process required another direct heating process for distillation and direct heated drying process.Based on the NER and NEEI values,it is clearly seen that the solar heating process may reduce the energy intensiveness of the biodiesel production process using traditional methods.The cost of biodiesel can considerably be reduced by following solar heating processes.This work has demonstrated the positive effects quantitatively.

Fig.8. GCMS profile of biodiesel produced from pongamia oil via solar reactor.

4.Conclusions

The study has confirmed that the solar reactor can be used as an efficient means of converting oil to biodiesel,in this case Pongamia oil.Further studies are required in order to confirm the vehicular compatibility of the biodiesel.Different variables involved in the reaction were changed in order to arrive at optimum conditions.Optimum parameters for the production of biodiesel were found to be 350 r·min-1stirring speed,5 L·h-1flow rate,270 minutes reaction time,15:1 methanol/oil ratio and 1%(mass)catalyst loading.The fatty acid profile of the obtained biodiesel comprised of palmitic acid,stearic acid,oleic acid,hepatadecanoic acid and Arachidic acid.The magnitude of cost reduction by using Pongamia oil was significantly more than the increased expenses due to pumping of reactants and stirring.The energy efficiency calculations revealed that solar reactor has increased net energy ratio and the overall energy efficiency index has increased during scaling up of biodiesel production.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.10.010.