P.Vignesh *,A.R.Pradeep Kumar ,N.Shankar Ganesh ,V.Jayaseelan ,K.Sudhakar

1 Department of Mechanical Engineering,Indira Institute of Engineering and Technology,Tiruvallur,India

2 Department of Mechanical Engineering,Dhanalakshmi College of Engineering,Chennai,India

3 Department of Mechanical Engineering,Kingston College of Engineering,Vellore,India

4 Department of Mechanical Engineering,Prathyusha Engineering College,Chennai,India

5 Faculty of Mechanical and Automotive Engineering Technology,Universiti Malaysia Pahang,26600 Pekan,Pahang,Malaysia

6 Automotive Engineering Centre,Universiti Malaysia Pahang,26600 Pekan,Pahang,Malaysia

Keywords:Biofuel Methy esters from fatly acids Renewable diesel Kinetics Catalyst Transesterification

ABSTRACT The need for sustainable fuels has resulted in the production of renewables from a wide range of sources,in particular organic fats and oils.The use of biofuel is becoming more widespread as a result of environmental and economic considerations.Several efforts have been made to substitute fossil fuels with green fuels.Ester molecules extracted from processed animal fats and organic plant materials are considered alternatives for the use in modern engine technologies.Two different methods have been adopted for converting esters in vegetable oils/animal fats into compounds consistent with petroleum products,namely the transesterification and the hydro-processing of ester bonds for the production of biodiesel.This review paper primarily focuses on conventional and renewable biodiesel feedstocks,the catalyst used and reaction kinetics of the production process.

1.Introduction

The automobile and fuel industries are under threat to produce green fuels due to a lack of crude oil supplies and a rise in society’s environmental consciousness [1].In comparison to conventional fuels,vehicles using renewable fuels would have lower carbon emissions [2].The European Commission has introduced a biofuel production plan.Alternate fuel,such as biodiesel,has been used for 10%of conventional fuels in Europe by 2020,according to this plan[3].First-generation biodiesel,a mixture of methyl esters from fatty acids (FAME),was made using low-value raw materials to obtain cheaper and cleaner fuel [4].Massive quantities of chemicals are required for the production of FAME.Production costs can be reduced by using enzymes as biocatalysts since fewer chemicals are required.

It is also possible to skip the development stage because free fatty acids (FFA) do not have to be removed.The FAME produced by the enzyme is known as the E-FAME or FAME enzyme[5].FAME tends to dilute the lubricating oil of the engine,particularly when the particulate matter was frequently regenerated.It might be essential when biodiesel is operated on a continuous basis.So far,combining of biodiesel with conventional diesel fuel has also been constrained [6].Another alternative could be the use of hydro-treated vegetable oils.In order to achieve fuel properties similar to conventional diesel fuel,vegetable oils must not only be hydro-treated but also shortened by the length of the C-chain.The outcome product is known as Hydro-treated vegetable oil(HVO) [7].HVO was distinguished by a high quantity of cetane and other combustion properties,comparable to crude diesel fuel.No incompatibility with reagentsis evident relative to FAME,and HVO does not appear to dilute lubricating oils [8].HVO ’s production focuses mainly on palm oil and waste as feed ingredients,such as animal fats[9].Also,HVO has a better emissions behavior,especially in relation to particulate matter compared to standard diesel,due to a lack of aromatic compounds and better mixture formation.HVO is fully soluble with traditional diesel and can also be used as a direct fuel substitute [10].

1.1.Biodiesel and renewable diesel -Comparison

The green diesel consists of O2-free paraffin and sulfur.Renewable diesel fuel has a maximum calorific value and a higher cetane number.Catalytic hydro-processing did not,however,leave any by-products;glycerine is released during transesterification [11].Green diesel has a low sulfur content as the desulfurization process is carried out during hydro-processing.Green diesel is widely made from rapeseed oil and palm oil.There are also various sources of renewable oil used in the production of green diesel[12].Table 1 demonstrates the essential properties of green diesel,FAME biodiesel,and petro-diesel.A description of the process of production,merits,and demerits of FAME and Renewable biodiesel are given in Table 2.FAME biodiesel and Green diesel have very distinctive characteristics.FAME biodiesel and green diesel are low in sulfur compared to petro-diesel and almost zero [13].By comparing renewable fuel with FAME biodiesel,techno-economic studies have shown that both economic processes depend on unit capacity and raw material prices [14].The unit capacity of fewer than 1 million tons per year has been reported as contributing to the negative trend at present.In the refinery,the production of green diesel by hydrodeoxygenation is an economical solution with a capacity of more than 3 million tons per year [1].

1.2.Raw materials used for producing biodieseland renewable diesel

A variety of lipid sourcescan be used to produce biodiesel.Vegetable oils,animal fats,marine algae,and used cooking oils may be the source of the lipids[15].Approximately 80%-90%of the price of biodiesel production relates to raw materials.The raw material available must be supplied at a reasonable price to ensure the continuous production of biodiesel products [16].Edible oil has also been commonly used as a primary source of biodiesel.In the world,edible oil makes up 99%of biodiesel.The typical edible oil used for biodiesel is soya (20%),sunflower oil (15%) and palm oil (5%),and rapeseed oil (60%) [17].Legal and environmental problems using vegetable oil/animal fat as the primary feedstock for biodiesel.The new advances are the production of biodiesel from waste oils,organic fats,and non-edible oils [18].The use of higher-grade raw materials would involve more effective processing of green diesel and the use of more robust catalysts to manufacture betterquality FAME biodiesel [19].

2.FAMe

Fatty acid methyl ester oils are widely known as biodiesel,a sustainable,non-toxic fuel used in conventional CI engines.FAME was synthesized with the reactions of triglyceride and methane transesterification [20].Transesterification is the mechanism by which an ester is transferred to another material by means of an alkoxy moiety transfer [21].Vegetable oils are an effective substitute for diesel fuel because they are infinite in nature,are mostly produced locally and are environmentally sustainable.Both unsaturated fat substances can be used in the production of biodiesel,such as plant lipids or animal fats [22] FAMEbiodiesel extraction process,as seen in Fig.1.

Table 1 Properties for petro-diesel,biodiesel,and HVO [1]

Organic oils,such as Karanja,Neem,Mahua,Jatrophacurcas or Polanga,which are non-consumable,are found to be ideal for the manufacture of biodiesel under the tested conditions.In terms of the few essential elements,vegetable oil has an incredible capacity to replace oil over the long term [23].Table 3 provides a concise report on edible and non-edible products from various countries.

Table 2 Comparison of FAME Biodiesel and HVO [20]

Fig.1. FAME biodiesel extraction process.

Table 3 Edible and non-edible biodiesel products from different countries [23]

2.1.Pre-treatment process of biodiesel

Pre-treatment of vegetable oil/animal fat has been done through adsorption.Adsorption was accomplished by a range of adsorbents including high-quality activated carbon,pulverized coconut coir,and bleaching soil mixtures,and pulverized carbon and coconut coir mixtures[24].Fig.2 illustrates the different types of the pretreatment process.In the first stage,the oil was filtered to eliminate food contaminants or suspended solids and then heated to 120-135°C for 60 minutes to remove the water level.A certain amount of heated and filtered organic oil was then placed in a series of beaker glasses [25].Various forms of adsorbents and different quantities of organic oil have been added.Seven adsorption experiments included:1 untreated investigation,3 with varying levels of activated carbon(8%,16%,and 20%),1 with 12%activated carbon and 14%bleached soil mixture,1 with 16%activated carbon,and 1 with 3% coconut coir mixture with 4.5%.Every beaker was heated to 120 °C and stirred at a constant rate of 85 r·min-1for 60 minutes [26].Upon adsorption,the mixture was extracted to separate the adsorbent from the solvent.The bio-oil was then analyzed in order to assess the quality of the FFA,the volume of water,and the density [27].

Fig.2. Various types of the pre-treatment process.

2.2.Catalyst for transesterification

Transesterification was performed through enzymatic,noncatalytic,and catalytic processes.In addition,the process of noncatalytic transesterification was performed under supercritical conditions [28].These conditions require higher temperatures and pressures,which are not commercially feasible.Catalytic transesterification processes are also more commonly used in the production of biodiesel and are more typically preferred [29].Homogeneous catalysis can be simple,enzymatic,or acidic,where the necessary catalysis consists of alkaline,as seen in Fig.3.

Fig.3. Classification on transesterification catalyst method [24].

Base catalysts have remarkable transesterification activity rates at low temperatures and at low pressure.Sulfuric,sulphonic,and hydrochloric acids are the primary catalysts for acids [30].Although homogeneous acids and bases have specified chemical properties,heterogeneous acids and bases have particular properties.However,acid or base at Bronsted or Lewis sites may be classified as heterogeneous catalysts [31].The base-catalyzed transesterification process has been the most viable,heterogeneous method for several years [9].It has various processes for converting triglycerides into FAME by releasing glycerol as a byproduct;Experts are looking for potential catalysts.The type of feedstock,reaction kinetics,and post-reaction processing also influence the catalyst used [32].

2.2.1.Homogenous transesterification (acid-catalyzed) process

The process of transesterification with liquid acid-catalyzed is less efficient than the base-catalyzed process.The acid-catalyzed homogeneous reaction is approximately 400 times slower than that which has been homogeneously catalyzed[33].The efficiency of the acid catalyst in the feedstock is not significantly affected by the FFAs.In addition,both acid catalysts can be esterified and transesterified simultaneously[34].A significant advantage of acid catalysts is that bio-diesel can be generated directly in low-cost lipid feedstock,usually linked to high FFA[35,36].The ester molecule carbonyl group with a liquid acid catalyst is protonized during transesterification,leading to the carbocation.Alcohol must then undergo a nucleophilic reaction to form an intermediate tetrahedral[37].This tetrahedral releases glycerol by-product for the production and regeneration of acid catalysts.Fig.4 displays the complete reaction cycle for the transesterification of the organic oil acid catalyst [38].Diglycerides,triglycerides and acidic liquid catalysts can also be used in the same process.Table 4 indicates a few homogeneous catalytic acids/alkalines reported in the literature [39].

2.2.2.Homogenous transesterification (alkali catalyzed) process

Transesterification reactions are catalyzed by homogenous and heterogeneous catalysts.The main alkaline catalysts used are CH3-ONa,NaOH,and KOH[40].Three steps have been taken to evaluate the mechanism of alkali-catalytic transesterification[41].Alkalinecatalytic vegetable oil transesterification occurs more rapidly than acid-catalytic reactions,as shown in Fig.5.Moazeniet al.[36]elucidated the method of transesterification of base-catalyzed vegetable oils.

Fig.4. Acid catalyst transesterification process [33].

Fig.5. Alkali catalyst transesterification process [35].

The best transesterification catalyst is characterized by its moisture content and free fatty acid consistency.The strong alkaline catalysts for transesterification are less than 1% free of fatty acids.This is due to the sodium and potassium saponification reactions of free fatty acids that neutralize the alkaline catalytic element.The manufacture of this excess soap improves the reaction times and seems to be soluble in glycerine.Acid catalysts are also extremely susceptible to moisture in feed materials.Acid catalyst induced by corrosion restricts the use of FAME biodiesel[37-39].The catalyst is transformed into methanol by continuous stirring during transesterification of alkaline catalytic methanol in the reactor.The oil is injected into the feedstock reactor and then converted to the combination of alcohol/catalysis.The end result is constantly combined with 4 hours,416 K,and air pressure.Efficient transesterification leads to two liquid phases:ester and glycerol [42,43].

2.2.3.Heterogeneous transesterification process (acid-base catalyzed)

Homogeneous catalysts have shown better transesterification performance for the production of biodiesel.Problems associated with homogeneous catalysts include high energy consumption,the production of unwanted soap by-products in FFA reactions,the removal of the homogeneous catalyst from the mixture is very expensive,and the use of large quantities of wastewater during catalytic removal and purification[44,45].Heterogeneous catalysts could be an attractive alternative.These catalysts can be more easily separated from the reaction products[46].The use of heterogeneous acid catalysts avoids unintended saponification reactions.They make the transesterification of high organic FFA oils and fats such as food processing waste cooking oil[47].Biodiesel synthesis with the help of solid catalysts can result in lower cost of production[48]due to the reusability of the catalyst and the potential for simultaneous transesterification and esterification.

Strong transesterification catalysts can be generated from biomass materials with viable catalytic performance for the transesterification of biodiesel.The use of these natural catalysts would increase the economic and environmental effects of transesterification [49].An environmentally friendly approach to the production of biodiesel can also be used.The use of this natural material saves time for the preparation of the catalyst[50].During transesterification,calcite and dolomite have a strong catalytic activity of between 91 and 95%,respectively,owing to the production of calcium oxide during the calcination process [51].

The CaO and CaO2enhanced by Li catalysts provide outstanding catalytic efficiency and are very stable.In order to minimize engine issues,the catalyst must be isolated from FAME biodiesel by using liquid catalysts[52].High levels of ash are formed by alkaline catalysts when corrosion is caused by an acid catalyst [53].As transesterification with homogeneous catalysts requires expensive isolation and purification,transesterification often entails heterogeneous solid catalysts,including metal oxides and zeolites [54].Table 5 presents a number of examples of heterogeneous acid/alkaline research findings in the literature.

2.2.4.Enzymatic catalyst transesterification

Lipase enzyme transesterification is desirable and motivating due to easy substance isolation,minimal wastewater treatment requirements,rapid glycerol recovery,and lack of side reactions[55].Proper implementation of lipase poses a number of technological challenges for pseudo-homogeneous systems,such as residual enzyme contamination of the product and economic costs.In the end,the enzyme is used in the immobilization process to address this issue in order to reduce costs reliably and thus improve product quality.If free enzymes are used in the production of biodiesel,the enzyme component may be partially restored during the glycerol phase [56-58].However,the build-up of glycerol reduces the amount of future reuses.A variety of lipasemediated transesterification experiments have been proposed for the processing of biodiesel from solvent-free systems.In both of these methods,methanol tends to be poorly soluble in the oil feedstock;and too much methanol in the system as it enhances the detrimental activity of lipase [59-61].Kumaret al.[62,63]suggested that methanol be introduced step by step,becausemethanol solubility is higher in alkyl esters than in oil,thereby reducing enzyme deactivation.

Table 4 Homogeneous acid and alkali-catalyzed transesterification

Table 5 Heterogeneous acid and alkali-catalyzed transesterification

In addition,the release of glycerol can also avoid reactions by reducing the diffusion of substrates and products by oil or organic solvents[63].Triglycerides of alcohol enzymes of organic solvents,hexane and fuel solutions have been studied.The approach to biodiesel enzymes was most beneficial than the chemical solution.However,the cost of lipase would be a major obstacle to the industrialization of the lipase-mediated production of biodiesel [64].Two methods are thought to reduce the production of lipase.Reduce lipase’s cost by producing new lipase,optimizing fermentation,and enhancing downstream treatment [65].Another approach to increasing/enhancing the existence of lipase can be accomplished by immobilizing the enzyme,enhancing its response to alcohol,etc.[66].The relation of the related development of lipase-mediated biodiesel is seen in Table 6.Figs.6 and 7 indicate the method of transesterification using an enzymatic catalyst and the chemical process.

Table 6 Comparison of different biodiesel production mediated by lipase

Fig.6. Enzymatic catalyst transesterification method (flow chart) [62].

Fig.7. Enzymatic catalyst transesterification method (chemical process) [59].

2.2.5.Supercritical alcohol transesterification

A modern approach to the development of biodiesel is the transesterification of vegetable oils using supercritical noncatalytic methanol [67].Transesterification occurred in minutes under supercritical conditions,although the standard method of catalytic transesterification takes several hours [68].Transesterification of usually heterogeneous (two phases of liquid) alcoholbased triglycerides at normal processing temperature due to incomplete miscibility of non-polar and polar elements.Nevertheless,under supercritical conditions,the mixture has become a single homogeneous phase,accelerating the process as there is no inter-phase mass exchange to maintain the reaction rate [69].

This method depends on the temperature and pressure reaction above the critical temperature and pressure resulting in a singlephase mixture [70].The transesterification of supercritical alcohol is seen in Fig.8.Supercritical methanol is usually manufactured at an 80 atmospheric pressure and temperature between 300 °C to 400 °C.Owing to high pressures and temperatures,a supercritical technique does not require a catalyst and decreases the cost of separation,oil quality,and reaction time to about 5 minutes[71].This approach extends to a wide variety of feedstocks and has been shown to produce high-quality glycerol.Temperature is the primary concern of supercritical transesterification[72].The comparison of various outputs of biodiesel using a supercritical alcohol method is seen in Table 7.

Table 7 Supercritical alcohol transesterification reaction for various feedstocks

2.2.6.Ultrasonic irradiation transesterification method:

Ultrasonic irradiation is used as a transesterification process to overcome the minimum mass transition of the two immisciblereactants,oil and methanol [73].Fig.9 demonstrates the transesterification process using ultrasonic radiation.The sound waves are transmitted to the macro probe shaping the reactive mixture at the required frequency,which can allow the phase limits between the feedstock and the methanol to develop a wellbalanced emulsion [74].Transesterifying reactions are detected more rapidly as a result of improved mixing,reduced use of methanol and reduced use of catalysts at lower temperatures.

Fig.8. Supercritical alcohol transesterification [70].

Fig.9. Ultrasonic irradiation transesterification [74].

Low-level ultrasonic irradiation can be effective for alcohol and triglyceride transesterification.Ultrasonic uses mechanical energy to combine and initiate reactions [75].Ultrasound is a very useful method for increasing response rates in many processing systems.Conversion was effectively improved,yield increased,reaction mechanisms modified and/or biochemical,thermal and electrochemical reactions initiated [76].Ultrasound increases the rate of chemical reactions and yields from the transesterification of animal fats and vegetable oils to biodiesel.Ultrasonic transesterification has some advantages due to slower reaction time and energy consumption compared to traditional methods [77].Mechanical stirring and ultrasonic cavitation systems use 670 and 800 W·kg-1of energy for transesterification of 5 kg of organic oil/fat [78].The comparison of different production of biodiesel by ultrasonic irradiation transesterification is shown in Table 8.

2.3.Post-treatmentof biodiesel

Biodiesel extracted was further processed at the final stage,often involving two phases:laundry/polishing and distillation.Wash/polish has been compulsory,and the distillation of biodiesel is optional [79].The word wash is used when water is used for washing and polishing when powder and ion exchange resins or any other means are used.Large amounts of glycerin or soap are washed or cleaned in newly extracted biodiesel to comply with the ASTM standard [80].As the term implies,the distillation process distills gasoline into a colorless methyl ester.

2.3.1.Washing or polishing

In oils and bases (catalysts) fatty acid binds to a common substance called soap and water.Compounds such as soap,in which the acid hydrogen has been replaced by a metal ion,are also known as salts[81].The reason for such compounds is that products such as sodium hydroxide or potassium hydroxide are not evenly dispersed,resulting in the formation of charged particles in a manner that gives rise to Na+,OH-or K+,and OH-.Neither protons nor electrons shall be equally dispersed.Thus,H+can be replaced by Na+or K+with the same charge here,and the stray quantity of glycerin becomes the final product during the seedling process [82-84];soap and glycerine can be exmust be used for biodiesel applications to conform with D6751 ASTM biodiesel specifications.soap/glycerincan be extracted by cleaning with water or by mixing biodiesel with magnesium silicate using advanced soap-absorbing chemicals [85].This phase should be versatile in order to extract excess soap or glycerine [86].If there is a production failure,a well-planned and well-designed polishing process may solve the problems.It is easier to associate operating costs with the results of wash water and dry polishing (with magnesium silicate) [87].

In fact,the cost of washing with water in 15 MMgy(million gallons per year)plants ranges from 6%to 8%per gallon.This includes expenses relating to water,storage or waste,electricity,and labor.However,the rate of polishing is 9%-12% per gallon [88].This includes recycling,disposal/depositing,and labor expenses.Water wash is preferred where there is either a surplus of water or a means of washing it,such as a nearby ethanol plant or a wastewater treatment plant,or where the wastewater disposal policy wasn’t as stringent as any other location [89].In a small scalebiodiesel plant larger than 15 MMgy,it makes sense to propose a water wash facility with specific water treatment capability and filtrate disposal.Production plants with a production capacity of less than 15 MMgy are also better equipped with a polishing system [90].Fig.10 describes the method of biodiesel purification with washing.

Cold soak filtration is a comparatively recent upgrade over the ASTM 6751 standard a few years ago[91].Cold soak filtering happens as biodiesel is cooled and reheated at room temperature.The cooling and reheating process results in the development of mushy crystal content known as sterols[92].This content is not consistent with diesel and obstructs the engine fuel filter [93].Cold soak filtration with water wash can also be treated beyond the washing stage.In the case of polishing,cold filtration can be adjusted at the same time as the polishing stage [94].

2.3.2.Biodiesel distillation

Biodiesel distillation would include the numerous changes presently suggested in the ASTM specifications for untreated productsand most of the potential requirements for chemicals,salts,and so on [95,96].Fig.11 describes the distillation process.Distillation is used for a variety of uses,from increasing the red colour of maize oil to extracting high concentrations of sulphur and metal from yellow or other animal fats[97].Biodiesel distillation is the purest type of fuel.Unreactive oil derivatives mono-,di-and triglycerides,additives,catalysts,and pigmentation-are used as a base and must be drained or transported during distillation with crude glycerine.This range is between 220 °C and 270 °C [98].The method of distilling biodiesel operates at very high temperatures compared with other areas of the production process.The thermal fluid heater is the primary heater of this process that is cost-effective [99].Since the thermal oil heater is designed to withstand high temperatures in advance,it can run at high temperatures.The heating fluid used as heat transfer fluid should be able to withstand too high temperatures;otherwise,higher temperatures would disintegrate the heatedliquid and require expensive heat fluid replacement.Heat transfer systems,including heat exchangers and condensers,should comply with the specifications of the US Mechanical Engineers Society or be designed to meet ASME specifications due to high temperatures [100].There is also a possibility of flickering due to high temperatures of biodiesel during the process [101].Each liquid with rapid exposure to air flashes at high temperatures and vapors.Caution should be taken most of the time during the design process to resolve these concerns in order to ensure proper and safe operation [102].

Table 8 Ultrasonic irradiation transesterification reaction for various feedstock

Fig.10. Washing type biodiesel purification [83].

Fig.11. Distillation type biodiesel purification [95].

3.Kinetics Study of Biodiesel

Diglycerides(DG)and Monoglycerides(MG)serve as intermediate compounds in transesterification,while FAME and glycerol(GL)form during triglyceride (TG) transesterification due to three consecutive reversible reactions [103].

Transesterification reactions occur more rapidly where the ratio of methanol to oil is above the stoichiometric value.Due to a large amount of alcohol,the conversion of Diglycerides and Monoglyceride to methyl ester is facilitated.The process of transesterification is still subject to research studies.The chemical reaction in which each compound is extracted from the first order can be seen in the second kinetic order.

whereTis specific reaction rate,Xcis concentration (mol-1),kiis reaction rate constant (forward reaction) (L·mol-1·min-1·g-1),k-iis reaction rate constant (reverse reaction) (L·mol-1·min-1·g-1).

Transforming the equations mentioned above into differential equations,

3.1.1st order kinetic reaction

Reaction time and temperature effects are measured to assess the kinetic transesterification reaction [104].Changes in catalytic concentrations are known to be negligible during the reaction.Reversible reactions can be ignored in this case[105].If the reversible reaction is ignored,the forward reaction can be viewed as a singlephase reaction.The rate of conversion for the irreversible first pseudo-order may be:

The concentration of methanol in oil is higher and unsustainable.The concentration of methanol is supposed to be stable and the reaction equilibrium changes during the processing of the product.

The expression for the conversion rate is as follows:

Consideration is given to the initial stage(t=0)and to the concentration of TG is [TG]0,t=tis concentrated in [TG]t.The next equation consists of the following:

Ast=0,ln[TG]0=0 as [TG]0=0.1 mol·L-1,while [TG]tis the yield for biodiesel.The mass balance for the pseudo-first order,

whereCFAMEis the conversion of FAME

Integrating the above equation

3.2.2nd order kinetic reaction (ir-reversible)

The reaction follows the second-order kinetics rate at whichkis the rate constant for the 2nd order reaction:

Further development of this equation and the relationship with the conversion of TG is given as follows:

With respect to time,the above equation is integrated,

3.3.2nd order kinetic reaction (reversible)

The first reaction involved in the production of biodiesel is a reversible reaction and follows the second-order kinetics applied in the process.Biodiesel reactions occur when each mole of vegetable oil undergoes a process of formation of 1 mole of glycerin and 3 moles of methyl ester based on the Langmuir-Hinshelwood model [106].The Langmuir-Hinshelwood mechanism consists of nine phases (3 phases of absorption,3 phases of reaction and 3 phases of desorption) and indicates the reaction between the reactants.

Integrating above equation

Arrhenius Equation reaction rate constant is a temperaturedependent term that could be used to measure the activation energy and the pre-exponential factor.

Kinetic research on heterogeneous catalysts is not currently being performed extensively,as homogeneous catalysts are a promising catalyst commonly used in the standard biodiesel industry.The heterogeneous catalyst has yet to be studied[107].Therefore,the study of kinetic data for all types of catalytic systems offers information to enhance the selective response path [108].The Kinetic results for various homogeneous and heterogeneous catalysts shown in Table 9.It can be concluded that the use of heterogeneous catalysts will take longer to react than homogeneous catalysts [109].

4.Biodiesel Standards and Specifications

Biodiesel specifications began to develop in the mid-1990s to facilitate the use of biodiesel based on alkyl esters and their variation as vehicle fuels.In 1999,ASTM International adopted the provisional PS12 biodiesel standard [110].The first standard ASTM D6751 was introduced in 2002.Biodiesel standards EN 14214 were finalized in Europe in October 2003.US and Euro standards are of universal importance;they are,in general,the root cause of biodiesel standards elsewhere in the world.Implementations differ depending on the specifications of US and Euro biodiesel [111].

ASTM D6751 points out requirements for biodiesel mixtures for distilled fuels in the USA.While the B100 standard has been published,it does not apply to pure biodiesel used as a vehicle fuel[112].Instead,the portion of biodiesel could be combined to produce blends of biodiesel and diesel fuel [113].Since 2012,the ASTM D6751 standard has featured two different grades of biodiesel:Grade 2-B and Grade 1-B,with improved regulation of monoglyceride and cold soak filtration.ASTM has issued two standards for biodiesel/diesel fuel blends for vehicles [114]:

? In 2008,the diesel oil standard ASTM standard specification,ASTM D975,was amended to allow the blending of up to 10%biodiesel into conventional diesel.

? ASTM D7467 is a blend of biodiesel from B11 to B20 specifications.

Unblended FAME diesel fuel and some high-level biodiesel blends have been developed in Europe,while low-level mixtures are given by EN 590,the EU diesel fuel specification:

? EN 14214 sets the FAME diesel standards for diesel engines.B100 can be used unblended in a diesel engine or combined with diesel fuel for blending in conjunction with EN 590 or other applicable specifications,as compared to ASTM D6751.EN14214:2012 has introduced a number of improvements,including an increase in the concept of heating oil specifications and B10 blending adjustments.Different environmental classes based on the monoglyceride substance have also been established [115].

? Biodiesel/diesel fuel blends up to B5 are listed in EN 590.EN 590:2004 allowed FAME to blend up to 10%of diesel fuel,while EN 590:2009 raised the permissible FAME content to 15%[116].

? Launched in 2015,EN 16709 contains blends B20 and B30 for commercial fleet use.FAME part of EN 16709 ensure compliance with EN 14214 and comply with EN 590 for the diesel component [117].

Table 9 The Kinetic results for various homogeneous and heterogeneous catalysts

? EN 590,EN 16734-like standard covering mixtures up to B10 in 2016 [118].

Biodiesel regulations and test methods are aligned with those for petroleum diesel in accordance with ASTM D6751 and EN 14214 in Table 10.ASTM D6751 and EN 14214 these identify the key characteristics of biodiesel fuel—primary biodiesel blends for blending and clean diesel fuel.

ASTM D6751 describes biodiesel as monoalkyl esters with long chains with fatty acids obtained from organic oils and fats [119].There is no description of the sort of alcohol used.As a result,mono-alkyl esters can be developed with any alcohol provided they conform with the comprehensive fuel design specifications.Unless the gasoline were to be mono-alkyl esters of long chain fatty acids,all elements other than additives would possibly have been omitted.The European biodiesel guidelines,EN 14214,are simpler and refer only to mono-alkyl esters made of methanol,FAME [120].

North America,European and Asian engine makers have already approved the B100 specifications for diesel/diesel fuel mixtures[120].These instructions are identical to EN 14214,although there are several significant differences between them [121]:

? Restriction of blends to a maximum of B5,

? Increasing the stability of oxidation from B100 to 12 h,

? Reduce from 0.05% sulphate ash to 0.003% and apply an ash level of 0.004%,

? Implementing the restriction of ferrous corrosion,

? Limitation of viscosity,iodine concentration,and flashpoint.

5.Various Approach for Renewable or Green Dieselproduction

Renewable diesel is a biodiesel fuel produced by hydrodeoxygenated feedstock and also known as hydrogenated vegetable oil derived from palm oils,waste oils,and fats[122].Renewable diesel provides the food and agriculture industry with the opportunity to synthesize fuel from waste.Hydro-treated vegetable oils are well-established agricultural technologies.The factors which affect the hydro-treatment process include temperature,pressure,catalyst,hydrogen,and oil ratio [123].The triglyceride molecule is separated by the hydrogen reaction by the oxygen removal in three parts.The elimination of oxygen can produce diesel-like hydrocarbon molecules [124].Sustainable Petro-diesel diesel can be combined without any issues in terms of fuel efficiency or technological change in diesel engines.Organic hydro-processing oils/fats are a direct synthesis of the C15-C18 chain of paraffin [125].Hydro-deoxygenated vegetable oils are an effective substitute for crude diesel fuel.In diesel,aromatic compounds are considered to be essential.Clean combustion is not ideal for flavourings,but green diesel is safe compared to the efficiency of petro-diesel combustion.Various technologies for renewable diesel are available[126].Fig.12 demonstrates the process of producing renewable diesel.

Table 10 Various biodiesel standards and test methods

Fig.12. Green diesel production process [81].

Fig.13. Hydro-deoxygenation process [85].

Fig.14. Fischer-Tropsch method [90].

5.1.Hydro-deoxygenating of organic oils and animal fats

Renewable diesel is described as a replacement for diesel,an alternative source of renewable organic oils and fats(vegetable/animal).The major differences between FAME and HVO are:green biodiesel (HVO) originates from the catalyzed hydro-processing of organic oils and animal fats.FAME is formed by the transesterification process [123].Catalytic hydro-processing is widely used in petroleum refining and improves the hydrogen/carbon ratio,the decrease in hetero-atom and metal concentrations,and the heating value of refinery products [127].High-quality vegetable diesel is needed for the processing of catalytic hydrocarbons in standard diesel engines[128].The process of renewable diesel usually consists of two steps:catalytic hydrotreating process for the processing of standard paraffin and catalytic isomerization to produce a mixture of n-and isoparaffin [129,130].Fig.13 demonstrates the process of hydro-deoxygenation.

5.2.Fischer-Tropsch (FT) approach for renewable diesel

Fischer -Tropsch system is used to extract hydrocarbon from coal and convert it into carbon monoxide and hydrogen.The FT process is widely used for the processing of biomass,gas,and lubricating oils [131].The Fischer -Tropsch mechanism converts syngas into higher hydrocarbons.The process,which is an integral component of liquid gas,produces oil substitutes for synthetic lubricating oil and synthetic gasoline,typically from shale,natural gas,or biomass [132].There are two primary types of FT technology available:low-temperature FT (LTFT) and high-temperature FT (HTFT).LTFT requires the use of cobalt-dependent and lowtemperature catalysts.The low-temperature FT process of hydrocarbon and wax condensate is mainly direct paraffin with a small portion of olefins and oxygenates.Subsequent refining of olefins and oxygenates by catalytic hydrocracking of wax to naphtha and diesel in relatively mild conditions is possible[133].HTFT uses iron-based catalytic systems and higher (350-400 °C) temperatures.The HTFT is mainly used for the treatment of liquid fuel,such as LTFT.The most critical qualities of HTFT distillate are its cold flow characteristics,which are long-lasting due to its solid paraffin and low-density materials [134].

Fig.15. White diesel process [100].

Table 11 Properties of renewable diesel [149]

Substantial progress has been made on the development of sustainable biofuels by Fischer-Tropsch.The slurry reactor is widely used as a synthetic reactor in the production of the residual biomass of Fischer-Tropsch.Iron and cobalt catalysts can be used for these applications[135].Cobalt catalysts are cheaper,longer-lived,and stronger,whereas iron catalysts are more impure-tolerant and cost-effective.FT liquids are derived from various forms of biomass.Fischer-Tropsch biofuels have the same energy quality,viscosity,and flashpoint as conventional fuels.It is a high-quality fuel with desirable characteristics for use in diesel engines [136-138].Fig.14 illustrates the Fischer-Tropsch process.

Fischer-Tropsch biodiesel density varies from 0.70 g·ml-1to 0.80 g·ml-1as per global biodiesel specifications.The aromatic content is also very limited and leads to cleaner combustion (0-0.2%,mass).The amounts of particulate matter and NOxare lower[139].Furthermore,the biodiesel supplied by Fischer-Tropsch is low in sulfur (＜8×10-6) emissions.It should be noted that the amount of cetane (50 to 95) in Fischer-Tropsch diesel indicates improved self-ignition efficiency.Its oxidation stability is also high since the induction time reported is ＜70 hours[140].FAME biodiesel thus does not need antioxidant additives,because low levels of natural antioxidants make FAME biodiesel less oxidant-stable.Fischer-Tropsch’s heating value varies from 40 to 43 MJ·kg-1above the biodiesel and diesel requirements,rendering it a good replacement for diesel.The flashpoint is weak,and there is an increased risk of auto-combustion [141].

5.3.White diesel approach for renewable diesel

White diesel is a 100% catalytic hydro-treatment method of waste cooking oil that requires a single catalytic hydroprocessing step [142].White diesel is a sulfur-free,flavor-free paraffin fuel.White diesel has marginal sulfur concentrations in heteroatomic elimination as an objective for catalytic hydrotreatment at ＜1.85×10-6.It also has a lower density than standard diesel and biodiesel [143].This ensures that a higher volume of diesel has to be supplied with the same amount of energy as diesel.However,it can reduce the volume of smoke due to lower fuel density.The cetane value of white diesel tends to be very high,and some are similar to other biofuels [144].Fig.15 explains the process of producing white diesel.White diesel oxidation stabilization increased by ＜22 h in order to meet diesel and biodiesel standards.White diesel does not require any antioxidant additives such as diesel fuel containing small quantities of natural antioxidants to maintain sufficient oxidation stability.White diesel has a very safe flash point (120 °C) [145].

5.4.Hybrid approach for renewable diesel

In addition to the hydro-treatment of vegetable oils for diesel production,it can also be co-processed with raw petroleum resources [146].This facilitates the use of current hydroprocessing and refining equipment technologies by incorporating liquid biomass into typical hybrid fuel processes [147].Various types of vegetable oil can be used to co-process oil streams,such as oil palm,rapeseed,coconut,Karanja,jatropha,and waste cooking oil.Plant oil components improve global hybrid fuel production with certain co-processing studies [148].

Hybrid diesel density varied from 0.80 g·ml-1to 0.90 g·ml-1,while distillate temperature (v/v) varied from 350 °C to 400 °C[141].In comparison,within the diesel specifications,the kinetic viscosity range (2.0-5.0 cSt) is considerably higher,and the net heat value(45-49 MJ·kg-1)ranges considerably[149].The properties of cold flow range considerably from 15 to 200°C and from 20 to 250°C,depending on the catalyst and the operating parameters used [150].The co-hydrocracking method can ultimately be best suited,as hybrid diesel with a higher temperature of cold flow aids in isomerize reactions[151].Details of the properties of Renewable diesel are shown in Table 11.

6.Conclusions

The literature review includes several biodiesel from a wide range of feedstocks and its production process.It also illustrates the characteristics of the different types of biodiesel.The following are significant conclusions from the study.

? Biodiesel is usually produced by transesterification (FAME) of animal fat,vegetable oils,and waste cooking oils or by hydrodeoxygenation (renewable).

? Biodiesel and green diesel are both highly beneficial and complementary.Either fuel can replace a few percent of the oil market owing to the abundance of these oils in the regions with the most substantial gain.

? The limited supply of feedstock sufficient to meet future demand for both fuels remains a significant concern in the future,not withstanding all efforts to establish additional feedstock.

? More biodiesel or green diesel production could be a matter of concern on shifts in agricultural commodity prices,while this effect may not be entirely evident.

? Renewable diesel,primarily made up of shortening chain and isomerized molecules,are suitable for the aviation market because it would benefit from superior cold-flow properties and reduced environmental effects.

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.

Acknowledgements

The corresponding author sincerely thank Er.V.G.Raajendran MLA,Chairman -Indira groups of Institutions,Dr.N.Velvizhi,Principal -Indira Institute of Engineering and Technology,Mr.M.Parivallal,Head of Department Mechanical Engineering,for the motivation and support to complete this paper successfully.