Biodiesel Production from Vegetable Oil: A Review

Abdelrahman MAA, Yassin AAA, Hussein IH, Mirghani MES and Karama AB

Published on: 2020-05-15

Abstract

Biodiesel is an alternative, renewable and sustainable source of energy that has become more attractive recently due to environmental concern and diminishing petroleum reserves. This article is focused on reviewing the production of biodiesel from vegetable oil. Biodiesel is produced by Tran’s esterification, catalytic and non-catalytic Tran’s esterification, in addition to assisted techniques such as ultrasonic and microwave assisted Tran’s esterification. Also, the factors that affecting the biodiesel production such as feedstock quality, free fatty acid (FFA) content and moisture content, molar ratio, temperature and agitation speed have been reported. Moreover the article discussed the biodiesel standard, stability and economics.

Keywords

Biodiesel; Vegetable oil and Tran’s esterification

Introduction

In developing countries energy consumption has been increased due to increasing the economic growth, since imported petroleum is the source of this energy, increasing use of petroleum will increase the local air pollution and global warming. A special attention to alternate petroleum by employing vegetable oils as sources for diesel fuels is thus warranted. Biodiesel is produced by reacting vegetable oils with short chain alcohols in the presence of an alkali catalyst to transform the triglyceride molecules into smaller molecules of fatty acid methyl or ethyl esters [1-5]. Biodiesel can be produced from a great variety of feedstock depending on the origin, availability and quality of feedstock; the common vegetable oils are (e.g. soybean, palm, cottonseed, jatropha, canola, safflower, etc.) as well as fat and waste oils (e.g. used frying oils) [6]. In the last two decades, Sudan became one of the petroleum producing countries. However, due to the cessation of the southern part, the country lost a major part of its petroleum reserves; as a result Sudanese uprising was initiated on 19 December 2018; one reason of this uprising is the shortage of fuels. This should intensify the search for alternative, renewable resources to replace the fossil fuels.

Biodiesel Production from Vegetable Oils

Vegetable oils have always been used for both edible purposes and for a wide range of industrial applications, such as Biodiesel, illumination oil, soaps, cosmetics, pharmaceuticals, emulsifiers, lubricants and greases, drying and semi drying oils in paint etc. [7]. Since vegetable oils are renewable, biodegradable and environmentally friendly, they have become important feedstock for producing biodiesel [8-19]. The most common vegetable oils for producing biodiesel in the United States of America and Brazil is soybean, in Southeast Asia is palm oil, in India is Jatropha, in Eastern Europe is Rapeseed [11]. Due to increasing demand for edible oils, non-edible oils became more attractive feedstocks for biodiesel shows some feedstock’s for biodiesel production and their physiochemical properties. Vegetable oils in general have a high viscosity which is a major cause of poor fuel atomization and operational problems such as engine deposits which can be reduced by blending with petro diesel, pyrolysis, micro-emulsification and Tran’s esterification. Only transesterification reaction gives products known as biodiesel [6]. Biodiesel specifications such as cetane number, energy content and density are almost similar to the specifications of petroleum based diesel fuel [12,13]. Shows physiochemical properties of biodiesel from different feedstock’s and the yield under the production (Tables 1 and 2).

Tran’s esterification Reaction

The transesterification reaction consists of a sequence of reversible reactions. The triglyceride is converted firstly to diglyceride, then to monoglyceride and finally to glycerol. Equations (1), (2), (3) and (4) show the Trans esterification reactions of vegetable oil using alcohol to form esters and glycerol [14-18].

Where TG: Triglycerides; DG Diglycerides MG Monoglycerides.

k1, K2, k3, forward reactions constants; k4, k5, k6, backward reactions constants.

In transesterification using methanol (for example), the meth oxide radicals (CH3O-) is an active species of catalysis. The activity of the catalyst depends on the amount of meth oxide radicals available for the reaction [19]. For sodium or potassium hydroxide, meth oxide solution is prepared in situ as shown in Equation (5).

This reaction also produces water which remains in the system that will increase the formation of free fatty acids (FFA) and soap due to hydrolysis of triglycerides and alkyl esters. Saponification also occurs if a strong base (NaOH or KOH) is present in the system Equation (6) and Equation (7) However, FFA reacts with alcohol by acid catalyzed esterification to form ester; this reaction is very useful for treating oils with high FFA (Equa (8) [20,21]. Although using sodium and potassium meth oxides cannot completely avoid soap formation with high FFA feedstocks, little saponification of esters and triglycerides occurs when meth oxide solutions was prepared water-free because meth oxides behave as weak Lewis bases [22].



Trans esterification Methods

Trans esterification is either catalytic which consist of base, acid or enzyme catalyzed reactions or non-catalytic Trans esterification; the latter is commonly known as supercritical Trans esterification [8-22].

One-step Alkali Base Catalyzed Trans esterification

The FFA is the key parameter to identify the suitable process of biodiesel production. Oil with low FFA (<2%) was Trans esterified to methyl ester using the conventional alkali catalyst [23].

Two-step Acid-base Catalyzed Trans esterification

Oil with high FFA (>2%) will produce soap, and the separation of products will be difficult; this will decrease the biodiesel produced. An alternative is acid-catalyzed esterification of the oil; compared to the base-catalyzed Trans esterification, the reaction is slower. As a result, a two-step process was investigated for high FFA oils; Firstly FFA is decreased to < 2% then the second step is alkali Tran’s esterification [24-29].

Ultrasonic Assisted Trans esterification

Ultrasound energy has applications in a number of fields, such as preparation of Nano-materials, food processing, polymer chemistry, electrochemistry, green chemistry and biodiesel production. Also, ultrasound energy can be used for kinetics enhancement for chemical reactions such as esterification which can reduce high FFAs before Tran’s esterification process [30-33]. Ultrasonic waves are energy application of sound waves which are vibrated more than 20,000 per second. Three effects are associated with ultrasound irradiation; firstly it causes a variation of sonic pressure that result in rapid fluid movement. The second is cavitation’s formation, which is a major factor; cavitation bubbles will be formed, rapid growth and violent collapse as a result of liquid break down due to the application of a large negative pressure gradient on the liquid. This phenomenon is responsible for most of the significant chemical effects. The third effect of ultrasound is acoustic streaming mixing [34-36]. Ultrasonic assisted Tran’s esterification is very useful because it takes a shorter reaction time and consumes less energy than the conventional mechanical stirring method [37-40].

In-situ Transesterification

In-situ extraction (simply termed as reactive extraction) is another method for producing biodiesel that has the potential to reduce the processing cost. In this process, extraction and Tran’s esterification proceed in one single step, with alcohol acting both an extraction solvent and a Trans esterification reagent [41, 42]. Determined the reaction parameters and the optimum conditions of producing biodiesel by reactive extraction from J.curcas seeds; higher oil yield was associated with the smaller particle sizes. The effect of different particle sizes of J.curcas seed on the oil yield has studied [43-44]. Reported that the main reason for increasing oil yield by decreasing the particle size is due to increasing the specific surface area of oilseed that is interacting with the solvent [45]. Reported that drying the oilseeds before the reaction will reduce the alcohol consumption during in situ transesterification in order to obtain high efficiency.

Supercritical Transesterification

The supercritical transesterification is a highly promising process for both trans esterification of triglycerides and methyl esterification of FFAs since no catalyst is required; it is simpler in purification and takes lower reaction time, but requires a high temperature and pressure [46-48]. Was developed a mechanism of hydrolysis of esters in sub/supercritical water, upon which a reaction mechanism of supercritical method of vegetable oil transesterification was proposed depending on the relation between pressure and temperature upon the thermo physical properties of the solvent such as dielectric constant, specific weight, viscosity and polarity [49].

Supercritical process is achieved when a fluid is subjected to temperatures and pressures in excess of its critical point. Has studied the critical temperatures and critical pressures of various alcohols which are shown (Table 3). At subcritical state of alcohol, reaction rate is so low and gradually increases as increasing of either temperature or pressure until the supercritical condition is reached which has a favorable influence on the yield of ester conversion. (Figure 1) shows the supercritical alcohol transesterification method at 575 K that was yield of conversion rises from (50 to 95) % for first ten minutes. The fatty acid composition of the feed stocks has a significant impact on the properties of the biodiesel obtained; the fatty acid profile of the resulting biodiesel is identical to that of the feed stocks. The presence of polyunsaturated fatty esters leads to low oxidation stability and low cetane numbers; high content of fully saturated fatty esters has a negative effect on cold flow properties. Therefore, partial hydrogenation of polyunsaturated fatty acid methyl esters to monosaturated compounds will increase the fuel quality because it was reported that monounsaturated methyl esters such as methyl oleate (18:1) and methyl palmitoleate (16:1) give the best properties for biodiesel produced. Partial hydrogenation reactions catalyzed metal-based cataly have been tested to improve biodiesel quality, but resulted in low monosaturated fatty acid methyl ester selectivity. On the other hand, copper catalysts have good selectivity in partial hydrogenation of highly unsaturated fatty acid methyl esters; reducing the degree of unsaturation without increasing the fully saturated methyl esters [50-55]. However, transesterification for biodiesel production followed by partial hydrogenation of fatty acid methyl esters will significantly increase the cost of biodiesel production. Thus a supercritical one-pot process was developed by Shin to combine supercritical transesterification and partial hydrogenation in one step [56-63] (Figure 1).

Table 1: Biodiesel Production and their Physiochemical Properties.

 

Type of oil

 

Density (g/cm3)

 

Flash point (oC)

 

Kinematic viscosity at 40 oC (cSt)

 

Acid Value (mg KOH/g)

 

Heating Value (MJ/kg)

 

References

Soybean

0.91

254

32.9

0.2

39.6

[78,113,114,130]

Rapeseed

0.91

246

35.1

2.92

39.7

[65,73,130]

Sunflower

0.92

274

32.6

-

39.6

[114,20,34]

Palm

0.92

267

39.6

0.1

-

[27,112,36]

Peanut

0.90

271

22.72

3

39.8

[120,90,130]

Corn

0.91

277

34.9a

-

39.5

[130]

Cotton

0.91

234

18.2

-

39.5

[129,36,34]

Jatropha

0.92

225

29.4

28

38.5

[72,116,122]

Pongamia piñata

0.91

205

27.8

5.06

34

[87]

 

 

Microwave Assisted Transesterification

Microwave irradiation has been widely applied in organic synthesis [64]. It provides a comfortable, safe and clean way of working with chemical reactions besides requiring very less energy input for heating compared to the conventional heating [65]. Recently, microwave irradiation has been used in transesterification reactions as an alternative heating system that accelerates the transesterification in a short reaction time and reduces both the separation time and the quantity of by-product [66-68]. Many authors have studied the homogenous base catalyzed transesterification reactions assisted by microwave irradiation in batch conditions; A yield of 93.7 % was obtained from rapeseed oil using a power of 1200 W at 40 C in one minute by a conversion of greater than 96% was obtained [69] in six minutes with KOH catalyst at 60 C using a power of 500 W in a 50 ml reactor obtained 98.4% from safflower oil in six minute at 60 C using a power of 300 W in a 500 ml reactor. Moreover have studied the effect of microwave in reducing the FFA using homogeneous catalyst; the acidity of J. curcas oil was decreased from 14% to about 1% using sulfuric acid in a 500 ml reactor at the power of 110 W for 35 minutes [66-71].


Figure 1: Supercritical Method using Different Alcohols at 575 K.

Factors Affecting the Transesterification Reaction

A number of parameters affect the transesterification reaction which is governed by reaction conditions. It is necessary to optimize the parameters to avoid having an incomplete reaction or yield reduction [72]. Transesterification depends on feedstock’s quality, alcohol type, alcohol-to-oil ratio, catalyst type, catalyst-to-oil ratio, reaction temperature, agitation rate, FFA and water content [73, 74].

Feedstock Quality

Improper handling and inappropriate storage conditions deteriorate seed oil quality gradually, hence water content will increase. A significant increase in FFA levels takes place when the oil is exposed to open air and sunlight for a long period of time.

FFA and Moisture Content

Increasing FFA and moisture content always affected negatively; which is resulting in causing soap formation, catalyst consumption and reducing catalyst effectiveness, moreover resulting in low conversions. In addition, more catalyst is required to neutralize FFAs for oils having higher FFA contents [75, 76]. Observed that the presence of water has a negative effect greater than that of FFAs, so the feed stocks should be water free.Have reported that even a small amount of water in the feedstock’s or from esterification reaction might reduce the methyl ester yield and cause soap formation.Has reported that the presence of water has a negative effect on the yield of methyl esters in base catalyzed and acid catalyzed methods. However, the presence of water and FFA has no effect on the methyl ester yield in supercritical methanol method as shown in (Figure 2, 3) [77-79].

Table 2: Physiochemical Properties and Yield of Biodiesel from Different Feedstocks.

 

Type of Oil

 

Kinematic viscosity at 40 oC (cSt)

 

Density (g/cm3)

 

Saponification number

 

Iodine value

 

Acid Value (mg KOH/g)

 

Cetane number

 

Heating Value (MJ/kg)

 

Yield %

 

References

 

Soybean

 

4.08

 

0.885

 

201

 

138.7

 

0.15

 

52

 

40

 

>95

 

[130,132,58,64,68,126]

Rapeseed

4.3- 5.83

0.88

-

-

0.25-0.45

49- 50

45

95-96

[127,128]

 

Sunflower

4.9

0.88

200

142.7

0.24

49

45.3

97.1

[132,127]

 

 

 

 

 

 

 

 

 

 

Palm

4.42

0.86-0.9

207

60.07

0.08

62

34

89.23

[130,132,118]

Peanut

4.42

0.883

200

67.45

-

54

40.1

89

[120,130,132

Corn

3.39

0.88- 0.89

202

120.3

-

58-59

45

85-96

[132]

Cotton

4.07

0.875

204

104.7

0.16

54

45

96.9

[127,128]

Jatropha curcas

4.78

0.8636

202

108.4

0.496

61-63

40-42

98

[116,132]

Pongamia piñata

4.8

0.883

-

-

0.62

60-61

42

97-98

[87,9,7]

Figure 2: Biodiesel Yield as a Function of Water Content in Tran’s esterification process.

Figure 3: Effects of Cotton Seed Oil to Methanol Molar Ratio of the Methyl Esters Yield using Supercritical Transesterification Method at 513 K.

Molar Ratio

The alkyl ester yield is increases with increasing the molar ratio of oil to alcohol; the stoichiometric ratio for transesterification requires three moles of alcohol and one mole of triglyceride to produce three moles of fatty acid methyl ester and one mole of glycerol (see section 2.6 Equation (2.4)) . Lower molar ratios of alcohol to oil require more reaction time; higher molar ratios result in greater ester production with short time but the recovery is decreased due to the poor separation of glycerol. However, excess amounts of alcohol are required to shift the reaction to the product hence, a molar ratio of 1:6 oil-alcohols is sufficient for the transesterification of vegetable oils [80]. Has studied the effects of molar ratio on the methyl esters yield of cottonseed oil in the supercritical methanol method at 513 K; it was found that 1:41 vegetable oil to methanol is the best molar ratio as shown in Fig 2,3.

Catalyst Type

Homogeneous catalysts are commonly applied in the vegetable oils transesterification. Base homogenous catalyzed (Noah and KOH) transesterification is still the most widely used method in biodiesel production. Sodium meth oxide (NaOCH3) and potassium meth oxide (KOCH3) are also preferable catalysts for large continuous flow production processes. Acid homogeneous catalysts are also used but require more reaction time as compared to the alkaline based transesterification reaction [81]. The transesterification of vegetable oils can also employ heterogeneous catalysts, since they can be recycled and used several times; there is a better separation of the final product, and there will be no need for further purification steps when used in a continuous process [81,82].

Table 3: Critical Temperatures and Critical Pressures of Various Alcohols.

 

Alcohol

 

Critical temperature

(K)

 

Critical pressure

(MPa)

 

Methanol

Ethanol

1-Propanol

1-butanol

 

512.2

516.2

537.2

560.2

 

8.1

6.4

5.1

4.9

 

Alcohol Type

Methanol (polar and shortest chain alcohol) and ethanol (renewable and environmentally friendly) are widely used alcohols for biodiesel production [83, 84]. Alcohol solubility plays an important role in the acid catalysed reactions; it was observed that higher reaction rates were obtained by longer chain alcohols, but the opposite effect was observed in base catalysed reactions; this was noted in a study by Nye [85]. On the effect of increasing molecular weight and boiling points of alcohols.

Reaction Time

Increasing reaction time will increase the conversion rate. In the initial phase when the alcohol is mixed and dispersed in the oil, the reaction is slow and then the reaction proceeds at a faster rate until maximum yield is obtained. However, a reduction in products yield which caused by the backward reaction of transesterification will occur with increasing reaction time. This result in esters loss and causes more fatty acids to form soaps. For base catalysed transesterification, the yield of biodiesel reaches its maximum at a reaction time of 120 min or less. Usually, base catalyst exhibit reactivity higher than acid catalyst, hence more reaction time is expected with acid catalysed transesterification. The reaction time needed during the conversion of triglycerides to biodiesel may range from 18 to 24 hours [86-92].

Agitation Rate

Mixing between the alcohol and vegetable oils are very important in the transesterification process since, the triglyceride and alcohol phases are not miscible and produce two liquid layers. Mechanical mixing is applied to increase the reactants contact and mass transfer rate. Therefore, variations in mixing strength are expected to alter the kinetics of the transesterification reaction have studied the transesterification reaction with 180, 360 and 600 rpm; incomplete reaction with 180 rpm, the yield of methyl ester was same with 360 and 600 rpm [93,94].

Reaction Temperature

The kinetic energy in the reaction process increases with increasing temperature, since high temperatures give energy to the molecules to move faster; therefore, it is easier to break the carbon bond in the glycerides with the help of alcohol and catalyst during the transesterification process [95]. Normally the transesterification reaction is conducted at a temperature close to the boiling point (for methanol 60 C to 70 C) and atmospheric pressure. Further increases in temperature were reported to have a negative effect on the conversion [80, 96]. Furthermore it was reported that the optimum reaction condition for biodiesel production from vegetable oils are: reaction temperature is 65 C, agitation rate is 400 rpm and reaction time is 2 hr.

Specific Gravity

Decreasing the specific gravity of the final biodiesel produced to a lower value is an indication of completion of reaction and removal of heavy glycerine [97]. Have studied the influence of molar ratio, temperature and catalyst amount on the specific gravity of the biodiesel; 56:1 methanol to oil molar ratio with 100% catalyst (quantity based on oil weight) at 303 K in four hours is the best condition to reduce the specific gravity from 0.912 to 0.864

Biodiesel Purification

When the transesterification reaction process is completed; two layers were observed; the product (biodiesel) and the heavy by product (glycerol) which is settles at the bottom of the reactor. Although two layers can be observed within ten minutes; the mixture is allowed to settle for several hours (usually to overnight). Centrifuge can used to accelerate the separation of two phases in some cases. Due to the soap formation and the contamination of both biodiesel and glycerol with unreacted catalyst, alcohol and oil during the transesterification process; the crude biodiesel should be purified. After separation from the glycerol phase; residual catalyst, water, excess alcohol, free glycerol and soap which contaminated with crude biodiesel can be removed by washing with a warm distil water for several times since the alcohol, glycerol, residual sodium salts and soap are soluble in water [98].

Biodiesel Standards

Biodiesel can be used as a direct fuel replacement, or as blends with petroleum. The American Society for Testing and Materials (ASTM) standard D6751-09, and the Committee of Standardization in Europe (CEN) standard EN 14214 in Europe stipulate several quality specifications that must be met before biodiesel can be made available for commerce. While, many of these specifications are dependent upon process conditions impacting the Trans esterification reaction, several specifications are also feedstock’s dependent such as cold flow properties and oxidative stability [99-106].

Biodiesel Stability

Biodiesel stability has studied in the previous work. Biodiesels produced from vegetable oils and other feedstocks have been found to be very susceptible to oxidation and thermal degradation. According to European biodiesel standard EN-14214 the minimum requirement of oxidation stability in terms of induction period (IP) is six hours by the Rancimat method (EN-14112). The chemical reactivity of fatty oils and their esters is divided into oxidative and thermal stability. Basically, the key of oxidation stability is the chemical structure. More oleic double bonds in the fatty acid chain of the triglyceride provide better oxidation stability. However, the moreconjugated double bonds in the fatty acid chain, the poorer the oxidation stability. On the other hand, highly saturated fats generally have high oxidation [107]. Tocopherols are naturally occurring antioxidants in vegetable oils (alpha, beta, gamma and delta). Alpha tocopherols provide protection to the oil against photo oxidation (oxidation under visible light). Beta tocopherols are found at very low concentrations in oils and their functions are not fully known. Gamma and delta tocopherols protect oils against autoxidation [108]. However, processing of biodiesel deactivates tocopherols that were originally present in the vegetable oil feed; Synthetic antioxidants were added to enhance biodiesel stability without affecting the properties of biodiesel such as viscosity, density, carbon residue and sulphated ash [109,110].

Biodiesel Economics

In spite of the favourable impact of biodiesel on the environment, the economic aspect of biodiesel production is a still a barrier for its development due to the lower price of petroleum and the high cost of vegetable oil as a main raw material for biodiesel production especially, when the vegetable oil used is edible since the cost of raw material represents about (60-80%) of the total production cost. On the other hand, biodiesel production increases the competition with the edible oil market by increasing the cost of both edible oils and biodiesel. However, one way of reducing the biodiesel production costs is to use the less expensive feed stocks such as inedible oils [111-133].

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