Technical aspects of biodiesel production from vegetable oils via catalytic transesterfication

Brianna Ball

Winter 2016

Biochemistry Co-op, Bachelor of Science University of Guelph

295784 29th Line Lakeside, Ontario N0M 2G0 May 9th, 2016 Enoka Wijekoon, PhD Department of Molecular and Cellular Biology University of Guelph Guelph, Ontario N1G 2W1 Dear Dr. Enoka Wijekoon, This report, entitled “Technical aspects of biodiesel production from vegetable oils via catalytic transesterfication” was prepared as my COOP*1000 Work Report for the Household Hazardous Waste Division of the Regional Municipality of Peel. This is my first work term report. The purpose of this report is to investigate the process and the chemistry behind the production of the alternative fuel biodiesel. The Household Hazardous Waste Division provides multiple locations for residents of the Region of Peel to safely dispose of residential and business hazardous waste that could be harmful to people, animals and the environment. The Household Hazardous Waste Division, in which I was employed, is supervised and managed by Grace McKenzie, Matthew Stevens and Matthew Gregorchuk who are primarily involved with managing the day to day and future incoming hazardous waste. This report was written entirely by me and has not received any previous academic credit at this or any other institution. I would like to thank Grace McKenzie, Matthew Stevens and Matthew Gregorchuk for providing me with valuable advice and resources, including leads to informative textbooks and a personal experience in producing biodiesel. I received no other assistance. Sincerely, Brianna Ball ID: 0853550

1

University of Guelph

College of Biological Science

Technical aspects of biodiesel production from vegetable oils via catalytic transesterfication

Household Hazardous Waste Division, Regional Municipality of Peel

Brianna Ball

ID Number: 0853550

Winter 2016

Biochemistry Co-op, Bachelor of Science

April 16th, 2016

2

Abstract

The ongoing search for a suitable alternative fuel in the transport sector has raised

increasingly more attention as the world pollution problems and the depletion of fossil

fuels has become more obvious. The necessity for a renewable energy source with less

environmental degradation and more energy efficiency has become important in recent

years. Biodiesel obtained from vegetable and animal oils are gaining potential as blending

components or direct replacements of diesel fuel. Biodiesel fuel is comprised of a mixture

of monoalkyl esters of long chain fatty acids. Transesterfication is the most common

technique to produce biodiesel fuel with the goal of decreasing viscosity of the vegetable

oil. In this paper, various methods’ to produce biodiesel are reviewed with an emphasis

on catalytic transesterfication. The two most common catalysts in transesterfication are

homogeneous liquids and heterogeneous solids are presented at length. An overview of

the most basic technology surrounding the production of biodiesel involving the

transesterfication reaction, separation of byproducts, the removal of the catalysts as well

as the removal of alcohol and free fatty acids, are also discussed.

Keywords: Alternative fuel; Biodiesel; Transesterfication; Catalytic; Vegetable oil

Table of Contents:

1. Introduction……………………………………………………………………………..3

2. Biodiesel……………………………………………………………………………..…5

3. Biodiesel feedstocks…………………………………………………………………….6

4. Transesterfication……………………………………………………….………………8

4.1 Catalytic Transesterfication………………………………………………………..10

4.1.1 Homogeneous catalytic transesterfication……………………………..…..10

4.1.1.1 Homogeneous base catalytic transesterfication………………….11

3

4.1.1.2 Homogeneous acid catalytic transesterfication………..…………12

4.1.2 Heterogeneous catalytic transesterfication…………………….………...…13

4.1.2.1 Heterogeneous solid base catalytic transesterfication………........14

4.1.2.2 Heterogeneous solid acid catalytic transesterfication……………15

5. Basic Technology………………………………………………………….…………..16

6. Conclusion…………………………………………………………………………….18

Acknowledgements……………………………………………………………...……….19

References……………………………………..…………………………………………19

List of Tables:

Table 1: Physical Properties of Biodiesel…………………………………………………6

Table 2: Comparison of acid and base catalysts displaying ester yields of used oils……13

Table 3: Homogeneous and heterogeneous catalysts comparison ………………………14

List of Figures:

Figure 1. Transesterfication of triglycerides with alcohol………………………………...8

Figure 2. Effect of ratio of methanol to Waste Cooking Oil on the conversion of biodiesel

at 30ºC and 0.75 wt% KOH……………………………………………………………...10

Figure 3. Side reaction in biodiesel using base-catalytic transesterfication a)

saponification b) hydrolysis……………………………………………………………...12

Figure 4: Simplified block flow diagrams for a typical continuous homogeneous

catalyzed process………………………………………………………………………... 17

1. Introduction

The conventional fossil fuel resources, such as coal, petroleum and natural gas,

are fulfilling the demand for a continuous supply of energy. The finite reserves of

petroleum around the world are decreasing daily causing an improved interest in finding a

suitable alternative fuel (Sivaprakasam and Saravanan 2007). Biofuel is a liquid or

gaseous fuel that is an alternative energy source that has potential for becoming a

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successful option for the transport sector (Demirbas 2007). Biofuel is produced from the

always-present biomass feedstock, which is comprised of vegetable matter that is

converted during the process of photosynthesis. An advantage of biomass is the

versatility of the range of organic matter from the bio-feedstock’s that can be converted

into gaseous, liquid and solid fuels from thermochemical and biological processes. The

biodegradable feedstocks are all environmentally friendly by being renewable,

sustainable, and having a neutral carbon life cycle, while supporting agriculture and green

industries (Yusuf et al. 2011).

The inventor of the first diesel engine, Rudolf Diesel, first commenced the

approach of the use of biofuels in diesel engines using peanut oil in 1900 at the World

Exhibition in Paris. However, research and development activities towards biofuels

seized because of the abundant supply of the inexpensive petroleum diesel in the

beginning of the 19th century (Yusuf et al. 2011). The scarcity of petroleum-based fuels,

the accelerated degradation of the environment and the increasing emissions of

greenhouse gases are now making biofuels more attractive as a friendly renewable

substitute (Demirbas 2007).

The types of biofuels include bioethanol, biomethanol, biodiesel and biohydrogen.

Biodiesel is the most widely accepted biofuel thus far due to its efficient adaptability to

diesel engines. Biodiesel made from vegetable or animal fats can be a direct replacement

of diesel fuel or a just a partial replacement as a blending component for internal

combustion engines due to its chemical structure (Demirbas 2009). Therefore, no

modification is necessary to the existing models of engines.

5

The Region of Peel has implemented an innovative program involving the car and

light truck fleet of the Waste Management Division to be converted to the use of

biodiesel. Residents and small business owners of the Region of Peel are able to drop off

up to 120 L of used cooking oil at the Community Recycling Centres. A four-step process

occurs to convert the waste vegetable oil into the final product of useable biodiesel,

involving transesterfication, separation, cleaning and filtering.

2. Biodiesel

Biodiesel comprises of a combination of methyl esters of long-chain fatty acids

varying in carbon chain length and number of double bonds such as palmitic, stearic,

oleic, linoleic and linolenic (Anand et al. 2010). The most common production of

biodiesel is by transesterfication of a triglyceride with an alcohol in the presence of a base

catalyst (Stephenson et al. 2010). The conventional alcohols used as acyl acceptors are

methanol and ethanol, other alcohols are acceptable however the former are the least

expensive and more reactive (Yusuf et al. 2011).

Physical properties of biodiesel are given in Table 1. Biodiesel has a higher

ability to biodegrade then petroleum diesel while still maintaining a stable reactivity level

with zero toxicity. Biodiesel has a clear amber-yellow physical appearance, and shares a

similar viscosity range to petroleum diesel however differs in flash point ranges from 423

K and 337 K for petroleum diesel (Yusuf et al. 2011).

A nomenclature has been adopted to distinguish the specific concentration of

biodiesel in a mixture with petrodiesel (Yusuf et al. 2011). When biodiesel is 100% pure,

the refereed terminology is B100, also termed “neat” fuel. Biodiesel blends are known as

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the BXX nomenclature, the XX is representative of the quantity of biodiesel in the blend.

For example, B60 blend is 60% biodiesel and 40% petrodiesel (Demirbas 2009).

Common name Biodiesel (bio-diesel) Common chemical name Fatty acid (m)ethyl ester Chemical formula range C14 – C24 methyl esters or C15-25H28-48O2 Kinematic viscosity range (mm2/s. at 313K)

3.3 -5.2

Density range (kg/m3, at 288K) 860-894 Boiling point range (K) >475 Flash point range (K) 420-450 Distillation range (K) 470-600 Vapor pressure (mm Hg, at 295K) <5 Solubility in water Insoluble in water Physical appearance Light to dark yellow, clear liquid Odor Light musty/ soapy odor Biodegradability More biodegradable than petroleum

diesel Reactivity Stable, but avoid strong oxidizing agents

Table 1: Physical properties of biodiesel (Demirbas 2009)

3. Biodiesel feedstocks

Common raw materials of biodiesel are rapeseed oil, canola oil, soybean oil,

sunflower oil, and palm oil. Animal sources including beef and sheep tallow, and cooking

oil are also widely accepted as sources of raw materials (Demirbas 2009).

The wide popularity surrounding vegetable oils is because of its renewability and

its environmental benefits. Vegetable oils can potentially supply an infinite resource with

an energy count close to that of diesel fuel. Therefore, biodiesel is the sole alternative fuel

so that minimal concentrations of biodiesel/diesel blends will be compatible with

conventional engines (Demirbas 2007). The production of Global vegetable oil increased

32 million tons in 10 years, from 56 million tons in 1990 to 88 million tons in 2000. The

7

increase of this consumption was distributed among the numerous oils (Kralova and

Sjöblom 2010).

Vegetable oils and animal fats are composed of triglyceride molecules in which a

single glycerol molecule is attached through ester bonds to three fatty acid groups. The

fats and oils contain properties of water-insolubility and hydrophobicity (Demirbas

2009).

To produce biodiesel, various lipids can be used, such as: a) waste vegetable oil;

b) virgin vegetable oil, most commonly used oils are soybean and rapeseed oils, however

other sources such as mustard, palm, sunflower, hemp and algae show potential; c)

animals fats and d) nonedible oils, including jatropha, neem, castor and tall oil (Demirbas

2009).

The direct use of vegetable oils can cause engine malfunctions in the long term

from the higher viscosity resulting in incomplete combustion and carbon accumulation.

The greater viscosity arises from the increased oxygen content in biodiesel contributing

to an increase in polar interactions of the fatty acid esters (Smith et al. 2008). For a fuel to

undergo full combustion, there is a minimal requirement of the fuel to be in existence

with a stoichiometric amount of oxygen. However, since the fuel is not oxygenated, the

structural amount of oxygen is not sufficient for a full combustion reaction. During

combustion, the physical oxygen content of fuel intensifies the combustion efficiency due

to increased interactions of oxygen with the fuel. Therefore, for vegetable oil the cetane

number and combustion efficiency are larger compared to those for diesel fuel (Kralova

and Sjöblom 2010).

Multiple methods have been considered to reduce the viscosity of vegetables oils

8

including dilution, micro-emulsification, pyrolysis, catalytic cracking and

transesterfication. Transesterfication is one of the more common techniques because the

reaction is reversible, therefore allowing the equilibrium to be pushed towards the

products side with the excess of alcohol (Demirbas 2009).

4. Transesterfication

The Region of Peel currently uses a method of transesterfication of triglycerides

(vegetable oil) with an alcohol and a base catalyst to produce a quantity of biodiesel that

is compatible with the diesel engine. Transesterfication involves neutralizing free fatty

acids on a triglyceride molecule or a complex fatty acid, thus removing the glycerin and

producing an alcohol ester (Tremblay 2008). Figure 1 illustrates the transesterfication

reaction.

Figure 1. Transesterfication of triglycerides with alcohol (Demirbas 2009)

Transesterfication is an equilibrium reaction that follows a stepwise procedure

starting with the conversion of triglycerides to diglycerides, followed by monoglycerides

and ending in glycerol. This conversion yields one mole of ester for each reduction of

glyceride, with a total count of 3 methyl esters per one triglyceride molecule

(Abbaszaadeh et al. 2012).

Since the reaction is an equilibrium reaction excess alcohol is needed to shift the converted to diglycerides and then diglycerides are converted tomonoglycerides followed by the conversion of monoglycerides toglycerol. In each step an ester is produced and thus three estermolecules are produced from one molecule of triglycerides [16].The transesterification reaction requires a catalyst such as sodiumhydroxide to split the oil molecules and an alcohol (methanol orethanol) to combine with the separated esters. Out of these threemethods transesterification is the most viable process adoptedknown so far for the lowering of viscosity. It also gives glycerolas a byproduct which has a commercial value. Among all thesealternatives, transesterification seems to be the best choice as thephysical characteristics of fatty acid (m)ethyl esters (biodiesel) arevery close to those of diesel fuel and the process is relatively sim-ple. In the esterification of an acid, an alcohol acts as a nucleophilicreagent; in the hydrolysis of an ester, an alcohol is displaced by anucleophilic reagent. This alcoholysis (cleavage by an alcohol) ofan ester is called transesterification [77]. Fig. 4 shows the transe-sterification reaction of triglycerides with alcohol. A catalyst is usu-ally used to improve the reaction rate and yield. Because thereaction is reversible, excess alcohol is used to shift the equilibriumto the product side.

Acid catalyst and alkali catalyst are used depending upon thenature of the oil used for biodiesel production. Another catalystin study is lipase. Lipase has advantage over acid and alkali catalystbut its cost is a limiting factor for its use at large scale productionof biodiesel. Choice of acid and alkali catalyst depends on the freefatty acids (FFA) content in the raw oil. FFA should not exceed acertain amount for transesterification to occur by an alkali catalyst.Fig. 5 shows enzymatic biodiesel production by interesterificationwith methyl acetate in the presence of lipase enzyme as catalyst.

The emergence of transesterification can be dated back to asearly as 1846 when Rochieder described glycerol preparationthrough the ethanolysis of castor oil [78]. Since that time alcohol-ysis has been studied in many parts of the world. Other researchershave also investigated the important reaction conditions andparameters in the alcoholysis of triglycerides such as fish oils, tal-low, soybean, rapeseed, cottonseed, sunflower, safflower, peanut,and linseed oils [79–84].

4. Biodiesel from triglycerides via transesterification

The transesterification reaction proceeds with catalyst or with-out any catalyst by using primary or secondary monohydric ali-phatic alcohols having 1–8 carbon atoms. Among the alcohols

that can be used in the transesterification reaction are methanol,ethanol, propanol, butanol and amyl alcohol. Methanol and ethanolare used most frequently. Ethanol is a preferred alcohol in thetransesterification process compared to methanol because it is de-rived from agricultural products and is renewable and biologicallyless objectionable in the environment [10]. However methanol ispreferable because of its low cost and its physical and chemicaladvantages (polar and shortest chain alcohol). Table 9 shows com-parison of various methanolic transesterification methods.

Triacylglycerols (vegetable oils and fats) are esters of long-chaincarboxylic acids combined with glycerol. Carboxylic acids {R–C(@O)–O–H} can be converted into methyl esters {R–C(@O)–O–CH3} by the action of a transesterification agent. The parametersaffecting the methyl ester formation are reaction temperature,pressure, molar ratio, water content, and free fatty acid content.It was observed that increasing the reaction temperature had afavorable influence on the yield of ester conversion. The yield of al-kyl ester increased when the oil-to-alcohol molar ratio was in-creased [6,29].

Fatty acid ðR1COOHÞ þ alcohol ðROHÞ¡ester ðR1COORÞþwater ðH2OÞ ð1Þ

Triglycerideþ ROH¡diglycerideþ RCOOR1 ð2ÞDiglycerideþ ROH¡monoglycerideþ RCOOR2 ð3ÞMonoglycerideþ ROH¡glycerolþ RCOOR3 ð4Þ

Transesterification consists of a number of consecutive, revers-ible reactions. The triglyceride is converted stepwise into diglycer-ide, monoglyceride, and, finally, glycerol (Eqs. (1)–(4)) in which1 mol of alkyl esters is removed in each step. The reaction mecha-nism for alkali-catalyzed transesterification was formulated asthree steps [6]. The formation of alkyl esters from monoglyceridesis believed to be the step that determines the reaction rate sincemonoglycerides are the most stable intermediate compound [29].

CH2–OOC–R1

CH–OOC–R2

CH2–OOC–R3

+ 3ROH Catalyst

R1–COO–R

R2–COO–R

R3–COO–R

+

CH2–OH

CH–OH

CH2–OH

Triglyceride Alcohol Esters Glycerol

Fig. 4. Transesterification of triglycerides with alcohol.

CH2–OOC–R1

CH–OOC–R2

CH2–OOC–R3

+ 3RCOOCH3

R1–COO–R

R2–COO–R

R3–COO–R

+

CH2–OOCCH3

CH–OOCCH3

CH2–OOCCH3

Lipase

Fig. 5. Enzymatic biodiesel production by interesterification with methyl acetate.

Table 9Comparison of various methanolic transesterification methods

Method Reaction temperature(K)

Reaction time(min)

Acid or alkali catalytic process 303–338 60–360Boron trifluoride–methanol 360–390 20–50Sodium methoxide–catalyzed 293–298 4–6Non-catalytic supercritical

methanol523–573 6–12

Catalytic supercritical methanol 523–573 0.5–1.5

20 A. Demirbas / Energy Conversion and Management 50 (2009) 14–34

9

balance towards the completion of biodiesel. Acceptable alcohols that will drive the

reaction forward are methanol, ethanol, propanol, butanol, and amyl alcohol

(Sivaprakasam et al. 2007). Methanol is a preferred alcohol because it has the shortest

chain, is the most polar of all the alcohols and has preferably cheaper rates. Ethanol is

also widely popular because of its environmental benefits of being renewable and derived

from agriculture products (Demirbas 2009).

The stoichiometric ratio of methanol and oil required to produce biodiesel is 3:1,

however the majority of the time the quantity of alcohol is increased to push the reaction

towards the products side (Phan and Phan 2008). For example, Cetinkaya and

Karaosmanoglu (2004) stated that the ratio of methanol to oils has to have a minimum of

5:1 to be effective, any lower ratio and the equilibrium will not proceed to create methyl

esters (proposed product).

Phan and Phan (2008) illustrates in Figure 2 the duration of the conversion of

biodiesel and the effects that the ratio of methanol to oil has at a temperature of 30ºC with

the assistance of a base catalyst, 0.75 wt% KOH. As shown in Figure 2, the methanol/oil

molar ratio of 5:1 had the smallest conversion weight percent for the duration of the

reaction time. It is shown in this study; after 80 minutes 88-90% of reactants had been

converted to biodiesel under the optimum ratios of methanol/oil was 7:1 – 8:1.

There are many variables of the transesterfication process including, temperature,

type of catalyst and its concentration, the mixing rate along with the alcohol to oil ratio

that impacts the conversion productivity and reaction time (Tremblay et al. 2008).

10

Figure 2. Effect of ratio of methanol to Waste Cooking Oil on the conversion of biodiesel at 30ºC and 0.75 wt% KOH (Phan and Phan 2008).

4.1 Catalytic Transesterfication

Often alcohol and triglycerides are not able to form a uniform nature with a single

phase; therefore the efficiency of the transesterfication proceeds slowly. To expedite the

conversion process, vegetable oils can be transesterfied by introducing heat and a catalyst

(Demirbas 2009). In biodiesel production there are two general classifications of

catalysts, homogeneous (catalyst remains in the same phase as reactants) and

heterogeneous (catalyst remains in a different phase). The suitable selection of a catalyst

for transesterfication is an important specification to keep production costs of biodiesel to

minimum (Abbaszaadeh et al. 2012).

4.1.1 Homogeneous catalytic transesterfication. Homogeneous catalytic transesterfication

contains two categories, acid and base catalysts. The homogenous process requires a post

reaction procedure that involves biodiesel being separated from the catalyst and the by-

product (Abbaszaadeh et al. 2012). Deciding between a base catalyst versus an acid

where mester: weight of ester collected (g); moil: weight of the oilsample (g); MWoil: averaged molecular weight of oil sample.

MWoil ¼ 3"X

i

ðMWi "%miÞ þ 38;

MWi: molecular weight of fatty acid i; %mi: percentage of fatty acid iin the raw material. MWester: averaged molecular weight of fattyacid ester.

MWester ¼X

i

ðMWi "%miÞ þ 14:

2.3. Analysis

Fatty acid quantitative was determined by using a HitachiG-5000A GC (column length: 30 m; diameter: 0.25 mm, film thick-ness: 0.25 lm). The physical properties of the raw samples, biodie-sel and its blends with diesel were measured by using ASTMstandard methods, including density (D1298), kinetic viscosity(D445), flash point (D93), cloud point (D2500), pour point (D97),distillation curve (D86) and carbon residue (D189), calorific value(D240) and acid value (D664).

3. Results and discussion

3.1. Transesterification of WCO with methanol

3.1.1. Effect of ratio of methanol to WCOThe methanol/oil ratio is one of the most important factors

affecting the yield of biodiesel. Although stoichiometric ratio re-quires 3:1, the transesterification is commonly carried out withan extra amount of alcohol in order to shift the equilibrium tothe proposed product, methyl ester. According to Centikaya andKaraosmanoglu [6], for example, transesterification is insufficientat the ratios of methanol/oils below 5:1. Furthermore, the metha-nol/oil ratio is associated with operating parameters such as thetype of catalyst used and the quality of oils. The optimum ratioof methanol/used frying oils was, for instance, 4.8 in the presenceof sodium hydroxide [13]; while it could be up to 250 in the pres-ence of acidic catalysts [45].

Shown in Fig. 1 is the effect of the methanol/oil ratio on the con-version of biodiesel at a temperature of 30 !C in the presence of0.75 wt% KOH. The conversion reached a value of above 50% in just20 min. Increasing the ratio from 5:1 to 8:1 increased the conver-sion. It rose from 50% for the ratio of 5:1 to 64% for the ratio of 8:1.

The difference in the conversion between the ratios of 5:1 and 8:1was about 24% in the first 60 min and slightly decreased to 13–16%for the last 60 min. The difference in the conversion was less than2% when the methanol/WCO ratio increased from 8:1 to 9:1.

A further increase in the methanol/WCO ratio above 9:1 causeda reduction in the conversion. It was 82% for the ratio of 12:1 com-pared to 88% for the ratio of 8:1 after 80 min as illustrated in Fig. 1.The reduction could be because the excess of methanol could inter-fere with the separation of ester product and by-products byincreasing solubility of glycerol. Consequently, part of the dilutedglycerol remained in the ester phase, leading to foam formationand therefore apparent lost ester product. In addition, the excessof methanol could also drive the combination of ester productand glycerol into mon-glycerides [11]. This indicated that the opti-mum molar ratio of methanol/WCO was 7:1–8:1, giving a biodieselyield of approximately 88–90% after 80 min. The optimum ratio inthis study was in accordance with that obtained from other inves-tigators [1].

Increasing the ratio also enhanced a settling process. The set-tling time took hours for the molar ratios below 7:1 while it wasonly approximately 30 min for the ratios of 7:1 and 8:1. The esterlayer isolated in the cases of 7:1 and 8:1 was yellowish and trans-parent while it was translucent for the other cases. This indicatedthat there was a certain amount of un-reacted glycerides dilutedin the ester phase at the ratios below 7:1.

3.1.2. Effect of concentration of catalystFelizardo et al. [13] revealed that the optimum concentration of

sodium hydroxide was 0.6 wt%. This value was much lower thanthe finding of Georgogianni et al. [18]. Leung et al. [25] also studiedthe effect of NaOH concentration on biodiesel derived from neatCanola and used frying oil. The results showed that the optimumvalue of NaOH concentration for neat Canola oil and used fryingoil was 1.0 wt% and 1.1 wt%, respectively. It could be concludedthat the concentration of alkali catalyst is strongly dependent onthe type of oils used.

Considering data from literature reviews, the concentration ofKOH was tested in a range of 0.5–1.5 wt% of WCO. Fig. 2 showsthe effect of concentration of KOH on the conversion at the meth-anol/WCO ratio of 8:1. Increasing KOH concentration from 0.5 wt%to 0.75 wt% increased the conversion. It was 82% and 90% at0.5 wt% KOH and 0.75 wt% KOH, respectively during 120 min.However, the conversion reduced to 75% in the case of1.5 wt% KOH. This could be explained by the fact that the forma-

Time (min)

0 20 40 60 80 100 120 140

Conv

ersio

n (%

wt)

40

50

60

70

80

90

100

5:1 6:1 7:18:19:112:1

Fig. 1. Effect methanol/WCO ratio on the conversion of biodiesel at a temperatureof 30 !C and 0.75 wt% KOH.

Time (min)

0 20 40 60 80 100 120 140

Conv

ersio

n (%

wt)

40

50

60

70

80

90

100

1.50%

0.50%

0.75%1.00%

1.25%

Fig. 2. Effect of KOH concentration on the conversion of biodiesel at the methanol/WCO ratio of 8:1 and a temperature of 30 !C.

3492 A.N. Phan, T.M. Phan / Fuel 87 (2008) 3490–3496

11

catalyst depends on the quality of the vegetable oil, the free fatty acid (FFA) count and

the water contamination (Sivaprakasam and Saravanan 2007).

4.1.1.1 Homogeneous base catalytic transesterfication. The base catalysts commonly

include hydroxides, alkaline metal alkoxides, and sodium or potassium carbonates. Base-

catalyzed processes are commercially more preferred because of the simpler set up

required. The process includes a direct conversion rate of approximately 98% or more

with no intermediate steps at a low temperature (70ºC) and with normal atmospheric

pressure of 1 atm (Sivaprakasam and Saravanan 2007).

When a base is used as a catalyst, an alkoxide group is formed which becomes the

effective catalyst that drives the thermochemical reaction forward. The alkoxide reaction

is shown below, in which R’ is a short alkali group, and R’O-- is the actual catalyst

(Tremblay et al. 2008):

!"! + !!!"⟷ !!!! + !!!

It is also important to account for the two side reactions of utilizing a base catalyst, which

are saponification and hydrolysis. When the oils contain a large quantity of FFA and

water, the excess base catalyst will react to produce soaps that decrease the overall output

of biodiesel and consumes the catalyst (Shah et al. 2004). Since the base catalyst is

consumed in saponification, this decreases the efficiency of the transesterfication and

makes separation of ester yields and glycerol difficult from the increased viscosity (Phan

and Phan 2008). As shown in Figure 3, excess FFA in the oil reacts with the base catalyst

to produce soap and water, followed by hydrolysis reaction of the conversion of alkyl

esters back into FFA (Abbaszaadah et al. 2012).

12

Figure 3. Side reaction in biodiesel using base-catalytic transesterfication a) saponification b) hydrolysis (Abbaszaadeh et al. 2012). 4.1.1.2 Homogeneous acid catalytic transesterfication. Saponification and hydrolysis

mentioned earlier can be avoided by using an acid catalyst. Common acid catalysts

include sulfuric acid, sulfonic acids, and hydrochloric acid. If water and FFA content is

high in the triglycerides, then acid-catalyzed transesterfication is preferable since there is

less sensitivity to these problems (Sivaprakasam and Saravnan 2007). The triglycerides

are vigorously mixed with the combination of the alcohol and acid catalyst, this forces the

transesterfication and separation to proceed spontaneously, therefore the alcohol acts as

both a solvent and an esterification reagent. The Brønsted acid catalysts produce

exceptional yields of alkyl esters, however a high alcohol/oil ratio and catalyst

concentration is needed to increase conversion efficiency (Abbaszaadeh et al. 2012).

Nye et al. (1983) studied the transesterfication of waste cooking oils from

cafeterias comparing an acid and base catalyst. Sulfuric acid and KOH were used to

catalyze numerous types of alcohol with partially hydrogenated soybean oil and

margarine. The ester yields of the two catalysts are shown in Table 2 (Enweremadu and

Mbarawa 2009), the yields were calculated after the product was able to settle at 25ºC for

two days followed by separation of byproducts and filtration. The remainder was re-

filtered after another 7 days at 25ºC. The results show that the yield of all esters other

then methyl esters have a higher weight percent for acid-catalyzed esters compared to

condition; (ii) high conversion can be achieved in a minimal time,(iii) high catalytic activity, (iv) widely available and economical[50,51]. In general, base catalytic transesterification processes arecarried out at low temperatures and pressures (333–338 K and1.4–4.2 bar) with low catalyst concentrations (0.5–2 wt.%) [52,21].

Typical process conditions for homogeneous base catalysis aremoderate: stoichiometeric alcohol/oil molar ratio (or slightly high-er), reaction at alcohol reflux temperature, atmospheric pressure orsmall overpressure, low reaction times (typically 1 h) and low cat-alyst concentration (Table 3).

The limits of this process are due to the sensitivity to purity ofreactants, free fatty acid content, as well as to the water concentra-tion of the sample. When the oils contain significant amounts offree fatty acids and water content, they cannot be converted intobiodiesels but to a lot of soap [49]. Free fatty acids of oil react withthe basic catalyst to produce soaps that inhibit the separation of

biodiesel, glycerin and wash water that cause to more wastewaterfrom purification [49]. Because water makes the reaction partiallychange to saponification, the basic catalyst is consumed in produc-ing soap and reduces catalyst efficiency. The soap causes an in-crease in viscosity, formation of gels which reduces ester yieldand makes the separation of glycerol difficult [53]. Therefore, sidereactions such as saponification and hydrolysis must be kept to aminimum. Saponification produces soap and water as shown inFig. 4a, from excess FFA in the feedstock. Also, a second hydrolysisreaction causes conversion of alkyl esters, using base catalysts, toFFA as shown in Fig. 4b.

4.1.1.2. Homogeneous acid catalytic transesterification. An alterna-tive way of processing triglycerides for biodiesel production is touse an acid catalyst. Acid catalytic transestrification of biodieselcan economically compete with base catalytic process using virginoil, especially when the former uses low-cost feedstocks [54]. Sul-furic acid, hydrochloric acid, and sulfonic acid are usually preferredas acid catalysts. Acid-catalyzed transesterification starts by mix-ing the oil directly with the acidified alcohol, so that separationand transesterification occur in single step, with the alcohol actingboth as a solvent and as esterification reagent [55]. The use of ex-cess alcohol effects significant reductions in reaction time requiredfor the homogeneous acid catalyzed reaction. Hence, Bronsted acidcatalyzed transesterification requires high catalyst concentrationand a higher molar ratio to reduce the reaction time [53]. Mianoet al. [56] investigated the various amount of molar ratio of meth-anol and trifluoroacetic acid catalyst on methyl ester content andspecific gravity of reaction. The optimum conversion was achievedat the molar ratio of 20:1. Under this molar ratio, the methyl estercontent and specific gravity of reaction product were 98.5%and 0.878%, respectively, after 5 h of reaction time (Fig. 5). They

Fig. 3. The process flowchart of homogeneous catalytic transestrification process.

Table 3Typical reaction conditions for biodiesel production using homogeneous basecatalysts [18].

Feedstocks Refined triglycerids (FFA<0.5 wt.%; H2O < 0.06 wt.%),anhydrous short-chain alcohols (generallymethanol)

Alcohol/oil molar ratio 3:1–9:1 (usually 6:1)Oil/co-solvent molar

ratio0.2–0.4

Temperature 20–75 !C (usually 60–65 !C)Pressure 0.14–0.41 MPaCatalyst NaOH and KOHCatalyst concentration

(oil basis)0.25–2 wt.%

Stirring speed 300–600 rpmReaction time 1–4 h (usually 1 h) for >98% conversion

Fig. 4. Side reaction in biodiesel production including (a) saponification to create soaps and (b) FFA formation from hydrolysis [49].

142 A. Abbaszaadeh et al. / Energy Conversion and Management 63 (2012) 138–148

13

base-catalyzed. One of the disadvantages of acid-catalyzed transesterfication is the longer

time to completion for acid catalysts compared to base catalyst.

Other disadvantages include, equipment corrosion, a requirement of a higher

molar ratio of alcohol/oil, higher reaction temperature, and sensitivity to water

contamination that inhibits ester conversion, the effects of these disadvantages could

become more costly (Canakci and Gerpen 1999).

Table 2. Comparison of acid and base catalysts displaying ester yields of used oils (Enweremadu and Mbarawa 2009).

4.1.2 Heterogeneous catalytic transesterfication. The use of heterogeneous catalysts is an

attractive alternative that offers economic benefits. Unlike homogeneous catalysts,

heterogeneous catalysts operate in a different phase and can be easily separated and

recycled from the desired products (Smith et al. 2009). Therefore, there is a decrease in a

high consumption of energy from the eradication of multiple refining steps that are no

longer necessary (Abbaszaadeh et al. 2012). This process diminishes the need for an

abundant separation and purification step while providing a continuous flow reaction that

Alcohol Molar Ratio (alcohol:oil)

Temperature (ºC)

Time (h) Catalyst Ester yield (wt%)

2 days at 25ºC

7 days at 25ºC

Methanol 3:6:1 65 40 0.1% H2SO4 79.3 64.0 Methanol 50 24 0.4% KOH 91.9 85.3 Ethanol 3:6:1 73 40 0.1% H2SO4 66.9 54.8 Ethanol 50 24 0.4% KOH 28.9

1-propanol 3:5:1 90 40 0.1% H2SO4 92.2 76.2 1-propanol 50 24 0.4% KOH 42.7 2-propanol 3:5:1 80 40 0.1% H2SO4 78.7 54.4 2-propanol 50 24 0.4% KOH 51.2 1-butanol 3:6:1 105 40 0.1% H2SO4 78.1 61.9 1-butanol 50 24 0.4% KOH 59.5

2-ethoxyethanol

4:2:1 125 40 0.1% H2SO4 53.5 39.2

2-ethoxyethanol

50 24 0.4% KOH 37.0

14

keeps the catalyst at a constant concentration. Table 3 gives a general comparison

between homogeneous and heterogeneous catalytic transesterfication. The comparison

displays the differences in factors involved with the transesterfication method such as, the

rate of the reaction, the process of post transesterfication, and the effect that any

contaminants may have on the catalysts (Helwani et al. 2009). Heterogeneous catalytic

methods tend to be time consuming, inefficient and difficult to mass transfer. Factors

affecting catalytic activity are pore size and volume, active site concentration on the

surface of the catalyst, and specific surface area (Lin at al. 2011).

Factors Homogeneously Catalysis Heterogeneously catalysis Reaction rate Fast and high conversion Moderate conversion After treatment Catalyst cannot be

recovered, must be neutralized leading to waste chemical production

Can be recovered

Processing methodology Limited use of continuous methodology

Continuous fix bed operation possible

Presence of water/free fatty acids

Sensitive Not sensitive

Catalyst reuse Not possible Possible Cost Comparatively costly Potentially cheaper Table 3. Homogeneous and heterogeneous catalysts comparison (Helwani et al. 2009).

4.1.2.1 Heterogeneous solid base catalysts. The more common solid-base catalyst are

generally more active then solid-acid catalyst, these include alkaline metal oxides,

hydrotalcites, and basic zeolites. Basic oxides are easily accessible, low-priced and have

restricted solubility in polar media, all advantageous motivation for industrial production.

(Baerolocher et al. 2007). The group II oxides and hydroxides increase in strength in the

order Mg > Ca > Sr > Ba. Calcium derived bases, such as CaO, are very promising

because of its simple requirement for mild reaction conditions, its ability to last a long

15

catalyst lifetime and contain a higher activity. However, the conversion rate is very slow

in producing biodiesel (Helwani et al. 2009).

Basic zeolites are crystalline structures that are composed of a framework made of

silicon, aluminum and oxygen that contain channels inside where water, cations, and

small molecules may occupy (Baerolocher et al. 2007). Similar to when a homogeneous

Brønsted basic catalyst is used and an alkoxide group is formed, when a basic zeolite is

used as a heterogeneous basic solid catalyst an alkoxide catalytic species also forms (Di

Serio et al. 2008). With an increase in electropositive matter of the exchanged cation, the

basicity of the alkali ion exchanged zeolite increases as well (Helwani et al. 2009).

Hydrotalcites, also known as layered double hydroxides (LDHs) with the formula

Mg6Al2(OH)16CO3•H2O and are usually manufactured by coprecipitation of soluble metal

salts. LDH are a group of compounds comprised of charge balancing anions in the

interlayer region of positively charged layers. The method to making LDH can strongly

influence the basicity of the mixed oxides, such as temperature, pH, and duration of the

hydrothermal treatment applied to improve crystallinity (Smith et al. 2009). The yield of

esters for LDH is relatively low, however can be improved if the specific surface of the

solid pore is enhanced and its participle size is reduced (Helwani et al. 2009).

4.1.2.2 Heterogeneous solid acid catalysts. Similar to a homogeneous acid catalyst, when

the FFA content is high in waste vegetable oils, it is preferable to use a heterogeneous

solid acid catalyst. Despite having lower amounts of activity, large-scale commercial

processes have raised attention to solid acid catalysts because of the variety of acid sites

with varying strengths of Brønsted or Lewis acidity (Helwani et al. 2009). The

replacement of liquid acid for solid acid catalyst will potentially remove environmental

16

dangers from contamination of liquid acids as well as improve the corrosion problems of

the machinery. Among the solid acid catalysts include sulfated zirconia, tungstated

zirconi, acidic zeolites, and heteropoly acids. Nevertheless, it is a necessity to have a

much higher reaction temperature to support a large yield conversion rate because of the

drop in activity for transesterfication. Accordingly, the higher temperatures pose

economic problems as well as a difficulty to control the thermal stability (Helwani et al.

2009).

5. Basic Technology

In current industrial biodiesel production, a batch or continuous system with the

presence of homogeneous catalysts is used to supply an output of biodiesel. Figure 4

shows a simplified block diagram for a continuous homogeneous catalyzed process

(Bournay et al. 2005). First, the waste vegetable oil is usually preheated in a heat

exchanger at 50-70ºC with continuous stirring to increase the flow property while

decreasing the viscosity. The process involves dissolving the homogeneous catalyst, most

commonly sodium hydroxide, in methanol before adding to the waste vegetable oil. The

catalyst/alcohol is then transferred to the reaction chamber along with the already

preheated vegetable oil. Once the reaction is complete, the mixture settles under gravity.

The byproduct glycerin is heavier then the esters produced, causing glycerin to settle

down onto the bottom layer. This presents two distinct phases, the glycerin phase is

removed and the esters are refined by wash columns for the recovery of excess catalysts,

glycerol and sodium soaps. After the biodiesel layer is dried, often with the use of silica

gel, the fuel is ready for blending with diesel in various mixtures compatible for engine

operations (Barnwal and Sharma 2005).

17

Fig 4. Simplified block flow diagrams for a typical continuous homogeneous catalyzed process (Bournay et al. 2005)

Using a homogeneous base catalysts such as sodium hydroxide, leaves the sodium

to be recovered from the glycerin as sodium glycerate, sodium methylate and sodium

soaps. To purify the glycerol, acidic neutralization occurs. For example, hydrochloric

acid is used to neutralize the salts producing an aqueous solution of glycerol containing

sodium chloride. Following neutralization, the sodium soaps can be decanted as fatty

acids from the glycerol phase (Bournay et al. 2005).

The critical steps in producing biodiesel to protect the diesel engines from harm

are: separation of glycerin, removal of catalysts, removal of alcohol and FFA, as well as a

complete transesterfication reaction. Accordingly, these parameters drive up the cost of

production (Barnwal and Sharma 2005).

2. Biodiesel production through transesterification

2.1. Transesterification reaction

The transesterification of triglycerides to fatty acidmethyl esters (FAME) with methanol is a balanced andcatalyzed reaction, as illustrated in Fig. 1. An excess ofmethanol is required to obtain a high degree of conversion.

The conventional catalysts in natural oil transesterifica-tion processes are selected among bases such as alkaline oralkaline earth hydroxides or alkoxides [6]. However,transesterification could also be performed using acidcatalysts, such as hydrochloric, sulfuric and sulfonic acid,or using metallic base catalysts such as titanium alcoholatesor oxides of tin, magnesium, or zinc. All these catalysts actas homogeneous catalysts and need to be removed from theproducts after the methanolysis step.

2.2. Conventional processes

Several commercial processes for FAME production havebeen developed. In conventional industrial biodieselprocesses, vegetable oil methanolysis is achieved using ahomogeneous catalyst system operated in either batch orcontinuous mode. Sodium hydroxide or sodium methylate isoften used as catalyst. Sodium is recovered after thetransesterification reaction as sodium glycerate, sodiummethylate, and sodium soaps in the glycerol phase.

An acidic neutralization step with, for example, aqueoushydrochloric acid is required to neutralize these salts. In thatcase, glycerol is obtained as an aqueous solution containing

sodium chloride. Depending on the process, the finalglycerol purity is about 80–95%.

When sodium hydroxide is used as catalyst, sidereactions forming sodium soaps generally occur. This typeof reaction is also observed when sodium methylate isemployed and traces of water are present. The sodium soapsare soluble in the glycerol phase and must be isolated afterneutralization by decantation as fatty acids. The loss ofesters converted to fatty acids can reach as high as 1% of thebiodiesel production. These operations are illustrated inFig. 2.

2.3. New heterogeneous catalyst process

Much effort has been expended on the search for solidacid or basic catalysts that could be used in a heterogeneouscatalyzed process [7–10]. Some solid metal oxides such asthose of tin, magnesium, and zinc are known catalysts butthey actually act according to a homogeneous mechanismand end up as metal soaps or metal glycerates.

In this new continuous process, the transesterficationreaction is promoted by a completely heterogeneouscatalyst. This catalyst consists of a mixed oxide of zincand aluminium, which promotes the transesterificationreaction without catalyst loss. The reaction is performedat higher temperature and pressure than homogeneouscatalysis processes, with an excess of methanol. This excessis removed by vaporization and recycled to the process withfresh methanol.

The desired chemical conversion, required to producebiodiesel at the European specifications, is reached with twosuccessive stages of reaction and glycerol separation in orderto shift the equilibrium of methanolysis. The flow sheet ofthis process is presented in Fig. 3.

The catalyst section includes two fixed bed reactors, fedwith vegetable oil and methanol at a given ratio. Excess ofmethanol is removed after each reactor by partial evapora-tion. Then, esters and glycerol are separated in a settler.Glycerol outputs are gathered and the residual methanol isremoved by evaporation. In order to obtain biodiesel at theEuropean specifications, the last traces of methanol andglycerol have to be removed. The purification section of

L. Bournay et al. / Catalysis Today 106 (2005) 190–192 191

Fig. 1. Overall reaction for vegetable oils methanolysis.

Fig. 2. Global scheme for a typical continuous homogeneous catalyzed

process. Fig. 3. Simplified flow sheet of the new heterogeneous process, Esterfif-HTM.

18

6. Conclusion

The transport sector is continually being threatened by the diminishing fossil fuel

reserves and the awareness of the consequential effects of the green house gas emissions

on the environment. Thus, there is a pressure to develop an alternate fuel that is

renewable, biodegradable and a direct replacement for the preexisting fuel into the

conventional technology already in use. Fuels derived from biomass, more notably

biodiesel, can solve this problem with the advantage of being carbon neutral.

Biodiesel, derived from numerous sources of vegetable oils and animal fats, is

defined as the monoalkyl esters of fatty acids. Multiple techniques are available to reduce

the viscosity of vegetable oils to increase combustion efficiency in unmodified diesel

engines. The most common process is catalytic transesterfication in the presence of either

a homogeneous or heterogeneous catalysts. The preference between the catalysts is

dependent upon the quantity of water contamination and free fatty acids in the vegetable

oil.

The homogeneous base catalyzed transesterfication reaction is the most common

commercial method due to the lower production expenses required and the efficient

conversion rate in comparison to heterogeneous catalyzed transesterfication. However,

the high consumption of energy required to separate the homogeneous catalyst from the

ester yield have increased attention towards heterogeneous catalysts.

A solid heterogeneous catalyst is anticipated to increase in popularity in the future

due to its efficiency and the potential to reuse the catalyst in the following reaction. The

use of solid catalysts may reduce the cost of production and be more advantageous for the

environment.

19

Acknowledgements

The author acknowledges the support of the Household Hazardous Waste sector in the

Waste Management division in the Region of Peel.

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