Accepted Manuscript Title: The Effects of Catalysts in Biodiesel Production: A Review Authors: I.M. Atadashi, M.K. Aroua, A.R. Abdul Aziz, N.M.N. Sulaiman PII: DOI: Reference:

S1226-086X(12)00233-X doi:10.1016/j.jiec.2012.07.009 JIEC 1004

To appear in:

Please cite this article as: I.M. Atadashi, M.K. Aroua, A.R.A. Aziz, N.M.N. Sulaiman, The Effects of Catalysts in Biodiesel Production: A Review, Journal of Industrial and Engineering Chemistry (2010), doi:10.1016/j.jiec.2012.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

i cr us

The Effects of Catalysts in Biodiesel Production: A Review (Article No: IE2011-183)

an

I.M. Atadashi, M.K. Aroua*, A.R. Abdul Aziz, N.M.N. Sulaiman Chemical Engineering Department, Faculty of Engineering, University Malaya 50603 Kuala Lumpur, Malaysia

M

*Corresponding author. Tel: +603-79674615, Fax: +603-79675319

ed

Email address: [email protected]

Abstract

pt

Biodiesel fuel has shown great promise as an alternative to petro-diesel fuel. Biodiesel production is widely conducted through transesterification reaction, catalyzed by homogeneous catalysts or heterogeneous catalysts. The most notable catalyst used in

ce

producing biodiesel is the homogeneous alkaline catalyst such as NaOH, KOH, CH3ONa and CH3OK. The choice of these catalysts is due to their higher kinetic reaction rates. However because of high cost of refined feedstocks and difficulties associated with use of

Ac

homogeneous alkaline catalysts to transesterify low quality feedstocks for biodiesel production, development of various heterogeneous catalysts are now on the increase. Development of heterogeneous catalyst such as solid and enzymes catalysts could overcome most of the problems associated with homogeneous catalysts. Therefore this study critically analyzes the effects of different catalysts used for producing biodiesel using findings available in the open literature. Also, this critical review could allow identification of research areas to explore and improve the catalysts performance commonly employed in producing biodiesel fuel. Key words: Biodiesel, Production, alkaline catalyst, acid catalyst, solid catalyst

1   

Page 1 of 40

i cr us

catalysts

1. Introduction

heterogeneous

catalysts.

an

CH3ONa, and CH3OK are more often used in producing

development. The World’s economic growth is affected by

biodiesel [13]. The catalytic performance of these catalysts and

climatic change, fuel price hike, and the gradual depletion of

their ability to perform under moderate conditions has led to

M

fossil fuel reserves. Therefore, to increase energy security for

their choice [14]. Among these homogeneous alkaline

economic development, the need to search for an alternative

catalysts, CH3ONa is most active, providing biodiesel yield

source of energy such as biodiesel is necessary [1,2]. Biodiesel

ed

above 98wt% in short reaction time (30min) [15,16]. However

is renewable, sustainable, biodegradable, and emits low

because of low price, industrial biodiesel production process

greenhouse gases [3,4]. As well, the oxygen content of 11–15%

mostly employs NaOH and KOH [14]. The process involving

pt

in the molecular structure speed up the combustion process in

these catalysts needs high-quality feedstocks, thus the free fatty

compression ignition engines and decreases pollutants such as

acid (FFAs) level of the feedstocks should not exceed 3wt%,

ce

soot, fine particles, and carbon monoxide (CO) [5,6]. Thus,

beyond which the reaction will not occur. In addition, water

biodiesel is a potential substitute to replace/supplement petro-

Ac

content of the feedstocks is critical, as a result the feedstocks used in alkali-catalyzed transesterification have to anhydrous

Biodiesel fuel is produced via transesterification of

[14,17]. Thus, presence of water leads to hydrolysis of oils to

refined vegetable oil, waste cooking oil, and used frying oil

FFAs. Figure 2 shows water hydrolysis of fats and oils to form

using alkaline catalysts [9-12] as shown in Figure 1. The

free fatty acid. The FFAs react with alkaline catalysts to

nature of catalyst employed during transesterification reaction

produce soaps formation. Figure 3 presents soaps formation in

is crucial in converting triglycerides to biodiesel. As a result

homogeneous

different catalysts have being explored for converting

alkali-catalyzed

transesterification.

Soaps

formation consumes the catalyst, deactivates it and makes

triglycerides to biodiesel fuel. The catalysts usually employed

biodiesel purification process difficult [13,18]. Therefore,

to catalyze transesterification reaction are homogeneous 2   

Conventionally,

homogeneous alkaline catalysts such as NaOH, KOH,

Energy is the prime mover for socio-economic

diesel fuel [7,8].

and

Page 2 of 40

cr

i alcohol

an

CH2 - OH | CH - OH | CH2 - OH

M

Alkyl Esters

glycerin

ed

Triglyceride

O || R 4 - O - C - R1 + O || R 4 - O - C - R2 + + O || R4- O - C - R3

us

O || CH2 - O - C - R1 | | O | || CH - O - C - R2 + 3 R4OH ↔ | (KOH) | O | || CH2 - O - C - R3

Ac

CH2 - O - CO - R1 | | | CH - O - CO - R2 + | | | CH2 - O - CO - R3

ce

pt

Figure 1: Transesterification reaction of triglycerides via alkaline catalyst

Triglyceride

H2O

Water

O || R1- C -OH Free fatty acid

CH2 - OH | | O | || CH - O - CO - R2 + HO - C-R1 | | | CH2 - O - CO- R3 Diglyceride

+ KOH

Potassium hydroxide

R1 – COOK + H2O (Soap)

Water

Figure 3: Soap formation in homogeneous alkali-catalyzed

Free fatty acid

Figure 2: Water hydrolysis of fats and oils to form free fatty acid (FFAs)

3   

→

Page 3 of 40

i cr us

homogeneous catalysts such as; simple catalyst recovery,

huge quantity of FFAs and water needs sound technology [19].

catalyst reusability, simple product purification, less energy

However, high cost of refined feedstocks result in high price of

and water consumption, less added cost of purification, and

biodiesel compared to diesel fuel [15,20]. The cost of refined

simple

feedstocks, account to over 70% of the overall cost of biodiesel

heterogeneous catalysts used especially solid alkaline catalysts

M

an

preparation of biodiesel by low quality feedstocks containing

glycerol

recovery

etc.

Besides,

most

of

the

have provided high yields [26], though faced with problem of

feedstocks such as: waste cooking oils, used cooking oil,

leaching [27]. Also, the stability of enzymes catalysts in non-

greases (yellow and brown), and non-edible oils have being

aqueous media is significant to its excellent catalytic activity,

investigated [13,22]. The price of low quality feedstocks such

this improves tranessterification and esterification during

as waste cooking oil is 2-3 times lower than refined oils.

biodiesel production [28], and providing high biodiesel yield

pt

ed

production [21]. As a result, different kinds of low quality

(95wt%) [29]. However, the problem mostly associated with

and water contents. This features makes their processing

enzymes catalysts is the cost of the enzymes, but

challenging [23,24]. Therefore to augment their processing

immobilization of the catalyst could mitigate the cost [30].

difficulties, acid-catalyzed transesterification is first employed

Therefore, to achieve biodiesel that is economically feasible,

Ac

ce

Nonetheless, the feedstocks contains higher amount of FFAs

to decrease the content of FFAs before performing alkali-

development of active and cheap catalysts for effective

catalyzed transesterification [19]. Thus adopting two-step

transesterification of different kinds of feedstocks is necessary

transesterification technique could provide large biodiesel

[31]. In this regards, this study extensively examined and

conversion of up to 98% [25].

reported the effects of different catalysts in producing biodiesel

Recently heterogeneous catalysts such as solid catalysts and

enzyme

catalysts

transesterification Heterogeneous

are

reaction

catalysts

employed for

offers

to

producing many

fuel.

catalyze biodiesel.

advantages

over

4   

Page 4 of 40

i cr us

2. Biodiesel Production

2.2 Feedstocks for Biodiesel Production.

2.1 Techniques for biodiesel production

Biodiesel production is achieved via different kinds of

feedstocks. The nature of feesdstock used is dependent on the

techniques such as direct/blends [32,33], microemulsion

geographical position and climate of the place. For instance

[34,35], pyrolysis [36,37] and transesterification [38,39]. As

Europe employs sunflower and rapeseed oils, palm oil

M

an

Biodiesel is usually produced through different

predominates in tropical countries, soybean in United States

adopted technique for producing biodiesel, this method usually

and canola oil in Canada [43]. Singh and Singh [44] reported

needs refined feedstocks containing less FFAs content

the major feedstocks employed in producing biodiesel are

otherwise it will result to much soaps formation. For alkali-

cotton seed, palm oil, sunflower, soybean, canola, rapeseed,

catalyzed transesterification, if the feedstocks contains high

and Jatropha curcas. Additionally, Zhang et al. [45] remarked

pt

ed

stated earlier, alkali-catalyzed transesterification is the most

that employing feedstocks such as waste frying oils, non-

transesterification [40]. Hideki et al. [14] and Ramadhas et al.

edible oils, and animal fats, as feedstocks could be useful in

[25] recommended acid value of feedstocks to be less than

producing biodiesel. Although, Banerjee and Chakraborty

4.0mg KOH/g before performing alkaline transesterification.

[46] stated that FFAs contents in the waste cooking oil

Ac

ce

amount of FFAs then it has undergo pretreatment steps before

Although, Canakci and Gerpen [41] and Mittelbach [42] stated

should be kept within certain limit for reaction involving both

that before alkali-catalyzed transesterification, the acid value of

acid- and alkali-catalyzed transesterification reactions.

a feedstock has to be 2.0 mg KOH/g. Nonetheless, the use of

Otherwise these substances may cause severe difficulties in

heterogeneous catalysts in biodiesel production has reduced the

refining of biodiesel products. Table 1 presents the

effects of using low quality feedstocks, especially enzymes

recommended

catalysts that has the potential of converting FFAs into

transesterification method. While Table 2 shows FFAs

biodiesel, besides high purity by-products [14].

contents of different vegetable oils. In addition, Table 3

FFAs

values

5   

Page 5 of 40

for

alkali-catalyzed

cr

i Reference

presents FFAs levels of most of the feedstoks used to produce biodiesel. The cost of feedstocks decreases as FFAs content increases. In case of industrial biodiesel production, there is

an

FFAs recommended (%)

us

Table 1: Level of FFAs recommended for alkali-catalyzed transesterification