Vegetable Oils as Biodiesel

6Vegetable Oils as Biodiesel

M. J. T. Reaney, P. B. Hertz, and W. W. McCalley

Agriculture and Agri-Food Canada

Saskatoon, Saskatchewan, Canada

1. INTRODUCTION

Renewable fuels are increasingly being used to displace fuels from non-renewable

sources, though the goals of using these bio-based fuels are often complex. In

Austria, where much of the pioneering commercial development occurred, bio-

diesel development was driven by the need to: (1) increase security of energy

supply for the transportation sector by having a renewable source at hand, (2)

have an environmentally friendly fuel available for the diesel combustion engine,

(3) reduce health and security risks, and (4) provide the customer with a reliable

fuel at a reasonable cost-benefit ratio (1).

Generally, renewable fuels are produced to reduce greenhouse gas emissions,

improve combustion of fuels, and to extend supplies of fossil fuels, although their

production may also be used to subsidize the production of agricultural commod-

ities and improve the balance of trade for countries that produce little fossil fuel.

Biodiesel is a renewable fuel that is usually narrowly defined as esters of lower

alcohol and fatty acids, where the fatty acids are derived from vegetable oil, animal

fat, or tallow (2). In spite of the restrictive nature of this definition, it is possible to

include a wide range of molecules derived from living organisms in fuels for com-

pression ignition engines (3). In light of the diverse potential for using biocompo-

nents for fuels, a section of this chapter will be dedicated to specific molecules

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

223

derived from fatty acids and their derivatives that are used as biodiesel fuel and fuel

components. Pyrolysis oils, derived by heating biomass in the absence of oxygen,

are derived from a broad range of molecules including carbohydrates, proteins, phe-

nolic compounds, and lipids. Their formation and use is beyond the scope of this

discussion.

Fatty acid derivatives are used in diesel fuel primarily and include a narrow

range of current and proposed applications (1). Commercial fuels are predomi-

nantly methyl esters of fatty acids or blends of fatty acid methyl esters with

conventional diesel fuels from fossil sources. Although other esters have been

studied as fuels, the relative cost benefits and ease of preparing and using methyl

esters have allowed these sources to dominate current commercial production (1).

Other options are being explored, including conversion of vegetable oils to

n-alkanes to improve combustion (4–6) and developing diesel technology that

combusts triacylglycerol oils directly (7–9). These latter technologies bear discus-

sion but are not part of the major global commercialization effort that is currently

under way.

2. BIODIESEL QUALITY

Diesel fuels are complex mixtures of hydrocarbons defined by physical and chemi-

cal properties. Petroleum diesel fuels are based on molecules with 9 to 20 carbon

atoms and a boiling range between 170�C and 350�C (10). These fuels are produced

by sequential chemical treatments and refining of heavy petroleum oils followed by

distillation. In general, specifications for fuels are inclusive so as not to exclude

compositions with similar operational characteristics. However, environmental con-

cerns regarding toxic emissions from diesel engines have led to legislation that has

forced manufacturers to modify diesel fuel chemistry (11).

Fatty acid methyl esters and other esters of fatty acids and a lower alcohol can be

added at a low ratio to most diesel fuels without substantially changing fuel char-

acteristics. It has been reported that in blends containing 30 percent biodiesel, low-

temperature flow properties are not greatly affected (12), but at higher blend levels,

the properties of the methyl ester may affect the properties of the fuel. With few

exceptions, pure biodiesel does not meet minimum low-temperature requirements

and may exceed manufacturers’ maximum viscosity for diesel fuels.

2.1. Physical Parameters

Official methods of physical analysis used to characterize conventional diesel are

applicable and meaningful when applied to biodiesel and provide useful informa-

tion. Biodiesel chemistry leads to a number of physical characteristics that are

unique when compared with diesel fuels. Most biodiesel preparations have higher

viscosity, density, initial boiling point, final boiling point, cold-filter plugging point,

and flash point than conventional diesel fuels. Virtually all of these characteristics

are due to the high average molecular weight of the component esters of biodiesel.

224 VEGETABLE OILS AS BIODIESEL

Boiling point and flash point, for example, are related to vapor pressure. Fatty acid

saturation does not appear to impact surface tension significantly.

Physical properties may affect fuel characteristics and combustion in a number

of ways. Diesel fuel must be efficiently atomized to effect combustion. Surface ten-

sion and viscosity of diesel fuel are properties that determine atomization charac-

teristics (19). Allan and Watts (19) compared the atomization characteristics of 15

biodiesel fuels with number two diesel. They found that molecular weight was the

main property affecting atomization. As an example, the mean oil droplet diameter

of coconut oil methyl esters, which are composed of 12 and 14 carbon chains, was

not significantly different from diesel fuel, whereas methyl esters, which had pre-

dominantly 18 carbon chains, produced droplets with mean diameters 20–30

percent larger than diesel fuel. Rapeseed oil, with a mean carbon chain length of

20 carbons, increased droplet size by 40 percent over that of diesel fuel. The larger

size of atomized droplets of biodiesel may account for some of the differences in

emissions observed from diesel fuels. Vapor pressure and, consequently, the flash

point of all diesel fuels is largely determined by the molecular species with the low-

est molecular weight present in the fuel. As most of the diesel supply infrastructure

was built for a fuel with a high flash point, it is advantageous that properly made

biodiesel has an elevated flash point (2, 14–18).

2.2. Chemical Parameters

Official methods of chemical analysis of conventional diesel are often not adequate

to characterize biodiesel. Tests for the levels of sulfur and aromatic components in

biodiesel are useful but usually reveal that the concentrations of compounds con-

taining these atoms or functional groups are very low. Analysis of biodiesel chem-

istry can reveal characteristics conferred by the source of the oil, the method of

manufacture, and duration of storage (20, 21). For example, free and bound glycer-

ol is measured to ascertain if biodiesel has been completely formed during synth-

esis. Fatty acid content, residual soaps, iodine value, peroxide value, and fatty acid

composition all may reflect the quality of biodiesel (Table 1) but are unimportant

and inapplicable in conventional diesel fuel quality determination.

Significant differences in fuel standards also exist among different countries (2,

14–18; Table 1). In the European Union (EU), member countries have adopted a

standard requiring an iodine value of less than 115 (15, 16), 120 (14), or 125

(18). This iodine value reflects the upper extreme iodine value of canola (low erucic

acid rapeseed) oil. The American Society for Testing Measures (ASTM) and Italian

National Standards Body (UNI) standards do not include iodine value (2, 17) and

thus allow higher iodine value oils such as soy and sunflower.

Chemical analyses of biodiesel may be accomplished through standard methods

as recommended by ASTM and other organizations, but these measures do not

determine all aspects of biodiesel chemistry (20, 21). Knothe (21) extensively

described methods of measuring trans-esterification progress and assuring biodiesel

quality, and suggested that no single test could assure quality. Online methods, such

as NIR (near infrared spectroscopy), could be used to rapidly screen products to

BIODIESEL QUALITY 225

TABLE 1. International Standards for Biodiesel.

AUSTRIA FRANCE GERMANY ITALY SWEDEN USA

Standard/ Test ON C1191 Journal DIN E 51606 UNI 10635 SS 15 54 36 D-6751-02

Specification Value Unit (14)� Officiel (15)� (16)� (17)� (18)� (2)�

Date 1 July 1997 14 Sep 1997 Sep 1997 21 April 1997 27 Nov 1996 10 Jan 2002

Application FAME�� VOME�� FAME�� VOME�� VOME�� FAME��

Density 15�C g/cm3 0.85–0.89 0.87–0.90 0.875–0.90 0.86–0.90 0.87–0.90 —

Viscosity 40�C mm2/s 3.5–5.0 3.5–5.0 3.5–5.0 3.5–5.0 3.5–5.0 1.9-6.0

Distillation I.B.P �C — — — >300 — —

Distillation 95% �C — <360 — <360 — —

Flashpoint �C >100 >100 >110 >100 >100 >130

CFPP �C <0/�15 — <0/�10/�20 <0/�15 <�5 —

Pourpoint summer �C — <10 — — — —

Total Sulfur % mass <0.02 — <0.01 <0.01 <0.001 —

CCR 100% % mass <0.05 — <0.05 — — <0.050

CCR 10% % mass — <0.3 — <0.5 — —

Sulfate ash % mass <0.02 — <0.03 — — <0.02

(Oxide) Ash % mass — — — <0.01 <0.01 —

Water content mg/kg — <200 <300 <700 <300 <500

Impurities total mg/kg — — <20 — <20 —

Cetane No. — >49 >49 >49 — >48 >47

Neutral No. mgKOH/g <0.8 <0.5 <0.5 <0.5 <0.6 <0.80

Methanol cont. % mass <0.20 <0.1 <0.3 <0.2 <0.2 —

Ester content % mass — >96.5 — >98 >98 —

Monoacylglycerol % mass — <0.8 <0.8 <0.8 <0.8 —

Diacylglycerol % mass — <0.2 <0.4 <0.2 <0.1 —

Triacylglycerol % mass — <0.2 <0.4 <0.1 <0.1 —

Free glycerol % mass <0.02 <0.02 <0.02 <0.05 <0.02 <0.020

Total glycerol % mass <0.24 <0.25 <0.25 — — <0.240

Iodine No. — <120 <115 <115 — <125 —

Phosphorous mg/kg <20 <10 <10 <10 <10 <10

Alkaline met. NA/K mg/kg — <5/5 <5 — <10/10 —

�Reference No.��Abbreviations: FAME: Fatty Acid Methyl Ester, VOME: Vegetable Oil Methyl Ester, CFPP: Cold Filter Plugging Point, CCR: Conradson Carbon Residue.

10 9 8 7 6 5 4 3 2 1 0 ppm

10 9 8 7 6 5 4 3 2 1 0 ppm

10 9 8 7 6 5 4 3 2 1 0 ppm

B Stage 1 Methyl Ester

A Canola Oil

4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4

4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4

C Stage 2 methyl Ester

Figure 1. Proton NMR (400 MHz) spectra: a) spectrum of bulk refined canola oil in deuterated

chloroform (CDCl3) with the expansion of the region containing protons from glycerine (3.4–

4.8 ppm); b) spectrum of single stage reaction of canola oil with methanol and KOH in CDCl3 with

the expansion of the region containing protons from glycerine (3.4–4.8 ppm); c) spectrum of two

stage reaction of canola oil with methanol and KOH in CDCl3 with the expansion of the region

containing protons from glycerine (3.4–4.8 ppm).

227

identify those materials that warrant further analyses. Samples that failed NIR scru-

tiny would be subject to more stringent testing methods, such as gas chromatogra-

phy (GC), to determine the exact problem with the fuel. Mittlebach (20) suggested

that GC methods provided most of the needed information for determining biodie-

sel quality.

In many specifications, there is redundancy in biodiesel testing. The ASTM spe-

cification is unique because it does not limit methanol concentration, but the spe-

cified flashpoint is only possible if the methanol content is very low (2). Other

specifications define both methanol content and flash point. Some biodiesel stan-

dards are also redundant in the measurement of mono-, di-, and triacylglycerols

as separate analyses from the determination of free and bound glycerol (15–18).

The quantitation of individual acylglycerols is achieved by liquid or gas chromato-

graphy and may require sample preparation and a significant period for analysis.

Free and bound glycerol can be determined by relatively simple colorimetric iodo-

metry (20, 21). In the future, rapid spectrophotometric methods will likely lead to

online analyses, as predicted by Knothe (21). Online methods will be based on tech-

niques that have sufficient signal-to-noise ratio to determine the presence of low

levels of contaminants while accurately measuring target components. Figure 1a

shows the 400 MHz NMR spectrum of canola oil, whereas Figure 1b and 1c

show the first and second stage of a two-stage trans-esterification, respectively.

The inset to all figures shows the expansion of the spectrum from 3.5 ppm to

4.5 ppm. The loss of the proton signals between 4.1 and 4.4 and their replacement

by a singlet at 3.7 ppm is indicative of the replacement of glycerol with methanol.

The NMR method allows direct quantitation of glycerol and methanol without deri-

vatization. New NMR technology based on microcells could allow development of

a cost-effective instrument for rapid and precise determination of glycerol (22).

Although NMR technology remains costly, HPLC methods involving gel permea-

tion chromatography (GPC) are also able to separate acylglycerides. Figure 2a

shows the separation of four standards using a gel permeation column where larger

molecules elute first followed by smaller molecules. Therefore, the elution order is

triacylglycerols, diacylglycerols, monoacylglycerols, and, finally, methyl esters,

which coelute with fatty acids. In Figure 2b, the residual acylglycerols in the first

stage of a biodiesel synthesis reaction are clearly visible. These components are not

detected in the second stage of the reaction. The largest problem with both NMR

and GPC is low sensitivity. The liquid chromatography method relies on a light

scattering detector, which does not provide a linear response and may only be

used for qualitative analysis.

2.3. Toxicological Parameters

Methyl and ethyl esters of fatty acids have been used in consumer products for a

long time and are generally recognized as safe in specific applications (23, 24).

However, the new use of biodiesel as a fuel has led to tests to determine its potential

toxicity in new applications.


Acute oral toxicity was determined in fasted male and female albino rats to be

greater than 5000 mg/kg body weight for both methyl and ethyl esters of canola oil

(25). Dermal toxicity was tested on albino rabbits. Applying levels of up to

2000 mg/kg body weight was found to have no observable effect for systemic toxi-

city (25). The treatment produced only slight and temporary erythrema (redness)

and edema (swelling).

Canola OilDiglycerideMethyl EsterStanderd Mix

A

B

C

Stage 1 Methyl Ester

Stage 2 Methyl Ester

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Minutes

0.00

200.00

400.00

0.00

200.00

400.00

0.00

200.00

400.00

600.00

800.00

Figure 2. Gel permeation chromatograms: a) chromatogram of four standards of triglyceride

(triolein), diglyceride (diolein), and methyl ester standards as detected by evaporative scattering

light detection; b) chromatogram of single stage reaction of canolaoil with methanol and KOH; c)

chromatogram of a two stage reaction of canola oil with methanol and KOH.

BIODIESEL QUALITY 229

Acute aquatic toxicity was determined on both Daphnia magna and rainbow

trout (25). The Daphnia magna became trapped in oil sheen and the concentration

that produced 50% death was determined to be 99 ppm (compared with table salt at

3.7 ppm and diesel at 1.4 ppm).

Rainbow trout survived 48 hours of treatment with 100 ppm and 300 ppm canola

methyl and ethyl esters, but were in poor condition (25). Biodiesel exhibits acute

toxicity in aquatic systems but its rapid degradation and low overall toxicity make it

greatly preferred to diesel fuel in environmentally sensitive areas.

3. BIODIESEL AND DIESEL EMISSIONS

3.1. Local Emissions

The U.S. National Institute for Occupational Safety and Health (NIOSH) has iden-

tified diesel exhaust as a ‘‘potential occupational carcinogen’’ (26). Particulate mat-

ter in diesel exhaust has been identified as a potential risk factor. Biodiesel

emissions are generally found to be more benign than diesel fuel, but it cannot

be said that all components of the emissions are reduced compared with conven-

tional diesel fuels. The results can be especially confusing as diesel fuels can differ

greatly depending on the source of crude oil and the refining process that was used

to produce the fuels (10). Simply stated, biodiesel emissions compare favorably

with emissions from most diesel fuels, but the fuels used for comparisons have

varied among tests. In addition, as fuel specifications are changed, often in response

to legislation, combustion characteristics of ‘‘conventional’’ diesel fuels are also

changing. Older data obtained using fuels with higher sulfur and aromatic components

and lower cetane numbers may no longer represent current industrial practice.

Furthermore, biodiesel is variable depending on the source of the fuel and its age

(27). For example, Monyem and Van Gerpen (27) observed decreased carbon mon-

oxide and hydrocarbon emissions from oxidized neat biodiesel when compared

with unoxidized biodiesel. In addition, cetane number is inversely proportional to

iodine value (28) and proportional to biodiesel age (29).

Hydrocarbon, carbon monoxide, and particulate matter are all reduced in both

direct and indirect inject diesel engines fueled with biodiesel when compared

with diesel, whereas nitrogen oxide emissions are increased (30–33).

In addition to the measurements on regulated emissions mentioned above, unre-

gulated emissions and, in particular, aldehyde and polyaromatic hydrocarbon emis-

sions, have been studied extensively. When compared with engines operating on

conventional diesel, formaldehyde and acrolein were found to increase by 40%

in engines operating on biodiesel fuel while polyaromatic hydrocarbons greatly

decreased (33). Although mutagenicity was not attributed to specific compounds,

diesel fuel exhaust was found to have greater total mutagenic potential in modified

Ames testing when compared with biodiesel fuel exhaust (33).

The combined emissions improvements obtainable with 100% biodiesel provide

an advantage in enclosed areas where the use of this fuel mitigates the potentially


toxic diesel fumes. This toxicity of biodiesel exhaust was thoroughly tested by the

Lovelace Respiratory Research Institute (34). The authors concluded that ‘‘no pro-

nounced toxicity resulted from the exposure of rats to biodiesel exhaust emissions

at any concentration.’’ Histological analysis revealed small changes in the lungs of

female rats treated with the highest level of exhaust indicating an adverse exposure

to high-level exhaust. The study also noted some mutagenic activity in both the par-

ticulate and semivolatile fractions. The authors of the study assert that ‘‘the no-

adverse-effect-level for this study of inhaled biodiesel exhaust emission was the

intermediate exposure level’’ (34).

Blends of biodiesel with conventional diesel fuel have also produced substantial

improvements in combustion products similar to those reported for pure biodiesel.

Several research groups (35–37) have investigated the potential for construction of

a fuel sensor that may be able to detect biodiesel (and ethanol) content in fuel and

adjust the engine during operation to minimize NOx emissions. Although they have

met with some technical difficulties, such a detector is likely practical. With

improvements obtainable through combinations of biodiesel, low-sulfur diesel,

and emission controls including oxidation catalysts and adaptive fuel systems, it

may be possible to meet stringent new standards.

3.2. Global Emissions

The impacts of biodiesel on global greenhouse gas emissions have been extensively

and repeatedly studied. The exercise of Life Cycle Assessment (LCA) is a form of

accounting that is used to determine the net impact of a process or series of pro-

cesses. In an early LCA assessment of biodiesel production, Krahl et al. (38)

reported a positive output of energy from the cultivation of winter canola grown

under European conditions. Other assessments have followed and generally reveal

a ratio of one unit of fossil energy used in the production of two to four units of

renewable energy embodied in the biodiesel fuel (38–41). The major energy con-

sumption in biodiesel production revealed by LCA includes field tillage, synthetic

fertilizer (N, P, K, and S) production, oilseed processing to recover oil, and

methanol used in trans-esterification (38–41). Substantial improvements in LCA

are possible. For example, herbicide-resistant crops may be grown using minimum

tillage technology to save substantially on fuel consumption (42). Nitrogen-fixing

organisms like soybean have an enhanced positive LCA mainly because of their

minimal need for nitrogen fertilizer.

It may be assumed that LCA is not static and that numerous improvements may

be made to improve the overall assessment. In Canada, substantial improvements to

the ratio of yield to energy input have been attained through the adoption of low-

tillage systems made possible by the introduction of herbicide-tolerant crops (42).

Oilseed crops have been developed using hybrid seed technology that displays yield

improvement over nonhybrids (43). Agronomic practices are continuously improv-

ing the efficiency of fertilizer use and decreasing field emissions.

Improvements in the LCA of biodiesel may be realized by using ethanol in bio-

diesel production. The LCA of ethanol is positive (39, 40) but not as positive as

BIODIESEL AND DIESEL EMISSIONS 231

biodiesel (38–41). As ethanol comprises a greater portion of the biodiesel fuel com-

pared with methanol, it may be seen to displace a portion of the higher LCA fuel. It

is important to note that substitution of methanol with ethanol in biodiesel produc-

tion yields more fuel. Therefore, the impact of substituting ethanol for methanol is a

decrease in total greenhouse gas emissions due to the larger potential volume of

fuel produced, but the LCA assessment on a fuel weight basis is not greatly

affected.

Life Cycle Assessment of canola and other oilseeds could be substantially

improved by increasing seed oil content while decreasing overall plant phosphorous

and nitrogen content. Canola meal is particularly high in phosphorous in the form

of phytate (inositol hexaphosphate) (44). Phytate is indigestible by nonruminants

and can impair digestion by making mineral nutrients, particularly zinc, unavail-

able. It is likely that a reduction in phosphorous in canola meal or a conversion

of the phytate phosphorous to other forms would have a net positive impact on

LCA as it would be easier to use the meal as feed. To date, corn varieties released

with low phytate characteristics have had reduced yields when compared with high-

er phytate counterparts (45). A similar reduction in canola phytate level with con-

sequent energy savings may also be possible through conventional breeding or

through genetic engineering. Although it is not desirable to lower the protein con-

tent of canola meal, it is possible to reduce the total meal production with respect to

oil. This has been accomplished with the development of yellow-seeded canola

varieties that have significantly higher oil content and a higher ratio of oil to protein

TABLE 2. Biodiesel Production Capabilities, North America 2002.�

Name Location Capacity Feedstock

Griffin Industries Butler, Kentucky 6.8 million L /year Waste Oil & Grease

and animal fats

Ocean Air Lakeland, Fl 38 million L /year Waste Oil & Grease

Pacific Biodiesel Kahului, Hawaii 567,000 L /year Waste Oil & Grease

Stepan Co. Joliet, Il UNK Soy Oil

Ag Environmental Lenexa, Kansas 22.7 million L /year Soybeans

Imperial Western

Products

Coachella, CA 46 million L /year Waste oil and grease/

and soy

West Central

Co-operative

Ralston, Iowa 45 million L /year Soybeans

Columbus Foods Chicago, Illinois 5.7 million L /year

Biodiesel Industries Las Vegas, Nevada 18 million L /year Waste oil and grease

American Bio-Fuels/

Green Star

Products

Adelanto, CA 10 million L /year Soybeans

Montana Biodiesel Missoula, Montana 1 million L /year

Proctor and Gamble

Co.

Cincinnati, OH Unknown

Government of

Saskatchewan

Saskatoon

Saskatchewan

8 million L /year Refined Canola Oil

�Survey performed by telephone October 2002 by the senior author.


than conventional dark seed coat varieties (46). These varieties also have higher

average meal protein contents than the brown seed coat counterparts. Owing to

its high molecular weight, rapeseed (high erucic acid rapeseed) oil produces less

glycerol and requires less methanol for trans-esterification than canola oil. The

best combinations of plant traits would improve LCA of canola biodiesel well

beyond current practice.

Soybean, corn and sunflower oil content can also be manipulated by plant breed-

ing (47, 48). Historic attempts to increase oil content of soy have lowered protein

production (47). If the domestic demand for soybean biodiesel increases in

North America, diversion of plant photosynthate from meal to oil production

may prove advantageous and limit market distortions.

3.3. Global Biodiesel Production

The Austrian Biofuels Institute currently maintains an ongoing database on the

global biodiesel industry (49). Their current statistics reveal that most biodiesel

is obtained from canola (low erucic acid rapeseed; 84%) and sunflower (13%).

European biodiesel production dominated world production in 2000 and Europeans

have chosen canola oil as the mainstay for manufacturing biodiesel. However,

recent facility construction (50) and legislative action (51) in North America may

lead to rapid increases in biodiesel production (Table 2). The State of Minnesota

has passed legislation that mandates the inclusion of 2% biodiesel in all diesel

fuel sold in the state (51). European and American biodiesel production is rapidly

increasing. In 2000, U.S. production was just 22 ML (Table 3).

4. RESOURCES FOR BIODIESEL PRODUCTION

Currently, most biodiesel is synthesized from higher quality vegetable oils, includ-

ing canola, sunflower, and soy. More recently, used frying oil and tallow have

TABLE 3. Biodiesel Production in Selected Countries

(Approximate Use in 2000).

Region Biodiesel

Annual Biodiesel Production (millions of liters)

North America

USA 22

Canada <1

Europe

Austria 22

Belgium 90

France 275

Germany 230

Italy 90

Sweden 11

RESOURCES FOR BIODIESEL PRODUCTION 233

increasingly been used as sources of oil for biodiesel production (Table 2) (1).

Modern biodiesel facilities are usually designed with capability to cleanly process

multiple-feed stocks (MFS) to accept lower cost oils. The capability of dealing with

lower cost feed stocks is also important in recovering esters and acids that are often

entrained in glycerol during processing. Modern MFS biodiesel plants can convert

almost 100% of fatty acids and esters to usable biodiesel (1).

New sources of oil are continually being suggested for biodiesel production.

Rubber seed (Hevea brasiliensis) produces unsaturated oil that has little value

and is often not recovered but could be effectively used to produce biodiesel

(52). Jatropha curcus, a bushy tree that grows quickly and produces a fruit with

a high-oil seed, has been touted as an energy plant for tropical climates (53).

Research has also been conducted into the development of unique high-

glucosinolate producing Brassica sp. hybrids (54). These species are suggested

as industrial crops because the meal they produce is inedible and would not enter

feed markets, but the glucosinolates in the meal provide the potential for use

of the meal or meal extracts as pesticides. The tuber, yellow nut-sedge (Cyprus

esculentus L.), is also a rich source of oil, and blends of this oil with diesel fuels

have been recommended (55).

Northern regions with short growing seasons have limited total production

potential. Typical yields of canola and flaxseed in Canada, for example, average

1300 kg/ha (56), whereas in Europe, yields of these crops are often double this

amount (57); the EU average yield for canola was 2700 kg/ha in 1998. The primary

limitation in growing more productive crops is the cold winter conditions that pre-

vent the introduction of winter varieties. Enormous potential exists for increasing

the production of oilseed in northern climates (e.g., Europe, North America, and

Northern Asia) through the introduction of higher yielding species.

5. PRODUCTION TECHNOLOGY

Biodiesel is produced from vegetable- and animal-based oils by esterification or

trans-esterification with a lower alcohol (3). Most vegetable oils and animal fats

are predominantly triacylglycerols (TAGs). Although TAGs may be incorporated

in diesel fuels without chemical modification, these compounds increase fuel visc-

osity, are poorly combusted, and tend to prematurely foul upper cylinder engine

parts. Therefore, chemical processes of converting fats and oils to alkyl esters of

monohydric alcohols are now in common use to produce a fuel with lower viscosity

that may be used as a direct replacement for diesel fuel. The core synthetic process

of most biodiesel production technologies is trans-esterification (3). In this process,

animal fat or vegetable oil consisting mostly of acylglycerols is reacted with a

catalyst and alcohol. The reaction that ensues consumes the acylglycerols and

liberates glycerol and alkyl esters of lower alcohols. Reaction 1 shows the trans-

esterification of a triacylglycerol with three moles of alcohol releasing one mole

of glycerol and three moles of ester. Ma and Hanna (3) reviewed biodiesel produc-

tion, and this article is suggested as an additional reference.


Reaction 1

CH2

CH

OOCR

CH2

OOCR

OOCR

CH2

CH

OH

CH2

OH

OH

+ 3R′OH + 3R′-OOCR

where

R ¼ long chain alkyl group (Example: (CH2)7(CH)2(CH2)7CH3)

R0 ¼ short chain alkyl group (Example: CH3)

5.1. Oil

Oil quality and composition are important determining factors in optimizing ester-

ification strategy. Oils may contain numerous components that will affect the effi-

ciency of esterification. Common oil contaminants include fatty acids, partial

acylglycerols, phospholipids, unsaponifiables, and water (20, 21).

Fatty acids react with alkaline catalysts to form catalytically inactive soaps (3).

The chemical reaction consumes one mole of fatty acid per mole of alkaline cata-

lyst. Although fatty acid composition of the starting material varies, the content

determined by titration reflects the amount of catalyst that would be consumed in

a chemical reaction. By calculation, it may be determined that one gram of fatty

acid (expressed as oleic acid) will react with about 0.2 g of anhydrous potassium

hydroxide or 0.14 g of anhydrous sodium hydroxide. Often, additional catalyst

must be added to esterify a vegetable oil containing higher levels of fatty acids

(3). Conversely, acid catalysts are not inactivated by fatty acids (3). In a unique

reaction, fatty acids produced during biodiesel manufacture are actually used as

a catalyst in their own esterification (see below).

Partial acylglycerols, including mono- and diacylglycerols (DAGs), often occur

in high concentrations in the same reaction mixtures as fatty acids (58). These

materials can be thought of as interim products of the biodiesel manufacturing pro-

cess and do not normally present significant problems with reactions. However,

monoacylglycerols (MAGs) can increase the viscosity and melting point of oils

(59). Furthermore, lower concentrations of alcohol would be required to esterify

oils that are enriched in partial acylglycerols. One mole of glycerol mono-oleate

(356 g/mole) may react with one mole of methanol to produce one mole of methyl

ester (296 g/mole) and one mole of glycerol. In this reaction, the mass ratio of alco-

hol to MAG is lower than that produced in a reaction with a TAG. The yield of

glycerol is higher than that produced in reaction with the TAG. As emulsifiers,

MAGs may also serve to accelerate the formation of esters.

Phospholipids are often removed in refining steps prior to biodiesel synthesis

processes (3). Refining steps for phospholipid extraction often include the

industrial-standard degumming with acids or water that form oil-insoluble materials,

which are readily removed by centrifugation. These methods have the disadvantage

of removing a portion of the oil, which is lost to further reaction. Other methods

include removing the gums while the oil is dissolved in a hexane solution (60).

PRODUCTION TECHNOLOGY 235

This process may mitigate some neutral oil losses. It is predictable from chemistry

that the impact of the presence of phospholipid during trans-esterification is mostly

a contamination of the glycerol phase with compounds that include glycerol phos-

phate, inositol, and basic compounds (choline, serine) derived from the phospholi-

pid. The authors are not aware of studies describing the presence of these products

in the glycerol derived from trans-esterification of unrefined oils. In our experience,

oils with high levels of phospholipid (50–300 ppm) are readily trans-esterified

using alkaline catalysts. The remaining oil is depleted in phosphorous by the reac-

tion, and the resulting methyl esters contain less than 10 ppm phosphorous after

refining (unpublished results). The impact of this process on glycerol production

is not known.

Unsaponifiable materials are rarely considered as significant components in bio-

diesel, though they may constitute several percent of the fuel weight (61). Unsapo-

nifiables are primarily alcohols and hydrocarbons that do not affect the overall

trans-esterification reaction. Unsaponifiable components do not require alcohol

for esterification and decrease the total alcohol used in trans-esterification to a

very small extent. The presence of unsaponifiable matter in biodiesel raises ques-

tions regarding the fate of these materials during combustion. In most oils, the unsa-

ponifiable fraction is composed mainly of sterols that are higher in molecular

weight than other fuel constituents. To the authors’ knowledge, little is known of

the fate of sterols during combustion in a diesel combustion cycle (62). However,

research has been conducted on the effects of unsaponifiables on cetane number.

The slight changes that were observed were less than the ASTM repeatability

and reproducibility specifications.

Biological oils are often contaminated with water in emulsion (3). Although

some water is tolerable in the synthesis of esters, its presence is generally not pre-

ferred. Water can act with alkali catalysts to saponify esters that form soaps.

Water can also change the equilibrium of esterification with acidic catalysts favor-

ing the formation of free fatty acids over that of esters. Water has the potential to

react with 16 times its weight with TAG or ester, assuming the ester or TAG is of

oleic acid. As will be described later, this reaction can be mitigated even if water is

present.

Oil composition specifically affects the esterification strategy and also the poten-

tial uses of the ester. Esters rich in highly saturated fats and high-melting trans-fatty

acids, for example, may need to be esterified or trans-esterified at elevated tempera-

tures. (3). The product esters of these saturated compounds will typically have ele-

vated melting points (59). Such esters are often not well suited for use in fuels that

may be exposed to lower temperature (62). With dilution in fuel, low-melting esters

or solvent esters of saturated fatty acids may be included in fuels suitable for use at

low temperatures (62). Unsaturated fats have lower freezing points and are often

preferred in lower temperature applications (63). Polyunsaturated fats are prone

to oxidation and polymerization. The changes induced by oxidation may alter the

quality of biodiesel deriving from these fats (64). Polymerization can increase the

fuel viscosity and alter combustion characteristics (27, 64). In addition to the

increase in viscosity with oil oxidation, in an accelerated aging test, acid value


and specific gravity also increased (64). The antioxidants a-tocopherol and tert-

butylhydroquinone (TBHQ) were partially effective in mitigating effects of accel-

erated aging (64).

The ratio of alcohol to oil in ester is determined by the molar weights of both the

alcohol and the oil. Lower molecular weight fatty acids comprise a smaller portion

of an ester product than higher molecular weight substances (Table 4). For common

fatty acids, changes in chain length have the greatest impact on ester weight,

whereas changes in saturation have little impact. Biodiesel made with oils rich in

palmitic acid will require more alcohol than biodiesel made with oils rich in oleic or

higher alcohols. Erucic acid, found in rapeseed oil, requires 10% less alcohol in the

synthesis of its esters than oleic acid.

5.2. Alcohol

Many choices of alcohol are possible, including methanol, ethanol, isopropanol, n-

propanol, butanol (all isomers), and pentanol (all isomers) (65). In general,

improved conversion is achieved by adding extra alcohol (3). Higher molecular

weight alcohols have lower polarity. Phase separations become more difficult

with higher alcohols (66). Mixing higher alcohols with methanol has been used

to improve reaction completion and phase separation (66). Adding glycerol and

water can have the same effect (67). Small amounts of water are tolerable only

if a glycerol phase forms (68). Otherwise, soaps form slowly in a reversible

reaction. In reactions where water is present, the key to avoiding saponification

is to minimize reaction time and have water only in the glycerol phase. The amount

of alcohol contributing to the weight of the ester increases with the molecular

weight of the alcohol. Table 5 indicates the contribution of various alcohols that

may be used in esterification of triolein. Approximately 5% weight contribution

increase in the proportion of alcohol to ester is contributed with each additional

carbon atom.

5.3. Catalyst

5.3.1. Alkali In base catalysis of fatty acids, little or no esterification occurs and

saponification is essentially irreversible. Therefore, it is necessary to optimize con-

TABLE 4. Impact of Oil Type on Alcohol Required

for Trans-Esterification.

Oil Methanol

Oil Type moles/kg kg/kg Oil

Palm oil 1.17 0.112

Canola oil� (LEAR) 1.13 0.108

Rapeseed oil (HEAR) 1.04 0.100

�Moles/kilogram is similar for canola, soy, corn, sunflower, and flax.


ditions so that hydrolysis (saponification) is minimized. Water present in the oil,

alcohol, or catalyst may result in saponification (3). As the reaction proceeds and

sufficient glycerol is formed to separate from solution, water will dissolve prefer-

entially in the glycerol phase (67). The separation of the glycerol phase can prevent

substantial lipid hydrolysis even when substantial amounts of water are present

(67, 68). In alkali catalyzed reactions, the catalyst is presumed to be metal alkylate

of the alcohol. For example, potassium hydroxide reacts with methanol to reversi-

bly form potassium methoxide and water. The water of reaction is not sufficient to

produce significant quantities of fatty acids through hydrolysis (3).

Reaction 2:

OH� þ H3COH � H2O þ H3CO�

Reaction 3:

H3CO� þ RCOOR0 � RCOOCH3 þ OR0�

5.3.1.1. NaOH vs KOH Sodium hydroxide has a lower molecular weight than

KOH, and thus it also has more moles per gram (Table 6). The potassium ion is

larger and forms slightly more soluble salts, including more oil-soluble ethoxide.

On a mole basis, KOH is a more efficient catalyst than NaOH. Weight for weight

both catalysts are equivalent in reaction rate. However, water and fatty acids are

known to inactivate both catalysts with one mole of fatty acids reacting with one

TABLE 6. Comparison of Alkaline Catalysts Required for Efficient

Trans-Esterification.

Moles Base

Catalyst Purity Mol. Wt. (100 g/oil)

0.25% KOCH3 0.95 70 0.0034

0.25% NaOCH3 0.95 54 0.0044

1.0% KOH 0.9 56 0.0161

1.0% NaOH 0.9 40 0.0225

TABLE 5. Alcohol Molecular Weights and Percent Weight

of Three Moles of Alcohol to One Mole of Pure Triolein.

Alcohol

Molecular Weight

(g/mole)

Ratio (3 mole alcohol/

1 mole triolein; w/w)

Methanol 32 11

Ethanol 46 15

Propanol 60 20

Butanol 74 25

Pentanol 88 30


mole of each catalyst. In this reaction, it requires significantly more fatty acid to

inactivate sodium hydroxide than potassium hydroxide.

5.3.1.2. NaOCH3 vs NaOH Where oils and alcohols contain little water, it is pos-

sible to use alkoxide catalysts to produce alkyl esters from vegetable oils. In anhy-

drous systems, as little as 0.1% to 0.2% alkoxide by weight of oil can efficiently

catalyze trans-esterification, but higher concentrations are normally reported (3).

Alkoxides also have the advantage of minimizing the base content of the glycerol

product. For example, a trans-esterification process using 0.25% potassium meth-

oxide would require approximately 0.034 moles of methoxide per kg of oil, whereas

1% potassium hydroxide would require 0.16 moles per kg of oil (Table 6). The

relatively small level of ions simplifies the requirements of glycerol refining. An

additional benefit of alkoxides is the avoidance of water. Typically commercial

hydroxides contain small amounts of water and form additional water in reaction

with alcohol to form the alkoxide. By definition, commercial methoxides are anhy-

drous. Water produced by or released from catalyst in biodiesel manufacture will

eventually be recovered with methanol evaporation. If the water content of the alco-

hol is substantial, a rectification distillation column and a zeolite-drying column are

required to distill dry alcohol for further esterification (69). Methoxide catalysts can

be used to avoid adding water to the reaction and to prevent water accumulation in

the alcohol. Alcohol recovered from alkoxide-catalyzed reactions may be dried

using a zeolite-packed column.

5.3.2. Acid Esters are readily synthesized from fatty acids and alcohols in the

presence of acid catalysts. In general, a large excess of alkylating reagent is

required to drive the equilibrium. Preferred acids include sulfuric acid and hydro-

gen chloride, although p-toluenesulfonic acid is also used (3). Acid-catalyzed ester-

ification of fatty acids is relatively rapid and is usually performed at the reflux

temperature of the alcohol. Small amounts of water are known to reverse the reac-

tion, and thus methods that require large additions of alcohol are often used (3, 70).

However, methods that significantly lower water content or provide a second

phase that contains the water can lead to esterification with smaller amounts of

alcohol (67).

As a result of the need for acid-resistant alloys and other equipment required for

acid esterification, the process is typically more capital intensive than base trans-

esterification. The higher capital costs associated with the use of acidic catalysts are

usually offset by the ability of the process to accept lower cost feedstocks (1).

Acidic catalysts may be used to recover soap byproducts of alkali-catalyst based

trans-esterification processes (3, 71). In these processes, acid is used to convert

soap to free fatty acids and then to esters (see below).

Many oil products are available that cannot be esterified directly using alkali cat-

alysts as they have sufficient fatty acid content to neutralize an alkali catalyst, yet

have high ester contents. An acid esterification step may be employed in advance of

base catalyzed esterification. These pre-esterification methods need only provide

sufficient acid and alcohol to esterify the fatty acids present (3, 68, 71), although


some methods use large molar excesses of alcohol (72). Acid-catalyzed trans-

esterification of acylglycerols with alcohol is a relatively slow reaction when

compared with acid-catalyzed esterification. Trans-esterification and release of

glycerol is avoided by maintaining abbreviated reaction times. In essence, the bulk

of the acylglycerol oil is not reacted and, therefore, inert during the pre-esterification

reaction. These pre-esterification methods will be discussed in detail below.

5.3.3. Exotic Ideally, catalysts are long lasting, hasten reactions, lower energy

inputs, and simplify product refining. Alkali catalysts, used as the mainstay of

many biodiesel production processes, are entrained in glycerol released by the

process and are lost for further catalysis. Alkali catalysts are neutralized by

fatty acids forming soaps with limited alkalinity that do not act as catalysts.

This latter property limits the utility of basic catalysts to applications with low acid

value oils. Liquid acid catalysts may be reacted with oils with high acid numbers

but, like basic catalysts, these materials tend to be soluble in the glycerol phase.

Most innovative approaches to catalysis involve new heterogeneous (solid) cat-

alysts that can be repeatedly used to manufacture biodiesel, although a few novel

approaches describe using homogeneous catalysts. For example, Demmering (72)

was able to use oleic acid as a catalyst for biodiesel synthesis with reaction condi-

tions of 240�C for 3 hours. Georghiu (73) was able to generate esters using an orga-

notitanate catalyst at just 0.15% of the reaction mixture using a reaction time of

2.5 hours at 220�C. Acidic resins are also capable of forming esters but may also

produce ethers as a coproduct. Lundquist (74) describes esterification using acidic

resins at 110�C. The reaction produced a significant amount of dimethyl ether in

spite of adjusting reaction conditions to minimize ether formation. Resin-based

acid catalysts may also be used in pre-esterification of acidic oils. Jeromin et al.

(75) achieved a reduction of acid value from 10 to less than 1. The elevated

temperature, high autogenic pressure, and exotic catalysts often combine to detract

from the economics of these novel catalysts. Improved catalysts are anticipated.

Ultimately, enzymatic esterification holds great promise in biodiesel production.

Foglia et al. (76) and Haas (77) have worked to develop enzymatic methods for

production of alkyl esters. Foglia et al. (76) found that 10% lipase by weight of

TAG stirred at 200 rpm for 5 hours at 45�C was sufficient for high conversion of

oils to alkyl esters. As a result of the current high cost and low activity of enzymes,

the reaction described was limited in scale. Wu et al. (78) have been able to

improve and regenerate lipase activity by washing the catalyst with alcohols with

three or more carbon atoms. However, the enzymatic reactions and their improve-

ments as described did not lead to a process that could economically yield commer-

cial quantities of fuel. Enzymes will not be components of commercial biodiesel

synthesis until significant improvements have been developed.

5.4. Ester Production Processes and Strategies

An abundance of intellectual property has been developed to improve production of

fatty acid esters. The goals of most inventions are to increase the yield of product,


use lower quality materials, ease refining requirements, or minimize inputs such

as energy, catalyst, or alcohol. Simple principles of chemistry would seem to limit

the efficiency of all processes (i.e., esterification is reversible) but combinations

of principles have been used to design processes and reactions that achieve

almost 100% efficient conversion of materials. Basic catalyzed reactions often

appear to go beyond theoretical yields; but this is caused by the separation of gly-

cerol into a second phase during the reaction. This phenomenon prevents feedback

inhibition of glycerol on ester synthesis. Soaps do not accumulate to high levels in

the reaction as they dissolve in water, lower alcohol, and glycerol. Removal of

soaps in a glycerol phase can greatly assist in the refining required for biodiesel

production.

Some total processes define the extraction and refining of the oilseed within their

invention. Stidman et al. (79) describes a process for extracting oils from soybean

and converting those oils to methyl esters. The process includes standard steps for

degumming, caustic refining, and bleaching followed by esterification. The process

finishes with glycerin removal and a washing step.

In one process, the inventor has reported that sugars from oilseed meals are read-

ily fermented to form ethanol (80). With soybeans, the concentration of fermentable

sugars can constitute 12% of the meal weight. These sugars could readily be fer-

mented to produce enough ethanol for esterification of oil from the seed. However,

it is improbable that seeds with higher oil contents would produce sufficient ethanol

from fermentation to esterify the oils present.

5.4.1. Unique Reaction Sequence Advantageous methods have been develop-

ed to improve esterification processes. In an early development, Glossop (81) re-

ported the removal of fatty acids from high acid oils by washing with methanol

or another solvent that was not miscible with oil in a countercurrent extractor. After

washing, the oil phase had low free fatty acids and was suitable for base trans-

esterification. The methanol phase was treated with acid to esterify the fatty

acids.

It is often found to be more difficult to produce esters of ethanol and higher alco-

hols than methanol. Dreger (66) overcame this inefficiency in forming esters by

using two-stage trans-esterification and two alcohols, one of which is methanol.

The reaction produced a mixed ester, but the yield was significantly improved.

5.4.2. Unique Reaction Conditions Billenstein et al. (82) reported 498-g

tallow plus 27 g of 30% sodium methylate were combined in a reactor and success-

fully reacted with alcohol vapor at >210�C.

Klok and Verveer (83) were able to achieve extensive esterification with the

minimum addition of alcohol. According to their description, 40-kg soy oil and

6.2-kg methanol were heated to 65�C and 0.95-kg 30% sodium methoxide was

added. The mixture was stirred for 1.5 h then settled and the glycerol separated.

Subsequently, a further 0.2 kg of 30% sodium methoxide was added to the ester

layer. The reaction mixture was heated and methanol recovered from the methyl

ester layer by distillation.


Soapstock is a novel low-cost material that is rich in fats. Soapstock is generated

during alkali refining of vegetable oils, where oils are mixed with sodium or potas-

sium hydroxide solutions (84). This mixture is centrifuged to yield a low acidity,

light-refined oil phase and a heavy phase composed of soap, neutral lipid, and

water. Haas et al. (85) developed a process for converting these lipids into esters.

In the process, the soap solution is treated with additional alkali to complete the

saponification, and then it is dried. The dried solids may be esterified by the addi-

tion of inorganic acid catalysts and alcohol. Reaney (86) also approached the con-

version of soapstock to esters. In the patented technology, soapstock was treated

with acid and an alcohol, where the alcohol was selected from among alcohols

that remained insoluble in salt water. The process efficiently generates separate

phases of water and oils without evaporation. Fatty acids and alcohol in the mixed

upper layer are condensed to esters through treatment with heat. The resulting

product, including mixed esters and glycerides, may be trans-esterified using

base catalysis to remove glycerol. Preferred alcohols for conducting this reaction

include n-propanol, i-propanol, n-butanol, and isopentanol alcohol. Mixtures of

these alcohols, called fusel oils, may be obtained as byproducts from the production

of ethanol by fermentation.

The early stages of most biodiesel reactions proceed slowly, but the slow reac-

tion is readily overcome when the reaction is induced to form a single phase.

Boocock (87) described methods of accelerating methyl ester synthesis by adding

ether cosolvents. In one example, 100 g of soy oil was reacted with 28 mL of

methanol in the presence of 35 mL of tetrahydrofuran and 1 g of sodium hydroxide.

The reaction proceeded rapidly and separated in 20 min. After the reaction was

complete, the solvents were flashed off the methyl ester phase and a small amount

of glycerol separated. The ester yield was 90.1%.

Stern et al. (71) recovered free fatty acids from alkaline esterification. Fatty acids

are converted to soaps in trans-esterification. When the alkaline glycerol layer

forms, the soaps partition into the glycerol layer leaving a high-quality methyl ester.

The alkaline glycerol is then neutralized with acid and the soaps are converted to

fatty acid in the process. The glycerol and fatty acids are mutually insoluble and

separate. When separated from the glycerol, the fatty acids are recovered as a con-

centrate. Stern et al. (71) recommend a process of mixing the fatty acids with gly-

cerol and heating to more than 200�C. The acidity of the mixture is sufficient to

allow the formation of esters from the glycerol and fatty acid mixture. The esters

may be added to future batches for alkaline trans-esterification. This method

increases yield of the reaction while allowing some tolerance for fatty acid content

in the oil and fatty acid production during processing. Stern et al. (71) described

production of methyl esters by trans-esterification with 95% ethanol and recovery

of the lost fatty acids by reaction with glycerol. Stern et al. (71) also proved that a

high yield is possible with alkali-based trans-esterification even using 7% acidity

palm oil. In this reaction, the level of base required for the reaction was increased

from 0.4% to 1.2% by weight. The method may require an additional amount of

alkaline catalyst to overcome acids encountered in the reaction. The reaction

requires equipment for handling acidic materials but has the advantage that the total


DECANTERTANK 2

REACTOR 2

DECANTERTANK 1

REACTOR 1METHANOLCATALYST

OIL & FAT

CATALYST

RECT.COLUMN

RECT.COLUMN

METHANOLTO STORAGE

CRUDE GLYCERINE80 − 85%

MOLECULARSIEVES

METHYL ESTER99%+

McDonald Patent 6,262,285 FlowFigure X

Courtesy of Crown Iron Works, Co.

Figure 3. Biodiesel production schematic as taught in US Patent 6,262,285. Flow diagram shows the vessels and devices included in

the invention required to mix, react, heat, separate glycerine, recover solvents, and dry solvents for reuse.

243

material reacting under acidic conditions is less than 10% of the total reaction

volume.

McDonald (69) reported a total system for ester synthesis that described a pro-

blem that is seldom alluded to in other published methods (Figure 3). Alcohol

recycled in biodiesel synthesis commonly accumulates water from catalysts, leaks,

and fresh alcohol. If the water content is low, a column containing molecular sieves

may remove it, but with higher amounts of water present, a process for drying the

alcohol is required. The method described by McDonald (69) also avoids a water

wash step for fatty acid methyl esters altogether by decanting the glycerol in con-

tinuous decanters and recovering the methanol from the methyl ester directly.

Lepper and Friesenhagen (67) were able to esterify oils rich in free fatty acids by

adding alcohol, glycerol (or other polyhydric alcohol), and p-toluenesulfonic acid

(or other acid catalyst). The unique contribution of this technology is the use of a

polyhydric alcohol to form a second phase that does not occur if the reaction does

not release glycerol. Fatty acids react quickly to form esters, and the water formed

by the reaction is largely partitioned into the glycerol phase. The reaction products

include a phase of alkyl esters of lower alcohols and glycerol mixed with a phase of

glycerol, catalyst, and methanol. The alkyl ester phase is readily trans-esterified

using alkaline catalysts to form fatty acid esters suitable for use as biodiesel. The

process relies on inexpensive means to refine or reuse the polyol to remain efficient.

Lepper and Friesenhagen (67) reported that the polyol was effectively recycled nine

times (67).

Wimmer (88) described the need in base catalysis to compensate for the con-

sumption of base by fatty acids. In their patent, they claim the need for 0.025 moles

of base catalyst for each 100 g of fatty acid ester. Fatty acids are overcome by the

addition of more catalyst, on an equivalent molar amount, to the fatty acids in the

free state. Clearly, this method can be used with some fatty acids being present in

the oil; but it is not adaptable to oils with elevated fatty acid contents.

All biodiesel plants risk emission of the alcohol used in trans-esterification

reactions, evaporation, and other processes. Emissions are typically restricted by

government regulations and sophisticated recovery processes are required to meet

these regulations. Traditionally, methanol can be recovered from plant emissions by

condensation, absorption into organic solvents (e.g., triethylene glycol), or adsorp-

tion onto activated carbon. In a unique process, Granberg and Schafermeyer (89)

have designed an agitated column that uses a chemical reaction to recover methanol

entrained in inert gases used in a biodiesel process. Their reaction intimately mixes

the inert gas with a catalyst and a ‘‘fatty source.’’ The reaction that ensues converts

the vapor to nonvolatile alkyl esters and essentially recovers most of the waste alco-

hol vapor from plant exhaust. In this process, Granberg and Schafermeyer (89) indi-

cated a reduction of alcohol from 16,000 ppm to less than 80 ppm.

Oils with high acid values may be esterified with synthetic heterogeneous cata-

lysts. Jeromin et al. (75) successfully reduced the free fatty acid content of coconut

oil in reaction columns containing Lewatit SPC 1118 BG ion exchange resin

(Sybron Chemicals Inc., Birmingham, NJ) and a related product. They claimed

methods for pre-esterification of crude fat or oil of vegetable or animal origin in


an admixture with methanol, in a mole ratio of methanol to free fatty acid content of

the fat or oil of 10 : 1 to 50 : 1 over a fixed bed, heterogeneous catalyst. Although

this method holds the promise of directly trans-esterifying the resultant mix of

esters and alcohol, the authors proceed to evaporate the alcohol to produce oils

with low acid numbers.

Kawahara and Ono (90) reviewed methods of pre-esterifying oils with high acid

numbers and found that these methods often produced poor quality esters. They

attributed the low quality to impurities that were readily extracted by washing

the esters with alcohol immediately after pre-esterification.

5.5. Summary of Biodiesel Production Technology

Clearly there are numerous technologies for biodiesel production and improve-

ments to increase efficiency and lower costs. Evaluating these processes and

comparing the costs of biodiesel production processes can and should be accom-

plished based on objective criteria such as comprehensive empirical data and com-

parative analysis of approaches. Many improvements to biodiesel technology may

be considered as components that may be added or inserted into existing processes.

For the purpose of this discussion, we consider the impact of these improvements

on the core process of trans-esterification and alcohol recycling described by

McDonald (69). The two-stage process described in this patent includes a complete

engineering diagram with information on pumps, mixers, and solvent recovery sys-

tems. With appropriate insertion of processing data and engineering information,

complete economic and operation models may be devised to emulate the process

and determine potential advantages of new processes versus old.

As most biodiesel glycerol separations can occur quickly, the McDonald patent

reports replacement of typical centrifuge technology with less capital-intensive

decanters. Discharging from the decanter will be the lightest, therein the least con-

taminated, methyl ester from the top discharge and inversely, the heaviest compo-

nent, glycerol, from the bottom of the decanter. As the methyl ester and methanol

mixture from the top of the decanter is free of glycerol, the wash step commonly

found in standard processing can be eliminated. The elimination of the wash water

step improves energy efficiency, yield, and alcohol recovery.

The many methods of pre-esterification would complement the described pro-

cess by allowing the use of acylglycerols with higher fatty acid content. The ben-

efits of such methods may then be determined independent of a total process.

Cosolvents such as tetrahydrofuran would increase the need for energy used in

evaporation of solvents and may increase the need for vapor compression to con-

dense a mixed phase. The ether solvents described by Boocock (91) would add to

the total energy consumed in biodiesel production and would detract from the effi-

ciency of the McDonald design. However, the advantage of using cosolvents in pre-

esterification, described later by Boocock (91), could be manifold if lower cost high

free fatty acid materials are used. Pre-esterification without the need for evaporation

may eventually prove the most efficient method of processing oils with high levels

of free fatty acids.


McDonald’s last step is then to recover the methanol using vacuum and rectify-

ing the methanol stream using molecular sieves. Inclusion of the molecular sieves

significantly lowers energy costs when compared with traditional distillation, as

steam is only used during the regeneration of the sieves and steam usage in the rec-

tification column is greatly reduced because a lower operating temperature is

required as a result of the use of vacuum. The ingenious vapor recovery system

developed by Granberg and Schafermeyer (89) could reduce the cost of condensing

alcohol and could be incorporated into most biodiesel manufacturing plants, but it

would not provide specific advantages in the recovery of ether solvents. If this tech-

nology were incorporated in the McDonald design, this could replace significant

portions of the alcohol vapor condensation equipment and energy inputs.

It is possible that the need for water removal from methanol is not necessary if

the entire system is maintained in a dry state and dry catalysts are used. For

example, sodium methoxide brings less water to the reaction than the comparable

hydroxide, and commercial methanol can be obtained in an essentially dry state. In

the case where dry oil is available for processing, it may be advantageous to pro-

duce biodiesel using the dry ingredients and monitor water levels in recovered

methanol.

Many glycol liquids form a two-phase mixture with biodiesel, and these com-

pounds can be contacted with biodiesel to remove polar contaminants (92). The

process has many advantages in refining biodiesel but glycols tend to have high

boiling points and, thus, are difficult to recycle. The use of glycols as absorptive

media described by Bam et al. (92) might consume significant amounts of energy

and capital costs in the distillation to recover the liquid glycol products.

Single-stage reactions are normally inefficient, as they require the addition of

large amounts of alcohol to drive the reaction to completion. In a single-stage reac-

tion, it is normal to add 5–10 times the amount of alcohol required for a stoichio-

metric reaction. Trans-esterification in a two-stage reaction is advantageous when it

allows the reduction of alcohol used for trans-esterification to 2–4 times stoichio-

metric requirements. A single-stage reaction using methanol and a typical vegetable

oil with a molecular weight of 885 g/mole (glycerol trioleate) uses between 483 g

and 976 g of extra methanol. In a two-stage reaction, the extra methanol is reduced

to 108–325 g. In addition, as methyl esters are soluble in both glycerol and cosolvents,

a two-stage reaction has lower losses of these materials. In any case, the methyl

esters and fatty acids dissolved in alcohol are not lost and they may be recovered.

6. UTILIZATION TECHNOLOGY

6.1. Lubrication

At current oil crop production levels and fuel consumption rates, biodiesel is unli-

kely to replace more than a very small portion of total diesel fuel consumed glob-

ally (1). However, biodiesel has many potential niche markets where its low toxicity

and improved emissions can provide value that outweighs the added costs of using

this fuel. Toxicity and biodegradability tests have determined that biodiesel is a


preferred fuel for environmentally sensitive areas where fuel spillage poses an

undue risk. This advantage is especially important in inland waterways and national

parks. Fuels comprising 20% to 100% biodiesel also have special applications

where the oxygen content of the fuel significantly reduces a broad spectrum of

potentially toxic emissions. Applications include the use of ultra-low-sulfur-fuel

blends with biodiesel and oxidation catalysts in mines to reduce the production

of particulates and carbon monoxide. Additional technology is required to reduce

NOx emissions that accompany the combustion of this fuel.

In addition to fulfilling market demand, governments (1, 51) have also enabled

the development of new markets by mandating specific levels of biodiesel in fuels.

Primarily, these mandated fuel markets develop demand for domestic agriculture,

reduce reliance on foreign fuel sources, and improve the balance of trade. An unex-

pected improvement in overall fuel economy can be realized with the addition of

low levels of biodiesel to certain fuels.

Concern for the environment has resulted in moves to significantly reduce the

noxious components in emissions when fuel oils are burned. Attempts are being

made to minimize sulfur dioxide emissions and, as a consequence, a strategy to

minimize the sulfur content of fuel oils has been implemented. Although typical

diesel fuel oils have, in the past, contained 1% or more of sulfur (expressed as ele-

mental sulfur) by weight, environmental legislation in the United States has

required that sulfur content of diesel fuel be less than 0.05% (11). These levels

will be reduced to 15 ppm or less to protect new exhaust catalyst after-treatment

devices. In Europe, various jurisdictions have moved to lower sulfur content. In

Sweden, for example, taxation of higher sulfur, lower cetane fuels is elevated to

reflect to their respective environmental cost (93).

The reduction in the sulfur content of diesel fuel is correlated with lubricity pro-

blems. It is generally accepted that the reduction in sulfur is also accompanied by a

reduction in polar oxygen and nitrogen-containing compounds as well as polycyclic

aromatic compounds (10). As these compounds are responsible for fuel boundary

lubricating ability, their loss in severe refining results in low-sulfur fuels with

reduced lubricity. Thus, low-sulfur content is not necessarily indicative of a lubri-

city problem, but it has become the measure of the degree of refinement of the fuel.

In comparison with less refined diesel fuels, ultra-low-sulfur diesel fuels have been

found to induce an increase in sliding adhesive wear and fretting wear of pump

components including rollers, cam plates, couplings, lever joints, and shaft drive

journal bearings. Reducing the level of one or more of the diesel fuel components

that contributes to lubricity reduces the ability of the fuel oil to lubricate the injec-

tion system of the engine. The engine’s upper cylinder liner and top piston rings

also suffer from reduced fuel lubricity. When diesel fuels are compared using lubri-

city bench tests, including the high-frequency reciprocating rig (HFRR) or the

Munson roller on cylinder lubricity evaluator (M-ROCLE), small additions of bio-

diesel have been found to reduce wear areas and friction coefficients for unadditized

low-sulfur fuels (94–96). This additional lubricity translates into significantly

reduced (10–50%) engine wear, as measured by accumulation of wear metals in

crankcase oil, and improvements (2–13%) in fuel economy in field tests (94–101).

UTILIZATION TECHNOLOGY 247

The consequences of lowered lubricity include fuel injection pump failures rela-

tively early in the life of an engine. Fuel injection system failures due to low-lubri-

city fuel occur more frequently in high-pressure rotary distributor pumps, although

inline pumps and injectors also experience reduced life. Poor lubricity problems in

diesel fuel oils are likely to be exacerbated by the future engine developments

aimed at further reducing emissions, which are predicted to have more exacting

lubricity requirements than present engines. For example, the advent of high-pres-

sure unit injectors is anticipated to increase fuel oil lubricity requirements. Poor

lubricity can lead to wear problems in the engine upper cylinder areas and in other

mechanical devices, such as valve guides dependent for lubrication on the natural

lubricity of fuel oil. Diesel fuels exhibiting higher coefficients of friction in lubri-

city bench tests can be expected to increase engine upper piston ring drag on

cylinder walls, thus increasing fuel consumption as well as engine wear.

Hertz et al. (95–101) have reported a lengthy series of diesel engine vehicle field

studies with biodiesel lubricity additives applied at rates from 30% (soy methyl

ester) to 0.5% (canola methyl ester). Unadditized as well as additized commercial

summer and winter seasonal low-sulfur diesel fuels were referenced. Engine wear

was inferred from ICP spectrometry and ferrographic analysis of the used engine oil

wear particles as well as examination of oil filter debris. Both indirect and direct

injection engines were compared, with and without exhaust gas recirculation and

turbo charging. In these studies, it was found that biodiesel additization to low-

lubricity diesel fuels resulted in significant iron wear reductions, typically in the

10% to 50% range. The benefit depended on the engine design, the lubricity of

the reference diesel fuel, the additization rate, and the efficacy of the additive. A

typical 0.5% canola methyl ester treatment in commercial diesel fuels resulted in

50% to 57% ICP iron wear reductions in a VW TDI engine (100). In this study,

0.5% canola methyl ester additization increased field fuel economy from 2% at

moderate ambient temperatures to 13% under arctic winter conditions.

Numerous lubricity additives for fuel oils have been described (101). Caprotti

et al. (102) disclosed an additive that is comprised of an ester of a carboxylic

acid and an alcohol, wherein the acid has from 2 to 50 carbon atoms and the alcohol

has one or more carbon atoms, e.g., glycerol monooleate. Although general mix-

tures are contemplated, no specific mixtures of esters were described.

Furey, in U.S. Patent No. 3,273,981 (103), disclosed a lubricity additive as being

a mixture of A þ B, wherein A is a polybasic acid or a polybasic acid ester made by

reacting the acid with C1��C5 monohydric alcohols; and B is a partial ester of a

polyhydric alcohol and a fatty acid, for example, glyceryl monooleate, sorbitan

monooleate, or pentaerythitol monooleate. The mixture finds application in jet

fuels.

Beimesch and Zehler (104) described the uses of two esters with different visc-

osity in diesel fuel to reduce smoke emissions and increase fuel lubricity. In one

preferred embodiment of that invention, methyl octadecenoate, a major component

of biodiesel, was included in the formula. Similarly, Dilworth (105) also described

a fuel composition comprising middle distillate fuel oil and two additional

lubricating components; those components being (1) an ester of an unsaturated


monocarboxylic acid and a polyhydric alcohol and (2) an ester of a polyunsaturated

monocarboxylic acid and a polyhydric alcohol having at least three hydroxyl

groups.

The approach of using a two-component lubricity additive was pioneered by

Fainman (106). He described an additive and a liquid hydrocarbon fuel composition

consisting essentially of a fuel and a mixture of two straight-chain carboxylic acid

esters, one having a low molecular weight and the other having a higher molecular

weight.

In U.S. Patent 5,713,965, Foglia et al. (76) describe the synthesis of alkyl esters

from animal fats, vegetable oils, rendered fats, and restaurant grease. The resultant

alkyl esters are reported to be useful as additives to automotive fuels and lubricants.

The addition of alkyl esters of fatty acids derived from vegetable oleaginous seeds

were recommended at rates between 100 ppm to 10,000 ppm to enhance the lubri-

city of motor fuels in U.S. Patent 5,599,358, (107). Similarly, a fuel composition

was disclosed by Stoldt and Harshida (108) comprising low-sulfur diesel fuel

and esters from the trans-esterification of at least one animal fat or vegetable oil

triacylglycerol.

Lang et al. (109) investigated the lubricity of methyl, ethyl, 2-propyl and 1 butyl

esters of canola, linseed, rapeseed, and sunflower oils. A statistical test of the results

revealed effects of oil used and alcohol on the resultant lubricity measured. Canola

and rapeseed esters were superior to linseed esters in reducing wear scar formation

followed by the esters of sunflower. Esters of isopropyl alcohol had the greatest

impact on reducing the coefficient of friction. Munson and Hertz (95, 96) surveyed

the lubricity of a series of vegetable-based esters at 1% treatment rates in

hydrotreated low-sulfur No.1 diesel fuel. The reported M-ROCLE lubricity num-

bers, based on the ratio of wear area stress to coefficient of friction, were found

superior for canola methyl ester, linseed ethyl ester, and rapeseed methyl ester.

The lubricity number response for canola-based esters was found to be a semiloga-

rithmic function of treatment rates. Soy and sunflower methyl esters performed

poorly as a result of their respective higher coefficients of friction and larger

wear areas.

6.2. Low Temperature Behavior

In temperate climates, diesel fuel must remain fluid at temperatures below the mini-

mum expected temperature for the season. Through much of North America, winter

diesel fuels have low temperature flow points below �30�C. Diesel fuels with low

pour points have lower viscosity and often lack lubricity (110). These fuels typi-

cally provide little lubricity. For example, Noureddini (111) reports that biodiesel

fuels that are simple esters of various vegetable oils have poor flow characteristics

below a temperature of �2�C. To overcome this difficulty, a solvent consisting of

mixed ethers of glycerol is added to the biodiesel. The resultant fluid has a low tem-

perature cloud point below �32�F (�36�C). The pour point of this fluid is still

above that necessary to effectively add to many winter diesels, as it may be neces-

sary to pour the fuel component at temperatures as low as �45�C.

UTILIZATION TECHNOLOGY 249

Soybean ester fuel blends may also be produced that have improved low

temperature performance (62). Thirty percent blends of soybean methyl esters

with kerosene began crystallization at �14.7�C, whereas the 2-butyl ester began

crystallization at �30.2�C. These solutions do not have sufficient low temperature

performance for inclusion in many low temperature applications.

Vegetable oils have also been considered in applications as fuels and attempts to

maintain solutions of triacylglycerol oils at low temperatures have also been

attempted. Blends of vegetable oil and lower alcohols have suitable viscosity for

use in fuels, but they separate into two phases as temperature is lowered (62). High-

er molecular weight alcohols can be used to lower the temperature of phase separa-

tion. In a soy oil number 2 diesel blend with ethanol, phase separation was deferred

to �16�C whereas the cold filter plugging point was lowered to �24�C.

6.2.1. Hydrotreated Vegetable Oils n-Alkane Fuels Tall oil, animal fats,

yellow grease, and vegetable oils can be converted to combustible fuels suitable

for diesel engines by hydrotreating, i.e., treating with hydrogen over catalysts at

elevated temperatures (5, 6). The impact of hydrotreating was complete decarboxy-

lation and hydrogenation of all alkene and hydroxyl functionality. Decarboxylation

occurring during the reaction shortens the chain length of fatty acids by one and

hydrogenation assures that the converted products are mostly odd chain length n-

alkanes. Glycerol undergoing this conversion is primarily converted to propane.

Water and carbon dioxide are the primary byproducts and they embody none of

the energy present in the starting material. The n-alkane products have the desirable

characteristic of high cetane values, which allows their use as fuel combustion

improvers. However, these materials have the undesirable trait of high melting

point, which limits their use in fuels suited for low-temperature applications.

The test results show that n-paraffin (1) linearly raises the cetane of unadditized

diesel without a top-end limit; (2) can linearly raise the cetane of ‘‘nonresponsive’’

diesel blends; (3) is synergistic with traditional cetane improvers; (4) has a low

sulfur content; and (5) increases endproduct volume added (4, 5). When n-alkanes

and cetane enhancers were added to a diesel fuel with a cetane number of 32 to

raise the cetane number to 43, a 10% reduction in carbon dioxide emissions was

achieved.

6.2.2. Triacylglycerols Triacylglycerols (TAGs) may be combusted directly in

diesel engines, and technology exists to efficiently use these resources without

trans-esterification (8). Diesel engines may be modified to operate directly on vege-

table oil, but low temperature performance can be difficult. This is overcome by

heating the fuel and insulating fuel lines (112). As viscosity is the main difficulty

in operating on TAG oils, treatments to lower viscosity have been attempted. Heat-

ing TAG oil has been partially successful in allowing combustion of the fuel with-

out engine modifications, but engine failure has occurred when fueling with heated

TAG (113). Fuel viscosity can be reduced by blending soybean oil with other fuels

or by forming emulsions (7). This approach still may result in inadequate engine

performance due mostly to blocked injectors.


Certain short-chain triacylglycerols (SCT) produced by Cuphea viscosissima

(113) could be used as sources of low viscosity triacylglycerols. There is no current

abundant and commercial short-chain triacylglycerol source available but in the

future new plant domestication, genetic engineering, or breeding efforts may lead

to SCT biodiesel fuels.

Most biodiesel synthesis processes produce a fuel that has a slight contamination

with acylglycerols, and this fact is recognized in most current biodiesel standards

(Table 1). In spite of their low concentrations, these acylglycerols may contribute

disproportionately to fuel lubricity.

7. COPRODUCT USE

During methyl ester production, glycerol is produced at approximately 10% of the

weight of most fats and oils, with exceptions being fats with higher average mole-

cular weights, which produce less glycerol, and fats with lower molecular weights,

which produce more glycerol. As the source for most biodiesel produced to date has

been canola, sunflower, and soy oils, the glycerol production is approximately 10%

of biodiesel production. An increasing percentage of the world’s glycerol supply is

coming from the production of biodiesel. As biodiesel production increases, new

markets must be found to deal with the rising production levels of the byproduct

glycerol.

In addition to traditional uses, which are reviewed elsewhere, glycerol may be

used in animal feed, reaction solvents, and chemical synthesis (113–115). Glycerol

containing strong alkali may be used in the synthesis of conjugated linoleic acid

(114). It is expected that substantial new demand will be generated for glycerol

in the production of diacylglycerols that are being promoted for their beneficial

dietary effects. Glycerol can also be converted through chemical and biological pro-

cesses to polyglycerol ethers, glycerol acetates, isopropylidene glycerol, glycerol

ethers, and propylene glycol (115). As the value of glycerol falls, the opportunities

to use glycerol will necessarily expand.

Salt is a coproduct of biodiesel production. Usually, salts are formed in the step

of glycerol use or during glycerol distillation. The salts formed are dependent on

the catalyst used in ester formation and the acid used to neutralize the glycerol.

Sodium- or potassium-based catalysts can be reacted with phosphoric, hydrochlo-

ric, sulfuric, carbonic, or other acids to produce the respective phosphates, chlorides,

sulfates, carbonates, or other salts. Preferred technologies include the use of

potassium hydroxide and phosphoric acid to generate a final product of potassium

phosphate, which is suitable for fertilizer. Sodium salts are more difficult for land

disposal, but they may be suited to production of specific commercial products.

Sodium chloride is the resultant salt in processes developed by Lurgi and Crown

Iron Works. Although chloride ions are corrosive on equipment, advanced materials

available from these technology suppliers allow cost-effective recovery of glycerol

and brines.

COPRODUCT USE 251

8. THE FUTURE

If current trends continue, biodiesel will grow to become the largest market for

triglyceride oil, expanding in size beyond current markets for food, feed, and

industrial products. Research will be required to improve biodiesel production

technology at all levels. With innovation, the footprint of agriculture land required

for biodiesel production will be minimized. Energy inputs for biodiesel production

will also be minimized, whereas protein and glycerol coproducts of biodiesel

production will be improved to allow total use. In spite of the diversion of food

crops to energy production, it is anticipated that technology development will

enable agriculture production to meet future needs.

The steps taken to the future will be small at first. Over the next several years,

feedstocks, including yellow grease, soybean, corn, and sunflower oil, will be the

foundation of profitable biodiesel production. However, in the long term, the

demand for oil will drive the development of new biodiesel sources. Canola pro-

duces a desirable ratio of oil to meal; but it cannot fix nitrogen, whereas soy will

inevitably be found to produce too much protein and too little oil to be considered

over the long term for global biodiesel production. Breeding existing crops and

introducing new species will produce new biodiesel crops for our future.

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Documents

Vegetable Oils as Biodiesel