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Continuous esterification for biodiesel production from palm fatty acid

distillate using economical process

S. Chongkhong*, C. Tongurai, P. Chetpattananondh

Department of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand

a r t i c l e i n f o

Article history:

Received 4 January 2007

Accepted 12 July 2008

Available online 10 September 2008

Keywords:

Biodiesel

Palm fatty acid distillate (PFAD)

Continuous esterification

Economical process

a b s t r a c t

An overflow system for continuous esterification of palm fatty acid distillate (PFAD) using an economicalprocess was developed using a continuous stirred tank reactor (CSTR). Continuous production compared

to batch production at the same condition had higher product purity. The optimum condition for the

esterification process was a 8.8:1:0.05 molar ratio of methanol to PFAD to sulfuric acid catalyst, 60 min of

residence time at 75 C under its own pressure. The free fatty acid (FFA) content in the PFAD was reduced

from 93 to less than 1.5%wt by optimum esterification. The esterified product had to be neutralized with

10.24%wt of 3 M sodium hydroxide in water solution at a reaction temperature of 80 C for 20 min to

reduce the residual FFA and glycerides. The components and properties of fatty acid methyl ester (FAME)

could meet the standard requirements for biodiesel fuel. Eventually the production costs were calculated

to disclose its commercialization.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel is renewable clean bio-energy as it can be producedfrom vegetable oils and has no sulfur content. The quality of bio-

diesel is similar to diesel fuel; thus it becomes a promising alter-

native to diesel fuel[1].

At present, the main drawback for the commercialization of

biodiesel is its higher cost than petroleum-based diesel. The high

cost of biodiesel is due to its being produced mostly from expensive

high-quality virgin oil with a low content of free fatty acid (FFA)

[2,3]. A way of reducing biodiesel cost is to use less expensive

feedstock containing high FFA, recycled or waste oils and by

products of the refining vegetable oils. With feedstock having high

FFA, biodiesel production is processed in two steps. The first step is

to reduce the FFA content of the oil by esterification. The second

step is transesterification, which converts the triglyceride portion

of the oils to mono-alkyl ester and glycerol [47].Esterification is an acid-catalyzed chemical reaction involving

FFA and alcohol which yields fatty acid alkyl ester and water (H2O).

Methanol is the most commonly used alcohol because of its low

cost and high reactivity as compared to longer-chain alcohols[2,8].

Biodiesel production in the presence of enzyme catalysts can ach-

ieve satisfactory ester yield under mild conditions. However, the

process is costly and needs longer reaction times and higher cata-

lyst concentrations. For example, the enzymatic production of

biodiesel from cotton seed oil observed 97% yield after 24 h at 50C

with a reaction mixture containing 32.5% t-butanol, 13.5% meth-

anol, 54% oil and 0.017 g enzyme/gram of oil using a one step fixedbed continuous reactor [9]. The favorable acid catalyst is sulfuric

acid (H2SO4) because of its low cost[10,11].

Conventional continuous production of biodiesel usually oper-

ates under high pressure and temperature. The supercritical

process requires not only high pressure and temperature but also

a high amount of alcohol. The optimum condition of continuous

biodiesel production using supercritical methanol with a tube

reactor is a 40:1 molar ratio of alcohol to vegetable oil, 25 min of

residence time, 35 MPa and 310 C[12].

Biodiesel is conventionally produced from feedstock having FFA

content less than 20 wt% [212]. In this work, palm fatty acid

distillate (PFAD), a by product from the production of consumable

palm oil, with an FFA content of 93 wt% was used as a feedstock for

biodiesel production. The study is focused on the economicalprocess: using low cost PFAD and mild operating conditions with

continuous overflow production. The material balance of the

process and biodiesel quality testing are included.

2. Materials and methods

2.1. Chemical

PFAD was obtained from Chumporn Palm Oil Industry Public

Company Limited, Thailand. It is a light yellow solid at room

temperature. The fatty acid profile of PFAD is given in Table 1. The

FFA level of 93 wt% is much greater than the 1% limit for sufficient* Corresponding author. Tel.: 66 74287055; fax: 66 74212896.

E-mail address: [email protected](S. Chongkhong).

Contents lists available atScienceDirect

Renewable Energy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / r e n e n e

0960-1481/$ see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.renene.2008.07.008

Renewable Energy 34 (2009) 10591063
mailto:[email protected]://www.sciencedirect.com/science/journal/09601481http://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.sciencedirect.com/science/journal/09601481mailto:[email protected]


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alkaline-catalyzed transesterification process. Consequently, ester-

ification was operated. All chemicals including methanol (99.5%),

sulfuric acid (98%), and sodium hydroxide (99%) are of commercial

grade.

2.2. Experimental design

Continuous production using an overflow process was simply

carried out under mild conditions, low reaction temperatures (65,

70 and 75 C) that made the system pressure slightly higher than

the atmospheric pressure. This process requires an uncomplicated

apparatus which is easily constructed and low cost. The

experiments were designed by the Taguchi method [13]. With

a three-level-four-factor array, L9(34), nine experiments were

investigated as shown inTable 2. Molar ratio of methanol to PFAD

(R), reaction temperature (T), reaction time (t) and catalyst amount

(S) were selected as independent variables.

2.3. Batch experiments

To save time and expenses the optimum condition for the batch

experiment was used as the initial condition for continuous

experiments. The batch experiments were conducted as described

below.

2.3.1. Esterification

Batch esterification was carried out in a laboratory scale. A hot

plate with a magnetic stirrer was used for heating the mixture in

the screw-capped bottle. The bottle was kept in an oil bath to

maintain the temperature. The mixture was stirred at the same

speed (300 rpm) for all test runs. After the esterification reaction,the product mixture was poured into a separating funnel and then

allowed to settle into two phases. The bottom FAME-layer was

separated and purified by the water washing process before being

analyzed for its composition by thin layer chromatography (TLC).

Three trial runs were carried out and the results presented are

average values.

2.3.2. Purification

The FAME product obtained from the optimum esterification

process still had about 1.4 wt% residual FFA, which needed

further purification. A neutralization process was used instead of

the distillation process. Neutralization was carried out using 3 M

of sodium hydroxide in water (NaOHH2O) solution. Then, 2%wt

salt (NaCl) was dissolved in the solution to remove soap forma-

tion and mixed with FAME. The mixture was heated to a reaction

temperature of 80 C and stirred at a speed of 350 rpm for 15

20 min. After that, the neutralized product was poured into the

Table 1

Fatty acid profiles of PFAD

Fatty acid Formula Structure %wt

Saturated fatty acids

Myristic C14H28O2 14:0 1.0

Palmitic C16H32O2 16:0 45.6

Stearic C18H36O2 18:0 3.8

Arachidic C20H40O2 20:0 0.3

Monounsaturated fatty acidsPalmitoleic C16H30O2 16:1 0.2

Oleic C18H34O2 18:1 33.3

Ecosenoic C20H38O2 20:1 0.2

Tetracosenoic C24H46O2 24:1 0.6

Polyunsaturated fatty acids

Linoleic C18H32O2 18:2 7.7

Linolenic C18H30O2 18:3 0.3

Table 2

Independent variables and levels of L9(34) for Taguchi method

Experimental run no. Independent variable values

T(C) T(min) R (mole/mole) S(%wt)

1 65 60 5.7:1 0.92

2 65 90 7.2:1 1.83

3 65 120 8.8:1 2.75

4 70 60 7.2:1 1.83

5 70 90 8.8:1 0.926 70 120 5.7:1 2.75

7 75 60 8.8:1 1.83

8 75 90 5.7:1 2.75

9 75 120 7.2:1 0.92

Fig. 1. A schematic diagram of a continuous unit for biodiesel production from PFAD using an economical process.

S. Chongkhong et al. / Renewable Energy 34 (2009) 105910631060


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separating funnel and then allowed to settle into two phases. The

soap phase that formed in the neutralization reaction wasremoved. The FAME phase was washed with water at 6080 C,

allowed to settle and heated to evaporate residual water.

2.4. Continuous experiments

The continuous experiments contain two processes: (1) an

esterification process, which is carried out in a continuous stirred

tank reactor (CSTR) and (2) a purification process, where FAME was

purified to meet the standard requirements for biodiesel fuel. A

schematicdiagram of the continuous experiments is shown in Fig.1.


On the basis of the results from Section 2.3.1, the reaction

operating parameters were set to a 8.8:1 molar ratio of methanol toPFAD, a 0.05:1 molar ratio of H2SO4 to PFAD, a reaction temperature

of 75 C under its own pressure (about 133 kPa) for residence time

of 60 min.

Fresh methanol (2.3 kg/h), recycled methanol (6.5 kg/h) and

H2SO4 (0.16 kg/h) were mixed first and pumped into a 22.4 L

CSTR. PFAD was fed into the CSTR after being heated to 75 C.

The mixture was mixed at a constant stirring rate of 400 rpm.

After that, the product mixture overflowed to the cooling water

tank before entering a 30 L separator. The esterified products

were allowed to settle into two phases. The top phase, consisting

of un-reacted methanol together with water formed during the

reaction, was then overflowed at the top of the 30 L separator.

The methanol phase was purified by distillation and then recy-

cled. The FAME phase underflowed at the bottom pipe line of the

separator. The FAME phase was passed into the evaporator to

recover traces of methanol and further treated in the purification

unit.

2.4.2. Purification

To meet biodiesel specifications, the FAME phase having

a residual FFA of about 1.4%wt was neutralized with 10.24%wt of3 M NaOHH2O solution (10.24 g of the solution/100 g of FAME

phase) on the basis of the results from Section2.3.2. The NaOH

H2O solution (0.8 kg/h) and the FAME phase (8.3 kg/h) were

preheated to 80 C before entering a 4.2 L CSTR. The mixture was

mixed at a 500 rpm stirring rate with a reaction temperature of

80 C for 20 min. After that, the neutralized products settled into

two phases in a 12.6 L separator. The FAME product overflowed at

the top of the separator and the soap phase was removed at the

bottom. The FAME product was passed into a 20 L water washing

tank with a water feed rate of 10 L/h and then overflowed into

a 20 L separator for water separation. Finally, the biodiesel

product was heated by an evaporator to remove the residual

water in the product.

2.5. Sample analysis

Thin layer chromatography equipped with a flame ionization

detector (TLC/FID) using an Itronscan MK-6s with Chromarods

type S-III quartz rod (Mitsubishi Kagaku Iatron) was used to

analyze the components of biodiesel. The sample was diluted in

hexane and 1 mL of the sample solution was spotted onto the

Fig. 2. FAME content obtained from a Taguchis experimental condition (Table 2) for

deciding the initial condition used in the continuous process.

Fig. 3. FAME and glycerides content in the neutralized product using the 3 M NaOHH2O solution at 80

C under atmospheric pressure for 15 min.

Table 3

Batch process yields of FAME from PFAD in comparison with continuous process

yields

Process Esterification Purification

% Yield % FAME % Yield % FAME

Batch 99 93.9 94.4 99.6

Continuous 97 97.3 94 99.7

Table 4

Operating costs for theannual production of 72,000 kg of biodiesel fromPFADin this

study

Description Cost

(baht)

Annual

use

Annual cost

(baht/year)

Unit cost

(baht/kg)

Raw materials

PFAD 11/kg 76,464 kg 841,100

Methanol 19/kg 20,115 kg 382,199

Sulfuric acid 8/kg 1404 kg 11,232

Sodium hydroxide 19/kg 756 kg 14,364

Subtotal raw materials 1,248,895 17.34

Utilities

Electricity 1/kga 72,000

Charcoal 3.4/kg 2898 kg 9853

Process water 0.012/L 86,400 L 1037

Subtotal utilities 82,890 1.15

Labor

Operating 200/day 72,000 1.00

Gross production cost 1,403,612 19.50

a Kilograms of biodiesel; 31,425 baht per USD (April 23, 2008).

S. Chongkhong et al. / Renewable Energy 34 (2009) 10591063 1061
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rods. The rods were immersed in a mixture of hexane:diethyl

ether:formic acid (50:20:0.3 v/v) until the mixture reached 8 cm,

and then in a mixture of benzene:hexane (1:1 v/v) until the

mixture reached 10 cm. After that, the rods were dried at 105 C

for 5 min and scanned with the TLC/FID. Scanning was carried out

under a hydrogen flow rate of 160 mL/min and an air flow rate of

2.0 L/min to produce a chromatogram. Peak areas of FAME,

triglycerides (TG), diglycerides (DG), monoglycerides (MG) and

residual FFA were calculated with a chromatography data systemChromStar and depicted as weight percentages (%wt) on a GL-free

basis.

3. Results and discussion

3.1. Batch experiments


The esterification reaction hardly occurred without a catalyst.

Nevertheless, a good reaction rate required only an appropriate

amount of the catalyst.Excess addition of the catalyst caused a dark

color of the product and more neutralizing solution was needed.

From our experiments, the yield of FAME obtained by using 0.92,

1.83 and 2.75%wt of H2SO4 was not significantly different. Theinteraction between the reaction temperature and reaction time

was studied by experimental run no. 3 and 5. With the reaction

temperature of 65 C, a longer reaction time of 120 min was

required to obtain the same FAME content with the reaction

temperature of 70 C and reaction time of 90 min (Fig. 2). The

interaction between the reaction time and molar ratio of methanol

to PFAD was represented by experimental run no. 7 and 9. With

a higher ratio of 8.8:1 a shorter reaction time of 60 min was enough

to gain the same FAME content by using a lower ratio of 7.2:1 and

reaction time of 120 min. In addition, experimental run no. 3, 5, 7

and 9 obtained quite the same maximum yield of FAME. These

mean that with a higher reaction temperature the esterification

reaction was completed in a shorter time. Similarly, with a higher

amount of methanol a shorter reaction time was required.However, the optimum operating condition was decided to be

experimental run no. 7 by using an 8.8:1 ratio for 60 min at 75 C

with 1.83 wt% H2SO4 because it possibly required the lowest energy

consumption.

3.1.2. Purification

Neutralization and saponification reactions took place as the

FAME phase was neutralized with the NaOHH2O solution. When

the solution increased, the glycerides (TGDGMG) content

decreased and the purity of FAME increased. Neutralization with

2.05%wt of the 3 M NaOHH2O solution could get rid of all the

residual FFA. To meet biodiesel specifications, 10.24%wt of 3 M

NaOHH2O solution was required to reduce glycerides and 100%

FAME was obtained when using 13.32%wt of the 3 M NaOHH 2Osolution (Fig. 3).

3.2. Continuous process

3.2.1. Esterification and purification

The optimum conditions for batch esterification and neutrali-

zation were used as the operating conditions for the continuous

process. The ester yields and purities for the batch and continuous

processes are shown inTable 3. The batch esterification yield was

2% higher than the continuous yield. On the other hand, the purity

of batch esterification was 3.4% lower than that of continuousesterification. For the purification process the yields and purities

obtained from the batch and continuous processes were not so

different.

3.2.2. Production costs analysis

Production cost (Table 4) was calculated based on the process

flow diagram shown inFig. 1. The gross production cost for making

biodiesel from PFAD is estimated to be 19.5 baht/kg. The cost of raw

materials constitutes the greatest component, while the cost of

PFAD is 60% of the overall production cost. However, the PFAD price

is cheaper than the used oil, crude palm oil and refined palm oil (2,

6 and 20 baht/kg cheaper, respectively). In addition, the process of

making biodiesel in this work is inexpensive as it is operated under

mild conditions (low temperature and pressure) and the capitalcost of the apparatus is low. Thus, the use of low cost PFAD feed-

stock can improve the economic viability of biodiesel.

3.3. Properties of PFAD biodiesel

A summary of the properties of PFAD biodiesel is presented in

Table 5along with rubber seed oil, mahua and soy soapstock bio-

diesel [4,7,14] and diesel. The results show that the two step

process, esterification and neutralization, enabled the PFAD bio-

diesel properties with respect to density, viscosity, flash point,

water content, ash content and acid value to meet biodiesel stan-

dards (Table 1). Especially the ash content of PFAD biodiesel is the

lowest compared to biodiesel from other feedstocks and diesel,

which means that PFAD biodiesel is cleaner fuel in terms of lowerparticulate matter and gas emission risk. But, the pour point of

PFAD biodiesel is a bit higher than the standard because the main

component of PFAD is 53.7% saturated FFA. However, the pour point

can be easily lowered by blending PFAD biodiesel with diesel.

Therefore, this PFAD biodiesel is a high potential alternative to

petroleum diesel for use in unmodified diesel engines. In addition,

it is widely known that the color of FAME obtained from the acid-

catalyzed esterification process is usually black. With our purifi-

cation process the color of the PFAD biodiesel was the preferred

clear brown.

4. Conclusion

An economical process for the continuous production of bio-diesel from low cost PFAD was established. The feature of the

Table 5

Properties of PFAD biodiesel in comparison with others

Properties Unit PFAD

biodieselaRubber seed

oil biodiesel

Mahua

biodiesel

Soy soapstock

biodiesel

Di esel Bi odiesel standar ds

ASTM D6751-02

Density at 15 C kg/m3 879 874 880 885 850 870900

Viscosity at 40 C mm2/s 4.87 5.81 3.98 4.302 2.60 1.96.0

Flash point C 180 130 208 169 68 130

Pour point C 14 8 6 NA 20 15 to 10

Water content % 0.03 NA 0.04

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