Biodiesel

Biochemical Engineering Journal xxx (2006) xxx–xxx

The effect of fatty acid concentration and water content
on the production of biodiesel by lipase
Sulaiman Al-Zuhair ∗ , Kishnu Vaarma Jayaraman, Smita Krishnan, Wai-Hoong Chan
School of Chemical Engineering, Faculty of Engineering, The University of Nottingham Malaysia Campus,
Jalan Broga, 43500 Semenyeh, Selangor D.E., Malaysia
Received6 March 2006; received in revised form 25 April 2006; accepted 25 April 2006

Abstract
The kinetics of the production of biodiesel by esterification of butyric acid with methanol, catalysed by lipase from Mucor miehei, was studied
in two types of systems, namely, n-hexane microaqueous and biphasic (n-hexane/water) containing different amounts of water. The experimental
results were fit to aPing-Pong mechanism and the constants found in the rate expression were determined. The results have shown that in a
microaqueous media, butyric acid does not inhibit the reaction in the range of initial concentrations considered. The effect of water content on the
initial rate of reaction and conversion were illustrated. It was shown that the initial rate of reaction increased as the initialwater content increased
up to 25% (v/v). However, the conversion was found to be higher at low initial water concentrations.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Biodiesel; Lipase; Butyric acid; Esterification; Ping-Pong kinetics

1. Introduction
Biodiesel, defined as monoalkyl fatty acid ester (preferentially methyl and ethyl esters), presents a promising alternative
fuel for usein compression-ignition (diesel) engines [1]. It has
been proven that biodiesel fuels have viscosities close to those of
diesel. In addition, although the volumetric heating values are
a little lower, they have high cetane numbers and flash points
[2]. Biodiesel is a strong candidate to replace petroleum diesel
as their characteristics are generally similar in addition to the
many attractiveadvantages of biodiesel over petroleum diesel.
These advantages include: it is plant- not petroleum-derived and
as such it is less toxic and comes from renewable sources; it is
biodegradable and relative to conventional diesel, its combustion products have reduced levels of particulates, carbon oxides,
sulfer oxides and under some conditions, nitrogen oxides [2,3].
The application of lipasefor esterification reaction in organic
media has increased significantly in the last decade [4]. For
the design of suitable reactors, control systems and process
optimisation, kinetic information on the rate of product formation and the effects of changes in operating conditions are

∗

Corresponding author. Tel.: +6 3 89248129; fax: +6 3 89248017.
E-mail address:alzuhair.sulaiman@nottingham.edu.my (S. Al-Zuhair).

essential. Modelling of lipase-catalysed reactions in organic
media has received relatively little attention and only limited
reports on the kinetics of esterification reactions are found in the
literature.
Most kinetic studies on lipase-catalysed esterifications considered short chain alcohols observed only alcohol inhibition
and described by a Ping-Pong kineticmodel [5] with competitive inhibition by the alcohol as shown in the following
equation:
υ=

Vmax
1+

KA
[A]

1+

[B]
KiB

+

(1)

KB
[B]

where υ is the initial rate of reaction, Vmax the maximum rate
of reaction, KA and KB are the binding constants for the fatty
acid (A) and the alcohol (B), and KiB is the inhibition constant
for the alcohol. Although it is generally agreedon the inhibition
effect of the alcohol, the effect of fatty acid concentration and
water content are disputed. Krishna and Karanth [4] proposed
that in microaqueous media, in addition to alcohol inhibition,
fatty acids have also an inhibition effect, especially for short
chains fatty acids (Eq. (2)). Fig. 1 shows a schematic representation of the Ping-Pong mechanism with competitive...