Environmental Engineering Reference
In-Depth Information
recently published by Al-Zuhair [16]. The methods of preparation of biodiesel can
be classified in: chemical catalytic (base- or acid catalysis), biocatalytic (enzyme
catalysis) and non-catalytic processes.
Biodiesel Produced by Chemical Catalytic Methods
a. Homogeneous catalysis
. The conventional and traditional methodology for the
production of biodiesel primarily involved the transesterification of the vegetable
oils using NaOH and KOH [17-21] or mineral Brönsted acids (sulphuric, phos-
phoric or hydrochloric acids) [22-24] as homogeneous catalysts and vegetable oils
or waste oils and fats as feedstock at relatively mild temperatures (50-80
◦
C). Few
reports on the production of biodiesel using a variety of homogeneous catalysts
including guanidines [25] and different amines as catalysts (yielding conversions
higher than 98% in a one-step reaction, minimizing the production of waste water)
[26] can also be found.
Regardless of the limitations of the methodology, the process is also far from
being environmentally friendly. The final mixture needs to be separated, neutralised
and thoroughly washed, generating a great amount of salt, soaps, and waste water
which need to be further purified or treated. The catalyst cannot also be recycled.
These several additional steps certainly put the total overall biodiesel production
costs up, reducing at the same time the quality of its main by-product (glycerol).
This phase needs to be separated from the biodiesel for further washing/drying to
remove the traces of glycerol aand from the fuel to comply with EU quality standard
regulations (EN 14214). The standard prescribes 0.02% or lower glycerol content
must be present in the biodiesel.
The acid catalysed homogeneous transesterification has not been widely investi-
gated compared to the alkali-catalysed process due to its slower reaction rates, the
need of harsher conditions (higher temperatures, methanol to oil molar ratios and
quantities of catalysts) and the formation of unwanted secondary products such as
dialkyl or glycerol ethers [15]. These drawbacks make impractical its successful
implementation with hardly any examples of commercial processes available [27].
b. Heterogeneous catalysis
. Several reports can be recently found on the production
of biodiesel involving other chemically catalysed protocols as greener alternatives
using vegetable oils using solid bases [28-32] and solid acids [24, 28, 33-37].
Di Serio et al. have recently reviewed the use of heterogeneous catalysts for
biodiesel production [28]. The advantages of the heterogeneously catalysed pro-
tocols from the green chemistry standpoint are that the catalyst may be recycled
and subsequently employed in the reaction. The biodiesel prepared has improved
properties compared to the homogeneously catalysed process. The elimination of
the pre-treatment steps and the minimisation of waste, avoiding the production of
waste salts, also improves the green credentials of the reaction. Excellent yields of
FAME/FAEE can be obtained under relatively mild conditions with many of these
heterogeneous catalysts. However, the separation, disposal or use of the glycerol
generated in the process as well as the washing of the crude biodiesel obtained
