Environmental Engineering Reference
In-Depth Information
Fig. 2.1
The three main steps
of trans esterification
There are various types of trans-esterification reactions. Some of these are:
i.
Homogeneous alkali catalyzed trans-esterification:
Trans-esterification can be catalyzed by using homogeneous and heterogeneous
catalyst. This is carried out in the same three steps. The alkali catalyzed reac-
tions for trans-esterification are faster than acid catalyzed reactions (Vicente
et al.
2004
)
ii.
Homogeneous acid catalyzed trans-esterification:
The liquid acid catalyzed reaction of trans esterification is not popular as com-
pared to base catalyzed reaction. This process is about four thousand times slower
as compared to base catalyzed reaction. The performance of this process isn't
affected much due to the presence of free fatty acids in feed stock. It can catalyze
both esterification and trans esterification. The great advantage of using this pro-
cess is direct formation of biodiesel from low cost lipids feedstocks, having high
free fatty acid concentration which is greater than 6 %. The common catalysts for
acid catalyzed reactions are hydrochloric acid, sulphuric acid, phosphoric acid,
boron tri fluoride and organic sulphonic acid (Lotero et al.
2005
).
iii.
Heterogeneous acid and base catalyzed transesterification:
The former reaction has greater performance for trans-esterification to gain bio
diesel but they consume high amount of energy. Heterogeneous catalyst in com-
parison utilizes less energy, can be separated from the product more easily, avoids
undesired saponification reactions and can use high free fatty acid feed stocks.
Biodiesel synthesis by using solid catalyst can lead to a cheap manufacture cost
because the catalyst can be re-used and esterification can be carried out along
with trans esterification simultaneously (Di Serio et al.
2007
; Garcia et al.
2008
).
iv.
Enzymatic trans esterification:
Enzymatic trans esterification using lipase has promising outcomes such as ease
in product separation, negligible water surplus requirements, ease in glycerol
regaining and nonexistence of side responses. It also faces some difficulties
like infectivity of product due to left over enzyme action and market price. To
fix these problems enzyme is used in immobilized condition hence allowing to
be used numerous times to decrease the cost and improve the quality of prod-
uct formed. When free enzymes are used in this process the enzyme activity is
incompletely recovered but the built up of glycerol also limits the number of
uses of free and immobilized enzymes (Nielsen et al.
2008
).
v.
Supercritical and subcritical alcohol trans esterification:
Trans-esterification of vegetable oils by super critical without the use of cata-
lyst offers a novel way of making bio-diesel. By using this process the trans
esterification is finished in minutes while other catalytic processes take several
