Environmental Engineering Reference
In-Depth Information
one currently predominant in industry, it does have important limitations in the energy
required to recover the glycerol by-product and in treating the alkali waste, as well as in the
interference of water and free fatty acids with reaction progress [18].
If either water or free fatty acids are present in unacceptably high amounts, acid catalysis
is preferable, typically employing hydrochloric, sulfuric, or phosphoric acids. While this
process proceeds several thousand-fold more slowly than the alkali-catalyzed process, it does
allow use of lower-quality oils [1].
2.2.4. Supercritical methanol catalysis
. Recently, researchers have attempted
transesterification in supercritical methanol, where supercritical fluids are solvent phases
obtained under high temperature and pressure that have both vapor and liquid characteristics
and in which many reactions have been found to proceed more readily than in subcritical
solvents [19]. Since supercritical methanol has a hydrophobic nature with a lower dielectric
constant than subcritical methanol, nonpolar triglycerides were well-solvated with
supercritical methanol to form a single-phase oil-methanol mixture. As a result, the oil to
methyl ester conversion rate increased dramatically in the supercritical state. Added
advantages were that free fatty acids in the oil could also be converted efficiently to the
methyl esters, improving product yield, and that purification of products following
transesterification was much simpler and more environmentally friendly due to the absence of
alkali (or acid) catalysts. Unfortunately, however, the process required a temperature of 3
50°C and pressure of 45MPa, as well as large quantities of methanol, with the result that
much optimization will be required before this approach is commercially competitive [20-22]
2.3. Enzymatic (Lipase-based) Transesterification
In efforts to improve upon the abiotic transesterification methods, considerable effort has
been devoted to the investigation of lipases, carboxylesterases that catalyze the hydrolysis and
synthesis of long-chain acylglycerols, for the synthesis of biodiesel [23]. These remarkable
enzymes currently constitute the single most widely-used biocatalyst class in biotechnology:
they often possess high chemoselectivity, regioselectivity, and stereoselectivity; they are
naturally extracellular enzymes and many are secreted in great quantity by fungi and bacteria,
allowing relatively simple purification from culture media; they require no cofactors; they
typically catalyze no undesirable side reactions; and the crystal structures of a number of
representatives have been solved, facilitating rational design approaches in optimizing
enzyme activity. The commercial lipase market is currently approximately 1 billion dollars
per year, involving applications in detergents and in the production of food ingredients and
enantiopure pharmaceuticals [24]; this status is fortunate for biodiesel applications because it
provides preexisting incentives for improvements in lipase production and activity. A chart
comparing the relative merits of the alkali-catalyzed and enzyme-catalyzed transesterification
processes for biodiesel production is shown in Table 4.
Commercial lipases on which much biodiesel research has been based include enzymes
from Mucor miehei (Lipozyme IM-20) and Candida antarctica (Novozym 435), as well as
from Pseudomonas cepacia, Candida rugosa, and Rhizopus delemar. Both extracellular and
intracellular lipases effectively catalyze a variety of triglyceride transesterification reactions,
and they have the potential to overcome all of the problems of chemical catalysis (Table 4).
On the other hand, the cost of lipase catalysts is significantly greater than that of alkali
catalysts [1].
