Biodiesel From Conventional Feedstocks - Biotechnology in China: Biofuels and Bioenergy - page 62

immobilization of such whole cells can be realized spontaneously during the
process of cell cultivation [ 25 ]. To further stabilize the whole-cell catalysts,
cross-linking treatment with glutaraldehyde solution was suggested and in this way
the immobilized whole cells can be reused for many batches. Further study has
revealed that the activity of such cross-linking immobilized cells was even
competitive to the commercial lipase Novozym 435. Acyl migration was also observed
during the whole-cell-mediated methanolysis for biodiesel production, which was
promoted with the increase in water content in the reaction mixture [ 26 - 28 ].
Although whole-cell biocatalysts overproducing intracellular lipase can be
expected to reduce the cost of lipase preparation considerably and offer a
promising prospect for industrial biodiesel production, many challenges such as
scaling up and process optimization need to be investigated further.
3.3.3 Liquid Lipase-Mediated Alcoholysis
Liquid lipase offers an alternative approach to enzyme-catalyzed biodiesel
production, though few reports are available. Compared with immobilized lipase,
soluble lipase possesses the advantages of faster reaction rate and lower cost;
hence, much attention has been paid to soluble lipase-mediated methanolysis for
biodiesel preparation in recent years [ 29 - 32 ]. Research has already showed that a
soluble lipase NS81006 could be capable of catalyzing the methanolysis of
triglycerides to produce biodiesel in an oil/water biphasic system and a biodiesel
yield of over 90% could be obtained after 8 h reaction. Although the recovery of
lipase and the enzyme's performance during continuous running need to be
evaluated further, the liquid lipase-mediated reaction provides an alternative
method of enzyme-mediated alcoholysis for biodiesel production.
3.4 Biodiesel Production in a Supercritical Fluid System
Supercritical fluid has properties different from gas or fluid, with its density similar
to fluid, its viscosity similar to gas, and its thermal conductivity and diffusion
coefficient between gas and fluid. Methanol is hydrophobic in supercritical con-
ditions, and triglyceride dissolves well in supercritical methanol. As a result,
biodiesel production in a supercritical system has advantages of fast reaction rate
and high conversion yield [ 33 , 34 ]. Since the supercritical fluid method is sensitive
to operating temperature and pressure variations, it is feasible to change the
operating conditions of the reaction to adjust the physical properties of super-
critical fluid, and thus affect the mass transfer, solubility, and reaction dynamics
performance correspondingly. However, the shortcomings associated with this
method are high operating temperature and pressure of about 350-400 C and
45-65 MPa, respectively, and the economics of its industrial application needs
further systematic evaluation [ 33 - 35 ].