Biofuels in Perspective

88 Biofuels
methoxide as catalyst under more drastic reaction conditions like higher pressure and high
temperatures. Under these reaction conditions also free fatty acids are converted into fatty
acid methyl esters. For the use as intermediates for fatty alcohol production the esters have
to be distilled. At the beginning of the biodiesel development these technologies, however,
were too expensive and needed too high investment costs, Therefore the so-called low
temperature and low pressure processes were developed, which use temperatures up to
60 ◦ C at ambient pressure and there is no need for distillation of the final product. These
processes can be used in very small production units, but today also production plants with
a capacity of 100,000 tons or more are using this technology. This is also the reason that
today there is an enormous number of small-scale producers existing worldwide, because
one can use simple equipment. However, these so-called ‘backstage’ or ‘garage’ producers
mostly don’t have any safety precautions or quality control of the product, furthermore they
cannot further process the glycerol layer, which contains excess methanol and is therefore
considered as special waste.
Moreover, in most cases, the biodiesel produced does not meet the high quality standards
defined in the European specifications EN 14214 or ASTM specifications D 6584, and therefore
it is possible that this quality can lead to serious injection pump or engine problems.
Therefore in the following only the technologies with industrial applications are
described.
There are different possibilities in classifying the different biodiesel production technologies.
One can distinguish according to the type of catalyst between homogenously or
heterogeneously catalyzed processes; one can distinguish according to the reaction conditions
between low and high temperature and pressure reactions, or between continuous or
batch operation. On the other hand it also possible to classify according to type of feedstocks.
The so-called single feedstock technologies are using half or fully refined vegetable
oils like rape seed, soybean, sunflower etc. With these technologies the content of free
fatty acids should be very low, so the formation of soaps is very limited. Normally alkaline
catalysts like sodium methoxide or potassium hydroxide is used, and the soaps formed as
side products during the reaction are either removed by water washing steps or recycled by
esterification with acid catalysts after work up of the glycerol phase. With this technology
also a small amount of other feedstocks like recycled frying oil or higher acidic palm oil
can be blended to the refined vegetable oils.
The so-called multi-feedstock technologies are also capable of processing feedstocks
with higher amounts of free fatty acids. Here a so-called preesterification of the free fatty
acids is necessary, or during a high pressure and temperature process all fatty material is
directly converted in FAME in one step. These processes could be capable of processing
any type of feedstocks, including acid oils, animal fat, high acidic palm oil or even fatty
acids. The reaction conditions can be easily adapted to the change of feedstock.
Though a differentiation of these two technologies is often not very easy, especially
with newer developments of technology, the terms single and feedstock technologies are
broadly used in the biodiesel terminology, and therefore will be used in the following.
5.8.1 Single Feedstock Technologies
The biggest biodiesel production units with a capacity of over 100,000 tons mainly use
fully refined vegetable oils with low content of water and free fatty acids. In that case