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 English Document
The road to pure plant oil in diesel engines?
Production: from rapeseed to oil
Technology
PPO production includes two subsections:
1. Producing the raw oil from the seed. This can be achieved in two ways:
a. Mechanical isolation, using cold pressing (small-scale oil presses).
b. Pressing/extracting: mechanical and physical-chemical isolation using a combination
of flattening and pressing and extraction via solvents (large-scale extraction using
hexane).
2. Refining the oil in order to remove undesired components.
In practice both processes are used to produce rapeseed oil for use as a vehicle fuel. The oil
produced can then be estered with methanol into biodiesel, but this study looks only at the
unestered oil.
Scale size
The scale of the oil presses producing biodiesel vary, from several thousands of tons of oil
per year, up to 100,000 or 150,000 tons per year. The oil-seed processing industry maintains a
rule of thumb that cold pressing is economically more beneficial when processing less than
500 tons of seed per day. In both cases the oil can be refined in order to remove unwanted
substances. Following the aforementioned rule of thumb, cold pressing is primarily used for
small-scale decentralised presses with production capacities of several thousands of tons per
year. However, there are also large-scale plants where this technique is used, e.g. Bio-oelwerk
Magdeburg12 .
Pressing/extracting is the standard production technology for manufacturing pure plant oil
for the foodstuff industry. It is also used in large-scale integrated biodiesel plants with their
own oil press, e.g. at Rheinische Bioester in Neuss.
Small-scale production
The press generally consists of a screw press. The seeds are slightly preheated to reduce the
viscosity, and are squashed in the press. With some types of press the exhaust area of the
press pulp is heated, in order to prevent blockages.
The oil obtained consists of several percent solid material that needs to be removed before
the oil can be used in road vehicles. Current cleaning techniques include filtration, centrifuge
or sedimentation. Sedimentation has the disadvantage that there is a relatively large loss of
oil. The cleaned oil should preferably be stored in a stainless steel tank, to prevent the acid in
the tank material damaging the oil, and to prevent degradation of the oil due to light.
The operation of the press aims for maximum isolation of the oil, as well as minimising the
amount of phosphor and solid particles in the oil. Solid particles are not conducive to using
the oil as a vehicle fuel and can lead to extra oil loss during separation. Phosphor is present
in the oil or seed in the form of phosphor lipids; the presence of which makes the oil more
sensitive to oxidation break-up and increases the hydration abilities (i.e. its ability to absorb
water). Phosphor is also an undesired element when using the oil as a vehicle fuel, as it can
lead to deposits and blockages in the engine and can contaminate catalytic converters. The table provides an indication of how these parameters are important and how operation can be
optimised.
Cold pressing and filtration isolates around 75% of the oil from the seed as a separate
fraction. The rest is left behind in the press pulp and the filter cake. This result, at an original
rapeseed oil content of 43%, produces 3.3 tons of rapeseed (from 1 hectare) and around 0.9
tons of oil – taking into account the dry substance level of the harvested seeds13 .
German experience shows that the quality of the oil produced in around half of these smallscale
decentralised oil presses does not meet the so-called ‘RK-Qualitätsstandard 05/2000’ (see Chapter 5) [Hassel, 2004], [Schuemann, 2003]. The
quality of the oil produced by these plants, if stored for a long time, is also clearly reduced
and, even if it initially met this standard, no longer does so if stored for any length of time
[Hassel, 2004].
Large-scale production
So-called oil-seed plants lightly press the seed, thus producing a pulp with a relatively high
oil content. The residual oil is then isolated from the pulp via extraction, whereby hexane is
often used. The remaining ‘scrap’ is then lightly ‘toasted’ to remove the remaining hexane,
and is then made into pellets.
The combination of pressing and extracting ensures that 98% of the oil is removed from the
seed. This return, at an original oil content of 43% in the rapeseed, produces around 1.2 tons
of oil from 3.3 tons of rapeseed (from 1 hectare), taking into account the dry substance level
of the harvested seeds14 .
Refining
Refining of the raw rapeseed oil is necessary via pressing/extracting, in order to bring the
fuel quality up to the required level. A somewhat simpler refining process is sufficient if the
raw oil is to be upgraded for use in the foodstuff industry. Certain substances can disturb
the balance when used in combustion engines and storing PPO, i.e. phosphor lipids and free
fatty acids [De Kock, 2004], see also Chapter 7. For foodstuff applications, additives such as
colourings and fragrances are not a problem when the oil is used in combustion engines, and
thus do not need to be removed.
Refining plant oil in order to produce PPO is not yet common practice. Large-scale PPO
production is not a fact, as yet. However, relatively simple refining is taking place at largescale
biodiesel plants, such as at Elbe-Öl Prignitz.
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