Development of prototype sedimentation & filtration systems for Jatropha oil (on-going activity, self-funded)

To enable a complete production chain of Jatropha oil in Mali, from plantation in a village to engine grade fuel.

MFC has developed locally made sedimentation systems for purification of Jatropha oil, and has experimented with filtration systems as well.


Vertical bag filter suitable for filtration of Jatropha oil, as demonstrated at MFC's stand at National Energy Week, December 2004.

Purification of plant oil (vegetable oil)

After cold pressing, Pure PLant Oil contains solids in the range of 1 - 13 percent by weight, depending on the kind of seed and the press conditions. Sedimentation, filtration or centrifugation can dobe performed to purify the raw oil. Also a combination of sedimentation followed by filtration or centrifugation is possible. It is vital to remove from all particles > 5 µm from the oil size when it is for use in modern engines, or for other technical purposes. It is not advisable to wait longer than 3 months between pressing and cleaning the oil, otherwise there can be filtration problems due to oxidisation of the oil. Below the basic principles of sedimentation & filtration are explained.


Sedimentation

Sedimentation is the simplest and cheapest way of cleaning by using the gravity of the earth. The density difference between the oil and the solid particles is used to separate the fluid/ solid mix. Cleaning by sedimentation is recommended only by small processing capacities up to maximal daily capacities of 1 t seed/d and 200-300l oil/d. For the reasons of space and time, it is not recommendable to have a sedimentation system for higher processing capacities. The disadvantage of sedimentation is a high oil loss in the sediment. Filtration and centrifugation are better solutions in order to get more yield of oil after the purification process.

A typical sedimentation system works by the overflow principal. In each tank (sedimentation step) the oil should stay 2 - 4 days at an ambient temperature of at least 20°C. Therefore it is necessary to have a single tank volume of several daily oil outputs. The sedimentation time depends on temperature because of the rapid increase in viscosity with falling temperature. Lower temperature means a longer sedimentation time. The pipe connection diameter between the tanks should be at least 20 to 30 mm to avoid high flow rates to keep possibilities for turbulence low.

A solution for a continuously working sedimentation system is the following. The sediment in the bottom of the tank can be led to a separating hopper. From the tank, a sludge pump can evacuate it. For this pumping it is recommended to have a valve between tank and hopper to avoid evacuating oil. The sediment can then be filtered to keep losses of oil low. It is also a good feedstock with an even higher content of oil than press cake.

With optimal conditions, sedimentation removes all particles > 8 µm from the raw oil. That is why the plant oil must cleaned by a security filter (e.g. bag filter or fine filter) after sedimentation when it is destined for use as fuel in a motor.


Filtration

The raw oil is forced through a membrane in the filtration process and the filter media blocks impurities. The membrane materials generally consist of fabric, paper and metal. The membrane pore diameter is chosen to be a little bit bigger for plate, cricket and leaf filters, to avoid fast blocking of the filter. That is why in the first filtering step, bigger particles pass the filter until the solids build up an extra membrane. This membrane of particles provides a finer filtration. So some filter units operate for a short time in a circle before the filtering process starts.

The temperature of the plant oil is very important. A certain amount glycerine will come out of the oil at low temperatures (<10°C), and block the filter fast. Also the flow of the oil through the filter decreases at low oil temperatures. It is recommended to filter plant oil with a temperature of at least 20 - 30 °C. But it should not overheated up to 60 °C or more because this causes higher oxidation and shorter storage times for the purified oil.

The filter membrane is made from thick woven materials such as polypropylene (PP), cotton, polyester (PE) or paper in Plate or Chamber filter. Paper can also be lying in addition to the membrane to make the cleaning easier. The dirt holding capacity is important, as for all filter types. A high dirt or sludge volume allows a longer operating time before the filter should be cleaned or even the filter media changed.

Plate filters are flexible and can be extended by adding more frames for bigger capacities. There are widely applied in the food industry (e.g. beer, wine production) The filter-cake discharging can be manual or automatic. For a manual filter cake discharging, around a half-hour daily work must be planned. The filter-cake has oil content of 35 to 50 % in such plate filters. You get 2-4 kg filter-cake from processing 100 kg rape seed.

Bag filters are constructed of non-woven materials such as polypropylene in the shape of a bag. Raw oil is placed in the bag and filtered through the bottom by gravity or a pump. The bag fills with filtered particles and is discharged.

Text by Erik Ferchau, adapted from the webpage of the Danish Folkecenter for Renewable Energy. For more information on Pure Plant Oil as a diesel substitute, please see www.folkecenter.dk