Making Biodiesel From Rubber Seed Oil
Rubber seed oil (RSO)-derived from Rubber tree (Hevea brasiliensis) seed cotyledons-is the most reasonable source of vegetable oil for large-scale production of Bio-diesel, if the primary concerns (where the need to avoid feeding our food to cars and engines is the most notorious) are to be eradicated once and for all.
Once upon a time, production of Bio-diesel was from any source where the most popular are our own sources of food. Ethanol and Bio-diesel fuel campaigners will not be wrong if they claim that we can pursue Bio-fuels without affecting our food supplies by increasing outputs of cultivated crops. But we were soon to face reality when the prices of food commodities began to increase. It was so tense that Bio-diesel or Ethanol are-in some quarters-associated with increase in price of food.
Today Bio-fuel campaign has taken a new dimension. Domestication of seed-bearing plants are being promoted. One of such plants is the once unpopular Jatropha Plant. Large scale investments, sensitization and extension work, are being used to encourage people to take to its cultivation. But it is not Jatropha seed we found more massive when placed on a scale of comparison with the rubber seed. This leads us to the advantages of rubber seed in the production of Bio-diesel.
Advantages of using rubber seed to produce Biodiesel
There are so many advantages associated with the use of rubber seed to produce Bio-diesel. A few of those advantages are as represented below.
1. Rubber seed is a rich source of essential fatty acids which should have made it a great dietary source of nutrients. But it is simply poisonous! Rubber seed contains cyanogenic glycoside which releases hydrogen cyanide in the presence of enzymes. This makes it a reserve for the production of non-edible products.
2. Rubber tree do not necessarily require cultivation of new plantations. Large plantations of Rubber trees meant for the production of rubber latex are rampant in Asia and Africa.
3. It is more profitable to farmers because they can now make extra profit from harvesting rubber seeds.
4. It will re-invogorate the economy of local communities which have abandoned their plantations due to low demand for rubber latex.
5. Rubber seed produces more oil, and exploring existing plantations around the world will go a long way in providing large percentage of world energy consumption.
Rubber trees grows spontaneously in the wild
Fatty acids in rubber seed oil
Rubber seed contains a minimum of 38.9% of oil. It is made up of 80.5% unsaturated fatty acid and 18.9% saturated fatty acid.
Unsaturated fatty acids are 18-C three Omega fatty acids comprising the two essential fatty acids:
Linoleic acid - 39.6% (Omega-6 fatty acid) Essential fatty acid
Oleic acid - 24.6% (Omega-9 fatty acid)
Linolenic acid - 16.3% (Omega-3 fatty acid) Essential fatty acid
While the saturated fatty acids are 16-C and 18-C fatty acids respectively:
Palmitic acid - 10.2%
Stearic acid - 8.7%
But rubber seed oil contains high percentage of free fatty acid (FFA) which is a major set-back in esterification.
Esterification of rubber seed oil
Bio-diesel production from normal oil requires esterification reaction of lipid(in this case Vegetable oil) with alcohol (usually ethanol or methanol) in the presence of alkaline catalyst.But the presence of high free fatty acids of up to 37.69% (according to studies carried out in three universities in Nigeria) can discourage the use of Rubber seed oil for Biodiesel.
Free fatty acids combines with alkaline catalyst (initiating saponification) to form soap which inhibits the separation of glycerol and esters. Hence, a remedy must be initiated to overcome this obstacle before the actual formation of the diesel. The remedy involves the introduction of the Fischer Esterification before alkaline-catalysed Trans-esterification.
Acid catalysed esterification of rubber seed oil
This is simply the purification of the rubber seed oil. It involves a reaction of the oil with alcohol in the presence of an acid to convert the free fatty acid contents to mono-alkyl esters (and water) thus reducing free fatty acid to less than 2%.
But acid-catalysed esterification has its own problem. The reaction of Alcohol R'-OH) with FFA (RCOOH) in the presence of an acid (H+) reversible and at equilibrium. This implies that, we may not be able to obtain the product of the reaction.
But 'Le Chatelier principle' shows that, if we increase one of the reactants (alcohol or oil) or remove one of the products (mono-ester or water) the equilibrium will shift to the right. Hence, it is essential that the amount of the alcohol used should be in excess. But, why don't we increase the amount of oil instead of the alcohol?
The amount of FFA (RCOOH) in the oil in the reaction must not be in excess because there must be enough alcohol to donate the alkyl group (R') to form the mono-esters(RCOOR'). Also, removal of reactant is not recommended because it is better to carry out separation of products at the end of the reaction.
Note: We must state that rubber seed oil is not the only type of vegetable oil with very high FFA content. Palm kernel oil (PKO) is also notorious.
Alkaline catalysed Trans-esterification of rubber seed oil
This is the actual process of formation of the Bio-diesel. Before starting the trans-esterification process, it is essential to take the following precautions:
1. Water must be completely separated from the product formed from acid esterification. Otherwise, water content may initiate saponification.
2. The alcohol must be dry.
3. The alkaline should be Conc. NaOH or KOH (Lye).
Bio-diesel is a mono-alkyl ester as such, the propan-1,2,3-triol must be separated from the three molecules of fatty acid in the triglyceride back-bone. The process involves dissolving the alkaline in the alcohol. The mixture is added to the oil (Triglyceride). And the Alkyl group of the alcohol (-R') substitutes the backbone to form mono-alkyl esters and glycerol. The stoichiometry is that one mole of triglyceride reacts with three moles of alcohol to form three moles of mono-alkyl ester (Biodiesel) and one molecule of glycerol.
The products must be separates to obtain pure Bio-diesel.
Note: Alkaline transesterification will be explained in details under Bio-diesel.
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