Jatropha oil, an alternative raw material for the production of biodiesel. Analysis of the pretreatment process.

As an alternative fuel, biodiesel must compete economically with conventional fuels. One way to reduce costs is to use less expensive feedstocks such as non-edible oils, animal fats, waste cooking oil and by-products of vegetable oil refining. Non-edible vegetable oils, mostly from seeds of trees and shrubs, can be a good alternative. Within them it is Jatropha oil.

Jatropha oil is toxic. Therefore, it has the advantage of not being edible and thus not competing with other uses like other oils that may be intended for human or animal consumption. However, it has the disadvantage of having a high content of free fatty acids (FFA). This requires a special pretreatment process to be used as a feedstock in the biodiesel production process.

The biodiesel production process consists of the following steps:

Graphic 1. Steps in the production process in the chemical industry. Biodiesel production process.

Starting with the oil, it is necessary to perform several pretreatment steps in order to obtain a raw material which fulfills the specifications for been used in the production of biodiesel. Graphic 2 shows the general steps of the pretreatment process of a crude oil in the biodiesel production process.

Graphic 1. Pretreatment process of an oil as raw material for the biodiesel production process.

Making an analysis of pretreatment steps in the particular case of Jatropha oil, we conclude the following:

• Degumming: Vegetable oils often contain gums such as phospholipids. These substances deactivate the transesterification catalyst, due to its content in phosphorus. The purpose of degumming is removing these gums.

In this case, the effect of the phospholipids in the esterification reaction has been investigated by adding them to the oil after the degumming or the refining step. Phospholipids were shown to show little influence on the final conversion of free fatty acids in the esterification. It can be concluded that in this particular case this step is not necessary.

• Neutralization. Due to the high FFA content (up to 40% by weight) in Jatropha oil, its elimination would not be advisable, since raw material is lost. The best option would be one of the proposals for raw materials with high content in AGL. The literature consulted showed that the most common method for reducing the FFA content is acid catalysis or esterification.

• Deodorization and bleaching: Because the goal of the process is to produce biodiesel, these operations will not be performed, since it is only recommended in the case of  oils for human consumption.

• Washing and drying: The two stages are necessary in all oils pretreatment, being of great importance drying due to the influence of the water on the kinetics of the transesterification reaction.

It is concluded that the pretreatment process for Jatropha oil case consists of the following phases:

Graphic 3. Steps of the pretreatment process for the Jatropha oil in the biodiesel production process

The esterification reaction

An AGL reversibly reacts with an alcohol in the presence of an acid catalyst to form ester and water. FFA by this reaction may be transformed into esters. The reaction product contains less amount of FFA and after separation of the water, can be used as an economical raw material for the transesterification.

Graphic 4. Esterification reaction using sulphuric acid as a catalyst.

Secondary reactions

FFA reactive with the basic catalysts of the transesterification  reaction (NaOH or KOH) to form soap (saponification reaction).

 Graphic 5. Saponification reaction of FFA in the presence of the transesterification catalyst.

Furthermore, the presence of moisture results in the hydrolysis of the triglycerides present in the oil, that should give AGL esters as a result and dilute the acid catalyst so that the reaction rate decreases. Being a reversible reaction, the presence of water, a reaction product, influences the conversion and the reaction rate.

Free fatty acids

The FFA content in Jatropha oil may exceed 40 % wt, above the values ​​for a desirable feedstock for transesterification, being recommended less than 0.5 % wt.

Alcohols

At this stage the most widely used alcohols are methanol and ethanol, being the first more widely used.

Catalysts

The esterification reaction does not occur in the absence of catalyst. The catalyst is acid and the catalysis may be:

 Homogeneous catalysis: with catalysts such as sulfuric, hydrochloric, sulphonic and phosphorus acids,  being the first the most used.

 Heterogeneous Catalysis: various solid acid catalysts have been studied such as D72 resin HM molecular sieve, SAPO-11, HZSM-5, H.beta and metatitanic acid (SO 4 2 / TiO 2), showing the latter a higher activity in the reaction .

Conditions of the esterification reaction

Doing a research, the usual conditions of the esterification reaction are shown in the following table:

 Table 1. Usual Conditions in the esterification reaction.

	Units	Range
METANOL PURITY	% wt.	98 - 99,8
CATALYST PURITY (H2SO4)	% wt.	98 - 99
PRESSURE	atm	1
TEMPERATURE	ºC	30-60
TIME	min	60-120
CATALYST CONCENTRATION	% wt.	1-10
METHANOL / OIL RATIO	v/v	0,16-12

Author: Silvia Fernández Castejón. Chemical Engineer. Complutense University of Madrid.

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