4. Conclusions
The conventional biodiesel refining downstream transesterification reactors, currently performed by decanting/washing or centrifugation/washing, can be conveniently replaced by a simple biodiesel purification method consisting of one single stage treatment with silica as adsorbent. The method seems a solution for biodiesel refining, saving water, money and process time.
With the proposed method it is possible to achieve a biodiesel composition complying the regulations on the analyzed impurities without a preliminary step to remove residual methanol, as it is the case with other adsorbents, and loading the adsorbent without a previous drying treatment. Optimum operating conditions are 0.2 bar, 90 C, contact times longer than 45 min and 1.1% charge of adsorbent. Working under these conditions makes it possible to totally eliminate the methanol and water from the medium without affecting the adsorption of other impurities. Water elimination does not affect the adsorption of polar impurities.
Silica Trisyl 3000 retains 23% of its weight when the process is performed at atmospheric pressure and nearly 235% when performed under vacuum conditions. This greatly improves the utilization of the adsorbent and reduces the cost of the process.
At the optimal process conditions silica behaves as a nonselective adsorbent with a high capacity of adsorption of the different types of impurities, which makes it an excellent adsorbent to purify biodiesel. The adsorption phenomenon is not limited to the formation of a theoretical ‘‘monolayer’’ of adsorbed impurities.
The phenomenon is more complex and includes the formation of multiple layers.