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Satish Lele
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A comparative study between chemical and enzymatic transesterification of oils with high free fatty acids

Only advantage of using Enzyme process is that you can use a Carbon Steel Equipments for Pretreatment Tank and BioDiesel Reactor. For Pretreatment Tank and BioDiesel Reactor, you have to use Stainless Steel for traditional Acid / Caustic process. This extra original investment cost is very small in total project cost. It will prove economic as operation costs are lower for traditional Acid / Caustic process.
The choice of catalyst for biodiesel production has attained significance as the demand of alternative fuels like biodiesel is growing rapidly in India. Most plants in India use chemically-catalysed transesterification (NaOH or KOH as catalyst). Experiments are being conducted from chemical-catalysed as well as enzyme-catalysed methods. Comparison using common influencing parameters such as oil/alcohol molar ratio, catalyst concentration and reaction duration is being studied. Requirement of certain solvents to enhance the reaction rate was tried in the enzyme-catalysed transesterification reaction. Biodiesel conversion of more than 96% was attained for chemically-catalysed transesterification (NaOH or KOH as catalyst), whereas the conversion rate was 85% for enzyme-catalysed method.
Further refinement in the enzyme-catalysed transesterification process is needed. The influencing parameters and absolute results of the analysis give the impression of superiority of enzymatic transesterification method for biodiesel production from high free fatty acid. However, the raw materials used in India, Palm Stearin and Animal Tallow, have very low free fatty acids and normal chemical-catalysed transesterification (NaOH or KOH as catalyst) is widely used.
Development of a suitable method for biodiesel production is a needy matter in the present scenario. Transesterification of non-edible oils with a suitable catalyst is the most promising method for biodiesel production. Presently studies are being conducted for comparison between chemical transesterification and enzymatic transesterification. Enzymatic transesterification is a comparatively novel method for production of biodiesel from high free fatty acid-contained oils, however it is tricky, expensive and needs careful handling. It is very good method if raw material is Used Cooking Oil.
Some enzymes have capability to catalyze transesterification of oils/fats (lipids), and are commonly known as lipases. Chemical-catalysed transesterification (NaOH or KOH as catalyst) reaction can not be used if acid value is above 2 mg KOH/gram of oil (FFA of 5%). An acid esterification pre-treatment is mandatory for high FFA-contained vegetable oils. If the FFA content is too high, one-step acid esterification with sulphuric acid (H2SO4) may not be sufficient which may lead to two- or three-stage pre-treatment process. The pre-treated oil will then undergo alkaline transesterification for biodiesel production. However, in India, high FFA oils (also called as Acid Oils, Fatty Acids) are used for manufacture of soaps. (Consumption in India, of such oil is 800,000 tons per year for manufacture of soaps)
Enzymes are biological catalysts which accelerate the rate of a chemical reaction without undergoing a permanent change in the structure. Lipase is an enzyme that catalyzes the breakdown or hydrolysis of fats. Since lipases are insensitive to FFA, the oil does not require a pre-treatment and both the esterification and transesterification take place simultaneously to produce biodiesel directly. In addition to this, moisture content in the raw oil does not degrade the reaction, in fact will enhance the reactivity of the enzyme. However, longer period of reaction and cost of enzymes restrict the wide acceptance of enzymatic transesterification for biodiesel production. The process parameters need to be optimized in enzymatic transesterification process to make it a viable method for biodiesel production.
Lipase immobilization technology provides a number of important benefits

  • Enzyme reuse
  • Easiness in separation of product from enzyme
  • Decrease in the inhibition rate.

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