Some Basic Questions about Biodiesel Production

Some Basic Questions about Biodiesel Production Jon Van Gerpen Department of Biological and Agricultural Engineering University of Idaho 2012 Collective Biofuels Conference Temecula, CA August 17-19, 2012 1

Views of Biodiesel Production How do we usually look at biodiesel production? Can we learn something from looking at it in a new way? o From a microscopic perspective. 2

How do we usually talk about biodiesel production? We always start with the chemical equation. O O CH 2 - O - C - R 1 CH 3 - O - C - R 1 O O CH 2 - OH CH - O - C - R 2 + 3 CH 3 OH -----------> CH 3 - O - C - R 2 + CH - OH (KOH) O O CH 2 - OH CH 2 - O - C - R 3 CH 3 - O - C - R 3 Triglyceride methanol mixture of fatty esters glycerin Transesterification 3

Biodiesel Production 3 basic steps Oil Methanol Catalyst Reactor Settling tank Clean up Sometimes we recognize that there are other steps in the process. Glycerin Finished biodiesel 4

Oil Biodiesel Production Schematic Methanol Catalyst Finished biodiesel Dryer Reactor Separator Methyl esters Neutralization and methanol removal Water washing Free fatty acids Acid Crude glycerin (85%) Glycerin (50%) Acidulation and separation Methanol removal Wet methanol Wet methanol Methanol storage Acid Water Methanol/water rectification Gray water Sometimes we want to look at Water individual processes. 5

Microscopic perspective To look at biodiesel production in a different way, let s look at what is happening on a small scale. Questions we should ask: o What affects the speed of reaction and why? o How can we guide the reaction to give higher yield and purity? 6

During the Reaction Methanol solubility in triglyceride Most biodiesel producers operate at atmospheric pressure close to the boiling point of methanol (65 C). Solubility is 10% but we add about 20%. 7

Binary phase diagram Concentration of oil in methanol phase Concentration of methanol in oil phase Cerce, T., S. Peter and E. Weidner. 2005. 8

Initial Reaction Conditions Oil Methanol + Catalyst Oil Due to its low solubility, 50-70% of the methanol is distributed throughout the oil as small droplets. The catalyst tends to be more attracted to the methanol. Methanol + Catalyst Oil Methanol + Catalyst Methanol in the oil phase is about 8-10%. Oil in the methanol droplets is <1%. 9

Later stages of the reaction Glycerol + catalyst Methyl esters The glycerol has low solubility in the oil and methyl esters. As more glycerol is produced during the reaction, it will attract the catalyst and form small droplets. Methyl esters Glycerol + catalyst Methyl esters Glycerol + catalyst 10

We have always referred to biodiesel production as a mass transfer limited reaction. But is this right? Oil Methanol + catalyst Methyl ester Reaction is limited by the rate at which oil can get to reaction zone in the methanol/catalyst droplets and the rate at which the methyl esters can get away. 11

Faster stirring If we stir faster, the reaction will occur faster. This is what we would expect of a masstransfer limited reaction. Noureddini, H. and D. Zhu. 1997. Kinetics of Transesterification of Soybean Oil. JAOCS 74(11):1457-1463. 12

Reaction time Transesterification reaction will proceed at ambient (25 C) temperatures but needs 4-8 hours to reach completion. Reaction time can be shortened to 2-4 hours at 40 C and 0.5-1 hours at 60 C. This generally follows the old rule-ofthumb that reaction rates double for every 10 C rise in temperature. 13

Higher temperature Higher temperature makes the reaction go faster. This makes it seem as if the reaction rate is limiting. Noureddini, H. and D. Zhu. 1997.

Balance between the relative significance of diffusion and reaction These generic curves show the concentration of oil as the primary reactant. CA/CAR 1.2 1 0.8 0.6 0.4 0.2 Reaction is slow and occurs throughout the droplet. Beta=1 Beta=3 Beta=5 Beta=10 0 0 0.2 0.4 0.6 0.8 1 r/r Reaction consumes oil as fast as it enters droplet. 15

Reaction zone 1.2 Reaction zone for slow reaction/fast diffusion. Entire droplet is reaction zone. 1 CA/CAR 0.8 0.6 0.4 0.2 Beta=1 Beta=3 Beta=5 Beta=10 0 0 0.2 0.4 0.6 0.8 1 r/r Reaction zone for fast reaction/slow diffusion. Outside surface of droplet only. Mass transfer limited! 16

Which case is appropriate for biodiesel transesterification? droplet The only way we can sort this out is with modeling the diffusion and reaction processes. Diffusion flux of species in and out of droplet. Internal diffusion flux within the droplet. + chemical reaction within the droplet depending on catalyst Need this piece! 17

Where can we get the chemical reaction rates? Boocock David Boocock, a chemical engineering professor at the University of Toronto, recognized the limitations of the twophase reaction. He proposed, and patented, the concept of adding a co-solvent, which makes the reaction mixture a single phase. Boocock, D.G.B, Single phase process for production of fatty acid methyl esters from mixtures of triglycerides and fatty acids, US Pat#6,642,399 18

Co-solvents Patented process now marketed by Biox corp. Cosolvent makes methanol, oil, methyl ester, and glycerol soluble. Complete reaction in minutes.