Some Basic Questions about Biodiesel Production

Transcription

1 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,

2 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

3 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 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

4 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

5 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

6 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

7 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

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

9 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

10 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

11 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

12 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 Kinetics of Transesterification of Soybean Oil. JAOCS 74(11):

13 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 hours at 60 C. This generally follows the old rule-ofthumb that reaction rates double for every 10 C rise in temperature. 13

14 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

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

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

17 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

18 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

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

20 Recent news 20

21 Single phase reaction There should be no mass transfer limitation. Measured reaction rates should be true chemical reaction rate. These kinetics have been measured: R. Doell, S.K. Konar, and D.G.B. Boocock, J. Am. Oil Chem. Soc., 2008, 85,

22 Single phase reaction rate Time to reach BG=0.24% T=20 C 13.5 min T=30 C 5.78 min T=40 C 2.59 min T=50 C 1.21 min T=60 C 0.59 min Van Gerpen, J. and B. He. Biodiesel Production and Properties, Chapter 15 in Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals,

23 Modeling results Concentration, mole/l TG DG MG The distribution of the main reactant, triglycerides, indicates a mixed reaction where both diffusion and chemical kinetics are important. 23

24 Modeling results Results are only preliminary! Important factors are not yet included. o We don t have good data for the solubility limits of mono- and diglycerides in methanol and oil. o Surface tension effects are not included. (Important!) 24

25 To speed up the reaction Need to break the methanol/catalyst droplets into smaller droplets to increase their surface area. 25

26 Droplet size Droplets in air have surface tension that acts like a flexible membrane that pulls the droplet into a sphere. Interfacial tension still draws droplets within another liquid into spheres. Liquid droplets within a liquid have the same force called interfacial tension that resists deformation. 26

27 Internal pressure The tension in the outer layer of the droplet from the interfacial tension causes an internal pressure in the droplet. 27

28 To break up droplets You need to have a high shear velocity field to distort the droplet against the pressure caused by the interfacial tension. 28

29 Effect of surfactants Surfactants collect at surfaces and reduce surface tension. Lower surface (and interface) tension makes it easier to produce smaller droplets. Schramm, L.L., Editor Emulsions. ACS 29

30 Surfactants Many biodieselrelated compounds are strongly surfaceactive. Especially monoglycerides. Kuhrt, N.H., E.A. Welch and F.J. Kovarik JAOCS 27:

31 Monoglycerides are natural products of transesterification. The reaction proceeds in three steps: TG + MeOH DG + methyl ester DG + MeOH MG + methyl ester MG + MeOH Glycerin + methyl ester The products of complete reaction are methyl esters and glycerin. The products of incomplete reaction include TG, DG, and MG. Monoglycerides, and to some extent, diglycerides, are surface active. 31

32 Effect of Stoichiometry on Monoglycerides (alcohol to oil ratio) For a single step reaction, at least 100% excess alcohol is required to get a complete reaction (based on Le Chatelier s principle). This corresponds to about 22% methanol, by weight. 32

33 Modeling can tell us Where the mono- and di-glycerides reside in the droplets or the continuous phase. How the methanol droplets get smaller as the reaction proceeds and then transition to glycerin droplets in the methyl ester continuous phase. Can we optimize the timing of agitation and settling to get the fastest overall process? Still a lot of work to do 33

34 Summary Biodiesel production is a deceptively simple process involving mass-transfer limited reactions and complex separations. There are many technology options for biodiesel production. The technology choice is a function of desired capacity, feedstock type and quality, alcohol recovery, and catalyst recovery. 34