From Wikipedia, the free encyclopedia

Biodiesel is the an alternative diesel fuel derived from natural, non-petroleum. Production involves the reaction of vegetable oils or animal fats with a short-chain aliphatic alcohol, typically methanol, or ethanol, to produce mono-alkyl esters. The most commonly-used reaction is transesterification, or alcoholysis, though new techniques are emerging. The application of this process in an industrial scale is what is meant by biodiesel production.

Contents 1 Steps in the process 1.1 Feedstock treatment 1.1.1 Debris removal 1.1.1.1 Degumming and phospholipid removal 1.1.2 Water removal 1.1.3 Free fatty acid treatment 1.2 Reaction 1.2.1 Batch process 1.2.2 Continuous process 1.2.2.1 Ultra- and high-shear in-line reactors 1.2.2.2 Ultrasonic reactors 1.3 Coproduct separation 1.3.1 Batch process 1.3.2 Continuous process 1.4 Purification 1.4.1 Water washing 1.4.2 "Dry" washing 1.4.3 Removal of alcohol and filtering 1.5 Reaction chemistry 1.5.1 Base-catalyzed Transesterification 1.5.1.1 Base-catalyzed transesterification mechanism 1.5.2 Supercritical transesterification 1.5.3 Acid-catalyzed esterification 1.6 See also 1.7 References 1.8 Further reading 1.8.1 External links

[edit] Steps in the process

The major steps required to synthesize biodiesel are as follows:

[edit] Feedstock treatment

[edit] Debris removal

If waste vegetable oil is used, it is filtered to remove dirt, charred food, and other non-oil material often found.

[edit] Degumming and phospholipid removal

If virgin oil is used,... sourced from plants, degumming and phospholipid removal must be done.^{[citation needed]}

[edit] Water removal

[edit] Free fatty acid treatment

A sample of the cleaned oil is titrated against a standard solution of base in order to determine the concentration of free fatty acids (RCOOH) present in the waste vegetable oil sample.

[edit] Reaction

[edit] Batch process

• Catalyst is dissolved in the alcohol using a standard agitator or mixer.

• The alcohol/catalyst mix is then charged into a closed reaction vessel and the biolipid (vegetable or animal oil or fat) is added. The system from here on is totally closed to the atmosphere to prevent the loss of alcohol.

The reaction mix is heated to speed up the reaction. Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters.

[edit] Continuous process

[edit] Ultra- and high-shear in-line reactors

Ultra- and High Shear in-line reactors allow to produce biodiesel continuously, therefore, reduces drastically production time and increases production volume. Ultra – Shear, up to three sets of rotor and stator which converts mechanical energy to high tip speed, high shear stress, high shear-frequencies. Droplet size range expected in the low micrometer until sub-micrometer range after one pass.

The reaction takes place in the high-energetic shear zone of the Ultra- and High Shear mixer by reducing the droplet size of the immiscible liquids such as oil or fats and methanol. Therefore, the smaller the droplet size the larger the surface area the faster the catalyst can react.

Ultra- and High Shear mixers are used for the pre-treatment of crude vegetable oil or animal fats such as:

• the dispersion of citric/phosphoric acid and crude oil within the de-gumming process to remove Phosphatides (Gums).
• the dispersion of caustic and de-gummed oil within the neutralization process to remove FFA (Free Fatty Acid)
• Furthermore, for the transesterification of pre-treated vegetable oil or animal fats into Methyl Ester.

Finally, for the water wash process of Methyl Ester. Water amount to be used in conjunction with high-shear is around 3%. Citric Acid amount is ~ 0.2% within the 1st water wash process.

[edit] Ultrasonic reactors

Using an ultrasonic reactor for biodiesel production drastically reduces the reaction time. Hence the process of transesterification can run in-line rather than using the time consuming batch processing. Industrial scale ultrasonic devices allow for the industrial scale processing of several thousand barrels per day. Cavitation has also been shown to have a similar effect.

[edit] Coproduct separation

The biodiesel reaction produces not only methyl esters, but also the coproduct, glycerol. This coproduct must be removed so that the biodiesel can be purified. Once regarded as a byproduct, new uses for glycerol are being found.

Glycerol separation is commonly done after the reaction is completed, though some production methods draw off byproduct in between reaction steps or while the reaction is occurring. Most separation techniques exploit the density differences of glycerol and methyl esters. Glycerol is more dense, settling into a distinct layer under the biodiesel. The glycerin by-product contains unused catalyst and soaps that are neutralized with an acid and sent to storage as crude glycerin (water and alcohol are removed later, chiefly using evaporation, to produce 80-88% pure glycerin).

[edit] Batch process

• The glycerin phase is much more dense than biodiesel phase and the two can be gravity separated with glycerin simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.

[edit] Continuous process

Mention centrifuge and coalescers.

[edit] Purification

Once the glycerin and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation. In other systems, the alcohol is removed and the mixture neutralized before the glycerin and esters have been separated. In either case, the alcohol is recovered using distillation equipment and is re-used. Care must be taken to ensure no water accumulates in the recovered alcohol stream.

[edit] Water washing

Once separated from the glycerin, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.

[edit] "Dry" washing

[edit] Removal of alcohol and filtering

If the residual methanol in impure biodiesel is completely removed (via distillation), the suspended impurities will naturally precipitate out of the solution. These impurities tend to coalesce and are easily removed out using conventional filtration.

[edit] Reaction chemistry

Animal and plant fats and oils, or lipids, are composed of triglycerides which are esters of fatty acids with the trihydric alcohol, glycerol. These lipids are reacted with short-chain alcohols, typically methanol or ethanol, to produce biodiesel.

[edit] Base-catalyzed Transesterification

In the transesterification process, the alcohol is deprotonated with a base to make it a stronger nucleophile. Shown below is an example of the transesterification reaction equation, shown in skeletal formulas:

Since biologically-sourced oils are used in this process, the alkyl groups of the triglyceride are not necessarily the same. Therefore, distinguishing these different alkyl groups, we have a more accurate depiction of the reaction:

R₁, R₂, R₃ : Alkyl group.

During the transesterification process, the triglyceride is reacted with alcohol in the presence of a catalyst, usually a strong alkaline (NaOH, KOH, or Alkoxides). The alcohol reacts with the fatty acids to form the mono-alkyl ester (or biodiesel) and crude glycerol. The reaction between the biolipid (fat or oil) and the alcohol is a reversible reaction so the alcohol must be added in excess to drive the reaction towards the right and ensure complete conversion.

Water is removed because its presence causes the triglycerides to hydrolyze to give salts of the fatty acids instead of undergoing transesterification to give biodiesel.

Normally, this reaction will proceed either exceedingly slowly or not at all. Heat, as well as an acid or base are used to help the reaction proceed more quickly. It is important to note that the acid or base are not consumed by the transesterification reaction, thus they are not reactants but catalysts.

Almost all biodiesel is produced using the base-catalyzed technique as it is the most economical process requiring only low temperatures and pressures and producing over 98% conversion yield (provided the starting oil is low in moisture and free fatty acids). For this reason only this process will be described below.

The following steps can be performed in a small, home-based biodiesel processor, or in large industrial facilities. The chemistry is similar in either case.

[edit] Base-catalyzed transesterification mechanism

This reaction is base-catalyzed. Any strong base will do, e.g. NaOH, KOH, Sodium methoxide, etc. Commonly the base (KOH,NaOH) is dissolved in the alcohol to make a convenient method of dispersing the otherwise solid catalyst into the oil. The ROH needs to be very dry. Any water in the process promotes the saponification reaction and inhibits the transesterification reaction.

A word on methoxide production: Claims that methoxide is produced by the reaction

KOH + ROH → RO^- + H₂O

are incorrect as the reaction constant is on the order of K_log -15. I.e. the reaction equilibrium is far to the left. While KOH and NaOH are strong bases, methoxide can only be produced by reacting e.g. sodium metal in alcohol, or by using sodium amide and an alkane. However, the following reaction mechanism using methoxide as an example are common in the literature as methoxide is an excellent base catalyst for this reaction.

Once the alcohol mixture is made, it is added to the triglyceride. The S_n2 reaction that follows replaces the alkyl group on the triglyceride in a series of reactions.

The carbon on the ester of the triglyceride has a slight positive charge, and the oxygens have a slight negative charge, most of which is located on the oxygen in the double bond. This charge is what attracts the RO^- to the reaction site

                        R1
   Polarized attraction |
RO-  ————————————————>  C=O 
                        |
                        O-CH2-CH-CH2-O-C=O
                              |        |
                              O-C=O    R3
                                |
                                R2

This yields a transition state that has a pair of electrons from the C=O bond now located on the oxygen that was in the C=O bond.

   R1
   |
RO-C-O- (pair of electrons)
   |
   O-CH2-CH-CH2-O-C=O
         |        |
         O-C=O    R3
           |
           R2

These electrons then fall back to the carbon and push off the glycol forming the ester.

   R1
   |
RO-C=O
 
+  
   -O-CH2-CH-CH2-O-C=O
          |        |
          O-C=O    R3
            |
            R2

Then two more RO groups react via this mechanism at the other two C=O groups. This type of reaction has several limiting factors. RO^- has to fit in the space where there is a slight positive charge on the C=O. So MeO- works well because it is small. As the R on RO- gets bigger, reaction rates decrease. This effect is called steric hindrance. This is why methanol and ethanol are typically used.

There are several competing reactions, so care must be taken to ensure the desired reaction pathway occurs. Most methods do this by using an excess of RO^-.

[edit] Supercritical transesterification

An alternative, catalyst-free method for transesterification uses supercritical methanol at high temperatures and pressures in a continuous process. In the supercritical state, the oil and methanol are in a single phase, and reaction occurs spontaneously and rapidly. ^[1] The process can tolerate water in the feedstock and free fatty acids are converted to methyl esters instead of soap, so a wide variety of feedstocks can be used. Further, purification is simplified as there is no need to remove residual catalyst. ^[2] High temperatures and pressures are required, but energy costs of production are similar or less than catalytic production routes. ^[3]

[edit] Acid-catalyzed esterification

Section dedicated to esterification production of Biodiesel

[edit] See also

• Biodiesel
• Dehulling
• Soap
• Transesterification
• Vegetable oil used as fuel
• Biomass to liquid

[edit] References

1. ^ Bunkyakiat, Kunchana; Et Al (2006). "Continuous Production of Biodiesel via Transesterification from Vegetable Oils in Supercritical Methanol". Energy and Fuels 20: 812-817. American Chemical Society. 
2. ^ Vera, C.R.; S.A. D'Ippolito, C.L. Pieck, J.M.Parera (2005-8-14). "Production of biodiesel by a two-step supercritical reaction process with adsorption refining". 2nd Mercosur Congress on Chemical Engineering, 4th Mercosur Congress on Process Systems Engineering. Retrieved on 2007-12-20. 
3. ^ Kusdiana, Dadan; Saka, Shiro. Biodiesel fuel for diesel fuel substitute prepared by a catalyst free supercritical methanol. Retrieved on 2007-12-20.

[edit] Further reading

• Gerpen, J.V., Biodiesel processing and production, Fuel Processing Technology, 2005, 86, 1097-1107.
• Ma, F. & Hanna, M.A., Biodiesel production: a review, Bioresource Technology, 1999, 70, 1-15.

[edit] External links

• Fast-Transesterification of Soybean Oil Using Ultrasonication
• Sonochemistry in Transesterification of Biodiesel
• Current State of Ultrasonic Processing for Fast Biodiesel Production
• www.nrel.gov NREL Report on Biodiesel Production Technology
• High Shear Mixing Reactor for Glycerolysis
• A Continuous Process for the Conversion of Vegetable Oils into Methyl Esters of Fatty Acids
• Biodiesel Production Technology - presentation