Biodiesel Production - Reaction

Biodiesel is produced by chemically reacting a fat or oil with an alcohol, in the presence of a catalyst. The product of the reaction is a mixture of methyl esters, which are known as biodiesel, and glycerol, which is a high value co-product. The process is known as tranesterification.

Triglycerides	+	3 Methanol	+	Catalyst	<--->	3 Methyl Ester	+	Glycerol
1000 kg		107.5 kg				1004.5 kg		103 kg

Technology - Alkali-Catalyzed Transesterification

The Figure above shows a schematic diagram of the process involved in biodiesel production from feedstocks containing low levels of free fatty acids (FFA). This includes soybean oil, canola (rapeseed) oil, and the higher grades of waste cooking oils. Alcohol, catalyst, and oil are combined in a reactor and agitated for approximately one hour at 60 deg. C. Smaller plants often use batch reactors but larger plants (>4 million liters/yr) use continuous flow processes involcing continuous stirred-tank reactos (CSTR) and plug flow reactors (PFR).

Following the reaction, the glycerol is removed from the methly esters. Due to the low solubility of glycerol in the esters, this separation generally occurs quickly and may be accomplished with either a settling tank or a centrifuge. The excess methanol tends to act as a solubilizer and can slow the separation. However, this excess methanol is usually not removed from the reaction stream until after the glycerol and methyl esters are seprated due to concern about reversing the tranesterification reaction. Water may be added to the reaction mixture after the tranesterification is compete to improve the separation of glycerol. The esters are sent to the cleanup or purification process, which consists of water washing, distillation, cacuum drying, and filtration. The basis properties of methyl esters produced from different types of vegetable oils are shown below.

Source Oil	Density g/cc @ 15.5 C	Viscosity cSt @ 40 C	Cetane No.	Heating Value MJ/kg	Cloud Point C
Palm	0.880	5.7	62	37.8	+13
Soybean	0.884	4.08	46.2	39.8	+2
Sunflower	0.880	4.6	49	38.1	+1
Tallow	0.877	4.1	58	39.9	+12
Canola	0.880	4.4	49.6	40.1	-1

Different Technologies to Produce Biodiesel

Variable	Alkaline Catalysis	Lipase Catalysis	Supercritical Alcohol	Acid Catalysis
Reaction temperature (deg C)	60-70	30-40	239-385	55-80
Free fatty acid in raw materials	Saponified products	Methyl Esters	Esters	Esters
Water in raw materials	Interference with reacation	No influence		Interference with reaction
Yield of methyl esters	Normal	Higher	Good	Normal
Recovery of glycerol	Difficult	Easy		Difficult
Purification of methyl	Repeated washing	None		Repeated washing
Production cost of catalyst	Cheap	Relatively expensive	Medium	Cheap

Akali-catalyzed transesterification has some limitations among which are that it is sensitive to FFA content of the feedstock oils. A high FFA content (>1 %w/w) will lead to soap formation which redures catalyst efficiency, causes an increase in viscosity, leads to gel formation and makes the separation of glycerol difficult. Also, the oils used in transesterification should be substantially anhydrous (0.06 %w/w). The presence of water gives rise to hydrolysis of some of the produced ester, with consequent soap formation.

The main parameters affecting the base-catalyzed transesterification process are: alcohol formulation, alcohol/oil molar ratio, catalyst formulation and concentration, reaction temperature, reaction time, agitation, and the presence of moisture and FFA.

Fuel Quality

The primary criterion for biodiesel quality is adherence to the appropriate standard. In the united States and Canada, this standard is ASTM D 6751-02 "Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels". Generally, the fuel quality of biodiesel can be influenced by several factors:

- The quality of the feedstock.
- The fatty acid composition of the parent vegetable oil or animal fat.
- The production process and the other materials used in this process.
- Post-production parameters.

Production Process Factors

Completion of Reaction

The most important issue during biodiesel production is the completeness of the tranesterification reaction. The basic chemical process that occurs during the reaction is indicated by the following sequence of events:

Triglyceride --> Diglyceride --> Monoglyceride --> Glycerol

The triglycerides are converted to diglycerides, which in turn are converted to monoglycerides, and then to glycerol. Each step produces a molecule of a methyl ester of a fatty acid. If the reaction is incomplete, then there will be triglycerides, diglycerides, and monlglycerides left in the reaction mixture. Each of these compounds still contains a glycerol molecule that has not been released. The glycerol portion of these compounds is referred to as bound glycerol. When the bound glycerol is added to the free glycerol, the sum is known as the total glycerol. The ASTM specification requires that the total glycerol be less than 0.24% of the final biodiesel product as measured using a gas chromatographic method described in ASTM D 6584. Since the glycerol portion of the original oil is usually about 10.4%, this level of total glycerol corresponds to 97.7% reaction completion.

The total glycerol is calculated as the sum of bonded glycerol (consists of triglycerides, diglycerides and monoglyceris) and free glycerol.

TG = FG + (0.255 x Monos) + (0.146 x Dis) + (0.103 x Tris)

In EN standard, the maximum allowable for monoglycerides, diglycerides and triglycerides are 0.80 wt%, 0.20 wt% and 0.20 wt%, respectively.

Free Glycerol

Free glycerol refers to the amount of glycerol that is left in the finished biodiesel. Glycerol is essentially insoluble in biodiesel so almost all of the glycerol is easily removed by settling or centrifugation. Free glycerol may contain either as suspended droplets or as the very small amount that is dissolved in the biodiesel. Alcohols can act as cosolvents to increase the solubility of glycerol in the biodiesel. Most of this glycerol should be removed during the water washing process. Water-washed fuel is generally very low in free glycerol, especially if hot water is used for washing. Distilled biodiesel tends to have a greater problem with free glycerol due to glycerol carry-over during distillation. Fuel with excessive free glycerol will usually have a problem with glycerol settling out in storage tanks, creating a very mixture that can plug fuel filters and cause combustion problems in the engine.

Residual Alcohol and Residual Catalyst

Since methanol and the alkaline catalysts are more soluble in the polar glycerol phase, most will be removed when the glycerol is separated from the biodiesel. However, the biodiesel typically contains 2-4% methanol after the separation, which may constitute as much as 40% of the excess methanol from the reaction. Most processors will recover this methanol using a vacuum stripping process. Any methanol remaining after this stripping process should be removed by the water washing process. Therefore, the residual alcohol level in the biodiesel should be very low. The ASTM standard limits the amount of alcohol to a very low level (<0.1%).

Most of the residual catalyst is removed with the glycerol. Like the alcohol, remaining catalyst should be removed during the water washing. Although a value for residual catalyst is not included in the ASTM standard, it will be limited by the specification on sulfated ash. Excessive ash in the fuel can lead to engine deposits and high abrasive wear levels.

Glycerin Treatment

The glycerin can be upgraded from 50-60% purity towards 75-85% purity by means of soap splitting.

The first step involves neutralization using an acid to remove catalyst and soaps. The reaction of an acid with soap will give FFA and salt while its reaction with the base catalyst gives salt and water. Since the FFA's are insoluble in the glycerol they will rise to the top that they can be skimmed off. Some salts which are insoluble in the glycerol will also precipitate out.

The second step invovles remvoal fo methanol. The methanol stream in the glycerol can be removed with flash evaporation or using falling film evaporators. Falling film evaporators have an advantage of keeping the contact time short and are best suited because of the temperature susceptibility of glycerol which can result in its decomposition. After removal of methanol the purity of glycerol will be approximately 86%.

In the third step, glycerol can be further be purified to 99.5% using a combination of adsorption, vacuum distillation and ion exchange processes.