Fat is a generic term for lipids, a class of compounds in biochemistry. You would know them as greasy, solid materials found in animal tissues and in some plants – oils that are solids at room temperature.

Vegetable oil is the fat extracted from plant sources. We may be able to extract oil from other parts of a plant, but seeds are the main source of vegetable oil. Typically, vegetable oils are used in cooking and for industrial uses. Compared to water, oils and fats have a much higher boiling point. However, there are some plant oils that are not good for human consumption, as the oils from these types of seeds would require additional processing to remove unpleasant flavors or even toxic chemicals. These include rapeseed and cottonseed oil.

Animal fats come from different animals. Tallow is beef fat and lard is pork fat. There is also chicken fat, blubber (from whales), cod liver oil, and ghee (which is a butterfat). Animal fats tend to have more free fatty acids than vegetable oils do.

Chemically, fats and oils are also called “triglycerides.” They are esters of glycerol, with a varying blend of fatty acids. The figure below shows a generic diagram of the structure without using chemical formulas.

A generic diagram of oils and fats.

Credit: BEEMS Module B4

So what is glycerol? It is also known as glycerin/glycerine. Other names for glycerol include 1,2,3-propane-triol, 1,2,3-tri-hydroxy-propane, glyceritol, and glycyl alcohol. It is a colorless, odorless, hygroscopic (i.e., will attract water), and sweet-tasting viscous liquid. The following figure shows the chemical structure in two different forms.

Chemical structure of glycerol.

Credit: BEEMS Module B4

So now we need to define what the fatty acids are. Essentially, fatty acids are long-chain hydrocarbons with a carboxylic acid. The following figure shows the generic chemical structure of a fatty acid with the carboxylic acid on it.

Generic carboxylic acid chemical structure.

Click for a text description of the image above.

This image provides a visual and textual explanation of the carboxylic acid functional group, commonly found in organic molecules such as fatty acids. It features two equivalent representations of the same chemical structure:

1. R–C=O with an –OH group attached to the carbon, which is the expanded structural form.
2. RCOOH, the condensed molecular formula.

In both cases, "R" denotes a long hydrocarbon chain, which can vary in structure. The image notes that this chain may be saturated, meaning it contains only single bonds between carbon atoms, or unsaturated, meaning it includes one or more double bonds.

The COOH group is identified as the acidic functional group—a defining feature of carboxylic acids. The image emphasizes that this group can be written in either the expanded or condensed form, both of which are chemically equivalent.

Credit: BEEMS Module B4

The figure below shows different fatty acid chemical structures. The chemical structures are shown as line chemical structures, where each point on the links is a carbon atom and the correct number of hydrogen atoms is dependent on whether there is a single or double bond. Fatty acids can be saturated (with hydrogen bonds) or unsaturated (with some double bonds between carbon atoms). Because of the metabolism of oilseed crops, naturally formed fatty acids contain even numbers of carbon atoms. In organic chemistry, carbon atoms have four pairs of electrons available to share with another carbon, hydrogen, or oxygen atom. Free fatty acids are not bound to glycerol or other molecules. They can be formed from the breakdown or hydrolysis of a triglyceride.

Other long-chain acids, such as steric, palmitic, oleic, and linoleic acids.

Credit: BEEMS Module B4

The fatty acids shown have slightly different properties. Palmitic acid is found in palm oil. The figure below shows the relationship of each fatty acid to its size and saturation. Palmitic and steric acids are saturated fatty acids, while oleic and linoleic acids are unsaturated with different amounts of double bonds. The figure below shows differing amounts of carbon atoms compared to the number of double bonds in the compound.

A series of fatty acids. The ratio represents the carbon atoms: double bonds in the compound.

Credit: BEEMS Module B4

The figure below shows the part of the triglyceride that is a fatty acid and the part that is glycerol, including chemical structures this time. The chemical structure shown here is a saturated triglyceride.

Chemical structure of triglyceride, pointing out the fatty acid parts and the glycerol part.

Credit: BEEMS Module B4

So, we’ve discussed what fats and oils are. Now, what is biodiesel? What is at least one definition? It is a diesel fuel that was generated from biomass. However, there are different types of biodiesel. The most commonly known type of biodiesel is a fuel comprised of mono-alkyl esters (typically methyl or ethyl esters) of long-chain fatty acids derived from vegetable oils or animal fats – this is according to ASTM D6551. An ASTM is a document that contains the standards for particular types of chemicals, particularly industrial materials. This is a wordy definition that doesn’t really show us what it is chemically.

So when we talk about an alkyl group, it is a univalent radical containing only carbon and hydrogen atoms in a hydrocarbon chain, with a general atomic formula of C_nH_2n+1. Examples include:

Another term we need to know about is an ester. Esters are organic compounds where an alkyl group replaces a hydrogen atom in a carboxylic acid. For example, if the acid is acetic acid and the alkyl group is the methyl group, the resulting ester is called methyl acetate. The reaction of acetic acid with methanol will form methyl acetate and water; the reaction is shown below. An ester formed in this method is a condensation reaction; it is also known as esterification. These esters are also called carboxylate esters.

Reaction of acetic acid with methanol to form methyl acetate and water.

Click for a text description of the image above.

This image illustrates a chemical esterification reaction, both in terms of structural formulas and molecular formulas, showing the formation of an ester from a carboxylic acid and an alcohol.

At the top, the structural formulas depict the reaction between acetic acid (CH₃COOH) and methanol (CH₃OH). The acetic acid molecule features a carboxyl group (–COOH), while methanol contains a hydroxyl group (–OH). These two reactants undergo a condensation reaction, where a molecule of water (H₂O) is eliminated, and a new bond forms between the acid and alcohol.

The product of this reaction is methyl acetate (CH₃COOCH₃), an ester, along with water. The reaction is shown as reversible, indicated by the double arrows (⇌), which is characteristic of esterification reactions under equilibrium conditions.

Below the structural representation, the molecular formulas summarize the same reaction:

${\mathrm{CH} }_{3 }\mathrm{COOH} + {\mathrm{CH} }_{3 }\mathrm{OH} \rightleftharpoons {\mathrm{CH} }_{3 }{\mathrm{COOCH} }_{3 }+ {\mathrm{H} }_{2 }\mathrm{O}$

Each compound is labeled:

• Acetic acid
• Methanol
• Methyl acetate
• Water

Credit: BEEMS Module B4

This is the basic reaction that helps to form biodiesel. The following figure shows the different parts of the chemical structure of the biodiesel, the methyl ester fatty acid, or fatty acid methyl ester (FAME).

The chemical structure of the typical biodiesel, methyl ester fatty acid, or FAME.

Credit: BEEMS Module B4

So, at this point, let’s make sure we know what we have been discussing. Biodiesel is a methyl (or ethyl) ester of a fatty acid. It is made from vegetable oil, but it is not vegetable oil. If we have 100% biodiesel, it is known as B100 – it is a vegetable oil that has been transesterified to make biodiesel. It must meet ASTM biodiesel standards to qualify for warranties sold as biodiesel and qualify for any tax credits. Most often, it is blended with petroleum-based diesel. If it is B2, it has 2% biodiesel and 98% petroleum-based diesel. Other blends include B5 (5% biodiesel), B20 (20% biodiesel), and B100 (100% biodiesel). We’ll discuss why blends are used in the following section. And to be clear: sometimes vegetable oil is used in diesel engines, but it can cause performance problems and deteriorate engines over time. Sometimes, vegetable oil and alcohol are mixed together in emulsions, but that it is still not biodiesel, as it has different properties from biodiesel.

So, if straight vegetable oil (SVO) will run in a diesel engine, why not use it? Vegetable oil is significantly more viscous (gooey is a non-technical term) and has poorer combustion properties. It can cause carbon deposits, poor lubrication within the engine, and engine wear, and it has cold-starting problems. Vegetable oils have natural gums that can cause plugging in filters and fuel injectors. For a diesel engine, the injection timing is thrown off and can cause engine knocking. There are ways to mitigate these issues, which include: 1) blending with petroleum-based diesel (usually < 20%), 2) preheating the oil, 3) making microemulsions with alcohols, 4) “cracking” the vegetable oil, and 5) using the method of converting SVO into biodiesel using transesterification. Other methods are used as well, but for now, we’ll focus on biodiesel from transesterification. The table below shows three properties of No. 2 diesel, biodiesel, and vegetable oil. As you can see, the main change is in the viscosity. No. 2 Diesel and biodiesel have similar viscosities, but vegetable oils have much higher viscosity and can cause major problems in cold weather. This is the main reason for converting the SVO into biodiesel.

So, how do we make biodiesel?

The method being described here is for making FAMEs biodiesel. The reaction is called transesterification, and the process takes place in four steps. The first step is to mix the alcohol for reaction with the catalyst, typically a strong base such as NaOH or KOH. The alcohol/catalyst is then reacted with the fatty acid so that the transesterification reaction takes place. The first figure below shows the preparation of the catalyst with the alcohol, and the second figure shows the transesterification reaction.

The catalyst is prepared by mixing methanol and a strong base such as sodium hydroxide or potassium hydroxide. During the preparation, the NaOH breaks into ions of Na+ and OH-. The OH- abstracts the hydrogen from methanol to form water and leaves the CH₃O- available for reaction. Methanol should be as dry as possible. When the OH- ion reacts with H+ ion, it reacts to form water. Water will increase the possibility of a side reaction with free fatty acids (fatty acids that are not triglycerides) to form soap, an unwanted reaction. Enzymatic processes can also be used (called lipases); alcohol is still needed and only replaces the catalyst. Lipases are slower than chemical catalysts, are high in cost, and produce low yields.

Once the catalyst is prepared, the triglyceride will react with 3 mols of methanol, so excess methanol has to be used in the reaction to ensure a complete reaction. The three attached carbons with hydrogen react with OH- ions and form glycerin, while the CH₃ group reacts with the free fatty acid to form the fatty acid methyl ester.

The figure below is a graphic of the necessary amounts of chemicals needed to make the reaction happen and the overall yield of biodiesel and glycerin. The amount of methanol added is almost double the required amount so the reaction goes to completion. With 100 lbs of fat and 16-20 lbs of alcohol (and 1 lb of catalyst), the reaction will produce 100 lbs of biodiesel and 10 lbs of glycerin. The reaction typically takes place at between 40-65°C. As the reaction temperature goes higher, the rate of reaction will increase, typically 1-2 hours at 60 °C versus 2-4 hours at 40°C. If the reaction is higher than 65°C, a pressure vessel is required because methanol will boil at 65°C. It also helps to increase the methanol-to-oil ratio. Doubling the ratio of 3 mols of alcohol to 6 mols will push the reaction to completion faster and more completely.

The following video shows a time-lapsed reaction of transesterification of vegetable oil into biodiesel. It also incorporates the steps after the reaction to separate out the biodiesel (9:44).

The figure below shows a schematic of the process for making biodiesel. Glycerol is formed and has to be separated from the biodiesel. Both glycerol and biodiesel need to have alcohol removed and recycled in the process. Water is added to both the biodiesel and glycerol to remove unwanted side products, particularly glycerol, that may remain in the biodiesel. The wash water is separated out similar to solvent extraction (it contains some glycerol), and the trace water is evaporated out of the biodiesel. Acid is added to the glycerol in order to provide neutralized glycerol.

As briefly discussed, the initial reactants used in the process should be as dry as possible. Water can react with the triglyceride to make free fatty acids and a diglyceride. It can also dissociate the sodium or potassium from the hydroxide, and the ions Na+ and K+ can react with the free fatty acid to form soap. The figure below shows how water can help to form a free fatty acid, and that free fatty acid can react with the Na+ ion to form soap. The sodium that was being used for a catalyst is now bound with the fatty acid and unusable. It also complicates separation and recovery. All oils may naturally contain free fatty acids. The refined vegetable oil contains less than 1%, while crude vegetable oil has 3%, waste oil has 5%, and animal fat has 20%. Animal fats are a less desirable feedstock.

Some of the processes used in making biodiesel are different from what we’ve discussed. The first of these processes we’ll discuss is solvent extraction.

In the process of making biodiesel through transesterification, we noted that biodiesel and glycerol are the products, with some water formation and unwanted potential soap formation. So, the products are liquid, but they are also immiscible (do not dissolve in each other) and have differences in specific gravity. The specific gravity of the products is shown in the table below.

In batch processing, gravity separation is used, and the products remain in the reactor; the reactor then becomes a settler or decanter. Once the reaction is finished, the product mixture then sits without agitation. After 4-8 hours, the glycerol layer settles at the bottom (because it has higher gravity) and the biodiesel settles at the top. However, if a continuous flow facility is utilized, the products separate too slowly in a settler, so a centrifuge is used. A centrifuge will spin the liquids at a very high speed, which helps to promote density separation. The figure below shows a few different types of industrial centrifuges that can be used for biodiesel separation.

One of the issues that can happen during separation is the forming of a layer containing water and soap, in between the glycerol and biodiesel. That will hinder the separation. Another issue is that glycerol contains 90% of the catalyst and 70% of the excess methanol. In other words, the glycerol fraction is kind of the “trashcan” layer of the process. The biodiesel layer also contains some contaminants, including soap, residual methanol, free glycerol, and residual catalyst. The catalyst in biodiesel is extremely problematic if introduced into fuel systems. One way to improve the separation is through water washing with hot water, as the contaminants are soluble in water, but the biodiesel is not. Water washing will remove contaminants such as soap, residual methanol, free glycerol, and catalysts. The water should be softened (had ions removed) and be hot (both the biodiesel and water should be at 60°C). Thorough mixing with the wash water is needed so that all the contaminants can be removed, but the mixing intensity should also be controlled so that emulsions do not form between the biodiesel and water. Sometimes acid is added in the wash process to separate out the soaps. However, the last portion of washing needs to be acid-free, so a step may need to be added to neutralize the glycerol.

There is more than one way to implement the washing process. For batch processes, two of the methods are: a) top spray and b) air bubbling (see figure below). For the top spray, a fine mist of water is sprayed top-down in a fine mist. The water droplets contact the biodiesel as the water flows down, separating out the impurities. Air bubbling is a method that uses air as a mobile phase. Air bubbles through a layer of water and carries water with it on the way up. As the air bubbles burst on the way up, water droplets are released and drop down on the biodiesel at the bottom, contacting the biodiesel and washing out impurities. It can be a relatively slow process; a combination of the two is also possible.

For continuous-flow processes, different equipment is used, which typically incorporates some sort of counter-current flow process. The lighter biodiesel is introduced at the bottom and the heavier water is introduced at the top, and as they flow the fluids contact each other so that the biodiesel at the top has impurities removed and the water flowing down out the bottom contains the contaminants. The figure below shows two types of counter-current units: a) counter-flow washing system and b) rotating disc extractor. Both units contain materials to increase the interaction between water and biodiesel. For the counter-flow system, packing increases the interaction, while for the rotating disk extractor, disks rotate around as the fluid flows through. These types of equipment are typically used on an industrial scale and need precise mechanical design and process control; these units cost much more than the other type of system.

The most problematic step in biodiesel production, however, is water washing. It requires heated, softened water, some method of wastewater treatment, and water/methanol separation. Methanol recovery from water is somewhat costly using methanol-water rectification. Water can also be removed by vacuum drying. One of the alternative methods for removing water is the use of absorbent materials such as magnesium silicate. One company that provides a process for doing this is Magnesol, which is produced by the Dallas Group. Once the magnesium silicate removes the water, it can be regenerated by heating it up and evaporating the water. Methanol must also be removed from the biodiesel; one method for doing this is flash vaporization of methanol.

So, which type of process should be used? Should it be a batch or continuous flow system? Smaller plants are typically batch (< 1 million gallons/yr). They do not require continuous operation 24 hours per day 7 days a week. The batch system provides better flexibility and the process can be tuned based on particular feedstocks. However, in a commercial, industrial setting, most likely a continuous flow system will be used because of increased production and high-volume separation systems, which will increase the throughput. There are automation and process controls, but this also means higher capital costs and the use of trained personnel. It is feasible to have hybrid systems as well.

The primary byproduct is glycerin (aka glycerine, glycerol). It is a polyhydric alcohol, which is sometimes called a triol. It is a colorless and odorless liquid, which is viscous (thick-flowing) and sweet-tasting. It is non-toxic and water-soluble. Parameters to test quality are purity, color, and odor. Glycerol properties and chemical information are shown in the table below.

There are several different applications that glycerol can be used for, including the manufacture of drugs, oral care, personal care, tobacco, and polymers. Medical and pharmaceutical preparations use glycerol as a means to improve smoothness, lubrication, and moisturize – it is used in cough syrups, expectorants, laxatives, and elixirs. It can also be substituted for alcohol, as a solvent that will create a therapeutic herbal extraction.

Glycerol can be used in many personal care items; it serves as an emollient, moisturizer, solvent, and lubricant – it is used in toothpaste, mouthwashes, skin care products, shaving cream, hair care products, and soaps. Glycerol competes with sorbitol as an additive; glycerol has a better taste and a higher solubility.

Since it can be used in medical and personal care products, glycerol can also be used in foods and beverages. It can be used as a solvent, moisturizer, and sweetener. It can be used as a solvent for flavors (vanilla) and food coloring. It is a softening agent for candy and cakes. It can be used as part of the casings for meats and cheeses. It is also used in the manufacture of shortening and margarine, filler for low-fat food, and thickening agents in liqueurs.

Glycerol is also used to make a variety of polymers, particularly polyether polyols. Polymers include flexible foams and rigid foams, alkyl resins (plastics) and cellophane, surface coatings, and paints, and as a softener and plasticizer.

Unfortunately, there is already enough glycerol produced for the glycerol market. Glycerol consumption in traditional uses is 450 million lb/yr, and traditional capacity is 557 million lb/yr. If we produce glycerol from making biodiesel, it has the potential to produce 1900 million lb/yr. Therefore, we need to find a new market for glycerol, or it will be wasted in some fashion.

There is research being done to find new uses for glycerol. This includes use in additional polymers as an intermediate, conversion to propylene glycol for antifreeze, production of hydrogen via gasification, as a boiler fuel (have to remove alkali), in an anaerobic digester supplement, and for algal fermentation to produce Omega-3 polyunsaturated fatty acids.

To ensure quality biodiesel, there are standards for testing the fuel properly to see that it meets specifications for use. ASTM (an international standards and testing group) has a method to legally define biodiesel for use in diesel engines, labeled ASTM D6751. The table below shows the test methods necessary for all the expected standards for biodiesel.

There are advantages and disadvantages to using biodiesel compared to ultra-low sulfur diesel. It has a higher lubricity, low sulfur content, and low CO and hydrocarbon emissions. This makes it good to blend with diesel from petroleum to be able to achieve the required specifications for ultra-low sulfur diesel because ultra-low sulfur diesel has poor lubricity. But as discussed previously, biodiesel has poor cold weather properties. It really depends on the location; for instance, if using biodiesel in the upper Midwest, there could be problems in the winter.

As with all materials, the production and quality of biodiesel is important. Most importantly, the transesterification reaction should reach completion for the highest production and quality. Due to the nature of the transesterification of triglycerides, a small amount of tri-, di-, and mono-glycerides remain. The figure below shows the changes in these compounds as the glycerides react to form biodiesel. Some terminology to be aware of: 1) bound glycerol is glycerol that has not been completely separated from the glyceride and is the sum of tri-, di-, and mono-glycerides and 2) total glycerol combines the bound glycerol with the free glycerol.

Glycerol content in biodiesel must be as low as possible, as ASTM standards state. The biodiesel will not technically be “biodiesel” unless ASTM standards are met, which means it is below the total glycerol specifications. High glycerol content can cause issues with high viscosity and may contribute to deposit formation and filter plugging. Crude glycerol is often a dark brown color and must be refined and purified before use elsewhere. In biodiesel preparation, brown layers will form, and, possibly, white flakes or sediments, formed from saturated mono-glycerides, will fall to the bottom of the tank the biodiesel is being stored in.

Biodiesel is also a great solvent, better than petroleum-based diesel. It can loosen carbon deposits and varnishes that were deposited by petro-diesel and can cause fuel-filter plugging when switching over to biodiesel. Filters should be changed after the first 1,000 miles with biodiesel.

Another issue is the cold weather properties of biodiesel. These properties include cloud point, pour point, and cold soak filtration. Biodiesel can form cloud points at a much higher temperature than petro-diesel, close to the freezing point. The cloud point is the temperature at which crystals begin to form; it can cause the biodiesel to gel and flow slower than it should. Once the pour point is reached (basically completely frozen), the fuel cannot move. It depends on the normal temperature of the climate as to whether the fuel can be used or blended with petrodiesel. What can complicate it more is the saturated or unsaturated fatty acid content. High saturated fatty acid content can lead to higher fuel stability but higher pour points. High unsaturated acid content can lead to lower pour points but less stability for storing. The figure below shows a pour point comparison of biodiesels made from various oils (including fatty acid content) compared to petrodiesel. Petro-diesel pour points are significantly lower than biodiesels.

Cetane number is also an important property for diesel fuels. Cetane number measures the point that the fuel ignites under compression, and this is what we want for a diesel engine. The higher the cetane number, the greater the ease of ignition. Most petro-diesel fuels have a cetane number of 40-50 and meet the ASTM specification for ASTM D975. In general, most biodiesels have higher cetane numbers, 46-60 (some as high as 100), and meet the specifications for ASTM D6751. Because of the higher cetane numbers of biodiesel, the engine running on biodiesel will have an easier time starting and have low idle noise. The table below shows the heat of combustions for various fuels along with their cetane number.

If full-strength biodiesel is used (i.e., B100), most engine warranties will not be covered. It will also require replacing rubber seals in older engines. Blends include B2, B10, and B20 (2%, 10%, and 20% biodiesel, respectively). Adding biodiesel as a blend with ultra-low sulfur should improve lubricity for ultra-low sulfur diesel fuel, which will improve engine wear. Emissions of hydrocarbons, CO, NO_x, and particulate matter are similar to petrodiesel fuels, although can be reduced in some cases.

Biodiesel is stored very similarly to petrodiesel. It is stored in clean, dark, and dry environments. It can be stored in aluminum, steel, fluorinated polyethylene, fluorinated polypropylene, and Teflon types of containers. It is best to avoid copper, brass, lead, tin, and zinc containers.

In another lesson, we will discuss the economics behind using biodiesel.

Summary

In this lesson, you’ve learned about how to make biodiesel from vegetable oils. You’ve had the opportunity to see how it is made and that it’s fairly simple to make. You’ve been provided with information on properties and how biodiesel is typically used. In a future lesson, you will be provided with information on the economics behind biodiesel and ethanol, as well as determining how much energy it takes to make biodiesel, versus the amount of energy that is produced. The additional information is important to determining the use of, and best practices for making, alternative fuels.

Lesson Objectives

By the end of this lesson, you should be able to:

• explain the chemistry of vegetable oil biodiesel, and several ways to make it;
• explain how to utilize biodiesel in a diesel engine;
• evaluate the best uses of biodiesel.

References

Scott W. Pryor ^1*, B. Brian H ², J.H. Van Gerpen ² 1. Department of Agricultural and Biosystems Engineering, North Dakota State University, 2. Department of Biological and Agricultural Engineering, University of Idaho, BEEMS Module B4, Biodiesel, USDA Higher Education Challenger Program, 2009-38411-19761. Contact: Scott Pryor, Scott.Pryor@ndsu.edu