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Biodiesel Chemistry
Biodiesel
Chemically, biodiesel (from transesterification) refers to mono alkyl esters of long chain fatty acids derived from natural oils. We’ll look a little closer into what exactly that means.
Ester
An ester is the product of combining an acid (abbreviated as R1-COOH) with an alcohol (abbreviated as R2OH). Esters in general are often abbreviated as R1-COOR2, where R1COO represents the residue of an oxygen acid (The residue is what’s left when the hydrogen is lost), and R2 represents an alkyl – from an alcohol that lost its OH group. So, through the combination, an H is lost, and an OH (although in reality the O could come from either the alcohol or the acid), yielding a water molecule (H2O), and the ester, made up of everything else remaining of the acid and alcohol except the H2O. Esters vary depending on the type of acid (R1COOH, often abbreviated as R1 for simplicity) and the type of alcohol (R2OH, often abbreviated R2).
Vegetable Oil
Vegetable oils are esters of glycerin (an alcohol, aka glycerol) and varying fatty acids. A glycerin molecule looks like:
CH2OH
|
CHOH
|
CH2OH
Figure 1 – Glycerin (glycerol)
Each vegetable oil molecule is a triglyceride, meaning it consists of three fatty acids (which can be of different types) connected to a glycerin backbone. So, while some esters consist of just one acid (R1), vegetable oil molecules have three acids combined with an alcohol. So, when speaking about vegetable oils in particular, the three acids are often referred to as R1, R2, and R3, and the alcohol as R4. A triglyceride (vegetable oil) can be drawn as:
CH2OOR1
|
CHOOR2
|
CH2OOR3
Figure 2 – A triglyceride (vegetable oil molecule)
The fatty acids involved would be R1OH, R2OH, and R3OH (the hydrogen atoms are lost when the acid is combined with the alcohol to make the triglyceride).
Alcohol
An alcohol is a molecule in which any carbon atoms have the maximum number of hydrogen atoms attached possible, except for one carbon atom which has an OH group connected. The simplest alcohol, and the one used most often in biodiesel production, is methanol. Methanol consists of only one carbon atom, with three hydrogen atoms attached, and one oxygen attached. The oxygen atom also has a hydrogen atom attached. Thus, methanol is often written as CH3OH. The next simplest alcohol is ethanol, which has one more carbon atoms, with two hydrogen atoms attached, in between the CH3 group and the OH group. Ethanol is written as either CH3CH2OH, or C2H5OH. The former method is often used to give more of a description of the structure (since it consists of a carbon with three hydrogen atoms attached, connected to another carbon with two hydrogens attached, connected to an oxygen with one hydrogen attached). These alcohols are abbreviated as ROH, where the R is the hydrocarbon chain (consisting of CnH2n+1), and determines what type of alcohol it is.
Typical Fatty Acids in vegetable oils
Below is a table of typical fatty acids found in alcohol, and some of their properties. The “Acronym” is a chemical abbreviation for the molecule. The first number refers to the number of carbon atoms in the chain, and the second number refers to the number of double bonds in the molecule. Thus, Linoleic acid, for example, is a fatty acid consisting of a chain of 18 carbon atoms, with two double bonds. Notice that the more double bonds in the acid, the lower the melting point. This is an important issue regarding the cold weather suitability of the oil or the biodiesel produced from the oil, and will be discussed further. The double bonds also lower the boiling point, which does not make a significant difference on the operability of the fuel since all biodiesel molecules have boiling points so high as to make vaporization not an issue.
Table 1 – Properties of Fatty Acids commonly found in vegetable oils 1
Fatty Acid Acronym Molecular Weight MeltºC/ºF BoilºC/ºF CetaneNumber Heat Combust(kg-cal/mole)
Caprylic 8:0 144.2 16.5/61.7 239.3/462.7
Capric 10:0 172.27 31.5/88.7 270.0/518.0 47.6 1453.07
Lauric 12:0 200.32 44.0/111.2 131.0/267.8 1763.25
Myristic 14:0 228.38 58.0/136.4 250.5/482.9 2073.91
Palmitic 16:0 256.43 63.0/145.4 350.0/662.0 2384.76
Stearic 18:0 284.43 71.0/159.8 360.0/680.0 2696.12
Oleic 18:1 282.47 16.0/60.8 286.0/546.8 2657.4
Linoleic 18:2 280.45 -5.0/23.0 230.0/446.0
Linolenic 18:3 278.44 -11.0/12.2 232.0/449.6
Erucic 22:1 338.58 33.0/91.4 265.0/509.0
Saturated Fatty Acids
A saturated fatty acid is one containing no double bonds. Since fatty acids are acids with a COOH group at the end, a saturated acid is one in which the rest of the carbon chain is an alkane – i.e., the carbon atoms in the chain have the maximum number of hydrogen atoms bonded possible (every bonding location is filled with a hydrogen atom, except for single bonds to neighboring carbon atoms). Stearic acid (18:0) is an example of a saturated fatty acid, as it has no double bonds. The chemical formula for stearic acid is CH3(CH2)16COOH.
Unsaturated Fatty Acids
An unsaturated fatty acid is one containing one or more alkene functional groups – those being hydrocarbons with double bonds between two carbon atoms. An alkene does not have the maximum number of hydrogen atoms possible on all of the carbon atoms, as two adjacent carbons have a double bond between them, and therefore one less hydrogen attached each. Oleic acid is an unsaturated acid of the same length as stearic acid, but with a double bond between two of the carbon atoms, and therefore two less hydrogen atoms. Molecules with double bonds are often written using an equals sign (“=”) to show where the double bond is. For example, oleic acid has its one double bond as the ninth carbon-carbon bond, counting from the chain most distant from the carboxyl group (COOH). Thus, oleic acid would be written as CH3(CH2)7CH=CH(CH2)7COOH. Thus, the molecule has a methyl group (CH3), then 7 carbons with single bonds between them, each having two hydrogens attached ((CH2)7), then the carbon that has one end of the double bond (leaving only room left for one hydrogen atom, so it’s a CH), the double bond connecting to another CH, followed by 7 more CH2 groups, and finally the carboxyly group (COOH). This molecule is exactly the same as the stearic acid molecule (CH3(CH2)16COOH, no double bonds), except for the double bond between the 9th and 10th carbon atoms (so the 9th carbon-carbon bond). Thus, in the middle of the molecule, a set of two CH2 groups is replaced by CH=CH (double bond between the carbons, only one hydrogen each).
This seemingly minor difference results in a significant change in some of the properties of the molecule, most notably the melting point. The cold flow properties of the oils, and the resulting molecules can thus be a nice method for introducing this topic of how minor differences in a molecule can have large effects, and in particular, double bonds lower the melting point of molecules (generally).
The oleic acid molecule could have another double bond added, which would turn it into linoleic acid (18:2). A third double bond would make linolenic acid. Many vegetable oils normally consist of a significant percentage of these particular 18 carbon acids with double bonds. When the oil is hydrogenated (usually through high temperatures, such as when oil is used in a fryolator), that is when some of these double bonds are lost, replacing a CH=CH group with CH2CH2. The acid (or the oil that the acid is a part of) acquires two more hydrogens, and loses a double bond. The result is an increasing of the melting temperature of the acid, or oil of which it is part. This is an important consideration as far as using waste vegetable oils as feedstocks for producing biodiesel. The more heavily used the oil is, the more hydrogenated it becomes, resulting in higher melting points for the molecules. Therefore, caution should be taken when using heavily used (hydrogenated) oils for making biodiesel, as the higher freezing/melting points of the molecules would result in a greater tendency of the fuel to clog fuel filters, or possibly gel entirely.
Transesterification
Chemically, transesterification is the process of exchanging the alkyl group (from an alcohol) of an ester with another alkyl, from a different alcohol. In the case of biodiesel, a vegetable oil ester is combined with a simple alcohol and a catalyst, resulting in the breakup of the triglyceride ester (three fatty acids connected to a single glycerol (alcohol)), and the joining of the fatty acids with the added simple alcohols. The glycerin alkyls are replaced with the alkyl of the added alcohol (i.e. methyl for methanol, ethyl for ethanol, etc.). The separated glycerol is the waste product. This reaction is shown below:
CH2OOR1 catalyst CH2OH
| â |
CHOOR2 + 3CH3OH à 3CH3OORx + CHOH
| |
CH2OOR3 CH2OH
Vegetable oil 3 Methanols Biodiesel Glycerin
Figure 3 – Transesterification
Rx is used since the biodiesel produced will consist of different types of mono-alkyl esters, because of the various fatty acids (R1, R2, R3) in the vegetable oil. The reaction can proceed both ways, so it is generally necessary to add an excess of methanol to force the reaction to the right. Since it is not desirable to have free methanol in the biodiesel fuel, it is then necessary to recover the methanol either by water washing, or a pressure-condensing method. The glycerin is more dense than the biodiesel, so it will gradually settle to the bottom in the reactor.
Biodiesel – Mono Alkyl Esters
As mentioned previously, biodiesel molecules are referred to as mono-alkyl esters, since they are esters with one alkyl (from the alcohol) per fatty acid, in contrast to the triglycerides in the vegetable oil, which had three fatty acids for each glycerol. If the alcohol used in making the biodiesel was methanol, then the biodiesel is referred to as a methyl ester. If the alcohol was ethanol, the biodiesel would consist of ethyl esters. Table 2 below shows a list of the methyl esters made from the fatty acids listed in Table 1, and their properties. Note that the methyl esters of fatty acids with more double bonds have lower melting points than those without double bonds, just as the fatty acids themselves do. Also notice that the melting points of the methyl esters are lower than the melting points of the fatty acids themselves. An interesting point of discussion is that the boiling points are not all affected similarly, from fatty acids being turned into mono alkyl esters. Note that methyl stearate has a much higher boiling point than stearic acid, while methyl linolenate has a much lower boiling point than linolenic acid. Fortunately, the boiling points don’t have any significant affect on the use of the chemicals as fuels.
Table 2 – Properties of Methyl Esters from Vegetable Oils 1
Methyl Ester AcidAcronym Molecular Weight MeltºC/ºF BoilºC/ºF CetaneNumber Heat Combust(kg-cal/mole)
Methyl Caprylate 8:0 158.24 193.0/379.4 33.6 1313
Methyl Caprate 10:0 186.30 224.0/435.2 47.7 1625
Methyl Laurate 12:0 214.35 5.0/41.0 266.0/510.8 61.4 1940
Methyl Myristate 14:0 242.41 18.5/65.3 295.0/563.0 66.2 2254
Methyl Palmitate 16:0 270.46 30.5/86.9 418.0/784.4 74.5 2550
Methyl Stearate 18:0 298.51 39.1/102.4 443.0/829.4 86.9 2859
Methyl Oleate 18:1 296.49 -20.0/-4.0 218.5/425.3 47.2 2828
Methyl Linoleate 18:2 294.48 -35.0/-31.0 215.0/419.0 28.5 2794
Methyl Linoleneate 18:3 292.46 -57.0/-70.6 109.0/228.2 20.6 2750
Methyl Erucate 22:1 352.60 222.0/431.6 76.0 6454
An important point to notice is that for the most part, the methyl esters with the lower (and therefore preferable) melting points, unfortunately have lower (and therefore less preferable) cetane numbers. Modern diesel engines generally require a cetane number of at least 40, preferably 45 or higher. So, while from looking at Table 2, we might think that the ideal biodiesel would be composed entirely of methyl linoleneate, due to the extremely low melting/freezing point (-70º F), we should also notice that the cetane number is far too low (20.6) for a fuel composed entirely of methyl linoleneate to be acceptable.
Another point of interest is that other alcohols produce biodiesel molecules with lower melting points. For example, isopropyl stearate has a melting point of 28º C, compared to 39.1º C for methyl palmitate. 3
Differences between various vegetable oils
Looking at the properties of the various methyl esters demonstrates that the properties of biodiesel – in particular the cold weather properties, could vary considerably depending on what oil it is made from. Table 3 below lists a few different vegetable oils, and the levels of various fatty acids they contain (bare in mind, in the oil the fatty acids are bound to glycerin as triglycerides. The table lists the fatty acids themselves, but is not meant to imply that they exist as free fatty acids in the oil). The fatty acid profiles are generalities, as various plants (and animal fats, such as the tallow included) do have variability among them, depending on growing conditions and other factors. A field in biodiesel research focuses on breeding varities of various plants for ideal fatty acid profiles. The table also includes the gel point, cloud point (these qualities are explained in section II.c of the Lesson Ideas portion of this document), and cetane number for methyl ester made from each oil.
Table 3 – Fatty Acid profile, and properties of methyl esters for various oils2
Rapeseed Canola Tallow Soybean
Myristic (14:0 0 0.0 3.0 0.0
Palmitic (16:0 2.2 4.0 23.3 9.9
Stearic (18:0 0.9 2.4 17.9 3.6
Oleic (18:1) 12.6 65.0 38.0 19.1
Linoleic (18:2) 12.1 17.3 0.0 55.6
Linolenic (18:3) 8.0 7.8 0.0 10.2
Elcosenoic (20:1) 7.4 1.3 0.0 0.2
Behenic (22:0) 0.7 0.4 0.0 0.3
Erucic (22:1) 49.9 0.1 0.0 0.0
Properties of Methyl Esters of the oils
Cetane number 61.8 57.9 72.7 54.8
Cloud Point ºC 0 1 16 3
Gel/Pour Point ºC -15 -9 16 -3
Data taken from
http://www.biofuels.fsnet.co.uk/comparison.htm Soap Formation
Soap can be made by combining sodium hydroxide (NaOH), water, and vegetable oil. The water separates the sodium hydroxide, resulting in free Na+ ions. The vegetable oil triglycerides are broken apart, separating the fatty acids and glycerin. The Na+ ions attach to the fatty acids in the same place that the alkyl groups attach during transesterification to produce biodiesel. The fatty acids with a sodium ion attached make a soap.
Since a base is used both for making soap from vegetable oil, and also as the catalyst for breaking apart the vegetable oil molecule during transesterification, care needs to be taken that one doesn’t inadvertantly make soap. The NaOH is combined with the alcohol to make sodium methoxide, which is then added to the vegetable oil. It is imperative that there be no water present in the methoxide mix, or at least as little as possible. This is because the water would break apart the NaOH molecule, producing free Na+ ions, which could then combine with fatty acids to produce soap. If the sodium is bound up in sodium methoxide, when the vegetable oil is broken apart, the methyl groups will preferentially bond with the fatty acids, rather than sodium – resulting in biodiesel rather than soap. Using too much NaOH can result in soap formation, due to the excess sodium joining the fatty acids after the vegetable oil molecules are separated.
With waste vegetable oils, free fatty acids are generally already present. These free fatty acids will essentially always combine with a sodium ion during the processing, resulting in saponification (soap formation). Unfortunately, that is an unavoidable result when using this base-catalysed transesterification process (and is a reason why some groups have developed methods of performing the reaction without a catalyst). Since these free fatty acids consume the catalyst, when waste vegetable oils are used, extra catalyst needs to be added to account for that. Otherwise, not enough catalyst would be left for breaking apart the triglycerides in the oil, as some would be consumed by the free fatty acids (FFAs). This is the reason for doing the titration when using a waste vegetable oil feedstock, so that extra catalyst can be added to account for the fact that some catalyst will be consumed by the FFAs.
When oils with free fatty acids are used, the free fatty acids will be turned into soaps by the catalyst. As a result, the yield of biodiesel is lower for these oils, and the soap needs to be removed (usually through water washing).
