**2. Supercritical alcohol process for the production of biodiesel from oils/ fats/lipids**

feedstock has its own problems of processing the biomass such as harvesting, drying and extraction of oil to produce biofuels. Various processing technologies are available to produce these bio fuels from different feed stock materials. This chapter focuses on the production of biodiesel from oils/fats and wet algal biomass through supercritical alcohol transesterification, novel methods for extraction of oil/lipids from wet algal biomass, liquefaction of whole algal biomass through hydrothermal extraction and liquefaction (HT E&L) and catalytic hydro‐ thermolysis to produce regular hydrocarbon fuels from oils using supercritical water. Before discussing the biofuels production, the sub and super critical technologies will be discussed.

As shown in figure 1, the four phases of a pure material or compound can be observed at different temperature and pressure conditions. When a compound is heated above its boiling point and below its critical point under pressure, it is called a subcritical fluid and when a compound is heated above its critical point is called as supercritical fluid. The sub critical and supercritical fluids possess different physical-chemical properties compared to their proper‐ ties at normal conditions. They are compressibility (like gases) due to reduced densities, increased polarity due to reduced dielectric constants and they have catalytic properties attained by variations in ion dissociation constants. Above the critical point the particular material obtains gas like densities, liquid like solvating properties and intermediate mass transfer kinetics [4]. By varying temperature and pressure, these enhanced capabilities of the sub and supercritical fluids are being used for environmentally benign selective separations, catalytic reactions for production or purification of various products. The commonly known supercritical fluids are water, CO2, ethanol, methanol, ethane, methane etc. In this chapter the utilization of water, methanol, and ethanol to produce various kinds of biofuels or fuel

**1.2. Sub and supercritical conditions**

164 Biofuels - Status and Perspective

intermediates will be discussed.

**Figure 1.** Phase diagram of a pure compound

Biodiesel is one of the first generation biofuel developed that is being used in present day transportation. Biodiesel is not a new source of alternative fuel, it has a long history. When Rudolph diesel invented the diesel engine, he also suggested that pure vegetable oils can be used as a fuel in those engines. Three decades later basic research has started to use modified vegetable oil as a fuel in the diesel engine. This modified vegetable oil can be called as biodiesel. It took almost a century after the invention of diesel engine to start extensive research on biodiesel and its use as a fuel. Biodiesel is a fuel derived from biomass such as vegetable oil, animal fat, algae or other renewable resources which consist of long chain alkyl esters. Biodiesel is a nontoxic, renewable, biodegradable, and eco-friendly fuel. Biodiesel produces lower emissions compared to that of regular petroleum based fuels. Biodiesel usage in the place of regular diesel fuel can reduce emissions such as SOx, CO, particulate matter and hydrocarbons in the exhaust gas and it is better than regular diesel fuel in terms of sulfur content, flash point, aromatic content, and cetane number. Biodiesel does not contribute to a net rise in the level of carbon dioxide in the atmosphere and has the capability of minimizing the intensity of the greenhouse effect. Biodiesel is more promising fuel because of its renew‐ ability, energy security and the high energy content consistent with that of petroleum based fuels. Biodiesel can be used as a fuel blend or as a substitute and will have similar properties to that of regular diesel. Several countries around the world have made it mandatory to sell regular diesel fuel with a blend biodiesel and gasoline with a blend of ethanol in order of environmental concerns to address. The blend concentration varies and can be denoted by different notations such as B100 (pure biodiesel), B50 (50% biodiesel, 50% petroleum diesel), B20 (20% biodiesel, 80% petroleum diesel), B10, B5 etc.

There are different processes to make biodiesel from renewable feedstock. These include but are not limited to pyrolysis, micro emulsions, dilution, catalytic cracking, and transesterifica‐ tion. Pyrolysis is a thermo chemical process that decomposes organic material in the absence of oxygen. In this process, the biomass will be converted into bio oil which is similar to crude oil. This oil will be further converted to small chain hydrocarbons via hydro treating and hydrocracking and then used as transportation fuels. Micro emulsions are isotropic mixtures of oil, water and a surfactant; which can be blended with petroleum diesel fuels, solvents such as alcohols and can be used as transportation fuels. Other methods like dilution and catalytic cracking and transesterification can also be used for the production of biodiesel.

Among all these processes, transesterification is one of the most economic and to produce biodiesel simplest way. Transesterification or alcoholysis is a process in which the triglycerides present in the oils chemically react with alcohol to produce alkyl esters with or without the aid of a catalyst. Alcohols like methanol, ethanol, propanol, butanol and amyl alcohol can be used for transesterification. When the transesterification occurs in the presence of methanol, it is known as methanolysis and the esters formed are known as fatty acid methyl esters. Ethyl esters, propyl esters and butyl esters will be produced when their respective alcohols are used in the transesterification process. On an industrial scale, methanol a 'refinery residue' is the primary alcohol used for the production of biodiesel. Ethanol is an agricultural product which is renewable, non-toxic, eco-friendly and can also be used for biodiesel synthesis. Figure 2 shows a simple mechanism of transesterification reaction with ethanol as alcohol.

**Figure 2.** Transesterification reaction: R1, R2 and R3 are long chain hydrocarbons which may be same or different

Biodiesel can be produced by various transesterification methods using alkali, acid or enzyme catalysts or by advanced methods such as microwave irradiation and ultrasonic transesterifi‐ cation. The alkali process gives a high purity, high yield biodiesel in a short span of reaction time but is not suitable for oils with high free fatty acid (FFA) content, for these oils, acid esterification followed by alkali transesterification can be employed to reduce the high FFA content and to improve the biodiesel yield. However, the longer reaction time and low catalyst recovery are problems in this process. Enzyme catalytic transesterification requires longer reaction times. All the methods mentioned above have their limitation and challenges such as longer reaction time, lower reaction rate, and weak catalytic activity.

To overcome these limitations, non-catalytic transesterification can be implemented to produce biodiesel under supercritical alcohol conditions [5]. Under supercritical conditions, the intermolecular hydrogen bonding in the alcohol molecule will be significantly decreased. As a result, the polarity and dielectric constant of alcohol are reduced allowing it to act as a free monomer. Alcohol at supercritical conditions can solvate the triglycerides to form a single phase of oil/alcohol mixture and yield fatty acid alkyl ester and diglycerides. Diglyceride is further transesterified to form ethyl ester and monoglyceride, which in the last step is then converted into alkyl ester and glycerol. Vegetable oils which include edible oils such as palm oil, sunflower oil, rice bran oil, rapeseed oil and non-edible oils such as jatropha oil, paradise oil, and pongamia oil can be used in biodiesel production. Waste cooking oil, algae and animal fats such as lard, tallow, yellow grease are also potential feedstock in the production of biodiesel. Table 1 shows the fatty acid profiles of some common biodiesel feed stocks collected from literature from Balat et al.,[6] and others. The fatty acid profiles of the same (particular) feed stocks may vary due to its cultivation conditions and extraction methods.


**Table 1.** Fatty acid profiles of various biodiesel feed stocks

primary alcohol used for the production of biodiesel. Ethanol is an agricultural product which is renewable, non-toxic, eco-friendly and can also be used for biodiesel synthesis. Figure 2

H5C2 + -OH +

High Temp

High Pressure (SCE)

HO

Triglyceride Ethanol Glycerin Ethyl Ester

Biodiesel can be produced by various transesterification methods using alkali, acid or enzyme catalysts or by advanced methods such as microwave irradiation and ultrasonic transesterifi‐ cation. The alkali process gives a high purity, high yield biodiesel in a short span of reaction time but is not suitable for oils with high free fatty acid (FFA) content, for these oils, acid esterification followed by alkali transesterification can be employed to reduce the high FFA content and to improve the biodiesel yield. However, the longer reaction time and low catalyst recovery are problems in this process. Enzyme catalytic transesterification requires longer reaction times. All the methods mentioned above have their limitation and challenges such as

To overcome these limitations, non-catalytic transesterification can be implemented to produce biodiesel under supercritical alcohol conditions [5]. Under supercritical conditions, the intermolecular hydrogen bonding in the alcohol molecule will be significantly decreased. As a result, the polarity and dielectric constant of alcohol are reduced allowing it to act as a free monomer. Alcohol at supercritical conditions can solvate the triglycerides to form a single phase of oil/alcohol mixture and yield fatty acid alkyl ester and diglycerides. Diglyceride is further transesterified to form ethyl ester and monoglyceride, which in the last step is then converted into alkyl ester and glycerol. Vegetable oils which include edible oils such as palm oil, sunflower oil, rice bran oil, rapeseed oil and non-edible oils such as jatropha oil, paradise oil, and pongamia oil can be used in biodiesel production. Waste cooking oil, algae and animal fats such as lard, tallow, yellow grease are also potential feedstock in the production of biodiesel. Table 1 shows the fatty acid profiles of some common biodiesel feed stocks collected from literature from Balat et al.,[6] and others. The fatty acid profiles of the same (particular)

**Figure 2.** Transesterification reaction: R1, R2 and R3 are long chain hydrocarbons which may be same or different

O

O

O

O

O

R3 C2H5

C2H5

C2H5

R1

R2

OH

OH

O

shows a simple mechanism of transesterification reaction with ethanol as alcohol.

O

166 Biofuels - Status and Perspective

<sup>O</sup> R2

O

O

R1

O

O

3

longer reaction time, lower reaction rate, and weak catalytic activity.

feed stocks may vary due to its cultivation conditions and extraction methods.

R3

The catalytic transesterification processes requires a lower amount of alcohol (1:9 oil to alcohol ratio), and mild temperatures (60o C) for the production of biodiesel. Alkali catalysts like potassium hydroxide, sodium hydroxide, sodium methoxide, potassium methoxide and acid catalysts like hydrochloric acid, phosphoric acid, and sulfuric acid can be used as catalysts in catalytic transesterification but catalyst separation, free fatty acid and water interference in the reaction, glycerol separation, and energy intensive are disadvantages. The use of different feed stocks, different alcohols greatly vary the processing conditions; the complete conversion may not be achieved with such changes in the process, reaction times could reach hours or days and separation of product becomes much more challenging.

In a non-catalytic supercritical alcohol process, the transesterification of triglycerides and the alkyl esterification of fatty acids will occur simultaneously with a shorter reaction time and reduced the energy consumption due to the simplified separation and purification steps. This process does not require any pre-treatment of the feed stock regardless of its fatty acid composition and profile. In non-catalytic supercritical transesterification the oil to alcohol ration varies between 1:40-45 depending upon the feed stocks fatty acid profile, 290-350o C temperature, and reaction pressure above saturation pressure [5, 10]. Introduction of cosolvent into the reaction mixture decreases the critical point of alcohol, increases the mutual solubility of the oil and alcohol at lower reaction temperatures and accelerates the reaction rate under supercritical alcohol conditions [11]. Normally methanol and ethanol are being used as alcohol to produce biodiesel. But longer chain alcohols like 1-butanol, 1-propanol and 1-octanol could also be used to produce biodiesel. The critical conditions of these alcohols are presented in Table 2.


**Table 2.** Transesterification alcohols and their critical conditions

The major influencing factors on the yields of biodiesel are type of alcohol, reaction tempera‐ ture, oil to alcohol ratio, reaction time and pressure. The critical temperatures increase with increase in chain length or molecular weight of the alcohol. At the same temperature, the acidity of longer chain alcohols tends to decrease resulting in slower reactivity or slightly more reaction time than the short chain alcohols. However the cold flow properties of the biodiesel produced with long chain alcohols are better than the biodiesel produced with short chain alcohols [12]. All these factors influence the selection of alcohol, as it affects both the economics and energetics of the process. The yield of biodiesel increases with the increase in reaction temperature above the critical conditions of the alcohols. Beyond the optimum temperature, the yield may start decreasing due to degradation of fatty acids at higher temperatures. Usually this also depends on the fatty acid profiles; as poly unsaturated fatty acids (PUFAs) are thermally unstable at higher temperatures. Feed stocks having more PUFAs may give higher yields at slightly lower temperatures than the feed stocks having less PUFAs [13]. The usual optimum reaction temperature ranges between 290-350o C, which also depending on the other reaction parameters.


**Table 3.** Variation in the yields of biodiesel with different alcohols

As mentioned earlier the long chain alcohols need higher reaction temperatures to get higher yields of biodiesel than the short chain alcohols. The molar ratios of oil to alcohol vary for different feed stocks with different alcohols. This usually ranges between 1:40-45 at optimum reaction temperature. A lower amount of alcohol negatively affects the yields as the reverse transesterification reaction tries to go backwards. On the other hand more alcohol also reduces the yields by changing the critical point of the mixture to higher temperatures, where the optimum temperature of the reaction is not sufficient to perform the forward reaction. This also imposes another economic barrier as this extra alcohol requires more energy to heat, and will need to be recycled after separation process [14]. Supercritical alcohol processing is very fast compared to conventional transesterification. The typical processing times of supercritical processes are 5-30 minutes depending upon the type of alcohol and reaction temperature. The reaction pressure also slightly increases the yield of biodiesel above its saturation pressure at a particular reaction temperature. But it is always a better option to use the lowest possible pressures; as high pressure demands more energy and capital investment. Variation in the yields of biodiesel and reaction times with respect to alcohol is shown in Table 3.
