Production of Green fuel (Biodiesel) from used cooking oil (UCO)

Abstract:

In Pakistan fuel prices are fluctuating upwards day by day and transportation cost and other daily utilities are also increasing with it. Waste cooking oil (UCO) is abused by small restaurants which has severe health effects and is finally discarded in the water course where it increases the cost of waste water treatment and causes environmental degradation. This can be converted into biodiesel by the process of trans-esterification. Biodiesel produced from this source is cheap and has an emission profile that is far much lesser than that of normal fossil fuel diesel. The biodiesel produced has properties similar to biodiesel produced from pure and virgin vegetable oil. Biodiesel produces is set according to the standards given by ASTM D6571 and European standards. Biodiesel that fulfill these standards could only be used in diesel engines without any modifications. This conversion will reduce the burden on the environment and will also stimulate the use of renewable energy resources as this is a green fuel.

Introduction:

Pakistan is a place where one can see prices of daily use are raising day by day due to increase in the prices of petroleum products especially petrol and diesel. These two petroleum products have enormous use in transportation. Almost all the heavy vehicles utilizes diesel as a fuel. Burning of this fuel produces a lot of pollutant gases that is destroying the environment of Pakistan day by day. Lack of awareness among the local population about the role of trees and environmental conservational elements is further boosting environmental degradation. In such chaotic scenario, there is need to introduce alternatives of petroleum products so that the dependence on the fossil fuel could be minimized.

Now a day there is an emerging trend of converting virgin cooking oil in biodiesel. But the problem in doing this is the production cost of the final product (biodiesel) is more (expensive) than the ordinary diesel which is off course a major disadvantage and discouraging factor. This thing is also unethical as people are not getting enough food to feed themselves and their families and virgin cooking oil is main food utility in Pakistan so that will be unfair.
However, another option is of converting used cooking oil into biodiesel. This is a byproduct which is discarded in water courses. Many big restaurants and hotels sell used cooking oils to smaller restaurants which over uses the used cooking oil. This is very dangerous for the health of local population and is the cause of many diseases as heating the oil for an extended period of time causes oxidation of the oil and gives rise to oxides. Many of these such as hydroperoxides, epoxides and polymeric substances have adverse health effects such as growth retardation, increase in liver and kidney size as well as cellular damage to different organs when fed to laboratory animals (1). Discarding the waste out in water waste increases the chemical oxygen demand (COD)(1). As a ‘liter of oil poured into a water course can pollute up to 1000 tanks of 500 liters’(2). This also increases the cost on treatment of the waste water so its conversion into biodiesel will be good for environment and local population. According to a research conducted by hussain and Boyace, biodiesel produced from pure virgin cooking oil and used cooking oil have no considerable difference (3). So making biodiesel from waste cooking is much better.

What is Biodiesel?

Biodiesel is a renewable, biodegradable, nontoxic and non-inflammable fuel that is used as an alternative to diesel(4)(3).

Biodiesel is defined as a fuel comprised of mono alkyl ester of long chain fatty acid derived from vegetable oil or animal fats (3).

It is produced by the process of trans-esterification with alcohol mainly methanol and potassium hydroxide/ sodium hydroxide/ sulphuric acid as a catalyst. However, immobilized lipases enzymes (4) and non-catalytic supercritical fluid method have also been applied for making biodiesel. Biodiesel produced from methanol is chemically known as fatty acid methyl ester (FAME). If the alcohol is ethanol, then it will become an ethyl ester.
So far in transesterification process methanol is used which is most favorable due to low price per liter as compared to ethanol and it is polar and have shortest carbon chain. However, ethanol should be preferred over methanol because it is derived from agricultural products, is renewable and is biologically less offensive for the environment (3). UCO conversion in biodiesel is more difficult because it contains water, food particles and free fatty acids (FFA). FFA is formed when oil is cooked as a result of which fatty acids break apart.

Reaction steps

Filtration and sample composition determination:

Used cooking oil (UCO) first has to be filtered in order to remove any burnt solid particles or food residues. filtration is done with a cotton cloth(5). Before carrying out filtration process a researcher allowed the UCO to stand for 2-3 days so that solid impurities settle down (1).Hussain and Boyce used 11 bar vacuum pressure to filter the oil (3). Oil was then measured for total polar content using a standard cooking oil tester (1). Gas chromatography can also be used to get fatty acid composition present in the sample oil (6).

Removal of water molecules:

Then UCO is heated at 100 – 110oC for 15 min with continuous stirring to remove the water present in the oil as water molecules destroy the catalyst used (1)(7).
Determination of FFA and Pretreatment of oil:

Then waste cooking oil is analyzed for the presence of free fatty acids (FFA). If the FFA content is below 1% then it could be used for biodiesel production using alkaline catalyzed process but if the FFA content is more than 3.8% then it can’t be used for the biodiesel production without acid catalyzed pretreatment process (8). Formation of soap and gel in the reactor is the main problems associated with the use of UCO with FFA content more than 3.8% (8).
However, to neutralize the free fatty acids accumulated in the WCO, extreme care has to be taken. This is because, during the neutralizing the free fatty acids, as both excess as well as insufficient amount of catalyst may cause soap formation (1). Hence to determine the correct amount of catalyst required, a titration must be performed on the oil being transesterified. One simple method is by using a chemical indicator called phenolphthalein.

In the titration, 0.1% of NaOH in distilled water is titrated against the titration sample which is essentially a solution of 10ml of Isopropyl alcohol and 1 ml of oil sample with 2-3 drops of indicator. The end point of the titration is marked when the titration sample turns pink (magenta), and stays pink for 10 seconds. The number of milliters of 0.1% NaOH solution needed is equal to the number of extra grams of pure sodium hydroxide catalyst needed to produce the proper reaction to make biodiesel from WCO (1). The conversion of FFA present in the UCO into FAME was calculated from the mean of acid value (Av) of the oil layer by the following equation (7).
Conversion (%) =[1- ((Av of oil))/((Av ofUCO)]×100
Then the reactants are weight according to the results from the previous step. Highest yield of biodiesel is obtained when oil to methanol at a ratio 1:6 is used (3). Acid catalyzed pretreatment for UCO with FFA content more than 3.8% is done – if required.

The catalyst:

Then we prepare our base catalyst (1%) by mixing it with alcohol (methanol) in order to make methoxide which reacts with all the FFA (if less than 3.8%) present in oil (9). Using base catalyst have problem because of side saponification reaction which creates soap and consumes the catalyst. This must be neutralized as they give rise to wastewaters and cannot be reutilized as they create difficulties in separation from biodiesel by formation of stable emulsions (6). The byproduct glycerol is obtained as an aqueous solution of comparatively low purity and the reaction becomes very sensitive to the presence of water and free fatty acids. However, the advantages of using base catalyst are also there. These bring low production cost, faster reaction speed and milder reaction conditions. KOH can also be used as a catalyst but 2 % is used. NaOH make more emulsion as compared to KOH.
Acid catalyst is not used by many researchers because when used in transesterification process it gives slower reaction rates and is also corrosive for the equipment (7).

As stated earlier that we can also use biocatalyst for conversion of UCO in biodiesel. These give higher yields of biodiesel. Different types of lipases are used as a biocatalyst which can be obtained from Candida Antarctica, Pseudomonas Cepacia and Thermomyces lanuginosus (6). However, high concentration of methanol interferes with the separation of biodiesel causing an apparent loss of product by increasing the solubility of the glycerol.
Mostly enzyme lipase B from Candida Antarctica is used. High methanol concentration inhibits the enzyme lipases B from Candida Antarctica. We can use macro-porous acrylic resin (10) or ceramic beads (11) as a support for the enzyme. This immobilizes the enzyme which will increase the stability of the enzyme, easy recovery of the enzyme and eliminates the side saponification reaction that occurs (10). Its main disadvantages are high cost, low reaction rate and propensity for deactivation over extended use or through thermal deactivation (10).
Conversion in biodiesel:

This mixture of alkaline catalyst and alcohol (methanol) is mixed with the UCO and all the contents are mixed again at 320 rpm – 350 rpm (3) and allowed to settle for some time. This process takes about 2 hours at 70oC with mechanical stirring but we can reduce this time to 5 minutes and increase the yield of biodiesel by using ultra sonication technique (7). Some researchers have also used microwave radiation from domestic microwave oven to achieve this conversion in 1-3 min(12). The reason for shorter time and higher yield is due to the high speed mixing and mass transfer between the methanol and oil, as well as the formation of a microemulsion resulting from the ultrasonic cavitation phenomenon (7). Some biofuel companies have collaborated with each other to perform esterification and transesterification simultaneously which have the potential to increase the yield, decrease waste and processes higher FFA feedstock (13). When using lipases as an enzyme, the reaction was carried out at 50oC at 150rpm (6).

Separation of biodiesel from glycerol:

Then the glycerol is separated from the biodiesel using a separating funnel or similar large scale apparatus. This is called phase separation. Biodiesel forms the upper layer and glycerol forms the lower layer. The biodiesel is then washed by using warm water and excess water is then removed by heating the biodiesel at 110oC for 30 minutes (7). For same purpose ,Hussain and Boyce, used 5% water followed by anhydrous magnesium sulphite to remove the water (3). According to Yokayo, biodiesel is washed three time to remove the impurities and in second wash phosphoric acid is added to facilitate the removal of soap (13). Wastewater from the washing step was previously discarded but now these people are exploring ways to use it as a liquid fertilizer as it contains potassium phosphate (13) .

Glycerol that is obtained is used by industry. Glycerol that is not of the industrial grade is used to increase biogas production in anaerobic digestion or could be used as a carbon source for denitrification (13).

Significance of transesterification

Reduction in viscosity:

Cooking oil could be used in diesel engines but due to high viscosity and poor volatility it becomes unfavorable. High viscosity deteriorates the atomization, evaporation and air-fuel mixture formation characteristics leading to improper combustion and higher smoke emission. Moreover this high viscosity generates operational problems like difficulty in engine starting, unreliable ignition and deterioration in thermal efficiency (3). Conversion in biodiesel shortens the carbon chain to one third of the original fatty chain and hence reduces the viscosity (14).

Density:

Cooking oil have a high density which makes it impossible to add in a car, weights extra in fuel tanks and have poor diesel injection system. By transesterification, the density is reduced making biodiesel easy injection in the engine (14).

Flash point:

The lowest temperature at which fuel emits enough vapours to ignite is called flash point. Transesterification reduces the flash point of the biodiesel produced as compared to UCO (14). However, the value of flash point of biodiesel is higher than that of ordinary diesel which makes its handling easier (14).

Characterization and testing

Biodiesel obtained is then characterized on the basis of few parameters. Characterization of biodiesel is very important as it help us to know the nature of the biodiesel produced. Characterization of biodiesel is done on the basis of few parameters such as quantity of biodiesel and total glycerol, free glycerol, density, viscosity (kinematic) at 400 C, flash point, carbon residue, calcium and magnesium, Centane number, acid value, specific value t 600F, cloud point, pH, and phosphorous content (8)(5). Once characterized and values determined, these are then compared with the American Society of Testing and Materials standards i.e. ASTM D 6571 or European standards EN 14214 depending on the requirements.

Property Standard values of ASTM D 6571
Density (g/cc) 0.87
Kinematic viscosity (mm2/s) 1.9-6.0
Calcium and magnesium combined 5 (max)
Centane number >47
Carbon residue (% mass)- for 100% pure sample 0.050
Acid number (mg KOH/g) 0.50 (max)
Cloud point (0C) –
pH 7
Flash point (0C) 93
Water and sediments (vol. %) 0.050 (max)
Sulfated ash (% mass) 0.020
Sulphur (% mass) 0,0015 (S15), 0.05 (S500)
Copper strip corrosion 0.020 (max)
Free glycerin (% mass) 0.020
Total glycerin (% mass) 0.240
Phosphorous content (% mass) 0.001 (max)
Methanol content (vol. %) 0.2 (max)
Oxidation stability (hours) 3 (min)
Distillation temperature.90% recovered (0C) 360 (max)
Table 1: ASTM standards for biodiesel (15)

Biodiesel as green fuel:

Green fuel refers to the fuel which produces lesser emission in the atmosphere. Biodiesel produces less carbon monoxide gas, particulate matter and volatile organic chemicals that causes smog and health problems. Burning of pure biodiesel without using any bled of it eliminates all the sulphur emission. Hydrocarbon emission from UCO is 35% less than the baseline diesel operation (1). Thermal performance of biodiesel from UCO is marginally less by1-1.85% as compared to normal diesel but was similar to the biodiesel from pure cooking oil (1). At higher loads the engine only suffers from nearly 1 to 1.5% brake thermal efficiency loss. From emission standpoint the NOx , CO2 and CO is same as that of normal diesel but hydrocarbon emission is lower (1) (16). Some researcher stated 78.5% – 80% reduction in CO2 emission and 100% reduction in SO2 (16)(6). From 1 L of UCO 750ml of biodiesel could be produced (9). It is used by blending it with local diesel fuel. Some researchers have emphasized on using 20% biodiesel and 80% diesel but some people have used it at 5% – 25% i.e. biodiesel (16). It is usually used in diesel engines that are manufactured after 1990. These diesel engines do not require any modification in them for using biodiesel.

Using biodiesel is one of the main successes towards sustainable environment. These fuels will provide new ways for our future generations to work on better ways than this thus protecting the environment and reducing the dependency over fossil fuels.

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