As fuel costs rise, novel alternatives are continually being sought to replace traditional crude oil and petrodiesel. Biodiesel is the promising alternative because it is also a renewable fuel.
History
Rudolf Diesel who invented the diesel engine showed for the Paris Exhibition in 1900 that a fuel could be prepared from peanut oil. At the time the French government was interested to know if peanut oil which was produced in their African colonies might be a viable commercial energy source.
So began the story of biofuels!
Biofuels: A Short Overview
Biofuels are produced from various organic products and wastes. The three most common forms are bioethanol, biodiesel and biomethane. Bioethanol is made from sugar, algae, sugar beet and wheat. Biodiesel is prepared from plant and animal lipid sources such as vegetable oil, animal fat and algal lipids. Biomethane is manufactured from waste organic materials, agricultural and domestic waste and sewage. Biodiesel is the main focus of our attention here.
What is Biodiesel?
As a compound it is described as non-petroleum based diesel consisting of short chain alkyl methyl and ethyl esters. It is used alone or blended with conventional petro-diesel in unmodified diesel engines.
Biodiesel Production
Biodiesel is produced by two approaches which either involve triglycerides or fatty acids. In the first approach, triglyceride is reacted with methanol or ethanol using acid or alkaline hydrolysis to produce a fatty acid methyl ester (FAME) and glycerol.
A triglyceride as the name suggests is a glycerol molecule to which is bound three fatty acid esters. In this approach, glycerol is the main byproduct of reaction where it forms 10% w/w of the final reaction with 90% w/w being biodiesel.
The second approach is to take a source of fatty acids and react with methanol or ethanol usually under acid hydrolysis to produce the same type of FAME. This process is just called esterification.
Biodiesel is defined in regulations by a European Standard for biodiesel fuel (EN 14214).
The Process
The basic process is:
- To collect oil and fat as a feedstock for the process
- Pretreatment of the oil by heating to 120ºF
- Early esterification reactions
- Transesterification reaction
- Atmospheric (non-pressure) distillation
- Washing and drying
- Biodiesel refinement
Feedstocks
Biodiesel is synthesized from animal fats which are waste from the meat processing industry and from vegetable oils. These feedstocks of glycerides are industrially found in oleaginous products such as cooking oil, soybean oil, rapeseed oil, waste greases, pork and beef lard, biomass from algae – in fact almost any vegetable oil or animal fat. Most feedstock sources are oleaginous crops (Girio et al., 2017).
Lard is often treated with the catalyst of potassium hydroxide (Berrios et al., 2009).
Poultry waste rendering process
Poultry wastes such as chicken feathers, blood, offal and other trimmings are crushed. They are heated in two ways depending on whether the dry or wet method is used. In both cases the product is pressed to generate a solid and a liquid. The solids are defatted to produce a high protein meal for feed whilst the fat is heated and concentrated to chicken fat. This fat is then used further for biodiesel production.
It is estimated that 4.78 million litres of chicken fat oil could generate 4.1 million litres of biodiesel.
The Rendering Process for Tallow
If animal fat is used say from the slaughterhouse, then it must be rendered down which is usually for 30 minutes at 260 to 270°C. The rendering process converts slaughter house animal byproducts usually carcasses into two product streams of fat (tallow) and high protein meal.
In most tallows, 50% of the total fatty acids are saturated (Adewale et al., 2014). Tallow is higher in palmitic and stearic acid which means it has a high melting point and viscosity.
Non-edible tallow has a high free fatty acid content. It is more expensive to convert to biodiesel than other feedstocks. Bhatti et al., (2008) studied various process parameters on biodiesel production from waste tallow using the acid catalysed transesterification process. They reported that under optimal processing conditions (60°C, oil/methanol molar ratio of 1:30, and 2.5 g of sulphuric acid) 5 g of mutton tallow yielded a 93.21 % of biodiesel after 24 hours.
The Use of Lard
Lard has also been used for transesterification using potassium hydroxide as a catalyst. In one study, the most suitable operational conditions were an agitation speed of 600 rpm, and 0.9 wt.% catalyst concentration. Furthermore, the product characterization was set out to meet the requirements laid down by the European Standard for biodiesel fuel (EN 14214) (Berrios et al., 2009).
Waste Frying Oils
Waste frying oil (WFO) is an exceptionally good source of biodiesel. Transesterification reactions can be carried out for 1 hour using WFOs, with methanol, and sodium hydroxide as catalyst. For oils with an acid value of 0.42 mg KOH/g, results show that a methanol/WFO ratio of 4.8 and a catalyst/WFO ratio of 0.6% gives the highest yield of methyl esters (Felizardo et al., 2006).
Impurities in Feedstock (Vegetable & Animal Fats).
It is noted that impurities in feedstock material impact their physicochemical behaviour. The high level of free fatty acids present in wastes derived from any source means that transesterification cannot be applied directly. The FFA level must be reduced by acid pretreatment and/or enzyme catalysed transesterification (Adewale et al., 2014).
So, pretreatments are necessary to remove the excess of water, free fatty acids and suspended solids of animal fats before transesterification. Some pretreatments for moisture reduction are heat drying, silica gel, calcium chloride or anhydrous sodium sulfate (Felizardo et al., 2006).
The High Sulphur Content of Biodiesel from Animal Fat
Sulphur is problematic with animal fat derived biodiesel. In the USA, biodiesel must only contain up to 15 ppm of sulphur. However some samples of beef tallow contain over 100 ppm of sulphur and chicken fat is no better. This sulphur is derived from sulphur-containing amino acids such as threonine, methionine and cysteine which are part of the proteins carried over from rendering.
Vacuum distillation is probably the best method of removal although the addition of zeolites including kaolin and alumina will also remove sulphur compounds.
Why is neat vegetable oil not used?
The major reason for not using a neat vegetable oil as fuel is its high viscosity, usually in the range of 28–40 mm2/s, which leads to operational problems in diesel engine including formation of deposits and injector coking due to poorer atomization upon injection into the combustion chamber. Transesterification of the oil reduces the viscosity of the oil to a range (usually 4–5 mm2/s) closer to that of petrodiesel.
Alcohols used In The Production of Biodiesel
Methanol is the most common alcohol used in transesterification. The other alcohols used but in increasingly smaller amounts are ethanol, propanol, iso-propanol and butanol.
Ethanol is less expensive than methanol and can be produced by its own bioproduction process which implies that biodiesel production using this alcohol is more bio-based than with methanol. Butanol can also be produced by a bio-based process
The process of methanolysis for vegetable oils and animal fats is a molar ratio of 6:1 of MeOH to oil, 0.5% wt.% alkali catalyst in a reaction vessel operating at 600 rpm stirrer speed, 60ºC reaction temperature and 1 hour reaction time. The classic reaction produced FAME and glycerol (Freedman et al., 1984).
Catalytic Reactions
Different catalysts are used for biodiesel production primarily in transesterification.
The alkaline base catalysts most typically used in transesterification reactions are alkalis (sodium hydroxide, sodium methoxide, potassium hydroxide, potassium methoxide, sodium amide, sodium hydride, potassium amide and potassium hydride).
The acid catalysts are acids (sulphuric acid, phosphoric acid, hydrochloric acid or organic sulfonic acid), heterogeneous catalysts like enzymes (lipases) and complex catalysts like silicates, zirconias, nanocatalysts, etc. (Ma & Hanna, 1999).
In a number of cases, catalysts such as p-toluenesulphonic acid (tosyl acid) or methylsulphonic acid are preferred.
Acid catalysts are claimed to be the most commonly employed (Su & Guo, 2014) but they are not as effective as homogeneous alkaline base catalysts. The alkaline catalysts are generally faster, less expensive and drive a more complete reaction than acids (Boocock et al., 1996a).
Glycerol – A Fat Rich Organic Material
Glycerol (or glycerine or 1,2,3-propanetriol) is a colourless, odourless, viscous and non toxic alcohol, which liquefies at 17.8C. The chemical formula of glycerol is C3H5(OH)3.
Glycerol can be obtained from:-
- biological fermentation,
- chemical synthesis from petrochemicals,
- hydrogenation of sucrose in the presence of a catalyst under high pressure and temperature,
- the production of bioethanol,
- by-product of the production of soap,
- the reaction of transesterification of vegetable and animal oils for biodiesel production
The glycerol derived from biodiesel production has impurities that increases the operational costs of these industrial processes. The main impurities in crude glycerol are methanol, salts of potassium and sodium, heavy metals and soap; and water, fatty acids and other organic impurities.
The concentration of these impurities in the crude glycerol, as well as some physicochemical parameters such as pH, density, colour, and concentration of organic matter, varies depending on the nature of the animal or vegetable oil used and on the industrial process used for the biodiesel production. Sulphates which are used as a catalyst in biodiesel production have become especially problematic for glycerol handling in anaerobic digesters because sulphate-reducing bacteria reduce sulphates to the extremely noxious gas, hydrogen sulphide.
Economics Of Glycerol Production
If the feedstock is oil and fat, every gallon of biodiesel produced generates 105 pounds of glycerol. The price of crude glycerol is very low because of the biodiesel industry and new processes using this material are needed.
Glycerol is readily biodegradable and contains a considerable amount of carbon. That makes it highly desirable as a useful co-substrate for anaerobic digestion to produce good quality biogas. It appears that any source of glycerol is feasible and that supplying a nitrogen source is the most effective co-feedstock to use.
Codigesting the glycerol with swine manure raises methane yields by 215 mL CH4/gCOD which was 125% more than when swine manure was digested alone giving a yield of 96 mL CH4/g COD (Astals et al., 2011).
Processing of Biodiesel
The separation of biodiesel from glycerol is often problematic. One method is to use membranes for separation. Ceramic membranes for microfiltration and ultrafiltration have been tested in a tangential filtration module (Gomes et al., 2010). Glycerol was retained with a 0.2 μm membrane. These membranes were prepared as tubular Al2O3/TiO2 ceramic construction.
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