Renewable Energy Technology 3(2+1)

Lesson 28. Production of Bio-Diesel

Liquid biofuel production uses both old (fermentation) and new (transesterification) technologies.  Based on production technologies, biofuels can be classified as first generation biofuels and second generation biofuels.  Conventional “first generation” ethanol is made by fermenting sugars from plants with high starch or sugar content into alcohol, using the same basic methods that brewers have relied on for centuries. The purest form of biodiesel is straight vegetable oil, but a more refined form uses a fairly simple process called transesterification to produce biodiesel.  “Second-generation” biofuel technologies employ more sophisticated processes to convert biomass into fuel.  These include enzymatic and other processes to convert cellulose from grasses and waste wood into ethanol and other fuels, and to process animal waste and fat, algae, and urban wastes into biodiesel.  Many developed countries have active bio-diesel programmes.  Currently bio-diesel is produced mainly from vegetable oils such as rapeseed, sunflower, soybean, palm oil etc. where as in India and African countries bio-diesel production is mainly from Tree Borne oils seeds such as jatropha oil, pungam etc.

Triglycerides (vegetable oils) as diesel fuels

Triglycerides of vegetable oils or fatty substances and their derivatives are considered as viable alternatives for diesel fuels.  Whereas, direct substitution of triglycerides for diesel fuels will lead to the problems such as higher viscosities, low volatilities etc.  The following processes help in making these vegetable oils and fatty substances compatible to petrodiesel fuels for various applications.

• pyrolysis

• microemulsification

• dilution and

• transesterification

 Pyrolysis

Pyrolysis refers to the process of thermochemical conversion of selected fuel materials in the absence of air or nitrogen. The liquid fractions of the thermally decomposed vegetable oil are likely to approach diesel fuels. The pyrolyzate had lower viscosity, flash point, and pour point than diesel fuel and equivalent calorific values.

Micro-emulsification

Micro-emulsions are defined as transparent, thermodynamically stable colloidal dispersions with droplet diameters from 100 to 1000 Å.  A micro-emulsion can be made of vegetable oils with an ester and dispersant (co-solvent), or of vegetable oils, an alcohol and a surfactant and a cetane improver, with or without diesel fuels.

Dilution

Dilution of vegetable oils can be accomplished with such materials as diesel fuels, solvent or ethanol.

Transesterification reaction

In the transesterification of vegetable oils, a triglyceride molecule of oil reacts with an alcohol in the presence of a strong acid or base, producing a mixture of fatty acids alkyl esters and glycerol. The fatty acid alkyl esters from the transesterification reaction are called as biodiesel. The stoichiometric transesterification reaction requires one mole of a triglyceride and three moles of the alcohol. Excess of the alcohol is used to increase the yields of the alkyl esters and to allow its phase separation from the glycerol formed.

Lab scale transesterification unit

Laboratory scale biodiesel setup of one litre capacity can be formed with a three-neck flask, water cooled condenser, thermometer, mechanical stirrer and water bath. The condenser was used to condense methanol vapour released from the reactor and return the condensed methanol to reactor. Thermometer was used to measure the reactants mixture temperature during the experiment and vigorous mixing of reactants was done by mechanical stirrer. 

Figure Lab scale transesterification unit

Studies can be conducted in lab scale biodiesel reactor to determine the optimum quantity of methanol, catalyst (NaOH), reaction temperature and reaction time required for the transesterification of selected vegetable oil by varying the concentration of methanol, NaOH concentration, reaction temperature and reaction time.  After optimizing the concentration of methanol and NaOH, re­action time and reaction temperature in alkali-catalysed transesterification of Jatropha curcus oil, experimental studies on large-scale production of biodiesel can be carried out.

Biodiesel pilot plant

After optimizing the concentration of methanol, NaOH, re­action time and reaction temperature in alkali-catalysed transesterification of Jatropha curcus oil in the laboratory set up, experiments can be carried out in biodiesel pilot plants of higher capacity. 

Figure Biodiesel pilot plant

Components of biodiesel pilot plant

The biodiesel unit consists of a biodiesel reactor, mechanical stirrer, oil lifting pumps, chemical mixing tank, steam generator and settling tanks. The biodiesel reactor is made of stainless steel and insulated to arrest heat loss so as to maintain the reaction temperature during the process. The function of mechanical stirrer is to make vigorous stirring of reactant mixtures in the reactor. Steam generator is used to supply heat to the reactor in order to maintain reaction temperature for transesterification reaction. Oil lifting pumps are used to pump the raw oil to the reactor and also to pump the reactant mixtures to glycerol settling tank for separation of biodiesel and glycerol. The chemical mixing tank having a stirrer is used to dissolve the catalyst in methanol. The chemical mixing tank is connected to biodiesel reactor.

The amount of raw material, catalyst and methanol are measured before the biodiesel production process and the end products namely glycerol and biodiesel can be measured after the reaction to assess the performance of the reactor and reaction effectiveness.

Process variables

 The most important variables that influence transesterification reaction and conversion are :

• reaction temperature

• alcohol addition

• catalyst type and concentration

Reaction temperature

The rate of biodiesel production is influenced by the reaction temperature.  The reaction temperature will be maintained just above the boiling point of type of alcohol added (60 to 70°C). 

Ratio of alcohol to oil

Another important variable affecting the yield of ester is the molar ratio of alcohol to vegetable oil.  A molar ratio of 6:1 is normally used to obtain methyl ester yields higher than 98% by weight.  Higher molar ratio of alcohol to vegetable oil interferes in the separation of glycerol.

Catalyst type and concentration

Transesterification process occurs faster in the presence of an alkaline catalyst.  Potassium hydroxide and sodium hydroxide are the most common catalyst used in transesterification process.  The optimized dosage of alkaline catalyst concentration is in the range of 0.5 to 1% by weight and will result 94 to 99% conversion of vegetable oil into esters. Further, increase in catalyst concentration does not increase the conversion and it adds to extra costs because it is necessary to remove it from the reaction medium at the end.