BIODIESEL PRODUCTION AND FUEL QUALITY-REVIEW: INTRODUCTION

Posted by Kathryn Schwartz on November 25, 2013
Trans-esterification

INTRODUCTIONDifferent Process for Manufacturing Biodiesel

Biodiesel is produced by chemically reacting a fat or oil with an alcohol, in the presence of a catalyst. The product of the reaction is a mixture of methyl esters, which are known as biodiesel, and glycerol, which is a high value co-product. The process is known as Trans-esterification, as shown in the equation below, where R1, R2, and R3 are long hydrocarbon Chains, Sometimes called fatty acid chains.


Biodiesel Production and-1

There are only five chains that are common in most vegetable oils and animal fats (others are present in small amounts). The relative Amounts of the five methyl esters determine the physical properties of the fuel, including the cetane number, Cold flow, and oxidative stability.

Biodiesel can be used neat and when used as a pure fuel it is known as B100. However, it is often blended with petroleum-based diesel fuel and when this is done the blend is designated “BXX” where XX is the percentage of biodiesel in the blend. For example, B20 is a blend of 20% biodiesel and 80% petroleum diesel fuel.

While virtually all commercial biodiesel producers use an alkali-catalysed process for the trans-esterification process, other approaches have been proposed including acid catalysis and enzymes. The use of acid catalysts has been found to be useful for pretreating high free fatty acid feed stocks but the reaction rates for converting triglycerides to methyl esters are very slow. Enzymes have shown good tolerance for the free fatty acid level of the feedstock but the enzymes are expensive and unable to provide the degree of reaction completion required to meet the ASTM fuel specification. Immobilization of the enzyme and use of multiple enzymes in sequence may provide future opportunities in this area.

The most common oil used in biodiesel production is soybean oil in U.S. A and rapeseed oil in Europe. However all vegetable oils animal fats can be used as feed stock. The chemical process involved in the biodiesel production is called esterification.

Fig1Biodiesel Production and-2
Figure 1 : Chemical reaction for bas catalyzed biodiesel production

The formation of at least one ester depicted in Figure 1 above from the figure, R is a short chain hydrocarbon in the alcohol; R1, R2, and R3 are fatty acid chains associated with oil or fat, which are largely palmitic, stearic, oleic and lin oleic acids for naturally occurring oils and fats (catalysts 1%).

Depending on the feedstock and alcohol used, a feed quality fat may be produced in place of the fertilizer component shown above, while the glycerine formed as a by – product can be used in the cosmetic and pharmaceutical industries. During the esterification process, the triglyceride in vegetable oil or animal fat reacts with an alcohol (normally methanol or ethanol) in the presence of catalyst that has already been mixed the alcohol to form alkyl ester (biodiesel) and glycerine. The alcohol is charged in excess to assist in quick.

Esterification is used as a simple method for adapting plant to the requirements or specifications of diesel engines and the mass balance of the process inputs and outputs is as follows.

Veg. oil (87%) + Alcohol (12%) ^ Biodiesel (80%) + Alcohol(4%) + Glycerine(9%) + Fertiliser (1%)………..(2)

Depending on the feedstock and alcohol used, a feed quality fat may produce in place of the fertilizer component shown above. While the glycerine formed as a by-product can be used in the cosmetic and pharmaceutical industries. During the esterification process, the triglyceride in vegetable oil or animal fat reacts with an alcohol (normally methanol or ethanol) in the presence of a catalyst that has already with an alcohol to form mono alkyl esters (biodiesel) and glycerine. The alcohol is charged in excess to assist in quick conversion. The fuel produced consists of a maximum of 6 – 7 fatty acids, depending on the feedstock used. The following three methods of producing biodiesel are common.
a. Base catalysed transesterification of oil using an alkaline (base) catalyst such as sodium hydroxide, potassium hydroxide and sodium methoxide;
b. Direct acid catalysed esterification using an acid catalyst such as sulphuric, sulphonic and hydrochloric acids;
c. Conversion of oil to fatty acids and then to alkyl esters through acid catalysis.

The first method is deemed to be most common and most economical and is used by commercial producers because it is fast. The second method is favoured for esterification of waste oils such as restaurant oils and frying oils due to their free fatty acid contents, which are not compatible with alkaline catalysts. A related most method used in the production of the so-called synthetic biodiesel is termed as the biomass-to liquid (BTL) process in which solid feedstock is converted into charcoal and a gas. The gas is purified and liquefied through the Fischer-Tropsch reaction and used as biodiesel.

Biodiesel production is considered to be technically simple and a “home-brewing” trial has been reported. A test-batch mini-processor for producing 1-2 considerably reduces the viscosity (the major disadvantage) of vegetable oil from a range of 11 – 17 times to just about twice that of petroleum diesel. Table 1 presented some fuel properties of two B100 biodiesel types (rapeseed methyl ester and rapeseed ethyl ester) and those of high-grade petro diesel.

Considerable amount of work has done at laboratory and filed/road levels to assess the performance of biodiesel and its blends in C1 engines.

Table 1 : Some fuel characterization data for biodiesel (B100) and No. 2 diesel (D2)
table1Biodiesel Production and-3

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