Heterogeneous Catalysts in Bio Diesel Production – A Review

Heterogeneous Catalysts in Bio Diesel Production – A Review

Shripad Diwakar 1 Rana Pratap Reddy2

Department of Mechanical Engineering Department of Mechanical Engineering BMSITM, Bengaluru RITM, Bengaluru

Abstract: – Bio diesel has attracted in recent years as a renewable fuel with less pollutant emissions as compared to diesel fuel on its combustion. Bio diesel fuel is made from vegetable oils, animal fats and microbial oils. The most common method of Bio diesel production is by means of Trans-esterification process, where, triglycerides present in the oil reacts with monohydric alcohol in the presence of a catalyst. Synthesis of Bio diesel necessarily needs a catalyst to attain the equilibrium in practical manner. Homogeneous acid catalysts are corrosive in nature and take more time for Bio diesel synthesis, alkaline catalysts requires the thorough rinsing with water to remove the leftover catalyst, resulting in waste water generation and consequently results in the loss of production. In order to overcome these constraints, this review article is focused on finding suitable Heterogeneous catalysts that can be easily separated and gives a high yield and conversion.

Keywords: Heterogeneous catalyst, Bio diesel, vegetable oils, Trans-esterification

1. INTRODUCTION:

   In recent years, biodiesel has gained international attention as a source of alternative fuel due to characteristics like high degradability, no toxicity, low emission of carbon monoxide, particulate matter and unburned hydrocarbons [1]

   Biodiesel is a mixture of alkyl esters and it can be used in conventional compression ignitions engines, which need almost no modification. As well, biodiesel can be used as heating oil and as fuel[2] ,So far,this alternative fuel has been successfully produced by transesterification of vegetable oils and animal fats using homogeneous basic catalysts (mainly sodium or potassium hydroxide dissolved in methanol). Traditional homogeneous catalysts (basic or acid) possess advantages including high activity (complete conversion within 1 h) and mild reaction conditions (from 40 to 65°C and atmospheric pressure). However, the use of homogeneous catalysts leads to soap production. Besides, in the homogeneous process the catalyst is consumed thus reducing the catalytic efficiency. This causes an increase in viscosity and the formation of gels. In addition, the method for the removal of the catalyst after reaction is technically difficult and a large amount of wastewater is produced in order to separate and clean the products, which increases the overall cost of the process. Thus, the total cost of the

   biodiesel production based on homogeneous catalysis, is not yet sufficiently competitive as compared to the cost of diesel production from petroleum. An alternative is the development of heterogeneous catalysts that could eliminate the additional running costs associated with the above mentioned stages of separation and purification. In addition, the use of heterogeneous catalysts does not produce soap through free fatty acid neutralization and triglyceride saponification. Therefore, development of efficient heterogeneous catalysts is important since opens up the possibility of another pathway for biodiesel production. The efficiency of the heterogeneous process depends, however, on several variables such as type of oil, molar ratio alcohol to oil, temperature and catalyst type. Thus, the objective of this review paper is to present a review of the effect of the variables on important characteristics of biodiesel such as methyl esters content. Some characterization techniques for both, biodiesel and heterogeneous catalysts will also be addressed.

2. TRANS-ESTERIFICATION PROCESS:

   Nowadays, there are four known methods to reduce the high viscosity of vegetable oils to enable their use in conventional compression ignitions engines: blending with diesel, pyrolysis, emulsification and trans- esterification. The pyrolysis and the emulsification, however, produce heavy carbon deposits, incomplete combustion, an increase of lubricating oil viscosity and undesirable side products such as alkanes, alkenes, alkadienes, aromatic compounds and carboxylic acids. Regarding the direct use of vegetables oils as fuel for combustion engines, this requires the engines to be modified [3] ,Also, the direct use of vegetables oils is not feasible due to their high viscosity and low volatility which affect the atomization and spray pattern of fuel, leading to incomplete combustion, severe carbon deposits, injector choking and piston ring sticking[4]. The alcohol used for trans-esterification is usually methanol. Thus, the most common way to produce biodiesel is by trans-esterification of triglycerides of refined/edible types of oils using alcohol, in presence of an acid or a basic catalyst[5]. The alcohol used for trans-esterification is usually methanol. The general scheme of trans esterification reaction is shown in Fig.1, where R is a mixture of various fatty acid chains[6]

   Fig 1: Reaction for Oil trans-esterification

3. BIO DIESEL PRODUCTION:

   Biodiesel in another definition is a non-petroleum based fuel consisting of alkyl esters derived from trans- esterification of triglycerides (TG) or by the esterification of free fatty acids (FFA) with low molecular weight alcohols. The fuel properties of the esters produced from the trans-esterification can vary depending on the types of vegetable oils used.

   Biodiesel production has become an area in which many researchers have increasing interest. This is due to its potential as an alternative fuel that offers a complementary strategy for sustainability. The most common approach of

   biodiesel production is by trans-esterification of vegetable oils and animal fats. This is a well established process introduced since 1853. Varieties of vegetable oils have been exploited in the past and present for biodiesel production with varying but promising results. The oils used include cotton seed, soybean, waste cooking, rapeseed, sunflower seed, Jojoba and jatropha curcas. The choices of alcohols used are mainly methanol, ethanol and butanol. The catalysts used in the transesterification include sodium hydroxide, potassium hydroxide, sulphuric acid supercritical fluids or enzymes such as lipases[6,7,8]

   The steps involved in the Bio diesel production is shown in Fig.2

   Fig 2: Block diagram of Bio Diesel Production process

4. CATALYSTS

   A catalyst is a chemical that helps to speed up the chemical process without actually participating in it. there are two types of catalysts (i) Homogeneous catalysts (ii) Heterogeneous catalysts. Homogeneous acid catalysts such as hydrochloric and sulphuric acids and homogeneous alkaline catalysts[10] such as potassium hydroxide, sodium hydroxide, sodium methoxide are used in Bio diesel synthesis. The major disadvantage of using homogeneous catalysts is the fact that these cannot be reused or regenerated, because the catalyst is consumed in the reaction and the separation of the catalyst from the products is difficult and requires more equipment and the process implies further stages of washing, which involves an increase in production costs, more ever, the process is not eco friendly, because large quantity of waste water is produced in the separation step.

   Based on the above things, developing new solid catalysts seems to be an appropriate solution to overcome the problems associated with homogeneous catalysts. Metal hydroxides, Metal complexes, metal oxides such as calcium oxide, magnesium oxide, zirconium oxide and supported catalysts have been investigated as solid catalysts. Because, solid catalysts, are not consumed or dissolved in the reaction and therefore can be easily

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   separated from the products. As a result, the products do not contain impurities of the catalyst and the cost of final separation can be reduced. The catalysts can also be readily regenerated and it is reused and more eco friendly because there is no need for acid or water treatment in the separation step. One of the major problems associated with the heterogeneous catalysts is the formation of three phases with alcohol and oil which leads to diffusion limitations thus lowering the rate of reaction.

5. Heterogeneous Catalysts

   It is appropriate to begin the discussion on heterogeneously catalyzed trans-esterification with a comparison of factors in relation to the corresponding homogeneously catalyzed reaction[6]. Such a comparison is summarized in Table 3. In heterogeneous catalysis, a number of operating parameters such as temperature, extent of catalyst loading, mode of mixing, alcohol/oil molar ratio, presence/absence of impurities in the feed stock and the time of reaction are important. In a similar manner, trans-esterification reaction can also be carried out under supercritical conditions and this method may also evolve as a viable alternative to catalytic routes[9]

   Table 3: Comparison of Homogeneous and Heterogeneous catalyzed trans-esterification

   S. No	Factors	Homogeneous catalysts	Heterogeneous catalysts
   1	Reaction rate	Fast and High conversion	Moderate conversion
   2	Processing methodology	Limited use of continuous methodology	Continuous fixed bed operation is possible
   3	Presence of water/ free fatty acids	Not sensitive	Sensitive
   4	Catalyst reuse	Not possible	Possible
   5	Cost	Comparatively costlier	Potentially cheaper

   1. Solid acid catalysts

      Solid acid catalysts have the potential to replace strong liquid acids to eliminate the corrosion problems and consequent environmental hazards posed by the liquid acids. However, the efforts at exploiting solid acid catalysts for trans-esterification are limited due to the pessimistic expectations on the possibility of low reaction rates and adverse side reaction. As a result, the factors governing the reactivity of solid catalysts have not been fully understood.[6]

   2. Zeolites

      Zeolites can be synthesized with extensive variation of acidic and textural properties. They can be synthesized to overcome the diffusional limitations so that optimum biodiesel production can be achieved1. Zeolites can also be modulated to exhibit hydrophobic characteristic without compromising its functionalized acidic sites. This can be done by incorporating certain organic species like heteropoly acids into their pore structures [10].

   3. Other solid acid catalysts:

      The other types of solid acid catalysts that were exploited for use in esterification and trans-esterification

      reaction studies in the past include tungsten oxides, sulphonted zirconia (SZ), sulphonated saccharides, Nafion resins, and organosulphonic functionalized mesoporous silicas. Even though solid acid catalysts have been applied effectively in the esterification of carboxylic acids, the use of these catalysts to obtain high conversion of triglycerides to biodiesel necessitates much higher reaction temperatures than base catalysts because of their lower activity for trans-esterification. Some resins, may be considered an exception as these catalysts catalyze appreciably both esterification and trans-esterification reactions under mild reaction conditions due to their high concentrations of acid sites. However, thermal stability becomes an issue when resin-type catalysts are used at higher temperatures in order to achieve higher reaction rates in an application such as reactive distillation. The other issue is associated with catalyst regeneration [6,10,11]

   4. Solid base catalysts

      1. Basic zeolites

         The base strength of the alkali ion exchanged zeolite increases with increasing electropositive nature of the exchanged cation. The occlusion of alkali metal oxide clusters in zeolite cages through the decomposition of impregnated alkali metal salts results in an increase in the basicity of these materials.

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         These exchanges can affect the water tolerant behavior of the basic zeolite system2. A micro-porous inorganic lithium containing zeolite has been shown to be a new generation solid base catalyst for transesterification. Most of these catalysts contain the basic sites (cation) generated by thermal decomposition of the supported salt. It has been shown that the conversion to methyl ester over NaX faujasite zeolite that was ion exchanged with more electropositive cations was higher than that of the parent zeolite5.the intermediate electro negativity of the solid is correlated with the yield of the methyl ester.

         This type of correlation has become one of the preferred methods for catalyst selection. Although some guidelines on these aspects are available, a general concept has yet to be developed to facilitate the catalyst selection for trans-esterification[11]

      2. Alkaline metal salt on porous support

   The non-loaded alumina support did not yield methyl oleate or glycerol within 1 h, and yielded only 7% methyl oleate over 12 h at 423 K. Alumina loaded with K2CO3, KF, LiNO3 and NaOH, on the other hand, produced glycerol and higher yield of methyl oleate over 1 h at 333K. This provides evidence that porous supports can be functionalized effectively by loading alkaline metal salts, with the exception of KOH/Al2O3 that reportedly exhibited low catalytic activity[12]

6. CONCLUSION:

   The development of heterogeneous catalysts is essential for the production of bio diesel due their advantages. The efficiency using heterogeneous catalysts, however, depends upon various parameters such as type of oil, molar ratio of alcohol to oil, temperature and type of catalyst etc.This review article, suggests that development of heterogeneous catalysts has been growing because increasing bio diesel consumption requires optimized production processes that are compatible with high production capacities and that features simplified operations, high yields and the absence of special chemical requirements and waste streams. From the commercial point of view, solid base catalysts are more effective than acid catalysts and enzymes.

7. REFERENCES:

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