Biomas Ando Bioenergy Production Of Biodiesel Biology Essay

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Biodiesel is gaining more and more importance as an attractive fuel due to the depleting fossil fuel resources. Biodiesel is a renewable substitute fuel for petroleum diesel fuel which is made from nontoxic, biodegradable, renewable sources such as refined and used vegetable oils and animal fats. Biodiesel is produced by transesterification in which oil or fat is reacted with a monohydric alcohol in the presence of a catalyst. The process of transesterification is affected by molar ratio of alcohol to oil, type of alcohol, nature and amount of catalysts, reaction time, and temperature. Various studies have been carried out using different oils as the raw material and different alcohols (methanol, ethanol, butanol), as well as different catalysts, notably homogeneous ones such as sodium hydroxide, potassium hydroxide, sulfuric acid, and supercritical fluids or enzymes such as lipases. Recent research has focused on the application of heterogeneous catalysts to produce biodiesel, as is the case zeoltias, because of their environmental and economic advantages. Heterogeneous catalysts are promising for the transesterification reaction of vegetable oils to produce biodiesel, could improve the synthesis methods by eliminating the additional processing costs associated with homogeneous catalysts. Moreover, they can be reused and regenerated. In the present paper various methods of production of biodiesel with different catalysts have been described.

Introduction

Biofuels are an important alternative to current energy demand nationally and globally. Use of products as oils or household waste in the production of ethanol, hydrogen and obtaining liquid fuels from pyrolysis and anaerobic digestion respectively and biodiesel production have grown steadily providing greater energy security. In general, the limitations to the development of each of these technologies reside in obtaining suitable catalysts. Furthermore, it must take into account the optimal reaction conditions (temperature, pressure and reagent systems) in order to obtain good results and which give a good economic viability technology [1].

Biodiesel

Biodiesel is composed of monoalkyl esters of long chain fatty acids derived from renewable raw materials (vegetable oils and / or animal fats), which can be used alone or blended with petroleum diesel. [2] Its combustion emitted to the atmosphere an amount of CO2 to be absorbed by another plant in the photosynthesis process (Figure 1). Thus, the use of a compression ignition engine with biodiesel does not modify the carbon cycle [3].

Figure 1. Basic cycle for biodiesel carbon [3].

The use of vegetable oils as fuel is not new, Rudolph Diesel (inventor of the diesel engine) already used peanut oil in its engines in 1930.

More than a century later, these engines support the use of biodiesel, which is only modified vegetable oil, with properties very similar to those of conventional diesel. It is possible to use directly in diesel engines both direct and indirect injection without modification. In fact, this product is currently used in over 25 countries around the world [4].

Transesterification reaction

The transesterification is also known as alcoholysis reaction between a vegetable oil or fat with an alcohol to form esters and glycerol (Figure 5) is also one of the most used commercially to produce biodiesel. [5]

Figure 5. transesterification reaction.

To complete a transesterification reaction stoichiometrically required a ratio of 3 molecules of alcohol per one molecule of oil. In practice, for a maximum performance, this ratio must be greater than the stoichiometric ratio since the reaction is reversible. Therefore, an excess of alcohol shift the equilibrium to the product side.

The transesterification reaction can be carried out with different kinds of alcohols such as methanol, ethanol, propanol, butanol and alkali catalysts such as acids and enzyme [6].

Parameters to be considered transesterification reaction

The transesterification reaction involves some critical parameters that strongly influence the final yield. The most significant variables are [7]:

• Free fatty acids and water content in the oil.

Water and free fatty acids are important in the production methods of acid-alkaline biodiesel. Commercial crude oils and fats contain free fatty acids provided in different percentages. Free fatty acids and water produce adverse effects in conventional oils, since the presence of these cause formation of soaps, consumes catalyst, decreases catalyst efficiency and product quality decreases, resulting in a low conversión [8-10] .

• Reaction temperature.

The transesterification occurs at various temperatures depending on the alcohol and oil used. While increasing the temperature increases the yield and the reaction time is reduced, it is recommended that the temperature does not exceed the boiling point of the alcohol [8-10].

• molar ratio alcohol / oil.

According to the stoichiometry of the reaction, the transesterification of triglycerides with alcohol requires that the molar ratio of alcohol / oil is 3:1. Since the transesterification reaction is reversible, the excess alcohol displaces the equilibrium in the direction of formation of esters. However, this may generate problems of phase separation, the yield decreases and increases production costs. The optimum molar ratio used at industrial level and the most common is 6:1 in the methanolysis of soybean oil, sunflower and cotton [8-10].

• type catalyst.

The type of catalyst depends on the type of oil used. If the oil has a high content of free fatty acids and moisture is recommended to use acid catalysts, because base catalysis favors the formation of soaps. However, the most basic catalysts are used in industry because they require temperatures and molar ratios alcohol / low oil, short reaction times and corrode less the equipment [8-10].

• Reaction time

The greater the reaction time, the reaction yield increases. It has been reported that increases the conversion of 87.81% to 88.90% with reaction times of 50 to 90 minutes, respectively, using castor oil [36]. As the catalyst, as mentioned above, the acidic catalysis requires more time to achieve good performance in contrast to the alkaline catalysis requires less reaction time [8-10].

Tecnologías para la producción de biodiesel

Acid and base catalysis

The process carried out by alkaline catalysis is relatively fast, but it is affected by water content and the free fatty acids present in the oils or fats. The free fatty acids can react with soaps and catalyst to form water.

The alkaline transesterification is most effective when the levels of free fatty acids is less than 1%. The process most commonly used because it requires temperatures of 60 ° C and 70 ° C and atmospheric pressure with an excess of alcohol achieving high conversion. This process requires little time and there is a direct conversion of biodiesel without any intermediate steps. The most common alkaline catalysts are used for this process are NaOH and KOH and the elimination of these catalysts is technically difficult and leads to additional cost to the final product [7,11-13]

Table 7 shows the typical conditions of a transesterification reaction by alkaline catalysis in the synthesis of biodiesel.

Table 7. Conditions in a transesterification reaction by alkaline catalysis

Raw material

Vegetable oils with low free fatty acids content (<0.5%) + short chain alcohol

Molar ratio alcohol/oil

6:1

Reaction time (min)

60

Reaction temperature (°C)

60-70

Reaction pressure (bar)

1.4-4.1

Catalyst concentration

NaOH ( the most common) de 0.5 a 2 % by weight of raw material

yield

(>95%)

Recovery of glycerol

Hard

Esters purification

Hard

Production cost of catalyst

Low

The processes carried out by acid catalysis have a slower reaction rate and require a high ratio of alcohol / oil (20:1 or more). Another major problem in the use of acidic catalysts is the formation of water remaining in the reaction [7,11-13].

Generally, the reactions are catalyzed by acidic catalysts used to convert free fatty acids to esters or ester-soaps as a pretreatment for the raw materials that contain high amounts of free fatty acids.

The acid catalysts used are sulfuric acid, phosphoric acid, hydrochloric acid and sulfonic acid.

In general, the acid catalyzed reactions are carried out at high molar ratios, high catalyst concentrations and high and low temperatures.

Table 8 shows the catalytic activity in the transesterification of oils by acid catalysis.

Table 8. Conditions in a transesterification reaction by acid catalysis.

Raw material

Vegetable oils with high free fatty acids contents (>1%) + alcohol de cadena corta

Molar ratio

20:1

Reaction time (min)

240

Reaction temperature (°C)

55-80

Reaction pressure (bar)

4

Catalys concentration

H2SO4

Yield

(>97%)

Recovery of glycerol

Hard

Esters purification

Hard

Production cost of catalyst

Low

Enzymatic Catalysis

Biodiesel also may be produced by biocatalytic processes transesterification in the presence of an enzyme such as lipase. This process has many advantages over the conventional catalysts, produces no by-products, requires moderate processing conditions (35 ° C-45 ° C) and the catalyst can be recycled [7,11-13].

Enzymatic reactions are not affected by the presence of free fatty acids or water containing the raw material. However, it has the disadvantage that the process has not been commercialized because of the high cost of production of the enzymes.

Table 9 shows the conditions in a process of transesterification catalyzed by a lipase.

Tabla . Conditions in a transesterification reaction by enzymatic catalysis.

Raw material

Vegetable oils or Animal fat

Reaction temperature (°C)

30-40

Yield

High

Recovery of glicerol

Easy

Esters purification

none

Procuction cost of catalyst

expansive

Supercritical metanol

Among the many proven technologies to improve the production of biodiesel are those which are used in supercritical alcohols. This process requires a short reaction time (4 minutes) to complete the transformation reaction. The operating conditions are the supercritical alcohol (temperature 350-400 ° C and pressure greater than 80 bar). This method avoids the complications of purification in later stages. However, it requires a high ratio of alcohol / oil (42:1) and the operating cost is very high [7,11-13].

Table 10 shows the conditions of transesterification in a supercritical process.

Tabla . Conditions in transesterification reaction by enzimatic catalysis

Alcohol

methanol

Molar ratio alcohol/oil

42:1

Reaction time (min)

7-15

Reaction temperature (°C)

340-385

Catalys concentration

none

Yield

98%

Recovery of glicerol

Hard

Free fatty acids

Methyl esters, water

heterogeneous catalysts

Considering the limitations that the traditional process of obtaining biodiesel presents, efforts have been made to obtain better catalysts. Thus, there are several studies reporting the heterogeneous solid catalyst activity used in the transesterification. Among these are oxides of tin, zinc and magnesium.

Calcium oxide has been used extensively as a basic catalyst since it raises many advantages such as long life as a catalyst, higher activity and moderate reaction conditions.

One of the advantages of the solid catalysts in comparison with the homogeneously catalyzed transesterification process is that the solid can tolerate extreme reaction conditions. Thus the temperature can range from 70 ° C to 200 ° C to achieve more than 95% yield using MgO, CaO and TiO2 [7]

Heterogeneous catalysts such as calcium oxide and calcium methoxide are insoluble in organic solvents. When these catalysts are used, the reaction becomes very slow because the three-phase mixture of oil-alcohol-poor catalyst for mass transfer [13].

Table 11 shows the typical conditions of transesterification catalyzed by heterogeneous catalysts.

Tabla . Conditions in transesterificaction reaction by heterogeneous catalyst.

Raw material

Vegetable oils with high free fatty acids contentsv+ alcohol de cadena corta

Molar ratio

5.3:1

Reaction temperature (°C)

180-220

Catalys concentration

CaO

Yield

Normal

Recovery of glycerol

Easy

Esters purification

Easy

Production cost of catalyst

Potentially cheaper

There are many more important factors in choosing a catalyst. This should be consistent, should catalyze the reaction in the shortest possible time and with acceptable yield and selectivity. And let's not forget the price and lifetime.

Moreover, in heterogeneous catalysis, it must be remembered that not all the surface of a catalyst is active. Generally catalysts are sought particle size as small as possible to maximize the exposure of their active centers. However, a pulverulent solid material is difficult to handle in a process of reaction in both industrial and laboratory since it often compacted under the reaction conditions making it difficult to mass transfer. That problem is solved by extending the surface area of the catalyst itself through its dispersion on porous materials such as zeolites, silica and / or activated carbon [14].

Among these porous materials stand out the zeolites, catalyst supports most curently used. This is because its porous structure has good properties to be used in ion exchange, adsorption and catalysis, in addition to its high thermal stability, mechanical and chemical.

Mainly been studied by presenting the environmental benefits and this makes their study is in full development.

Zeolites

Zeolites by definition, are crystalline aluminosilicates of alkali and alkaline earth metal cations (potassium, calcium and sodium). A frame consisting of tetrahedrons of [SiO4]4- and [AlO4]5- connected to each other in the corners by oxygen atoms [15].

The dimensions of the opening of the cavities may vary from 2.6 to 10 Amstrongs. There are about 40 types of natural zeolites and 100 obtained via chemical synthesis.

The microporosity of these solids is open and the structure allows mass transfer between the intracrystalline space and the surrounding medium.

Catalytic activity of zeolites.

Zeolites are catalysts used worldwide. They have received special attention because of their properties (microporous structure, chemical composition and easily varied ion exchange charge compensating cations) which makes them a group of materials widely used in catalytic processes such as the conversion of hydrocarbons (alkylation , dehydration, conversion of methanol to gasoline) inorganic oxidation reactions of H2S, carbon monoxide oxidation, dissociation of water and organic chemical reactions as biodiesel production [16].

Factors that influence the catalytic activity of zeolites are:

• The structure of the zeolite, determined by the shape and size of your pores.

• The type, size and charge of the cation.

• Degree of exchange.

• Ratio silicon / aluminum, which can affect activity and selectivity.

• The number of proton donors present in the structure.

• The presence of metallic components scattering activated state.

Zeolites as catalysts in the production of biodiesel.

Zeolites as catalyst have the characteristics of acidic sites. Moreover, vary in pore structures and surface properties attributed to their varying catalytic properties. Zeolite can accommodate a wide variety of cations such as Na+, K+, Ca2+, Mg2+ and many others, that attributed to its basic nature. The acid-base properties of zeolites are controlled by the kinds and quantities of ion-exchanged cations, and by the Si/Al ratio of the main zeolite framework [17-19].

according to the study of Suppes et al. [20], the incorporation of NaOx species in zeolite-X drastically increased the basic strength and concentration, which eventually enhanced the transesterification activity, and the yield of methyl ester from soybean oil increased from 22.7 to 94.2% when NaOx/NaX replaced K-X as a catalyst. Showed higher conversion of methyl esters (94.6% and 94.2%, respectively) at 120 °C for 24 h owing to their higher basicity and larger pore volume resuling in improved intra-particle diffusion. Optimum conditions were calcination of catalysts at 500°C, which resulted in the removal of water and carbon dioxide from its surface. At low temperature (60°C) conversion obtained was less (<85%) even after 24 h reaction time. At 100 °C reaction temperature, 92% conversion was achieved in 3 h with diglyceride and monoglyceride concentration of 4% and 2%, respectively. Presence of FFA had a significant effect on the reaction and reduced the methyl ester conversion to less than 13.7% even after 4 h reaction time.

it is noteworthy that such high yields were obtained at a methanol/oil molar ratio of 6:1, which is the lowest for heterogeneous biodiesel synthesis

A similar result was observed when KOH was impregnated on the Na-X zeolites [21]. The methyl ester yield reached 85% at 65°C after 8 h with a methanol/oil molar ratio of 10:1.

Zeolites (mordenite, beta, and X), were used as heterogeneous catalysts for transesteri-fication of sunflower oil [22], gave methyl ester contents of 93.5- 95.1 wt.% at 60 °C reaction temperature. However, the method employed for the preparation of the catalyst was longer. This required heating, drying, and calcining at 500 °C for 10 h, 120 °C for 12 h, and 550 â-¦C for 15 h respectively.

Chung et al. [23], studied removal of FFA from waste frying oil by esterification with methanol using various zeolite catalysts. The ZSM-5 (MFI), modern site (MOR), fauja site (FAU), beta (BEA) zeolites, and silicalite were employed with different Si/Al molar ratio in the reaction. The effects of acidic properties and pore structure of the zeolite catalysts were discussed relating to the conversion of the FFA. The MFI zeolite induced an improvement of the removal efficiency of FFA by cracking to the FFA in its pore structure due to its narrow pore mouth. The catalytic activity for FFA removal was lowered with decreasing of acid strength of the zeolites. The strong acid sites of zeolites induced the high conversion of FFA. The acid strength and pore structure of acidic zeolites affected the catalytic activity in FFA removal.

Conclusions

This review shows the large number of technologies available for the production of biodiesel and the growing interest in the development of heterogeneous catalyst. The heterogeneous catalysis features lower corrosiveness, environmental friendliness, easy catalyst recovery and high process integrity, all at levels superior to those of conventional catalyts.

The research and development of the zeolitic catalysts for biodiesel synthesis have focused mainly on improving its slow reaction rate up to the level of its homogeneous. The reaction performances were usually evaluated in the following respects: what level of FAME yield can be achieved within a given time frame, how low the reaction temperature is and how the methanol/oil molar ratio and catalyst amount can be reduced. Generally, a higher reaction temperature (100-250 °C) and/or methanol amount are required for the performance of the heterogeneous process to equal that of its homogeneous counterparts.

The selectivity of the reaction product is also important. The reaction should produce as much FAME as possible while minimizing production of mono- or di-glycerides. The deactivation of the catalyst is the second important issue. The regeneration method for this type of poisoning usually requires a specific washing procedure. The reaction tests in the presence of FFA and/or water are considered as crucial in examining the potential for commercialization, because the utilization of waste cooking oil is a key issue for process profitability and most waste oils contain FFA and water as impurities. The decrease of activity and soap formation should be avoided to qualify the tests.

It is expected that heterogeneous catalysts may be in commercial biodiesel production in a very near future.

Bibliografía

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