EFFECT OF WATER ACTIVITY ON TRANSESTERIFICATION KINETIC PARAMETERS OF COCONUT OIL WITH ETHANOL

ABSTRACT

This work is focused on determining quantitatively how water, even in its smallest amount, affects  transesterification kinetic  parameters  of  coconut  oil  with  ethanl;  with  the  view  of assessing whether it is preferable to use anhydrous ethanol with high price or ethanol containing a certain amount of water in biodiesel production process. Physicochemical properties of the coconut oil were also determined. The coconut oil produced was yellow and had the following values for viscosity, 21.24 ± 0.22mm2/s, relative density, 0.92 ± 0.001, flash point, 1900C, cloud point, +27, pour point, +25, refractive index, 1.45 ± 0.001 and moisture content, 0.1%.The average acid, iodine, peroxide and saponification values of the coconut oil were 0.523 ± 0.03 mgKOHg-1, 9.33 ± 0.04 mgIodineg-1, 0.00 meq/1000g and 270.26 ± 0.05 mgKOHg-1 respectively.The crude coconut oil was transesterified using serially diluted anhydrous ethanol with water activity 0.002, 0.022, 0.042, 0.062 and 0.102. NaOH was used at constant reaction conditions.The ethyl ester produced (biodisel) was light yellow with the following values for viscosity 2.66 ± 0.22mm2/s, relative density, 0.86 ± 0.001, flash point, 1420C, cloud point, +5, pour point, -3, refractive index 1.43 ± 0.001 and moisture content 1.7%. The average acid, iodine, peroxide, cetane number and saponification values of the biodiesel produced were 0.094 ± 0.002 mgKOHg-1, 1.53 ± 0.28 mgIodine g1, 0.160 ± 0.001 meq/1000g, 70.81 and 222.99 ± 0.10 mgKOH g-1 and 70.81 respectively. During kinetic study, it was observed that the sample with the least water activity (0.002) generated higher biodiesel yield than others. The reaction rates for biodiesel samples with water activities of 0.002, 0.022, 0.042, 0.062 and 0.102 were 0.0978, 0.0786, 0.0498, 0.0276 and 0.0054 (mg/g/m) respectively. The function of ethyl ester concentration with time when determined showed a first order reaction.The reaction rate decreased as water activity increased while the over – all reaction rate constant  K was found to be 2.4 × 10-2. The presence of water in the reacting mixture had a negative effect on the transesterification  reaction  progress  and  this  effect  has  been  quantitatively  presented.  The reaction kinetics gave a better understating of the techno-economic process, in other to avoid waste of reactants during biodiesel production process and maximize profit.

CHAPTER ONE

INTRODUCTION

As energy demands extremely increase while the energy sources are limited, a number of current studies focus on development of alternative fuels (Jha et al, 2007). One of such alternative fuels for combustion in compression-ignition (diesel) engines is the biodiesel. Biodiesel is defined as a fuel composed of mono-alkyl esters of long-chain fatty acids derived from renewable lipid feedstock such as vegetable oils or animal fats, for use in compression ignition (diesel) engines (National Biodiesel Board,

1996).   Furthermore,   biodiesel   is   oxygenated,   sulfur-less,   non-toxic   and   biodegradable   biofuel suggestively because their lipid sources possess these qualities. It is natural and renewable; producing less pollution than petro diesel (Wassell and Dittmer, 2006).

Biodiesel came to be established as an alternative fuel after it was discovered that vegetable oils originally used in the diesel engine were problematic (with poor fuel atomization, incomplete combustion, carbon deposition on the injector and piston etc) (Ramadhas et al., 2005). These engine problems that were due primarily to high viscosities and low volatilities of the vegetable oil were however, resolved through several chemical processes such as micro-emulsion, pre- heating the oil, pyrolysis, blending, transesterification and the use of viscosity reducers (Srivastava and Prassad, 2000; Peterson et al., 1991; Ma and Hanna, 1999; Muniyappa et al., 1996).  Among  these,  transesterification  was  considered  as  the  most  suitable  modification because technical properties of esters are nearly similar to diesel.

Transesterification is a chemical reaction involving oil or fat, and a short chain alcohol such as methanol or ethanol to yield fatty acid alkyl esters and glycerol ((Pinto et al., 2005; Thiruvengadaravi et al., 2009). Through transesterification, these vegetable oils are converted to the alkyl esters of the fatty acids present in the vegetable oil. These esters are commonly referred to  as  biodiesel.  Biodiesel  is  a  renewable,  biodegradable,  environmentally  benign,  energy efficient, substitution fuel which can fulfill energy security needs without sacrificing engine’s operational performance. Thus; it provides a feasible solution to the twin crises of fossil fuel depletion and environmental degradation. Biodiesel fuels have many attractive features such as that it is domestically produced, offering the possibility of reducing petroleum imports. The by-

products of combustion have reduced levels of harmful substances as compared to petroleum. Any fatty acid source may be used to prepare biodiesel. Thus, any animal or plant lipid should be a ready substrate for the production of biodiesel (Refaat, 2010). In the reaction, each mole of triglycerides reacts stoichiometrically with 3moles of a primary alcohol and yields 3moles of ester and 1mole of glycerol (Pilar et al., 2004). The actual mechanism of the reaction consists of a sequence of three consecutive and reversible reactions (Darnoko and Cheryan, 2000) in which di- and monoglycerides are formedas intermediates. In this reaction, a catalyst is needed in order to improve the reaction rate and yield. Under this process, tranesterification is carried out using homogeneous or heterogeneous catalysts which may be base, acid or enzymes (Krishnan and Dass, 2012). Conventionally, alkali-metal alkoxides are the most effective catalyst compared to the acidic catalyst. Sodium alkoxides are the most efficient catalyst used due to the fact that the reaction is completed in a short time under mild conditions of temperature and pressure (Pilar et al, 2004). Though transesterification methods have been widely used to reduce the viscosity and improve the fuel property of vegetable oils, the reaction is strongly influenced by several factors which include molar ratio of oil to alcohol, type and amount of catalyst, presence of water, free fatty acid (FFA) in oil samples, reaction temperature, reaction time and agitation speed. Among these  factors,  FFA and  moisture contents are  the  two  variables dictating  the  feasibility of transesterifying vegetable oils (Meher et al., 2006; Ma and Hanna, 1999). The presence of water even in small quantity inhibits the reaction progress (Bikou et al., 1999; Peterson et al., 1991).

Water activity is a measure of how efficiently the water present in reactants can take part in a chemical (physical) reaction. It is sometimes defined as “free”, “unbound”, “active” or “available water” in a system (Bell and Labuza, 2000). Simply stated, it is a measure of the energy status of the water in a system.  Water activity is defined as aw=p/powhere p and po  are the partial pressures of water above the food and a pure solution under identical conditions respectively. Therefore, pure distilled water has a water activity of exactly one. A portion of the total water content present in the reaction mixture is strongly bound to some specific sites. These sites may include the hydroxyl groups, the carbonyl and hydrogen bonds, ion-dipoles and other polar sites. The tightly bound water has no tendency to escape as a vapour and therefore exerts no partial pressure and has an effective water activity of zero. Some water is bound less tightly, but is still not available.  Water activity is clearly a function of composition but  is also a function of

temperature. Water activity instruments measure the amount of free (sometimes referred to as unbound or active) water present in a sample.

The coconut palm (Cocos nucifera L.) is one of the most important and useful palms in the world, it is an important crop in the agrarian economy of many countries of the world providing food, drink, shelter and raw materials for industries (Nair, et al., 2003). Coconut oil, which is extracted from the mature dried copra is edible and has been consumed in tropical countries for thousands of years (Bach and Babayan, 1982). Undoubtedly, it has had its main application in food and cosmetics industry, but research is currently on-going in the production of biodiesel through transesterification from it.

Kinetic study of transesterification in the last thirty years has been mainly on finding best fit of empirical data to simple models of reaction order. Freedman and his colleagues started work in this area in the 1980s. According to Komer (2002), the probability of a reaction can be predicted to about 78% using only kinetic factors. Kinetics is the study of rates (that is the change in concentration of either  reactants  or  products  with  time)  of  chemical  reactions,  it  includes investigations of how different experimental conditions can influence the speed of a chemical reaction to generate information about reaction mechanisms and transition states as well as construct mathematical models (Mary,2010). The aim of kinetic study is to make predictions about the composition of a reaction mixture as a function of time, to understand the processes that occur during the reaction and to identify what controls its rate.

This work will focus on adding knowledge to the research currently going on with coconut oil, by exploring the rate of reaction and how transesterification of coconut oil is affected by the presence of ‘active’ water (water activity) in the reaction-mixture. It will also focus on determining the kinetic parameters such as the over-all order of the reaction and reaction rate constant (K) and hence help in providing a reference point in future techno-economical study to assess whether it is preferable to use anhydrous ethanol with high price or ethanol containing a certain amount of water in biodiesel production process.

1.1        Coconut

The coconut palm (Cocos nucifera L.) is one of the most important and useful palms in the world, it is animportant crop in the agrarian economy of many countries of the world providing food, drink, shelter and raw materials for industries (Nair et al., 2003). It is a member of the palm family Arecaceae. It is the only accepted specie in the genus Cocos (Hahn, 1997). The coconut palm is undoubtedly the most economically important plant in the family, as it is used as both an ornamental and as a food crop.

The term coconut can be referred to the entire coconut palm including the seed, or the fruit, which is not  a  botanical  nut.  Coconut  palm  from  which  the  coconut  is  produced  can  be  found  growing throughout the tropical region of the world, with its origin not exactly known (Cornelius, 1996). Jackson (2006) explained that the origin of the coconut palm is obscured by the ability of the fruit to disseminate the  species  naturally  over  distances  of  thousands  of  miles.  The  exceptionally  wide  distribution  of coconut today is due to the influence of humans, having been carried from place to place by explorers and immigrants (Frank et al., 1995). Unlike many tropical fruits, coconuts are still grown largely by small landholders, although plantations have become more popular recently (Grimwood et al, 1975). Coconut palms have two natural subgroups simply referred to as “Tall” and “Dwarf”. Most commercial plantings use high yielding, longer lived Tall cultivars and each region has its own selections, e.g., ‘Ceylon Tall’, Indian Tall’, ‘Jamaica Tall’ (syn. ‘Atlantic Tall’), ‘Panama Tall’ (syn. ‘Pacific Tall’). The Tall cultivar group is sometimes given the name Cocos nuciferavar. typica and the dwarf cultivar group C.nuciferavar. nana (Perera et al., 2009). Though the coconut palm is not indigenous to Nigeria, but of the humid tropics, it is known to grow under diverse types of climate andis highly adaptable. They are usually grown along the sea coast and in plain grounds. They can be cultivated up to 1,000 m above sea level and it tends to grow best in places with a mean annual temperature of 25  -38 °C and an annual rain fall of 200 mm (Nair et al., 2003).   The cultivation of coconut is in scattered holdings and mostly in groves in the rainforest zone of Nigeria, so it is difficult to estimate the number of farmers that grow the crop. An estimated 36,000 ha is presently under cultivation mostly in Lagos and Rivers states and an estimated

1.2 million hectares of land is suitable for coconut cultivation (NIFOR, 2008). The West African tall (WAT) is the most extensively grown tall variety both as a plantation and compound crop. Traditionally, tall varieties are commercially cultivated and usually known by the places where they are cultivated. They grow to a height of 15-18 metres and their life span expands up to 60 to 75 years. They can be easily detected by the presence of balls at the base of the palm. They come to flowering 6 to 7 years after planting and produces large sized nut with good quality copra and oil content (67%) (Nair et al., 2003).

Coconut palm is an important economic crop because of the heavy demand for its products. In recent years coconut palm has gained importance as an economic crop hence the federal government of Nigeria vested the Nigerian Institute for Oil palm research with the mandate for coconut research in Nigeria and this resulted to the creation of the coconut research sub-station in Badagry, Lagos State, which was established in 1978 (Uwubanmwen et al., 2011). The sole responsibility of the sub- station is to research into the economy, ecology and biology of the coconut palm with the aim of improving yields, provide job opportunities for researchers and provide avenues for the increased production of the product to the end users. Consequently, coconut output for both export and local consumption will increase tremendously in the coming years. The role of coconut in food production, foreign exchange earnings, raw materials for industries, income and employment generation to millions of Nigerians including women and young people make it a very crucial asset for National Economic Development. In Africa  the  major  coconut  producing  countries  include  Tanzania,  Cote  d’Ivoire,  Kenya,  Madagascar, Ghana and Mozambique (FAO, 2010).

1.1.1Coconut Fruit

Botanically the coconut fruit is a drupe, not a true nut. Like other fruits it has three layers: exocarp, mesocarp and endocarp. The fruit endocarp usually is presumed to protect the developing seed from predation, desiccation, or  crushing.  The  exocarp  and mesocarp  make  up the  husk  of the  coconut. Coconuts sold in the shops of non-tropical countries often have had the exocarp (outermost layer) removed. The mesocarp or “shell” thus exposed is the hardest part of the coconut and is composed of fibers called coir which have many traditional and commercial uses. The shell has three germination pores  or  eyes  that  are  clearly  visible  on  its  outside  surface  once  the  husk  is  removed  (stoma).

Figure 1: A full sized coconut seed.

A full-sized coconut weighs about 1.44 kilograms (3.2 lb). It takes around 6000 full-grown coconuts to produce a tonne of copra (Bourke et al., 2009).

1.1.2   Description of the Coconut Oil (Copra Oil)

Coconut oil is edible oil that has been consumed in tropical countries for thousands of years (Bach and Babayan, 1982). Extracted from the dried copra of mature seeds, this white, glycerin – rich, semi-solid, lard-like fat is stable in air and remains bland and edible for several years. Coconut oil contains a high level of low molecular weight saturated fatty acids (SFA) (≈93%), the distinctive characteristic of lauric oil (Marina, et al., 2009). It is also a natural source of medium chain triglycerides (MCTs) (≈60%), especially C12:0 ≈50%. The term MCT refers to triglyceride which is composed of a glycerol backbone and three saturated fatty acids with chain length of 6-12 carbons. MCTs have been reported to be beneficial to human health (Norulaini et

al., 2009). Although, coconut oil is predominantly saturated fat, it does not have negative effect on cholesterol (Belle and et al., 1980).

It isused extensively in cosmetics, emulsions, vegetable shortening and lard-compound, margarine, salves, perfumery, flavoring, soap,candy, ice cream, candles, dye, tooth paste, paints, hydraulic fluids, lubricants and insecticides. It is an essential ingredient in the manufacture of synthetic rubber. It is used as a plasticizer in many products. It serves as a substitute for cocoa butter in the manufacture of chocolate. It is also being used as a fuel for diesel engines. Apart from being good for the skin and hair of a person, coconut oil has been found to be beneficial in case of the following ailments; stress, heart diseases, high cholesterol levels, too much weight, kidney problems, poor digestion, low metabolism, high blood pressure, low immunity, dental problems, diabetes, low bone density, cancer, premature aging, pancreatitis (Uwubanmwen et al.,

2011). While coconut possesses many health benefits due to its fiber and nutritional content, it is the oil that makes it a truly remarkable food, medicine and industrial raw material. Hence, the need to exploit coconut oil for use in biodiesel production.

1.2 Biodiesel and Other Biofuels

Generally, any material that is burned or altered to obtain energy and to heat or to move an object is referred to as fuel (World Encyclopedia, 2005). Similarly, biologically derived fuels or fuels produced from bioenergy sources are termed biofuels. Biodiesel  is a biofuel. Among biofuels are fuel wood, charcoal, livestock manure, biogas, biohydrogen, bioalcohols, microbial biomass, agricultural wastes and byproducts, energy crops and others (FAO, 2010). These materials are obtained from recently dead or lifeless organisms, plants and animals alike. Thus, there is a direct or indirect dependence of biofuels on the photosynthetic process.

Biodiesel and bioethanol represent the first generation biofuels whose biorefineries utilize readily processable bioresources such as sucrose, starches and plant oils (Marchetti et al., 2007). While plant oils  (and  animal  fats)  are  the  primary  source  of  biodiesel,  sucrose  and starch  serve  as  the  major feedstock for bioethanol production via fermentation. Biodiesel, bioethanol and indeed, liquid biofuels are predominantly used in the transport sector due to their high volumetric density and convenience of use just as with the liquid hydrocarbons (Agrawal et al., 2007). Biogas as well as other gaseous biofuels

can  find  application  in  lighting,  cooking  and  electricity  generation.  Biohydrogen  and  especially, bioethanol are of particular interest as fuel for fuel cells, which are used to produce energy.

Biofuels have peculiar features, some already pointed out under biodiesel introduction (section 1.0). Additionally, alternative  fuels  such  as  biofuels  tend  to reduce  dependency  on  fossil  fuels  globally. Biofuels are equally considered as offering many priorities, including sustainability, reduction of green- house  gas  emissions,  regional  development,  social  structure,  agriculture,  and  security  of  supply (Reijnder et al., 2009).

1.3  Biodiesel Production

Biodiesel production is a technically simple process (figure.2). There are a number of methods used in the production of biodiesel; some of the most widely used technologies in this context are blending, pyrolysis, micro emulsification and transesterification (Schwab et al, 1987). The use of supercritical method is currently attracting attention.

1.3.1Direct use of vegetable oil/blending

Direct use of vegetable oils and/or the use of blends of the oils have been carried out by so many researchers starting with Rudolf in 1900. Others include (Anon, 1982; Engler et al., 1983). Vegetable oil is much more viscous (thicker) than either petro-diesel or biodiesel. The purpose of diluting or blending straight vegetable oil (SVO) with other fuels and solvents is to lower the viscosity to make it thinner, so that it flows more freely through the fuel system into the combustion chamber. Blending SVO with petro-diesel is a process that still uses fossil-fuel; this may be cleaner than pure fossil fuel, but still not clean enough. People use various blends, ranging from 10 % SVO and 90 % petro-diesel to 90 % SVO and

10 % petro-diesel (Ziejewski et al., 1986). Much of the world uses asystem known as the “B” factor to state the amount ofbiodiesel in any fuel blend:

_ 100% biodiesel is referred to as B100, while

_ 20% biodiesel, 80% petro diesel is labeled B20

_ 5% biodiesel, 95% petro diesel is labeled B5

_ 2% biodiesel, 98% petro diesel is labeled B2 (Hossain et al., 2012).

The outcome of the use of the oils and there blends as alternative fuels were negative, even though they performed  satisfactorily  for  a  while.The  problems  include;  coking  and  trumpet  formation  on  the injectors to such an extent that fuel atomization does not occur properly or is even prevented as a result of plugged orifices, carbon deposits, oil ring sticking and thickening and gelling of the lubricating oil as a result of contamination by the vegetable oils. Direct use of vegetable oils and/or the use of blends of the oils have generally been considered to be not satisfactory and impractical for both direct and indirect diesel engines. The high viscosity, acid composition, free fatty acid content, as well as gum formation due to oxidation and polymerization during storage and combustion, carbon deposits and lubricating oil thickening are obvious problems (Ma and Hanna, 1999).

1.3.2. Micro-emulsions

To  solve  the  problem  of  high  viscosity  of  vegetable  oils,  micro-emulsions  with  solvents  such  as methanol, ethanol and 1-butanol have been studied. A micro-emulsion is defined as a colloidal equilibrium dispersion of optically isotropic fluid microstructures with dimensions generally in the 1±150 nm range formed spontaneously from two normally immiscible liquids and one or more ionic or non- ionic amphiphiles (Schwab et al., 1987). They can improve spray characteristics by explosive vaporization of the low boiling constituents in the micelles (Pryde, 1984). Short termperature performances of both ionic and non-ionic micro-emulsions of aqueous ethanol in soybean oil were nearly as good as that of No. 2 diesel, in spite of the lower cetane number and energy content (Goering et al., 1982b). Their durability  was  not determined.  Different percentages  of micro-emulsions have  been  prepared  and tested (Ziejewski et al., 1984). No significant deteriorations in performance were observed, but irregular injector needle sticking, heavy carbon deposits, incomplete combustion and an increase of lubricating oil viscosity were reported.

1.3.3. Thermal cracking (pyrolysis)

Pyrolysis, strictly defined, is the conversion of one substance into another using heat or by heating with the aid of a catalyst (Sonntag, 1979 b). It involves heating in the absence of air or oxygen (Sonntag, 1979

b) and cleavage of chemical bonds to yield small molecules (Weisz et al., 1979). Pyrolytic chemistry is difficult to characterize because of the variety of reaction paths and the variety of reaction products that may be obtained from the reactions that occur (Ma and Hanna., 1999). The pyrolyzed material can be vegetable oils, animal fats, natural fatty acids, methyl and ethyl esters of fatty acids. The pyrolysis of fats has been investigated for more than 100 years, especially in those areas of the world that lack deposits of petroleum (Chang and Wan, 1947; Sonntag, 1979b). Pioch et al studied the catalytic cracking of vegetable oils to produce biofuels in 1993. Copra oil and palm oil stearin were cracked over a standard petroleum  catalyst  SiO2/Al2O3   at  450°C  to  produce  gases,  liquids  and  solids  with  lower  molecular weights. The condensed organic phase was fractionated to produce biogasoline and biodiesel fuels. The chemical compositions (heavy hydrocarbons) of the diesel fractions were similar to gasoline and diesel fuel.  However,  the  absence  of oxygen  during  thermal  processing  also  removes  any  environmental benefits of using an oxygenated fuel. It produced some low value materials and, sometimes, more gasoline than diesel fuel (Ma and Hanna, 1999).

1.3.4  Supercritical Process

This  is  a  procedure  that  does  not use  a  catalyst  in  the  biodiesel  processor  for  the  production  of biodiesel. It is also a process of achieving homogenization during transesterification reaction. In this method, a high ratio of a supercritical alcohol (methanol) to oil at very high temperatures and pressures are used. The oil and the methanol are in one phase and reaction takes place in all spontaneity and rapidity. FFAs present are converted directly to methyl esters thus there is no need for deacidification. Moreover, the product does not require washing if free of soaps.The use of supercritical methanol is advantageous for feed stocks that contain water (like crude vegetable oils and waste oils) because water does not have a negative effect on conversion in transesterification with supercritical methanol (Kusidiana and Saka, 2004). However, high temperatures and pressures are required, as well as alcohol to oil ratios of 42:1 are major challenges for commercial application of this method in biodiesel production (Mittelbach and Rernschmidt, 2004).

1.3.5    Transesterification (Alcoholysis)

Conventionally, the most widely used method is transesterification (also known as alcoholysis). It requires either vegetable oils or animal fats, alcohol (especially, of short chain length), catalyst and certain necessary conditions such as temperature and optimum reaction time. The reaction is shown in

Figure 2. A catalyst isusually used to improve the  reaction rate and yield. Because the reaction is reversible, excess alcohol is usedto shift the equilibrium to the products side. Alcohols are primary and secondary monohydric aliphatic alcohols having 1±8 carbon atoms (Sprules and Price, 1950). Among the alcohols that can be used in thetransesterification process are methanol, ethanol, propanol, butanol and amyl alcohol. Methanol and ethanolare used most frequently,though ethanol is preferred because its less toxic, has renewable origin and its esters have higher heating value due to the extra carbon on the alcohol carbon chain, despite the low cost of methanol, shorter chain length, availability, reactivity  and polarity. To complete a transesterification stoichiometrically, a 3:1 molar ratio of alcohol to triglycerides is needed. In practice, the ratio needs to be higher to drive the equilibriumto a maximum ester yield. The reactions can be catalyzed by alkalis, acids, or enzymes. The alkali Include NaOH, KOH, carbonates and corresponding sodium and potassium alkoxides such as sodium methoxide, sodium ethoxide, sodium propoxide and sodium butoxide. Sulfuric acid, sulfonic acids and hydrochloric acid are usually used as acid catalysts. Lipases also can be used as biocatalysts (Ma and Hanna, 1999).

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