OPTIMIZATION OF BIODIESEL FROM COCONUT (Cocos nucifera) SEED OIL

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ABSTRACT

Coconut seeds was investigated for its use as biodiesel feedstock. Oil was extracted from coconut seeds using soxhlet extraction method where 67.2% yield of oil was obtained. Biodiesel synthesis was developed and optimized  using Box-Behnken design in Response Surface Methodology to study the effect of experimental variables such as methanol to oil ratio, catalyst concentration, reaction temperature and reaction time on the extracted oil from coconut seeds. The model shows optimum conditions of biodiesel yield of 79% were found at 6:1 alcohol/oil ratio, 1% catalyst concentration (KOH), reaction temperature of 650C and reaction time of 40 min. respectively. At the end of experimental design it was found that the catalyst concentration and reaction time significantly affect the biodiesel yield than the molar ratio among others under the range of values studied. The produced biodiesel was analyzed for its physicochemical and characterized for its fatty acid methyl ester (FAME) profile using GC-MS. The fuel properties of biodiesel obtained showed that except cetane number, diesel index and sulphur content that were higher than the recommended ASTM values all other determined properties were within the ASTM specification indicating that its quite suitable as an alternative source of fuel.

CHAPTER ONE

1.0    INTRODUCTION AND LITERATURE REVIEW

1.1   INTRODUCTION

The replacement of mineral fuel by biodiesel is one of the effective ways of solving the problem of saving and effective usage of energetic resources. Biodiesel is becoming an increasingly acceptable alternative to fossil diesel because of narrowing gap between worldwide oil production and consumption.

Also Nigeria’s vegetation and rainfall regime support agrarian activities that can produce feedstock for biofuel production. Sustainable biofuel production will create more jobs and stimulate related industries thus improving the socioeconomic industries of the country (Itodo et al., 2010).

The surge of interest in biodiesel has highlighted a number of positive environmental effects associated with its use. These potentialities include reduction in greenhouse gas emission, deforestation, pollution and the rate of biodegradation (US department of energy, 2003).

1.1.1 BIODIESEL

Biodiesel is a non-petroleum based fuel made from virgin or used vegetable oil (both edible and non-edible) and animal fat. The main sources or biodiesel can be non-edible oils obtained from plants species available in different countries. Direct application of vegetable oils as fuel for diesel engine is not possible due to its higher viscosity, hence reduction of vegetable oil viscosity is an urgent need. The viscosity of vegetable oils can be reduced by using different methods, namely blending, pyrolysis, micro-emulsification and transesterification (Peterson et al., 1991; Ma and Hanna, 1999; Muniyappa et al., 1996). However transesterification methods have been widely used to reduce the viscosity and improved the fuel property of vegetable oil. Transesterifiction is the process of biodiesel production which involves the reaction of fat/oil with alcohol in the presence of acidic, basic or enzymatic catalyst to form esters and glycerol (Agarwal, 2007).

Biodiesel generally is an ester produced from transeseterification by reacting vegetable oil with alcohol. It is biodegradable, non-inflammable, nontoxic and free of sulfur and aromatics. It shows favorable combustion emission profile producing less carbonmonoxide, sulfur oxides and unburned hydrocarbons than petroleum based diesel. These properties make diesel a good alternative fuel to petroleum based diesel oil (Zheng et al., 2006; Song et al., 2000).

The properties of biodiesel can be influenced by several factors such as fatty acid composition of the parent vegetable oil or animals fat, the quality of the feedstock in the production process and other materials used in the process as well as post-production materials. Biodiesel is a mixture of fatty acids with each contributing to the properties of the fuel (Knothe, 2005). The nature of fuel component ultimately determine the fuel properties in a particular biodiesel. The properties of biodiesel fuel that are determined by the structure of its component fatty esters include the following: density, viscosity, lubricity, cold flow properties cloud and pour point (Knothe, 2005). Other properties that affect biodiesel fuel properties include: flash point, specific gravity, acid number, moisture content (Weiksner et al, 2006).

1.1.2  VEGETABLE OILS

Vegetable oil also known as triglycerides consist of glycerides, an ester formed from glycerol molecules and fatty acids, involves straight vegetable oil comprised of 98 percent triglycerides and small amount of mono and diglycerol. Triglycerides are ester of three molecules of fatty acid and the glycerol which contain substantial amount of oxygen in their structure. The fatty acids vary in their carbon chain length and the number of double bond. A different type of oil has different fatty acids; The empirical formula and structure of various fatty acids present in vegetable oil are given in Table 1.1 below (Barnwal and Sharma, 2005).

TABLE 1.1  FATTY ACID COMPOSITION OF TRIGLYCERIDES
Fatty acid Chemical name of Fatty Acid Structure (xx.y) Formula
Lauric Dodecanoic 12:0 C12H24O2
Myristic Tetradecanoic 14:0 C14H28O2
Palmitic Hexadecanoic 16:0 C16H32O2
Stearic Octadecanoic 18:0 C18H36O2
Arachidic Eicosonoic 20:0 C20H40O2
Behenic Docosonaic 22:0 C22H42O2
Lingnoceric Tetradecasonoic 24:0 C24H48O2
Oleic Cis-9-Octadecanoic 18:1 C18H34O2
Linoleic Cis-9,Cis-12-Octadecadienoic 18:2 C18H32O2
Linolenic Cis-9-cis-12-cis-15 Octadecatrienoic 18:3 C18H30O2
Erucic Cis-13-Dicosenoic 22:1 C32H42O2

xx- indicates number of carbon and y indicates number of double bounds in fatty acid chain. Source – Barnwal and Sharma (2005)

1.2  LITERATURE REVIEW

 1.2.1  COCONUT (Cocos nucifera)

Fig 1.  Coconut palm (Cocos nucifera)

The coconut Palm (Cocos nucifera), is a member of the family Arecaceae (palm family). It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed or the fruit which botanically is a drupe not a nut. The spelling cocoanut is an archaic form of the word. The term is derived from 16th-century Portuguese and spanish coco, meaning “head” or

“skull”,from the three indentations on the coconut shell that resemble facial features (Hahn, 1997).

Found throughout the tropic and subtropic area, the coconut is known for its great versatility as seen in the many uses of its different parts, Coconuts are part of the daily diets of many people. Coconuts are different from any other fruits because they contain a large quantity of “water” and when immature they are known as tender-nuts or jelly-nuts and may be harvested for drinking. When mature, they still contain some water and can be used as seed nuts or processed to give oil from the kernel, charcoal from the hard shell and coir from the fibrous husk. The endosperm is initially in its nuclear phase suspended within the coconut water. As development continues, cellular layers of endosperm deposit along the walls of the coconut, becoming the edible coconut “flesh”. When dried, the coconut flesh is called copra. The oil and milk derived from it are commonly used in cooking and frying; coconut oil is also widely used in soaps and cosmetics. The clear liquid coconut water within is a refreshing drink. The husks and leaves can be used as material to make a variety of products for furnishing and decorating. It also has cultural and religious significance in many societies that use it. (Pearsall, 1999).

1.2.1 BIODIESEL PRODUCTION

In order for vegetable oils and fats to be compatible with the diesel engine, it is necessary to reduce their viscosity. This can be accomplished by breaking down triglyceride bonds, with the final product being referred to as biodiesel. There are at least four ways in which oils and fats can be converted into Biodiesel;

  1. Transesterification
  2. Blending
  3. Microemulsions

Among these processes, transesterification is the most commonly used method. The transesterification process is achieved by reaction of a triglyceride molecule with an excess of alcohol in the presence of a catalyst to produce glycerin and fatty esters.

 

The chemical reaction of base catalysed production is shown below;

 

H2C – OOC – R1                                                              H2C- OH         R1COOR

HC – OOC – R2   +3ROH                                  HC – OH   +   R2COOR

 

H2C – OOC – R3                                                        H2C- OH           R3COOR

Triglycerides             Methanol                                Glycerol           Biodiesel

Where R1, R2 and R3 are long chain hydrocarbons which may be the same or different with R = CH3

As seen above the transesterification is an equilibrium reaction in which excess alcohol is required to drive the reaction to completion. Fortunately the equilibrium constant favors the formation of methyl esters such that only a 6:1 molar ratio of methanol to triglycerides is sufficient for 95-98% yield of esters (Barnwal and Sharma, 2005). Methanol is the most commonly use alcohol because of it low cost and choice. (Gerpen, 2005).

The following are steps involved in based catalyzed production:

Preparation: care must be taken to monitor the amount of water and free fatty acids in the incoming oil. If the free fatty acid or water level is too high, it may cause problems of soap formation (saponification) and the separation of the glycerin by-product downstream.

Mixing of alcohol and catalyst: catalyst is dissolved in the alcohol using a standard agitator or mixer.

Reaction: The alcohol and catalyst mix is then charged into closed reaction vessel and the oil is added. The system from here is totally closed to the atmosphere to prevent loss of alcohol.

The reaction mix is kept above the boiling point of alcohol (around 70⁰C) to speed up reaction, though some researchers recommend the reaction to take place anywhere between room temperature to 55⁰C. For safety reasons, recommended reaction time varies from 1 to 8 hours under normal conditions the reaction rate will double with every 10⁰C increase in reaction temperature, excess alcohol is normally used to ensure total conversion of the fats and oils to its esters.(Biodiesel Fuel Fact Sheet, 2006).

Separation: Once the reaction is complete, two major products are found glycerin and biodiesel, each of the product has substantial amount of the excess methanol that are used in the reaction. The reacted mixture is sometimes neutralized at the step if needed. The  glycerin phase is much denser than biodiesel phase and the two can be separated by gravity with glycerin simply drawn off the setting vessel. (Biodiesel Fuel Fact Sheet, 2006).

Alcohol removal: once the glycerin and biodiesel phase have been separated, the excess alcohol in each phase is removed with rotary-evaporator.

Glycerin neutralization: the glycerin by-product contains unused catalyst and soap that are neutralized with an acid and sent to storage as crude glycerin.

Silica gel drying: At this step, silica is added to the biodiesel obtained to remove moisture and it is further filtered in sodium disulphide. This is usually the end of the production process, In some system the biodiesel is distilled in an addition step to remove small amount of color bodies to produce colorless biodiesel.

(Biodiesel Fuel Fact Sheet, 2006).

1.2.2   FACTORS AFFECTING BIODIESEL PRODUCTION

The yield of biodiesel in the process of transesterification is strongly influenced by several factors. The most important factors that influenced the yield of biodiesel from transesterification include molar ratio of alcohol and oil, catalyst, temperature, reaction time, presence of moisture and free fatty acid (FFA) in the oil sample.

  1. Effect of Molar Ratio: Molar ratio of alcohol plays vital role in biodiesel yield (Leung and Guo, 2006; Zhang et al, 2003; Ma and Hanna, 1999; Freedman et al., 1986), normally the transesterification reaction requires 3 mole of alcohol. For 1 mole of triglycerides to 3 mole of fatty acid ester and 1 mole of glycerol. Excess amount of alcohol increases conversion of oil into ester within a short time. So the yield of biodiesel increases with increase in the concentration of alcohol up to certain concentration. However, further increase of alcohol the content does not increase the yield of biodiesel but it also increase the cost of alcohol recovery (Leung and Guo, 2006). In addition to this, the ratio of alcohol content may vary with catalyst used i.e when we use alkali catalyst the reaction require 6:1 ratio of alcohol to oils or fats (Zheng et al, 2003).

Freedman et al. (1986)  study the molar ratio (from 1:1 to 6:1) on ester conversion with soy bean, sunflower peanut, cotton seed and the oils behaved similarly and achieved highest conversion (93-98%) at a 6:1 molar ratio. A ratio greater than 6:1 did not increase yield (beyond 98-99%). However this interfered with separation of the products after the reaction.

  1. Effect of Catalyst: Biodiesel formation is also affected by the type and concentration of catalyst, catalyst are classified as alkaline, acidic or enzymatic. Alkali catalyzed transesterification is much faster than acid catalyzed transesterification and it is most often used commercially. Transesterification occur approximately 4000 times faster in the presence of alkaline catalyst than those catalyzed by the same amount of acid catalyst (Sukumar et al., 2005). Since alkaline catalysts are less corrosive to industrial equipment than acidic catalyst, most transesterfication reactions are conducted with alkaline catalyst, sodium alkoxides were found to be more effective than sodium hydroxides although the low cost of NaOH has made it attract wide usage in large scale transesterificaton. According to Leung and Guo, (2006), the alkaline catalyst concentration in the range of 0.5- 1% by weight yields 94.99%. The catalyst and methanol are normally mixed first then added to the oil or fats, in additions to this, when the concentration of the catalyst increase with oil samples, the conversion of triglycerides into biodiesel also increases. On the other hand insufficient amount of catalyst leads to the incomplete conversion of fatty acid esters. However, optimal product yield (biodiesel) was achieved when the concentration of NaOH reaches 1.5 weight% at the same time, further increase of catalyst concentration proved to have negative impact on end product yield. Because addition of excess amount of alkali catalyst react with triglycerides to form more soap. (Leug and Guo, 2006; Gabelman and Hwang, 1999).

Vincente et al. (2004) reported higher yield with methoxide catalysts but the rate of reaction was highest for NaOH and lowest for KOCH3 at 650C methanol to oil of 6:1 and a catalyst concentration of 1% weight.

  • Effect of Reaction Temperature: Reaction temperature is another important factor that affect the yield of biodiesel. For example higher reaction temperature increases the reaction rate and shorten the reaction time due to the reduction in viscosity of oil. However increase in reaction temperature beyond the optimal level leads to decrease of biodiesel yield. Higher reaction temperature accelerate the saponification of triglycerides in the transesterficaition reaction, temperature should be below the boiling point of alcohol in order to prevent the alcohol evaporation.

Freedman et al. (1986) studied the transesterification of refined soy bean oil with methanol (6:1) and 1% NaOH catalyst at three different temperatures of 65, 45 and 32°C. After 1hour the ester formation was identical at 60°C and 45°C reaction temperatures but slightly lower at 32°C. (Agarwal, 2007).   The ranges of optimal reaction temperature may vary from 500C to 600C

depending upon the oils or fat used. (Eevera et al., 2009).

  1. Effect of Time: Reaction time also increases the conversion rate of biodiesel production. Freedman et al. (1986) observed that increase in fatty acid esters conversion occur where there is an increase in reaction time. The reaction is slow at the beginning due to mixing and dispersion of alcohol and oil. After that the reaction proceeds very fast. However the maximum ester conversion was achieved in less than 90min, further increase in reaction time does not increase the yield of product i.e biodiesel (Leung and Guo, 2006; Alamu et al, 2007). Besides longer reaction time leads to the reduction of end product (biodiesel) due to the reversible reaction of transesterification resulting in loss of esters as well as soap formation (Eevera et al, 2009; Ma et al, 1998).

Darnako and Cheryan (2006) observed that when palm oil was transesterified with 1 weight% KOH at 600C and molar ratio 1:6 (oil to methanol) it yielded 58.6% of Biodiesel at residence time of 40minutes which increased to 97.3% at a residence time of 60minutes.

V. EFFECT OF FREE FATTY ACID (FFA)

The presence of moisture cause partial saponification reaction which produces soap formation and lowers the yield of esters and renders the separation of esters and glycerol difficult. The methyl ester formed is removed by gravity separation of the catalyst and moisture by using silica gel.

If the oil contains FFA greater than 1% more of the alkali (NaOH) is required to neutralize the FFA which also reduces yield. The NaOH and sodium methoxide react with moisture and CO2 in the air and these reduce their effectiveness (Freedman et al., 1986; Agarwal, 2007).

The effect of FFA and water on transesterification of beef tallow with methanol was investigated by Ma and Hanna (1999), it was concluded that the water content of beef tallow should be kept below 0.06% w/w and FFA of the beef should be kept below 0.5% w/w in order to get the best conversion. Water content was a more critical variable in the transesterfication process than FFA (Ma and Hanna, 1999).

1.3   FUEL PROPERTIES OF BIODIESEL

Biodiesel consist of fatty acid ester and the structure of the fatty acids in the ester derived from the alcohol influence the fuel properties of biodiesel (Knothe, 2005).

The quality of biodiesel is important because it affects its use in internal combustion engines. The viscosity, specific gravity, acid number, moisture content, cloud point, pour point and flash point are properties of fatty esters that make the overall property of biodiesel as a fuel and must meet values established by international standards to be acceptable. (Itodo et al., 2010).

I. MOISTURE CONTENT

The presence of moisture in transport or storage tanks causes the methyl esters in biodiesel to degrade quickly resulting in further increase in acid number. Moisture causes the methyl ester in the biodiesel to undergo hydrolysis forming free fatty acid. Water in a B20 blend is also soluble with any remaining methanol and glycerin carried over from the manufacturing process. Overtime, this can cause stratification of the fuel (Weiksner et al., 2006).

II.  ACID NUMBER

Acid number is a good indicator of the level of free fatty acid in biodiesel. High tested value for acid number can be correlated to manufacturing of a fatty acid methyl ester (FAME) fuel from unrefined feedstock (i.e high in free fatty acid) and/or poor process control in the conversion of the feedstock oils or fat to a FAME fuel (i.e methanol carryover). High acid level in biodiesel can cause fuel system deposits and is another indicator that the fuel will act as a solvent resulting in the deterioration of rubber components of a fuel system (Weiksner et al., 2006).

 III. VISCOSITY

Viscosity is a measure of resistance to flow of a liquid. It affects the atomization (fuel spray) of biodiesel in internal combustion engines thus affecting its volumetric efficiency. Biodiesel is expected to have higher viscosity than petrol diesel. Alamu et al. (2007) reported a viscosity of palm kernel oil that was 1.684 to 1.712 times that of a petrol diesel.

The recommended ASTM kinematic viscosity at 40°C for biodiesel is 4.0-6.0 (US

Department of Energy, 2003).

iv.         SPECIFIC GRAVITY

The specific gravity is the most basic and important property of fuel because it affects some important performance indicators such as the cetane number and heating value of biodiesel. It is expected that the specific gravity of biodiesel should be higher than that of petroleum diesel (Yuan et al., 2004).

Alamu et al. (2007) reported a specific gravity value of palm kernel oil biodiesel as 1.033413 to 1.035419 times that of petrol diesel. The recommended specific gravity of biodiesel is 0.88 (US Department of Energy, 2003).

V.  COLD FLOW PROPERTIES

The cold flow properties include the cloud and the pour point. Cloud point is the temperature at which biodiesel begins to gel while the pour point is the lowest temperature at which the fuel can flow.

It is an important fuel storage property that affects engine performance because it can cause clogging of the fuel system of an engine. Generally biodiesel has higher pour point than petrol diesel (Graboski and McCornic, 1998).

VI. FLASH POINT

The flash point of biodiesel can be correlated directly with the quality of the fatty acid methyl esters fuel (B100) usually biodiesel has low volumetric heating value (about 12%) and high flash point. The flash point of biodiesel is between 15-25°C higher than those of diesel fuel (Knothe, 2005). The low flash point associated with biodiesel is caused by methanol carryover due to poor production. A blend of B20 biodiesel will deteriorate rubber components in a fuel system (Weiksner et al., 2006).

VII. CETANE NUMBER (CN)

The connection between the structure of fatty esters and exhaust emission was investigated by the National Renewable Energy Laboratory (NREL) in

Golden Colorado. (Knothe, 2005). They found that high levels of saturates (C14:0, C16:0, C18:0) raise CN reduce NOx emission and improve stability while more polyunsaturated (C18:2, C18:3) reduce CN, raise NOx emission and reduce stability. The presence of a long chain hydrocarbon in fatty acid alkyl ester and straight chain alkanes (such as hexadecane) gives biodiesel a high cetane number which make it suitable for use as a fuel in diesel engines.(Knothe, 2005).

1.4  ENVIRONMENTAL CONSIDERATION ON USE OF BIODIESEL

Biodiesel is considered carbon neutral because all carbon dioxide (CO2) released during consumption is sequestered from the atmosphere by photosynthesis via carbon cycle for the growth of energy crops (Barnwal and

Sharma, 2005). A test carried out by the United State Environment protection Agency (USEPA) with 100% biodiesel produced from soybean oil reveled that there was reduction in the emission of particulate matter by 40%, unburnt hydrocarbon by 68%, carbon monoxide (CO) by 40%, Sulphur (iv)oxide (SO2) by 100%, polycyclic aromatic hydrocarbons (PAHs) by 80%, carcinogenic nitrated PAHs by 90% on an average and in smoke capacity. This is due to the oxygenated nature of biodiesel where more oxygen is available for burning and reducing hydrocarbon emissions into the environment (Barnwal and Sharma 2006; ERII, 2006; Agarwal, 2007). The substitution of biodiesel for conventional fuel contributes to the reduction of the greenhouse gases (GHGs) emission such as CO thus helps in achieving international climate commitments. This is based on the assumption that the combustion of biofuel is CO2 neutral because the amount of CO2 accrued during combustion equal to the amount of that is sequestered during crop growth (Frondal and Peterson, 2007).

However the use of biofuel result in slight increase in nitrogen oxide (NOx like N2O, NO and NO2) due to high exhaust temperature which causes stratospheric ozone depletion (Barnwal and Sharma, 2006).

1.5 BY-PRODUCTS OF BIODIESEL

Biodiesel by-product glycerin contains unused catalysts and soaps that are neutralized with an acid and sent to storage as crude glycerin in some cases. The salt formed during the glycerin neutralization is recovered  for use as fertilizer and sometimes the salt is left as glycerin (Biodiesel Fuel Fact Sheet, 2007). Biodiesel is used in producing pure glycerin when alcohol and water are removed  to produce 80-88% pure glycerin and can be sold as pure glycerin (Biodiesel Fuel Fact Sheet, 2007).

The glycerin can be used in more sophistiscated  operation and the glycerol is used to distil to higher purity and sold into the cosmetics and pharmaceuticals market (Biodiesel Fuel Fact Sheet, 2007).

1.7 AIM AND OBJECTIVES

This research work is aimed at ascertaining conditions for optimum yield of  biodiesel from coconut oil and to determine the viability of the seed oil as potential source of biodiesel production.

The specific objectives of the research work include; a  To determine the percentage yield of coconut seed oil (CSO) using n-hexane as extraction solvent. b   To determine the physicochemical properties of the coconut seed oil. c   To optimize the condition variables for biodiesel from coconut oil. d    To analyze the fatty acid  methyl composition of CSO and COME. e   To determine the fuel properties of the produced COME.

1.8 SCOPE OF WORK

Coconut oil is an edible oil extracted from the kernel of matured coconuts harvested from coconut palm. Because of its high saturated fat content it is slow to oxidize thus resistant to rancidification lasting upto two years without spoiling.

Although several researchers reported works on optimization of biodiesel production from coconut oil through transesterificaton process but not using Response surface methodology to design and optimize result of analysis as in this work, so the variables that affect biodiesel production the most are studied in order to obtained the most efficient range of variables for biodiesel production..

1.9 JUSTIFICATION OF STUDY

The Nigeria’s vegetation and rainfall regime support agrarian activities that can produce feedstock for biofuel production, also the land can be used to produce non-food products including biodiesel for the domestic energy market to diminish imports. Much research has been done on biodiesel over the past decades after the oil crisis in 1973. At present, the concern about environmental regulations has been the major reason to look for alternatives fuel.

Transesterification method have been widely used to reduce viscosity and improve fuel properties of vegetable oil because of its low temperature and pressure conditions also maximum conversion with no intermediate reaction. The reaction variables  that affect the transesterication process the most i.e the molar ratio, catalyst type/concentration, temperature and time were studied using

Response surface methodology to optimize and analyze the biodiesel produce taking fatty acid content and moisture of the oil in consideration this has the potential of cost reduction and efficient utilization of resources.

The cocos nucifera are cultivated crops in the tropics and subtropic area in the western part of Nigeria. The coconut kernel gives an appreciable amount of oil when processed; therefore it can be properly utilized for biodiesel production. Though significant amount of biodiesel has been produced so far but there is a lack of full or partial replacement of fossil fuels that needs to be discussed.

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