Production Of Biodiesel Transesterification Engineering Essay

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Due to increasing environmental awareness, biodiesel is gaining worldwide recognition as a renewable fuel which may be used as an alternative or a possible replacement to diesel fuel without any modifications to the engine. Biodiesel fuels can be produced from ethanol and vegetable oil, both of which are agriculturally derived products. As such, they provide several advantages: they are renewable, they are safer, they are biodegradable, and they reduce engine exhaust emissions.

Since Rudolf Diesel's demonstration of groundnut oil as a diesel fuel at the Paris exposition of 1900, many others have emerged with various other alternatives. Today, there are a number of vegetable oils that have been tried as diesel alternatives but, with the exception of a few, none of them can be classed as satisfactory. Bio-oils have been tried as a substitute for diesel fuels. They include sunflower, soybean, peanut, palm oil, linseed, rapeseed, winter rape, cottonseed, and canola oil, [1]. Research continues with the aim of solving the two major problems of production and combustion. This problem arises due to the high viscosity of the oils, which means that the oils have to be preheated or diluted with lighter components before they could be used.

Small scale production of biodiesel is well established in developed countries. These systems rely on electrical immersion heaters for the biodiesel processor. However this creates problem in areas like Sierra Leone where there is electric power limitation and cannot be used in the bulk heating. Attempts have been made by a few producers to use gas or open fire for heating the oil processor. But this creates the problems of safety and control. So, alternative heating options have to be suggested for future use.

Literature review

Biodiesel background

It is necessary to understand the difference between biofuel and biodiesel. A biofuel is a solid, liquid or gaseous fuel made of biological material, usually plants. Thus biofuels are sustainable compared to fossil fuels which take about millions of years to replenish. Examples of biofuels are wood, whale oil, vegetable oil etc. On the other hand, biodiesel is a diesel like fuel made from vegetable oil that has gone under a chemical reaction called transesterification

During the early 80's, Arida et al [2] reported the production of coconut methyl esters in the Philippines. Some of the samples were mixed with diesel fuel in varying proportions and were tested as fuel on three types of diesel engines. The short-term tests did not show very much difference when compared to diesel fuel. Later, Mongkoltanatas [3] reported his results after operating a three cylinder, 24 horsepower Kubota truck diesel engine with coconut oil blended with kerosene fuel in a ratio of 20:1 by volume. Running the truck for a long distance and then opening it up for inspection and tests revealed that the major problem appeared to be the high freezing point if coconut oil was used in a high proportion.

Ziejewski et al [4], carried out comparative analysis tests using alkali refilled sunflower oil blended with diesel fuel. The tests were carried out on a direct injection, four cylinder, intercooled and turbocharged Allis-Chalmers, diesel engine. After running the engine for 268 hours, it was reported that there was excessive carbon build-up on the injector nozzle tips, the injector needle was sticking, and that there was abnormal lacquer and varnish build up on the third piston land.

Borgelt et al [5] reported their findings after carrying out similar types of tests on small single cylinder, pre-combustion chamber, naturally aspirated, air cooled diesel engines fuelled with crude de-gummed soybean oil diesel mixtures. The findings were similar to that of the engines powered with sunflower oil blends, as the amount of soy oil in the fuel increased the amount of combustion chamber carbon deposits and the crankcase oil dilution increased.

Barsic et al [6] studied the performance and emissions characteristics of a single cylinder, direct injection, naturally aspirated diesel engine operating on 100% sunflower oil, peanut oil, and 50% mixtures by volume of both with diesel fuel. The oils they were using were crude and not de-gummed. It was reported that the engine produced almost equal power or slight increases in power when operating on vegetable oils and oil -diesel mixtures. The increase in power was stated to be due to the higher density of the vegetable oils, which increases the fuel mass flow and thereby increases the fuel energy delivery. The higher viscosity of the vegetable oil fuels increases the fuel flow by reducing the internal pump leakages. At equal energy inputs, a slight decrease in engine performance was reported. Comparison of the performance concluded that the engine produced less power, had lower thermal efficiency, with slightly lower Nitrogen Oxides (NOx), slightly increased Carbon Monoxide (CO) and Hydrocarbons (HC), and particulate emissions in the exhaust.

Crookes et al [7] carried out engine ignition delay studies on a single and four cylinder diesel engines using vegetable oil and their emulsions with 10% of water. The results concluded that both engines performed well on all the test fuels and ignition delays were of the same order. It was also reported that the thermal efficiency was higher with vegetable oils, whilst the NOx concentration and smoke number were lower with the vegetable oil and lowest for the four cylinder engine run on vegetable oil emulsions.

Crookes et al [8] presented the behaviour of low cetane number bio-fuels in terms of performance and emissions. They concluded that neat and water emulsified vegetable oil (50% rapeseed oil + 50% soybean oil) exhibited similar performance and emission levels when compared to neat and emulsified diesel oils. They also state that emulsification tends to be effective in reducing the emissions of both NOx and smoke, particularly with the vegetable oil. The ignition delay tests showed that, under extreme conditions, the emulsified fuels exhibited slightly longer ignition delays. Smoke agglomerates collected from the engine showed that the mean particle sizes tend to be higher for the vegetable oil fuel compared to the 10% water emulsified diesel fuel. Perkins et al [9] reported results of the long-term durability tests of a three cylinder, 4-stroke, Dl Yanmar diesel engine. The fuels used in the tests were 100% methyl ester of winter rape oil, 100% No. 2 commercial diesel and a blend of both at 50:50 by volume. They concluded that, based upon the engine performance, wear (oil analysis), and injector deposits as indicators of engine durability, methyl esters of winter rape oil appeared to be at least equivalent to No.2 diesel fuel.

All et al [10] operated a Cummins N14-410 diesel engine on a fuel blend of 80:13:7% (by volume) of diesel fuel: methyl tallowate: ethanol. The engine durability tests were done according to the standard 200 hour EMA test cycle. They concluded that the engine performed satisfactorily for 148 hours, after which the injector nozzle failed. The power output, torque, brake specific fuel consumption and brake specific emissions remained almost constant throughout the test period. Engine oil viscosity change was reported to be insignificant after 100 hours of engine operation, and engine wear metals remained well within the recommended limits.

Pioch [11] used catalytically cracked tropical vegetable oils as bio fuels. Catalytic cracking of vegetable oils helps to reduce the molecular weight, thus, making it similar to diesel fuel. Reports of engine durability tests using cracked vegetable oils have not been found in the reviewed literature.

Transesterified cottonseed oil (methyl ester) blended with diesel fuel was tested on a turbocharged, open chamber diesel engine. The results show that the modified oil had viscosity characteristics reasonably close to those of some commercial diesel fuels. The transesterification process also raises the cetane number of the oil making it more comparable to the upper level of commercial fuels. The performance and emissions of the 50% cottonseed oil blended with 50% No.2 diesel fuel were essentially the same as with the baseline No.2 diesel fuel. However, during the low and high idle modes of engine operation, it was reported that improper vaporization of the vegetable oil part led to heavy carbon deposits inside the combustion chamber, filling heavily all the top piston ring grooves [12].

Production of Biodiesel- Transesterification

The most promising technique is the transesterification [13] (Fig.2). This is due to the ease of the process and the low cost associated with it. This is a chemical reaction in which mono hydroxy alcohols react with, triglycoside or fatty acid form alcohol ester (biodiesel) and glycerine, the rate of formation being catalytically accelerated. In a typical reaction, one mole of triglyceride reacts with three moles of methanol to produce three moles of fatty esters (biodiesel) and one mole of glycerol.

Vegetable oils and animal fats, although they may have different flavour and colour, have remarkably similar chemical structures. Generally, they are the esters of glycerol in which all three hydroxyl groups are esterified by saturated or unsaturated (C 12 to C 20) long chain fatty acids (LCAs). These triglycerides can be transesterified to lower the high viscosity of the oil or fat which otherwise may cause the coking of the injectors, oil ring sticking and thickening of lubricating oil. The high viscosity results from the high molar masses of the oils. The transesterification of vegetable oil or fat lowers the molar mass to one third that of triglyceride. Typically, the cleavage of the oil or fat reduces the molar mass from about 900 to 300 and the viscosity from 30-40 cSt to 3-5 cSt. The reaction can be catalyzed by either base or acid. The overall chemistry of transesterification with methanol is represented in the equation overleaf. Overall, it involves the interchange of the alkoxide group between an ester and an alcohol to give a new ester and a new alcohol. The overall reaction in the equation (Fig.1) consists of a number of consecutive and reversible reactions as follows:

a). The formation of diglycerides equation (1).

b). The formation of monoglycerides equation (2).

c). The formation of glycerol equation (3).

Fig.1 Transesterification of Biodiesel- Chemical reactions

C:\Users\Narendran\Desktop\process layout.jpg

Fig.2 Process layout of Transesterification

The vegetable oil and methoxide are thoroughly mixed at a temperature of about 55 deg. C for about an hour. After this reaction period the products are allowed to settle and the by-products (glycerol with some acids and soaps) are drawn off from the bottom of the vessel. The crude biodiesel is then 'washed' of residual acids and oils by contacting with fresh water. After decanting off the wash-water, the product has to be dehydrated - usually by a molecular sieve material in a contactor tower. The primary disadvantages of the process are the labour-intensity and cost of the washing stage, and the loss of unreacted methanol, which remains in the glycerol.

Conventional Biodiesel production

The vegetable oil and methoxide are thoroughly mixed at a temperature of about 55 deg. C for about an hour. After this reaction period the products are allowed to settle and the by-products (glycerol with some acids and soaps) are drawn off from the bottom of the vessel. The crude biodiesel is then 'washed' of residual acids and oils by contacting with fresh water. After decanting off the wash-water, the product has to be dehydrated - usually by a molecular sieve material in a contactor tower. The primary disadvantages of the process are the labour-intensity and cost of the washing stage, and the loss of unreacted methanol, which remains in the glycerol.

Graham Laming's process

Graham Laming [14] in the UK has developed a domed-top domestic hot water tank which could be used with any type of tank. This system [15] (Fig.3) can dewater the vegetable oil, process it into biodiesel and recover the leftover methanol from the biodiesel. It also avoids the water washing completely. The design incorporates a venture which is used both to suck the methoxide into the reactor and to circulate all the vaporized methanol and water through a condenser for dewatering and methanol recovery.

How Graham Laming's process works

Initially the tank is filled with prefiltered oil using the pump.

The oil is then circulated around the tank with the heater on to dry the oil. The filtered oil is fed into the reactor and heated to 50 degrees Celsius if dry or 90 degrees Celsius if wet.

The venturi sucks the air, drawing wet air through the condenser and drying out the air before returning it back to the tank. Here it comes into contact with the wet oil and takes away some moisture. This circulation carries on until the oil is hot and dry.

Oil is allowed to cool and a small amount of it is taken for titration to determine the amount of catalyst required. The methoxide is then pumped with the heater off.

The methoxide displaces the air and vapour. Methanol fumes caught in the distillate tank can be reused.

Now the reaction is underway and the pump is kept running to keep it well mixed. The reaction goes on for 2 hours.

After the end of reaction, the pump is switched off. The methoxide container is filled with water and the pump is switched back on for the venture to suck the water.

The glycerine and by-products are allowed to settle out for about 90 minutes, following which it is drained out.

Now the heater is turned on together with the pump. The biodiesel in the reactor is heated and circulated for the water to evaporate and the methanol to be caught in the distillate trap.

Biodiesel is transferred to a settling tank where any excess methanol and glycerine is removed.

GL's Eco-system biodiesel processor

Fig.3 Layout of Graham Laming's process

Heat exchangers

Heat transfer occurs between two fluids that are at different temperatures and are separated by solid walls. The device that is used to implement this heat exchange is called a heat exchanger. Heat exchangers are devices where two moving fluid streams exchange heat without mixing. The simplest form of a heat exchanger is a double-tube (also called tube-and- shell) heat exchanger. Heat exchangers find specific applications as in space heating, air-conditioning, power production, chemical processing and waste heat recovery.

Conventionally, a double-pipe heat exchanger would be used for small to medium duties and continuous systems. But, helical-coil heat exchangers (HCHE) are better in some ways.

In case of space constraints, where not enough straight pipe can be laid out.

When the flow rate is either very low or laminar, a shell-and-tube heat exchanger would become uneconomical, due to low heat transfer coefficients.

Construction of HCHE is quite easy.

Industrial type HCHE consists of a helical coil fabricated out of a metal pipe that is fitted in the annular portion of two concentric cylinders, as shown in Fig. The fluids flow inside the coil and the annulus, with heat transfer taking place at the coil wall. The design values of the heat exchanger have been determined by Ramachandra et al [15].

Although the proposed design of a helical coil wrapped around a pipe type of heat exchanger has been attempted by people in domestic installations, design calculations have not been found in reviewed literature. Therefore an attempt was made to design the same.

Previous project attempt

The current biodiesel production setup in Sierra Leone utilises palm kernel nut (PKN) oil. The daily production target is between 100 to 200 liters of biodiesel. Notably there is no generator or electric power, so it has been impracticable to base any of the systems relying on grid power.