MANUFACTURE OF BIODIESEL FUEL Field of the Invention

This invention relates to a method for the manufacture of biodiesel fuel from vegetable oils. Background to the Invention

The production of biodiesel fuel oil from vegetable oils typically involves, amongst other stages, the base catalysed transesterification of the oil to form esters and glycerol. The oils contains triglycerides - essentially a glycerine molecule with three long-chain fatty acids attached. During the esterification process, the triglyceride is reacted with alcohol, usually methanol, in the presence of a catalyst, usually a strong alkaline such as sodium hydroxide. The methanol reacts with the fatty acids to produce methyl esters and crude glycerol.

The amount of glycerol produced as a by-product is about 1 tonne for every ten tonnes of biodiesel. The UK Government has set a target of producing 20% of the nation's fuel from natural sources by the year 2020. If this were to be in the form of biodiesel, this would translate to 3.6 million tonnes of biodiesel and about 360,000 tonnes of glycerol. The current world market for glycerol is in the region of 150,000 tonnes per year, so the waste glycerol from UK sources alone will be nearly 2.5 times the current world consumption. It is very unlikely that sufficient new uses will be found for glycerol to consume the anticipated production as a by-product.

Waste glycerol from the production of biodiesel is contaminated with fatty acids and other related materials, and would require purification before it could be sold for industrial use. Since the market demand is insufficient, this is not feasible for all waste glycerol. Current ideas for disposal include burning and biological digestion. Glycerol has approximately 50% of the calorific value of fuel oil, and burning is therefore not an attractive option. Current disposal costs for waste glycerol are in the region of £30 per tonne. Summary of the Invention

According to the present invention, there is provided a method for the manufacture of bio diesel fuel from vegetable oils, comprising treating the oils

with methanol and a base catalyst to cause esterification yielding methyl fatty acid esters and glycerol, separating the glycerol from the estehfied oil, treating the glycerol with a catalyst to yield methanol, and using the methanol produced in the esterification of further vegetable oil. Preferably, the glycerol is reacted with a catalyst in the presence of a reducing gas such as carbon monoxide or hydrogen. The catalyst is suitably a mixed base metal oxide/precious metal. It is suitably presented in thin film form. The metal oxide may be, for example, zinc oxide, and the precious metal is suitably palladium. The catalyst is preferably operated at elevated tempera- tures, for example in the region of 300° C to 800° C, and so the glycerol will need to be in vapour form, but may need protection from thermal breakdown.

Pre-treatment of the glycerol may be required to remove contaminants which might adversely affect the catalytic reaction.

The methanol is typically produced by the reaction in vapour form and will therefore need to be condensed. Purification of the methanol may be needed before it can be used in the esterification process, for example to remove water.

Although at first sight the production of methanol in this way might not seem to be an economically attractive idea, as glycerol currently has a higher price than methanol, in practice it is expected that the following factors will favour the process:

1 . glycerol is dropping in price and will continue to fall due to the glut of glycerol on the market from the production of bio diesel.

2. The price of methanol is rising. 3. Reuse of glycerol on the side of bio diesel production will save on disposal costs, currently around £30 per tonne.

4. Recycling of glycerol as methanol will reduce on-site storage requirements and hence fire hazards.

5. Recycling will reduce tanker movements delivering methanol. 6. Greenhouse gas emissions in the production of methanol will be reduced.

Brief Description of the Drawing

The drawing represents diagrammatically a process for the manufacture of biodiesel according to one embodiment of the invention. Detailed Description of the Illustrated Embodiment In the drawing, vegetable oil, for example produced by milling or pressing rape seed or similar oil-bearing seed, is stored in an oil feed tank 1. it will be appreciated that oils from a range of different vegetable sources may be used, as well as recycled cooking oils. The oil feed tank 1 supplies oil to a reactor 2, which is also supplied with methanol from a methanol feed tank 3, and with so- dium hydroxide solution from a base supply tank 4. The esterified oil passes to a separation stage 5 where the glycerol is separated off for supply to a second vapour phase reactor 6. The biodiesel oil is removed by line 7 for further treatment (not shown). The reactor 2 will typically be a batch reactor. While the supply of sodium hydroxide and methanol are shown to be separate, in practice it may be desirable to mix the two before introduction into and mixing with the crude vegetable oil. A typical batch reaction might occupy 4 to 6 hours.

The reactor 6 contains the zinc oxide/palladium catalyst and is supplied with carbon monoxide from a supply vessel 8. The methanol vapour is led to a condenser 9 and the liquid methanol passes to a purification/drying stage 10 before being delivered to the methanol feed tank 3.

The process will employ heat recovery to ensure energy efficiency. For example, the esterification reaction requires heat input to maintain a temperature of around 50 ⁰ C, and the catalytic reaction requires to be carried out at elevated temperatures in excess of 300 ⁰ C.