One difficult aspect of a greener lifestyle involves disposal of used cooking fats. Most people either pour it down the drain, where it can lurk for years while conspiring to clog your pipes, or pour it in the yard, where it attracts pests of various sorts looking for a free meal. Recycling is obviously a better option, and to this end the BioBot 20 tabletop diesel processor – a (relatively) simple chemical reactor for converting used kitchen oils into biodiesel fuel at home – has been introduced by UK-based company Biobot.

How biodiesel is made

Widely used for a host of purposes, highly-efficient diesel engines power a good fraction of the world's transportation, industry, and power generation needs. Diesel fuel is denser than gasoline, and has 11 percent larger energy content per liter. Nearly a trillion liters of diesel fuel are used each year worldwide, which releases about 10 percent of the world's anthropogenic CO2 emissions.

Biodiesel fuels offer a greener alternative to the use of petroleum-derived fuels. Otherwise known as fatty-acid methyl ester (FAME), biodiesel is derived from waste vegetable oils, and is close to carbon-neutral in use. Worldwide, about 20 billion liters of biodiesel are made yearly, with the potential of a fivefold increase without diverting oil away from food uses. Compared to petrodiesel, biodiesel has better lubrication ability, higher cetane rating (less diesel knock), and essentially no sulfur, making it a desirable replacement fuel.

Transesterification converts A (methyl alcohol) and B (vegetable oil) into C (glycerin) and D (biodiesel) (Image: B. Dodson)

The process of making biodiesel is called transesterification. Vegetable oil is largely made of triglycerides, which contain three fatty acid esters bound to a single glycerine molecule. In the transesterification process, triglycerides are reacted with a mixture of methyl alcohol and sodium hydroxide so that the fatty acid esters break off from the glycerine molecule, and are capped with the methyl group from the methyl alcohol. Potassium hydroxide can also be used, and is preferred by many biodiesel producers.

The BioBot 20 tabletop diesel processor

Perhaps as much an educational tool as a practical way to produce biodiesel, the BioBot 20 tabletop diesel processor has a capacity of 20 liters per batch. It's operation is shown in the video below, but briefly you fill the reaction chamber with used vegetable oil, then heat the oil to a designated temperature while agitating the oil with a built-in hand operated mixer. When the oil comes to temperature, a small amount is tested to determine the amount of free fatty acids it contains. This determines the amount of sodium hydroxide catalyst is required to process the batch.

The desired amount of catalyst is added to four liters of pure, dry methanol, and the combination is mixed until the catalyst dissolves in the methanol, forming sodium methoxide. The sodium methoxide is stored in a special tank which pumps it into the reaction chamber so that it need not be handled any more than necessary (it is very corrosive).

The pumping takes place while the oil is hot and being agitated. The reaction proceeds slowly, often taking 12-24 hours to finish. At that point the glycerin has accumulated at the bottom of the reaction chamber, from which it is drained using a tap at the bottom of the reaction chamber. The remainder is biodiesel.

The raw biodiesel must be washed before use to remove soaps, excess methanol, residual sodium hydroxide, free glycerine and other contaminants. This is accomplished by washing it with water. In the BioBot 20, water is pumped to a spray mister at the top of the reaction chamber. Agitation during washing is not recommended, lest soap result from a batch having free fatty acids. The water does not dissolve in the biodiesel, but as it passes through it will pull out contaminants. The water wash process is repeated until the biodiesel is clear, at which point in time it is reheated to remove residual traces of water.

Is all this worthwhile? Depends on how you value your time, but if the vegetable oil is waste cooking oil, it at least is free. The four liters of methanol costs about US$3.50, while the sodium hydroxide might cost US$0.50 or so. If the biodiesel yield is 15 liters, this corresponds to less than €0.25/liter or US$1.00/gallon. There are other minor expenses, but overall brewing home-made biodiesel can be a profitable activity. The BioBot 20 sells for £415, or about US$655.

Source: Biobot

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