Making Biodiesel From Waste Vegetable Oil

What is Biodiesel?

Biodiesel is a fuel derived from a process known as transesterification whereby the oils produced by oliferous plants (typically in the UK we are talking about rapeseed or sunflower oil as the major sustainable sources) are combined under the correct conditions with a methoxide catalyst to cause separation of the oil into usable fuel oil and glycerol by-product.

In layman's terms, transesterification can be thought of as the process of converting one ester into another ester. An ester is a chemical combination of fatty acids attached to alcohol. Animal and vegetable fats, oils and biodiesel are examples of esters.

If both vegetable oil and biodiesel are esters, why is it not practical to use vegetable oil in a diesel engine instead of going through the process of creating biodiesel? In other words, why is there a need for transesterification?

The answer lies in the difference in viscosity, that is the thickness or resistance to flow, between the two esters. Vegetable oil has too high a viscosity for diesel engines, designed for fossil diesel, to cope with. This is because the constituent alcohol molecule of the vegetable oil ester, glycerol, is very large. Hence we need to reduce the thickness of the vegetable oil by replacing the glycerol with an alcohol that is smaller in molecular size, methanol, and thus create a different ester.

This is what the process of transesterification allows us to do. By converting the vegetable oil ester into the biodiesel ester, it separates the larger glycerol molecules from the fatty acids within the vegetable oil. The methanol combines with the fatty acids producing smaller methyl esters thus creating the more free flowing biodiesel.

Given that transesterification is the process of converting one ester into another, it has to be noted that the process is reversible. 

Benefits of Using Biodiesel

As we all know, the fossil fuels are a finite resource and will soar in price as the world's resources dwindle. Alternatives for road transport are not being given the impetus and investment that they deserve (hydrogen fuel cell technology is a prime example) and this represents a tremendous opportunity for the biodiesel industry to solve several problems with a series of simple strokes.

Firstly, biodiesel is completely sustainable. It is carbon neutral in that it releases the same amount of carbon dioxide into the atmosphere as it took out in the first place during the growth cycle. There are other major benefits in the use of biodiesel.

  1. 3 tonnes less carbon dioxide are liberated from storage in fossilised hydrocarbons
  2. 180g less sulphur oxides are produced - virtually zero emissions
  3. 20kg less nitrous oxides are produced
  4. 50kg less carbon monoxide is produced
  5. 40kg less particulates are produced - and biodiesel particulate emissions are NON-carcinogenic

Additionally, biodiesel fuel is 98% biodegradeable within 21 days.

Economically, there are also huge potential long term advantages in terms of producing cash-crops for farmers. Such utilisation of set aside and under-utilised land could increase agricultural sector employment by one person per 20 hectares dedicated to energy crops.

Professionally manufactured biodiesel is monitored by Customs and Excise as well as the Environment Agency. It conforms to DIN 51606 and EN 14214 and so is guaranteed to be effective in any diesel engine without modification.

The Process

The process of making biodiesel is known as transesterification and is achieved by adding methanol to vegetable oil. The process requires a catalyst to increase the rate of the chemical reaction between the methanol and vegetable oil. The catalyst used in the creation of biodiesel is an alkaline one, either Sodium Hydroxide or Potassium Hydroxide.

When the process is complete the catalyst can be recovered unaffected by the chemical reaction that it accelerated, along with the glycerol separated from the vegetable oil.

If waste vegetable oil is used then we have another situation to deal with. Waste vegetable oil will have been been reheated several times during the course of its usage. The reheating will cause some of the fatty acids bonded to the glycerol to break away and float freely in the vegetable oil - hence the name Free Fatty Acid (FFA). There are two ways of dealing with free fatty acids:

  1. Esterify the FFAs creating methyl esters then proceeding with the transesterification.
  2. Increase the amount of catalyst in the single transesterifaction process so that the additional catalyst neutralises the FFAs creating soap as an additional by-product.

Transesterification is a reversible reaction. This means that the process is working both ways simultaneously until a balance between the vegetable oil and biodiesel is reached. Consequently we need to ensure that the process continues the creation of biodiesel rather than stall once it reaches this point of equilibrium.

In commercial production we would tap off the output as it is created thus ensuring that there is a greater quantity of input vegetable oil to keep the reaction producing the biodiesel. For smaller scale production, however, it is more practical to use an increased volume of methanol to ensure that the reaction continues in the direction of producing biodiesel.

Step by Step from the Top

The commencement of the production process depends upon the type of oil employed, and whether it is fresh oil or used oils from the catering industry. In the case of the latter, a titration process takes place, the result of which determines the proportions of methanol to sodium hydroxide used in the preparation of the reaction catalyst. (Inadequate or omitted titration on used vegetable oil is the single biggest cause of fatty deposits in fuel filters).

There are then the following steps in the process of producing the biodiesel:

  1. Filtration of inbound waste oil
  2. Drying the fuel (i.e. removing water content, especially in the case of used oils)
  3. Transesterification (specifically, the separation of the methyl esters from the glycerol)
  4. Settling period
  5. Separation of the biodiesel fuel from the glycerine layer [containing glycerol, catalyst, soap and methanol]
  6. Washing the biodiesel fuel
  7. Filtration to 5 microns
  8. Drying the fuel again
  9. Final products of biodiesel fuel and the by-products

This is the picture of the process making of wasted vegetable oil-biodiesel from Utah Biodiesel Supply :

[source :]

Related post :

  1. Biofuel
  2. Biofuel : First Generation Biofuels
  3. Biofuel : Second Generation Biofuels
  4. Biofuel : Third Generation Biofuels
  5. Make Your Own Biogas Generator

1 comment:

Anonymous said...

Nice post :)