Hazards associated with biodiesel production

Regardless of the scale of operation of a biodiesel plant the hazards are the same: a combination of flammable, toxic and corrosion hazards depending on the stage of the process. In particular:

Methanol

This is a highly flammable and toxic liquid. It will freely burn in the open air or explode if confined in a vessel or room and ignited. Whilst all precautions should be taken to avoid leaks of flammable vapors into the workroom, leaks and spillages may still occur and it is necessary to take further precautions to reduce the likelihood of their ignition. In areas where such materials are handled, companies are required to identify the areas where flam­mable atmospheres may exist, for example due to a leak, and determine their likely extent. Such areas are classed as hazardous and should be classified into zones, depending upon the likelihood of their occurrence. In such areas ignition sources such as naked flames should be excluded and only suitably protected electrical equipment should be used. It should be noted that the presence of metha­nol can also render the product and any waste materials flammable, depending upon the way the biodiesel is manufactured.

The catalyst

Most of the catalysts used for biodiesel production are corrosive and some of them are violently water reactive, toxic, explosive and highly corrosive.

Feedstock oil

If clean and pure it should not be a health problem. However, if the source is unknown or of doubtful quality, then it should be treated as contaminated. Oils are a serious slipping hazard if spillage or contamination outside sealed vessels occurs. Oils can seep into lagging and many can self-combust following a period of chemical degradation. All oils are combustible and will add fuel to any developing fire.

Glycerol

This material is combustible (with a flash-point of 160°C). However, it may be contaminated with methanol and caustic, with their associated hazards, including a potential reduction in flash-point. Unless the initial by-product quality is reliably monitored, then prudence dictates that it should be regarded as contaminated until it has been suitably purified.

Biodiesel

If certified to EN 14241 : 2003  it may be regarded as combustible (its flash-point is approximately 150°C). It has rather unusual solvent properties, and will attack some common engineering polymers, including polyvinyl, natural rubber, some gasket and hose materials and metals, including copper, tin and zinc5. The effect can increase with heating and ageing of the biodiesel. It is also hygroscopic, and can absorb up to 1500 ppm water from the air. If even slightly contaminated with acid or alkali, biodiesel may be hydrolyzed to fatty acids and methanol. This reaction also occurs more slowly in the absence of water, so material stored for more than a couple of weeks may show evidence of a different flash­point than anticipated (which may be significantly lower). Unless this can be reliably prevented, then the product should be used as quickly as possible after production, or reclas­sified and stored and handled accordingly.

Wash water

This may be contaminated with acid, alkali and methanol. It should be treated as corrosive, toxic and flammable unless tests determine otherwise.

Reaction hazards

The main reaction hazards identified are in the preparation of the base catalyst, which can be by one or more of the following methods:

- The direct addition of sodium or potassium to methanol: this reaction is very exothermic and should be the subject of a rigorous risk assessment, particularly as molten sodium is spontaneously combustible in air and the reaction produces hydrogen as a by-product. Fortunately this route is not generally available to smaller/domestic producers.

- The addition of dried hydroxide or methoxide to methanol which is also very exother­mic (heat of dilution)

A much gentler heat of reaction is produced if the catalyst is supplied in methanol solution (although this has to be manufactured safely elsewhere), and further diluted to the required strength on plant. This is normally the preferred option for smaller companies.

A further reaction hazard occurs when concentrated mineral acid is mixed with water. Addition of water to acids often results in violent boiling and ejection of the acid from vessels. Acids should be added slowly to water with cooling and agitation.

  

General hazards

These include:

- Corrosion of processing equipment, building fabric, and supporting structures through exposure to caustic and acids leading to premature weakening and catastrophic failure. A by-product of the corrosion process is hydrogen,

- Biodiesel can soften and dissolve a variety of polymers commonly used in safety equipment, such as plastic aprons and rubber boots, causing premature failure.