The main way to extract essential oils from plant materials is by steam distillation, whereby the steam volatilises the oil which then condenses with the steam. Another system uses solvents that include gases such as carbon dioxide that has been highly pressurised to the point that it becomes a liquid: the start-up cost of this system is such that it is only suitable for the largest manufacturing entities. This leaves standard steam distillation technique as the only financially viable option for simple essential oil production. This steam distillation is currently performed in two ways.
The first method is the extremely primitive system that is ubiquitous through rural areas of Africa and Asia, whereby the plant material is placed in a vessel (often converted from oil-drums etc.) and boiled in water, with the condensate collected in an amazing variety of contraptions. The main problem here is that this method often means the quality of oil is poor because of the hydrolysis that occurs when the plant material is steeped in boiling water. In addition, yields can be variable, oils can vary dramatically in quality, stills have been known to run dry and explode and often large amounts of oil are not recovered etc.
The second method is to contain the plant material in a suspended vessel held above a boiling vessel so that dry steam can pass through - or to inject dry steam underneath. These are the more modern systems and are currently the major industrial methods of producing essential oils. These oils are more consistent in their yield and in their quality too and these extraction systems are both safe and efficient.
A suitable size for a portable still would be one with a charge basket of say around 500 litres. This means the dismantled equipment would fit in a small van/pick-up truck and could be transported easily to the site where the crop was growing. This is what used to happen throughout southern Europe during the early part of the 20th century, where copper stills were taken to the crop and boiled using locally available firewood. This size of still would take a substantial amount of a small farmer’s crop at each charge and for most oils, would complete the distillation in less than 2 hours. However using firewood, although it works, renders it difficult to achieve a rolling boil as this method is far less controllable than gas. A propane gas burner is better but if electricity is on site, this is much the preferred method. It is also more ecologically sound.
If crop production is harvested via a farmers co-operative and brought to a hub, then it may warrant siting larger stills at this central point to cope with the quantities involved. In this case, a simple larger version of the above still would work well and a charge capacity of 1000 litres would be easy to manufacture. It would need an effective sliding block and tackle system in place for lifting the charge basket into and out of the boiling vessel, but these are inexpensive and easy to fit.
It may also be considered cost effective to have an independent boiler to supply the steam. The steam is introduced through a sparge (a perforated tube sited in the base of the vessel) but apart from that, the still operates in an identical fashion to the ones where the water is boiled underneath.
The independent boiler system does have one significant drawback in that any freestanding steam boiler must be licensed and rigorously checked annually by qualified inspector for health and safety purposes. This may not be as strict in other countries as it is in the UK and in any case, the boiler manufacturers will have their own safety checks in place, but these inspections can be expensive and must be taken into account.
Finally, it is vitally important to know how to treat the oils once they have been produced so that they achieve maximum market price. The methods for treating the oils immediately they have been collected must be taught and training given to all operators and quality control staff.