Water
and Steam Distillation
With this type of distillation the plant material and
water are both found in the retort, but a perforated
grid is used to separate the two. Direct firing of the
still is one approach that is employed in field distillation,
however if this direct fire exposes the upper portion
of the retort to the heat of the direct fire the plant
material can become overheated. Thus injecting steam
into the water from an outside source is preferable.
Steam alone contacts the charge, because of the separation
of the plant material from the water, in this system.
With the steam being generated from the water within
the retort overheating or drying of the charge is avoided
as the temperature cannot rise above that of saturated
steam. Water and steam distillation, therefore represents
a typical case of distillation with saturated low pressure
steam.
Preparation of the plant material is much more important
in this method of distillation than in water distillation.
This is because the steam will only come in contact
with material as it rises. The plant material must be
spread uniformly if high yields of oil are to be maintained.
The charge must also be processed to a uniformed size
within the retort, and within narrow limits to prevent
packing while maximizing yields.
An advantage of water and steam distillation over water
distillation is that it gives less rise to products
of decomposition in the oil and in most cases is considered
a better method than water distillation, requiring less
fuel, shorter hours, and yields more oil even with a
low rate of vaporization.
The disadvantage of water and steam distillation lies
in the fact that as a result of the low pressure of
the rising steam, oils of high boiling range require
large quantities of steam for complete vaporization,
thus requiring long distillation times. In this process
much steam condenses in the plant charge, which becomes
increasingly wet, agglutinates, and will yield its oil
only very slowly. This is where direct steam distillation
is preferred.
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