Shortpath-Distillation
Short-Path Distillation is a thermal
separation process for thermal sensitive products. Short residence time
and low evaporation temperature will cause a minimum thermal stress to
the distilled product. Typical applications are high molecular organic
compounds particularly from the fields of chemistry, pharmaceutical and
food industry.
Principle of processDistillation is one of the most important
thermal separation methods. Short-Path Distillation is a continuous separation
process working under vacuum conditions. Evaporation takes place from a
heated wiped film. Caused by the pressure drop between the place of vaporization
and the vacuum system the operating pressure in typical wiped film evaporators
with external condenser is limited to some millibars. The considerably
lower pressure in the Short-Path-Evaporator is obtained by the short distance
for the vapors on their way from the evaporator surface to the condenser.
In addition, the cross section area of flow is equal to the evaporator
surface, so that there is only a minor pressure drop between evaporator
and condenser - a detail which is absolutely necessary for this process!
Typical pressure range is the "fine
vacuum range", e.g. between 1 and 0.001 mbar. At this pressure the required
evaporation temperature will decrease so far, that during the short residence
time no thermal decomposition of the product can occur.
Different from the traditional Thin
Film Evaporator design the condenser is located inside the Short Path Evaproator
body. There is no vapor line between evaporator and condenser. The large
volume of the vapors at very low pressure would lead to high vapor speed
even with large diameters. Because of the large pressure drop the design
with external condenser cannot be used below 1 mbar operating pressure.
Thinfilm Evaporator
with external condensor
A comparison with conventional pot
still distillation will show the special features of the process:
The raw product is heated in the evaporator
vessel until evaporation temperature, vapors of the low boiling component
are transferred to the cooler where they will be condensed. Distillate
from the condenser will be collected in a vessel. Boiling temperature can
be reduced according to the vapor pressure curve by connecting a vacuum
pump system.
In a still pot distillation there are
some disadvantages for heat sensitive products:
· long residence time caused
by the batch process
· poor vacuum at the place
of evaporation, because drops in pressure and static height of the liquid
have to be added to the suction pressure of the vacuum pump set.
The ultimate pressure is not determined
by the size of the vacuum pump set, but is limited by the conductivity
of the piping and the static height of the liquid in the evaporator vessel.
Evaporation from a thin film will
eliminate the influence of the static height of liquid. In a Wiped-Film
Evaporator (WFE) the product runs down on a heated cylindrical surface
on a cylindrical pipe. The liquid film is mixed continuously by a rotating
wiper system. Condensation takes place in a condenser located outside,
which is connected to a vacuum system. The obtainable vacuum is limited
to some mbar due to the pressure drop of the connection between evaporator
and condenser.
Shortpath Evaporator
with internal condensor
Using Short-Path Distillation will
eliminate this disadvantage. The condenser is located inside a cylindrical
evaporator. The distance for the vapors between evaporator and condenser
is extremely short. In fact, there is no pressure drop. If the distance
between evaporator and condenser is in the order of the average free path
length of the molecules, this process is also called "molecular distillation".
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