3 Short Path Distillation
3 Short Path Distillation
3 Short Path Distillation
Where q stands for the distillate rate related to the evaporator surface in
kilograms per square metre hour, p for the vacuum pressure in mbar, M for the
molecular weight of the vapour in grams per mole and T for the absolute
temperature in Kelvin
In practice a fine or high vacuum still is usually operated at ultimate
vacuum pressure
The parameters as heating temperature and/or throughput are
adjusted in such a way to get the desired distillate rate.
The observed vacuum level ensues; it can be derived from the
above equation.
Example: At a specific distillate rate of 25 kg/(m2×h), for √M/T≈0.8
the ultimate operating pressure in the still amounts to 0.02 mbar
Short path and molecular distillation were developed in the 1930s and 1940s
mainly in the UK and in the USA.
Different types of apparatus were developed.
A still with a disk, rotating at high speed, is named after Hickman, one of the
pioneers.
In this machine the centrifugal force is used to distribute the feedstock as a
thin film on a heated disk. Exposure time is less than a few seconds
Hickman still
The feedstock is fed at a constant rate into the centre of the rotating disk (11).
The disk is heated from the rear via heat radiation (9).
The volatiles are evaporated and condensed on a water cooled surface(10)
The distillate is discharged by means of a pump(6)
The residual product flows from the collecting ring surrounding the disk also
into a pump(6)for discharge
The cold trap(5) is cooled with liquid nitrogen
Short Path evaporator
Nowadays the most currently applied apparatus is a wiped film evaporator
with integrated condenser
Prior to entering the still the feedstock is preheated and degassed.
Feedstock stored at ambient conditions may contain 10 vol% of dissolved gas,
that Is in 100 l of liquid 10 bar × l of gas !
Most of this gas has to be removed prior to a fine or high vacuum distillation.
If no vacuum process precedes the short path distillation a degasser applies.
Short path distillation consists of the following types:
Wiped film molecular distillation unit and
Centrifugal molecular distillation unit
The degassed feed is then fed at the top of the cylindrical, heated jacket,
equally distributed at the circumference and flows downwards due to gravity as
a thin film where by it is continuously wiped.
Due to the heating the volatile components are evaporated and condensed at
the internal condenser. The non-volatile fraction, the residue is collected in an
inclined ring at the lower end of the cylinder and discharged through the residue
nozzle.
The distillate is collected at the lower end of the condenser.
The non condensable gas and some vapour are sucked through the condenser
and leave the apparatus also at the bottom.
For best heat transfer and circulation the material is distributed and wiped by
means of a rotor armed with elements contacting the film.
Blades made from PTFE (polytetrafluoroethylene as appropriate reinforced with
glass fibre) or made from steel, pushed by centrifugal force into the film are very
often in operation
The film is circulated in a bow wave in front of the blade
Rollers made from PTFE, loosely held on vertical rods and pushed by centrifugal
force into the film have excellent self-cleaning properties.
The rollers slide over the film surface
Simplicity : recommended for reactive products as polymers.
The distillation task determines the required evaporator size
Pilot plant testing is often necessary to quantify
A typical evaporator related feed rate is 100kg/(m2×h).
Depending on the product (viscosity, surface tension etc.) and the goal of the
distillation this value might be magnified by up to factor 3 or shrink by factor 0.3.
In the latter : diffusion limitation (when removing of a little amount of
remaining lights)
Blocks made from PTFE and profiled can be used to control the exposure time
of typically 3–180 s.
Depending on the orientation of the slots the thin film is pushed upwards or
downwards.
The rotor can be (totally or partially) equipped with an area covering splash
guard.
In this way any droplet carry-over from the evaporator to the condenser is well
avoided.
Special wiping elements as, for example high-speed swinging blades for the
drying of products are not commonly used as hardly any applications may
change : in the recovery of high boiling valuables from wastes
SPDU flowsheet
Simple fractionation
The produced vapours do not have contact with the liquid anymore.
The number of theoretical stages is 1 at a small distillate rate and may rise upto
2.2 at a higher evaporation rate (this is due to the composition change of the
thin film along the evaporator height)
Although the vapour pressure ratio may increase with decreasing boiling
temperature, the separation might not be sufficient
To increase the separation efficiency : A first short path evaporator is used to
distill off a fraction that meets the quality demands, but at a poor yield; too
much of the light fraction remains in the residue
This residue is then distilled in a second still where the light fraction is removed
completely unfortunately together with heavier components
This mixture is then recycled into the feedstream to the first short path
evaporator.
Application
Heat sensitive products are distilled under vacuum.
Besides the thermal sensibility also the viscosity, respectively the solid content of the material
influences the type of deployed evaporator.
To recover solvents from waste a batch evaporator might do the job
Natural or forced circulation evaporators are often used to concentrate products that tend to scale
on hot surfaces; the material is heated first and then evaporated by flashing.
A falling film evaporator consists of a vertically arranged bundle tube heat exchanger; it allows to
process upto huge quantities of material most often waste water as, for example mash from a
distillery.
The wiped film evaporator (grey) has a very wide field of applications only limited by the
investment costs. Equipped with special wiping elements it can also be used as dryer.
For gentle treatment the short path and molecular distillation (light grey) apply.
Application examples
Lactic acid is a polymer precursor for biodegradable plastic(grocery bags) but is also used in
pharmaceuticals, cosmetics and food applications.
After the fermentation the water is removed by falling film and/or wiped film evaporation
In the following the lactic acid has to be distilled overhead.
This is achieved at about 1 mbar in a short path still.
The purity of the lactic acid is of greatest importance.
The colour shall remain below a few Hazy even after exposure to heat.
To avoid any splashing of liquid material onto the condenser, and thus into the product, the
wiper system of the short path evaporator is equipped with special area covering splashguards.
Paraffin wax is required for candle-making , coatings of papers or cloths, modification of
bitumen, ingredients for cosmetics and even for chewing gum.
Wax fractions of distinct properties are produced by short path distillation.
The feedstock is distilled in a plant with two (or more) stages:
(i) separation of volatiles and
(ii)overhead distillation of the product.
Apart from the volatiles the feedstock and the product streams are solid at room temperature.
Such a plant requires adequate heat tracing of all piping.
Monoglycerides are used as emulsifier in the food industry (ice cream, bread etc.), in
pharmaceutical applications (toothpaste)
The reaction product is a mixture consisting of mono-,di and triglycerides.
Himo (High concentrated monoglyceride) can be produced by short path distillation.
The temperature is still low enough thus avoiding back reactions.
The material is degassed first to remove light ends like glycerol and fatty acids
Monoglyceride is distilled overhead in a subsequent step.
Di- and especially triglycerides are heavier and remain in the residue.
Himo is produced at about 200∘C and approximately 0.02 mbar.
Monoglyceride concentration
Omega-3 fatty acids as DHA (docosahexaenoic acid) and EPA (eicosapentaenoicacid) are known
for their wholesome impact on blood circulation.
Eskimo people do not often suffer heart attacks despite of the fatty nourishment. This is due to
the Omega-3 fatty acids present in the polar fish.
As the glycerides cannot be distilled they are first converted into methyl-and/orethyl-esters
Distillation is achieved at an ultimate pressure of approximately 0.01mbar and at medium
thermal oil heating temperature
Fractions with a high concentration of Omega-3 fatty acids are obtained
After the distillation the esters are converted back into glycerides before being filled, for
example into capsules.
Tall oil consists of several components: some moisture and lights, fattyacids, resin acids and
heavies.
All these components can be further used in different applications.
The lights can be separated in a falling film and/or wiped film evaporator. In a consecutive step
fatty and resin acids are distilled overhead.
The remaining residue solidifies immediately after discharge.
Monomers can be fractionated by short path distillation. To avoid any chemical reactions the
heating temperature might be low so that warm water is preferably used.
The purification of amides as many other products are performed in a typical two-step
operation: removal of light impurities in a wiped film evaporator and overhead distillation of the
product in a short path still. Heavy impurities remain in the residue
New Developments
Efficiency, the further reduction of light components in residual products, and the enhancement
of through put
The removal of a solvent from a valuable product can be achieved by short path distillation.
Driving force is the low pressure.
By reducing the partial pressure of the already condensed distillate the content of lights in the
residue product can further be minimised.
This can be achieved by use of chilled water instead of cooling water.
Another development consists of moistening or washing the condenser with a
liquid which is circulated
The vapours are condensed, mixed in the liquid,and the partial pressure
decreases accordingly.
The liquid has to be processed before being recycled. Essential larger quantities
of material can be treated with a newly developed plate molecular still
It is a combination of plate-falling film evaporator and short path still
In a vacuum chamber heated and cooled plates are fixed alternately; The feed is
equally distributed at the top of the heated plates.
The produced vapours are condensed on the cold surfaces.
At the bottom of the still distillates and residues are collected separately and
discharged.
Main advantages: no rotating parts and a large evaporation area per volume
vacuum chamber
Principles of plate molecular still
Reference
Bethge, D. (2010) Innovative Technik für die Molekulardestillation. Vakuum in Forschung und
Praxis, 22 (1), 30–31.
Short path distillation
Molecular distillation is a special liquid-liquid separation technology, which is different from the traditional
distillation principle according to boiling point difference.
It depends on the difference of molecular mean free path to separate different matters.
Molecular mean free path means the path between molecules striking two times. When the mixture liquid
flows along the heating board and are heated, light and heavy molecule will become into vapor phase.
Light and heavy molecule will have different moving distance when they are in vapor phase because they
have different mean free path.
Supposing that we put a condensation board at the proper position, Light molecule can reach the board
and will be condensed, then be guided along the condensation board; while the heavy molecule cannot
reach the condensation board and will be guided along the heating board. In this way, the objective of
separation has been made.
It lowers the boiling temperature and is an excellent method for gentle thermal treatment
of heat sensitive, high boiling products.
The Short Path Evaporator consist basically, of a cylindrical body with a heating jacket, a
rotor and condenser inside.
The vapor passes by the shortest route and with practically no pressure drop to the
internal condenser where it is precipitated on the tubes.
The non-volatile portion reaches the lower part of the evaporator and leaves it through
the bottom product outlet. The residual vapor and inert gases flow through the vacuum
nozzle to the vacuum system.
On the supporting structure of the rotor mobile precision wiper blades are mounted.
Application
Molecular distillation, also called short-path distillation, is particularly useful in isolating components of
heat-sensitive liquids or in separating substances with very high boiling points.
Oil and Fat Industry:
Separation of free fatty acids from vegetable oil Fractionation of tall oil
Concentration of glycerol Isolation of monoglyceride
Purification and deodorization of fish oil
Petrochemical Industry:
Separation of microcrystalline wax out of crude oil high vacuum pump residue
Fractionation of synthetic and petrochemical waxes to soft-and hard waxes
Applications
Environmental care:
Purification of spent lubricants, brake fluids, glycerol and transformer oils
Recycling of dimethylsulfoxide (DMSO) mother liquors ;Processing of mother liquors in pharmaceutical
industry and the production of advanced organic intermediates
Waste water concentration
Solvent recovery
Chemical Industry:
Purification of polyphenol compounds
Separation of aromatic amines from oxidized components and by products
Separation of long chain acid chlorides from impurities and free acids
Solvent separation from silicon oils
Concentration and purification of esters
Purification of organic esters
Concentration of herbicides and insecticides
Purification of candelilla wax and other natural waxes
Food Industry:
Purification Elimination of terpens and concentration of ethereal oils
Separation of solvents from flavors Concentration of juices or extracts Concentration of tocopheroles
Concentration and isolation of lactic acid Concentration of capsicum
Pesticides produce:
Purification and refining of pesticides and its intermediate
Polymer Industry:
Purification and concentration of monomers Purification and concentration of polymers Purification of
softeners
Painting industry:
Subtractive solvent of polyurethane partial polymer and separation of TDI monomer
Cosmetic industry:
Producing wool alcohols and wool acid of sheep oil Wiping off pesticides of cosmetic substrate
Perfume compound and perfume material:
Purification of synthetic and natural perfume compound and perfume material
Pharmaceutical and Food Industry:
Separation and purification of the valuable product from solvent and by products
Separation of extracting solvent from plant and herbs extracts
Volume reducing of fermentation broth
Concentration of sugar / Glycose solutions and fruit pulps.
Characteristics
Operation temperature is low (lower than boiling point), with reduced pressure (lower than
1Pa), heating time is short (several seconds) and the efficiency of separation is high.
Removing odor, color and impurities of mixture.
The course of separation is physical, preventing separated matter from pollution, especially
keeping original quality of natural extraction.
The separation degree is higher than the traditional distillation.
Operating at a process pressures in the range of 0.001 bar.