## Short Path Distillation: A Deep Dive (Around 1500 Words) Short path distillation (SPD) is a highly versatile technique for purifying and separating heat-sensitive compounds. Unlike traditional distillation methods, SPD operates under reduced pressure, significantly lowering the boiling points of liquids. This allows for the gentle and efficient purification of volatile organic compounds (VOCs), pharmaceuticals, natural products, and other delicate materials that might decompose at high temperatures. This in-depth exploration delves into the world of short path distillation, covering its principles, components, operational procedures, applications, and advantages. ### Unveiling the Principles: The Science Behind Short Path Distillation The core principle of SPD lies in **manipulating pressure** to achieve lower boiling points. By reducing the pressure within the distillation chamber, the energy required for a liquid to transition into a vapor state decreases. This allows for the distillation of heat-sensitive compounds at much lower temperatures compared to traditional methods at atmospheric pressure. Here's a breakdown of the key factors at play: * **Vapor Pressure:** Every liquid has a specific vapor pressure, which is the pressure exerted by its vapor when in equilibrium with the liquid at a given temperature. As temperature increases, the vapor pressure also rises. * **Reduced Pressure:** In SPD, a vacuum pump is employed to significantly lower the pressure inside the distillation chamber. This reduction in pressure directly impacts the vapor pressure of the liquid mixture. * **Lower Boiling Points:** With a lower vapor pressure required for boiling under reduced pressure, the boiling point of the liquid mixture also decreases. This allows for the gentle distillation of components without subjecting them to high temperatures that could potentially lead to decomposition. The efficiency of SPD hinges on two additional factors: * **Fractional Distillation:** Short path distillation often incorporates a **fractionating column** packed with a specialized material like glass beads or ceramic rings. This column provides additional surface area for vapor-liquid contact, enhancing the separation of different components within the mixture based on their relative boiling points. * **Condensation:** The vaporized components travel upwards in the distillation flask and encounter a condenser cooled by a circulating coolant, typically water or a chilled liquid. As the vapors come into contact with the cold condenser, they condense back into their liquid forms and collect in a receiving flask. ### The Anatomy of an SPD System: Essential Components A short path distillation system comprises several key components working in concert to achieve efficient purification: * **Distillation Flask:** This round-bottomed flask, typically made of borosilicate glass due to its heat resistance and chemical compatibility, holds the liquid mixture to be dis

1. Reactive and catalytic distillation
• Reaction and separation are performed one by one.
• Reactants reacts and products, byproducts and inerts are
separated later.
• By combining reaction and separation of product in one
equipment can get different advantages:
(1) Removal of product from the reaction mass at the reaction
conditions increases equilibrium conversion of reactant or
increases the extent of reaction.
(2) In reactive distillation, change in composition of the reaction
mass is made by continuously removing the product from the
reacting mass. To nullify the effect of this change, reaction
proceeds in the forward direction and produces more and
more products which improves equilibrium conversion.
(3) Separate equipment for separation is not required also
separate piping and instrument are not required.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

2. Commonly used techniques to combine reaction and separation
(1) Reactive distillation (Reaction + distillation)
(2) Membrane reaction (Reaction + Membrane
separation)
(3) Extractive reaction (Reaction + Extraction)
(4) Reaction with absorption (Reaction +
Absorption)
• In reactive distillation reaction and distillation
both takes place simultaneously if catalyst is
used in distillation it is called as catalytic
distillation.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

3. • In manufacturing of ethyl acetate, reaction between ethanol and acetic acid
and separation of crude ethyl acetate from reaction mass by distillation were
carried simultaneously in one equipment.
• Ethanol was used as excess reactant.
• Overhead vapor from reactive distillation column is a ternary azeotrope
having composition 82.16% ethylacetate, 8.4% ethanol, 9% water by mass.
• On condensation azetrope is converted into two layers, ester rich layer and
water ethanol solution layer.
• Part of ester rich layer is taken out as top product (distillate) and
remaining is recycled back as reflux.
• Waste water (water + sulfuric acid) is continuously taken out as bottom
product.
• Alcohol-water stream is taken out as intermediate product.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

4. • If solid catalyst is used, the same catalyst is
used as packing material of packed tower.
• The solid catalyst plays double role to enhance
the rate of chemical reaction as well as to
enhance the rate of mass transfer for
distillation.
• Reactive distillation which uses catalyst is
known as catalytic distillation.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

5. Disadvantages of Reactive distillation
(1) The processes which involve reaction and distillation,
both process will have an optimum condition. To
carry out both process simultaneously one has to
compromise either with reaction or with distillation.
(2) In the process of MTBE, optimum conditions for the
reaction and distillation are almost same. This can
be followed by reactive distillation.
(3) In the process of acetic acid by carbonylation of
methanol, reaction is carried out at in the presence of
homogeneous catalyst at 50 atm and 180 ˚C. After the
reaction, reactants, promoters and catalyst are
separated by distillation at near to atm pressure. In
this case reactive distillation cannot be applicable.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

6. Comparison of reactive distillation with conventional technology
• In conventional process first step is the
reaction step where reactants are in contact
with solid or liquid catalyst at appropriate
temperature and pressure.
• In second step, stream from the reactor is sent
to separation section where desired product is
separated from unconverted reactants,
catalyst, byproducts and inerts.
• When reactive distillation is applied in such
cases the flow sheet is changed.
• In the middle section reaction and distillation
takes place simultaneously while upper and lower
sections represent enriching and stripping section
of distillation or utilized for separation.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

7. Flow sheet of conventional process
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

8. Flow sheet of catalytic distillation
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

9. The advantage of reactive distillation can be explained by
considering its advantage over reaction and distillation step over
conventional process
(1) Advantage over reaction step
(i) For equilibrium limiting reaction, continuous
removal of one of the products, increases
equilibrium conversion and it converts
reversible reaction into irreversible reaction. It
provides high conversion.
(ii) In conventional process for exothermic and
endothermic reaction separate cooling and
heating is required. In reactive distillation heat
of reaction is utilized in separation or distillation
part.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

10. (2) Advantage over distillation step
(i) Advantage in capital cost reduction
(a) In reactive distillation associated piping, pump and
instrumets is not required which reduces cost.
(b) Heat of reaction reduces either condensation
duty or reboiling duty. Hence it reduces either
condenser size or reboiler size.
(c) In reactive distillation less amount of unreacted
reactant needs to be separated. It reduces number of
stages required in distillation column for same value of
reflux compared to conventional process. It reduces size
of condenser and reboiler.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

11. (2) Advantages related to reduction in operating cost
(i) Heat of reaction reduces either condensation
duty or reboiler duty, hence it reduces either
load of cooling system or the same of heating
system which reduces operating cost.
(ii) Operating cost required to reduce recycling of
reactants is reduced considerably.
(iii) Lesser amount of reflux is required which
decreases condenser heat duty, reboiler heat
duty and power required for pumping flux.
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

12. • Various contact devices used for catalytic distillation
(1) Tray column (Bed of catalyst placed either in
downcomers or above the trays)
(2) Packed tower (using random packing which
may contain catalyst)
(3) Packed tower (Using structural packing with
catalyst particles), catalyst particles are
sandwiched between supporting material.
(i) Bale packing
(ii) Structured packing (KATMAX)
Górak, A., and H. Schoenmakers eds.,
“Distillation: Operation and Application, ”

13. “Short Path Distillation”
Prepared by : Vidur P Sosa

14. • Short path distillation is a technique by which heat
sensitive chemicals can be distilled efficiently.
• Short path distillation is usually performed under
vacuum at a pressure less than 0.001 mbar
• It is a process in which distillation apparatus consist of
condenser and evaporator fitted in a single unit.
• The distance between evaporator and condenser surface
is only of few centimeters.
• Hence it provides lowest distillation temperature and
short residence time.
• Thus reduces possibility of thermal degradation of
product
Introduction

15. The principle of short path distillation can be understood from the figure below:
Principle
Fu, Z. J., et al., “Technology Research on Molecular Distillation Separation of Renewable Waste Lubricating Oil, ” Advanced Materials Research, 726, pp. 2979-2982
(2013).

16. 1. Residue Discharge
2. Heater jacket
3. Roller wiper
4. Space under vacuum
5. Internal condenser
6. Feed
7. Geared motor
8. Heating medium inlet
9. Heating medium outlet
10. Exhaust
11. Cooling medium inlet
12. Cooling medium outlet
13. Distillate discharge
Construction

17. • Feed is continuously fed at top through a rotating distributor plate.
• Distributor is rotated using motor.
• Distributor plate distributes feed uniformly on the heat transfer surface.
• Feed flows down on heat transfer surface by gravity and gets heated.
• Heat required can be provided by thermic fluid, steam or hot water.
• This heat causes partial vaporization of feed.
• Vapor travel through the distillation gap to condenser surface where heat is
removed by cooling coils.
• Cooling medium can be cooling water, chilled water, oil or brine.
Thakore, S. B., and B. I. Bhatt, “Introduction to Process Engineering and Design, ” 7th ed., McGraw-Hill. India, pp. 493-500(2007).
Working

18. • There are mainly two types of short path distillation units based on force used for
distribution of liquid feed:
1. Falling film short path distillation unit; which uses gravitational force to allow
the liquid to be distributed as a thin film on a vertical vaporizer surface .
2. Centrifugal short path distillation unit; which uses centrifugal force generated
by rotor to promote the formation of thin film.
• The separation principle of both type of distillation unit is same
• There is also a third kind of short path distillation unit based on working;
– Reactive short path distillation unit; which is a combination of reactor and a
short path distillation unit
– Reactive short path distillation can be of both; falling film or centrifugal unit
type.
Types of short path distillation units.
Górak, A., and H. Schoenmakers eds., “Distillation: Operation and Application,,” 1st ed., Elsevier Inc., Oxford, U.K., pp. 368-375 (2014)

19. • The parameters affecting the molecular diffusion are feed and condensation
temperature, feed rate, pressure and concentration of feed.
• With increase in temperature and decrease in pressure, the separation increases.
• By increasing the temperature, product purity increases but its yield decreases.
Kulkarni, S. J., “Molecular Distillation: A Review on Applications and Affecting Parameters, ” International Journal of Research (IJR),
2(1), pp. 310-313 (January 2015).
Factors affecting short path distillation

20. • Separation of fat soluble vitamins.
• Grape seed oil deacidification.
• Separation of fatty acids from vegetable oil.
• Increasing citral concentration in lemongrass oil.
• Removing phthalates from sweet orange oil.
• Separation of waxes, fats and natural oils.
• For fish oils, petroleum residues, solvent removals.
• In fractionation of milk fat.
• For enrichment of carotene.
• For recovery of polyphenols, tocopherols and sterols from deodorizer distillate.
-Górak, A., and H. Schoenmakers eds., “Distillation: Operation and Application, ” 1st ed., Elsevier Inc., Oxford, U.K., pp. 368-375 (2014).
-Thakore, S. B., and B. I. Bhatt, “Introduction to Process Engineering and Design, ” 7th ed., McGraw-Hill, India, pp. 493-500 (2007).
Application

21. • Polyphenols are identified as the natural compounds which impart antioxidant
property and thus improve shelf life.
• During physical refining of rapeseed oil, polyphenols gets separated and
concentrates in the distillate along with fatty acids, tocopherols, sterols, etc.
• Thus oil obtained as bottom product is lean in polyphenols and hence lags in
antioxidant property.
• So in order to obtain natural antioxidant, deodorizer distillate is treated in short path
distillation to separate them from free fatty acids which act as oxidants.
Short path distillation for recovery of Polyphenols from
deodorizer distillate
Meyer, F., et al., “Application of short path distillation for recovery of polyphenols from deodorizer distillate, ” European Journal of Lipid
Science and Technology, 113 11), pp. 1363-1374 (November 2011).

22. • The effect of various parameters was studied experimentally which is stated as
follows:
– At 110°C about 43% of the material was evaporated to the distillate phase
whereas at 170°C the total distillate yield was approximately 86%.
– A separation factor near one is equivalent to an inefficient separation.
– Separation efficiency between FFA and tocopherols increases with decrease in
evaporation temperature with decrease in distillate rate.
– Whereas when experiment were carried out for different feed rates (100-300
mL/h) at 110°C no significant change was observed
Meyer, F., et al., “Application of short path distillation for recovery of polyphenols from deodorizer distillate, ” European Journal of Lipid
Science and Technology, 113 11), pp. 1363-1374 (November 2011).
Short path distillation for recovery of Polyphenols from
deodorizer distillate

23. • Advantages
– Can be used for thermally sensitive substances.
– Components with high boiling points can be distilled easily due to high
vacuum.
– There is minimum pressure drop in the system due to short distance between
vaporizer and condenser unit.
– It requires relatively less maintenance.
– High purity of end products is obtained.
• Disadvantages
─ high vacuum is required for carrying out distillation.
─ High operating cost.
─ If residence time increases, than thermal decomposition of heat sensitive
material may occur.
─ The starting natural form is sometimes lost during process.
-Rizvi, S. H. ed., “Separation, Extraction and Concentration Processes in the Food, Beverage and Nutraceutical industry, ” 1st ed.
Woodhead Publishing Limited, Cambridge, U.K. (2010)
-Thakore, S. B., and B. I. Bhatt, “Introduction to Process Engineering and Design, ” 7th ed., McGraw-Hill, India, pp. 493-500 (2007).
-http://www.avta-us.com/basics.html referred on 29/09/2015
Advantages and Disadvantages

24. Pressure Swing Distillation
– Although known from good old days, only now pressure swing distillation
technique is gaining popularity for separation of a binary azeotrope over
heterogeneous azeotropic distillation and extractive distillation.
– The chief reason for not finding many industrial applications till date was the
lack of experimental VLE data for binary systems at different pressures.
– Two main advantages of pressure swing distillation are; energy saving by heat
integration of the system and no need to add an entrainer or a solvent for the
separation.
– The technique is based on the principle that the composition of almost all
azeotropes varies with the operating pressure.
– In industrially important water-solvent mixtures, the water content of the
azeotrope increases with increasing pressure.
– This variation provides a technique for separation of the components.
– A shift in composition of azeotropic mixture with respect to pressure can be
well understood by studying Figs. 8.52(a) and (b).
Thakore, S. B., and B. I. Bhatt, “Introduction to Process Engineering and Design, ” 7th ed., McGraw-Hill.
India, pp. 493-500(2007).

25. – Acetonitrile—ACN (also called methyl cyanide; formula: C2H3N) forms a
minimum boiling azeotrope with water at 1 atm and 77°C containing 83.5% by
mass or 69.0% by mole ACN.
– This azeotrope composition shifts to 60.0% by mole ACN at 3.02 bar a which
boils at 112.5°C.
– Using these data a two-column system can be designed to operate at two
different pressures to separate ACN and water from the mixture.
– Feed with lower concentration than 69.0 mole % ACN can be fed to the first
column operating at 1 atm.
– Distillate from the column with near azeotropic composition is the feed to the
second column operating at 3.02 bar a.
– Bottom products from the first and second column are water and ACN,
respectively.
– Distillate from the second column is recycled to the first column along with the
fresh feed.
Thakore, S. B., and B. I. Bhatt, “Introduction to Process Engineering and Design, ” 7th ed., McGraw-Hill. India, pp.
493-500(2007).

26. – An important point to note is that in heterogeneous azeotropic distillation,
bottom product from the first column is the solvent (low boiling component)
while in pressure swing distillation, bottom product is water (high boiling
component).
– In pressure swing distillation since both the columns operate at different
pressures, their heat exchangers can be readily thermally integrated.
– This will result in substantial savings in energy for distillation.
– However, such an integration results in complexity of the system which
requires adequate process control and instrumentation.
– Also hazards of handling flammable (and toxic) solvents at high pressure must
be given due considerations while designing such a system.

28. Thank You